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

Abstract

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 two mafic granulites yield P - T conditions of 750–805°C 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-Mg-spinel 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, CO 2 and magnesite were identified in fluid inclusions of the felsic granulite while one mafic granulite yielded fluid inclusions with CO 2 dominating besides CH 4 , H 2 O, graphite and siderite. The carbonic fluid is consistent with the implied low water activities. Possible CO 2 -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.