THE REACTION TITANITE + KYANITE = ANORTHITE + RUTILE AND TITANITE-RUTILE BAROMETRY IN ECLOGITES

Titanite and rutile are a common mineral pair in eclogites, and many equilibria involving these phases are potentially useful in estimating pressures of metamorphism. We have reversed one such reaction, CaTiSiO5 + Al2SiO5 = CaAl2Si2O8 + TiO2, Titanite Kyanite Anorthite Rutile using a piston-cylinder apparatus. Titanite + kyanite is the high-pressure assemblage and our results locate the equilibrium between 15.5-15.9, 17.7-17.9, 18.8-19.0, and 20.0-20.2 kb at 900, 1000, 1050, and 1100-degrees-C, respectively. The experiments require a positive dP/dT of between 20.5 and 23.5 bars/degrees-C for the reaction. We use the reversed equilibrium and two other reactions, 3Ca3Al2Si3O12 + 5TiO2 + 2SiO2 + H2O = Grossular Rutile Quartz 2Ca2Al3Si3O12(OH) + 5CaTiSiO5 Zoisite Titanite and Ca3Al2Si3O12 + 3TiO2 + SiO2 = 3CaTiSiO5 + Al2SiO5, Grossular Rutile Coesite Titanite Kyanite to calculate metamorphic conditions for three eclogite localities. Using these reactions in conjunction with garnet-clinopyroxene Fe2+-Mg exchange equilibria, conditions of metamorphism were 16 kb and 750-degrees-C for kyanite-eclogites from Glenelg, Scotland, 21 kb and 625-degrees-C for eclogite-facies mica schists from the Tauern Window, Austria, and 46 kb and 850-degrees-C for eclogite-facies biotite gneisses from the Kokchetav Massif, USSR. For the Scottish and Austrian eclogites, the pressures derived from the titanite-rutile reactions provide additional constraints on pressures for these localities, leading to precise estimates of metamorphic conditions. In the case of the Soviet Union eclogites, the results show that the silicate-oxide assemblage is consistent with the remarkable occurrence of diamond inclusions in the garnets. The results of this study suggest that titanite and rutile stably coexist in many eclogites and that titanite solid solutions are ideal or nearly so.