A revised thermodynamic model for ternary feldspars—an extension of the approach of Ghiorso (1984)—was made possible by (a) inclusion of volume data for ternary feldspars and (b) adjustment of the compositions (within analytical error) of some of the experimental data points (Seek, 1971a). The excess free-energy expressions of Newton et al. (1980) are used for the plagioclase binary join and those of Haselton et al. (1983) for alkali feldspar. Excess terms for the An-Or join and for ternary interaction were obtained from Seek's experiments by the method of linear least squares. The new model is consistent with most occurrences of natural ternary feldspars and with experiments on strongly ternary feldspars. Most ternary feldspars have negative excess volumes—an important constraint on pressure corrections in geothermometry.

The activity expressions of this model permit yet another formulation of the two-feldspar geothermometer that fully takes ternary solution into account; it yields three temperatures, one each for Ab, Or, and An equilibria. Few published analyses of feldspar pairs give concordant temperatures, probably because small errors in composition strongly affect the calculated temperatures. However, if the compositions of coexisting feldspars are allowed to vary within expected analytical error, three concordant temperatures can be calculated for feldspar pairs that were close to being in equilibrium. The program presented will also indicate pairs that could not have been in equilibrium. This thermometer is more useful than previous two-feldspar thermometers because it (1) fully accounts for ternary solid solution, (2) indicates whether the two feldspars could have been in equilibrium, and (3) gives further information about how the feldspar compositions may have been changed by such postcrystallization processes as alkali exchange and subsolidus exsolution. Our thermometer yields temperatures similar to those obtained using the formulation of Haselton et al. (1983) for pairs of feldspars close to binary compositions, and much more reasonable temperatures for strongly ternary compositions than any previous thermometer.

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