Garnet and clinopyroxene solid solutions, and geothermometry based on Fe-Mg distribution coefficient

The available thermodynamic mixing data of aluminosilicate garnet and clinopyroxene have been critically reviewed, and integrated with the thermochemical and selected experimental data to express the Fe-Mg distribution coefficient (K_D) between these phases as function of pressure, temperature and composition. The predicted compositional dependence of K_D agrees with the available experimental and observational data. Owing to the lack of adequate data on the mixing properties of Jadeite with diopside and hedenbergite, the geothermometric application of the model has to be currently restricted to the Na-poor bulk compositions. The temperature of a variety of rocks that have equilibrated under a wide range of P-T conditions have been estimated, and found to agree, on the average within 25°C, with other reliable temperature estimates of these rocks. The latter are, however, often in sharp disagreement with the temperatures determined on the basis of Råheim and Green's (1974, Contrib. Mineral. Petrol. 48, 179-203) experimental calibration of K_D as function of temperature and pressure on tholeiitic bulk composition.