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[1] Although water controls the biology and geology of the surface, hydrogen is perhaps the most poorly constrained compositional variable in the bulk Earth. Its concentration in the upper mantle appears to be controlled by its solubility as hydroxyl in the nominally anhydrous silicate phases, olivine, pyroxene, garnet, wadsleyite, and ringwoodite. Here we describe a series of experiments showing that the solubility of H2O in olivine at 12 GPa increases with temperature to 8900 ppm by weight at 1250°C and decreases at higher temperature with the onset of melting. Sample characterization by infrared spectroscopy indicates that the primary hydration mechanism is the substitution of 2H+ for Mg2+. Similar results obtained from samples coexisting with clinohumite (low-silica) and with clinoenstatite (high-silica) indicate that silica activity has minimal effect on hydration under these conditions. Single-crystal X-ray diffraction measurements constrain the volume of hydration and indicate significant M-site vacancies. Hydrogen thus appears to become a geochemically compatible element as depths approach 400 km.