Emergence of felsic crust and subaerial weathering recorded in Palaeoarchaean barite

Abstract

Reconstructing the emergence and weathering of continental crust in the Archaean eon is crucial for our understanding of early ocean chemistry, biosphere evolution and the onset of plate tectonics. However, considerable disagreement exists between elemental and isotopic proxies that have been used to trace crustal input into marine sediments, and data are scarce before 3 Ga. Here we show that chemical weathering modified the Sr isotopic composition of seawater as recorded in 3.52–3.20 Ga stratiform barite deposits from three different cratons. Using a combination of Sr, S and O isotope data, barite petrography and a hydrothermal mixing model, we calculate a Sr isotope evolution trend of Palaeoarchaean seawater that is much more radiogenic than the curve previously determined from carbonate rocks. Our findings suggest that evolved crust containing high Rb/Sr was subaerial and weathering into the oceans from approximately 3.7 ± 0.15 Ga onwards with impacts on ocean chemistry and the nutrient supply to the marine biosphere.