We conducted a series of experiments to determine how decreasing the fugacity of CO2 affects the diffusivity of C and O in calcite. Previous experimental studies have shown that the diffusivity of O in calcite is ~ 2 orders of magnitude greater in the presence of nearly pure H2O than in a pure-CO2 environment. We reduced the fCO2 at constant T and p by dilution with either N2 or H2O to determine whether merely reducing fCO2 or adding H increases the rate of diffusion. Single cleavage fragments of calcite were enclosed in gold capsules with 13C18O2 and various amounts of H218O or Cu3N, a source of N2. The capsules were heated in cold-seal hydrothermal vessels to 700 °C at 100 MPa for periods up to 79 d. After experimentation, the 18O/(18O + 16O) and 13C/(13C + 12C) were measured by depth-profiling with SIMS ion probes. The results indicate that even a small amount of H2O (xCO2 = 0.8) caused an increase in DO from 10- 15.8 to 10-13.8 cm2/s. Greater proportions of H2O did not further increase DO appreciably. DC changed little from 10-17 cm2/s at xCO2 = 1.0, although the data are scattered. The effect of N2 on DO is similar to that for H2O. The effect on DC is more pronounced, though, with DC increasing apparently linearly from 10-17 cm2/s at xCO2 = 1.0 to 10-15.3 cm2/s at xCO2 = 0.2. That result is consistent with a model for C isotope exchange and diffusion controlled by a reaction at the surface of calcite for which the diffusivity is inversely proportional to fCO2. The diffusion of O is clearly enhanced by the presence of H2O, but the effect is similar for N2.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- 1041 Stable isotope geochemistry (0454;
- 1042 Mineral and crystal chemistry (3620);
- 3620 Mineral and crystal chemistry (1042)