High-pressure dissociation of Fe3Al2Si3O12 garnet:: phase boundary determined by phase equilibrium experiments and calorimetry

Phase transition of Fe3Al2Si3O12 garnet (almandine) was examined at pressure of 20.3-23.5 GPa and temperatures of 100-1600 degrees C in the oxygen fugacity of the iron-wustite buffet, using a multi-anvil apparatus. It was observed that almandine dissociates into a mixture of wustite, corundum and stishovite. The equilibrium dissociation boundary was tightly constrained at 20-21 GPa in the temperature of 1200-1600 degrees C. For thermodynamic calculation of the dissociation boundary, enthalpy of formation of almandine from the mixture of fayalite, corundum and quartz was measured at 702 degrees C by solution calorimetry in controlled atmosphere. Using the measured enthalpy of formation, dissociation enthalpy (Delta H(298)degrees,,) of almandine into wustite, corundum and stishovite was obtained as 178.94 +/- 5.57 kJ/mol. This enthalpy value was combined with published entropies of the relevant phases for the boundary calculation. The calculated dissociation boundary was in excellent agreement with that determined by the phase equilibrium experiments, and has a slightly negative dP/dT slope. The dissociation of almandine into the constituent oxides at 20-21 GPa shows a clear contrast to that of pyrope which decomposes into perovskite and corundum solid solutions at about 27 GPa. The results on the transitions of almandine and pyrope suggest that a recent proposal on stability of pyrope-almandine rich Earner at pressures up to about 50 GPa is unlikely. (C) 1998 Elsevier Science B.V.