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Silicon isotopes of chemical sediments have received growing attention, given their applicability in the search for properties of ancient seawater. An important target is the reconstruction of secular changes in surface temperature of the Precambrian Earth, but interpretations are problematic since controls of the isotopic signals are potentially manifold. Here we report the existence of significant silicon isotope variability in chemically precipitated chert layers covering a continuous stratigraphic section across the ~ 3.42 Ga Buck Reef Chert (BRC) in the Barberton Greenstone Belt (South Africa). Black chert bands are variably but consistently higher in δ30Si (up to ca. 0.9) than co-existing translucent counterparts at the same stratigraphic level, which is interpreted as a primary feature acquired during deposition upon interaction between submarine discharging hydrothermal water and a stratified water body. Averaged δ30Si values range from + 0.3 (maximum + 0.5) for shallow-marine sediments in the lower part of the section to – 0.5 (minimum – 1.3) for deeper water deposits in the upper part when results from translucent and black cherts are taken together. An average shift of ca. 0.8 (maximum 1.8) thus accompanied the formation of chemical cherts in a single Archean basin. Cherts at the base of the section represent silicified felsic volcanics and volcaniclastics and have exclusively positive δ30Si values between + 0.1 and + 0.7.
Rare-earth-elements-and-yttrium patterns confirm the marine origin of the cherts and support the presumed basin evolution. From the combined evidence we infer that the δ30Si variations in the BRC reflect changes in the predominant origin of the silica, with terrigenous input supplying positive δ30Si to shallow waters and seafloor hydrothermal sources negative δ30Si to deeper levels. Our findings demonstrate the viability of silicon isotopes in cherts for reconstructing the evolution of ancient marine basins.
Silicon isotopes have considerable potential as proxy for (near-) surface processes and environmental conditions. However, unambiguous interpretations of isotope signatures in natural silica deposits are often hampered by a lack of independent quantitative information on isotopic fractionations operating under the environmental conditions of interest. We performed seeded silica precipitation experiments using flow-through reactors in the 10–60 °C temperature range to alleviate this problem. The principal objective was to quantify the silicon isotope fractionations during controlled precipitation of amorphous silica from a flowing aqueous solution. The experiments were designed to simulate silica deposition induced by a temperature drop, with particular relevance for (near-) surface hydrothermal systems associated with steep temperature gradients.
Monitored differences in silicon isotope ratios (30Si/28Si and 29Si/28Si) between input and output solutions demonstrated a systematic sequence in behavior. During an initial time interval, that is, before the reaction system reached steady state, the observed isotope shifts were influenced by dissolution of the seed material, the saturation state of the solution and the specific surface area of the seeds. After reaching steady state, the selective incorporation of silicon isotopes by the solid phase exhibited an explicit temperature dependency: the lighter isotopes were preferentially incorporated, and apparent fractionation magnitudes increased with decreasing temperature.
Calculated magnitudes of silicon isotope fractionations between precipitated and dissolved silica (Δ30Si = δ30Siprecipitate (calculated) – δ30Siinput solution) were –2.1 at 10 °C, –1.2 at 20 °C, –1.0 at 30 °C, –0.5 at 40 °C, 0.1 at 50 °C, and 0.2 at 60 °C (s.d. ≤ 0.6, based on replicate experiments). Hence, fractionation was nearly insignificant at temperatures ≥50 °C. Apart from this relationship with temperature, our results indicate that the effective Si isotope fractionation during precipitation from a solution is subject to changes in the saturation state, reactive surface area and flow regime. We therefore infer that, to a significant extent, solidfluid fractionation in natural (near-) surface environments is system dependent.