Hydrated Minerals in the Earth’s Mantle

The Earth’s deep-seated layers have different water contents. The transfer of water from the Earth’s crust to the mantle is mainly related to subduction processes of the oceanic lithosphere. Due to accumulation of water in nominally anhydrous minerals (NAMs: wadsleyite and ringwoodite), the mantle transition zone is the most important water reservoir, especially in comparison with the upper mantle. Upper mantle peridotite within subducted slabs inside the transition zone can preserve H2O at large depths in “alphabetic” DHMS phases (dense hydrous magnesium silicates). The lower mantle peridotite consists of 80 wt % Mg-perovskite (bridgmanite), 15 wt % ferropericlase, and 5 wt % Ca-perovskite (davemaoite). Among these minerals, the highest water content occurs for Ca-perovskite followed by Mg-perovskite and ferropericlase. Taking the huge mass of the Earth’s mantle into account, even the minimum water content that is concentrated in NAMs can exceed the amount of water in the present-day hydrosphere by several times in spite of degassing of the mantle as a result of volcanic activity, which was especially intense during the first 500 million years of the planet’s formation. The structure of minerals–possible water hosts has been characterized for deep geosphere conditions.