Temperature dependence and mechanism of hydrogen incorporation in olivine at 12.5-14.0 GPa

The concentration and incorporation mechanisms of hydrogen into (Mg, Fe)₂SiO₄ olivine have been studied under water saturated conditions at 12.5-14.0 GPa and 1100-2000°C. The hydrogen content of olivine (Fo₉₀-Fo₉₅) coexisting with enstatite and hydrous melt increases from about 4600 ppmw at 1100°C to a maximum of 6250 ppmw at 1200°C. Above 1400°C, the hydrogen content of olivine decreases non-linearly, reaching 160-240 ppmw in the 1800-2000°C range where there is a high melt fraction and no enstatite. Polarized infrared spectra of the recovered olivines exhibit OH absorption bands in both the 3450-3650 cm^-1 range (group I) and 3160-3450 cm^-1 range (group II). Results indicate that hydration of olivine is associated with both Mg and Si site vacancies. Microprobe analyses of the samples show a clear deficit of Si in olivine. The water storage capacity of olivine is probably no more than 1.5-2.0 times less than wadsleyite at 1400°C and transition zone pressures.