An experimental study of oxygen isotope partitioning between silica glass and CO_2 vapor

Abstract

The fractionation of oxygen isotopes between CO_2 vapor and silica glass was determined at a total pressure of ~0.5 bars at temperatures of 550-950°C. Experiments were conducted by equilibrating a small amount of CO_2 gas with a large amount of silica glass of known isotopic composition. Because most of the oxygen in the system is in the glass, its oxygen isotope ratio changes negligibly over the course of the experiment, and the fractionation factor (a) can be determined by measurement of the isotopic composition of CO_2 in the vapor at the end of the experiment. Results are independent of the grain size of the silica glass and of time in long duration runs; these are among the criteria used to conclude that isotopic equilibrium was achieved. The δ^(18)O value of CO_2 vapor is higher than that of coexisting silica glass at equilibrium. The fractionation factor decreases from 1.0042 ± 0.0002 at 550°C to 1.0022 ± 0.0002 at 950°C. Ln(α) is linear with 1/T over the temperature range we investigated, corresponding to a standard state enthalpy change for the isotopic exchange reaction of approximately -10 cal/mole. The reduced partition function ratio for silica glass is well described by multiplying that of crystalline quartz by 1.035. Comparison of our results with data in the literature on crystalline quartz suggests that silica glass is enriched in ^(18)O relative to quartz with which it is in isotopic equilibrium by 0.3-0.6 per mil over the temperature range we have investigated. This appears to confirm previous suggestions that oxygen isotopic fractionations between crystalline and amorphous materials of the same composition and similar short-range structures are small. In contrast to the CO_2-silica glass fractionation factor, the logarithm of the crystalline quartz-silica glass fractionation factor is expected to be roughly proportional to 1/T^2 over the temperature range of this study. Experiments were also conducted to determine the kinetics of oxygen isotopic exchange between CO_2 vapor and silica glass. The activation energy for apparent self-diffusion of oxygen in the glass is similar to those for diffusion of Ar and molecular CO_2, O_2 , and H_2O in silica-rich glasses. This suggests that isotopic exchange in our experiments occurs by diffusion of CO_2 molecules into the glass followed by exchange with the oxygen atoms of the glass structure and that the rate limiting step is the diffusion of the CO_2 molecules, not their reaction with the glass network.

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