When subjected to lower-mantle pressures and temperatures, natural anhydrous basalt containing 0.2 wt.% H2O forms a phase assemblage in which SiO2 stishovite is a significant carrier of hydrogen (up to 500 ppm H2O by weight, as hydroxide), whereas the coexisting (Mg, Fe, Al, Ca)SiO3 perovskite appears to be not (upper bound of 50 ppm (wt) H2O). Contrary to the devolatilization characteristically observed at lower pressures, we find that the abundance of H2O 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 ~102 times the amount of water present in today's atmosphere.