Primary Fe isotope signatures record oxidative precipitation in 3.2 Ga ferruginous siliciclastic sedimentary rocks deposited in a shallow ocean environment.

• Whole-rock Fe isotope ratios significantly vary in 3.2 Ga ferruginous rocks. • δ56Fe values correlate with Fe/Al and detrital matrix ratios in Fe-rich siltstones. • Fe isotope trend was consistent between magnetite- and carbonate-rich siltstones. • Small Fe isotope fractionation occurred during Fe2+-silicate precipitation. • Fe isotope signals preserve oxidative iron precipitation in a 3.2 Ga shallow ocean. Iron (Fe) isotopic compositions of Iron formations (IFs) have the potential to constrain the oceanic redox environment and marine biosphere on the early Earth. However, the interpretation of Fe isotope ratios in IFs is controversial and related to various factors, such as Fe sources, mode of primary precipitation, and subsequent mineral transformations. This paper presents whole-rock Fe isotope data for ca. 3.2 Ga unweathered ferruginous siliciclastic sedimentary rocks deposited in a shallow ocean in the lower part (unit MdI1) of the Moodies Group, Barberton Greenstone Belt, South Africa. We also experimentally examined Fe isotope effects during the precipitation of Fe2+-silicates (e.g., greenalite), proposed as primary Fe minerals in IFs. The Fe isotope data show significant variation (δ56Fe = −0.58 ‰ to +0.60 ‰) for different lithologies (i.e., magnetite-rich siltstone, carbonate-rich siltstone, sandy siltstone, and jaspilite). The δ56Fe values (δ56Fe = −0.54 ‰ to +0.60 ‰) of the magnetite-rich siltstones tend to decrease with decreasing Fe 2 O 3(T) /Al 2 O 3 ratios and matrix ratios (the percentage of detrital grains with a size of <30 μm). Carbonate-rich siltstones also fall on the same Fe 2 O 3(T) /Al 2 O 3 – δ56Fe and matrix ratio – δ56Fe trends as magnetite-rich siltstone. The synthetic experiment showed that isotope fractionation during anoxygenic Fe2+-silicate precipitation from dissolved ferrous Fe (Fe2+ (aq)) was much smaller (Δ56Fe Fe2+-silicate–Fe2+(aq) < +0.3 ‰) than that of oxidative precipitation. These results indicate that Fe isotopic variations in Fe-rich siltstones (magnetite- and carbonate-rich siltstones) are only explained by the oxidative precipitation of Fe2+ (aq) supplied from the deep ocean following Rayleigh-type fractionation. Low carbonate-C isotope ratios (δ13C carb = −5.8 ‰ to −3.7 ‰) of the Fe-rich siltstones show that magnetite and ankerite or Mg-siderite formed from a primary Fe3+-bearing mineral by oxidation of organic C after Fe burial. The consistent Fe 2 O 3(T) /Al 2 O 3 – δ56Fe trends between the magnetite- and carbonate-rich siltstones suggest that Fe reduction during diagenetic and/or metamorphic transformation processes of Fe-bearing minerals caused negligible changes in the whole-rock Fe isotope composition, possibly because of limited mobility of Fe2+ in the sediment. Consequently, the Fe isotope compositions predominantly record the primary precipitation process that occurred in the water column of a 3.2 Ga shallow ocean environment. [ABSTRACT FROM AUTHOR]