Self-diffusion of carbon and oxygen in calcite by isotope exchange with carbon dioxide

Abstract

Carbon self-diffusion coefficients were determined by exchange between calcite grains and a large reservior of C14-labeled CO2 at 250°–750°C. The progress of diffusion and the surface area of the grains were measured by the C14 uptake of the solid. Both carbon and oxygen self-diffusion coefficients were determined by exchange between calcite grains and a limited reservoir of C13- and O18-enriched CO2 at 650°–850°C. The progress of diffusion was measured by the change in isotope composition of the gas. On an Arrhenius plot of log D versus 103/T°K, the carbon self-diffusion coefficients are fitted to two straight lines. Carbon diffusivity determined by both methods in unannealed and annealed material at temperatures above 550°C are given by D = 1.3 × 103 exp(−88,000/RT)cm2/sec. Oxygen self-diffusion coefficients were higher than the corresponding carbon values by up to a factor of 4. Carbon diffusivity determined by the C14 method in unannealed calcite below 550°C are given by D = 4.6 × 10−16 exp(−17,000/RT)cm2/sec. Diffusivity in annealed material is lower by a factor of 50. The relative rates of oxygen and carbon self-diffusion indicate that there may be several mechanisms of oxygen migration in calcite in the high temperature region. A detailed examination of the C14 uptake results suggest that carbon self-diffusion is depth-dependent for the shallow penetration distances obtained in these experiments. Extrapolation of these results suggests that solid-state diffusion will not significantly alter the isotopic record of calcite under a wide range of geologic environments.