Phase Relations of Hydrous Peridotite: Implications for Water Circulation in the Earth's Mantle

Abstract

Using recent results from high-pressure experiments and thermodynamic calculations, phase diagrams of simplified hydrous peridotite in the system MgO-SiO2-H2O have been constructed to lower mantle conditions. Possible water subduction and circulation processes in the deep mantle are discussed on the basis of the constructed phase relations. For water transportation by subduction of peridotites, important phase relations are : (1) of antigorite (serpentine) at lower pressures (less than 10 GPa), and (2) of seven different high-pressure hydrous phases at higher pressures. In cold subducting slabs, water in antigorite is partially transferred to the hydrous phase A at depths greater than 160 km. With further subduction, water in phase A may be transported to the bottom of the transition zone via solid-solid reactions among seven high-pressure hydrous phases. If the slab temperature at 30 GPa is lower than 1000°C, hydrous phase D will carry water into the deep lower mantle. Along the cold slab geotherm, large fluid fluxes are predicted at shallow (300-km depth) and deep (700-km depth) levels, depending on the slab temperature. The depth distributions of dehydration reactions suggest that observed subduction zone seismicity could be related to dehydration reactions in the slab. The lower water activities in the fluid phases at deep mantle conditions imply that such fluid phases could dissolve significant amounts of silicate components. Therefore, fluid phases released by dehydration reactions at deeper levels should have different physical properties from those at shallower levels.