The assemblage WO₂ +H₂O as a steady-state hydrogen source in moderately reduced hydrothermal experiments

The values of f_H2 for the assemblage WO₂ + WO_2.72 + H₂O (designated as WO) have been measured in sealed Au capsules under an external pressure of 2 kbar CH₄ and between 650 and 800 °C using Ag-AgBr-HBr sensors of f_H2. The f_H2 values obtained can be represented by the equation

and are found to be slightly greater than those associated with the previously calibrated C-CH₄ buffer. The value at 2 kbar and 650 °C is approximately an order of magnitude lower than the theoretical value of the wiistite-magnetite-H₂O buffer. The assemblage has been found to generate reproducible steady-state f_H2 values at fixed P-T conditions. However, at the relatively high H₂ pressures developed by this assemblage, H₂ leakage through the Au capsule was unavoidable. This leakage can be mitigated by decreasing the f_H2 gradient across the Au capsule wall, but it is strongly recommended that f_H2 sensors be employed to monitor the actual redox conditions in experiments using buffers or assemblages that generate f_H2 values equivalent to the Co-CoO-H₂O buffer or greater. The WO₂ + wo_2.72 + H₂O assemblage can still be used as a redox buffer even though the system’s H₂ consumption is accomplished by H₂ leakage rather than chemical reaction.