Elasticity and sound velocities of polycrystalline Mg ₃Al ₂(SiO ₄) ₃ garnet up to 20 GPa and 1700 K

Elastic wave velocities of synthetic polycrystalline Mg ₃Al ₂(SiO ₄) ₃ garnet have been successfully measured to 20 GPa and temperatures up to 1700 K by ultrasonic interferometry combined with energy-dispersive synchrotron x-ray diffraction in a Kawai-type multi-anvil apparatus. Compressional (Vp) and shear (Vs) wave velocities as well as the adiabatic bulk (Ks) and shear (G) moduli exhibit monotonic increase with increasing pressure and decrease with increasing temperature, respectively. Two-dimensional (P-T) linear fittings of the present data yield the following parameters: K _S0 170.0(2) GPa, Ks/P 4.51(2), Ks/T -0.0170(1) GPa/K, G ₀ 93.2(1) GPa, G/P 1.51(2), and G/T -0.0107(1) GPa/K, which is in good agreement with the earlier results by Brillouin scattering and/or ultrasonic measurements at relatively low P-T conditions. The observed linear pressure and temperature dependence in both Vp and Vs is in contrast to the non-linear behavior of Vp and Vs for majorite garnet with the pyrolite composition, in particular for Vs.