Metal-silicate partitioning of W and Mo and the role of carbon in controlling their abundances in the Bulk Silicate Earth

TL;DR

This study investigates how tungsten and molybdenum partition between metal and silicate during Earth's core formation, emphasizing the role of carbon and oxidation states, to better understand Earth's compositional evolution.

Contribution

It provides new high-pressure, high-temperature experimental data and a comprehensive model showing how W and Mo partitioning depends on oxidation state, melt composition, and carbon content during core formation.

Findings

W is in a 6+ oxidation state in silicate melts.

W and Mo become more siderophile with increased carbon in metal.

Earth's early core was sulfur-depleted and carbon-enriched.

Abstract

The liquid metal-liquid silicate partitioning of molybdenum and tungsten during core formation must be well-constrained in order to understand the evolution of Earth and other planetary bodies, in particular because the Hf-W isotopic system is used to date early planetary evolution. We combine 48 new high pressure and temperature experimental results with a comprehensive database of previous experiments to re-examine the systematics of Mo and W partitioning. W partitioning is particularly sensitive to silicate and metallic melt compositions and becomes more siderophile with increasing temperature. We show that W has a 6+ oxidation state in silicate melts over the full experimental fO2 range of $\Delta$IW -1.5 to -3.5. Mo has a 4+ oxidation state and its partitioning is less sensitive to silicate melt composition, but also depends on metallic melt composition. DMo stays approximately constant with increasing depth in Earth. Both W and Mo become more siderophile with increasing C content of the metal, so we fit epsilon interaction parameters. W and Mo along with C are incorporated into a combined N-body accretion and core-mantle differentiation model. We show that W and Mo require the early accreting Earth to be sulfur-depleted and carbon-enriched so that W and Mo are efficiently partitioned into Earth's core and do not accumulate in the mantle. If this is the case, the produced Earth-like planets possess mantle compositions matching the BSE for all simulated elements. However, there are two distinct groups of estimates of the bulk mantle's C abundance in the literature: low (100 ppm), and high (800 ppm), and all models are consistent with the higher estimated carbon abundance. The low BSE C abundance would be achievable when the effects of the segregation of dispersed metal droplets produced in deep magma oceans by the disproportionation of Fe2+ to Fe3+ plus metallic Fe is considered.