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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
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DOI：10.1016/j.chemgeo.2024.122407.png)
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Abstract
En 中文
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 delta O-18 (11.3-14.9 parts per thousand) and high Delta'O-17 values (-0.05 to - 0.07 parts per thousand) attributed to precipitation of silica from hydrothermal fluids at lower temperatures than 200 degrees 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 delta O-18 oceans, but the exact delta O-18 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 delta Si-30 value of the pervasively silicified lavas and clastic sediments is positive (> +0.19 parts per thousand) and compositionally similar to published values of the seawater-precipitated cherts. This suggests comparable delta Si-30 values between Archean hydrothermal fluids and seawater. A gradual increase of delta O-18 values of silicified lavas and cherts from 10 to 13 parts per thousand to 15-20 parts per thousand 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 compositions measured in different granitoid generations in the Barberton area.
Keywords:
Chert
Silicified lava
Seafloor hydrothermalism
Palaeoarchean granitoids
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