Identification of biologically recycled continental materials in banded iron formations

Abstract:

The controversy on the origin of banded iron formations (BIFs) has lasted for many decades. Studies prior to the 1970s suggested that Fe in BIFs was supplied from continental riverine inputs[1], but discovery of midocean ridge hydrothermal systems in the 1970s and identification of positive Eu anomaly in BIF samples led to an alternative model where hydrothermal vents provided Fe in BIFs[2]. Although the latter model has became widely accepted, it should be noted that interpretations of Fe sources for BIFs using the abundance and isotopic composition of rare earth elements (REEs) are based on an assumption that transport and deposition of REEs and Fe were coupled.

We address the question of Fe sources and pathways for BIFs by combining stable Fe isotopes with radiogenic Nd isotopes as well as REE measurements to test proposals that Fe in BIFs was hydrothermally sourced. The samples investigated are from a type section of the Dales Gorge member of the 2.5 Ga Brockman Iron Formation, the world’s most extensive Superior-type BIF that represents the climax of BIF deposition in the geologic record. Large variations were observed in both Fe and Nd isotope compositions of the BIF samples, and there is a positive correlation between the bulk rock ε_Nd and δ⁵⁶Fe values. In addition, there is a negative corelation between ε_Nd and Sm/Nd ratios. In order to explain the observed correlations in those isotopic and elemental data, a two-component model, where mixing between a high ε_Nd, low Sm/Nd hydrothermal endmember and a low ε_Nd, low δ⁵⁶Fe, but high Sm/Nd continental endmember occurred prior to deposition of the BIF, is required. The low-δ⁵⁶Fe, high-Sm/Nd endmember is best explained by microbial dissimilatory iron reduction (DIR) in the coastal sediments, which fractionated Fe isotopes and REEs and released these components back to water column that were ultimately precipitated in BIFs. The range and distribution of ε_Ndvalues in the BIF samples suggest that the amount of continental Fe recycled by microbes was comparable to the amount of Fe provided by MOR hydrothermal activity in the ocean at 2.5 Ga, at least for the BIFs in Hamersley Group.

[1] Lepp H, Goldich SS (1964) Econ Geol 59(6):1025–1060.

[2] Bekker A, et al. (2010) Econ Geol 105(3):467–508