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Seminaires externes 2020 - 2021

Les séminaires externes ont lieu dans l’amphithéâtre du CRPG, en général le jeudi de 13h30 à 14h30 tous les 15 jours. Contact : Johan Villeneuve (johan.villeneuve@univ-lorraine.fr)

Jeudi 22 Octobre 2020 à 13h30 : Charlotte Prud’homme, Max Planck Institute

Caractérisation des variations climatiques dans les dépôts éoliens : une approche combinée géochimie-datation des carbonates continentaux

La mise en évidence de la réponse des environnements continentaux aux variations climatiques à différentes échelle de temps constitue un objectif de recherche majeur. En domaine continental, les dépôts éoliens constituent l’archive sédimentaire la plus étendue sur le continent Eurasiatique. Ces dépôts enregistrent les variations climatiques abruptes de la dernière période glaciaire sous la forme de séquences de loess-paléosols, et également sur des échelles de temps plus longues au cours de la transition Plio-pléistocène sous la forme de séquences sédimentaires. Une démarche de géochimie isotopique basée sur les isotope stables (18O et 13C) a été développée sur différents types de carbonates extraits de séquences loessiques typiques des milieux périglaciaires de l’Europe de l’Ouest (granules de vers de terre, Vallée du Rhin, Allemagne) et arides d’Asie Centrale (concrétions pédologiques, Kazakhstan). Ces travaux débouchent sur 1) la quantification des paléotempératures et paléoprécipitations au cours des interstades de Dansgaard-Oeschger en domaine continental et 2) sur une meilleure compréhension de la dynamique du système climatique en Asie Centrale au cours des cinq derniers millions d’année avec la mise en évidence d’une aridification associée à une intensification des processus éoliens, modulée par l’interaction entre les systèmes climatiques de moyennes (vent d’ouest) et hautes latitudes (anticyclone sibérien).

Jeudi 12 Novembre 2020 à 13h30 : Marine Paquet, IPG Paris

Highly siderophile elements in shergottite sulfides and the sulfur content of the martian mantle

Shergottite meteorites are ultramafic to mafic igneous rocks derived from partial melting of distinct regions of the martian mantle. As such, they trace magmatic processes, including fractional crystallization and mixing processes in Mars. New chalcophile (Cu, Se, Zn, Pb), siderophile (Ni, Co, W), and highly siderophile element (HSE : Au, Re, Pd, Rh, Pt, Ru, Ir, Os) abundance data are reported for sulfide assemblages in a suite of thirteen incompatible trace element depleted, intermediate and enriched shergottites, along with new whole-rock HSE abundance and 187Os/188Os data for seven shergottites. Sulfide grains in depleted and intermediate shergottites typically have the highest absolute abundances of the HSE, with broadly flat CI-chondrite normalized patterns. Enriched shergottite sulfide grains typically have highly variable Au, elevated Pd and Rh and are relatively depleted in Zn, Ir and Os. The new HSE whole-rock data for enriched (Northwest Africa [NWA] 7397, NWA 7755, NWA 11043), and intermediate shergottites (NWA 10961, NWA 11065, NWA 12241, and NWA 12536) are generally consistent with existing 187Os/188Os and HSE abundance data for these geochemical groupings. Enriched shergottites with > 1 ppb Os have measured 187Os/188Os ranging between 0.1296 and 0.1471, with variable Pd and Pt contents. Intermediate shergottites with > 1 ppb Os have chondrite-relative proportions of the HSE at 0.01 to 0.001 × CI chondrites and 187Os/188Os from 0.1284 and 0.1295. Sulfides are the major host of the HSE, and they control the behavior of the HSE during petrogenetic processes in shergottite magmas, enabling the determination of HSE compatibility for martian magmatism in the order : Os > Ir ≥ Ru ≥≥ Rh ≥ Pd ≥ Re ≥ Pt ≥ Au. Fractionation models of removal of an olivine-dominated cumulate recreate HSE patterns for the whole-rock shergottites. Enriched shergottites are best reproduced by 25 to 30% of fractionation from a degassed parent melt (250 ± 50 ppm of S), whereas depleted and intermediate shergottites can be explained by slightly lower fractionation (10 to 15%) from higher S content parent melts (350 ± 100 ppm of S). Sulfur contents in the melt 50% higher than these estimates yield earlier S-saturation during fractional crystallization, leading to an abrupt decrease of the more compatible HSE (Ru, Ir, Os), which is not observed. These results indicate that the martian mantle and partial melts from it, are probably not anomalously enriched in S, and instead are similar to slightly higher than those of the terrestrial mantle and its partial melts.

Jeudi 26 Novembre 2020 à 13h30 : Thorsten Kleine, Universität Münster

How did the terrestrial planets form ?

Understanding the origin of Earth is a fundamental but still unsolved question. Currently, two different modes of terrestrial planet formation are known. In the classic, "Wetherill-type" model, the terrestrial planets formed by accretion of Moon- to Mars-sized planetary embryos over a timescale of 100 million years (Ma). The embryos themselves formed in the inner solar system, so the terrestrial planets predominantly formed from ‘local’ material, with only little contamination from outer solar system bodies. More recently, an alternative model has been proposed, in which planets grew by accreting "pebbles" that drifted sunwards through the disk. Pebble accretion requires the presence of gas, so in this scenario Earth grew to near completion within the 5 Ma lifetime of the gaseous protoplanetary disk. In this talk I will show how isotope anomalies in meteorites can be used to distinguish between these two fundamentally modes of rocky planet formation.

Jeudi 03 Décembre 2020 à 13h00 : Pierre-Yves Arnould, OTELo

OTELo accompagne ses laboratoires aux enjeux de la science ouverte

Mercredi 9 Décembre 2020 à 13h30 : Sune Nielsen, Woods Hole Oceanographic Institution

Planetary core formation and the lunar giant impact : evidence from vanadium isotopes

Planetary differentiation processes and the conditions under which these take place are an integral part of understanding the early evolution of our solar system. Metal stable isotopes (e.g. Fe, Mo, Cr) have in the last few decades emerged as potential tools to further elucidate such processes due to small but significant isotope fractionation factors between metal and silicate. I will present the first vanadium isotope data set for Martian meteorites that reveal values slightly heavier than chondrites. Combined with the recently deduced heavy composition measured for bulk silicate Earth relative to chondrites, it is most likely that the silicate portion of both planets are consistent with cores that are isotopically light. I will also present new vanadium isotope data for lunar rocks that reveal these to be significantly lighter than bulk silicate Earth. The difference in vanadium isotope compositions between the Earth and Moon will be discussed in terms of different models of lunar formation.

Jeudi 10 Décembre 2020 à 13h30 : Véronique Le Roux, Woods Hole Oceanographic Institution

Water in the Earth’s mantle : story from the oceans abyss

Water is one of the most important constituents of the Earth’s mantle. Small variations in water concentrations affect the viscosity, electrical conductivity, and seismic velocity of mantle rocks, and influence the melting behavior of the mantle. However, our understanding of the processes that control H2O distribution in upper mantle rocks (peridotites) remains strikingly poor, especially in oceanic environments. Here, we present new secondary ion mass spectrometry (SIMS) data on the H2O contents of peridotite minerals recovered from the mid-Atlantic ridge. We report unique correlations between H2O contents, crustal ages, and mineral chemistry, allowing us to unravel a potentially widespread process of water enrichment in the oceanic lithosphere.

publié mardi 11 août 2015