An experimental study of the diffusion of C and O in calcite in mixed CO2-H2O fluid

The diffusivity of C and O in calcite in mixed CO 2 -H 2 O fluid was determined over the range in x CO ₂ from 1.0 to about 0.2 at 700 °C, 100 MPa, with selected experiments conducted at pressures to 250 MPa and temperatures of 600 and 800 °C. The diffusivity of C, D C , varies little with x CO ₂, although there is some evidence for a slight increase in DC from -5 × 10 −18 to -5 × 10 −17 cm 2 /s with decreasing x CO ₂. Our data and those of others are consistent with a model for DC ∝ 1/f CO₂ . Despite the large uncertainty, we observed that the diffusivity of O, DO, increases from ~2 × 10 −16 to ~5 × 10 −14 cm 2 /s with x CO ₂ decreasing from 1.0 to 0. There is a good correlation at 700 °C between log D O and log f H ₂ O regardless of the total pressure, matching the observations of previous workers. The data are consistent with a simple two-component model for the diffusion of O in calcite, one component for diffusion in the presence of CO 2 and one in the presence of H 2 O: D O = D O CO₂ + D O H₂O a H₂O . The activity of H 2 O is relative to the fugacity at 100 MPa, 700 °C. D O CO ₂ is 3.45 × 10 −16 , and D O H₂O is 3.8 × 10 −14 cm 2 /s. The data indicate that the rate of diffusion of C and O in calcite is controlled by reactions at the surface of calcite. Adsorption of H 2 O and the creation of vacancies at the surface account for the dependence of the diffusivity on the fugacity of the fluid components. There is little evidence that H itself diffuses into calcite. With this model and the values of D O in pure CO 2 (Labotka et al. 2000) and in pure H 2 O (Farver 1994), the value of D O is predicted over the temperature range 600-800 °C and p H₂O up to 300 MPa, the range of the data. Calculated closure temperatures for diffusive exchange of O between calcite and fluid are reduced by about 150 °C in the presence of an aqueous fluid.