Effect of dynamic melting on acoustic velocities in a partially molten peridotite

The mechanical effect of materials with a zero shear modulus aggregated with solids with non-zero shear moduli has been the primary motivation for why partially molten rocks should have lowered shear wave velocities. We propose that elastic softening associated with the process of melting must also be considered. In this view, seismic waves, propagating in regions of partial melting, perturb the thermodynamic state and drive solid to liquid and liquid to solid, thereby lowering the effective elastic moduli. We present measurements of Young’s modulus for a mantle peridotite, KLB1, at LVZ pressures and temperatures and seismic frequency that support this model. The model predicts that the softening of elastic modulus is controlled by the pressure dependence of melt fraction, ∂F/∂P, and not the percentage of melt present. In contrast to previous models, the P-wave velocity is decreased by nearly the same percentage as the S-wave velocity.