Titanium in phengite: a geobarometer for high temperature eclogites

Phengite chemistry has been investigated in experiments on a natural SiO2-TiO2-saturated greywacke and a natural SiO2-TiO2-Al2SiO5-saturated pelite, at 1.5-8.0GPa 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 (KMgTi[Si3Al]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 crus