Oxygen regime of granulite metamorphism: Modeling by the method of Gibbs free energy minimization

Using a new version of the Selektor-C program package for the minimization of Gibbs free energy, physicochemical modeling was conducted for real mineral assemblages from the rocks of the Okhotsk and Chogar complexes and the Larba block, which crystallized under granulite-facies conditions. Considering a two-reservoir fluid-rock system, model assemblages of metapelites and metabasites adequate to natural assemblages were reconstructed by the method of Gibbs potential minimization. The P-T parameters of crystallization, oxygen potential, and the composition of the deep fluid that produced the assemblages were estimated. It was shown that the character of oxygen behavior can be dual under granulite-facies metamorphic conditions: inert behavior in rocks enriched in magnetite and (or) hemoilmenite and perfectly mobile behavior (after D.S. Korzhinskii) in rocks devoid of these minerals. It was shown that the oxygen regime is controlled by the degree of complete or partial leveling of oxygen potential between the deep reduced fluid and the rock in agreement with their oxygen capacities.