Kinetics of Partial Melting in a Mantle Peridotite

Partial melting in response to a change in pressure, stress, or temperature induces a perturbation in strain owing to the volume difference between the solid and the liquid phases. Total chemical equilibrium may take hours to achieve in the laboratory. However, partial equilibrium may occur on a much shorter time. The extent to which the partial melting will lower the elastic modulus depends on how much of the volume change occurs on the time scale of the stress field. For volume changes that are fast compared to a stress wave, then the volume change will be effective in lowering the elastic modulus; slow volume changes will not affect the elastic modulus. We have investigated the kinetics of melting in KLB-1 at 2 GPa and 1000 - 1400 C. We monitor the volume of the sample with radiographic images taken at a rate of 20/second. Volume strain is resolved to 0.0001. The experimental protocol involves monitoring the effect of sudden changes in temperature of 50 - 100 C. Melting creates an enhanced volume strain in the sample. In samples both with and without melting we see a rise time of the volume strain of 1 - 3 seconds, consistent with the thermal diffusivity of the sample. Our observations are consistent with a melting time scale of less than one second. While total equilibrium will probably not occur so quickly, it is concluded that the bulk of the melting strain will. Thus, seismic velocities should be softened by the dynamics of melting.