Searching for the Great Oxidation Event in North America: A Reappraisal of the Huronian Supergroup by SIMS Sulfur Four-Isotope Analysis
Searching for the Great Oxidation Event in North America: A Reappraisal of the Huronian Supergroup by SIMS Sulfur Four-Isotope Analysis
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DOI：
10.1089/ast.2017.1722
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发表时间：
2018-05-01
期刊：
影响因子：
4.2
通讯作者：
Valley, John W.
中科院分区：
文献类型：
--
作者：
Cui, Huan;Kitajima, Kouki;Valley, John W.
古元古代休伦超群的沉积学观测表明，当时大气氧含量上升，这通常被称为大氧化事件（GOE），并通常伴随着硫同位素从质量无关分馏（MIF）到质量相关分馏（CO2）的转变。Papineau等人（2007年）对休伦超群中S三同位素的早期原位研究发现了一个弱的MIF-S跃迁。然而，这种转变的解释和地层位置是模糊的。本文首次利用二次离子质谱仪（西姆斯）对两个休伦钻孔岩芯中的S同位素进行了分析，计算了S-33和S-36。基于改进的精度和详细的岩相学，我们重新解释磁黄铁矿的优势，在所研究的部分，这是以前提出的早期自生的起源，由于区域变质作用。S-33（从-0.07 ‰到+0.38 ‰）和S-36（从-4.1 ‰到+1.0 ‰）的小但可解析的非零值在整个下休伦超群中持续存在。在本研究中既没有观察到明显的MIF-S信号，也没有观察到MIF-S转换。四种情况下，提出了小的非零S-33和S-36值在休伦的成因：区域变质作用的同质化，从老黄铁矿回收，岩浆流体稀释，和发生的黄铁矿。我们认为，在休伦的MIF-ESTA过渡的确切位置仍然没有解决。这种假定的过渡可能已被下休伦超群的沉积后过程所抹去，或者可能位于上休伦超群。我们的研究强调了集成扫描电子显微镜和二次离子质谱技术在深时间研究中的重要性，并表明不同的分析方法（散装与西姆斯）和成岩历史（主要与变质）之间的不同盆地可能会导致不一致的解释S同位素剖面的GOE继承在全球范围内。
Sedimentological observations from the Paleoproterozoic Huronian Supergroup are suggested to mark the rise in atmospheric oxygen at that time, which is commonly known as the Great Oxidation Event (GOE) and typically coupled with a transition from mass-independent fractionation (MIF) to mass-dependent fractionation (MDF) of sulfur isotopes. An early in situ study of S three-isotopes across the Huronian Supergroup by Papineau et al. (2007) identified a weak MIF-MDF transition. However, the interpretation and stratigraphic placement of this transition is ambiguous. In this study, all four S isotopes were analyzed for the first time in two Huronian drill cores by secondary ion mass spectrometer (SIMS), and both S-33 and S-36 were calculated. Based on improved precision and detailed petrography, we reinterpret the dominance of pyrrhotite in the studied sections, which was previously proposed as early authigenic in origin, as resulting from regional metamorphism. Small but analytically resolvable nonzero values of S-33 (from -0.07 parts per thousand to +0.38 parts per thousand) and S-36 (from -4.1 parts per thousand to +1.0 parts per thousand) persist throughout the lower Huronian Supergroup. Neither pronounced MIF-S signals nor a MIF-MDF transition are seen in this study. Four scenarios are proposed for the genesis of small nonzero S-33 and S-36 values in the Huronian: homogenization by regional metamorphism, recycling from older pyrite, dilution by magmatic fluids, and the occurrence of MDF. We argue that the precise location of the MIF-MDF transition in the Huronian remains unsolved. This putative transition may have been erased by postdepositional processes in the lower Huronian Supergroup, or may be located in the upper Huronian Supergroup. Our study highlights the importance of integrated scanning electron microscopy and secondary ion mass spectrometry techniques in deep-time studies and suggests that different analytical methods (bulk vs. SIMS) and diagenetic history (primary vs. metamorphic) among different basins may have caused inconsistent interpretations of S isotope profiles of the GOE successions at a global scale.