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Precambrian Si isotope compositions were largely controlled by Si chemical precipitation, input of Si with different isotope compositions (e.g., continental versus hydrothermal sources), and later alteration and diagenetic processes associated with silicification. In Precambrian banded iron formations (BIFs) and chert deposits there is a 4‰ spread of Si isotopes (δ³⁰Si), which stands in marked contrast to the narrow range (<0.5) measured in igneous rocks, highlighting the potential of using Si isotopes to reconstruct those processes that controlled the Precambrian marine Si cycle. However, Si isotope fractionation factors are poorly constrained. This prevents a faithful interpretation of Si isotope data in Precambrian records. In this work, we preformed a series of laboratory experiments that determine Si isotope fractionation factors during simulated processes directly relevant to Archean conditions. With the support of the experimentally determined fractionation factors we will be able to provide an interpretative framework for our future Si isotope studies of Archean age.

Experiments focused on Si isotope fractionation during co-precipitation of Fe-Si gels, and between amorphous Fe-Si gels and aqueous Si. All experiments were conducted in an artificially prepared medium that mimics Archean seawater (e.g. Si > 20 ppm, seawater-like matrix), rather than in a simple Fe-Si solution. Use of an Archean seawater analog is important because previous studies have revealed distinct Fe isotope fractionation behaviors in such a medium as compared to simples solutions. For Fe-Si co-precipitation experiments, our results show that δ³⁰Si values of amorphous Fe-Si co-precipitates, produced during oxidation of Fe²⁺ in the AAS at room temperature, are ~2‰ lower than the initial AAS (Δ³⁰Si_precip-AAS = -2.13 ± 0.18‰ (2σ)) (Fig. 1). The Si exchange experiments adopted the so-called three-isotope method, in which aqueous Si (AAS) was spiked with a 29Si-enriched tracer, and then allowed to equilibrate with isotopically normal amorphous Fe-Si gels. This is the first application of the three-isotope method to the Si isotope system that allows for rigorous evaluation of Si isotope exchange and determination of Si isotope fractionation factors at 100% isotope exchange. Two parallel sets of reactors were utilized. The first set used an Fe²⁺-free 29Si-spiked AAS, and second set used the ²⁹Si-spiked AAS containing ~1 mM aqueous Fe²⁺. Both solutions were reacted with the same Fe-Si gel at similar pH and ambient conditions in an anaerobic chamber. After ~30 days, the experiment using the Fe-free AAS had 50% Si isotope exchange between the solid and solution. In contrast, the experiment using the Fe-containing AAS had isotopically exchanged~80% in the same amount of time (Fig. 2). Extrapolation to 100% isotope exchange leads to an equilibrium fractionation factor (Δ³⁰Si_precip-AAS) of -2.52 ± 0.31‰ for the experiments without Fe²⁺ in the AAS, and a larger fractionation factor (Δ³⁰Si_precip-AAS) of -3.20 ± 0.30‰ for the experiments using 1 mM Fe²⁺ in the AAS (Fig. 2). Several important conclusions can be drawn from these results: (1) Si isotopes in Fe-Si co-precipitates, the assumed precursors of Archean cherts, are easily exchangeable with ambient aqueous Si solution; (2) the presence of aqueous Fe²⁺ in the solution promotes Si isotope exchange; (3) the presence of Fe²⁺ in the solution leads to larger Si isotope fractionation during isotope exchange between the solid and solution. This last conclusion may explain the observation that BIF-associated Precambrian cherts have Si isotope compositions more negative than those recorded in coeval pure cherts (Chakrabarti et al., 2012; Marin-Carbonne et al., 2014). Our results highlight the importance to take into account formation and diagenetic processes, which are largely ignored in the published literature, when interpreting Archean Si isotope data.

References Cited:
(1) Chakrabarti et al., 2012. Si isotope variability in Proterozoic cherts. Geochimica et Cosmochimica Acta (91), 187-201.
(2) Marin-Carbonne et al., 2014. The silicon and oxygen isotope compositions of Precambrian cherts: a record of oceanic paleo-temperatures? Precambrian Research (247), 223-234.