Formation of Al-rich titanite (CaTiSiO4O–CaAlSiO4OH) reaction rims on ilmenite in metamorphic rocks as a function of fH2O and fO2 |
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Authors: | Daniel Harlov Peter Tropper Wolfgang Seifert Timo Nijland Hans-Jürgen Frster |
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Institution: | aGeoForschungsZentrum Potsdam, Telegrafenberg, D-14473 Potsdam, Germany bDepartment of Earth and Atmospheric Sciences, Institute of Mineralogy and Petrography, University of Innsbruck, Innrain 52, A-6020 Innsbruck, Austria cNational Museum of Natural History Naturalis, PO Box 9517, NL-2300 RA Leiden, The Netherlands dInstitute of Earth Sciences, University of Potsdam, PF 601553, D-14415 Potsdam, Germany |
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Abstract: | Reaction rims of titanite on ilmenite are described in samples from four terranes of amphibolite-facies metapelites and amphibolites namely the Tamil Nadu area, southern India; the Val Strona area of the Ivrea-Verbano Zone, northern Italy, the Bamble Sector, southern Norway, and the northwestern Austroalpine Ötztal Complex. The titanite rims, and hence the stability of titanite (CaTiSiO4O) and Al–OH titanite, i.e. vuaganatite (hypothetical end-member CaAlSiO4OH), are discussed in the light of fH2O- and fO2-buffered equilibria involving clinopyroxene, amphibole, biotite, ilmenite, magnetite, and quartz in the systems CaO–FeO/Fe2O3–TiO2–SiO2–H2O–O2 (CFTSH) and CaO–FeO/Fe2O3–Al2O3–SiO2–H2O–O2 (CFASH) present in each of the examples. Textural evidence suggests that titanite reaction rims on ilmenite in rocks from Tamil Nadu, Val Strona, and the Bamble Sector originated most likely due to hydration reactions such as clinopyroxene + ilmenite + quartz + H2O = amphibole + titanite and oxidation reactions such as amphibole + ilmenite + O2 = titanite + magnetite + quartz + H2O during amphibolite-facies metamorphism, or, as in the case of the Ötztal Complex, during a subsequent greenschist-facies overprint. Overstepping of these reactions requires fH2O and fO2 to be high for titanite formation, which is also in accordance with equilibria involving Al–OH titanite. This study shows that, in addition to P, T, bulk–rock composition and composition of the coexisting fluid, fO2 and fH2O also play an important role in the formation of Al-bearing titanite during amphibolite- and greenschist-facies metamorphism. |
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Keywords: | Titanite Al–OH titanite Oxygen barometry Water barometry Tamil Nadu Ivrea-Verbano Zone Bamble Sector Ötztal Complex |
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