Two-phase fluid induced by N2 in metamorphic rocks

The first quantitative thermodynamic model of two-phase fluid induced by N2 and equilibrated with mineral assemblage in the system SiO2-TiO2-Al2O3-Fe2O3-MnO-MgO-CaO-Na2O-K2O-P2O5-H2O-CO2-N2 (STAFMMCNKPO-HCN) in the conditions of amphibolite facies rocks is proposed. Calculations are performed using the Gibbs free energy minimization technique. The fluid may consist of one phase, either aqueous solution (AS) or fluid with gas-like properties (FG), and also these two fluid phases (AS + FG) may co-exist, depending upon nitrogen content in the system. The values of total nitrogen at which AS co-exists with FG in the fluid were calculated at carbon saturation over a range in pressure and temperature of 3-7 kb and 550-650 °C. The model is capable of accurately describing various parameters of two fluid phases at each step of nitrogen increment in the system: density, partial pressure and mole fraction of constituents, pH, Eh, concentration of aqueous species. Increments of such an inert component as nitrogen cause the changes in the fluid resulting into subtle quantitative re-distribution of mineral phases in the assemblage plagioclase-biotite-quartz-ilmenite-garnet-apatite-graphite plus minor sillimanite and andalusite.