Water Content of the Dehydration Melting Layer in the Topmost Lower Mantle

The water-rich mantle transition zone and dry lower suggest that a dehydration melting layer can form at the 660-km depth boundary. However, water content of (), which dominates its gravitational stability melt fraction, remains poorly constrained. Here, hydrous silicate by mass balance calculations is investigated found significantly decreases with increasing temperature, but relatively insensitive to chemical composition (FeO SiO2 contents) coexisting phases. Melt should contain ?50 wt.% 1600 K (slab geotherm) or ?20 2000 (topmost geotherm). density <3.9 g/cm3, makes it buoyant. With fraction ?0.5 vol.%, expected reduce viscosity seismic velocity near slabs, may cause slab stagnation prohibit whole convection.