Chemical evolution of seawater in the Transvaal Ocean between 2426 Ma (Ongeluk Large Igneous Province) and 2413 Ma ago (Kalahari Manganese Field) |
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| Institution: | 1. Department of Physics and Earth Sciences, Jacobs University Bremen, Campus Ring 1, 28759 Bremen, Germany;2. Institute of Geosciences, Kiel University, Ludewig-Meyn-Straße 10, 24118 Kiel, Germany;1. University of Vienna, Department of Geodynamics and Sedimentology, Althanstr. 14, 1090 Wien, Austria;2. Nanjing University, Department of Earth Sciences, Xianlin Ave. 168, 21000 Qixia District, Nanjing, PR China;3. Jacobs University Bremen, Department of Physics and Earth Sciences, Campus Ring 1, 28759 Bremen, Germany;4. Universidade de Brasília, Instituto de Geociências, Campus Universitário Darcy Ribeiro, CEP 70910-900 Brasília, DF, Brazil;5. Max Planck Institute for Chemistry, Climate Geochemistry Department, Hahn-Meitner-Weg 1, 55128 Mainz, Germany;6. China University of Geosciences, State Key Laboratory of Geological Processes and Mineral Resources, Faculty of Earth Resources, Wuhan, PR China |
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| Abstract: | Drill core and outcrop samples of pure marine chemical sediments (banded iron formation (BIF), manganese formation (MnF), jaspilites, lutites, and cherts) from the transition of the ~2426 Ma old Ongeluk Formation into the 2413 Ma old Hotazel Formation, Transvaal Supergroup, South Africa, reveal remarkable changes of seawater chemistry in the Transvaal Ocean. Similar to pre-Ongeluk chemical sediments, the shale-normalized rare earths and yttrium (REYSN) patterns of jaspilites intercalated with the volcanic rocks of the Ongeluk large igneous province and directly overlying cherts do not show positive EuSN anomalies, indicating that high-temperature (>250 °C) hydrothermal fluids did not contribute significantly to the REY budget of ambient waters. However, a 10 cm drill core section in the lower Hotazel Formation is characterized by conspicuous positive EuSN anomalies, revealing temporary inflow of water masses strongly affected by high-temperature hydrothermal fluids. After this short episode, the REYSN pattern of Transvaal seawater returned to that of pre-Ongeluk times, showing heavy REYSN enrichment, positive LaSN, GdSN and YSN anomalies, but no CeSN or EuSN anomalies. Higher up in the stratigraphy, the Hotazel Formation shows negative CeSN anomalies in some of the lutites, BIFs and MnFs, reflecting Ce depletion in ambient seawater. All Hotazel lutite, BIF, and MnF samples studied show unradiogenic εNd(t) values (?0.5 ± 0.2 to ?2.4 ± 0.2), indicating a mostly continental REY source. The REY distribution and Nd isotope data combined suggest that oxidative terrestrial weathering of this continental crustal source supplied most of the dissolved REY to local “Transvaal seawater”. Precipitation of the Hotazel lutites, BIFs and MnFs with negative CeSN anomalies, therefore, suggests that oxic conditions prevailed on the Kaapvaal Craton and in Hotazel seawater already at ~2.413 Ga, i.e. 80 m.y. before the disappearance of mass-independent sulfur isotope fractionation (MIF-S) that defines the Great Oxidation Event at ~2.33 Ga. |
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