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 eNd(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.

中文翻译：

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2426 Ma（Ongeluk 大型火成岩省）和 2413 Ma 前（喀拉哈里锰矿区）之间德兰士瓦海海水的化学演化

摘要 从~2426 Ma 老 Ongeluk 组过渡到 2413 Ma 老 Hotazel​​ 组的纯海洋化学沉积物（带状铁层 (BIF)、锰层 (MnF)、jaspilites、lutite 和燧石）的钻芯和露头样品，南非德兰士瓦超群，揭示了德兰士瓦海洋海水化学的显着变化。与前 Ongeluk 化学沉积物类似，夹在 Ongeluk 大型火成岩省火山岩和直接上覆硅质岩中的页岩归一化稀土和钇 (REYSN) 模式没有显示正 EuSN 异常，表明高温 ( > 250 °C) 热液对周围水域的 REY 预算没有显着贡献。然而，Hotazel​​ 组下部 10 cm 钻芯段的特征是明显的 EuSN 正异常，揭示了受高温热液影响强烈的水团的暂时流入。在这短暂的事件之后，德兰士瓦海水的 REYSN 模式恢复到 Ongeluk 时代之前的模式，显示出大量的 REYSN 富集，LaSN、GdSN 和 YSN 正异常，但没有 CeSN 或 EuSN 异常。在地层较高的地方，Hotazel​​ 组在一些黄铁矿、BIF 和 MnF 中显示出负的 CeSN 异常，反映了环境海水中的 Ce 耗竭。研究的所有 Hotazel​​ 黄铁矿、BIF 和 MnF 样品均显示非放射性 eNd(t) 值（-0.5 ± 0.2 至 -2.4 ± 0.2），表明主要是大陆 REY 源。REY 分布和 Nd 同位素数据相结合表明，该大陆地壳源的氧化陆地风化将大部分溶解的 REY 提供给当地的“德兰士瓦海水”。因此，具有负 CeSN 异常的 Hotazel​​ 黄岩、BIF 和 MnF 的降水表明，在与质量无关的硫同位素分馏 (MIF) 消失之前，Kaapvaal 克拉通和 Hotazel​​ 海水中的含氧条件已经达到~2.413 Ga，即 80 my -S) 定义了约 2.33 Ga 的大氧化事件。