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Fe isotopes as a key to understanding fluid-rock processes during hydration of oceanic crust

铁同位素是了解洋壳水合过程中流体-岩石过程的关键

基本信息

批准号：
1634669
负责人：
Kenneth Sims
金额：
$ 38.85万
依托单位：
University of Wyoming
依托单位国家：
美国
项目类别：
Standard Grant
财政年份：
2016
资助国家：
美国
起止时间：
2016-08-01 至 2020-07-31
项目状态：
已结题

来源：
https://www.nsf.gov/awardsearch/showAward?AWD_ID=1634669&HistoricalAwards=false
关键词：
Fe isotopes key understanding fluid

项目摘要

Serpentinization, the reaction of olivine and pyroxene, the two most prevalent minerals in the mantle in the oceanic lithosphere, with aqueous fluids is the primary alteration reaction in the ocean crust. Through this reaction, H2O is taken up into the rock and bound in the structure of minerals that replace olivine and pyroxene as they react with magmatic fluids and/or seawater. Vast volumes of ocean crust have undergone serpentinization; and this reaction results in a myriad of interesting and poorly known processes, such as the development of ultra-mafic hosted hydrothermal vents that are home to unusual microbes living both on and below the seafloor in the deep sea. In subduction zones, serpentinized oceanic lithosphere dives down into Earth's mantle where increasing temperature causes serpentine to react and release its bound H2O, which then lowers the melting point of overlying rocks and causes magmatism and island arc volcanism, such as that which occurs in Japan and the Alaskan Aleutian Islands, and cycles H2O from surface reservoirs back into the mantle. Thus, understanding serpentinization processes is essential for our knowledge of how the Earth works, how ore deposits associated with convergent tectonic margins are formed, and how life can exist deep in the ocean crust far from light and the input from organic matter settling down from the sea surface. This research provides a new means to understand the early part of the serpentinization process, using the isotopes of Iron (Fe) in minerals that form and form from serpentine. Samples from cores from holes drilled into the ocean crust will be analyzed as will samples from the Josephine Ophiolite in California and from the Oman and New Caledonian ophiolites, all of which represent seafloor that has been thrust onto the continentals. Analyses of mineral phases will be performed by electron microprobe. Electron Energy-Loss Spectroscopy will be used to determine the ferric iron content of the serpentines; and Fe isotopes will be measured on a thermal ionization mass spectrometer. Analysis of the resulting data will be assisted by thermodynamic modeling and determinations of the various oxidation states of Fe. The main goals of this research are to investigate the processes by which Fe in serpentinizing crust moves, determine how magnetite, a major Fe bearing mineral, forms during serpentinization, and explore how non-traditional stable isotopes of Fe can be used to track the oxidation and mobility of Fe irrespective of the formation of magnetite. Specific hypotheses are that the Fe isotopic signature of chlorite and temolite rims around olivine will be close to zero per mil because little to no magnetite is produced in the reactions. If the signature is found to be light, then it is likely to indicate Fe fractionated as it moved through the solution phase to create magnetite. It is further predicted that the Fe isotopic signature of magnetite in serpentine veins is heavy compared to that in the bulk rock. Additional mineral specific fractionation questions will be examined and addressed. Broader impacts of the work include support of faculty at the University of Wyoming, which is an institution in an EPSCoR state (i.e., state that does not receive significant federal funding). It also involves graduate student training in cutting-edge technology and international collaboration with Australian scientists. To increase public awareness of the research and the science of marine geology, the awardees will work closely with the University of Wyoming Geology Museum to create a new series of exhibits on the seafloor and ocean science. The project has additional broader impact in that it informs the economic geology and formation of ore deposits fields.

蛇纹石化是大洋岩石圈地幔中最常见的两种矿物橄榄石和辉石与水流体的反应，是大洋地壳中最主要的蚀变反应。通过这种反应，H2O被吸收到岩石中，并结合在矿物结构中，取代橄榄石和辉石，因为它们与岩浆流体和/或海水反应。大量的海洋地壳经历了蛇纹石化;这种反应导致了无数有趣但鲜为人知的过程，例如超镁铁质热液喷口的发展，这些喷口是生活在深海海底和海底下的不寻常微生物的家园。在俯冲带，蛇纹石化的海洋岩石圈潜入地幔，温度升高导致蛇纹石反应并释放其结合的H2O，然后降低上覆岩石的熔点并导致岩浆作用和岛弧火山作用，如日本和阿拉斯加阿留申群岛发生的那样，并将H2O从表面水库循环回地幔。因此，了解蛇纹石化过程对于我们了解地球如何运作，与会聚构造边缘有关的矿床如何形成，以及生命如何在远离光线和海洋表面沉积的有机物质的输入的海洋地壳深处生存至关重要。这项研究提供了一个新的手段来了解蛇纹石化过程的早期部分，利用铁（Fe）同位素的矿物形成和形成的蛇纹。将对钻到海洋地壳中的钻孔的岩芯样品进行分析，还将对来自加州的约瑟芬蛇绿岩以及阿曼和新喀里多尼亚蛇绿岩的样品进行分析，所有这些样品都代表了被冲到大陆上的海底。矿物相的分析将通过电子探针进行。电子能量损失光谱将用于确定蛇纹石的三价铁含量;铁同位素将在热电离质谱仪上测量。所得数据的分析将有助于热力学建模和确定的各种氧化态的铁。本研究的主要目的是调查的过程中，铁在蛇纹化地壳的移动，确定磁铁矿，一个主要的含铁矿物，形成蛇纹化过程中，并探讨如何非传统的稳定同位素的铁可以用来跟踪的氧化和移动的磁铁矿的形成无关。具体的假设是，橄榄石周围的铁和铁云母边缘的铁同位素特征将接近每密耳为零，因为反应中几乎没有磁铁矿产生。如果发现特征是轻的，那么它可能表明Fe在通过溶液相移动以产生磁铁矿时被分馏。进一步预测，蛇纹石脉中磁铁矿的Fe同位素特征比块状岩石中的重。将审查和解决其他矿物特定分馏问题。这项工作的更广泛影响包括对怀俄明州大学教师的支持，怀俄明州是EPSCoR州的一个机构（即，没有收到大量联邦资金的州）。它还涉及研究生尖端技术培训和与澳大利亚科学家的国际合作。为了提高公众对海洋地质学研究和科学的认识，获奖者将与怀俄明州大学地质博物馆密切合作，创造一个关于海底和海洋科学的新系列展览。该项目还具有更广泛的影响，因为它为矿床的经济地质和形成提供了信息。