Details regarding the early evolution of the mantle-crust system are still poorly constrained due to the great scarcity of >3.7 Ga rocks in the geological record. The Napier complex (East Antarctica) is an Eoarchean craton that contains some of Earth’s oldest rocks. This complex recorded Meso- and Neoarchean metamorphism that reached extreme conditions corresponding to granulite facies at 2.5 Ga (1050-1120°C and 7-11 kbar). As a consequence, most samples exhibit disturbed radiogenic isotope systematics (e.g., Rb-Sr, Sm-Nd) and zircon crystals found in such samples are very complex rendering isotopic systematics interpretations challenging. The analytical methods employed in previous studies do not allow these complexities to be understood, which motivated the present contribution.

Here we studied two granulitic orthogneisses labelled 78285007 (Mount Sones) and 78285013 (Gage Ridge) that correspond to the oldest available rocks from the Napier Complex. Mount Sones displays typical characteristics of Archean tonalite-trondhjemite-granodiorite (TTG) suites (e.g., high Na₂O/K₂O, high Sr/Y, fractionated REE patterns with low heavy REE concentrations) with a normative composition intermediate between tonalite and trondhjemite whereas Gage Ridge has a composition closer to that of granite despite a strongly fractionated REE pattern and a pronounced positive Eu anomaly. We have conducted zircon texture assessment using cathodoluminescence and back-scattered electron images in annealed and not annealed crystals. We have subsequently combined U-Pb age profiling by laser-ablation inductively-coupled-plasma mass spectrometry (LA-ICP-MS) and Lu-Hf isotope systematics measurement by LA-MC-ICP-MS in these zircon crystals. Finally, we analysed ^146,147Sm-^143,142Nd isotopesystematics in corresponding whole-rock samples to better constrain the early history of their source.

Our results reveal that Mount Sones and Gage Ridgeorthogneisses formed at 3794 ± 40 and 3857 ± 39 Ma, respectively, with initial ɛ_Hf of -2.6 ± 1.5 and -3.6 ± 2.5, respectively. Sm-Nd isotope measurements indicate a μ¹⁴²Nd of -8.7 ± 3.9 and a ɛ_Nd of -2.0 ± 0.3 at 3794 Ma for Mount Sones, whereas Gage Ridge exhibits a μ¹⁴²Nd of -12.1 ± 6.2 and a disturbed ¹⁴⁷Sm-¹⁴³Nd systematics. Taken altogether our results indicate that the oldest granitoids of the Napier Complex formed by reworking of 4456-4356 Ma mafic protocrust(s). Our inferred petrogenesis is similar to what has been proposed for other Eoarchean terranes worldwide (e.g., Itsaq Gneiss Complex, the Acasta Gneiss Complex, the Nuvvuagittuq Supracrustal Belt, and the North China craton). We propose that Hadean protocrusts were massively reworked in the Eoarchean to form cratons which, in turn, would account for both the absence of Hadean crust in the geological record and its little influence throughout the Archean despite crustal growth models proposing that ≤ 25% of present-day volume of continental crust was formed by the end of the Hadean.

How to cite: Guitreau, M., Boyet, M., Paquette, J.-L., Gannoun, A., Konc, Z., Benbakkar, M., Suchorski, K., and Hénot, J.-M.: The Hadean origin of the Archean Napier Complex (East Antarctica), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16896, https://doi.org/10.5194/egusphere-egu2020-16896, 2020.

Banded Iron Formations (BIF’s) show a typical layering of Fe minerals and quartz that is observed at various scales ranging from micrometers to meters. Millimeter sized micro bands are commonly interpreted as annual layers so larger bands require decades or millennia to form, whereas micrometer sized nano bands have been interpreted to represents sub annual and even diurnal cycles. Because the mineralogical composition of BIF’s is not primary and because single-phase Fe(III) silica gel forms when Fe(III) (oxyhydr-)oxide precipitates in Si rich water, secondary processes are often invoked to explain the banding. However, trace element and isotope data point towards distinct sources for the Fe and Si rich bands, which is difficult to reconcile with a single phase starting material. In addition, the correlation of banding over long distances is inconsistent with most secondary models. Both primary and secondary models struggle to explain the versatile nature of the banding. I will present a conceptual model that could explain BIF layering at all scales and the more widespread formation of granular iron formations (GIF’s) in the Paleoproterozoic.

The concept builds on primary precipitation models postulating that banding forms due to some form of periodicity such as cyclic Fe or nutrient supply to the shelf. Fe(III) is mainly produced by phototrophic iron oxidizing bacteria. These photoferrotrophs are adapted to very low light levels corresponding to about 1% of the light level required by oxygen producing phototrophs allowing them to thrive deep down in the water column. The depth of Fe(III) production is mainly controlled by water turbidity which controls how deep photosynthetically available radiation (PAR) penetrates into the water column. Eutrophic conditions result in turbidity induced by the biomass itself resulting in shallow Fe(III) production depth and the formation of Fe rich bands. During oligotrophic stages, Fe(III) is only produced deep down in the water column, so that silica rich bands can form. In this case, Fe(III)-silica co-precipitation is not an issue because silica precipitates in the Fe(III) free upper water column. Reactive transport modelling shows that besides upwelling and nutrient supply, alternating Fe(III) production depth are mainly associated with changing light conditions. Hence the model predicts annual layering, but also local occurrences of diurnal cycles. Larger periodicities could be associated with: 1) nutrient supply patterns; 2) formation and clearing of atmospheric haze; or 3) additional sources of turbidity in the water column such as silicate particles, MnO₂ particles or metal sulfides. These additional sources of turbidity become more important in the Paleoproterozoic and could be responsible for the more widespread occurrences of GIF’s, indicative of Fe(III) production above storm wave base. The additional factor light, is quite versatile in producing periodicities at variable scales.

How to cite: Herwartz, D.: Is the banding in iron formations controlled by water transparency?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8817, https://doi.org/10.5194/egusphere-egu2020-8817, 2020.

The long-lived radiogenic isotope systems Lu-Hf and Sm-Nd have been widely used by geochemists to study magma sources and crustal residential times of (igneous) rocks in order to understand how early crust formed and to model the production rate and volume of continental crust on global and regional-scales during the last ~4.4 Ga. However, while throughout most of Earth’s history Nd and Hf isotope signatures in terrestrial rocks are well correlated due to their very similar geochemical behavior, some of Earth’s oldest rocks show an apparent inconsistency in their Nd and Hf isotope signatures. While Hf isotopes in early Archean rocks are generally (near) chondritic, Nd isotope signatures can be distinctly super- or sub-chondritic. The super-chondritic Nd isotope values in Eoarchean samples would suggest that these rocks are derived from a mantle reservoir depleted by prior crust extraction. The chondritic Hf isotope values, on the other hand, support a mantle source from which no significant volume of crust had been extracted. While a range of different processes, some of them speculative, might explain this Hf-Nd isotope paradox, recent research [1, 2] has shown that relatively simple, post-magmatic, open-system processes can explain decoupling of the typically correlative Hf-Nd isotope signatures. This talk will focus on the importance of identifying Nd-bearing accessory minerals in (Archean) rocks to understand how the Sm-Nd isotope system is controlled and how in situ isotope and trace element analyses by LA-(MC)-ICP-MS in combination with detailed petrographic observations help to understand when and via which processes the two isotope systems become decoupled. Reconstructing the isotopic evolution of the different isotope systems since formation of the protoliths has important implications for our understanding of early crust formation and questions some of the proposed current models for early crust extraction from the mantle.

[1] Hammerli et al. (2019) Chem. Geol 2; [2] Fisher et al. (2020) EPSL

How to cite: Hammerli, J.: Understanding the role of accessory minerals in the Sm-Nd isotopic evolution of ancient rocks: An in-situ LA-(MC)-ICP-MS approach, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9103, https://doi.org/10.5194/egusphere-egu2020-9103, 2020.

One of the fundamental tenets of geochemistry is that the Earth’s crust has been extracted from the mantle creating a crustal reservoir enriched—and a mantle depleted—in incompatible elements. The Hf-Nd isotope record has long been used to help understand the timing of this process. Increasingly, however, it has become apparent that these two isotope records do not agree for Earth’s oldest rocks. Hf isotopes of zircon from juvenile, nominally mantle-derived rocks throughout the Eoarchean have broadly chondritic initial isotope compositions and indicate large-scale development of the depleted mantle reservoir started no earlier than ~ 3.8 Ga. In contrast, the long-lived Sm-Nd isotope record shows large variation in Nd isotope compositions. Most notably, Paleo- and Eoarchean terranes with chondritic initial Hf isotope compositions have significantly radiogenic Nd isotope compositions indicative of the development of a widespread depleted mantle reservoir very early in Earth’s history which, by extension, requires extraction of significant volumes of enriched crust. These two isotope systems, therefore, indicate two fundamentally different scenarios for the early Earth and has been called the Hf-Nd paradox. However, an important unresolved question remains: Do these records represent primary isotopic signatures or have they been altered by subsequent thermomagmatic processes? We have been able to provide clarity in the Hf isotope record by analyzing zircon from Eo- and Paleoarchean magmatic rocks by determining its U-Pb crystallization age and linking this to its corresponding Hf isotope composition. We can do this unambiguously—even in complex polymetamorphic gneisses—with the laser ablation split stream (LASS) technique whereby we determine U-Pb age and Hf isotope composition simultaneously in a single zircon volume. The existing Nd isotope data, in contrast, are all from bulk-rock analyses. These analyses are potentially problematic in old, polymetamorphic rocks because of the inability to link the measured isotopic composition to a specific age. In addition, the REE budget in these rocks is hosted by accessory phases that can be easily mobilized during later metamorphic and magmatic events. We can now use the LASS approach in REE rich phases (e.g., monazite, titanite, allanite, apatite) to determine U-Pb age and Nd isotope composition in a single analytical volume. New Nd isotope data from the Acasta Gneiss Complex (Fisher et al., EPSL, 2020) show that REE-rich accessory phases are not in isotopic equilibrium with their bulk rock compositions and clearly demonstrate mobilization after initial magmatic crystallization. This post-magmatic open-system behavior may well explain the disagreement in the Hf-Nd isotope record in high-grade polymetamorphic terranes like Acasta. In less complicated, lower-grade rocks, such as in the Pilbara terrane, these REE-rich phases yield consistent U-Pb and Sm-Nd age and isotope compositions indicating that the Nd isotope system in these rocks has remained closed since formation. Of particular note, in the Pilbara samples, the Hf and Nd isotope systems have consistent, broadly chondritic, initial Hf and Nd isotope compositions. In these less-complicated samples, where the Sm-Nd isotope system has remained closed, the Hf and Nd isotope systems agree and there is no Hf-Nd paradox.

How to cite: Vervoort, J., Fisher, C., and Salerno, R.: Resolving the Hf-Nd paradox of early Earth crust-mantle evolution, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13269, https://doi.org/10.5194/egusphere-egu2020-13269, 2020.

Outgassing of an early magma ocean on Earth plays a dominant role in determining the composition of its secondary atmosphere, and hence bears on the potential for the emergence of life. The stability of gaseous species in such an atmosphere reflects the redox state of the magma ocean. However, the relationship between oxygen fugacity (fO₂) and the oxidation state of the most abundant polyvalent element, Fe, in likely magma ocean compositions is poorly constrained. Here we determine Fe²⁺/Fe³⁺ ratios as a function of fO₂ in peridotite liquids, experimentally synthesised by aerodynamic laser levitation at 1 bar and 2173 K. We show that a magma ocean with Fe³⁺/∑Fe akin to that of contemporary upper mantle peridotite (0.037) would have had fO₂ 0.5 log units higher than the Fe-“FeO” equilibrium. At this relative fO₂, a neutral CO₂-H₂O-dominated atmosphere of ~ 150 bar would have developed on the early Earth, taking into account the solubilities of the major volatiles, H, C, N and O in the magma ocean. Upon cooling, the Earth’s prebiotic atmosphere was likely comprised of CO₂-N₂, in proportions and at pressures akin to that on presently found on Venus.

How to cite: Sossi, P., Burnham, A., Badro, J., Lanzirotti, A., Newville, M., and O'Neill, H.: A Venus-like atmosphere on the early Earth from magma ocean outgassing, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11217, https://doi.org/10.5194/egusphere-egu2020-11217, 2020.

Geochronological data in zircon from Archaean tonalite–trondhjemite–tonalite (TTG) gneisses is commonly difficult to interpret. A notable example are TTG gneisses from the Lewisian Gneiss Complex (LGC), northwest Scotland, which have metamorphic zircon ages that define a more-or-less continuous spread through the Neoarchaean, with no clear relationship to zircon textures. These data are generally interpreted to record discrete high-grade events at c. 2.7 Ga and c. 2.5 Ga, with intermediate ages reflecting variable Pb-loss. Although ancient diffusion of Pb is commonly invoked to explain such protracted age spreads, trace element data in zircon may permit identification of otherwise cryptic magmatic and metamorphic episodes. Although zircons from the TTG gneiss analyzed here show a characteristic spread of Neoarchaean ages, they exhibit subtle but key step changes in trace element compositions that are difficult to ascribe to diffusive resetting, but which are consistent with emplacement of regionally-extensive bodies of mafic magma. These data suggest suprasolidus metamorphic temperatures persisted for 200 Myr or more during the Neoarchaean. Such long-lived high-grade metamorphism is supported by data from zircon grains from a nearby monzogranite sheet. These preserve distinctive trace element compositions suggesting derivation from a mafic source, and define a well-constrained U–Pb zircon age of c. 2.6 Ga that is intermediate between the two previously proposed discrete metamorphic episodes. The persistence for hundreds of millions of years of melt-bearing lower crust was probably the norm during the Archaean.

How to cite: Johnson, T., Taylor, R., and Clark, C.: Persistence of melt-bearing Archean lower crust for >200 m.y.— An example from the Lewisian Complex, northwest Scotland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12837, https://doi.org/10.5194/egusphere-egu2020-12837, 2020.

The early evolution of continental crust, particularly its lower layer, during the first 2.0 billion years of Earth history remains enigmatic. Here, we present the first coupled in-situ U-Pb, Lu-Hf and O isotope data for the Precambrian zircons from fourteen deep-crustal xenoliths from five localities in the North China craton. The results show that: (1) the oldest (3.82−3.55 Ga) known lower crustal rocks were survived in the southern part of this craton; (2) the Eo-Paleoarchean zircons have predominant sub-chondritic Hf isotope compositions and elevated δ¹⁸O values, suggesting Lu-Hf fractionation and crust-hydrosphere interactions on the Earth can be traced back to Eoarchean or even earlier; (3) a secular change in zircon O isotopes documents an increase in recycling rate of surface-derived materials into magmas at the end of Archean, which, in turn, is possibly linked to modern style subduction processes and maturation of the crust at that time.

How to cite: Ma, Q., Xu, Y.-G., Huang, X.-L., and Zheng, J.-P.: Eoarchean to Paleoproterozoic crustal evolution in the North China Craton, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4504, https://doi.org/10.5194/egusphere-egu2020-4504, 2020.

More and more convection codes now consider the apparition of melt when the temperature of the mantle exceeds a considered solidus temperature. How melt is treated when it appears varies a lot from one code to another. The convection code StagYY has been using an implementation in which molten eclogite is produced out of melting of mixed mantle. The melt is then teleported above ("erupted") or below ("intruded") the basaltic crust. In a recent study by Jain et al. 2019, we have shown that it is possible to also self-consistently generate continental crust (so-called TTG rocks) if the basaltic crust is entrained in the mantle and remolten. In nature, this only happens if a lot of water is present in the recycled basalt so a numerical treatment of water is necessary.

In this poster, we discuss the details of a new implementation of melting in which each cell of the convection domain is divided in several groups of different composition. Each group has a different solidus and liquidus temperature according to the composition and the water content. The solidus temperature is computed using an interpolation between composition and water concentration end members instead of using an extrapolation from the solidus temperature, as it is usually done. This ensures that TTGs form at a realistic melt fraction and provides a different view on how the continental crust of the early Earth might have formed.

How to cite: Rozel, A., Mojzsis, S., Guitreau, M., Manjón Cabeza Córdoba, A., Ballmer, M., and Tackley, P.: Implementation of partial melting with a water- and composition-dependent solidus temperature adapted to TTG formation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8964, https://doi.org/10.5194/egusphere-egu2020-8964, 2020.

Numerical models of mantle convection help our understanding of the complex feedback between the plates and deep interior dynamics through space and time. Did the early Earth have plate tectonics, a stagnant lid, or something in between? The surface dynamics of the early Earth remain poorly understood. Current numerical models of mantle convection are constrained by present-day observations, but the behavior of the hotter, early Earth prior to the onset of plate tectonics is less certain. The early Earth may have possessed a large hot magma ocean trapped near the core-mantle boundary after formation during differentiation, and likely containing different elements from the surrounding mantle. We examine how composition-dependent properties in the deep mantle affect convection dynamics and surface mobility in high Rayleigh number models featuring plastic yielding. Our Newtonian models indicate that increased conductivity or decreased viscosity flattens basal topography while also increasing the potential for surface yielding. We vary the viscosity, thermal conductivity, and internal heating in a compositionally distinct basal magma ocean and explore the compositional topography, insulation effects and surface stresses for non-Newtonian rheology. Models are run using a variety of crustal compositions, such as the inclusion of primordial continental material before the onset of plate tectonics. We monitor the surface for plate-like behavior. Since convective vigour is very strong in the early Earth, specialized tracer methods are employed for increased accuracy. In our models, Stokes flow solutions are obtained using a multigrid method specifically designed to handle large viscosity contrasts and non-Newtonian rheology.

How to cite: O'Farrell, K. A., Trim, S., and Butler, S.: Mobile or not mobile: exploring the linkage between deep mantle composition and early Earth surface mobility, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10638, https://doi.org/10.5194/egusphere-egu2020-10638, 2020.

Singhbhum Craton, eastern India, exposes some of the oldest known composite Paleoarchean granitoids. These granitoids range from sodic TTGs to evolved, potassic granites.  The whole process of their formation, starting from nucleation of a juvenile continent to its evolution and final stabilization is documented. The central part of the craton started nucleating with the formation of 3.45–3.40Ga juvenile (zircon εHf_t=+0.6 to +7.1) TTGs. These TTGs characterized by slightly depleted HREE and Y, negligible Eu-anomaly (Eu/Eu*=0.90 to 1.00) and moderate Sr/Y (25–64), consistent with derivation from a low-K mafic crust at a pressure near the lower end of the garnet stability field, causing subordinate garnet retention in the residue and negligible role of plagioclase. During 3.32Ga, deeper melting of a juvenile mafic crust (zircon εHf_t=+1.3 to +5.7) caused emplacement of a second generation of TTG. Deeper melting is suggested by depleted HREE and Y, and high Sr/Y (52–155), implying significant amount of residual garnet retention. Subsequently at 3.28 and 3.25Ga, melting of moderately old to juvenile (zircon εHf_t=-1.9 to +4.5), mostly TTG sources at variable depths generated potassic, LILE-enriched, high-silica granites. Intrusion of these potassic granites resulted in a stable and buoyant crust that marked the final Cratonization of the Singhbhum Craton. The sequence of events is interpreted in terms of repeated intracrustal melting and granitoid generation in a gradually thickening oceanic plateau with a progressive change in granitoid source from mafic to felsic in composition. Combination of rock assemblage, regional geology, and structural pattern also supports intraplate nature of the magmatism in Singhbhum Craton, which might have been a significant mechanism of crustal growth worldwide during Paleoarchean.

How to cite: Mitra, A., Dey, S., Zong, K., Liu, Y., and Mitra, A.: Paleoarchean crustal evolution of the Singhbhum Craton, eastern India: Insights from granitoid petrology and zircon U-Pb and Lu-Hf systematics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1142, https://doi.org/10.5194/egusphere-egu2020-1142, 2020.

Available zircon ages indicate that the plutonic protoliths of Neoarchean TTG (tonalitic-trondhjemitic-granodioritic) gneisses in the Eastern Block were emplaced at two phases, with the earlier one at 2.75-2.65 Ga and the younger one at 2.55-2.50 Ga. Although the 2.75-2.65 Ga rock associations are only exposed in the Luxi and Qixia areas, the ~2.7 Ga igneous event must have occurred across the whole Eastern Block and was a major crustal accretionary or mantle-extraction event that formed a thick mafic crust beneath the whole Eastern Block based on the following lines of evidence:

(1) The 2.75-2.65 Ga TTG rocks in the Luxi granite-greenstone terrane have positive εHf(t) values (+2.7 to +10.0), with most zircon Hf model ages close to the rock-forming ages, which provides robust evidence that the ~2.7 Ga event that formed the 2.75-2.65 rock associations was a crustal accretion (mantle extraction) event, not a crust-reworking event.

(2) The 2.55-2.50 Ga TTG rocks in the Eastern Block possess mildly positive to slightly negative εHf(t) values, with most zircon Hf model ages pointing to 2.8-2.6 Ga, similar to rock-forming ages of the 2.75-2.65 Ga TTG gneisses in the Luxi granite-greenstone terrane, suggesting that the 2.55-2.50 Ga rocks in the Eastern Block were mainly derived from the partial melting of an early Neoarchean (2.75-2.65 Ga) juvenile crust that formed at ~2.7 Ga. As the 2.55-2.50 Ga TTG gneisses are ubiquitous over the whole Eastern Block, the 2.7 Ga event must have occurred over the whole Eastern Block, forming an early Neoarchean juvenile crust that experienced partial melting or reworking to form the 2.55-2.50 Ga TTG rocks.

(3) TTG rocks are generally considered to have been derived from the partial melting of a thickened mafic crust (eclogite or rutitle/garnet-bearing amphibolite). This means that an early Neoarchean (2.75-2.65 Ga) juvenile crust formed by the ~2.7 Ga event should be a mafic-dominant crust, which is either a lower continental crust or an oceanic crust. In this case, the ~2.7 Ga event in the Eastern Block may have represented a Large Igneous Province event that formed the main body of the Eastern Block. This study was financially supported by the sub-project of a NSFC Major Project, entitled “Continental Crust Growth-Stabilization and Initiation of the Early Plate Tectonics” (Project Code: 41890831) and HKU Seed Fund for Basic Research (201811159089).

How to cite: Zhao, G.: Ages and Hf isotopes of igneous zircons from Neoarchean TTG gneisses in the Eastern Block, North China Craton: Tectonic implications , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1544, https://doi.org/10.5194/egusphere-egu2020-1544, 2020.

About 2.5 Ga ago, two distinct mantle sources for basalts developed: one with a lower mantle potential temperature (MPT) being today relatively depleted and feeding the mid-ocean ridges, and one with a higher MPT being relatively enriched and pluming today's ocean-island-basalt (OIB) volcanism (Condie et al., 2016). Previous to that, basalts record rather uniform MPTs corresponding to today's higher-temperature OIB reservoir. The cooler mantle domain started forming, when the slowly cooling thermally uniform mantle reached a MPT of 1550-1500 °C (Condie, 2018). We attribute this “Great Thermal Divergence” (Condie et al., 2016) to a transition from non-layered to layered mantle convection. For primitive mantle compositions, a 1530-adiabat propagates precisely to the high-temperature slope break of the 660 phase transition at about 1800 °C/23 GPa. Mantle with MPT higher than that does not experience the suppression of convective passage through the lower-upper mantle boundary, which results from the negative slope of the ringwoodite-to-perovskite-plus-periclase transition. We propose that mantle convection prior to 2.5 Ga was capable of stirring the whole mantle. A 660 phase transition with a negative slope formed only 2.5 Ga ago and thus established a thermomechanical boundary layer that allowed the formation of two thermally distinct mantle reservoirs.

Condie, K. et al. (2016): A great thermal divergence in the mantle beginning 2.5 Ga: Geochemical constraints from greenstone basalts and komatiites. Geoscience Frontiers, 7, 543-553.

How to cite: Nagel, T., Petersen, K. D., and Vesterholt, A.: The Great Thermal Divergence and the slope break of the 660 phase transitions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13615, https://doi.org/10.5194/egusphere-egu2020-13615, 2020.

We present new time series partial melting experiments performed on a natural enstatite chondrite (EL6), aimed at investigating the textural and geochemical changes induced by silicate-metal equilibration during early planetary differentiation. The starting material of our experiments consisted of small fragments (ca. 50 mg) obtained from the interior of the enstatite chondrite MCY 14005 (MacKay Glacier, Antarctica), collected during the XXX° Italian Expedition in Antarctica (PNRA). Experiments were performed in graphite capsules at a pressure of 1 GPa, at temperature ranging from 1100 to 1300 °C, with run durations from 1 to 24 h. The initial phase assemblage of the enstatite chondrite, mostly composed by granular enstatite and Fe-Ni metal (up to 400 µm in size) with minor amounts of sulphides and plagioclase, undergoes significant changes with increasing temperature and run duration. At 1100 °C, no silicate melt is produced and subsolidus reactions occur at the contact between the metal and silicate phases. At 1200 °C, small amounts of silicate melt are produced at the grain boundaries and enstatite grains in contact with the melt grow Fe-enriched rims. The metal portions are characterized by two immiscible liquid phases that exhibit rounded shapes when in contact with the silicate melt, whereas smaller (micrometric) liquid metal spheres occur isolated within the silicate melt throughout the experimental charges. These features are already observed in the 1 h experiment but become increasingly evident with increasing run duration, and at higher temperatures. In the experiments performed at 1300 °C, the amount of silicate melt increases and new silicate minerals form (olivine and low-Ca-pyroxene).

Enstatite chondrites are characterized by an oxygen isotope composition similar to that of the bulk Earth and Moon, and are considered to have initially formed in the terrestrial planetary zone of the solar nebula. For this reason, they represent a suitable material to investigate the early planetary differentiation processes that occurred in the proto-Earth system. Preliminary results from our experiments indicate that, at the investigated oxygen fugacity (1-2 log units below the IW buffer), the Fe-Si exchange between the metal and silicate phases allows the formation of silicate melt and silicate phases such as olivine and low-Ca-pyroxene. At the same time, the change in shape of the metal grains (increasingly circular/spherical with increasing temperature) and the overall reduction of their number density with increasing experimental time point to rapid aggregation of the metal phase and, possibly, to fast silicate-metal differentiation in small planetesimals.

How to cite: Masotta, M., Folco, L., Ziberna, L., and Myhill, R.: Partial melting of an enstatite chondrite at 1 GPa: Implications for early planetary differentiation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9935, https://doi.org/10.5194/egusphere-egu2020-9935, 2020.

The biogenicity of proposed stromatolites from deformed greenschist/amphibolite facies Eoarchean (ca. 3.71 Ga) rocks of the Isua Supracrustal Belt (ISB) in West Greenland, is debated  [1,2; cf. 3]. To assess their promise as primary sedimentary structures – as opposed to artefacts of strain localization in layered ductile rocks – we report new field mapping at the discovery site of Nutman et al. (2016) to guide micro- and macro-structural investigations and geochemical sampling. Discontinuous field relations preclude confident assignment of these outcrops as being structurally overturned as originally argued. The structures are not deformed conical stromatolites, but instead linear inverted ridges aligned with azimuths of local and regional fold axes, and parallel to linear structures. Combined major element (e.g., Ca, Mg, Si) scanning μXRF maps, and electron back-scattered diffraction (EBSD) patterns on fresh surfaces cut perpendicular and parallel to the ridges show that the structures lack any internal laminae. Seeming internal layering previously inferred for these features instead arises from variable weathering of outcrop surfaces that otherwise conceal structureless quartz ± dolomite granoblastic cores. These asymmetric boudins sit between semi-continuous competent layers of enveloping quartzite in a calc-silicate schist. Boudinage fabrics reflect viscosity contrasts of the different ductile layers during deformation, and are thus not of primary origin. Collectively, our results show that such structures were probably never stromatolites, but are instead the expected result of a tectonic fabric that preserves no fine-scale primary sedimentary structure.

[1] Nutman, A.P. et al. 2016, Rapid emergence of life shown by discovery of 3,700-million-year-old microbial structures: Nature, v. 537, p. 535–538; [2] Nutman, A.P. et al., 2019, Cross-examining Earth’s oldest stromatolites: Seeing through the effects of heterogeneous deformation, metamorphism and metasomatism affecting Isua (Greenland) ∼3700 Ma sedimentary rocks: Precambrian Research, v. 331, p. 105347; [3] Allwood, A.C. et al. 2018, Reassessing evidence of life in 3,700-million-year-old rocks of Greenland: Nature, doi: 10.1038/s41586-018-0610-4.

How to cite: Zawaski, M., Kelly, N., Orlandini, O. F., Nichols, C., Allwood, A., and Mojzsis, S.: Chemical and structural analysis of proposed ca. 3.7 Ga stromatolites from the Isua Supracrustal Belt (West Greenland) - a reappraisal, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6044, https://doi.org/10.5194/egusphere-egu2020-6044, 2020.

The c. 3.5 Ga Dresser Formation of the East Pilbara Craton (Western Australia) contains large amounts of blackish barite. These rocks produce an intense sulfidic odor when crushed, resulting from abundant primary fluid inclusions. In part, the black barites are interbedded with sulfidic stromatolites. Using Raman spectroscopy, microthermometry, and two different online GC–MS approaches, we characterized in detail the chemical composition of the barite-hosted fluid inclusions. Our GC–MS techniques were based on (i) thermodecrepitation at 150-250°C and (ii) solid phase microextraction (SPME)–GC–MS at reduced temperature (50°C), thereby minimizing external contamination and artefact formation. Major fluid inclusion classes yielded mainly H₂O, CO₂, and H₂S in varying abundance, along with minor amounts of COS and  CS₂, N₂, and CH₄ (< 1%). Notably, we also detected a wide range of volatile organic compounds, including short–chain ketones and aldehydes, thiophenes, and various organic (poly)sulfides. Some of these compounds (CH₃SH, acetic acid) have previously been invoked as initials agents for carbon fixation under primordial conditions, but up to now their presence had not been observed in Precambrian materials. Based on our findings, we hypothesize that hydrothermal seepage of organic and inorganic compounds during Dresser times provided both, catabolic and anabolic substrates for early microbial metabolisms.

How to cite: Thiel, V., Duda, J.-P., van den Kerkhof, A. M., Reitner, J., and Mißbach, H.: Volatile organic compounds in barite-hosted fluid inclusions from the 3.5 Ga old Dresser Formation, Western Australia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9161, https://doi.org/10.5194/egusphere-egu2020-9161, 2020.

Use of trace and rare earth element concentration of terrigenous sedimentary rocks to deduce the composition of their source rocks in the hinterland is a very common and efficient practice. The results of geochemical analysis of the metaquartzarenites located at the basal part of Bababudan and Sigegudda belt, late Archean greenstone sequences of western Dharwar craton show that the sediments were most possibly supplied from Paleo to Mesoarchean granitoids of western Dharwar Craton. Rare earth element patterns of these basal quartzites display fractionated REE pattern in variable degree (La_N/Yb_N =1.47-10.63) with moderate to highly fractionated LREE (La_N/Sm_N=2.67-8.93) and nearly flat to slighly elevated HREE (Gd_N/ Yb_N=0.62-1.29) and a significant Eu negative anomaly (avg. Eu/Eu*=0.67). In general, presence of negative Eu anomaly in clastic rocks reflect the widespread occurrence of granitic rocks in the source area, which possess negative Eu anomaly. On the other hand, mechanical enrichment of zircon (having negative Eu anomaly, high HREE concentration and low La_N/Yb_N), if present, will hamper the whole REE pattern of the sediments and necessarily, do not actually mimic the source composition. Here, in our study, the Th/Sc vs Zr/Sc diagram show mineral Zircon has been concentrated by mechanical concentration in the sedimentary rocks. Few quartzite samples which have high Zr content typically exhibit low La_N/Yb_N values, reflecting pivotal role of mineral zircon in controlling the REE pattern of the sediments. Hence, in this case, we should be cautious in interpreting of the Eu negative anomaly of the basal quartzites for meticulously identifying their source rock composition. More geochemical and other analytical approaches are required in this regard.

How to cite: Mitra, A. and Dey, S.: Eu anomaly- reliability of the proxy in inferring source composition of clastic Sedimentary rocks: A case study from western Dharwar craton, Karnataka, India, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21526, https://doi.org/10.5194/egusphere-egu2020-21526, 2020.

Physics-based geodynamic modeling of mantle convection provide a unifying framework for solid-Earth sciences, explicitly linking together disparate fields such as tectonophysics, tomographic imaging, basin analysis, mantle mineralogy, geomorphology, global geodesy and the long-term chemical and thermal evolution of the mantle. Studying the evolution of mantle convection in time is particularly powerful as it reduces trade-offs, increase the possible linkages and the opportunities to cross-test hypotheses. But since mantle convection evolves over geologic timescales, its future evolution is precluded from us and we must focus on its past history.

Here I will show how geodynamic modeling of past mantle flow can be combined with tomographic imaging and geologic observations, highlighting the strengths of this approach and some of its potential pitfalls. I will use a series of case studies, starting from simple analytical solutions for channelized flow in the South Atlantic and Caribbean regions. I will move on to an application of sequential assimilation to the South China Sea, ending with computationally demanding large-scale numerical optimizations of past mantle flow.

How to cite: Colli, L.: Combining tomographic images and geodynamic modeling of past mantle flow: from simple analytical solutions to numerical inverse methods, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6134, https://doi.org/10.5194/egusphere-egu2020-6134, 2020.

The formation of large igneous provinces is a focus of geoscientists and is a major scientific issue in mantle dynamics. A broad consensus holds that the Emeishan large igneous province (ELIP) was generated by an upwelling mantle plume. However, recent geological and seismic studies have challenged this notion. In this study, I redraw and reanalyze previous tomographic images and use images of three velocity perturbation profiles crossing the ELIP. I collected abundant high-quality teleseismic data and performed common conversion point (CCP) stacking of receiver functions in the mantle transition zone (MTZ) of the ELIP. The tomographic images show a high-velocity anomaly of a northeastward-subducted slab-like body beneath the ELIP, which might be a relic of the Paleo-Tethys oceanic lithosphere. Images from CCP stacking of receiver functions indicate that the subducted slab of the Paleo-Tethys oceanic lithosphere retained an imprint on the X-discontinuity and the 410 and 660 km discontinuities. Based on my assessment, the subducted slab might have induced return mantle flow or large-scale mantle upwelling, which possibly played an important role in the formation of the ELIP.

How to cite: He, C.: A subducted slab of the Paleo-Tethys oceanic lithosphere associated with the formation of the Emeishan large igneous province, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1467, https://doi.org/10.5194/egusphere-egu2020-1467, 2020.

The thermal and compositional structure of the lithospheric keels underlying the Precambrian cratonic cores of the continents may shed light on their evolution and long-term stability. A number of seismic studies have found significant 3D seismic heterogeneity in cratonic lithosphere, which is enigmatic because temperature variations in old shields are expected to be small and seismic sensitivity to major-element compositional variations is limited. Previous studies show that metasomatic alteration may lead to significant variations in shield velocities with depth. Here we perform a grid search for thermo-chemical structures including metasomatic compositions, to model Rayleigh-wave phase velocities between 20 and 160 s for the northeastern part of North America comprising the Superior craton, the largest Archean craton in the world, and surrounding Proterozoic belts. We find smooth variations in thermal structure that include variations in thermal thickness within the Superior and decreasing thickness towards the edges of the shield. Four types of distinct compositional structures are required to match the long-period phase velocities. The different types appear to correlate with: (i) the unaltered oldest cores of the Superior, (ii) Archean and Proterozoic lithosphere modified by rifting and plume activity, and two distinct types of subduction signatures: (iii) an Archean/Paleo-Proterozoic signature that includes a high-velocity eclogite layer in the mid-lithosphere and (iv) a post Paleo-Proterozoic signature characterised by strongly altered shallow mantle lithosphere. Thus, processes that have affected the formation and modification of cratonic lithosphere and were previously recognised in xenoliths appear to have also left large-scale imprints in seismic structure.

How to cite: Goes, S., Eeken, T., Altoe, I., Petrescu, L., Foster, A., Pedersen, H., Arndt, N., Darbyshire, F., and Bouilhol, P.: Thermochemical structure of cratons from Rayleigh wave phase velocities, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5380, https://doi.org/10.5194/egusphere-egu2020-5380, 2020.

A better understanding of the Earth’s compositional structure is needed to place the geochemical record of surface rocks into the context of Earth accretion and evolution. Cosmochemical constraints imply that lower-mantle rocks may be enriched in silica relative to upper-mantle pyrolite, whereas geophysical observations support whole-mantle convection and mixing. To resolve this discrepancy, it has been suggested that subducted mid-ocean ridge basalt (MORB) segregates from subducted harzburgite to accumulate in the mantle transition zone (MTZ) and/or the lower mantle. However, the key parameters that control basalt segregation and accumulation remain poorly constrained. Here, we use global-scale 2D thermochemical convection models to investigate the influence of mantle-viscosity profile, planetary-tectonic style and bulk composition on the evolution and distribution of mantle heterogeneity. Our models robustly predict that, for all cases with Earth-like tectonics, a basalt-enriched reservoir is formed in the MTZ, and a harzburgite-enriched reservoir is sustained at 660~800 km depth, despite ongoing whole-mantle circulation. The enhancement of basalt and harzburgite in and beneath the MTZ, respectively, are laterally variable, ranging from ~30% to 50% basalt fraction, and from ~40% to 80% harzburgite enrichment relative to pyrolite. Models also predict an accumulation of basalt near the core mantle boundary (CMB) as thermochemical piles, as well as moderate enhancement of most of the lower mantle by basalt. While the accumulation of basalt in the MTZ does not strongly depend on the mantle-viscosity profile (explained by a balance between basalt delivery by plumes and removal by slabs at the given MTZ capacity), that of the lowermost mantle does: lower-mantle viscosity directly controls the efficiency of basalt segregation (and entrainment) near the CMB; upper-mantle viscosity has an indirect effect through controlling slab thickness. Finally, the composition of the bulk-silicate Earth may be shifted relative to that of upper-mantle pyrolite, if indeed significant reservoirs of basalt exist in the MTZ and lower mantle.

How to cite: Yan, J., D. Ballmer, M., and J. Tackley, P.: The evolution and distribution of recycled oceanic crust in the Earth’s mantle: Insight from geodynamic models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20964, https://doi.org/10.5194/egusphere-egu2020-20964, 2020.

Early periods of Earth's history are of great interest for the evolution of plate tectonics. For instance, neither the formation of lithospheric plates nor the nature of Archean plate tectonics is well known. As a remnant of the magma ocean period, a compositionally dense layer at the core-mantle boundary is assumed to interact with the convective flow of the Earth's mantle forming todays LLSVPs. Since plate motions are strongly coupled to the convection of mantle material, stabilizing effects of compositionally dense material have a profound impact on mantle convection and plate tectonics and will be of major importance for its evolution.
To investigate the influence of a dense basal layer, we use a numerical approach employing thermo-chemical mantle convection models with self-consistent plate generation. Considering different possible scenarios of the post magma ocean period we analyze the influence of different parameters, i.e. the density contrast between the dense basal material and the ambient mantle and the volume of the enriched layer.
Generally we observe that a stagnant lid forms which is initially mobilized episodically before turning to a permanently mobile surface. However, the temporal evolution of the episodic stage is considerably altered due to the presence of dense basal material. The time when an episode occurs, is determined by the mechanism which induces this mobilization. The mechanism itself is controlled by the density and volume of the enriched layer. Therefore, we distinguish between four different initiation mechanisms, which occur for different configurations of the density and volume of enriched material.

How to cite: Hellenkamp, P., Stein, C., and Hansen, U.: LLSVPs of primordial origin: Implications for the evolution of plate tectonics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21471, https://doi.org/10.5194/egusphere-egu2020-21471, 2020.

The dynamic topography of the core-mantle boundary (CMB) provides important constraints on dynamic processes in the mantle and core. However, inferences on CMB topography are complicated by the uneven coverage of data with sensitivity to different length scales and strong heterogeneity in the lower mantle. Particularly, a trade-off exists with density variations, which ultimately drive mantle flow and are vital for determining the origin of mantle structures. Here, I review existing models of CMB topography and lower mantle density, focusing on seismological constraints (Koelemeijer, 2020). I develop average models and vote maps with the aim to find model consistencies and discuss what these may teach us about lower mantle structure and dynamics.

While most density models image two areas of dense anomalies beneath Africa and the Pacific, their exact location and relationship to seismic velocity structure differs between studies. CMB topography strongly influences the retrieved density structure, which partially helps to resolve differences between recent studies based on Stoneley modes and tidal measurements. CMB topography models vary both in pattern and amplitude and a discrepancy exists between models based on body-wave and normal-mode data. As existing models typically feature elevated topography below the Large-Low-Velocity Provinces (LLVPs), very dense compositional anomalies may be ruled out as possibility.

To achieve a similar consistency as observed in lower mantle models of S-wave and P-wave velocity, future studies should combine multiple data sets to break existing trade-offs between CMB topography and density. Important considerations in these studies should be the choice of theoretical approximation and parameterisation. Efforts to develop models of CMB topography consistent with both body-wave and normal-mode data should be intensified, which will aid in narrowing down possible explanations for the LLVPs and provide additional insights into mantle dynamics.

Koelemeijer, P. (2020), “Towards consistent seismological models of the core-mantle boundary landscape”. Book chapter in revision for AGU monograph "Mantle upwellings and their surface expressions", edited by Marquardt, Cottaar, Ballmer and Konter

How to cite: Koelemeijer, P.: Towards consistent seismological models of the core-mantle boundary landscape, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3518, https://doi.org/10.5194/egusphere-egu2020-3518, 2020.

Examination of the least-contaminated rocks of the Jurassic Karoo flood basalt province indicates considerable compositional variability in the mantle source. New and previously published Sr, Nd, and Pb isotopic data are suggestive of two main categories of mantle reservoirs: one coincides with the field of depleted mantle (DM) -affinity oceanic crust and the other has low initial eNd (+3.3 to 0.3) and high ⁸⁷Sr/⁸⁶Sr (0.7039 to 0.7057) and Δ8/4 (92 to 138) typical of enriched mantle 1 (EM1) -affinity oceanic crust. Previous studies have proposed the DM type reservoir included domains affected by subduction-related fluids and recycled oceanic components (e.g. Heinonen et al., 2014). The EM1 type reservoir probably also contained subducted crustal components, but the geochemical data are suggestive of an additional primitive mantle (PM) type component (Turunen et al., 2019).

Importantly, the two reservoirs can be geochemically linked to a recently identified bilateral compositional asymmetry in the volumious Karoo flood basalts (Luttinen, 2018): The DM type  reservoir is the most likely source of Nb-depleted flood basalts in the southeastern Karoo subprovince (Lebombo rifted margin and Antarctica), whereas the EM1-PM type reservoir has been identified as the principal source of the Nb-undepleted flood basalts in the northwestern subprovince (Karoo-Kalahari-Zambezi basins). The boundary between the flood basalt subprovinces and the occurrences of the DM-affinity and EM1-PM-affinity rocks overlie the Jurassic location of the margin of the Jurassic sub-African LLSVP. Magmas derived from the EM1-PM type reservoir were largely emplaced above the deep mantle anomaly, whereas those derived from the DM type reservoir were emplaced outside the footprint of the LLSVP.

Based on isotopic similarity, the EM1-PM type reservoir of the Karoo province may record the same overall LLSVP material as the Gough component in the zoned Tristan da Cunha plume (e.g. Hoernle et al., 2015). Furthermore, it is possible that the DM type reservoir of the Karoo province, which has been interpreted to represent depleted upper mantle heated by mantle plume, could also represent a plume component and that the bilateral Karoo flood basalt province as a whole could thus register melting of a large zoned plume source associated with the margin of the sub-African LLSVP.

References

Heinonen, J.S., Carlson, R.W., Riley, T.R., Luttinen, A.V., Horan, M.F. (2014). Subduction-modified oceanic crust mixed with a depleted mantle reservoir in the sources of the Karoo continental flood basalt province. Earth and Planetary Science Letters 394, 229–241. http://dx.doi.org/10.1016/j.epsl.2014.03.012

Hoernl, K., Ronde, J., Hauff, F., Garbe-Schönberg, D., Homrighausen, S., Werner, W., Morgan, J.P. (2015).  How and when plume zonation appeared during the 132 Myr evolution of the Tristan Hotspot. Nature Communications 6:7799. doi: 10.1038/ncomms8799

Luttinen, A.V. (2018). Bilateral geochemical asymmetry in the Karoo large igneous province. Scientific Reports 8:5223. doi:10.1038/s41598-018-23661-3

Turunen, S.T., Luttinen, A.V., Heinonen, J.S., Jamal, D.L. (2019). Luenha picrites, Central Mozambique – Messengers from a mantle plume source of Karoo continental flood basalts? Lithos 346–347. https://doi.org/10.1016/j.lithos.2019.105152

How to cite: Luttinen, A., Heinonen, J., Turunen, S., Carlson, R., and Horan, M.: Were Karoo flood basalts derived from a LLSVP-related plume source?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18210, https://doi.org/10.5194/egusphere-egu2020-18210, 2020.

Significant effort has been made to characterize the diversity of geochemical components sampled by oceanic hotspot volcanoes and mid-ocean ridges, and progress has been made to identify the origin of these geochemical components. However, the locations of the key mantle domains sampled at hotspots—EM1 (enriched mantle I), EM2 (enriched mantle II), HIMU (high ‘μ’, or high ²³⁸U/²⁰⁴Pb), and an ancient, high ³He/⁴He component—remain poorly constrained. In turn, the lack of spatial constraints on the locations and geometries of these reservoirs limits understanding of how geodynamic processes (e.g., subduction, mantle convection) operate to modify the Earth’s interior.

We provide an updated compilation the most extreme EM (lowest ¹⁴³Nd/¹⁴⁴Nd) and HIMU (highest ²⁰⁶Pb/²⁰⁴Pb) compositions in lavas from 46 oceanic hotspots with available data, and the highest ³He/⁴He compositions from 44 hotspots globally. We examine the geographic distributions of hotspots hosting extreme geochemical components at the Earth’s surface. We also explore how tilted plume conduits relate the geochemical distributions in hotspots at the Earth’s surface with the two inferred Large Low Shear Wave Velocity Provinces (LLSVP) in the deep mantle.

We find that the most extreme EM signatures are identified in southern hemisphere hotspots, and northern hemisphere hotspots exhibit more geochemically-depleted compositions. Critically, hotspots with the most extreme HIMU compositions show a very different distribution, and are found in, or near, the tropical latitudes. The EM and HIMU domains thus exhibit a clear spatial separation in the deep Earth.

In order to evaluate whether EM and HIMU domains are spatially associated with the LLSVPs, we compare the magnitude of EM and HIMU signatures with minimum hotspot distances from the LLSVP margins. While EM-hosting hotspots show a clear geographic affinity for the LLSVPs, new data make it apparent that HIMU-hosting hotspots show no geographic association with the LLSVPs, further supporting to the spatial decoupling of EM and HIMU mantle domains.

Hotspots hosting ancient high ³He/⁴He domains exhibit a spatial relationship with the LLSVPs (doi: 10.1029/2019GC008437), suggesting that the EM and high ³He/⁴He domains may coexist in the LLSVPs. While the degree of the EM signature exhibits no relationship with hotspot buoyancy flux, maximum high ³He/⁴He values correlate with hotspot buoyancy fluxes, consistent with the hypothesis that high ³He/⁴He mantle reservoirs are hosted in dense regions in the LLSVPs sampled by only the hottest and most buoyant plumes.

These results raise several key questions. First, if subduction of oceanic crust and sediment generate HIMU and EM reservoirs, then why are they spatially separated?  Why are EM2 domains concentrated in the southern hemisphere, and why are they limited to being inside or near the LLSVPs? Why are EM and high ³He/⁴He domains both geographically associated with the LLSVPs, and are they spatially separated within the LLSVPs so that the low ³He/⁴He of the former does not overprint the high ³He/⁴He of the latter? If elevated ³He/⁴He originates in the core, consistent with negative ¹⁸²W anomalies in high ³He/⁴He lavas, why are high ³He/⁴He plumes associated with the LLSVPs?   

How to cite: Jackson, M., Becker, T., and Steinberger, B.: Geochemical and seismological constraints on the locations and geometries of deep mantle reservoirs, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10547, https://doi.org/10.5194/egusphere-egu2020-10547, 2020.

Earth’s oldest preserved crustal archive, the Jack Hills zircon of Western Australia, has been controversial to interpret in terms of the onset of plate tectonics. Here we conduct time series analysis on hafnium isotopes of the Jack Hills zircon and reveal an array of statistically significant cycles that are reminiscent of plate tectonics, i.e., subduction. At face value, such cycles may suggest early Earth conditions similar to today—the uniformitarian “day one” hypothesis. On the other hand, in the context of expected secular changes due to planetary evolution and geological observations, the cycles could instead imply that modern plate tectonic subduction inherited convective harmonics already facilitated by an early phase of stagnant-lid delamination—the “lid-to-plates” hypothesis. Either way, any model for the initiation of plate tectonics must begin in Hadean time.

How to cite: Mitchell, R. N., Spencer, C. J., Kirscher, U., and Wilde, S. A.: Modern plate tectonic cycles are inherited from Hadean mantle convection, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21211, https://doi.org/10.5194/egusphere-egu2020-21211, 2020.

Basalts are generated by adiabatic decompression melting of the upper mantle, and thus provide spatial and temporal records of the thermal, compositional, and dynamical conditions of their source regions. Uniquely constraining these factors through the lens of melting is challenging given the complexity of the melting process. To limit the a priori assumptions typically required for forward modeling of mantle melting, and to assess the robustness of the modeling results, we combine a Markov chain Monte Carlo sampling method with the forward melting model REEBOX PRO [1] simulating adiabatic decompression melting of lithologically heterogeneous mantle. Using this method, we invert for mantle potential temperature (Tp), lithologic trace element and isotopic composition and abundance, and melt productivity together with a robust evaluation of the uncertainty in these system properties. We have applied this new methodology to constrain melting beneath the Reykjanes Peninsula (RP) of Iceland [2] and here extend the approach to Iceland’s Northern Volcanic Zone (NVZ). We consider melting of a heterogeneous mantle source involving depleted peridotite and pyroxenite lithologies, e.g., KG1, MIX1G and G2 pyroxenites. Best-fit model sources for Iceland basalts contain more than 90% depleted peridotite and less than 10% pyroxenite with Tp ~125-200 °C above ambient mantle. The trace element and Pb and Nd isotope composition of the depleted source beneath the Reykjanes Peninsula is similar to DMM [3], whereas depleted mantle for the NVZ is isotopically distinct and more trace element enriched. Conversely, inverted pyroxenite trace element compositions are similar for RP and NVZ and are more enriched than previously inferred, despite marked differences in their Pb and Nd isotope composition. We use these new constraints on the Iceland source to investigate their relative importance in basalt genesis along the adjoining Reykjanes and Kolbeinsey Ridges. We find that the proportion of pyroxenite diminishes southward along Reykjanes Ridge and is seemingly absent to the north along the Kolbeinsey Ridge. Moreover, abundances of inverted RP and NVZ depleted mantle also diminish away from Iceland and give way to a common depleted source for the North Atlantic. These findings further illuminate the along-strike variability in source composition along the North Atlantic ridge system influenced by the Iceland melting anomaly, while reconciling geochemical, geophysical and petrologic constraints required to rigorously test plume vs. non-plume models.

[1] Brown & Lesher (2016); G^3, v. 17, p. 3929-2968

[2] Brown et al. (2020); EPSL, v. 532, 116007

[3] Workman and Hart (2005); EPSL, v.231, p. 53-72

How to cite: Brown, E. and Lesher, C.: Constraining Mantle Source Conditions at Iceland and Adjoining Ridges Using Markov Chain Monte Carlo Inversion, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22469, https://doi.org/10.5194/egusphere-egu2020-22469, 2020.

In this study we explored the contrasted plate tectonic reconstructions proposed for the proto-South China Sea and SE Asia. We implemented four different end-member plate models into global geodynamic models to test their predicted mantle structure against tomography. All models reproduced the Sunda slabs beneath Peninsular Malaysia, Sumatra and Java and the proto-South China Sea (PSCS) slabs beneath present Palawan, northern Borneo, and offshore Palawan; some models also predicted slabs under the southern South China Sea. PSCS slabs generated from double-sided PSCS subduction and earlier Borneo rotation generated a slightly better fit to tomography but pure southward PSCS subduction was also viable. A smaller Philippine Sea plate (PSP) with a short ~1000 km restored northern slab (i.e. Ryukyu slab) was clearly superior to a very long >3000 km slab. Mantle flows generated from our geodynamic models suggest strong upwellings under Indochina during the late Eocene to Oligocene. Our models generated strong downwellings under the South China Sea in the late Cenozoic that did not support a deep-origin ‘Hainan plume’. 

The following plate models variants were assimilated in the geodynamic models: (1) southward vs. double-sided PSCS subduction; (2) early Borneo counterclockwise rotations during the Oligocene to Early Miocene vs. later rotations (mid- to Late Eocene and Early Miocene); (3) a smaller Philippine Sea plate restored with a shorter ~1000 km northern slab vs. a longer >3000 km slab. This study assimilates four different plate models into the numerical model TERRA (Bunge et al., 1998). We digitally re-built in GPlates (Boyden et al., 2011) the implemented the plate models as a set of continuously closing plates in order to generate a global self-consistent velocity field to be assimilated into the convection models. The temperature fields were converted to seismic velocities assuming a Pyrolite composition and equilibrium mineralogy. We quantify the correlation between our geodynamic models and seismic tomography within SE Asia. For the tomography models S40RTS and LLNL-G3Dv-JPS we explicitly accounted for their finite resolution (Ritsema et al., 2011; Simmons et al. 2019).

How to cite: Lin, Y.-A., Colli, L., and Wu, J.: Where are the proto-South China Sea slabs? SE Asia plate tectonic and mantle flow insights from TERRA global mantle convection models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12407, https://doi.org/10.5194/egusphere-egu2020-12407, 2020.

The Caribbean region has been proposed as a candidate for outflow of asthenospheric mantle, from a shrinking Pacific region to an expanding Atlantic region. If this flow exists it should be associated to a dynamic topography gradient across the region. Estimating dynamic topography requires constraining the thicknesses and densities of sediment, crust and lithosphere to remove their isostatic response from the total topography. Dynamic topography has been studied globally in areas of ‘normal’ oceanic lithosphere but the Caribbean region, characterized by overthickened oceanic lithosphere, has not been fully analyzed due to the challenges of estimating crustal thicknesses.

Thanks to the wealth of seismic reflection, as well as borehole data, the basement relief and bulk sediment density in the Caribbean are well constrained. We performed a structural inversion of free air gravity anomalies, constrained by seismic refraction data, to established an improved Moho surface which provides more detail than existing global models such as Crust 1.0. With the improved basement and Moho relief, we computed residual basement depth. We obtained a ~300 m dynamic topography high on the Pacific-side of the Caribbean, gradually decaying to 0 m to the east near the Aves ridge.

This result supports the hypothesis of Pacific outflow through the Caribbean. Assuming a ~200 km thick asthenosphere and a flow velocity a few to a few tens of cm/yr, as suggested by tomographic imaging and regional magmatism, our results suggest the viscosity is ~5*10¹⁸ Pa s.

How to cite: Chen, Y.-W., Colli, L., Bird, D. E., Wu, J., and Zhu, H.: Asthenosphere viscosity in the Caribbean region constrained by gravity anomalies, seismic structure and regional magmatism, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12682, https://doi.org/10.5194/egusphere-egu2020-12682, 2020.

Two dimensional numerical models of mantle convection in a cylindrical shell provide a possible geodynamic explanation for cold patches in the mantle below India and Mongolia as detected by seismic tomography. We investigate the influence of very high viscosities at mid-mantle and lower-mantle depths, as proposed by Mitrovica and Forte (2004) and Steinberger and Calderwood (2006), on mantle convective flow.  Models are considered with and without mineral phase transitions.  Our viscosity profiles are depth dependent with deep mantle viscosities increasing to values of 300 times the viscosity of the upper mantle, and then decreasing dramatically on approaching the core-mantle boundary.   The decrease of viscosity near the CMB mobilizes the overall mantle-wide flow despite very high mid-mantle viscosities.  However, cold detached slabs sinking below continental collisions become captured by the high viscosity interior and circulate slowly for times exceeding 200 Myr.  The separation of time scales for mantle-wide flow vs slab circulation, is a consequence of the high viscosity of the mid-mantle.

How to cite: Jarvis, G.: Slab Remnant Recycling and Mantle-Wide Convection: A Separation of Time Scales, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2032, https://doi.org/10.5194/egusphere-egu2020-2032, 2020.

Polarities of seismic reflection of P and S-waves at the discontinuity at the top of  D" are usually assumed to indicate the sign of the velocity contrast across the D" reflector. For reflections in paleo-subduction regions the S-wave reflections off D" (SdS) are the same as ScS and S, indicating a positive velocity contrast at the reflector. In recent years, an opposite polarity of PdP waves (P-reflection at the D" discontinuity) has been observed in some regions, partly dependent on travel direction, partly dependent on distance. This would indicate a velocity reduction in P-waves where a velocity increase is detected in S-waves. This phenomenon can be explained with the presence of post-perovskite below the top of D", but azimuthal dependence of PdP polarities can be better explained with anisotropy. Here we re-analyse PdP and SdS wave polarities and, when modelling the polarities and amplitudes using Zoeppritz equations, we find that a ratio of dVs/dVp= R of larger than 3 reverses polarities of P-waves in the absence of anisotropy, i.e. we find a polarity of PdP that would point to a velocity decrease while modelling a velocity increase. The S-polarity stays the same as S and ScS and does not change even with large R. Values of R up to 4.1 have been reported recently, so these cases do exist in the lower mantle. Using a set of 1 million models with varying minerals and processes across the boundary, we carry out a statistical analysis (Linear Discriminant Analysis, LDA) and find that there is a marked difference in mantle mineralogy to explain R values larger and smaller than 3, respectively. The regime of cases with R-value larger than 3 is mostly due to an increase in MgO and post-perovskite across the discontinuity. In regions where high R is observed, alternate explanations of lowermost mantle composition versus anisotropy can then be tested by measuring polarities in different azimuths.

How to cite: Thomas, C., Cobden, L., and Jonkers, A.: A new look at polarity information in D" reflections, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18402, https://doi.org/10.5194/egusphere-egu2020-18402, 2020.

The Saffman-Taylor instability, a fluid dynamical phenomenon, occurs when a less viscous
fluid is injected into a more viscous fluid, leading to the development of radial miscible viscous
fingers. Approximately five horizontal fingers radiate away from the Icelandic plume at a
depth range of 100 km–200 km. These fingers are manifest as shear wave velocity anomalies in
full-waveform tomographic models. The best resolved fingers lie beneath the British Isles and
beneath western Norway, extending ∼1,000 km away from the Icelandic plume conduit. The
number and wavelength of miscible viscous fingers are controlled by Péclet number (i.e. the
ratio of advective and diffusive transport rates), mobility ratio (i.e. the ratio of fluid viscosities)
and thickness of the horizontal layer into which the fluid is injected. Observational estimates
for the Icelandic plume suggest the Péclet number is O(10⁴), the mobility ratio is at least 20–
50 and the asthenospheric channel thickness is 100 ± 20 km. Appropriately scaled laboratory
experiments play a key role in developing a quantitative understanding of the spatial and
temporal evolution of the Icelandic plume planform. During laboratory experiments, Péclet
number is varied primarily by changing the flow rate as well as the altering the thickness of
the horizontal layer. Viscosity contrasts are generated by using glycerol and water mixtures
which are miscible, like plume material with ambient mantle. However there is no temperature
contrast in the experiments, which is probably significant in the mantle. Comparison between
scaled analogue experiments and observed values suggests the fluid dynamics may be more
complex than the Saffman-Taylor instability alone. Additional processes including influence
of temperature, interaction with the base of the lithospheric plate or small-scale convection,
along with the Saffman-Taylor instabillity, may be the origin of the fingers imaged by seismic
tomography.

How to cite: Galbraith-Olive, P. H., White, N., and Woods, A.: Radial Miscible Viscous Fingering of the Icelandic Plume, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-524, https://doi.org/10.5194/egusphere-egu2020-524, 2020.

Geodynamic reconstructions are largely based on information contained in mafic igneous rocks, including dykes and sills. The age and isotope-geochemical characteristics of such rocks are inevitable for understanding of geodynamic history of the Proterozoic cratons. The regions in Siberian Craton, where Precambrian mafic dyke swarms are known are following: Anabar Shield and Olenek Uplifts, Aldan-Stanovoi Shield, SE area of Siberian Craton, and smaller Uplifts on the SW margin of Siberian Craton.

The Udzha paleo-rift is located in the northern part of Siberian Craton between Anabar and Olenek Uplifts is also associated with mafic dyke swarm. These dykes cross-cut the pre-Neoproterozoic sedimentary successions. The age of the largest dyke in Udzha paleo-rift (Great Udzha Dyke) presented by medium-grained dolerite was determined to be 1386 ± 30 Ma (Malyshev et al., 2018).

We present new data of Sr, Nd and Pb isotopic composition on the Udzha paleo-rift dykes, determined by TIMS. The initial isotopic composition of Pb in the dykes was obtained using the leaching method by Savatenkov et al., 2019. The Sr isotopic composition of the dykes demonstrates substantial variation (εSr varies from 8.4 to 110.4). We do not consider this fact as a result of crust contamination, because Nd isotopic composition does not vary significantly (εNd varies from -1.4 to 0.7). Obtained results indicate that initial for the Udzha paleo-rift dykes melts were generated from two mantle reservoirs of DM and EMII-type. The initial Pb isotopic composition of the dykes reveals EMII source participation in the melts generation too (²⁰⁶Pb/²⁰⁴Pb varies from 16.133 to 16.266, ²⁰⁷Pb/²⁰⁴Pb varies from 15.343 to 15.458). The presence of enriched component is likely associated with lithospheric mantle, metasomatized by fluids, derived from subducted terrigenous material.

The studies were supported by the Russian Science Foundation project No. 19-77-10048.

References

Malyshev, S. V., Pasenko A. M., Ivanov A. V., Gladkochub D. P., Savatenkov V. M., Meffre S., Abersteiner A., Kamenetsky V. S. & Shcherbakov V. D. (2018): Geodynamic Significance of the Mesoproterozoic Magmatism of the Udzha Paleo-Rift (Northern Siberian Craton) Based on U-Pb Geochronology and Paleomagnetic Data. – Minerals, 8(12), 555

Savatenkov V. M., Malyshev, S. V., Ivanov A. V., Meffre S., Abersteiner A., Kamenetsky V. S., Pasenko A. M. (2019): An advanced stepwise leaching technique for derivation of initial lead isotope ratios in ancient mafic rocks: A case study of Mesoproterozoic intrusions from the Udzha paleo-rift, Siberian Craton. – Chemical Geology, 528, 119253

How to cite: Shpakovich, L., Malyshev, S., and Savatenkov, V.: Pb, Nd, Sr isotopic composition of the Mesoproterozoic mafic intrusions (Udzha paleorift, Northern Siberia), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11081, https://doi.org/10.5194/egusphere-egu2020-11081, 2020.

Elucidating the role of deep mantle plumes in mantle convection is challenging because their influence on seismic waveforms – which could be used to map their location – is subtle. Previous seismic studies have mainly focused on waveform modelling and inversion (i.e. tomography). In this study we instead consider the potential visibility of mantle plumes using array methods. We investigate, in particular, how plumes deviate seismic energy from the great-circle path. This requires a multidisciplinary approach: first, we perform geodynamic modelling to generate thermochemical plumes, and convert them to “seismic” plumes via thermodynamic modelling of mineral physics data. Next, spectral element methods are used to model the interaction of seismic waves with the plumes and generate synthetic seismograms. These seismograms are divided into arrays and we generate slowness-backazimuth plots for each array. With recent advances in computational methods and resources, we investigate wave behaviour at previously unattainable frequencies.  We find that plumes do indeed cause seismic waves to change direction, although the exact behaviour may be frequency-dependent, and at low frequencies we observe waves apparently bending around the plume conduit.  We consider how and where these results may be applied to real seismic arrays, to provide new constraints on the location and structure of mantle plumes.

How to cite: Cobden, L., Afanasiev, M., Deschamps, F., Stockmann, F., Thomas, C., Rost, S., and Fichtner, A.: Probing mantle plumes using seismic arrays, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7491, https://doi.org/10.5194/egusphere-egu2020-7491, 2020.

A clear understanding of the transition from a liquid magma ocean (MO) to a convective solid mantle is still lacking. Part of the problem is that there is still no clear view of all the physical phenomena at play during this crucial stage. As the MO cools down, the formation of a solid and therefore very viscous lithosphere at its surface has often been considered to trigger a new pattern of motion where convection occurs below the lithosphere which remains stagnant. However, when the liquid thermal boundary layer at the top of the MO cools down, it first becomes a mushy lithosphere through which melt and exsolved gas bubbles can still percolate to the surface. Using laboratory experiments of thermal convection in colloidal suspensions, we study the formation of this mushy lithosphere and its different regimes of deformation and coupling to mantle convection. We observe that deformation of the lithosphere can include « heat pipe » formation at high heat, melt and volatile flux. On the other hand, rapid thermal contraction of the lithosphere can cause buckling, leading to subduction. Transition from MO to solid-state convection could involve both processes in succession , or in competition, depending on the temperature and volatiles conditions. 

How to cite: Davaille, A. and Massol, H.: From Magma Ocean turbulent convection to lithosphere formation and mantle convection: insights from laboratory experiments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10746, https://doi.org/10.5194/egusphere-egu2020-10746, 2020.

The Middle-Late Permian Emeishan large igneous province (ELIP), located in the western margin of Yangtze craton, SW China, is regarded as the result of the impingement of a mantle plume onto the lithosphere. However, little is known about the petrogenesis of Late Permian basalts in Sichuan Basin, which were previously considered to be located outside the ELIP. Here we report new petrographic, major elements, trace elements and isotopic data (Sr-Nd-Pb) for Late Permian basalts in the boreholes from the northwestern Sichuan Basin. These basaltic rocks are characterized by low SiO₂ contents (47.17-49.40 wt.%), high TiO₂ contents (3.38-4.11 wt.%) and Ti/Y ratio (539-639), moderate total alkalis contents (Na₂O+K₂O, 3.36-6.01 wt.%) and Mg^# values (40.93-46.04), which geochemically resemble the Emeishan high-Ti basalts. These rocks are enriched in large ion lithophile elements (LILEs) and light rare earth elements (LREE), and have (La/Yb)_N ranging from 9.95 to 11.78, showing that typical oceanic island basalt (OIB)-like normalized patterns. The fractionation of MREE to HREE suggests that the basalts were generated by low degree of partial melting within the garnet stability field. Low initial ⁸⁷Sr/⁸⁶Sr ratios (0.70572-0.70676; t=260 Ma), Pb isotopic ratios [²⁰⁶Pb/²⁰⁴Pb_(t) (18.062-18.637), ²⁰⁷Pb/²⁰⁴Pb_(t) (15.574-15.641), ²⁰⁸Pb/²⁰⁴Pb_(t) (38.33-38.98)], and slightly high εNd(t) values (-0.03 to +1.34) indicate that the magma formed from a deep mantle source that may possibly be a mantle plume and have negligibly been affected by crustal contamination. This inference is further supported by high Nb/U ratios (20.56-25.70), low Th/Nb (0.17-0.19) and Th/Ta ratios (2.77-3.14), and no visible Nb and Ta anomalie. In addition, thermal history reconstruction using paleogeothermal indicators in the study area shows that the Lower Paleozoic to Middle Permian formations experienced an intensive thermal event and abnormal high heat flow value reached 118.0 mW/m² at the Late Permian, which may be due to the mantle plume magma upwelling. The geochemical and geothermal characteristics all demonstrate that these basalts were probably generated in response to the Emeishan mantle plume. Thus, we conclude that the ELIP may have larger areal extent and has been played an important role on the thermal evolution of source rocks in the Sichuan basin.

How to cite: Liu, X. and Qiu, N.: Late Permian basalts in the northwestern Sichuan Basin, SW China: Implications for the geodynamics and thermal effect of the Emeishan mantle plume, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6821, https://doi.org/10.5194/egusphere-egu2020-6821, 2020.

How to cite: Dongmo wamba, M., Romanowicz, B., Montagner, J.-P., and Barruol, G.: A narrow plume conduit anchored in the lower mantle beneath la Réunion hotspot, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9135, https://doi.org/10.5194/egusphere-egu2020-9135, 2020.

How to cite: Mandea, M., Dehant, V., and Cazenave, A.: GRACEFUL: Probing the deep Earth interior by synergistic use of observations of the magnetic and gravity fields, and of the rotation of the Earth, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13515, https://doi.org/10.5194/egusphere-egu2020-13515, 2020.

Radio signals from distant quasars allow us to determine Earth's rotation variations with exquisite accuracy. These observations can be used to estimate the amplitudes, frequencies and damping constants associated with Earth's rotational modes, particularly the Free Core Nutation (FCN) and the Free Inner Core Nutation (FICN). These estimates suggest, however, fluid core viscosities many orders of magnitude higher than expected, or rms magnetic fields at the core-mantle boundary (CMB) incompatible with downward continuation of the observed surface field. Aiming at resolve this difficulty, we have developed a proof-of-concept model where we incorporate an approximate fluid-dynamical treatment of the core flow associated with the FCN and the FICN. We show that, at least for the FCN, no abnormally high viscosities or magnetic fields are required. The model might provide in fact a robust, independent estimate of the rms magnetic field strength in the fluid core. Additionally, the model illustrates the importance of considering inter-mode resonances involving inertial modes (i.e. Coriolis-restored) and the rotational normal modes.

How to cite: Triana, S., Trinh, A., Rekier, J., and Dehant, V.: Ohmic dissipation induced by Earth's nutation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11740, https://doi.org/10.5194/egusphere-egu2020-11740, 2020.

Nonlinear analysis in a rotating Rayleigh-Benard system of electrically conducting fluid is studied numerically in the presence of externally applied horizontal magnetic field with rigid-rigid boundary conditions [1, 2]. This DNS approach is carried near the onset of convection to study the flow behaviour in the limiting case of Prandtl number [2]. The flow topology is verified with respect to the Euler number. The fluid flow is visualized in terms of streamlines, limiting streamlines, isotherms and heatlines. The dependence of the Nusselt number on the Rayleigh number, Ekman number, Elsasser number is examined.

References:

[1] S. Chandrasekhar, Hydrodynamic and Hydromagnetic Stability, 1961, Oxford University Press, London.

[2] P.H. Roberts and C.A. Jones, The onset of magnetoconvection at large Prandtl number in a rotating layer I. Finite Magnetic Diffusion, Geophysical and Astrophysical Fluid Dynamics, 92, 289-325 (2000).

How to cite: Rani, H. P., Rameshwar, Y., Brestensky, J., and Filippi, E.: Nonlinear Convection of Electrically Conducting Fluid in a Rotating Magnetic System, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21322, https://doi.org/10.5194/egusphere-egu2020-21322, 2020.

The magnetohydrodynamics of Earth has been explored at the University of Maryland through experiments and numerical models. Experimentally, the interaction between Earth's magnetic fields and its outer core is replicated using a three-meter spherical Couette device filled with liquid sodium that is driven by two independently rotating concentric shells and a dipole magnetic field applied from external electromagnets. Currently, this experiment is being prepared for design modifications that aim to increase the helical flows in the poloidal direction in order to match the turbulence of convection-driven flows of Earth. The experiment currently has 33 hall probes measuring the magnetic field, 4 pressure probes, and torque measurements on each sphere. We supplement the experiment with a numerical model, XSHELLS, that uses pseudospectral and finite difference methods to give a full picture of the velocity and magnetic field in the liquid and stainless steel shells. However, its impracticable to resolve all the turbulence. Our ultimate goal is to implement data assimilation by synchronizing the experimental observations with the numerical model, in order to uncover the unmeasured velocity field in the experiment and the full magnetic field as well as to predict the magnetic fields of the experiment. Through numerical simulations (XSHELLS) and data analysis we probe the behavior of the experiment in order to (i) suggest the best locations for new measurements and (ii) find what parameters are most feasible for data assimilation. These computational studies provide insight on the dynamics of this experiment and the measurements required to predict Earth's magnetic field. We gratefully acknowledge the support of NSF Grant No. EAR1417148 & DGE1322106.

How to cite: Burnett, S., Schaeffer, N., Ide, K., and Lathrop, D.: Exploring Alternative Instrumentation in the Three-Meter Spherical Couette Experiment, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10723, https://doi.org/10.5194/egusphere-egu2020-10723, 2020.

We have studied theoretically the weakly nonlinear analysis in a rotating Rayleigh-Benard system of electrically conducting fluid in the presence of applied horizontal magnetic field with free-free boundary conditions [1]. This theoretical approach is carried near the onset of convection to study the flow behavior at the occurrence of cross rolls, which are perpendicular to the applied magnetic field. The nonlinear problem is solved by using the Fourier analysis of perturbations up to the O(ε⁸) to study the cross rolls visualization [2,3]. The dependence of the Nusselt number on the Rayleigh number, Ekman number, Elsasser number is extensively examined. The fluid flow is visualized in terms of kinetic energy, potential energy, streamlines, isotherms, and heatlines.

References :

[1] P. H. Roberts and C. A. Jones , The Onset of Magnetoconvection at Large Prandtl Number in a Rotating Layer I. Finite Magnetic Diffusion, Geophysical and Astrophysical Fluid Dynamics, Vol. 92, pp. 289-325 (2000).

[2] H.L. Kuo, Solution of the non-linear equations of the cellular convection and heat transport,  Journal of Fluid Mechanics,  Vol.10, pp.611-630 (1961).

[3] Y. Rameshwar, M. A. Rawoof Sayeed, H. P. Rani, D. Laroze, Finite amplitude cellular convection under the influence of a vertical magnetic field, International Journal of Heat and Mass Transfer, Vol. 114, pp.  559-577 (2017).

How to cite: Rameshwar, Y., Srinivas, G., Rani, H. P., Brestensky, J., and Filippi, E.: Convection of Electrically Conducting Fluid in a Rotating Magnetic System: Cross rolls, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21632, https://doi.org/10.5194/egusphere-egu2020-21632, 2020.

The Earth is submitted to the gravitational effect of different objects,  resulting  in  small  variations  of  the  orientation  of  its  axis  rotation.   The  precession corresponds to the rotation of the body spin axis around the normal to the elliptic plane. The primary flow forced by precession in a sphere is mainly a tilted solid body rotation, a flow of uniform vorticity. In this study we focused on the pseudo-resonance between the precessional forcing  and  the  spin-over  mode,  detected  as  a  peak  of  amplitude  of  the  norm  of the  vorticity  of  the  fluid.   We  show  the  influences  of  both  the  geometry and the application of an uniform external magnetic field on the external boundary, onto this pseudo-resonance.  The major purpose is to validate a semi-analytical model to allow its interpolation to planetary bodies.  We compared the semi-analytical model [Noir and C ́ebron, 2013] with numerical simulations performed with XSHELLS [Schaeffer, 2013],  which give us the components of the fluid vorticity in a precessing frame. We compared also the spin-over mode coefficients, used to simulate the viscous  effect  on  the  model,  with  two  methods :  an  empirical  equation  and  the numerical solver Tintin [Triana et al., 2019], taking into account the solid inner-core size (η=RI/R).  The differential rotation between the flow and the container, obtained with the model and the XSHELLS simulations, show us a verygood agreement especially for a small Ekman number (E= 10^−5), thus the spin-over mode coefficients for small E and η≤0.5.  An increase of the inner-core size  implies  a  decrease  of  the  resonance  amplitude  caused  by  the  supplementary Ekman layer added at the Inner Core Boundary (ICB); nevertheless thecolatitude (αf) and the longitude (φf) of the fluid don’t change significantly.The  application  of  a  uniform  magnetic  field  at  the  CMB  implies  a  decrease of the resonance amplitude, but also a modification of the mean rotation axis direction.  Indeed, the coupling between the viscous flow and the magnetic field induces a modification of the αfand φf, which follow the main direction angle of the magnetic field axis.  We observe small discrepancies between the simulations (XSHELLS and Tintin) and the model but the behavior following different parameters (Po,α angle,Ro,η,β angle, Λ) is well understood.  As a result, we applied the models at few parameter ”realistic values” of planetary objects like terrestrial planets but also ice’s satellites.

References

[Noir and C ́ebron, 2013]  Noir,  J.  and  C ́ebron,  D.  (2013).    Precession-driven flows in non-axisymmetric ellipsoids.Journal of Fluid Mechanics, 737:412–439.

[Schaeffer, 2013]  Schaeffer, N. (2013).  Efficient spherical harmonic transforms aimed  at pseudospectral numerical  simulations.Geochemistry, Geophysics,Geosystems, 14(3):751–758.

[Triana et al., 2019]  Triana, S. A., Rekier, J., Trinh, A., and Dehant, V. (2019).The coupling between inertial and rotational eigenmodes in planets with liq-uid cores.Geophysical Journal International.

How to cite: Laguerre, R., Houliez, A., Cébron, D., and Dehant, V.: Steady flows in the core of precessing planets : effects of the geometry and an applied magnetic field. , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14058, https://doi.org/10.5194/egusphere-egu2020-14058, 2020.

The release of latent heat and lighter materials during inner core solidification is the driving force of the liquid iron flow in the outer core which generates the Earth's magnetic field. It is well known that the behaviour of the magnetic field varies over long time scales. Two clearly identifiable regimes are recognized, (i) superchrons and (ii) periods of hyperactivity (Biggin et al. 2012). Superchrons are characterized by an exceptionally low reversal rate of the magnetic pole and are associated with a low core mantle boundary (CMB) heat flux. Hyperactive periods are defined by a high reversal rate and have a high CMB heat flux.

Here we investigate whether the occurrence of these two regimes is related to radial variations in inner core seismic structure. Using seismic body-wave observations of compressional PKIKP-waves (Irving & Deuss 2011, Waszek & Deuss 2011, Lythgoe et al. 2013)., we construct a model of inner core anisotropy by comparing the difference between travel times for polar and equatorial rays. Anisotropy is the directional dependence of wave velocity and is determined by the structure of iron crystals in the inner core, hence changes in seismic anisotropy are due to changes in inner core crystal texture. We invert for radial changes in anisotropy while allowing for lateral variations and find that a model of the inner core containing five layers best fits our data. The model contains an isotropic uppermost inner core and four deeper layers with varying degrees of anisotropy.

Texture differences of the inner core iron crystals have been linked to changes in the solidification process of the inner core (Bergman et al. 2005), i.e. the motor of outer core flow. Therefore, the observed anisotropy variation, caused by variations of inner core solidification, might be related to changes in the behaviour of the magnetic field. Using an inner core growth model (Buffett et al. 1996) we convert depth to time for a range of inner core nucleation ages between 3.0 and 0.5 Ga (Olsen 2016). We find a remarkable correlation between the solidification time of the seismically observed layers and the occurrence of the magnetic regimes for two inner core ages; one with a nucleation at 1.4 Ga and one at 0.6 Ga, corresponding to an average CMB heat flux of 7.6 TW and 16.7 TW respectively.

Although we currently cannot differentiate between these two inner core ages considering our results alone, they do show that a relation between inner core structure and the behaviour of the magnetic field is possible, and suggest that seismic observations of inner core structure might provide new and independent insights into the magnetic field and its history.

How to cite: de Jong, J., de Groot, L., and Deuss, A.: Observing the signature of the magnetic field's behaviour in the radial variation of inner core anisotropy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20504, https://doi.org/10.5194/egusphere-egu2020-20504, 2020.

Determining the high pressure and temperature behavior of iron (Fe) provides valuable insight into the evolution and dynamics of the Earth’s core. Shock compression using lasers can achieve extreme pressure and temperature conditions simultaneously. The duration of the extreme conditions state is on the order of nanoseconds. This is a challenge for in situ measurements of the shocked material’s properties. In this work, we shock-compressed polycrystalline iron at the Matter in Extreme Conditions End Station at the Linac Coherent Light Source, SLAC National Accelerator Laboratory and performed in situ X-ray diffraction (XRD) measurements with sub-picosecond time resolution. The final aim of these experiments is the study of stress of texure in Fe under extreme conditions of pressure and temperature. The presentation will highlight the strategies for such experiment and data processing and present our premilinary results.

How to cite: Merkel, S., Hok, S., Bolme, C., Gleason, A., and Mao, W.: Understanding strength and texture in Fe at planetary core pressures and temperatures: insights from laser compression experiments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22379, https://doi.org/10.5194/egusphere-egu2020-22379, 2020.