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.

9804864Ravizza Funds被要求研究与(1)塞浦路斯的Troodos蛇绿岩、(2)阿曼的Smail蛇绿岩和(3)加利福尼亚的Franciscan混杂岩有关的隐伏金属沉积物的主要元素、微量元素和Os同位素化学。这项研究的主要目的是确定这些沉积物是否可以用来重建海水Os同位素组成过去的变化。从更广泛的意义上讲，这项工作是朝着建立跨越显生界时间的海水Os同位素记录的长期目标迈出的第一步，可以与海相锶同位素曲线进行比较。利用从现代洋盆中回收的金属沉积物重建新生代海相Os同位素记录的成功为这一研究提供了动力。为了将海相Os同位素记录延伸到新生代以外，包括中生代和古生代，必须利用现在露头的古代海相沉积物。然而，目前还没有系统的努力来确定哪些岩性可能适合于这一目的，以及什么标准可以用来确定这些沉积物实际上保存了Os同位素过去变化的记录。这项研究计划的基础假设是，遮蔽沉积物样品的主要和微量元素成分与已经证明记录海水Os同位素组成的类似现代沉积物非常相似，很可能是海水Os同位素组成过去变化的可靠记录。因此，这项研究的重点是含金属沉积物，特别是那些由浮力热液羽流在扩张的海脊附近产生的沉积物。在该提案中，审查了现有的数据，这些数据证明了作为海水Os同位素变化记录器的金属沉积物和远洋粘土的完整性。塞浦路斯蛇绿岩上沉积物的初步数据表明，这些被遮挡的沉积物保留了沉积微量元素特征，并可能产生有意义的数据，限制中白垩世海水(都罗纪至坎帕尼亚)的Os同位素组成。这一想法将通过研究塞浦路斯几个区段的同时代沉积物和阿曼的类似样品进行经验检验，以确定不同的沉积区段是否产生相同的Os同位素组成随时间变化的模式。将对蒙特利组的层状硅质岩进行类似的调查，以评估这些经历了更广泛的成岩和构造作用的早侏罗世沉积是否也能提供海水Os同位素组成过去变化的记录。这项工作将对解释海洋锶和Os同位素记录具有重要意义，这些记录反过来又影响将地球化学循环与显生宙气候联系起来的模型。显生宙的锶同位素记录被引用为证据，支持造山作用通过硅酸盐风化反应增加对大气二氧化碳的吸收而导致降温的假设。新生代海洋Os记录中“更接近大陆”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锶旋回的详细调查则不支持这一解释。最终，确定海相锶和锶记录中的最大值和最小值在整个显生界是否大致重合，将为评估这些海洋同位素记录是否可用于重建硅酸盐风化地质历史提供一种经验手段。