ABSTRACT

A model incorporating experimentally determined fluid-crystal cation exchange equilibrium constants is proposed to explain compositional variations in clinopyroxene from Japanese skarns. The proposed “Fluctuation Model” can account for the observed compositional heterogeneity in both individual and aggregate clinopyroxene grains on a thin section scale. The model assumes formation from a hydrothermal solution whose mean concentrations of magnesium, manganese and iron components remained essentially constant throughout crystal growth period at constant temperature and pressure. Compositional heterogeneity developed as a result of local spatial and temporal fluctuations about the mean solution component concentrations. The degree of fluctuation in the proportion of component i in the hydrothermal solution, as defined by the percent coefficient of variation, CV_i, is linearly related to the mean mole percent, M_i, of the component in the hydrothermal solution by the equation CV_i=0.25×(100-M_i). Various compositional patterns off naturally occurring skarn clinopyroxenes can be closely reproduced by the model.