Reactive infiltration of hydrous melt above the mantle transition zone

Seismic studies repeatedly image a low-velocity layer overlying the mantle transition zone in tectonic settings ranging from subduction zones to continental cratons. This layer has been hypothesized to result from the presence of a dense partial melt formed by dehydration melting as relatively wet transition zone material is advected by convection currents into the low-water-solubility upper mantle. Here we examine this hypothesis by considering the dynamic infiltration of a low-viscosity reactive hydrous melt into a high-viscosity ambient solid. The thickness of the melt layer is strongly controlled by return flow induced in the surrounding viscous solid, and in steady state a dynamic equilibrium is achieved where the melt lens is restrained from collapse by the exterior mantle. Melt layers with thicknesses in excess of 10 km develop for a wide range of mantle parameters and develop on timescales equivalent to the lifespan of an ocean basin.