Fig. 1. Subduction of oceanic lithosphere drives material circulation in the Earth’s mantle, providingan important conduit for the deep-water cycle and the coevolution of the hydrosphere and solid Earth. Dehydration of water-bearing stishovite, CaCl₂-type silica, and seifertite (DHS) during subduction results in the addition of water to mantle rocks, and this can significantly affect mineral phase relations, melting temperatures, and seismic velocities. Importantly, hydrous CaCl₂-type silica can continuously carry water into the base of the mantle at mantle temperatures over 3000 K.⁶ The O₂-dominated volatiles decomposed from the released water or newly formed high-pressure peroxides can oxygenate the surrounding mantle and promote melting.^7–9