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

International audienceThis 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