GD – Geodynamics

GD2.1 – Multi-disciplinary perspectives on mantle-surface and plume-plate interactions through time and space

EGU22-3756 | Presentations | GD2.1 | Highlight

Large-scale drainage disequilibrium in central Australia

Gregory Ruetenik, John Jansen, Mike Sandiford, and Robert Moucha

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.

EGU22-354 | Presentations | GD2.1 | Highlight

Constraining Neogene Mantle Dynamics of Western Mediterranean Region Encompassing Iberia by Quantitative Modeling of Basalt Geochemistry

Chia-Yu Tien, Nicky White, John Maclennan, and Benedict Conway-Jones
Dynamic topography is the surface expression of sub-plate mantle convective processes. In recent years, there has been considerable interest in combining a wide range of geophysical, geological and geomorphic observations with a view to determining the amplitude, wavelength and depth of mantle thermal anomalies. Here, we are interested in exploring how quantitative modelling of major, trace and rare earth elements can be used to constrain the depth and degree of asthenospheric melting for a mantle peridotitic source. Our focus is on a region that encompasses the Iberian Peninsula where previous research suggests that long-wavelength topography is supported by a significant sub-plate thermal anomaly which is manifest by reduced shear-wave velocities. Stratigraphic and fluvial studies imply that this dynamic support is a Neogene phenomenon. We analyzed 48 Neogene basaltic rocks that were acquired from Iberia in September 2019 and combined these analyses with previously published datasets. Both major element thermobarometry and rare earth element inverse modelling are used to determine the asthenospheric potential temperature and lithospheric thickness. These values are compared with those estimated from calibrated shear-wave tomographic models. Our geochemical results indicate that potential temperatures and lithospheric thicknesses are 1300-1375 °C and 50-80 km, respectively. These values broadly agree with calibrated tomographic models which yield values of 1300-1360 °C and 45-70 km. We conclude that a region encompassing Iberia is dynamically supported by a combination of warm asthenosphere and thinned lithosphere.

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.

EGU22-13092 | Presentations | GD2.1

Dynamic topographic observations of Antarctica and its fringing oceanic basins

Aisling Dunn, Nicky White, Megan Holdt, and Robert Larter

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.

EGU22-373 | Presentations | GD2.1

Dynamic Topography of the Australian Continent and its Margins

Philippa Slay, Nicholas White, and Simon Stephenson

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.

EGU22-9038 | Presentations | GD2.1

Quaternary magmatism above a slab tear, Northern Andes of Colombia

Camilo Conde-Carvajal, Andreas Kammer, Michael Avila-Paez, Sofia Cubillos, Alejandro Piraquive, and Albrecht von Quadt

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.

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.

EGU22-226 | Presentations | GD2.1

Exhumation signals and forcing mechanisms in the Southern Patagonian Andes (Torres del Paine and Fitz Roy plutonic complexes)

Veleda Astarte Paiva Muller, Christian Sue, Pierre Valla, Pietro Sternai, Thibaud Simon-Labric, Joseph Martinod, Matias Ghiglione, Lukas Baumgartner, Frédéric Herman, Peter Reiners, Cécile Gautheron, Djordie Grujic, David Shuster, and Jean Braun

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.

EGU22-5992 | Presentations | GD2.1 | Highlight

Mantle dynamics and intraplate orogeny: The Atlas of Morocco

Riccardo Lanari, Claudio Faccenna, Claudio Natali, Ebru Sengul, Giuditta Fellin, Thorsten Becker, Oguz Gogus, Nasser Youbi, and Sandro Conticelli

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.

EGU22-11947 | Presentations | GD2.1

Mantle contribution to surface heat flow of stable europe

Laetitia Le Pourhiet, Thomas Francois, and Laurent Jolivet

EGU22-5561 | Presentations | GD2.1

Investigating the effects of plate-driving forces on observed surface deformation using global mantle flow models 

Arushi Saxena, Juliane Dannberg, and Rene Gassmoeller

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.

EGU22-2113 | Presentations | GD2.1

Imaging the meso-scale structure of the upper mantle beneath the southern and central Atlantic ocean

Barbara Romanowicz, Federico Munch, Max Rudolph, and Sujoy Mukhopadhyay

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.

EGU22-6571 | Presentations | GD2.1 | Highlight

Parallel volcanic chains generated by plume-slab interaction

Ben Mather, Maria Seton, Simon Williams, Joanne Whittaker, Rebecca Carey, Maëlis Arnould, Nicolas Coltice, Angus Rogers, Saskia Ruttor, and Oliver Nebel

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.

EGU22-12422 | Presentations | GD2.1

Plume push force: a relevant driver of plate tectonics that can be constrained by horizontal and vertical plate motions

Ingo Stotz, Berta Vilacís, Jorge N. Hayek, Hans-Peter Bunge, and Anke M. Friedrich

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.

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.

EGU22-9199 | Presentations | GD2.1

Plume-Fracture Zone interactions in the NE Atlantic 

Lea Beloša, Carmen Gaina, Sara Callegaro, Adriano Mazzini, Christine Meyzen, Stephane Polteau, and Michael Bizimis

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.

EGU22-5259 | Presentations | GD2.1

Plume conduits rooted at the core-mantle boundary beneath the Réunion hotspot

Mathurin Dongmo Wamba, Barbara Romanowicz, Jean-Paul Montagner, and Frederik Simons

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.

GD2.4 – Geochemical and geodynamic perspectives on the origin and evolution of deep-seated mantle melts and their interaction with the lithosphere

EGU22-6723 | Presentations | GD2.4

Deep ultra-hot melting in cratonic mantle roots

Carl Walsh, Balz Kamber, and Emma Tomlinson

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.

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) (XMg). 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 XMg ≈ 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.

EGU22-3326 | Presentations | GD2.4

Deep seismic reflection profile with big-size dynamite shots reveals Moho and mantle reflection: tracking continental evolution

Mingrui Li, Rui Gao, Jianbo Zhou, Simon A Wilde, Hesheng Hou, Xiaomiao Tan, and Yanlin Zhu

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.

EGU22-3284 | Presentations | GD2.4

Seismogenesis in granite under brittle-plastic transition condition

Jae Hoon Kim and Jin-Han Ree

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.

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.

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.

EGU22-449 | Presentations | GD2.4

Internal structure of zircon from gneisses of the Ingozero massif of the Achaean TTG complex as an evidence of its more than 3.1 Ga formation age and complicated history of superimposed processes (Kola region, Russia).

Elena Nitkina and Tatiana Kaulina

EGU22-516 | Presentations | GD2.4 | Highlight

A relatively pristine C-like component in the eastern Anatolian asthenosphere

Alican Aktağ, Kaan Sayit, Bradley J. Peters, Tanya Furman, and Jörg Rickli

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.

EGU22-4994 | Presentations | GD2.4

Low He content of the high 3He/4He Afar mantle plume: Origin and implications of the He-poor mantle

Ugur Balci, Finlay M. Stuart, Jean-Alix Barrat, and Froukje M. van der Zwan

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.

EGU22-5450 | Presentations | GD2.4

Exotic magmatism from the western branch of the East African Rift: insights on the lithospheric mantle source.

Francesca Innocenzi, Sara Ronca, Stephen F. Foley, Samuele Agostini, and Michele Lustrino

The northernmost sector of the western branch of the East African Rift (EAR) includes the young (~40-50 ka [1]) volcanic province of Toro Ankole, characterized by the presence of exotic volcanic products such as carbonatites, melilitites, kamafugites and foidites [2]. Among these, the occurrence of kamafugites (kalsilite-bearing volcanic rocks [3]) is noteworthy, as Toro Ankole represents the type locality for these compositions, found in only two other localities worldwide. The Toro Ankole volcanic province developed along the margin of the Archean Tanzanian craton, and its magmatic products show the influence of metasomatic processes and phases developed in the thick continental lithosphere. Indeed, MARID-like metasomatism is proposed in literature, with the formation of a veined mantle [4].

A multidisciplinary approach, based on a detailed petrographic, mineral chemical, geochemical and isotopic (Sr, Nd, Pb and B) study, has been carried out on 53 samples, which include not only lavas and tuffs, but also holocrystalline and wall rock xenoliths. Two types of lava may be identified: the first is represented by carbonatites and silico-carbonatites, characterized by low SiO2 (4.89-21.78 wt%) and low alkali (0.44-2.03 wt%) and high CaO (25.17-47.57 wt%), while the second most peculiar lithotypes is represented by kamafugites; katungites (melilite-rich kalsilite-olivine-bearing volcanic rocks), mafurites (kalsilite-rich melilite-olivine-bearing) and ugandites (olivine-rich kalsilite-melilite-bearing). The kamafugites are strongly SiO2-undersaturated and moderately ultrabasic, potassic to ultrapotassic volcanic rocks, with high MgO (6.08-22.20 wt%) and CaO (up to 15.46 wt%). They consist of phenocrysts of clinopyroxene and olivine set in a hypo-holocrystalline fine-grained groundmass made up of microliths of clinopyroxene, olivine, perovskite, kalsilite, nepheline, leucite, melilite, phlogopite, carbonates and opaques.

The xenolith cargo shows wide range of compositions, varying from clinopyroxenite to glimmerite, with low modal abundance of opaques and perovskite in agreement with the literature data that generally report a lack of olivine and orthopyroxene in the mineral assemblage [5]. The common presence of phlogopite, abundant clinopyroxene and carbonate-rich veins indicate the presence of veined lithosphere [6]. This is consistent with the isotopic data for lavas and xenoliths (87Sr/86Sr = 0.70480-0.70563 and 143Nd/144Nd = 0.512515-0.512575), which outlines an enriched and complex mantle source. 206Pb/204Pb is extremely variable, with values from the holocrystalline xenolith (19.99-19.27) being slightly higher than lava samples (19.28-19.63). The d11B values for lavas and xenoliths, show a wide range, varying from DMM-like values (-6 and -8‰) to more variable OIB-like values (down to -12 and up to -3‰; [7]), through to positive values (up to +6.6‰ in the lavas). These latter also exhibit the highest Sr isotopic ratios of the dataset, pointing to the possible occurrence of old and altered oceanic crust and/or serpentinite in the mantle source.

Bibliography

[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, 9th 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.

EGU22-9813 | Presentations | GD2.4

The architecture of the lithospheric mantle controlled the emplacement of the Central Atlantic Magmatic Province

Andrea Boscaini, Andrea Marzoli, Hervé Bertrand, Massimo Chiaradia, Fred Jourdan, Manuele Faccenda, Christine Meyzen, Sara Callegaro, and Lina Serrano Durán

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.

EGU22-13248 | Presentations | GD2.4

Source and evolution of metasomatizing liquids in orogenic peridotites: evidence from multiphase solid inclusions 

Jana Kotkova, Renata Čopjaková, and Radek Škoda

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.

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 Fo86.5.  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.

EGU22-1260 | Presentations | GD2.4 | Highlight

Manifestation of various stages PGE mineralization in the different locations Ospa-Kitoy ophiolite massif (East Sayan, Russia).

Olga Kiseleva, Evgeniya Ayriants, Dmitriy Belyanin, and Sergey Zhmodik

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.