Ultrasonic Acoustic Velocities During Partial Melting of a Mantle Peridotite KLB‐1

Knowledge of the elastic properties of partially molten rocks is crucial for understanding low‐velocity regions in the interior of the Earth. Models of fluid and solid mixtures have demonstrated that significant decreases in seismic velocity are possible with small amounts of melt, but there is very little available data for testing these models, particularly with both P and S waves for mantle compositions. We report ultrasonic measurements of P and S velocities on a partially molten KLB‐1 sample at mantle conditions using a multi‐anvil device at a synchrotron facility. The P, S, and bulk sound velocities decrease as melting occurs. We find that the quantity, ∂lnVS/∂lnVB (where VB is the bulk sound velocity) is lower than mechanical models estimate. Instead, our data, as well as previous data in the literature, are consistent with a dynamic melting model in which melting and solidification interact with the stress field of the acoustic wave. Key Points Acoustic velocities of P and S waves are measured on a mantle peridotite, KLB‐1, at 1.5 GPa as temperature is raised through the solidus The observed values of ∂lnVS/∂lnVB are consistent with models that include solid‐melt interaction with the acoustic wave Literature data for samples of olivine plus basalt melt over a pressure and temperature range also imply dynamic melting