Studies in diffusion,II. Oxygen in phlogopite mica

Self-diffusion of oxygen in a natural phlogopite mica (annite 4%) has been measured under hydrothermal conditions at 2000 bars pressure and from 500 to 800°C using water enriched in¹⁸O. Diffusional transport is dominantly parallel to the c crystallographic axis. A linear Arrhenius plot was obtained with a pre-exponential term = (1.03 ± 0.38) × 10^?9cm²sec^?1 a and an activation energy of 29 ± 2kcal/g-atom O. The difference in transport rate between oxygens in the OH groups and those in tetrahedral sites is small to non-existent unless the OH oxygens diffuse much more slowly than the other oxygens, which we consider unlikely. A typical phlogopite crystal, 0.2 mm thick by 1 mm across will lose radiogenic argon faster than it will exchange oxygen at temperatures above 435°C, but the reverse holds at lower temperatures if the diffusion mechanism can be extrapolated to temperatures below 500°C. Such a crystal will lose only 5% of its argon if held at 380°C for 1 m.y., but could exchange 27% of its oxygen in that time. The rate at which phlogopite will undergo deformation by diffusional creep does not appear to be controlled by oxygen diffusion.