Oxygen-isotope fractionation in titanium-oxide minerals at low temperature

Abstract

Oxygen-isotope results for natural and synthetic Ti-oxide minerals are presented. Rutile-water oxygen-isotope fractionation factors (α) of 1.0061 ± 0.0006 (1σ) at 22 °C and 1.0030 ± 0.001 ( 1σ) at 50°C have been obtained for synthesis experiments under a variety of conditions. These results are in close agreement with recent, semiempirical predictions and with values determined using naturally occurring rutile. Nevertheless, the true fractionation factors may lie toward the upper bounds of the error limits, given the possibilities that (1) isotopic equilibrium may not have been achieved in some experiments and that (2) hydration water may not have been removed completely from some samples prior to analysis. Anatase-water oxygen-isotope fractionation factors of 1.0087 ± 0.0015 (1σ) at 22°C and 1.0049 ± 0.0005 (1σ) at 50°C were obtained from synthesis experiments in which sulphate was not added to the starting solutions. Higher values of α for anatase-water vs. rutile-water are in accord with predictions based on theoretical and empirical considerations. However, fractionation factors increased dramatically to maxima of 1.0143 at 22°C and 1.0081 at 50°C for anatase-water syntheses in which sulphate ions were added to the starting solutions. The large increase in the δ 18O values of anatase produced in such solutions may result from donation of, or exchange with, high- 18O sulphate oxygen during formation of Ti-O-OH-SO 4 complexes. The range of fractionation factors obtained from the syntheses is mirrored by the range obtained for natural samples. This similarity suggests that the isotopic composition of anatase is controlled not only by its temperature of formation and the isotopic composition of associated water, but also by the mechanism of formation. Moreover, in cases where rutile formed by inversion of less stable anatase, rutile appears to at least partially inherit its isotopic composition from the precursor polymorph. However, coprecipitated quartz and rutile (formed directly, that is, in the absence of an anatase inversion) has potential as an oxygen-isotope geothermometer for low-temperature environments.