High pressure partitioning behavior of Mo and W and late sulfur delivery during Earth’s core formation

Abstract

Mo and W in the bulk silicate Earth and their partitioning behavior between molten metal and silicate can be used to constrain the thermochemical conditions during Earth’s core-mantle differentiation. In order to improve our understanding of core-forming conditions, we performed a series of superliquidus metal-silicate partitioning experiments on Mo and W at 40–93 GPa and 3000–4700 K in laser-heated diamond anvil cells. Under the extended P-T conditions directly relevant to terrestrial core formation in a deep magma ocean, we find that pressure and temperature have profound yet opposing effects on their partitioning, and a significant amount of O dissolved in the metal. Based on an activity model for liquid Fe-rich metal, it is observed that O enhances the solubility of both Mo and W in the metal, whereas S makes W significantly less siderophile than Mo. Combining our new data with those of the literature, we modeled the effects of pressure, temperature and metal composition on partitioning, and applied them to a multi-stage accretion model. While our model with homogeneous S accretion successfully explains the abundance of Mo, it underestimates that of W and therefore overestimates Mo/W ratio in Earth’s mantle, regardless of the oxidation conditions prevailing during core formation. On the other hand, mantle observables (Mo and W abundances, Mo/W ratio) can be reproduced simultaneously if S is supplied to the Earth towards the end of accretion. This corroborates previous work at lower pressures, and agrees with heterogeneous accretion models where the late volatile-rich delivery was envisaged to explain various isotopic signatures of terrestrial bodies. Nonetheless, this conclusion does not discriminate between reducing and oxidizing conditions.