The Urucum iron- and manganese formation (IF-MnF) in the Santa Cruz Formation, Brazil, was deposited in a glacially influenced, late Neoproterozoic depositional environment. It has proven to be a reliable and robust archive for the late Neoproterozoic, allowing unique insights into the composition of seawater during an interval that is characterized by dramatic changes in the Earth’s litho-, hydros-, atmo-, and biosphere, including several episodes of low-latitude glaciations. Here we present highly siderophile element data of the Urucum IF-MnF to evaluate elemental sources that affected the Neoproterozoic seawater from which the Urucum IF-MnF precipitated. High uncertainties associated with current dating attempts of this formation overlapped with the Marinoan glaciation (∼650–635 Ma) as well as the Gaskiers glaciation event (∼580 Ma). A Re-Os regression line defined by iron-, manganese-, and chert-rich samples of the Urucum formation is interpreted as an isochron, yielding a refined age estimate of 577 ± 38 Ma and an initial 187Os/188Os ratio of 0.122 ± 0.003. The proposed depositional age overlaps with previous published age data of the Urucum IF-MnF, but relates the Urucum IF-MnF to the Ediacaran Gaskiers glaciation and overlaps with the age of the Shuram CIE (∼567 Ma). An initial 187Os/188Os ratio of 0.122 ± 0.003 is interpreted to represent pristine Neoproterozoic seawater and supports strong hydrothermal input, probably related to rifting, that promoted ferruginous conditions in the Urucum seawater.

中文翻译：

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新元古代铁层的高嗜铁元素和 Re-Os 同位素系统学及其与冰川作用事件的时间关系


巴西圣克鲁斯组的 Urucum 铁和锰地层 （IF-MnF） 沉积在受冰川影响的晚新元古代沉积环境中。它已被证明是新元古代晚期的可靠而强大的档案，可以对地球岩石圈、水圈、大气圈和生物圈发生巨大变化的时期的海水成分有独特的见解，包括几次低纬度冰川作用。在这里，我们提供了 Urucum IF-MnF 的高嗜铁元素数据，以评估影响 Urucum IF-MnF 沉淀的新元古代海水的元素来源。与该地层的当前测年尝试相关的高度不确定性与马里诺冰川作用（∼650-635 马）以及加斯基尔斯冰川作用事件（∼580 马）重叠。由 Urucum 地层富含铁、锰和燧石的样品定义的 Re-Os 回归线被解释为等时线，得出 577 ± 38 马的精炼年龄估计值，初始 187Os/188Os 比值为 0.122 ± 0.003。拟议的沉积年龄与以前公布的 Urucum IF-MnF 年龄数据重叠，但将 Urucum IF-MnF 与 Ediacaran Gaskiers 冰川作用联系起来，并与 Shuram CIE 的年龄重叠（∼567 马）。最初的 187Os/188Os 比值为 0.122 ± 0.003，被解释为代表原始的新元古代海水，并支持强烈的热液输入，可能与裂谷有关，这促进了 Urucum 海水中的铁质条件。