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Séminaires externes 2017-2018

Les séminaires auront lieu dans l’amphithéâtre du CRPG, en général, le jeudi de 13h30 à 14h30 tous les 15 jours. Contact : Etienne Deloule (deloule@crpg.cnrs-nancy.fr)

Jeudi 22 mars 2018 à 13h30 : Martin Bizzarro (Centre for Star and Planet Formation, University of Copenhagen) Evidence for extremely rapid magma ocean crystallization and crust formation on Mars

The formation of a primordial crust is a critical step in the evolution of terrestrial planets but the timing of this process is poorly understood. The mineral zircon is a powerful tool for constraining crust formation as it can be accurately dated with the U-Pb system and is resistant to subsequent alteration. Moreover, the high concentration of Hf in zircon allows for the utilization of the 176Lu-176Hf decay system to determine the nature and formation timescale of its source reservoir. Ancient igneous zircons with ages of 4430 Ma have been reported in martian meteorites believed to represent regolith breccias from the southern highlands of Mars. These zircons are present in evolved lithologies interpreted to reflect re-melted primary martian crust thereby potentially providing unique insights into early crustal evolution on Mars. Here, we report concomitant high-precision U-Pb ages and Hf-isotope compositions of ancient zircons from the NWA 7034 martian regolith breccia. Seven zircons with mostly concordant U-Pb ages define 207Pb/206Pb dates ranging from 4476.3±0.9 Ma to 4429.7±1.0 Ma, including the oldest directly dated material from Mars. All zircons record unradiogenic initial Hf-isotope compositions inherited from an enriched, andesitic-like crust extracted from a primitive mantle no later than 4547 Ma. Thus, a primordial crust existed on Mars by this time and survived for 100 Myr before it was reworked, possibly by impacts, to produce magmas from which the zircons crystallized. Given that formation of a stable primordial crust is the end product of planetary differentiation, our data require that the accretion, core formation and magma ocean crystallization on Mars was completed <20 Myr after Solar System formation. These timescales support models suggesting rapid magma ocean crystallization leading to a gravitationally unstable stratified mantle, which subsequently overturns resulting in decompression melting of rising cumulates and extraction of a primordial basaltic to andesitic crust.

Jeudi 29 mars 2018 à 13h30 : Claudio Rosenberg (iSTeP, UPMC Paris) Relation entre raccourcissement, érosion, exhumation et largeur de la Chaîne Alpine, au cours de la collision.

Erosion exerts a major control on the structural evolution of collision wedges. One aspect that has often been investigated in models and natural case studies is the effect of erosion on the width of orogens. The higher the efficiency of erosion and the smaller the width of the wedge, hence of the orogen. The Alps are a well suited natural laboratory to assess such relationships during their collisional history. Calculating and compiling the amount of collisional shortening along 6 distinct orogen-scale cross sections shows that the partitioning of shortening between upper and lower plates varies along-strike in the Central and in the Eastern Alps. We investigate the relationship between the width of the thickened accreted lower plate in relation to the inferred amount of collisional shortening and exhumation. Assuming that the present-day, along-strike increase in the amount of collisional shortening represents the structural evolution that affected the collisional wedge through time, it may be concluded that the cross-sectional area of the accreted lower plate diminishes during ongoing shortening, indicating that the erosional flux outpaced the accretionary flux. Higher amounts of collisional shortening systematically coincide with smaller widths of the accreted lower plate and dramatic increases of the reconstructed eroded rock column. Higher amounts of shortening also coincide with larger amplitudes of orogen-scale upright folds, with larger amounts of exhumation and with larger exhumation rates. These relationships suggest that erosion did play a major role in reducing by >30 km the thickness of the orogenic wedge, thus allowing for the long-term localization of shortening and its accommodation by upright orogen-scale folds. Long-term climate differences cannot explain the observed, alternating changes of width by a factor of almost 2 along straight segments of the orogen, on length-scales of less than 200 km. Sedimentary or paleontological evidences supporting such paleo-climatic differences are lacking, suggesting that erosion was the not the major factor controlling the width of the orogen.

Jeudi 5 avril 2018 à 13h30 : Cornellis Dullemond (université d’Heidelberg) Formation of chondrules and transport of particles in protoplanetary disks

The formation of chondrules and the transport of chondritic material in the protosolar nebula remain major open questions. I will discuss both these topics in my talk. On the first topic I will discuss my work on chondrule formation in impact splashes. Using numerical simulations of a ballistically expanding cloud of hot lava droplets, I found that impact splashes are a natural way to explain the high densities needed during chondrule formation to keep the volatile elements such as Na and K inside chondrules. On the second topic I will discuss some of the latest findings on particle transport in protoplanetary disks obtained from recent spectacular observations of protoplanetary disks with the ALMA telescope array. These observations show that these disks consist of dust rings, and that these rings indicate (a) low turbulent mixing and (b) strong trapping of dust particles. These findings appear to be in agreement with the low degree of mixing found from the meteoritic record in our own solar system.

Jeudi 19 Avril 2018 à 13h30 : Marc Blanchard (GET Toulouse) Modélisation ab initio des processus de fractionnement des isotopes stables : le cas du zinc

L’analyse des mécanismes contrôlant la composition isotopique des espèces naturelles et l’utilisation de ces compositions isotopiques pour élucider les processus naturels constituent un domaine central de la géochimie moderne. Un point clé pour l’interprétation de ces compositions est de connaître les facteurs de fractionnement isotopique en conditions d’équilibre. Ces constantes peuvent être calculées en utilisant une approche thermodynamique statistique basée sur les propriétés vibrationnelles des composés (calculs ab initio utilisant la théorie de la fonctionnelle de la densité). Ces outils théoriques permettent de relier les propriétés isotopiques à la spéciation et à la cristallochimie, et offrent donc la possibilité d’étudier les mécanismes moléculaires fondamentaux contrôlant les fractionnements isotopiques. L’approche computationnelle complète ainsi parfaitement le travail expérimental avec pour objectif de mieux appréhender les milieux naturels. Cet exposé montrera comment les calculs ab initio contribuent à la géochimie isotopique avec l’exemple des isotopes du zinc pour les minéraux et les espèces aqueuses en conditions de basse température.

Jeudi 14 juin 2018 à 13h30 : Anne Magali Seydoux ( Université de Saint Etienne) Nouveaux développement avec la sonde atomique tomographique

Jeudi 21 juin 2018 à 13h30 : Susanne Wampfler (Center for Space and Habitability (CSH) University of Bern) Stable isotope ratios - a promising link between astrochemistry and cosmochemistry

Understanding the formation and evolution of the Solar System is a key goal in both astronomy and cosmochemistry. While astronomical observations inform us about the physical and chemical processes taking place during the formation of stars and their planetary systems, cosmochemistry provides us with direct information about the composition and conditions in the protosolar nebula. However, linking the results from both fields is challenging, because the most pristine Solar System materials are mainly accessible in solid form, either as cometary ices or rocky material in meteorites, whereas astronomical observations primarily probe the gas around protostars and in protoplanetary disks.

Stable isotope ratios of the volatile elements hydrogen, nitrogen, and oxygen are among the most promising tools for linking the processes observed in star-forming regions to the properties of the Solar System. I will explain how we measure isotopic ratios in star-forming regions from (sub-)millimeter observations, and then present our latest results on nitrogen isotope fractionation around protostars. I will conclude with our recent efforts to link these results with isotopic measurements of meteorites and comets.

Les séminaires déjà présentés cette année

publié jeudi 13 septembre 2012