Deciphering the Distribution of Partial Melt in the Asthenosphere with Converted Seismic Waves

The global distribution of partial melt in the asthenosphere remains uncertain, but recent constraints on mantle velocity gradients obtained with Sp converted phases are providing new insight. In regions of the mantle where seismic shear velocities are relatively low, Sp phases indicative of positive shear velocity gradients at depths of 100-150 km depth are common. Beneath the Anatolian region, common conversion point stacking of Sp phases reveals strong positive velocity gradients at depths of 100-150 km that correspond to the base of an asthenospheric low velocity layer imaged in multiple tomographic models. Interpretation of this shear velocity gradient as the onset of hydrous partial melting, where partial melt at shallower depths reduces velocities, is consistent with primary magma equilibration conditions from basaltic samples in the region. Beneath Alaska, common conversion point stacking of Sp phases indicates a strong positive velocity gradient at depths of ~150 km. This gradient appears not only in the mantle wedge above the lithosphere that is subducting beneath southern Alaska, but also beneath the subducting plate. If this velocity gradient also indicates the lower boundary of a partial melt layer, its distribution indicates that the onset of partial melting does not depend on the presence of fluids fluxing from the top of the slab. To provide a global assessment of the presence this type of positive velocity gradient, Sp receiver functions were stacked at 430 globally distributed stations. In regions of the mantle where shear velocities are less than 4.35 km/s at 150 km depth in the GLAD-M15 model of Bozdag et al. (2016), Sp stacks are dominated by cases containing strong positive velocity gradients at depths of 120-150 km. However, the velocity gradient weakens as velocities at 150 km depth increase, indicating a global spatial correlation of the velocity gradient and low velocity mantle. This association, coupled with the possibility that the positive velocity gradient represents the onset of hydrous partial melting, suggests that partial melt is present in the upper mantle beneath more than a third of the Earth's surface.