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 generating dehydration melting feed the base of the lithosphere forming the 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.