Water, melting, and the deep Earth H₂O cycle

Hydrous melting driven by changes in H₂O storage capacity may occur in a variety of settings in the mantle, including in oceanic basalt sources and in deeper regions above and below the transition zone. The 50-200 ppm H₂O in the upper mantle likely derives from a blend of sources that may include residues of hydrous partial melting, either in the deep mantle and/or beneath arcs or oceanic islands. Relative to the large storage capacity in the transition zone, low storage capacities above and below may lead to hydrous melting for material upwelling through 410 km or downwelling through 670 km. The apparently very low storage capacity of the lower mantle (<20 ppm H₂O) may force melting even if downwelling rocks have normal upper mantle H₂O (50-200 ppm) concentrations. Very low storage capacity in the lower mantle, if verified experimentally, presents a challenge to the view that the H₂O-rich sources of oceanic island basalts reside in the lower mantle.