Oxygen isotope fractionation in TiO2 polymorphs and application to geothermometry of eclogites

Abstract

Oxygen isotope fractionation in TiO2 polymorphs has been calculated by the modified increment method. The results suggest that rutile is enriched in18O relative to brookite but depleted in18O relative to anatase. Due to the same crystal structure, oxygen isotope partitioning in the TiO2 polymorphs is determined by the cation-oxygen interatomic distances. The theoretical calibrations involving rutile are in fair agreement with known experimental measurements and empirical estimates. Application of the theoretical quartz-rutile calibration to geothermometry of natural eclogite assemblages indicates the preservation of isotopic equilibrium at high temperatures. The isotopic temperatures calculated are only slightly lower than the non-isotopic temperatures, indicating the slow rates of exchange for oxygen diffusion in rutile. The kinetics of exchange for oxygen diffusion in rutile is accordingly estimated by reconciling the differences between the isotopic and the non-isotopic temperatures. The rates of exchange for oxygen diffusion in rutile should be smaller than those for hornblende, but may be equal to or greater than those for diopside.