THERMODYNAMIC ANALYSIS OF QUADRILATERAL PYROXENES - PART II: MODEL CALIBRATION FROM EXPERIMENTS AND APPLICATIONS TO GEOTHERMOMETRY

Abstract

The thermodynamics of quadrilateral pyroxene solutions have been analysed with the ternary non-convergent site-disorder model developed in Part I. Solution parameters have been refined for this model with a non-linear least squares technique to fit experimentally determined phase equilibria for the assemblages orthopyroxene-clinopyroxene (opx-cpx), pigeonite-augite (pig-aug), and opx-pig-aug. Calculated phase relations agree, within error, with experiments. Predicted cation distributions and enthalpies of solution for opx are also in agreement with measurements. Predicted cation distributions for cpx are more disordered than indicated by most available measurements. Two types of pyroxene thermometers are presented: (a) a single-pyroxene (aug) thermometer, and (b), a two-pyroxene thermometer which approximates the temperature of an observed pig-aug or cpx-opx assemblage as that of its nearest model tieline. Pyroxene pairs from two granulite suites, whose compositions were projected to the ternary system by the method of Lindsley and Andersen (1983), yield temperatures that are 25° C higher by thermometers (a) and (b) than determined from Lindsley's (1983) graphical thermometer. Temperatures calculated for projected compositions of coexisting pig-aug are within 20° C of run temperatures in experiments by Grove and Bryan (1983).