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.