Optimized standard state and solution properties of minerals

Abstract

 Internally consistent thermodynamic data for endmembers and solid solutions derived in Part I of this work are used to predict phase relationships and compositions of solid solutions in good agreement with direct experimental observations that were not used in the calibration. This “external” consistency with a large body of experimental observations increases confidence in applications to both petrogenetic grid and thermobarometric calculations. The predicted position of the univariant FMAS equilibrium Gt+Cd=Opx+Si+Qz occurs between 7.8 kbar – 700° C and 10.6 kbar – 1100° C, in excellent agreement with available phase equilibrium reversals, and with thermobarometric results for both higher-P Opx+Si+Qz bearing and lower-P Cd bearing granulites. In these assemblages, garnet composition is an excellent geobarometer and Al₂O₃ content of Opx is an excellent geothermometer, almost independent of other compositional variables. Comparison of temperature estimates from different exchange and net-transfer reactions for a number of samples representative of high-grade terranes demonstrates the ability of carefully chosen portions of Fe−Mg minerals to preserve information regarding a high temperature steady state during their evolution. The equilibrium Fs+Ok=Alm, based on the Al₂O₃ content of Opx, offers the most robust thermometry for the Opx−Gt assemblage because of its relative insensitivity to late Fe−Mg exchange. Applications of this thermometer indicate that in many samples the Al₂O₃ content of Opx yields very similar temperatures to Gt−Cd Fe−Mg exchange. For some high temperature samples, these temperatures are up to 150° C higher than calculated with the Gt−Opx Fe−Mg exchange thermometer.