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Comportement des éléments lithophiles lors de la formation du noyau terrestre

Abstract : Core formation is a pivotal event in early Earth history, causing a major segregation of elements according to their affinity with either metal (siderophile) or silicate (lithophile) phases. The Bulk Silicate Earth (BSE) shows a depletion in siderophile elements coherent with a core differentiation within a magma ocean. The affinity of these elements is a function of the physical and chemical conditions (P, T, fo2, chemical composition) of metal-silicate differentiation. The behaviour of any one element varies over the formation of the core within the magma ocean; hence, the elemental distribution of Earth's Building Blocks between the two reservoirs cannot be reproduced by considering a unique behaviour of each element.In order to determine the concentration of an element in the core and the BSE, it is necessary to characterize the behaviour of the element and its evolution during Earth's accretion. In comparison to siderophile elements, lithophile elements form a minority of core-mantle differentiation studies so far. However, significant disagreement can be found concerning the concentrations of these elements among published models of BSE composition. It has also been proposed that a late sulphur accretion event could be responsible for a significant amount of lithophile elements (Sm, Nd, U, Th, K) entering the core. In order to determine the effect of core formation on the distribution of these elements between the two reservoirs, we performed ~60 new high-pressure, high-temperature experiments of metal-silicate partitioning for several elements (Uranium, Thorium, Rare Earth Elements (REE), Alkali elements) with the aim of better understanding their behaviour during Earth's accretion. These studies allow us to bring new constraints on Earth's accretion models and to refine our understanding of the core and BSE composition.We show that the (Th/U)BSE is better reproduced when considering EL chondrites as Earth's Building Blocks rather than EH. The same ratio allows us to constrain the maximum oxygen concentration of the core to no more than 4 wt%. We confirm that these elements play a small role in the core thermal history, with their concentration in the metal remaining negligible.We also demonstrate that the BSE cannot be enriched in Refractory Lithophile Elements (RLE) by more than 2.1 times their concentrations in CI chondrites, when the Earth is made of Building Blocks mixing in agreement with isotopic studies. This shows a clear incompatibility between several models of BSE composition and isotopic observations.The study of U, Th and REE also allowed us to re-evaluate the impact of a late sulphur event at the end of Earth's accretion, which would neither lead to fractionation of Sm and Nd nor allow a significant amount of U and Th to enter the core to start the geodynamo.Finally, we widen the range of studied elements to include volatile elements. A study on potassium shows that this element concentration in the core cannot exceed 57 ppm. The core would then contain 15% of the total K budget of the Earth. This content is highly dependent on the timing of volatilisation processes. We then propose that the Earth is a result of the accretion of already volatile-depleted building blocks. Giant impacts would have a minor role in the volatile depletion observed in the BSE today.
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  • HAL Id : tel-02611688, version 1

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Pierre Faure. Comportement des éléments lithophiles lors de la formation du noyau terrestre. Sciences de la Terre. Université Clermont Auvergne, 2019. Français. ⟨NNT : 2019CLFAC076⟩. ⟨tel-02611688⟩

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