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Etude expérimentale du omportement des Terres Rares en milieu réducteur : applications à la nébuleuse solaire et à la formation des chondrites à enstatite

Abstract : Studying chondrites as a primitive material of the solar system permit to understandformation and differentiation of the Earth and the original material of the accretion.Among the chondrites classes, the enstatite chondrites (E.C), a main candidate for Earthprimitive material, are associated with reducing conditions. One of the main problematicsaround these meteorites is the origin of their oldhamite (CaS) that concentrate more thanhalf of their REE (rare earth elements) budget (up to 1000 times CI chondrites).The first part of this study presents melting experiments done with mixtures ofsynthetic and natural powders doped in REE at 1300 and 1400_C, low pressure andlow oxygen fugacity. Comparing experimental samples to natural samples shows thathigh natural abundances are not reproduced experimentally and reject a simple igneousorigin for CaS in these meteorites. Despite this observation, natural and experimentalpatterns show anomalies in the same elements: Eu and Yb. These anomalies are mainlypositive in the experiments and also for some natural patterns. XANES analyses onYb and Sm show the presence of 2+ Yb and Sm together with their 3+ valence state.Models of oldhamite/melt, FeS/melt and enstatite/melt shows that the intensity of theanomalies are dependant of this valence change under different redox conditionsIn the second part of this study we analysed the condensates associated to the sameexperiments. We developed a treatment method for the deposits in order to analyse themwith the ICP-MS and the SEM. We constituted a database about REE concentrationsin the condensates as a function of the condensation conditions. From this database wediscuss the volatility of the REE under reducing conditions: Eu, Yb and Sm are the mostvolatiles. We conclude with a simple model of REE behaviour under the reducing solarnebula conditions, this model can explain the formation of CaS in E.C and their budgetin REE.
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Nathan Ingrao. Etude expérimentale du omportement des Terres Rares en milieu réducteur : applications à la nébuleuse solaire et à la formation des chondrites à enstatite. Sciences de la Terre. Université Clermont Auvergne, 2019. Français. ⟨NNT : 2019CLFAC109⟩. ⟨tel-03244683⟩

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