Measurement of oxygen grain boundary diffusion in natural, fine-grained, quartz aggregates

Grain boundary diffusion measurements using fine-grained, natural, monomineralic aggregates offer several distinct advantages over techniques previously employed. Pure quartz aggregates (Arkansas novaculite), with 1.2 and 4.9 μm diameter grains, were annealed at 450–800°C and 100 MPa confining pressure in 18O-enriched water. Profiles of 18O ( 18O + 16O) with depth from the surface were measured using an ion microprobe, and data were collected from an area 68 μm in diameter in order to obtain a well-averaged value for many grain boundaries. Using graphical solutions appropriate to the boundary conditions employed, the product of the average grain boundary diffusion coefficient ( D′) and effective boundary width (δ) is obtained and is independent of the grain size, geometry, and grain boundary tortuosity. Arrhenius parameters for the 1.2 and 4.9 μm grain size samples are: D′ 0 δ = 2.6 and 3.4 × 10 −17 m 3/sec, and Q = 27 ± 1 and 26 ± 3 kcal/mol, respectively. Measured values of D′ δ were about three times greater for the 4.9 μm aggregate, although this might be due in part to thermal cracking while going to temperature. The activation energy of both samples, ~ 27 kcal/mol ( 113 kJ/mol), is significantly less than that for volume diffusion of oxygen in quartz, but greater than that for ionic diffusion in a static fluid. Measured D′ δ values are within the range of most previous estimates. For a representative effective grain boundary width of 1 nm, the oxygen grain boundary diffusion coefficients are 4 to 6 orders of magnitude greater than oxygen volume diffusion coefficients in quartz single crystals, and ~6 orders of magnitude less than ionic diffusion coefficients in a static fluid, over the temperature range of the experiments.