Stability limits of hydrous minerals in sediment and mid‐ocean ridge basalt compositions: Implications for water transport in subduction zones

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<jats:p>Phase relationships in hydrated natural sediment and mid‐ocean ridge basalt (MORB) compositions were investigated experimentally from 6 to 15 GPa and 700°C to 1400°C in order to determine the stability of hydrous phases in subducting slabs and to constrain reactions resulting in the release of water from subduction zones to the mantle. Phengite and hydrous aluminum silicates (topaz‐OH and phase egg) are stable in the sediment composition at depths greater than 150 km and constitute H<jats:sub>2</jats:sub>O reservoirs. Similarly, lawsonite is an H<jats:sub>2</jats:sub>O reservoir in MORB composition at depths greater than 150 km. According to mass balance calculations based on experimental run products, the weight proportions of phengite and H<jats:sub>2</jats:sub>O at 6 GPa in the sediment are about 50% and 2%, respectively. At pressures from 8 to 12 GPa, topaz‐OH (Al<jats:sub>2</jats:sub>SiO<jats:sub>4</jats:sub>(OH)<jats:sub>2</jats:sub>) appears as the hydrous mineral, and the sediment retains about 0.5–0.7 wt % H<jats:sub>2</jats:sub>O. At higher pressures, phase egg (AlSiO<jats:sub>3</jats:sub>(OH)) appears as the hydrous phase with a large stability field. The H<jats:sub>2</jats:sub>O content in the sediment is about 0.5 wt% at pressures greater than 12 GPa. In natural MORB compositions, lawsonite is the hydrous phase at pressures lower than 10 GPa. About 1 wt%, H<jats:sub>2</jats:sub>O in the MORB composition is retained by lawsonite at 6 GPa and 800°C. Above 6 GPa, a lawsonite‐out reaction is controlled by compositional changes of garnet and clinopyroxene. If the oceanic crust descends with an overlying sediment layer, water can be carried down into the transition zone by the sediment. The amount of transported water is controlled by the chemical compositions of the subducting sediments. Water in the MORB layer can be stored to 300 km in lawsonite for a cold subduction zone geotherm. Water release at depths greater than 200 km through progressive lawsonite breakdown can hydrate the overlying mantle, causing the generation of a peridotite region including hydrous phases (chondrodite, clinohumite, phase A, and phase E). If this hydrous peridotite is dragged down by the descending oceanic crust, water can be transported into the transition zone by the subduction process. The amount of transported water is controlled by the thermal state of the slab and the chemical composition of MORB.</jats:p>

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