Effects of changing solution chemistry on Fe/Fe isotope fractionation in aqueous Fe-Cl solutions

The range in ⁵⁶Fe/⁵⁴Fe isotopic compositions measured in naturally occurring iron-bearing species is greater than 5‰. Both theoretical modeling and experimental studies of equilibrium isotopic fractionation among iron-bearing species have shown that significant fractionations can be caused by differences in oxidation state (i.e., redox effects in the environment) as well as by bond partner and coordination number (i.e., nonredox effects due to speciation).To test the relative effects of redox vs. nonredox attributes on total Fe equilibrium isotopic fractionation, we measured changes, both experimentally and theoretically, in the isotopic composition of an Fe²⁺-Fe³⁺-Cl-H₂O solution as the chlorinity was varied. We made use of the unique solubility of FeCl₄⁻ in immiscible diethyl ether to create a separate spectator phase against which changes in the aqueous phase could be quantified. Our experiments showed a reduction in the redox isotopic fractionation between Fe²⁺- and Fe³⁺-bearing species from 3.4‰ at Cl⁻] = 1.5 M to 2.4‰ at Cl⁻] = 5.0 M, due to changes in speciation in the Fe-Cl solution. This experimental design was also used to demonstrate the attainment of isotopic equilibrium between the two phases, using a ⁵⁴Fe spike.To better understand speciation effects on redox fractionation, we created four new sets of ab initio models of the ferrous chloride complexes used in the experiments. These were combined with corresponding ab initio models for the ferric chloride complexes from previous work. At 20 °C, 1000 ln β (β = ⁵⁶Fe/⁵⁴Fe reduced partition function ratio relative to a dissociated Fe atom) values range from 6.39‰ to 5.42‰ for Fe(H₂O)₆²⁺, 5.98‰ to 5.34‰ for FeCl(H₂O)₅⁺, and 5.91‰ to 4.86‰ for FeCl₂(H₂O)₄, depending on the model. The theoretical models predict ferric-ferrous fractionation about half as large (depending on model) as the experimental results.Our results show (1) oxidation state is likely to be the dominant factor controlling equilibrium Fe isotope fractionation in solution and (2) nonredox attributes (such as ligands present in the aqueous solution, speciation and relative abundances, and ionic strength of the solution) can also have significant effects. Changes in the isotopic composition of an Fe-bearing solution will influence the resultant Fe isotopic signature of any precipitates.