Abstract Precambrian banded iron formations (BIF) are chemical sedimentary deposits whose trace element signatures have been widely used to interrogate the chemical composition and redox state of ancient seawater. Here we investigated trace element signatures in BIF of the 3.22 Ga Moodies Group, Barberton Greenstone Belt (South Africa), which are interbedded with near-shore siliciclastic sedimentary rocks and represent one of the oldest known shallow-water occurrences of BIF. Unusual rare earth element (REE) signatures, notably with pronounced negative Ce anomalies in shale-normalized spectra, have been previously reported for chemical sediments of the Moodies Group, which we confirm here through an expanded dataset for Moodies BIF spanning three different localities. We find negative Ce anomalies as low as 0.2 Ce/Ce⁎ that are associated with unusual enrichment of LREE relative to HREE in the sample set. While total REE abundances and certain REE features appear strongly related to the concentration of detrital indicators (e.g., Zr), and are likely primary, other features, notably LREE enrichment, cannot be explained as a primary feature of the sediment. This is better explained by later addition of REE from a LREE-enriched but Ce-depleted fluid that generated the significant negative Ce anomalies observed in surface samples of Moodies Group BIF. This REE addition event influenced both Sm-Nd and La-Ce isotope systematics, the latter yielding an isochron of 60 ± 32 Ma, thus constraining the timing of emplacement of the negative Ce anomalies to the past 100 Ma, possibly upon surface exposure of the Barberton Greenstone Belt to wetter conditions during the Cenozoic. Our findings constitute a cautionary tale in that even the most immobile elemental redox proxies may be more sensitive to post-depositional modification than previously thought, and demonstrate the clear advantage offered by paleoredox proxies coupled to radiometric geochronometers to enable the direct dating of ancient signals of Earth surface oxygenation.

中文翻译：

---

La-Ce 年代学揭示的古太古代带状铁地层中的沉积后 REE 迁移率：古代氧化信号的警示故事

摘要 前寒武纪带状铁地层 (BIF) 是化学沉积矿床，其微量元素特征已被广泛用于研究古代海水的化学成分和氧化还原状态。在这里，我们研究了巴伯顿绿岩带（南非）的 3.22 Ga Moodis Group 的 BIF 中的微量元素特征，它们与近岸硅质碎屑沉积岩互层，代表了已知最古老的 BIF 浅水矿点之一。之前已经报道了穆迪集团化学沉积物的异常稀土元素 (REE) 特征，特别是在页岩归一化光谱中具有明显的负 Ce 异常，我们在此通过跨越三个不同地点的穆迪 BIF 扩展数据集确认了这一点。我们发现负 Ce 异常低至 0。2 Ce/Ce⁎ 与样品组中轻稀土相对于重稀土的异常富集有关。虽然 REE 总丰度和某些 REE 特征似乎与碎屑指标（例如 Zr）的浓度密切相关，并且可能是主要特征，但其他特征，尤其是轻稀土元素的富集，不能解释为沉积物的主要特征。这可以通过后来从富含轻稀土但缺乏 Ce 的流体中添加 REE 来更好地解释，该流体产生了在 Moodis Group BIF 的表面样本中观察到的显着负 Ce 异常。该 REE 添加事件影响了 Sm-Nd 和 La-Ce 同位素系统，后者产生了 60 ± 32 Ma 的等时线，从而将 Ce 负异常的就位时间限制在过去 100 Ma，可能是在新生代巴伯顿绿岩带地表暴露于更潮湿的条件下。我们的发现构成了一个警示故事，因为即使是最固定的元素氧化还原代理也可能比以前认为的对沉积后的修改更敏感，并证明了古氧化还原代理与辐射地球计时仪耦合所提供的明显优势，以实现对古代信号的直接测年。地球表面氧化。