Partial Melting, The Low Velocity Zone, and the LAB

The Earth’s low velocity zone, ubiquitous in the shallow oceanic mantle, is associated with the base of the lithosphere and a low viscosity zone. Recent observations of receiver function arrivals indicate a sharp discontinuity involving a decrease in both P and S velocities with depth demarking the lithosphere – asthenosphere boundary (LAB). While partial melting has often been viewed as the cause of the lowered velocity, the traditional mixing model of a solid and liquid indicates that the effect on velocity may be too small considering the small amount of liquid that can remain trapped in a partially molten zone (<2%). The magnitude of the effect is controlled by the volume fraction of melt, F. The expected gradual change in melt volume with depth precludes a seismic discontinuity such as required for the lithosphere asthenosphere boundary (LAB).

 The inadequacy of these models to sufficiently lower the shear velocity and provide a seismic discontinuity with small amounts of melt challenges the association of low velocity zone and melting. We assert that the dynamic process of melting and solidifying in response to the varying stress field of the seismic wave will significantly lower both the P and the S wave velocities with only small amount of melt present. Seismic velocity depends on the ratio of stress to strain, but is insensitive to the origin of the strain. Melting involves a change in volume, which induces a strain, and is driven by the stress field. The melting process will affect the elastic moduli in proportion to the pressure derivative of the melt fraction (dF/dP). Thus, the velocities can change discontinuously at the depth of the solidus even though F changes continuously.

Thermodynamic models such as MELTS require significant softening of the elastic moduli in the achievement of equilibrium for small amounts of melt in a peridotite. The relevant issue becomes: how much softening will occur on the time scale of the seismic period? We report results of experiments using a high flux synchrotron x-ray source examining samples at pressure, temperature, chemical composition, and seismic frequency that are present in the Earth’s low velocity zone. We conclude that seismic softening will occur for small amounts of partial melt with the possibility of a LAB discontinuity associated with the in situ solidus.