• journal_title：American Mineralogist
• Contributor：T.C. Labotka ; D.R. Cole ; L.R. Riciputi
• Publisher：Mineralogical Society of America
• Date：2000-
• Format：text/html
• Language：en
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：85
• issue：3-4
• firstpage：488
• section：Articles

The diffusivities of C and O in calcite were determined in a pure CO₂ atmosphere at 100 MPa and temperatures ranging from 600 to 800 °C. The calcite crystals were preannealed and H₂O was excluded from the system to determine the self-diffusion coefficients. The CO₂ consisted of 99% ¹³C and 90% ¹⁸O. After heating for 7–147 d, diffusion profiles were measured with the use of secondary ion mass spectrometry. The results indicate that the diffusivity of C is D_C = 7.77 × 10⁻⁹ exp (−166 ±16 kJ/mol/RT) cm²/s and of O is D_O = 7.5 × 10⁻³ exp (−242 ± 39 kJ/mol/RT) cm²/s. In comparison with other determinations of diffusivities in calcite, diffusion of O under the experimental conditions is consistent with vacancy migration in the intrinsic region, and diffusion of C seems to occur by diffusion of carbonate anions. Increased pressure appears to reduce the activation energy and the value of D₀, and the presence of H₂O greatly increases the diffusivity of O without appreciably changing the activation energy. Closure temperatures calculated for isotopic exchange by diffusion predict that C isotope compositions of calcite are preserved during cooling in most geologic environments, but that O isotope compositions in H₂O-rich environments are preserved only in rapidly cooling environments, such as contact metamorphic aureoles.