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 ∼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.

Plain Language Summary: A dehydration melting layer is predicted near the 660‐km depth boundary by the phase transition of hydrous ringwoodite in the mantle transition zone to dry bridgmanite and ferropericlase in the lower mantle. However, the water content in the melt, which controls the melt density and thus gravitational stability, remains poorly constrained. In this study, the water content in hydrous melt under 660‐km depth conditions is estimated as a function of temperature. At 2000 K, corresponding to topmost lower mantle geotherm, hydrous silicate melt should contain ∼20 wt.% water, which will cause the melt to be gravitationally unstable. The melt fraction at 660‐km depth is estimated to be >0.5 vol.%, which may significantly reduce rock viscosity and seismic velocity.

Key Points: Water content in hydrous silicate melt decreases with increasing temperature. Water content in hydrous melt is relatively independent of chemical composition and coexisting phases. The dehydration melt at 660‐km depth contains 20–50 wt.% depending on temperature conditions.