The beginnings of hydrous mantle wedge melting

This study presents new phase equilibrium data on primitive mantle peridotite (0.33 wt% Na ₂O, 0.03 wt% K ₂O) in the presence of excess H ₂O (14.5 wt% H ₂O) from 740 to 1,200°C at 3.2-6 GPa. Based on textural and chemical evidence, we find that the H ₂O-saturated peridotite solidus remains isothermal between 800 and 820°C at 3-6 GPa. We identify both quenched solute from the H ₂O-rich fluid phase and quenched silicate melt in supersolidus experiments. Chlorite is stable on and above the H ₂O-saturated solidus from 2 to 3.6 GPa, and chlorite peridotite melting experiments (containing ~6 wt% chlorite) show that melting occurs at the chlorite-out boundary over this pressure range, which is within 20°C of the H ₂O-saturated melting curve. Chlorite can therefore provide sufficient H ₂O upon breakdown to trigger dehydration melting in the mantle wedge or perpetuate ongoing H ₂O-saturated melting. Constraints from recent geodynamic models of hot subduction zones like Cascadia suggest that significantly more H ₂O is fluxed from the subducting slab near 100 km depth than can be bound in a layer of chloritized peridotite ~ 1 km thick at the base of the mantle wedge. Therefore, the dehydration of serpentinized mantle in the subducted lithosphere supplies free H ₂O to trigger melting at the H ₂O-saturated solidus in the lowermost mantle wedge. Alternatively, in cool subduction zones like the Northern Marianas, a layer of chloritized peridotite up to 1.5 km thick could contain all the H ₂O fluxed from the slab every million years near 100 km depth, which suggests that the dominant form of melting below arcs in cool subduction zones is chlorite dehydration melting. Slab P-T paths from recent geodynamic models also allow for melts of subducted sediment, oceanic crust, and/or sediment diapirs to interact with hydrous mantle melts within the mantle wedge at intermediate to hot subduction zones.