Sound velocities measurement on MgSiO_3 akimotoite at high pressures and high temperatures with simultaneous in situ X-ray diffraction and ultrasonic study

摘要

Elastic wave velocities of MgSiO_3 akimotoite polycrystalline samples have been measured at pressures up to 25.7 GPa and temperatures to 1500 K by a combination of in situ X-ray diffraction and ultrasonic interferometry techniques in a large volume Kawai-type multianvil apparatus (KMA). The elastic moduli of akimotoite and their pressure and temperature dependences are determined by a 2D linear fitting analysis of the present data, yielding: K_S = 219.4(7) GPa, ?K_S/?P = 4.62(3), ?K_S/?T = -0.0228(4) GPa/K, G_0 = 132.1(7) GPa, ?G/?P = 1.63(4), ?G/?T = -0.0225(4) GPa/K. The bulk and shear moduli at ambient conditions are generally consistent with the result of a previous Brillouin study. However, significant nonlinear behaviors of the elastic moduli were observed at higher temperatures, indicating that the velocities derived from the linear fitting analysis are overestimated for the actual mantle conditions. Using the present new experimental data, we compared the elastic velocities of various high-pressure forms of MgSiO_3 under the mantle conditions. The results demonstrate a large velocity difference between akimotoite and perovskite, which may be relevant to the complex seismic structures near the bottom of the mantle transition zone.