Hydration of mantle olivine under variable water and oxygen fugacity conditions

The incorporation of H into olivine is influenced by a significant number of thermodynamic variables (pressure, temperature, oxygen fugacity, etc.). Given the strong influence that H has on the solidus temperature and rheological behavior of mantle peridotite, it is necessary to determine its solubility in olivine over the range of conditions found in the upper mantle. This study presents results from hydration experiments carried out to determine the effects of pressure, temperature, and the fugacities of H2O and O2 on H solubility in San Carlos olivine at upper mantle conditions. Experiments were carried out at 1–2 GPa and 1,200 °C using a piston-cylinder device. The fugacity of O2 was controlled at the Fe0–FeO, FeO–Fe3O4, or Ni0–NiO buffer. Variable duration experiments indicate that equilibration is achieved within 6 h. Hydrogen contents of the experimental products were measured by secondary ion mass spectrometry, and relative changes to the point defect populations were investigated using Fourier transform infrared spectroscopy. Results from our experiments demonstrate that H solubility in San Carlos olivine is sensitive to pressure, the activity of SiO2, and the fugacities of H2O and O2. Of these variables, the fugacity of H2O has the strongest influence. The solubility of H in olivine increases with increasing SiO2 activity, indicating incorporation into vacancies on octahedral lattice sites. The forsterite content of the olivine has no discernible effect on H solubility between 88.17 and 91.41, and there is no correlation between the concentrations of Ti and H. Further, in all but one of our experimentally hydrated olivines, the concentration of Ti is too low for H to be incorporated dominantly as a Ti-clinohumite-like defect. Our experimentally hydrated olivines are characterized by strong infrared absorption peaks at wavenumbers of 3,330, 3,356, 3,525, and 3,572 cm−1. The heights of peaks at 3,330 and 3,356 cm−1 correlate positively with O2 fugacity, while those at 3,525 and 3,572 cm−1 correlate with H2O fugacity.