Effects of water on P-V-T equation of state of pyrope

Abstract

High-pressure single-crystal/powder synchrotron X-ray diffraction was carried out on a hydrous pure magnesium pyrope (Mg3Al2Si3O12) containing 900ppmw H2O, synthesized at 4.0GPa and 1300K. The pressure-volume (P-V) single-crystal data from room pressure to 9.81GPa at ambient temperature were fitted by a third-order Birch-Murnaghan equation of state (BM-EoS) yielding a unit-cell volume of V0=1505.14±0.38Å3, an isothermal bulk modulus of K0=160±3GPa and its pressure derivative K′0=5.2±0.4. When fixing K'0=4.0, the data yielded V0=1504.58±0.32Å3 and K0=166±2GPa. The pressure-volume-temperature (P-V-T) EoS of the synthetic hydrous pyrope was also measured at temperatures up to 900K and pressures up to 16.75GPa, using a diamond anvil cell in conjunction with in situ synchrotron angle-dispersive powder X-ray diffraction. The P-V data at room temperature and in a pressure range of 0.0001–14.81GPa were then analyzed by a third-order BM-EoS and yielded V0=1505.35±0.25Å3, K0=161±2GPa, K′0=5.0±0.3. With K'0 fixed to 4.0, we also obtained V0=1505.04±0.29Å3 and K0=167±1GPa. Consequently, we fitted the P-V-T data with the high-temperature third-order BM-EoS approach and obtained the thermoelastic parameters of V0=1505.4±0.3Å3, K0=162±1GPa, K′0=4.9±0.2, the temperature derivative of the bulk modulus (∂K0/∂T)P=−0.018±0.004GPaK−1, and the thermal expansion coefficient at ambient conditions α0=(3.2±0.1)×10−5K−1. These properties were consistent with the thermal pressure EoS analysis. These new results on hydrous pyrope were also compared with previous studies of anhydrous pyrope. The main effect of hydration on pyrope is to decrease K0 and increase K'0 by increasing the vacancies or unoccupied volume in the structure. The entire dataset enabled us to examine the thermoelastic properties of important mantle garnets and this data has further applications for modeling the P-T conditions in the upper mantle of the Earth’s interior using deep mineral assemblages.