Water in the Earth's mantle

Water plays an important role in most processes within the Earth's mantle, e.g. transport phenomena, differentiation and seismic properties. This paper reviews the various aspects of water in the mantle, with a special emphasis on the water content in nominally anhydrous minerals, especially lower-mantle minerals. The saturation of the upper mantle with respect to water is calculated as a function of pressure, based on available water-solubility data obtained for upper-mantle minerals. The result indicates that the upper mantle is saturated at pressures between 2 and 4 GPa for bulk water contents of 250 to 700 ppm wt., as retrieved from measurements on mid-ocean ridge basalts (MORBs) and ocean island basalts (OIBs). Whereas up to 4000 ppm wt. of H2O could be dissolved in the upper mantle at pressures corresponding to 410 km depth, such a value is less than the 1.5-2.5 wt.% solubility stored in the hydrous phases of the transition zone. Water solubility in mantle perovskite is still controversial, because of the difficulty of synthesizing samples free of impurities. Reported data indicate that water solubility in perovskite decreases with increasing temperature and Al content. Water partitions preferentially into ferropericlase rather than into perovskite, and its water solubility increases with the incorporation of trivalent cations.