The melt fraction induced by the dehydration melting at the base of upper mantle

The mantle transition zone (MTZ) is widely considered to be water rich, whereas the upper mantle has a much lower water storage capacity. Materials from the Earth’s surface are transported to the bottom of mantle transition zone and topmost lower mantle by slab subduction, resulting in the global upwelling of the mantle transition zone materials to shallow regions as counterflows. Since minerals in the mantle transition zone are considered to be water-rich, dehydration melting could occur near the 410-km discontinuity when the water-rich materials are transported to the low-water-storage-capacity upper mantle. However, the amount of the melt produced by the dehydration melting process remains poorly constrained. Here, we performed high-pressure phase equilibrium experiments under the conditions just above the mantle transition zone (at 13 GPa and 1800 K) using peridotite + 1 wt.% H₂O, which represents the bulk compositions of a water-rich mantle transition zone. Our results show a very high melt fraction ~ 10 wt.% (equivalent to 10.21 vol.%) produced by the dehydration melting process near the 410-km discontinuity, far exceeding the minimum melt fraction required to significantly reduce seismic velocities. Because of the low density and low viscosity, most melts formed near the 410-km discontinuity should migrate upwards rapidly to shallow regions. They may accumulate near the lithosphere-asthenosphere boundary, causing the rheological weakening of the asthenosphere.