Oxygen diffusion in a fine‐grained quartz aggregate with wetted and nonwetted microstructures

Abstract

Oxygen grain boundary and bulk diffusion rates have been measured in a 1.2‐μm grain diameter quartz aggregate (Arkansas novaculite) preannealed in wetting and nonwetting fluids to determine the effect of microstructure. Samples were preannealed at 800°C and 150 MPa confining pressure for 6 to 12 days with pure water, pure CO2, or water + NaCl (∼6M) to produce equilibrium microstructures. The samples were then annealed at 450° to 800°C and 100 MPa confining pressure with 98%18O‐enriched water to determine the diffusion rates. Profiles of18O/(18O+16O) with depth from the surface were measured using an ion microprobe, and the product of the grain boundary diffusion coefficient (D′) and effective boundary width (δ) or the bulk diffusion coefficient (Dbulk) determined. Compared to the starting material, the D′δ values for samples texturally equilibrated with pure CO2(nonwetted microstructure) and pure water are about the same and about 4 times greater, respectively. Bulk diffusivities in samples preannealed with 6MNaCl (wetted microstructure) are 4 to 5 orders of magnitude greater than the bulk diffusivities in samples preannealed with pure CO2(nonwetted). In addition, the D'δ value for a sample texturally equilibrated first with 6MNaCl and then with pure CO2is about a factor of 3 greater than the value for the starting material, demonstrating that the textural equilibration process is essentially reversible. The results of this study indicate that transport rates are strongly influenced by the wetting characteristics of coexisting fluids in a polycrystalline aggregate. In nonwetted samples, changes in the effective grain boundary width or the formation of small isolated pores at triple junctions can change D′δ values. However, these effects are minor compared to the change in transport rate for samples with nonwetted versus wetted microstructures. The much faster rates for samples with a wetted microstructure are believed to be due to oxygen transport along the interconnected channels by ionic diffusion in a static fluid. The bulk diffusivities obtained for samples with a wetted microstructure are consistent with previously determined values for oxygen exchange at an igneous intrusive contact.