Thermodynamics of plagioclase-melt equilibria in the system albite-anorthite-diopside

An analysis of thermochemical and phase equilibrium data for high plagioclase solid solutions, glasses, and liquids in the system albite-anorthite-diopside produces an internally consistent model of mixing properties of crystals and melts. The mixing behavior of high temperature plagioclases is described adequately by positive heats of mixing and the aluminum-avoidance entropy model of Kerrick and Darken (1975). This description predicts plagioclase exsolution with compositions in the region of Bøgglid and Huttenlocher intergrowths but provides no evidence for the peristerite gap. Liquids in the region of the plagioclase saturation surface of the system albite-anorthite-diopside are shown to have entropies of mixing significantly less positive than predicted by the "two-lattice" model of Weill et al. (1980a), which appeared adequate for the pyroxene saturation surface of that system. Along the albite-anorthite binary these deviations are not consistent with aluminum avoidance in the liquid, but can be described by an empirical "regular" excess entropy term which allows a satisfactory calculation of the albiteanorthite binary liquidus loop. The magnitude of this negative excess entropy term can be explained by the assumption of strong Ca-Al charge balance (without restriction of Al-O-Al contacts) which leads to a vanishingly small entropy arising from Na-Ca mixing.