Barium-isotopic constraints on the origin of post-Marinoan barites
Barium-isotopic constraints on the origin of post-Marinoan barites
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DOI：
10.1016/j.epsl.2019.05.018
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发表时间：
2019-08-01
影响因子：
5.3
通讯作者：
Horner, Tristan J.
中科院分区：
文献类型：
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作者：
Crockford, Peter W.;Wing, Boswell A.;Horner, Tristan J.
对马里诺后盖碳酸盐层内重晶石层中三氧同位素比率的测量提供了一些最有力的证据，表明马里诺冰川作用是一场雪球地球事件。然而，这些重晶石层位的起源仍未解决。为了限制Ba来源以及这些层位的形成机制，我们分析了加拿大西北部、挪威北部、巴西和中国南部的后马里诺世重晶石矿床的Ba同位素组成。我们通过对近 100 个现代和古代附加重晶石测量进行 Ba 同位素调查来增强这些分析，包括来自远洋（或“海洋”）、热液、陆地、元古代地层和冷泉环境的样本。与现代冷泉或陆地重晶石不同，我们发现全球分布的后马里诺重晶石表现出相对较窄的同位素范围，表明混合良好、实际上无限的Ba来源。此外，后马里诺时代沉积物表现出与现代海相重晶石相似的平均钡同位素组成，我们将其解释为海相钡来源的证据。结合现有的地球化学、地质和新的 Ba 同位素调查数据，我们得出结论，重晶石层中的 Ba 源自马里诺雪球地球期间积累的充分混合、富含 Ba 但缺乏 SO4 的储层。然后，这个深巴储层通过海洋环流或下面的碳酸盐岩白云石化作用向上输送，并与冰川后融水表层中大陆风化产生的硫酸盐接触。因此，除了提供一种合理的机制来生成后马里诺时代重晶石层的全球同步沉积，以协调所有现有的地球化学和地质数据之外，我们的结果还证明了Ba同位素在询问整个沉积记录中神秘重晶石矿床的起源方面的效用。 (C) 2019 Elsevier B.V. 保留所有权利。
Measurements of triple oxygen isotope ratios in barite horizons within post-Marinoan cap carbonates have provided some of the most compelling evidence that the Marinoan glaciation was a Snowball Earth event. However, the origin of these barite horizons remains unresolved. To constrain the Ba sources, and thus formation mechanisms of these horizons, we analyzed the Ba isotope composition of post-Marinoan barite deposits from Northwest Canada, Northern Norway, Brazil and South China. We augment these analyses with a Ba isotope survey of almost 100 modern and ancient additional barite measurements, including samples from pelagic (or 'marine'), hydrothermal, terrestrial, Proterozoic stratiform and cold seep environments. Unlike modern cold seep or terrestrial barites, we find that globally-distributed post-Marinoan barites exhibit a relatively narrow isotopic range, suggesting a well-mixed, effectively limitless Ba source. Moreover, post-Marinoan deposits exhibit a similar mean Ba isotope composition to modern marine barites, which we interpret as evidence of a marine Ba source. Considered alongside existing geochemical, geological, and new Ba isotope survey data, we conclude that Ba in barite horizons was sourced from a well-mixed, Ba-replete but SO4-poor reservoir that accumulated during the Marinoan Snowball Earth interval. This deep Ba reservoir was then transported upward-either by ocean circulation or dolomitization of underlying cap carbonates-and was brought into contact with continental weathering-derived sulfate in a post-glacial meltwater surface layer. Thus, in addition to providing a plausible mechanism for generating globally-synchronous deposition of post-Marinoan barite horizons that reconciles all existing geochemical and geological data, our results demonstrate the utility of Ba isotopes to interrogate the origin of enigmatic barite deposits throughout the sedimentary record. (C) 2019 Elsevier B.V. All rights reserved.