Titanium in phengite: a geobarometer for high temperature eclogites

• Published in: Contributions to mineralogy and petrology 2010, Vol.159 (1), p.1-24
• Main Authors: Auzanneau, Estelle, Schmidt, M. W., Vielzeuf, D., D Connolly, J. A.
• Format: Article
• Language: English
• Subjects: Continental crust; Crystallization; Earth and Environmental Science; Earth Sciences; Geochemistry; Geology; High temperature; Metamorphic rocks; Mineral Resources; Mineralogy; Original Paper; Petrology; Quartz; Sciences of the Universe; Temperature; Titanium; Titanium dioxide
• ISSN: 0010-7999; 1432-0967
• DOI: 10.1007/s00410-009-0412-7

Phengite chemistry has been investigated in experiments on a natural SiO 2 –TiO 2 -saturated greywacke and a natural SiO 2 –TiO 2 –Al 2 SiO 5 -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 + R 2+  = Al IV  + Al VI ) coupled with the substitution: Al VI  + Si = Ti + Al IV . The latter exchange is best described using the end-member Ti-phengite (KMgTi[Si 3 Al]O 10 (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.