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

Abstract

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 Fe₂O_3(T)/Al₂O₃ ratios and matrix ratios (the percentage of detrital grains with a size of <30 μm). Carbonate-rich siltstones also fall on the same Fe₂O_3(T)/Al₂O₃ – 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 Fe₂O_3(T)/Al₂O₃ – 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.