Os Isotopic Variations in Obducted Metalliferous Sediments

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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.