Phase-equilibrium constraints on titanite and rutile activities in mafic epidote amphibolites and geobarometry using titanite-rutile equilibria

Activities of titanite (Ttn, CaTiSiO5) and/or rutile (Rt, TiO2) phase components were calculated for 45 well-characterized natural titanite- or rutile-undersaturated epidote-amphibolites by using the equilibria: (i) 3 anorthite + 2 zoisite/clinozoisite + rutile + quartz = 3 anorthite + titanite + water (referred to as TZARS) and (ii) anorthite + 2 titanite = grossular + 2 rutile + quartz (referred to as GRATiS). In titanite-bearing and rutile-absent samples a(Rt) is 0.75 +/- 0.26. In titanite-absent, rutile-bearing samples a(Ttn) is 0.89 +/- 0.16. Mean values derived for a(Rt)/a(Ttn) are 0.92 +/- 0.12 for rutile + titanite-bearing samples and 0.42 +/- 0.27 for samples lacking both titanite and rutile. Use of these values with TZARS yields pressure estimates for epidote-amphibolites that differ on average by < 0.5 kbar from those recorded by established mineral barometers, even where both titanite and rutile are lacking. Despite rather large uncertainties in the average values obtained for a(Rt), a(Ttn) or a(Rt)/a(Ttn), application of TZARS yields pressure estimates that agree with independent estimates to within +/- 0.5 kbar for titanite- and/or rutile-saturated samples, and to within +/- 0.8 kbar for samples that contain neither Ti-phase. The accuracy and precision of the TZARS barometer are comparable to that of many well-calibrated barometers. TZARS offers a much-needed barometer for mafic rocks metamorphosed at epidote-bearing amphibolite and blueschist facies conditions. In addition, the results provide a basis for application of other thermobarometers, such as Ti-in-zircon, where rutile activity is required as input.