Fe and Si isotopic composition of banded iron formation determined by femtosecond laser ablation

The investigation of stable Fe and Si isotopes in banded iron formations (BIFs) can refine our understanding of the Precambrian Fe and Si cycle in the ancient ocean. BIFs are the product of initial precipitation from seawater and subsequent depositional, diagenetic and metamorphic processes. In order to study these processes we have determined Fe isotopes ratios in Fe-oxides and Si isotopes ratios in cherts from two BIFs of different ages and composition. The development of a UV-femtosecond laser ablation system coupled to a MC-ICP-MS provides us now with a tool to determine δ^{56}Fe and δ^{30}Si values in situ at a spatial resolution of 30μm and less with a precision of 0.1‰. The formation of BIFs involves redox reactions, dissolution and subsequent precipitation of Fe resulting in the fractionation of Fe isotopes. The hematite-chert BIF of the Proterozoic Transvaal Supergroup shows a maximal range in δ^{56}Fe from -1.17 to -0.27‰ for hematite in a single microband within the same stratigraphical level. A similar range is observed in several sequenced microbands while the average Fe isotopic composition remains surprisingly constant with δ^{56}Fe being -0.41±0.13‰ (2σ). The carbonate-magnetite-chert BIF of the Archean Shurugwi Greenstone exhibits a similar heterogeneity in the Fe isotopic composition for magnetite in single magnetite-rich bands showing a variation of up to 0.40‰ in δ^{56}Fe while the average of the bands is again uniform with δ^{56}Fe being +0.73±0.10‰ (2σ). The observed isotopic inhomogeneities are in part the result of isotopic zonation of the individually analysed magnetite crystals. The small-scale variability in the Fe isotope composition suggests relocation of Fe during diagenesis and metamorphism associated with recrystallization and crystal growth. This process took place locally and varied in both intensity and reaction rate at a sub- millimetre scale. However, the overall process and the Fe sources remained steady over at least several periods of microband formation as no significant secular change in the Fe isotope composition is observed. The Si isotopic composition of chert layers interspacing the magnetite or hematite microbands seems to be robust to post-depositional processes and may carry primary signatures [1]. First results indicate an overall homogeneous Si isotopic composition of the chert-bands in both samples. Chert-bands in the Proterozoic sample appear to be homogeneous inside a band, as well as along several sequenced bands. The Archean sample exhibits a similar picture but is in average 0.6‰ lighter in δ^{30}Si compared to the Proterozoic sample. Some single bands differ from the average by up to 0.6‰. However, the Si source and the overall process seem to be steady over the formation of several sequenced chert-bands. These results show that the study of Fe and Si isotopes on a high-spatial resolution can discover sources and diagenetic effects in BIFs and enhance our understanding of Precambrian Fe and Si cycles. [1] André, L. et al. (2006), {EPSL} 245, 162-173.