A ternary solid solution model for omphacite and its application to geothermobarometry of eclogites from the Middle Adula nappe (Central Alps, Switzerland)

Abstract

A ternary solid solution model for omphacite with the end‐members jadeite (NaAlSi2O6), diopside (CaMgSi2O6) and hedenbergite (CaFeSi2O6) was derived from experimental data from the literature. The subregular solution model, fitted by linear programming, is best suited to omphacites with very little aegirine component in common eclogites. Applying this solution model to the calculation of equilibrium phase diagrams of eclogites from the Adula nappe (Central Alps, Switzerland) results in large stability fields for common eclogite assemblages (garnet+omphacite+quartz+H2O±kyanite). Within this field the compositions of garnet and omphacite show very little variation. A precise determination of the peak‐pressure and temperature is not possible. The occurrence of amphibole, overgrowing the peak‐pressure assemblage in fresh eclogite, suggests retrograde re‐equilibration, still under eclogite facies conditions. The computation of isopleths for garnet and pyroxene end‐members allows the estimation of the pressure and temperature conditions of this re‐equilibration event (19–21 kbar, c. 700 °C).