Thermal equation of state of garnets along the pyrope-majorite join

Abstract

P-V-T relations of two garnet samples along the pyrope (Py)-majorite (Mj) join (Py n Mj 1− n , where subscripts indicate molar percent) were measured at pressure and temperature conditions up to 11 GPa and 1163 K, respectively, with energy-dispersive synchrotron X-ray diffraction in a cubic-anvil, DIA-6 type apparatus (SAM-85). For each volume measurement, non-hydrostatic stress was determined from relative shifts of the diffraction lines of NaCl, within which the sample was embedded. Heating to ∼ 1100 K reduced the strength of NaCl below 0.1 GPa, making the measurements nearly hydrostatic. The recovered samples were examined by transmission electron microscopy (TEM) to detect irreversible changes that may affect data quality. For the cubic garnet Py 62Mj 38, no irreversible changes took place during the experiment. A fit using the third-order Birch-Murnaghan equation of state yielded ambient cell volume V 0 = 1509.2(3) Å 3 (113.62 cc/mol), isothermal bulk modulus K T0 = 160(3) GPa, and its pressure derivative K′ T0 = 4.9(5). Various thermal equations-of-state analyses gave consistent results for the temperature derivative of bulk modulus ( ∂K T ∂T ) P = −0.020(1) GPa K −1 pressure derivative for thermal expansion ( ∂α ∂P ) T = −7.8 × 10 −7 GPa −1 K −1 , with the ambient volumetric thermal expansion α 0 = 2.5 × 10 −5 K −1. For the tetragonal sample Py 21Mj 79, TEM indicated that micro-twin domains were significantly coarsened after the P-V-T experiment. Stress relaxation may have occurred during the coarsening, thereby compromising the P-V-T data. In addition, the tetragonal distortion in this sample was too small to be resolved by the energy-dispersive technique. Therefore, data for the latter sample were not suitable for accurate equation-of-state analysis. Previous data on pyrope were compiled, analyzed, and compared with the new data on the majoritic garnet. The entire data set enabled us to examine systematics of the thermoelastic properties of garnets along the majorite-pyrope join. These data will have important applications for modeling the transition zone of the mantle. Published by Elsevier Science B.V.