Sound velocity measurements at mantle transition zone conditions of pressure and temperature using ultrasonic interferometry in a multianvil apparatus

Technological developments using a 1000-ton uniaxial split-cylinder apparatus enable elastic wave velocities to be measured using millimeter-sized polycrystalline and single crystal specimens by ultrasonic interferometry. Measurements on polycrystalline Lucalox alumina to 10 GPa at room T show very good agreement with extrapolations of previous data at low pressures (<lGPa). Specimens recovered from the high pressure experiments are undamaged and re-usable; travel times measured during successive pressure cycles are reproducible within 0.5%. Sound velocities of the olivine (α) and beta-phase (β) polymorphs of Mg₂SiO₄ are reported to P≥12 GPa at room T on polycrystalline specimens previously hot-pressed in a multianvil apparatus. The new velocity data for olivine are lower by 1 % for P waves and 2% for S wave at transition zone pressures than finite-strain extrapolations of low pressure data, but exhibit good agreement to 10 GPa with recent Brillouin scattering and ultrasonic data for single crystals. The velocity data for the beta phase at 12 GPa agree within 1% with finite strain extrapolations of previous data to 3 GPa for similar specimens. Linear fittings to K_s and G vs. pressure yield dK_s/dP = 4.4 and dG/dP = 1.3 for olivine and dKg/dP = 4.2 and dG/dP =1.5 for the beta phase. The longitudinal modulus C₂₂ of San Carlos olivine and the shear modulus C₅₅ of San Carlos olivine and forsterite were measured to pressures above 10 GPa at room T. Examination of the specimens recovered from -10 GPa revealed no internal cracks and no increase in the dislocation density. Good agreement is observed for the C₂₂ of San Carlos olivine and C₅₅ of forsterite as compared to previous studies. The new ultrasonic data indicate that C₅₅ of San Carlos olivine remains linear with pressure to 13.5 GPa and does not exhibit the pronounced curvature reported earlier. Preliminary results on polycrystalline forsterite demonstrate the feasibility of performing ultrasonic velocity measurements to simultaneous pressure of 10 GPa and temperature of 1000°C. A specially designed P-T path in pressurization/heating and decompression/cooling ensures the integrity of the sample at these conditions.These technological advances make it now possible to measure acoustic velocities at the P and T conditions of the Earth’s mantle transition zone.