Transport of water into the lower mantle: Role of stishovite

Abstract

When subjected to lower‐mantle pressures and temperatures, natural “anhydrous” basalt containing 0.2 wt.% H 2 O forms a phase assemblage in which SiO 2 stishovite is a significant carrier of hydrogen (up to 500 ppm H 2 O by weight, as hydroxide), whereas the coexisting (Mg, Fe, Al, Ca)SiO 3 perovskite appears to be not (upper bound of 50 ppm (wt) H 2 O). Contrary to the devolatilization characteristically observed at lower pressures, we find that the abundance of H 2 O in residual stishovite increases from ∼100 to ∼400 ppm by weight upon partially melting the high‐pressure mineral assemblage at 28–60 GPa. We infer that the trace concentration of Al within residual stishovite increases upon partial melting, thereby increasing the coupled abundance of H in this crystalline phase. The “anhydrous” component of subducted oceanic crust can thus recycle a significant amount of water into the lower mantle over the age of the Earth, with subducted stishovite potentially returning ∼10 2 times the amount of water present in today's atmosphere.