Elastic wave velocity measurement in multi-anvil apparatus to 10 GPa using ultrasonic interferometry

Elastic wave velocities of dense fine-grained polycrystalline Al₂O₃ were measured to 10 GPa at room temperature by ultrasonic interferometry within a multi-anvil apparatus. This technique features the use of a WC cube as the acoustic buffer rod allowing the location of 40 MHz LiNbO₃ transducers in a stress-free environment, an in-situ pressure indicator (Bi and/or ZnTe), and a wide frequency range (20 to 70 MHz) for precise (10^-3) measurement of travel times. The specimens recovered from the high-pressure experiments are undamaged and re-usable; and the travel times measured during successive pressure cycles are reproducible within 0.4%. Room pressure velocity measurements for Lucalox alumina yield V_P = 10.92 ± 0.05 km s^-1 and V_S = 6.39 ± 0.03 km s^-1. The average pressure derivatives of P and S wave velocities are dV_P/dP = 5.4 × 10^-2 km s^-1 GPa^-1 and dV_S/dP = 2.2 × 10^-2 km s^-1 GPa^-1 in the pressure range of 3-9.6 GPa, which are consistent with previous results from single-crystal and polycrystal specimens at much lower pressures. This study demonstrates the feasibility of measuring the acoustic wave velocity of mantle minerals at the pressures corresponding to the Earth's transition zone using ultrasonic interferometry in multi-anvil apparatus.