OXYGEN DIFFUSION RATES IN QUARTZ EXCHANGED WITH CO2

Oxygen self diffusion rates were determined in quartz samples exchanged with O-18-enriched CO2 between 745 and 900-degrees-C and various pressures, and the diffusion profiles were measured using an ion microprobe. The activation energy (Q) and preexponential factor (D0) at P(CO2) = P(tot) = 100 bar, for diffusion parallel to the c-axis are 159 (+/- 13) kJ/g atom and 2.10 (+0.75/-0.55) x 10(-8) cm2/s. This rate is approximately 100 times slower than that obtained from hydrothermal experiments and 100 times faster than a previous 1-bar quartz-O2 exchange experiment. The oxygen diffusion rate measured at 0.6 bar, 888-degrees-C, and at 900-degrees-C in vacuum is in agreement with the previous 1-bar exchange experiments with O-18(2). The effect of higher CO2 pressures is small. At 900-degrees-C, the diffusion rate exchanged with CO2 is = 2.35 x 10(-15) cm2/s at 100 bar, 2.24 x 10(-15) cm2/s at 3.45 kbar and 8.13 x 10(-15) cm2/s at 7.2 kbar. There is probably a diffusing species, other than oxygen, that enhances the oxygen diffusion rate in these quartz-CO2 systems, relative to that occurring at very low pressures or in a vacuum. The effect of this diffusing species, however, is not as strong as that associated with H2O. Preserved oxygen isotope fractionations between coexisting minerals in a slowly cooled, high-grade metamorphic terrane will vary depending upon whether a water-rich phase was present or not. Closure temperatures will be approximately 100-degrees-C higher in rocks where no water-rich phase was present during cooling. The measured fractionations between coexisting minerals in metamorphic rocks may potentially be used as a sensor of water presence during retrogression.