Oxygen diffusion rates in quartz exchanged with CO 2

Abstract

Oxygen self diffusion rates were determined in quartz samples exchanged with 18O-enriched CO 2 between 745 and 900°C and various pressures, and the diffusion profiles were measured using an ion microprobe. The activation energy ( Q) and preexponential factor ( D 0 ) at P(CO 2) = P(tot) = 100 bar, for diffusion parallel to the c-axis are 159 ( ± 13) kJ/g atom and 2.10 (+0.75/ −0.55) × 10 −8 cm 2/s. This rate is approximately 100 times slower than that obtained from hydrothermal experiments and 100 times faster than a previous 1-bar quartz-O 2 exchange experiment. The oxygen diffusion rate measured at 0.6 bar, 888°C, and at 900°C in vacuum is in agreement with the previous 1-bar exchange experiments with 18O 2. The effect of higher CO 2 pressures is small. At 900°C, the diffusion rate exchanged with CO 2 is = 2.35 × 10 −15 cm 2/s at 100 bar, 2.24 × 10 −15 cm 2/s at 3.45 kbar and 8.13 × 10 −15 cm 2/s at 7.2 kbar. There is probably a diffusing species, other than oxygen, that enhances the oxygen diffusion rate in these quartz-CO 2 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 H 2O. 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°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.