Mechanisms of hydrogen incorporation and diffusion in iron-bearing olivine

Abstract

The incorporation and diffusion of hydrogen in San Carlos olivine (Fo₉₀) single crystals were studied by performing experiments under hydrothermal conditions. The experiments were carried out either at 1.5 GPa, 1,000°C for 1.5 h in a piston cylinder apparatus or at 0.2 GPa, 900°C for 1 or 20 h in a cold-seal vessel. The oxygen fugacity was buffered using Ni–NiO, and the silica activity was buffered by adding San Carlos orthopyroxene powders. Polarized Fourier transform infrared (FTIR) spectroscopy was utilized to quantify the hydroxyl distributions in the samples after the experiments. The resulting infrared spectra reproduce the features of FTIR spectra that are observed in olivine from common mantle peridotite xenoliths. The hydrogen concentration at the edges of the hydrogenated olivine crystals corresponds to concentration levels calculated from published water solubility laws. Hydrogen diffusivities were determined for the three crystallographic axes from profiles of water content as a function of position. The chemical diffusion coefficients are comparable to those previously reported for natural iron-bearing olivine. At high temperature, hydrogenation is dominated by coupled diffusion of protons and octahedrally coordinated metal vacancies ${\left( {V^{\prime\prime}_{{{\text{Me}}}} } \right)},$ where the vacancy diffusion rate limits the process. From the experimental data, we determined the following diffusion laws (diffusivity in m² s⁻¹, activation energies in kJ mol⁻¹): $D_{{V_{{{\text{Me}}}} [100],[010]}} = 10^{{ - 4.5 \pm 4.1}} \exp [ - (204 \pm 94)/RT]$ for diffusion along [100] and [010]; $D_{{V_{{{\text{Me}}}} [001]}} = 10^{{ - 1.4 \pm 0.5}} \exp [ - (258 \pm 11)/RT]$ for diffusion along [001]. These diffusion rates are fast enough to modify significantly water contents within olivine grains in xenoliths ascending from the mantle.