Valence and metal/silicate partitioning of Mo: Implications for conditions of Earth accretion and core formation

- Mo dissolves in silicate melt as 4+ at the P-T-fO2 conditions of early Earth.
- Mo becomes significantly less siderophile at high PT conditions.
- The number of D(Mo) metal/silicate experiments at P>10 GPa is 2× higher.
- D(Mo) metal/silicate is lowered substantially by Si, especially for Fe-C liquids.
- Equilibration of the mantle with core explains the Mo content of Earth's mantle.

To better understand and predict the partition coefficient of Mo at the conditions of the deep interior of Earth and other terrestrial planets or bodies, we have undertaken new measurements of the valence and partitioning of Mo. X-ray absorption near edge structure (XANES) K-edge spectra for Mo have been measured in a series of Fe-bearing glasses produced at 1 bar and higher PT conditions. High pressure experiments have been carried out up to 19 GPa in order to better understand the effect of pressure on Mo partitioning. And, finally, a series of experiments at very low fO2 conditions and high Si content metallic liquids has been carried out to constrain the effect of Si on the partitioning of Mo between metallic liquids and silicate melt. The valence measurements demonstrate that Mo undergoes a transition from 4+ to 6+ near IW−1, in general agreement with previous 1 bar studies on FeO-free silicate melts. High pressure experiments demonstrate a modest pressure dependence of D(Mo) metal/silicate and, combined with previous results, show a significant decrease with pressure that must be quantified in any predictive expression. Finally, the effect of dissolved Si in Fe-rich metallic liquid is to decrease D(Mo) significantly, as suggested by previous work in metallurgical systems. The effect of pressure, temperature, oxygen fugacity, metallic liquid composition, and silicate melt composition can be quantified by using multiple linear regression of available experimental data for Mo. Our XANES results show that Mo will be 4+ at conditions of core formation, so only experiments carried out at fO2 of IW−1 and lower were used in the regressions. Application of predictive expressions to Earth accretion shows that D(Mo) decreases to values consistent with an equilibrium scenario for early Earth core-mantle. The Mo content of the primitive upper mantle (PUM) can be attained by metal-silicate equilibrium involving S-, C-, and Si-bearing metallic liquid, and peridotite silicate melt along the peridotite liquidus near 45 GPa and 3600 °C, late in the accretion process. This conclusion is insensitive to late giant impacts unless the degree of equilibration is very low (<5%).