Elasticity of polycrystalline pyrope (Mg₃Al₂Si₃O₁₂) to 9 GPa and 1000 °C

Acoustic wave velocities for synthetic polycrystalline pyrope (Mg₃Al₂Si₃O₁₂) were measured to 9 GPa and temperatures up to 1000 °C by ultrasonic interferometry combined with energy-dispersive synchrotron X-ray diffraction in a cubic-anvil DIA-type apparatus (SAM-85). Specimen lengths at high pressures (P) and temperatures (T) are directly measured by X-radiographic methods. Elastic wave travel times and X-ray diffraction data were collected after heating and cooling at high pressures to minimize effect of non-hydrostatic stress on the measurements. A linear fit to the high P and T data set yields the elastic bulk and shear moduli [K_S = 175 (2) GPa; G = 91 (1) GPa] and their pressure and temperature derivatives [ K^′_S = 3.9 ± 0.3; G′ = 1.7 ± 0.2 and (∂K_S/∂T)_P = -18 (2) MPa/K; (∂G/∂T)_P = -10 (1) MPa/K]. In a separate analysis, the pressure-volume-temperature data collected during these acoustic experiments were fit to a high temperature Birch-Murnaghan (HTBM) equation [with K′ fixed at 3.9] and to each isothermal P-V-T data yielding (∂K_T/∂T)_P = -22 (2) MPa/K and (∂K_T/∂T)_P = -20 (5) MPa/K, respectively. Comparison of Py₁₀₀ data with those other Py-Mj compositions indicates that the thermo elastic properties are insensitive to majorite content in the garnet along the pyrope-majorite join.