Banded iron formations (BIFs) are particularly important when considering the surface environments of the early Earth, which were significantly different from the surface environments observed today. This is manifested in the mineralogy of these formations, which are indicative of different redox conditions. Additionally, BIFs are interesting in that they were deposited while major changes were occurring in the atmosphere and in the biosphere 2.8 to 1.8 billion years ago. Iron isotopic studies can reveal the details of the formation of BIFs, which allows for the reconstruction of redox and climatic conditions at the time of deposition.
BIFs formed as a result of the oxidation of iron by oxygen, which was likely generated by the evolution of cyanobacteria. This was followed by the subsequent precipitation of iron particles in the ocean. Observed variations in the iron isotopic composition of BIFs span the entire range observed on Earth, with δFe values between -2.5 and +1‰. The cause of these variations are hypothesized to occur for three reasons. The first relates to the varying mineralogy of the BIFs. Within the BIFs, minerals such as hematite, magnetite, siderite, and pyrite are observed. These minerals each having varying isotopic fractionation, likely as a result of their structures and the kinetics of their growth. The isotopic composition of the BIFs is indicative of the fluids from which they precipitated, which has applications when reconstructing environmental conditions of the ancient Earth. It has also been suggested that BIFs may be biologic in origin. The range of their δFe values fall within the range of those observed to occur as a result of biologic processes relating to bacterial metabolic processes, such as those of anoxygenic phototrophic iron-oxidizing bacteria. Ultimately, the improved understanding of BIFs using iron isotope fractionations would allow for the reconstruction of past environments and the constraint of processes occurring on the ancient Earth. However, given that the values observed as a result of biogenic and abiogenic fractionation are relatively similar, the exact processes of BIFs are still unclear. Thus, the continued study and improved understanding of biologic and abiologic fractionation effects would be beneficial in providing better details regarding BIF formation.