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.