Experimental study of ¹⁸O ¹⁶O partitioning between crystalline albite, albitic glass and CO₂ gas

Oxygen isotope partitioning between gaseous CO₂ (~ 1 bar) and crystalline albite and albitic glass has been measured at 750-950°C, using the gas-mineral exchange techniques of O'Neil and Epstein (1966) and Stolper and Epstein (1991). Convergence of oxygen isotope ratios of CO₂ and avoidance of surface-correlated fractionation effects is achieved in long runs (>200 days), using relatively coarse grain sizes. Equilibrium CO₂-crystalline albite oxygen isotope fractionation factors are: 4.74 ± 0.22 at 750°C, 3.77 ± 0.23 at 850°C and 3.36 ± 0.21 at 950°C. These values compare well with calculations based on the experimental calcite-albite data of Clayton et al. (1989) and the CO₂-calcite fractionation factors determined by Chacko et al. (1991) and Rosenbaum (1994). Our results, thus, provide independent support for the high pressure calcite-mineral fractionation factors of Clayton et al. (1989). An estimate of the reduced partition function ratio for albite derived from the CO₂-albite data using the reduced partition function ratio of CO₂ (Richet et al., 1977) differs by ~2% from that proposed by Clayton and Kieffer (1991). CO₂-albite exchange experiments of relatively short duration give disequilibrium fractionation factors. Oxygen diffusion coefficients calculated from these experiments, however, are comparable with previous determinations of oxygen diffusion in feldspars under nominally anhydrous conditions and support the hypothesis that isotopic exchange is diffusion-controlled. Equilibrium oxygen isotope fractionation factors determined for CO₂-albitic glass are identical within experimental uncertainty to those determined for CO₂-crystalline albite, thus indicating that fractionation between crystalline and glassy albite is unresolvable at the 0.1 %. level. In contrast, additional measurements of oxygen partitioning between CO₂ and silica glass confirm the results of Stolper and Epstein (1991) and imply that at equilibrium silica glass is ¹⁸O-enriched relative to quartz.