J. metamorphic Geol. 1983, 1, 331-336 A new geobarometer for the assemblage biotite-muscovite- chlorite-quartz ROGER POWELL, Department of Earth Sciences, University of Leeds, Leeds LS2 SJT, UK JANE A. EVANS,* Department of Geology and Mineralogy, University of Oxford, Parks Road, Oxford, UK Abstract. The equilibrium thermodynamics of the reaction : 4KMgA1Si40, o( OH), + Mg5A1,Si30, o( OH), muscovlte chlorite muscovite biotite = KA13Si,0,,(OH), + 3KMg3AISi,0,0(OH), + 7Si0, + 4H,O quartz fluid can be formulated as a geobarometer. Thermo- dynamic data for the Mg-A1 celadonite end- member of muscovite is derived from the reversed mineral equilibria experiments of Velde (I 965) using the calculation method of Powell (1978) giving AfH(1298) = -5823.5 kJ, S(1298)=0.3060kJK-' V(1298)= 13.8kJkbar-'. and For the above reaction: AGO =212.6-0.6116T- 1.02P+4F",,(kJ), (T in K, P in kbar) and the equilibrium constant is composed of activities formulated using ideal mixing on sites. Consideration is given to the evaluation of uncertainties in pressures calculated using the geobarometer. Preliminary testing suggests that the geobarometer has considerable potential. Much wider testing is now required. Key-words: biotite; chlorite; geobarometry; Mg-A1 celadonite; muscovite INTRODUCTION Metamorphic temperatures are usually reason- ably well established from geothermometry and * Now at Institute of Geological Sciences, 64-78 Grays Inn Road, London, WCI 02634929/83/120(r0331 $02.00 Q 1983 Blackwell Scientific Publications from petrogenetic grid considerations. On the other hand, metamorphic pressures are usually poorly constrained particularly at low grade. Minerals which have octahedrally coordinated aluminium tend to have a higher pressure stability than minerals which have tetrahedrally co- ordinated aluminium. Reactions between min- erals with octahedral A1 and minerals with tetrahedral Al might be expected to have low dP/dTs, and thus might be put into service as geobarometers. The muscovite (sensu luto) end- member, KMgA1Si40,,(OH)2, Mg-A1 cela- donite (here referred to as celadonite), has no tetrahedrally coordinated A1 and thus might be expected to become more significant in pro- portion in muscovites at higher pressures. This correlation is indeed observed, suggesting that reactions involving celadonite might be used as geobarometers. Thermodynamic data for cela- donite can be extracted from the experimental data of Velde (1 965). With these data, many new reactions involving, among others, quartz, alkali feldspar (orthoclase end-member), muscovite (muscovite and celadonite end-members), biotite (phlogopite end-member) and chlorite (clino- chlore end-member), can be considered. Thermodynamic data Velde (1965) performed a series of experiments on quartz-K-fFldspar-muscovite-biotite assemblages in systems of a variety of compositions. Results of the experiments in the Fe-free system are used here. The starting materials were synthetic minerals, oxide mixes or gels. Only the runs involving synthetic minerals can be expected to reflect the equilibrium relations. These experi- ments were performed on bulk compositions on the muscovite-celadonite join starting with either single phase muscovite or with muscovite, biotite, K-feldspar and quartz. Reaction to muscovite, biotite, K-feldspar and quartz or to single phase muscovite, respectively, properly brackets the stability of muscovite of the specified bulk 33 1