Ultrasonic wave velocities of pyrope-majorite garnets at high pressure and high temperature

Pyrope (Py) - majorite (Mj) garnet specimens with compositions of Py62Mj38, Py50Mj50, were hot-pressed in a uniaxial split-sphere apparatus (USSA-2000) and characterized by X-ray diffraction, density measurements, electron microprobe as well as ultrasonic wave velocity measurements. Their elastic properties were studied using ultrasonic interferometry under high-pressure and simultaneous high-pressure and high-temperature conditions. High-pressure ultrasonic interferometry measurements to 9 GPa at room temperature were carried out on the specimens in the Kennedy-Walker type Uniaxial Split-Cylinder Apparatus (USCA-1000). Compressional and shear wave travel times as functions of pressure were measured at room temperature. The compressional and shear wave velocities, VP and VS, and the adiabatic bulk moduli (KS and G) and their pressure derivatives (KS' and G) were obtained from the travel time measurements. Using high P-T ultrasonic interferometry techniques in conjunction with the synchrotron X-radiation at Brookhaven National Laboratory, simultaneous measurements of P-V-VP-VS-T for the mantle garnets to 8 GPa and 873 K were conducted and elastic moduli and their pressure and temperature derivatives [(∂KS/∂T)P and (∂G/∂T)P] of the garnets were obtained for the first time. Elastic parameters of the specimens obtained from high-pressure room temperature experiments and those from high P-T experiments are consistent with those reported in previous studies. High P-T ultrasonic velocity and X-ray diffraction measurements provide a set of data that can be processed independent of pressure and give us a way to evaluate the precision of pressure calibration based on thermal equation of state of the pressure calibrants in X-ray diffraction experiments.; Elastic moduli and the their pressure derivatives of the garnets obtained from ambient conditions and high pressures in this research offers insight into the elasticity systematics of majorite-pyrope garnets. Combined with the results from several other compositions along the pyrope-majorite join (Chen et al., 1998, Gwanmesia, et al., 1998), these new data demonstrate that both the bulk and shear moduli decrease monotonically as Mg2+ and Si4+ replace Al3+, resulting in net decrease of 2.5% for KS and 3% for G from pyrope (Mg3Al 2Si3O12) to majorite (Mg4Si4O 12). The pressure derivative Ko' = (∂K S/∂P)T increases by about 26% while Go ' = (∂G/∂P)T increases by about 20% across this composition join. Neither the elastic moduli nor their pressure derivatives appear to be influenced by the change in crystallographic symmetry from cubic to tetragonal that occurs near 75 mol% majorite (Parise et al., 1996; Heinemann et al., 1996). (Abstract shortened by UMI.)...