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Fractionnement élémentaire et isotopique des éléments sidérophiles : implications pour l’accrétion des éléments volatils sur Terre

Abstract : The accretion of volatile elements on Earth continues to be debated despite intense research focusing on the topic. Geochemical and cosmochemical observables have been explained by a variety of scenarios ranging from (1) an accretion of the volatile elements during the main phases of Earth’s formation to (2) an accretion of volatile-poor material followed by a late accretion of volatile-rich material after core formation ceased, called the late veneer. The mantle signature of elements that are both volatile and siderophile record volatile-related processes as well as differentiation-related processes. Studying the behaviour of such elements at metal–silicate equilibrium at high pressure and high temperature allows to separate the effect of differentiation from volatile-related processes on their abundances, and hence to discriminate scenarios capable of explaining the observed abundances.In this thesis work, the aim is to determine one or a variety of accretion scenarios of the Earth explaining the abundances of moderately volatile and siderophile elements in the mantle. Experiments were performed in piston cylinder and multianvil presses from 2 to 20 GPa and 1700 to 2600 K in order to measure the elemental and isotopic behaviour of a selection of moderately siderophile and volatile elements. Studying the partitioning of Sn, Cd, Bi, Sb and Tl allowed to determine the effect of temperature, pressure, fO2, and S content of the metal on the affinity of these elements for metal relative to silicate. The partitioning of these elements is reconstructed across Earth’s accretion, at appropriate pressure and temperature conditions. The studied elements present enhanced siderophility, preventing reconciliation of the experimental data with observed mantle abundances. A scenario involving partial equilibration of the mantle with the core, as well as a late delivery of volatile elements in the last 10 to 20 % of Earth’s accretion, as a large impactor, is able to explain the abundances of the studied elements, in agreement with other studies based on distinct observables. Around 3 wt.% of S in the core is needed to explain the Sn abundance. A late veneer consisting of 0.5 % of Earth’s mass is also required to explain the Bi abundance in the mantle. The study of Sn isotopic fractionation between metal and silicate yields a significant factor of ~0.3 ‰ at 2 GPa and 2000 K (∆122/118Snmetal−silicate), suggesting that Sn isotopes could be fractionated at core formation conditions, enriching the mantle in light isotopes. Carbonaceous chondrites present an identical Sn isotopic composition to the silicate Earth, making them the most likely contributors to the source of volatile elements on Earth, according to this study. These results are consistent with the favoured accretion scenario suggested by the elemental fractionation part of this study. As elemental and isotopic fractionations of siderophile elements between metal and silicate are strongly dependent on the metal composition, the effect of the Ni content of the metal on Fe isotopic fractionation was tested. The experiments performed in this study (22 experiments, from 0 to 70 % Ni in the metal) do not allow to detect any effect of Ni on the Fe fractionation factor. Therefore, this study brings new insight regarding the timing, process and origin of the volatile delivery on Earth, as well as information on the effect of core composition on the behaviour of siderophile elements during Earth’s differentiation.
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Submitted on : Friday, June 4, 2021 - 11:24:09 AM
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Edith Kubik. Fractionnement élémentaire et isotopique des éléments sidérophiles : implications pour l’accrétion des éléments volatils sur Terre. Sciences de la Terre. Université de Paris, 2020. Français. ⟨NNT : 2020UNIP7097⟩. ⟨tel-03249578⟩



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