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

P. Kapp, C. E. Manning, Peter Tropper

Research output: Contribution to journalArticlepeer-review

22 Scopus citations

Abstract

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 aRt is 0.75 ± 0.26. In titanite-absent, rutile-bearing samples aTtn is 0.89 ± 0.16. Mean values derived for aRt/aTtn 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 aRt, aTtn or aRt/aTtn, 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.

Original languageEnglish (US)
Pages (from-to)509-521
Number of pages13
JournalJournal of Metamorphic Geology
Volume27
Issue number7
DOIs
StatePublished - Aug 10 2009

Keywords

  • Activity
  • Epidote amphibolites
  • Geobarometry
  • Rutile
  • Titanite

ASJC Scopus subject areas

  • Geology
  • Geochemistry and Petrology

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