Displays

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

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

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

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

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, https://doi.org/10.5194/egusphere-egu2020-18156, 2020.

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, https://doi.org/10.5194/egusphere-egu2020-9784, 2020.

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, https://doi.org/10.5194/egusphere-egu2020-9164, 2020.

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, https://doi.org/10.5194/egusphere-egu2020-8384, 2020.

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, https://doi.org/10.5194/egusphere-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, https://doi.org/10.5194/egusphere-egu2020-8817, 2020.

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

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

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

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

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

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

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

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

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

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, https://doi.org/10.5194/egusphere-egu2020-6942, 2020.

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

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

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, https://doi.org/10.5194/egusphere-egu2020-520, 2020.

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

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

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

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

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

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, https://doi.org/10.5194/egusphere-egu2020-13737, 2020.

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

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

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

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

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

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

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

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, https://doi.org/10.5194/egusphere-egu2020-3542, 2020.

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, https://doi.org/10.5194/egusphere-egu2020-13603, 2020.

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

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

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

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, https://doi.org/10.5194/egusphere-egu2020-8297, 2020.

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

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

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

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

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

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

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, https://doi.org/10.5194/egusphere-egu2020-13558, 2020.

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

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

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

(^*correspondence: arathy.ravindran@geo.unibe.ch)

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. 

References:

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, https://doi.org/10.5194/egusphere-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 (La_N/Yb_N =1.47-10.63) with moderate to highly fractionated LREE (La_N/Sm_N=2.67-8.93) and nearly flat to slighly elevated HREE (Gd_N/ Yb_N=0.62-1.29) and a significant Eu negative anomaly (avg. Eu/Eu*=0.67). In general, presence of negative Eu anomaly in clastic rocks reflect the widespread occurrence of granitic rocks in the source area, which possess negative Eu anomaly. On the other hand, mechanical enrichment of zircon (having negative Eu anomaly, high HREE concentration and low La_N/Yb_N), if present, will hamper the whole REE pattern of the sediments and necessarily, do not actually mimic the source composition. Here, in our study, the Th/Sc vs Zr/Sc diagram show mineral Zircon has been concentrated by mechanical concentration in the sedimentary rocks. Few quartzite samples which have high Zr content typically exhibit low La_N/Yb_N values, reflecting pivotal role of mineral zircon in controlling the REE pattern of the sediments. Hence, in this case, we should be cautious in interpreting of the Eu negative anomaly of the basal quartzites for meticulously identifying their source rock composition. More geochemical and other analytical approaches are required in this regard.

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