Measurements of acoustic wave velocities at P-T conditions of the Earth's mantle

Abstract

A method is being developed to determine acoustic wave velocities in polycrystalline samples at pressures and temperatures of the Earth's mantle using an MA-8 multianvil apparatus. Ultrasonic sound waves, generated by a LiNbO 3 transducer, are transmitted through one of the eight tungsten carbide anvils and a platinum buffer rod to the sample which is situated at the center of the multianvil high P-T assembly. Travel times are determined by interferometry using the phase comparison method. Currently, the main uncertainty in determining velocities arises from estimating the changes in sample length at high pressure and temperature. Potentially, however, both compressional and shear wave velocities can be measured simultaneously using this technique. This leads to the possibility of determining the compressibility and resulting changes in sample length with pressure in a self consistent manner on the same sample, thus greatly improving the accuracy of the determined velocities. The technique has the potential for improving our knowledge of acoustic velocities in single phase and polyphase aggregates relevant to the Earth's mantle without the need for large extrapolations of experimental data.