Titanium in phengite: a geobarometer for high temperature eclogites |
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Authors: | Estelle Auzanneau M W Schmidt D Vielzeuf J A D Connolly |
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Institution: | 1. Institute for Mineralogy and Petrology, ETH, 8092, Zurich, Switzerland 2. Laboratoire Magmas et Volcans, OPGC, CNRS et Université Blaise Pascal, 5 rue Kessler, 63038, Clermont-Ferrand, France 3. CINaM, CNRS, Aix-Marseille University, Campus de Luminy, 13288, Marseille, France
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Abstract: | Phengite chemistry has been investigated in experiments on a natural SiO2–TiO2-saturated greywacke and a natural SiO2–TiO2–Al2SiO5-saturated pelite, at 1.5–8.0 GPa and 800–1,050°C. High Ti-contents (0.3–3.7 wt %), Ti-enrichment with temperature, and a
strong inverse correlation of Ti-content with pressure are the important features of both experimental series. The changes
in composition with pressure result from the Tschermak substitution (Si + R2+ = AlIV + AlVI) coupled with the substitution: AlVI + Si = Ti + AlIV. The latter exchange is best described using the end-member Ti-phengite (KMgTiSi3Al]O10(OH)2, TiP). In the rutile-quartz/coesite saturated experiments, the aluminoceladonite component increases with pressure while
the muscovite, paragonite and Ti-phengite components decrease. A thermodynamic model combining data obtained in this and previous
experimental studies are derived to use the equilibrium MgCel + Rt = TiP + Cs/Qz as a thermobarometer in felsic and basic
rocks. Phengite, rutile and quartz/coesite are common phases in HT-(U)HP metamorphic rocks, and are often preserved from regression
by entrapment in zircon or garnet, thus providing an opportunity to determine the T–P conditions of crystallization of these rocks. Two applications on natural examples (Sulu belt and Kokchetav massif) are presented
and discussed. This study demonstrates that Ti is a significant constituent of phengites that could have significant effects
on phase relationships and melting rates with decreasing P or increasing T in the continental crust. |
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