A mechanism for low-extent melts at the lithosphere-asthenosphere boundary

Recent studies have imaged sharp vertical drops in shear wave velocity at the lithosphere-asthenosphere boundary (LAB). In some regions, the magnitude of the negative velocity gradient at the LAB is too large to be explained by changes in temperature alone. This study demonstrates that small amounts of partial melt in the shallow asthenosphere are a viable model for this sharp seismic boundary. In particular, we examine melting in the upper asthenosphere at the edge of thick cratonic lithosphere, using the example of eastern North America where a sharp LAB velocity gradient has been observed. Finite element modeling of asthenospheric flow at an abrupt lateral decrease in lithosphere thickness indicates that this geometry, together with lateral plate motions, produces edge-driven convection and asthenospheric upwelling at the continental margin. A key component of this work is a comparison of the locations and extents of melting produced by using different models for the depression of the peridotite solidus with varying H2O content. In addition, we develop a simplified parameterization of the H2O-undersaturated peridotite solidus for a constant degree of H2O saturation in nominally anhydrous minerals. The patterns of mantle flow produced by our numerical modeling and various solidus parameterizations predict less than 0.1 wt % to 2.8 wt % (0.01-3.3 vol %) melting at depths between 102 and 126 km for an asthenosphere with a mantle potential temperature of 1350 degrees C and 150 ppm H2O, or between 91 km and a maximum of 200 km for a mantle at 1350 degrees C and 450 ppm H2O. If the asthenosphere has a mantle potential temperature <= 1340 degrees C or contains less than 150 ppm H2O at 3 GPa, no melting will occur. This process of generating melt in the asthenosphere to produce a sharp vertical velocity gradient at the LAB is viable in other locations where convective upwelling occurs in the shallow asthenosphere although it is dependent on asthenospheric potential temperature, composition, and H2O content. Because the asthenosphere may be heterogeneous in composition and H2O content, the onset of melting below the LAB may fluctuate with time and space, as may the magnitude of the shear velocity drop at the LAB.