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

Iron isotope fractionation between aqueous iron and siderite has been measured using abiotic synthesis experiments at similar to20 degreesC. 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 10(3)Inalpha(Fe(II)-Siderite)=+0.48 +/- 0.22parts per thousand (2sigma). This experimentally determined fractionation factor is significantly smaller (by similar to1.5parts per thousand) than that calculated from vibrational frequencies and Mossbauer 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 -1.7parts per thousand and +0.3parts per thousand [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 delta(56)Fe values (-1.5parts per thousand to -0.5parts per thousand), may reflect precipitation from Fe(II)(aq) whose origin was hydrothermal. In contrast, natural siderites that have higher delta(56)Fe 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. (C) 2004 Elsevier B.V. All rights reserved.