Using shockless ramp compression to investigate melting in the Earth's mantle.

A major feature of the Earth’s interior structure is the mantle transition zone (MTZ), which lies between 410-660 km depths (12-20 GPa), defined by discontinuous increases in seismic velocity associated with mineral phase transitions. Geophysical observations from seismic tomography show slower than average seismic velocities just above and below the MTZ. It is proposed that due to the convective flow of the mantle, these regions of low velocities indicate the presence of partially molten rock near the upper and lower boundaries of the MTZ. Testing this hypothesis requires experimental determination of velocity for silicate melts at high pressures and elevated temperatures. Static studies of silicate melts at one-atmosphere and silicate glasses at high-pressure exhibit little compositional or density dependence for velocity. Whether this observation holds at MTZ conditions has not been demonstrated. To investigate the composition-velocity relationship at elevated pressure and temperature, we conduct shockless dynamic compression (ramp) of glasses along the SiO₂-MgSiO₃ compositional join to >20 GPa on the pulsed-power machine Thor at Sandia National Laboratories. These experiments probe an isentropic pressure-temperature path, similar to that of the mantle geotherm, and will illuminate the seismic properties of partial melts, if they exist in the MTZ.