Abstract:
Tungsten partitioning between liquid metal and liquid silicate, D(W), from 1723-2673 K, 0.5-18 GPa, and over a wide range of metal and silicate compositions provides constraints on planetary core formation models. We find that W partitioning is extremely sensitive to the carbon content of the metal alloy, becoming about an order of magnitude more siderophile at carbon saturation. Activity-composition corrections based on interactions between W-C and Fe-C in metal solution shift calculated D(W) to more lithophile values, and calculated oxygen fugacities (fO2) to more oxidizing values, than uncorrected data. W exists as a highly charged cation in silicate solution, displaying a mixture of oxidation states from + 4 to + 6 in experiments at fO2 of ~ 0.5-2 log units below the iron-wüstite buffer. At constant fO2, the average calculated valence decreases with pressure from ~ + 5.5 at 0.5 GPa to ~ + 4 at 11-18 GPa. As a result of its high oxidation state, W partitioning is strongly dependent on silicate melt polymerization and fO2. In contrast to some previous results, we find that D(W) may decrease slightly in response to increasing temperature in the pressure range of our experiments. Pressure exerts a non-monotonic effect: D(W) increases with pressure up to ~ 3 to 4 GPa, but decreases at higher pressures. Previous models for the effects of pressure and temperature on W partitioning that conflict with our results appear to result from a conflation of the intensive parameters of pressure, temperature, and carbon content. The mantle abundance of W could have been set by single-stage metal-silicate equilibration along the liquidus in a deep peridotite magma ocean at pressures from 20-50 GPa, and at oxygen fugacities consistent with the mantle's present iron budget (IW- 2 to - 2.5). Equilibration at higher pressure is viable if the core-forming metal contained a significant, but not unreasonable, abundance of carbon (~ 2 wt.%). Recent continuous accretion models involving multi-stage metal-silicate equilibration in a deepening magma ocean with progressive oxidation of the silicate remain permissible given our new treatment of W partitioning data.
