ISOTOPIC CONSTRAINTS ON THE SOURCES OF FE IN BANDED IRON FORMATIONS

Deposition of the large, Superior-type Banded Iron Formations such as the ~2.5 Ga sequences in the Hamersley and Griquatown West/Transvaal basins of the Pilbara and Kaapvaal cratons, respectively, seem likely to reflect the confluence of 1) development of large stable continental shelves, 2) supply of copious quantities of aqueous Fe(II) to the oceans, and 3) establishment of strong redox gradients in shallow marine environments.  Iron isotope compositions of siderite- and magnetite-facies BIFs support a major Fe contribution from marine hydrothermal sources, and in some cases adjacent siderite and magnetite bands appear to have formed in Fe isotope equilibrium from a common Fe(II)-bearing fluid.  In other cases, however, magnetite appears to have formed from a fluid that had very low ⁵⁶Fe/⁵⁴Fe ratios, which most likely reflects dissimilatory Fe(III) reduction by bacteria.  Primary hematite has quite variable Fe isotope compositions, ruling out a continental source during weathering, but instead likely reflects variable oxidation of upwelling Fe(II)-rich deep marine waters.  Siderite appears to be dominated by hydrothermal Fe sources, although some uncertainty exists due to significant effects of carbonate stoichiometry on Fe isotope fractionation factors and the paucity of experimental calibrations.

Iron isotope variations in BIFs provide strong evidence of extensive redox cycling of Fe in shallow marine shelf environments.  Although the role for biology in BIF genesis has generally focused on Fe(II) oxidation, dissimilatory Fe(III) reduction of ferric oxide/hydroxide precipitates and formation of biogenic magnetite may have been the major biological role in BIF formation.  Given biomarker evidence for oxygenic photosynthesis by 2.7 Ga, it seems most likely that the biological role during oxidation was indirect, through production of O₂-bearing surface waters, although anaerobic photosynthetic Fe(II) oxidation, or even UV-photooxidation may have occurred.  The infrequent occurrence of Superior-type BIFs in the geologic record may reflect distinct periods of time where geologic conditions produced environments that were favorable for massive Fe redox cycling by bacteria.