Transport of water into the lower mantle: Role of stishovite

When subjected to lower-mantle pressures and temperatures, natural 'anhydrous' basalt containing 0.2 wt.% H{sub 2}O forms a phase assemblage in which SiO{sub 2} stishovite is a significant carrier of hydrogen (up to 500 ppm H{sub 2}O by weight, as hydroxide), whereas the coexisting (Mg, Fe, Al, Ca)SiO{sub 3} perovskite appears to be not (upper bound of 50 ppm (wt) H{sub 2}O). Contrary to the devolatilization characteristically observed at lower pressures, we find that the abundance of H{sub 2}O in residual stishovite increases from {approx}100 to {approx}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 {approx} 10{sup 2} times the amount of water present in today's atmosphere.