The effect of water activity on calculated phase equilibria and garnet isopleth thermobarometry of granulites, with particular reference to Tongbai (east-central China)

Granulites form at low water activities whose exact value is often unknown. This complicates the calculation of equilibrium assemblage phase diagrams and metamorphic pressure (P) and temperature (T) conditions by isopleth thermobarometry, because the fluid characteristics distinctly affect the results. To evaluate the significance of this influence, granulites from Tongbai, east-central China are investigated in this study; equilibrium-assemblage calculations and multiequilibrium calculations are combined with fluid-inclusion studies to determine peak metamorphic P-T conditions together with water activities. Calculations using the Theriak/Domino and the TWQ programs on a felsic and twomafic granulites yield P-T conditions of 750-805C at 0.75-0.93 GPa and water activities of 0.10-0.65 for the equilibration of the mineral assemblages. Armored relics of Zn-rich Fe-Mgspinel included in garnet of the felsic granulite point to an earlier metamorphic stage at lower pressures on the same P-T loop. Garnet porphyroblasts from two samples contain primary fluid inclusions comprising vapor and daughter crystals. Using a Raman probe, CO2 and magnesite were identified in fluid inclusions of the felsic granulite while one mafic granulite yielded fluid inclusions with CO2 dominating besides CH4, H2O, graphite and siderite. The carbonic fluid is consistent with the implied low water activities. Possible CO2-sources are fluids from marbles associated with the granulites and fluids or magmas from the mantle. Equilibrium assemblage calculations reveal a distinct influence of the water activity on both the stability of considered phases and the location of compositional isopleths of the rock-forming minerals. We demonstrate that an estimate of the water activity using the Theriak/Domino and the TWQ programs in combination with Raman spectroscopy is essential to perform accurate thermobarometry in granulites