Contrib Mineral Petrol (1992) 110:346-354 Contributions to Mineralogy and Petrology 9 Springer-Verlag 1992 Geobarometers involving elinopyroxene, garnet, plagioelase, ilmenite, rutile, sphene and quartz: estimation of pressure in quartz-absent assemblages A. Mukhopadhyay, A. Bhattacharya, and L. Mohanty Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur 721 302, India Received October 9, 1990/Accepted September 12, 1991 Abstract. Enthalpy of formation and third law entropy for andradite (Ca3Fe2Si3012) and hedenbergite (CaFeSi206) at standard state have been retrieved from experimental P- T-f(O2) data on the stability rela- tions of these phases. The data for hedenbergite were combined with the thermodynamic data for related phases (Berman 1988) to formulate the geobarometers based on the reactions CaFeSi206 + 2CaA12Si208 + 2FeTiO3 = Ca3A12Si3012 + Fe3A12Si3012 + 2TIO2 (CPG - IR), 3CaFeSizO6 + 3CaA12Si208 + 3FeTiO3 = Ca3A12Si3012 + 2Fe3AI2Si3012+3CaTiSiO5 (CPG - IS) and 3CaFeSi206 + 3CaA12Si2Os = 2CaaA12Si3012 + Fe3A12Si3012 + 3SIO2 (CPG - O). The geobarometers involving rutile and sphene are proposed for the first time. In several granulite terranes, pressures have been estim- ated from the three geobarometers for basic granulites and charnockitic suite of rocks containing the above as- semblages. In clinopyroxene-garnet-plagioclase-quartz-il- menite-rutile assemblages, pressures obtained from CPG- IR are within + 500 bars of pressures calculated from the CPG - Q barometer. However, if rutile is absent, the pressures computed from the CPG-IR barometer are 300 to 1200 bars higher than those calculated from the CPG- Q barometer. For maximum differences in pressures ob- tained from the two equilibria, TiO2 activity [a(TiO2)] in the rocks is calculated to be 0.8. The sphene-bearing geobarometer (CPG-IS) also registers pressures compar- able ( < + 500 bars) to those obtained from the CPG-Q barometer. The close agreement in pressure values ob- tained from the CPG-IR and the CPG-IS equilibria with those registered by the CPG-Q geobarometer in assem- blages containing quartz implies that the rutile- and sphene-bearing geobarometers which do not involve quartz, can also be applied to estimate pressures from quartz-absent assemblages-assemblages which are not amenable to quantitative geobarometry using existing formulations. Offprint requests to: A. Bhattacharya Introduction Geobarometers based on the assemblages orthopyroxene (clinopyroxene)-garnet-plagioclase-quartz are widely em- ployed to estimate pressures in garnetiferous basic granulites and rocks of the charnockitic suite. In char- nockites of enderbitic and tonalitic compositions, which are fairly widespread, and in mafic granulites, quartz is often absent, i.e., silica activity [a(SiO2) ]. < 1 and, in such assemblages pressures computed from pyroxene (Cpx)- garnet (Gt)-plagioclase (Plag)-quartz (Qtz) geobarometers are higher than the "real" pressure values. This under- scores the need to estimate pressures for quartz-absent rocks from other reactions, preferably those which do not involve quartz. In the present study, two new geobaro- meters have been formulated based on the reactions CaFeSi206 + hedenbergite (hed) = Ca3A12Si3Ol2 + Fe~A12Si3012 grossular (gr) almandine (aim) 3CaFeSi206 + 3CaA12Si208 + 3FeTiO3 3 hedenbergite 3 anorthite 3 ilmenite 2CaAl2Si2Os + 2FeTiO3 2 anorthite (an) 2 ilmenite (il) + 2TIO2 2 rutile (A) = Ca3AI2Si3012 + 2Fe3AI2Si3012 + 3CaTiSiO5 grossular almandine 3 titanite (tit) (B) Ilmenite (Ilm), rutile (Ru) and sphene (Sph) are commonly present as accessories in basic granulites and the char- nockitic suite of rocks. Pressure values have been computed from the two equilibria using the internally consistent and updated compilation of thermochemical data for pure phases after Berman (1988) and updated activity-composition rela- tionships for clinopyroxene, plagioclase, garnet and il- menite. The pressure values obtained from (A) and (B) have been checked against those obtained from the