CALCULATION OF FRACTIONATION FRACTORS FOR CARBON AND OXYGEN ISOTOPIC EXCHANGE IN SYSTEM CALCITE-CARBON DIOXIDE-WATER

Partition-function ratios for H2O18/H2O16, CO182/CO162, CaCO183/CaCO163, C13O2/C12O2, and CaC13O3/CaC12O3 have been calculated. Water vapor and carbon dioxide are treated as ideal gases, but vibrational anharmonicity effects are taken into account. The calcite partition function ratios were calculated by adopting the Giulotto-Loinger model for the acoustical, optical, and librational lattice modes; the orbital-valence force-field method was used for the carbonate ion vibrational frequencies. For oxygen isotopic exchange between CO2 and calcite, the calculated fractionations can be expressed as 1000 ln αo = -3.2798 + (1.0611 × 104/T) - (1.8034 × 106/T2), for the temperature interval 0-550°; for carbon exchange, the corresponding equation is 1000 ln αc = -2.4612+ (7.6663 × 103/T) - (2.9880 × 106/T2). The calculated fractionation factors for isotopic exchange in the systems carbon dioxide-calcite and carbon dioxide-water agree with the observed ones. The agreement is less satisfactory for the oxygen isotopic fractionations between calcite and water above 100°.