Abstract:
Iron isotope fractionation between aqueous iron and siderite has been measured using abiotic synthesis experiments at ∼20 °C. Applying a Rayleigh distillation model to three experiments in which siderite (FeCO3) was continuously precipitated from solutions over the course of tens of hours, the derived fractionation factor is 103lnαFe(II)–Siderite=+0.48±0.22‰ (2σ). This experimentally determined fractionation factor is significantly smaller (by ∼1.5‰) than that calculated from vibrational frequencies and Mössbauer shifts [Geochim. Cosmochim. Acta 64 (2000) 849–865; Geochim. Cosmochim. Acta 65 (2001) 2487–2497], and lies on the high end of that inferred from natural mineral assemblages, where the Fe(II)aq–siderite fractionation factor was inferred to lie between −1.7‰ and +0.3‰ [Contrib. Mineral. Petrol. 144 (2003) 523–547]. The measured Fe(II)aq–siderite fractionation from this study suggests that siderite from Archean and Proterozoic rocks, which have moderately negative δ56Fe values (−1.5‰ to −0.5‰), may reflect precipitation from Fe(II)aq whose origin was hydrothermal. In contrast, natural siderites that have higher δ56Fe values apparently require non-hydrothermal sources of Fe and/or different pathways of formation, or may reflect the effects of cation substitution on the Fe(II)aq–carbonate fractionation factor.
