The beginnings of hydrous mantle wedge melting

Abstract

This study presents new phase equilibrium data on primitive mantle peridotite (0.33 wt% Na2O, 0.03 wt% K2O) in the presence of excess H2O (14.5 wt% H2O) from 740 to 1,200°C at 3.2–6 GPa. Based on textural and chemical evidence, we find that the H2O-saturated peridotite solidus remains isothermal between 800 and 820°C at 3–6 GPa. We identify both quenched solute from the H2O-rich fluid phase and quenched silicate melt in supersolidus experiments. Chlorite is stable on and above the H2O-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 H2O-saturated melting curve. Chlorite can therefore provide sufficient H2O upon breakdown to trigger dehydration melting in the mantle wedge or perpetuate ongoing H2O-saturated melting. Constraints from recent geodynamic models of hot subduction zones like Cascadia suggest that significantly more H2O 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 H2O to trigger melting at the H2O-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 H2O 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.