A Strength Softening Phase Transition Observed in Shocked (Mg_0.92,Fe_0.08)SiO₃ Perovskite at About 83GPa

We report the experimental data of Hügoniot longitudinal sound velocity V_L for natural (Mg_0.92,Fe_0.08)SiO₃ enstatite sample at about 40--140GPa, consisting of three new data and five previously reported data but revised by our new Hügoniot equation of state parameters. Three segments, separated by two discontinuities, appear in the V_L--pH (shock pressure) plot. Analyses show that the first discontinuity at about 64GPa, with a sharp increase of V_L of about 21%, is judged to be a phase transition from enstatite to Pbnm perovskite (PV); while the second one at about 83GPa, with a dramatic decrease of V_L of about 23%, is likely caused by a subtle structural change from Pbnm PV to tetragonal PV, accompanied by material strength softening due to melting of oxygen sublattices. This strength softening evidence is obtained first from shock wave experiments, and probably has profound implications for probing into the origin of low seismic velocity anomaly in the Earth's lower mantle and thus constraining the geophysical and geochemical models for the Earth's lower mantle.