The W-WO (sub 2) oxygen fugacity buffer (WWO) at high pressure and temperature; implications for f (sub O2) buffering and metal-silicate partitioning

Synchrotron X-ray diffraction data were obtained to simultaneously measure unit-cell volumes of W and WO (sub 2) at pressures and temperatures up to 70 GPa and 2300 K. Both W and WO (sub 2) unit-cell volume data were fit to Mie-Gruneisen equations of state; parameters for W are K (sub T) =307 (+ or -0.4) GPa, K' (sub T) =4.05 (+ or -0.04), gamma (sub 0) =1.61 (+ or -0.03), and q=1.54 (+ or -0.13). Three phases were observed in WO (sub 2) with structures in the P2 (sub 1) /c, Pnma, and C2/c space groups. The transition pressures are 4 and 32 GPa for the P2 (sub 1) /c-Pnma and Pnma-C2/c phase changes, respectively. The P2 (sub 1) /c and Pnma phases have previously been described, whereas the C2/c phase is newly described here. Equations of state were fitted for these phases over their respective pressure ranges yielding the parameters K (sub T) =238 (+ or -7), 230 (+ or -5), 304 (+ or -3) GPa, K' (sub T) =4 (fixed), 4 (fixed), 4 (fixed) GPa, gamma (sub 0) =1.45 (+ or -0.18), 1.22 (+ or -0.07), 1.21 (+ or -0.12), and q=1 (fixed), 2.90 (+ or -1.5), 1 (fixed) for the P2 (sub 1) /c, Pnma, and C2/c phases, respectively. The W-WO (sub 2) buffer (WWO) was extended to high pressure using these W and WO (sub 2) equations of state. The T-f (sub 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 (sub 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 (KD) 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 KD 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.