The effect of water activity on calculated phase equilibria and garnet isopleth thermobarometry of granulites, with particular reference to Tongbai (east-central China) THOMAS BADER 1, * ,LEANDER FRANZ 2 ,CHRISTIAN DE CAPITANI 2 and LIFEI ZHANG 1 1 Key Laboratory of Orogenic Belts and Crustal Evolution, MOE, School of Earth and Space Sciences, Peking Universita ¨t, Yiheyuan Road No. 5, Beijing 100871, P.R. China *Corresponding author, e-mail: thomas.bader@vodafone.de 2 Mineralogisch-Petrographisches Institut, Universita ¨t Basel, Bernoullistrasse 30, 4056 Basel, Switzerland 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 com- bined 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. Key-words: Tongbai, granulite, phase diagram, water activity, fluid inclusion. 1. Introduction The determination of metamorphic temperatures (T) and pressures (P) by equilibrium assemblage phase diagrams and isopleth thermobarometry presumes equilibrium of the considered mineral and mineral assemblage, respectively, with the bulk-rock and, apart from rarely occurring dry metamorphism, with a fluid phase. Consequently, the pre- cision and reliability of calculated T and P critically depend on both. The bulk-rock composition might either be derived from XRF analysis or point counting and microprobe data of considered minerals (e.g. Zeh, 2001; Hoschek, 2004). The character of the fluid phase is harder to determine. Hints on the fluid’s nature (i.e. hydrous vs. carbonic) and its amount (i.e. limited vs. excess) are given, if primary fluid inclusions are preserved in peak meta- morphic phases, because they indicate both the presence and the composition of the fluid phase. However, if primary fluid inclusions are absent, the fluid composition has to be assumed. The problem particularly concerns granulites, which form at low water activities (a(H 2 O)) where a free, H 2 O-dominated fluid phase may be absent (e.g. Newton, 1986; White & Powell, 2002). Correspondingly, granulite facies fluids discovered so far are either carbonic (e.g. Touret, 1971, 2001; Santosh & Omori, 2008; Santosh et al., 2008) or high-salinity brines (Newton et al., 1998). If primary fluid inclusion have not been modified after entrapment, a(H 2 O) determined by microthermometry should be a suitable input for the cal- culations. In other cases, one has to find a method to determine a(H 2 O) together with T and P, because the assumption of a specific a(H 2 O)-value or a C-H-O ratio would increase the uncertainties of the P-T estimates. In this study we estimate T, P, and a(H 2 O) for mafic and felsic granulites from Tongbai in east-central China using equilibrium-assemblage calculations. The results are 0935-1221/13/0025-2351 $ 8.55 DOI: 10.1127/0935-1221/2013/0025-2351 # 2013 E. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart Eur. J. Mineral. 2014, 26, 5–23 Published online January 2014