Effects of bond environment on equilibrium iron isotope fractionation in aqueous iron chloride and iron sulfide solutions

Isotopic fractionation (56Fe/54Fe) up to -3.6‰ among naturally occurring iron-bearing species is often attributed to environmental redox effects. However, isotope theory also predicts fractionations due to differences in bond partner and coordination number. Here we examine, theoretically and experimentally, the relative effects of changing bond environment upon equilibrium mass-dependent iron isotope fractionation, using the aqueous iron chloride system as an example of an iron-ligand system.;Multiple theoretical models were developed for each ferric and ferrous chloride complex, using Unrestricted Hartree Fock and hybrid Density Functional Theory with different basis sets, from which beta's, the reduced partition function ratios, were calculated. 1000 ln beta decreases as the number of Fe-Cl bonds per complex increases.;Fractionations were measured experimentally in a series of low pH solutions of ferric chloride at chlorinities ranging from 0.5 to 5.0 M. Advantage was taken of the unique solubility of FeCl4- in immiscible diethyl ether to create a separate spectator phase, used to monitor changing fractionation in the aqueous solution. Delta56Feaq-eth = delta56Fe (total Fe remaining in aqueous phase) - delta56Fe (FeC4- in ether phase) was determined for each solution via MC-ICPMS analysis. Speciations models showed that species with higher Cl/Fe ratios become more abundant with increasing chlorinity, resulting in lower 1000 ln beta in the aqueous solution.;Both experiments and theoretical calculations of Delta56Fe aq-eth show a downward trend with increasing chlorinity: Delta 56Feaq-eth is greatest at low chlorinity, where FeCl 2(H2O)4+ is the dominant species, and smallest at high chlorinity where FeCl3(H2O) 3 is dominant. The experimental Delta56Feaq-eth , ranges from 0.8‰ at [Cl-]=0.5M to 0.0‰ at [Cl-] = 5.0M, a decrease in aqueous-ether fractionation of 0.8‰. Ab initio models predicted decreases in Delta56Fe aq-eth, ranging from 1.0 to 0.7‰, depending on model.;Experiments combining ferric and ferrous chloride in the aqueous solution showed an average reduction in the redox isotopic signature by ∼0.3‰%0/M Cl- for chlorinities rising from 1.5 to 5.0 M compared with a total 3.4 ‰ 56Fe/54Fe isotope ferric-ferrous fractionation at [Cl-] = 1.5M. Equilibrium was demonstrated using spiked reversal experiments.;Preliminary modeling of ferrous sulfides showed similar trends in Fe fractionation. We also looked at S fractionation in the ferrous sulfides, finding that beta for S isotopes in the ferric iron protein rubredoxin (Fe(S-cys) 4) were larger than the ferrous protein.;Our results show that oxidation state is likely to be the dominant factor controlling equilibrium Fe isotope fractionation in solution, but nonredox attributes (i.e., speciation in the solution) may also have significant effects and need to be considered when interpreting Fe isotopic signatures in the geologic record.