Experimental determination of Fe isotope fractionation between aqueous Fe(II), siderite and “green rust” in abiotic systems

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 (FeCO 3) was continuously precipitated from solutions over the course of tens of hours, the derived fractionation factor is 10 3ln α Fe(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 δ 56Fe values (−1.5‰ to −0.5‰), may reflect precipitation from Fe(II) aq whose origin was hydrothermal. In contrast, natural siderites that have higher δ 56Fe 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.