The W-WO2 oxygen fugacity buffer (WWO) at high pressure and temperature: Implications for f(o2) buffering and metal-silicate partitioning

摘要

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 Kr = 307 ( 0.4) GPa, K-T = 4.05 (+/- 0.04), yo = 1.61 ( 0.03), and q = 1.54 ( 0.13). Three phases were observed in WO2 with structures in the P2(1)/c, Pnma, and C2c space groups. The transition pressures are 4 and 32 GPa for the P211 c-Pnma and Pnma-C2I c phase changes, respectively. The P2,/c and Pnma phases have previously been described, whereas the C2I c phase is newly described here. Equations of state were fitted for these phases over their respective pressure ranges yielding the parameters KT = 238 ( 7), 230 ( 5), 304 ( 3) GPa, K-j, = 4 (fixed), 4 (fixed), 4 (fixed) GPa, yo = 1.45 (+/- 0.18), 1.22 ( 0.07), 1.21 ( 0.12), and q =1 (fixed), 2.90 (+/- 1.5), 1 (fixed) for the P2(1)/c, Pnma, and C2I c phases, respectively. The W-W02 buffer (WWO) was extended to high pressure using these W and WO2 equations of state. The T-f02 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 foe than the iron-wilstite (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 (KE,) for the Fe-W exchange reaction. This approach captures the non-linear pressure dependence of W metal-silicate partitioning using the W WO-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-IVV 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.