Ultrasonic measurements of the elastic wave velocities of peridotite KLB-1 at mantle P and T

In situ measurements of sound velocities and densities on individual phases of Earth minerals at high pressure and temperature have provided important data to interpret the seismic structure at depths. When using these data to test compositional models of the mantle (e.g., pyrolite and piclogite), seismic properties of the mineralogical aggregates have to be calculated by using averaging schemes based on the proportion and elasticity of each phase. More importantly, the chemical interactions among various mantle phases are difficult to be accurately included. Since the composition of peridotite KLB-1 closely matches the composition of pyrolite, measurement of it's velocities at relevant mantle conditions will provide the most direct comparison with seismic data. In this study, well-sintered KLB-1 aggregates suitable for ultrasonic measurements were hot-pressed at various P-T conditions up to those of the transition zone. The recovered samples were characterized by X-ray diffraction, optical, and scanning electron microscopy (SEM) and found to be uniform in grain size (around 10 micron), free of macrocracks and to have bulk densities close to theoretical values (small amount of pores/microcracks observable in SEM scale). P and S wave velocities have been accurately measured up to 7 GPa and 800°C using a combined ultrasonic and X-radiation technique with direct measurement of sample length at high P and T. The success of this experiment makes it possible to extend these measurements to the conditions of the transition zone on aggregate samples to directly discriminate the composition of the Earth's mantle.