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Phase‐equilibrium constraints on titanite and rutile activities in mafic epidote amphibolites and geobarometry using titanite–rutile equilibria
Author(s) -
KAPP P.,
MANNING C. E.,
TROPPER P.
Publication year - 2009
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1111/j.1525-1314.2009.00836.x
Subject(s) - titanite , rutile , mafic , geology , epidote , geochemistry , anorthite , mineralogy , quartz , metamorphic rock , chlorite , paleontology
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, 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.