FeSiO4H2 stabilized at subducting slab conditions: A geologically viable water carrier into the Earth's lower mantle

Hydrous minerals hold the key to unlocking the enduring mystery of the water cycle deep inside the Earth. Tremendous efforts have been devoted to identifying geologically viable minerals meeting stringent pressuretemperature-density stability requirements for descent into deep Earth, and such pursuits remain active. Here, we identify two hydrous iron silicates, alpha- and beta-FeSiO4H2, formed by a reaction of Earth-abundant FeSiO3 and H2O and stabilized at the pressure-temperature conditions in cold subducting slabs. These phases have a sufficiently high density for a stable descent into the Earth's lower mantle, and then decompose to release water after reaching equilibrium with the mantle geotherm. Moreover, Mg(Fe)SiO4H2 solutions are found to be more stable than the pure substances and can serve as effective carriers to transport substantial amounts of water to lower-mantle regions via the cold subduction zones. These findings establish a viable and robust material basis for the deep-Earth water cycle, with major implications for elucidation of many prominent geological processes.