The W-WO2oxygen fugacity buffer (WWO) at high pressure and temperature: Implications forfO2buffering and metal-silicate partitioning

Abstract

Synchrotron X-ray diffraction data were obtained to simultaneously measure unit-cell volumes of W and WO2 at pressures and temperatures up to 70 GPa and 2300 K. Both W and WO2 unit-cell volume data were fit to Mie-Gruneisen equations of state; parameters for W are K T = 307 (±0.4) GPa, K ′T = 4.05 (±0.04), γ = 1.61 (±0.03), and q = 1.54 (±0.13). Three phases were observed in WO2 with structures in the P 21/ c , Pnma , and C 2/ c space groups. The transition pressures are 4 and 32 GPa for the P 21/ c-Pnma and Pnma-C 2/ c phase changes, respectively. The P 21/ c and Pnma phases have previously been described, whereas the C 2/ c phase is newly described here. Equations of state were fitted for these phases over their respective pressure ranges yielding the parameters K T = 238 (±7), 230 (±5), 304 (±3) GPa, K ′T = 4 (fixed), 4 (fixed), 4 (fixed) GPa, γ = 1.45 (±0.18), 1.22 (±0.07), 1.21 (±0.12), and q = 1 (fixed), 2.90 (±1.5), 1 (fixed) for the P 21/ c , Pnma , and C 2/ c phases, respectively. The W-WO2 buffer (WWO) was extended to high pressure using these W and WO2 equations of state. The T - f O2 slope of the WWO buffer along isobars is positive from 1000 to 2500 K with increasing pressure up to at least 60 GPa. The WWO buffer is at a higher f O2 than the iron-wustite (IW) buffer at pressures lower than 40 GPa, and the magnitude of this difference decreases at higher pressures. This implies an increasingly lithophile character for W at higher pressures. The WWO buffer was quantitatively applied to W metal-silicate partitioning by using the WWO-IW buffer difference in combination with literature data on W metal-silicate partitioning to model the exchange coefficient ( K D) for the Fe-W exchange reaction. This approach captures the non-linear pressure dependence of W metal-silicate partitioning using the WWO-IW buffer difference. Calculation of K D along a peridotite liquidus predicts a decrease in W siderophility at higher pressures that supports the qualitative behavior predicted by the WWO-IW buffer difference, and agrees with findings of others. Comparing the competing effects of temperature and pressure the results here indicate that pressure exerts a greater effect on W metal-silicate partitioning.