Carlson and Lindsley (1988) presented a model for the thermodynamic properties of the Mg2Si2O6-CaMgSi2O6 join that describes most of the known subsolidus phase relations up to 1600 °C and 60 kbar. They recognized that devising a model to cover this wide range required the adoption of a number of compromises, none of which violates the explicit data available for that join. The preferred values for their model parameters generate an extraordinarily complex P-T diagram, especially at pressures below 5 kbar (Fig. 1). Much of that complexity is illustrated by the 1-atm T-X diagram (Fig. 2a). The two coincidences of Pig and Opx compositions (one stable at 1387 °C, the other metastable at 1303 °C for 1 atm) result from complex intersections of the Opx and Cpx free-en-ergy curves. Complexities involving PEn result from the choice to make it ideal, in contrast to the strong nonidealities of Opx and Cpx. The very wide field for Opx at high temperatures results from the relatively low temperature for the inversion of Di to “orthodiopside.” Another effect of that low temperature is the predicted breakdown of Pig to Aug + Opx with increasing temperature at 1389 °C. They were aware of these complexities, but similar features arose in all variations on the model that satisfactorily replicated the experimental data over the entire range of pressures from 1 atm to 60 kbar. Because the goal of their modeling was to provide a thermodynamic framework encompassing the full range of experimental data on the Fe-free join, the complexities appearing at low pressure and high temperature were accepted as an unavoidable compromise. Those complexities were regarded as inconsequential to the principal goal of their model, inasmuch as the complexities occur predominantly above the solidus and therefore would not be stable.

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