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

海洋岩石圈中地幔中最常见的两种矿物橄榄石和辉石与含水流体发生的蛇纹岩化反应是海洋地壳的主要蚀变反应。通过这种反应，水被带入岩石，并结合在矿物结构中，这些矿物在与岩浆流体和/或海水反应时取代橄榄石和辉石。大量的海洋地壳经历了蛇纹岩化；这种反应导致了无数有趣而鲜为人知的过程，比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，蛇纹岩化的海洋岩石圈潜入地幔，温度升高导致蛇纹岩反应并释放其结合的水，然后降低上覆岩石的熔点，引起岩浆活动和岛弧火山活动，例如发生在日本和阿拉斯加阿留申群岛的火山活动，并将水从地表储层循环回地幔。因此，了解蛇纹石化过程对于我们了解地球是如何工作的，与汇聚构造边缘相关的矿床是如何形成的，以及生命是如何在远离光线的海洋地壳深处存在的，以及从海面沉降下来的有机物是如何输入的。本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阶段提供了一种新的手段。从海洋地壳钻孔中提取的岩心样本将被分析，从加利福尼亚的约瑟芬蛇绿岩、阿曼和新喀里多尼亚蛇绿岩中提取的样本也将被分析，所有这些蛇绿岩都代表了被推入大陆的海底。矿物相的分析将由电子探针进行。电子能量损失谱法将用于测定蛇纹石中三铁的含量；和铁的同位素将在热电离质谱仪上测量。结果数据的分析将通过热力学建模和铁的各种氧化态的测定来辅助。本研究的主要目的是研究铁在蛇纹石化地壳中的运动过程，确定蛇纹石化过程中主要含铁矿物磁铁矿的形成过程，并探索如何利用铁的非传统稳定同位素来跟踪铁的氧化和迁移，而不考虑磁铁矿的形成。具体的假设是，绿泥石和橄榄石周围的铁同位素特征将接近于每毫升零，因为在反应中几乎没有产生磁铁矿。如果发现信号较轻，那么很可能表明铁在通过溶液阶段产生磁铁矿时发生了分馏。进一步预测了蛇纹岩脉中磁铁矿的铁同位素特征较大块岩石重。将审查和解决其他矿物特定分选问题。这项工作的更广泛的影响包括对怀俄明大学教师的支持，怀俄明大学是EPSCoR州的一所机构（即，没有收到大量联邦资金的州）。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，创建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油田的形成提供了信息。

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.