Submarine volcanism along shallow ridges did not drive Cryogenian cap carbonate formation

Geology Pub Date : 2024-02-08 DOI:10.1130/g51884.1
A. Dutkiewicz, R. D. Müller
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Abstract

The termination of Neoproterozoic “Snowball Earth” glaciations is marked globally by laterally extensive neritic cap carbonates directly overlying glacial diamictites. The formation of these unique deposits on deglaciation calls for anomalously high calcium carbonate saturation. A popular mechanism to account for the source of requisite ocean alkalinity is the shallow-ridge hypothesis, in which initial spreading ridges surrounding fragments of Rodinia, assumed to be dominated by volcanic margins, were formed at sea level. The shallow ridges are inferred to have promoted widespread deposition and alteration of glassy hyaloclastite—a source of alkalinity. We test this hypothesis by quantifying the prevalence of shallow ridges along Pangea’s passive continental margins, and by assessing Neoproterozoic reconstructions of continents. We find that the most frequently occurring depth range for incipient mid-ocean ridges is 2.1 ± 0.4 km. Ridges with initial elevations of approximately sea level are rare and have anomalous crustal thicknesses >14 km that only occur proximal to large igneous provinces (LIPs). Hyaloclastite is uncommon on mid-ocean ridges as it is generally restricted to water depths of <200 m for tholeiitic basalts, instead forming mostly on intraplate seamounts. Additionally, ocean drilling recently found hyaloclastite to be insignificant along the outer Vøring Plateau (offshore Norway)—an exemplar of a volcanic margin. Reconstructions of Rodinia and associated LIPs demonstrate that volcanic margins potentially hosting minor hyaloclastites were scarce during the late Neoproterozoic. We conclude that the shallow-ridge hypothesis fails to explain the formation of cap carbonates and suggest that other mechanisms such as enhanced continental weathering may be largely responsible.
浅海脊沿岸的海底火山活动并没有推动低碳盖碳酸盐的形成
新近纪 "雪球地球 "冰川期结束时,全球范围内都会出现直接覆盖在冰川二迭岩上的横向广泛的海相盖碳酸盐岩。这些独特沉积物的形成需要异常高的碳酸钙饱和度。浅海脊假说是解释所需海洋碱度来源的一个流行机制,根据该假说,假定以火山边缘为主的罗迪尼亚碎片周围的最初扩张海脊是在海平面上形成的。据推断,浅海脊促进了玻璃质透明岩的广泛沉积和蚀变--这是碱度的来源。我们通过量化泛大陆被动大陆边缘浅海脊的普遍程度,并评估新近纪大陆的重建情况,验证了这一假设。我们发现,初生洋中脊最常出现的深度范围是 2.1 ± 0.4 千米。初始海平面高度近似海平面的海脊很少见,地壳厚度大于 14 千米的异常海脊仅出现在大型火成岩带(LIPs)附近。大洋中脊上的透辉玄武岩并不常见,因为透辉玄武岩通常局限于水深小于200米的海域,而主要形成于板块内海山。此外,最近的大洋钻探发现,外弗林高原(挪威近海)--火山边缘的典范--沿岸的透明玄武岩微不足道。罗迪尼亚及相关LIPs的重建表明,在新近纪晚期,可能容纳少量透明岩的火山边缘非常稀少。我们的结论是浅脊假说无法解释帽状碳酸盐岩的形成,并认为大陆风化增强等其他机制可能是主要原因。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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