Coupled silicon-oxygen isotope fractionation traces Archaean silicification

Silica alteration zones and cherts are a conspicuous feature of Archaean greenstone belts worldwide and provide evidence of extensive mobilisation of silica in the marine environment of the early Earth. In order to understand the process(es) of silicification we measured the silicon and oxygen isotope composition of sections of variably silicified basalts and overlying bedded cherts from the Theespruit, Hooggenoeg and Kromberg Formations of the Barberton Greenstone Belt, South Africa.The δ³⁰Si and δ¹⁸O values of bulk rock increase with increasing amount of silicification from unsilicified basalts (-0.64%<δ³⁰Si<-0.01% and +8.6%<δ¹⁸O<+11.9%) to silicified basalts (δ³⁰Si and δ¹⁸O values as high as +0.81% and +15.6%, respectively). Cherts generally have positive isotope ratios (+0.215<δ³⁰Si<+1.05% and +10.9<δ¹⁸O<+17.1), except two cherts, which have negative δ³⁰Si values, but high δ¹⁸O (up to +19.5%).The pronounced positive correlations between δ³⁰Si, δ¹⁸O and SiO₂ imply that the isotope variation is driven by the silicification process which coevally introduced both ¹⁸O and ³⁰Si into the basalts. The oxygen isotope variation in the basalts from about 8.6% to 15.6% is likely to represent temperature-dependent isotope fractionation during alteration. Our proposed model for the observed silicon isotope variation relies on a temperature-controlled basalt dissolution vs. silica deposition process.