Abstract
A thermodynamic model for the enstatite-diopside join consistent with all experimental observations has been developed. Unlike previous models, this model reproduces exactly the phase relations determined by Carlson (1988) in the enstatite-rich portion of the join at 1 bar and 1295–1425 °C. The model fits the high-pressure data up to 60 kbar as well as previous models. However, only this model is consistent with the data at 100–152 kbar. Although the new model is similar in complexity to the models used before, the improvements were achieved by including second-order parameters related to the differences in heat capacity and compressibility. The model predicts the stability of all enstatite polymorphs, including the previously unrecognized stability of high clinoenstatite at temperatures below melting. The stability of orthopyroxene at 1 bar and high temperatures is confirmed. The ability of the model to predict the compositions of two coexisting pyroxenes at 100-160 kbar is essential for deciphering the mineral and chemical composition and the evolution of the Earth’s upper mantle.
