GD – Geodynamics

GD1.1 – Early Earth: Dynamics, Geology, Chemistry and Life in the Archean Earth

EGU2020-16896 | Displays | GD1.1

The Hadean origin of the Archean Napier Complex (East Antarctica)

Martin Guitreau, Maud Boyet, Jean-Louis Paquette, Abdelmouhcine Gannoun, Zoltan Konc, Mhammed Benbakkar, Krzysztof Suchorski, and Jean-Marc Hénot

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 ɛHf 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 ɛNd 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,, 2020.

EGU2020-18156 | Displays | GD1.1

Large Oxygen and hafnium isotopic variations in zircon from the Saglek Block (North Atlantic Craton) document reworking of mature supracrustal rocks as early as 3.5 Ga

Emilie Thomassot, Vezinet Adrien, Graham Pearson, Richard Stern, Yan Luo, and Chiranjeeb Sarkar

The most ancient rocks in the geological record provide insights into the processes that shaped the evolution and composition of the first continental masses. Due to both the scarcity and the polymetamorphic history of exposed Eoarchean (>3.5 Ga) crust, the study of early geodynamic processes is very challenging and most of our knowledge has been learned from only a few localities on Earth.

The present study focuses on felsic meta-igneous rock from the Saglek Block (North Atlantic Craton), a locality where recent zircon U-Pb dating studies indicate earliest crust formation in the Eoarchean (Komiya et al., 2017; Sałacińska et al., 2018; Vezinet et al., 2018). We performed in situ oxygen isotopes measurement (SIMS analyses) in zircon grains that have been carefully selected from CL-imaging for the good preservation of their internal structure and for their pristine composition in rare Earth element. We then performed U-Pb/Hf isotopes by laser ablation split stream (LASS)-ICP-MS. The results indicate 3 distinct crystallization events: (1) an Eoarchean event at ca 3.86 Ga; (2) an early Paleoarchean metamorphic event at ca. 3.5 Ga, and (3) a Neoarchean event (ca. 2.7-2.8 Ga) with zircon domains showing complex zoned overgrowths. While the 3.86 Ga magmatic domains display mantle-like δ18O(+4.9±0.2‰ to +6.8.0±0.2‰, n=30), large O-isotope fractionation (δ18Ovalues up to +9‰) characterise the Paleoarchean metamorphic event. Such elevated δ18O signatures provide unequivocal evidence for hydrosphere–crust interactions and reworking processes resulting in metamorphic zircon growth at ca. 3.5 Ga, namely 1 Ga before the Archean-Proterozoic transition (Vezinet et al., 2019).

Interestingly, the two oldest age groups have chondritic to sub-chondritic εHfi values: +1.0 ± 2.2 to –5.5 ± 1.8 whereas large variations in Hf isotope composition (εHfi value from –11.2 ± 2.5 to –20.3 ± 1.5) are found in the 2.8–2.7 Ga zircon domains. Such intra-sample heterogeneities implies a significant perturbation of Hf-isotope composition during metamorphic events related to mixing of fluid with inherited (older) Hf isotope source. In the light of these results, we will discuss the potential consequences of isotope perturbation on whole-rock isochrones interpretation.


Komiya, T., et al. "A prolonged granitoid formation in Saglek Block, Labrador: Zonal growth and crustal reworking of continental crust in the Eoarchean." Geoscience Frontiers 8.2 (2017): 355-385.

Sałacińska, A., et al. "Complexity of the early Archean Uivak Gneiss: Insights from Tigigakyuk Inlet, Saglek Block, Labrador, Canada and possible correlations with south West Greenland." Precambrian Res. 315 (2018): 103-119.

Vezinet, A, et al. "Hydrothermally-altered mafic crust as source for early Earth TTG: Pb/Hf/O isotope and trace element evidence in zircon from TTG of the Eoarchean Saglek Block, N. Labrador. EPSL 503 (2018): 95-107.

Vezinet, A., et al. "Extreme δ18O signatures in zircon from the Saglek Block (North Atlantic Craton) document reworking of mature supracrustal rocks as early as 3.5 Ga." Geology 47.7 (2019): 605-608.

How to cite: Thomassot, E., Adrien, V., Pearson, G., Stern, R., Luo, Y., and Sarkar, C.: Large Oxygen and hafnium isotopic variations in zircon from the Saglek Block (North Atlantic Craton) document reworking of mature supracrustal rocks as early as 3.5 Ga, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18156,, 2020.

EGU2020-9784 | Displays | GD1.1

How can REE-bearing minerals help us refine our understanding of crustal evolution and Archean tectonics?

Emilie Bruand, Craig Storey, and Mike Fowler

Delineating the evolution of the Earth’s dynamics and interactions between its different silicate reservoirs (ocean crust, continental crust, mantle) is key to understanding planetary differentiation and the conditions of surface habitability. Today, plate tectonic processes play a major role in creating and destroying the Earth’s crust, and modifying its silicate mantle. For this reason the Earth is unique in the solar system. Reconstructing its long-term evolution is, however, extremely difficult since the Hadean record is essentially missing and most Archean rocks have experienced reworking and overprinting of their original signatures. 

In this presentation, we will explore the constraints available with isotopic and chemical information from REE-bearing minerals in magmas that appear at different times during Earth history. We present, new geochemical data on these phases from a compilation of granitoids that cover a large span of the geological record from the Archean to the Phanerozoic. We demonstrate that trace element analysis and detailed petrographic work can give direct information about the petrogenesis of the host magmas even when these granitoids have been affected by metamorphism. Other studies focusing on rutile have shown that it records important information on metamorphic conditions in the Archean. On the other hand, and also helpfully, all three minerals are resistant to secondary processes and erosion, and thus may also offer significant archives of pertinent information in the detrital rock record. Development of such petro-geochemical tools could deliver complementary information to that provided by zircon and have significant potential for provenance studies and for tracing the secular evolution of the Earth.

How to cite: Bruand, E., Storey, C., and Fowler, M.: How can REE-bearing minerals help us refine our understanding of crustal evolution and Archean tectonics?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9784,, 2020.

EGU2020-9164 | Displays | GD1.1

Evolution of the Earth’s polar wind escape from mid-Archean to present

Kristina Kislyakova, Colin Johnstone, Manuel Scherf, Helmut Lammer, Mats Holmström, Maxim Khodachenko, and Manuel Güdel

The evolution of habitable conditions on Earth is tightly connected to the evolution of its atmosphere which, in turn, is strongly influenced by atmospheric escape. We investigate the evolution of the the polar wind outflow from the magnetic cusps which is the dominant escape mechanism on the Earth. We perform Direct Simulation Monte Carlo (DSMC) simulations and estimate the upper limits on escape rates from the Earth's cusps starting from three gigayears ago (Ga) to present assuming the present-day composition of the atmosphere. We perform one additional simulation with a lower mixing ratio of oxygen of 1% to account for the conditions shortly after the Great Oxydation Event (GOE). We account for the evolution of the magnetic field of the Earth by adjusting the polar opening angle and the location of the magnetosphere's substellar point.

Our results present an upper limit on the escape rates, but they indicate that polar wind escape rates for nitrogen and oxygen ions were likely much higher in the past.  We estimate the maximum total loss rates due to polar wind of 2.0x1018 kg and 5.2x1017 kg for oxygen and nitrogen, respectively. According to our results, the main factors that governed the polar wind outflow in the considered time period are the evolution of the XUV radiation of the Sun and the atmosphere's composition. The evolution of the Earth's magnetic field plays a less important role. We conclude that although the atmosphere with the present-day composition can survive the escape due to polar wind outflow, a higher level of CO2 between 3.0 and 2.0 Ga is likely necessary to reduce the escape.

How to cite: Kislyakova, K., Johnstone, C., Scherf, M., Lammer, H., Holmström, M., Khodachenko, M., and Güdel, M.: Evolution of the Earth’s polar wind escape from mid-Archean to present, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9164,, 2020.

EGU2020-8384 | Displays | GD1.1 | Highlight

Streamer discharges in the atmosphere of Primordial Earth

Martin Bødker Enghoff, Nikolaos Segkos, Sasa Dujko, Olivier Chanrion, and Christoph Köhn

Motivated by the Miller-Urey experiment suggesting that lightning may have contributed to the origin of life on Earth through the formation of amino acids and carbonic acids, we here investigate the occurrence of electric discharges in the atmosphere of Primordial Earth. We focus on the early stages of lightning in the atmosphere of Primordial Earth, the so-called streamers, thin ionized plasma channels.

We study electron avalanches and potential avalanche-to-streamer transitions by modeling the motion of electrons with a particle-in-cell Monte Carlo code in gas mixtures of H2O:CH4:NH3:H2=37.5%:25%:25%:12.5% [S. L. Miller. Production of Some Organic Compounds under Possible Primitive Earth Conditions. Am. Chem. Soc., 77:9, pp. 2351-2361 (1955)] and N2:CO2:H2O:H2:CO=80%:18.89%:1%:0.1%:0.01% [J. F. Kasting. Earth’s Early Atmosphere. Science, 259:5097, pp. 920-926 (1993)] suggested for Primordial Earth approx. 3.8 Ga ago in different electric fields and for different levels of background ionization mimicking the photoionization process. We compare the evolution of the electron density,  electric field, and electron energies with those for Modern Earth. Finally, we will discuss which conditions favour streamer inception, as well as consequences for discharges on Primordial Earth.

How to cite: Enghoff, M. B., Segkos, N., Dujko, S., Chanrion, O., and Köhn, C.: Streamer discharges in the atmosphere of Primordial Earth, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8384,, 2020.

EGU2020-12841 | Displays | GD1.1

Molecular Dynamics simulations indicate solvation and stability of single-strand RNA at the air/ice interface, supporting a primordial RNA world on Ice

Steven Neshyba, Ivan Gladich, Penny Rowe, Maggie Berrens, and Rodolfo Pereyra

Outstanding questions about the RNA world hypothesis for the emergence of life on Earth concern the stability and self-replication of prebiotic aqueous RNA. Recent experimental work has suggested that solid substrates and low temperatures could help resolve these issues. Here, we use classical molecular dynamics simulations to explore the possibility that the substrate is ice itself. We find that at -20 C, a quasi-liquid layer at the air/ice interface partially solvates a short (8-nucleotide) RNA strand such that the phosphate backbone anchors to the underlying crystalline ice structure though long-lived hydrogen bonds. The hydrophobic bases, meanwhile, are seen to migrate toward the outermost layer, exposed to air. Our simulations also reveal two key kinetic differences with respect to aqueous RNA. First, hydrogen bonds between solvent water molecules and phosphate diester moieties, believed to shield the RNA from hydrolysis, are much longer-lived for RNA on ice, compared to aqueous RNA at the same temperature. Second, contact between solvent water and ribose 2-OH’ groups, considered a precursor to nucleophilic attack by deprotonated 2-OH’ on the phosphate diester, is significantly less frequent for RNA on ice. Both differences point to lower susceptibility to hydrolysis of RNA on ice, and therefore increased opportunities for polymerization and self-copying compared to aqueous RNA. Moreover, exposure of hydrophobic bases at the air/ice interface offers opportunities for reaction that are not readily available to aqueous RNA (e.g., base-pairing reaction with free nucleotides diffusing across the air/ice interface). These findings thus offer the possibility of a role for an ancient RNA world on ice distinct from that considered in extant elaborations of the RNA world hypothesis. This work is, to the best of our knowledge, the first molecular dynamics study of RNA on ice.

How to cite: Neshyba, S., Gladich, I., Rowe, P., Berrens, M., and Pereyra, R.: Molecular Dynamics simulations indicate solvation and stability of single-strand RNA at the air/ice interface, supporting a primordial RNA world on Ice , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12841,, 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,, 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,, 2020.

EGU2020-13269 | Displays | GD1.1

Resolving the Hf-Nd paradox of early Earth crust-mantle evolution

Jeff Vervoort, Chris Fisher, and Ross Salerno

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,, 2020.

EGU2020-11217 | Displays | GD1.1

A Venus-like atmosphere on the early Earth from magma ocean outgassing

Paolo Sossi, Antony Burnham, James Badro, Antonio Lanzirotti, Matt Newville, and Hugh O'Neill

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,, 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,, 2020.

EGU2020-6942 | Displays | GD1.1

The lithologic composition of Earth’s emerged lands reconstructed from the chemistry of terrigenous sediments

Nicolas D. Greber, Nicolas Dauphas, and Matous P. Ptàček

Despite its importance for the biological and atmospheric evolution of our planet, the question how the lithological composition of Earth’s landmasses evolved from around 3.5 Ga to present is still a matter of considerable debate. Furthermore, the type of rocks that build the continents are an expression of the prevailing rock forming mechanisms and thus the geodynamic regime that was at work. Therefore, a good understanding of the lithological and chemical composition of Archean continents is crucial to gain a comprehensive picture of Earth’s evolution.

Lately, the view that Earth’s continents were dominated by basaltic rocks until around 3.0 Ga became increasingly popular. The subsequent rapid transformation from mafic to felsic continents has been used to argue for the onset of plate tectonics at 3.0 Ga and to suggest that the change in the chemical composition of the emerged continents initiated the Great Oxidation Event. Here we present a summary of our work over the past three years (Greber et al., 2017, Greber and Dauphas 2019, Ptáček et al., accepted) that challenges this view. Reconstructing the composition of past continents is difficult because erosion and crustal reworking may have modified the geologic record in deep time, so direct examination of the nature of igneous rocks could provide a biased perspective on the nature of the continents through time. A less biased record is provided by terrigenous sediments that average the composition of rocks exposed to weathering on emerged lands. We use the Ti isotopic, major and trace element composition of fine grained terrigenous sediments (shales) as a proxy for the average composition of the emerged continents in the past. Our model shows that since 3.5 Ga, the landmasses that were subjected to erosion were dominated by felsic rocks. Furthermore, our reconstructed relative abundance of felsic, mafic and komatiitic rocks in the Archean is close to that currently observed in Archean terrains. The combination of Ti isotopes and element abundances also indicates that the rocks exposed to weathering in the Archean resemble that of modern type calc-alkaline rocks and that tholeiitic rocks (e.g. Icelandites) were of subordinate importance.

To summarize, the lithological composition of the Paleoarchean continents should no longer be used as argument against the existence of subduction zones at that time. Instead, their nature rather supports that some form of subduction process was already operating since the early Archean.

References:  Greber N.D., et al (2017), Science 357, 1271–1274; Greber N.D. and Dauphas N. (2019), GCA 255, 247–264; Ptáček M.P., Dauphas N. and Greber N.D. (accepted), EPSL.

How to cite: Greber, N. D., Dauphas, N., and Ptàček, M. P.: The lithologic composition of Earth’s emerged lands reconstructed from the chemistry of terrigenous sediments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6942,, 2020.

EGU2020-4504 | Displays | GD1.1

Eoarchean to Paleoproterozoic crustal evolution in the North China Craton

Qiang Ma, Yi-Gang Xu, Xiao-Long Huang, and Jian-Ping Zheng

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,, 2020.

EGU2020-520 | Displays | GD1.1

Neoarchean tectonics: insight from the deformation of the Archean basement of North China Craton

Yongjiang Liu, Jing Li, Weimin Li, Sanzhong Li, and Liming Dai

The controversy over the Archean tectonic regimes has lasted several decades focusing around horizontal and vertical tectonics, the two classical tectonic models for Archean times. Thus, more studies of the early crustal growth and tectonic evolution are requisite for better understanding geodynamic regimes in the early Precambrian. The North China Craton is one of the major Archean to Paleoproterozoic cratons in the world and oldest craton in China, which preserves a large amount of ancient basement and abundant structures showing the early earth tectonics.

In this study, we have carried out detailed structural analysis of two down-slip ductile shear zones which developed in eastern Anshan area and provided an example for revealing of Neoarchean vertical tectonics in the study area. There were also develop many structures of dome and keel style in the North China Craton, such as Qian ’an, Qingyuan areas.

Based on abundant structural evidence and previous studies, we infer that the vertical tectonics is still the dominant model for Neoarchean crust growth and tectonic evolution in Anshan area. The formation of dome and keel structure, and the deformation of the down-slip ductile shear zones may have resulted from the sagduction of the banded iron formations and synchronous Archean granite dome emplacement, supporting a vertical tectonic regime in Archean times.

How to cite: Liu, Y., Li, J., Li, W., Li, S., and Dai, L.: Neoarchean tectonics: insight from the deformation of the Archean basement of North China Craton, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-520,, 2020.

EGU2020-8964 | Displays | GD1.1

Implementation of partial melting with a water- and composition-dependent solidus temperature adapted to TTG formation

Antoine Rozel, Stephen Mojzsis, Martin Guitreau, Antonio Manjón Cabeza Córdoba, Maxim Ballmer, and Paul Tackley

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,, 2020.

EGU2020-10638 | Displays | GD1.1

Mobile or not mobile: exploring the linkage between deep mantle composition and early Earth surface mobility

Keely A. O'Farrell, Sean Trim, and Samuel Butler

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,, 2020.

EGU2020-13737 | Displays | GD1.1

Cyclic tectono-magmatic evolution of TTG source regions in plum-lid tectonics

Ria Fischer, Lars Rüpke, and Taras Gerya

Geological-geochemical evidence points towards higher mantle potential temperature and a different type of tectonics, known as plume-lid tectonics, in the early Earth. In order to investigate tectono-magmatic processes associated with plume-lid tectonics and the formation of felsic TTG-like crust, we conduct a series of 3D high-resolution magmatic-thermomechanical models at elevated mantle temperature corresponding to Archean conditions. The numerical experiments show two distinct phases in coupled cyclic tectono-magmatic crust-mantle evolution: a long quiet growth phase followed by a short catastrophic overturn phase. Results of the detailed model analysis presented here suggest that

1) low- and medium-pressure TTGs are formed at the bottom of the crust during both phases; growth and overturn phase. The formation of low- and medium-pressure TTGs is linked with Moho depth and the ratio changes during crustal growth or thinning.

2) To form high-pressure TTGs an entirely different mechanism is required as hydrated basaltic rocks need to be buried below the crust. Cold eclogitic drips can be excluded as a valid mechanism due to their low temperatures and rapid sinking into the deep mantle, instead we suggest delamination or subduction as the main process for high-pressure TTG production.

How to cite: Fischer, R., Rüpke, L., and Gerya, T.: Cyclic tectono-magmatic evolution of TTG source regions in plum-lid tectonics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13737,, 2020.

EGU2020-1142 | Displays | GD1.1

Paleoarchean crustal evolution of the Singhbhum Craton, eastern India: Insights from granitoid petrology and zircon U-Pb and Lu-Hf systematics

Aniruddha Mitra, Sukanta Dey, Keqing Zong, Yongsheng Liu, and Anirban Mitra

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 εHft=+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 εHft=+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 εHft=-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,, 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,, 2020.

The world’s oldest rocks of demonstrable volcano-sedimentary origin comprise the Archean “supracrustal belts”, in which they occur as variably deformed enclaves within ancient metamorphosed granite-granitoid gneiss terranes. The Inukjuak Domain in northern Québec is part of the Archean Minto Block in the northwestern Superior Province of Canada. Eoarchean (ca. 3800-3780 Ma) rocks of the Nuvvuagittuq supracrustal belt (NSB) and the Ukaliq supracrustal belt (USB) are the best known of numerous supracrustal enclaves within this domain. Sample IN14032 represents a quartzite, interpreted as a quartz-pebble metaconglomerate from the USB. The main mineral assemblage is anthophyllite + muscovite + quartz + rutile + zircon. Owing to the pervasive greenschist-facies retrogression of the sample it was not possible to constrain P-T conditions using phase equilibrium calculations; however, the Zr-in-rutile geothermometer provides a tight constraint on T. A total of 41 rutile analyses were done by electron microprobe at the University of Innsbruck. Zr contents of rutile range from 407 ppm to 914 ppm and yielded T of 660-730°C at an assumed pressure of 0.6 GPa and the calculated mean T is 670°C ± 40°C (2s). U-Pb dating of rutile from this sample using the ion microprobe at Heidelberg University following Schmitt & Zack (2012) yielded ages of 2500-2600 Ma, which correlate well with the youngest zircon ages from this sample, consistent with the lower closure T for Zr diffusion in rutile (<600°C).

Similarly, supracrustal rocks from the Nuuk region of West Greenland preserve a record of surficial processes in the early Archean (>3600 Ma). Within the lithologies of the enclave a minor anthophyllite-garnet rock (sample GR114) with chemical characteristics suggesting a sedimentary protolith was identified. The main mineral assemblage of this sample is garnet + anthophyllite + hornblende + biotite + plagioclase + K-feldspar + quartz. Evidence for a later metamorphic overprint is given by the growth of a second generation of biotite and plagioclase as well as diffusive modification of the garnet composition along fractures. Phase equilibrium calculations of the main matrix assemblage yielded average P-T conditions of 580 ± 40°C and 0.6 ± 0.1 GPa. Zr-in-rutile geothermometry of rutile inclusions in garnet yielded increasing T from 610 ± 30°C in the core to 670 ± 30°C in the rims. U-Pb dating of rutile from this sample yielded discordant ages of 2400-1400 Ma. The upper intercept yields an age of ca. 2500 Ma, which again correlates again well with previous U-Pb zircon ages around 2700 Ma whereas the lower intercept at ca. 1000 Ma is indicative of a Grenville-age overprint.

The rutile U-Pb ages combined with Zr-in-rutile geothermometry show that Neoarchean metamorphism reached upper amphibolite-facies conditions (580-670°C) in both supracrustal localities in accordance with previous P-T estimates and U-Pb zircon ages. In addition, the sample from Akilia island yields hitherto unknown evidence of a later-stage Grenville metamorphic (high-greenschist-lower amphibolite-facies) overprint.


Schmitt, A. K., & Zack, T. (2012). High-sensitivity U–Pb rutile dating by secondary ion mass spectrometry (SIMS) with an O2+ primary beam. Chemical Geology, 332, 65-73.

How to cite: Tropper, P., Schmitt, A., Mojzsis, S., and Manning, C.: Rutile petrochronology of Eo-Archaean metasediments from the southern Inukjuak domain, Québec (Canada) and Akila island (SW Greenland), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3542,, 2020.

EGU2020-13603 | Displays | GD1.1

Feasibility of plate tectonics during the Archean: Insights from 3D numerical thermo-mechanical modelling

Andrea Piccolo, Nicholas Arndt, Richard White, and Kaus Boris

Slab-pull forces are considered the major driving forces of the present-day plate tectonic. Their efficiency relies on the buoyancy contrast between asthenosphere and subducting plate and on the strength of the latter. Subduction is not only pivotal for understanding the dynamics of plates but also represents the only modern geodynamic setting that produces significant amount of juvenile continental crust and allows exchange between the mantle, lithosphere and atmosphere.

One of the most important unsolved questions is related to the onset of plate tectonics, which is inherently linked to feasibility of the subduction during the early in Earth history. During the Archean, the mantle potential temperature was higher than nowadays, which promoted extensive mantle melting and possibly a weaker lithosphere. The intense magmatism associated with the high mantle potential temperature generated highly residual lithospheric mantle that was more buoyant than the underlying asthenosphere. Altogether these factors may have inhibited the dynamic effect of slab pull and prevented modern style tectonic during the Archean. However, the Archean mantle potential temperature is still not well constrained, and many of these theoretical considerations have not been fully tested by integrating petrological forward modelling into 3D numerical geodynamic modelling.

In our contribution, we focus on the feasibility of modern style plate tectonic as a function of the mantle potential temperature and the composition and structure of the lithosphere. We compute representative phase diagrams that represents the composition of mantle lithospheric and its complementary crust as a function of the mantle potential temperature and integrate them into large-scale 3D numerical experiments. The numerical setup is constructed assuming the existence of a set plates interacting with each other. We prescribe the principal plate boundaries and allow the model to spontaneously evolve as function of the thermal ages of the prescribed plate, testing the effect of continental terrains and oceanic plateau on overall geodynamic evolution. The overall goal is to understand the feasibility of plate tectonics at high mantle potential temperature and to estimate the amount of fluid released by the subduction processes, which provide useful insights on the formation of continental crust.

How to cite: Piccolo, A., Arndt, N., White, R., and Boris, K.: Feasibility of plate tectonics during the Archean: Insights from 3D numerical thermo-mechanical modelling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13603,, 2020.

EGU2020-13615 | Displays | GD1.1

The Great Thermal Divergence and the slope break of the 660 phase transitions

Thorsten Nagel, Kenni Dinesen Petersen, and Anders Vesterholt

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,, 2020.

EGU2020-8297 | Displays | GD1.1

Fate of Carbon During the Formation of Earth’s Core

Ingrid Blanchard, Eleanor Jennings, Ian Franchi, Zuchao Zhao, Sylvain Petitgirard, Nobuyoshi Miyajima, Seth Jacobson, and David Rubie

Carbon is an element of great importance in the Earth, because it is intimately linked to the presence of life at the surface, and, as a light element, it may contribute to the density deficit of the Earth’s iron-rich core. Carbon is strongly siderophile at low pressures and temperatures (1), hence it should be stored mainly in the Earth’s core. Nevertheless, we still observe the existence of carbon at the surface, stored in crustal rocks, and in the mantle, as shown by the exhumation of diamonds. The presence of carbon in the crust and mantle could be the result of the arrival of carbon during late accretion, after the process of core formation ceased, or because of a change in its metal–silicate partitioning behavior at the conditions of core formation (P >40 GPa – T >3500 K). Previous studies reported metal–silicate partitioning of carbon based on experiments using large volume presses up to 8 GPa and 2200°C (2). Here, we performed laser-heated diamond anvil cell experiments in order to determine carbon partitioning between liquid metal and silicate at the extreme conditions of Earth’s core–mantle differentiation. We recovered our samples using the Focused Ion Beam technique and welded a 3 μm thick slice of each sample onto a TEM grid. Major elements were analyzed by electron microprobe, whereas the concentrations of carbon in the silicate were analyzed by nanoSIMS. We thus have obtained metal–silicate partitioning results for carbon at PT conditions relevant to planetary core formation, where C remains siderophile in all experiments, but partition coefficients are up to two orders of magnitude lower than in low PTexperiments. We derive a new parameterization of the pressure–temperature dependence of the metal–silicate partitioning of carbon and apply this in a state-of-the-art model of planet formation and differentiation (3,4) that is based on astrophysical N-body accretion simulations. Results show that BSE carbon concentrations increase strongly starting at a very early stage of Earth’s accretion and, depending on the concentration of carbon in accreting bodies, can easily reach or exceed estimated BSE values.


(1) Dasgupta et al., 2013. Geochimica et Cosmochimica Acta 102, 191-212

(2) Li et al., 2016. Nature Geoscience 9, 781–785

(3) Rubie et al., 2015. Icarus 248, 89–108

(4) Rubie et al., 2016. Science 353, 1141–1144


How to cite: Blanchard, I., Jennings, E., Franchi, I., Zhao, Z., Petitgirard, S., Miyajima, N., Jacobson, S., and Rubie, D.: Fate of Carbon During the Formation of Earth’s Core , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8297,, 2020.

EGU2020-9935 | Displays | GD1.1

Partial melting of an enstatite chondrite at 1 GPa: Implications for early planetary differentiation

Matteo Masotta, Luigi Folco, Luca Ziberna, and Robert Myhill

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,, 2020.

EGU2020-6044 | Displays | GD1.1 | Highlight

Chemical and structural analysis of proposed ca. 3.7 Ga stromatolites from the Isua Supracrustal Belt (West Greenland) - a reappraisal

Mike Zawaski, Nigel Kelly, Omero Felipe Orlandini, Claire Nichols, Abigail Allwood, and Stephen Mojzsis

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,, 2020.

EGU2020-13558 | Displays | GD1.1

Thermal stability of metalorganic compounds on volcanic olivine

Joanna Brau, Marco Matzka, Philippe Schmitt-Kopplin, Norbert Hertkorn, Werner Ertel-Ingrisch, Bettina Scheu, and Donald Bruce Dingwell

Previously unknown class of metalorganic compounds revealed in meteorites [1] also found on the surfaces of silicate phases such as olivine, may have been involved in the emergence of life.  Here, the thermal stability of such organic compounds has been experimentally investigated under conditions which simulate those extant on the early Earth. We have studied olivines from the Hawaiian eruptions of 1959 and 2018. Individual mineral grains have been hand-picked to be free of secondary phases such as pyroxene or melt. We use a high temperature gas-tight tube furnace under CO-CO2 gas mixture at temperatures ranging from 950°C to 1350°C and oxygen fugacity ranging from 10-12 to 10-10 bar, within the stability field of olivine. The samples were contained in Pt crucibles and held for dwell times of 1 to 64 h. Quenching was performed by lifting the samples vertically out of the tube furnace. Using EPMA (electron microprobe analyzer) and RAMAN spectroscopy, we have mapped the state of the olivine samples. We observe that the composition of the individual mineral grains remains stable and homogeneous with thermal treatment. We are also investigating the role of impurities and cracks in the natural olivine and synthetic forsterite that might influence our study. The metalorganic cargo of these olivines has been analyzed using FT-ICR-MS (Fourier Transform ion cyclotron mass spectrometry). Preliminary results reveal systematic changes or organic molecular composition depending on time and heat of thermal treatment whose origins will be discussed.

[1] A. Ruf, B. Kanawati, N. Hertkorn, Q. Yin, F. Moritz, M. Harir, M. Lucio, B. Michalke, J. Wimpenny, S. Shilobreeva, B. Bronsky, V. Saraykin, Z. Gabelica, R. D. Gougeon, E. Quirico, S. Ralew, T. Jakubowski,  H. Haack, M. Gonsior, P. Jenniskens, N. W. Hinman, P. Schmitt-Kopplin. (2017) Previously unknown class of metalorganic compoundsrevealed in meteorites. PNAS 114 (2017) 2819-2824.

How to cite: Brau, J., Matzka, M., Schmitt-Kopplin, P., Hertkorn, N., Ertel-Ingrisch, W., Scheu, B., and Dingwell, D. B.: Thermal stability of metalorganic compounds on volcanic olivine, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13558,, 2020.

EGU2020-9161 | Displays | GD1.1

Volatile organic compounds in barite-hosted fluid inclusions from the 3.5 Ga old Dresser Formation, Western Australia

Volker Thiel, Jan-Peter Duda, Alfons M. van den Kerkhof, Joachim Reitner, and Helge Mißbach

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,, 2020.

EGU2020-21119 | Displays | GD1.1

The Hf-Nd dichotomy: constraints from felsic, mafic and ultramafic rocks in the western Dharwar Craton, India

Arathy Ravindran, Klaus Mezger, and Srinivasan Balakrishnan

The Hf-Nd dichotomy: constraints from felsic, mafic and ultramafic rocks in the western Dharwar Craton, India

Arathy Ravindran1*, Klaus Mezger1, S. Balakrishnan2

1Institut für Geologie, Universität Bern, Bern, Switzerland

2Department of Earth Sciences, Pondicherry University, Puducherry, India


The small extend of exposed Hadean-Paleoarchaean (>3.2 Ga) rocks in the global record poses a major challenge in interpreting Earth’s early crust-mantle evolution. This results in major uncertainty in the degree and extent of heterogeneity of the Archaean mantle (e.g. Nebel et al., 2014). Isotope systems like 176Lu-176Hf and 147Sm-143Nd are powerful tools in tracing the degree of mantle depletion and the influence of concomitant continental crust formation. However, these isotope systems are apparently decoupled in Archaean ultramafic rocks (e.g. Hoffmann and Wilson, 2017). Hence, the Hf-Nd isotope dichotomy in ultramafic rocks requires a detailed study of cratonic areas hosting granitoids spatially associated with greenstone belts and ultramafic rocks, as it is the case in the western Dharwar Craton (~3.4 Ga) of India.

The 3.25 Ga old rhyolitic to basaltic rocks of the craton that have flat, mantle-like REE patterns also have 147Sm-143Nd and 176Lu-176Hf signatures ‘coupled’ along a trend ɛ176Hf = 1.55 * ɛ143Nd + 1.21 (Vervoort et al., 2011). The minor depletion recorded in these rocks is a result of mixing at different levels between a 3.6 Ga old mafic crust (Ravindran et al., 2020) and the contemporary depleted mantle. The tonalite-trodhjemite-granodiorite (TTG) gneisses have similar isotope ratios and their petrogenesis involved the mafic crust until 3.3 Ga, after which reworked crust was the major component. Komatiitic rocks (MgO=15-30%; Na2O+K2O <1%; (Gd/Yb)N=0.6-1.8) with an age of 3.35 Ga have high and variable initial ɛHf (+3 to +20) compared to their initial ɛNd (+1.0 to +3.5). These ultramafic rocks have decoupled Hf-Nd signatures which is uncommon for the mafic and felsic rocks in the craton. This further shows that the mantle composition was more heterogeneous in the early Archaean than today. It is also possible that the presence of garnet in the mantle source was an important parameter which influenced the composition of the early Archaean crust. 



Hoffmann, J. E., Wilson, A. H., 2017. Chem. Geo. 455, 6-21

Nebel, O., Campbell, I. H., Sossi, P. A., Van Kranendonk, M. J., 2014. Earth. Planet. Sci. Lett. 397, 111-120

Ravindran, A., Mezger, K., Balakrishnan, S., Kooijman, E., Schmitt, M., Berndt, J., 2020. Prec. Res. 337

Vervoort, J., Plank, T., Prytulak, J., 2011. Geochim. Cosmochim. Acta 75, 5903-5926

How to cite: Ravindran, A., Mezger, K., and Balakrishnan, S.: The Hf-Nd dichotomy: constraints from felsic, mafic and ultramafic rocks in the western Dharwar Craton, India , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21119,, 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 (LaN/YbN =1.47-10.63) with moderate to highly fractionated LREE (LaN/SmN=2.67-8.93) and nearly flat to slighly elevated HREE (GdN/ YbN=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 LaN/YbN), 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 LaN/YbN 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,, 2020.

GD2.1 – Mantle dynamics, structure and evolution: Combining geochemical, mineralogical and seismological constraints with geodynamics

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,, 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,, 2020.

EGU2020-5380 | Displays | GD2.1

Thermochemical structure of cratons from Rayleigh wave phase velocities

Saskia Goes, Thomas Eeken, Isabella Altoe, Laura Petrescu, Anna Foster, Helle Pedersen, Nick Arndt, Fiona Darbyshire, and Pierre Bouilhol

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,, 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,, 2020.

Grain scale diffusive processes are involved in the rheology at convective timescales, but also at the transient timescales of seismic wave propagation, solid Earth tides and post-glacial rebound. Seismic and geodetic data can therefore potentially provide constraints on lower mantle properties such as grain size that are unconstrained otherwise. Current models of the transient viscosity of the lower mantle infer an absorption band of finite width in frequency. Seismic models predict a low frequency end to the absorption band at timescales corresponding to the longest normal modes of about an hour. By contrast, geodetic models infer the onset of an absorption band at these frequencies to cover anelastic deformation at timescales up to 18.6 years. A difficulty in extracting frequency dependence from mode and tide data is its convolution with depth dependence.

To circumvent this problem we select a distinct set of seismic normal modes and solid Earth body tides that have similar depth sensitivity in the lower mantle. These processes collectively span a period range from 7 minutes to 18.6 years. This allows the examination of frequency dependent energy dissipation over the lower mantle across 6 orders of magnitude. To forward model the transient creep response of the lower mantle we use a laboratory-based model of intrinsic dissipation that we adapt to the lower mantle mineralogy. This extended Burgers model represents an empirical fit to data principally from olivine, but also MgO and other compounds. The underlying microphysical model envisions a sequence of processes that begin with a broad plateau in dissipation at the highest frequencies after the onset of anelastic behavior, followed by a broad absorption band spanning many decades in frequency. The absorption band transitions seamlessly into viscous behavior. Since dissipation both for the absorption band and for (Newtonian) viscous behavior is due to diffusion along grain boundaries there can be no gap between the end of the absorption band and onset of viscous behavior.

Modeling of the planetary response to small strain excitation necessitates consideration of inertia and self gravitation. The phase lag due to solid Earth body tides therefore does not correspond directly to the intrinsic dissipation measured in the laboratory as material property. We have developed a self consistent theory that combines the planetary response with time-dependent anelastic deformation of rocks. Results from a broad range of forward models show that at lower mantle pressures periods of modes fall onto the broad plateau in dissipation at the onset of anelastic behavior. This explains the apparent frequency independence or even negative frequency dependence observed for some normal modes. At longer timescales, solid Earth tides fall on the frequency-dependent absorption band. This reconciles seemingly contradictory results published by seismic and tidal studies. Observations at even longer timescales are needed to constrain the transition from absorption band to viscous behavior.

How to cite: Faul, U. and Lau, H.: Viscoelasticity of the lower mantle from forward modeling of normal modes and solid Earth tides, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10789,, 2020.

EGU2020-21471 | Displays | GD2.1

LLSVPs of primordial origin: Implications for the evolution of plate tectonics

Philipp Hellenkamp, Claudia Stein, and Ulrich Hansen

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,, 2020.

EGU2020-5577 | Displays | GD2.1

How thermochemical piles initiate plumes at their edges

Björn Heyn, Clinton Conrad, and Reidar Trønnes

Deep-rooted mantle plumes are thought to originate from the margins of the Large Low Shear Velocity Provinces (LLSVPs) at the base of the mantle. Visible in seismic tomography, the LLSVPs are often numerically modeled as dense and viscous thermochemical piles. Although the piles force lateral mantle flow upwards at their edges, it is not clear if, and how, plumes are predominantly initiated at the pile margins. In this study, we develop numerical models that show a series of plumes periodically rising from the margin of an approximately 300 km thick dense thermochemical pile, with each plume temporarily increasing the pile’s local thickness to almost 370 km due to upward viscous drag from the rising plume. When the plume is pushed towards the pile center by the lateral mantle flow, the viscous drag on the dense material at the pile margin decreases and the pile starts to collapse back towards the core-mantle boundary (CMB). This causes the dense pile material to extend laterally along the CMB (about 150 km), locally thickening the lower thermal boundary layer on the CMB next to the pile, which initiates a new plume. The resulting plume cycle is reflected in both the thickness and lateral motion of the local pile margin within a few degrees of the pile edge, while the overall thickness of the pile is not affected. The frequency of plume generation is mainly controlled by the rate at which slab material is transported to the CMB, and thus depends on the plate velocity and the sinking rate of slabs in the lower mantle. Within Earth, this mechanism of episodic plume initiation may explain the suggested link between the positions of hotspots and Large Igneous Provinces (LIPs) and the LLSVP margins. Moreover, a collapse of the southeastern corner of the African LLSVP, and subsequent triggering of plumes around the spreading pile material, may explain the observed clustering of LIPs in that area between 95 and 155 Ma.

How to cite: Heyn, B., Conrad, C., and Trønnes, R.: How thermochemical piles initiate plumes at their edges, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5577,, 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,, 2020.

EGU2020-18210 | Displays | GD2.1

Were Karoo flood basalts derived from a LLSVP-related plume source?

Arto Luttinen, Jussi Heinonen, Sanni Turunen, Richard Carlson, and Mary Horan

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.


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.

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.

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,, 2020.

EGU2020-10547 | Displays | GD2.1

Geochemical and seismological constraints on the locations and geometries of deep mantle reservoirs

Matthew Jackson, Thorsten Becker, and Bernhard Steinberger

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,, 2020.

EGU2020-21211 | Displays | GD2.1

Modern plate tectonic cycles are inherited from Hadean mantle convection

Ross N. Mitchell, Christopher J. Spencer, Uwe Kirscher, and Simon A. Wilde

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,, 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,, 2020.

EGU2020-8741 | Displays | GD2.1

Do olivine crystallisation temperatures faithfully record mantle temperature variability?

Simon Matthews, Kevin Wong, Oliver Shorttle, Marie Edmonds, and John Maclennan

Crystallisation temperatures of primitive olivine crystals have been widely used as both a proxy for, or an intermediate step in calculating, mantle temperatures. The olivine-spinel aluminium-exchange thermometer has been applied to many samples from mid-ocean ridges, ocean islands and large igneous provinces, yielding considerable variability in primitive olivine crystallisation temperatures. We supplement the existing data with new crystallisation temperature estimates for Hawaii, in the range 1282±21 - 1375±19°C.

Magmatic temperatures may be linked to mantle temperatures if the thermal changes during melting can be quantified. Melting lowers the temperature of co-existing magma and solid mantle, owing to the latent heat of melting. The magnitude of this cooling depends on melt fraction, itself controlled by mantle temperature, mantle lithology and lithosphere thickness. All of these parameters are likely to vary both spatially and temporally. For robust quantification of mantle temperature variability, the controls of lithosphere thickness and mantle lithology on crystallisation temperatures must be isolated.

We develop a multi-lithology melting model that can predict crystallisation temperature. The model allows mantle temperature, lithospheric thickness, and fractions of mantle lherzolite, pyroxenite and harzburgite to be varied. Inverting the model using a Bayesian Monte Carlo routine enables assessment of the extent to which crystallisation temperatures require variations in mantle temperature. We find that the high crystallisation temperatures seen at mantle plume localities do require high mantle temperatures. However, in the absence of further constraints on mantle lithology or melt productivity, we cannot robustly infer variable plume temperatures in either the present-day or throughout the phanerozoic. This work demonstrates the limit of petrological thermometers when other geodynamic parameters are poorly known.

How to cite: Matthews, S., Wong, K., Shorttle, O., Edmonds, M., and Maclennan, J.: Do olivine crystallisation temperatures faithfully record mantle temperature variability?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8741,, 2020.

EGU2020-4258 | Displays | GD2.1

Low-frequency seismic properties of olivine-orthopyroxene mixtures

Tongzhang Qu, Ian Jackson, and Ulrich Faul

Although the seismic properties of polycrystalline olivine have been the subject of systematic and comprehensive study at seismic frequencies, the role of orthopyroxene as the major secondary phase in the shallow parts of the Earth’s upper mantle has so far received little attention. Accordingly, we have newly prepared synthetic melt-free polycrystalline specimens containing different proportions of olivine (Ol, Fo90) and orthopyroxene (Opx, En90) by the solution-gelation method. The resulting specimens, ranging in composition between Ol95Opx5 and Ol5Opx95 composition, were mechanically tested by torsional forced oscillation at temperatures of 1200 ºC to 400 ºC accessed during staged cooling under a confining pressure of 200 MPa. The microstructures of tested specimens were evaluated by BSE, EBSD and TEM. The forced-oscillation data, i.e. shear modulus and associated strain-energy dissipation at 1-1000 s period, were closely fitted by a model based on an extended Burgers-type creep function. This model was also required to fit data from previous ultrasonic and Brillouin spectroscopic measurements at ns-µs periods. Within the observational window (1-1000 s), the shear modulus and dissipation vary monotonically with period and temperature for each of the tested specimens, which is broadly comparable with that previously reported for olivine-only samples. There is no evidence of the superimposed dissipation peak reported by Sundberg and Cooper (2010) for an Ol60Opx40 specimen prepared from natural precursor materials and containing a melt fraction of 1.5%. The higher orthopyroxene concentrations are associated with systematically somewhat lower levels of dissipation and corresponding weaker modulus dispersion. The new findings suggest that the olivine-based model for high-temperature viscoelasticity in upper-mantle olivine requires only modest modification to accommodate the role of orthopyroxene, including appropriate compositional dependence of the unrelaxed modulus and its temperature derivative.

How to cite: Qu, T., Jackson, I., and Faul, U.: Low-frequency seismic properties of olivine-orthopyroxene mixtures, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4258,, 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,, 2020.

EGU2020-12682 | Displays | GD2.1

Asthenosphere viscosity in the Caribbean region constrained by gravity anomalies, seismic structure and regional magmatism

Yi-Wei Chen, Lorenzo Colli, Dale E. Bird, Jonny Wu, and Hejun Zhu

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,, 2020.

EGU2020-8428 | Displays | GD2.1

Numerical Insights into the Formation and Stability of Cratons

Charitra Jain, Antoine Rozel, Emily Chin, and Jeroen van Hunen
Geophysical, geochemical, and geological investigations have attributed the stable behaviour of Earth's continents to the presence of strong and viscous cratons underlying the continental crust. The cratons are underlain by thick and cold mantle keels, which are composed of melt-depleted and low density peridotite residues [1]. Progressive melt extraction increases the magnesium number Mg# in the residual peridotite, thereby making the roots of cratons chemically buoyant [2, 3] and counteracting their negative thermal buoyancy. Recent global models have shown the self-consistent production of Archean continental crust by two-step mantle differentiation [4]. These models exhibit intense recycling of crust with delamination and eclogitic dripping in the first 500 million years and this behaviour is similar to the "plutonic-squishy lid'' that has been suggested for the early Earth. However, no stable continents form and no major regime transition from "vertical tectonics'' towards "horizontal tectonics'' is observed. This points to the missing ingredient of cratonic lithosphere in these models, which could act as a stable basement for the crustal material to accumulate on and may help initiate plate tectonics. Based on the bulk FeO and MgO content of the residual peridotites, it has been proposed that cratonic mantle formed by hot shallow melting with mantle potential temperature, which was higher by 200-300 °C than present-day [5]. We will introduce Fe-Mg partitioning between mantle peridotite and melt to track the Mg# variation through melting, and parametrise craton formation using the corresponding P-T formation conditions. Grain-size evolution, which has been shown to influence mantle rheology [6] is another mechanism that may contribute towards cratonic strength and will be explored using self-consistent global geodynamic models.
[1] Boyd, F. R. High-and low-temperature garnet peridotite xenoliths and their possible relation to the lithosphere- asthenosphere boundary beneath Africa. In Nixon, P. H. (ed.) Mantle Xenolith, 403–412 (John Wiley & Sons Ltd., 1987).
[2] Jordan, T. H. Mineralogies, densities and seismic velocities of garnet lherzolites and their geophysical implications. In The Mantle Sample: Inclusion in Kimberlites and Other Volcanics, 1–14 (American Geophysical Union, Washington, D. C., 1979).
[3] Schutt, D. L. & Lesher, C. E. Effects of melt depletion on the density and seismic velocity of garnet and spinel lherzolite. Journal of Geophysical Research 111 (2006).
[4] Jain, C., Rozel, A. B., Tackley, P. J., Sanan, P. & Gerya, T. V. Growing primordial continental crust self-consistently in global mantle convection models. Gondwana Research 73, 96–122 (2019).
[5] Lee, C.-T. A. & Chin, E. J. Calculating melting temperatures and pressures of peridotite protoliths: Implications for the origin of cratonic mantle. Earth and Planetary Science Letters 403, 273–286 (2014).
[6] Hall, C. E. & Parmentier, E. M. Influence of grain size evolution on convective instability. Geochemistry, Geophysics, Geosystems 4, 469 (2003).

How to cite: Jain, C., Rozel, A., Chin, E., and van Hunen, J.: Numerical Insights into the Formation and Stability of Cratons, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8428,, 2020.

EGU2020-11819 | Displays | GD2.1

Thermal and chemical properties of the mantle transition zone from seismic observations

Saskia Goes, Chunquan Yu, Ross Maguire, Elizabeth Day, Rob van der Hilst, Jeroen Ritsema, and Jing Jian

The mantle transition zone (MTZ), bounded by 410 and 660 discontinuities, is a key region to understand the thermal, chemical, and dynamical evolution of the mantle. Mantle dynamics is primarily thermally driven and the topography of 410 and 660 has been widely used to infer the temperature of the MTZ. However, in a number of recent studies, we have found that properties of transition-zone discontinuities may also provide insight in the distribution of compositional heterogeneity. We will present preliminary results from a global study of PP and SS precursors using a curvelet-based seismic array processing technique, where we successfully extract P660P signals, which are traditionally difficult to observe, over a wide distance range. Comparison with thermodynamic models suggests that on a global scale, amplitude trends of SS and PP precursors from both 410 and 660 are consistent with predictions from a pyrolitic mantle transition zone. We also find that global variation in MTZ thickness has a positive correlation with velocity anomalies within the MTZ. Both of them are likely controlled by thermal anomalies, consistent with mineralogical phase transitions of the olivine system. In an application of this method to data from Hawaii however, we found evidence of compositional variations, consistent with the analysis of tomographic images below a few other hotspots. Further compositional heterogeneity linked to recent subduction has been found from a receiver-function study below the US. Results thus indicate a quite well mixed background mantle with more heterogeneity in areas of recent up-and downwelling.

How to cite: Goes, S., Yu, C., Maguire, R., Day, E., van der Hilst, R., Ritsema, J., and Jian, J.: Thermal and chemical properties of the mantle transition zone from seismic observations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11819,, 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,, 2020.

EGU2020-18402 | Displays | GD2.1

A new look at polarity information in D" reflections

Christine Thomas, Laura Cobden, and Art Jonkers

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,, 2020.

EGU2020-10995 | Displays | GD2.1

Constraints on D″ beneath the North Atlantic region from P and S traveltimes and amplitudes

stephanie durand, Christine Thomas, and Jennifer Jackson

EGU2020-8187 | Displays | GD2.1

Interaction of a mantle plume and a moving plate: insights from numerical modeling

Sascha Brune, Marzieh Baes, Taras Gerya, and Stephan Sobolev

The impingement of a hot buoyant mantle plume onto the lithosphere can result in either breaking of the lithosphere, which might results in subduction initiation or in under-plating of the plume beneath the lithosphere. Key natural examples of the former and latter are formation of subduction along the southern margin of Caribbean and northwestern South America in the late Cretaceous as well as the hotspot chains of Hawaii, respectively. In previous studies the interaction of a buoyant mantle plume with lithosphere was investigated either for the case of stationary lithosphere or for moving lithosphere but ignoring the effect of magmatic weakening of the lithosphere above the plume head. In this study we aim to investigate the response of a moving lithosphere to the arrival of a stationary mantle plume including the effect of magmatic lithospheric weakening. To do so we use 3d thermo-mechanical models employing the finite difference code I3ELVIS. Our setup consists of an oceanic lithosphere, mantle plume and asthenosphere till depth of 400 km. The moving plate is simulated by imposing a kinematic boundary condition on the lithospheric part of the side boundaries. The mantle plume in our models has a mushroom shape. The experiments differ in the age of the lithosphere, rate of the plate motion and size of the mantle plume. For different combinations of these parameters model results show either (1) breaking of the lithosphere and initiation of subduction above the plume head or (2) asymmetric spreading of the plume material below the lithosphere without large deformation of the lithosphere. We find that the critical radius of the plume that breaks the lithosphere and initiates subduction depends on plume buoyancy and the lithospheric age, but not on the plate speed. In general, the modeling results for the moving plate are similar to the results for a stationary plate, but the shapes of the region of the deformed lithosphere differ.

How to cite: Brune, S., Baes, M., Gerya, T., and Sobolev, S.: Interaction of a mantle plume and a moving plate: insights from numerical modeling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8187,, 2020.

EGU2020-18461 | Displays | GD2.1

Core Phases Observed with AlpArray

On Ki Angel Ling, Simon Stähler, Domenico Giardini, Kasra Hosseini, and The AlpArray Working Group

In most seismic tomographic models, the first P and/or S wave data generated by regional and teleseismic events are used to conduct tomographic inversion. Despite the abundance and precise measurement of the first body wave arrival times, the non-uniform distribution of their ray path leads to a lower resolution in the mantle below 1000km in depth. Curiously, there are particularly few ray paths sampling the lowermost mantle below dense seismic arrays, due to the limited incidence angle range of P and S waves. Previous studies have demonstrated the importance of core phases, resulting from reflection and/or conversion of seismic waves at the core discontinuities, in seismic tomography by improving the ray path coverage and constraining the structures in the lower mantle. Therefore, adding core-grazing phases (Pdiff, Sdiff) as well as core phases (e.g. PKP, PKIKP, SKS) in tomography could deliver high-resolution tomographic images of deep mantle structures in poorly resolved regions and may even reveal undiscovered features.

To increase the topographic resolution in the Alpine region, the AlpArray Initiative deployed about 250 temporary stations alongside the local permanent stations in the European Alps forming a greater AlpArray seismic network. This large-scale network provides a dense sampling rate and high-quality seismic data across the region, which gives us a unique opportunity to observe core phases coming from all directions in such a large aperture. We investigate the visibility of core phases observed with AlpArray and find that it is uniquely suited to observe high order core phases (P’P’, PcPPcPPKP, PKPPKPPKP) from sources in Alaska, Japan, and Sumatra in a distance range of 60-110 degrees. We show some array processing methods to improve the resolution of seismic observation and examine the waveforms in different frequency ranges. We find significant deviations in core phase amplitudes from predictions which are most likely linked to other structures directly above the core mantle boundary and can serve to test tomographic models in this depth region. The insight gained from this modelling is used to discuss the usability of core phases in future tomographic studies.

How to cite: Ling, O. K. A., Stähler, S., Giardini, D., Hosseini, K., and AlpArray Working Group, T.: Core Phases Observed with AlpArray , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18461,, 2020.

EGU2020-9346 | Displays | GD2.1

Full-waveform analysis of core-mantle boundary structure using adjoint methods

Anselme F.E. Borgeaud, Maria Koroni, and Frédéric Deschamps
This study presents a new approach for investigating the structure of the core-mantle boundary (CMB)
topography based on full-waveforms and adjoint methods. We compute intermediate period (10-20 seconds)
spectral-element seismograms using existing models of core-mantle boundary topography and we analyse the
sensitivity of relevant seismic phases. Our study adds new information about effects of CMB structure on
exact synthetics and observable traveltimes of seismic body waves by means of sensitivity kernels. It also
highlights the difficulty of imaging the boundary due to the strong trade-off between velocity and topography
variations, addressed by many previous investigators.
Given the significance of CMB and its importance for many disciplines in geophysical research, there have
been many studies trying to understand and geographically map the variations of topography and velocity
above this boundary. However, the vast mantle area wherein seismic waves travel before and after they
interact with the CMB makes the identification of desired seismic phases somehow difficult. In addition, the
observable traveltimes can be hard to interpret as a result of the boundary’s topography only, due to the
approximate inverse methods and limited modelling methodologies. Despite considerable progress made the
past years, there is still a necessity for improving the understanding of effects of core-mantle boundary and
D″ structure on recorded waveforms.
For our analyses, we perform comparisons between time shifts due to topography made on full-waveform
synthetics to ray theoretical predictions in order to assess methods usually deployed for imaging CMB.
Then, we calculate the corresponding sensitivity kernel for time windows around the theoretical arrival of
each phase. We focus on diffracted, core reflected and refracted P and S waves. The sensitivity kernels
depict the finite-frequency nature of these waves and possible contributions from other phases unpredictable
by ray theory. Results show that for most phases ray theory performs acceptably with some accuracy loss,
however comparisons of the effect of velocity variations to topography on traveltimes are discouraging due
to the low sensitivity to the latter.
The conclusions drawn by our traveltime and sensitivity analyses are twofold. Firstly, using spectral-
element waveforms, the seismic phases which are frequently found in literature can be thoroughly investigated
and better understood, since their traveltime sensitivity through mantle and core is explicitly shown. The
full-waveform analysis allows us to assess the usability of phases which are informative for core-mantle
boundary structure and its topography. Secondly, we propose that using the analysed phases simultaneously
in a full-waveform inversion scheme will improve imaging of the CMB, while also allowing to jointly invert
for velocity variations along the D″ layer, which is generally poorly understood. From this study, we want
to promote advanced techniques of full-waveform inversion for improving CMB and lower mantle models.

How to cite: Borgeaud, A. F. E., Koroni, M., and Deschamps, F.: Full-waveform analysis of core-mantle boundary structure using adjoint methods, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9346,, 2020.

EGU2020-12035 | Displays | GD2.1

Investigation into the influence of the Réunion plume on the Central Indian Ridge

Clément Vincent, Jung-Woo Park, Sang-Mook Lee, Jonguk Kim, and Sang-Joon Pak

EGU2020-524 | Displays | GD2.1

Radial Miscible Viscous Fingering of the Icelandic Plume

Patricia Hannah Galbraith-Olive, Nicky White, and Andy Woods

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

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,, 2020.

It is generally accepted that the Tunguska event in Siberia on 30 June, 1908 resulted from an explosion of cosmic body. However, there is no common agreement that this bolide really existed. Moreover, registered ultra low frequency (ULF) magnetic oscillations in Kiel, Germany on 27-30 June 1908 [1] had a correlate with the 'acoustic halo' (ULF) of a solar flare [2].

Large low-shear velocity provinces (LLSVPs) are linked to so-called blobs located atop the Earth's outer core [3]. It was shown the Earth's D"-layer core-mantle boundary was perturbed by both the solar flare and an anomalous lunar-solar tide during the Tunguska 1908 event [2]. Therefore, gravitational/magnetic lunar-solar perturbations could have triggered a plume/hotspot/LIP activation by means of a LLSVPs convection.

It was suggested that planetary hotspots chains are interconnected [4]. Indeed, during the Tunguska event brightest glows were observed over the Eifel volcano and more weak one over the Yellowstone volcano (both volcanoes are associated with hotspots) [5]. In addition, day by day a slowly lifting of the earth round the diabase stones was registered in Tasmania from 7 June till 29 June, 1908 [6]. This lifting was independent from atmospheric temperature variations and terminated as soon as a blast took place in the caldera of Tunguska paleovolcano on 30 June, 1908 [5, 6]. Observations in Tasmania remained a mystery for a long time. Recently scientists discovery the Cosgrove hotspot had moved from Eastern Australia to Tasmania [7]. In our opinion, the Cosgrove did not lose its activity fully 9 My ago as previously assumed: the Darwin crater in Tasmania originated about of 803 ka years and large volume ejected glasses in/around this small crater contradicts to the impact origin [5, 8]. Therefore, we consider the underground activation of Cosgrove hotspot as a cause of surface uplift in Tasmania from 7 to 30 June 1908.

As in Tasmania, moving mantle hotspots were registered in Eastern Siberia [9]. Probably, hotspots in Tasmania (near Pacific LLSVPs) and in the Tunguska basin (near Perm LLSVPs) are interconnected. Because common hotspots thermal energy was released in/by the Tunguska paleovolcano explosion on 30 June 1908, the fluidal pressure of the Cosgrove hotspot under Tasmania was reduced, resulting in the termination of surface uplift. Since meteorites could not have caused the earth uplift in Tasmania, the impact hypothesis for the Tunguska phenomenon can be excluded. All data favor an endogenic origin of this event due to lunar-solar perturbations and the whole-mantle convection.

[1]. Weber L. (1908) Astronomische Nachrichten, 178, 23. [2]. German B. (2010) EPSC2010-430. [3]. Duncombe J. (2019) Eos, 100. [4]. Courtillot V. (1990) ISBN 9780813722474, 401. [5]. German B. (2019) ISBNs 9783981952605(in Russian)/9783981952612(in English). [6]. Scott H. (1908) Nature, 78(2025), 376. [7]. Davies D. (2015) Nature, 525, 511. [8]. Haines P. (2005) Australian Journal Earth Sciences, 52, 481. [9]. Rosen O. (2015) ISBN 9785902754954, 148.

How to cite: German, B. R.: Is a whole-mantle convection the key to solving the Tunguska 1908 problem?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1662,, 2020.

EGU2020-4621 | Displays | GD2.1

The effect of composite rheology on mantle convection models with plate-like behavior

Maelis Arnould and Tobias Rolf

The coupling between mantle convection and plate tectonics results in mantle flow patterns and properties which can be characterized with different seismic methods. In particular, the presence of mantle seismic anisotropy in the uppermost mantle suggests the existence of mineral Lattice-Preferred Orientation (LPO) caused by asthenospheric flow. Dislocation creep, which implies non-Newtonian mantle rheology, has been identified as a deformation mechanism responsible for such LPO leading to seismic anisotropy. While it has been proposed that the use of a composite rheology (with both diffusion and dislocation creep) significantly impacts the planform of convection and thus the resulting tectonic behavior at the surface, large-scale mantle convection studies have typically assumed diffusion creep (Newtonian rheology) as the only deformation mechanism, due to computational limitations.

Here, we investigate the role of composite rheology on mantle convection with self-consistent plate-like behavior using the code StagYY in 2D annulus (Hernlund and Tackley, 2008). We quantify the spatial distribution of dislocation creep in the mantle in models characterized by different transitional stresses between Newtonian and non-Newtonian rheology. Such models are built on previous viscoplastic cases featuring Earth-like plate velocities, surface heat flow and topography with Newtonian rheology (Arnould et al., 2018). We then investigate how composite rheology impacts the planform of convection and the style of plate-like behavior.



Hernlund, J. W., & Tackley, P. J. (2008). Modeling mantle convection in the spherical annulus. Physics of the Earth and Planetary Interiors, 171(1-4), 48-54.

Arnould, M., Coltice, N., Flament, N., Seigneur, V., & Müller, R. D. (2018). On the scales of dynamic topography in whole‐mantle convection models. Geochemistry, Geophysics, Geosystems, 19(9), 3140-3163.

How to cite: Arnould, M. and Rolf, T.: The effect of composite rheology on mantle convection models with plate-like behavior, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4621,, 2020.

EGU2020-5252 | Displays | GD2.1

Geochemical features of Meso-Neoproterozoic dolerite sill on the South-East margin of the Siberian Craton.

Aleksandr Savelev, Andrei Khudoley, and Sergey Malyshev

Meso-Neoproterozoic dolerite sills and dykes of the south-east margin of the Siberian Craton are commonly known as linked to the Sette-Daban LIP-related event (Ernst, 2014). They are localized in the Maya-Kyllakh zone which represent moderately deformed sedimentary cover of the craton. The mafic intrusions are numerous and variable in size, but the best studied are large sills up to 200 m thick. Smaller intrusions are identified to be related to the same magmatic event according to their appearance and structural position.

There are several U-Pb and Sm-Nd isochron isotopic dates for the rocks of the Ulakhanbam complex, giving a range of values from 930 to 1000 Ma. Although there is an overlap of several dates within error, sills become younger westward from 974-1005 Ma in the east part of the study area to 932-946 Ma in its west part. Due to wide range of ages they likely represent at least 2 different magmatic events, although long-term event is possible as well. To resolve this issue, new accurate dates are needed.

Chemical composition of mafic intrusions is not uniform also varying from east to west. The average Y, Zr, Nb, La, Ce, and Nd concentrations in the intrusions from the east part of the study area are approximately two times higher than in the western ones. The separation into two groups is also observed in triple discriminatory diagrams according to Sm, Ti, V, and Sc. However, εNd(T) values vary from 2.3 to 7.5 without clear correlation with chemical composition. Thus, the revealed patterns basically support interpretation with occurrence of two stages of magmatic activity, the first of which is characterized by enrichment of REEs and other elements.

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

How to cite: Savelev, A., Khudoley, A., and Malyshev, S.: Geochemical features of Meso-Neoproterozoic dolerite sill on the South-East margin of the Siberian Craton., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5252,, 2020.

EGU2020-11081 | Displays | GD2.1

Pb, Nd, Sr isotopic composition of the Mesoproterozoic mafic intrusions (Udzha paleorift, Northern Siberia)

Lidiia Shpakovich, Sergey Malyshev, and Valeriy Savatenkov

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.


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,, 2020.

EGU2020-19405 | Displays | GD2.1

Evidence of increased density of LLVPs from vote map constrained density inversion

Wolfgang Szwillus, Joerg Ebbing, and Bernhard Steinberger

The Large Low Velocity Provinces (LLVP) are two antipodal regions of reduced seismic velocity that extend about 800 km into the mantle from the core-mantle boundary. The LLVPs might affect the generation of plumes and organize large-scale plate motions.

However – except for the reduced velocity – almost all properties of the LLVPs are the subject of vigorous debate. The LLVPs could simply be hot upwellings, or they could be chemically different from normal mantle. They could be a transient feature, exist since the Early Earth or be the result of continuous accumulation as a result of plate tectonics. To some extent, determining the density of the LLVPs could help to distinguish between these scenarios. However, most seismic methods are only weakly sensitive to density and so far both negative and positive density anomalies have been proposed based on seismology. A more direct means of assessing the density structure comes from inverting the gravity field.

While density inversions are inherently non-unique, this can be somewhat alleviated by constraining the geometry of potential sources of the gravity anomalies. In this contribution, we use vote maps to constrain the geometry. A vote map is based on a collection of seismic tomographies and highlights areas of agreement between the seismic tomographies.

We find that the LLVPs possess a slight positive density anomaly between 0.1 and 0.6 %. The variation results from how the lithosphere is treated, since we use both an isostatic model and seismically determined Moho depths, with the isostatic model resulting in smaller LLVP densities. The combination of increased density and reduced velocity can only be explained if the LLVPs are somewhat chemically different from ‘normal’ pyrolitic mantle. Using petrophysical data bases we estimate that an enrichment of 1-1.5% iron oxide content together with a temperature increase of 260 – 380 K with respect to an adiabatic temperature curve can explain the density increase and velocity decrease. Alternatively, the LLVPs would have to contain 40-60 % Mid-Oceanic Ridge Basalt and be 870 – 960 K hotter in order to explain our findings.

How to cite: Szwillus, W., Ebbing, J., and Steinberger, B.: Evidence of increased density of LLVPs from vote map constrained density inversion, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19405,, 2020.

EGU2020-7491 | Displays | GD2.1

Probing mantle plumes using seismic arrays

Laura Cobden, Michael Afanasiev, Frederic Deschamps, Fabienne Stockmann, Christine Thomas, Sebastian Rost, and Andreas Fichtner

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,, 2020.

EGU2020-22295 | Displays | GD2.1

Radial thermo-chemical structure beneath Western and Northern Pacific inferred from seismic waveform inversion

Frederic Deschamps, Kensuke Konishi, Nobuaki Fuji, and Laura Cobden

The Earth’s deep mantle seismic structure is dominated by two large low shear-wave velocity provinces (LLSVPs) located beneath Africa and the Pacific. These structures have been observed by many studies and data sets, but their nature, purely thermal or thermo-chemical, is still debated. Due to trade-off between temperature and composition, maps of shear-wave velocity anomalies (dlnVS) alone are unable to discriminate between purely thermal and thermo-chemical hypotheses. Seismic shear-wave attenuation, measured by the quality factor QS, strongly depends on temperature and may bring additional information on this parameter, allowing to resolve the trade-off between temperature and composition. Here, we invert seismic waveform data jointly for radial models of dlnVS, and QS at two different locations beneath the Pacific, and from a depth of 2000 km down to the core-mantle boundary (CMB). At the Northern Pacific (NP) location, sampling a region around 50º N latitude and 180º E longitude, around VS and QS remain close to the PREM values, representing the horizontal average mantle, throughout the investigated depth-range, with dlnVS ~ -0.1% and QS ~ 300 (compared to QPREM = 312). At the Western Pacific (WP) location, sampling the western tip of the Pacific LLSVP and the Caroline plume, both VS and QS are substantially lower than PREM. Importantly, dlnVS and QS sharply decrease in the lowermost 500 km, from -0.6 % and 255 at 2500 km, to -2.5% and 215 close to the CMB. We then show that WP models cannot be explained by thermal anomalies alone, but require excess in iron of 3.5 to 4.5 % from the CMB up to 2600 km, and about 0.4 to 1.0 % at shallower depths. This later enrichment may be due to the entrainment of small amounts of the Pacific LLSVP material by the Caroline plume. The values of QS we observe give an estimate of the temperature anomalies, around 300-400 K close to the CMB, and 150 K at shallower depths. By contrast, NP models may have a purely thermal origin and can be explained by a temperature excess of about 50 K.

How to cite: Deschamps, F., Konishi, K., Fuji, N., and Cobden, L.: Radial thermo-chemical structure beneath Western and Northern Pacific inferred from seismic waveform inversion, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22295,, 2020.

It is common to perform 2-dimensional simulations of mantle convection in spherical geometry, either with (r, theta) axisymmetry or the (r, phi) spherical annulus geometry (Hernlund and Tackley, PEPI 2008). 

A problem with both of these is that the geometrical restriction forces deformation that is not present in 3 dimensions. Specifically, in a 2-D spherical approximation, a downwelling is forced to contract in the plane-perpendicular direction, requiring it to extend in the 2 in-plane directions. In other words, it is "squeezed" in the plane-perpendicular direction.  If the downwelling has a high viscosity, as a cold slab does, then it resists this forced deformation, sinking much more slowly than in three dimensions, in which it could sink with no deformation. This can cause unrealistic behaviour and scaling relationships for high viscosity contrasts. 

This problem can be solved by subtracting the geometrically-forced deformation ("squeezing") from the strain-rate tensor when calculating the stress tensor. Specifically, components of in-plane and plane-normal strain rate that are required by and proportional to the vertical (radial) velocity are subtracted, a procedure that is here termed "anti-squeeze". It is demonstrated here that this "anti-squeeze" correction results in sinking rates and scaling relationships that are similar to those in 3-D geometry whereas without it, abnormal and physically unrealistic results can be obtained for high viscosity contrasts. This correction has been used for 2-D geometries in the code StagYY (Tackley, PEPI 2008; Hernlund and Tackley, PEPI 2008) since 2010.

How to cite: Tackley, P.: "Anti-squeeze" for Mantle Convection Simulations in Two-Dimensional Spherical Geometry, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10108,, 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,, 2020.

EGU2020-11689 | Displays | GD2.1

Strain-weakening rheology in Earth’s lower mantle: a multi-scale numerical endeavour

Gregor J. Golabek, Anna J. P. Gülcher, Marcel Thielmann, Paul J. Tackley, and Maxim D. Ballmer

Rocks in the Earth’s interior are not homogeneous but consist of different mineralogical phases with different rheological properties. Deformation of heterogeneous rocks is thus also heterogeneous, and strongly depends on the rheological contrasts and spatial distribution of the mineral phases. In Earth’s lower mantle, the main rock constituents are bridgmanite (Br) and smaller amounts of ferropericlase (Fp). Bridgmanite is substantially stronger than ferropericlase [1]. Recent studies propose that lower mantle rheology is highly dependent on the relative mineral abundances and distribution of these two phases [1,2]. It has been suggested that for bridgmanite-depleted compositions, the viscosity decreases with accumulating strain due to the interconnection of the weaker ferropericlase. This implies that deformation may localize in the lower mantle, potentially aiding the formation and preservation of compositionally distinct and “hidden” reservoirs away from these regions of localized deformation [3]. Therefore, understanding the rheological nature of Br-Fp aggregates on a small-scale is crucial for assessing the dynamics of global mantle convection. Here, we address this objective with multi-scale numerical approaches.  

Using a numerical-statistical approach, a connection between ferropericlase morphology and effective rheology of Earth’s lower mantle has recently been established [4]. Results show that bulk-rock weakening depends on the topology of the weak phase as well on its rheology, but also that significant rheological weakening can already be achieved when ferropericlase does not (yet) form an interconnected weak layer.

In a second suite of models, we implement a macro-scale description of strain-weakening based on the micro-scale solutions found in [4] in a global mantle convection model to test the first-order effect of strain weakening on convection dynamics in the lower mantle. We present 2D numerical models of thermochemical convection in a spherical annulus geometry [5] that include a new implementation of tracking the strain ellipse at each tracer through time. We further allow lower mantle materials to rheologically weaken once a certain strain threshold has been reached. Preliminary results indicate that strain localizes along both up- and downwellings in the lower mantle and that rheological weakening has a stabilizing effect on these conduits. 

This multi-scale approach is essential for addressing lower-mantle rheological behavior and our results form an important step towards addressing the feasibility of isolated, long-lived geochemical reservoirs in Earth’s lower mantle.

[1] Yamazaki and Karato (2001), Am. Mineral. 86, 385-391. [2] Girard et al. (2016), Science 351, 144-147. [3] Ballmer et al. (2017), Nat. Geosci. 10, 236-240. [4] Thielmann et al. (2020), Geochem. Geophys. Geosyst., doi:10.1029/2019GC008688. [5] Hernlund and Tackley (2008), Phys. Earth Planet. Int. 171, 48–54.

How to cite: Golabek, G. J., Gülcher, A. J. P., Thielmann, M., Tackley, P. J., and Ballmer, M. D.: Strain-weakening rheology in Earth’s lower mantle: a multi-scale numerical endeavour, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11689,, 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,, 2020.

EGU2020-9135 | Displays | GD2.1

A narrow plume conduit anchored in the lower mantle beneath la Réunion hotspot

Mathurin Dongmo wamba, Barbara Romanowicz, Jean-Paul Montagner, and Guilhem Barruol

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,, 2020.

EGU2020-13705 | Displays | GD2.1

The formation, preservation and seismic signatures of chemical heterogeneities in the lower mantle

Anna J. P. Gülcher, Maxim D. Ballmer, Paul J. Tackley, and Paula Koelemeijer

Despite stirring by vigorous convection over billions of years, the Earth’s lower mantle appears to be chemically heterogeneous on various length scales. Constraining this heterogeneity is key for assessing Earth’s bulk composition and thermochemical evolution, but remains a scientific challenge that requires cross-disciplinary efforts. On scales below ~1 km, the concept of a “marble cake” mantle has gained wide acceptance, emphasising that recycled oceanic lithosphere, deformed into streaks of depleted and enriched compositions, makes up much of the mantle. On larger scales (10s-100s of km), compositional heterogeneity may be preserved by delayed mixing of this marble cake with either intrinsically-dense or intrinsically-strong materials. Intrinsically dense materials may accumulate as piles at the core-mantle boundary, while intrinsically viscous domains (e.g., enhanced in the strong mineral bridgmanite) may survive as “blobs” in the mid-mantle for large timescales, such as plums in the mantle “plum pudding”1,2. While many studies have explored the formation and preservation of either intrinsically-dense (recycled) or intrinsically-strong (primordial) heterogeneity, only few if any have quantified mantle dynamics in the presence of different types of heterogeneity with distinct physical properties. 

To address this objective, we use state-of-the-art 2D numerical models of global-scale mantle convection in a spherical-annulus geometry. We explore the effects of the (i) physical properties of primordial material (density, viscosity), (ii) temperature/pressure dependency of viscosity, (iii) lithospheric yielding strength, and (iv) Rayleigh number on mantle dynamics and mixing. Models predict that primordial heterogeneity is preserved in the lower mantle over >4.5 Gyr as discrete blobs for high intrinsic viscosity contrast (>30x) and otherwise for a wide range of parameters. In turn, recycled oceanic crust is preserved in the lower mantle as “marble cake” streaks or piles, particularly in models with a relatively cold and stiff mantle. Importantly, these recycled crustal heterogeneities can co-exist with primordial blobs, with piles often tending to accumulate beneath the primordial domains. This suggests that the modern mantle may be in a hybrid state between the “marble cake” and “plum pudding” styles. 

Finally, we put our model predictions in context with recent discoveries from seismology. We calculate synthetic seismic velocities from predicted temperatures and compositions, and compare these synthetics to tomography models, taking into account the limited resolution of seismic tomography. Convection models including preserved bridgmanite-enriched domains along with recycled piles have the potential of reconciling recent seismic observations of lower-mantle heterogeneity3 with the geochemical record from ocean-island basalts4,5, and are therefore relevant for assessing Earth’s bulk composition and long-term evolution. 

1 Ballmer et al. (2017), Nat. Geosci., 10.1038/ngeo2898
2 Gülcher et al. (in review), EPSL: Variable dynamic styles of primordial heterogeneity preservation in Earth’s lower mantle
3 Waszek et al. (2018), Nat. Comm., 10.1038/s41467-017-02709-4
4 Hofmann (1997), Nature, 10.1038/385219a0;
5 Mundl et al. (2017), Science, 10.1126/science.aal4179

How to cite: Gülcher, A. J. P., Ballmer, M. D., Tackley, P. J., and Koelemeijer, P.: The formation, preservation and seismic signatures of chemical heterogeneities in the lower mantle, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13705,, 2020.

GD3.1 – Earth's core structure, dynamics and evolution: observations, models, experiments

EGU2020-10540 | Displays | GD3.1

Linking the core heat content to Earth's accretion history

Renaud Deguen and Vincent Clési

The composition of Earth's mantle, when compared to experimentally determined partitioning coefficients, can be used to constrain the conditions of equilibration - pressure P, temperature T, and oxygen fugacity fO2 - of the metal and silicates during core-mantle differentiation.
This places constraints on the thermal state of the planet during its accretion, and it is tempting to try to use these data to estimate the heat content of the core at the end of accretion. To do so, we develop an analytical model of the thermal evolution of the metal phase during its descent through the solid mantle toward the growing core, taking into account compression heating,   viscous dissipation heating, and heat exchange with the surrounding silicates. For each impact, the model takes as initial condition the pressure and temperature at the base of the magma ocean, and gives the temperature of the metal when it reaches the core. The growth of the planet results in additional pressure increase and compression heating of the core. The thermal model is coupled to a Monte-Carlo inversion of the metal/silicates equilibration conditions (P, T, fO2) in the course of accretion from the abundance of Ni, Co, V and Cr in the mantle, and provides an estimate of the core heat content at the end of accretion for each geochemically successful accretion. The core heat content depends on the mean degree of metal-silicates equilibration, on the mode of metal/silicates separation in the mantle (diapirism, percolation, or dyking), but also very significantly on the shape of the equilibration conditions curve (equilibration P and T vs. fraction of Earth accreted). We find that many accretion histories which are successful in reproducing the mantle composition yield a core that is colder than its current state. Imposing that the temperature of the core at the end of accretion is higher than its current values therefore provides strong constraints on the accretion history. In particular, we find that the core heat content depends significantly on the last stages of accretion. 

How to cite: Deguen, R. and Clési, V.: Linking the core heat content to Earth's accretion history, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10540,, 2020.

To understand the processes involved in the deep interior of the Earth and explaining its evolution, in particular the dynamics of the Earth’s fluid iron-rich outer core, only indirect satellite and ground observations are available. They each provide invaluable information about the core flow but are incomplete on their own:

-        The time dependent magnetic field, originating mainly within the core, can be used to infer the motions of the fluid at the top of the core on decadal and subdecadal time scales.

-        The time dependent gravity field variations that reflect changes in the mass distribution within the Earth and at its surface occur on a broad range of time scales. Decadal and interannual variations include the signature of the flow inside the core, though they are largely dominated by surface contributions related to the global water cycle and climate-driven land ice loss.

-        Earth rotation changes (or variations in the length of the day) also occur on these time scales, and are largely related to the core fluid motions through exchange of angular momentum between the core and the mantle at the core-mantle boundary.

Here, we present the main activities proposed in the frame of the GRACEFUL ERC project, which aims to combine information about the core deduced from the gravity field, from the magnetic field and from the Earth rotation in synergy, in order to examine in unprecedented depth the dynamical processes occurring inside the core and at the core-mantle boundary.

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,, 2020.

EGU2020-11740 | Displays | GD3.1

Ohmic dissipation induced by Earth's nutation

Santiago Triana, Antony Trinh, Jeremy Rekier, and Veronique Dehant

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,, 2020.

EGU2020-875 | Displays | GD3.1

Anisotropic turbulent diffusivities and rotating magnetoconvection problems

Enrico Filippi and Jozef Brestenský

Earth’s core Physics inspires the magnetoconvection models. Turbulent state of the core can increase the viscosity, the thermal diffusivity and also the magnetic diffusivity. The change of magnetic diffusivity is also called β-effect and it is important in dynamo mechanisms. Moreover, the turbulence suggests that the dynamics can be more complicated than it is usually presented. For instance, due to turbulence the diffusivity coefficients could be anisotropic as it was described in some recent studies, which stress how anisotropy in many cases facilitate convection and in other cases inhibits it. For example, if there is anisotropy some types of convection can occur also with very small values of Ekman numbers, which are usual for the Earth’s core. This is important because the convection can be the main cause of dynamo action. We present several rotating magnetoconvection models in horizontal plane layer with gravity and rotation axis in vertical direction and homogeneous magnetic field in horizontal direction. Different models correspond to different cases of anisotropic diffusivities. In other words, we consider several anisotropic models: one with anisotropy in all diffusivities and other models with various combinations of anisotropic and isotropic diffusivities. Comparisons with other former models (e.g. with isotropic case, p-case, partial anisotropy case when only magnetic diffusivity is isotropic, and f-case, full anisotropy case with all diffusivities anisotropic) are thoroughly performed. In all models we consider two distinct kinds of anisotropy, Stratification Anisotropy – SA, determined by direction of single gravity (buoyancy) force and Braginsky-Meytlis one – BM, determined by directions of magnetic field and rotation axis. All systems described by these models are prone to instabilities, so analysis in term of normal modes and search for preferred modes are very useful to study such systems. We focus our attention on stationary modes and SA anisotropies. Furthermore, we distinguish two sub-cases of SA anisotropy: atmospheric – Sa, if the diffusion in the vertical direction is greater than in the horizontal ones and oceanic – So, if opposite holds. In Sa (So) anisotropy the convection is in major cases facilitated (inhibited). This fact suggests that it is important to study Sa as well as So anisotropies in the Earth’s core. Our main results concern cases of anisotropic diffusivities, when preferred modes give new dynamics (unexpected in isotropic case) in the system in which geodynamo can work. 

How to cite: Filippi, E. and Brestenský, J.: Anisotropic turbulent diffusivities and rotating magnetoconvection problems, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-875,, 2020.

EGU2020-21322 | Displays | GD3.1

Nonlinear Convection of Electrically Conducting Fluid in a Rotating Magnetic System

Hari Ponnamma Rani, Yadagiri Rameshwar, Jozef Brestensky, and Enrico Filippi

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.


[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,, 2020.

EGU2020-10723 | Displays | GD3.1

Exploring Alternative Instrumentation in the Three-Meter Spherical Couette Experiment

Sarah Burnett, Nathanaël Schaeffer, Kayo Ide, and Daniel Lathrop

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,, 2020.

EGU2020-21632 | Displays | GD3.1

Convection of Electrically Conducting Fluid in a Rotating Magnetic System: Cross rolls

Yadagiri Rameshwar, Gudukuntla Srinivas, Hari Ponnamma Rani, Jozef Brestensky, and Enrico Filippi

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,, 2020.

EGU2020-14058 | Displays | GD3.1

Steady flows in the core of precessing planets : effects of the geometry and an applied magnetic field.

Raphael Laguerre, Aymeric Houliez, David Cébron, and Véronique Dehant

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.


[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,, 2020.

Traditionally, searching the Slichter mode (the longest-period mode of the Earth's free oscillations 1S1) is based on the data of the superconducting gravimeters of the international GGP network. Currently this network is included in the International Geodynamics and Earth Tide Service (IGETS).

The sensitivity limit of the best superconducting gravimeters is about 1 nGal and not sufficient for direct observation of the Slichter mode even after the significant earthquakes. To reduce the detection threshold, the researchers used the “stacking” procedure — an joint data processing of the several instruments, but the different sensitivity level of the gravimeters prevents the achievement of maximum efficiency.

We have developed an asymptotically optimal algorithm based on the maximum likelihood method that takes into account the features of the Slichter mode and seismic noise. An important feature of the algorithm is its ability to evaluate the splitting parameter b which determines the distance between the side singlets of the triplet, simultaneously with the mode period T. The use of a non-linear inertial converter allows to take into account the non-Gaussian noise of real data. The use of the Neumann-Pearson criterion makes also possible to determine confidence level of detection: the false alarm probability and the correct detection probability, depending on the signal-to-noise ratio).

The algorithm was tested on synthetic data. A computer experiment has shown that the algorithm can detect the Slichter mode for a signal-to-noise ratio of 10-4. The algorithm was used to search the Slichter mode after the largest earthquakes based on the data of the IGETS network.

The results of the analysis are reported.

This work is supported by the Russian Foundation for Basic Research under Grant No Grant No 19-05-00341.

How to cite: Milyukov, V., Vinogradov, M., Mironov, A., and Myasnikov, A.: Detection and estimation of the Slichter mode based on the data of the IGETS superconducting gravimeters network using the asymptotically optimal algorithm, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11614,, 2020.

EGU2020-20504 | Displays | GD3.1

Observing the signature of the magnetic field's behaviour in the radial variation of inner core anisotropy

Janneke de Jong, Lennart de Groot, and Arwen Deuss

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,, 2020.

EGU2020-13416 | Displays | GD3.1

Inner core scattering estimates inferred from PKiKP coda waves

Dmitry Krasnoshchekov

Lateral variations in structure and composition with a scale length of about several kilometers are thought to be one of the reasons for strong seismic attenuation in the Earth’s solid inner core. These fine-scale heterogeneities are probably best constrained by scattered coda of body waves pre-critically reflected from the inner core boundary (PKiKP). Here we analyze 9 arrays of sources and receivers to detect weak PKiKP coda on short-period and broadband seismic records in the range of epicentral distances from 6 to 94 degrees. 6 PKiKP bounce points scan the IC surface below Central Asia, 2 – under the Arctic region, and one – under Southeastern Asia. We observe the IC scattered coda in the Hilbert envelope of the PKiKP beam built by linear summation of 1.3 – 5 Hz bandpass frequency filtered vertical records of array channels. Assuming the detected PKiKP codas result from scattering through the volume of the uppermost inner core, we estimate the Qc quality factor by fitting of the observed PKiKP codas with a standard model – the classical method invoked in crust and mantle studies. We find the quality factors are between 400 and 500 with no distinct geographical dependence.

How to cite: Krasnoshchekov, D.: Inner core scattering estimates inferred from PKiKP coda waves, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13416,, 2020.

EGU2020-22379 | Displays | GD3.1

Understanding strength and texture in Fe at planetary core pressures and temperatures: insights from laser compression experiments

Sébastien Merkel, Sovanndara Hok, Cynthia Bolme, Arianna Gleason, and Wendy Mao

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,, 2020.

GD4.1 – Crust-Mantle Lithosphere-Asthenosphere Interplay, Structure, Deformation and Dynamics

Madeira and the Canary Islands, located in the eastern North Atlantic, are two of many examples of hotspot surface expressions. Their tracks have been reconstructed to past locations close to the south-western part of the Iberian Peninsula and north-western Africa, respectively. Furthermore, due to their close proximity, an interconnected origin of these two hotspots has been proposed but details remain unclear. A better understanding of the crust and upper mantle structure beneath these islands is needed to investigate this potential connection.

The subsurface structure has an influence on the stress field, which can be investigated studying seismic anisotropy patterns of the region. Seismic anisotropy leads to variations in the speed of seismic waves as a function of the direction of wave propagation. In the crust an orientation in the direction of maximum stress is observed, commonly being parallel to the alignment of fractures or cracks. In the upper mantle the orientation is influenced by mantle flow. A widely used method to identify anisotropy is the observation of shear-wave splitting of data from teleseismic events. In case of multiple anisotropic layers, including measurements from local events it is possible to distinguish crustal from upper mantle influences.

As part of the SIGHT project (SeIsmic and Geochemical constraints on the Madeira HoTspot), we carried out the first detailed study of seismic anisotropy beneath both archipelagos, using teleseismic SKS and local shear-wave splitting measurements of data collected from land stations of seismic networks located on Madeira and the Canary Islands.

Significant changes, both in orientation and delay time, can be observed on short length-scales on the order of tens of kilometres, matching major geological features such as, for example, the major rift zone on Madeira island. In a further step, we compare these results to previous studies of crustal and upper mantle anisotropy focusing on north-western Africa and the Iberian Peninsula to investigate the nature of the lithospheric corridor between the present day hotspot positions and the Atlas-Gibraltar region.

This is a contribution to project SIGHT (Ref. PTDC/CTA-GEF/30264/2017). The authors would like to acknowledge the financial support FCT through project UIDB/50019/2020 – IDL.

How to cite: Schlaphorst, D., Silveira, G., and Mata, J.: Investigating Seismic Anisotropy of the Madeira and Canaries Hotspots Using Teleseismic and Local Shear-Wave Splitting with the SIGHT project, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9340,, 2020.

EGU2020-8071 | Displays | GD4.1