The Elasticity of Synthetic Polycrystalline Complex Garnet to 9.5 GPa and 1000K Determined Using Ultrasonic Interferometry

Abstract:

Petrological studies show that the transition zone of the Earth’s mantle (depth of 410-660 km) is composed of 40-70% garnet by volume. Extensive knowledge of the elastic properties of complex garnet, of chemical composition pertinent to the region and at relevant high pressures and temperatures, is necessary to constrain the composition of the region and to explain the steep compressional wave (P-wave) and shear wave (S-wave) velocity gradients observed from seismic studies. We have measured the acoustic wave velocities of synthetic polycrystalline complex garnet of a suitable pyroxene composition (Mg_1.875Ca_0.75Fe_0.375 Na_0.5Al_0.5Si₄O₁₂) at simultaneous pressures (P) up to 9.5 GPa and temperatures (T) up to 1000 K in a DIA-type apparatus using ultrasonic interferometry in conjunction with energy-dispersive synchrotron X-ray diffraction and X-ray imaging. Travel times of the P- and S-waves, X-ray image data, and X-ray diffraction data were collected during cooling cycles to minimize the effect of non-hydrostatic stress on the specimen. Elastic adiabatic bulk (K_S) and shear (G) moduli data were fit to polynomial equations in P and T, yielding (∂K_S/∂P)_T = 4.6 (1) and (∂G/∂P)_T = 1.27 (3) for the pressure derivatives of the elastic K_S and G, respectively, and (∂K_S/∂T)_p = -15.0 (7) MPa/K and (∂G/∂T)_P = -10.4 (2) MPa/K for the temperature derivatives of K_S and G, respectively. The results are consistent with those of previous studies on garnets of varying compositions, and in general suggest that the pressure and temperature dependence of the elastic bulk and shear moduli of garnet may be insensitive to the garnet composition.