Sound velocities of peridotite and the role of orthopyroxene on Lehmann and X discontinuities

The current sound velocities of peridotite are primarily determined based on the individual mineral properties under mantle temperature and pressure conditions. To provide directly experimental constraints on the sound velocities of peridotite at high pressure and high temperature, we simulated peridotite using olivine-orthopyroxene aggregates and employed ultrasonic interferometry to measure the sound velocities of these aggregates at room temperature up to 12 GPa, and at up to 873 K and 8 GPa. The experimental results demonstrate that sound velocities increase with pressure but decrease with orthopyroxene content and temperature. Empirical equations have been established to describe the relationship between sound velocities and factors such as pressure, temperature, and orthopyroxene content in peridotite. By integrating the seismic characteristics of the Lehmann and X discontinuities, we found that discontinuities with 1%−2% velocity jumps can be explained by the orthopyroxene content in the mantle. Discontinuities exhibiting 3%−6% velocity jumps are predominantly observed in hot and wet regions, which can be attributed to the localized enrichment of orthopyroxene resulting from reactions between olivine and silica-rich fluids generated by melting of eclogite. Thus, the phase transition from orthopyroxene to high-pressure clinopyroxene is proposed as a potential explanation for both the L and X discontinuities.