Coupled δ56Fe and δ18O analysis of the Frere iron formation, Western Australia, and Paleoproterozoic ocean circulation

• Published in: Precambrian research 2024-08, Vol.410, p.107461, Article 107461
• Main Authors: Raye, Urmidola, Pufahl, Peir K., Banerjee, Sandeep, Ricard, Estelle, Kurtis Kyser, T.
• Format: Article
• Language: English
• Subjects: Fe isotopes; Iron formation; O isotopes; Paleoproterozoic; Sequence Stratigraphy; Shelf Circulation
• ISSN: 0301-9268
• DOI: 10.1016/j.precamres.2024.107461

•The 1.89 Ga Frere granular Fe formation was deposited on a redox stratified shelf.•Trend in whole-rock and magnetite δ56Fe data tracks changes in sea level.•The δ56Fe of iron formations are a robust tracer for Paleoproterozoic ocean circulation.•(Ce/Ce*)PAAS holds a primary seawater signature in spite of terrestrial input.•Primary δ18O values of minerals were overprinted by diagenesis. The Paleoproterozoic (ca.1.89 Ga) Frere Formation is a ca. 300-m-thick sedimentary succession of five stacked, Fe- and silicate-rich shallowing upwards cycles or parasequences that were deposited during marine transgression. Deposition in an oxygen-stratified water column is supported by the occurrence of magnetite-rich mudstones (anoxic and distal) beneath shallower, hematite- and chert-rich grainstones (suboxic and proximal). Minor (LREE/HREE)PAAS depletion and positive Ce anomalies in grainstones support this interpretation. Sequence stratigraphic relationships suggest that the Frere Formation accumulated on an unrimmed continental shelf associated with coastal upwelling of Fe. Large variations in oxygen isotope ratios (δ18O) of magnetite (–0.6 ‰ ≤ δ18O ≤ +2.8 ‰, n = 12) and quartz (+8.8 ‰ ≤ δ18O ≤ +20.1 ‰, n = 11) from lithofacies composing the Frere Formation indicate that pristine δ18O of minerals were overprinted by diagenesis. Highly variable δ18O values calculated at 200 °C in equilibrium with quartz (–3.0 to + 8.3 ‰) and magnetite (+8.0 to + 12.7 ‰) suggest that fluids with heterogeneous δ18O evolved through oxygen isotope exchange between minerals and porewater in accumulating sediment during diagenesis. The Fe isotope ratios (δ56Fe) of bulk rock (–0.50 ‰ ≤ δ56Fe ≤ +1.55 ‰, n = 20) and magnetite separates (–0.09 ‰ ≤ δ56Fe ≤ +2.06 ‰, n = 11) from deeper anoxic and shallow suboxic paleoenvironments define a decreasing trend through stratigraphically stacked parasequences. The high δ56Fe values of magnetite (+0.7 ‰ to + 2.06 ‰) characterizing the base of the Frere Formation is interpreted to reflect equilibrium isotopic fractionation between dissolved Fe2+aq and freshly precipitated Fe-(oxyhydr)oxide (Fe(OH)3) resulting from sluggish recharge of Fe2+aq from deeper shelf environments. These high δ56Fe values of magnetite from shallow paleoenvironments may also reflect microbial dissimilatory Fe reduction at the seafloor. As sea level rose, continued deepening and encroachment of a well-mixed water mass increased the oxidation of Fe2+aq, which ultimately increased the