Iron isotopes constrain biologic and abiologic processes in banded iron formation genesis

Abstract

The voluminous 2.5 Ga banded iron formations (BIFs) from the Hamersley Basin (Australia) and Transvaal Craton (South Africa) record an extensive period of Fe redox cycling. The major Fe-bearing minerals in the Hamersley–Transvaal BIFs, magnetite and siderite, did not form in Fe isotope equilibrium, but instead reflect distinct formation pathways. The near-zero average δ 56Fe values for magnetite record a strong inheritance from Fe 3+ oxide/hydroxide precursors that formed in the upper water column through complete or near-complete oxidation. Transformation of the Fe 3+ oxide/hydroxide precursors to magnetite occurred through several diagenetic processes that produced a range of δ 56Fe values: (1) addition of marine hydrothermal Fe 2 + aq , (2) complete reduction by bacterial dissimilatory iron reduction (DIR), and (3) interaction with excess Fe 2 + aq that had low δ 56Fe values and was produced by DIR. Most siderite has slightly negative δ 56Fe values of ∼ −0.5‰ that indicate equilibrium with Late Archean seawater, although some very negative δ 56Fe values may record DIR. Support for an important role of DIR in siderite formation in BIFs comes from previously published C isotope data on siderite, which may be explained as a mixture of C from bacterial and seawater sources. Several factors likely contributed to the important role that DIR played in BIF formation, including high rates of ferric oxide/hydroxide formation in the upper water column, delivery of organic carbon produced by photosynthesis, and low clastic input. We infer that DIR-driven Fe redox cycling was much more important at this time than in modern marine systems. The low pyrite contents of magnetite- and siderite-facies BIFs suggests that bacterial sulfate reduction was minor, at least in the environments of BIF formation, and the absence of sulfide was important in preserving magnetite and siderite in the BIFs, minerals that are poorly preserved in the modern marine record. The paucity of negative δ 56Fe values in older (Early Archean) and younger (Early Proterozoic) BIFs suggests that the extensive 2.5 Ga Hamersley–Transvaal BIFs may record a period of maximum expansion of DIR in Earth’s history.