Theoretical estimates of equilibrium chromium-isotope fractionations

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

Division of Geological and Planetary Sciences, Caltech

Pasadena, CA 91125 USA


Equilibrium Cr-isotope (53Cr/52Cr) fractionations are calculated using published vibrational spectra and both empirical and ab initio force-field models. Reduced partition function ratios for chromium isotope exchange, in terms of 1000•ln(b53-52), are calculated for a number of simple complexes, crystals, and the Cr(CO)6 molecule. Large (> 1‰) fractionations are predicted between coexisting species with different oxidation states or bond partners. The highly oxidized [Cr6+O4] anion will tend to concentrate 53Cr when in equilibrium with compounds containing Cr3+ or Cr0. Substances containing chromium bonded to strongly-bonding ligands like CO will concentrate 53Cr relative to compounds with weaker bonds, like [CrCl6]. Substances with short Cr-ligand bonds (Cr-C in Cr(CO)6, Cr-O in [Cr(H2O)6]3+ or [CrO4]) will also tend to concentrate 53Cr relative to substances with longer Cr-ligand bonds ([Cr(NH3)6]3+, [CrCl6], and Cr-metal). These systematics are similar to those found in an earlier study on Fe-isotope fractionation (Schauble et al., 2001). The calculated equilibrium fractionation between Cr6+ in [CrO4] and Cr3+ in either [Cr(H2O)6]3+ or Cr2O3 agrees qualitatively with the fractionation observed during experimental (probably kinetic) reduction of [CrO4] in solution (Ellis et al., 2002), although the calculated fractionation (~6-7 ‰ at 298 K) does appear to be significantly larger than the experimental fractionation (3.3-3.5 ‰). Our model results suggest that natural inorganic Cr isotope fractionation at the Earth’s surface may be driven largely by reduction and oxidation processes.