Contrib Mineral Petrol (1985) 89:370 378 Contributions to Mineralogy and Petrology 9 Springer-Verlag1985 Energetics of hydration of cordierite and water barometry in cordierite-granulites A. Bhattacharya and S.K. Sen Department of Geology and Geophysics Indian Institute of Technology, Kharagpur 721302, India Abstract. The Gibbs free energy and volume changes attendant upon hydration of cordierites in the system magnesian cordierite-water have been extracted from the published high pressure experimental data at PH~o =Ptot~l, assuming an ideal one site model for H20 in cordierite. Incorporating the dependence of A G and AV on temperature, which was found to be linear within the experimental conditions of 500~176 and 1- 10,000bars, the relation between the water content of cordierite and P, T and fH~Ohas been formulated as ~crd __ H20-- [exp-~{64,775-32.26T+G~l'2ro-P(9xlO-4T-O.5142)}]§ The equation can be used to compute H20 in cor- dierites at PH~o < 1. Our results at different P, T and partial pressure of water, assuming ideal mixing of H20 and CO 2 in the vapour phase, are in very good agreement with the experimental data of Johannes and Schreyer (1977, 1981). Applying the formulation to determine X ~ in the H20 garnet-cordierite-sillimanite-plagioclase-quartz granu- lites of Finnish Lapland as a test case, good agreement with the gravimetrically determined water contents of cordierite was obtained. Pressure estimates, from a thermodynamic modelling of the Fe-cordierite - al- mandine - sillimanite - quartz equilibrium at Pn~o=0 and Pn~o=Ptotal, for assemblages from South India, Scottish Caledonides, Daly Bay and Hara Lake areas are compatible with those derived from the garnet- plagioclase-sillimanite-quartz geobarometer. Introduction The high grade assemblage cordierite-garnet-plagiocla- se-sillimanite-quartz with cordierites of varying water contents offers an opportunity for estimating fH2o dur- ing metamorphism. Experimental investigations on Mg- cordierites at PH2o=Ptotal (Schreyer and Yoder 1964; Newton 1972; Holdaway 1976; Duncan and Green- wood 1977; Johannes and Schreyer 1977; Mirwald and Offprint requests to: A. Bhattacharya Schreyer 1977; Mirwald et al. 1979) and P~2o<Ptotal (Johannes and Schreyer 1977, 1981) have shown that the stability of hydrous cordierite with respect to its high pressure equivalent garnet+sillimanite+quartz +H20 is a function of its water content. It has also been found that the amount of H20 incorporated in the cordieritc structure at any P and T is determined by the f~f~ in the co-existing fluid phase. Hence if the relation between f~2o and x~do can be uncovered from a thermodynamic analysis of the experimental data, both the parameters can be estimated at a known P and T. The bulk of the extant laboratory data was derived under the condition PH2o=Ptotal or an20=l. For PH2o<Ptotal conditions, which are so important in in- terpreting the natural assemblages, the volume of ex- perimental data is rather small. Hence it is desirable to apply thermodynamic methods for tackling the prob- lem of water barometry under a range of pressure and temperature. Such a thermodynamic analysis can serve as a complement to the experimental findings as well as a potent theoretical tool for water barometry. The relevant thermodynamic parameters will have to be extracted from the experimental data on the stability of hydrous cordierites at P~2o= Ptotal because of the extensive coverage under these conditions. Next they can be checked against and extended to laboratory data under Pn~o< Ptotal conditions. Previous attempts at such thermodynamic modell- ing of the interrelation between P, T, f~2o and X ~ra H20 have assumed that there is no volume change of cor- dierite upon hydration (Newton and Wood 1979; Mar- tignole and Sisi 1981; Lonker 1981). But crystal struc- ture studies on beryl, isostructural with cordierite, shows that hydrated beryl has a larger cell volume than its anhydrous counterpart (Gibbs et al. 1968). This in- dicates that thermodynamic modelling assuming the same molar volume for hydrous and anhydrous cor- dierite is likely to lead to erroneous f~o estimates. Neither have such models been tested or applied to hydration equilibria of pure Mg-cordierites at PH20 < Ptotal conditions. In the present communication we have extracted, from a theoretical analysis of high pressure experimen- tal data on Mg-cordierites at Pn2o=P~otal, the free en- ergy and volume change on hydration as a function of temperature and pressure. These data have been em-