a b s t r a c t A new method is developed to control water fugacity at a fixed pressure and temperature. We use two divariant phase assemblages (clinohumite-periclase-forsterite and brucite-periclase) in the MgOSiO 2 -H 2 O system and the chemical reactions among co-existing phases buffer the water fugacity. In order to avoid a reaction between the water fugacity buffer and the specimen, a double-capsule assemblage was designed such that the water fugacity buffer was separated from the specimen by an inner metal jacket permeable to hydrogen and impermeable to other components. This method was applied to investigate the incorporation mechanisms of water in single crystals of San Carlos olivine as a function of water fugacity at 5 GPa and 1273 K. The chemical environment of the olivine crystal was controlled by the water fugacity buffer, the Ni-NiO oxygen fugacity buffer, and the olivine-orthopyroxene silica activity buffer. The establishment of chemical equilibrium was demonstrated by the presence of all relevant phases in the buffering reactions. The effect of water fugacity on water concentration in olivine was investigated using Fourier transform infrared (FTIR) spectroscopy with a polarized light. The total water concentration is nearly linearly dependent on water fugacity, indicating that the dominant incorporation mechanism of water in olivine involves two hydrogen atoms substituted in a Mg-site vacancy. The proposed method of the water fugacity buffer can be applicable to investigate the role of water in various important properties including water solubility and partitioning between mantle minerals, rheological properties, electrical conductivity, as well as solidus temperatures
