资源环境科技发展态势分析平台

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 (C_H2O^melt), which dominates its gravitational stability and melt fraction, remians poorly constrained. Here I investigated the C_H2O^melt of hydrous silicate melt by mass balance calculations and found that C_H2O^melt 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 ∼50 wt.% water at 1600 K (slab geotherm) or ∼20 wt.% water at 2000 K (topmost lower mantle geotherm). The density of the hydrous melt is <3.9 g/cm³, which makes it buoyant. With a melt fraction of ≳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.