Kinetics of melting in peridotite from volume strain measurements

• Effective thermal expansion is measured from X-ray images of the sample at high pressure and temperature.
• The onset of partial melting in a KLB1 peridotite sample was marked by a significant increase in effective thermal expansion by a factor of 2–3.
• The time constant of the melting that caused the increase in thermal expansion is less than 1 s.
• Melting dynamics will probably significantly lower seismic velocities in partially molten regions, with only small (1–2%) melt present.

Partial melting in the Earth’s mantle involves a combination of processes such as ionic diffusion and solid–liquid transition. These processes yield an array of characteristic times for the melting kinetics. Both the magnitude and time constant of melt-induced volume strain affect seismic velocity, attenuation, and buoyancy. In situ monitoring of samples during melting are enabled by multi-anvil high-pressure devices coupled with synchrotron X-ray radiation. In this study, the volume strain induced in a sample from melting is measured as a function of time using X-ray images. We find a doubling of the effective thermal expansion for only 2% melt both from our data and from thermodynamic models of peridotite. The characteristic time of melting is determined to be less than one second. These findings have a direct impact on the effect of melting on seismic velocities.