Os Isotopic Variations in Obducted Metalliferous Sediments

(1)塞浦路斯的Troodos蛇绿岩，(2)阿曼的Semail蛇绿岩，(3)加利福尼亚的Franciscan杂岩，(3)相关的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沉积物是否可以用来重建过去海水中Os同位素组成的变化。更广泛地说，这项工作是建立一个跨越显生宙的海水Os同位素记录的长期目标的第一步，可以与海洋Sr同位素曲线进行比较。利用现代大洋盆地恢复的含金属沉积物成功重建新生代海相Os同位素记录，为本研究提供了动力。为了将海相Os同位素记录扩展到新生代以外的中生代和古生代，必须利用目前露头的古海相沉积物。然而，目前还没有系统的努力来确定哪种岩性可能适合于这一目的，以及可以使用什么标准来确定这些矿床实际上保存了过去Os同位素变化的记录。本研究计划的假设基础是，已被证明可以记录海水的Os同位素组成的倒伏沉积物样品，其主要和微量元素组成与类似的现代沉积物非常相似，可能是过去海水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，特别是那些由扩张脊附近的浮力热液羽流落下的沉积物。本文对现有的证明含金属沉积物和远洋粘土作为海水Os同位素变化记录物完整性的资料进行了综述。塞浦路斯超蛇绿岩沉积物的初步数据表明，这些隐伏沉积物保留了沉积微量元素特征，并可能提供有意义的数据来限制中白垩纪海水（Turonian - Campanian）的Os同位素组成。这一想法将通过检查塞浦路斯几个剖面的同时期沉积物和阿曼的类似样品进行经验检验，以确定不同的沉积剖面是否产生相同的Os同位素组成的时间变化模式。将对蒙特雷组的层状燧石进行类似的调查，以评估这些受到更广泛的成岩作用和构造作用影响的早侏罗世沉积物是否也可能提供过去海水中Os同位素组成变化的记录。这项工作将对海洋Sr和Os同位素记录的解释产生重要影响，进而影响将地球化学旋回与显生宙气候联系起来的模式。显生宙Sr同位素记录被引用为支持造山作用通过硅酸盐风化反应增加大气二氧化碳吸收导致降温的假说的证据。新生代海相同位素组成趋向于“更像大陆”的总体趋势支持这一解释，但对过去和现在海相Sr旋回的详细研究并不支持这一解释。最终，确定显生宙海洋Sr和Os记录的最大值和最小值是否大致一致，将提供一种经验手段，评估这些海洋同位素记录是否为重建硅酸盐风化的地质历史提供有用的代用指标。

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.