Elasticity of polycrystalline pyrope (Mg 3Al 2Si 3O 12) to 9 GPa and 1000 °C

Abstract

Acoustic wave velocities for synthetic polycrystalline pyrope (Mg 3Al 2Si 3O 12) 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 Py 100 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.