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

Abstract

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 H 2 O content. In addition, we develop a simplified parameterization of the H 2 O-undersaturated peridotite solidus for a constant degree of H 2 O 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°C and 150 ppm H 2 O, or between 91 km and a maximum of 200 km for a mantle at 1350°C and 450 ppm H 2 O. If the asthenosphere has a mantle potential temperature ≤1340°C or contains less than 150 ppm H 2 O 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 H 2 O content. Because the asthenosphere may be heterogeneous in composition and H 2 O 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. Copyright 2010 by the American Geophysical Union.