IRON ISOTOPE FRACTIONATION IN THE SYSTEM FE(III)-FE(II)-HEMATITE-MAGNETITE-FE CARBONATE, AND IMPLICATIONS FOR THE ORIGIN OF BANDED IRON FORMATIONS

            Banded Iron Formations (BIFs) record the largest range in ⁵⁶Fe/⁵⁴Fe ratios yet measured for ancient terrestrial rocks, and dFe values decrease in the order hematite ~ magnetite > siderite > ankerite.  New data for Fe isotope fractionation during formation of magnetite and Fe carbonates through reductive dissolution of hydrous ferric oxide by Geobacter sulfurreducens and Shewanella putrefaciens indicate that Fe carbonates will have ⁵⁶Fe/⁵⁴Fe ratios that are lower than Fe(II) in solution or the ferric oxyhydroxide starting material, whereas magnetite will have Fe isotope compositions ~ equal that of Fe(II) in solution; this is the same relative order of  dFe values as observed in BIFs.  Iron isotope fractionation factors measured during formation of magnetite by Geobacter sulfurreducens were dominated by kinetic effects in the initial ~20 days, followed by approach to equilibrium over the long term (164 days).  Long-term (~ 1 ½ years) experiments with Shewanella putrefaciens are interpreted to reflect Fe(II)-Fe carbonate fractionations that are close to those of equilibrium conditions

            Integration of our new data with previous results reported for Fe isotope fractionation produced by anoxygenic photosynthetic Fe(II) oxidation allows us to constrain Fe isotope fractionation factors in biologic systems for Fe(III)-Fe(II)-Hematite-Magnetite-Fe Carbonate.  Comparison of biologic fractionation in the system Fe(III)-Fe(II)-Ferric Oxide with that in “equivalent” abiotic systems suggests that there is an Fe isotope “vital” effect that is unique to biologically processed Fe.  The values for Fe(II)-mineral fractionations measured in the Geobacter sulfurreducens and Shewanella putrefaciens experiments differ from those inferred from natural samples or calculated from spectroscopic data, raising the possibility that Fe isotope fractionations in the system Fe(II)-magnetite-Fe carbonate, when mineral formation is catalysed by biologic activity, may reflect “vital” effects as well.

            Relative to expected Fe isotope compositions of the major sources for Fe(II) in the oceans in the Archean, the most likely fingerprint for a role by biology during the genesis of BIFs are the moderately positive dFe  values measured for some hematite samples that may be considered primary.