The oxidation state of tungsten in silicate melt at high pressures and temperatures

Abstract

Although at 1atm pressure the oxidation state of W in silicate melts is 6+at oxygen fugacities from air down to several logfO2 units below Fe-FeO (IW) equilibrium, Cottrell et al. (2009) suggested that, at pressures above 6GPa, W becomes predominantly 4+in this oxygen fugacity range. Wade and Wood (2005), using a similar, but expanded metal-silicate partitioning dataset found, however, no evidence for an oxidation state change. In order to resolve the issue we collected tungsten L3 edge XANES spectra of a series of synthetic tungsten- bearing glass standards and of silicates from a range of high-pressure (1.5 to 25GPa) metal/silicate partitioning experiments. Glass standards were made at 1atm pressure and equilibrated at oxygen fugacities spanning a range from approximately 5.5 log units below the Fe-FeO buffer (IW-5.5) to Air. Metal-silicate partitioning experiments were performed at oxygen fugacities between IW-6.2 and IW-1 and at pressures between 1.5 and 25GPa. At low pressures and oxygen fugacities above IW-3.5, W exists in the silicate melt almost exclusively as W6+ (identical L3-edge energy to WO3) with the progressive reduction to W4+ completed by about IW-6. The XANES spectra of W from experiments at 6, 7 and 25GPa are completely consistent with those from 1atm and 1.5GPa experiments and with that of WO3. We conclude that there is no change of oxidation state with increasing pressure to 25GPa and that modelling of the oxygen-fugacity dependence of core formation requires use of a+6 oxidation state of W.