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 <0.5kbar 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 ± 0.5kbar for titanite- and/or rutile-saturated samples, and to within ± 0.8kbar 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.