Calculation of oxygen isotope fractionation in anhydrous silicate minerals

Thermodynamic oxygen isotope factors for anhydrous silicate minerals have been calculated by means of the modified increment method. The obtained order of 18O enrichment in common rockforming minerals is quartz > albite ≥ K-feldspar > sillimanite ≥ leucite > andalusite > Jadeite > kyanite > anorthite ≥ cordierite > diopside > wollastonite > zircon ≈ garnet > olivine. Two sets of self-consistent fractionation factors between quartz and the silicate minerals and between the silicate minerals and water have been respectively obtained for a temperature range of 0 to 1200°C. The present results on the quartz-mineral systems are in good agreement with high-temperature data derived from semi-empirical calibrations and calcite-exchange experiments, demonstrating that the present calculated fractionation factors for mineral pairs are applicable to isotopic geothermometry and as a test of disequilibrium in natural mineral assemblages over all temperature ranges of geological interest. The silicate-water fractionations obtained from the present calculations also match those from hydrothermal experiments (except 3-isotope exchange results). The present calculations provide a potential insight into the quantitative dependence of oxygen isotope fractionation upon chemical composition and structural state of minerals. Minerals may be depleted in 18O with increasing pressure as a result of the change in crystal structures.