Fig. 3.

Iron isotope trends of selected Proterozoic ironstones. (A) Relative frequency histogram of δ⁵⁶Fe values. For the Chuanlinggou ironstone, these values show a cluster around 0.8‰ with gradually decreasing relative frequency toward more negative values. For comparison, the dark blue line represents the δ⁵⁶Fe distribution predicted for Fe(III) (oxyhydr)oxides produced by progressive oxidation according to Rayleigh distillation processes (e.g., ref. 68), assuming an initial Fe(II) source with δ⁵⁶Fe = 0‰ and an instantaneous fractionation of 0.8‰ for the combined processes of Fe(II) oxidation and precipitation as Fe(III) (oxyhydr)oxides. For the McClure ironstone, δ⁵⁶Fe values show a similar cluster around 0.8‰, but the tail of the distribution toward lower δ⁵⁶Fe values is not as well-developed. For the Galeros ironstone, δ⁵⁶Fe values cluster around 0‰ with a narrow range, interpreted to indicate quantitative oxidation of an unfractionated Fe(II) source. (B) Cross-plot diagram showing the relationship between Mn enrichment (Mn/Fe) versus δ⁵⁶Fe values, a proxy for degree of oxidation. For the Chuanlinggou ironstone, these data show that more negative δ⁵⁶Fe values are typically Mn-enriched and likely indicate a greater degree of water column oxidation. This trend is also less developed for the McClure ironstone, which lacks negative δ⁵⁶Fe values, and is absent in the Galeros ironstone.