Stability of H2O‐Rich Fluid in the Deep Mantle Indicated by the MgO‐SiO2‐H2O Phase Relations at 23 GPa and 2,000 K

Abstract The Earth's mantle contains significant amounts of water in the form of hydroxyl in hydrous minerals, nominally anhydrous minerals, and hydrous silicate melts. H 2 O fluid is thought to be present only in the shallow regions because it will always dissolve tens of weight percent of silicates by forming hydrous silicate melt in the deep mantle. Here I investigated the phase relations in the MgO‐SiO 2 ‐H 2 O system by high‐pressure experiments at a pressure of 23 GPa and a temperature of 2,000 K, corresponding to the conditions at the bottom of the mantle transition zone and the topmost lower mantle. The experimental results indicate that hydrous melt can contain more than 90 wt.% of H 2 O, that is, it becomes H 2 O‐rich fluid when coexists only with stishovite. In contrast, silicate‐rich hydrous melt is formed when the system is enriched with MgO component. Therefore, H 2 O‐rich fluid may be stabilized in locally SiO 2 ‐enriched rocks even at the topmost lower mantle, acting as a water source for the deep lower mantle by slab subduction. The H 2 O fluid also provide a possible cause for the occurrence of natural ice‐VII originated from 660 km depth.