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

Activities of titanite (Ttn, CaTiSiO 5 ) and ⁄ or rutile (Rt, TiO 2 ) 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, rutilebearing 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.