Fe isotope fractionation on FeS formation in ambient aqueous solution

Iron sulfide phases are the ultimate repository of iron and reduced sulfur in sediments. The sulfur isotope geochemistry of pyrite has had much to tell us about modern and ancient Earth environments and it is likely that Fe isotopes will too, once fractionations for key processes are known. We report the results of an experimental study of Fe isotope fractionation on precipitation of FeS, synthetic mackinawite, from excess aqueous Fe(II) solutions by addition of sodium sulfide solution at 2-40 °C. The results show a significant kinetic isotope effect in the absence of a redox process. No detectable effect of temperature on the fractionation factor was observed. The Fe isotope fractionation for zero-age FeS is ΔFe(II)-FeS = 0.85 ± 0.30‰ across the temperature range studied, giving a kinetic isotope fractionation factor of αFe(II)-FeS = 1.0009 ± 0.0003. On ageing, the FeS in contact with aqueous Fe(II) becomes progressively isotopically heavier, indicating that the initial fractionations are kinetic rather than equilibrium. From published reaction mechanisms, the opportunity for Fe isotope fractionation appears to occur during inner sphere ligand exchange between hexaqua Fe(II) and aqueous sulfide complexes. Fe isotope fractionation on mackinawite formation is expected to be most significant under early diagenetic situations where a readily available reactive Fe source is available. Since FeS(aq) is a key reactive component in natural pyrite formation, kinetic Fe isotope fractionations will contribute to the Fe isotope signatures sequestered by pyrite, subject to the relative rate of FeS2 formation versus FeS-Fe(II)(aq) isotopic equilibration.