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Séminaires externes 2018-2019

Lundi 3 septembre 2018 à 13h : Katie Smart Insights into Archean volatile cycles and magmatism from diamonds and kimberlites

Diamonds from the 2.7 - 3 Ga Witwatersrand supergroup (South Africa) provide a unique window into carbon and nitrogen cycles on Mesoarchean Earth. Isotope compositional information from the diamonds suggest the presence of recycled nitrogen and relatively oxidized carbon in the upper mantle prior to 3 Ga. The carrier magma for the Witwatersrand diamonds remains speculative due to the scarcity of global kimberlite magmatism before 1.2 Ga, a feature that can be linked to the secular cooling of Earth.

Jeudi 27 septembre 2018 à 13h30 : Osvaldo P. Gonzalez Maurel Triggering of Neogene-Quaternary mafic eruptions in the northern Chilean Andes

The western margin of South America is the active magmatic arc of the Andean Central Volcanic Zone (CVZ) formed on a thick continental crust (70-74 km). The primitive magma composition is difficult to determine due to the scarcity of true basalts, with basaltic-andesite being the least-evolved volcanic product. This lack of mafic rocks is directly related to the significant crustal contribution to the parental magmas during their ascent. Most mafic magmas identified in Neogene and Quaternary volcanoes in the CVZ are isolated eruptions of basaltic-andesite to andesite lavas. Evidence of such rare mafic eruptions is concentrated in the Western Cordillera of northern Chile between 21º10’S to 22º50’S as lava fields erupted from stratovolcanoes and/or scoria cones. Based on new automated mineralogical analyses, whole rock geochemical data, and Sr and Nd isotope ratios of the aforementioned mafic volcanism and combined with the geological framework, yields information about the magmatic evolutionary processes and its spatio-temporal evolution, and even constrains a potential source for the parental magmas. According to macroscopic observations and microscopic petrography, combined with automated mineralogical analyses, all studied rocks are quite similar (overall colour, grain size), that is : porphyritic, containing four main mineral phases as phenocrysts (0.2 - 2.5 mm), plagioclase, orthopyroxene, clinopyroxene and/or olivine. Three main types of mafic rocks were identify based on the ferromagnesian assemblages : (i) olivine-bearing, (ii) olivine- and two-pyroxene-bearing, and (iii) two-pyroxene-bearing. In contrast, whole-rock geochemical and Sr and Nd isotopic data reveal a compositional heterogeneity for the parental magmas (52.5 - 64.8 wt% SiO2 ; 1.76 - 6.33 wt% MgO ; 2 - 382 ppm Cr ; 3 - 80 ppm Ni ; 87Sr/86Sr = 0.70554 - 0.70875 ; 143Nd/144Nd = 0.512208 - 0.512513). The petrogenesis of these rare mafic rocks is resembled by the interplay of magma mixing, assimilation and fractional crystallization (AFC), and assimilation during turbulent ascent (ATA) processes. Mineralogy and geochemistry indicate temporal compositional changes, including the processes involved in the magmatic evolution in time, nevertheless the final composition of the mafic input erupted is related to the spatial framework of the Altiplano-Puna Magma Body (APMB). The spatial distribution of the studied volcanoes define that the least differentiated lava fields are concentrated at the borders of the large partial melt body, where the minimum thickness of the vertical mush column is located. To erupt these Neogene-Quaternary mafic rocks, the parental magmas required a combined effect of a restricted degree of interaction with the APMB melts, which is linked to a rapid ascent due to thermal ductility and faulting features in the mid-upper crust.

Jeudi 11 octobre 2018 à 13h00 : Stacy Carolin, University of Insbruck Speleothems reveal Western Asia rainfall response to abrupt millennial events and orbital forcing, 190-70 kaBP

Western Asia, from the Levant across the Tigris-Euphrates river basin and through Iran, is comprised of several areas labeled as high overall water risk. Dust storms are also a particular health hazard in the area. More palaeoclimate records are needed to improve our understanding of rainfall variability beyond the instrumental record in this region, which currently relies on knowledge gained mostly from lake records and loess deposits. Here we present a speleothem record from northeast Iran spanning 190-70 ka. The site today receives a small amount of rainfall in the winter to spring months, 250 mm total annually, making caves in the region under modern conditions particularly dry. Oxygen isotopes in the calcite layers are also difficult to interpret due to the general location of the cave, which is far inland, causing precipitation to form from water vapor that may have traveled along many rainout pathways and originated from various source waters. Nonetheless, we present a stalagmite oxygen isotope record that reveals shifts to more positive values on a millennial scale that are strikingly similar to the Greenland stadial events. We use additional metal elements (local rainfall proxies) that have precipitated within the speleothem calcite, and a limited number of speleothem oxygen isotope records available in other parts of Western Asia to interpret the cause of the oxygen isotope enrichment in northeast Iran and the associated rainfall changes. The northeast Iran record also reveals significant precession-length period variations, with higher precession in phase with lighter oxygen isotope values. We use atmospheric climate model experiments and several speleothem records with precession-length period variations across Asia to interpret these signals as rainfall pattern or amount changes in either the winter or summer season. Finally, we note the sole deviation of the low-resolution oxygen isotope record from its linear relationship with precessional forcing, which occurs between 155-135 ka. We suggest that the relationship has broken down here due to the presence of large Eurasian ice sheets, which likely had a significant impact on the atmospheric circulation system that typically carries moisture from the Atlantic across Europe and the Mediterranean into Iran.

Jeudi 8 novembre 2018 à 13h30 : Alessio Sanfilippo, (Dep. of Earth and Environmental Sciences, University of Pavia & IGG-CNR Pavia, Italy) : Melting an ancient, depleted mantle at Mid Ocean Ridges

The use of combined radiogenic isotope systematics (e.g., Os, Nd, Hf, Sr, Pb) on mantle peridotites and erupted melts reveals an extreme heterogeneity of the asthenospheric mantle, which is nowadays considered to consist of portions re-enriched in fusible elements and portions an old, refractory oceanic lithosphere survived from re-homogeneization in the asthenosphere. This contribution infer that ancient (> 1 Ga), ultra-depleted domains in the upper mantle have the ability to produce melts, and that these “ultra-depleted” and “radiogenic” melts have a fundamental -and so far unrecognized - contribution in the basalts chemistry.

Jeudi 15 novembre 2018 à 13h30 : James Farquhar (University of Maryland, USA) : Four S-isotope provide information about reaction pathways (equilibrium vs kinetic effects) and geochemical processes

This talk will review the basic principles that control the nature of sulfur isotope variability in terrestrial environments extending from the modern atmosphere to the ancient atmosphere, sediments in deep geological time, and recycling of material from ancient sediments to solid-Earth reservoirs. It will be focused on some recent work with ice core sulfate (collaboration with Joel Savarino and Elsa Gautier) and some experiments related to transfer of the S isotope signal to pyrite in microbial experiments we have done.

Lundi 19 novembre 2018 à 11h : Romain Delunel, U. Bern Applications récentes et futures des nucléides cosmogéniques dans les archives (et flux) sédimentaires

Depuis une vingtaine d’années, l’utilisation des nucléides cosmogéniques produits in situ tels que le beryllium-10 s’est imposée comme une méthode unique pour la datation absolue des formations superficielles quaternaires d’origine fluviatile, glaciaire ou encore gravitaire. En outre, la concentration en nucléides cosmogéniques dans les sédiments, actuels ou anciens, permet de tracer les flux sédimentaires et de quantifier les taux de dénudation moyens pluriséculaires à l’échelle du bassin versant.
Après une présentation de cette approche méthodologique, je vous présenterai les principaux enseignements issus d’une compilation des taux de dénudation pour plus de 350 bassins versants alpins. Enfin, je proposerai une application possible de cette méthode afin de quantifier, dans l’espace et le temps, les contributions respectives des processus naturels et des perturbations anthropiques dans l’évolution pédo-géomorphologique des systèmes naturels et anthropisés.

Mercredi 21 novembre 2018 à 13h : Christoph Burkhardt, Institut für Planetologie - Münster (Allemagne) Solar system genealogy — from grains to planets

Understanding the formation and evolution of our solar system through the analysis of meteorites took great leaps forward in the last years. This progress is in particular driven by the discovery of minute nucleosynthetic isotope anomalies in meteorites and their components. These anomalies are conservative in nebular and planetary processes and thus provide a unique tool for unraveling genetic relationships among planetary materials. In this talk I will provide an overview of the state of the art in this emerging field, with a focus on the inferred early solar system structure and the dynamics of volatile delivery to Earth and Mars.

Jeudi 29 novembre 2018 à 13h : Marc Monnereau (IRAP Toulouse) Modélisation de Vesta et de Cérès, et ségrégation métal-silicate

En septembre 2012, la sonde Dawn quitte Vesta qu’elle a étudiée pendant plus d’un an pour rejoindre Céres autour de laquelle elle tourne encore. Cette première partie de la mission confirme Vesta comme le corps parent des météorites HED (gabbros et pyroxénites cumulatives). Ces témoins d’une activité magmatique très précoce (i.e. datant des premiers millions d’années de la formation du système solaire), est probablement due à la présence de l’isotope radioactif 26Al en abondance suffisante au moment de la formation des corps rocheux primitifs pour permettre leur fusion. La différenciation de Vesta et la formation des HED ont été d’abord vues comme le résultat de la cristallisation d’une fusion totale du corps. Pourtant, la migration de la principale source chaleur, contenue dans le premier minéral fondu, le plagioclase, rend ce scénario assez difficile. On montre cela via un modèle de migration des magmas, basé sur les équations de la compaction, couplé à un diagramme d’équilibre de phase simplifié (olivine-anorthite-quartz). Cela permet de calculer l’évolution de la minéralogie en fonction du temps et de la profondeur. Les résultats montrent que les eucrites et les diogénites pourraient être une caractéristique commune des petits corps les plus gros (quelques centaines de km de diamètres) formés très tôt dans l’histoire du système solaire.

Jeudi 6 décembre 2018 à 13h : Catherin Annen, Bristol University Accumulation, différentiation et cristallisation des magmas dans la croûte continentale

Le magmatisme est un phénomène complexe dont la compréhension nécessite de prendre en compte ses dimensions spatiales et temporelles. Les données géophysiques, géochronologiques et expérimentales permettent d’élaborer des modèles conceptuels qui peuvent être testés avec des simulations numériques. Celles-ci indiquent que les vitesses de transfert et de mise en place des magmas dans la croûte sont déterminantes pour la génération des chambres magmatiques. Les calculs de transfert de chaleur montrent que la plupart des corps ignés superficiels se sont mis en place trop lentement pour que de grandes quantités de magmas aient pu s’accumuler dans les niveaux supérieurs de la croûte. En conséquence, il est vraisemblable que l’essentiel de la différentiation magmatique opère à des niveaux plus profonds en milieu et base de croûte. La compréhension des conditions d’accumulation des magmas présente un intérêt non seulement académique, mais également sociétal, dans la mesure où elle affecte les modèles de genèse de certaines ressources minières ainsi que notre interprétation des précurseurs d’éruptions volcaniques.

publié mercredi 14 août 2013