EMPIRICAL DETERMINATION OF OXYGEN ISOTOPE FRACTIONATION FACTORS FOR TITANITE WITH RESPECT TO ZIRCON AND QUARTZ

Abstract

Measurements of oxygen isotope fractionations between coexisting igneous titanite (Ttn) and zircon (Zrc) have been used to formulate a self-consistent, empirical calibration of equilibrium oxygen isotope partitioning: 1000 ln(Zrc Ttn) 1.02 10 6 /(T 2) This calibration is based on the average measured (Zrc-Igneous Ttn) 1.2 0.3‰ (n 27 rocks) and a closure temperature of titanite to oxygen diffusion of approximately 650°C. The average measured fraction-ation between zircon and metamorphic titanite is 2.1 0.4‰ (n 5 rocks). These results show that (Zrc-Ttn) can be used to distinguish igneous vs. metamorphic (or hydrothermal) titanite. The new zircon-titanite fractionation has been combined with published experimental and empirical data to provide a new quartz-titanite fractionation curve: 1000 ln(Qtz Ttn) 3.57 10 6 /(T 2) This new calibration is consistent with (Qtz-Igneous Ttn) data from coexisting mineral pairs in granitic rocks. Modeling diffusional exchange of oxygen during cooling in a typical granitic rock, using the new calibration, results in the same (Qtz-Igneous Ttn) as measured (4.5 0.43‰, n 25 rocks). Both measured (Zrc-Igneous Ttn) and (Qtz-Igneous Ttn) differ from fractionations calculated using the semiempirical increment method. The consistency of empirical (Zrc-Igneous Ttn) in a range of geologic environments allows the calculation of a more accurate fractionation factor for titanite.