The origin and fate of subslab partial melts at convergent margins

Abstract

Channelized thin seismic low-velocity zones (LVZs) are observed beneath subducting slabs, extending from the lithosphere-asthenosphere boundary to the mantle transition zone (MTZ). While LVZs above slabs are well-explained by slab dehydration and flux melting, the origin and fate of these enigmatic subslab LVZs - persisting far deeper than expected - remain elusive. Here we use geodynamic modeling to show that subduction-induced wet upwellings from a water-bearing MTZ that generates dehydration melting feed the base of the lithosphere forming seismically detected LVZs. A similar process may occur when a pre-existing partially molten layer atop the 410-km discontinuity is displaced upward in response to subduction. These thin partially molten layers are successively entrained alongside the subducting slab and recycled back to MTZ depths. This mechanism supports a globally widespread scenario where subduction-induced upwelling of a water-rich MTZ can account for the observed mantle heterogeneities. Since minute quantities of melts may dramatically reduce viscosity, this process is likely to have non-negligible implications for the Earth's dynamics and recycling of the lithosphere into the deep mantle.