Silicon isotope and trace element constraints on the origin of ∼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, similar to 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 parts per thousand to +1.3 parts per thousand) 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 delta Si-30 (-2.4 parts per thousand to +0.6 parts per thousand) and are characterised by positive Eu, La and Y anomalies and overall depletions in lithophile trace elements. Silicon isotope systematics in these orthochemical deposits are explained by (1) mixing between hydrothermal fluids and seawater, and/or (2) fractionation of hydrothermal fluids by subsurface losses of silica due to conductive cooling. Rayleigh-type fractionation of hydrothermal fluids was largely controlled by temperature differences between these fluids and seawater. Lamina-scale Si isotope heterogeneity within individual chemical chert samples up to 2.2 parts per thousand is considered to reflect the dynamic nature of hydrothermal activity. Silicified volcanogenic sediments lack diagnostic REE+Y anomalies, are enriched in lithophile elements, and exhibit a much more restricted range of positive delta Si-30 (+0.1 parts per thousand to +1.1 parts per thousand), which points to seawater as the dominant source of silica. The proposed model for Si isotope variability in the Early Archaean implies that chemical cherts with the most negative delta Si-30 formed from pristine hydrothermal fluids, whereas silicified or chemical sediments with positive delta Si-30 are closest to pure seawater deposits. Taking the most positive value found in this Study (+1.3 parts per thousand), and assuming that the Si isotope composition of seawater is governed by input of fractionated hydrothermal fluids, we infer that the temperature of similar to 3.5 Ga seawater was below similar to 55 degrees C. (C) 2009 Elsevier Ltd. All rights reserved.