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

The water-rich mantle transition zone and dry lower mantle suggest that a dehydration melting layer can form at the 660-km depth boundary. However, the water content of the melting layer (CH2Omelt), which dominates its gravitational stability and melt fraction, remains poorly constrained. Here, the CH2Omelt of hydrous silicate melt by mass balance calculations is investigated and found that CH2Omelt significantly decreases with increasing temperature, but is relatively insensitive to chemical composition (FeO and SiO2 contents) and coexisting phases. Melt at 660-km depth should contain similar to 50 wt.% water at 1600 K (slab geotherm) or similar to 20 wt.% water at 2000 K (topmost lower mantle geotherm). The density of the hydrous melt is <3.9 g/cm(3), which makes it buoyant. With a melt fraction of greater than or similar to 0.5 vol.%, the melting layer is expected to significantly reduce the viscosity and seismic velocity near slabs, which may cause slab stagnation and prohibit whole mantle convection.

Automated summary

The study indicates the presence of a dehydrated melting layer in the deep mantle, where the water content significantly decreases with increasing temperature. This melting layer at a depth of 660 kilometers may lead to slab stagnation and inhibit whole mantle convection.