EGU General Assembly 2022  

The physical cause, or origin, or generation of plate tectonics, especially how it arises from a convecting mantle on Earth (and not apparently on our solar system's other terrestrial planets) is one of the big questions in geophysics, and has  haunted (or taunted) the author  for the last 30+ years.  Although he's tried to drive the question to basic physical causes for plate boundary formation in a cold stiff lithosphere,  he's certainly taken his share of wrong turns.  His earliest attempts to understand these processes from fluid lab experiments (while a postdoc at WHOI) only achieved (1) making gallons of fluids that look much like mucus, and (2) proof that he was a lousy experimentalist.  But in the intervening decades, he's burrowed deeper into smaller and smaller scales to understand how microscale physics of mineral grains influence plate boundary formation at large scales.  This led to the most recent theory of grain damage that allows for formation of weak boundaries, corresponds to field and laboratory observations of mylonitic behavior, and has applications from the onset of early plate tectonics, to passive margin collapse, to slab segmentation and necking.   The most recent theory incorporates how mineral phases (olivine and pyroxene) mix with each other at the grain scale, and this has allowed a close comparison to new rock deformation experiments on grain mixing and shear localization, which opens up many new questions and predictions for more experiments and observations.  

How to cite: Bercovici, D.: Searching for the origin of plate tectonics, leaving no grain unturned, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9705, https://doi.org/10.5194/egusphere-egu22-9705, 2022.

Earthquakes provide a crucial way of probing the deformation style, strength, and stress state of the lithosphere.  In this talk, I will outline ways in which we can use careful analysis and precise seismological observations of earthquakes, particularly those at moderate magnitudes (M ~5-6), to map out how stress is supported in the lithosphere, and how the rheology of the lithosphere can vary in both space and time, summarising our current understanding of the controls on the distribution of earthquakes.  I will draw on examples from a range of regional studies, and outline what conclusions we can draw about the geological and geodynamic controls on the distribution of earthquakes in each region, and the variation on the style of deformation within the lithosphere.  I will also discuss areas in which our current understanding of the distribution of earthquakes remains unable to explain some observations, and challenges for the future.

How to cite: Craig, T.: Probing the rheology of the lithosphere using earthquake seismology, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13373, https://doi.org/10.5194/egusphere-egu22-13373, 2022.

This presentation describes the structure and morphologies associated with seafloor spreading in the Red Sea inferred from bathymetric, gravity, magnetic and seismic data. We show that the orientation of the structures is consistent with an Arabia-Nubia Euler pole located within the 95% confidence of Ar-Rajehi et al, (2010) Euler pole and with the tectonic model initially proposed by Girdler (1984). At the Red Sea scale, our model shows that a spreading axis extends along its entire length, even though it is mostly covered by allochthonous Middle Miocene salt and Late Miocene minibasins flowing inward from the margins. In the northern Red Sea, oceanic basement is only exposed through small windows within the salt, forming a series of deeps. The seafloor segments symmetrically bisect the new ocean in the south. Right-stepping transform faults that cluster near Jeddah, Zabargad and Ikhwan Islands offset the ridge axis as spreading is getting more oblique towards the Euler Pole. The northern, central and southern Red Sea segments display a well-developed mid-ocean ridge flanked by landward-dipping volcanic basement, typical of slow spreading ridges. In the northern magma poor spreading segment, mantle exhumation is likely at the transition between continental and oceanic crust. Transpression and transtension along transform faults accounts for the exhumation of the mantle on Zabargad Island as well as the collapse of a pull-apart basin in the Conrad deep.

We propose a new structural model for the Red Sea constrained by the geodetic rules of tectonic plates movements on a sphere. Finally, we discuss the effect of the Danakil microplate on the ridge morphology and show that the Arabia-Nubia-Danakil triple junction is likely located further north than previously described, around 18±0.5°N, where we observe a shift in the ridge axis orientation as well as in the spreading orientation.

How to cite: Delaunay, A., Alafifi, A., Baby, G., Fedorik, J., Tapponnier, P., and Dyment, J.: Structure and Morphology of the Mid-Ocean-Ridge in the Red Sea, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1405, https://doi.org/10.5194/egusphere-egu22-1405, 2022.

How to cite: Jolivet, L., Allanic, C., Becker, T., Bellahsen, N., Briais, J., Davaille, A., Faccenna, C., Lasseur, E., and Romanowicz, B.: Continental rifts and mantle convection: Insights from the East African Rift and a new model of the West European Rift System, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1696, https://doi.org/10.5194/egusphere-egu22-1696, 2022.

Corrugated detachments are fundamental crustal structures found in many extensional systems and plate tectonic boundaries, including mid-oceanic ridges and rifted margins. Direct observations of the complete geometry of extensional detachments are rare and our understanding of detachment fault structures and the mechanisms of development of high-angle normal faults and their rotation to lower angles mainly relies on proxy observations, for example seismicity trends, and numerical modelling.

We present interpretations of a high-resolution 3D seismic reflection survey from the hyperextended domain of the Porcupine Basin, Offshore West of Ireland. The 3D data image a highly reflective corrugated surface, the P reflector, that we interpret as an extensional detachment preserved in its slip position that likely developed at the top mantle surface during Jurassic hyperextension of the basin. Within the 3D data, the P reflector covers an area 95 km long and 35 km wide and has a domal shape that is elongate in the N-S direction with a crest at ~6.3 s two way travel time. It is the first time to our knowledge that 3D seismic data has imaged a complete detachment in the hyperextended area of a rifted margin, including its domal shape, the breakaway structures, and the linkage between the steep and shallow segments of the detachment. The resolved texture and geometry of the detachment and its relationship with overlying faults provide a basis for refining current models of detachment formation accommodating extreme extension.

Steep west-dipping faults mark the western frontal margin of the detachment. The steep faults pass eastward into shallower, predominantly west-dipping faults that appear to merge downwards with the P reflector. The P reflector has pronounced E-W corrugations, interpreted to indicate the detachment slip vector. The reflector is also characterised by abrupt changes in dip across N-S transverse ridges. These ridges are spaced on average 10 km apart, they coincide with lines of intersection between the P reflector and large overlying faults, and they often mark the termination of detachment corrugations. We interpret these ridges as recording former locations of the western boundary of the detachment so that they indicate a step-wise westward propagation of the P reflector. While it is generally accepted that detachments develop by oceanward propagation, we suggest that the faceted nature of the detachment indicates that this process is a punctuated one and that the clearly imaged transverse ridges record the oceanward stepping of the detachment with the initiation of a new family of steep faults.

We propose a new concept for the growth of detachments that may be applicable to other detachments that accommodate extreme extension, for example at mid-oceanic slow and ultra-slow spreading ridges.

How to cite: Lymer, G., Childs, C., and Walsh, J.: Punctuated propagation of a corrugated extensional detachment offshore West of Ireland, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1799, https://doi.org/10.5194/egusphere-egu22-1799, 2022.

The Afar Depression forms a triple junction between three rift systems: the Red Sea Rift, the Gulf of Aden Rift and the Main Ethiopian Rift. Rifting began in the Oligocene after the eruption of the Ethiopian Flood Basalts. It represents a unique modern example of hotspot-influenced continental breakup. Its emerged position allows detailed field and remote sensing investigations. Important mapping efforts in the area during the 60s and 70s provided very valuable input for the understanding of the local geology but also for the development of global tectonic, volcanological and sedimentary concepts in continental rift settings.

This study presents the compilation of a new geological map which covers the complete Afar depression and includes its Phanerozoic sedimentary and magmatic cover. The map is based on extensive literature research, remote sensing and fieldwork. The geological history of the Afar Depression has also been reviewed. The map evidences the complexity of the rift system with the interaction of distinct tectonic plates, blocks, rift segments, sedimentary basins and volcanic areas that evolve through time and space. This integrative geological map and review is used to reassess and discuss aspects of the style, evolution, kinematics and dynamics of this rift system. Studying this unique modern example of active rifting will help in the better comprehension of rift processes and passive margin development worldwide.

How to cite: Rime, V., Foubert, A., Atnafu, B., and Kidane, T.: New mapping of the Afar Depression: towards the better understanding of rift dynamics in a hotspot-influenced continental rift zone, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2208, https://doi.org/10.5194/egusphere-egu22-2208, 2022.

Rifting in Ethiopia is predominantly driven by magmatic intrusion into the rifting crust. Unravelling the dynamics of lithospheric melt migration and storage is paramount to understanding the late-stage development of continental rifts. In particular, extensive geophysical observations of the structure and composition of rifting crust must be supported by petrology to provide a complete picture of rift-related magmatism. We present major element, trace element, and volatile element compositional data for olivine-hosted melt inclusions from the Boku Volcanic Complex (BVC), a monogenetic cone field in the north Main Ethiopian Rift. Through combined CO₂-density-calibrated Raman spectroscopy and secondary ion mass spectrometry we assess the total CO₂ concentrations within the melt inclusions allowing us to estimate pressures of entrapment via CO₂-H₂O solubility models. Our results show that primitive BVC melts carry up to 0.58 wt% CO₂ (mean ~0.2 wt%), with as much as half of the CO₂ in the melt inclusion present within shrinkage bubbles. Volatile solubility models suggest that these melts are stored over a narrow range of depths (10-15 km), consistent with geophysical data and implying the existence of focussed zone of magma intrusion at mid-crustal depths. The expansive range of trace element concentrations in the inclusions illustrate that, at the time of entrapment, compositional heterogeneity remains extant, and melts must therefore be stored in discrete magmatic bodies with limited mixing. Our results have implications for understanding the interplay between magma intrusion and extensional tectonics during continental break-up, such as magmatic compensation of crustal thinning and the thermo-mechanical effects of melt emplacement into the rifting crust.

How to cite: Wong, K., Ferguson, D., Wieser, P., Morgan, D., Edmonds, M., Tadesse, A. Z., and Yirgu, G.: Petrological evidence for focussed mid-crustal magma intrusion in the Main Ethiopian Rift, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2290, https://doi.org/10.5194/egusphere-egu22-2290, 2022.

The continental deposits of the Triassic basins developed along the eastern margin of the Central and North Atlantic show a similar sedimentological evolution, as those of the western margin resulting from the interaction of various processes.

The examples chosen in this work are those of the Mohammedia-Benslimane-ElGara-Berrechid basin MBEB in the Moroccan meseta that we studied in detail in the field, and that we tried to compare with Portugal which is on the same East Atlantic margin.

At the begininig of the Mesozoic, the northwestern part of the African continent was affected by an initial fracturing associated with the early stages of the opening of the Central Atlantic (Atlantic rift) during which several Moroccan Triassic basins are open.

The Mohammedia-Benslimane-ElGara-Berrechid basin is part of the Moroccan western Triassic province, which corresponds to all the basins of the Moroccan Atlantic margin in direct relation with the Atlantic rift. In this basin, an asymmetric rift is set up on the old Hercynian structures during the Carnien-Norien, the paroxysm is reached at the Trias-Lias passage with the installation of basalts (CAMP: Central Atlantique Magmatic Province).

During rifting (syn-rift stage in the Upper Triassic), the MBEB basin experienced three major phases of sediment filling. The first phase is purely continental, the first deposits to arrive in the opening basin are of proximal fluvial origin. Subsequently, the decrease of the paleopente and the rise of the base level generated paleoenvironmental changes in the basin (2nd phase), and the deposition system evolved towards distal environments. During the third phase, the syn-rift sedimentary series recorded a marine incursion in the late Triassic with saliferous sedimentation. This marine intervention is deduced from the presence of a thick saliferous series with a large lateral extension and whose isotopic ratios of sulfur and bromine contents indicate their marine origin. These marine waters are probably of Tethysian origin and are also linked to the opening of the Proto-Atlantic.

In Portugal, the Upper Triassic is represented by two formations in the north of the Lusitanian basin (Palain, 1976): Silves Fm which is fluvial sandstone and Dagorda Fm which includes first dolomites and then evaporites. In this Portuguese basin, the proximal-distal fluvial transition took place at the Norien-Rhétien limit. This also rift-type basin was filled with continental fluvial and alluvial clastic rocks of the Silves Formation, largely derived from the adjacent Iberian highlands of the Meseta. Locally, black shales are present at the top of the Silves and may represent the first marine incursion into the basin.

The comparison between the two basins shows that they followed a similar evolution at the base and in the middle of the series but at the top the MBEB basin presented thick layers of evaporites while that of Portugal presented mainly dolomites attributed to paralic facies.

Palain, C., 1976. Une série détritique terrigene; 'les grès de silves'; Trias et Lias inférieur du Portugal. Mem. Serv. Geol. Portugal, p. 25 (377 pp.)

How to cite: Essamoud, R., Afenzar, A., and Belqadi, A.: Triassic sedimentation on the Eastern Atlantic margin: two examples from Moroccan Meseta and Portugal, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3259, https://doi.org/10.5194/egusphere-egu22-3259, 2022.

Multiple geoscientific studies along the Main Ethiopian and Eastern rifts have revealed that extension via magma intrusion now prevails over plate stretching as the primary mechanism for strain accommodation throughout the crust and mantle lithosphere. However, problematic in this picture is where the Main Ethiopian and Eastern rifts meet, across the low-lying, broadly-rifted, and as-yet poorly-studied Turkana Depression which separates the elevated Ethiopian and East African plateaus. We have so far revealed through body-wave tomography (Kounoudis et al., 2021), that the Depression does not lack mantle dynamic support in comparison to the plateaus, suggesting a significantly thinned crust, resulting from superposed Mesozoic and Cenozoic rifting, most likely explains its low elevations. Slow uppermost-mantle wavespeeds imply the presence of either melt-intruded mantle lithosphere or ponded asthenospheric material below lithospheric thin-spots induced by the region’s multiple rifting phases. To better illuminate the Depression’s lithosphere-asthenosphere system, we conduct a surface-wave analysis to image crust and uppermost-mantle structure using data from the NSF-NERC funded Turkana Rift Arrays Investigating Lithospheric Structure (TRAILS) project broadband seismic network. In particular, we investigate the presence of melt, whether the lithosphere is melt-rich, melt-poor, and/or if ponded zones of asthenosphere exist below variably thinned lithosphere. Group velocity dispersion curves, measured using data from local and regional earthquakes, yield the first high resolution fundamental mode Rayleigh-wave group velocity maps for periods between 4 and 40s for the Turkana Depression. In collaboration with the ongoing TRAILS GPS project, we explore how these results relate to present-day versus past episodes of extension.

Kounoudis, R., Bastow, I.D., Ebinger, C.J., Ogden, C.S., Ayele, A., Bendick, R., Mariita, N., Kiangi, G., Wigham, G., Musila, M. & Kibret, B. (2021). Body-wave tomographic imaging of the Turkana Depression: Implications for rift development and plume-lithosphere interactions. G3, 22, doi:10.1029/2021GC009782.

How to cite: Kounoudis, R., Bastow, I., Ebinger, C., Ogden, C., Ayele, A., Bendick, R., Mariita, N., Kianji, G., Musila, M., and Sullivan, G.: The Crust and Uppermost-Mantle Structure of the Turkana Depression: Insights from Surface-Wave Analysis, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4683, https://doi.org/10.5194/egusphere-egu22-4683, 2022.

During the formation of rifted continental margins, a rift evolves through a number of stages that produce major sedimentary basins and distinct rifted margin domains. While these domains have been classified based on the resulting structures and crustal thickness seen in geophysical data, the evolution of the fault network that produces these domains is not as well understood. Further, margin architecture may be influenced by erosion and sedimentation. Previous studies have qualitatively examined how faults respond to sedimentation during rifting, but there has not been a quantitative study on how variable surface processes efficiency affects fault network properties and the effect this has on rift evolution.

In this study we use a two-way coupling between the geodynamic code ASPECT (Kronbichler et al., 2012) and the surface processes code FastScape (Braun and Willett, 2013) to run 12 high-resolution 2D rift models that represent asymmetric, symmetric, and wide rift types (Neuharth et al., in review). For each rift type, we vary the surface process efficiency by altering the bedrock erodibility (K_f) from no surface processes to low (K_f = 10^-6 m^0.2/yr), medium (10^-5), and high efficiency (10^-4). To analyze these models, we use a novel quantitative fault analysis toolbox that extracts discrete faults from our continuum models and correlates them through space and time (https://github.com/thilowrona/fatbox). This toolbox allows us to track faults and their properties such as the number of faults, their displacement, and cumulative length, to see how they evolve through time, as well as how these properties change given different rifting types and surface processes efficiency.

Based on the evolution of fault network properties, we find that rift fault networks evolve through 5 major phases: 1) distributed deformation and coalescence, 2) fault system growth, 3) fault system decline and basinward localization, 4) rift migration, and 5) continental breakup. Each of these phases can be correlated to the rifted margin domains defined from geophysical data (e.g., proximal, necking, hyperextended, and oceanic). We find that surface processes do not have a large impact on the overall evolution of a rift, but they do affect fault network properties by enhancing strain localization, increasing fault longevity, and reducing the total length of a fault system. Through these changes, they can prolong rift phases and delay continental breakup with increasing surface process efficiency. To summarize, we find that surface processes do not change the overall evolution of rifts, but they do affect fault growth and as a result the timing of rifting.

Braun, J., and Willett, S.D., 2013, A very efficient O(n), implicit and parallel method to solve the stream power equation governing fluvial incision and landscape evolution: Geomorphology, v. 180–181, p. 170–179, doi:10.1016/j.geomorph.2012.10.008.

Kronbichler, M., Heister, T., and Bangerth, W., 2012, High Accuracy Mantle Convection Simulation through Modern Numerical Methods.: Geophysical Journal International, v. 191, doi:doi:10.1111/j.1365-246x.2012.05609.x.

Neuharth, D., Brune, S., Wrona, T., Glerum, A., Braun, J., and Yuan, X.P., (in review at  Tectonics), Evolution of rift systems and their fault networks in response to surface processes, [preprint], doi: https://doi.org/10.31223/X5Q333

How to cite: Neuharth, D., Brune, S., Wrona, T., Glerum, A., Braun, J., and Yuan, X.: Evolution of rift systems and their fault networks in response to surface processes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5758, https://doi.org/10.5194/egusphere-egu22-5758, 2022.

The palaeobathymetric evolution of rifted margins during continental breakup is complex. We investigate the subsidence of Late Triassic to Early Jurassic evaporitic sequences in the proximal and distal parts of the Scotian margin that formed during the opening of the Central Atlantic Ocean.

We use a 3D flexural backstripping technique, which incorporates decompaction and post-breakup reverse thermal subsidence modelling applied to key stratigraphic intervals through the Jurassic down to the Late Triassic base salt. The isostatic evolution of rifted margins depends on crustal thinning, lithosphere thermal perturbation and melt production during rifting and breakup. Quantitative analysis of seismic reflection and gravity anomaly data together with subsidence analysis have also been used to determine crustal thickness variations and ocean–continent transition structure, and to constrain the along strike variability in breakup related magmatism and crustal composition.

Reverse post-breakup subsidence modelling to the Late Triassic base salt restores this horizon at breakup time to near sea level in the proximal domains of the Scotian margin where the continental crust was only slightly thinned during rifting. In contrast, predicted palaeobathymetry of the base salt surface restored to breakup time is greater than 2 to 3 km in the distal parts of the margin where the continental crust was highly thinned (<10km) close to the ocean-continent-transition. One possible interpretation of this is that while the proximal salt underwent post-rift thermal subsidence only, the distal salt was deposited during the latest stage of rifting focused along the distal domains of the Scotian margin, where it underwent additional tectonic subsidence from crustal thinning. This observed difference between the subsidence of proximal and distal salt has been observed elsewhere on the South Atlantic margins (e.g., the Angolan Kwanza margin) and illustrates the complexity of the subsidence and palaeobathymetric evolution of distal rifted margins during breakup.

The deposition of Triassic evaporites occurred before and after the emplacement of the Central Atlantic Magmatic Province (CAMP). The impact of the CAMP on rifting, crustal structure and palaeobathymetric evolution of the Nova Scotia remains to be determined. We do not exclude an additional positive dynamic topography effect at breakup time related to the CAMP magmatic event.

How to cite: Tugend, J., Kusznir, N., Mohn, G., Deptuck, M., Kendell, K., Keppie, F., Morrison, N., and Dmytriw, R.: Palaeobathymetric evolution of the Nova Scotia rifted margin during the Central Atlantic Ocean opening, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5938, https://doi.org/10.5194/egusphere-egu22-5938, 2022.

Early Carboniferous igneous rocks are widespread in the Southern Urals. We have obtained new stratigraphic and isotopic data on plutonic and volcanic complexes, allowing us to determine correlation of their age and to construct a new geodynamic model.

The prevailing tectonic setting in the Southern Urals during the Early Carboniferous was sinistral transtension. Volcanic and plutonic complexes in transtensional zones were synchronously formed along large submeridional orogen-parallel strike-slip faults, but are particularly abundant within two N–S-trending zones: Magnitogorsk and East Ural.

The upper Tournaisian–lower Visean sequence in the Magnitogorsk zone consists mainly of moderately alkaline volcanic rocks, basalt and rhyolite are predominant, but pyroclastic, volcano-sedimentary, terrigenous, and carbonate rocks are also widespread. The middle Visean sequence consists of moderately alkaline basalt, andesite, dacite including lavas, tuffs and tuffites. The thickness of the Lower Carboniferous volcanic group varies from 1200 to 5500 m. The age of the volcanic rocks has been proved by findings of foraminifera in limestone interbeds. The oldest volcanic rocks appear in upper Tournaisian, while the youngest are found in the middle upper Visean. New U–Pb zircon dating using SHRIMP is now in progress.

Volcanic rocks in the East Ural zone occur within a few tectonic sheets. The sequence consists of lavas and tuffs of basalt, basaltic andesite, andesite and rhyolite. The total thickness of the sequence varies from 800 to 1500 m. The age of the sequence is determined by findings of fossil plants as middle Visean.

We studied eight plutons in the Magnitogorsk and six in the East Ural zones. Most of them record several intrusive phases. The composition of the rocks varies from gabbro to granodiorites and granites from normal to moderately alkaline series. We combined our new isotopic data on zircons (SHRIMP) with published ages and came to the following conclusions.

• Two main stages of Early Carboniferous plutonism can be distinguished in the Southern Ural. The first began simultaneously in both zones at the Devonian/Carboniferous boundary (ca. 356–357 Ma) and then changed to volcanic activity at around 346 Ma in the Magnitogosk zone and at around 340 Ma in the East Ural zone, respectively. The second stage began after the termination of volcanic activity and corresponds to 334–327 Ma interval in both zones. So, stages of active volcanism and plutonism alternate in time.
• Early Carboniferous rifting began with intrusion of plutons, usually associated with transtensional zones under oblique collision. The subsequent volcanic stage corresponds to local extension. The next stage of plutonism began just after volcanism termination and marked a cessation of tectonic activity.

The reported study was funded by RFBR and Czech Science Foundation according to the research project № 19-55-26009. Centre of collective usage ‘Geoportal’, Lomonosov Moscow State University (MSU), provided access to remote sensing data.

How to cite: Pravikova, N., Tevelev, A., Kazansky, A., Kosheleva, I., Sobolev, I., Borisenko, A., Koptev, E., Shestakov, P., and Žák, J.: Early Carboniferous rifting in the Southern Urals: New isotopic dating of plutonic and volcanic complexes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6172, https://doi.org/10.5194/egusphere-egu22-6172, 2022.

The East African Rift System (EARS) is the largest active continental rift on Earth. Inherited lithospheric strength variations have played a large role in forming the system’s current geometry. The partly overlapping eastern and western EARS branches encompass the large Victoria continental microplate that rotates counter-clockwise with respect to Nubia, in striking contrast to its neighboring plates.

Both the forces driving rifting in the EARS as a whole and the rotation of Victoria in particular are debated. Whereas some studies largely ascribe the rifting to horizontal mantle tractions deriving from plume-induced flow patterns (e.g., Ghosh et al., 2013), or to more equal contributions of mantle tractions and gravitational potential energy (e.g., Kendall and Lithgow-Bertelloni, 2016), recent work by Rajaonarison et al. (2021) points to a dominant role for lithospheric buoyancy forces in the opening of the rift system. Similarly, other numerical modeling (Glerum et al., 2020) has shown that Victoria’s rotation can be induced through drag of the major plates along the edges of the microplate transmitted along stronger lithospheric zones, with weaker regions facilitating the rotation, without the need for plume-lithosphere interactions (e.g., Koptev et al., 2015; Calais et al., 2006).

With unprecedented data-driven, regional spherical geodynamic numerical models spanning the EARS and the upper 660 km of mantle, we aim to identify the individual contributions of lithosphere and mantle drivers of deformation in the EARS and of Victoria’s rotation. Observational data informs the model setup in terms of crustal and lithospheric thickness, sublithospheric mantle density structure and plate motions. Comparison to separate observations of the high-resolution model evolution of strain localization, melting conditions, horizontal stress directions, topography and horizontal plate motions allows us to identify the geodynamic drivers at play and quantify the contributions of large-scale upper mantle flow to the local deformation of the East African crust.

Calais et al. (2006). GSL Special Publications, 259(1), 9–22.

Ghosh et al. (2013). J. Geophys. Res. 118, 346–368.

Glerum et al. (2020). Nature Communications 11 (1), 2881.

Koptev et al. (2015). Nat. Geosci. 8, 388–392.

Rajaonarison et al. (2021). Geophys. Res. Letters, 48(6), 1–10.

How to cite: Glerum, A., Brune, S., and Ben Mansour, W.: Geodynamic Drivers of the East African Rift System, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7155, https://doi.org/10.5194/egusphere-egu22-7155, 2022.

Fossil carbonate reefs are common along rifts and rifted passive margins. They provide valuable paleoecological and paleogeographical information. Moreover, porous reef buildups are targeted as potential oil and gas reservoirs and sites for gas storage.

The Red Sea and Gulf of Suez contain several generations of reef deposits: (1) syn-rift Early and Middle Miocene reefs that formed along the eroded footwalls of normal faults, and (2) post-rift Pliocene-Holocene coastal reefs that split apart, subsided, and aggraded to form carbonate platforms by salt-driven raft tectonics. The Late Miocene lacks reefs due to evaporitic conditions. This study focuses on the uplifted Early-Middle Miocene reef deposits, which outcrop sporadically along the Arabian and African margins of the Red Sea, particularly the northern half, over a distance of ~1000 km. They are exhumed along the coastal plain at elevations of 50-150 meters. We studied several reefs on the Arabian side and carried out age determination implementing a revised planktonic foraminifera zonation and paleoenvironmental interpretation. We also used satellite images to identify and map similar exhumed reefs on the African side.

The Miocene reefs are located along the eroded footwalls of normal fault scarps that form the first or second marginal half grabens, usually sitting unconformably over the basement. The flat reef and back-reef lagoonal facies are often removed by erosion, but the dipping thick fore-reef talus breccias are preserved. The breccias are an unsorted mix of coral reef and back reef debris and also contain basement clasts. The linear fore-reef talus deposits follow along the fault scarps, revealing paleo-valleys incised into the hanging wall. Placing the reef on the basin-scale helps us distinguish the tectonic influence, accompanied by climate and eustatic sea-level variation, on shallow marine carbonates during rifting.

Mapping all published, newly discovered, and inferred outcrops along the African and Arabian coast of the Red Sea allow us to develop a new tectono-sedimentary model for reef evolution in the syn-rift setting. The proposed model explains the absence of the reef outcrops in the southern areas of the Arabian Red Sea and predicts subsurface zones where reef growth possibly took place. Nature of the contact between reef carbonates and the underlying Precambrian basement in conjunction with the consistently preserved fore-reef zone disclose the uplift history and erosion events prior and post reef growth. In addition, following the reef distribution, we developed a syn-rift paleogeographic model of the Red Sea.

How to cite: Pensa, T., Afifi, A., Delaunay, A., and Baby, G.: Tectonic control on the reef evolution in the Red Sea syn-rift basin, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7186, https://doi.org/10.5194/egusphere-egu22-7186, 2022.

Within rifted margins, the necking domain corresponds to the area where drastic reduction in basement thickness leads the crust to attain a wedge-shape. The crustal thinning occurs along detachment fault systems typically recording displacements in the order of 10s of kilometers. These systems commonly shape the crustal taper and eventually the taper break, where crustal thickness is thinned to 10 km or less. In recent years, it has become clear that evolutionary models for detachment fault systems remain unsatisfactory as the well-known principles for smaller magnitude fault systems are not fully applicable to these large-magnitude systems. Consequently, the detailed responses in the foot- and hanging walls and associated basin sedimentation within detachment fault systems and necking domains remain poorly understood compared to those observed in extensional half-graben basins.

We use interpretation of 3D- and 2D seismic reflection data from the Mid-Norwegian rifted margin to discuss the effects of lateral interaction and linkage of extensional detachment faults on the necking domain configuration. We investigate how the structural evolution of these detachment faults interact with the effects of isostatic rollback to produce complex 3D geometries and control the configuration of the associated supradetachment basins. The study area demonstrates how successive incision may induce a complex structural relief in response to faulting and folding. In the proximal parts of the south Vøring and northeastern Møre basins, the Klakk and Main Møre Fault Complexes form the outer necking breakaway complex and the western boundary of the Frøya High. We interpret the previously identified metamorphic core complex within the central Frøya High as an extension-parallel turtleback-structure. The now eroded turtleback is flanked by a supradetachment basin with two synclinal depocenters resting at the foot of the necking domain above the taper break. We attribute footwall and turtleback exhumation to Jurassic-Early Cretaceous detachment faulting along the Klakk and Main Møre Fault Complexes. The study area further demonstrates how detachment fault evolution may lead to the formation of younger, successively incising fault splays locally. Consequently, displacement may occur along laterally linked fault segments generated at different stages in time. Implicitly, the detachment fault system may continue to change configuration and therefore re-iterate itself and its geometry during its evolution.

How to cite: Gresseth, J. L. S., Osmundsen, P. T., and Péron-Pinvidic, G.: Evolution of detachment fault systems within necking domains: insights from the Frøya and Gossa Highs, mid-Norwegian margin, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8003, https://doi.org/10.5194/egusphere-egu22-8003, 2022.

How to cite: Ferrante, G., Rivalta, E., and Maccaferri, F.: Spatio-temporal evolution of rift volcanism driven by progressive crustal unloading, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8663, https://doi.org/10.5194/egusphere-egu22-8663, 2022.

Marginal Seas are extensional basins formed in a convergent setting near active subduction zones. They are characterized by a short life (<25 Ma), as well as unstable and changing directions of seafloor spreading. However, the underlying processes involved in their formation from rifting to seafloor spreading initiation are still debated (supra-subduction convection/extension, slab-pull). This problem is further compounded by the fact that our understanding of continental breakup is primarily derived from the evolution of magma-poor and magma-rich Continent-Ocean Transitions (COT) of the Atlantic margins.

In this contribution, we characterize the tectono-magmatic processes acting during continental breakup by investigating the COT structures of three main Marginal Seas located in the Western Pacific, namely the South China Sea, the Coral Sea and the Woodlark Basin. All three examples formed under rapid extension rates and propagation of seafloor spreading. Although each marginal basin has its uniqueness, we show that these three marginal basins are characterized by a narrow COT (typically <~20 km), documenting the sharp juxtaposition of continental crust against igneous oceanic crust. The COT of the three basins shows that final extension is accommodated by the activity of one major low-angle normal fault. This extension is contemporaneous with important magmatic activity expressed by volcanic edifices, dykes and sills emplaced in the distalmost part of these margins. Such narrow COT suggests that a rapid shift from rifting to spreading.

The rapid localization of extensional deformation in a narrow area has major implications for partial melting generation. The evolution of extensional structures is controlled by the interplay of lithospheric thinning, asthenosphere upwelling and decompression melting. High extension rate prevents conductive cooling and lead to focus volcanic activity in a narrow area evolving rapidly in space and time to magmatic accretion. Causes for the fast extensions rates of Marginal Sea rifting are likely controlled by kinematic boundary conditions directly or indirectly controlled by nearby subduction zones. Such mode of breakup is probably not limited to marginal Seas but only enhanced in such settings.

How to cite: Mohn, G., Ringenbach, J.-C., Nirrengarten, M., Tugend, J., McCarthy, A., and Lei, C.: Continental breakup style of Marginal Seas, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8715, https://doi.org/10.5194/egusphere-egu22-8715, 2022.

Breakup volcanism along rifted passive margins is highly variable in time and space. The factors controlling magmatic activity during continental rifting and breakup are not resolved and controversial. Here we use numerical models to investigate melt generation at rifted margins with contrasting rifting styles corresponding to those observed in natural systems. Our results demonstrate a surprising correlation of enhanced magmatism with margin width. This relationship is explained by depth-dependent extension, during which the lithospheric mantle ruptures earlier than the crust, and is confirmed by a semi-analytical prediction of melt volume over margin width. The results presented here show that the effect of increased mantle temperature at wide volcanic margins is likely over-estimated, and demonstrate that the large volumes of magmatism at volcanic rifted margin can be explained by depth-dependent extension and very moderate excess mantle potential temperature in the order of 50-80 °C, significantly smaller than previously suggested.

How to cite: Lu, G. and Huismans, R.: Melt volume at Atlantic volcanic rifted margins controlled by depth-dependent extension and mantle temperature, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9420, https://doi.org/10.5194/egusphere-egu22-9420, 2022.

Permian-Triassic rifts of the West Siberian basin compose one of the largest continental rift systems in the world. The Koltogor-Urengoy and Khudosey rifts of meridional strike are the main structures in the eastern part of the basin and are filled mainly by basaltic lavas with clastic sediments. However, in the central part of the West Siberian plate felsic lavas are widespread along with mafic volcanics. Here we present the detailed data on composition of lavas, whole-rock geochemistry, geophysical features and U-Pb ages from the Frolov-Krasnoleninsky region in the central part of the West Siberian basin.

Within the studied region, Permian-Triassic rifts of NW and NE strike are predominant. The main structure is Rogozhnikov-Nazym graben of NW strike, composed of rhyolite-dacitic lavas.  According to the seismic data, this volcanic area comprises multiple local eruptive centers (1-5 km in diameter). Lavas constitute the major part of the volcanic pile, while tuffs are subordinate (up to 15%). Deep boreholes did not reach the base of volcanic sequence, but its thickness exceed 0.5 km.

The main geochemical features of the Rogozhnikov-Nazym volcanics are: 1) acidic composition and increased alkali content; 2) signs of supra-subduction setting: Ta-Nb and Pb anomalies; 3) high ratios of all incompatible trace elements. According to these features, volcanic rocks of the Rogozhnikov-Nazym graben were formed in the setting of post-collisional extension. Furthermore, coeval felsic lavas are widespread in smaller structures of the Frolov-Krasnoleninskiy region and demonstrate similar geochemical characteristics.

We obtained 9 U-Pb (SHRIMP) ages from felsic lavas of the Rogozhnikov-Nazym graben and other rift structures. All samples yielded ages in the range from 254±2 to 248.2±1.3 Ma (Late Permian – Early Triassic). Thus, volcanic activity in the Frolov-Krasnoleninsky region was nearly synchronous to the main phase of Siberian Traps magmatism in the Siberian platform.

Volcanic rocks of the Frolov-Krasnoleninsky region constitute rifts of NW strike (mainly felsic lavas, including the Rogozhnikov-Nazym graben) and NE strike (mainly mafic lavas, geochemically similar to the Siberian Traps basalts). We suggest that orientation of rifts inherits two conjugate strike-slip fault systems, which mark the W-E compression during the preceding collisional event in the Early-Middle Permian, and the mechanism of extension is similar to pull-apart model. The contrasting composition of volcanics can be caused by different-depth zones of magma generation.

The Permian-Triassic volcanics are overlain by continental coal-bearing coarse-grained volcanoclastic sediments of the Chelyabinsk Group (Middle Triassic – Early Jurassic). These deposits fill the local depressions in the paleotopography. The Middle Jurassic clastic Tyumen Formation overlays both volcanic rocks and Chelyabinsk Group, covers almost the entire territory of the Frolov-Krasnoleninsky region and marks the initiation of post-rift subsidence in the West Siberian basin.

How to cite: Smirnova, M., Latyshev, A., Panchenko, I., Kulikov, P., Khotylev, A., and Garipov, R.: Permian-Triassic rifts of the West Siberian basin: evidence of voluminous felsic volcanic activity, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9480, https://doi.org/10.5194/egusphere-egu22-9480, 2022.

NE Brazil is scarred by a number of aborted rift basins that developed from the same extensional stresses that lead to the opening of the South Atlantic. Extension started in Late Jurassic times, with the formation of an AfroBrazilian Depression south of the Patos Lineament, and continued through the Early Berriasian along two NS trending axes of deformation: Recôncavo-Tucano-Jatobá (RTJ) and Gabon-Sergipe-Alagoas (GSA). In the Late Berriasian - Early Barremian, rifting jumped North of the Pernambuco Lineament to progress along the NE-SW trending Cariri-Potiguar (CP) axis. In the Late Barremian, approximately coinciding with the opening of the Equatorial Atlantic, rifting aborted along the RTJ and CP axes and continued along the GBA trend eventually resulting in continental break-up. Extension-related magmatic activity seems to have been restricted to break-up along the marginal basins, although dyke swarms bordering the Potiguar basin (Rio Ceará-Mirim) seem to be associated to early extension stages in NE Brazil and three subparallel dolerite dykes, with K-Ar dates of 105±9 Ma, were inferred indirectly from aeromagnetic and outcrop data East of the RTJ axis. Aiming at better understanding the structure and evolution of the syn-rift basins of NE Brazil, a total of 20 seismic stations were deployed between October 2018 and January 2021 along the CP and RTJ trends. The deployment, funded by the national oil company Petrobras, included both broadband and short-period stations borrowed from the Pool de Equipamentos Geofísicos do Brasil. These stations complemented a number of permanent broadband stations belonging to the Rede Sismográfica do Brasil. Receiver functions were obtained for each of the seismic stations from teleseismic P-wave recordings and S-wave velocity models were developed from their joint inversion with dispersion velocities from an independent tomographic study. In the RTJ basins, our results show that the crust is about 41 km thick and displays a thick (5-8 km) layer of fast-velocity material (> 4.0 km/s) at its bottom; in the Potiguar basin, our results show a thinner crust of about 30-35 km underlain by an anomalously slow (4.3-4.4 km/s) uppermost mantle. We argue that those anomalous layers are the result of syn-rift and/or post-rift magmatic intrusions, which would have had the effect of increasing velocity at lower crustal levels under the RTJ basins and decreasing velocity at uppermost mantle depths under the Potiguar basin. If correct, ou interpretation would imply that, in spite of an overall lack of evidence at shallow levels, deep magmatic processes have played a role in the formation and evolution of the syn-rift basins of NE Brazil.

How to cite: Julià, J., Döring, M., and Barbosa, T.: Crustal architecture under the NE Brazil syn-rift basins from receiver functions: Evidence of deep magmatic processes., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9962, https://doi.org/10.5194/egusphere-egu22-9962, 2022.

Since the middle Triassic the long-lived convergent margin of western Gondwana evolved from a relatively steeply inclined into a flat lying slab setting that combined an extensional regime on the backarc side with the telescoping of crustal slices at the continental margin. In the Northern Andes the opening of Late Triassic basins is practically contemporaneous with the outwedging of lower crustal slices, that often alternate with intrusive sheets of S-type granites and mark the limit to a  non-metamorphic roof. A tectonic coupling between backarc collapse and the escape of lower crustal slices can be examined in detail in the northwestern flank of the Sierra Nevada de Santa Marta, a northern-most outlier of the North Andean basement. Remnants of a Late Triassic graben fill attest here to a block tilted toward the hinterland. Its tri-partite sedimentary sequence recycled material sourced from external parts of the continental margin. The basement of a more foreland-oriented block of the Sevilla belt is affected by outward-verging folds, which have formed under greenschist facies conditions in its upper and lower amphibolite conditions in its lower part. The succeeding Inner Santa Marta Metamorphic Belt consists of a stack of high-grade metamorphic basement slices separated by siliciclastic wedges metamorphosed under lower amphibolite conditions. The soles of the basement slices consist of migmatites with remobilized granitic pods and resulting folds oriented in a dip-slip direction. These structures are overprinted by a flattening and a second migmatitic event, which records peak P-T conditions of a lowest crustal level. Accordingly, they contain inclusions of ultramafic rocks. The time-equivalent correspondence between a supracrustal  backarc extension and a foreland-directed stacking of crustal slices suggests some similarity to the model  of a low-viscosity channel of a thickened orogenic crust. An important difference of this flat-slab setting resides, however, in a wholesale mobility of a strongly heated crust that constitutes the backarc and frontal position of this active margin.

How to cite: Avila Paez, M. A., Kammer, A., Conde Carvajal, C. A., Piraquive Bermudez, A., and Gomez Plata, C. N.: Backarc rifting as a response to a crustal collapse at the western Gondwana margin: The Triassic tectonic setting of the Sierra Nevada de Santa Marta, Northern Andes of Colombia, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10866, https://doi.org/10.5194/egusphere-egu22-10866, 2022.

Rifted margins are the result of the successful process of thinning and breakup of continents leading to the formation of new oceanic lithosphere. Observations on rifted margins are now integrating an increasing amount of multi-channel seismic data and drilling of several Continent-Ocean Transitions. Based on large scale geometries and domains observed on high-quality long-offset seismic lines, we illustrate a simple classification based on mechanical behavior and magmatic production. Therefore, rifted margins are not divided into opposing types, but described as a combination and continuum that can evolve through time and space from ductile to brittle mechanical behavior on one hand and from magma-poor to magma-rich on the other hand.

For instance, margins such as the Mauritania-Senegal Basin evolve north to south from a magma-poor to a magma-rich margin. Margins such as the Vøring one suffered different rifting episodes evolving from ductile deformation in the Devonian to more brittle and magma-poor rifting in the Cretaceous prior to a final magma-rich breakup in the Paleogene.

Thanks to these examples and to some others, we show the variability of the rifted margins worldwide but also along strike of a single segment and through time along a single margin in order to explore and illustrate some of the forcing parameters that can control the initial rifting conditions but also their evolution through time.

How to cite: Sapin, F.: Rifted margins classification and forcing parameters, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11260, https://doi.org/10.5194/egusphere-egu22-11260, 2022.

The breakup of Pangaea in Early Mesozoic times initiated first in the Central Atlantic region, where Triassic to Early Jurassic lithosphere extension led to continental breakup and oceanic accretion. The Central Atlantic rifted margins of NW Africa and eastern North America exhibit complex along-strike variations in structural configuration, crustal geometries, and magmatic budget at breakup. Quantifying these lateral changes is essential to understand the tectonic and geodynamic processes that dominated rifting and continental breakup. The existing seismic refraction lines along the African side and its American conjugate provide good constraints on the 2D crustal architecture of several Central Atlantic margins. However, they are insufficient to quantify the ambiguous lateral variations.

This work examines the central segment of the Moroccan Atlantic margin, which is named here the Sidi Ifni-Tan Tan margin. Using 2D seismic reflection and well data, we quantify the stratigraphic and structural architecture of the margin. We then use this to constrain 2D and 3D gravity models, to predict crustal thickness and types. Ultimately, our results are integrated with previous findings from the conjugate Nova Scotia margin, on the Canadian side, to propose a rift to drift model for this segment of the Central Atlantic and discuss the tectonic processes that dominated rifting and decided the fate of continental breakup.

How to cite: Gouiza, M.: Rift to drift evolution and crustal structure of the Central Atlantic: the Sidi Ifni-Nova Scotia conjugate margins, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11336, https://doi.org/10.5194/egusphere-egu22-11336, 2022.

The East African Rift System (EARS) is the classic example of an active continental rift associated with extension, deformation, lithosphere thinning, and generation of magmas from different mantle domains and depths. Magmatism and tectonics have always been closely linked and their mutual relationships concern many processes such as the kinematics and rates of extension, the passive versus active role of mantle upwelling and magma genesis. In addition, the spatial and temporal variations of the geochemical signature of magmas varies in response to different mantle domains contributing to their genesis (subcontinental lithosphere, asthenosphere and deeper mantle sources).

In this study we carefully screened an exhaustive geochemical database of basalts (including authors’ unpublished data) emplaced in the EARS to decipher the possible connection between different mantle domains, and the evolution and tectonic characteristics of the EARS. The geochemical data were subdivided according to spatial and temporal criteria: from a spatial point of view, the samples were ascribed to five groups, namely Afar, Ethiopia, Turkana depression, Kenya and Tanzania. From a temporal point of view, the magmatic activity of the EARS was subdivided into three main temporal sequences: 45-25 Ma, 25-10 Ma and 10-0 Ma.

The geochemical signature and radiogenic isotopes (Sr, Nd, Pb) of the selected basalts reveal significant spatial and temporal variations and permits to place important constraints on the contribution of subcontinental lithosphere, asthenosphere, and lower mantle in magma genesis

How to cite: Braschi, E., Tommasini, S., Corti, G., and Orlando, A.: Spatial and temporal variation of magmatism in the East African Rift System: influence of tectonics and different mantle domains, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11973, https://doi.org/10.5194/egusphere-egu22-11973, 2022.

When continental lithosphere is extended to break-up it forms two conjugate passive margins. In many instances, these margins are asymmetric: while one is wide and extensively faulted, the conjugate thins more abruptly and exhibits little faulting. Recent studies have suggested that this asymmetry results from the formation of an oceanward-dipping sequential normal fault array and rift migration leading to the observed geometry of asymmetric margins. Numerical models have shown that fault sequentiality arises as a result of asymmetric uplift of the hot mantle towards the hanging wall of the active fault. The preferential localization of strain reinforced by strain weakening effects is random and can happen on either conjugate. However, along the long stretch of the South Atlantic margins, from the Camamu-Gabon to the North Santos-South Kwanza conjugates, the polarity can be very well correlated with the distance of the rift to nearby cratonic lithosphere. Here, we use numerical experiments to show that the presence of a thick cratonic root inhibits asthenospheric flow from underneath the craton towards the adjacent fold belt, while flow from underneath the fold belt towards the craton is favoured. This enhances and promotes sequential faulting and rift migration towards the craton and resulting in a wide faulted margin on the fold belt and a narrow conjugate margin on the craton side, thereby determining the polarity of asymmetry, as observed in nature.

How to cite: Gudipati, R., Pérez-Gussinyé, M., Andres-Martinez, M., Neto-Araujo, M., and Phipps Morgan, J.: Passive margin asymmetry and its polarity in the presence of a craton, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12619, https://doi.org/10.5194/egusphere-egu22-12619, 2022.

The choice of crustal and mantle densities in numerical geodynamic models is usually based on convention. The isostatic component of the topography is, however, in most if not all cases not calibrated to fit observations resulting in not very well constrained elevations. The density distribution on Earth is not easy to constrain because it involves multiple variables (temperature, pressure, composition, and deformation). We provide a review and global analysis of the topography of the Earth showing that elevation of stable continents and active mid-ocean ridges far from hotspots on average is +400 m and -2750 m respectively. We show that density values for the crust and mantle, commonly used for isostatic modeling result in highly inaccurate prediction of topography. We use thermodynamic calculations to constrain the density distribution of the continental lithospheric mantle, sub-lithospheric mantle, the mid-ocean ridge mantle, and review data on crustal density. We couple the thermo-dynamic consistent density calculations with 2-D forward geodynamic modelling including melt prediction and calibrate crustal and mantle densities that match the observed elevation difference. Our results can be used as a reference case for geodynamic modeling that accurately fits the relative elevation between continents and mid-ocean ridges consistent with geophysical observations and thermodynamic calculations. 

How to cite: Theunissen, T., Huismans, R. S., Lu, G., and Riel, N.: Relative continent/mid-ocean ridge elevation: a reference case for isostasy in geodynamics, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12955, https://doi.org/10.5194/egusphere-egu22-12955, 2022.

A holistic understanding of rift initiation, evolution, and variation is made complicated by the difficulties of deep seismic imaging, limited modern examples of continental rifting, and few accessible outcrops of fossil rifted margins. In particular, The temporal structural and rheological evolution of the mantle lithosphere during riftingis poorly constrained. The mantle lithosphere rheology controls lithospheric strength at initiation, but how deformation is partitioned between the crust and mantle,  and how the paths for melt migration from the asthenosphere to the rift surface evolve during rifting is fundamental for our understanding of the rift-to-drift evolution .
Here, we use elastoplastic-viscoelastoplastic modeling in concert with published deep seismic profiles of Atlantic rifted margins and geological insights from the Lanzo peridotite outcrops in the Alps to propose a new mode of extensional tectonics in the subcontinental mantle. We run a series of dynamic models varying initial conditions and mechanisms of deformation localization in the mantle lithosphere consistent with mechanisms of ductile shear zone formation observed at slow spreading centers. Models and geophysical surveys show homologous, sigmoidal reflectors in the mantle, a reversal of fault vergence as seafloor spreading develops, exhumation of the mantle, and increasing magmatic accretion. Geological evidence, along with the coincidence of magmatic accretion and extensional structures in the mantle, suggests that faults in the mantle may serve as conduits for melt, resulting in bright reflectors on seismic profiles.

How to cite: Montiel, N., Massini, E., Lavier, L., and Müntener, O.: Characterizing mantle deformation processes during the rift-to-drift transition at magma-poor margins, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13043, https://doi.org/10.5194/egusphere-egu22-13043, 2022.

A common observation in plate tectonics is the successive stages of rifting and associated crustal and lithospheric thinning, and subsequent convergence and inversion of sedimentary basins. Rates and style of inversion often vary across the sedimentary basins, influenced by changing stress and thermal fields, different convergence directions, and also controlled by inherited structures, all of which determine the localization and style of the resulting deformation. However, the dynamic feedbacks between lithospheric tectonics and surface processes, and their 3D expressions have not been studied in details by previous models, even though erosion and sediment distribution exerts a significant control on differential vertical movements and thermal evolution.

In this study, we investigate strain partitioning during extension and subsequent structural inversion, and tackle the coupling between tectonics, mantle melting and surface processes. To do so, we apply the 3D thermo-mechanical code I3ELVIS (Gerya 2015; Munch et al. 2020), which is based on staggered finite differences and marker-in-cell techniques to solve the mass, momentum and energy conservation equations for incompressible media. The models also take into account simplified melting processes, as well as erosion and sedimentation by diffusion.

We compare the modeling results with seismic and well data from the Mediterranean back-arc basins, such as the Alboran, Tyrrhenian and Pannonian Basins. The temporal variation of different plate convergence and slab retreat velocities lead to the extensional formation, recent structural inversion and related differential vertical motions of these basins. In fossil extensional basins, plate convergence has ultimately overprinted the former basin structure, and lead to the rise of young orogens, i.e. the Pyrenees or Great Caucasus.

How to cite: Oravecz, É., Balázs, A., Gerya, T., May, D., and Fodor, L.: Structural inversion of sedimentary basins: insights from 3D coupled thermo-mechanical and surface processes models and observations from the Mediterranean, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-542, https://doi.org/10.5194/egusphere-egu22-542, 2022.

The Troodos ophiolite of Cyprus hosts a fossil spreading ridge at the Solea graben, whose last magmatic activity has been previously dated to 94.3±0.5 Ma. To study the evolution of a dying ridge, we collected structural geologic data and oriented specimens from the mantle section (serpentinized peridotites, pegmatitic dykes, pyroxenite and wehrlite intrusions) and lower crust (layered gabbros and massive gabbros) for paleomagnetic and rock magnetic analyses. Our results revealed a systematic pattern of horizontal axis rotations (i.e., tilt) in the region to the west of the Solea spreading axis, involving upper crust, lower crust, and upper mantle. Horizontal axis rotations vary in magnitude between ~20° to ~90° within the studied area, with the largest tilts observed to the west of the exposed mantle section at Mt. Olympus, and the smallest tilts observed near the NNW-SSE trending Troodos Forest-Amiandos fault system. This rotation pattern conflicts with previous interpretations considering the Troodos Forest-Amiandos fault as an oceanic detachment, and rather indicates the existence of deep-rooted listric faults that dismembered the Solea spreading ridge after the final phase of spreading.

Paleomagnetic directions from serpentinized peridotites indicate that serpentinization occurred both before and during dismemberment of the ridge by listric faulting. As these directions also record a well-studied regional 90° counter-clock-wise rotation of the Troodos ophiolite, we constrained the timing of ridge dismemberment and associated serpentinization between ~94 Ma and the beginning of the regional microplate rotation in the Turonian, hence encompassing a relatively short period of time of 2-4 Myr that well coincides with hydrothermal alteration in nearby plagiogranites dated at ~92–90 Ma.

How to cite: Advokaat, E., Maffione, M., Burton-Johnson, A., and Dekkers, M.: Anatomy of a dying spreading ridge: paleomagnetic evidence for horizontal axis rotations in the Troodos Ophiolite, Cyprus, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-957, https://doi.org/10.5194/egusphere-egu22-957, 2022.

Continental extension at mature rifts systems focuses along spreading segments where dominant magmatic activity, diking and minor faulting assist plate divergence. Such processes make adjacent spreading segments grow but also interact at zones where the spreading is transferred from one segment to another. A great variety of tectonic structures has been observed at transfer zones, encompassing parallel strike-slip faults (bookshelf faulting) or conjugate systems of en-echelon oblique faults. Transfer zones can also become transform plate boundaries once continental breakup occurs. However, the role of magma in influencing the deformation at rift-rift transfer zones is unclear as direct observations are rare. In this study, we address this open question by exploiting high-resolution Pléiades-1 tri-stereo imagery to produce the first 1 m DEM of the Afrera Plain transfer zone, between the Erta Ale and Tat Ali spreading segments in Northern Afar. This dataset has been used to conduct a detailed structural analysis of both tectonic and magmatic features and explore their geometrical and spatial relationships. We observed different trends and kinematics: Dikes opens with an extension oriented ~N65°E, consistent with the regional extension; tectonic features have instead an extensional component with direction varying between ~N46°E and ~N68°E. Riedel shears and measurements of fractures opening directions indicate that tectonic deformation occurs along two families of NW-SE- and NS-striking oblique faults having right-lateral and left-lateral components, respectively. At the same time, spatial relationships between faults and lava flows also indicate that magmatic and tectonic activity co-exist in the transfer zone. We explain these observations by two different strain fields acting in the Afrera Plain during magmatic and amagmatic phases. During magmatic phases, dikes open orthogonal to the spreading direction responding to the regional extension. Conversely, during amagmatic phases, the transfer zone is dominated by the interaction between the two spreading segments with counterclockwise rotations of the strain field and shear motions accommodated by conjugate fault systems.

How to cite: La Rosa, A., Pagli, C., Hurman, G., and Keir, D.: Analysis of high-resolution Digital Elevation Model (DEM) of the Afrera Plain (Afar) reveals relationship between magmatism and tectonics in a rift transfer zone, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1175, https://doi.org/10.5194/egusphere-egu22-1175, 2022.

        The tectonic evolution of the North Atlantic Ocean has been extensively studied using a variety of geological, geophysical, and plate reconstruction techniques. Recently, deformable plate tectonic reconstructions, built using the GPlates software, have become an increasingly used method for studying the plate kinematics, deformation, and subsequent crustal thickness evolution of tectonic regimes. For the North Atlantic Ocean in particular, deformable plate models have proven to be useful for studying the kinematic evolution of continental blocks (e.g. Flemish Cap and Galicia Bank) and the partitioning of strain within sedimentary basins (e.g. Orphan Basin). However, despite these advancements, previously published deformable plate models have included limitations that can be geologically unsatisfying. Some notable examples include, but are not limited to, uniform crustal thickness assumptions at model start times, and the rigid nature of continental blocks and model boundaries that define the limits of where deformation takes place.  

        Using the interplay of GPlates and its python programming module, pyGPlates, we present a new deformable plate modelling strategy and application within the North Atlantic Ocean. In contrast to previous studies, this approach considers deformation within continental blocks and the reconstruction of present day crustal thickness estimates calculated via gravity inversion. In addition, we also demonstrate the minimized impact of rigid landward model boundaries using this approach and the resultant ability to reconstruct rift domain boundaries a priori. The results of this study provide insight into the pre-rift (200 Ma) crustal thickness template of the North Atlantic and the evolution of relevant continental blocks during rift-related deformation. Furthermore, this work also highlights the potential impact of Appalachian and Caledonian terrane boundaries on the distribution and extent of rifting experienced along the Newfoundland, Ireland, and West Iberian offshore rifted margins.  

How to cite: King, M. and Welford, J. K.: Reconstructing deformable continental blocks and crustal thicknesses back through time within the North Atlantic Ocean, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1310, https://doi.org/10.5194/egusphere-egu22-1310, 2022.

With increased geophysical scrutiny of the NE Newfoundland-Irish margin pair (North Atlantic), the previously assumed conjugate relationship and rift-perpendicular extension between the Flemish Cap and Goban Spur are increasingly questioned. We present multichannel reflection profiles along the Flemish Cap, the Porcupine Bank, and the Goban Spur, along which structural domains (proximal, necking, hyperextended, and/or exhumed mantle domains included) are defined. Features of each structural domain along these profiles on the Flemish Cap and the Goban Spur are strikingly different, whereas similar structural features are observed in the necking domains along seismic profiles on the Porcupine Bank and the Flemish Cap. The variability in basement features suggests oblique rifting between the Flemish Cap and the Goban Spur-Porcupine Bank region, as well as a connection between the Porcupine Bank and the Flemish Cap during Early Jurassic rifting. This understanding is consistent with crustal thickness evolution calculated from a deformable plate reconstruction model that is locally updated based on seismic interpretation constraints and previously published plate reconstructions. The updated deformable plate model shows varying extension obliquity between the Porcupine Bank, Goban Spur, and Flemish Cap, which are strongly influenced by inherited Caledonian and Variscan structures, resulting in the conclusion that the Flemish Cap and the Goban Spur were not conjugate margins prior to the opening of the modern North Atlantic Ocean.

How to cite: Yang, P. and Welford, J. K.: Conjugate no more: redefining the pre-rift relationship between the Flemish Cap and the Goban Spur prior to the North Atlantic opening, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2780, https://doi.org/10.5194/egusphere-egu22-2780, 2022.

The Wilson Cycle of closing and opening of oceans is often schematically portrayed with ‘empty’ oceanic basins. However, bathymetric and geophysical observations outline anomalous topographic features, such as microcontinents and oceanic plateaus, that can be accreted when oceans close in subduction. This implies that numerous rifted margins have formed in regions characterized by the presence of previously accreted continental terranes. The main factors controlling where and how such continental rifts localize in relation to the inherited compressional structures is yet to be explored properly. Potential factors that can influence the evolution and structural style of a rift in such a tectonic setting include the thermo-tectonic age of the accretionary orogen, the number and type (size, rheology) of accreted terranes, the nature of terrane boundaries, as well as the velocity of rifting.

We use 2D finite-element thermo-mechanical models to investigate how the number and size of accreted terranes as well as the duration of tectonic quiescence between orogenesis and extension (i.e., the amount of time available for the thermal re-equilibration of the thickened lithosphere) affect the style of continental rifting. Our results can further understanding of how rifted margins formed after accretionary orogenesis are influenced by the compressional stage such as the Norwegian rifted margin, where the late-Paleozoic to Mesozoic rifting occurred after the early Paleozoic Caledonian orogeny.

We test two hypotheses. According to our first hypothesis, the location of the rift is dependent on the age of the accretion. If extension directly follows accretion, we expect the thick lithosphere of the orogen to be strong in a brittle sense, causing extension to localise adjacent to the orogen. In contrast, if the onset of extension happens after a period of tectonic quiescence, the accretionary orogen has time to heat up and viscously weaken, allowing it to localize deformation more efficiently. We test this hypothesis by varying the amount of time available for thermal re-equilibration.

Secondly, we hypothesize that the degree to which the compressional structures such as terrane boundaries in the accretionary stack reactivate depends on the size and complexity of the accreted assembly (through the number and size of the accreted terrains) as well as the strength of shear zones. We test this hypothesis by varying the number of terranes accreted prior to rifting.

Our preliminary results show that the subduction interface is reactivated in an extensional regime, but without a period of quiescence the reactivation is temporary and rifting occurs in the unthickened foreland basin area.

How to cite: Erdős, Z., Buiter, S., and Tetreault, J.: The influence of accretionary orogenesis on rift dynamics, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3220, https://doi.org/10.5194/egusphere-egu22-3220, 2022.

The intracontinental belt of the High Atlas is an aborted rift system along NW Africa, which formed during the Mesozoic break-up of Pangaea and was inverted during the Alpine Orogeny. Although the inversion and orogeny build-up have been extensively studied, the Triassic to Jurassic rifting, synchronous to the opening of the Atlantic and the Tethys, is still poorly understood. True orthogonal rifting is proposed to occur in the Triassic to Early Jurassic, while the end of rifting is controversial and believed to be controlled by oblique extension. Restoration of the Atlantic-Tethys triple junction suggests sinistral motion between Iberia and Africa being active during the Middle Jurassic, which reactivated pre-existing NE-SW trending Hercynian weaknesses in transtension mode. This led to the formation of a series of pull-apart basins involving the basement and localised volcanic activity.

The Atlas system is an excellent field analogue to analyse the role of strike-slip tectonics in extensional systems, especially in the early stages of rifting. Despite the late Cenozoic (Alpine) inversion, the well-exposed syn-rift structures and sediments have been weakly affected by the broad contractional event.

Our study aims to investigate the kinematic and geometry of the oblique rifting phase, the strain variation lengthwise in the Atlas rift system, the relationship between the orthogonal rift structures, the strike-slip structures, and the synchronous volcanism. In this contribution, we will highlight the fieldwork results, which we used to constrain the restoration of the rift sytem, quantify extension vs. transtension, and produce a conceptual model of how strike-slip tectonics can influence the early stages of a rift system.

How to cite: Vasileiou, A., Gouiza, M., Mortimer, E., and Coliier, R.: Analysis of strike-slip tectonics in extensional systems: the case of the Moroccan Atlas system, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3597, https://doi.org/10.5194/egusphere-egu22-3597, 2022.

Quantifying the spatial and temporal evolution of fault systems is crucial in understanding plate boundary deformation and the associated seismic hazard, as well as to help georesources exploration in sedimentary basins. During the last decade, 3D lithospheric-scale geodynamic models have become capable of simulating the evolution of complex fault systems, from the onset of rifting to sea-floor spreading. But since these models describe faults as finite-width shear zones within a deforming continuum, additional efforts are needed to isolate and analyse individual faults, so we can understand the entire life span of normal fault networks.

Here we present 3D numerical forward models using the open-source community software ASPECT. Our thermo-mechanical models include visco-plastic rheology, strain softening as well as lithospheric and asthenospheric layers to capture rift evolution from inception to continental break-up. We quantify normal fault evolution at the surface of the model with a method that describes fault systems as 2D networks consisting of nodes and edges. Building on standard image analysis tools such as skeletonization and edge detection, we establish a hierarchical network structure that groups nodes and edges into components that make up individual evolving faults. This allows us to track fault geometries and kinematics through time enabling us to analyse the growth, linkage and disintegration of faults.

We find that the initial fault network is formed by rapid fault growth and linkage, followed by competition between neighbouring faults and coalescence into a mature fault network. At this stage, faults accumulate displacement without a further increase in length. Upon necking and basin-ward localisation, the first generation of faults shrink and disintegrate successively while being replaced by newly emerging faults in the rift centre. These new faults undergo a localisation process similar to the initial rift stage. We identify several of these basin-ward localisation phases, which all feature this pattern. In oblique rift models, where the extension direction is not parallel to the rift trend, we observe strain partitioning between the rift borders and the centre, with strike-slip faults emerging in the centre even at moderate obliquity. Analysing the spatio-temporal evolution of modelled faults thus allows us to map their entire life span to observed stages of rift system evolution.

How to cite: Brune, S., Wrona, T., Neuharth, D., Glerum, A., and Naliboff, J.: Life and Death of Normal Faults: Quantitative Analysis of Fault Network Evolution in 3D Rift Models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6294, https://doi.org/10.5194/egusphere-egu22-6294, 2022.

The northwestern margin of the Iberian Peninsula (western Bay of Biscay) is a unique place that gathers several outstanding geological features in a relatively reduced area. Here, a former hyperextended continental margin developed in proximity to a triple point, underwent a subsequent partial tectonic inversion yielding the present Cantabrian margin. For all these reasons, the northwest area of ​​Iberia can be considered as a natural laboratory for the study of the role of tectonic inheritance in the evolution of the extensional continental margins and their subsequent inversion. However, and largely due to the lack of interest from exploration companies, the northwestern margin of Iberia presented a great deficit of geophysical and geological information. Both scientific interest and the lack of information provided the main reasons for the MARIBNO amphibious project (2019-2022). This project is being carried out by a multidisciplinary geoscientific team leaded by the Complutense University of Madrid with the acquisition of offshore and onshore data. The main objectives are focused on the study of the crustal structure, the tectonic control by the structure prior to the alpine stages and the mapping and characterization of the crustal domains, combining geological and geophysical criteria.

A one month-long geophysical cruise was carried out aboard the BO Sarmiento de Gamboa (Spanish Research Council, CSIC) in September-October of 2021. Data acquisition was divided in two cruise legs: The WAS Leg consisted in the acquisition wide-angle seismic data (WAS) along 3 transects with simultaneous offshore-onshore recording in 3 component short-period instruments: Transect WAS-1 (∼320 km) recorded in 14 OBS and 11 land seismometers, Transect WAS-2 (∼260 km) recorded in 12 OBS and 10 land seismometers and Transect WAS-3 (∼255 km) recorded in 9 OBS and 12 land seismometers. The seismic source consisted in an airgun array with 4660 ci and 90 seconds of shot interval. The MCS leg consisted in the acquisition of 2D multichannel seismic reflection data (MCS) along 14 transects (∼1500 km) recorded on a digital streamer with a 12.5 m channel-interval. Several streamer configurations were deployed with 480, 240 and 168 channels and the seismic source consisted in an airgun array with 1960 ci. During both legs, continuous marine acquisition of multibeam bathymetry, gravity, geomagnetics and ultra-high resolution seismic data also were carried out. MARIBNO project is still underway, and the data are being processed and interpreted. Acquired information will be complemented and combined with the additional acquisition of onshore gravity and magnetic data and the information from several geological field mapping studies on seismic transects throughout the Cantabrian Mountains. Here we show some preliminary results and the current development of the MARIBNO amphibious project.

How to cite: Muñoz-Martín, A., Granja-Bruña, J.-L., De la Fuente-Oliver, M. A., Druet, M., De Vicente, G., Gallastegui Suárez, J., and Maestro, A. and the MARIBNO WORKING GROUP: MARIBNO amphibious project: Structure of the northwestern Iberian margin and role of the inherited tectonics in the Alpine extension and inversion, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6886, https://doi.org/10.5194/egusphere-egu22-6886, 2022.

Extensional detachment faults, core complexes and supradetachment basins play major roles in the evolution of 3D rifted margin architecture. The successive incision of basement from early to late stages in the margin evolution is rarely explained in 3D. One reason for this is likely the lack of a unifying model for how very large faults grow and link laterally, and how this, in turn, links to the temporal evolution of the margin. As fault shape exerts a fundamental control on syn-rift basin architecture, the 3D evolution of detachment faults is critical to understand sedimentation in associated basins.

In the proximal margin offshore Norway, one control on lateral variation appears to be the differential exploitation of `extraction´ structures that evolved above the ductile crust. This controlled flips in fault polarity under the proximal margin, and lateral transitions from supradetachment- to half-graben style, Late Paleozoic-Triassic basins. Extensional culminations and core complexes were associated with this deformation pattern at depth.

The growth of an extensional fault past a displacement of a few kilometers will involve a change in 3D fault shape related to the isostatic rollback of parts of the fault plane. As displacement magnitude varies along the fault plane, so will the amount of extensional unloading and associated isostatic compensation. With increasing extension this will enforce a particular shape on the fault plane, with an extensional culmination developing in the area of maximum displacement, and synclinal recesses evolving on the flanks. With continued extension, the culmination evolves into a core complex. Necking domains, where faults propagate into the ductile middle crust appear to be prime locations for this type of faulting. As large-magnitude faults combine into domain-bounding breakaway complexes, this results in intermittent occurrences of core complexes along the main breakaways and lateral transitions into steeper megafaults and fault arrays. At the Mid-Norwegian margin, we interpret the Jurassic-Cretaceous North Møre and south Vøring basins to illustrate this type of evolution. Components of strike-slip may modify this type of pattern, as illustrated by  continental core complexes exposed in areas such as Death Valley and western Norway.

How to cite: Osmundsen, P. T., Péron-Pinvidic, G., Gresseth, J. L., and Braathen, A.: 3D evolution of extensional detachment faults and their effect on the architecture of rifts and rifted margins, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7480, https://doi.org/10.5194/egusphere-egu22-7480, 2022.

The Neuquén Basin is a major Mesozoic sedimentary depocenter located in the foreland of the Andes Mountains in Argentina. The basin hosts world renown inversion systems that have been the target of georesource exploration for the last three decades. The Huincul High is a structurally and economically prominent ~270km long, E-W trending feature that formed by the accretion of exotic Paleozoic terranes influencing subsequent Mesozoic deformation in the basin. Exploration in Huincul High has been mainly focused on the shallow part of the inversion structures leaving a limited understanding of  deep structural architecture and early tectonic evolution. With this research, for the first time a set of 4 3D seismic reflection surveys covering an area of 1400km2 have been analysed and integrated with stratigraphic information from 15 exploratory wells to provide new insights into the tectonostratigraphic and kinematic evolution of the western reaches of the Huincul High.

Detailed horizon and fault interpretation revealed Late Triassic, isolated, 10-50km long NE-SW to NW-SE trending half grabens. These extensional systems are attributed to the Late Triassic cessation of the Andean subduction to the west and intraplate extension regime ensuing. Thickness map of the Lower Jurassic Los Molles unit shows the development of an extensive ~50km  long ~15km wide NE-SW depocentre at that time. It is proposed that Andean subduction was renewed at that time, moving the Neuquén Basin into a backarc environment with hotter, weaker continental lithosphere thinned by mantle underflow which might have caused ductile flexural sag and minimal brittle faulting.

Prominent NE-SW cylindrical inversion anticlines ~17km across and well-developed harpoon structures are observed in the hangingwall of reactivated  ~50km long, NE-SW trending, extensional faults. Growth strata analysis shows thinning of Middle to Lower Cretaceous strata over the crest of these folds suggesting a phase of  NW-SE compression at this time. This compressional phase is attributed to the increase in Andean subduction rate and shallowing of the subduction dip, as the Neuquén Basin is moved into a foreland setting. Fault displacement analysis suggests that the reactivated faults were formed as separate fault segments at the time of extension in the Late Triassic. Additionally, analysis indicates that faults segments with increased reactivation show prominent hangingwall inversion anticlines.

Dip-steered coherency extractions along the Early Cretaceous Vaca Muerta Formation showed en echelon NW-SE transtensional faults occurring directly above Late Triassic non inverted faults; decoupled by the underlying shaly and mechanically weak Los Molles unit. These observations point to a post-inversion tectonic event that might coincide with reconfiguration of subducted plates changing the principal stress orientation and causing strike slip reactivation.

These results highlight the importance of  structural inheritance of a pre-existing  fault architecture in the development of  consequent inversion, and how mechanically weak units can inhibit fault propagation during the later compressional events, acting as a decoupling layer.  A detailed evolutionary model is proposed for the western reaches of the Huincul High which envisages crustal weakening and thermal sag to explain the thickening of the Early Jurassic strata previous to the main Cretaceous inversion.

How to cite: Antonov, I., Scarselli, N., and Adam, J.: Tectonic and geometric assessment of inversion systems in the Huincul High, Neuquén Basin (Argentina) – the role of structural inheritance and mechanical stratigraphy, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8244, https://doi.org/10.5194/egusphere-egu22-8244, 2022.

Footwall fault scarp-degradation produces sediments resulting in gravity-driven syn-rift wedge-shaped deposits located on the immediate hangingwall. To understand which aspects control footwall scarp-degradation we propose a model suggesting where, why, and how degradation occurs. We compare five offshore 3D seismic surveys acquired on the Northern Carnarvon Basin (North West Shelf of Australia) calibrated with well data to assess these questions. Two 3D seismic surveys (i.e., Panaeus 2001 East and Fortuna) are located on the Dampier Sub-basin, proximal to the Western Australia coastline and three (i.e., Thebe, Bonaventure and Agrippina) in a more distal position on the Exmouth Plateau. Data show that degradation is more pronounced on the distal surveys compared to the proximal ones. On the proximal surveys, the sedimentation rate is greater than in the distal ones, and footwall scarp-degradation is less pronounced. Answering these questions will help us to predict the style and the amount of footwall scarp-degradation in similar extensional settings.

How to cite: Martinez, C., Chiarella, D., Jackson, C. A.-L., and Scarselli, N.: Regional-scale proximal to distal footwall scarp-degradation variability of extensional faults, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8726, https://doi.org/10.5194/egusphere-egu22-8726, 2022.

Throughout Earth’s history, the movement, suturing, and rifting of tectonic plates in the Wilson cycle often takes advantage of lithospheric weaknesses and pre-existing plate boundaries. Continental rifting and subduction initiation represent arduous phases of this cycle for plate divergence and convergence, respectively, where strain is not yet focused into a narrow and mature plate boundary. Here we present an analysis of the Puysegur margin to demonstrate how past tectonic regimes create inherited lithospheric structures that facilitate subsequent stages of the Wilson cycle.

The Puysegur margin is a young subduction zone and forms the northern segment of the Australian-Pacific plate boundary south of New Zealand, which has evolved from divergence to strike-slip and recently to oblique convergence, all in the last ~45 million years. Magnetic anomalies and curved fracture zones located south of the Puysegur segment show the divergent phase involved seafloor spreading and the formation of new oceanic lithosphere. However, these features are not present in the upper Pacific plate at the latitudes of the Puysegur margin, and the lack of quality seismic images in this region hampered our understanding of the local crustal structure, which was assumed to be a northward extension of the oceanic domain. A deep penetrating multichannel reflection (MCS) and ocean-bottom seismometer (OBS) dataset was acquired in 2018 with the R/V Langseth and provided new high-quality seismic images of the crustal structure along the Puysegur margin.

Our seismic images reveal that the overriding Pacific plate contains stretched continental crust with magmatic intrusions, which formed from rifting between Zealandia continental plateaus during AUS-PAC plate divergence. This stretching phase was highly asymmetric and resulted in the opening of the Solander Basin. Rifting was more advanced to the south, yet never proceeded to breakup and seafloor spreading as previously thought. A new southern continent-ocean transition is inferred from potential field data, marking the boundary between stretched continental crust and new oceanic crust formed during the extensional phase.

Along-strike heterogeneity with mixed continental and oceanic domains and asymmetric rift architecture along the Puysegur margin were critical features for following tectonic regimes. Increasingly oblique plate motions sparked strike-slip motion, which localized near the pre-existing spreading center in the south, but along the western edge of the rift zone in relatively unstretched crust at the Puysegur margin in the north. Translational motion juxtaposed weak ~10 Myr old oceanic lithosphere with buoyant continental crust across the strike-slip boundary. Incipient subduction transpired as oceanic lithosphere from the south forcibly underthrust continent lithosphere at an oblique collision zone.

We suggest that subduction initiation at the Puysegur Trench was enabled by inherited buoyancy contrasts and structural weaknesses that were imprinted into the lithosphere during earlier phases of continental rifting and strike-slip along the plate boundary. In the global evolution of plate tectonics, strike-slip might be the key component to achieving the Wilson cycle, as it is the most efficient mechanism to offset terranes and juxtapose lithospheric domains of contrasting properties across broad regions, thus generating advantageous conditions for subduction initiation and subsequent closure of oceanic basins.

How to cite: Shuck, B., Van Avendonk, H., Gulick, S., Gurnis, M., Sutherland, R., Stock, J., and Hightower, E.: A Cenozoic Wilson cycle along the Puysegur Margin, New Zealand: The role of rift architecture and strike-slip dynamics enabling subduction initiation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10197, https://doi.org/10.5194/egusphere-egu22-10197, 2022.

The Gulf of Mexico opened as a Late Triassic-Mid Jurassic continental rift that was first largely covered by the Mid-Jurassic Louann Salt and later split apart by a triangular-shaped oceanic crust. Salt in the Gulf of Mexico largely hampers the imaging and interpretation of underlying pre-salt and crustal geometries, which are fundamental for assessing the early kinematic evolution of the margin. To better define these deep geometries and their lateral variations, we built three seismic-based crustal-scale cross-sections across the Florida-Yucatan conjugate margins, in the areas where the Mid-Jurassic salt unit is thinner.

Seismic-based cross-sections image the architecture of rifting and the geometries of the continental and oceanic crusts and the transition between them (ocean-continent transition, OCT). They show a meaningful along-strike variation: the South Florida-East Yucatan area is characterized by a narrower rifted continental crust that evolves sharply to oceanic crust whereas in the North Florida and central-western Yucatan areas, the rifted continental crust is wider and the transition to the oceanic crust corresponds to a narrow magmatic or exhumed mantle domain. In the rifted continental crust, seismic profiles image doubly-verging basement faults organized into decoupled and coupled rift domains. The geometrical and cross-cutting relationships between these basement faults, the Louann Salt and the underlying pre-salt sequence indicates a progressive migration of rifting from proximal to distal domains and from the central and north-eastern to the south-eastern Gulf of Mexico.  

Bulk continental crust extension was determined using the area balancing method. Estimated horizontal extension values vary from a minimum of ∼120 km in the South Florida-East Yucatan conjugate to a minimum of ∼240 km in the North Florida-Central Yucatan conjugate, being systematically larger in the northern margin. Crustal domains identified in the cross-sections were laterally correlated and westwards extended considering gravity and magnetic anomalies data to build a regional-scale, crustal domains map of the Gulf of Mexico. This map, together with the crustal extension estimates, has been used as the reference to carry out a plate-scale reconstruction of the Gulf of Mexico from the early rifting stages to the end of oceanic spreading.

Based on our observations and considering previous models, we propose that the study area evolved from an early rift involving magmatism, to a magma-poor margin, with continental break-up (OCT formation) being characterized by mantle exhumation and associated magmatism along the North Florida and central-western Yucatan areas.

How to cite: Izquierdo-Llavall, E., Ringenbach, J. C., Sapin, F., Rives, T., Callot, J.-P., and Nielsen, C.: Crustal structure and along-strike variations in the Gulf of Mexico conjugate margins: From early rifting to oceanic spreading, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11453, https://doi.org/10.5194/egusphere-egu22-11453, 2022.

Rift-related inheritance plays a key role in orogenic building, by controlling the thermal state and the position of major sedimentary and crustal decollement levels. As recognized by various authors, a switch from thin- to thick-skinned style of deformation in reactivated rifted-margin during convergence occurs where the necking domain of the margin is involved in the subduction. This is observed in the External Crystalline Massifs (Aar, Mont-Blanc, Belledonne, Pelvoux, Argentera) located at the transition between the external and the internal domains of the western Alps corresponding also to the proximal-distal transition (necking domain) of the former Jurassic margin. Necking reactivation during Alpine convergence is accommodated by shear zones, rooted in the ductile middle crust, propagating  the deformation toward the external domain.  This Alpine overprint, which led to a lower greenschist metamorphism (ca. 330°C) in the External Crystalline Massifs, raise the question of the preservation of the rift-related, pre-alpine structures in the western Alps, and their use as fossil-analogues of present-day necking domains.

A case study is the internal Mont-Blanc massif, where preserved pre-rift to syn-rift (Triassic to Mid-Jurassic) cover is observed below the internal nappes, and on top the crustal basement (Mont-Blanc granite). The contact between these deposits and the underlying basement is a fault zone, made of a cataclastic basement overlaid by a black gouge. Above the contact, remnants of allochthonous pre-rift deposits and delaminated carbonates are observed. The syn-rift sandstones (Grès Singuliers Fm), which are either in contact with the fault or located above the pre-rift deposits, contain reworked clasts of cataclasite. Above the contact, in the cataclastic basement, some crinoid-rich sediments of likely Pliensbachian age fill open cracks. Taken together, these observations strongly point to the preservation of a pre-alpine, rift-related detachment fault of Jurassic age.

The petrographical and geochemical analysis of the exhumed fault indicates strong hydration-assisted deformation. In the cataclasite, feldspars breakdown and important element transfer (especially Ba, F, Si, Pb, Zn and REE) suggest fluid circulation in an open system. The black gouge matrix is mostly made of illite, likely recrystallized during the Alpine overprint. In addition, different generations of syn-kinematic veins are observed in the detachment. The first type, composed of graphite precipitated at ~400°C in the cataclasite. Syn-kinematic quartz and quartz hyalophane (Ba-rich feldspars) in the cataclasite and gouge were formed from a fluid above 170°C a salinity of ~9 wt.% NaCl-equivalent. The mobilized elements are the same as those involved in pre-alpine Pb-Zn (Ba-F) ore-deposits of the internal Mont-Blanc (Amône, Mont-Chemin, Catogne), suggesting a genetic link between rift-related faults and mineralisations.

Despite partial Alpine metamorphic overprint, the early tectonic, sedimentary and geochemical records of this rift-related detachment fault are very well preserved, making a good analogue of present-day necking domains. The example of Mont-Blanc massif gives an opportunity to study all these aspects in detail, in particular to understand fluid-mediated element mobility during rifting. Finally, it can be used to better understand the final stages of reactivation of the necking domain in a mature orogenic system.

How to cite: Dall'asta, N., Hoareau, G., Manatschal, G., and Ribes, C.: Preservation of the necking domain in orogens: a case study of the Mont-Blanc massif (Western Alps, France), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11778, https://doi.org/10.5194/egusphere-egu22-11778, 2022.

Late Miocene calc-alkaline intrusions in the back-arc of Southern Patagonia mark an eastward migration of the arc due to accelerated subduction velocity of the Nazca plate or slab flattening preceding active ridge subduction. Amongst these intrusions are the emblematic Torres del Paine (51°S) and Fitz Roy (49°S) plutonic complexes, crystalised at ca. 12.5 and ca. 16.5 Ma, respectively (Leuthold et al., 2012; Ramírez de Arellano et al., 2012). Both intrusions are located at the eastern boundary of the Southern Patagonian Icefield and form prominent peaks with steep slopes that are ~3 km higher in elevation than the surrounding low-relief foreland. Their exhumation has been proposed as a response to glacial erosion and associated glacial rebound since ca. 7 Ma (Fosdick et al., 2013), and/or by regional dynamic uplift between 14 and 6 Ma due to the northward migration of subducting spreading ridges (Guillaume et al., 2009). Here we present a new data set of apatite and zircon (U-Th)/He from both plutonic complexes, numerically modelled to unravel their late-Neogene to Quaternary thermal histories. Our results show three rapid cooling periods for the Fitz Roy intrusion: at ca. 9.5 Ma, at ca. 7.5 Ma, and since ca. 1 Ma. For Torres del Paine, inverse thermal modelling reveals short and rapid cooling at ca. 6.5 Ma followed by late-Quaternary final cooling. The 10 Ma cooling signal only evidenced in the northern plutonic complex (Fitz Roy) may represent an exhumation response to the northward migrating subduction of spreading ridge segments, causing localized dynamic uplift. Thus, the absence of exhumation signal before 6.5 Ma in the southern part (Torres del Paine) suggest that the spreading ridge subduction must have occurred before its 12.5 Ma emplacement. On the other hand, rapid cooling by similar magnitude in both plutonic complexes between ca. 7.5–6.5 Ma, likely reflects the onset of late-Cenozoic glaciations in Southern Patagonia. Finally, the late-stage Quaternary cooling signals differ between Torres del Paine and Fitz Roy, likely highlighting different exhumation responses (i.e. relief development vs. uniform exhumation) to mid-Pleistocene climate cooling. We thus identify and distinguish the causes of rapid exhumation periods in the Southern Patagonian Andes, and propose a first Late Miocene exhumation pulse due to subduction of spreading ridge dynamics, and two Late Cenozoic exhumation episodes due to regional climate changes that have shaped alpine landscapes in this region.

References:

Leuthold J., et al. 2012. Time resolved construction of a bimodal laccolith (Torres del Paine, Patagonia). EPSL.

Ramírez de Arellano C., et al. 2012. High precision U/Pb zircon dating of the Chaltén Plutonic Complex (Cerro Fitz Roy, Patagonia) and its relationship to arc migration in the southernmost Andes. Tectonics.

Fosdick J. C., et al. 2013. Retroarc deformation and exhumation near the end of the Andes, southern Patagonia. EPSL.

Guillaume B. 2009. Neogene uplift of central eastern Patagonia: Dynamic response to active spreading ridge subduction? Tectonics.

How to cite: Paiva Muller, V. A., Sue, C., Valla, P., Sternai, P., Simon-Labric, T., Martinod, J., Ghiglione, M., Baumgartner, L., Herman, F., Reiners, P., Gautheron, C., Grujic, D., Shuster, D., and Braun, J.: Exhumation signals and forcing mechanisms in the Southern Patagonian Andes (Torres del Paine and Fitz Roy plutonic complexes), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-226, https://doi.org/10.5194/egusphere-egu22-226, 2022.

How to cite: Tien, C.-Y., White, N., Maclennan, J., and Conway-Jones, B.: Constraining Neogene Mantle Dynamics of Western Mediterranean Region Encompassing Iberia by Quantitative Modeling of Basalt Geochemistry, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-354, https://doi.org/10.5194/egusphere-egu22-354, 2022.

Mantle convection generates transient vertical motion at the surface, which is referred to as dynamic topography. The bulk of topography and bathymetry is isostatically supported by variations in the thickness and density of both the crust and the lithosphere which means that dynamic topography generated by sub-plate density anomalies needs to be isolated from these dominant isostatic signals. Australia’s isolation from plate boundaries and its rapid northwards translation suggest that long-wavelength dynamic topography is primarily controlled by the interplay between plate motion and sub-plate convection. Along the eastern seaboard of Australia, the coincidence of elevated topography, positive long-wavelength free-air gravity anomalies and Cenozoic basaltic magmatism imply that a combination of asthenospheric temperature anomalies and thinned lithosphere generate and maintain regional topography. Distributions of onshore and offshore intraplate magmatism reflect both plate motion and convective instabilities. Compilations of deep seismic reflection profiles, wide-angle surveys and receiver function analyses are used to determine crustal velocity structure across Australia. Residual (i.e. dynamic) topographic signals are isolated by isostatically correcting local crustal structure with respect to a reference column that sits at sea level. The resultant pattern of dynamic topography is consistent with residual bathymetric anomalies from oceanic lithosphere surrounding Australia. Significant positive dynamic topography occurs along the eastern seaboard and in southwest Australia (e.g. Yilgarn Craton). These signals are corroborated by independent geologic evidence for regional uplift.

How to cite: Slay, P., White, N., and Stephenson, S.: Dynamic Topography of the Australian Continent and its Margins, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-373, https://doi.org/10.5194/egusphere-egu22-373, 2022.

Lithosphere removal beneath orogenic plateaus are transient events that must often be inferred from the absence of evidence: for example, unexplained topographic uplift in the geologic record, or the absence of high-velocity mantle lithosphere. Even when foundering events do leave traces of their occurrence on the surface, the low preservation potential of such evidence leaves incomplete and ambiguous records. Distinctive features include isotopically juvenile magmatism and transient surface subsidence that form localized, internally drained hinterland basins and playas. However, basaltic volcanism and related lacustrine sediments are rarely well preserved, and this limits our ability to evaluate the role of lithosphere removal in orogenesis to only select localities. To develop a more comprehensive record of this process, and facilitate comparisons between regions with copious surface and/or geophysical evidence of lithospheric foundering with regions where the evidence is scant, whether poorly preserved or not yet recognized, we present the detrital record from young strata in internally-drained hinterland basins as a proxy for foundering-related magmatism. The detrital samples include unconsolidated to poorly consolidated lacustrine sediment of the Bidahochi paleolake from the Colorado Plateau, which is associated with the isotopically juvenile (positive epsilon Nd) Hopi Buttes Volcanic field; Oligocene siltstone from the Pamir Plateau with juvenile isotopic signature (positive epsilon Hf); and Eocene-Oligocene sandstone from several localities on the Tibetan Plateau. Integration of isotope geochemistry, trace element geochemistry, and thermochronology of detrital zircon and apatite presents a promising approach to reconstruct a continuous record of low-volume magmatism, both eroded and preserved. Ti-in-zircon thermometry, Ce-U-Ti oxybarometry, and REE proxies for depth of magmatic differentiation potentially provide a means of distinguishing zircon crystals associated with hinterland magmatism from that associated with arc magmatism. Using these datasets, we consider whether lithospheric foundering can be associated with recognizable patterns that are similar across orogens, and whether geochemical shifts in hinterland magmatism reveal first-order differences in the temporal scale of lithosphere removal in different orogens. 

How to cite: He, J. and Kapp, P.: Evaluating scant surface evidence of deep lithosphere removal: Towards a more comprehensive record, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-443, https://doi.org/10.5194/egusphere-egu22-443, 2022.

Although seismic tomography has provided important constraints on the long-wavelength structure of the mantle and its planform of convection, much is yet not well understood about the dynamic interaction of tectonic plates and deep mantle circulation at intermediate wavelengths (i.e., below plate-scale). In particular, a better understanding of the seismic structure of the oceanic upper mantle could potentially help unraveling the relationships between different scales of mantle convection, hotspot volcanism, and surface observables (e.g., MORB geochemistry, gravity gradients and bathymetry). We here present a new tomographic model of the shear-wave velocity and radial anisotropy structure beneath the central and southern Atlantic ocean constructed from the inversion of surface and body waves waveforms down to 30s period. Preliminary results confirm the existence of quasi-periodically distributed low-velocity regions in the upper mantle (200–350 km depth) organized in horizontally elongated bands some of which are parallel to the direction of absolute plate motion, as previously found in a lower resolution global tomographic models SEMum2 (French et al., 2013) and SEMUCB_WM1 (French and Romanowicz, 2014). Many of these elongated structures overlie vertically elongated plumelike conduits that appear to be rooted in the lower mantle, located, when projected vertically to the surface, in the vicinity of major hotspots.  However, there is no direct vertical correspondence between the imaged plumelike conduits and hotspots locations suggesting a complex interaction between the upwelling flow and the lithosphere/asthenosphere system. We discuss possible relations of this structure with trace element geochemistry of the corresponding hotspots.

How to cite: Romanowicz, B., Munch, F., Rudolph, M., and Mukhopadhyay, S.: Imaging the meso-scale structure of the upper mantle beneath the southern and central Atlantic ocean, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2113, https://doi.org/10.5194/egusphere-egu22-2113, 2022.

Age-progressive volcanic “hotspot” chains result from the passage of a tectonic plate over a thermochemical mantle plume, thereby sampling the otherwise-inaccessible lowermost mantle. A common feature in oceanic hotspot tracks is the occurrence of two parallel volcanic chains with an average separation of ~50 km (e.g., Loa and Kea chains in Hawaii). Some other tracks (including Tristan-Gough, Shona, the Line Islands, Wake seamounts, Tuvalu and Cook-Austral) feature a 200-400 km spacing, but the origin of such widely-spaced melting zones in the mantle remains unknown. Here, we explore 3D Cartesian geodynamic models of thermochemical plume ascent through the upper mantle. We explore various distributions of intrinsically-dense eclogitic material across the plume stem. For a wide range of eclogite distributions, the plume pools in the depth range of 300~410 km, where the excess density of eclogite is greater than above and below, as also predicted by Ballmer et al., EPSL 2013. This “Deep Eclogitic Pool” then splits up into two lobes that feed two separate shallow plumelets, particularly at high eclogite contents in the center of the underlying plume stem. The two plumelets feed two separate melting zones at the base of the lithosphere, which are elongated in the direction of plate motion due to interaction with small-scale convection. This “forked plume” morphology can account for hotspot chains with two widely-spaced (250~400 km) tracks and with long-lived (>5 My) coeval activity along each track. Forked plumes may also provide an ideal opportunity to study geochemical zonation of the lower-mantle plume stem, as each plumelet ultimately samples the opposite side of a deep plume conduit that potentially preserves spatial heterogeneity from the lowermost mantle. We compare this model to geochemical asymmetry evident along the Wake, Tuvalu and Cook-Austral double-chain segments, which make up the extensive (>100 Ma) Rurutu-Arago hotspot track. The preservation of a long-lived NE-SW geochemical asymmetry along the Rurutu-Arago double chain indicates a deep origin, likely from the southern boundary of the Pacific large low shear-velocity province. Our findings highlight the potential of the hotspot geochemical record to map lower-mantle structure over space and time, complementing the seismic-tomography snapshot.

How to cite: Ballmer, M. and Finlayson, V.: Widely-spaced Double Hotspot Chains due to Forked Plumes sample Lower Mantle Geochemical Structure, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3461, https://doi.org/10.5194/egusphere-egu22-3461, 2022.

It has been hypothesized that Australia is experiencing long-wavelength uplift and subsidence in response to intraplate stresses and/or dynamic topography (e.g. Beekman et al., 1997; Czarnota et al., 2013). In central Australia, intraplate stresses are of particular interest due to the presence of several enigmatically long-lived (500+ Myr) Bouguer anomalies of magnitude + 150 mgal. Additionally, a recent study by Jansen et al. (2022) showed that the Finke river, which drains away from a large gravity high, is actively responding to cyclic changes in uplift. Here, transient uplift and subsidence of up to ~150 m may be driven by the the flexural response to variable in-plane stresses in the presence of large loads embedded within the lithosphere.  The in-plane stress changes may be associated with shear at the base of the lithosphere and therefore inherently linked to plate velocity and mantle dynamics.
     Here, we explore mechanisms of uplift in central Australia and investigate their signatures within the geomorphic record through numerical modeling and χ analysis. We observe strong χ variations across drainage divides associated with gravity anomalies, which we link to episodic transitions from exorheic to endorheic drainage during periods of uplift and subsidence.  Landscape evolution models that incorporate flexural uplift in response to time-transient variations in horizontal stresses suggest that depositional patterns, spatial χ variations, and river profiles can be explained by this uplift mechanism.  In a more general sense, these results demonstrate that the cyclic loss and gain of drainage area during periods of endorheism and exorheism can result in drastic, sudden changes in χ which correspond to waxing and waning of basinal areas.

How to cite: Ruetenik, G., Jansen, J., Sandiford, M., and Moucha, R.: Large-scale drainage disequilibrium in central Australia, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3756, https://doi.org/10.5194/egusphere-egu22-3756, 2022.

Mid-plate volcanoes are well known as hotspots. They represent the surface signature of mantle plumes, nevertheless their origin and their role in geodynamics are still a challenge in the Earth sciences. Even though plate tectonics and mantle plumes were discovered at the same time, the latter cannot be explained by the former. Plumes’ birth, life and death play a fundamental role on the evolution of life on Earth and on plate-tectonic reorganization. La Réunion hotspot is known as one of the largest on the Earth, that created the Deccan volcanic traps in India (almost 2 million km²) and the death of more than 90% of life on the Earth including dinosaurs ~65Ma ago. So far the origin of the mantle plumes and their role in geodynamics are still unclear in Earth sciences. In that respect, we use the dataset from the French-German RHUM-RUM experiment around La Réunion hotspot (2012-2013), from IRIS data center and FDSN to extensively investigate the deep structure of the plume along its complete track from its birth to its present stage, as well as from the upper mantle to the lowermost mantle. Several shear-wave anomalies are resolved underneath Indian Ocean and the upper mantle beneath this region is fed by mantle plume rising from the core-mantle boundary. The lower mantle thermochemical dome associated to the South-African Large Low-Shear Velocity Province (LLSVP) is found to be composed of several conduits. Plume branches are highlighted at ~900 km depth. Thermal instability and thermochemical heterogeneities in the D" layer are likely the principal reasons of the plumes birth at the core-mantle boundary, and therefore an indicator of long-life of the Réunion hotspot.

How to cite: Dongmo Wamba, M., Romanowicz, B., Montagner, J.-P., and Simons, F.: Plume conduits rooted at the core-mantle boundary beneath the Réunion hotspot, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5259, https://doi.org/10.5194/egusphere-egu22-5259, 2022.

Geodynamic models based on seismic tomography have been utilized to understand a wide range of physical processes in the Earth's mantle ranging from lithospheric stress states to plate-mantle interactions. However, the influence of various model components and the associated physical properties of the mantle on the observed surface deformation is still an open question and requires further research. In this study, we develop global mantle flow models based on high-resolution seismic tomography to quantify the relative importance of the plate driving and resisting forces on the surface motions. Our models include temperature and density variations based on seismic tomography, lithospheric structure, and the observed locations of subducted slabs, using the geodynamics software ASPECT. We use a diffusion/dislocation creep rheology with different parameters for the major mantle phases. To facilitate plate-like deformation, we prescribe weak plate boundaries at the locations given by global fault databases. We resolve the resulting strong viscosity variations using adaptive mesh refinement such that our global models have a minimum resolution of <10 km in the lithosphere. We analyze the influence of slab viscosity, plate boundary friction, asthenospheric viscosity, and plate boundary geometry on reproducing the observed GPS surface velocities. Our parameter study identifies model configurations that have up to 85% directional correlation and a global velocity mean within 10% difference with the observed surface motions. Our results also suggest that the modeled velocities are very sensitive to the plate boundary friction, particularly to variations in viscosity, dip angles, and the plate boundary geometry, i.e., open vs closed boundaries, or localized vs. diffused deformation zones. These models show the relative influence of plate-driving forces on the surface motions in general, and in particular the importance of using accurate models of plate boundary friction for reproducing observed plate motions. In addition, they can be used as a starting point to separate the influences of lithospheric structure and mantle convection on surface observables like strain rate, stress field, and topography.

How to cite: Saxena, A., Dannberg, J., and Gassmoeller, R.: Investigating the effects of plate-driving forces on observed surface deformation using global mantle flow models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5561, https://doi.org/10.5194/egusphere-egu22-5561, 2022.

Most orogenic belts are close to convergent plate margins. However, some orogens are formed far away from plate boundaries, as a result of compressional stress propagating within plates, basal loading, or a combination of thereof. We focus on the Atlas of Morocco, which is such an intraplate orogeny and shows evidence of mantle driven uplift, and plume-related volcanism. How these processes interact each other is still poorly constrained and it provides clues about intraplate stress propagation, strain localization, and lithospheric weakening due to mantle dynamics. 

We present three sets of observations constructed by integrating previous data with new analyses. Crustal and thermal evolution constraints are combined with new analyses of topographic evolution and petrological and geochemical data from the Anti-Atlas volcanic fields. Our findings reveal that: i) crustal deformation and exhumation started during middle/late Miocene, contemporaneous with the onset of volcanism; ii) volcanism has an anorogenic signature with a deep source; iii) a dynamic deep mantle source supports the high topography. Lastly, we conducted simple numerical tests to investigate the connections between mantle dynamics and crustal deformation. This leads us to propose a model where mantle upwelling and related volcanism weaken the lithosphere and favor the localization of crustal shortening along pre-existing structures due to plate convergence.

How to cite: Lanari, R., Faccenna, C., Natali, C., Sengul, E., Fellin, G., Becker, T., Gogus, O., Youbi, N., and Conticelli, S.: Mantle dynamics and intraplate orogeny: The Atlas of Morocco, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5992, https://doi.org/10.5194/egusphere-egu22-5992, 2022.

Deep mantle plumes are buoyant upwellings rising from the Earth’s core-mantle boundary to its surface, and describing most hotspot chains. Mechanisms to explain dual chains of hotspot volcanoes for the Hawaiian-Emperor and Yellowstone chains fail to explain the geochemical similarity and large distances between contemporaneous volcanoes of the Tasmantid and Lord Howe chains in the SW Pacific. Using numerical models of mantle convection, we demonstrate how slab-plume interaction can lead to sustained plume branching over a period of >40 million years to produce parallel volcanic chains that track plate motion. We propose a three-part model: first, slabs stagnate in the upper mantle, explaining fast upper mantle P-wave velocity anomalies; second, deflection of a plume conduit by a stagnating slab splits it into two branches 650-900 km apart, aligning to the orientation of the trench axis; third, plume branches heat the stagnating slab causing partial melting and release of volatiles which percolate to the surface forming two contemporaneous volcanic chains with slab-influenced EM1 signatures. Our results highlight the critical role of long-lived subduction on the evolution and behaviour of intraplate volcanism.

How to cite: Mather, B., Seton, M., Williams, S., Whittaker, J., Carey, R., Arnould, M., Coltice, N., Rogers, A., Ruttor, S., and Nebel, O.: Parallel volcanic chains generated by plume-slab interaction, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6571, https://doi.org/10.5194/egusphere-egu22-6571, 2022.

The north Andean block evidences by its shallow to intermediate seismicity a juxtaposition of a southern, relatively steeply dipping slab segment with a correlating volcanic arc and a northern flat slab domain, where a margin-parallel volcanic arc became extinct since the Late Miocene. The clear-cut offset of the seismic pattern suggests the presence of a slab tear, which has its correlative morphological expression by a distinct lineament in the Cauca Valley and separates, within the Eastern Cordillera of Colombia, a southern narrow antiformal cordilleran tract from a northern composite belt with an axial depression that constitutes the High Plain of Bogotá. Faults are consistently blind and associated with tight, basement-cored folds with inverted limbs at the mountain front and distinct domes separated by marginal synclines. These structures belong to a young deformation phase as they were superposed on older cylindrical fold trains. Their ductile deformation style may be associated with a thermal anomaly as evidenced by abnormally high Ro data. In order to assess the age of this folding we extracted zircons from a rhyolitic dike that straddles a marginal syncline of a major dome. U-Pb age data indicate a recycling of these crystals from a Neoproterozoic volcanoclastic sequence that composes the basement of this marginal part of the Cordillera. Euhedral overgrowths yield, however, Quaternary ages that we tentatively associate to the advance of the outer bend of the flat slab to its present position.

How to cite: Conde-Carvajal, C., Kammer, A., Avila-Paez, M., Cubillos, S., Piraquive, A., and von Quadt, A.: Quaternary magmatism above a slab tear, Northern Andes of Colombia, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9038, https://doi.org/10.5194/egusphere-egu22-9038, 2022.

Typically, the change in lithospheric thickness associated with fracture zones relates directly to the vigor of secondary convection or mantle flow patterns. Therefore, one might expect that mantle flow considerably boosted by the presence of a mantle plume would easily overcome the lithospheric steps created at fracture zone locations. However, to date, there are no studies to verify this assumption. Numerical models based on an example from the SW Indian Ridge suggest that the axial flow driven by a plume (the Marion plume) is indeed likely to be curtailed by the long-offset fracture zones¹.

We have investigated the interactions between the Jan Mayen fracture zone and Iceland mantle plume in the NE Atlantic by considering (a) the lithospheric and asthenospheric regional configuration and (b) the geochemistry of rocks produced by submarine volcanism.

Several global lithospheric models indicate a thinning of the lithosphere on both sides of the Jan Mayen Fracture transform, despite the difference in age of the two adjacent oceanic basins. However, the tomographic models indicate a gap in the asthenospheric flow at the lithosphere-asthenosphere depth under Jan Mayen transform fault, and only a narrow northward channel of this flow is visible under the westernmost part of the fracture zone.

Vesteris seamount is an alkaline seamount placed in the central part of the Greenland Basin, located ca. 480 km west from slow-spreading Mohn's ridge and ca. 250 km north from the Jan Mayen Fracture Zone. Vesteris is a solitary volcanic center far away from an active ridge regime with an eruptive age ranging from 650 – 10 ka ². Here we report new results from geochemical analysis of several samples dredged during the East Greenland Sampling campaign EGS-2012 from the flanks of Vesteris. Whole-rock major and trace elements, together with isotopes and olivine phenocryst mineral data, are used to decipher the source of volcanism at Vesteris Seamount.

The Sr-Nd-Pb isotopic signatures indicate that Vesteris volcanism is unrelated to the Iceland mantle plume. Low NiO concentrations in highly forsteritic olivines from Vesteris alkali basalt suggest that the source was dominantly peridotitic. Rare Earth Elements profiles indicate very low degrees of partial melting of a deep mantle source in the presence of residual garnet.

Vesteris seamount was formed in a location of a relatively steep gradient of the lithospheric-asthenospheric boundary and close to the northward mantle flow that is carving the Greenland thick lithosphere. The results suggest that the Iceland mantle flow may not have crossed the Jan Mayen Transform Fault; instead, the seamount tapped into a mantle reservoir in the Greenland Basin that preserved the complex history of the Greenland craton and adjacent terranes.   REFS. (1) Georgen and Lin, 2003, Plume-transform interactions at ultra-slow spreading ridges: Implications for the SW Indian Ridge, G-cubed, doi:10.1029/2003GC000542; (2) Mertz & Renne, 1995, Quaternary multi-stage alkaline volcanism at Vesteris Seamount (Norwegian—Greenland Sea): evidence from laser step heating 40Ar/39Ar experiments, Journal of Geodynamics, doi:10.1016/0264-3707(94)E0001-B.

How to cite: Beloša, L., Gaina, C., Callegaro, S., Mazzini, A., Meyzen, C., Polteau, S., and Bizimis, M.: Plume-Fracture Zone interactions in the NE Atlantic, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9199, https://doi.org/10.5194/egusphere-egu22-9199, 2022.

Earth's surface moves in response to a combination of tectonic forces from the thermally convective mantle and plate boundary forces. Plate motion changes are increasingly well documented in the geologic record and they hold important constraints. However, the underlying forces that initiate such plate motion changes remain poorly understood. I have developed a novel 3-D spherical numerical scheme of mantle and lithosphere dynamics, aiming to exploit information of past plate motion changes in quantitative terms. In order to validate the models and single out those most representative of the recent tectonic evolution of Earth, model results are compared to global plate kinematic reconstructions. Additionally, over the past years a pressure driven, so-called Poiseuille flow, model for upper mantle flux in the asthenosphere has gained increasing geodynamic attention–for a number of fluid dynamic arguments. This elegantly simple model makes a powerful testable prediction: Plate motion changes should coincide with regional scale mantle convection induced elevation changes (i.e., dynamic topography). For this the histories of large scale vertical lithosphere motion recorded in the sedimentary record holds important information.

Here, I will present analytical results that help to better understand driving and resisting forces of plate tectonics – in particular the plume push force. Moreover, numerical results indicate that mantle convection plays an active role in driving plate motions through pressure driven upper mantle flow. Altogether, theoretical and observational constrains provide powerful insights for geodynamic forward and inverse models of past mantle convection.

How to cite: Stotz, I., Vilacís, B., Hayek, J. N., Bunge, H.-P., and Friedrich, A. M.: Plume push force: a relevant driver of plate tectonics that can be constrained by horizontal and vertical plate motions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12422, https://doi.org/10.5194/egusphere-egu22-12422, 2022.

Constraining the dynamic topography of Antarctica and its surrounding seas is required in order to gauge the pattern of mantle convection beneath this continent. However, such studies are limited by this continent’s geographical remoteness, by the lack of bedrock exposure and by extensive glaciation. Oceanic residual depth measurements provide a well-established proxy for offshore dynamic topography. Here, over 400 seismic reflection profiles have been interpreted to calculate residual depth measurements throughout the oceans that surround Antarctica. These measurements have been carefully corrected for sedimentary loading and, where possible, for crustal thickness variations. When combined with previous global compilations, these new residual depths significantly improve spatial resolution across the region, providing excellent constraints on dynamic topographic basins and swells. In the continental realm, an improved understanding of dynamic topography will help to quantify temporal and spatial variations in ice sheet stability. Volcanism and slow shear wave velocity anomalies beneath the continent indicate dynamic support.  By mapping offshore dynamic topography to a higher resolution, greater context is provided for future onshore studies.

How to cite: Dunn, A., White, N., Holdt, M., and Larter, R.: Dynamic topographic observations of Antarctica and its fringing oceanic basins, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13092, https://doi.org/10.5194/egusphere-egu22-13092, 2022.

First ⁴⁰Ar/³⁹Ar isotopic age data for gold hydrothermal veinlet-vein mineralization of the late Mesozoic Ketkap-Yuna igneous province (KYuIP) of the Aldan shield (AS) confirm the geological relation of this type of mineralization with the early Cretaceous sub-alkali magmatism. The combination of geological characteristics and U-Pb dating of magmatites indirectly enabled us to determine the age and highly productive bi-metasomatic «massif-skarn» type of mineralization associated with sub-alkali magmatogenic formations of the province.

Isotopic datings of magmatites and gold mineralization of the KYuIP and other late Mesozoic igneous provinces of the Aldan shield show age conformity of ore-bearing magmatites and ores accompanying them (fig. 1, 2). A relative, in comparison to provinces of the tectonic-magmatic activation (TMA) of the Western and Central Aldan, delay in time of occurrences of the KYuIP late Mesozoic magmatism and gold mineralization related to it, and the difference in volume ratios of formational types of magmatic formations in different provinces can be explained by the characteristics of tectonic structure of the region.

We have distinguished two large areas of the late Mesozoic TMA of the AS differing in the timing of polyformational magmatism and concomitant mineralization of different types, and in dominating formational type of magmatites: Western–Central-Aldan on the one hand, and Eastern-Aldan on the other (fig. 1-3). The first is characterized by a long-term development of magmatic activity from the Berriasian to the early Albian (≈ 30 Ma), and prevalence of leucitite–alkali(foid)-syenite formation; the second is characterized by occurrences of magmatism in a period twice as smaller (≈ 15 Ma), and domination of subalkaline diorite-granodiorite-granite formation.

The termination of the late Mesozoic magmatism in both areas was sub synchronous. The “set” of magmatogenic formations within them is also similar: leucitite–alkali(foid)-syenite with alkali granites, monzonite(subalkaline shonkinite)-syenite and subalkaline diorite-granodiorite-granite. A typical feature of the Eastern-Aldan area of the TMA consists in Coniacian-Santonian burst of alkali volcanoplutonism, which manifested in the KYuIP after a long (about 30 Ma) period of amagmatism.

How to cite: Polin, V., Zvereva, N., Travin, A., and Ponomarchuk, A.: Ketkap-Yuna igneous province gold mineralization age, ore-bearing complexes formational types, and different occurrence time of the late Mesozoic magmatism in different parts of the Aldan shield, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-163, https://doi.org/10.5194/egusphere-egu22-163, 2022.

The results of studying the granulite belts of the Earth show the presence of two types of granulite metamorphism in them: high-pressure and high-temperature ones.

     High-pressure granulites are characterized by P-T trends in the form of clockwise curves. According to widespread opinion,  the granulite metamorphism with such trends characterizes the areas that were formed as a result of the tectonic thickening of the crust due to continent-continent collisions that correspond to the model of the Himalayan type.

     High-temperature granulites are characterized by counterclockwise trends. For the formation of such granulites, researchers involve the mechanism of mantle underplating or the introduction of large volumes of intrusions under stretching. This model requires a mantle plume, which transports hot mantle material to the base of the crust.

  Thus, granulites with contrasting P-T trends, "orogenic" and "anorogenic" may be present inside the same belt. High-temperature granulites are superimposed on the dominant high-pressure ones. The time interval between these discrete events is not clearly defined and can be estimated in several tens of millions of years.

      Let's consider these two types of metamorphism against the background of the events of the supercontinental cycle (SC). Its structure consists of two stages: proper-continental (one continent-one ocean) and intercontinental (several continents-several oceans). In turn, the stages divide into phases. The first agglomeration phase of the proper-continental stage is characterized by compaction of already collected continental fragments. After the supercontinental culmination, the next, destruction phase begins, which precedes and prepares the break-up of the supercontinent. Its main content is continental rifting and the formation of the basic intrusions. The content of the first phase of the second stage consists of the break-up of the supercontinent, the formation of spreading zones and passive margins of young oceans. The next convergent phase of this stage is the assembly of the new supercontinent, the formation of subduction zones and the closure of young oceans as a result of numerous collisions.

     Based on the collision model of high-pressure granulite metamorphism, it is obvious that its formation will occur in this convergent phase of the SC, when, as a result of continent-continent collisions, a new supercontinent is assembled.

     Conditions for high-temperature granulite metamorphism in a tension environment arise in the phases of destruction and break-up of this supercontinent when plume processes are actively manifested as a result of the heat blanket effect.

      The analysis of the modern world factual material on supercontinental cyclicity for 3 billion years of the Earth history, conducted by the author, generally confirms the above correlation of the evolution of metamorphism during the development of granulite belts with events of SC.

Thus, these two types of granulite metamorphism, which fit into the structure of the super continental cycle, are indicators of geodynamic conditions of the corresponding stages and phases of the SC and show a complex interaction in the course of their manifestation of two geodynamic styles - the tectonics of lithospheric plates and mantle plumes.

How to cite: Bozhko, N.: On the manifestation of two types of granulite metamorphism during supercontinental cyclicity, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-362, https://doi.org/10.5194/egusphere-egu22-362, 2022.

We have carefully studied an unusual sulfide-bearing garnet megacryst from the ever-surprising Cenozoic Shavaryn-Tsaram basaltic cone (Tariat Platou, Mongolia). Similar sulfide inclusions in minerals constituting mantle xenoliths and clinopyroxene megacrysts related to alkali basalts were already known (Peterson and Francis, 1977, Chaussidon et al, 1989, Ionov et al, 1992) but they have never been found in garnet megacrysts. Since these garnets are believed to be mantle-derived material, their sulfide inclusions provide information on the deep sulfur cycle.

The sulfide-rich garnet megacryst from Shavaryn Tsaram pyroclastic strata is a chip of a large (up to 3 cm) cracked and partly quenched glassy crystal (fig. 1A, fig.1B) with melt pockets (Aseeva et al, 2021) inside (fig. 1C).

Sulfide inclusions are primary, isometric, elongated, and orientated towards crystal growth with a distinctive arrangement (3D X-ray images, Skyscane, fig. 2A). Swarms of inclusions contour the growth planes typical for the deltoidal icositetrahedron (fig. 2B).

Sulfide inclusions mainly consist of Ni-bearing pyrrhotite (1.66-2), scarce chalcopyrite (fig.3A and B), and rarely of pentlandite. Incompletely crystallized droplets of MSS (monosulfide solid solution) occur periodically as thin crystal pyrrhotite and pentlandite intergrowths (fig. 3C). These MSS inclusions are thought to be a product of the sulfide melt exsolution caused by undercooling (Chaudison et all, 1989).

The multi-isotope sulfur composition of these sulfide inclusions has been studied to define whether the sulfur source is crustal or mantle-derived. Thus, their δ³⁴S values account for 0.2-0.4‰, δ³³S for 0.1-0.2‰, and Δ³³S for 0.00-0.03‰, which is characteristic of mantle, meteoric, MORB, and volcanic settings. As for the host garnet, its oxygen isotope composition (Δ¹⁸О 5.4 to 5.8‰) also suggests the volcanic origin of these sulfides.

Submicron surface analysis (Bruker Dimension Icon and Solver NT-MDT) reveals the linear-globular structure of garnet (fig. 4A). Being probable nuclei, nearly 1 μm globules compose layers of garnet. We assume that garnet crystal formed via epitaxial growth from the gas phase. Garnet megacryst linear structures consisting of globules differ significantly from the metamorphic garnet crystal lattice (fig. 4B). Sulfur redundancy causes sulfide droplets, immiscible with silicate material (fig. 4C), to gather and form bulbs on top of a growing crystal due to surface tension (fig. 4C). 

The following conclusions may be drawn: 1. Sulfide inclusions in alkali basalt-associated garnet megacrysts are primary. 2. Sulfides hosted in garnet are mantle-derived according to isotopic data. 3. Garnet megacryst formation was caused by epitaxial growth.

How to cite: Aseeva, A., Ignatyev, A., Karabtsov, A., Ruslan, A., Sinev, A., Velivetskaya, T., Vysotskiy, S., and Ushkova, M.: Sulfide inclusions in alkali basalt-associated garnet megacrysts shed light on the mysterious megacryst nature, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-379, https://doi.org/10.5194/egusphere-egu22-379, 2022.

Eastern Anatolia (Eastern Turkey) resides in the Alpine-Himalayan orogenic belt and hosts the Eastern Anatolian Volcanic Province (EAVP), one of the volumetrically most important volcanic provinces within the circum-Mediterranean region. Previous studies have revealed that the predominant portion of EAVP is composed of the products of the sub-continental lithospheric mantle (SCLM) metasomatized during subduction of the Neo-Tethyan slab. The wide distribution of the lithospheric signatures in EAVP lavas has led to the availability of a large number of geochemical information regarding the regional SCLM in eastern Anatolia. In contrast, the nature of the asthenospheric mantle of eastern Anatolia remains poorly constrained due to scarcity of the asthenosphere-derived melts and lack of detailed information on the source components it comprises. Hence, this study aims primarily to put constraints on the chemical nature of asthenosphere beneath eastern Anatolia by a detailed characterization of its end-members.  

In this study, we provide new trace element and Sr-Nd-Hf-Pb isotope data from Quaternary Elazığ volcanism. This volcanism, entirely represented by mafic alkaline basaltic rocks, is one of the most recent members of EAVP, and its chemistry provides compelling evidence for a predominate asthenosphere origin. Modellings suggest that these mafic volcanics are largely free of crustal assimilation; their geochemical signatures, hence, closely reflect their source regions. Their trace element and Sr-Nd-Hf-Pb isotope systematics are consistent with derivation from an asthenospheric mantle source domain containing approximately 70% recycled oceanic lithologies with the characteristics of the C-like mantle component. However, minor contributions from depleted component (DM; ca. 20%) and an enriched component representing metasomatically modified SCLM (ca. 10%) are also needed to explain their total range of isotope data. With these findings, we propose that the C-like material is dispersed within the asthenosphere, and has mixed with the depleted mantle matrix beneath eastern Anatolia. The SCLM domains, on the other hand, occur as detached pods, following the lithospheric delamination in the region. Having triggered by the extensional dynamics during Quaternary, upwelling of the hot asthenosphere resulted in the melting of the C-DM and SCLM domains. Subsequently, the C-DM melts interacted with the SCLM-type melts, eventually generating the Elazığ volcanism.

How to cite: Aktağ, A., Sayit, K., Peters, B. J., Furman, T., and Rickli, J.: A relatively pristine C-like component in the eastern Anatolian asthenosphere, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-516, https://doi.org/10.5194/egusphere-egu22-516, 2022.

Most researchers believe that large igneous provinces (LIPs) are formed by adiabatic melting of heads of ascending mantle plumes. Because the LIPs have existed throughout the geological history of the Earth (Ernst, 2014), their rocks can be used to probe the plume composition and to decipher the evolution of deep-seated processes in the Earth’s interior.

The early stages of the LIPs evolution are discussed by the example of the eastern Fennoscandian Shield, where three major LIP types successively changed each other during the early Precambrian: (1) Archean LIP composed mainly of komatiite-basaltic series, (2) Early Paleoproterozoic LIP made up mainly of siliceous high-Mg series, and (3) Mid-Paleoproterozoic LIP composed of picrites and basalts similar to the Phanerozoic LIPs (Sharkov, Bogina, 2009). The two former types of LIPs derived from high-Mg depleted ultramafic material practically were extinct after the Mid-Paleoproterozoic, whereas the third type is survived till now without essential change. The magmas of this LIP sharply differed in composition. Like in Phanerozoic LIPs, they were close to E-MORB and OIB and characterized by the elevated and high contents of Fe, Ti, P, alkalis, LREE, and other incompatible elements (Zr, Ba, Nb, Ta, etc.), which are typical of geochemically enriched plume sources.

According to modern paradigm (Maruyama, 1994; Dobretsov, 2010; French, Romanowiсz, 2015, etc.), formation of such LIPs is related to the ascending thermochemical mantle plumes, generated at the mantle-liquid core boundary due to the percolation of the core’s fluids into overlying mantle. Thus, these plumes contain two types of material, which provide two-stage melting of the plume’s heads: adiabatic and fluid-assisted incongruent melting of peridotites of upper cooled margins (Sharkov et al., 2017).

These data indicate that the modern setting in the Earth’s interior has existed since the Mid Paleoproterozoic (~2.3 Ga) and was sharply different at the early stages of the Earth’s evolution. What was happened in the Mid Paleoproterozoic? Why thermochemical plumes appeared only at the middle stages of the Earth’s evolution? It is not clear yet. We suggest that this could be caused by the involvement of primordial core material in the terrestrial tectonomagmatic processes.  This core survived from the Earth’s heterogeneous accretion owing to its gradual centripetal warming accompanied by cooling of outer shells (Sharkov, Bogatikov, 2010).

References

Dobretsov, N.L. (2008). Geological implications of the thermochemical plume model. Russian Geology and Geophysics, 49 (7), 441-454.

Ernst, R.E. (2014). Large Igneous Provinces. Cambridge Univ. Press, Cambridge, 653 p.

French, S.W., Romanowicz, B. (2015). Broad plumes rooted at the base of the Earth’s mantle beneath major hotspots. Nature, 525, 95-99.

Maruyama, S. (1994). Plume tectonics. Journal of Geological Society of Japan, 100, 24-49.

Sharkov, E.V., Bogina, M.M. (2009). Mafic-ultramafic magmatism of the Early Precambrian (from the Archean to Paleoproterozoic). Stratigraphy and Geological Correlation, 17, 117-136.

Sharkov, E.V., Bogatikov, O.A. (2010). Tectonomagmatic evolution of the Earth and Moon // Geotectonics 44(2), 83-101.

Sharkov, E., Bogina, M., Chistyakov, A. (2017). Magmatic systems of large continental igneous provinces. Geoscience Frontiers 8(4), 621-640

How to cite: Sharkov, E.: The Late Cenozoic global activation of tectonomagmatic processes as a result of physico-chemical processes in the solidifying Earth’s core?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-968, https://doi.org/10.5194/egusphere-egu22-968, 2022.

More than 200 metasomatised peridotite xenoliths containing phlogopite, amphibole and ilmenite from the Komsomolskaya pipe are garnet and spinel harzburgites or dunites, and clinopyroxene-enriched lherzolites with garnets (up to 12.5 wt.% Cr₂O₃) and clinopyroxenes (up to 5 wt.% Na₂O). Low-Cr varieties are Fe-enriched pyroxenites, phlogopite metasomatic veins and type A, B eclogites. Minerals were studied by electron microprobe and LA-ICP-MS which revealed their geochemical groupings and their distribution in the mantle section.

     Results indicate that the lithospheric mantle beneath the Komsomolskaya pipe is layered and relatively heated. Heated peridotites at the lithosphere base (7-6 GPa) are enriched in Fe and are porphyroclastic, deformed types and rare polymict breccias. The cold group at 6.0-5.5 GPa (34 mW/m²) are depleted peridotites with sub-Ca garnets. Cpx-fertilized varieties belong to the middle part of the mantle section. Amphiboles range from Cr-hornblendes to edenites (2-6 GPa), showing K-Ti enrichment. Picroilmenites yield two pressure intervals from 6.5 to 5.0 GPa and from 5.0 to 4.0 GPa, forming two differentiation branches. Eclogites are mainly related to the lower part of the section with a peak at pressures of 4-6 GPa.

Trace elements of melts that formed harburgitic garnets-pyroxenes rever to oceanic MORB like melt interaction with peridotites. The subcalcic S-type garnets are similar to subduction-related melts (S-type REE) with troughs in HFSE. Adakite-like hybrid metasomatism formed Na, Al-rich pyroxenes with peaks in Sr and HFSE. K-bearing pyroxenes and amphiboles refer to shoshonitic metasomatism.

Trace elements for Cpx of re-fertilized mantle peridotites have high LREE, Nb-Ta troughs and peaks in Zr, Th, Sr, U and are related to carbonatite –alkaline melts. Protokimberlite (essentially carbonatitic) interaction produced HFSE-enrichment. Type B eclogites show more subduction-related features with HFSE troughs while type A eclogites are closer to hybrid and peridotitic signatures. We suggest six types of major metasomatic agents.  The ⁴⁰Ar/³⁹Ar ages of phlogopites are in the 440-690 Ma range, with some at 1.6 Ga, suggesting multistage metasomatism.  Supported by  RFBR grant 19-05-00788

How to cite: Ashchepkov, I., Ntaflos, T., Medvedev, N., Yudin, D., Makovchuk, I., and Salikhov, R.: The multistage metasomatized mantle beneath Alakit: evidence from mantle xenoliths from Komsomolskaya kimberlite pipe, Yakutia, stages of mantle evolution, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1176, https://doi.org/10.5194/egusphere-egu22-1176, 2022.

A study of chrome-spinels and PGE mineralization (PGM) from the podiform chromitites has been carried out on the area of four locations of the Ospa-Kitoy ophiolite massif (northern and southern branches East Sayan ophiolite). It has been established that different PGM assemblages formed at different stages of formation of the Ospa-Kitoy ophiolite massif, at various temperature and fluid regimes, are present at four sites. The chromite pods show both disseminated and massive structures. There are veins of massive chromitites, 0.01-0.5 m thick and 1-10 m long, rarely disseminated, schlieren, and rhythmically banded ores, which are discordant to the host ultramafic rocks. (Os-Ir-Ru) alloys occur as inclusions in the Cr-spinel or intergrowth with them (fig 3a). In addition, FePt3 alloys are found in the PGM assemblage. In such grains, decomposition structures of solid solutions represented by osmium lamellas can be observed. Polyphase PGM assemblage: (Os, Ir, Ru), (Ni, Fe, Ir),  (Ir, Ru, Pt)AsS, CuIr2S4, (Os, Ru)As2, Rh-Sb,  PtCu, and Pd5Sb2 are localized in serpentine, in close association with sulfides, sulfoarsenides, arsenides of nickel.

Figure 1. Chromitite bodies and PGE mineralization in Ospa-Kitoy ophiolite massif: 1 – Harh mountain (north branch of the ophiolites); 2 –  lake Sekretnoye (apically Zun-Ospa river); 3 – stream Zmeevikovyi (south branch of the ophiolites); 4 – Harh-Ilchir site (south flank Harh mountain).

Figure 2. Composition of  Os-Ir-Ru alloys: 1 – Harh mountain, 2 – lake Sekretnoye site, 3 – stream Zmeevikovyi.

Based on chemical and microtextural features of the PGM´s and assemblage with magmatic and hydrothermal minerals in the chromitites, it is established that each studied location of chromitites at different stages of PGM formation are exhibited. High-temperature magmatic Os-Ir-Ru alloys are widely exhibited in the Harh and Zmeevikovyichromitites. In the Harh-Ilchir site, there is no magmatic PGM and are established sulfoarsenides and arsenides Ru, Ir, which are formed from the residual fluid phase in the late magmatic stage. Chromitites in the lake Sekretnoye MPG are contained high-temperature magmatic (Os-Ir-Ru) alloys, and there are signs of PGE remobilization with Os⁰ , Ru⁰ , (Ir-Ru) alloys. Remobilization processes during serpentinization and fluid interaction of peridotites and chromitites.

In addition, it should note that the PGM assemblage of the Zmeevikovyi and Harh-Ilchir locations has been undergone by influence metamorphogenic fluids with increased activity of O₂, As, Sb. and these minerals can be formed directly in hypergenic environments. PGM҆'s such as (Ru, Rh, Pt)Sb, Rh-Sb were created at this stage.

Figure 3. BSE images of primary and secondary PGM: Harh location: а) individual grain of magmatic (Os-Ir-Ru) with microinclusion native Os; b) remobilized polyphase aggregate native Os, (Ir-Ru) (CuIr₂S₄); location Sekretnoye lake: с) inclusion magmatic (Os-Ir-Ru) in the chromite grain; d) remobilized polyphased aggregate (Ir-Ru), (Rh-Sb); location stream Zmeevikovyi: e) idiomorphic magmatic grain (Os-Ir) replaced by (Ir,Ru)AsS, with separation remobilized (Os,Ir);  Harh-Ilchir site: f) inclusion of Pd₅Sb₂ in the heazlewoodite (Hzl).

Analytics  made in Analytical Centre SB RAS. Supported by RFBR  19-05-00764а and  Russian Ministry of Education and Science.

How to cite: Kiseleva, O., Ayriants, E., Belyanin, D., and Zhmodik, S.: Manifestation of various stages PGE mineralization in the different locations Ospa-Kitoy ophiolite massif (East Sayan, Russia)., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1260, https://doi.org/10.5194/egusphere-egu22-1260, 2022.

We designed the mantle transects using the PTXFO2 diagrams  (Ashchepkov et al., 2010; 2013; 2017) constructed (Fig. 1) for mantle columns beneath kimberlites of  Kaapvaal and the Congo-Kasai cratons.

The PTXFO2 diagrams (Ashchepkov et al., 2013) in South Africa were constructed using mainly analyses of garnets, eclogitic minerals and inclusions in diamonds in open publications. The sub-calcic type garnets mainly refer to the ancient low-temperature geotherms (35 mv/m2) and higher-temperature inclusions of eclogite-pyroxenite type, giving convective geotherms crossing conductive ones, which reflects the migration process of apparently hybrid melts. 

Roberts Victor is a Mesozoic pipe 95Ma  famous due to the abundance of various eclogite xenoliths. Many eclogites in the SCLM show P-Fe# trends that are typical of ascending and differentiating magmas. Such “basaltic eclogites” may show typical features of their magmatic origin (Fig.1A). They may create channels within the peridotitic lithosphere starting from the deep subduction stages.  These irregularities formed during subduction stages and due to later plumes could explain the irregular distribution of eclogites in kimberlite pipes and abundance in  Roberts Victor (Jacob et al., 2005; Huang et al., 2014) and practical absence in others.

 In the mantle of Luaxe and Cuilo pipes (Fig.1 B, C) the minerals give highly variable conditions representing the multistage metasomatic processes. The oxygen conditions are good for diamonds  The mantle column reveals a long ilmenite trend and the presence of abundant eclogites (Zinchenko et al., 2021; Nikitina et al., 2014; Ashchepkov et al., 2012).

In the sub-meridional mantle transect through the South Kaapvaal and Zimbabwe cratons, mainly dunitic at the basement ancient cores of cratons like in Lesotho and Central part of  Zimbabwe mantle is relatively depleted and low temperature.   In the marginal parts like near Premier pipe, Venetia in Limpopo and Orapa in Magondi belt the amount of the pyroxenitic and eclogitic materials drastically rises and the temperature regimes and oxidation state rise because these zones are more transparent for the melts. These zones are often highly diamondiferous and the largest diamonds are occurring in these regions and pipes (Fig. 2).

In the mantle section through the pipes of the so-called diamond-bearing corridor of the Lucapa within the northeastern part of the Congo craton (Fig. 3), the immersion of the least oxidized and more productive horizon represented mainly by depleted peridotite material and much less oxidized is gradually thinking and in the to the southwest is recorded in the lower part. The temperatures in the lower part are also decreasing. This determines the sharp increase in the diamond grades of kimberlite pipes in this direction. But commonly this transect represents a relatively smooth homogeneous structure, the lithosphere of the craton's mantle distinguishes outflow clusters corresponding to thickenings of pips and kimberlite clusters that have arisen within the limits of separately permeable zones that occur at the intersection of deep faults.

RFBR grant 19-05-00788.  Supported by Ministry of Science and Higher Education.

How to cite: Ashchepkov, I., Zinchenko, V., Ivanov, A., and Logvinova, A.: Mantle transects in South and Central Africa according to data of mantle xenocrysts and diamond inclusions.  , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1306, https://doi.org/10.5194/egusphere-egu22-1306, 2022.

The system CaCO₃-MgCO₃ has been used since the '60s for reconstructing the petrogenesis of carbonated lithologies, notably of carbonatite magmas possibly generated in the Earth's mantle. Yet, experimental results at high temperatures and pressures remain contradictory, and a thermodynamic model for the carbonate liquid in this binary is still lacking.

We experimentally investigated the melting of aragonite and magnesite to pressures of 12 GPa, and of calcite-magnesite mixtures at 3 and 4.5 GPa, and at variable Mg/(Mg+Ca) (X_Mg). Results show that the melting of aragonite, and of magnesite have similar slopes, magnesite melting ≈ 30 °C higher than aragonite. The minimum on the liquidus surface is at X_Mg ≈ 0.35-0.40, 1200 °C at 3 GPa, and 1275 °C at 4.5 GPa, which, when combined with data from Byrnes and Wyllie (1981) and Müller et al. (2017), imply that minimum liquid composition remains approximately constant with pressure increase. We present the first thermodynamic model for CaCO₃-MgCO₃ liquids, retrieved from the experimental data available. Although carbonate liquids should be relatively simple molten salts, they display large non-ideality and a three-component (including a dolomite component), pressure dependent, asymmetric solution model is required to model the liquidus surface. Attempts to use an end-member two-component model fail, invariably generating a very wide magnesite-liquid loop, contrary to the experimental evidence.

The liquid model is used to evaluate results of experimentally determined phase relationships for carbonated peridotites modelled in CaO-MgO-SiO₂-CO₂ (CMS-CO₂), and CaO-MgO-Al₂O₃-SiO₂-CO₂ (CMAS- CO₂). Computations highlight that the liquid composition in the CMS-CO₂ and CMAS-CO₂ and in more complex systems do not represent "minimum melts" but are significantly more magnesian at high pressure, and that the pressure-temperature position of the solidus, as well as its dP/dT slope, depend on the bulk composition selected, unless truly invariant assemblages occur. Calculated phase relationships are somewhat dependent on the model selected for clinopyroxene, and to a lesser extent of garnet.

Byrnes A.P. and Wyllie P.J. (1981) Subsolidus and melting relations for the join CaCO₃-MgCO₃ at 10 kbar. Geochim. Cosmochim. Acta 45, 321-328

Müller I.A., Müller M. K., Rhede D., Wilke F.D.H. and Wirth R. (2017) Melting relations in the system CaCO₃-MgCO₃ at 6 GPa. Am. Mineral. 102, 2440-2449.

How to cite: Poli, S., Zhao, S., and Schmidt, M. W.: An experimental determination of the liquidus in the system CaCO3-MgCO3 and a thermodynamic analysis of the melting of carbonated mantle melting, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1531, https://doi.org/10.5194/egusphere-egu22-1531, 2022.

Mantle xenoliths provide a clear evidence of interaction with low-degree mantle melts, however, this evidence is mostly geochemical, manifested by incompatible element enrichment, or mineralogical, manifested by already crystallised phases (e.g. amphibole, phlogopite) as a result of this interaction.

Despite decades of research, the composition of low-degree melts generated in lithospheric mantle are still not very well-known. In situ characterisation of such melts is hampered due to their modification during the ascent as well as rapid alteration and weathering at the surface, while experiments are hampered by difficulties to produce and analyse very low-degrees (<2-3%) melts.

In this study we report a rare sample of well-preserved low-degree melts within a peridotite xenolith GGU473178 from Majuagaa kimberlite in West Greenland. We report alkali-carbonatitic-chloride melt pools and veins that may represent primary low-degree partial melts and products of their in situ crystallisation.

Melt pools are largely composed of carbonate (predominantly dolomite) and contain spinel, apatite, phlogopite as well as minor amounts of Fe-Ni sulphides, barite and halite.

Euhedral crystals of spinel present in these melt pools contain large usually round aggregates of mineral inclusions, which we explain as former melt pools captures by spinel. Mineral assemblage found in these spinel inclusions is consistently composed of ferropericlase, dolomite, alkali-rich carbonate and apatite, which is indicative of a strongly silicate-undersaturated alkali-carbonatitic melt that contains chlorine and phosphorous. Due to the almost complete absence of SiO₂, ferropericlase (instead of olivine) crystallises in equilibrium with dolomite and alkali-rich carbonate, implying incredibly low degrees of melting, when essentially only carbonated component is melted, or carbonate-silicate liquid immiscibility, previously reported for spinel lherzolite and garnet wehrlite xenoliths (Frezzotti et al., 2002; Soltys et al., 2016).

References

Frezzotti, M. L., Touret, J. L. R., and Neumann, E. R., 2002, Ephemeral carbonate melts in the upper mantle: carbonate-silicate immiscibility in microveins and inclusions within spinel peridotite xenoliths, La Gomera, Canary Islands: European Journal of Mineralogy, v. 14, no. 5, p. 891-904.

Soltys, A., Giuliani, A., Phillips, D., Kamenetsky, V. S., Maas, R., Woodhead, J., and Rodemann, T., 2016, In-situ assimilation of mantle minerals by kimberlitic magmas — Direct evidence from a garnet wehrlite xenolith entrained in the Bultfontein kimberlite (Kimberley, South Africa): Lithos, v. 256-257, p. 182-196.

How to cite: Kiseeva, E. S., Kamenetsky, V. S., and Nielsen, T. F. D.: In situ low-degree melts in peridotite xenolith from Majuagaa kimberlite, West Greenland, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1932, https://doi.org/10.5194/egusphere-egu22-1932, 2022.

Plateaus and isochronous and integral ages of ³⁹Ar/⁴⁰Ar xenocrysts and phlogopite grains from kimberlite xenoliths can be used to determine the ages of mantle processes (Hopp et al., 2008) and decipher the genesis of diamond-forming processes. Dating of deep xenoliths of kimberlites of the Siberian craton reveals a significant spread (Pokhilenko et al., 2012; Ashchepkov et al., 2015) from the Archean to the time close to the age of the host kimberlites, mainly Devonian. The most ancient ages for Udachnaya Daldyn fields for phlogopites from xenoliths of spinel harzburgites of the highest level belong to the late Archean (2.6-2.0) - early Proterozoic 1.7 -1.5 Ga. In the Alakite field, all ages are younger from 1,87 – 1,05- 0,928 - 0,87 Ga and belong to the metasomatic history of the Rodinia continent mantle. Close dates are set for xenoliths from the Obnazhennaya pipe (Kalashnikova et al. 2017).

Fig.1 PT  Udachnaya pipe. Symbols: Op: ToC(Brey, Kohler, 1990)-P(GPa)(McGregor, 1974). Cpx: 2.ToC-P(GPa)(Nimis, Taylor, 2000); 3.ToC (Nimis, Taylor, 2000 with ed. author)–P(GPa)(Ashchepkov et al., 2011); 4. eclogites ; 5. inclusions in diamond; Gar: 6.ToC (O'Neill, Wood, 1979) -P(GPa) (Ashchepkov et al., 2010Gar), 7. For eclogite garnets Chromite: 8,  inclusions in diamond; 9. chromite ToC (O'Neill, Well, 1987)-P(GPa) (Ashchepkov et al., 2010Gar), 7. For eclogite garnets Chromite: 8, inclusions in diamond; 9. chromite ToC (O'Neill, Well, 1987)-P(GPa) (Ashchepkov et al ., 2010Chr); 10 the same for inclusions in diamond; 11. Film Tom (Taylor et al., 1998)- P(GPa) (Ashchepkov et al., 2010 ilm)

Our data on micas by the ³⁹Ar/⁴⁰Ar method often reveal complex configurations of spectra. The micas of the xenocrysts of the Alakite field give several peaks, ranging from the most high-temperature and ancient, which corresponded to the upper Proterozoic - Vendian and Paleozoic, and only the lowest temperature peaks with a high Ca/K ratio corresponded to the ages of kimberlite introduction. Some peaks may be associated with the thermal effects of the Vilyusky plume (Kuzmin et al., 2012). The lowest temperature peaks, which are close in age to the time of kimberlite formation, which is confirmed by the high ³⁸Ar/³⁹Ar ratios of the gas released at the low-temperature stage, can be used very approximately for dating kimberlites, however, the release of other gases at low-temperature stages significantly increases the measurement error. All of them correspond to the interval 440 -320 Ma. The pipes Mir, Internationalnaya, Ukrainianskaya - 420, Yubileynaya -342, Botuobinskaya -352 Ma). Some definitions practically coincide with Rb/Sr ages (Griffin et al., 1999, Agashev et al., 2005, Kostrovitsky et al., 2008; Zaitsev, Smelov, 2010) and probably represent mixing lines. For many xenocrysts (Feinsteinovskaya, Ukrainskaya, Yubileynaya, Krasnopresnenskaya tr.), the interval from 600 to 500 Ma is manifested, which corresponds to the stage of the Laurasia supercontinent breakdown. The presence of relatively low-temperature plateaus with ancient ages, and high-temperature young ones implies that some stages can be correlated with the mantle history of the mineral.  RFBR grant 19-05-00788.  Supported by Ministry of Science and Higher Education.

Fig.2 PT  Sytykanskaya pipe

How to cite: Iudin, D., Ashchepkov, I., and Travin, A.: Ages of micas from xenoliths and xenocrysts of kimberlites of the Siberian Craton determined by 39Ar/40Ar method, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2177, https://doi.org/10.5194/egusphere-egu22-2177, 2022.

The nickel content in pyropes interested researchers due to the possibility to create an algorithm for calculating the temperature boundaries of its joint crystallization with diamonds. Griffin proposed such a calculation algorithm, which was named the diamond-bearing corridor with his name [1]. Determination of nickel content in pyropes on microanalysts is difficult for several reasons. The first reason is the limit of detection of nickel in pyropes, which is very high for the determination of this element on electron microscopes (at least 15 ppm). And then, such an analysis is possible at a quantitative level with a probe beam current of 300nA and an analysis time of 3 minutes. The study of the correlation ratios of nickel with other elements in pyropes allowed us to determine two elements that have a significant correlation with the nickel content in pyropes - these are titanium and manganese and their content in pyropes is acceptable for quantitative determination. On the ion microanalyzer, more than two hundred analyses were performed for pyropes from the kimberlite pipes Botuobinskaya and Nyurbinskaya, the remaining determinations were made from kimberlites tr. Jubilee and tr. Victory with the use of a new technique for the determination of nickel in pyropes in the microanalyzer JXA-8230. In total, 443 definitions of nickel in pyropes were performed at the quantitative level. Such definitions made it possible to calculate the functional dependence of nickel contents on titanium and manganese contents. The STATISTICS program is used for such calculations (Fig. 1).

Fig. 1. Map of level lines (nickel manganese titanium for 443 definitions) with the calculation of functional dependence

The calculation of nickel contents in pyropes makes it possible to fully use the Griffin geothermometer to determine the number of pyrope grains from the diamond-bearing corridor area.

• Griffin W.L., Ryan C.G. Trace elements in indicator minerals: Area selection and target evaluation in diamond exploration. J. Geochem. Explor., 1995. Vol. 53., pp, 311-357

How to cite: Ivanov, A.: Precise calculations of Nickel content in pyropes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3014, https://doi.org/10.5194/egusphere-egu22-3014, 2022.

Most of earthquakes occur below 10-km depth in the Korean Peninsula. For example, the focal depth of the Mw 5.5 Gyeongju Earthquake in 2016, the largest instrumental earthquake in South Korea since scientific earthquake monitoring started in 1978, is about 14 km with hypocentral basement rocks of granitoid and temperature of 370°C (thus, brittle-plastic transition condition). A study on ancient granitoid shear zones with the similar temperature condition will aid in understanding the seismogenesis in the brittle- plastic transition regime. The Yecheon shear zone is an NE- to NNE-striking right-lateral shear zone cross-cutting Mesozoic granitoid belt in South Korea. The deformation temperature of the main shear zone was estimated to be about 350 ℃. In the southeastern margin of the shear zone, protomylonites change gradually into mylonites and then abruptly into ultramylonites toward southeast. Quartz and feldspar grains both of protomylonite and mylonite deform by dislocation creep and brittle fracturing, respectively. Greenish ultramylonite consists of quartz-, feldspar-, muscovite- and epidote-rich layers within matrix of quartz, muscovite and epidote. The protomylonite commonly displays a composite S-C foliation. The deflecting S-foliation of mylonite toward ultramylonite is sharply truncated by the boundary between mylonite and ultramylonite. Thin (several mm to several cm) greenish layers occur in protomylonite subparallel to mylonitic foliation or cross-cutting the foliation at a low angle. They also show injection structure with flow banding and cataclastic deformation along the protomylonite boundary. The greenish layer consists of fragments of protomylonite and matrix of very fine-grained quartz, feldspar, muscovite and epidote. Epidote grains of ultramylonite and greenish layers replace phengitic mica, biotite and plagioclase and show graphic texture. Together with epidote formation, chloritization of biotite and albitization of K-feldspar are prominent in the greenish layers. The growth of hydrothermal minerals including epidote and chlorite within the greenish layers and shear band along the C-foliation indicates fluid circulation in the layers. We interpret the greenish layers were generated during seismic events in fluid-rich conditions and thus seismic event may be caused by pore pressure build up. Once the greenish layers develop, deformation was localized along the layers due to much reduced grain size in interseismic periods, and the greenish layers became ultramylonite with further grain-size reduction.

How to cite: Kim, J. H. and Ree, J.-H.: Seismogenesis in granite under brittle-plastic transition condition, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3284, https://doi.org/10.5194/egusphere-egu22-3284, 2022.

The deep structure of orogenic belts and cratons has become an important part to track evolution and innovation of tectonics. The extremely thick crust and overlying deposition bring obstacles to the deep structure of the orogenic belt and ancient block. Deep seismic reflection profile is globally regarded as an advanced technology to perspective the fine structure of the crust and the top of the upper mantle, especially using large-size dynamite shots. In the 1990s, international scholars used deep seismic reflection profiles to find inclined reflections penetrating from the lower crust to the upper mantle (Calvert et al., 1995; Cook et al., 1999). They believe that these reflections are related to ancient subduction events(or fossil subduction). At the beginning of this century, Chinese scholars began to carry out similar experiments in the Tibet Plateau, Sichuan Basin and Songliao basin. Using big-size dynamite shots, they also found the Moho under the extremely thick crust of the Tibet Plateau and the mantle reflection under the ancient block (Gao et al., 2013, 2016; Zhang et al., 2015). In 2016, with the support of China Geological Survey Project,we arranged a seismic reflection profile around the Scientific Deep Drilling SK-2 Well in the middle of Songliao basin. According to the data processing results of all five big-size dynamite shots and four medium-size dynamite shots of the profile, we obtained a 127.3km long single-fold reflection profile, revealing the reflection characteristics of the lower crust, Moho and its upper mantle in the study area. The Moho structure distributed nearly horizontally at a depth of 33km (estimated by the average crustal velocity of 6km/s) is clearly obtained, and the mantle reflection extending obliquely from Moho to 80km-depth is found. We believe that this dipping mantle reflection represents an ancient subduction relic under the Songnen block.

Calvert, A. J., Sawyer, E. W., Davis, W. J., & Ludden, J. N.  Archaean subduction inferred from seismic images of a mantle suture in the Superior Province. Nature,1995, 375(6533), 670–674.

Cook,F. A., van der Velden, A. J., Hall, K. W., Roberts, B. J.Frozen subduction in Canada’s Northwest Territories: lithoprobe deep lithospheric reflection profiling of the western Canadian Shield. Tectonics 1999,18, 1–24.

Gao R, Chen C, Lu Z W, et al.New constraints on crustal structure an d Moho topography in Central Tibet revealed by SinoProbe deep seismic reflection profiling. Tectonophysics, 2013, 606:160 - 170.

Gao, R., Chen, C., Wang, H. Y., Lu, Z. W., et al.Sinoprobe deep reflection profile reveals a neo-Proterozoic subduction zone be neath Sichuan basin. Earth & Planetary Science Letters, 2016,454(18):86-91

Zhang, X. Z.,Zheng, Z.,Gao, R., et al. Deep reflection seismic section evidence of subduction collision between Jiamusi block and Songnen block. Journal of Geophysics, 2015,58 (12): 4415-4424

How to cite: Li, M., Gao, R., Zhou, J., Wilde, S. A., Hou, H., Tan, X., and Zhu, Y.: Deep seismic reflection profile with big-size dynamite shots reveals Moho and mantle reflection: tracking continental evolution, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3326, https://doi.org/10.5194/egusphere-egu22-3326, 2022.

The NE of the Siberian platform in Udzha and Anabar river locate the richest alluvial placers of diamonds Since the discovery of placers > 700 were found, but no industrial bodies. A study of kimberlite magmatism has established that there are kimberlites of three ages on the territory of the North-East of the Siberian platform – middle Paleozoic (single), lower Triassic (few) and Jurassic-Cretaceous (prevailing). The latter are almost non-diamond-bearing.

The nearest kimberlite fields of Kuranakh and Tomtor are poor in diamonds. Some placers in the basin of the Udzha river, the right tributary of the Anabar, contain Cr-rich (≤14 wt.% Cr2O3) sub-calcic pyrope garnet associated with diamond. Comparison of kimberlite indicator minerals (KIMs) from the basins of Udzha and Chemara (its right tributary) shows similarity and a large diversity of pyropes, mostly of lherzolitic type. Cr- diopsides found in the Devonian collector suggest a close kimberlite source.

Mainly eclogitic placer diamonds are abundant in the upper reaches of the Chimara river in the northeastern part of the region. They occur in Permian, Jurassic, and Neogene rocks and in Quaternary alluvium where they coexist with pyrope and ilmenite. The diamonds in this region have mostly eclogitic features (Shatsky et al., 2015).

Reconstructions using monomineral thermobarometry (Ashchepkov et al., 2010) for the sources of pyrope and diamond show that the areas of the Anabar and Udzha placers share the similarity in the structure of mantle roots since 7.5 GPa, with a convective branch at the base.

The P-Fe trend for the Jurassic is slightly inclined, which is typical of the Kuranakh field. For the Devonian kimberlites, non-inclined trends are typical. The subcontinental lithospheric mantle (SCLM) beneath the Udzha basin is rich in pyroxenitic garnets as typical for the Anabar region.

There are 3 intermediate collectors of pyropes and associated diamonds: Permian, Jurassic and Neogenic and alluvium.  A study of the chemistry and thermobarometry of the kimberlite indicator minerals show some variations which possibly indicate different kimberlite sources ( Fig.1).

The detailed trace element geochemistry of the KIM from Udzha and Chemyra rivers show high variations and systematic differences.

Fig.1. PTX diagrams for kimberlite indicator minerals (KIM) from three correctors in the Udzha basin.

Fig.2 TRE distributions for KIM from Udzha alluvium

Fig.3 TRE distributions for KIM from Udzha alluvium

Supported by  RBRF grant 19-05-00788.

How to cite: Vavilov, M., Afanasiev, V., Ashchepkov, I., Baranov, L., and Vera, E.: Possible sources of the alluvial diamonds Udzha basin, northern Anabar region near kimberlite Tomtor field, Yakutia., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3368, https://doi.org/10.5194/egusphere-egu22-3368, 2022.

For the first time, such an indicator as the Ti content in garnets was used as a criterion for the study of heterogeneity of the lithospheric mantle (LM) beneath the Yakutian kimberlite province (YaKP). Comparison of the compositions of garnet from pipes of most fields (18 out of 21) of YaKP was carried out. The study was based on representative collections of garnets from kimberlite concentrates, as well as literature and own data on the composition of garnets from mantle xenoliths from the Upper Muna pipes and northern fields adjacent to the Anabar shield, as well as from the Udachnaya, Dal’nyaya and Obnajennaya pipes. Three groups of YaKP fields with different Ti content (Fig. 1) and Mg# values ​​in garnets have been identified - 1) southern diamondiferous fields - high TiO₂ content (0.26-0.50 wt%) and high Mg# value (80.6-82.6%); an exception is the Mirninsky field (0.13 wt.% TiO₂); 2) the dominant number of northern fields (10 in total) is a low TiO₂ content (0.06-0.26 wt%) (Fig. 2) and a relatively high value of Mg# (78.8-81.7%, middle - 80.2%); 3) three northern fields (Chomurdakh, Ogoner-Yuryakh and Toluopka) - high TiO₂ content (0.53-0.78 wt.%) (Fig. 3) and low Mg# (76.9-78.3%). The trace element composition of garnets from the third group testifies to their mainly equilibrium magmatic crystallization (Fig. 4). It is assumed that the garnet-bearing rocks, due to the relatively low lithospheric mantle (LM) thickness in the marginal part of the Siberian Craton, were subjected to almost complete metasomatic processing by melt-fluids of the asthenospheric mantle. The obtained data on the composition of garnets allowed the authors to clarify the reason for the different compositions of kimberlites in the southern and northern fields of YaKP. The authors believe that the predominantly high-Ti composition of the kimberlites of the northern fields, despite the low-Ti composition of the LM rocks, reflects the primary composition of the kimberlite melt-fluid of asthenospheric origin. The relatively small thickness of the LM beneath the northern fields limited the degree of assimilation by kimberlite melt of high-Mg rocks of LM and initiated an increase in asthenosphere activity, which led to the formation of high-Ti kimberlites, high-Ti alkaline basalts, and alkaline-carbonatite massifs here. Supported by RBRF grant 19-05-00788

How to cite: Kostrovitsky, S., Yakovlev, D., Ashchepkov, I., and Tappe, S.: Inhomogeneity of the composition of lithospheric mantle beneath the Yakutian kimberlite province, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3753, https://doi.org/10.5194/egusphere-egu22-3753, 2022.

Xenoliths in kimberlites and other volcanic rocks are our best window into the subcranotic lithospheric mantle. Chemical overprinting associated with melt-rock interactions is almost ubiquitous in these mantle xenoliths [1]. Such local changes in chemistry may be recorded by the formation of compositional zoning in minerals. Studies of major and trace element zoning provide important information about the nature and time scales of metasomatic processes and thermal events in the upper mantle.

Usually, garnets from peridotite xenoliths have pronounced zoning, whereas olivine and pyroxenes are homogeneous. Currently, only zoning in garnets of sheared and coarse peridotite xenoliths from kimberlites of the Kaapvaal craton (southern Africa) and the minette neck The Thumb (North American craton) has been studied in detail (e.g., [2,3]). There is no detailed study on major-, minor- and trace-element zoning in garnets of peridotite xenoliths from kimberlites of the Siberian craton.

In our study, we provide a detailed description of complex major- and trace-element zoning patterns in garnets of two unique fresh sheared peridotites from the Udachnaya kimberlite pipe (Siberian craton). The mantle residence pressure and temperature of the peridotites UV-3/05 (lherzolite) and UV-33/04 (harzburgite) are 6.4 GPa and 1350°C [4] and 6.0 GPa and 1320°C [5], respectively.

The profiles of minor and major elements are complex and symmetric. The profiles change their slope signs (positive/negative) several times. It should be noted that the Ni content increases from the cores to the rims. The chondrite-normalized REE patterns show a continuous change from the cores to the rims. The cores display sinusoidal patterns (LREE enrichment peaking at Sm), whereas patterns of the rims are ‘normal’ (with HREE enriched by 15–19× chondrite abundances for Gd through Lu).

The profiles are consistent with the formation of garnet overgrowths and increasing temperature, followed by diffusive equilibration between the rims and cores over hundreds or thousands of years. Using melt-garnet distribution coefficients of trace elements, we showed that the metasomatic melt, which caused the formation of the garnet overgrowths, had a genetic link to the kimberlite magmatism that formed the Udachnaya pipe. The profile lengths of Zr, Ce, Sm, Eu, Gd, and Hf are longer than the profile lengths of Tb, Dy, Ho, Er, Tm, Yb, and Lu. This indicates that the composition of the melt changed (from composition in equilibrium with upper mantle peridotite to kimberlitic composition) during its percolation through the mantle, as predicted by the theory proposed by Navon and Stolper (1987).

This study was supported by the Russian Science Foundation (grant No 18-77-10062).

References: [1] Pearson, D.G. and Wittig, N., 2014, Treatise on Geochemistry, 255-292. [2] Griffin et al., 1989, Geochim. Cosmochim. Acta, 53(2), 561-567. [3] Smith et al., 1991, Contrib. Mineral. Petrol., 107(1), 60-79. [4] Golovin et al., 2018, Chem. Geol., 261-274. [5] Agashev et al., 2013, Lithos, 160, 201-215. [6] Navon, O. and Stolper, E., 1987, J. Geol., 95(3), 285-307.

How to cite: Solovev, K., Sharygin, I., and Golovin, A.: Zonation in garnets from the Udachnaya pipe: heating and melt infiltration in the lithospheric mantle of the Siberian craton, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3997, https://doi.org/10.5194/egusphere-egu22-3997, 2022.

Basalts from high flux intra-plate volcanism (Iceland, Hawaii, Samoa) are characterised by ³He/⁴He that are significantly higher than those from the upper mantle sampled at mid-ocean ridges.  The prevailing paradigm requires that a largely undegassed deep Earth is enriched in primordial noble gases (³He, ²⁰Ne) relative to degassed convecting upper mantle.  However, the He concentration and ³He/²⁰Ne ratio of high ³He/⁴He oceanic basalts are generally lower than mid-ocean ridge basalts (MORB). This so called ‘He paradox’ has gained infamy and is used to argue against the conventional model of Earth structure and the existence of mantle plumes.  While the paradox can be resolved by disequilibrium degassing of magmas it highlights the difficulty in reconstructing the primordial volatile inventory of the deep Earth from partially degassed oceanic basalts.

Basalts from 26 to 11°N on the Red Sea spreading axis reveals a systematic southward increase in ³He/⁴He that tops out at 15 Ra in the Gulf of Tadjoura (GoT). The GoT ³He/⁴He overlaps the highest values of sub-aerial basalts from Afar and Main Ethiopian Rift and is arguably located over modern Afar plume.  The along-rift ³He/⁴He variation is mirrored by a systematic change in incompatible trace element (ITE) ratios that appear to define two-component mixing between E-MORB and HIMU.  Despite some complexity, hyperbolic mixing relationships are apparent in ³He/⁴He-K/Th-Rb/La space.  Using established trace element concentrations in these mantle components we can calculate the concentration of He in the Afar plume mantle.  Surprisingly it appears that the upwelling plume mantle has 5-20 times less He than the convecting asthenospheric mantle despite the high ³He/⁴He (and primordial Ne isotope composition). This contradicts the prevailing orthodoxy but can simply be explained if the Afar mantle plume is itself a mixture of primordial He-rich, high ³He/⁴He (55 Ra) deep mantle with a proportionally dominant mass of He-poor low ³He/⁴He HIMU mantle. This is consistent with the narrow range of Sr-Nd-Os isotopes and ITE ratios of the highest ³He/⁴He Afar plume basalts, and is in marked contrast to high ³He/⁴He plumes (e.g. Iceland) that do not have unique geochemical composition. The HIMU signature of the Afar plume basalts implies origin in recycled altered oceanic crust (RAOC). Assuming that no He is recycled and using established RAOC U and Th concentrations, the low He concentration (< 5 x 10¹³ atoms/g He) of the He-poor mantle implies that the slab was subducted no earlier than 70 Ma and reached no more than 700 km before being incorporated into the upwelling Afar plume. We suggest that the Afar plume acquired its chemical and isotopic fingerprint during large scale mixing at the 670 km transition zone with the Tethyan slab, not at the core-mantle boundary.

This study implies that large domains of essentially He-poor mantle exist within the deep Earth, likely associated with the HIMU mantle compositions. Further, it implies that moderately high-³He/⁴He (< 30 Ra) mantle plumes (e.g. Reunion) need not contain a significant contribution of deep mantle, thus cannot be used a priori to define primitive Earth composition.

How to cite: Balci, U., Stuart, F. M., Barrat, J.-A., and van der Zwan, F. M.: Low He content of the high 3He/4He Afar mantle plume: Origin and implications of the He-poor mantle, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4994, https://doi.org/10.5194/egusphere-egu22-4994, 2022.

[1] Boven et al., 1998, J. Afr. Earth Sci., 26, 463-476.

[2] Holmes and Harwood, 1932, Quarterly J. Geol. Soc., 88, 370-442.

[3] Le Maitre, 2002, Cambridge University Press.

[4] Rosenthal et al., 2009, Earth Planet. Sci. Lett., 284, 236-248.

[5] Link et al., 2008, 9^th Int. Kimb. Conf., 1-3.

[6] Foley, 1992, Lithos, 28, 435-453.

[7] Agostini et al., 2021, Sci. Rep., https://doi.org/10.1038/s41598-021-90275-7.

How to cite: Innocenzi, F., Ronca, S., Foley, S. F., Agostini, S., and Lustrino, M.: Exotic magmatism from the western branch of the East African Rift: insights on the lithospheric mantle source., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5450, https://doi.org/10.5194/egusphere-egu22-5450, 2022.

Spinel peridotite xenoliths have been found in Cenozoic basalts from the Nuomin and Keluo areas in the northern Daxinganling. The Mg content of olivine in the mantleperidotite indicates that the upper mantle in the study area is partially refractory. According to the olivine content and Fo diagram, a part of peridotite xenoliths fell in the Archean and Proterozoic mantle regions, which reveals that there are remnants of ancient lithospheric mantle in the lithospheric mantle of the study area. In the study area, harzburgite and lherzolite show high oxygen fugacity values (FMQ + 1.95-3.15), which is in sharp contrast to the low oxygen fugacity values of the relatively reduced ancient lithospheric mantle. It is possible that the Paleozoic paleo Asian Ocean and Mesozoic paleo Pacific subducted successively under the Xingmeng orogenic belt, resulting in the oxidation of the lithospheric mantle at that time. K ₂O (1% ～ 6%) is found in the reaction edge of mantle xenoliths. It is considered that the mantle in the study area has experienced multiple periods of K-rich meltactivity, and the source of K-rich melt may be related to the crust source material recycled by subduction.

How to cite: Liu, J. and Li, H.: Oxygen fugacity characteristics of lithospheric mantle peridotite in northern Xingmeng orogenic belt, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5941, https://doi.org/10.5194/egusphere-egu22-5941, 2022.

The Tuva-Mongolian microcontinent and Khamardaban terrane are known as major tectonic units accreted to the Siberian paleocontinent. We report ²⁰⁷Pb/²⁰⁶Pb ages of 2.44–2.22 Ga for sources of Late Cenozoic volcanic rocks from the Tunka volcanic zone and of 1.63–1.31 Ga for those from the Khamardaban zone. The new ages are consistent with Precambrian geological events that are characteristic of the area and contradict the existing opinion about the Early Paleozoic collisional connection between these tectonic units inferred from dating of syn-collisional granites.

On the one hand, we constrain ore-forming processes in the Gargan block of the Tuva-Mongolian microcontinent and in the south of the Siberian paleocontinent between 2.45 and 1.4 Ga and between 1.3 and 0.25 Ga, respectively [Rasskazov et al., 2010]. The latest Pb-separating event in the Gargan block was followed by the generation of restite ultrabasic Ilchir belt that bounds the block from the south [Kiseleva et al., 2020]. So, we trace the boundary between the Gargan block and Ilchir belt to magma sources of the Tunka and Khamardaban zones that reasonably denote the root part of the Khamardaban terrane, accreted to the Tuva-Mongolian microcontinent and Siberian paleocontinent 1.63–1.31 Ga ago (Figure). On the other hand, we emphasize the importance of ore-forming events in the Gargan block, launched about 2.45 Ga, simultaneously with source generation in the Tunka zone. Basalts of this zone include xenoliths of fassaitic clinopyroxenites that show wide variations in the oxidation–reduction state. We suggest that fassaite (diopside) mineralization was due to interaction between orthopyroxene and calcite: (Mg, Fe)₂Si₂O₆ + CaCO₃ → (Mg, Ca)₂Si₂O₆ + CO₂ + FeO. Orthopyroxene of high-Mg spinel harzburgite xenoliths from Khobok River lavas (Tunka basin) shows SiO₂ content as high as 58.7 wt. %, while fassaite from pyroxenite xenoliths has SiO₂ content as low as 49 wt. %. Fassaitization of orthopyroxenites and harzburgites, obviously, releases both iron and silica. These components are found as amorphous Fe–Si phases in metasomatite xenoliths with low Mg/Si and Al/Si ratios [Ailow et al., 2021]. From data obtained, we speculate that fassaitization was an effective crust-mantle process of 2.4–2.2 Ga that could provide both the deep-seated Fe–Si mineralization and the generation of ferruginous quartzites displayed in the Great Oxidation Event.

Ailow Y. et al. // Lithosphere. 2021. V. 21, No. 4. P. 517–545.

Kiseleva O.N. et al. // Minerals. 2020. V. 10. P. 1077.

Rasskazov S.V. Brandt S.B., Brandt I.S. Radiogenic isotopes in geologic processes. Springer, 2010. 306 p.

How to cite: Rasskazov, S., Chuvashova, I., Saranina, E., Yasnygina, T., and Ailow, Y.: Crustal versus mantle events of 2.44–2.22 and 1.63–1.31 Ga at the junction between Khamardaban terrane, Tuva-Mongolian microcontinent, and Siberian paleocontinent: Petrogenetic consequences, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6686, https://doi.org/10.5194/egusphere-egu22-6686, 2022.

The persistence of Archaean cratons for >2.5Ga was aided by thick, mechanically strong, and cool lithospheric mantle keels up to 250km deep. It is widely accepted that the cratonic mantle, dominated by depleted harzburgite, lherzolite and dunite, was formed by extensive melt extraction from originally fertile mantle peridotite. Models seeking to explain the formation of deep cratonic mantle in the garnet and diamond stability fields, initially sought to answer how such rocks could form in-situ at high temperatures and pressures and envisaged large-scale thermochemical plume upwellings. More recently, mineralogical and geochemical observations, namely the high Cr content of garnet and low whole rock HREE concentrations in cratonic harzburgites, have led to the conclusion that the deep cratonic mantle couldn’t have originally melted in the garnet stability field.  Mechanical stacking of shallowly depleted oceanic lithosphere was therefore proposed to have thickened the depleted lithosphere cratonic roots. In this process, the spinel facies minerals are envisaged to transform into the garnet stability field.

Here we present the first results of combined thermodynamic and geochemical modelling at temperatures high enough to reconcile the very refractory residues. We found that the requirement for initially shallow melting is no longer supported. Deep (150-250km), ultra-hot (>1800°C), incremental melting can produce the mineralogical and geochemical signatures of depleted cratonic harzburgites. The modelling also implies a link between areas of extreme depletion in the deep lithospheric mantle and the genesis of Earth’s hottest lavas (Al-enriched komatiite) by re-melting depleted harzburgite. Diamond inclusion minerals have a well-documented skew to the most refractory compositions found in cratonic peridotite. We propose that these ultra-depleted, highly reducing regions of the lithospheric root possess the highest diamond formation and preservation potential.

How to cite: Walsh, C., Kamber, B., and Tomlinson, E.: Deep ultra-hot melting in cratonic mantle roots, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6723, https://doi.org/10.5194/egusphere-egu22-6723, 2022.

In terms of Pb isotope ratios, melting anomalies of Central and East Asia show no high μ (HIMU, high ²³⁸U/²⁰⁴Pb) signature that was generated on the Earth about 2 Ga ago and was caused by sulfide sequestration of Pb from the mantle to the core [Hart and Gaetany, 2006]. In such particular environment, we use Pb isotope data on Late Phanerozoic volcanic rocks to develop general systematics of their sources through definition of initial viscous protomantle reservoirs with low μ and elevated μ signatures (LOMUVIPMAR and ELMUVIPMAR, respectively) that imply a solidification time of the mantle in the Hadean magma ocean between 4.54 and 4.44 Ga ago. We suggest that the protomantle reservoirs retained specific Pb isotope signatures in the early, middle, and late epochs of the Earth's evolution (4.54–3.6, 2.9–1.8, and  <0.7 Ga ago, respectively) [Rasskazov et al., 2020]. In this presentation, we report the first representative Pb isotope data on the ELMU signature of Late Cenozoic rocks from the Dariganga volcanic field, Southeast Mongolia. Pb isotope secondary-isochron patterns of volcanic rocks show protomantle material that was not differentiated between 4.474 and 4.444 Ga (i.e. directly ascended from a deep mantle reservoir in the Cenozoic). In addition, the material was also differentiated in the deep mantle at about 3.69, 2.16, and 1.74 Ga. Pb isotope data on volcanic fields of North China are indicative for lateral change from the ELMU to LOMU signature (Figure). We infer that sources of volcanic rocks from Southeast Mongolia and North China display the primary inhomogeneity of the deep mantle that was generated in the Hadean magma ocean from its initial solidification as early as 4.54 Ga to its final respond of 4.44 Ga.   

Hart, S.R. &  Gaetani, G.A. (2006). Mantle paradoxes: the sulfide solution. Contrib. Mineral. Petrol., 152, 295–308.

Rasskazov, S., Chuvashova, I., Yasnygina, T., & Saranina, E. (2020). Mantle evolution of Asia inferred from Pb isotopic signatures of sources for Late Phanerozoic volcanic rocks. Minerals, 10 (9), 739. 

How to cite: Chuvashova, I., Rasskazov, S., Sun, Y., Yasnygina, T., and Saranina, E.: Lateral change of  ELMU–LOMU sources for Cenozoic volcanic rocks from Southeast Mongolia and North China: Tracing zonation of solidified Hadean magma ocean, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6724, https://doi.org/10.5194/egusphere-egu22-6724, 2022.

The method of cluster (R and G methods) analysis of the allocation of the cluster (CG) and chemical-generic (CGG) groups of average values of the compositions of diamond indicator minerals (DIM) (Ivanov, 2017) was used, supplemented with data on the frequency of occurrence (FO) of habitus forms, twins and clusters of diamond crystals of three kimberlite pipes of Angola – Catoca, Luele and Txiuzo (Ganga et al., 2021). Clustering of MSDS by their chemical composition was carried out on the basis of chemical-genetic classifications of Dowson J. and Soboleva N.V. for garnets (Dowson et al., 1975; Sobolev, 1973) and Garanin V. K. for Cr-diopsides, of ilmenite and chromite (Garanin et al., 1991).

It is found that FO CG/СGG of МSD are indirect and inverse significant correlation with FO habitus forms, twins and adhesion of diamond crystals of these kimberlite pipes. This is demonstrated by histograms of the joint distribution of CG DIM and habitus forms, twins and splices of diamond crystals from geological samples of kimberlites at their deposits (Fig. 1).

The fractions of octahedra (O) and transition habit crystals (OD) decrease in parallel with a decrease in the proportions of CG G9, G10 pyropes and an increase in G1 and G2a, an increase in the proportions of CGG 2b and 4b picroilmenites. The shares of rhombododecahedron, including dodecahedrons (RD), grow with the growth of the shares of CG pyropes G1a, G2a, as well as CGG picroilmenites 2b and CGG Cr-diopsides S2 and S5. The shares of twins (Tw), splices (Agr) and polycrystalline bead (PC) decrease in the studied tubes with a decrease in the shares of CG pyropes G10, G10a and an increase in the shares of CGG picroilmenites 2b and 4b and CGG Cr-diopsides S2 and S5 (Fig. 1). The presence of Ti and Fe compounds, which are part of DIM in elevated concentrations, in the process/medium of diamond crystal formation contributes to the formation of habitus forms OD and RD (D - dodecahedrons) during dissolution associated with low-chromium pyropes CG G1 and G2. Medium-high chromium pyropes CG G10 and G10a are associated with octahedral habitus (O) diamond crystals and their spinel counterparts (TwSp), whose shares they control.

Petrogenetic affiliation of CG/CGG MSD to various associations of deep mantle rocks allows us to identify the most favourable conditions and environments for the origin and growth of diamonds (high FO of O+TwSp+Tw) and environments (conditions) of their dissolution (high FO of OD+RD+Th+C). Interesting that the diamond grade calculated diamond deposits (Ct /T) is positively correlated with FO (Ar g+Tw), SGG S6 picroilmenites and SG G10 garnets, but the FO (RD+OD) has a negative effect on diamond grade, which allows determining the degree of the fertility of the mantle sources by DIM diamond.

How to cite: Zinchenko, V. and Ivanov, A.: Correlation of habitus forms, twins and aggregates of diamond crystals with the composition of its indicator minerals from kimberlite pipes of Angola, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6809, https://doi.org/10.5194/egusphere-egu22-6809, 2022.

Information about the compositions of primitive kimberlite melts is important for understanding the petrogenesis of kimberlites. Reconstruction of the composition of these melts is very difficult because the melts greatly changed their compositions via assimilation of mantle and crust xenogenic materials and degassing during ascent.

To reconstruct the composition of the kimberlite melt of the Bultfontein pipe (Kaapvaal craton, South Africa), the mineral assemblage of secondary melt inclusions in olivines of mantle peridotite xenoliths from the pipe has been studied. The depths of equilibrium of the studied peridotites range from 120 to 150 km.

The inclusions occur along the healed cracks in the olivine grains. Twenty-five daughter minerals were found in the inclusions by Raman spectroscopy and scanning electron microscopy. Liquids and gases were not detected. The inclusions are mainly made up of carbonates (calcite CaCO₃, magnesite MgCO₃, dolomite CaMg(CO₃)₂, eitelite Na₂Mg(CO₃)₂, nyerereite Na₂Ca(CO₃)₂, gregoryite (Na,K,Ca)₂CO₃, K-Na-Ca-carbonate (K,Na)₂Ca(CO₃)₂, shortite Na₂Ca₂(CO₃)₃) or carbonates with additional anions (nahcolite NaHCO₃, bradleyite Na₃Mg(PO₄)(CO₃), northupite Na₃Mg(CO₃)₂Cl, burkeite Na₆CO₃(SO₄)₂, tychite Na₆Mg₂(CO₃)₄(SO₄)). Halides (halite NaCl, sylvite KCl), sulfates (glauberite Na₂Ca(SO₄)₂, thenardite Na₂SO₄, aphthitalite K₃Na(SO₄)₂), phosphate (apatite Ca₅(PO₄)₃(F,Cl,OH)), oxides (rutile TiO₂, magnetite FeFe₂O₄), sulfide (heazlewoodite Ni₃S₂) and silicates (phlogopite KMg₃AlSi₃O₁₀(F,Cl,OH), tetraferriphlogopite KMg₃FeSi₃O₁₀(F,Cl,OH), richterite Na₂Ca(Mg,Fe,Mn,Al)₅[Si₄O₁₁](OH,F)₂) are also present in the inclusions.

These inclusions are considered to be relics of a near‐primary or primitive kimberlitic melt that later formed the Bultfontein pipe. The observed mineral assemblage indicates that the captured melt had an alkali-carbonatitic composition and was rich in Cl and S.

This work was supported by the Russian Foundation for Basic Research (grant No. 20-35-70058).

How to cite: Tarasov, A., Golovin, A., and Sharygin, I.: Reconstruction of the composition of the kimberlite melt of the Bultfontein pipe, Kaapvaal craton, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6844, https://doi.org/10.5194/egusphere-egu22-6844, 2022.

The article considers the geological position of the Zhidoy massif and its age. The scheme of magmatism of the massif has been developed. The graphs of paired correlations of petrogenic elements in massif rocks which had a consistent trend in composition are given for validation purposes. The present article provides graphs of REE spectra and the spider diagram of rare elements concentration in the massif rocks. Pyroxenites are the early rocks of the massif, which are the ores for titanium. Titanium is concentrated in three minerals: titanium magnetite, ilmenite and perovskite. The main type of titanium ores, perovskite, is known only in the Zhidoy massif. A conclusion about the mantle sources of the primary magma of the massif
is drawn based on the geochemistry of the isotopes Sr and Nd.

Fig. 1 Spectra of rare-earth elements in rocks of the Zhidoy massif (chondrite-normalized).
Symbols: 1−pyroxenite, 2−ijolite, 3−syenite, 4−phenite

Fig. 2 Spider-diagram of the Zhidoy massif rocks

Conclusions
1. Three varieties of ore pyroxenites have been defined−titanium-magnetite, ilmenite and perovskite ore.
2. The petrochemical diagrams show a common trend in the composition of rock- forming elements, indicating the homomorphism of the rocks and their crystallization from a single primary magma.
3. Geochemical data also confirm the genetic relation of the Zhidoy massif.
4. Mantle source, the depurated mantle for the primary magma of the Zhidoy massif, has been determined on the basis of isotope data.

How to cite: Sotnikova, I., Vladykin, N., and Alymova, N.: Zhidoy Alkali-Ultramafic Rock and Carbonatite Massif: GeochemicalFeatures, Its Sources And Ore-Bearing, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7771, https://doi.org/10.5194/egusphere-egu22-7771, 2022.

International Ocean Discovery Program (IODP) Expedition 357 drilled 17 shallow sites spanning ~10 km in the spreading direction (from west to east) across the Atlantis Massif oceanic core complex (OCC, Mid-Atlantic Ridge, 30°N). Exposed mantle in the footwall of the Atlantis Massif OCC is predominantly nearly wholly serpentinized harzburgite and subordinate dunite. Altered peridotites are subdivided into: (I) serpentinites, (II) melt-impregnated serpentinites, and (III) metasomatized serpentinites. Type I serpentinites show no evidence of melt-impregnation or metasomatism apart from serpentinization and local oxidation. Type II serpentinites have been intruded by gabbroic melts and are distinguishable in some cases based on macroscopic and microscopic observations, e.g., mm-cm scale mafic-melt veinlets, rare plagioclase (˂0.5 modal % in one sample) or by the local presence of secondary (replacive) olivine after orthopyroxene; in other cases, ‘cryptic’ melt-impregnation is inferred on the basis of incompatible element enrichment. Type III serpentinites are characterized by silica metasomatism manifest by alteration of orthopyroxene to talc and amphibole, anomalously high anhydrous SiO₂, and low MgO/SiO₂. Two fundamental features of the mantle serpentinites are identified: (1) A pattern of increasing melt-impregnation from west to east; and (2) a link between melt-impregnation and metamorphism. In regard to (1), whereas a dominant fluid- rock alteration (mostly serpentinization) is distinguished in the western serpentinites, a dominant mechanism of melt-impregnation is recognized in the central and eastern serpentinites. Melt-impregnation in the central and eastern sites is characterized by enrichment of incompatilble elements, Cr-spinel with anomalously high TiO₂ (up to 0.7 wt.%) and olivine forsterite (Fo) compositions that range to a minimum of Fo_86.5.  With respect to (2), in contrast to unmetamorphosed Cr-spinel of western site Type I serpentinized peridotites, spinel of the melt-dominated central and eastern peridotites record metamorphism, which ranges from sub-greenschist (<500°C) to lower amphibolite (>600°C) facies. Low grade, sub-greenschist facies metamorphism resulted in Mg and Fe²⁺ exchange between Cr-spinel and olivine resulting in Cr-spinel with anomalously low Mg# (cationic Mg/(Mg+Fe²⁺)). Higher grade amphibolite facies metamorphism resulted in Al-Cr exchange and the production of Fe-chromite and Cr-magnetite. Heat associated with magma injection and subsequent melt-impregnation resulted in localized contact metamorphism. High degrees of melt extraction are evident in low whole-rock Al₂O₃/SiO₂ and low concentrations of Al₂O₃, CaO, and incompatible elements. Estimates of the degree of melt extraction based on Cr# (cationic Cr/Cr+Al, up to ~0.4) of unaltered Cr-spinel and modeled whole rock REE patterns, suggest a maximum of ~18-20% non-modal fractional melting. As some serpentinite samples are ex-situ rubble, the magmatic histories at each site are consistent with derivation from a local source (the fault zone) rather than rafted rubble that would be expected to show more heterogeneity and no spatial pattern. In this case, the studied sites may provide a record of enhanced melt-rock interactions with time, consistent with proposed geological models for OCC formation.  

How to cite: Whattam, S. A.: Spatial patterns of fluid- and melt-rock processes and link between melt-impregnation and metamorphism of Atlantis Massif peridotites (IODP Expedition 357), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9186, https://doi.org/10.5194/egusphere-egu22-9186, 2022.

Unravelling the processes taking place during the genesis of kimberlites, their ascent through the sub-cratonic mantle and their emplacement in the crust is challenging, as kimberlites are mixtures of mantle-derived and magmatic components, rarely preserving pristine evidence of their original nature. Furthermore, their intense state of alteration makes it difficult to access the textural-compositional record of information engraved in the phase constituents. In this study, fresh samples of kimberlites and related mantle-derived xenocrysts-xenoliths from the Udachnaya pipe (Siberia) were investigated to reconstruct the pressure-temperature-time-composition (P-T-t-X) framework of the sub-cratonic lithosphere at the time of kimberlite emplacement. Routine and high-precision electron microprobe analyses of olivine, phlogopite and spinel from different facies of the Udachnaya pipe (intrusive coherent, hypabyssal and pyroclastic, sensu Scott Smith et al., 2013) showed that specific phase assemblages are associated with each evolutionary stage of the kimberlite. Olivine composition, in particular, is extremely variable, ranging from high-Fo and high-Ni (Fo93; NiO = 0.45 wt%) to low-Fo and low-Ni (Fo85; NiO = 0.10 wt%), but also to high-Fo and low-Ni (Fo>93; NiO <0.05 wt%) terms, often encompassing the whole compositional spectrum in a single sample and/or showing marked zoning within the individual crystals. 
A comparison between the main constituents of the Udachnaya kimberlite and those of the mantle xenoliths sampled during ascent, complemented by detailed major-trace element profiles on olivine crystals, was put forward to: (i) discriminate between the mantle-derived xenocryst cargo and the magmatic assemblage; (ii) model the P-T-fO2 path of kimberlites; (iii) speculate about their ascent rate; (iv) model the interactions between kimberlite-related fluid/melts and the Siberian sub-cratonic lithosphere.

REFERENCES
Scott Smith, B.H., Nowicki, T.E., Russell, J.K., Webb, K.J., Mitchell, R.H., Hetman, C.M., ... & Robey, J.A. (2013). Kimberlite terminology and classification. In Proceedings of 10th international kimberlite conference (pp. 1-17). Springer, New Delhi.

How to cite: Casetta, F., Abart, R., Ntaflos, T., Ashchepkov, I., and Coltorti, M.: Mantle-derived cargo vs. magmatic growth: ascent path, dynamics of the Udachnaya kimberlite and interactions with the Siberian sub-cratonic lithosphere, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9214, https://doi.org/10.5194/egusphere-egu22-9214, 2022.

Large Igneous Provinces (LIPs) represent exceptionally brief (<1 Ma) voluminous magmatic events that punctuate Earth history, frequently leading to continental break-up, global climate changes and, eventually, mass extinctions. Most LIPs emplaced in continental settings are located near cratons, begging the question of a potential control of thick lithosphere on mantle melting dynamics. In this study we discuss the case of the Central Atlantic Magmatic Province (CAMP), emplaced in the vicinity of the thick lithospheric keels of the Precambrian cratons forming the central portion of Pangea prior to the opening of the Central Atlantic Ocean. In particular, we focus on CAMP magmas of the Prevalent group, ubiquitous all over the province, and of the Tiourjdal and High-Ti groups, emplaced (respectively) at the edges of the Reguibat and Leo-Man shields in north-western Africa, and the Amazonian and São Luis cratons in South America. As imaged by recent tomographic studies, there is a strong spatial correlation between most CAMP outcrops at surface and the edges of the thick cratonic keels. Geochemical modelling of trace element and isotopic compositions of CAMP basalts suggests a derivation by partial melting of a Depleted MORB Mantle (DMM) source enriched by recycled continental crust (1-4%) beneath a lithosphere of ca. 80 km. Melting under a significantly thicker lithosphere (>110 km) cannot produce magmas with chemical compositions similar to those of CAMP basalts. Therefore, our results suggest that CAMP magmatism was produced by asthenospheric upwelling along the deep cratonic keels and subsequent decompression-induced partial melting in correspondence with thinner lithosphere. Afterwards, lateral transport of magma along dykes or sills led to the formation of shallow intrusions and lava flows at considerable distances from the source region, possibly straddling the edges of the cratonic lithosphere at depth. Overall, the variations of the lithospheric thickness (i.e., the presence of stable thick cratonic keels juxtaposed to relatively thinner lithosphere) appear to play a primary role for localizing mantle upwelling and partial melting during large-scale magmatic events like the CAMP.

How to cite: Boscaini, A., Marzoli, A., Bertrand, H., Chiaradia, M., Jourdan, F., Faccenda, M., Meyzen, C., Callegaro, S., and Serrano Durán, L.: The architecture of the lithospheric mantle controlled the emplacement of the Central Atlantic Magmatic Province, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9813, https://doi.org/10.5194/egusphere-egu22-9813, 2022.

The Kamthai carbonatites form part of the Sarnu-Dandali alkaline complex (SDAC) of Rajasthan, western India. The SDAC is one of several alkaline intrusive complexes emplaced prior to the Deccan continental flood basalt eruptions. Considered as one of the earliest Deccan-Reunion plume related magmatic activities, the rocks of the complex hold clues to many tectonomagmatic processes such as plume-lithosphere interaction, mantle melting prior to flood basalt volcanism, and carbonatite-plume relationship, apart from the outstanding questions pertaining to the origin of carbonatites themselves, and their association with alkaline silicate rocks. To understand some of these processes vis-à-vis the evolution of the complex, we have carried out a detailed field, petrographic, geochronological (⁴⁰Ar/³⁹Ar), geochemical, and Sr-Nd-Pb-C-O isotopic investigation. Phlogopites from carbonatites yield an age of 68.6 Ma, identical to the ages determined for the three associated phonolite dykes. Interestingly, an earlier study reports the presence of older (89-86 Ma) subvolcanic and volcanic bodies in the complex, thus suggesting recurrent alkaline magmatism. Carbonatites of Kamthai occur as veins, dykes, and small plugs, along with dykes/plugs of ijolite, nephelinite, syenite and phonolite etc. The SDAC intrudes into the basement made up of Malani Rhyolites. The stable C-O isotopic compositions of unaltered carbonatites (δ¹³C_PDB= -6.6 to -4.6 ‰; δ¹⁸O_SMOW=5.5 to 9.5 ‰), which are predominantly calcite carbonatites, not only confirm the magmatic nature of the rocks but also show evidence of fractional crystallization. The chondrite-normalized rare-earth element patterns of the carbonatites and alkaline silicate rocks show LREE enriched patterns, with the former possessing abnormally high contents of LREE. The average (⁸⁷Sr/⁸⁶Sr)_i and ε_Nd(t=68.5 Ma) for carbonatites are 0.7043±0.0001 and 2.4±0.2, respectively, which are indistinguishable from those for the alkaline silicate rocks (⁸⁷Sr/⁸⁶Sr)_i= 0.7045±0.0003; ε_Nd(t)=2.4±0.4), which suggests common parentage. All these data point towards a petrogenetic link between the 68.6 Ma carbonatites and alkaline silicate rocks of the SDAC, either through liquid immiscibility or fractional crystallization of a common parental magma. Overlapping initial Sr-Nd isotopic ratios of these rocks with those of the least contaminated Deccan lava flows and the Reunion island rocks suggest a possible genetic link between the SDAC and the Deccan-Reunion plume. 

How to cite: Mahala, M. K. and Ray, J. S.: Age and geochemistry of the Kamthai carbonatites, Rajasthan, western India, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10951, https://doi.org/10.5194/egusphere-egu22-10951, 2022.

Reconstruction of kimberlite melt composition is especially important to understand the processes of mantle-derived magmatism and the Earth’s mantle evolution. This task seems to be very complicated because mantle melts during ascent and emplacement changed their initial characteristics due to degassing and contamination by both mantle and crustal xenogenic materials. Moreover, mantle magmatic rocks are often subjected to secondary alteration. Melt inclusions in minerals of mantle xenoliths can preserve information about the initial characteristics of mantle melts.

Here, we present the results of a study on secondary crystallized melt inclusions in olivines in two partially serpentinized xenoliths of sheared peridotites (AKM-42n and AKM-56) from the Komsomolskaya-Magnitnaya pipe (Upper Muna field, Siberian craton). The mantle residence P–T conditions of AKM-42n and AKM-56 are 6.4 GPa and 1380°C, and 6.7 GPa and 1395°C, respectively.

We identified twenty-one daughter minerals in the melt inclusions using confocal Raman spectroscopy and scanning electron microscopy coupled with energy-dispersive X-ray microanalysis. The minerals within the inclusions are presented by chlorides (sylvite KCl and halite NaCl), silicates (tetraferriphlogopite KMg₃Fe³⁺Si₃O₁₀(OH,F)₂, phlogopite KMg₃AlSi₃O₁₀(OH,F)₂, olivine (Mg,Fe)₂SiO₄, clinopyroxene (Ca,Mg,Fe)₂Si₂O₆, and monticellite Ca(Mg,Fe)SiO₄), carbonates (nyerereite (Na,K)₂Ca(CO₃)₂, shortite Na₂Ca₂(CO₃)₃, eitelite Na₂Mg(CO₃)₂, dolomite CaMg(CO₃)₂, calcite CaCO₃, and magnesite MgCO₃), carbonates with additional anions (burkeite Na₆CO₃(SO₄)₂ and tychite Na₆Mg₂(CO₃)₄(SO₄)), sulphates (aphthitalite K₃Na(SO₄)₂ and thenardite Na₂SO₄), fluorapatite Ca₅(PO₄)₃F, sulfides (pyrrhotite Fe_1-xS and djerfisherite K₆(Fe,Ni,Cu)₂₅S₂₆Cl) and magnetite FeFe₂O₄.

The studied melt inclusions are considered to be relics of a near‐primary or primitive kimberlite melt that formed the Komsomolskaya-Magnitnaya pipe. The assemblage of the daughter minerals indicates that the melt had an alkali-carbonatitic composition and was enriched in Cl and S.

This work was supported by the Russian Foundation for Basic Research (grant No. 20-35-70058) and the Russian Science Foundation (grant No 18-77-10062).

How to cite: Kalugina, A., Sharygin, I., Solovev, K., Golovin, A., and Dymshits, A.: Composition of the kimberlite melt of the Komsomolskaya-Magnitnaya pipe (Upper Muna field, Siberian craton), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10980, https://doi.org/10.5194/egusphere-egu22-10980, 2022.

The problem of the lithospheric mantle structure under ancient cratons and their evolution attracts researchers in connection with the question of the diamond genesis. The petrological way is based on the mineral composition studying in xenoliths from the mantle depths. The Mirny kimberlite field belongs to the diamond-bearing kimberlite fields in the center of the Siberian craton. The authors studied a collection of mantle xenoliths from the Mir pipe (57 samples). The samples were classified as peridotites (Grt lherzolites) and pyroxenites (Grt websterites, Grt clinopyroxenites and eclogites).

Lherzolites from the Mir pipe are characterized by a high degree of alteration; olivine and orthopyroxene are replaced by serpentine in many samples (up to 50–70%). Websterite rocks are different by the presence of orthopyroxene and clinopyroxene, while clinopyroxene may contain lamellae of exsollution structures. Garnet websterites are distinguished by orange-reddish color of garnet, dark green color of pyroxene and dominanting medium-large-grained hypidiomorphic-granular textures; porphyroblastic and granoblastic textures (up to mosaic) are also observed. In garnet clinopyroxenites rutile is usually present in the form of thin (5–20 µm) needles in garnet and clinopyroxene. Eclogites are characterized by orangish or pinkish garnet color and granoblastic structure.

Garnets from lherzolites and websterites are also characterized by a relatively high Mg# content (75–83) and low TiO₂ contents (up to 0.2 wt %). It belongs to the lherzolite paragenesis by content CaO (3.68 - 5.35 wt.%) and Cr₂O₃ (0.07-3.7 wt.%). Eclogites are characterized by high-calcium (3.78 - 9.46 wt.%) and high-iron (7.77 - 17.20 wt.%) composition of garnet getting into the ​​wehrlite paragenesis area. None of the garnet studied compositions belongs to the high-chromium dunite - harzburgite paragenesis. Also garnets from the lithospheric mantle under the Mirny kimberlite field are characterized by a low-Ti garnet composition (up to 0.7 wt.%). Thus, the lithospheric mantle under the Mirny kimberlite field differs from the lithospheric mantle under other diamondiferous fields (for example, Udachnaya kimberlite pipe). The Mirny mantle xenoliths are characterized by the pyroxenites widespread development (up to 50%), the low-Ti composition and deformed lherzolites absence. These features indicate the minimal silicate metasomatic alteration in the lithospheric mantle under the Mirny field (in contrast to the center of the Siberian craton). The isotopic oxygen composition in garnet and clinopyroxene was also determined. The δ¹⁸O value varies in Cpx from 5.7-5.8‰ in clinopyroxenites and 6.1-6.1‰ in eclogites. On the whole, minerals from pyroxenites demonstrate δ¹⁸O values exceeding mantle values, which suggests a wide development of melting processes in the lithospheric mantle in the south of the Siberian craton Craton and the formation of megacrystalline pyroxene cumulates. In some cases, metamorphic recrystallization leads to oxygen isotope equilibrium between garnet and clinopyroxene. For minerals from eclogites higher values ​​of δ¹⁸O are noted, which may indicate the origin of eclogites from subducted oceanic crust, the presence of a subduction component in the process of formation of the lithospheric mantle.

The research was supported by Russian Science Foundation grant №20-77-00074.

How to cite: Kalashnikova, T., Kostrovitsky, S., Solovieva, L., Sinitsyn, K., and Yudintseva, E.: Garnets from xenolith in Mir kimberlite pipe: chemical composition and genesis, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11936, https://doi.org/10.5194/egusphere-egu22-11936, 2022.

Orogenic garnet peridotites exhumed in ultrahigh-pressure-ultrahigh-temperature terranes represent windows into material transfer in deep subduction zones. Multiphase solid inclusions (MSI) trapped in garnet proved to be important tracers of metasomatism by crustal-derived fluids. Our study of the MSI from the Saxothuringian basement in the Bohemian Massif, European Variscan Belt, allowed identifying the source and evolution of the liquids metasomatized the mantle rocks. As the MSI could not be re-homogenized due to a high content of volatiles, their bulk composition was estimated considering the proportions, phase densities and chemical composition of the constituent minerals.

The MSI occur in an annulus at garnet rim of garnet lherzolite and harzburgite, and throughout garnet in garnet pyroxenite. The major phases of the MSI include amphibole, barian mica and carbonate (dolomite, magnesite). Minor phases are clinopyroxene, orthopyroxene, garnet II, spinel, apatite, monazite, thorianite, graphite, pentlandite, scheelite and sulphides. The proportion of hornblende systematically decreases from pyroxenite and close harzburgite and lherzolite to more distal mantle rocks, where clinopyroxene and garnet II occur instead. By contrast, the amount of barium-bearing phases (barian mica, Ba-Mg carbonate norsethite, barian feldspar) and carbonates increases in the same direction.

Major element composition of garnet pyroxenite, including enrichment in alkalies and barium, approaches carbonate-silicate melts similar to kimberlites.  Trace element signatures indicate that it is a rare example of low-degree supercritical liquid derived from a mixed crust-mantle source frozen in the mantle. The MSI hosted by garnet in pyroxenite represent a residual solute-rich liquid after high-pressure fractional crystallization of the parental melt, enriched in alkalies (Na, K), highly incompatible elements (LILE – Ba, Sr; Th, U), LREE, Ti, W and volatiles (CO₂, Cl, F, P). The MSI in peridotites allow tracing the changes of this metasomatizing liquid during its reactive infiltration into peridotite through silicate crystallization as well as interaction with mantle minerals distinct in lherzolite and harzburgite (garnet±clinopyroxene). The liquid evolved from more silicic, solute-rich to more diluted carbonate-rich, with gradual enrichment in LILE (K, Ba) and volatiles (CO₂, Cl) and LREE fractionation, similar to evolution of kimberlitic to carbonatitic melts through differentiation by fractional crystallization.  

Here we demonstrate that the MSI trapped in garnet can be used as a unique tool for tracing chemical evolution of the liquids metasomatizing the mantle wedge. Importantly, these results are valid even in the case of the interaction of the trapped material (MSI) with the host garnet, as this potential contamination mainly concerns Al, Si and Cr while majority of the other elements used for petrogenetic implications remained unaffected

How to cite: Kotkova, J., Čopjaková, R., and Škoda, R.: Source and evolution of metasomatizing liquids in orogenic peridotites: evidence from multiphase solid inclusions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13248, https://doi.org/10.5194/egusphere-egu22-13248, 2022.

Earth’s topography is supported by both crustal and sub-crustal density variations. Dynamic topography results from the vertical displacement of the Earth’s surface due to processes operating within the mantle. Thus, isolating and quantifying observable dynamic topography can yield valuable information about mantle dynamics. An observationally-based approach can be used to investigate dynamic topography by calculating residual depth anomalies in the oceanic realm and residual topographic anomalies on the continents. To constrain the residual topographic contribution that arises from sub-crustal processes it is necessary to correct for crustal and sedimentary loading. We identify and correct for both forms of loading by exploiting a variety of seismologic datasets that include seismic reflection profiles, wide-angle/refraction surveys and receiver functions. We present a revised global compilation of oceanic residual depth measurements (n = 10,846) and continental residual topographic measurements (n = 3,897). This compilation represents a significant improvement in terms of the quantity and spatial distribution of measurements. In the oceanic realm, the correction methodology has been revised in two ways, which has improved resolution and accuracy. First, the crustal correction now accounts for variations in bulk density as a function of crustal thickness. Secondly, the quartz and clay content of the sedimentary column has been adjusted, which improves the quality of the sedimentary correction. The revised global compilation is used to generate a spherical harmonic representation of observable dynamic topography out to degree 40 (i.e. ~ 1000 km). The resultant power spectrum demonstrates that dynamic topography varies linearly with inverse wavenumber. Our global results are consistent with independent geologic markers of uplift and subsidence.

How to cite: Holdt, M., White, N., and Stephenson, S.: Constraining global dynamic topography using a revised observational database, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-345, https://doi.org/10.5194/egusphere-egu22-345, 2022.

The growth of the lithospheric mantle following tectonic thinning of the plate is well understood and is a primary consequence of the conductive cooling that is a key driver of mantle convection. However, it is less well understood how the lithosphere interacts with and responds to sub-plate temperature anomalies within the the underlying convecting mantle. Here, we investigate the evolution of the lithospheric mantle during and after intraplate magmatism. First, we examine the thickness of lithosphere beneath oceanic intraplate magmatic provinces that are < 10 Ma in age. Modelling of major oxide and rare earth element concentrations, alongside seismic tomography, indicate that these provinces lie upon lithosphere that is significantly thinner than expected given the age of underlying lithosphere. For example, Hawaii overlies lithosphere that is 50–80 km thick, despite the age of the plate suggesting a thickness of > 100 km. Next, we explore the lithospheric thickness beneath ancient intraplate magmatic provinces that have a record extending back to Jurassic times. Geochemical modelling demonstrates that, like recent magmatism, these provinces were also erupted atop thinner than expected lithosphere. Seismic tomography provides a further constraint on the thickness of the lithosphere by constraining the thermal structure of the upper mantle. By exploiting these tomographic images we show that the lithospheric mantle beneath ancient seamounts gets progressively thicker as a function of their eruption age. Importantly however, the lithosphere is consistently thinner by up to 40 km than would be expected if the plate cooled and thickened from a mid ocean ridge without perturbation. Finally, we extend our analysis to ancient continental large igneous provinces (LIPs). LIPs are massive accumulations (>100,000 km³) of magmatic material that are emplaced within a short period of geological time (1-2 Ma). We show that the lithospheric thickness beneath ancient continental LIPs increases as a function of time since eruption, following a similar relationship to oceanic LIPs. Our results suggest that the emplacement of LIPs causes ubiquitous thinning of the lithospheric mantle to thicknesses of 40–80 km, followed by systematic, progressive re-thickening via conductive cooling. Furthermore, they suggest that continental lithospheric mantle re-thickens to depths of > 200 km, supporting the idea that cratons can be destroyed by LIP emplacement and reformed following the end of eruption. Thinning and re-thickening of the lithospheric mantle during and after intraplate magmatism demonstrates that the lithosphere-asthenosphere boundary is routinely perturbed by sub-plate mantle convection. An understanding of LIP formation and its effect on the lithospheric mantle is necessary to reveal causal links with mass extinctions, continental break-up, and regional epeirogenic events.

How to cite: Stephenson, S. and Ball, P.: Destruction and Regrowth of Lithospheric Mantle by Emplacement of Large Igneous Provinces, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-524, https://doi.org/10.5194/egusphere-egu22-524, 2022.

Seismic tomographic models based only on wave velocities have limited ability to differentiate between a compositional or temperature origin for the Earth's 3D structure variations. Complementing wave velocities with attenuation (conversion of energy to heat) can help make that distinction, which is fundamental to understand mantle convection evolution. For example, a thermal origin for the lower mantle large low shear velocity provinces (LLSVPs) will point to them being short-lived anomalies, whereas a compositional origin will point to them being long-lived, forming stable 'anchors' and influencing the pattern of mantle convection. So far, only global 3D attenuation models built using seismic body waves and surface waves have been available for the upper mantle. Here, we use whole Earth oscillations or normal modes to measure 3D variations in mantle attenuation, which allow us to include focussing and scattering without the need for approximations. We achieve this by jointly measuring 3D variations in velocity and attenuation using splitting functions, which are depth-averaged models of how a mode 'sees' the Earth. 

Splitting functions are linearly dependent on heterogeneous structure and can be easily incorporated in tomographic models. We measured 14 anelastic splitting functions and used those to build a 3D global model of attenuation for the whole mantle. For comparison purposes, we have also constructed a 3D shear-velocity model using the same number of modes and model parametrization. In the upper mantle, we find high attenuation in the low velocity spreading ridges, which suggests a thermal origin and agrees with previous surface wave studies. In the lower mantle, we find the highest attenuation in the 'ring around the Pacific' high velocity region, which is thought to be the 'graveyard' of subducted slabs, and not in the LLSVPs beneath Africa and the Pacific. We compare our 3D attenuation model to the wave-speeds and attenuation predictions of a laboratory-based viscoelastic model. Our comparison indicates that the higher attenuation seen in the slab regions can be explained by a small grain-size in combination with cold temperatures, while the lower attenuation in the LLSVPs can be explained by a large grain-size in combination with high temperatures. Grain-size is related to viscosity in diffusion creep, which would mean that the LLSVPs have larger viscosity making them long-lived stable features, while the graveyard of slabs would have a lower viscosity making them shorter lived. 

How to cite: Talavera-Soza, S., Cobden, L., Faul, U. H., and Deuss, A.: Global 3D model of mantle attenuation using normal modes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-815, https://doi.org/10.5194/egusphere-egu22-815, 2022.

Continental rifting processes are influenced by viscous coupling of the deforming lithosphere to asthenospheric flow, as well as magma that migrates upward from the upper asthenosphere. Over the past few decades, significant advances have been made in finite element numerical methods that enable modeling of lithospheric deformation, viscous coupling to asthenospheric flow, and melt generation in the upper asthenosphere. In this work, we present new developments based in the NSF Computational Infrastructure for Geodynamics finite element code ASPECT (Advanced Solver for Problems in Earth’s Convection) that allow users to investigate lithospheric deformation, asthenospheric flow, and melt generation in the upper asthenosphere. Users have the options to constrain their initial temperature and density conditions with laterally varying lithospheric thickness, layers of crustal thickness, and shear wave seismic velocity models in the sublithospheric mantle. We present case studies from regions along the East African Rift System that demonstrate these capabilities. 

How to cite: Stamps, D. S., Njinju, E., Kwagalakwe, A., Naliboff, J., and Rajaonarison, T.: Continental Rifting Advances Using 3D Computational Modeling of Lithospheric Deformation, Asthenospheric Flow, and Deep Melt Generation with ASPECT, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-963, https://doi.org/10.5194/egusphere-egu22-963, 2022.

Mantle convection has profound effects on the Earth’s surface, such as inducing vertical motion, which is commonly termed dynamic topography. Sophisticated mantle convection models have been used to study current and past dynamic topography. But many input parameters, like complex rheologies and thermo-chemical flow properties remain poorly known, requiring ad-hoc model parameterization and long range extrapolation. This makes it attractive to explore simple analytic models of upper mantle flow. The existence of a weak asthenosphere allows one to explore upper mantle in the context of Poiseuille/Couette flow. The latter provides an geodynamically plausible link between flow properties and dynamic topography. Here we construct simple upper mantle flow models parameterized in terms of sources/sinks (plumes/slab) of Poiseuille/Couette flow. Our approach provides physical insight into the pattern of upper mantle flow, makes it easy to assess uncertainties of key model parameters, such as poorly resolved asthenospheric thickness and viscosity, and can be extended back in time, given first-order estimates of plume and subduction flux deduced from geological records. Importantly, it demands low computational cost relative to a time dependent geodynamic models. We present results for the Atlantic realm, and link our estimates of upper mantle flow history to Base Hiatus Surfaces (BHS) recently developed by Friedrich etal., (2018), Vibe etal., (2018), Carena etal., (2019),  Hayek metal., (2020) and Hayek metal., (2021). The latter serve as proxy for inferring past dynamic topography variations. We also relate our calculations to seismically inferred anisotropy, as a further proxy for upper mantle flow. Our results indicate that asthenospheric flow pattern can be explained through the concept of source to sink and that this flow type is testable against first order seismic and geologic observables.

References: 

Carena, S., Bunge, H. P., & Friedrich, A. M. (2019). Analysis of geological hiatus surfaces across Africa in the Cenozoic and implications for the timescales of convectively-maintained topography. Canadian Journal of Earth Sciences, 56(12), 1333-1346.

Friedrich, A. M., Bunge, H. P., Rieger, S. M., Colli, L., Ghelichkhan, S., & Nerlich, R. (2018). Stratigraphic framework for the plume mode of mantle convection and the analysis of interregional unconformities on geological maps. Gondwana Research, 53, 159-188.

Hayek, J. N., Vilacís, B., Bunge, H. P., Friedrich, A. M., Carena, S., & Vibe, Y. (2020). Continent-scale Hiatus Maps for the Atlantic Realm and Australia since the Upper Jurassic and links to mantle flow induced dynamic topography. Proceedings of the Royal Society A, 476(2242), 20200390.

Hayek, J. N., Vilacís, B., Bunge, H. P., Friedrich, A. M., Carena, S., & Vibe, Y. (2021). Correction: Continent-scale Hiatus Maps for the Atlantic Realm and Australia since the Upper Jurassic and links to mantle flow-induced dynamic topography. Proceedings of the Royal Society A, 477(2251), 20210437.

Vibe, Y., Friedrich, A. M., Bunge, H. P., & Clark, S. R. (2018). Correlations of oceanic spreading rates and hiatus surface area in the North Atlantic realm. Lithosphere, 10(5), 677-684.

How to cite: Wang, Z., Bunge, H.-P., Stotz, I., Vilacis Baurier, B., Hayek Valencia, J. N., and Friedrich, A.: Asthenospheric flow estimates in the Atlantic realm based on Poiseuille/Couetteflow models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1226, https://doi.org/10.5194/egusphere-egu22-1226, 2022.

How to cite: Muluneh, A., Brune, S., Kidane, T., Pagli, C., Keir, D., and Corti, G.: Complex strain accommodation mechanisms during rift linkage: an example from the central Afar, East Africa, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3343, https://doi.org/10.5194/egusphere-egu22-3343, 2022.

This presentation examines how the growing inventory of geologic, geophysical and geomorphic observations constrains amplitudes, wavelengths and histories of mantle convection. Ocean age-depth residuals have become a cornerstone in understanding loci, amplitudes and wavelengths of modern sub-plate support of Earth’s oceanic lithosphere. Improvements in mapping lithospheric structure, especially from seismology, means that quantifying modern sub-plate support of continents is increasingly tractable. The continents offer a great opportunity to constrain temporal and spatial evolution of sup-plate support because of the plethora of available geologic and geomorphic observations. A challenge is to disentangle, often dominant, lithospheric processes that generate uplift or subsidence (e.g. shortening, extension) to extract information about histories of sub-plate processes from geologic and geomorphic observations. This presentation focusses on how data and theory can be combined to quantify [1] modern and recent sub-plate support of the continents, [2] evolution of sub-plate support through time, and [3] test geodynamic models that predict dynamic topography.

Highlights from recent work include the use of collocated long wavelength gravity and shear wave velocity anomalies, mafic magmatism and drainage patterns to identify tracts of uplifted continental topography that are maintained by sub-plate support. These observations indicate that chemical and sedimentary fluxes through drainage networks are likely governed by sub-plate support in many places. Observational and theoretical constraints on uplift and subsidence histories of continents and their margins are presented. These include global paleobiological observations of uplifted marine rock, backstripped stratigraphy (e.g. New Jersey margin, Mauritanian basin), inversion of drainage patterns (e.g. North America, Africa) and landscape evolution modelling. This work indicates that re-assessment of the longevity and evolution of Earth’s surface topography, basin formation and histories of glacio-eustasy is required. Reasons for why large-scale vertical lithospheric motions, often associated with dynamic support, are likely to be recorded in modern and ancient continental landscapes are explored using spectral analysis of drainage patterns and physics-based modelling of erosional thresholds. Examples of how seismology, anelastic parameterisations, thermobarometry, geodesy, stratigraphy and geomorphic observations can be reconciled to constrain Cenozoic to Recent histories of upper mantle support are presented. Finally, examples of using geologic observations to test predictions from geodynamic models are given. Opportunities and challenges associated with assessing contributions of sub-plate support to evolution of Earth’s surface, particularly those associated with crust and lithospheric mantle densities, are discussed. Available observations show that Cenozoic mantle convection has been a significant, in places dominant, driver of continental evolution. It has generated and maintains epicontinental seaways, sedimentary basins, continental plateaux, and has determined routing of water and sediment across continents and biodiversity. It is an important driver of Earth surface evolution and is extremely likely to have left its trace throughout the geologic and geomorphic record.

How to cite: Roberts, G. G. and Fernandes, V.: Global geologic and geomorphic observations of mantle convection, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4069, https://doi.org/10.5194/egusphere-egu22-4069, 2022.

Absolute plate motion (APM) estimates are key to understand the driving forces of plates, particularly the role of the sublithospheric mantle flow, which has recently gained renewed recognition as a dominant driver. Tectonic plates that lack a subducting boundary (e.g., South America and Nubia) are prime examples of dynamics governed by mantle flow. Both the aforementioned plates, however, lack the hotspot space/age coverage required for high-resolution, well-constrained APM estimates. Here we resort to highly-resolved data sets of relative plate motions (RPM) across a number of spreading ridges in order to extract information on APM changes through geological time. Our analyses involve first mitigating the impact of noise in RPM data sets via Bayesian inference. This allows us to identify time periods that feature a relatively high probability of staging RPM changes. By extending these analyses to several neighboring plates, we can assess whether any of them is likely to feature an APM change through geological time. We apply such a method to RPM data sets in the Atlantic realm and identify three time-intervals for changes in the APMs of the Nubia and South America plates. Our analyses are complemented by a quantitative assessment of the forces required to generate such APM changes.

How to cite: Espinoza, V. and Iaffaldano, G.: Mid-Cenozoic absolute plate motion changes in the South Atlantic from relative plate motion analyses., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4206, https://doi.org/10.5194/egusphere-egu22-4206, 2022.

A quantitative understanding of Earth's deep compositional structure remains elusive. Geophysical and geochemical observations illuminate heterogeneous features on various scales in the lower mantle: however, the origin and interaction of such heterogeneities are not yet fully explained in the context of global mantle dynamics. Conversely, numerical geodynamic models predict a wide range of viable scenarios of mantle convection and heterogeneity preservation. In the "marble cake" end-member mantle model, slabs of Recycled Oceanic Crust (ROC) are subducted and deformed but never fully homogenized in the convecting mantle. In the "plum pudding" model, MgSiO₃-rich primordial material may resist convective entrainment due to its intrinsic strength. Only few geodynamic studies have explored the effects of subducted ROC properties on mantle dynamics while also accounting for the influence of primordial heterogeneity. Furthermore, predictions from numerical models need to be tested against geophysical data. However, current imaging techniques poorly resolve the lower mantle and may be unable to distinguish between both end-member models above.

Here, we use the finite-volume code StagYY to model mantle convection in a 2D spherical-annulus geometry. We investigate the style of heterogeneity preservation as a function of two parameters: the intrinsic density and the intrinsic strength (viscosity) of basalt at lower-mantle conditions.  Additionally, we employ the thermodynamic code Perple_X and the spectral-element code AxiSEM to compute, respectively, seismic velocities and synthetic seismograms from the predictions of our models.

We obtain two main regimes of mantle convection: low-density basalt leads to a well-mixed, "marble cake"-like mantle, while dense basalt aids the preservation of primordial blobs at mid-mantle depths as in a "plum pudding". Intrinsically viscous basalt also promotes the preservation of primordial material. These trends are well explained by smaller convective vigour of the mantle as intrinsically dense (and viscous) piles of basalt shield the core. In order to test these model predictions, we convert model temperatures and compositions to thermoelastic properties for two characteristic models of each regime. These are then used to compute synthetic seismic velocity models, through which we simulate wave propagation using AxiSEM. Finally, we  discriminate between these two end-members by comparing statistical properties of the corresponding ensembles of synthetic seismograms. Our results highlight how the interaction of mantle materials drives the long-term thermochemical evolution of terrestrial planets. Furthermore, they provide a framework for relating the style of heterogeneity preservation in the Earth's lower mantle with specific features of the seismic waveforms.

How to cite: Desiderio, M., Gülcher, A. J. P., and Ballmer, M. D.: The interplay between recycled and primordial heterogeneities: constraints on Earth mantle dynamics via numerical modeling, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5307, https://doi.org/10.5194/egusphere-egu22-5307, 2022.

Mantle plumes are commonly envisioned as thin, buoyant conduits rising vertically from the core-mantle boundary (CMB) to the earth's surface, where they produce volcanic hot spots. Most hotspots are located in the sparsely instrumented oceans, creating poor prospects for the seismic resolution of thin conduits in the deep mantle. 

The RHUM-RUM experiment remedied this issue around the hotspot island of La Réunion by instrumenting 2000x2000 km2 of seafloor for 13 months with 57 broadband ocean-bottom seismometers (OBS). We present a 3-D P-wave tomography model computed from the RHUM-RUM waveform data, supplemented by a global data set of P-diffracted measurements and a selection of ISC picks. Multifrequency travel times were measured on the waveforms and inverted in a finite-frequency framework. We achieve high image resolution beneath the Indian Ocean hemisphere, and especially beneath La Réunion, from upper mantle to CMB.

We observe the Large Low-Velocity Province (LLVP) rising 800 km above the CMB, forming a cusp beneath South Africa. A low-velocity branch undulates obliquely from this cusp region towards the uppermost mantle beneath La Réunion. Hence La Réunion's connection to the lower mantle is more complex than previously envisioned, being neither a thin vertical conduit nor projecting down to an edge of the LLVP. The deep-mantle connections of the Afar and Kerguelen hotspots emerge from the same LLVP cusp beneath South Africa and extend towards the surface through tilted low-velocity branches. 

Our results provide the first high-resolution image of a western Indian Ocean plume cluster from the surface to the CMB. This represents a key advance for linking geophysical, geodynamic and geochemical observations.

How to cite: Tsekhmistrenko, M., Sigloch, K., Hosseini, K., and Barruol, G.: A tree of Indo-African mantle plumes imaged by seismic tomography, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6182, https://doi.org/10.5194/egusphere-egu22-6182, 2022.

The rheological properties of­­­ Earth’s lower mantle materials are key for mantle dynamics and  planetary evolution. The main rock-forming minerals in the lower mantle are bridgmanite (Br) and smaller amounts of ferropericlase (Fp). Bridgmanite minerals are intrinsically much stronger than ferropericlase minerals, resulting in significant variations in lower-mantle rheological behavior depending on the quantity and degree of interconnectivity of the weak phase. The resulting effective bulk rock viscosity decreases with accumulating strain when the weaker Fp minerals become elongated and eventually interconnected. This implies that strain localization may occur in Earth’s lower mantle, which would in turn influence the pattern of mantle flow and could potentially aid the preservation of compositionally distinct, “hidden” reservoirs. So far, there have been no studies on global-scale mantle convection in the presence of such strain-weakening (SW) rheology.

Here, we present 2D numerical models of thermo-chemical convection in spherical annulus geometry including a new strain-weakening (SW) rheology formulation for lower-mantle materials. This macro-scale SW rheology is based on micro-scale rheological behavior found in prior studies, and combining rheological weakening and healing terms. We determine the effects of SW rheology on the planform of mantle flow, the mixing of chemical reservoirs, and the dynamics of mantle plumes.

We find that, in particular, plume conduits are weakened and act as lubrication channels which allow for the rapid ascent of mantle material. Their thermal anomalies and geometries are significantly different than those of mantle plumes which are not rheologically weakened. Moreover, larger thermochemical piles at the base of the mantle are stabilized by SW rheology, with implications for preservation of chemically-distinct materials over long timescales. Finally, we put our results into context with observations and existing hypotheses on the style of Earth's mantle convection and mixing. Most importantly, we suggest that the new kind of plume dynamics may explain the discrepancy between expected and observed thermal anomalies of deep-seated mantle plumes on Earth.

How to cite: Gülcher, A., Golabek, G., Thielmann, M., Ballmer, M., and Tackley, P.: Narrow, fast, and "cold" mantle plumes on Earth explained by strain-weakening rheology in the lower mantle, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7608, https://doi.org/10.5194/egusphere-egu22-7608, 2022.

How to cite: Jagt, L., Talavera-Soza, S., and Deuss, A.: Normal Mode Constraints on Anelastic Structures in the Earth's Lower Mantle, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8883, https://doi.org/10.5194/egusphere-egu22-8883, 2022.

A crucial goal in geodynamics is the development of time-dependent earth models so that poorly known mantle convection parameters can be tested against observables gleaned from the geologic record. To this end one must construct model trajectories to link estimates of the current heterogeneity state to future or past flow structures via forward or inverse mantle convection models. Unfortunately, the current heterogeneity state which is derived from seismic imaging methods is subject to substantial uncertainty due to the finite resolution of seismic tomography. These uncertainties are likely to considerably affect the computed flow trajectory, in what is known as the butterfly effect. Here we study mantle convection models to assess the effects of varying initial conditions on the evolution of mantle flow. We perform twin experiments (Lorenz 1965), that is, we compute convection calculations with identical flow parameters but different initial temperature fields. A base temperature field is generated by allowing a mantle convection calculation to evolve until a statistical steady state is reached. This temperature field is then used to initialize our reference case. We proceed to modify this reference temperature field in a number of different forms to reflect tomographic choices of damping and smoothing. In all cases we track the divergence of the perturbed models from the reference model. Furthermore, we test the efficiency of surface velocity assimilation, following from the work of Colli et al (2015), in locking two convecting systems and driving their divergence to a minimum.

We also introduce a framework for the comparison of model output with geological observables. To this end, we perform a comparison between the dynamic topography maps of our reference and perturbed models. We calculate simple traditional metrics such as RMSE, correlation, difference fields and Taylor diagrams. Such traditional grid-point based error measures, however, suffer from the “double-penalty” problem and as such we introduce scale-decomposition methods that allow a computation of correlation, RMSE and ratios of variances for every spatial scale (see Surcel et al (2015), Casati et al (2005) for examples). Furthermore, we introduce object-based verification measures that identify and match uplift and subsidence objects in the dynamic topography maps for both reference and perturbed models similar to what a human observer would identify. Borrowing from the wealth of work in meteorology, we calculate SAL scores (Wernli et al 2008) and a Critical Success Index (Schaefer 1990). Finally, for successfully matched objects, a Procrustes shape analyis (Michaes et al 2007) is performed to compare the similarities in area, shape, orientation and intensity, after which a final score is calculated based on these properties. We believe that the measures introduced here represent the next step in geodynamics as mantle convection models become increasingly complex and more focus is placed on matching model observations with the geological record.

How to cite: Taiwo, A., Bunge, H.-P., and Schuberth, B.: Mantle Flow Trajectories in the Presence of Poorly Constrained Initial Conditions: Analysis of an Ensemble of Models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8964, https://doi.org/10.5194/egusphere-egu22-8964, 2022.

Mantle convection is a fundamental driving force of plate tectonics. It is commonly perceived that mantle convection is difficult to constrain directly. However, the convection process affects the Earth surface imprints the geological record. In particular, the positive surface deflections driven by mantle convection create erosional/non-depositional environments, which induce gaps in the stratigraphic record (i.e., an absence or thinning of a sedimentary layer). Modern digital geological maps allow us to map the largest of such un/-conformable surfaces at continental scale systematically.
We report our continent-scale hiatus mapping in geological series across America, Europe, Africa, and Australia, from the Upper Jurassic onward. We find significant differences in the spatial extent of hiatus patterns across and between continents, which is on the order of 2000 – 3000 km in diameter. These surfaces change at geological series, ten to a few tens of millions of years (Myrs). This duration is significantly shorter than the timescale of mantle convection of about 100 – 200 Myrs, implying that different timescales for convection and topography in convective support must be an integral component of time-dependent geodynamic Earth models. Our results call for intensified collaboration between geodynamicists and geologists to test geodynamic Earth models by assembling relevant geological observations at continental scales.

How to cite: Vilacís, B., Hayek, J. N., Bunge, H.-P., Friedrich, A. M., and Carena, S.: Tracing upper mantle flow patterns through continent-scale hiatus surfaces in the Indo-Atlantic Realms since the Upper Jurassic, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9794, https://doi.org/10.5194/egusphere-egu22-9794, 2022.

The post-perovskite (pPv) phase is often invoked as an explanation for seismic observations of discontinuities, anisotropy and anti-correlation between velocities in the lower mantle. Accurate interpretations of these features in terms of pPv are important, as the phase transition provides a much-needed temperature probe in the lowermost mantle. Robust observations of this phase transition have the potential to constrain the temperature of and heat flow across the core-mantle boundary and thus provide estimates of the heat budget and thermal evolution of the Earth.

Traditionally, the presence of post-perovskite (pPv) has been inferred from observations of seismic discontinuities in the lowermost mantle. However, these only give a very patchy image of lateral variations in the presence of pPv due to the heterogeneous coverage of seismic data. In addition, interpretations are complicated by the fact that the properties and stability field of pPv remain uncertain from a mineral physics point of view.

Here, we describe different proxies for the presence of post-perovskite, proposed based on global seismic tomography. To investigate their accuracy, we utilize synthetic tomography models derived from geodynamic modelling in combination with mineral physics and we compare the predicted presence to the true occurrence of pPv in the model. By using both high-resolution geodynamic models as well as filtered models that have been corrected for the limited resolution of seismic tomography, we can investigate whether a proxy works in theory (on the high-resolution versions) and also in practice (on the filtered models). We will discuss how we may be able to constrain the stability field of pPv based on comparisons with published tomographic models and make recommendations as to what has to improve in seismic tomography to make different proxies work.

How to cite: Koelemeijer, P., Pagu, A., Schuberth, B., and Davies, R.: Proxies for the presence of post-perovskite in the lowermost mantle based on seismic tomography and geodynamic modelling, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9966, https://doi.org/10.5194/egusphere-egu22-9966, 2022.

The mantle transition zone (TZ) is expected to influence convective flow, but neither its structural characteristics nor dynamic effects have been conclusively constrained. Lateral temperature variations modulate the topography of associated seismic discontinuities at approximately 410 and 660 km depth (‘410’ and ‘660’). These discontinuities are related to mineral phase transitions and thus also sensitive to composition. Consequently, discontinuity topography can potentially provide insight on temperature and even phase composition at depth. It has been recognized that, in addition to phase transitions in olivine polymorphs, the transition of garnet to lower mantle minerals may impact particularly the ‘660’ at higher temperatures. However, the volume of material affected by this garnet transition and its dynamic implications have not yet been quantified.

We address this question by predicting synthetic seismic structure and discontinuity topography of the TZ based on the temperature field of a 3-D mantle circulation model (MCM) for a range of relevant bulk compositions and associated mineralogy models. The models differ in complexity in terms of the number of incorporated oxide-components and include pyrolite, depleted mantle and mechanical mixing (MM) models. We thus create a suite of relevant hypothetical realizations of TZ seismic structure and major discontinuities.

Our theoretical approach allows us to systematically investigate the effects of varying mineralogy, in combination with a dynamically constrained temperature field, on TZ structure. We explicitly relate major phase transitions as given by the mineralogical tables to specific topographic features of the ‘410’ and ‘660’ and quantify the relative impact of the different phases. Analyzing a number of statistical measures for our synthetic discontinuity topographies provides theoretical predictions on possible distribution and magnitude of real-world depth variations. Our study thus provides a framework for dynamically informed interpretations of seismically derived TZ structure in terms of mantle temperature and composition. It moreover gives insights on the potential dynamic behavior of the TZ by constraining the importance of garnet in our theoretical models.

We find that garnet only occurs in regions with excess temperatures above 150 - 300 K, depending on phase composition. This leads to ~ 3 % garnet at the ‘660’ in a pyrolite mantle and ~ 1 % in MM. Absolute base temperatures could however be higher (or lower) than predicted by the MCM’s geotherm. For different plausible background temperature fields the garnet proportion at the ‘660’ could vary between ~ 1 and 39 % in pyrolite, while remaining largely unaffected in MM. Since not all warmer than average but only the hottest mantle regions see the garnet transition, dynamic effects of the ‘660’ might be even more complex than previously assumed.

How to cite: Papanagnou, I., Schuberth, B. S. A., and Thomas, C.: Geodynamic predictions of seismic structure and discontinuity topography of the mantle transition zone, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11433, https://doi.org/10.5194/egusphere-egu22-11433, 2022.

In this work we investigate the genesis of widespread continental basaltic volcanism and the transition to localised magmatic segments at the Afar Rift-Rift-Rift triple junction. Basing on major and trace elements we investigated the thick (up to 1500m) and widespread (~55.000 km²) Lower (4.5-2.6 Ma) and Upper Stratoids (2.6-1.1 Ma) Series and the subsequent, less voluminous and focalised, Gulf Series (1.1-0.6 Ma). Our results, together with published geophysical and stratigraphical evidence, allow us to interpret the evolution of the Red Sea rift and the associated break-up process in Southern and Central Afar. The three Series are characterised by E-MORB magmatism and residual amphibole (K, Rb trough and Ba, Nb-Ta peak), with subordinately pyroxenite (Rb peak, Ba trough and MREE fractionation), in their mantle source, suggesting partial melting of the diffusely metasomatized sub-continental mantle. Marked differences in garnet-compatible trace elements reveal a deeper melting column for the Upper Stratoids (Tb_N/Yb_N > 1.7) with respect to the Lower Stratoids and the Gulf Series (Tb_N/Yb_N < 1.7), indicating distinct mantle sources for the three Series. Lower values of the incompatible element ratios Th/Nb, Th/Zr and La_N/Sm_N of the Gulf Series with respect to the Upper Stratoids indicate a higher degree of partial melting for the Gulf Series mantle source. The spatial variation in the volume and sources of Afar magmatism between 4.5-0.6 Ma correlates well with spatial changes in the locus of strain with two distinct episodes of rifting: (1) The late Miocene rifting episode (7-2.6 Ma), associated with thinned lithosphere and the Hadar Basin formation (3.8-2.9 Ma), erupted the Lower Stratoids in South Afar; (2) The Pleistocene rift (2.6-0.01 Ma), relocated in Central Afar, erupted the Upper Stratoids first (~2.6-1.1 Ma) and, subsequently, along with the stretching of the lithosphere and focalization of the rift, the Gulf Series (~1.1 Ma). Accordingly, our data supports the interpretation that the Afar strain localisation and associated magmatism migrated north-eastward from South to Central Afar through time, potentially in response to triple junction tectonics.

How to cite: Tortelli, G., Gioncada, A., Pagli, C., Braschi, E., Gebru, E., and Keir, D.: Stratoids flood basalt volcanism at the Afar rift: new insights from trace elements geochemistry, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11623, https://doi.org/10.5194/egusphere-egu22-11623, 2022.

Tomographic-geodynamic model comparisons are a key component in studies of the present-day thermodynamic state of the mantle. A fundamental prerequisite for quantitatively meaningful comparisons is “tomographic filtering” of the geodynamic model. This means that geodynamically predicted mantle structures have to be modified to account for the spatially variable resolving power of tomographic images, i.e. to mimic the effects of uneven data coverage and regularization. Different approaches for tomographic filtering are available, but it is so far unclear which one will be the method of choice in the context of computationally demanding retrodictions of past mantle flow.

Here, we investigate the impact of the possible filtering approaches in a fully synthetic framework. For the first time in a mantle circulation model (MCM), we simulate 3D-wavefields and seismograms for an entire tomographic earthquake catalogue with over 4,200 events using SPECFEM3D_GLOBE. We use both classic filtering with the resolution operator R, as well as the recently introduced “generalized inverse projection” (GIP; Freissler et al. 2020) to generate tomographically filtered versions of the MCM.

In the GIP method, the generalized inverse operator of a given tomographic image is applied to synthetic seismic data predicted from the geodynamic model, as well as to potential data errors, to obtain the filtered MCM plus the propagated error. Important to note, the same generalized inverse operator is applied to an observed data set to build the tomographic model. A physically accurate prediction of synthetic data, here realized with the seismograms from numerical wave propagation, thus enables GIP filtering to consistently reproduce the tomographic imaging process. This is an important methodological advantage over classic filtering with R, where an unphysically reparametrized version of the MCM is filtered directly in model space and seismic data errors can not be considered.

In our study, GIP-filtered models are computed with cross-correlation S-wave traveltime residuals from the synthetic seismograms, as well as with banana-doughnut kernel and ray-theoretical traveltime predictions. The differently filtered models are compared against each other using statistical measures. By taking the GIP-filtered model that is based on the 3D-wavefield simulations as a reference, we can quantify the impact of reparametrization in classic filtering versus the lack of exact wave physics when using less accurate methods for traveltime predictions in the GIP filtering. Additionally, all filtered models can be compared to the underlying original structure of the MCM.

Detailed knowledge of tomographic filtering effects with different strategies is required prior to efforts on the associated uncertainty quantification in data-driven geodynamic retrodictions of mantle evolution.

How to cite: Freissler, R., Schuberth, B. S. A., and Zaroli, C.: The relevance of full 3D-wavefield simulations for the tomographic filtering of geodynamic models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11686, https://doi.org/10.5194/egusphere-egu22-11686, 2022.

Geodynamic inverse models that aim at retrodicting past mantle evolution require accurate estimates of its thermodynamic present-day state. Tomographic models are in principle well suited to provide this information. However, a fundamental problem that impacts the quality of the retrodiction arises from their inherently limited resolving power and the fact that the magnitudes of seismic heterogeneity are difficult to constrain owing to the necessity to regularize the inversions (e.g. by norm damping). To get a better understanding of the magnitudes of heterogeneity in the mantle, one option is to predict seismic velocity variations from the temperature field of forward mantle circulation models (MCMs) in combination with thermodynamic models of mantle mineralogy.  Temperature is not a free parameter in these models, but rather constrained by the underlying conservation equations and relevant input parameters. If the geodynamic models are run at earth-like Rayleigh number, temperature variations are expected to feature realistic magnitudes, which, together with the mineralogical mapping, should lead to realistic magnitudes of seismic heterogeneity. This has been investigated in previous studies by computing secondary predictions for the MCMs, such as seismic body wave traveltimes and geoid undulations. A complicating factor, however, is the trade-off between thermal and compositional variations that both may affect the seismic velocities. A further complexity arises from the fact that the elastic velocities of the mineralogical model need to be corrected for the effects of anelasticity, the parameters of which are poorly known.  Thus, a range of seismic velocity values may still be possible for a given temperature. 

Here, we explore the possibility to use Earth's normal mode spectrum to narrow the range of plausible magnitudes of seismic heterogeneity in the mantle. To this end, we compute free-oscillation spectra with full coupling of modes below 3.5 mHz in our geodynamic models.  In our analysis, we consider different measures to investigate whether the normal mode data may provide complementary information to earlier assessments of MCMs based on body waves. In addition to the direct misfit between spectra of real and synthetic data, the variance of a large number of stacked multiplets can be used to constrain the even degree covariance of lateral heterogeneity under certain assumptions. Using different realizations of seismic MCM structure that differ in terms of the anelastic temperature to velocity mapping, we will analyse the potential of normal mode data to put tighter constraints on the magnitudes of heterogeneity.

How to cite: Schuberth, B., Strutz, D., and Schneider, A.: Earth's free-oscillation spectrum as a tool to assess mantle circulation models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12414, https://doi.org/10.5194/egusphere-egu22-12414, 2022.

The determination of the pressure and temperature (P-T) history experienced by mantle xenoliths, especially the pressure, is essential for elucidating the physicochemical layering structure of the uppermost mantle. However, the lack of continuous reactions between solid solution minerals with large volume changes in spinel-lherzolites makes it difficult to apply conventional geobarometry based on mineral chemistry. Here, elastic geobarometry (Angel et al., 2014; Angel et al., 2017), a complementary technique for determining equilibrium P-T conditions of rocks, was applied to spinel inclusions in olivine in a spinel-lherzolite xenolith.

To utilize elastic geobarometry, reliable equations of state (EoS) for the host mineral and inclusion are essential. Although the EoS for mantle olivine is well constrained by Angel et al. (2018), detailed studies on the EoS for spinel are scarce. Therefore, we firstly conducted a comprehensive review of previous studies investigating the temperature and/or pressure dependence of volume, bulk modulus, and heat capacity, and then determined the EoS for end member spinel using EoSfit7c (Milani et al., 2017).

Next, using Raman spectroscopy, we attempted to estimate the residual pressure of spinel inclusions (P_inc) trapped in olivine in a mantle xenolith from Ennokentiev, Sikhote-Alin, Far Eastern Russia (see Yamamoto et al. (2012) for the chemical composition of the sample). As a result, the peaks of the spinel inclusions were always shifted to higher wavenumbers than those of the unstrained reference spinel crystal from the same xenolith, but only E_g (~410 cm^-1) and A_1g (~750 cm^-1) peak positions could be measured with sufficient accuracy for quantitative analysis of residual pressure. When P_inc was estimated using relation between spinel peak position and pressure reported by Chopelas and Hofmeister (1991), the data obtained from the center of the inclusion showed positive P_inc from both A_1g and E_g peaks, and they agreed within error. However, it is desirable to use the A_1g peak for the calculation of P_inc because 1) the E_g peak has low Raman scattering intensity, 2) depending on the crystal orientation of the host olivine, the E_g peak of spinel could interfere with the B_3g peak of olivine, and 3) the E_g peak is expected to be sensitive to the differential stress because the P_inc calculated from the E_g peak obtained from the edge of the inclusion is unusually higher than that calculated from the A_1g peak. Since positive residual pressures were obtained from all the inclusions investigated, by combining the EoS of spinel constrained in this study and measured P_inc, spinel inclusions trapped in olivine can be expected to be a new method for estimating the depth provenance of spinel-bearing peridotite.

References

Angel et al. (2014) Am Mineral, 99, 2146-2149; Angel et al. (2017) Am Mineral, 102, 1957-1960; Angel et al. (2018) Phys Chem Miner, 45, 95-113, Chopelas and Hofmeister (1991) Phys Chem Miner, 18, 279-293; Milani et al. (2017) Am Mineral, 102, 851-859; Yamamoto et al. (2012) Tectonophysics, 554-557, 74-82.

How to cite: Hagiwara, Y., Angel, R., Gilio, M., Yamamoto, J., and Alvaro, M.: Evaluation of elastic geobarometry of spinel inclusions in olivine and its application to mantle xenoliths, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-199, https://doi.org/10.5194/egusphere-egu22-199, 2022.

Copper deposits or sulfide enrichment have been found along the crust-mantle transition zones in ophiolites and along the oceanic Moho. However, scarcity of suitable exposures limits our knowledge on the migration of chalcophile metals across the subcontinental crust-mantle boundary. This study aims to provide new constraints on the migration of sulfide-associated chalcophile metals at the transition between the subcontinental mantle peridotites of the Balmuccia massif and lower crustal gabbronorites of the Mafic Complex (Ivrea-Verbano Zone, NW Italy).

An ~80-m-thick zone composed of interlayered pyroxenites and gabbronorites (Contact Series; CS) showing igneous contact with the mantle peridotites was sampled along the Val Sesia river, near the Isola village. We investigated a transect from the mantle peridotites (rich in pentlandite) through the CS to the lower crustal gabbronorites (rich in pyrrhotite or pyrite). The CS zone comprises three sampling sites located 0–5 m (CS1), 65–70 m, and 75–80 m from the mantle peridotites and is characterized by the along-transect Mg# variations (Mg# of 71–57). The mantle peridotites are sulfide poor (average of 0.12 vol.‰), in contrast to the CS rocks (up to 7.8 vol.‰). The enhanced sulfide abundances in mafic rocks of the CS correlate with higher S, Cu, Ag, and Cd contents. This sulfide- and chalcophile-rich metal zone within the CS ends ~75 m away from the margin of mantle peridotites implying a probable thickness of the enrichment zone. Sulfides from mantle peridotites and CS1 are pyrrhotite-(troilite)-chalcopyrite-(cubanite)-pentlandite assemblages of magmatic origin, which is supported by δ³⁴S ranging from –0.6‰ to +1.8‰ (average of 0.0‰; cf., Oeser et al., 2012 – Chemical Geology).

The in-situ Fe isotope signatures of polyphasic sulfide grains from CS1 show a strong fractionation between the various phases. The δ⁵⁶Fe values of pyrrhotites are negative ranging from –0.8‰ to 0.0‰, whereas chalcopyrite exhibit positive values of 1.3–1.7‰. The mass balance calculations of the δ⁵⁶Fe for the bulk composition of the sulfide grains from CS1 show unfractionated (magmatic or mantle) values of 0.0 ± 0.2‰ (cf., Craddock et al., 2013 – EPSL).

The stagnant melts at the crust-mantle boundary extensively react with the mantle yielding enrichment in sulfides and chalcophile elements, which is known to yield enrichment in sulfides (Ciazela et al., 2018 - GCA; Patkó et al., 2021 - Lithos). However, the contact between the Balmuccia mantle peridotites and the lower continental crust of the Mafic Complex is highly heterogeneous with alternating layers of pyroxenites and gabbronorites. These layers may have formed from distinct magma batches as suggested by the along-transect Mg# variations. Therefore, the mechanism of observed enrichment in sulfides and chalcophile elements probably involves several stages of melt-peridotite and melt-pyroxenite reactions. These might explain the exceptionally large ~75-m-thick sulfide-rich horizon observed at the CS. Our results indicate that substantial chalcophile metal inventory is trapped at the CS. Assuming they behave the same at the Moho level, this would explain the relative deficit of these elements in the continental crust when compared its bulk composition to the composition of primitive mantle melts.

This research was funded by the NCN Poland (2018/31/N/ST10/02146)

How to cite: Pieterek, B., Ciążela, J., Tribuzio, R., Matusiak-Małek, M., Muszyński, A., Strauss, H., Lazarov, M., Weyer, S., Horn, I., Kuhn, T., and Nowak, I.: New constraints on the origin of metal enrichment at the crust-mantle boundary from the Ivrea-Verbano Zone, NW Italy, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-328, https://doi.org/10.5194/egusphere-egu22-328, 2022.

Holes CM1A and CM2B of the International Continental Scientific Drilling Program (ICDP) Oman Drilling Project (OmanDP, https://www.omandrilling.ac.uk/)  drilled  through  the Moho  Transition  Zone  (MTZ).  CM1A is composed  of layered gabbro (0–160 meters below surface, mbs), dunite (160–310 mbs), and harzburgites (310–405 mbs), whereas CM2B contains dunite (20–120 mbs) and harzburgites (120–300 mbs). The drillholes provided an unprecedented opportunity to study the behavior of metals in the MTZ, where arriving primitive MORB melts  extensively  react  with  the  mantle.  Here,  melts, typically  enriched  with  sulfur and  chalcophile  elements,  are supposed to enrich the mantle and lower crust with sulfides (Gonzalez-Jimenez et al., 2020 – Ore Geol. Rev.; Ciążela et al., 2018 - GCA).          

            Modal sulfide content increases downwards the gabbro sequence from ~0.004 vol.‰ to ~1.0 vol.‰ but decreases again from 0.8 vol.‰ to 0.01 vol.‰ in the lower part of the MTZ and in the harzburgite of the upper mantle. This is reflected in the S concentration increasing from 341 ± 17 ppm, 2sd (standard deviation = σ) to  832  ±  37  ppm,  2sd,  in  the  gabbro  section  and  decreasing  downwards  from  the middle part of  Moho into harzburgites from 475 ± 21, 2sd ppm to 63 ± 3 ppm, 2σ. The sulfides in olivine gabbro from MTZ are mostly (56–87% of all sulfides) pyrrhotite-pentlandite-chalcopyrite assemblages indicating the magmatic origin. Sulfides in layered gabbro sequence are consisted of similar magmatic assemblages (36-100%) with minor chalcopyrite, bornite, heazlewoodite, chalcocite, millerite, siegenite and sphalerite with secondary origin. In dunite and harzburgite sequences sulfides are exclusively hydrothermal.

Based on EMPA and LA-ICPMS measurements, Zn, Co and Cu seem to reach their maximum concentrations in magmatic sulfides from the MTZ. Although, no significant differences are observed between the Fe isotope signatures in magmatic pyrrhotites from the lower crust (–0.73 to –0.24, 2sd [‰] of δ⁵⁶Fe) and the MTZ (–0.73 to –0.53, [‰] of δ⁵⁶Fe), we found different δ⁵⁶Fe for pyrrhotite (–0.24‰) and chalcopyrite +0.36‰ within the same sulfide grain. The bulk signature of δ⁵⁶Fe for this grain is –0,12‰ being in accordance with the mass balance calculated δ⁵⁶Fe 0.025‰ ± 0.025‰ of the mantle (Craddock et al., 2013 – Earth Planet. Sci. Lett).

            The  enrichment in sulfides and selected metals (Zn, Co, Cu) towards the  MTZ  might  result  from  melt-mantle  reaction  as  we  proposed previously for the slow-spread oceanic lithosphere based on the Kane Megamullion Ocean Core Complex (Ciążela et al., 2018 - GCA).  In the CM1A/2B ultramafic rocks: dunites and harzburgites, most sulfides are, however, secondary, formed by the same secondary fluids which caused the pervasive serpentinization. To verify whether these sulfides replaced the primary magmatic sulfides or were brought from late-stage seawater-derived fluids, we plan to measure sulfur in whole-rocks and in situ and more iron isotopes in sulfides in situ. Preliminary δ⁵⁶Fe signature isotope data give us evidence for magmatic origin of the sulfides from upper part of the MTZ section.

How to cite: Marciniak, D., Jakub, C., Ana, J., Bartosz, P., Jürgen, K., Harald, S., Marina, L., Ingo, H., Ewa, S., Marta, P., and Konrad, B.: Metal migration and ore minerals across the crust-mantle transition zone (Oman DP ICDP holes CM1A, CM2B), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-517, https://doi.org/10.5194/egusphere-egu22-517, 2022.

In the Alpine orogen of the north Aegean region, the eastern Rhodope Zone consists of widespread high-grade metamorphic basement exposed in Bulgaria and Greece. In this high-grade basement, the lithologically variegated upper unit contains meta-ultramafic bodies, which are considered as dismembered Precambrian meta-ophiolite association (Kozhoukharova 1984). In the same unit, the voluminously predominant amphibolites, having mafic igneous precursors of boninitic-tholeiitic affinity, are in turn considered of Precambrian-Paleozoic island arc origin (Haydoutov et al. 2004), or part of the amphibolites of Ordovician age have back-arc origin (Bonev et al. 2013). The upper unit, together with the overlying Circum-Rhodope belt Jurassic ophiolite, constitutes the hanging wall of the Eocene extensional system consisting of meta-granitoids with Carboniferous protoliths in the footwall. Here, we report on the geochemistry of the amphibolites from the upper unit in Bulgaria and Greece, and discuss their composition and tectonic setting, which might shed a light on the mid-late Paleozoic-early Mesozoic tectonic architecture of the region.

The amphibolites occur intercalated with para- and ortho-metamorphic lithologies within the upper unit. Texturally, they are represented mainly by massive or banded amphibolite and garnet-bearing amphibolite. The bulk mineral assemblage contains amphibole and plagioclase ± quartz ± garnet ± epidote-clinozoisite ± chlorite ± sphene ± rutile, which resulted from the main metamorphic overprint in amphibolite-facies and variable retrogression to greenschist-facies. The meta-mafic rocks cover the range of basalt to andesite composition, with elevated MgO, variable alkali and low-K contents, having mainly tholeiitic to weak calc-alkaline affinity. The range of TiO₂ defines two groups of high-Ti (>1%) and low-Ti (<1%) meta-mafic rocks. Mostly flat to slightly LREE-depleted chondrite-normalized patterns characterize the high-Ti group, which overlaps N-MORB and E-MORB compositions. The low-Ti group exhibits pronounced LREE-depleted and fractionated REE patterns, rarely U-shaped boninitic-like pattern. N-MORB-normalized trace element profiles define high LILE/HFSE ratios, moderate to strong HFSE and HREE depletion of the low-Ti group, and close to N-MORB to slightly enriched HFSE-HREE trend of the high-Ti group. A negative Nb anomaly characterizes part of the low-Ti group, whereas other samples from both groups show no Nb anomalies and have contents higher than N-MORB. On various trace element discrimination diagrams the majority of high-Ti group meta-mafic rocks display clear MORB affinity and few samples plot in the WPB field of oceanic island tholeiites, whereas low-Ti meta-mafic rocks show island arc tholeiite (IAT) affinity or have transitional MORB/IAT signature. 

The compositional diversity of the meta-mafic rocks from the upper unit with MORB, transitional MORB/IAT and IAT affinity, in turn call for the origin of the protoliths in a paired ocean ridge-island arc environment, and thus could hints their supra-subduction zone origin in an island arc/back-arc setting.

References

Bonev, N., Ovtcharova-Schaltegger, M., Moritz, R., Marchev, P., Ulianov, A. 2013. Geod Acta 26, 3-4, 207-229.

Haydoutov, I., Kolcheva, K., Daieva, L., Savov, I., Carrigan, Ch.  2004. Ofioliti, 29, 2, 145-157.

Kozhoukharova, E. 1984. Geologica Balc., 14, 4, 9-36.

Acknowledgements: The study was supported by the NSF Bulgaria KP-06-N54/5 contract.

How to cite: Bonev, N., Dotseva, Z., and Filipov, P.: Geochemistry and tectonic significance of meta-ophiolitic mafic rocks in the high-grade metamorphic basement of the eastern Rhodope Zone, Bulgaria-Greece, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1863, https://doi.org/10.5194/egusphere-egu22-1863, 2022.

The Wum maar is located in the Oku Volcanic group, part of continental sector of the Cameroon Volcanic Line (CVL) in west Africa, which consists of volcanoes active from Eocene to recent. The continental part of the CVL is located on the metamorphic-igneous basement of the Neoproterozoic Central African Orogenic Belt (CAOB), which originated during Gondwana assembly. Some of the CVL lavas contain spinel-facies peridotite and pyroxenite xenoliths giving insight into the mantle lithosphere underlying the CAOB.

We studied xenolith suite (19 xenoliths) from the Wum maar, comprising 14 lherzolites and 5 websterites. The half of lherzolites (7) consist of minerals with fertile composition (olivine Fo₈₉, orthopyroxene Al 0.16-0.19 atoms per formula unit, clinopyroxene Al 0.28-0.31 a pfu, spinel Cr# 0.08-0.13). Clinopyroxene is REE-depleted and has ⁸⁷Sr/⁸⁶Sr ratios of 0.7017-0.7021. A reconnaissance study of crystal preferred orientation (CPO) by EBSD shows that at least in part of the rocks the clinopyroxene fabric is very weak, suggesting that its crystallization post-dates the primary deformation event recorded by the olivine-orthopyroxene framework. A smaller part of lherzolites (5) contains clinopyroxene the CPO of which fits that of the olivine-orthopyroxene framework, is LREE-enriched and has ⁸⁷Sr/⁸⁶Sr ratios of 0.7027-0.7028. One of these lherzolites contains amphibole (pargasite), which forms aggregates and schlieren and texturally is later than olivine-pyroxene host. CPO of amphibole, ortho- and clinopyroxene is decoupled from that of olivine in that rock. Two lherzolites have slightly depleted mineral compositions (olivine Fo_90-91, orthopyroxene Al 0.15 apfu, clinopyroxene Al 0.25 a pfu, spinel Cr# 0.18).

Websterites are dominated by orthopyroxene (Al 0.20-0.21 a pfu) whereas clinopyroxene (Al 0.30-0.31) is subordinate, and is characterized by LREE-depletion and ⁸⁷Sr/⁸⁶Sr ratios of 0.7019-0.7020. Spinel occurring in websterites is aluminous (Cr# 0.04-0.06), in some samples subordinate olivine (Fo₉₀) occurs. One of the xenoliths consists of millimetric monomineral layers of pyroxenes and olivine chemically identical to those occurring in websterites.  

The mineral chemical data coupled with mineral fabrics suggest that lherzolites with LREE-depleted clinopyroxene could have originated by late crystallization caused by melt metasomatism. The metasomatic agent is probably best represented by websterites, which contain LREE-depleted clinopyroxene with similar, depleted ⁸⁷Sr/⁸⁶Sr of 0.7019-0.7020 (compare to DM value of 0.7026, Workman and Hart 2005), confirming earlier findings of refertilization of the regional lithospheric mantle by highly depleted melts (Tedonkenfack et al. 2021). The addition of amphibole was connected with recrystallization of ortho- and clinopyroxene and with significant change of its ⁸⁷Sr/⁸⁶Sr signature to more radiogenic values.

Funding. This study originated thanks to the project of Polish National Centre of Research NCN 2017/27/B/ST10/00365 to JP. The bilateral Austrian-Polish project WTZ PL 08/2018 enabled extensive microprobe work.

References:

Tedonkenfack SST, Puziewicz J, Aulbach S, Ntaflos T., Kaczmarek M-A, Matusiak-Małek M, Kukuła A, Ziobro M: Lithospheric mantle refertilization by DMM-derived melts beneath the Cameroon Volcanic Line – a case study of the Befang xenolith suite (Oku Volcanic Group, Cameroon). Contributions to Mineralogy and Petrology 176: 37.

Workman RK, Hart SR (2005) Major and trace element composition of the depleted MORB mantle (DMM). Earth and Planetary Science Letters 231: 53-72.

How to cite: Puziewicz, J., Aulbach, S., Kaczmarek, M.-A., Kukuła, A., Ntaflos, T., Matusiak-Małek, M., Tedonkenfack, S. S. T., and Ziobro-Mikrut, M.: Preliminary data on mantle xenoliths from the Wum maar, Oku Volcanic Group, Cameroon Volcanic Line (West Africa), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1865, https://doi.org/10.5194/egusphere-egu22-1865, 2022.

Sulfides hosted by peridotites from Befang (Oku Volcanic Group, Cameroon) xenolith suite can play an important role in tracking migration of strategic metals such as Au, Ag, or Cu through the subcontinental lithospheric mantle (SCLM) beneath the Central African Orogenic Belt. Most peridotites are lherzolites, which are subdivided into two main groups differing by crystallographic preferred orientation (CPO) and rare-earth element (REE) composition of clinopyroxene. Group I is characterized by light REE (LREE)-depleted clinopyroxene (re-)crystallized during percolation of metasomatic melt. Group II contains LREE-enriched clinopyroxene with the CPO representing deformation before percolation of the melt (Tedonkenfack et al., 2021). Lherzolites of group I  are interpreted to be metasomatized by MORB-like melts coming from  Depleted MORB Mantle (DMM). Peridotites of  group II are interpreted to be a protolith for the group I ones.

The sulfides form oval to slightly elongated grains enclosed usually in orthopyroxene, or rarely in clinopyroxene and olivine. They are composed of pyrrhotite (Po), pentlandite (Pn), and chalcopyrite (Ccp). Pyrrhotite is mostly predominant, whereas Pn forms exsolution lamellae in Po or massive crystals separating Po from Ccp. Chalcopyrite is present on the rims of grain or penetrates through the entire grain, occasionally containing cubanite exsolutions. The Group I lherzolites contain more sulfides (up to 0.031 vol.‰), with larger grains (range: 14−250 µm, 57 µm on average) compared to the Group II sulfides (up to 0.002 vol.‰, range: 12−45 µm, 27 µm on average respectively). Sulfides from Group I are richer in Po, and especially Ccp (Po₇₇Pn₁₂Ccp₁₁ on average) compared to Group II (Po₇₂Pn₂₃Ccp₄ on average). Ni/(Ni+Fe) in pyrrhotite from Group I (0.14–0.43) is more heterogeneous compared to group II (0.20–0.37).

Enrichment in Po and Ccp in the Befang Group I xenoliths suggests a significant role of melts in transporting sulfur and metals. Observed refertilization by DMM-derived melts may affect the chalcophile and highly siderophile metal budget of the SCLM. The degree of refertilizaton seems to depend on temperature and therefore is moderate in Befang (up to 0.031 vol.‰) with moderate temperatures of orthopyroxene-clinopyroxene equilibration (938–997°C; Tedonkenfack et al, 2021). In lower temperatures of Opx-Cpx equilibration (810–970°C), we observe higher sulfide abundances (up to 0.062 vol.‰), whereas in higher temperatures (1010–1120°C) lower sulfide abundances (up to 0.00048 vol.‰; Mazurek et al., 2021).

This study was supported by the Diamond Grant project 093/DIA/2020/49.

References

Mazurek, H., Ciazela, J., Matusiak-Małek, M., Pieterek, B., Puziewicz, J., Lazarov, M., Horn, I., Ntaflos, T.: Metal enrichment as a result of SCLM metasomatism? Insight from ultramafic xenoliths from SW Poland., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15992, https://doi.org/10.5194/egusphere-egu21-15992, 2021

Tedonkenfack SST., Puziewicz J., Aulbach S., Ntaflos T., Kaczmarek M-A., Matusiak-Małek M., Kukuła A., Ziobro M.: Lithospheric mantle refertilization by DMM-derived melts beneath the Cameroon Volcanic Line – a case study of the Befang xenolith suite (Oku Volcanic Group, Cameroon). Contributions to Mineralogy and Petrology, 176: 37.

How to cite: Mazurek, H., Matusiak-Małek, M., Ciazela, J., Pieterek, B., Puziewicz, J., and Tedonkenfack, S. S. T.: Metal enrichment in refertilized subcontinental lithospheric mantle: insight from the ultramafic xenoliths from the volcanic rocks of the Oku Volcanic Group (Cameroon), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2217, https://doi.org/10.5194/egusphere-egu22-2217, 2022.

In the upper mantle, volatiles control its composition, partial melting conditions, as well as the ascent rate of the formed melts. As consequence, volatile composition of the mantle is, in turn, recorded in the melts and, therefore, in the erupted basaltic rocks. Despite their importance, origin, budget, and fluxes of the volatiles in the upper mantle are poorly constrained. It is well known that the main input of mantle volatiles, such as carbon (C) and sulphur (S), represents components released from the subducting slab, e.g., oceanic rocks and sediments, whose have characteristic isotopic signatures. In this view, studies of isotopic ratios of volatiles of subduction-related magmatic rocks could be used to identify the chemical components released by the subducting slab metasomatizing the upper mantle. To confirm this hypothesis, we investigated the major and trace element composition, as well as the C and S elemental contents and isotopic ratios of subvolcanic and volcanic rocks of the Vardar ophiolites of North Macedonia, which represent remnants of the Mesozoic Tethyan oceanic lithosphere formed in supra-subduction zone tectonic settings.

The ophiolites were sampled at Lipkovo and Demir Kapija localities, in the northern and southern part of North Macedonia, respectively. Based on whole-rock major and trace element composition, two main groups of rocks can be distinguished: i) Group 1 rocks, which are subalkaline basalts with backarc affinity and ii) Group 2 rocks, which are calc-alkaline basalts with arc affinity. The petrogenetic modelling based on trace and Rare Earth Elements, indicates that Group 1 mantle sources were affected by limited metasomatic processes by slab-released components, in particular aqueous fluids and sediment melts, whereas the Group 2 mantle sources were strongly metasomatized by sediment melts and adakitic melts. Accordingly, the Group 1 rocks exhibit C-enriched and S-depleted isotopic signature, indicating a minor involvement of melts from the subducting sediments. On the other hand, the C-depleted and S-enriched isotopic signatures of the Group 2 rocks suggest a major involvement of melts derived from the subducting sediments rich in organic matter and sulphate phases Therefore, both geochemical and isotopic data of the subvolcanic and volcanic samples of the North Macedonia ophiolites show that the sub-arc mantle sources are more affected by slab-released fluids than those of the backarc basin, which are more distal from the trench. Thus, combining the geochemical and isotopic data of subvolcanic and volcanic samples of complex geological framework can contribute to reconstruct the geodynamic scenarios, such as that of the Vardar ophiolites in the Dinaric-Hellenic belt. In addition, this approach may be useful to better understand the global geodynamic cycles of volatiles reconstructing their origin, budget, and isotopic composition, and understand the impacts on climate and environment from local to global scale.

How to cite: Brombin, V., Barbero, E., Saccani, E., Precisvalle, N., Lepitkova, S., Milevski, I., Ristovski, I., Milcov, I., Dimov, G., Bonadiman, C., and Bianchini, G.: Basaltic rocks from the Vardar ophiolite (North Macedonia): new insights on the metasomatism of sub-arc upper mantle using geochemical and stable isotope data, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3733, https://doi.org/10.5194/egusphere-egu22-3733, 2022.

Well-developed corona structures are observed and described in detail in the cumulate troctolites from Chainigund village, Kargil. The gabbro-troctolite unit is situated 5 km NW of Kargil city and consists of gabbros, troctolites, and anorthosites with doleritic dykes cross-cutting the unit at places. The host gabbros are fresh and display both fine and coarse-grained varieties. Troctolites occur as pods and veins within the gabbro and are composed of plagioclase (77-80 vol%), olivine (10-16 vol%), pleonaste spinel (6-8 vol%), amphiboles (2 -3 vol%) and opaques (0.5-2vol %). Both olivines and plagioclases are unzoned with spectacular coronas around the olivines (Fo 74.9-76.7) at the contact with plagioclase feldspar (An90.6-95.2). From center outwards, the discontinuous reaction series consists of the following members: Olivine, enstatitic orthopyroxene, magnesio-hornblende (Amph1) enclosed by a symplectitic rim of pargasite (Amph2) and pleonaste spinel and concludes at the plagioclase interface i.e. Ol-Opx-Amph1-Amph2-Spl-Plg. The mineral textures of the corona structure indicate formation in the presence of an interstitial fluid trapped between cumulus olivine and plagioclase. The reaction of this fluid with the olivine resulted in a rim of peritectic orthopyroxene around olivine which was subsequently replaced to form Amph 1 between the orthopyroxene and plagioclase. This is evident by the horse-shoe shaped outline and intermingling boundary shared by orthopyroxene and Amph 1. The formation of outer Amph 2 and spinel symplectite layers could be attributed to the replacement of precursor clinopyroxene and plagioclase at high temperatures (1050-1150° C ± 40° C). The Amph-Spl symplectites, presence of oxidizing conditions (magnetite and ilmenite), discontinuous reactions and local or short-range diffusion phenomena thus indicate that the corona structures are a result of metasomatic interaction of cooling magma with the previously formed minerals.

Keywords: Corona structures; troctolite gabbro; olivine- plagioclase contact; Kargil; Ladakh; India.

How to cite: Harshe, S., Jonnalagadda, M., Duraiswami, R., Benoit, M., Grégoire, M., and Karmalkar, N.: Formation of corona structures from the troctolitic gabbros of Chainigund, Kargil, Ladakh, NW Himalayas, India: Petrological implications, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3865, https://doi.org/10.5194/egusphere-egu22-3865, 2022.

The 3.5-0.5 Ma Devès volcanic field consists mainly of nepheline basanite rocks. The underlying Variscan basement is a part of the western Moldanubian Zone (an allochton of the European Variscan orogen, probably the Gondwana margin). The Devès volcanic field is located in the “southern” mantle domain of Massif Central (MC), which consists of fertile lithospheric mantle (LM) little affected by partial melting [1]. These characteristics probably resulted from intense metasomatism by melts coming from the upwelling asthenosphere [2].

Despite the rich literature dealing with the LM beneath the Devès volcanic field, some textural and geochemical details remain obscure. We studied a large xenolith population (n – 21) from Mt.Briançon (NW of the Devès volcanic field) with extensive use of EMPA and LA-ICP-MS in order to obtain a comprehensive and representative data set, and here present the preliminary findings.

The Mt.Briançon xenoliths are typically oval in shape and vary in size from 4 to 13 cm. The host rocks are tuff and scoria deposits. The xenoliths are mostly anhydrous spinel lherzolites rich in clinopyroxene (cpx, modal content up to 28%) and scarce harzburgites. One xenolith consists of olivine clinopyroxenite in contact with peridotite. The peridotites exhibit serial texture or different stages of porphyroclastic texture. In some xenoliths elongated spinel is arranged in streaks.

Most of the three major phases in the peridotites are homogenous at the grain and xenolith scale. Olivine Fo is typically 88.5-90.4% in the whole suite, and NiO content is 0.35-0.43 wt.%. Orthopyroxene (opx) has Mg# 0.89-0.91 and 0.128-0.217 atoms of Al per formula unit (apfu). Cpx has Mg# 0.88-0.91 and Al content of 0.208-0.316 apfu and spinel Cr# is highly variable in the whole suite (0.09-0.28). In contrast, one harzburgite (sample 4025) has olivine with higher Fo (~91.2%), opx with higher Mg# (~0.92) and lower Al content (0.111-0.116 apfu), cpx with Mg# ~0.92 and Al content of ~0.145 apfu, and spinel Cr# of ~0.43 and Mg# of ~0.75.

The main observed REE pattern in peridotite cpx is relatively flat Lu-Eu and slightly, but variably depleted in lighter REE. In several xenoliths cpx exhibits various REE patterns, transitioning from LREE-depleted to relatively flat or slightly LREE-enriched, while a few samples contain cpx with REE abundances moderately increasing Lu-Sm and steeply increasing towards La. The majority of peridotite opx REE patterns are moderately decreasing in Lu-Sm and more steeply decreasing towards La, whereas a less common opx pattern is similar to the previous one in Lu-Nd, but much less depleted in lighter REE. This opx coexists with LREE-rich cpx.

This study confirms that the LM beneath Mt.Briançon is mostly lherzolitic and quite fertile in terms of major elements. Ongoing work, utilizing the diversity of lithologies and pyroxene REE patterns, combined with detailed major-element and REE thermometry and with textural observations, will provide detailed insights into the microstructural, thermal and metasomatic history of the LM beneath the MC.

This study was funded by Polish National Science Centre to MZM (UMO-2018/29/N/ST10/00259).

References

[1] Uenver-Thiele L. et al. (2017). JPetrol 58, 395–422.

[2] Puziewicz J. et al. (2020). Lithos 362–363, 105467.

How to cite: Ziobro-Mikrut, M., Puziewicz, J., Aulbach, S., Ntaflos, T., and Matusiak-Małek, M.: Preliminary characteristics of mantle xenoliths from Mt. Briançon (Massif Central, France) - missing information about the lithospheric mantle beneath Devès volcanic field, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4351, https://doi.org/10.5194/egusphere-egu22-4351, 2022.

The Bakony-Balaton Highland Volcanic Field (BBHVF), where Neogene alkali basalts and their pyroclasts host a great number of upper mantle xenoliths, is situated in the western part of the Pannonian Basin. One of the barely investigated xenolith localities of the BBHVF is Mindszentkálla. In the BBHVF, most of the xenoliths have lherzolitic modal composition, however, the Mindszentkálla locality is dominated by harzburgites. In addition to the homogeneous coarse-grained harzburgite xenoliths, we collected composite and multiple composite (with more than two different domains) xenoliths that represent small-scale heterogeneities. Harzburgite, interpreted as the host rock, is crosscut by dunitic, orthopyroxenitic, apatite-bearing websteritic, and amphibole-phlogopite-bearing veins.

To understand the evolution of the conspicuously complex mantle beneath Mindszentkálla, in situ major and trace element analyses were carried out on all rock-forming minerals. The major element chemistry of silicate minerals in the harzburgite wall rock and dunite veins show lower basaltic element (Fe, Mn, Ti, Na) contents with respect to the orthopyroxenitic and websteritic veins. The rare earth elements display flat or spoon-shaped patterns in the harzburgitic clinopyroxenes, whereas the websteritic clinopyroxenes and the amphiboles of the amphibole-phlogopite vein are enriched in light rare earth elements.

The observed textural and geochemical features indicate that the Mindszentkálla xenoliths could have gone through significant mineralogical and compositional modifications in at least two events. During the first event, the lherzolitic mantle was metasomatized most likely by a silica-rich melt, which could have resulted in orthopyroxene-rich peridotitic lithology. The metasomatizing Si-rich melt is likely related to a former subduction event.

The second metasomatic event led to the formation of dunite, orthopyroxenite, apatite-bearing websterite, and amphibole-phlogopite-bearing veins. These lithologies are likely the products of interactions between volatile-enriched, asthenosphere-derived basaltic melts and the peridotite wall rock, or they represent the high-pressure crystallization of such melts. The ascent of these mafic melts may have happened shortly before the xenolith entrapment during the Neogene basaltic volcanism.

How to cite: Patkó, L., Kovács, Z., Liptai, N., Aradi, L. E., Berkesi, M., Ciazela, J., Hidas, K., Garrido, C. J., and Kovács, I. J.: Deciphering multiple metasomatism beneath Mindszentkálla (Bakony-Balaton Highland Volcanic Field, western Pannonian Basin) revealed by upper mantle peridotite xenoliths, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4679, https://doi.org/10.5194/egusphere-egu22-4679, 2022.

Keywords: magma-poor rifted margin, refertilization, partial melting, mantle-melt interaction

Although magmatic processes are of primary importance for the understanding of lithospheric breakup, many first order questions remain, such as: how, when, where and how much magma is produced during final rifting; what are the conditions and controlling processes of magma production; how does magma percolate and interact with the lithospheric mantle; and how and when does magma focus, how is it extracted and how does it interact with the extensional processes during final rifting and breakup? Answering to these questions is a prerequisite to understand lithospheric breakup and formation of a new plate boundary, which is among the least understood plate tectonic processes at present.

In this study we present preliminary petrological results from mantle rocks dredged from the SW Australia ocean-continent transition (OCT, Diamantina zone). We analyzed pyroxene and spinel compositions from these peridotites to identify mantle domains and mantle-melt reactions during rifting and breakup. The chemical composition of clinopyroxenes shows two distinct populations: a first generation characterized by low (Sm/Yb)_N ratios and no Eu anomalies, while a second generation shows interstitial textures and flat HREE patterns with a deep negative Eu anomaly. These two populations of clinopyroxenes suggest that the peridotites from the Diamantina zone record two distinct events: a first cooling event that is followed by magma infiltration. This is further supported by equilibrium temperatures calculated on the two clinopyroxene generations showing that the first population equilibrated at lower temperatures (900°C ± 30°C) corresponding to a subcontinental geotherm, while the second generation equilibrated at higher temperatures (1100°C ± 100°C), and was likely liked to the entrapment of MORB-type melts in the plagioclase stability field at low pressure (~5kbar) during magma infiltration.  

The exhumation path of the Diamantina peridotites determined in our study is similar to those of refertilized peridotites from the present-day Iberia and fossil Alpine Tethys OCTs, suggesting that refertilisation processes occurring at magma-poor rifted margins during final rifting and breakup are not dependent from the inherited nature of the subcontinental mantle.

How to cite: Ballay, M., Ulrich, M., and Manatschal, G.: Magmatic processes at rifted margins: Preliminary results from peridotites of the Diamantina zone (SW Australia), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5414, https://doi.org/10.5194/egusphere-egu22-5414, 2022.

In Nd-Hf isotopic space the great majority of the global abyssal peridotites plot in the field defined by global MORBs. However, Hf isotope ratios by far exceeding those in ridge basalts, are locally observed in abyssal peridotites showing that the Earth’s mantle is more heterogeneous that inferred from ridge basalts [1]. Mantle peridotites exposed at the Doldrums Fracture Zone at the Mid Atlantic Ridge (7-8° N) reveal that such heterogeneity coexists on a kilometre-scale. Abyssal peridotites from the northern part of the Doldrums FZ domain can be grouped into residual peridotites and melt-modified (refertilized) samples [2]. New Nd-Hf isotopic data show that the refertilized peridotites preserve highly radiogenic Hf values (εHf up to 101) associated with MORB-like Nd isotopes (εNd up to 12), reflecting partial resetting of ancient highly depleted mantle by recent melt-rock interaction. On the other hand, despite a very depleted incompatible element compositions, the residual peridotites have Nd-Hf isotope ratios similar to the local MORB (εNd = 7-12 and εHf =12-19). They most likely reflect highly depleted mantle that has been entirely reset by reaction with extracted or retained melts, and hence developed with only modest incompatible element depletion until recent melting at the Mid Atlantic ridge axis, which led the strong incompatible element depleted of these peridotites. The kilometre-scale association of such isotopically heterogeneous domains suggests that the upper mantle exposed in this portion of Atlantic formed by a combination of ancient melting and melt-rock reaction processes, preceding its emplacement below the present-day Mid Atlantic ridge axis.

 [1] Stracke, A., et al., 2011. Abyssal peridotite Hf isotopes identify extreme mantle depletion. Earth and Planetary Science Letters, 308(3-4), pp.359-368. [2] Sani, C., et al., 2020. Ultra-depleted melt refertilization of mantle peridotites in a large intra-transform domain (Doldrums Fracture Zone; 7–8° N, Mid Atlantic Ridge). Lithos, 374, p.105698.

How to cite: Sani, C., Sanfilippo, A., Payve, A. A., Genske, F., and Stracke, A.: Kilometre-scale isotopic heterogeneity in abyssal peridotites from the Doldrums Fracture Zone (Mid Atlantic Ridge, 7-8ºN)., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5617, https://doi.org/10.5194/egusphere-egu22-5617, 2022.

Cu-rich sulfide deposits of economic importance in ophiolites such as Troodos in Cyprus or Semail in Oman often occur along the crust-mantle transition zones (e.g. Begemann et al., 2010). Although secondary sulfides formed during serpentiniztion now prevail, the relicts of primary magmatic sulfides indicate the igneous nature of enrichment in sulfides at the oceanic Moho level. Crust-mantle transition zones in situ in the oceans are suggested to be enriched in sulfides and many chalcophile (e.g. Cu, Zn, Pb, Se, Te) metals via melt-mantle reaction (Ciazela et al., 2017; 2018). The enrichment in sulfides seems to be ubiquitous along the crust-mantle transition zone (Ciazela et al., 2018) and might be expected even at the continental Moho. This is possible as sulfides precipitate during melt-mantle reaction independently on pressure. The process seems to work at low pressures of the oceanic crust-mantle transition zone (0.1–0.2 GPa) (Marciniak et al., this session; Ciazela et al., 2018), medium pressures of the continental crust-mantle transition zone (~1.0 GPa) (Pieterek et al., this session), and in high pressures related to various melt-metasomatized mantle xenoliths (up to 2.5 GPa) (Mazurek et al., this session; Patkó et al., 2021). Metal refertilization due to variable melt-peridotite reactions at the crust-mantle transition zone and along melt channels in the upper mantle may affect the local, regional, and even global metal mass balance of the oceanic and continental lithosphere. The distribution of mantle sulfides is heterogeneous. The zones of enrichment in metals occur mostly at the crust-mantle transition or in melt-modified mantle rocks along melt channels in the upper mantle. These zones are important for subsequent ore formation in secondary processes. In the oceans, especially along slow-spreading ridges, shallow magmatic sulfide horizons are penetrated by hydrothermal fluids operating along faults to form massive sulfides on the seafloor. On land, the re-mobilization of the mantle sulfides horizons by sulfide-undersaturated melts or by buoyant CO₂ bubbles can contribute to the formation of porphyry and related epithermal mineral deposits.

Begemann F., Hauptmann A., Schmitt-Strecker S. and Weisgerber G. (2010) Lead isotope and chemical signature of copper from Oman and its occurrence in Mesopotamia and sites on the Arabian Gulf coast. Arab. Archaeol. Epigr. 21, 135–169.

Ciazela J., Dick H. J. B., Koepke J., Pieterek B., Muszynski A., Botcharnikov R. and Kuhn T. (2017) Thin crust and exposed mantle control sulfide differentiation in slow-spreading ridge magmas. Geology 45, 935–938.

Ciazela J., Koepke J., Dick H. J. B., Botcharnikov R., Muszynski A., Lazarov M., Schuth S., Pieterek B. and Kuhn T. (2018) Sulfide enrichment at an oceanic crust-mantle transition zone: Kane Megamullion (23°N, MAR). Geochim. Cosmochim. Acta 230, 155–189.

Patkó L., Ciazela J., Aradi L. E., Liptai N., Pieterek B., Berkesi M., Lazarov M., Kovács I. J., Holtz F. and Szabó C. (2021) Iron isotope and trace metal variations during mantle metasomatism: In situ study on sulfide minerals from peridotite xenoliths from Nógrád-Gömör Volcanic Field (Northern Pannonian Basin). Lithos 396–397, 106238.

How to cite: Ciazela, J., Pieterek, B., Marciniak, D., Mazurek, H., Patko, L., and Slaby, E.: Melt metasomatism and enrichment in metals in the uppermost Earth’s mantle, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6901, https://doi.org/10.5194/egusphere-egu22-6901, 2022.

Uncommon Ba-Cl-rich phases including Ba-Cl micas and Cl-phosphates have been found in garnet pyroxenites as a part of the matrix or in polyphase inclusions in garnets. Polyphase inclusions are rich in carbonates (dolomite, magnesite, norshetite), phosphates (Cl-apatite, goryainovite (Ca₂PO₄Cl), monazite) and other silicates (spinel, amphibole, orthopyroxene, clinopyroxene, margarite, aspidolite, scapolite, cordierite). The inclusions appear as chains crosscutting garnet crystals and their presence is not linked with any chemical zoning in the host garnet.

The Ba-Cl-rich mica has composition ranging from Ba-rich phlogopite to chloroferrokinoshitalite and to oxykinoshitalite. The mica present in the matrix correspond to Ba-rich phlogopite with low Cl contents and occur together with celsian and low-Cl hydroxyl apatite. The mica in the polyphase inclusions ranges to almost pure chloroferrokinoshitalite and oxykinoshitalite endmembers and coexists either with Cl-apatite (Cl = 1.2 apfu) or rarely goryainovite containing up to 2.5 wt% of SrO. This is second world occurrence of goryainovite and first evidence that Ca can be partially replaced by Sr in this mineral.

Special attention was paid to the composition trends of the Ba-Cl-rich micas. These are mainly related to the XFe ratio, which correlates positively with Cl, Ba, and Al and negatively with Si and Na. Positive correlation of Cl with Ba and XFe leads to the formation of mica with composition Ba_0.95K_0.03Fe_2.69Mg_0.37Al_1.91Si_2.02Cl_1.98, XFe_0.88, which is the most Cl-rich mica so far described from natural samples (10.98 wt% Cl) and is very close to the theoretical formula of chloroferrokinoshitalite BaFe₃Al₂Si₂O₁₀Cl₂. The positive correlation of Ba with Al and their negative correlation with Si and K is corresponding to the coupled substitution Ba₁Al₁K_-1Si_-1 linking the composition of phlogopite and kinoshitalite. Composition trend related with the Ti-content shows that Ti correlates positively with Ba but negatively with Cl, XFe, and with the sum of Mg and Fe. It implies that Ti is incorporated into mica in coordination with O (Ti₁O₂(Mg,Fe²⁺)_-1(OH)_-2) and it leads to the formation of oxykinoshitalite (BaMg₂TiSi₂Al₂O₁₂). Since the incorporation of either Cl or Ti + O correlates with XFe content of mica, XFe ratio can be the crucial factor controlling the ability of mica to incorporate Cl into its crystal lattice. In some cases, two micas with contrasting composition corresponding closer to chloroferrokinoshitalite or oxykinoshitalite coexist in one polyphase inclusion, demonstrated by distinct content of XFe, Ti and Cl (for example: XFe_0.20:0.77, Ba_0.48:0.63, Ti_0.35:0.02, Cl_0.27:1.45). This could imply the existence of an immiscibility between the composition trends of chloroferrokinoshitalite and oxykinoshitalite .

Such Ba, Cl and K-rich phases are atypical for garnet pyroxenite. Their presence may be caused by the injection of fluid/melt of crustal source during subduction and subsequent exhumation processes or may be related to earlier mantle metasomatism. The presence of Cl-rich phases together with carbonates indicates extremely high activity of Cl and CO₂ in the metasomatizing fluid/melt that interacted with garnet pyroxenites.

How to cite: Zelinková, T., Racek, M., and Abart, R.: Compositions of Ba-Cl-rich micas and other uncommon phases related to metasomatism of garnet pyroxenite (Gföhl unit of the Moldanubian Domain, Bohemian Massif), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7935, https://doi.org/10.5194/egusphere-egu22-7935, 2022.

Peridotite xenoliths are of great interest for research, since their composition is closest to the simulated compositions of the upper mantle, and they also make it possible not only to determine the conditions for the formation of these rocks, but also the degree of metasomatic processing of the diamondiferous keel, as well as the thickness and distribution area of diamondiferous rocks in the lithospheric mantle.

The Middle Paleozoic (D3-C1) diamondiferous kimberlite pipe Komsomolskaya-Magnaya was chosen as the object of research. This is one of the diamondiferous pipes of the Siberian platform, which contains many unchanged xenoliths of peridotite rocks.

We studied a collection of 180 peridotite xenoliths of the Komsomolskaya-Magnitnaya pipe, of which 104 belong to dunite-harzburgite paragenesis, 74 to lherzolite and 4 websterites. Also, we studied a large number of minerals from the concentrate material of the Komsomolskaya-Magnitnaya kimberlite pipe.

A high proportion (~ 30%) of peridotites with high magnesian olivines (Mg #> 93 mol%) indicates the presence of a block of highly depleted rocks in the lithospheric mantle.

We noted a high proportion of garnets with S-shaped REE distribution spectra (~ 60%), as well as garnets belonging to the harzburgite-dunite paragenesis in accordance with the CaO-Cr₂O₃ diagram. It indicates a moderate role of metasomatic changes associated with silicate melts, as well as interaction with carbonatite melts enriched in LREE.

In addition, kimberlite indicator minerals (KIM) (garnets, chrome spinels, ilmenites) were studied, sampled directly from 7 geophysical anomalies, 6 new kimberlite bodies, and kimberlite pipes Interkosmos, Kosmos-2, 325 years of Yakutia, belonging to the Upper Muna field. These data provide more information on the composition of the lithospheric mantle within the entire Upper Muna field.

For several kimberlite bodies, a high proportion of KIM of the diamond association is noted, however, for most kimberlite bodies, signs of a high degree of secondary metasomatic processes are noted, which negatively affect the preservation of diamond in the lithospheric mantle.

Cr-spinels from various kimberlite bodies of the Upper Muna field attract special attention. In addition to the typical peridotite Cr-spinels, there are Cr-spinels that follow the magmatic trend (Sobolev, 1974) and have extremely low contents of aluminum and titanium. The genetic identity of these Cr-spinels is still unknown.

Was done precise pressure (P)-temperature (T) estimation using single-clinopyroxene thermobarometry (Nimis, Ta). Was obtained mantle paleogeotherm.  Data was received about surface heat flux ~34–35mW/m2, 225–230 km lithospheric thickness, and 110–120 thick “diamond window” for the Upper Muna field (Dymshits et al, 2020).

• Dymshits A. M., Sharygin I. S., Malkovets V. G., Yakovlev I. V., Gibsher A. A., Alifirova T. A., Vorobei S. S., Potapov S. V., Garanin V. K. Thermal state, thickness, and composition of the lithospheric mantle beneath the Upper Muna kimberlite field (Siberian Craton) constrained by clinopyroxene xenocrysts and comparison with Daldyn and Mirny fields // Minerals. 2020. V. 10. P. 549.
• Sobolev N.V., Deep inclusions in kimberlites and the problem of the composition of the upper mantle // Novosibirsk: Nauka, 1974.

How to cite: Iakovlev, I., Malkovets, V., and Gibsher, A.: Features of the composition and structure of the lithospheric mantle of the Upper Muna field., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9103, https://doi.org/10.5194/egusphere-egu22-9103, 2022.

Nea Roda and Gomati ultramafic bodies (east Chalkidiki, north Greece) consist of both Cr- and Al- podiform chromitites, which are highly altered. Their PGE geochemistry and subsequently PGE-mineralogy (PGM) demonstrate abnormal element concentrations with an enrichment in PPGE (Pd, Pt), leading to high Pd/Ir ratios. Secondary PGM and base metal assemblages are dominated by Sb and As, whereas primary phases form sulphides. At a more mature stage, desulphurization of the aforementioned phases led to formation of native metals. Diopside hosted within diopsidite and chromitite show both an alkaline melt- and a fluid- rock interaction, depicted by LREE enrichment. The temperature of the metasomatic fluids was lower than 600^oC, as recorded by chlorite and garnet geothermometry. A raise in fluid mobile elements (FME: B, Sb, Li, As, Cs, Pb, U, Ba and Sr) is noted in the whole rock and clinopyroxene analysis. All these characteristics along with the distinctive spinel textures (porous, zoned grains) point to a metasomatic event during subduction that led to the post-magmatic modification of the chromitites and the mantle section causing a LREE, Pb, As, Sb, Pd and Pt enrichment. 

How to cite: Koutsovitis, P., Sideridis, A., Tsitsanis, P., Zaccarini, F., Tsikouras, B., Hauzenberger, C., Grammatikopoulos, T., Bindi, L., Garuti, G., and Hatzipanagiotou, K.: Mantle metasomatism recorded upon bimodal chromitites (E. Chalkidiki, Greece): a tool to unravel metasomatic processes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9331, https://doi.org/10.5194/egusphere-egu22-9331, 2022.

Southern Sweden (Scania region) is located in the peripherical parts of the East European Craton (EEC). In the Mesozoic, up to three pulses of volcanic activity took place between 191 and 110 Ma (Bergelin et al., 2006, IJES; Tappe et al., 2016, GCA). Some of the alkali basaltoids carry ultramafic, mafic and felsic xenoliths (Rehfeldt et al, 2007, IJES). In this study, we focused on the evolution of the lithospheric mantle sampled by anhydrous, spinel-facies lherzolites, harzburgites, and subordinate dunites.

Based on the Fo content in olivine, the peridotites were classified into three groups. Group X peridotites are characterized by Ca-rich olivine (890-1470 ppm) with Fo=91.1-91.7.  Enstatite has Mg#=91.5-91.9 and Al=0.16-0.22 atoms per formula unit (apfu), while the Cr-augite has Mg#= 90.8-91.2 and Al=0.21-0.28 apfu. Clinopyroxene is chemically homogenous in terms of trace elements and is LREE-enriched with positive Eu-anomaly. The Nd and Sr isotopic ratios in clinopyroxene are ¹⁴³Nd/¹⁴⁴Nd=0.512548 (εNd=2.63) and  ⁸⁷Sr/⁸⁶Sr=0.704237, respectively. Olivine in group Y peridotites is Ca-poor (<951 ppm) and has Fo=89.5-91.1, enstatite has Mg#=89.7-91.7, and Al content of 0.084-0.169 apfu. The Cr-diopside has Mg#=90.8-93.5 and Al=0.118-0.232 apfu. Trace element patterns in clinopyroxene allow subdivision of this group into two subgroups: subgroup Y1 – with heterogeneous LREE-enriched clinopyroxene, and subgroup Y2 – with homogenous LREE-enriched clinopyroxene; both groups are characterized by a positive Eu anomaly, but in subgroup Y1 it is significantly more pronounced. The Nd and Sr isotopic ratios in clinopyroxene from subgroup Y1 are ¹⁴³Nd/¹⁴⁴Nd=0.512624–0.512644 (εNd=4.13-4.52) and ⁸⁷Sr/⁸⁶Sr=0.703027–0.703100, therefore significantly more depleted than group X. In group Z peridotite the Fo content in olivine is 88.1-89.1, the Mg# in enstatite is 89.1-89.5 and its Al content is 0.19-0.20 apfu. The Mg# of Cr-diopside is 88.5-89.4 and the Al content is 0.24-0.25 apfu. The trace elements contents in clinopyroxene is homogenous and the REE pattern is flat at values double that in the primitive mantle.         

 The highest equilibration temperatures were estimated for the group X xenoliths, where T_WES=1101-1110 °C (Witt-Eickschen and Seck, 1991, CMP) and T_BK=1214-1241 °C (Brey and Köhler, 1990, JoP).  The temperatures calculated for group Y xenoliths are T_WES=875-1033 °C and T_BK=872-1027 °C and do not significantly differ between subgroups. Temperatures recorded by the group Z sample are T_WES=1040-1056 °C and T_BK=1065-1081 °C.

The composition of group X peridotites suggests their metasomatism by a high-temperature mafic melt resembling the basaltoids from Scania. Alternatively, they may represent high-pressure cumulates, as suggested by their coarse-grained texture. The group Y peridotites record cryptic metasomatism of a significantly depleted peridotite (melt extraction ranging typically between 25 and 30%) by a carbonatitic melt. The carbonatitic metasomatic agent was fractionating chromatographically from REE-, Th- and U-rich in subgroup Y2 to -poor in those elements in subgroup Y2. The group Z peridotite possibly represents depleted peridotite which was further metasomatized by a mafic melt. The lithospheric mantle beneath the marginal part of EEC has a complex composition, which is however different from a typical cratonic mantle.

Founded by Polish National Science Centre grant no. UMO-2016/23/B/ST10/01905 and WTZ PL 08/2018.

How to cite: Matusiak-Małek, M., Mikrut, J., Puziewicz, J., Kukuła, A., Ntaflos, T., Aulbach, S., Johansson, L., and Grégoire, M.: Composition of lithospheric mantle beneath southern margin of East European Craton evidenced by peridotitic xenoliths from Scania, S Sweden., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9457, https://doi.org/10.5194/egusphere-egu22-9457, 2022.

The Mirdita Ophiolite in northern Albania forms a ~240 km long and ~40 km wide zone within Dinaric-Hellenic belt. It marks suture after Neo-Tethyan Ocean closure. The chemical diversity of volcanic crustal rocks led to its division into two zones: the eastern one is interpreted to have Supra-Subduction Zone (SSZ) origin, whereas the western zone exhibits Mid-Ocean Ridge (MOR) affinity. More than a dozen of ultramafic massifs occur along the entire length of the ophiolite.

In this study we focus on chemical diversity of peridotites from two adjacent massifs, Kukes and Puka, which have SSZ and MOR affinities, respectively. The Kukes Massif is composed of a sequence from harzburgites at its base to clinopyroxene-poor dunites at the top, followed by pyroxenitic and peridotitic cumulates at the mantle/crust transition zone. The Puka massif is a mantle dome, composed of harzburgites and plagioclase/amphibole lherzolites (locally mylonitzed) and it is interpreted as a former oceanic core complex (OCC; Nicolas et al. 2017). Both massifs are pervasively penetrated by pyroxenitic and gabbroic veins and are serpentinised to variable degree.

Chemical composition of minerals varies between samples and lithologies, as well as between massifs. Olivine from the Kukes harzburgites has higher Fo values and NiO contents than that from dunites (Fo_89.5-92 and NiO 0.31-0.52 wt.% vs. Fo_88.1-91.2 and NiO 0.15-0.30 wt.%, respectively). Clinopyroxene has Mg#_92.5-95.1 and Al=0.03-0.08 apfu in harzburgite, while interstitial dunite clinopyroxene has Mg#_94-98 and Al below 0.03 apfu. Harzburgite orthopyroxene has Mg#_90.1-91.8 and Al=0.03-0.08 apfu. Chromian-spinel has Cr#_0.55-0.72 and Mg#_0.46-0.56 in harzburgites and Cr#_0.63-0.86 and Mg#_0.25-0.48 in dunites, moreover in dunites it often exhibits chemical zonation with Cr# increasing to core. Chemical composition of minerals changes gradually in the scale of single outcrop, with Fe content increasing toward veins.

The Puka peridotites have more enriched composition. Olivine has Fo_87.8-90.8 and NiO=0.25-0.43 wt. %, clinopyroxene has Mg#_90.1-93.3 and Al=0.05-0.15 apfu, orthopyroxene has Mg#_88.5-91.0 and Al=0.03-0.1 apfu, while spinel has Cr#_0.38-0.55 and Mg#_0.42-0.57, with single sample of Cr#_0.60-0.75 and Mg#_0.33-0.52. Plagioclase is Ca-rich (77-95 An), amphibole – occurring in some lherzolites – has composition of pargasite-tremolite.

Differences in lithological and chemical composition are visible between peridotites from both massifs, which correspond with diversity of crustal rocks and suggest that also mantle sections of the ophiolite record different origin. Peridotites from Kukes are harzburgites and dunites pointing to their refractory nature. The depleted peridotites were further affected by intensive magmatic veining. Infiltration of the melt triggered gradual enrichment in Fe of the silicates and chemical zonation of spinel. This process is well visible in dunites, where changes of Fe contents can be followed on distances of few meters. As metasomatic modification has a limited range, most of chemical differences have to be related with different protolith, but further studies are required to reconstruct rocks evolution.

Protolith of Puka peridotites is more fertile compared with Kukes, but reaction between veins and host lherzolite was not observed, and mylonitization led to Al depletion in pyroxenes.

This study was financed from scientific funds for years 2018-2022 as a project within program “Diamond Grant” (DI024748).

How to cite: Mikrut, J., Matusiak-Małek, M., and Puziewicz, J.: Preliminary insights into lithological and chemical diversity in Mirdita Ophiolite peridotite massifs – Kukes and Puka case studies, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9560, https://doi.org/10.5194/egusphere-egu22-9560, 2022.

Many aspects of structure and thermal state of >200 km thick cratonic lithospheric mantle (CLM) remain unclear because of insufficient sampling and uncertainties of pressure (P) and temperature (T) estimates. An exceptionally detailed record of equilibration temperature and composition for the central Siberian craton in the 60–230 km depth range was obtained using new and published petrographic and in-situ chemical data for ~200 garnet peridotite xenoliths from the Udachnaya kimberlite. The thermal profile is complex with samples between 35 and 40 mW/m² model conductive geotherms as well as hotter layers in the middle and at the base (190–230 km) of the CLM. A previously unknown mid-lithospheric zone includes rocks up to 150° hotter than ambient geotherm, with high modal garnet and cpx, low-Mg# and melt-equilibrated REE patterns. We posit that hot domains with enriched compositions may form at depths where ascending melts stall (e.g., due to loss of volatiles and/or redox change) and react with wall-rock harzburgites. By contrast, we find no rocks rich in volatile-rich metasomatic amphibole, mica or carbonate, nor layers composed of peridotites with distinct melt-extraction degrees. The CLM base contains both coarse and variably deformed rocks heated and re-worked (Mg#_Ol down to 0.86) by localized interaction with asthenospheric melts.

How to cite: Ionov, D., Liu, Z., Nimis, P., Xu, Y., and Golowin, A. V.: Xenolith-based thermal and compositional lithospheric mantle profile of the central Siberian craton, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13175, https://doi.org/10.5194/egusphere-egu22-13175, 2022.

Dyke swarms intruding the mantle–continental crust transition of the Adria plate as documented by the Ivrea-Verbano Zone (IVZ, Southern Alps) represent a unique opportunity to investigate the evolution of mantle melts from Late Paleozoic to Mesozoic in the post-collisional Variscan realm. Thus, we present new petrological and geochemical data of dyke swarms cropping out in the Finero Phlogopite Peridotite mantle unit. Dykes are from a few cm to >1 m thick and cut at a high angle the mantle foliation.

The dyke swarms are composed of cumulus phlogopite-bearing amphibole peridotite, hornblendite, diorite and anorthosite. Many dykes are composite, showing variable proportions of melanocratic and leucocratic layers. Volatiles overpressure during the late magmatic stage is testified by plastic flow and development of a porphyroclastic structure by deformation of early cumulates and by the widespread segregation of a fine-grained mica matrix. The dyke swarms show mineralogical and geochemical features varying between two end-member series.

A dyke series is characterized by Al-rich pargasite (Al₂O₃ up to 18 wt.%) and phlogopite, associated with apatite, calcite, sulphides and sometimes sapphirine. The amphiboles show i) large LILE and LREE contents, ii) negative Nb, Ta, Zr and Hf anomaly and iii) isotopic oxygen composition heavier than the mantle interval, which support the occurrence of recycled continental crust components in the parent melts and impart an overall “orogenic” affinity.

The second series mainly consists of Al-poorer pargasite, phlogopite and albite (An 8-10), associated with apatite, monazite, ilmenite, zircon, Nb-rich oxides and carbonates. Mineral compositions and assemblages indicate that the parent melts were strongly enriched in Fe, Na, H₂O, P and C. Amphiboles are still enriched in LILE and LREE, but show extreme enrichments in Nb, Ta, Zr and Hf. As a whole, the petrochemical features point to an “anorogenic” alkaline affinity. Zircons from the “anorogenic” dykes are mostly anhedral, with homogenous internal structure or sector zoning. The strongly positive εHf_t (average of +10) of zircons and the Sr isotopic composition of amphiboles (0.7042) point to a derivation of such “anorogenic” melts from mildly enriched mantle sources. Concordant ²⁰⁶Pb/²³⁸U zircon ages for “anorogenic” dykes vary from 221 ± 9 Ma to 192 ± 8 Ma. Some dykes show both “orogenic” and “anorogenic” affinities, thus recording different pulses of mantle melts and metasomatic overprinting. As a whole, the dyke swarms show a transition from “orogenic” to “anorogenic” affinity indicating re-opening of dykes’ conduits for the melt ascending, pointing to a progressive change of the mantle sources of the Mesozoic magmatism of the Southern Alps.

How to cite: Ogunyele, A. C., Giovanardi, T., Bonazzi, M., Mazzuccheli, M., De Carlis, A., Cipriani, A., and Zanetti, A.: Transition from “orogenic-like” to “anorogenic” geochemical affinity in Mesozoic post-collisional magmatism: evidence from alkali-rich dykes from Ivrea-Verbano Zone (Southern Alps), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13428, https://doi.org/10.5194/egusphere-egu22-13428, 2022.

Many coastal cities are an early casualty in climate-related coastal flooding because of processes resulting in land subsidence and thus enhanced relative sea-level (RSL) rise. Much of the Atlantic coast of North America has been sinking for thousands of years, at a maximum rate of ~20 cm per century as a consequence of solid Earth deformation in response to deglaciation of the Laurentide ice sheet (between ~18,000 and ~7,000 years ago) [e.g. Love et al., Earth's Future, 4(10), 2016]. Karegar et al. [Geophysical Research Letters, 43(7), 2016] have shown that vertical land motion along the Atlantic coast of the USA is an important control on nuisance flooding. A key finding in this study is that while glacial isostatic adjustment (GIA) is the dominant process driving land subsidence in most areas, there can be large deviations from this signal due to the influence of anthropogenic activity impacting hydrological processes. For example, between Maine (45°N) and New Hampshire (43°N), the GPS data show uplift while geological data show long-term subsidence. The cause of this discrepancy is not clear, but one hypothesis is increasing water mass associated with the James Bay Hydroelectric Project in Quebec [Karegar et al., Scientific Reports, 7, 2017].

The primary aim of this study is to better constrain and understand the processes that contribute to contemporary and future vertical land motion in this region to produce improved projections of mean sea-level change and nuisance flooding. The first step towards achieving these aims is to determine a GIA model parameter set that is compatible with observations of past sea-level change for this region. We make use of two regional RSL data compilations: Engelhart and Horton [Quaternary Science Reviews, 54, 2012] for northern USA and Vacchi et al. [Quaternary Science Reviews, 201, 2018] for Eastern Canada, comprising a total of 1013 data points (i.e., sea level index points and limiting data points) over 38 regions distributed throughout our study region. These data are well suited to determine optimal GIA model parameters due to the magnitude of other signals being much smaller, particularly in near-field regions such as Eastern Canada. We consider a suite of 32 ice history models that is comprised mainly of a subset from Tarasov et al. [Earth and Planetary Science Letters, 315–316, 2012] as well as the ICE-6G and ANU models. We have computed RSL for these ice histories using a state-of-the-art sea-level calculator and 440 1-D Earth viscosity models per each ice history model to identify a set of Earth model parameters that is compatible with the observations.

How to cite: Parang, S., Milne, G. A., Karegar, M. A., and Tarasov, L.: Towards an improved understanding of vertical land motion and sea-level change in eastern North America, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-50, https://doi.org/10.5194/egusphere-egu22-50, 2022.

The western Russian Arctic was partially covered by the Eurasian ice sheet complex during the Last Glacial Maximum (~26 ka BP) and is a focus area for Glacial Isostatic Adjustment (GIA) studies. However, there have been few GIA studies conducted in the Russian Arctic due to the lack of high quality deglacial relative sea-level (RSL) data. Recently, Baranskaya et al. (2018) released a quality-controlled deglacial RSL database for the Russian Arctic that consists of ~400 sea-level index points and ~250 marine and terrestrial limiting data that constrain RSL since 20 ka BP. Here, we use the RSL database to constrain the 3D Earth structure beneath the Russian Arctic, with consideration of the uncertainty in ice model ICE-7G_NA, which is assessed via iteratively refining the ice model with fixed 1D Earth model to achieve a best fit with the RSL data. Also, the uncertainties in 3D Earth parameters and RSL predictions are investigated.

We find an optimal 3D Earth model (Vis3D) improves the fit with the deglacial RSL data compared with the VM7 1D model when fixed with the ICE-7G_NA ice model. Similarly, we show improved fit in the White Sea area, where 1D model shows notable misfits, with the refined ice model ICE-7G_WSR when fixed with VM7 Earth model. The comparable fits of ICE-7G_NA (Vis3D) and ICE-7G_WSR (VM7) implies that the uncertainty in the ice model might be improperly mapped into 3D viscosity structure when a fixed ice model is employed. Furthermore, fixed with refined ice model ICE-7G_WSR, we find an optimal 3D Earth model (Vis3D_R), which fits better than ICE-7G_WSR (VM7), and the magnitude of lateral heterogeneity decreases significantly from Vis3D to Vis3D_R.  We conclude that uncertainty in the ice model needs to be considered in 3D GIA studies.

How to cite: Li, T., Peltier, W. R., Stuhne, G., Khan, N., Baranskaya, A., Shaw, T., Wu, P., and Horton, B.: The inclusion of ice model uncertainty in 3D Glacial Isostatic Adjustment modelling: a case study from the Russian Arctic, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-852, https://doi.org/10.5194/egusphere-egu22-852, 2022.

GIA is a global process, because of gravitational effects, its interplay with earth rotation, and the large spatial extent of ice-sheet and ocean loading. However, mainly due to the presence of heterogeneities in the structure of crust and upper mantle, modelling of GIA signals often requires a regional approach. This is particularly true in the light of continuous advances in earth observation techniques, that allow increasingly accurate determination of land deformation, coastal sea level change, and mass balance of glaciers and ice sheets.

This talk will address a number of open issues related to regional GIA models, such as the effect of transient and non-linear rheologies, and the complementary role of forward and semi-empirical approaches, with an eye on the needs of the geodetic, sea level and cryosphere communities.

How to cite: Riva, R.: Regional GIA: modelling choices and community needs, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-918, https://doi.org/10.5194/egusphere-egu22-918, 2022.

A challenge to understanding Late Quaternary glaciation history is the mechanism(s) responsible for the asymmetry in an individual glaciation cycle between the slow pace of glaciation and the more rapid pace of deglaciation (e.g., Broecker and Van Donk, 1970). It is increasingly clear that a major contributor to the rate of global deglaciation is the instability of marine terminating ice streams. Recent analyses by Velay-Vitow et al. (2020) suggest that these instabilities were often triggered by ocean tides of anomalously high amplitude. Examples include the Hudson Strait Ice Stream responsible for Heinrich Event 1 (H1) and the Amundsen Gulf Ice Stream. Here, we analyse the instability of the Laurentian Channel and St Lawrence River Channel ice stream system. Our analysis begins with the recognition of highly significant misfits of up to 60 m at ~9,000 calendar years ago between deglacial relative sea-level histories inferred by Vacchi et al. (2018) at sites along the St Lawrence River Channel and those predicted by the ICE-6G_C (VM5a) and ICE-7G_NA (VM7) models of the Glacial Isostatic Adjustment process. We suggest that these disagreements between models and data may be due to the St Lawrence River Channel ice stream becoming unstable during the deglaciation of the Laurentide Ice Sheet (LIS) due to the hypothesized tidal mechanism for ice stream destabilization. We investigate a sequence of scenarios designed to provide a best estimate of the timing of this event. Since this ice stream penetrated deeply into the interior of the LIS and was connected to the Laurentian Channel ice stream, the instability of the latter was required in order for destabilization of the St Lawrence River channel ice stream to be possible. We explore the consistency of the implied sequence of events with the observational constraints.

How to cite: Peltier, R., Li, T., Stuhnne, G., Velay-Vitow, J., Vacchi, M., Englehart, S., and Horton, B.: Resolving the Influence of Ice Stream Instability on Postglacial Relative Sea-Level Histories: the case of the St Lawrence River Channel Ice Stream, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1343, https://doi.org/10.5194/egusphere-egu22-1343, 2022.

During the last decade there has been an increasing demand to improve models of present-day loading processes and glacial-isostatic adjustment (GIA). This is especially important when modelling the GIA process in tectonically active regions like the Pacific Northwest, Patagonia or West Antarctica. All these regions are underlain by zones of low-viscosity mantle. Although one-dimensional earth models may be sufficient to model local-scale uplift within these regions, modeling of the wider-scale deformation patterns requires consideration of three-dimensional viscosity structure that is consistent with other geophysical and laboratory findings. It is this wider-scale modeling that is necessary for earth-system model applications as well as for the validation or reduction of velocity fields determined by geodetic observation networks based on GNSS, for improving satellite gravimetry, and for present-day sea-level change as paleo sea-level reconstructions.

There are a number of numerical GIA codes in the community, which can consider lateral variations in viscoelastic earth structure, but a proper benchmark focusing on lateral heterogeneity is missing to date. Accordingly, ambiguity remains when interpreting the modelling results. The numerical codes are based on rather different methods to solve the respective field equations applying, e.g., finite elements, finite volumes, finite differences or spectral elements. Aspects like gravity, compressibility and rheology are dealt with differently. In this regard, the set of experiments to be performed has to be agreed on carefully, and we have to accept that not all structural features can be considered in every code.

We present a tentative catalogue of synthetic experiments. These are designed to isolate different aspects of lateral heterogeneity of the Earth's interior and investigate their impact on vertical and horizontal surface displacements, geocenter and polar motion, gravity, sea-level change and stress. The study serves as a follow up of the successful benchmarks of Spada et al. (2011) and Martinec et al. (2018) on 1D earth models and the sea-level equation. The study was initiated by the PALSEA-SERCE Workshop in 2021 (Austermann and Simms, 2022) and benefits from discussions inside different SCAR-INSTANT subcommittees, the IAG Joint Study Group 3.1 “Geodetic, Seismic and Geodynamic Constraints on Glacial Isostatic Adjustment", the IAG Subcommission 3.4 “Cryospheric Deformation" and PALSEA.

References:

Austermann, J., Simms, A., 2022 (in press). Unraveling the complex relationship between solid Earth deformation and ice sheet change. PAGES Mag., 30(1). doi:10.22498/pages.30.1.14

Martinec, Z., Klemann, V., van der Wal, W., Riva, R. E. M., Spada, G., Sun, Y., Melini, D., Kachuck, S. B., Barletta, V., Simon, K., A, G., James, T. S., 2018. A benchmark study of numerical implementations of the sea level equation in GIA modelling. Geophys. J. Int., 215:389-414. doi:10.1093/gji/ggy280

Spada, G., Barletta, V. R., Klemann, V., Riva, R. E. M., Martinec, Z., Gasperini, P., Lund, B., Wolf, D., Vermeersen, L. L. A., King, M. A. (2011). A benchmark study for glacial isostatic adjustment codes. Geophys. J. Int., 185:106-132. doi:10.1111/j.1365-246X.2011.04952.x

How to cite: Klemann, V., Austermann, J., Bagge, M., Barlow, N., Freymueller, J., Huang, P., Ivins, E. R., Lloyd, A., Martinec, Z., Milne, G., Rovere, A., Steffen, H., Steffen, R., van der Wal, W., Yousefi, M., and Zhong, S.: Benchmark of numerical GIA codes capable of laterally heterogeneous earth structures, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1447, https://doi.org/10.5194/egusphere-egu22-1447, 2022.

Glacial isostatic adjustment (GIA) describes the viscoelastic response of the solid Earth to ice-sheet and ocean loading. GIA models determine the relative sea-level based on the viscoelastic deformations of the Earth interior including self-gravitation due to the loading of the water redistribution between ocean and ice and rotational effects. Choosing an Earth structure that adequately reflects the viscoelastic behavior of a region remains a challenge. For a specific region, the viscosity stratification can be inferred from present-day geodetic measurements like sea-level, gravity change and surface displacements or from paleo observations of former sea level. Here, we use a suite of geodynamically constrained 3D Earth structures that are derived from seismic tomography models and create regionally adapted 1D Earth structures to investigate to what extent regional, radially symmetric structures are able to reproduce the solid Earth response of a laterally varying structure. We discuss sea-level variations during the deglaciation in the near field (beneath the former ice sheet) and peripheral regions (surrounding the ice sheet) with focus on North America and Antarctica as well as Oregon and Patagonia. The suite of 3D Earth structures vary in transfer functions from seismic velocity to viscosity, i.e., in Arrhenius law and viscosity contrast between upper mantle and transition zone. We investigate how the relative sea-level predictions of the model suite members are affected due to the simplification of the Earth structure from 3D to 1D.

In general, our results support previous studies showing that 1D models in peripheral regions are not able to reproduce the 3D models’ predictions, because the response depends on the deformational behavior beneath the adjacent ice sheet and the local structure (superposition). Furthermore, the analysis of the model suite members shows different response behaviors for the 1D and 3D cases, e.g., suite members with weaker dependence of viscosity on seismic velocity can predict lowest RSL for the 3D case, but largest RSL for the 1D case. This indicates the relevance of the 3D structure in peripheral regions. 1D models in the near field are more capable to reproduce 3D model response behavior. But also here, deviations indicate that the lateral variations in the Earth structure beneath the ice sheet influence local relative sea-level predictions. 

How to cite: Bagge, M., Klemann, V., Steinberger, B., Latinovic, M., and Thomas, M.: Peripheral and near field relative sea-level predictions using GIA models with 3D and regionally adapted 1D viscosity structures, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1479, https://doi.org/10.5194/egusphere-egu22-1479, 2022.

How to cite: Ishiwa, T., Okuno, J., and Suganuma, Y.: Excess ice loads prior to the Last Glacial Maximum in the Indian Ocean sector of East Antarctica derived from sea-level observations and GIA modeling, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1568, https://doi.org/10.5194/egusphere-egu22-1568, 2022.

The lithospheric thickness beneath and around Greenland varies from a few tens of kilometres in offshore regions to several tens of kilometres (up to 200 – 250 km) in land areas. But, due to different datasets and techniques applied in geophysical studies, there are large differences between the different geophysical lithosphere models. As an example, lithosphere models from seismological datasets show generally larger values (above 100 km), while models using gravity or thermal datasets tend to be thinner (values mostly below 100 km). To model the deformation associated with the melting of the Greenland Ice Sheet a detailed lithosphere model is required. Nevertheless, seismologically obtained lithosphere models are the ones usually applied in these so-called glacial isostatic adjustment (GIA) models. Besides, GIA models can be used to provide additional constraints on the lithospheric thickness.

Results from most 3D GIA models are compared to observed vertical velocities only, while horizontal velocities are known to be sensitive to the lateral variations of the Earth (e.g., lithospheric thickness). But, horizontal velocities from incompressible GIA models, which are commonly used, are not suitable due to the neglect of material parameter changes related to the dilatation. Compressible GIA models in turn can provide more accurate estimates of the horizontal and vertical viscoelastic deformations induced by ice-mass changes. Here, we use a variety of lithospheric thickness models, obtained from gravity, thermal, and seismological datasets, in a three-dimensional compressible GIA Earth model. The GIA model will be constructed using the finite-element software ABAQUS (Huang et al., under review in GJI) and applying recent ice history models Huy3 and GLAC-GR2a for Greenland in combination with the Little Ice Age deglaciation model by Kjeldsen et al. (2015). We will compare various lithosphere models, including their impact on the modelled 3D velocity field, and compare these against independent GNSS (Global Navigation Satellite System) observations.

References:

Huang, P., Steffen, R., Steffen, H., Klemann, V., van der Wal, W., Reusen, J., Wu, P., Tanaka, Y., Martinec, Z., Thomas, M. (under review in GJI): A finite element approach to modelling Glacial Isostatic Adjustment on three-dimensional compressible earth models. Geophysical Journal International. Under review.

Kjeldsen, K., Korsgaard, N., Bjørk, A. et al. (2015): Spatial and temporal distribution of mass loss from the Greenland Ice Sheet since AD 1900. Nature 528, 396–400, https://doi.org/10.1038/nature16183.

How to cite: Steffen, R., Steffen, H., Huang, P., Tarasov, L., Kjeldsen, K. K., and Khan, S.: Three-dimensional velocity variations due to ice mass changes in Greenland – Insights from a compressible glacial isostatic adjustment model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1807, https://doi.org/10.5194/egusphere-egu22-1807, 2022.

When inferring mantle viscosity by modelling the effects of glacial isostatic adjustment (GIA) a necessary constraint is the external forcing by surface loading. Such forcing is usually provided by a glaciation history, where the mass-conserving sea-level changes are considered solving the sea-level equation. The uncertainties of glaciation history reconstructions are quite large and the choice of a specific history strongly influences the deformation response obtained by GIA modelling. The reason is that any history is usually based on a certain Earth rheology, and mantle viscosity inversions using such models tend to resemble the viscosity structure used for the glaciation history (Schachtschneider et al., 2022, in press). Furthermore, uncertainties of glaciation histories propagate into the respective GIA modelling results. However, to quantify the impact of glaciation history on GIA modelling remains a challenge.

In this study we investigate the effect of uncertainties in glaciation histories on GIA modelling. Using a particle-filter approach we study the effect of spatial and temporal variations in ice distribution as well as the effect of total ice mass. We quantify the effects on a one-dimensional viscosity stratification and derive measures to which extent changes in sea-level pattern and surface deformation depend on variations in ice loading.

References:

Schachtschneider, R., Saynisch-Wagner, J., Klemann, V., Bagge, M., Thomas, M. 2021. Nonlin. Proc. Geophys., https://doi.org/10.5194/npg-2021-22

How to cite: Schachtschneider, R., Saynisch-Wagner, J., Klemann, V., Bagge, M., and Thomas, M.: The effect of uncertain historical ice information on GIA modelling, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4475, https://doi.org/10.5194/egusphere-egu22-4475, 2022.

How to cite: Pallisgaard-Olesen, G., Pedersen, V. K., Gomez, N., and Mitrovica, J. X.: Sea level response to Quaternary erosion and deposition in Scandinavia, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4969, https://doi.org/10.5194/egusphere-egu22-4969, 2022.

In geodynamic studies, most Finite-Element (FE) models in the commercial FE software Abaqus use elastic foundations at internal boundaries. This method works well for incompressible and so-called material-compressible material parameters but it is unclear if it works sufficiently well for implementing compressibility, especially in a 3D spherical model. The latter is of importance in investigations of glacial isostatic adjustment (GIA). A possible alternative method is based on a combination of explicit gravity loading with non-linear geometry (NLGEOM parameter in Abaqus) (Hampel et al., 2019). This method would remove the need to make a stress transformation to get the correct GIA stresses, and automatically accounts for the change in internal buoyancy forces that arises when allowing for compression, according to the Abaqus Documentation. We compared the method for (in)compressible flat (~half-space) FE models with existing numerical half-space and spherical (in)compressible codes and tested the applicability of this method in a spherical FE model. We confirm that this method works for multi-layer incompressible flat FE models. We furthermore notice that horizontal displacement rates of incompressible flat FE models match those of spherical incompressible GIA models below the current GNSS (Global Navigation Satellite System) measurement accuracy of 0.2-0.3 mm/a, but only for ice sheets that are smaller than 450 km in extent. For compressible models, disagreements in the vertical displacement rates are found between the flat NLGEOM model and the compressible Normal Mode code ICEAGE (Kaufmann, 2004). An extension of the NLGEOM-gravity method to a spherical FE model, where gravity must be implemented in the form of body forces combined with initial stress, leads to a divergence of the solution when viscous behaviour is turned on. We thus conclude that the applicability of the NLGEOM method is so far limited to flat FE models, and in GIA investigations for flat models the applicability further depends on the size of the load (ice sheet, glacier).

References:

Hampel, A., Lüke, J., Krause, T., & Hetzel, R., 2019. Finite-element modelling of glacial isostatic ad-
justment (GIA): Use of elastic foundations at material boundaries versus the geometrically non-linear
formulation, Computers & geosciences, 122, 1–14.

Kaufmann, G. (2004). Program Package ICEAGE, Version 2004. Manuscript. Institut für Geophysik der Universität Göttingen.

How to cite: Reusen, J., Huang, P., Steffen, R., Steffen, H., van Calcar, C., Root, B., and van der Wal, W.: The use of Non-Linear Geometry (NLGEOM) and gravity loading in flat and spherical Finite Element models of Abaqus for Glacial Isostatic Adjustment (GIA), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5146, https://doi.org/10.5194/egusphere-egu22-5146, 2022.

A new finite element method called FEMIBSF is presented that is capable of modelling Glacial Isostatic Adjustment (GIA) on compressible earth models with three-dimensional (3D) structures. This method takes advantage of the classical finite element techniques to calculate the deformational and gravitational responses to the driving forces of GIA (including body forces and pressures on Earth’s surface and core-mantle boundary, namely CMB). Following Wu (2004) and Wong & Wu (2019), we implement the GIA driving forces in the commercial finite element software Abaqus and solve the equation of motion in an iterative manner. Different from those two studies, all formulations and calculations in this study are not associated with spherical harmonics but are performed in the spatial domain. Due to this, FEMIBSF is free from expanding the load, displacement, and potential into spherical harmonics with the short-wavelength components (of high degree and order) neglected. We compare the loading Love numbers (LLNs) generated by FEMIBSF with their analytical solutions for homogeneous models and numerical solutions for layered models calculated by the normal-mode approach/code, ICEAGE (Kaufmann, 2004), the iterative body force approach/code, IBF (Wong & Wu, 2019) and the spectral-finite element approach/code, VILMA-C (Martinec, 2000; Tanaka et al., 2011). We find that FEMIBSF agrees well with analytical and numerical LLN results of these codes. In addition, we show how to compute the degree-1 deformation directly in the spatial domain with the finite element approach and how to implement it in a GIA model using Abaqus. Finally, we demonstrate that the CMB pressure related to the gravitational potential change in the fluid core only influences the long-wavelength surface displacement and potential such as the degree-2 component.

References

Kaufmann, G. (2004). Program Package ICEAGE, Version 2004. Manuscript. Institut für Geophysik der Universität Göttingen.

Martinec, Z. (2000). Spectral–finite element approach to three-dimensional viscoelastic relaxation in a spherical earth. Geophysical Journal International, 142(1), 117-141.

Tanaka, Y., Klemann, V., Martinec, Z. & Riva, R. E. M. (2011). Spectral-finite element approach to viscoelastic relaxation in a spherical compressible Earth: application to GIA modelling. Geophysical Journal International, 184(1), 220-234.

Wong, M. C. & Wu, P. (2019). Using commercial finite-element packages for the study of Glacial Isostatic Adjustment on a compressible self-gravitating spherical earth–1: harmonic loads. Geophysical Journal International, 217(3), 1798-1820.

Wu, P. (2004). Using commercial finite element packages for the study of earth deformations, sea levels and the state of stress. Geophysical Journal International, 158(2), 401-408.

How to cite: Huang, P., Steffen, R., Steffen, H., Klemann, V., van der Wal, W., Reusen, J., Tanaka, Y., Martinec, Z., and Thomas, M.: A finite element approach to modelling Glacial Isostatic Adjustment on three-dimensional compressible earth models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6013, https://doi.org/10.5194/egusphere-egu22-6013, 2022.

In this study, we examine the effect of transient mantle creep on the prediction of glacial isostatic adjustment (GIA) signals. Specifically, we compare predictions of relative sea level change from GIA from a set of Earth models in which transient creep parameters are varied in a simple Burgers model to a reference case with a Maxwell viscoelastic rheology. The model predictions are evaluated in two ways: first, relative to each other to quantify the effect of parameter variation, and second, for their ability to reproduce well-constrained sea level records from selected locations. Both the resolution and geographic location of the relative sea level observations determine whether the data can distinguish between model cases. Model predictions are most sensitive to the inclusion of transient mantle deformation in regions that are near-field and peripheral relative to former ice sheets. This sensitivity appears particularly true along the North American west coast in the region of the former Cordilleran Ice Sheet, which experienced rapid sea-level fall following deglaciation between 14-12 kyr BP. Relative to the Maxwell case, Burgers models better reproduce this rapid phase of regional postglacial sea level fall. As well, computed goodness-of-fit values in this region show a clear preference for models where transient deformation is present in the whole or lower mantle, and for models where the rigidity of the Kelvin element is weakened relative to the rigidity of the Maxwell element. In contrast, model predictions of relative sea-level change in the far-field show little or weak sensitivity to the inclusion of transient deformation.

How to cite: Simon, K. M., Riva, R. E. M., and Broerse, T.: Identifying geographical patterns of transient deformation in the geological sea level record, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6236, https://doi.org/10.5194/egusphere-egu22-6236, 2022.

The Antarctic ice mass loss is accelerating due to recent global warming. Changes in Antarctic ice mass have been observed as the gravity change by GRACE (Gravity Recovery and Climate Experiment) satellites. However, the gravity signal includes both the component of the ice mass change and the component of the solid Earth response to surface mass change (Glacial Isostatic Adjustment, GIA). Evaluating the GIA-induced gravity change requires viscoelastic Earth structure and ice history from the last deglaciation.

Antarctica is characterized by lateral heterogeneity of seismic velocity structure. West Antarctica shows relatively low seismic velocities, suggesting low viscosity regions in the upper mantle. On the other hand, East Antarctica shows relatively high seismic velocities, suggesting thick lithosphere. Here we examine the sensitivities of GIA-induced gravity change in Antarctica to upper mantle viscosity and lithosphere thickness using spherically symmetric Earth models.

Results indicate that the gravity field change depends on both the upper mantle viscosity profile and the lithosphere thickness. In particular, the long-wavelength gravity field changes become dominant in the adoption of viscoelastic models with a low viscosity layer beneath the elastic lithosphere. The same trend is also shown in the adoption of viscoelastic models with a thick lithosphere, and there is a trade-off between the structure of the low viscosity layer and the thickness of the lithosphere. This trade-off may reduce the effect of the lateral variations in Earth structure beneath Antarctica on the estimate of Antarctic ice sheet mass change.

How to cite: Irie, Y., Okuno, J., Ishiwa, T., Doi, K., and Fukuda, Y.: Dependence of GIA-induced gravity change in Antarctica on viscoelastic Earth structure, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6829, https://doi.org/10.5194/egusphere-egu22-6829, 2022.

The Antarctic Ice Sheet is the largest and most uncertain potential contributor to future sea level rise. Understanding involved feedback mechanisms require physically-based models. Confidence in future projections can be improved by models that can reproduce past ice sheet changes, in particular over the last deglaciation. The complex interaction between ice, bedrock and sea level plays an important role in ice sheet instability with a large variety of characteristic response time scales dependent on the heterogeneous Earth structure underneath Antarctica and the ice sheet dynamics.

We have coupled the VIscoelastic Lithosphere and MAntle model (VILMA) to the Parallel Ice Sheet Model (PISM v2.0, www.pism.io) and ran simulations over the last two glacial cycles. In this framework, VILMA considers both viscoelastic deformations of the solid Earth by considering a three-dimensional rheology and a gravitationally self-consistent mass redistribution in the ocean by solving for the sea-level equation. PISM solves for the stress balance for a changing bed topography, which is updated in 100 years coupling intervals and which can directly affect ice sheet flow and grounding line dynamics.

Here, we show first results of coupled PISM-VILMA simulations scored against a database of geological constraints including sea level index points. We discuss sensitivities of model parameters and climatic forcing in preparation for a larger parameter ensemble study. This project is part of the German Climate Modeling Initiative PalMod.

How to cite: Albrecht, T., Winkelmann, R., Bagge, M., and Klemann, V.: Deglaciation of the Antarctic Ice Sheet modeled with the coupled solid Earth – ice sheet model system PISM-VILMA, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7609, https://doi.org/10.5194/egusphere-egu22-7609, 2022.

 The Antarctic Ice Sheet (AIS) is the largest ice sheet on Earth that has known important mass 
 changes during the last 20 kyrs. These changes deform the Earth and modify its gravity field, 
 a process known as Glacial Isostatic Adjustment (GIA). GIA is directly influenced by the mechanical
 properties and internal structure of the Earth, and is monitored using Global Navigation Satellite 
 System positioning or gravity measurements. However, GIA in Antarctica remains poorly constrained  
 due to the cumulative effect of past and present ice-mass changes, the unknown history of the past
 ice-mass change, and the uncertainties of the mechanical properties of the Earth. The viscous 
 deformation due to GIA is usually modeled using a Maxwell rheology. However, other geophysical
 processes employ Andrade (tidal deformation) or Burgers (post-seismic deformation) laws that could 
 result in a more rapid response of the Earth. We investigate the effect of using these
 different rheology laws to model GIA-induced deformation in Antarctica.  

Employing the ALMA and TABOO softwares, we use the Love number and Green functions formalism to
compute the surface motion and the gravity changes induced by the past and present ice-mass redistributions.
We use the elastic properties and the radial structure of the preliminary reference Earth model (PREM) and the
viscosity profile given by Hanyk (1999). The deformation is computed for the three rheological laws mentioned
above using ICE-6G and elevation changes from ENVISAT (2002-2010) to represent the past and present changes
of the AIS, respectively. 

We obtain that the three rheological laws lead to significant Earth response within a 20 kyrs time interval since
the beginning of the ice-mass change. The differences are the largest between Maxwell and Burgers rheologies
during the 500 years following the beginning of the surface-mass change. Regarding the response to present
changes in Antarctica, the largest discrepancies are obtained in regions with the greatest current melting rates,
namely Thwaites and Pine Island Glacier in West Antarctica. Uplift rates computed twelve years after the end of
the present melting using Burgers and Andrade rheologies are five and two times larger than those obtained
using Maxwell, respectively. 

How to cite: Boughanemi, A. and Mémin, A.: Glacial Isostatic Adjustment in Antarctica : a rheological study, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7906, https://doi.org/10.5194/egusphere-egu22-7906, 2022.

The Last Interglacial (LIG; MIS 5e) period (130 - 115 ka) saw the last time in Earth’s history that polar temperatures reached 3 - 5 °C above pre-industrial values causing the Greenland and Antarctic ice sheets to shrink to sizes smaller than those of today. Similar polar temperature increases are predicted in the coming decades and the LIG period could therefore help to shed light on ice-sheet and sea-level responses to a warming world. 

LIG estuarine sediments preserved in the North Sea region are promising study sites for identification of the Antarctic ice sheet's relative contribution to LIG sea level, as well as for the reconstruction of both the magnitude and rate of LIG sea-level change during the interglacial. For these purposes, sea-level records in the region must be corrected for the impacts of glacial isostatic adjustment (GIA) which is primarily a consequence of two components: the evolution of terrestrial ice masses during the Penultimate Deglaciation (MIS 6), predominantly the near-field Eurasian ice sheet, and the viscoelastic structure of the solid Earth. 

The relative paucity of geological constraints on characteristics of the MIS 6 Eurasian ice sheet makes it challenging to evaluate its effect on sea level in the North Sea region. In order to model the Eurasian ice extent, thickness, and volume during the Penultimate Deglaciation we use a simple ice sheet model (Gowan et al. 2016), calibrated against models of the Last Glacial Maximum. By employing a gravitationally consistent sea-level model (Kendall et al. 2005), we generate a large ensemble of GIA outputs that spans the uncertainty in parameters controlling both the viscoelastic earth model and the evolution of global ice sheets during the Penultimate Deglaciation. By performing spatial sensitivity analysis with this ensemble, we are able to demonstrate the relative importance of each parameter in controlling North Sea GIA. Our comprehensive approach to exploring uncertainties in both the global ice sheet evolution and solid earth response provides significant advances in our understanding of LIG sea level.

How to cite: Pollard, O., Barlow, N., Gregoire, L., Gomez, N., Cartelle, V., Ely, J., and Astfalck, L.: Investigating the Sensitivity of North Sea Glacial Isostatic Adjustment during the Last Interglacial to the Penultimate Deglaciation of Global Ice Sheets, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8112, https://doi.org/10.5194/egusphere-egu22-8112, 2022.

Subsidence is a land use problem in the western and northern Netherlands, especially where both shallow soft soil subsidence and deeper subsidence components, including glacio-isostatic adjustment (GIA), add up. The aim of this study is to improve the estimation of the GIA component within the total subsidence signal across the Netherlands during the Holocene, using coastal plain paleo-water level markers. Throughout the Holocene, the GIA induced subsidence in the Netherlands has been spatially and temporally variant, as shown by previous studies that used GIA modelling and geological relative sea-level rise reconstructions. From the latter work, many field data points are available based on radiocarbon dated coastal basal peats of different age and vertical position. These reveal Holocene relative sea-level rise to have been strongest in the Wadden Sea in the Northern Netherlands. This matches post-glacial GIA subsidence (forebulge collapse) as modelled for the Southern North Sea, being located in the near-field of Scandinavian and British former ice masses.

In this study, geological data analysis of RSL and other paleo-water level data available from the Dutch coastal plain for the Holocene period is considered in addition. The analysis takes the form of designing and executing a 3D interpolation (kriging with a trend: KT), where paleo-water level Z_(x,y,age) is predicted and the field-data points are the observations (Age, X, Y and Z as knowns). We use a spatio-temporal 3D grid that covers the Dutch coastal plain, and reproduces and unifies earlier constructed sea level curves and high-resolution sampled individual sites (e.g. Rotterdam). The function describing the trend part of the interpolation separates linear and non-linear components of relative water level rise, i.e.: long-term background subsidence and shorter-term GIA subsidence signal and postglacial water level rise. The kriging part then processes remaining subregional patterns. The combined reconstruction thus yields a spatially continuous parameterization of regional trends that (i) allows to separate subsidence from water level rise terms, and (ii) is produced independently of GIA modelling to enable cross-comparison. Results are presented for the coastal plain of the Netherlands ([SW] Zeeland – Rotterdam – Holland – Wadden Sea – Groningen [NE]). The percentage of the total coastal-prism accommodation space that appears due to subsidence, from the south to the north of the study area increases by 20%. Holocene-averaged subsidence rates from the first analysis ranged from ca. 0.1 m/kyr (Zeeland) to 0.4 m/kyr (Groningen), which is 5-10 times larger than present-day GPS/GNSS-measured rates.

The research presented in this abstract is part of the project Living on soft soils: subsidence and society (grantnr.: NWA.1160.18.259). 

How to cite: de Wit, K., van de Wal, R. S. W., and Cohen, K. M.: Reconstructing large scale differential subsidence in the Netherlands using a spatio-temporal 3D paleo-groundwater level interpolation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8350, https://doi.org/10.5194/egusphere-egu22-8350, 2022.

Regional sea-level change and the deformation of the solid Earth can lead to important feedbacks on the long- and short-term evolution and stability of ice sheets. A rigorous manner of accounting for these feedbacks in model-based ice-sheet reconstructions and projections, is to establish a two-way coupling between an ice-sheet and a sea-level model. However, the individual requirements of each of these two components — such as a global, long ice sheet load history or a high ice-model resolution over critical sectors of an ice sheet — are at present not easy to combine in terms of computational feasibility. Here, we present a coupling between the ice-sheet model UFEMISM, which solves a range of approximations of the stress balance on a dynamically adaptive irregular triangular mesh, and the gravitationally self-consistent sea-level model SELEN, which incorporates the glacial isostatic adjustment for a radially symmetric, viscoelastic and rotating Earth, including coastline migration. We show global simulations over glacial cycles, including the North American, Eurasian, Greenland, and Antarctic ice sheets, and compare its performance and results against commonly used alternatives.

How to cite: Bernales, J., Berends, T., and van de Wal, R.: An adaptive-triangular fully coupled 3D ice-sheet–sea-level model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9485, https://doi.org/10.5194/egusphere-egu22-9485, 2022.

Interacting feedbacks play an important role in governing the stability of the Greenland Ice Sheet under global warming. Here we study the interaction between the positive melt-elevation feedback and the negative feedback from glacial isostatic adjustment (GIA), and how they affect the ice volume of the Greenland Ice Sheet on long time scales. We therefore use the Parallel Ice Sheet Model (PISM) coupled to a simple solid Earth model (Lingle-Clark) in idealized step-warming experiments. Our results suggest that for warming levels above 2°C, Greenland could become essentially ice-free on the long-term, mainly as a result of surface melting and acceleration of ice flow. The negative GIA feedback can mitigate ice losses and promote a partial recovery of the ice volume.

Exploring the full factorial parameter space which determines the relative strength of the two feedbacks reveals that four distinct dynamic regimes are possible: from stabilization, via recovery and self-sustained oscillations to the irreversible collapse of the Greenland Ice Sheet. In the recovery regime an initial ice loss is reversed and the ice volume stabilized at 61-93% of the present day volume. For certain combinations of temperature increase, atmospheric lapse rate and Earth mantle viscosity, the interaction of the GIA feedback and the melt-elevation feedback leads to self-sustained, long-term oscillations in ice-sheet volume with oscillation periods of tens to hundreds of thousands of years and oscillation amplitudes between 15-70% of present-day ice volume. This oscillatory regime reveals a possible mode of internal climatic variability in the Earth system on time scales on the order of 100,000 years that may be excited by or synchronized with orbital forcing or interact with glacial cycles and other slow modes of variability.

How to cite: Zeitz, M., Haacker, J. M., Donges, J. F., Albrecht, T., and Winkelmann, R.: Interacting melt-elevation and glacial isostatic adjustment feedbacks allow for distinct dynamic regimes of the Greenland Ice Sheet, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9968, https://doi.org/10.5194/egusphere-egu22-9968, 2022.

Geodetic time series from autonomous GNSS systems distributed across Antarctica are revealing unexpected patterns and startling rates of crustal deformation due to GIA.  Linked with seismic mapping and derived rheological properties of the Antarctic crust and mantle, and with new modeling capabilities, our understanding of the timescales of GIA response to ice sheet change is swiftly advancing.  Rapid GIA response allows for cryosphere-solid earth interactions that can alter ice sheet behavior on decadal and centennial timescales.  Continued progress in understanding how such feedbacks may influence future contributions of polar ice sheets to global sea level change requires continuing and expanding our geodetic observations. What frameworks can lead to implementation of this goal?  U.S. and international science vision documents pertaining to geodynamics, the changing cryosphere and sea level, all point to international collaborative efforts as the way to achieve ambitious science goals and extend observational capacities in polar regions.  SCAR research programmes facilitated the network vision and collaborative relations that led to the POLENET (POLar Earth observing NETwork) network of geophysical and geodetic instruments during the International Polar Year 2007-08. Can the SCAR INSTANT programme provide a framework for collaborative initiatives between national Antarctic programs to form a sustainable model to support acquisition of the observations required to meet community science objectives?  Let’s consider the ‘grass roots’ actions by the science community needed to push international, interdisciplinary science frameworks forward.

How to cite: Wilson, T. J.: GNSS Observations of Antarctic Crustal Deformation – International Framework for Future Networks?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10610, https://doi.org/10.5194/egusphere-egu22-10610, 2022.

Recent studies suggest the hotspot currently under Iceland was located beneath eastern Greenland at ~40 Ma BP and that the upwelling of hot material from the Iceland plume towards Greenland is ongoing. A warm upper mantle has a low viscosity, which in turn causes the solid Earth to rebound much faster to deglaciation. In the area of the Kangerlussuaq glacier, a large GPS velocities residual after removing predicted purely elastic deformations caused by present-day ice loss suggests the possibility of such fast rebound to little ice age (LIA) deglaciation. Here we investigate the lithospheric thickness and the mantle viscosity structure beneath SE-Greenland by means of model predictions of solid Earth deformation driven by a low viscosity mantle excited by the LIA deglaciation to the present day. From the comparison of such modeled deformations with the GPS residual, we conclude that 1) a rather thick lithosphere is preferred (90-100 km) 2) and the upper mantle most likely has a viscosity that changes with depth. Assuming a two layer upper mantle, it is not well constrained which part of the upper mantle has to be low, with a preference for low viscosity in the deeper upper mantle.

To understand such results we implemented forward modelling with more realistic earth models, relying on improvements in seismic models, petrology and gravity data. This yields 3D viscosity maps that can be compared to inferences based on the 1D model and forms the basis for 3D GIA models. The conclusion based on the 1D model can be explained with 3D Earth models. In the area of the Kangerlussuaq glacier the seismic derived viscosities prefer a higher viscosity layer above a lower viscosity one. This stems from the slow decrease in viscosity with depth. The layer that is characterized as shallow upper mantle still contains shallow regions with low temperatures, while the deeper upper mantle reaches low viscosities. Generally, for GIA earth models the “higher above lower” viscosity layering is unusual. However, the analysis of the 1D model clearly shows this to be one of the preferred model regions, in combination with a large lithosphere thickness of 100 km. This is a notable result that draws attention to the importance of shallow layering in GIA models. 

How to cite: Barletta, V. R., van der Wal, W., Bordoni, A., and Khan, S. A.: Effect of Icelandic hotspot on Mantle viscosity in southeast Greenland, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10884, https://doi.org/10.5194/egusphere-egu22-10884, 2022.

The redistribution of the near-surface solid Earth due to glacial isostatic adjustment (GIA), which is the ongoing response of the solid Earth due to changes in the ice-ocean load following the Last Glacial Maximum, has a direct impact on the inferred Antarctic Ice Sheet (AIS) mass balance from gravimetric data acquired during the Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On (GRACE-FO) missions.

However, sparse in-situ observation networks across Antarctica have led to the inability to effectively constrain the GIA effect. Here, we analyze the mass change patterns across Antarctica via independent component analysis (ICA), a statistics-based blind source separation method to extract signals from complex datasets, in an attempt to reduce uncertainties in the glacial isostatic adjustment (GIA) effects and improve understanding of AIS mass balance.

The results reveal that GIA signal could be directly separated from GRACE/GRACE-FO observations without introducing any external model.  Although the GIA signal cannot be completely isolated, the correlation coefficients between ICA-separated GIA, and the ICE-5G and ICE-6G models are 0.692 and 0.691, respectively. The study demonstrates the possibility of extracting GIA effects directly from GRACE/GRACE-FO observations.

How to cite: Shi, T., Fukuda, Y., Doi, K., and Okuno, J.: Separating of Glacial Isostatic Adjustment (GIA) across Antarctica from GRACE/GRACE-FO observations via Independent Component Analysis (ICA), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10942, https://doi.org/10.5194/egusphere-egu22-10942, 2022.

Earth’s topography and bathymetry is shaped by a complex interplay between solid-Earth processes that deform the Earth from within and the surface processes that modify the outer shape of the Earth. At the surface, an ultimate baselevel set by global sea level marks the defining transition from erosion to deposition. Over geological time scales, this baselevel has resulted in a distinct hypsometric distribution (distribution of surface area with elevation), with the highest concentration of surface area focused in a narrow elevation range near present-day sea level.

This particular feature in Earth’s hypsometry makes the global land fraction very sensitive to changes in sea level. Indeed, a sea-level change will result in a significant change in the land fraction as dictated by the hypsometric distribution, thereby modulating the very same sea-level change. However, it remains unexplored exactly how sea-level changes have modified the global land fraction over past glacial cycles and into the future.

Here we analyse how Earth’s hypsometry has changed over the last glacial cycle as large ice sheets waxed and waned particularly in Scandinavia and North America. These changes in global ice volume resulted in a significant global excursion in sea level, modulated regionally by solid-Earth deformation, gravitational effects, and effects from Earth’s rotation. These changes modified Earth’s hypsometry, and therefore the global land fraction at any given time. Consequently, we can map out how Earth’s hypsometry has influenced global mean sea level (GMSL) over time. To examine this relationship between Earth’s hypsometry and sea level further, we look to the deep future, to a scenario where both the Greenland Ice Sheet and the Antarctic Ice Sheets will melt away completely over multi-millennial timescales. This scenario is not meant to represent a realistic future scenario per se, but it allows us to define the hypsometric GMSL correction needed for any GMSL that the Earth has experienced recently or will experience in the future.

How to cite: Pedersen, V. K., Gomez, N., Pallisgaard-Olesen, G., Garbe, J., Aschwanden, A., Winkelmann, R., and Mitrovica, J.: The influence of Earth’s hypsometry on global sea level through a glacial cycle and into the future, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11569, https://doi.org/10.5194/egusphere-egu22-11569, 2022.

New research aims to improve relative sea-level (RSL) projections for the Norwegian coast. The main objectives are to: i) collect observations of past RSL changes, ranging from the end of the last ice age to the last century, ii) develop a high-quality database of post-glacial sea-level index points (SLIPs) for the Norwegian coast, and to iii) improve our understanding of past and future vertical land motion using glacial isostatic adjustment (GIA) modelling. To now, our collection of new empirical data has focussed on three significant, but enigmatic RSL histories that are not adequately reproduced in existing GIA models: very recent stillstands and transgressions documented by historical tide gauge records, rapid transgressions during the early- to mid-Holocene Tapes period, and abrupt transgressions during the latest Pleistocene Younger Dryas chronozone. Ongoing field sampling is focussed on developing high-resolution RSL trends from salt marshes, isolation basins, and raised beaches, using multiple biostratigraphic and geochemical proxies (i.e. micropaleontology, macrofossils, x-ray fluorescence, C/N) and dating techniques (i.e. Pb-210, Cs-137, C-14, tephrochronology, geochemical markers). Results from various localities spanning the Norwegian coast provide robust constraints for the timing and rate of RSL change during the Younger Dryas and Tapes chronozones. Additional results providing new estimates of very recent RSL trends in southwest Norway are presented by Holthuis et al. (Late Holocene sea-level change and storms in southwestern Norway based on new data from intertidal basins and salt marshes; Session CL5.2.2). These new and emerging constraints are being integrated into a post-glacial RSL database that incorporates high-quality data from the entire Norwegian coastline. Over 1000 SLIPs have been assembled from published studies. These existing data were updated using current radiocarbon calibration curves, high-resolution digital elevation models, new field observations, and new quantitative estimates of relevant uncertainties. Ongoing GIA modelling is utilizing the new RSL database, a glaciological model that freely simulates ice sheet changes, as well as geodetic and ice margin chronology constraints, to develop rigorous uncertainty estimates for present and future GIA along the Norwegian coast.

How to cite: Lakeman, T. R., Nixon, F. C., Romundset, A., Simpson, M. J. R., Svendsen, J. I., Vasskog, K., Bondevik, S., Milne, G., and Tarasov, L.: Improving past and future relative sea-level constraints for the Norwegian coast, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12689, https://doi.org/10.5194/egusphere-egu22-12689, 2022.

Uncertainty in present-day glacial isostatic adjustment (GIA) rates represent at least 44% of the total gravity-based ice mass balance signal over Antarctica. Meanwhile, physical couplings between solid Earth, sea level and ice dynamics enhance the dependency of the spatiotemporally varying GIA signal on 3D rheology. For example, the presence of low-viscosity mantle beneath melting marine-based ice sheet sectors such as the Amundsen Sea Embayment may delay or even prevent unstable grounding line retreat. Improved knowledge of upper mantle thermomechanical structure is therefore required to refine estimates of current and projected ice mass balance.

Here, we present a Bayesian inverse method for mapping shear wave velocities from high-resolution adjoint tomography into thermomechanical structure using a calibrated parameterisation of anelasticity at seismic frequency. We constrain the model using regional geophysical data sets containing information on upper mantle temperature, attenuation and viscosity structure. The Globally Adaptive Scaling Within Adaptive Metropolis (GASWAM) modification of the Metropolis-Hastings algorithm is utilised to allow efficient exploration of the multi-dimensional parameter space. Our treatment allows formal quantification of parameter covariances, and naturally permits us to propagate uncertainties in material parameters into uncertainty in thermomechanical structure.

We find that it is possible to improve agreement on steady state viscosity structure between tomographic models by approximately 30%, and reduce its uncertainty by an order of magnitude as compared to a forward-modelling approach. Direct access to temperature structure allows us to estimate lateral variations in lithospheric thickness, geothermal heat flow, and their associated uncertainties.

How to cite: Hazzard, J., Richards, F., Roberts, G., and Goes, S.: Reducing Uncertainty in Upper Mantle Rheology, Lithospheric Thickness and Geothermal Heat Flow Using a Bayesian Inverse Framework to Calibrate Experimental Parameterisations of Anelasticity, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12967, https://doi.org/10.5194/egusphere-egu22-12967, 2022.

This article presents a comprehensive benchmark study for the newly updated and publicly available finite element code CitcomSVE for modeling dynamic deformation of a viscoelastic and incompressible planetary mantle in response to surface and tidal loading. A complete description of CitcomSVE’s finite element formulation including calculations of the sea-level change, polar wander, apparent center of mass motion, and removal of mantle net rotation is presented. The 3-D displacements and displacement rates and the gravitational potential anomalies are solved with CitcomSVE for three benchmark problems using different spatial and temporal resolutions: 1) surface loading of single harmonics, 2) degree-2 tidal loading, and 3) the ICE-6G GIA model. The solutions are compared with semi-analytical solutions for error analyses. The benchmark calculations demonstrate the accuracy and efficiency of CitcomSVE. For example, for a typical ICE-6G GIA calculation with a 122-ky glaciation-deglaciation history, time increment of 100 years, and ~50 km (or ~0.5 degree) surface horizontal resolution, it takes ~4.5 hours on CPU 96 cores to complete with about 1% and 5% errors for displacements and displacement rates, respectively. Error analyses shows that CitcomSVE achieves a second order accuracy, but the errors are insensitive to temporal resolution. CitcomSVE achieves the parallel computation efficiency >75% for using up to 6,144 CPU cores on a parallel supercomputer. With its accuracy, computing efficiency and its open-source public availability, CitcomSVE is a powerful tool for modeling viscoelastic deformation of a planetary mantle in response to surface and tidal loading problems. 

How to cite: Zhong, S., Kang, K., Aa, G., and Qin, C.: CitcomSVE: A Three-dimensional Finite Element Software Package for Modeling Planetary Mantle’s Viscoelastic Deformation in Response to Surface and Tidal Loads, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13136, https://doi.org/10.5194/egusphere-egu22-13136, 2022.

The Earth’s mass redistribution due to deglaciation and recent ice sheet melting causes changes in the Earth’s gravity field and vertical land motion in Greenland. The changes are because of ongoing mass redistribution and related elastic (on a short time scale) and viscoelastic (on time scales of a few thousands of years) responses. These signatures can be used to determine the mantle viscosity. In this study, we infer the mantle viscosity associated with the glacial isostatic adjustment (GIA) and long-wavelength geoid beneath the Greenland lithosphere. The viscosity is determined based on a spatio-spectral analysis of the Earth’s gravity field and the land uplift rate in order to find the GIA-related gravity field. We used and evaluated different land uplift data, i.e. the vertical land motions obtained by the Greenland Global Positioning System (GPS) Network (GNET), GRACE and Glacial Isostatic Adjustment (GIA) data. In addition, a  combined land uplift rate using the Kalman filtering technique is presented in this study. We extract the GIA-related gravity signals by filtering the other effects due to the deeper masses i.e. core-mantle (related to long-wavelengths) and topography (related to short-wavelengths). To do this, we applied correlation analysis to detect the best harmonic window. Finally, the mantle viscosity using the obtained GIA-related gravity field is estimated. Using different land uplift rates, one can obtain different GIA-related gravity fields. For example, different harmonic windows were obtained by employing different land uplift datasets, e.g. the truncated geoid model with a harmonic window between degrees 10 to 39 and 10 to 25 showed a maximum correlation with the GIA model ICE-6G (VM5a) and the combined land uplift rates, respectively. As shown in this study, the mantle viscosities of 1.6×10²² Pa s and 0.9×10²² Pa s for a depth of 200  to 650  km are obtained using ICE-6G (VM5a) model and the combined land uplift model, respectively, and the GIA-related gravity potential signal.

How to cite: Bagherbandi, M., Amin, H., Wang, L., and Shirazian, M.: Mantle viscosity derived from geoid and different land uplift data in Greenland, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13323, https://doi.org/10.5194/egusphere-egu22-13323, 2022.

The investigation of planetary cores is of great interest to those seeking to better understand magnetic fields and the life-processes of planets. Like many large-scale systems, planetary cores are unable to be modelled perfectly by numerical simulations or physical experiments. However, it is of constant importance to improve numerical and experimental methods and designs to better replicate full-scale processes. Many previous studies have over-looked the effects of the inhomogeneous insulation from the Earth's mantle on convection in the core. A few numerical studies have taken this effect into consideration for rotating Rayleigh-Benard convection (RBC) in spherical geometries. Experimental models are desirable to further understand the motion of fluid in the center of planets; however, due to physical limits, spherical systems are difficult to recreate experimentally. Therefore, cylindrical geometries are useful to study varied thermal flux on sidewalls both experimentally and numerically. While some studies have numerically and experimentally considered changes in temperature along the sidewall, there has been little consideration for variations in heat flux, which is the more physically appropriate boundary condition. 

The present study seeks to explore rotating RBC in a cylindrical domain with sidewalls inhomogeneously insulated in an experimentally-achievable system. It is experimentally plausible that the material of a cylindrical cell could varying in thickness, and therefore thermal conductivity, or have patches of heating and/or cooling attached to the sidewall to vary the thermal flux on the side boundaries. To imitate this numerically, a sinusoidal pattern of increasing and decreasing heat flux is applied to the sidewall in two cases: one whereby heat flux fluctuates between positive and negative, and another whereby the heat flux is strictly positive. Additionally the mode and amplitude of the wave is considered. The mode will either match the mode of the system with insulating sidewall conditions or have a larger wavelength to better simulate planetary cores. The amplitude is increased as necessary to achieve significant results. For simplicity, the top and bottom boundary conditions are fixed temperature.

Changes in heat transport and temporal behavior are measured with a global Nusselt number, Nu, time series. Additional variables such as mean zonal flow, number and location of convection rolls, and transitions to time-dependence are considered. Results indicate that large-wavelength heat flux on the sidewalls causes two modes to inhabit the system, existing on opposite sides of the cylinder: the mode natural to the homogeneously insulated system exists where heat flux is high and a large-wavelength mode dominates where heat flux is lower. However, the implementation of heat flux along the sidewalls with the same wavelength of the insulated system results in near-time independence as the amplitude increases. These results indicate that variation in heat flux boundary conditions can cause significant changes in rotating RBC behavior. Experimental studies could be used to validate or refute these conclusions. Overall, it is clear that numerical studies of molten planetary cores heterogeneously heated by mantles must take these irregularities into consideration to improve our understanding of core convection. 

How to cite: Peifer, J., Bokhove, O., and Tobias, S.: Changes in pattern formation and behavior in rotating Rayleigh-Benard convection due to inhomogeneous thermal insulation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-134, https://doi.org/10.5194/egusphere-egu22-134, 2022.

It was often shown how the anisotropy (due to turbulence) in the Earth’s outer core strongly influences some convection processes very important in the Core Dynamics. For instance, it was described how some instabilities in rotating magnetoconvection, described as usually by the analysis in term of normal modes, depend strictly on the anisotropic diffusion. Thus, we developed many models concerning the marginal modes (stationary and oscillating modes) of rotating magnetoconvection with different cases of anisotropy in the viscosity, thermal and magnetic diffusivities. In all cases, an anisotropy greater in the vertical direction parallel to gravity (“atmospheric anisotropy”) facilitates the convection, while an anisotropy greater in horizontal directions (“oceanic anisotropy”) inhibits some types of convection. This is linked with the balance among Magnetic, Archimedean and Coriolis forces in the Earth’s outer core.  

After recalling these former results concerning marginal modes, we present new results concerning the most unstable modes, namely the ones with maximum growth rate, with isotropic and anisotropic diffusivities.

Firstly, the state of the art about this topic in isotropic conditions is reminded, then our new approach on it is presented. We show that assuming a time-dependence only in the temperature perturbation (we call it T-case), like it was done in some former works, does not describe properly these modes in the Earth’s outer core. Indeed, this implies that some types of convection would occur only with some values of the dimensionless numbers unrealistic for the Earth (e.g., with too huge values of the Ekman numbers). We study the most general isotropic case (and we christen it G-case), namely the most unstable modes of convection with temperature, velocity and magnetic perturbations time-dependent. In this case the convection is much more facilitated than in the T-case: it occurs with much smaller values of Ekman and Elsasser numbers. Another model (named by us Q-case) with very small magnetic Prandtl number, namely with magnetic diffusivity much greater than viscosity, is considered. The Q-case results are very similar to the G-case ones. We demonstrate (and indicate) that Q and G cases can hold for the Earth (and for other planets).

We show that the anisotropy strongly influences the most unstable modes. Indeed, like in the marginal ones, the atmospheric anisotropy facilitates the occurrence of the most unstable modes convection, while the oceanic one inhibits it. Furthermore, we prove that, in contrast with isotropic case, in case of strong oceanic anisotropy the differences between Q and G cases can be significant for the Geodynamo.

Our approach allows to easily deal with very huge wave numbers and Rayleigh numbers as well as with very small Ekman numbers, what is usually not possible in the standard geodynamo simulations. This aspect and the growth rates search are useful to look for possible connections with small length and time scale analysis of the Geomagnetic field. 

How to cite: Filippi, E. and Brestenský, J.: The most unstable modes in rotating magnetoconvection with anisotropic diffusion in the Earth’s outer core, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-189, https://doi.org/10.5194/egusphere-egu22-189, 2022.

Although the solidity of Earth’s inner core is evidenced by normal mode data, the direct observation of inner core shear waves (J-waves) has remained challenging for decades due to their small amplitudes. Previous studies have presented evidence of J-waves in different seismic datasets (e.g., Okal and Cansi Y, 1998; Deuss et al., 2000; Cao et al., 2005; Wookey and Helffrich, 2008), however, the observability seems to be highly dependent not only on distance, but also on the location of the source and receiver, suggesting that amplification from specific 3D structures in the deep Earth is necessary to elevate the phase above noise for certain ray paths. Waszek and Deuss (2015) and Tkalčić and Phạm (2018) also found J-waves in global stacks and global correlation wavefield respectively, but these average over all possible source-receiver geometries and inner core structure.

To improve phase identification and discrimination, we use an approach that combines the array method of slant stacking and polarization filtering to enhance linearly polarized signals with the expected slowness and incident angle. We apply this technique on the data of the AlpArray Seismic Network, a large-scale seismic network in Europe that consists of over 600 broadband stations with a mean station spacing of 30-40km. An arrival consistent with PKJKP (in reference travel time, slowness, and polarization) is found from events in the source region reported by Cao et al. (2005). We present an overview of PKJKP candidate paths over distance based on observations with AlpArray. We also examine whether these observations correspond to specific depths or azimuths and investigate the effects of anisotropy or other three-dimensional earth structures​​​​​​.

How to cite: Ling, O. K. A., Stähler, S., Kim, D., Giardini, D., and AlpArray Working Group, T.: Observations of Inner Core Shear Waves with AlpArray, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1088, https://doi.org/10.5194/egusphere-egu22-1088, 2022.

Various types of waves exist in the Earth’s core. Waves associated with the magnetic field can leave a signature in the observed geomagnetic field, which may allow one to infer properties of the core. Among those, a balance of magnetic, Coriolis and pressure forces forms a type of waves known as Magneto-Coriolis (MC) waves. Previous studies of MC wave have mostly been focused on the ideal limit (without magnetic diffusion and viscous dissipation) with a columnar ansatz for the flow field. In this study, we investigate this problem by retaining the magnetic diffusion and three-dimensional flows in a full sphere. With several choices of axisymmetric background magnetic field, we analyse various branches of normal modes. The dependence of the normal mode's structure on the background field is clearly seen. A westward propagating branch with perfect columnar flows is found for some background B. We have also found eastward propagating modes constituted by flows with weaker columnarity. With the choice of Elsasser number Λ=1 (Coriolis and magnetic forces of similar magnitude), for axisymmetric background fields we find most of the MC modes have decay rates comparable or larger than their frequencies.

How to cite: Luo, J., Jackson, A., and Marti, P.: Waves in the Earth’s core. 2: Diffusive Magneto-Coriolis waves., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2363, https://doi.org/10.5194/egusphere-egu22-2363, 2022.

Light elements play a key role in the chemical and physical processes of planetary Fe-rich metallic cores [1].  H and Si are believed important candidates in planetary cores and previous estimates indicate as much as 0.6 wt% H and 13 wt% Si in the Earth’s core [2, 3]. However, existing studies are on Fe-H or Fe-Si binary systems and knowledge on Fe-Si-H ternary at high pressure and temperature is still limited [4, 5]. We conducted a series of experiments to understand the impact of hydrogen on Fe-Si alloy system. Fe-Si alloys with three compositions, Fe-9Si (9 wt% Si), Fe-16Si (16 wt% Si), and FeSi (33.3 wt% Si), reacted with H separately up to 125 GPa and 3700 K in diamond-anvil cells by combining pulsed laser heating with high-energy synchrotron X-ray diffraction. Results show little H solubility in B20 and B2 phases of FeSi (0.3 wt% and <0.1 wt% H, respectively) up to 62 GPa, which is significantly smaller than H solubility in Fe metal (1.8 wt% H) [6]. The low H solubility in these phases is likely because of their highly distorted interstitial sites which are not favorable for H incorporation. We found that the low-Si alloys (Fe-9Si and Fe-16Si) convert into FeH_x (fcc or dhcp), FeSi (B20 or B2), and Fe-Si-H ternary phases up to 125 GPa and 3700 K. Particularly, a Fe₅Si₃H_x phase is stable below 43 GPa and the cubic FeH₃ can appear after reactions above 100 GPa. These results indicate that H alters the behavior of the Fe-Si system severely. Considering the various sizes and masses of planets in the solar and exoplanetary systems, the planetary cores can have a wide range of Si contents. If Fe-droplets in early magma ocean contain much Si, Si could limit the amount of H incorporated in the core. On the other hand, for cores with low Si, crystallization at the solid-liquid core boundary may result in formation of separate H-rich and Si-rich crystals in the solid core, potentially inducing heterogeneities in the region [7]. 

References:

1. Shahar, A., et al., What makes a planet habitable? Science, 2019. 364(6439): p. 434-435.

2. Tagawa, S., et al., Experimental evidence for hydrogen incorporation into Earth’s core. Nature Communications, 2021. 12(1): p. 2588.

3. Hirose, K., B. Wood, and L. Vočadlo, Light elements in the Earth’s core. Nature Reviews Earth & Environment, 2021. 2(9): p. 645-658.

4. Terasaki, H., et al., Hydrogenation of FeSi under high pressure. American Mineralogist, 2011. 96(1): p. 93-99.

5. Tagawa, S., et al., Compression of Fe–Si–H alloys to core pressures. Geophysical Research Letters, 2016. 43(8): p. 3686-3692.

6. Pépin, C.M., et al., New iron hydrides under high pressure. Physical review letters, 2014. 113(26): p. 265504.

7. Deuss, A., Heterogeneity and anisotropy of Earth's inner core. Annual Review of Earth Planetary Sciences, 2014. 42: p. 103-126.

How to cite: Fu, S., Chariton, S., Prakapenka, V., Chizmeshya, A., and Shim, S.-H.: Phase Relations in the Fe-Si-H Ternary up to 125 GPa and 3700K: Implications for the Structure and Chemistry of Planetary Cores, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3229, https://doi.org/10.5194/egusphere-egu22-3229, 2022.

Increasing seismic evidence has accumulated, suggesting that the Earth’s outer core consists of distinct layers of low P-wave velocities relative to the Preliminary Reference Earth Model (PREM) in the top and bottom of the liquid core. Seismically detected low velocity in the outer core could be linked with the stratification, essential for understanding the geodynamo and thermochemical evolution of the liquid core. However, a consistent globally-averaged radial structure of the outer core has not been obtained due to the incomplete coverage of sampling body waves. To remedy this problem, we explore the seismic structure of Earth's outer core by employing a new theoretical and observational concept termed coda correlation wavefield. We construct the global correlogram in the 15-50 sec period range by stacking cross-correlations of the long-duration coda waves from the selected ten large earthquakes. We then assemble a dataset of prominent correlation features from the global correlogram that are sensitive to the outer core. The waveforms of these features are fit by computing synthetic correlograms through various outer core models. The obtained optimal model displays P-wave velocities in both the outer core's top and bottom, consistent with Coda Correlation Reference Earth Model (CCREM) and reduced relative to PREM. The low seismic speeds in the top of the outer core could likely imply the formation of a thermal and/or compositional stratification.

How to cite: Ma, X. and Tkalčić, H.: CCMOC: A New View of the Earth's Outer Core Through the Global Coda Correlation Wavefield, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3349, https://doi.org/10.5194/egusphere-egu22-3349, 2022.

Global seismic imaging of the Earth's interior has come a long way in exploring and understanding the Earth’s internal structure and dynamics with the worldwide proliferation of seismographs. However, investigating planetary interiors, including detections of their deep structures, remains challenging because of the limited number of seismographs that are and will be deployed in the foreseeable future. Besides, the existing imaging methods based on observations of a direct seismic wavefield from seismic sources require the emergence of the seismic waves with distinguishable amplitudes. That condition restricts the seismic station locations for practical wave reflections or refractions from internal planetary interfaces to a limited angular distance range from the source.

Here, we explore a new way to image deep planetary interiors, especially the planetary cores, using a single seismograph. We first develop a novel procedure for constructing global inter-source correlograms and show that they contain many prominent features sensitive to the internal planetary structures. We demonstrate that a single station is sufficient to produce a global correlogram for the Earth. We then utilize a single-station correlogram and show the steps for detecting and quantifying the Earth’s and Martian cores interfaces. This provides a new paradigm for imaging deep planetary interiors on global scales.

How to cite: Wang, S. and Tkalčić, H.: Imaging of Deep Planetary Interiors from Inter-source Correlations via a Single Seismograph, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3362, https://doi.org/10.5194/egusphere-egu22-3362, 2022.

Top-down solidification has been suggested in the liquid cores of small planets, moons, and large asteroids. An iron snow is then thought to exist, consisting of the crystallization of free iron crystals at the top of these cores and of their settling in a stably stratified ambient, until they remelt in a hotter, deeper region. This inward crystallization and associated buoyancy flux may sustain dynamo action by convection below the remelting depth. However, thermal evolution models are up-to-now oversimplified, assuming a constant-in-time and homogeneous-in-space buoyancy flux at the bottom of the snow zone. We have shown from analog experiments that the buoyancy flux is heterogeneous in time and space, with intense snow events, corresponding to an explosion of frazil-ice,  separated by quiescent periods where the snow zone supercools. We found that a wide range of crystal sizes exists, with large crystals overshooting the convection region and challenging the thermodynamic equilibrium hypothesis underlying the evolution models. The spatio-temporal variability of the energy source obviously impacts the shape and intensity of the generated magnetic field, which may provide alternative explanations for the observed and surprising features of Mercury's and Ganymede's magnetic fields.

How to cite: Huguet, L. and Le Bars, M.: A laboratory model for iron snow in planetary cores, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3740, https://doi.org/10.5194/egusphere-egu22-3740, 2022.

The Earth’s rotation is not perfectly steady: both its rotation rate (its spin rate) and its orientation in space change in time due to the gravitational pull of the Sun and Moon. The precession-nutation response of the Earth to this external tidal forcing depends strongly on the planet’s deep interior structure. 
In particular, the existence of the Earth’s liquid outer core is known to produce a resonance in the nutation signal at a near-diurnal frequency (as measured in the Earth-bound rotating frame). Physically, this resonance corresponds to the excitation of free mode whereby the liquid core experiences a global rotation of uniform vorticity, hence its name: Free Core Nutation (FCN). 

In parallel, experimental and theoretical studies of fluid dynamics have since long demonstrated that rotating fluids can support oscillatory motions known as inertial waves, which are due to the restoring effect of the Coriolis force. In planetary situations where the fluid domain is bounded by solid boundaries, these oscillations become global, so that they are sometimes referred to as inertial modes. The Spin-Over Mode (SOM), is the simplest of these inertial mode, with uniform vorticity. Because of this and the fact that the SOM, like the FCN, has a near-diurnal frequency, the two modes have often been identified as one and the same. In a former study, we showed that the FCN is in fact a generalization of the SOM to the case of a (non-steadily) freely rotating planet (Rekier et al 2020). 

In the present work, we analyse the relation between the SOM and the FCN in more details by showing how the two modes can, in fact, coexist together in a planet subjected to external gravitational forcing. We also show that the proximity between the frequencies of the SOM and the FCN can have a significant effect on the shape and the intensity of the FCN resonance – represented by the transfer function for nutations – when viscous and/or electromagnetic coupling is introduced at the planet’s Core-Mantle Boundary (CMB). In particular, we estimate that this can cause an increase of ∼1 day in the (retrograde) period of the resonance as measured in the inertial frame. 

We conclude with a discussion on some of the implications of our findings for the nutations of other planetary objects like Mars and the Moon.

Reference:

• Rekier, J., Trinh, A., Triana, S. A., & Dehant, V. (2020). Inertial modes of a freely rotating ellipsoidal planet and their relation to nutations. The Planetary Science Journal, 1(1), 20

How to cite: Rekier, J.: The Spin-Over Mode of freely rotating planets and its relation to their Free Core Nutation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3972, https://doi.org/10.5194/egusphere-egu22-3972, 2022.

The South Atlantic Anomaly (SAA) is a region at Earth’s surface where the intensity of the magnetic field is particularly low. Accurate characterization of the SAA is important for both fundamental understanding of core dynamics and the geodynamo as well as societal issues such as the erosion of instruments at surface observatories and onboard spacecrafts. Here, we propose new measures to better characterize the SAA area and center, accounting for surface intensity changes outside the SAA region and shape anisotropy. Applying our characterization to a geomagnetic field model covering the historical era, we find that the SAA area and center are more time dependent, including episodes of steady area, eastward drift and rapid southward drift. We interpret these special events in terms of the secular vari‑ation of relevant large‑scale geomagnetic flux patches on the core–mantle boundary. Our characterization may be used as a constraint on Earth‑like numerical dynamo models.

How to cite: Amit, H., Terra-Nova, F., Lézin, M., and Trindade, R.: Non-monotonic growth and motion of the South Atlantic Anomaly, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4635, https://doi.org/10.5194/egusphere-egu22-4635, 2022.

We present pymagglobal, a simple to use python interface for global geomagnetic field models. Pymagglobal was developed to provide easy access to global, spherical harmonics based magnetic main field models over historical and paleomagnetic times. The software readily handles cubic-spline based geomagnetic field models stored in the same file format as gufm1 or the CALSxk model series out of the box. Models in other file formats can be incorporated with minimal effort using the python backend. The python interface can, e.g., give model curves for any location, time series of dipole moment or spherical harmonic coefficients or grids and maps of magnetic field components. 

Pymagglobal can be installed by a single command and comes with a command line interface and a GUI, that allows easy extraction and visualization of information from the models. Additionally, the python backend can be used to access the models, for example to generate synthetic data or refer to them in your own analysis. Emphasis is put on documentation and accessibility. The package is available via a git repository  and a custom website at https://git.gfz-potsdam.de/sec23/korte/pymagglobal.

How to cite: Schanner, M. A., Mauerberger, S., and Korte, M.: A python interface for global geomagnetic field models: pymagglobal, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4857, https://doi.org/10.5194/egusphere-egu22-4857, 2022.

Geomagnetic field models are essential in the study of the physical processes that contribute to the Earth’s magnetic field. There are several groups that build models of the Earth’s magnetic field. These models essentially differ in the magnetic data and mathematical methods used during the model estimation, and in the represented sources of the geomagnetic field. It is then the users who choose the models that are most suitable for the study of the geophysical signals of interest. However, there is currently no single platform where field models are collected in a standardised way, and that provides information which helps users to find the best models for their purposes.

Here, we present the geomagnetic field model called CHAOS that is developed and regularly updated by the Technical University of Denmark. CHAOS provides estimates of the recent time-dependent and static internal magnetic fields, and the external magnetospheric field during quiet geomagnetic conditions. It is derived from magnetic data collected by the Swarm, CHAMP, Ørsted, SAC-C, CryoSat-2 satellite missions supplemented by ground observatory data. It is updated approximately every 4 months with the latest ground and satellite data; the current version CHAOS-7.9 covers the time from 1997 to November 2021.

The model is distributed in various formats. For the time-dependent internal field, B-spline coefficients for each spherical harmonic are provided in a similar format as traditionally used for the gufm1 historical field model and the CALS7K millennial timescale models. It is also provided in the shc-file format, which was developed and adopted for distributing spherical harmonic models determined in connection with the Swarm magnetic satellite mission. This format allows reconstruction of spline-based models from a dense sampling of the time series of the spherical harmonic coefficients and is easier for non-experts to use. A piecewise polynomial Matlab version is also available. For reading and evaluating the CHAOS model, we provide Fortran, Matlab and Python software. In particular, we have recently developed the ChaosMagPy Python package, which allows the CHAOS model (and other spherical harmonic field models) to be easily evaluated and visualized.

Although the shc-file format and ChaosMagPy have been developed primarily in support of the Swarm mission and the CHAOS model, they can be used more broadly for time-dependent spherical harmonic field models or serve as a starting point for the development of new tools that enable cross-disciplinary sharing of data and models.

How to cite: Kloss, C., Finlay, C. C., and Olsen, N.: Tools for sharing and evaluating the CHAOS geomagnetic field model and the shc-file format for time-dependent spherical harmonic models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6265, https://doi.org/10.5194/egusphere-egu22-6265, 2022.

An ever-expanding catalogue of satellite data has laid the foundations for new studies of Earth’s secular variation and acceleration. Studies that encode a-priori the axial rigidity conferred to core flows by the Earth’s rapid rotation have revealed novel fast dynamics and improved estimates for the magnetic field strength inside the core. Within this context, a new formalism christened “plesio-geostrophy” (PG) was developed by Jackson and Maffei (Proc. Roy. Soc. A, 476(2243), 2020) with the purpose of describing core dynamics in a regime closer to Earth's conditions. This model makes use of axial integration of the equations of fluid motion and magnetic induction to collapse all three-dimensional quantities into two-dimensional scalars. We report on new results within the PG formalism.

We consider the dynamics of a conducting, inviscid fluid in a full sphere subject to various background magnetic fields. The eigenmodes sustained by the Coriolis and Lorentz forces split into two branches: a fast and a slow one. We characterise these eigenmodes and compare their structure and frequency to fully three-dimensional results. Previous studies are extended by incorporating the effects of horizontal magnetic diffusion.

How to cite: Holdenried-Chernoff, D., Jackson, A., and Maffei, S.: A comparison between the magnetohydrodynamical modes of plesio-geostrophy and fully 3D calculations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6447, https://doi.org/10.5194/egusphere-egu22-6447, 2022.

It has been proposed that thermoelectric (TE) currents may be important in the vicinity of planetary core boundaries (Stevenson 1987, EPSL; Giampieri & Balogh 2002, P&SS). However, TE-induced core dynamics remain largely unstudied. To address this, we have conducted a series of laboratory experiments of turbulent Rayleigh-Bénard convection with a vertical magnetic field in a cylindrical cell filled with liquid gallium. Thermal measurements are taken at a fixed buoyancy forcing with varying Lorentz force. When buoyant inertia dominates, a large-scale overturning circulation cell develops, which imposes strong lateral temperature gradients onto the tank's top and bottom boundaries. In experiments equipped with electrically conducting boundaries, the large-scale circulation slowly precesses in azimuth when thermoelectrically induced Lorentz forces become comparable to buoyant inertial forces. Moreover, TE introduces an asymmetry in the system: this novel magnetoprecessional mode reverses its traveling direction when the magnetic field polarity is reversed. Extrapolating our results to Earth's core, we estimate the required net Seebeck coefficient to generate TE dynamics at CMB conditions. Furthermore, because TE-driven flows reverse direction as the magnetic field reverses, we hypothesize that thermoelectricity can provide a natural symmetry breaker by driving CMB (or ICB) core flows in opposite directions between normal and reversed geomagnetic field polarities. To test our hypothesis, we need to better constrain the electrical, thermal conductivity, and Seebeck coefficient of the CMB (or ICB), and gather observational evidence of geomagnetic secular variation during field reversals. This study is reported in Xu et al. 2022, JFM

How to cite: Xu, Y., Horn, S., and Aurnou, J.: A laboratory study of turbulent magnetoconvection: Could thermoelectricity induce asymmetry in geomagnetic secular variation?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6590, https://doi.org/10.5194/egusphere-egu22-6590, 2022.

The recent Kalmag and archaeomagnetic ArchKalmag14K models together represent the global geomagnetic field model evolution over the past 14000 years and resolve temporal scales of the order of a month over the last 122 years. They are obtained through the sequential assimilation of archeomagnetic and volcanic data, and survey, observatory and satellite data, respectively. Both these models provide full posterior information about the core field, and in the case of Kalmag also about other magnetic sources such as the lithospheric or some tidal fields. These models are made accessible online through different physical and statistical quantities associated with them. In this presentation, we will detail our modeling strategy, the type of results we are getting with it, and how the community can access and use our models by an online interface at https://ionocovar.agnld.uni-potsdam.de/Kalmag/ and https://ionocovar.agnld.uni-potsdam.de/Kalmag/Archeo/.

How to cite: Baerenzung, J., Schanner, M. A., Korte, M., Saynisch, J., and Holschneider, M.: The Kalmag and ArchKalmag14K geomagnetic field models: their derivation principle, properties and availability, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7290, https://doi.org/10.5194/egusphere-egu22-7290, 2022.

We bring together two important features of planetary cores: 1) wave propagation in the fluid and 2) topography of the fluid-solid interface. On one hand, inertial waves contribute to the maintenance of quasi geostrophic motions or to the formation of elongated structures in rotating turbulence. On the other hand, topography of the core-mantle boundary has been prososed in various seismological and geodynamical studies and can modify the fluid flow in the core, for example, by altering global fluid modes. Here, we focus on inertial waves excited by topography.

We present results from a combined numerical and experimental investigation of inertial wave motion which is forced by an oscillating topography. To allow comparison with the theory of linear inertial waves, we use a complex topography characterised by a single wavenumber in the spectral domain. Both, the wavenumber and the frequency of the oscillations are varied, allowing us to characterise the transport of kinetic energy at different length scales as well as the interactions of direct and reflected inertial waves. 

How to cite: Burmann, F. and Noir, J.: Inertial waves excited by topography, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8071, https://doi.org/10.5194/egusphere-egu22-8071, 2022.

The internal magnetic field of Mercury is best described by a northward offset dipole with almost zero obliquity. Its offset, weakness, axisymmetry and lack of secular variation still poses a challenge to dynamo theory. After NASA’s Mariner 10 flybys in the 1970’s and MESSENGER’s orbital mission in 2011-2015, BepiColombo performed a flyby at Mercury in October 2021. For the first time, magnetic field measurements are obtained from the southern hemisphere by the fluxgate magnetometer MPO-MAG. We will present an overview of the flyby data and compare the new in-situ data to magnetospheric models obtained from the previous missions to the innermost terrestrial planet. Does the flyby data reveal any secular variation? Has the dipole offset changed? These are some of the questions we will discuss with this unprecedented magnetometer data. We will close with a discussion on what is to be expected from the orbital phase of BepiColombo. 

How to cite: Heyner, D., Carr, C., Auster, U., Richter, I., Kolhey, P., Exner, W., Mieth, J., Plaschke, F., Pump, K., Wicht, J., Langlais, B., Berghofer, G., Schmid, D., Baumjohann, W., Fischer, D., Horbury, T., Magnes, W., Masters, A., Slavin, J., and Glassmeier, K.-H. and the MPO-MAG Team: BepiColombo at Mercury: First close-in magnetic field measurements from the southern hemisphere, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8509, https://doi.org/10.5194/egusphere-egu22-8509, 2022.

Since the discovery of Mercury’s peculiar magnetic field it has raised questions about the underlying dynamo process in its fluid core. The global magnetic field at the surface is rather weak compared to other planetary magnetic fields, strongly aligned to the planet's rotation axis and its magnetic equator is shifted towards north. Especially the latter characteristic is difficult to explain using common dynamo model setups. One promising model suggests that a thermal stably stratified layer right underneath the core-mantle boundary is present. As a consequence the magnetic field deep inside the core is efficiently damped by passing through the stably stratified layer due to the skin effect. Additionally, the non-axisymmetric parts of the magnetic field are vanishing, too, such that a dipole dominated magnetic is left at the planet’s surface. In this study we present new direct numerical simulations of the magnetohydrodynamical dynamo problem which include a stably stratified layer on top of the outer core, which can also reproduce the shift of the magnetic equator towards north. We revisit a model configuration for Mercury’s dynamo action, which successfully reproduced the magnetic field features, in which core convection is driven by thermal buoyancy as well as compositional buoyancy (double-diffusive convection). While we find that this model configuration produces Mercury-like magnetic field only in a limited parameter range (Rayleigh and Ekman number), we show that also a simple codensity model is sufficient over a wide parameter range to produce Mercury-like magnetic fields.

How to cite: Kolhey, P., Heyner, D., Wicht, J., Gastine, T., and Plaschke, F.: Dynamo models reproducing the offset dipole of Mercury’s magnetic field, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8532, https://doi.org/10.5194/egusphere-egu22-8532, 2022.

Many studies have suggested silicon as a candidate light element for the cores of Earth and Mercury. However, the effect of silicon on the melting temperatures of core materials and thermal profiles of cores is poorly understood, due to disagreements among melt detection techniques, uncertainties in sample pressure evolution during heating, and sparsity of studies investigating the combined effects of nickel and silicon on the phase diagram of iron. In this work (Dobrosavljevic et al. 2022), we develop a multi-technique approach for measuring the high-pressure melting and solid phase relations of iron alloys and apply it to Fe_0.8Ni_0.1Si_0.1 (Fe-11wt%Ni-5.3wt%Si), a composition compatible with recent estimates for the cores of Earth and Mercury.

This approach combines results (20-83 GPa) from two in-situ techniques: synchrotron Mössbauer spectroscopy (SMS) and synchrotron x-ray diffraction (XRD). Melting is independently detected by the loss of the Mössbauer signal, produced exclusively by solid-bound iron nuclei, and the onset of a liquid diffuse x-ray scattering signal. The use of a burst heating and background updating method for quantifying changes in the reference background during heating facilitates the determination of liquid diffuse signal onsets and leads to strong reproducibility and excellent agreement in melting temperatures determined separately by the two techniques. XRD measurements additionally constrain the hcp-fcc phase boundary and in-situ pressure evolution of the samples during heating.

We apply our updated thermal pressure model to published SMS melting data on fcc-Fe and fcc-Fe_0.9Ni_0.1 to precisely evaluate the effect of silicon on melting temperatures. We find that the addition of 10mol% Si to Fe_0.9Ni_0.1 reduces melting temperatures by ~250 K at low pressures (<60 GPa) and flattens the hcp-fcc phase boundary. Extrapolating our results, we constrain the location of the hcp-fcc-liquid quasi-triple point at 147±14 GPa and 3140±90 K, which implies a melting temperature reduction of 500 K compared with Fe_0.9Ni_0.1. The results demonstrate the advantages of combining complementary experimental techniques in investigations of melting under extreme conditions.

Reference:

Dobrosavljevic, V. V., Zhang, D., Sturhahn, W., Zhao, J., Toellner, T. S., Chariton, S., Prakapenka, V. B., Pardo, O. S., Jackson, J. M. (2022). Earth and Planetary Science Letters (in press).

How to cite: Dobrosavljevic, V., Zhang, D., Sturhahn, W., Zhao, J., Toellner, T., Chariton, S., Prakapenka, V., Pardo, O., and Jackson, J.: Melting and phase relations of Fe-Ni-Si determined by a multi-technique approach, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8916, https://doi.org/10.5194/egusphere-egu22-8916, 2022.

The geodynamo inside the liquid core is part of the Earth’s rotation. We discovered that electric currents in the heat exchanging liquid core need to follow the handedness of the spiraling liquids given by Coriolis force. Coriolis force splits the buoyant heat exchanging liquid into the two, north and south hemispheres, each with its unique handedness of spiraling convection systems. Convection spiraling model of the core fluid revealed that any planetary dynamo with a liquid conducitng core must have a two-component bimodal structure magnetic contribution, where, for Earth, the southern hemisphere is always associated with a dominating normal polarity component and northern hemisphere with a dominating component of reverse magnetic polarity. We show that the geodynamo would have a non-random distribution of the probability of generation of dynamo’s magnetic polarity, depending on a difference in a degree of buoyancy vigorousness between the two hemispheres.  In this work, the individual treatment of normal and reversed polarity durations revealed that while before 80 Ma geodynamo was generating predominantly normal polarity durations, after the Tertiary transition at ~ 60 Ma, the geodynamo produced predominantly reverse polarity durations. This observation of predominance of magnetic polarity durations is constrained by the existing temperature models near the core/mantle boundary (CMB) and we show a novel connection how a lower mantle temperature distribution may reorganize its convection pattern in the core and change the stability of the dipolar field in favor of a specific polarity.

How to cite: Kletetschka, G.: Chirality of the Geodynamo from the Core’s buoyancy and Sense of Spinning, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9908, https://doi.org/10.5194/egusphere-egu22-9908, 2022.

Seismic anisotropy observations indicate the presence of an innermost and outermost inner core, but the origin of this structure is unknown. Records of the past geomagnetic field provide a means to probe inner core evolution by establishing when growth started. The Ediacaran (~565 million-year-old) geodynamo was near collapse, with a strength 10 times weaker than that of the present-day consistent with model predictions for the field before the onset of inner core nucleation. But the timing of the key transition to stronger intensities typical of the Phanerozoic Eon, needed for establishing an exact onset age, has been unclear. We present single crystal paleointensity results from anorthosites of the early Cambrian (~532 million-year-old) Glen Mountains Layered Mafic Complex (Oklahoma). Data from single plagioclase crystals bearing single domain magnetite and titanomagnetite inclusions yield a time-averaged dipole moment of 3.5 +/- 0.9 x 10²² A m², 5 times greater than that recorded in the Ediacaran Period. This rapid field recovery is the expectation at the start of inner core growth, as new thermal and compositional sources of buoyancy to power the geodynamo become available. We will discuss thermal models, which together with our new paleointensity results, allow us to constrain growth of the inner core and when its structure may have changed.

How to cite: Zhou, T., Tarduno, J., Cottrell, R., and Nimmo, F.: Early Cambrian renewal of the geodynamo and the origin of inner core structure, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10532, https://doi.org/10.5194/egusphere-egu22-10532, 2022.

Taking advantage of the Swarm three-satellite magnetic field mission by ESA, launched on 22 November 2013 and still orbiting, and ground observatory magnetic data, we determine a spatiotemporal regional model for the geomagnetic field using the R-SCHA technique over the area comprising the South Atlantic Anomaly (SAA). The SAA is the region above the South Atlantic and South America where the geomagnetic field intensity is much lower than expected by a simple dipolar field. Its origin is deep in the outer core and is likely due to a reverse magnetic flux area that has been increasing in the last four centuries. On the basis of this model, we observe 1) the recent evolution of the anomaly from 2014 up to date, with a focus on its “tails” towards South Africa and West Pacific, 2) some features that can be related to important properties of the main geomagnetic field, such as its secular variation and the occurrence of geomagnetic jerks.

How to cite: Campuzano, S. A., De Santis, A., and Pavón-Carrasco, F. J.: Regional geomagnetic field model over the area comprising the South Atlantic Anomaly, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11293, https://doi.org/10.5194/egusphere-egu22-11293, 2022.

Given their small sizes and low central pressures, the cores of most asteroids are expected to have started crystallising at the core mantle boundary (CMB) instead of at their centre, as is the case for the Earth. This so-called top-down crystallisation is thermally unstable but compositionally stable, making the conditions for dynamo generation more difficult to achieve. Nevertheless, modern observations of Ganymede show an active magnetic field, where it has been suggested that solidification occurs away from the CMB as an iron snow. This model proposes that iron crystals grow in a snow zone and subsequently sink into the interior and melt, releasing dense fluid that drives convection and a magnetic field. However, whether this process could have occurred in asteroid cores is uncertain due to the significantly smaller adiabatic temperature difference between the CMB and the centre of their cores. This weak temperature gradient may also prevent crystallisation away from the CMB. Therefore, the power for a compositional dynamo may result from an increase in convective velocities caused by the formation of dense crystals at the CMB or turbulence caused by the settling of the crystals themselves.

To investigate these possibilities, we employ analogue tank experiments to explore the possible mechanisms driving convection during inward asteroid core crystallisation. An ammonium chloride solution is cooled from above with a layer of buoyant propanol separating the solution from the cold plate to prevent the growth of crystals on this boundary. Instead, the crystals form below the buoyant layer in a ‘snow zone’. We vary the temperature difference across this buoyant layer to investigate the different regimes that may exist. At each driving temperature difference, we measure the velocity fields of any fluid flow within the ammonium chloride solution using particle imaging velocimetry. This enables us to compare the convective velocities with and without crystallisation as well as develop scaling laws to apply the results of these experiments to models of core thermal evolution.

We find that the mean convective speeds increase by over an order of magnitude when the fluid is crystallising. This increase in speed is driven by an increase in the bulk density of the fluid in the snow zone due to the presence of a small crystal fraction. While the motion of crystals themselves do not induce any turbulence in the fluid due to their small size, they act to locally increase the density of the fluid, causing dense, crystal-rich plumes to emanate from the snow zone, which drive faster convective speeds throughout the fluid. This result provides a new mechanism for dynamo generation in inwardly crystallising cores, especially if remelting of falling iron crystals is delayed until deep within the core’s interior, as has recently been proposed for Mars, or if there is a nucleation barrier that causes significant undercooling before the onset of crystallisation. We also measure the temperature and composition as a function of depth within the tank, from which we may assess whether thermal equilibrium can be assumed when modelling snow zones in cores.

How to cite: Dodds, K., Bryson, J., Neufeld, J., and Harrison, R.: Exploring the dynamics of inward core solidification using analogue tank experiments, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11668, https://doi.org/10.5194/egusphere-egu22-11668, 2022.

Earth-based measurements of Mercury's libration amplitude have been used previously to establish the existence of Mercury's liquid core and to estimate its size. However these previous works have not yet taken into account the internal core flows that can be induced by rotational variations such as librations. In the present study, we use a numerical linear model to investigate the effect that these internal flows might have on Mercury's libration amplitude and other observables. In particular we find that the inclusion of a stably stratified layer at the top of the core – the existence of which has been suggested by thermal evolution and numerical dynamo models – in most cases prohibits the transmission of any motion from the top of the core to its deeper parts and vice versa.

How to cite: Seuren, F., Triana, S. A., Rekier, J., Van Hoolst, T., and Dehant, V.: The influence of a stratified core on Mercury's librations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12178, https://doi.org/10.5194/egusphere-egu22-12178, 2022.

Generation and reversal of the Earth’s magnetic field have remained one of the most controversial topics.  It is well known that the Earth’s magnetic field is generated by dynamo action in the liquid iron outer core. This mechanism explains how a rotating, convecting, and electrically conducting fluid sustains a magnetic field.

In this study, we investigate the kinematic dynamo action associated with the well-known ABC-flow (see Dombre et al. [1986]). We focus on the “A = B = C = 1. Its dynamo properties have been assessed in 1981 by Arnold et al. [1981]. It belongs to fast dynamo action: a flow which achieves exponential magnetic field amplification over a typical time related to the advective timescale and not the ohmic diffusive timescale (in which case it is referred to as a “slow dynamo”).

We use DNS method for solving the kinematic dynamo problem, for which a solenoidal magnetic field evolution is governed under a prescribed flow by the induction equation.

In this work, we propose a deep learning method to solve the inverse dynamo problem by estimating the velocity field from the magnetic field. We train our deep learning algorithm from the velocity field and the magnetic field values obtained from the above flow model. Once the algorithm parameters are trained, the optimized algorithm is tested for the velocity field estimation from magnetic field. 

How to cite: Mouhali, W., Jun, J.-Y., and Lehner, T.: Velocity field reconstruction by Machine Learning during kinematic dynamo process, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13478, https://doi.org/10.5194/egusphere-egu22-13478, 2022.

Singhbhum Craton, eastern India, exposes an array of Paleoarchean granitoids (e.g., TTGs and diorites, transitional TTG, and K-rich granite) ranging in age from ~3.53─3.25 Ga, thus making it a suitable archive for understanding crustal architecture and dynamics during that era. Granitoids cover the core of the craton as a composite dome and are fenced by keels of contemporaneous iron ore bearing greenstone belts from east, west, and south giving rise to a dome-and-keel architecture.  Change in granitoid chemistry and isotope signature over time and space can provide a window into the change of crustal evolution mechanism as well as geodynamics of the crust formation if put into a robust tectonic framework. Most of such earlier studies addressed the secular evolution of granitoid chemistry and isotopic changes as an expression of a shift in tectonic paradigms. This tectonic shift is explained broadly as a response to a progressively cooling earth. However, the timing of the transition (advent of a new tectonic setting) varies globally; hence, each craton needs to be studied separately and without any prior bias.

Spatial variation represented by contour diagrams from the cratonic core show two distinct areas exposing dominantly 3.35–3.25 Ga high-silica, low-magnesiam, high K₂O/Na₂O (K/Na>0.60) granitoids of shallow crustal origin, indicated by their low pressure-sensitive ratios (eg. Eu/Eu*, Sr/Y, Gd/Er, La/Yb). These two areas are surrounded by older intermediate granitoids (>3.35 Ga TTGs). Based on the spatial distribution, it is being suggested that these spatial arrangement of granitoids are related to “partial convective overturn (PCO)” process where the >3.35 Ga TTG basements were subjected to greenstone load while they were soft. As a result some part of the newly formed softer >3.35 Ga TTG crust melted as these overburdens helped in bringing the TTGs to a potential melting depth. The greenstones then sank into the partially molten TTGs along steep-dipping sinistral shear zones by forming synformal keels. The moderate- to- low-pressure TTG-derived partial melts then rose to the shallower level and formed the 3.35–3.25 Ga high-silica, low-Mg# potassic granitoids.

Preserved rock record in the Singhbhum Craton indicates that the granitoid magmatism started at ~3.47 Ga with emplacement of high-silica, low alumina tonalite, characterized by low Sr/Y, (Gd/Er)_N, (La/Yb)_N, Eu/Eu* and Sr. The 3.47 to 3.32 Ga TTG record from the Singhbhum Craton show a progressive increase in Al₂O₃, Sr/Y, (Gd/Er)_N, (La/Yb)_N, Eu/Eu* and Sr and decrease in Na₂O. The increase in the pressure-sensitive ratios reached peak during 3.32 Ga and then started decreasing until ~3.28 Ga followed by another increase during ~3.28 to ~3.25 Ga before ceasing of Paleoarcehan magmatism in the Singhbhum Craton. Such variation in geochemical tracers is explained in terms of episodic crustal thickening by periodic mantle upwelling and associated delamination along with time-progressive changes in bulk chemical composition of the continental crust from mafic to felsic.

How to cite: Mitra, A. and Dey, S.: Time-space evolution of an ancient continent, a window to crustal evolution: Insight from granitoids of Singhbhum Craton, eastern India, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-594, https://doi.org/10.5194/egusphere-egu22-594, 2022.

Biogenicity and taphonomy of the early life fossil records are debated as most of the previous studies focussed mainly on isotopes geochemistry. The non-metamorphosed Paleoproterozoic (~2.1 Ga) sedimentary succession of the Francevillian Basin (Gabon) contains the oldest complex multicellular organisms embedded in black shale facies. Several studies have confirmed the biogenicity of these soft-bodied organisms. Here, we used multi-proxy techniques to show that the preservation of these macro-organisms happened in a close system that limits interaction with their host rocks, which leads to their good preservations. The macro-organisms are present in different shapes and sizes: lobate (L), elongate (E), tubular (T), segmented (S), and circular (C), and are often associated with bacterial mats. Except for the C form, most of the other specimens are pyritized. Sulfur isotopes data confirms that pyritization occurred by bacterial sulfato-reduction during early diagenesis. We compare the clay mineral assemblages between the pyritized specimens and the late-diagenetically formed pure pyritized concretions in the sediments because the early pyritization process could not explain the taphonomic preservation alone. Our clay mineralogical data show that the specimens are dominated mainly by randomly mixed layer Illite-smectite (IS MLMs), illite, and chlorite relative to the host rocks. The abundance of IS MLMs indicates incomplete illitization of smectite, potassium deficiency, and limited mineral reactions in a semi-close local chemical system within the fossils.  In addition, the authigenic chlorites are more iron-rich and show vermicular habitus. By contrast, the pyritized concretions mainly consist of well-crystallized illite and less iron-rich chlorite, while the smectite phases are absent. These results confirmed that the diagenetic reaction is controlled by interaction with an open late diagenetic system. We concluded that taphonomic preservation of the ancient fossil record resulted from the early diagenetic growth of pyrite crystals during bacterial sulfato reduction in the fossils, which creates a semi-closed system that drastically reduced fluid-rock interactions with the host sediments.

How to cite: Ngwal Ghoubou Ikouanga, J. A., Fontaine, C., M. Bankole, O., Laforest, C., Riboulleau, A., Trentesaux, A., Boissard, C., Somogyi, A., Meunier, A., and El Albani, A.: Taphonomy of early life: Role of organic and mineral interactions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-955, https://doi.org/10.5194/egusphere-egu22-955, 2022.

The Precambrian comprises the vast majority of Earth’s history. Preserved archives contain essential information about the first few billion years for planetary evolution of our planet. Despite covering a large part of the history of our planet, these outcrops are not so abundant due to erosion and frequently occur in disparate areas. In order to relate them and to establish a timeline of geological events in a world lacking biochronology, we rely on accurate radio-isotopic age determinations. These are, however, rather scarce and still leave several hundreds of million years long time intervals undated. In this study, we present U-Pb age determinations from volcanic and sedimentary units of the Paleoproterozoic Transvaal Supergroup, South Africa. The Transvaal Supergroup is an exceptionally well preserved sequence and therefore accounts for a very large amount of geochemical data. Due to its capacity to produce large data sets the preferred technique in U-Pb zircon geochronology for ancient sediments is LA-ICP-MS. It allows the aqcuisition of maximum depositional ages (MDA) in a fast way and at a relatively low cost. However, the large analytical uncertainty preclude the temporal resolution to distinguish between different processes in such old rocks. Moreover, the standard dating procedure rarely includes zircon treatment via chemical abrasion to mitigate common problems such as open system behavior due to radioactive decay damage related Pb loss. In consequence, interpreted ages might be severely disturbed and may yield MDA’s that are tens to hundreds of million years too young. As an alternative, the much more work-intensive CA-ID-TIMS technique allows the obtention of more accurate and more precise ages, preferably using zircon grains that have previously been screened for their LA-ICP-MS U-Pb age.

 Our new combined LA-ICP-MS and CA-ID-TIMS data indicates that the glaciogenic Makganyene Formation has a MDA of ~2.42 Ga. Younger age clusters at around ~2.2 Ga from LA-ICP-MS dating disappear with chemical abrasion and have to be interpreted as artifacts of radiation-damage related Pb loss. These new results have important implications for both environmental evolution during the Neoarchean/Paleoproterozoic, as well as for the regional geology. The Makganyene diamictites are thought to represent the oldest Paleoproterozoic glaciation in South Africa. The data also corroborate the hypothesis that the directly overlying-to-locally-interfingered mafic volcanic Ongeluk Formation is ~200 Ma older than the volcanic rocks ~2250 Ma Hekpoort Formation in the East Transvaal basin. We therefore reject the long-standing correlation between both units, as previously published.

We demonstrate that LA-ICP-MS is not capable to provide a robust and reliable MDA’s in ancient clastic sediments. CA-ID-TIMS analysis provides dates of significantly higher accuracy, because the chemical abrasion is minimizing Pb-loss in the crystal. Therefore, for studies relying on U-Pb zircon geochronology, we encourage the application of CA-ID-TIMS in the youngest populations previously identified with the LA-ICP-MS. This is particularly important for establishing reliable maximum depositional ages in sedimentary rocks.

How to cite: Senger, M. H., Davies, J., Ovtcharova, M., Beukes, N., Gumsley, A., Gaynor, S. P., Ulyanov, A., and Schaltegger, U.: U-Pb zircon geochronology combining both in-situ and bulk-grain techniques in the Transvaal Supergroup, South Africa., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1666, https://doi.org/10.5194/egusphere-egu22-1666, 2022.

Much of the continental lithosphere developed during the Archean, which was an Eon of change in terms of global geodynamics and geochemical cycles. Uncovering the causal links between crust forming processes and prevailing geodynamic mechanisms is crucial for understanding the origins and composition of the present-day continental lithosphere. Pristine Archean crust is scarce yet can be found in cratons worldwide. Many of these occurrences comprise rocks of the tonalite-trondhjemite-granodiorite (TTG) suite, which represent a prevalent component of the Archean continental crust. TTGs are generally considered to have formed by partial melting of amphibolite or eclogite source rocks that had basaltic precursors originally extracted from a depleted mantle (e.g., [1]). The age of the source rocks (i.e., the time between the basalt extraction from the mantle and TTG formation) can be determined from the initial radiogenic isotope compositions of TTGs, provided that the P/D ratio of the source can be reliably estimated and is significantly different from that of the depleted mantle.

Based on this principle, we estimated the age of basaltic sources of TTGs from cratons of different age and paleogeography from initial ⁸⁷Sr/⁸⁶Sr compositions determined by in-situ Sr isotope analysis of primary igneous apatite (LA-MC-ICPMS). The ⁸⁷Sr/⁸⁶Sr of these apatites show that prior to 3.4 Ga TTGs were derived from relatively old mafic sources and that the average time between formation of basaltic material from the mantle and subsequent remelting under amphibolite to eclogite facies conditions decreased drastically during the Paleoarchean. This secular change indicates a rapid global increase in the efficiency of TTG production or the emergence of a new TTG-forming process at c. 3.4 Ga [2].

In this contribution we explore this hypothesis by comparing the ⁸⁷Sr/⁸⁶Sr signature of the TTGs with their trace-element compositions, as well as with ¹⁷⁶Hf/¹⁷⁷Hf zircon data for these rocks and contemporary TTGs from other studies. This combined geochronological, isotope and geochemical analyses will provide new constraints on the age of TTG sources during the Archean and will allow investigation into the nature and probable causes of the apparent rejuvenation at 3.4 Ga, as indicated by Sr isotopes.

[1] Hoffmann, J.E. et al. (2011) Geochim. Cosmochim. Acta 75, 4157-4178.

[2] Caton, S., et al., (in review) Chem. Geol.

How to cite: Musiyachenko, K., Smit, M., Caton, S., B. Emo, R., Kielman-Schmitt, M., Kooijman, E., Scherstén, A., Halla, J., Bleeker, W., Hoffmann, J. E., Prakash Pandey, O., Ravindran, A., Maltese, A., and Mezger, K.: Secular change in the age of TTG sources during the Archean from in-situ Sr and Hf isotope analysis by LA-MC-ICPMS, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3181, https://doi.org/10.5194/egusphere-egu22-3181, 2022.

The present-day Earth exhibits subduction-driven plate tectonics, which is a surface expression of processes happening in the deep interior. For the early Earth, following the magma ocean solidification stage, a variety of tectonic regimes have been proposed albeit without any consensus: heat-pipe tectonics, plutonic-squishy lid, stagnant lid. Furthermore, the rheological changes required to make the (supposedly gradual) transition to modern style plate tectonics on Earth remain hotly debated. Also, different estimates of mantle potential temperature (Herzberg et al., 2010; Aulbach and Arndt, 2019) for the Archean have been proposed.

Recently, it has been proposed that sediments accumulated at continental margins as a result of surface erosion processes could have acted as a lubricant to stabilise subduction and aid with the initiation of plate tectonics after the emergence of continents around 3 Ga (Sobolev and Brown, 2019). Before that time, the flux of sediments to the ocean was very limited. It was further suggested that subduction zones were already present at that time but were likely initiated only above hot mantle plumes. This tectonic regime of regional plume-induced retreating subduction zones was very different from the modern type of plate tectonics, but nevertheless might have been efficient in production of early continental crust and recycling of water and pre-existing crust into the deep mantle.

In this work, we test this hypothesis of surface-erosion controlled plate tectonics preceded by plume-induced retreating subduction tectonic regime in global convection models by introducing magmatic weakening of lithosphere above hot mantle plumes. We also adapt the effective friction coefficient in brittle deformation regime to mimic the lubricating effect of sediments. Furthermore, these models employ a more realistic upper mantle rheology and are capable of self-consistently generating oceanic and continental crust while considering both intrusive (plutonic) and eruptive (volcanic) magmatism (Jain et al., 2019). We also investigate the influence of lower mantle potential temperatures on crust production and compare our models with geological data.

When compared to models with just diffusion creep, the models with composite rheology (diffusion creep and dislocation creep proxy) result in more efficient mantle cooling, higher production of continental crust, and higher recycling of basaltic-eclogitic crust through delamination and dripping processes. These models also show higher mobilities (Tackley, 2000), which have been previously shown for diffusion creep models only with low surface yield stress values (Lourenço et al., 2020). Preliminary results from models initialised with lower mantle potential temperatures show an affect on the initial growth of TTG rocks over time. However, no considerable differences in terms of total crust production or mantle cooling are observed.

How to cite: Jain, C. and Sobolev, S.: Using composite rheology models to explore the interplay between continent formation, surface erosion, and the evolution of plate tectonics on Earth, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4850, https://doi.org/10.5194/egusphere-egu22-4850, 2022.

The origins of Eoarchean peridotites found in the Itsaq Gneiss Complex (IGG) of southern West Greenland represent a crucial record of igneous and geodynamic processes on the early Earth. The igneous and geodynamic origins of these rocks have, however, been the subject of controversy, with some researchers arguing that they represent the first known slivers of mantle emplaced by tectonic processes in the crust and others contending that they represent cumulates associated with the local basalt units. The geodynamic context for the formation of these rocks has also been disputed, with some researchers arguing that they formed in a horizontal tectonic setting analogous to a modern subduction zone, while others propose a vertical tectonic origin for all Eoarchean rocks. Here, we provide new insights into the history of these peridotites using multiple sulfur isotope signatures combined with Hf isotope compositions. Anomalously high εΗf values in some IGC peridotites identified in previous studies [1], as well as in metabasalts with boninite-like compositions [2] found in the Isua Supracrustal Belt (ISB) within the IGC, point to contributions from a mantle source already depleted in the Hadean [2]. The multiple sulfur isotope data of the IGC peridotites found south of the ISB reveal small but significant Δ³³S anomalies, consistent with incorporation of surface-derived material of Archean age or older. Furthermore, correlations between sulfur isotope data and major and trace element abundances as well as initial Hf isotope values of IGC peridotites support the hypothesis that high-degree melt depletion occurred under hydrous conditions, followed by variable degrees of melt metasomatism. The involved fluid and melt components precipitated sulfides that incorporated surface-derived sulfur with different depositional origins. We propose that these findings are best explained by a horizontal tectonic regime similar to modern arc settings.

1. van de Löcht, J., et al., Preservation of Eoarchean mantle processes in ∼3.8 Ga peridotite enclaves in the Itsaq Gneiss Complex, southern West Greenland. Geochimica et Cosmochimica Acta, 2020. 280: p. 1-25.

2. Hoffmann, J.E., et al., Highly depleted Hadean mantle reservoirs in the sources of early Archean arc-like rocks, Isua supracrustal belt, southern West Greenland. Geochimica et Cosmochimica Acta, 2010. 74(24): p. 7236-7260.

How to cite: Lewis, J., Hoffmann, J. E., Schwarzenbach, E. M., Strauss, H., Li, C., Münker, C., and Rosing, M. T.: Sulfur and Hafnium Isotope evidence for Early Horizontal Tectonics in Eoarchean Peridotites, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5226, https://doi.org/10.5194/egusphere-egu22-5226, 2022.

The nature of Paleoarchean (>3.2 Ga) crustal accretion continues to be debated, in particular the onset and timing of subduction-like processes. Crust of this age is typically characterised by dome-and-keel geometry that is widely interpreted to be related to “sagduction” or the episodic dripping of denser, mafic volcanics into the mantle around buoyant silicic cratonic nuclei. This occurs during regional scale crust-mantle overturn events.

The exceptional preservation of the East Pilbara Terrane (EPT) has been instrumental in the development of this model and its role in Paleoarchean continental crust formation. The Emu Pool Supersuite (~3324-3290 Ma) represents a phase of voluminous silicic magmatism that has been attributed to overturn and sagduction within the EPT (e.g. Wiemer et al., 2018). However, the widespread occurrence of magmatic-hydrothermal Cu and Mo mineralisation, reported to be linked to this magmatic episode, have received little attention. Comparisons to Phanerozoic porphyry Cu-Mo deposits have been drawn (e.g. Barley & Pickard, 1999), which is intriguing as such porphyry-type deposits have a clear genetic link to arc magmatism and subduction processes as they require hydrous, Cl-rich magmatism (e.g. Tattich et al., 2021).

To date the chronological relationships of the magmatic-hydrothermal deposits to the major dome forming silicic magmatism is poorly constrained. In one deposit, hydrothermal activity is constrained by ¹⁸⁷Re-¹⁸⁷Os geochronology (Stein et al., 2007) to late to post Emu Pool Supersuite magmatism, yet this interpretation is hampered by issues relating to the λ¹⁸⁷Re uncertainty. Furthermore, interpretation of Paleoarchean geodynamics and magmatic evolution generally relies on micro-beam zircon U-Pb geochronological analyses, typically reported at single ²⁰⁷Pb/²⁰⁶Pb date precision at >±10 Myrs (2s), and presents challenges for accurately resolving geological processes and events.

We demonstrate that high-precision CA-ID-TIMS (Chemical Abrasion-Thermal-Ionisation Mass Spectrometry) zircon U-Pb geochronology, utilising ATONA low-noise detectors, can now routinely obtain precision of  ~<±200 kyrs (2s) on ²⁰⁷Pb/²⁰⁶Pb dates of single zircon or fragments at ~3.3 Ga. By combining detailed field relationships, with unprecedented temporal precision, we show that the Mo-Cu hydrothermal mineralisation can be demonstrably linked to their host plutons and formation timescales can even be constrained to ~1 Myrs, comparable to Phanerozoic porphyry deposits. This study identifies that magmatic-hydrothermal systems were not synchronous across the EPT. Instead they occurred over >7 Myrs during the early phase of Emu Pool Supersuite and silicic magmatism within domes.

Whilst the geodynamic trigger for Mo and Cu magmatic-hydrothermal mineralisation at ~3.3 Ga remains enigmatic, we highlight their timing and occurrence should be accommodated within Paleoarchean geodynamic models. Furthermore, the results illustrate the potential of modern high-precision U-Pb geochronology to routinely examine Paleoarchean magmatic records at timescales that closely approximate known plutonic accretion rates within the Phanerozoic.

References

Barley, & Pickard, (1999) Precambrian Research, 96, 41-62

Stein et al., (2007) Geochimica et Cosmochimica Acta, 71

Tattitch et al., (2021) Nature communications, 12, 1-11.

Wiemer et al., (2018) Nature Geoscience, 11, 357-361.

How to cite: Thijssen, A., Tapster, S., and Parkinson, I.: Pinpointing Paleoarchean magmatic-hydrothermal events during the geodynamic and crustal evolution of the East Pilbara Terrane, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7947, https://doi.org/10.5194/egusphere-egu22-7947, 2022.

The Earth’s lithosphere, atmosphere, and biosphere interact with one another primarily at the surface of our planet, with the lithospheric coupling arising primarily from large-scale, long-period topographic evolution driven by deep mantle processes. Global numerical modelling of mantle convection in 3D with mobile continents in a modern plate tectonic regime has been previously demonstrated (Coltice et al., 2019). Improvements on such models can provide a useful tool for investigating the effects of large scale and long term changes in Earth’s tectonic regime on the surface.

We present preliminary results in 2D spherical geometry using newly implemented additions to the existing mantle convection code StagYY (Tackley, 2008). A free surface representation using a marker chain enables higher surface resolution and the possibility of future implementation of surface processes on a global scale (Duretz et al., 2016). Initial conditions based on previous work on self-consistent continent generation enables modelling of continents with realistic rheology and structure (Jain et al., 2019).

The successful development of these tools enables further study of the evolution of the surface as a result of tectonic changes. A key goal is the modelling of the transition from a pre-plate tectonic regime to modern plate tectonics, as may have occurred in the Neoproterozoic (Stern, 2018). The tectonic changes of this period were also associated with other radical changes in the atmosphere and biosphere, such as the Cryogenian glaciations, and the Cambrian explosion. Models of topographic evolution may be used in conjunction with climate models or models of biological evolution to study the coupling between these systems as a part of the emerging field known as Biogeodynamics (Gerya et al., 2020).

How to cite: Gray, T., Tackley, P., Gerya, T., and Stern, R. J.: Global scale numerical modelling of the transition to modern day plate tectonics, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8653, https://doi.org/10.5194/egusphere-egu22-8653, 2022.

Several geochemical tracers (S, C, O, Xe) underwent irreversible global changes during the Precambrian, and in particular during the Great Oxygenation Event (GOE), between the Archean and Proterozoïc eons [1]. Xenon is of particular interest as it presents a secular isotopic evolution during the Archean that ceased around the time of the GOE. In this regard Xe is somewhat analogous to mass-independent fractionation sulfur (MIF-S) in that it can be used to categorically identify Archean atmospheric components [2]. Xe isotopes in the modern atmosphere are strongly mass-dependent fractionated (MDF-Xe), with a depletion of the light isotopes relative to the heavy ones. There was a continuous Xe isotope evolution from primitive Xe to modern Xe that ceased between 2.6 and 1.8 Ga [2] and this evolution has been attributed to coupled H⁺-Xe⁺ escape to space [3].

The purpose of this project is to document the Xe composition of the paleo-atmosphere trapped in well-dated hydrothermal quartz fluid inclusions with ages covering the Archean-Proterozoic transition to better constraint its link with the GOE.

We have measured an isotopically fractionated Xe composition of 2.0 ± 1.8 ‰ relative to modern atmosphere at 2441 ± 1.6 Ma, in quartz vein from the Seidorechka sedimentary formation (Imandra-Varzuga Greenstone belt, Russia). A slightly younger sample from the Polisarka sedimentary formation (Imandra-Varzuga Greenstone belt, Russia) of 2434 ± 6.6 Ma does not record such fractionation and is indistinguishable from the modern atmospheric composition. A temporal link between the disappearance of the Xe isotopes fractionation and the MIF-S signature at the Archean-Proterozoic transition is clearly established for the Kola Craton. The mass-dependent evolution of Xe isotopes is the witness of a cumulative atmospheric process that may have played an important role in the oxidation of the Earth's surface [3], independently of biogenic O₂ production that started long before the permanent rise of O₂ in the atmosphere [4].

[1] Catling & Zahnle, 2020, Sciences Advances 6, eaax1420. [2] Avice et al., 2018, Geochimica et Cosmochimica Acta 232, 82-100 [3] Zahnle et al., 2019, Geochimica et Cosmochimica Acta 244, 56-85. [4] Lyons et al., 2014, Nature 506, 307-315.

How to cite: Ardoin, L., Broadley, M., Almayrac, M., Avice, G., Byrne, D., Tarantola, A., Lepland, A., Saito, T., Komiya, T., Shibuya, T., and Marty, B.: The end of the atmospheric xenon Archean’s evolution: a study of the Great Oxygenation Event period, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9527, https://doi.org/10.5194/egusphere-egu22-9527, 2022.

The lithologic and chemical composition of the continental crust impacts Earth atmosphere and environment through e.g. weathering feedbacks and nutrient supply. However, despite being important for  the biological and atmospheric evolution of our planet, the question of how the lithological composition of Earth’s landmasses evolved from around 3.5 Ga to present is still a matter of considerable debate.

Here I will present a summary of the work that has been conducted by my colleagues and myself over the past five years and that improved our understanding of the chemical and lithological evolution of Earth landmasses since 3.5 Ga. Reconstructing the composition of past continents is difficult because erosion and crustal reworking may have modified the geologic record in deep time, so direct examination of the nature of igneous rocks could provide a biased perspective on the nature of the continents through time. A less biased record is likely provided by terrigenous sediments that average the composition of rocks exposed to weathering on emerged lands and we therefore use major and trace element concentrations and stable isotope compositions of shales as a proxy for the average composition of the emerged continents in the past. Applying a three-component mixing model to the sediment record shows that since 3.5 Ga, the landmasses that were subjected to erosion were dominated by felsic rocks. Furthermore, our reconstructed relative abundance of felsic, mafic and komatiitic rocks in the Archean is close to that currently observed in these ancient terrains. While our model does not suggest a strong change in the lithologic composition of Earth continents, we find a secular change in the average major and trace element concentration, with incompatible elements being more depleted and compatible elements being more enriched in the old landmasses.

How to cite: Greber, N. D.: The lithologic composition of Earth’s emerged lands reconstructed from the chemistry of terrigenous sediments, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10352, https://doi.org/10.5194/egusphere-egu22-10352, 2022.

The tectonic processes responsible for the formation of early Earth felsic crust (predominantly composed of tonalite-trondhjemite-granodiorite, or TTGs) inform the global regime of mantle convection that operated at this time. Many models have been proposed to explain the formation of Archean TTGs, including melting of downgoing crust in hot subduction zone settings, or melting of crust that is buried by lava flows and founders into the mantle. Formation in a subduction zone setting would imply at least some form of mobile-lid tectonics on the early Earth, while TTG formation via crustal burial and foundering does not require subduction or plate tectonics, and can thus occur in a stagnant-lid regime.  

Regardless of tectonic setting, TTGs can only form if hydrated basaltic protocrust melts before it experiences metamorphic dehydration. Previous work has argued that this constraint may preclude a subduction origin to TTGs. Regional scale numerical models have found that slabs sink quickly and steeply through the mantle at Archean mantle temperatures, such that they dehydrate before melting. However, these models do not consider evolution of grainsize in the mantle interior and in plate boundaries. Using numerical models of mantle convection with grain damage, a mechanism for generating mobile-lid convection via grain size reduction, I show that a sluggish, drip-like style of subduction emerges at early Earth conditions. This subduction style is a result of plate boundaries becoming effectively stronger with increasing mantle temperature, and leads to significant slab heating at shallow depths.

To test whether TTGs can form from this style of sluggish subduction, I use scaling laws developed from numerical models combined with a simple model of the evolution of the vertical temperature profile through a slab. Results show that the slower sinking speed of slabs caused by grain size evolution in plate boundaries allows for crustal melting for a much wider range of mantle temperatures and subducting plate thicknesses than if the effects of grain size evolution were ignored. Overriding plate thickness is also important, with thin overriding plates favored for TTG formation. These results have important implications for the settings where subduction could generate Archean TTGs, and for potential episodicity in TTG formation resulting from both short- and long-term episodicity in subduction.  

How to cite: Foley, B.: Generation of Archean TTGs by slab melting during sluggish, drip-like subduction, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10873, https://doi.org/10.5194/egusphere-egu22-10873, 2022.

The timing of the onset of plate tectonics on Earth remains a topic of strong debate, as does the tectonic mode that preceded modern plate tectonics. Understanding possible tectonic modes and transitions between them is also important for other terrestrial planets such as Venus and rocky exoplanets. Recent two-dimensional modelling studies have demonstrated that impacts can initiate subduction during the early stages of terrestrial planet evolution - the Hadean and Eoarchean in Earth’s case (O’Neill et al. 2017). Here, we perform three-dimensional simulations of the influence of ongoing multiple impacts on early Earth tectonics and its effect on the distribution of compositional heterogeneity in the mantle, including the distribution of impactor material. We compare two-dimensional and three-dimensional simulations to determine when geometry is important. Results show that impacts can induce subduction in both 2-D and 3-D and thus have a great influence on the tectonic regime. The effect is particularly strong in cases that otherwise display stagnant-lid tectonics: impacts can shift them to having a plate-like regime. In such cases, however, plate-like behaviour is temporary: as the impactor flux decreases the system returns to what it was without impacts. Impacts result in both greater production of oceanic crust and greater recycling of it, increasing the build-up of subducted crust above the core-mantle boundary and in the transition zone. Impactor material is mainly located in the upper mantle, at least at the end of the modelled 500 million year period. This is modified when impactors are differentiated into metal and silicate: the dense metal blobs sink to the CMB. In 2-D simulations, in contrast to 3-D simulations, impacts are less frequent but each has a larger effect on surface mobility, making the simulations more stochastic. These stronger 2-D subduction events can mix both recycled basalt and impactor material into the lower mantle. These results thus demonstrate that impacts can make a first-order difference to the early tectonics and mantle mixing of Earth and other large terrestrial planets, and that three-dimensional simulations are important so that effects are not over- or under-predicted.

How to cite: Tackley, P. and Borgeat, X.: Hadean/Eoarchean plate tectonics and mantle mixing induced by impacts: A three-dimensional study, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12521, https://doi.org/10.5194/egusphere-egu22-12521, 2022.

Photosynthesis, the metabolic route for conversion of solar to chemical energy, could be present in any planetary system provided with the only three required ingredients: a light source, water, and carbon dioxide.

The ExoPhot project aims to study the relation between photosynthetic systems and exoplanet conditions around different types of stars (i.e. stellar spectral types) by focusing on two aspects: Assessing the photosynthetic fitness of a variety of photopigments (either real or theoretical) as a function of stellar spectral type, star-exoplanet separation, and planet atmosphere composition; and delineating a range of stellar, exoplanet and atmospheric parameters for which photosynthetic activity might be feasible. In order to tackle this goals, this project is studying the evolutionary steps that led to the highly evolved chlorophylls and analogues, and assessing the feasibility or likelihood to trigger photosynthetic activity in an exoplanetary system.

Based on the Darwinian theory of common ancestors, the first (photosynthetic) organism should have had simple oligopeptides, oligonucleotides and alkyl amphiphilic hydrocarbons as primeval membranes. Therefore, it should have had simple pigments. We propose that there could exist geochemical conditions allowing the abiotic formation of a simple pigment which might become sufficiently abundant in the environment of an exoplanet. Besides, we show that the proposed pigment could also be a precursor of the more evolved pigments known today on Earth by proposing, for the first time, an abiotic chemical route leading to tetrapyrroles not involving pyrrole derivatives.

Juan García de la Concepcióna,* Pablo Marcos-Arenala, Luis Cerdánb, Mercedes Burillo-Villalobosc, Nuria Fonseca-Bonillaa,María-Ángeles López-Cayuelad, José A. Caballeroe, and Felipe Gómez Gómeza

aCentro de Astrobiología (CSIC-INTA), Ctra. de Ajalvir km. 4, Torrejón de Ardoz, 28850 Madrid, Spain; bInstituto de Ciencia Molecular (ICMoL), Universidad de Valencia, 46071 Valencia, Spain.;cInstituto Nacional de Técnica Aeroespacial, 28850 Torrejón de Ardoz, Madrid, Spain.; dÁrea de Investigación e Instrumentación Atmosférica,Instituto Nacional de Técnica Aeroespacial, 28850 Torrejón de Ardoz, Madrid, Spain.; eCentro de Astrobiología (CSIC-INTA), ESAC, camino bajo del castillo, 28691 Villanueva de la Cañada, Madrid, Spain

How to cite: García de la Concepción, J. and Marcos-Arenal, P.: ExoPhot: Phot0, a plausible primeval pigment on Earth and rocky exoplanets, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13571, https://doi.org/10.5194/egusphere-egu22-13571, 2022.

The Earth is cooling down and its surface heat flux is the highest among all the terrestrial planet of the Solar System. The total heat loss (Q) is due to the energy released by the secular cooling of our planet (C) and of the radiogenic heat (H) produced by the radioactive decays of the radioelements contained therein. Can geoneutrino disentangle these two contributions?

Since while decaying, the uranium, thorium and potassium radioisotopes contained in the Earth release geoneutrinos in a well-fixed ratio, we can attempt to answer affirmatively to this question. Indeed, geoneutrinos are able to pass through most matter without interacting, so they can bring to surface useful information about the Earth’ deep interior. Concretely, measuring the geoneutrino flux at surface hence translates in estimating H and in turn constraining C once that Q is known.

The only two experiments which collected data in the last 15 years are KamLAND (Japan) and Borexino (Italy). By combining theoretical models and experimental flux with a sophisticated analysis, we inferred valuable insights on mantle radioactivity and of contribution of H to the Earth’s energy budget. We estimated a total radiogenic heat accounting for H = 20.8^+7.3_-7.9 TW and, by subtracting this value from the total heat power of the Earth, we derived a secular cooling C = 26 ± 8 TW. The obtained results are discussed and framed in the puzzle of the diverse classes of formulated Bulk Silicate Earth models, analyzing their implications on planetary heat budget and composition.

The effectiveness in investigating deep earth radioactivity demonstrated by geoneutrino studies confer them a prestigious role in the comprehension of geodynamical processes of our planet.

How to cite: Strati, V., Bellini, G., Inoue, K., Mantovani, F., Serafini, A., and Watanabe, H.: Studying the Earth’s heat budget with geoneutrinos, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-490, https://doi.org/10.5194/egusphere-egu22-490, 2022.

            Ferropericlase is the second most abundant phase of Earth’s lower mantle and is also considered to be one of the main constituents of the mantles of super-Earth exoplanets. Since ferropericlase is more ductile compared to silicates (Girard et al. 2016), it is expected to control the rheological behavior of mantle aggregates which governs solid-state convection of planetary mantles. The mechanical behavior of polycrystalline aggregates is strongly affected by the presence of grain boundaries. Despite previous work on MgO grain boundaries (e.g. Verma & Karki 2010; Hirel et al. 2019), little is yet known about the properties and mobility of ferropericlase grain boundaries at pressure conditions of deep planetary interiors.

            In this study, we carried out atomistic simulations based on the density functional theory to model the structures, energies and spin states of iron of a series of [001] symmetrical tilt grain boundaries in ferropericlase as a function of pressure. Based on these results, we investigated the mechanical behavior of the Σ5 tilt grain boundary by applying simple shear increments to the simulation cell to trigger grain boundary migration. Here, we will present the different mechanisms of grain boundary migration and the evolution of the ideal shear strengths up to a pressure of 400 GPa. Our results show that the mechanical strength of the grain boundaries and the directionality of their motion strongly varies with increasing pressure. Especially at pressure conditions of super-Earth exoplanets, significant grain boundary weakening is observed with increasing depth.  Implications for the deformation of ferropericlase at conditions of Earth’s and super-Earth’s mantles will be finally discussed.

How to cite: Ritterbex, S. and Tsuchiya, T.: Ab initio investigation of the intercrystalline mechanical behavior of ferropericlase at extreme pressures of planetary mantles, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1140, https://doi.org/10.5194/egusphere-egu22-1140, 2022.

In the early solar system radiogenic heating by ²⁶Al and collisions are the two prominent ways expected to modify the internal composition of icy planetesimals, building blocks of comets, by removing highly volatile compounds like CO, CO₂ and NH₃. However, observations indicate that even large comets like Hale-Bopp (R ≈ 35 km) can be rich in these highly volatile compounds [1].
Here we constrain under which conditions icy planetesimals experiencing both internal heating and collisions can retain pristine interiors [2]. For this purpose, we employ both the state-of-the-art finite difference marker-in-cell code I2ELVIS [3] to model the thermal evolution in 2D infinite cylinder geometry and a 3D SPH code [4] to study the interior heating caused by collisions among icy planetesimals. For simplicity we assume circular porous icy planetesimals with a low density (≈ 470 kg/m³) based on measurements for comet 67P/Churyumov-Gerasimenko [5].
For the parameter study of the thermal history we vary (i) icy planetesimal radii, (ii) formation time and the (iii) the silicate/ice ratio. For the latter we keep the mean density fixed and change the porosity of the icy planetesimal. For the impact models we use porous, low-strength objects and vary (i) target and (ii) projectile radii, (iii) impact velocity as well as (iv) impact angle. Potential losses of volatile compounds from their interiors are calculated based on their critical temperatures taken from literature [6]. Our combined results indicate that only small or late-formed icy planetesimals remain mostly pristine, while early formed objects can even reach temperatures high enough to melt the water ice.

REFERENCES
[1] Morbidelli & Nesvorný, In: The Trans-Neptunian Solar System. 25–59 (2019). [2] Golabek & Jutzi, Icarus 363, 114437 (2021). [3] Gerya & Yuen, Phys. Earth Planet. Int. 163, 83-105 (2007). [4] Jutzi, Planet. Space Sci. 107, 3–9 (2015). [5] Sierks et al., Science 347, 1044 (2015). [6] Davidsson et al., Astron. Astrophys. 592, A63 (2016).

How to cite: Golabek, G. and Jutzi, M.: Modification of icy planetesimal interiors by early thermal evolution and collisions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1433, https://doi.org/10.5194/egusphere-egu22-1433, 2022.

According to the canonical model, the Moon was formed in the aftermath of a giant impact, when the proto-Earth was struck by a Mars-size impactor leading to a debris disk from which the Moon accreted. This event is thought to have been sufficiently energetic to cause wholesale melting of the Moon. Solidification of the resulting Lunar Magma Ocean (LMO) involves plagioclase flotation and formation of an anorthositic crust that blankets the residual LMO. This crust may form directly through plagioclase flotation or involve more complex reprocessing mechanisms. Extensive fractional crystallization of the LMO likely led to formation of a residual KREEP component in the crust, enriched in K, REE, P and other incompatible elements relative to the bulk Moon, whose signature has been recognized in several lunar samples (e.g.  feldspathic basalt).

The experimentally-constrained liquid lines of descent of a range of plausible LMO compositions bear strong resemblances to one another, crystallizing in the sequence olivine -> opx -> cpx + plagioclase -> quartz + Fe-Ti oxide. Crystallisation of olivine ± orthopyroxene prevails, depending on the composition, between 61-77 PCS (percent solidified), followed by the concomitant appearance of plagioclase + cpx at 1230±30 ^oC. Crystallisation of plagioclase marks the point at which the crystallisation sequences diverge owing to differences in bulk composition (e.g. refractory element content), which in turn influence phase saturation. Existing experiments on liquid lines of descent lack resolution, in particular at the point of quartz and Fe-Ti oxide saturation. Moreover, these experiments rarely proceed to the extent required to produce a KREEP component. In this work, we aim to more precisely determine the phase relations during crystallisation of the uppermost LMO, and assess possible mechanisms of formation of the KREEP component.

An isobaric series (8 - 5kbar) of six experiments on the bulk silicate Moon composition of O’Neill (1991) yields a crystallization sequence beginning at 1250 ^oC with olivine ± opx ± Cr-sp (69 PCS), followed by plagioclase and clinopyroxene at 1200 ^oC (77 PCS). Our mineral and melt major and trace-element abundances constrain the terminal stages of LMO crystallisation. Melt compositions remain near 45 wt% SiO₂ during the final crystallization stage while FeO increases from 12 wt% (bulk) to 20 wt% at plagioclase saturation. The Al₂O₃ and CaO budget is controlled by plagioclase crystallization (but not cpx) as the An# is as high as 97. We report mineral/melt partitioning coefficients for La, Ce, Nd, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Y, Zr, Th and U for plagioclase, pigeonite and high-Ca clinopyroxene and use the lattice strain model to evaluate these, also in the context of literature data. These partition coefficients are therefore the most suitable for understanding the origin of the KREEP component.  

Preliminary results suggest KREEP forms only after 99 PCS due to the evolved melt and the relatively rapid cooling rate of the surface magma ocean once crystal fraction is high. The last stage of eutectic crystallisation should lead to gabbroic rocks as the final crystallisation product.  

How to cite: Ofierska, W., Schmidt, M., Sossi, P., and Liebske, C.: Crystallisation of the upper lunar magma ocean and implications for KREEP and crust formation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1746, https://doi.org/10.5194/egusphere-egu22-1746, 2022.

Planetary differentiation in small bodies is believed to be ruled by several partial end-states that were dominated by low degrees of partial melting and melt segregation, before arriving at the formation of rocky planets. Having a better understanding of non-equilibrium melting processes in undifferentiated chondritic materials is critical to characterize planetary differentiation processes and the formation of rocky planets and differentiated asteroids. In this context, partial melting experiments of natural chondrites can provide unique insights into the petrological evolution associated with early planetary differentiation of planetesimals. For this study, we performed partial melting experiments using fragments from the ordinary chondrite DAV01001. Experiments were performed in a piston-cylinder at 1 GPa pressure, at temperatures from 1100 to 1300 °C and for 24 hours run duration. Reducing conditions were imposed by the use of graphite capsules. The experimental products were analysed using electron microprobe and synchrotron radiation computed microtomography (SR-µCT).

DAV01001 is an equilibrated L6 ordinary chondrite that has still visible relic chondrules and contains olivine (Fo₇₅), low-Ca pyroxene (En₇₇Fs₂₁Wo₂), high-Ca pyroxene (En₄₇Fs₈Wo₄₅), albitic plagioclase (An₁₃Ab₈₁Or₆), metal, troilite, chromite, and apatite. Upon heating, metal and troilite disappear at 1100 °C forming two immiscible phases, one made of pure metal with variable amounts of Ni, the other made of a metal-sulphide liquid of variable composition. Chromite starts melting at 1100 °C and disappears at 1300 °C. Silicatic melt forms already at 1100 °C as a result of the melting of plagioclase. With increasing temperature, the pyroxene and olivine begin to melt and shift the composition of the liquid towards trachy-andesitic (1200 °C) and basaltic trachy-andesitic to andesitic (1300 °C) compositions. Melting of olivine and pyroxene is accompanied by the crystallisation of both phases. The newly-formed olivine has a composition varying from Fo₈₀ to Fo₅₉, becoming progressively enriched in Fe and Ca and depleted in Ni at increasing temperature. The newly-formed pyroxene has a variable Ca content, and is enriched in Al and Cr and depleted in Fe and Mn. The new-grown olivine and pyroxene crystals have a strong affinity with chondritic/primitive achondrites compositions, in contrast to the melts that have a good affinity to a bulk HED composition. Overall, the combination of melting and crystallisation fixes the amount of silicatic liquid to a rather constant value of 10% vol.

SR-µCT was used to create 3D reconstructions of the experimental samples, in order to evaluate the efficiency of metal segregation at increasing degrees of partial melting. At increasing temperature, no change in the object density (number of 3D particles divided by the sample volume) is observed but only a progressive increase of the roundness and sphericity of the particles. This suggests that, even in presence of an interconnected liquid silicate phase (~10% vol), the coalescence of the metal phases does not occur spontaneously and other forces such as rotational spin or deformation are needed to segregate metal under these conditions.

How to cite: Iannini Lelarge, S., Masotta, M., Folco, L., Mancini, L., and Pittarello, L.: Non-equilibrium melting of partially differentiated asteroids: insights from partial melting experiments on L6 chondrite DAV01001, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3089, https://doi.org/10.5194/egusphere-egu22-3089, 2022.

• The Apollo lunar samples reveal that Earth and the Moon have strikingly similar isotopic ratios, suggesting that these bodies may share the same source materials. This leads to the "standard" giant impact hypothesis, suggesting the Moon formed from a partially vaporized disk that was generated by an impact between the proto-Earth and a Mars-sized impactor. This disk would have had high temperature (~ 4000 K) and vapor mass fraction of ~20 wt %. However, impact simulations indicate that this model does not mix the two bodies well, making it challenging to explain the isotopic similarity. In contrast, more energetic impacts, such as a collision between two half Earth-sized objects, could mix the two bodies well, naturally solving the problem. These impacts would produce much higher disk temperatures (6000-7000K) and higher vapor mass fractions (~80-90 wt%). These energetic models, however, may have a challenge during the Moon accretion phase. Our analyses suggest that km-sized moonlets, which are building blocks of the Moon, would experience strong gas drag from the vapor portion of the disk and fall onto Earth on a very short timescale. This problem could be avoided if large moonlets (>1000 km) form very quickly by the process called streaming instability, which is a large clump formation mechanism due to spontaneous concentration of dust particles followed by gravitational collapse. We investigate this possibility by conducting numerical simulations with the code called Athena. Our 2D and 3D hydrodynamic simulations show that moonlet formation by streaming instability is possible in the Moon-forming disk, but their maximum size is approximately 50 km, which is not large enough to avoid the strong gas drag. This result supports the Moon formation models that produce vapor-poor disks, such as the standard model. We will further discuss implications for moons in the solar system and extrasolar systems (exomoons). 

How to cite: Nakajima, M., Atkins, J., Simon, J. B., and Quillen, A. C.: Moon Formation via Streaming Instability, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3311, https://doi.org/10.5194/egusphere-egu22-3311, 2022.

During the differentiation of terrestrial planets, the metal phase from the impactor core segregates from the silicate phase of the magma ocean. This buoyant mass forms a turbulent thermal and settles toward the proto-core. During this descent, thermal and chemical exchange occurs at the boundary between the metallic and silicate phases. Based on laboratory fluid dynamic experiments mimicking the settling of the metallic thermal turbulent, we develop a Lagrangian approach of the mixing from the experimental velocity field. We are able to track the evolution of the material elongated as lamellae by the turbulent stirring. We have characterised the elongation rate, the aggregation of lamellae, and the probability density function of the elongation and concentration, which are not accessible from direct measurements in the experiments. We have also investigated the effect of the Reynolds number and density ratio on these quantities. These results will allow us to develop a new predictive model of the mixing and chemical transfer in thermal turbulent to better understand the equilibrium between metals and silicates during the accretion of terrestrial planets.

How to cite: Huguet, L. and Deguen, R.: Lagrangian approach of the mixing in a turbulent thermal, and implications for metal-silicate equilibrium during Earth's formation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3776, https://doi.org/10.5194/egusphere-egu22-3776, 2022.

Accurate measurements of a planet's mass, radius and age (provided for example by the PLATO mission and follow-up measurements) together with compositional constraints from the stellar spectrum can help us to deduce potential evolutionary pathways that rocky planets can evolve along, and allow us to predict the range of likely atmospheric properties that can then be compared to observations.

However, for the evolution of composition and mass of an atmosphere, a large degeneracy exists due to several planetary and exterior factors and processes, making it very difficult to link the interior (and hence outgassing processes) of a planet to its atmosphere. The community therefore thrives now to identify the key factors that impact an atmosphere, and that may lead to distinguishable traces in planetary, secondary outgassed atmospheres. Such key factors are for example the planetary mass (impacting atmospheric erosion processes) or surface temperature (impacting atmospheric chemistry, weathering and interior-atmosphere interactions).

Here we investigate the signature that a planet evolving into plate tectonics leaves in its atmophere due to its impact on volcanic outgassing fluxes and volatile releases to the atmosphere - leading possibly to distinguishable sets of atmospheric compositions for stagnant-lid planets and plate tectonics planets. These preliminary findings will need to be further investigated with coupled atmosphere-interior models including various feedback mechanisms such as condensation and weathering as well as atmospheric escape to space.

How to cite: Noack, L. and Brachmann, C.: Is planetary resurfacing a key factor for outgassing and gas speciation on rocky planets?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4128, https://doi.org/10.5194/egusphere-egu22-4128, 2022.

Both the Moon¹ and Mars² are known to have significant degree-1 variations in their crustal thicknesses, with the Moon's far side and Mars's southern hemisphere having far thicker crusts than their respective opposing hemispheres. A number of potential mechanisms have been proposed to explain these dichotomies, including large impacts in both cases^3,4, radiant heat from the Earth⁵ (in the case of the Moon), and large-scale volcanism⁶ (in the case of Mars). However, the effectiveness of these mechanisms are limited by the difficulty of sustaining a large hemispheric difference during the tens to hundreds of Ma of crustal formation. Both planets' lithospheres are examples of a fluid-dynamical boundary layer known as a stagnant lid, caused by temperature-dependent viscosity in a convecting system. We consider the effect of pressure on the viscosity of magma oceans and mantles, finding that under certain circumstances a spherically-symmetric stagnant lid is linearly unstable to asymmetric perturbations. The fastest-growing wavenumbers of this instability is degree 1, meaning that a small initial asymmetry may grow into a full-scale hemispherical dichotomy. We then numerically examine the stability of these asymmetric states, finding that they may last for hundreds of Ma. We also compare to the case of Mercury, a similarly-sized planet with no such crustal dichotomy, to determine if our analysis matches observations.

¹ Wieczorek, M.A., Jolliff, B.L., Khan, A., Pritchard, M.E., Weiss, B.P., Williams, J.G., Hood, L.L., Righter, K., Neal, C.R., Shearer, C.K., McCallum, I.S., Tompkins, S., Hawke, B.R., Peterson, C., Gillis, J.J. & Bussey, B. 2006 The Constitution and Structure of the Lunar Interior. Reviews in Mineralogy and Geochemistry 60, 221–364.

² Thiriet, M., Michaut, C., Breuer, D. & Plesa, A.-C. 2018 Hemispheric dichotomy in lithosphere thickness on mars caused by differences in crustal structure and composition. Journal of Geophysical Research: Planets 123 (4), 823–848.
Weiss, Benjamin P. & Tikoo, Sonia M. 2014 The lunar dynamo. Science 346 (6214), 1198

³ Garrick-Bethell, I., Perera, V., Nimmo, F. & Zuber, M.T. 2014 The tidal-rotational shape of the Moon and evidence for polar wander. Nature 512 (7513), 181–184.

⁴ Andrews-Hanna, J.C., Zuber, M.T. & Banerdt, W.B. 2008 The borealis basin and the origin of the martian crustal dichotomy. Nature 453 (7199), 1212–1215.

⁵ Roy, A., Wright, J.T. & Sigurðsson, S. 2014 Earthshine on a young moon: Explaining the lunar farside highlands. The Astrophysical Journal Letters 788 (2), L42.

⁶ Golabek, G.J., Keller, T., Gerya, T.V., Zhu, G., Tackley, P.J. & Connolly, J.A.D. 2011 Origin of the martian dichotomy and tharsis from a giant impact causing massive magmatism. Icarus 215 (1), 346–357.

How to cite: Watson, C., Neufeld, J., and Michaut, C.: Asymmetric growth of planetary stagnant lids, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4758, https://doi.org/10.5194/egusphere-egu22-4758, 2022.

The application of the short-lived radiogenic ¹⁸²Hf/¹⁸²W-system (t_1/2 = 8.9 Ma [1]) is a good approach to study early differentiation processes or potential involvement of long-term isolated and/or core-influenced mantle domains as components for ocean island basalts (OIB) [2,3].

Several examples of OIB worldwide (e.g., Hawaii, Samoa and Iceland) exhibit a negative He-W correlation [2], possibly connected to the incorporation of primordial material characterized by high ³He/⁴He ratios and negative µ¹⁸²W (¹⁸²W/¹⁸⁴W deviation of a sample from laboratory standards in parts per million). Anomalous W isotope compositions in combination with elevated ³He/⁴He ratios have previously been connected to seismically anomalous structures in the lowermost mantle, so-called “(mega) ultra-low velocity zones” [3]. Recently, such a structure was discovered beneath the Marquesas Archipelago [4]. This volcanic island chain is located in the South Pacific, in proximity of the Marquesas Fraction Zone. Its formation process is not yet fully understood. Based on high ³He/⁴He ratios in combination with other geochemical characteristics, such as Sr, Nd and Pb isotopes, a deep-lying mantle source has been suggested [5].

In this study, we have analysed seven samples from two islands of the Marquesas Archipelago, which exhibit ³He/⁴He ratios up to 14.4 Ra [5]. µ¹⁸²W ranges from -3.6 ±3.1 to 4.7 ±8.5. Hence, despite elevated ³He/⁴He in some of the samples, none of them display resolved negative ¹⁸²W anomalies and thus, no negative He-W correlation is observed. Interpretations for the decoupling of He-W systematics in samples from the Marquesas Archipelago will be discussed.

References:

[1] Vockenhuber et al., 2004, Phys. Rev. Lett.

[2] Mundl et al., 2017, Science

[3] Mundl-Petermeier et al., 2020, Geochim. Cosmochim. Acta

[4] Kim et al., 2020, Science

[5] Castillo et al., 2007, Chem. Geol.

How to cite: Herret, M.-T., Mundl-Petermeier, A., Castillo, P., and Kim, D.: Tungsten isotope implications for the source of ocean island basalts from the Marquesas Archipelago, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4890, https://doi.org/10.5194/egusphere-egu22-4890, 2022.

The position of the critical point determines the top of the liquid-vapor coexistence dome, and is a physical parameter of fundamental importance in the study of high-energy shocks, including those associated with large planetary impacts. For most major planetary materials, like oxides and silicates, the estimated position of the critical point is below 1 g/cm³ at temperatures above 5000 K. Here we compute the position of the critical point of one of the most ubiquitous materials: MgO. For this, we perform first-principles molecular dynamics simulations. We find the critical density to be in the 0.4 - 0.6 g/cm³ range and the critical temperature in the 6500 - 7000 K range. We investigate in detail the behavior of MgO in the subcritical and supercritical regimes and provide insight into the structure and chemical speciation. We see a change in Mg-O speciation towards lower degrees of coordination as the temperature is increased from 4000 K to 10000 K. This change in speciation is less pronounced at higher densities. We observe the liquid-gas separation in nucleating nano-bubbles at densities below the liquid spinodal. The majority of the chemical species forming the incipient gas-phase consist of isolated Mg and O atoms and some MgO and O₂ molecules. We find that the ionization state of the atoms in the liquid phase is close to the nominal charge, but it almost vanishes close to the liquid-gas boundary and in the gas phase, which is consequently largely atomic.

This research was supported by the European Research Council under EU Horizon 2020 research and innovation program (grant agreement 681818–IMPACT to RC). This research was performed by access to supercomputing facilities via eDARI stl2816 grants, PRACE RA4947 grant, Uninet2 NN9697K grant.

How to cite: Bögels, T. and Caracas, R.: The critical point and the supercritical regime of MgO, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5002, https://doi.org/10.5194/egusphere-egu22-5002, 2022.

How volatiles were incorporated in the building blocks of planets and small bodies in the protosolar nebula remains an outstanding question. Some scenarios invoke the formation of planetesimals from a mixture of refractory material and amorphous ice in the outer nebula while others argue that volatiles have been incorporated in clathrate or pure condensate forms in these solids. Here we study the fate of volatiles (H₂O, CO, CO₂, CH₄, H₂S, N₂, NH₃, Ar, Kr, Xe, and PH₃) initially delivered in the forms of amorphous ice or pure condensates to the protosolar nebula. We investigate the radial distribution of these volatiles via a transport module coupled with an accretion disk model. In this model, multiple icelines are considered, including the condensation fronts of pure condensates, as well as those of clathrates when enough crystalline water is available at given time and location. Our simulations show that a significant fraction of volatiles can be trapped in clathrates only if they have been initially delivered in pure condensate forms to the disk. Under specific circumstances, volatiles can be essentially trapped in clathrates but, in many cases, the clathrate of a given species coexists with its pure condensate form. Those findings have implications for the compositions of giant planets and comets.

How to cite: Schneeberger, A., Mousis, O., Aguichine, A., and Lunine, J.: Evolution of the reservoir of volatiles in the protosolar nebula, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5270, https://doi.org/10.5194/egusphere-egu22-5270, 2022.

The radiogenic elements K, Th, and U are large contributors to the heating inside a terrestrial planet. Because they act incompatible in solid mantle rocks, they prefer to gather in partial melt, which is generally less dense than the surrounding material and rises upwards. While rising, the melt transports the radiogenic heat sources and other incompatible elements towards the surface, where over time they accumulate inside the crust. The amount of the transported incompatible elements is heavily dependent on their degree of incompatibility in mantle rocks and therefore their mineral/melt partition coefficients. Despite the fact that partition coefficients can change by multiple orders of magnitudes from 0-15 GPa along a peridotite solidus (Schmidt and Noack, 2021), they were generally taken as constant in mantle evolution models due to a lack of high-pressure models and experimental data.

Based on the thermodynamic approach of Blundy et al. (1995), Schmidt and Noack (2021) modelled partition coefficients for sodium in clinopyroxene/melt from 0-15 GPa. As sodium has a very low strain in the M2 lattice site of clinopyroxene and is therefore very compatible, its partition coefficients can act as a reference to model the other elements from. In this study, we take the approach of Schmidt and Noack (2021) to model the partition coefficients of the above-mentioned heat producing elements and volatiles at local P-T conditions for partial melting events inside the mantle of terrestrial planets. We insert local bulk partition coefficients for an adequate mantle rock composition into a 1D interior evolution model of Mars. By comparing the results of the redistribution to models with constant partition coefficients, we can assess the impact of the locally calculated partition coefficients on the accuracy of models which deal with the thermal evolution of a planet and the enrichment of heat producing elements and volatiles inside the crust.

Blundy, J. et al. (1995): Sodium partitioning between clinopyroxene and silicate melts, J. Geophys. Res., 100, 15501-15515.

Schmidt, J.M. and Noack, L. (2021): Clinopyroxene/Melt Partitioning: Models for Higher Upper Mantle Pressures Applied to Sodium and Potassium, SysMea, vol 13 nr 3&4, to be published.

How to cite: Schmidt, J. M. and Noack, L.: Applying locally calculated partition coefficients for radiogenic heat sources and volatiles to interior evolution models of terrestrial planets, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5850, https://doi.org/10.5194/egusphere-egu22-5850, 2022.

Every planet is singular, with scars and bumps at their surface. One planet, one history. But the physics at play is common to them, connecting planetary bodies together. Tectonics is a common theme of what we can observe on planets of the solar system, and a central question for explanets. More than 20 years of geodynamic modelling has resulted in  identifying a diversity of tectonic regimes for mantle convection, from very active, like heat-pipe (Monnereau and Dubuffet, 2002 among others) and squishy lid (Lourenço et al., 2020) to almost inert, like stagnant lid (Schmeling and Jacoby, 1982). Tectonics is an emergent property deriving from the intimate structure and composition of a planet. It is also a fundamental piece shaping the surface environment. This presentation will attempt to give an overview of tectonic regimes of planets and propose typical evolutional scenari, connecting structural and compositional histories from the depth to the surface.

References

Lourenço, D. L., Rozel, A. B., Ballmer, M. D., & Tackley, P. J. (2020). Plutonic‐squishy lid: A new global tectonic regime generated by intrusive magmatism on earth‐like planets. Geochemistry, Geophysics, Geosystems, 21, e2019GC008756.

Monnereau, M., & Dubuffet, F. (2002). Is Io's mantle really molten?. Icarus, 158, 450-459.

Schmeling, H., & Jacoby, W. R. (1982). On modelling the lithosphere in mantle convection with non-linear rheology. Journal of Geophysics, 50, 89-100.

How to cite: Coltice, N.: An overview of modeled dynamic histories of rocky planets, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6028, https://doi.org/10.5194/egusphere-egu22-6028, 2022.

Introduction: Hydrogen isotopic compositions (D/H or 𝛿D) in chondrites are a powerful tool for deciphering the source of water delivered to terrestrial planets (1). CM-type carbonaceous chondrites contain up to ~10wt.% H₂O, retained as OH in phyllosilicates. The D/H ratio of phyllosilicates (a direct proxy for water) in chondrites cannot be determined directly using whole rock measurements, because their matrices also accreted D-rich organics which are mixed with D-poor phyllosilicates at the sub-micrometer scale. To address this issue, water D/H has been estimated by in-situ measurements of both D/H and C/H in hydrated chondrites, which define a mixing line in a D/H vs. C/H plot. The intercept gives the isotopic composition of the phyllosilicate alone (1). However, SIMS measurements of water D/H using this method can be compromised by (i) contamination and (ii) limited dispersion of the phyllosilicates/organics ratio measured with a large primary beam.

Methods: We addressed both issues using the Wash U NanoSIMS50 which allows us to obtain coordinated isotopic and elemental data with high-spatial resolution. H⁻,D⁻ with ¹²C⁻,¹²C¹⁴N⁻,¹²C¹⁵N⁻,²⁸Si⁻ are collected using magnetic-field peak-jumping in “Combined Analysis” mode. Centering of the secondary ions beam in Cy and P2/P3 planes of the secondary column changes between the low and high masses, resulting in misaligned ion images. So, we used AutoHotkey scripts to send a different Cy voltage for every B-field set up through the virtual keyboard of the NanoSIMS. To separate phyllosilicate-rich from organic-rich pixels, we assume that D/H is not simply a linear function of C/H, but in general D/H is approximated by a function using all measured species: . The true phyllosilicate composition [C,N,Si,H] is estimated from the data and is then used to estimate the water D/H composition from the linear regression model. NanoSIMS isotopic analyses were carried out in a matrix area of the CM Maribo and our analytical conditions were the same as outlined in (2).

Results: First, we calculated a 𝛿D value of −178±46‰ (2σ) for the phyllosilicates in Maribo using the D/H vs. C/H correlation from the resized pixels. This value is higher than previous measurements using SIMS [𝛿D ≈ −420 to −270‰, (2, 3)], demonstrating that D/H ratio of phyllosilicate cannot be simply determined using the D/H vs. C/H line in this matrix area. Second, we calculated the 𝛿D value of the phyllosilicates in Maribo using all the measured species and the linear regression model described above. We found that the phyllosilicate D/H is best correlated for dominant contributions of N, Si and H (b=0.14, c=0.58 and d=−0.86) and minor contributions of C (a=0.06). We calculated a 𝛿D value of −286+/-60‰. This value is consistent with those previously determined by SIMS, demonstrating that our method can be used to precisely determine the water D/H on very small areas.

(1) Alexander C.M.O’D. et al. (2012) Science, 337, 721–723.

(2) Vacher L.G. and Ogliore R.C. (2022) 53rd LPSC, 2653.

(3) van Kooten E.M.M.E. et al. (2018) GCA, 237, 79–102.

(4) Piani L. et al. (2021) EPSL, 567, 117008.

How to cite: Vacher, L. G. and Ogliore, R. C.: Determination of Water D/H in Hydrated Chondrites using NanoSIMS Imaging, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6584, https://doi.org/10.5194/egusphere-egu22-6584, 2022.

The Martian dichotomy features a ~25 km difference in crustal thickness and ~5 km contrast in topography between the southern highlands and northern lowlands [1]. Among various origin hypothesis, a southern impact [2,3] creates a magma pond which, upon cooling, induces crustal thickening and thereby forms the crustal dichotomy within 10s of million years.

Our previous study [4], which utilizes a head-on parametrized impact in 2D geometry, shows that an impact-induced magma pond in the southern hemisphere is able to not only create a thickened crust in the south, but also a satisfying volcanic history with localized melt production in the equatorial region at geologically recent time.  Depleted material, formed from crystallization of the magma pond, spreads and underplates the thicker and colder Northern lithosphere undisturbed by the impact, reinforcing the lesser extent of volcanism in the northern hemisphere. Our resultant mantle structure is consistent with existing simulation efforts that focus on the post-dichotomy formation evolution history [5], and in addition gives the context of how such thermochemical structure is developed.

In order to include a more realistic impact scenario, we use smoothed particle hydrodynamics (SPH) simulations [6] to model the first 24-36 hours of a giant impact between proto-Mars and its impactor. The SPH result is then transferred to the mantle convection code StagYY [7], as an initial thermal condition, to simulate the long-term evolution of the crust and mantle for the subsequent 4.5 billion years. We systematically vary the impactor size, impact velocity and pre-impact Martian mantle temperature. Our preliminary results show that a 45-degree impact does not form a Martian dichotomy-like crustal structure, while a 15-degree impact is a better match.  With a realistic impact, the mechanisms reported in our parametrized impact study still hold.

References:

[1] Watters, T., McGovern, P., & Irwin III, R. (2007). Hemispheres Apart: The Crustal Dichotomy on Mars. Annual Review Of Earth And Planetary Sciences, 35(1), 621-652.

[2] Reese, C., Orth, C., & Solomatov, V. (2011). Impact megadomes and the origin of the martian crustal dichotomy. Icarus, 213(2), 433-442.

[3] Golabek, G., Keller, T., Gerya, T., Zhu, G., Tackley, P., & Connolly, J. (2011). Origin of the martian dichotomy and Tharsis from a giant impact causing massive magmatism. Icarus, 215(1), 346-357.

[4] Cheng, K.W., Tackley, P.J., Rozel, A.B., Golabek, G.J. (2021). Martian Dichotomy: Impact-induced Crustal Production in Mantle Convection Models, Abstract [DI35B-0023] presented at 2021 Fall Meeting, AGU, New Orleans, LA, 13-17 Dec.

[5] Plesa, A., Padovan, S., Tosi, N., Breuer, D., Grott, M., & Wieczorek, M. et al. (2018). The Thermal State and Interior Structure of Mars. Geophysical Research Letters, 45(22), 12,198-12,209.

[6] Emsenhuber, A., Jutzi, M., Benz, W. (2018). SPH calculations of Mars-scale collisions: The role of the equation of state, material rheologies, and numerical effects. Icarus, 301, 247-257

[7] Tackley, P. (2008). Modelling compressible mantle convection with large viscosity contrasts in a three-dimensional spherical shell using the yin-yang grid. Physics Of The Earth And Planetary Interiors, 171(1-4), 7-18.

How to cite: Cheng, K. W., Rozel, A., Ballantyne, H., Jutzi, M., Golabek, G., and Tackley, P.: Forming the Martian dichotomy with realistic impact scenarios, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6591, https://doi.org/10.5194/egusphere-egu22-6591, 2022.

      The crystallization of the Basal Magma Ocean (BMO) sets the stage for the long-term evolution of terrestrial planets and may leave behind large-scale thermochemical structures in the lower mantle. Previous work shows that a FeO-enriched molten layer or basal magma ocean (BMO) is stabilized at the core-mantle boundary of large rocky planets such as Earth for a few billion years. The BMO itself is expected to freeze by fractional crystallization (FC) because it cools very slowly. However, the fate of BMO cumulates has not yet been systemically explored.

To explore the fate of the BMO cumulates in the convecting mantle, we explore 2D geodynamic models with a moving-boundary approach. Flow in the mantle is explicitly solved, but the thermal evolution and related crystallization of the successively crystallizing BMO (i.e., below the moving boundary) are fully parameterized. The composition of the crystallizing cumulates is self-consistently calculated in the FeO-MgO-SiO₂ ternary system according to Boukaré et al. (2015). In some cases, we also consider the effects of Al₂O₃ on the cumulate density profile. We then investigate the entrainment and mixing of BMO cumulates by solid-state mantle convection over billions of years as a function of BMO initial composition and volume, BMO crystallization timescales, distribution of internal heat sources, and mantle rheological parameters (Rayleigh Number and activation energy). We vary the initial composition of BMO by manipulating the bulk molar fraction of FeO, MgO, and SiO₂, e.g. considering BMO compositions such as pyrolite, lower-mantle partial melts of pyrolite (after 50% batch crystallization), or Archean Basalt.

For all our model cases, we find that most of the cumulates (first ~90% by mass) are efficiently entrained and mixed through the mantle. However, the final ~9% of the cumulates are too dense to be entrained (either fully or partially) over the age of the Earth, and rather remain at the base of the mantle as a strongly FeO-enriched solid layer. Unless the initial thickness of the BMO is ≤100 km, this strongly enriched and intrinsically dense layer should cover the CMB globally. We highlight that this outcome of BMO fractional crystallization is inconsistent with the geophysical constraints. Our results suggest that the BMO was either very small initially or did not crystallize by end-member FC. An alternative mode of crystallization may be driven by an efficient reaction between a highly-enriched last-stage BMO with the overlying mantle. Such reactive crystallization may be much faster than FC of the BMO, as it is driven by chemical disequilibrium instead of (slow) planetary cooling.

How to cite: Ismail, M. and Ballmer, M.: Fractional Crystallization Of The Basal Magma Ocean: Consequences For Present-day Mantle Structure, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6651, https://doi.org/10.5194/egusphere-egu22-6651, 2022.

The formation mechanism of Jupiter is still uncertain, as multiple volatile accretion scenarios can reproduce its metallicity [1-4]. The Galileo mission allowed in situ measurements of the abundances of several elements (Ar, Kr, Xe, C, N, S and P), which exhibit a uniform enrichment of 2 to 5 times the protosolar abundance, and a subsolar abundance has been measured for O. Recent measurements for N and O by the Juno mission confirmed the supersolar abundance of N, but indicated that the abundance of O may also be supersolar [5]. Elemental abundances measured in the Jupiter's atmosphere are key ingredients to trace the origin of various species.
Here, we investigate the possible timescale and location of Jupiter's formation using measurements of molecular and elemental abundances in its envelope. To do so, we use a 1D accretion disk model to compute the properties of the protosolar nebula (PSN) that includes radial transport of trace species, present in the form of refractory dust, a mixture of ices and their vapors, to compute the composition of the PSN [6]. We focus on the radial transport of volatile species by advection-diffusion combined with the effect of icelines, computed as sublimation/condensation rates. Initialy, the disk is uniformly filled with H₂O, PH₃, CO, CO₂, CH₄, CH₃OH, NH₃, N₂, H₂S, Ar, Kr and Xe [6,7], corresponding to the main bearers of C, N, O, P, S, Ar, Kr and Xe.
As the PSN evolves, solid particles drift inward due to gas drag. Volatile species are thus efficiently transported to their respective icelines, where they sublimate. This results in supersolar abundances of volatile elements in the inner part of the PSN. We find that the composition of Jupiter’s envelope can be achieved by accretion of enriched gas only, or a mixture of gas and solids, depending on the viscosity of the PSN. In both cases, the composition of the PSN matches the one measured in Jupiter’s envelope in timescale that are compatible with a formation by core accretion or gravitational collapse.

[1] Gautier, D., Hersant, F., Mousis, O., et al. 2001, ApJL, 550, L227.
[2] Mousis, O., Ronnet, T., and Lunine, J. I. 2019, ApJ, 875, 9.
[3] Öberg, K. I. and Wordsworth, R. 2019, AJ, 158, 194.
[4] Miguel, Y., Cridland, A., Ormel, C. W., et al. 2020, MNRAS, 491, 1998.
[5] Li, C., Ingersoll, A., Bolton, S., et al. 2020, Nature Astronomy, 4, 609.
[6] Aguichine, A., Mousis, O., Devouard, B., and Ronnet, T. 2020, ApJ, 901, 97.
[7] Lodders, K., Palme, H., & Gail, H.-P. 2009, Landolt Börnstein, 4B, 712

How to cite: Aguichine, A., Mousis, O., and Lunine, J.: Formation of Jupiter's envelope from supersolar gas in the protoplanetary disk, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6692, https://doi.org/10.5194/egusphere-egu22-6692, 2022.

How to cite: Spaargaren, R., Ballmer, M., Mojzsis, S., and Tackley, P.: The effects of terrestrial exoplanet bulk composition on long-term planetary evolution, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7484, https://doi.org/10.5194/egusphere-egu22-7484, 2022.

The Moon deforms in response to tidal forcing exerted by the Earth, the Sun and, to a lesser extent, by other planetary bodies. Their observations from ground-based and space-borne instruments, as well as Lunar surface missions, provide one of the most significant constraints that can be employed to unravel the deep interior (Williams et al. [2014], Williams and Boggs [2015]). The tidal forcing generates periodic variations of the harmonic degree-2 shape and gravity that depend on the internal composition and structure of the Moon. These changes in shape and gravity of the Moon are described by three geodetic parameters, called Tidal Love numbers (TLNs). Because of their low degree, these TLNs are sensitive to the structure of the deep interior (e.g., Khan et al. [2004]). Apart from the geodetic constraints, the Moon and Mars (e.g. Zweifel et al. [2021]) are the only other bodies besides the Earth for which seismic data are available. Seismic studies using the Apollo Passive Seismic Experiment (PSE) constrain the seismic wave velocity distribution and therefore give a glimpse of the Moon’s interior structure (Garcia et al. [2011], Weber et al. [2011]). However, at greater depth, seismic data do not provide sufficient resolution on the velocity profile, leaving the near-centre Moon structure uncertain. Other studies based upon geophysical constraints (Khan et al. [2004], Harada et al. [2014, 2016], Matsumoto et al. [2015]) and the re-analysis of the Apollo seismic data suggested the existence of an attenuated region called low viscosity zone (LVZ) originated from a melting layer at the core-mantle boundary (Khan and Mosegaard [2001], Weber et al. [2011], Harada et al. [2014], Rambaux et al. [2014]).

Based on geodetic observations and seismic studies, we perform Monte Carlo simulations for combinations of thicknesses, densities and viscosities for two classes of Moon’s models, one including an undifferentiated core and one including an inner and outer core, with both classes assuming an LVZ at the core-mantle boundary. By comparing predicted and observed tidal deformation parameters we find that the existence of an inner core cannot be ruled out. Furthermore, by deducing temperature profiles for the LVZ and the mantle following Earth assumptions, we obtain stringent constraints on the radius, viscosity, and density of the LVZ. We also infer the first estimation for the outer core viscosity, for our two possible scenarios.

How to cite: Briaud, A., Fienga, A., Melini, D., Rambaux, N., Mémin, A., Spada, G., Saliby, C., Hussmann, H., and Stark, A.: Constraints on the Moon’s deep interior from tidal deformation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7919, https://doi.org/10.5194/egusphere-egu22-7919, 2022.