Partial melting and the low-velocity zone

The effect of partial melting on the elastic properties of polycrystalline material is a strong function of the shape of the melt zones. If the lowest melting constituents are concentrated in narrow zones, such as grain boundaries, a small amount of melt, ≈ 1%, can easily explain the upper mantle low-velocity zone, for both shear waves and compressional waves. The Eshelby-Walsh theory for the effective elastic moduli for a material with oblate spheroidal inclusions is used to calculate the compressional and shear velocities in a solid matrix with penny-shaped melt zones as a function of melt concentration and aspect ratio. Previous calculations, based on MacKenzie's (1950) and Sato's (1952) results for spherical inclusions, show a very weak dependence of velocity on melt concentration. The Eshelby-Walsh theory satisfactorily explains the velocities in the two-phase ice-brine system