Oxygen and silicon isotopic compositions of Archean silicified lava and cherts of the Onverwacht Group: Implication for seafloor hydrothermalism and the nature of recycled components in the source of granitoids

Cherts are commonly used as proxies for Archean environmental conditions such as oceanic temperature. However, because cherts encompass a wide variety of silica-rich rocks, including post-depositional silicified of clastic sediments or chemical precipitates from seawater, reconstructions of the Archean environment based on their O and Si isotopic compositions remain controversial. In this study, we present triple O isotope analyses associated with SIMS O and Si isotope measurements of Palaeoarchean pervasively silicified lavas and clastic sediments, which have been less studied than the associated cherts within the Onverwacht Group (Barberton Greenstone Belt, South Africa). We also provide triple O isotopic compositions of seawater-precipitated cherts. The O and Si isotope composition of silicified clastic sediments and lavas is not considerably variable through the Onverwacht succession. These lithologies display relatively low δ18O (11.3–14.9 ‰) and high Δ’17O values (−0.05 to - 0.07 ‰) attributed to precipitation of silica from hydrothermal fluids at lower temperatures than 200 °C. Using these results, we reconstruct the approximate O isotope composition of the hydrothermal fluids. Water-rock interaction models suggest that this fluid composition is consistent with the existence of low δ18O oceans, but the exact δ18O value of the Palaeoarchean ocean remains elusive. Seawater-precipitated cherts show a mixture of hydrothermal and seawater-like triple O isotope signatures, confirming that cherts are not a straightforward proxy for constraining Archean seawater composition or temperature. The average δ30Si value of the pervasively silicified lavas and clastic sediments is positive (> +0.19 ‰) and compositionally similar to published values of the seawater-precipitated cherts. This suggests comparable δ30Si values between Archean hydrothermal fluids and seawater. A gradual increase of δ18O values of silicified lavas and cherts from 10 to 13 ‰ to 15–20 ‰ observed along the Onverwacht succession is ascribed to local lithospheric cooling occurring between 3.47 Ga and 3.3 Ga, while Si isotope compositions remained mostly unchanged in the silica-rich rocks. As silicified lavas are likely part of materials that melted to generate Palaeoarchean granitoids, the evolutional difference between O and Si isotopes may have characterised the silicified rocks that were recycled in different generations of granitoid sources. This may explain the variation in O isotopic com