Silicon isotopes and trace elements in chert record early Archean basin evolution

• We report systematic δ30Si variations in a sequence of chemically precipitated cherts from the ~ 3.42 Ga Buck Reef Chert.
• Combined δ30Si and REE + Y results reflect a shift from terrigenous to seafloor hydrothermal silicon input into a deepening marine basin.
• Our data signal the potential variability of silicon isotope compositions of cherts deposited in individual Archean basins.

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.