Banded iron formations (BIF) preserve the elemental and isotopic signatures of the seawater in which they precipitate from. These reconstructions are supported by negligible changes in REE patterns in seawater throughout Earth’s history (Ernst & Bau, 2021). Preservation of seawater signatures in BIF can be demonstrated by NdCN/YbCN values > 1 (i.e., where CN denotes chondrite-normalised), enrichment of LREECN relative to HREECN, and by the presence of a W-type tetrad effect on REE abundances. However, most paleo studies focus on pristine BIF units and exclude hypogene and/or supergene altered BIF from sample suites due to the risk of primary signatures being disturbed by alteration fluids. While reasonable to assume that the precipitation of secondary minerals can affect bulk-rock signatures, conducting fraction-specific analyses of altered BIF greatly expands sample availability for paleo reconstructions and improves our understanding of the evolution of our oceans through time, where primary minerals are preserved. BIF mineralogy consists of three main fractions: carbonates (e.g., siderite), iron oxides (e.g., magnetite, hematite, and goethite) and silicates (e.g., quartz and Fe-silicates). Through sequential extraction techniques each fraction is targeted using reagents (sodium acetate, ammonium oxalate and HF-HNO3 digestions, respectively) to analyse major and trace element and isotopic signatures (Oonk et al., 2017). This study focusses on fraction-specific analysis of the ca. 2.7 Ga Weld Range BIF, Yilgarn Craton, Western Australia - with samples ranging from least-altered BIF (likened to pristine BIF) to progressively hypogene- and/or supergene-altered BIF. Sequential extractions have shown that seawater signatures are preserved across the carbonate, iron oxide and silicate fractions of the Weld Range BIF. Coupled with undisturbed textural relationships within the silicate fraction, the NdCN/YbCN values > 1 and the W-type tetrad effect supports a primary seawater origin and primary signatures. However, seawater signatures within the carbonate and iron oxide fractions are not considered to be primary due to secondary mineral textures, and lack of NdCN/YbCN values > 1 and W-type tetrad effects. Instead, these signatures are likely the product of hypogene alteration fluids, which contain an Archean seawater mixing component alongside magmatic-derived fluids (Duuring et al., 2018). This study shows that primary mineralogical fractions in altered BIF can serve as a reliable proxy and reservoir for understanding ancient ocean chemistry.

Reference(s)

Ernst & Bau (2021), Gondwana Research 91, 97-111 Oonk, Tsikos, Mason, Henkel, Staubwasser, Fryer, Poulton & Williams (2017) Chemical Geology 474, 17-32

Duuring, Hagemann, Banks & Schindler (2018) Ore Geology Reviews 93, 211-254.