Seismic evidence for a global low-velocity layer within the Earth's upper mantle

Within the upper mantle, the seismic discontinuity at 410-km depth marks the top of the transition zone and is attributed to pressure-induced transformation of olivine into wadsleyite mineral assemblage. Just above the 410-km discontinuity, a layer characterized by low seismic wave velocities has been identified regionally(1,2). This low velocity layer shows poor lateral continuity and is thought to represent partial melting induced by local effects, such as the dehydration of subducted crust(1) or the dehydration of water-bearing silicates beneath continental platforms in association with mantle plumes(2). However, some models predict that the low-velocity layer should extend globally, because the weaker water storage capacity of upper mantle minerals should induce partial melting of water-bearing silicates throughout this region(3,4). Here we report seismic observations from 89 stations worldwide that indicate a thick, intermittent low-velocity layer is located near 350 km depth in the mantle. The low-velocity layer is not limited to regions associated with subduction or mantle plumes, and shows no affinity to a particular tectonic environment. We suggest that our data image the thickest parts of a more continuous global structure that shows steep lateral variations in thickness. The presence of a global layer of partial melt above the 410-km discontinuity would modify material circulation in the Earth mantle and may help to reconcile geophysical and geochemical observations(3).