Optimal mixing properties of calcic and subcalcic amphiboles: Application of Gibbs' method to the Sanbagawa schists, SW Japan

Optimal activities of amphibole are obtained by means of a new application of Gibbs' method to the assemblage amphibole-epidote-chlorite-plagioclase-quartz-water in the system are derived from the amphibole chemistries via the differences relative to a set of the reference conditions. Amphibole solid solutions are modeled by the regular solution composed of seven end-members (tr-ts-ed-gln-mrb-fact-otr), and the optimal Margules parameters are obtained from hundreds of natural data sets of amphiboles in the Sanbagawa schists in the range 300-550 °C, 0.2-1.1 GPa. The temperatures obtained by Gibbs' method are consistent with the temperatures by the hornblende-plagioclase thermometer with the absolute deviation of about 30 °C. The P-T conditions of calcic and subcalcic amphiboles are plotted in the stability fields of actinolite, hornblende, winchite, and barroisite of the greenschists and epidote-amphibolite facies conditions. Calculations for the representative zonings of amphiboles in the Sanbagawa schists suggest distinctive decompression P-T paths.