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 d30Si and d18O values of bulk rock increase with increasing amount of silicification from unsilicified basalts (-0.64‰ < d30Si < -0.01‰ and + 8.6‰ < d18O < + 11.9‰) to silicified basalts (d30Si and d18O values as high as + 0.81‰ and + 15.6‰, respectively). Cherts generally have positive isotope ratios (+ 0.21‰ < d30Si < + 1.05‰ and + 10.9 < d18O < + 17.1), except two cherts, which have negative d30Si values, but high d18O (up to + 19.5‰).
The pronounced positive correlations between d30Si, d18O 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.
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