OXIDATION POTENTIAL AND THE COMPOSITION OF METAMORPHIC FLUID AS A SOLUTION TO THE INVERSE PROBLEM OF CONVEX PROGRAMMING

Abstract

Using the Selektor-C software program package, oxidation potential and the composition of metamorphogenic fluid were determined for mineral assemblages from nine samples of granulite-grade metamorphic rocks by solving the inverse problems of convex programming. The calculated and real mineral assemblages are in good agreement with respect to the composition and association of minerals, which is compelling evidence for the attainment of chemical equilibrium (minimum of Gibbs free energy) under given P-T conditions. Based on the dual solution of the inverse problem, a new approach was proposed for the estimation of the oxidation potential of fluid and mineral assemblages, which can be used to determine oxygen potential for almost any mineral association, independent of the presence of magnetite, ilmenite, or graphite. It was found that magnetite-free mineral associations are characterized by highly reducing conditions corresponding to oxygen potentials close to the CCO buffer. The external metamorphic fluid that was present during granulite-facies metamorphism was probably formed in the graphite stability field. The results of calculations for the model aqueous electrolyte solution-mineral assemblage suggest high SiO2 solubility in the metamorphogenic fluid. Therefore, the process of granulite metamorphism may be a potent geochemical factor of the redistribution and transportation of silica from lower to upper crustal levels.