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首页> 外文学位 >Evidence for climate and weathering in siliciclastic sedimentary rocks of the 3.2 GA Moodies Group, Barberton greenstone belt, South Africa.
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Evidence for climate and weathering in siliciclastic sedimentary rocks of the 3.2 GA Moodies Group, Barberton greenstone belt, South Africa.

机译：南非巴伯顿绿岩带3.2 GA Moodies组硅质碎屑沉积岩中的气候和风化证据。

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摘要

The oldest preserved terrestrial sedimentary rocks—the Moodies Group—provide a unique opportunity to study Earth's earliest surface environment and how potentially unique conditions may have influenced weathering and the generation of siliciclastic sediments,; A study of modern plutoniclastic sediment generation in different climates was used to understand the effects of climate on weathering. Sediment and bedrock samples were collected from the and southern Sierra Nevada Mountains and Mojave Desert and from the temperate northern Sierra Nevada and Klamath Mountains, California. With increasing precipitation, feldspar is destabilized due to weathering compared to quartz; potassium feldspar is more resistant to weathering than plagioclase but does show destabilization in temperate climates. Temperate-climate sands show a loss of CaO and Na2O compared to K2O and Al2O3. With the exception of biotite and hornblende, the mineralogy of medium-grained stream sand reflects that of medium-grained weathering profile material in both climates.; Archean weathering rinds on pebbles from the Moodies alluvial-fluvial conglomerate record geochemical interactions between the atmosphere and early Earth surface. Rinds developed during weathering and transport based on the truncation of rinds at pre-depositional cracks. Microprobe analysis shows evidence of hydrolysis: potassium feldspar and sericite altered to kaolinite during weathering. Rinds contain higher concentrations of Fe-Mg carbonate, indicative of reducing conditions and high pCO2. The co-existence of the two processes—both hydrolysis and reduction—in one micro-weathering system implies anoxic and elevated pCO 2 conditions in a well-aerated, fluvial system, and an anoxic and CO 2-rich early atmosphere.; Alluvial and braided fluvial deposits north of the Inyoka Fault were analyzed petrographically and geochemically to define their provenance and to then assess their degree of weathering. Conglomerate, sandstone, and shale data give varying pictures of provenance due to differential decomposition during weathering. Shale geochemistry gives the most accurate provenance, although rare-earth elements and Th, Sc, Zr, Cr, and Ti together give the best results. The Moodies Group is remnant of aggressive weathering: labile source materials decomposed to clays and the CIA for Moodies shale is 85, above global average. A combination high temperature, rainfall, and pCO2 likely offset the inhibitory weathering effects of a plant-free environment.
机译：保存最久的陆地沉积岩Moodies组为研究地球最早的地表环境以及潜在的独特条件如何影响风化和硅质碎屑沉积物提供了独特的机会。通过研究不同气候下现代现代碎屑沉积物来了解气候对风化的影响。从内华达山脉和南部山脉和莫哈韦沙漠以及加利福尼亚的温带内华达山脉北部和克拉马斯山脉收集沉积物和基岩样品。与石英相比，随着降水的增加，长石由于风化而不稳定。长石钾比斜长石更具耐候性，但在温带气候下确实表现出不稳定。与K 2 O和Al 2 O 3 2 O的损失>。除黑云母和角闪石外，中粒流砂的矿物学反映了两种气候下中粒风化剖面材料的矿物学。穆迪斯冲积河流相砾岩的卵石上的太古代风化作用记录了大气与地球早期表面之间的地球化学相互作用。在风化和运输过程中，基于沉积前裂缝处果皮的截断而形成的果皮。微量探针分析显示了水解的证据：长石中的钾长石和绢云母在风化过程中变为高岭石。外皮中含有较高浓度的Fe-Mg碳酸盐，表明还原条件和高 p CO 2 。在一个微风化系统中，水解和还原这两个过程并存，意味着在一个通风良好的河流系统中，缺氧和升高的 p CO 2 条件，以及富含缺氧和CO 2 的早期大气。对Inyoka断层以北的冲积和辫状河床沉积物进行了岩石学和地球化学分析，以确定其出处，然后评估其风化程度。由于风化过程中的差异分解，砾岩，砂岩和页岩数据提供了不同的物源图。尽管稀土元素与Th，Sc，Zr，Cr和Ti一起提供了最佳结果，但页岩地球化学提供了最准确的出处。 Moodies集团残留着侵蚀性的风化作用：不稳定的原料分解为粘土，Moodies页岩的CIA指数为85，高于全球平均水平。高温，降雨和 p CO 2 的组合可能抵消了无植物环境的抑制风化作用。

著录项

  • 作者

    Hessler, Angela Marie.;

  • 作者单位

    Stanford University.;

  • 授予单位 Stanford University.;
  • 学科 Geology.
  • 学位 Ph.D.
  • 年度 2001
  • 页码 258 p.
  • 总页数 258
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类 地质学;
  • 关键词

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