Abstract
Garnet, an important mineral component in Earth's interior, usually contains minor water as structural OH, but experimental data on OH incorporation in the mineral as a function of chemical and thermodynamic parameters remain rare. In particular, the role of oxygen fugacity (fO2) that is variable in the Earth has not been well assessed. We examine the effects of fO2 on the incorporation of OH at 1-3 GPa and 800-1000°C, by H-annealing gem-quality spessartine-rich, grossular-rich, and pyrope-rich garnets. For each type of annealed garnet, the OH peak positions and spectral shapes are the same, suggesting the same OH incorporation mechanism(s). Under identical conditions, spessartine-rich and grossular-rich garnets are rich in OH, with solubility orders of magnitudes greater than for pyrope-rich garnets. Over the studied conditions, the solubility of OH increases with pressure and temperature but decreases with fO2. The negative correlation of fO2 and OH solubility is probably due to the Fe3+ to Fe2+ reduction at more reduced conditions and the coupled OH incorporation in the structure. The effects of fO2 on OH solubility are more significant in Fe-rich garnets. The ascertained small OH solubility in peridotite garnets, together with comparisons with relevant studies, suggest that mantle garnet is unlikely to be a major water carrier. In combination with literature-reported positive correlation of fO2 and OH solubility in olivine, the partition coefficient of OH between garnet and olivine in the upper mantle is expected to be redox-dependent.