COMAGMAT - A FORTRAN PROGRAM TO MODEL MAGMA DIFFERENTIATION PROCESSES

The purpose of this paper is to present a general model (COMAGMAT) for the calculation of equilibrium temperatures and phase relations at a given extent of crystallization or melting in natural magmatic systems. This model is based on a set of empirical expressions that describe mineral-melt equilibria for major and trace elements in terms of pressure (up to 12 kbar), temperature, and liquid compositions for systems ranging from primitive basalts to dacites. These expressions are in the form of empirically calibrated mineral-melt geothermometers for Olivine, Augite, Pigeonite (Opx), Plagioclase, Ilmenite, and Magnetite used to develop the algorithm simulating multiply saturated magmatic melts. The results of the program are in the form of calculated liquid lines of descent, plus the equilibrium mineral proportions and compositions. The phase equilibria calculations form the core of a model that allows the user to simulate processes ranging from simple isobaric crystallization to in situ differentiation processes resulted from crystal settling, and polybaric fractionation.