Abstract

An internally consistent set of thermodynamic models for pyroxenes, olivine, and quartz in the CMFS system has been calibrated simultaneously with orthopyroxene cation distributions, orthopyroxene solution enthalpies, and phase equilibria for coexisting pyroxenesand coexisting pyroxene + olivine + quarlz. New experiments on pyroxene + olivine+ quartz equilibria constrain the models. Preferred values of model parameters are used to generate component activities that are consistent with our choice of apparent standard-state energies. These values are modified from the compilation of Helgeson et al. (1978)to fit newer experimental data and to incorporate coefficients of compressibility and thermal expansion.

Calculated geothermometers include those based on equilibria involving one, two, or three pyroxenes. Isothermal, isobaric augite solvs limbs are displaced to higher Ca cohtentsrelative to Lindsley's (1983) graphs. Calculated geobarometers include those based on assemblages of olivine + qvartz + one, two, or three pyroxenes. The calculated temperature for coexisting olivine + qtrartz + orthopyroxene + augite + pigeonite is 818±20 °C, in agreement with previous experiments the pressure for this assemblage is given by P(inkbar)=9.1533.8XMgPig. Silica activity as determined from olivine-pyroxene exchange equilibria is shown to be a function not only of P, T, and Fe/(Fe + Mg) ratio of the assemblage, but also of the olivine (and pyroxene) Ca content, particularly for the olivine-augite exchange.

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