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Uranium and thorium partitioning in the bulk silicate Earth and the oxygen content of Earth’s core

Abstract : This study investigates the partitioning of U and Th between molten metal and silicate liquid (DUandDTh) during Earth’score-mantle differentiation. We report new Th partition coefficients between chondritic silicate melt and various Fe-rich alloysin the system Fe-Ni-C-S-Si as determined by experiments in a multi-anvil apparatus at 3–8 GPa, 2073–2373 K, and oxygenfugacities from 1.5 to 5 log units below the iron-wu ̈stite (IW) buffer. By compiling all existing data on molten metal-silicateliquid partitioning of U and Th, we develop global models of U and Th partitioning between the mantle and core throughoutEarth’s accretion. The calculated concentrations in the Bulk Silicate Earth (BSE) are in agreement with previous studies(UBSE= 11.42 ± 0.45 ppb and ThBSE= 43.20 ± 1.73 ppb), whereas the contents of these radioactive elements in the Earth’score remain negligible. Compared to recent geochemical estimations, the calculated (Th/U)BSEsupports EL rather than EHenstatite chondrites as the reduced building blocks of the Earth. Furthermore, we demonstrate that Th is much more sensitivethan U to the oxygen content of the metallic phase. To reproduce the Th/U ratio of the BSE within its uncertainties, the oxy-gen content of the Earth’s core must be lower than 4.0 wt%. By combining other existing constraints, this suggests that thecore contains 2.0–4.0 wt% O. The calculations of U and Th concentrations and Th/U in the BSE developed herein can be usedas new constraints for determining the concentrations of other refractory lithophile elements in the BSE as soon as theirmetal-silicate partition coefficients are well constrained over the conditions of core segregation.
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P. Faure, Mohamed Ali Bouhifd, M. Boyet, Geeth Manthilake, V. Clesi, et al.. Uranium and thorium partitioning in the bulk silicate Earth and the oxygen content of Earth’s core. Geochimica et Cosmochimica Acta, Elsevier, 2020, 275, pp.83-98. ⟨10.1016/j.gca.2020.02.010⟩. ⟨hal-02504992⟩



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