Nominally anhydrous minerals and Earth’s deep water cycle

Deep reservoirs of water incorporated as hydroxyl into solid silicate minerals of the Earth’s interior may contain the majority of the planet’s hydrogen and have acted as buffers to maintain ocean volume and continental freeboard over geologic time. Two tenths of one weight percent H2O in subducted oceanic crustal material and subsequently released to the hydrosphere from mid-ocean ridge basalt is sufficient to recycle the total ocean volume once over 4.5 billion years. It is possible that actual fluxes are several times this amount. The nominally anhydrous minerals of the transition zone (410-660 km depth) may serve as a large internal reservoir. New and recent data on molar volumes and elastic properties indicate that hydration has a larger effect on shear velocities than does temperature within their respective uncertainties. Based on these new data, seismic velocities in this region are consistent with significant hydration (one-half percent or more H2O by weight in a pyrolite-composition mantle. The data indicate that lateral velocity variations in the Transition Zone (TZ) may reflect variations in hydration rather than variations in temperature, at least in regions distant from subduction zones.