Hydrogen incorporation and retention in metamorphic olivine during subduction: Implications for the deep water cycle

Incorporation of hydrogen into metamorphic olivine during dehydration reactions in the subducting oceanic lithosphere provides a mechanism to replenish the deep mantle with water. Fourier transform infrared spectroscopy of metamorphic olivines formed at 2.5 GPa and 550 degrees C through the reaction antigorite + brucite = olivine + chlorite + water shows water contents between 100 and 140 ppm H2O associated exclusively with silicon vacancies, similar to the highest values found in peridotite xenoliths. Brucite involvement in the olivine-forming reaction ensures H2O saturation and a low Si activity, favoring hydrogen incorporation into Si vacancies. The mapped water distribution in olivine is consistent with growth zoning and there is no evidence of water gain or loss. Thus, even for metamorphic timescales of several million years at 550 degrees C, no ionic diffusion modification is observed, in agreement with recent experimental findings. Metamorphic olivines formed by this dehydration reaction may contribute considerable amounts of water to the deep water cycle. Additionally, olivine with abundant H in Si vacancies are expected to be rheologically weaker than anyhdrous mantle olivine, and might provide a weak interface between slabs and mantle wedges in subduction zones at conditions beyond the stability of hydrous phases.