Os Isotopic Variations in Obducted Metalliferous Sediments

逆冲含金属沉积物中的锇同位素变化

• 批准号: 9804864
• 负责人: Gregory Ravizza
• 金额: $ 17.54万
• 依托单位: Woods Hole Oceanographic Institution
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 1998
• 资助国家: 美国
• 起止时间: 1998-08-01 至 2001-01-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=9804864&HistoricalAwards=false
• 关键词: Os; Isotopic; Variations; Obducted; Metalliferous

9804864RavizzaFunds are requested to investigate the major element, trace element and Os isotopic chemistry of obducted metalliferous sediments associated with (1) the Troodos ophiolite in Cyprus, (2) the Semail ophiolite in Oman, and (3) the Franciscan melange in California. The principal objective of this study is to determine whether these deposits can be used to reconstruct past variations in the Os isotopic composition of seawater. More broadly construed, this work is the first step toward the long-term goal of creating a seawater Os isotope record which spans Phanerozoic time, and can be compared to the marine Sr isotope curve.Successful efforts using metalliferous sediments recovered from the modern ocean basin to reconstruct the cenozoic marine Os isotope record provide the impetus for this study. In order to extend the marine Os isotope record back in time beyond the Cenozoic to include the Mesozoic and Paleozoic, ancient marine sediments now exposed in outcrop must be utilized. However, no systematic effort has yet been made to determine which lithologies might be suitable for this purpose, and what criteria could be used to establish that these deposits, in fact, preserve a record of past variations in the Os isotopic.The hypothesis which underlies this research program is that obducted sediment samples which have major and trace element compositions which closely resemble analogous modern sediments, already shown to record the Os isotopic composition of seawater, are likely to be reliable records of past variations in the Os isotopic composition of seawater. Therefore the focus of this study is on metalliferous sediments, specifically those sediments which are the product of fall-out from buoyant hydrothermal plumes proximal to spreading ridges. In the proposal the existing data which demonstrate the integrity of metalliferous sediments and pelagic clays as recorders of seawater Os isotope variations are reviewed. Preliminary data are presented from supra-ophiolite sediments in Cyprus, which show that these obducted deposits preserve depositional trace element signatures, and are likely to yield meaningful data constraining the Os isotopic composition of mid-Cretaceous seawater (Turonian to Campanian). This idea will be tested empirically by examining coeval deposits from several sections in Cyprus and analogous samples from Oman, in order to determine whether different sedimentary sections yield the same pattern of temporal variation in Os isotopic composition. A similar investigation of bedded cherts from the Monterey formation will be conducted in order to evaluate whether these Early Jurassic deposits, which have been subjected to more extensive diagenesis and tectonism, might also provide a record of past variations in the Os isotopic composition of seawater.This work will have important implications for interpretation of the marine Sr and Os isotope records, which in turn influence models linking geochemical cycles to Phanerozoic climate. The Phanerozoic Sr isotope record has been cited as evidence supporting the hypothesis that orogenesis causes cooling by increased uptake of atmospheric carbon dioxide by silicate weathering reactions. The general trend toward "more continent-like" isotopic compositions in the Cenozoic marine Os record supports this interpretation, but detailed investigation of the past and present marine Sr cycle do not. Ultimately, determining whether maxima and minima in the marine Sr and Os records are roughly coincident throughout the Phanerozoic will provide an empirical means assessing whether these marine isotopic records are useful proxies for reconstruction the geologic history of silicate weathering.

9804864 Ravizza基金被要求调查与（1）塞浦路斯的Troodos蛇绿岩，（2）阿曼的Semail蛇绿岩和（3）加州的Franciscan混杂岩有关的仰冲含金属沉积物的主元素、微量元素和Os同位素化学。 本研究的主要目的是确定这些存款是否可以用来重建过去的海水中的Os同位素组成的变化。 更广泛地解释，这项工作是第一步，建立一个长期的目标，海水Os同位素记录，跨越中生代，并可以比较的海洋Sr同位素曲线。成功的努力，利用含金属沉积物恢复现代海洋盆地新生代海洋Os同位素记录提供了动力，这项研究。 为了将海洋Os同位素记录的时间范围从新生代扩大到包括中生代和古生代，必须利用现在露出地表的古海洋沉积物。 然而，还没有系统的努力，以确定哪些岩性可能适合于这一目的，什么标准可以用来建立，这些存款，事实上，保存了过去的锇同位素变化的记录。假设这一研究计划的基础是仰冲沉积物样品的主要和微量元素组成，非常类似于类似的现代沉积物，已经显示出记录海水的Os同位素组成，可能是过去海水的Os同位素组成变化的可靠记录。 因此，本研究的重点是含金属沉积物，特别是那些靠近扩张洋脊的浮力热液羽流沉降物的沉积物。在建议中，现有的数据表明，完整的含金属的沉积物和远洋粘土记录海水的锇同位素变化进行了审查。 初步数据来自塞浦路斯的超蛇绿岩沉积物，这表明这些仰冲矿床保存沉积微量元素的签名，并可能产生有意义的数据约束的Os同位素组成的白垩纪中期海水（Turonian到Campanian）。 这一想法将通过检查来自塞浦路斯的几个部分和来自阿曼的类似样品的同时代沉积物进行经验测试，以确定不同的沉积部分是否产生相同的模式的时间变化的Os同位素组成。 对Monterey组层状硅质岩进行类似的研究，以评估这些经历了更广泛的成岩作用和构造作用的早侏罗世沉积物是否也可能提供海水Os同位素组成过去变化的记录。这项工作将对解释海洋Sr和Os同位素记录具有重要意义。这反过来又影响了将地球化学循环与中生代气候联系起来的模型。 早古生代的Sr同位素记录被引用作为证据，支持造山作用通过硅酸盐风化反应增加大气二氧化碳的吸收而导致冷却的假设。 新生代海洋Os记录中“更像大陆”的同位素组成的总体趋势支持这一解释，但对过去和现在海洋Sr循环的详细调查不支持。 最终，确定是否在海洋Sr和Os记录的最大值和最小值大致一致，整个中生代将提供一个经验的手段，评估这些海洋同位素记录是否是有用的代理重建硅酸盐风化的地质历史。