1 NASA Astrobiology Institute, University of Wisconsin-Madison , Madison, Wisconsin, USA .
2 Department of Geoscience, University of Wisconsin-Madison , Madison, Wisconsin, USA .
Astrobiology. 2018 May;18(5):519-538. doi: 10.1089/ast.2017.1722.
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 ΔS and ΔS 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 ΔS (from -0.07‰ to +0.38‰) and ΔS (from -4.1‰ to +1.0‰) 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 ΔS and ΔS 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. Key Words: Great Oxidation Event (GOE)-Secondary ion mass spectrometer (SIMS)-Paleoproterozoic-Sulfur isotopes-Mass independent fractionation (MIF). Astrobiology 18, 519-538.
古元古代休伦超群的沉积学观察结果表明，当时大气中的氧气含量上升，这通常被称为大氧化事件（GOE），并且通常与硫同位素从质量独立分馏（MIF）到质量依赖分馏（MDF）的转变相关。Papineau 等人对休伦超群的 S 三同位素进行的早期原位研究确定了一个较弱的 MIF-MDF 转变。然而，对这种转变的解释和地层位置存在歧义。在这项研究中，首次通过二次离子质谱仪（SIMS）分析了两个休伦超群钻芯中的所有四个 S 同位素，并且计算了ΔS 和 ΔS。基于改进的精度和详细的岩相学，我们重新解释了研究剖面中磁黄铁矿的主导地位，此前曾提出该磁黄铁矿起源于“早期自生”，实际上是由区域变质作用造成的。在整个下休伦超群中，ΔS（从-0.07‰到+0.38‰）和 ΔS（从-4.1‰到+1.0‰）的小但可分析分辨的非零值仍然存在。在这项研究中，没有看到明显的 MIF-S 信号或 MIF-MDF 转变。对于休伦超群中较小的非零ΔS 和 ΔS 值的成因，提出了四种情景：区域变质作用的均匀化、来自较老黄铁矿的再循环、岩浆流体的稀释以及 MDF 的发生。我们认为，休伦超群中 MIF-MDF 转变的确切位置仍未解决。这种假定的转变可能已被下休伦超群的后生过程所抹去，或者可能位于上休伦超群中。我们的研究强调了在深时研究中综合扫描电子显微镜和二次离子质谱技术的重要性，并表明不同盆地之间不同的分析方法（整体与 SIMS）和成岩历史（原生与变质）可能导致了全球范围内大氧化事件序列的 S 同位素剖面的不一致解释。关键词：大氧化事件（GOE）-二次离子质谱仪（SIMS）-古元古代-硫同位素-质量独立分馏（MIF）。天体生物学 18,519-538.
