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Synchrotron X-ray diffraction data were obtained to simultaneously measure unit-cell volumes of W and WO 2  at pressures and temperatures up to 70 GPa and 2300 K. Both W and WO 2  unit-cell volume data were fit to Mie-Grüneisen equations of state; parameters for W are  K  T  = 307 (±0.4) GPa,K  T′   =     4.05   (   ±  0.04   )$K_{\rm{T}}^{_\prime }\, = \,4.05\left( { \pm 0.04} \right)$   , γ 0  = 1.61 (±0.03), and q = 1.54 (±0.13). Three phases were observed in WO 2  with structures in the  P 2 1 / c ,  Pnma , and  C 2/ c  space groups. The transition pressures are 4 and 32 GPa for the  P 2 1 / c-Pnma  and  Pnma-C 2/ c  phase changes, respectively. The  P 2 1 / 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    $K_{\rm{T}}^{_\prime }\, = \,4$   , 4 (fixed), 4 (fixed) GPa, γ 0  = 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 2 1 / c ,  Pnma , and  C 2/ c  phases, respectively. The W-WO 2  buffer (WWO) was extended to high pressure using these W and WO 2  equations of state. The  T - f  O 2   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  O 2   than the iron-wüstite (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.

The W-WO 2 oxygen fugacity buffer (WWO) at high pressure and temperature: Implications for f O 2 buffering and metal-silicate partitioning