OXYGEN DIFFUSION IN QUARTZ - DEPENDENCE ON TEMPERATURE AND WATER FUGACITY

Oxygen self-diffusion coefficients (D) were determined in single crystals of Brazilian quartz by measuring O-18 concentration profiles with an ion microprobe. In the alpha-quartz field at 450-590-degrees-C and 100-MPa confining pressure, for diffusion parallel to c, the Arrhenius relation is described by the parameters D(O) = 2.9.10(-5) m2 s-1 and Q = 243 +/- 17 kJ (g-at. O)-1. The diffusion rate was also measured in beta-quartz from 5- to 350-MPa confining pressure and at 700-degrees-C as a function of water, hydrogen and oxygen fugacities as well as for different proton activities. There is good correlation of D with water fugacity, no correlation with oxygen nor hydrogen fugacities, and no correlation with proton activity over the range for which this parameter could be fixed independent of the water fugacity. The available evidence suggests that the oxygen-bearing species responsible for the rapid transport of oxygen under hydrothermal conditions is molecular water. The rate controlling step for oxygen diffusion may be either the rate of migration of water molecules, or the rate of oxygen exchange between the water molecules and the quartz structure.