Résumé
The effect of pressure on hydrogen diffusivity in San Carlos olivine (Fo90) single crystals was determined from hydrogenation experiments in a multi-anvil press at 6, 9 and 12 GPa, and at high temperature (900-1300°C), for various durations. Crystallographically oriented prisms of gem-quality San Carlos olivine were hydrogenated under controlled oxygen fugacity (Ni-NiO) and silica activity (10% Opx). Polarized Fourier-transform infrared spectroscopy was used to quantify the hydroxyl distribution in the samples parallel to the crystallographic axes. The diffusivity of hydrogen is consistent with a single diffusion mechanism, proton-vacancy coupled diffusion, which is dominated by the presence of trivalent ions (infrared band doublet at 3357-3329 cm−1). The inferred chemical diffusion coefficients are slower than in olivine hydrogenated at lower pressure (≤ 3 GPa) for the same diffusion mechanism and temperatures. Under the given experimental conditions, diffusion along the [001] axis is slightly faster than along [100] or [010]. Despite the pressure effect, hydrogen diffusion coefficients in the entire upper mantle remain fast enough to alter hydrogen concentrations at the grain scale, although too slow to enable km-scale re-equilibration of hydrogen-rich or hydrogen-poor anomalies. Melt migration is thus required to enable large-scale re-equilibration of H heterogeneities.