Abstract
High pressure high temperature transition of almandine garnet (Fe3Al2Si3O12) was examined at 20-23. 5 GPa at 1200-1600°C in the oxygen fugacity of iron-wustite buffer, using a multi-anvil apparatus. It was found that almandine dissociates into a mixture of wustite, corundum and stishovite. The dissociation boundary is placed at 20-21 GPa at 1200-1600°C with a slightly negative slope. Enthalpy of formation of almandine was measured by high temperature solution calorimetry and thus enthalpy of the almandine dissociation was obtained. The thermodynamically calculated dissociation boundary agrees with that determined by the phase equilibrium experiments. This dissociation boundary may be used as a pressure standard at around 20 GPa at high temperature. The present results in combination with those on pyrope suggest that almandine-pyrope garnet is stable in a depth range down to the upper part of the Earth's lower mantle.
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