Empirical determination of oxygen isotope fractionation factors for titanite with respect to zircon and quartz

Measurements of oxygen isotope fractionations between coexisting igneous ti tanite (Ttn) and zircon (Zrc) have been used to formulate a self-consistent , empirical calibration of equilibrium oxygen isotope partitioning: 1000 1n alpha (Zrc-Ttn) = 3.57 x 10(6)(T-2) This calibration is based on the average measured Delta (Zrc-Igneous Ttn) = 1.2 +/-0.3 parts per thousand (n = 27 rocks) and a closure temperature of titanite to oxygen diffusion of approximately 650 degreesC. The average mea sured fractionation between zircon and metamorphic titanite is 2.1 +/-0.4 p arts per thousand (n = 5 rocks). These results show that Delta (Zrc-Ttn) ca n be used to distinguish igneous vs. metamorphic (or hydrothermal) titanite . The new zircon-titanite fractionation has been combined with published expe rimental and empirical data to provide a new quartz-titanite fractionation curve: 1000 1n alpha (Qtc-Ttn) = 3.57 x 10(6)/(T-2) This new calibration is consistent with Delta (Qtz-Igneous Ttn) data from c oexisting mineral pairs in granitic rocks. Modeling diffusional exchange of oxygen during cooling in a typical granitic rock, using the new calibratio n, results in the same Delta (Qtz-Igneous Ttn) as measured (4.5 +/-0.43 par ts per thousand, n = 25 rocks). Both measured Delta (Zrc-Igneous Ttn) and D elta (Qtz-Igneous Ttn) differ from fractionations calculated using the semi empirical increment method. The consistency of empirical Delta (Zrc-Igneous Ttn) in a range of geologic environments allows the calculation of a more accurate fractionation factor for titanite. Copyright (C) 2001 Elsevier Sci ence Ltd.