A model for Cryogenian iron formation

•Widely separated Cryogenian iron formations share both depositional mechanisms and iron sources.•Iron was sourced from contemporary margin sediments dominated by basaltic detritus.•Cryogenian iron formations are uniquely associated with the Sturtian Snowball Earth episode.•δ56Fe reflects redox characteristics of the chemocline not the chemical evolution of the water mass.

The Neoproterozoic Tatonduk (Alaska) and Holowilena (South Australia) iron formations share many characteristics including their broadly coeval (Sturtian) ages, intimate association with glaciogenic sediments, and mineralogy. We show that these shared characteristics extend to their neodymium (εNd) and iron isotope (δ56Fe) systematics. In both regions δ56Fe values display a distinct up-section trend to isotopically heavier values, while εNd values are primitive and similar to non-ferruginous mudstones within these successions. The δ56Fe profiles are consistent with oxidation of ferruginous waters during marine transgression, and the εNd values imply that much of this iron was sourced from the leaching of continental margin sediments largely derived from continental flood basalts. Rare earth element data indicate a secondary hydrothermal source for this iron.

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