HR: 1340h
AN: MR13C-1405    [Abstracts]
TI: Effect of fCO2 on the Diffusion of C and O in Calcite at 700 °C, 100 MPa
AU: * Labotka, T
EM: tlabotka@utk.edu
AF: Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN 37996-1410, United States
AU: Cole, D
EM: coledr@ornl.gov
AF: Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, United States
AU: Fayek, M
EM: fayek@cc.umanitoba.ca
AF: Department of Geological Sciences, University of Manitoba, Winnipeg, MB R3T 2N2, Canada
AU: Chacko, T
EM: Tom.Chacko@ualberta.ca
AF: Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AT T6G 2E3, Canada
AB: 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 f_CO2 at constant T and p by dilution with either N₂ or H₂O to determine whether merely reducing f_CO2 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 (x_CO2 = 0.8) caused an increase in D_O from 10^{- 15.8} to 10^-13.8 cm²/s. Greater proportions of H₂O did not further increase D_O appreciably. D_C changed little from 10^-17 cm²/s at x_CO2 = 1.0, although the data are scattered. The effect of N₂ on D_O is similar to that for H₂O. The effect on D_C is more pronounced, though, with D_C increasing apparently linearly from 10^-17 cm²/s at x_CO2 = 1.0 to 10^-15.3 cm²/s at x_CO2 = 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 f_CO2. The diffusion of O is clearly enhanced by the presence of H₂O, but the effect is similar for N₂.
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1042 Mineral and crystal chemistry (3620)
DE: 3620 Mineral and crystal chemistry (1042)
SC: Mineral and Rock Physics [MR]
MN: 2007 Fall Meeting