During the subduction of oceanic lithosphere, water is liberated from minerals by progressive dehydration reactions1,2 and is thought to be critical to several geologically important processes such as island-arc volcanism3, intermediate-depth seismicity4 and chemical exchange between the subducting lithosphere and mantle5. Although dehydration reactions would yield supercritical fluid water in most slabs, we report here that the stable phase of H2O should be solid ice VII in portions of the coldest slabs. The formation of ice VII as a dehydration product would affect the generation, storage, transport and release of water in cold subduction zones and equilibrium conditions of dehydration would shift, potentially affecting the dephts of seismogenesis and mag-magenesis. Large amounts of pure ice VII might accumulate during subducion and, as a sinking slab warms, eventual melting of the ice would release large amounts of water in a small region over a short period of time, with a significant positive volume change. Moreover, the decreasing availability of fluid water, owing to the accumulation of ice VII and its subsequent reaction products in a cooling planetary interior (for example, in Mars or the future Earth), might eventually lead to a decline in tectonic ectivity or its complete cessation.
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