Silicon isotope and trace element constraints on the origin of similar to 3.5 Ga cherts: Implications for Early Archaean marine environments

Silicon (Si) isotope variability in Precambrian chert deposits is significant, but proposed explanations for the observed heterogeneity are incomplete in terms of silica provenance and fractionation mechanisms involved. To address these issues we investigated Si isotope systematics, in conjunction with geochemical and mineralogical data, in three well-characterised and approximately contemporaneous, ∼3.5 Ga chert units from the Pilbara greenstone terrane (Western Australia). We show that Si isotope variation in these cherts is large (-2.4‰ to +1.3‰) and was induced by near-surface processes that were controlled by ambient conditions. Cherts that formed by chemical precipitation of silica show the largest spread in δ