Reequilibration of fluid inclusions: Bulk-diffusion

Abstract

Bulk diffusion of fluid components through quartz, which is a nominally anhydrous mineral, is one of a series of processes that may alter fluid inclusions in crystals. Diffusion of water through quartz has been considered an important factor for re-equilibration of fluid inclusions. However, the solubility of water in quartz, the nature of the diffusing water-related molecules, and diffusion coefficients have not been adequately determined to calculate reliable rates of inclusion alteration. A new three-dimensional mathematical diffusion model is presented to characterize bulk diffusion in quartz with randomly distributed fluid inclusions. This model assumes an infinite external fluid source and treats fluid inclusions as instantaneous point sources. The computer program “ReqDif” from the software package “FLUIDS” ( http://fluids.unileoben.ac.at) can be used to calculate the effect of diffusion with self-defined parameters. Using a diffusion coefficient of 10 − 12 m 2 s − 1 (± 60%), the rate of H 2O loss or gain of most inclusions is relatively high, adapting them within several 10,000 of years to new pore fluid conditions. However, diffusion according to this model may be restricted and partly inhibited by several important factors, such as the low porosity in metamorphic rock and relative low temperatures in sedimentary and diagenetic rocks. The imprecision of estimated diffusion coefficients, and the undetermined accuracy do not allow reliable diffusion calculations. Diffusion along micro-cracks and dislocations is a more efficient process to transport fluid components through quartz.