Theoretical estimates of equilibrium chlorine-isotope fractionations

Edwin A Schauble, George R. Rossman and Hugh P. Taylor, Jr.

Division of Geological and Planetary Sciences
California Institute of Technology, MS 170-25, Pasadena, CA 91125

Geochimica et Cosmochimica Acta




Equilibrium chlorine-isotope (37Cl/35Cl) fractionations have been determined using published vibrational spectra and force-field modeling to calculate reduced partition function ratios for Cl-isotope exchange. Ab initio force fields calculated at the HF/6-31G(d) level are used to estimate unknown vibrational frequencies of 37Cl-bearing molecules, whereas crystalline phases are modeled using published lattice-dynamics models. Calculated fractionations are principally controlled by the oxidation state of Cl and its bond-partners. Molecular mass (or the absence of C-H bonds) also appears to play a role in determining relative fractionations among simple Cl-bearing organic species. Molecules and complexes with oxidized Cl (i.e., Cl0, Cl+, etc.) will concentrate 37Cl relative to chlorides (substances with Cl-). At 298 K, ClO2 (containing Cl4+) and [ClO4]- (containing Cl7+) will concentrate 37Cl relative to chlorides by as much as 27 per mil and 73 per mil, respectively, in rough agreement with earlier calculations (Kotaka and Kakihana, 1977; Richet et al., 1977, Urey 1947) Among chlorides, 37Cl will be concentrated in substances where Cl is bonded to +2 cations (i.e., FeCl2, MnCl2, micas and amphiboles) relative to substances where Cl is bonded to +1 cations (like NaCl) by ~2-3 per mil at 298 K; organic molecules with C-Cl bonds will be even richer in 37Cl (~5-9 per mil at 298 K). Precipitation experiments (Eggenkamp et al., 1995), in combination with our results, provide an estimate for Cl-isotope partitioning in brines, and suggest that silicates (to the extent that their Cl atoms are associated with nearest-neighbor +2 cations analogous with FeCl2 and MnCl2) will have higher 37Cl/35Cl ratios than coexisting brine (by ~2-3 per mil at room temperature). Calculated fractionations between HCl and Cl2, and between brines and such alteration minerals, are in qualitative agreement with both experimental results and systematics observed in natural samples. Our results suggest that Cl-bearing organic molecules will have markedly higher 37Cl/35Cl ratios (by 5.8 per mil to 8.5 per mil at 295 K) than coexisting aqueous solutions at equilibrium. Predicted fractionations are consistent with the presence of an isotopically heavy reservoir of HCl that is in exchange equilibrium with Cl-aq in large marine aerosols, as inferred by Volpe (1998).