Elastic wave velocities of (Mg_0.91Fe_0.09)₂SiO₄ ringwoodite under P-T conditions of the mantle transition region

Experimental techniques have been developed to measure elastic wave velocities of hot-pressed polycrystalline samples at high pressure and high temperature using a combination of ultrasonic interferometry and in situ synchrotron X-ray observation in conjunction with a large-volume Kawai-type multianvil apparatus (KMA). With these techniques, we have succeeded in precise measurements of the sound velocities of ringwoodite at pressures to ∼19 GPa and at temperatures to 1673 K, equivalent to the conditions of the middle part of the mantle transition region. A linear fitting of the present data on polycrystalline ringwoodite with a composition of (Mg_0.91Fe_0.09)₂SiO₄ yielded following parameters: K_0S = 186(1) GPa, ∂K_S/∂P = 4.3(1), ∂K_S/∂T = -0.018(1), G = 119(1) GPa, ∂G/∂P = 1.2 (1), and ∂G/∂T = -0.015 (1), which are in good agreement with earlier results at high temperatures and ambient pressure, and those at room temperature and high pressures. Significant non-linear decreases in Vp and Vs were noted at temperatures higher than 1200 K, however, suggesting that the elastic wave velocity measurements at temperatures corresponding to the actual mantle are needed to compare the laboratory data with the seismologically derived sound velocities.