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.

蛇纹石化是海洋岩石圈地幔中两种最常见的矿物橄榄石和辉石与水相流体的反应，是海洋地壳中的主要蚀变反应。通过这种反应，H2O 被吸收到岩石中并结合在矿物结构中，当橄榄石和辉石与岩浆液和/或海水发生反应时，它们会取代橄榄石和辉石。大量的洋壳已经经历了蛇纹石化；这种反应导致了无数有趣且鲜为人知的过程，例如超镁铁质热液喷口的发展，这些热液喷口是生活在深海海底上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发生反应并释放其结合的 H2O，然后降低上覆岩石的熔点并引发岩浆作用和岛弧火山活动，例如在日本和日本发生的情况。阿拉斯加阿留申群岛，并将水从地表水库循环回地幔。因此，了解蛇纹石化过程对于我们了解地球如何运作、与聚合构造边缘相关的矿床如何形成、生命如何存在于远离光线的海洋地壳深处以及从海洋沉积下来的有机物质的输入等方面的知识至关重要。海面。这项研究利用蛇纹石形成和形成的矿物中的铁 (Fe) 同位素，为了解蛇纹石化过程的早期部分提供了一种新方法。将分析来自钻入海洋地壳的孔的岩心样本，以及来自加利福尼亚州约瑟芬蛇绿岩以及阿曼和新喀里多尼亚蛇绿岩的样本，所有这些样本都代表已被推向大陆的海底。矿物相的分析将通过电子显微探针进行。电子能量损失光谱将用于测定蛇纹石的三价铁含量； Fe同位素将在热电离质谱仪上测量。热力学建模和铁的各种氧化态的测定将有助于对所得数据的分析。这项研究的主要目标是研究蛇纹石化地壳中铁的移动过程，确定磁铁矿（一种主要含铁矿物）在蛇纹石化过程中如何形成，并探索如何使用铁的非传统稳定同位素来追踪氧化和 Fe 的迁移率，与磁铁矿的形成无关。具体假设是，橄榄石周围的绿泥石和闪沸石边缘的铁同位素特征将接近于每密耳零，因为反应中几乎不产生磁铁矿。如果发现特征很轻，则很可能表明 Fe 在穿过溶液相形成磁铁矿时发生分馏。进一步预测，蛇纹岩脉中磁铁矿的铁同位素特征比块体岩石中的铁同位素特征重。将检查和解决其他特定矿物的分馏问题。这项工作的更广泛影响包括怀俄明大学教师的支持，该大学是 EPSCoR 州（即没有获得大量联邦资助的州）的一所机构。它还涉及尖端技术的研究生培训以及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，获奖者将与怀俄明大学地质博物馆密切合作，举办一系列关于海底和海洋科学的新展览。该项目还具有更广泛的影响，因为它为经济地质和矿床形成提供了信息。