Excess Volume and Exsolution in Pyrope-Grossular Garnet

X-ray diffraction (XRD) was used to measure the unit cell parameters of pyrope (Mg3Al2Si3O12), grossular (Ca3Al2Si3O12) and four intermediate solid solutions synthesized at ~6 GPa and 1400°C in multi-anvil (MA) apparatus. Intermediate garnet solid solutions on this join show regular asymmetric positive excess volumes, but roughly 2-3 times bigger than previous reports from garnet hydrothermally synthesized in piston cylinder apparatus. The binary Margules equation used to fit this excess volume data gives parameters = 2.0±0.1 cm3/mol, which is about half as big as = 4.3±0.1 cm3/mol. Garnets synthesized in diamond anvil cells (DAC) with wider XRD peaks than those synthesized in MA have more nearly symmetrical excess volumes about 3 times larger than those previous piston cylinder syntheses. The large size difference between divalent Mg and Ca is the key reason for the non-ideal mixing properties of pyrope-grossular garnet solid solution. Partial order/disorder of Ca-Mg in the X site may be responsible for the interesting, variable mixing phenomena among different synthesis methods. The large excess volumes we measured from MA-synthesized garnets imply that the solvus of pyrope-grossular garnet will become experimentally accessible at high pressures, perhaps less than 10 GPa, unlike the expectations derived from the much smaller excess volumes of hydrothermally grown garnets. X-ray diffraction (XRD) methods and diamond anvil cells were used to measure the unit cell parameters of multi-anvil (MA) grown garnets: pyrope, grossular, and four intermediate solid solutions up to ~600°C and ~10 GPa. The unit cell parameters of these synthetic garnets increase with temperature and decrease with pressure. Thermal expansion coefficients of garnets in this series were calculated from the unit cell volumes' change with temperature. They are all in the range from ~2.0-2.8*10-5 K-1, and uniformly increase with temperature but differ with garnet compositions. At high temperature, the calculated thermal expansion coefficients of end-member pyrope and grossular in this study are larger than those reported by Skinner (1956). The compressional properties of these MA-synthesized pyrope and grossular are comparable with previous reports (Finger 1978; O'Neill et al., 1989; Zhang et al., 1998 and 1999; Pavese et al., 2001; Jiang et al., 2004). The Birch-Murnaghan EOS yields Κ0=165.4±1.8GPa, with Κ0’ fixed to be 5.92 for grossular and Κ0=172.5±2.0GPa for pyrope, with Κ0’ fixed to be 4.4. The bulk moduli of garnet with intermediate composition are all 155~160GPa, smaller than the end-members, showing no significant compositional dependence, as is consistent with the fact that garnets on this join have large positive excess volume, which makes them more compressible at high pressure. Application of these results indicates that the excess volumes in the pyrope-grossular series remain high even at high P and T. Multi-anvil (MA) technique was used to study the grossular-pyrope garnet solid solution for conditions of pressure and temperature stability against exsolution. Two garnet phases Py90Gr10 and Py40Gr60 with wt.% composition ratio 1:1, the expected consolute composition, were heated at 6GPa and different temperatures. XRD measurement results showed that these two garnet phases converged completely to one phase with composition ~Py65Gr35 at 1200°C, indicating that the critical temperature of pyrope-grossular garnet solvus is lower than 1200°C, lower than some literature modeling results (Haselton and Newton, 1980). At 8GPa, long term heating experiments for both convergence and divergence showed that two garnet phases with composition around ~Py82Gr18 and ~Py62Gr38 were equilibrated with each other at 1200°C. These garnet pairs represent the positions of the pyrope-grossular garnet solvus' two limbs at 1200°C and 8 GPa. Convergence experiments at 1100 °C and 8 GPa also showed changing composition of a widely-separated compositionally different garnet pair. However the equilibrium composition at 1100 °C and 8 GPa failed to be constrained by divergence heating experiments because the relatively low temperature and much slow diffusion rate of Mg/Ca cation exchange in garnet. Observations of garnet immiscibility at < 10 GPa reported here suggest that the MA-garnet excess volumes represent internal equilibrium values. Deduction from our new two phase equilibrium experiments shows that pyrope-grossular solvus has a higher critical temperature in the range 800-900°C at 1 bar compared to previous thermodynamic models (T < 600°C) (Ganguly et al., 1996), suggesting that at pressure as high as 2GPa, exsolution in garnet can happen at a higher temperature than previous thought, which is strongly supported by the high temperature (800-860ºC) exsolution in garnet samples found from natural metagabbro, South Harris (Cressey,1978), and an immiscible garnet pair in pyrope-rich garnet crystal collected from Garnet Ridge, Arizona was reported by Wang et al. (2000)