Pressure and temperature dependence of the elasticity of pyrope–majorite [Py60Mj40 and Py50Mj50] garnets solid solution measured by ultrasonic interferometry technique

Compressional (P) and shear (S) wave velocities have been measured for two synthetic polycrystalline specimens of pyrope–majorite garnets [Py60Mj40 and Py50Mj50] by ultrasonic interferometry to 8GPa and 1000K, in a DIA-type cubic anvil high pressure apparatus (SAM-85) interfaced with synchrotron X-radiation and X-ray imaging. Elastic bulk (KS) and shear (G) moduli data obtained at the end of the cooling cycles were fitted to functions of Eulerian strain to third order yielding pressure derivatives of the elastic moduli (∂KS/∂P)T=4.3 (3); (∂G/∂P)T=1.5 (1) for Py60Mj40 garnet and (∂KS/∂P)T=4.4 (1); (∂G/∂P)T=1.3 (1) for Py50Mj40 garnet. Both (∂KS/∂P)T and (∂G/∂P)T are identical for the two garnet compositions and are also consistent with Brillouin scattering data for polycrystalline Py50Mj50. Moreover, the new pressure derivatives of the elastic moduli are equal within experimental uncertainties to those of end-member pyrope garnet from ultrasonic studies [Gwanmesia, G.D., Zhang. J, Darling, K., Kung, J., Li, B., Wang, L., Neuville, D., Liebermann, R.C., 2006. Elasticity of polycrystalline pyrope (Mg3Al2Si3O12) to 9GPa and 1000°C. Phys. Earth Planet. Inter. 155, 179–190] and from Brillouin spectroscopic studies [Sinogeikin, S.V., Bass, J.D., 2002a. Elasticity of majorite and majorite–pyrope solid solution to high pressure: implications for the transition zone. Geophys. Res. 9(2), 1017], thereby demonstrating that the pressure derivatives of the elastic moduli are independent of the physical acoustics technique employed and unaffected by substitution of Si for Mg and Al within the Py–Mj solid solution in the range (Py100–Py50) of the present measurements. Temperature dependence of the elastic obtained from linear regression of entire P–T–K and P–T–G data are (∂KS/∂T)P=−14.6 (4)MPa/K; (∂G/∂T)P=−9.4 (4)MPa/K for Py60Mj40 garnet, and (∂KS/∂T)P=−14.6 (4)MPa/K; (∂G/∂T)P=−9.33 (2)MPa/K for Py50Mj50 garnet. These values are essentially identical for the two compositions and are also in excellent agreement with the Brillouin scattering data of [Sinogeikin, S.V., Bass, J.D., 2002b. Elasticity of pyrope and majorite–pyrope solid solutions to high temperatures. Phys. Earth Planet. Inter. 203, 549–555] for Py50Mj50 garnet; however, they are higher than those for pyrope by about 23% for (∂KS/∂T)P and 11% for (∂G/∂T)P [Gwanmesia, G.D., Zhang. J, Darling, K., Kung, J., Li, B., Wang, L., Neuville, D., Liebermann, R.C., 2006. Elasticity of polycrystalline pyrope (Mg3Al2Si3O12) to 9GPa and 1000°C. Phys. Earth Planet. Inter. 155, 179–190].