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EAGER: Collaborative Research: Testing the marine carbonate recycling hypothesis

EAGER：合作研究：测试海洋碳酸盐回收假说

基本信息

批准号：
1747706
负责人：
Fangzhen Teng
金额：
$ 4.96万
依托单位：
University of Washington
依托单位国家：
美国
项目类别：
Standard Grant
财政年份：
2017
资助国家：
美国
起止时间：
2017-08-01 至 2018-07-31
项目状态：
已结题

来源：
https://www.nsf.gov/awardsearch/showAward?AWD_ID=1747706&HistoricalAwards=false
关键词：
EAGER Collaborative Research Testing marine

项目摘要

This EArly-concept Grant for Exploratory Research (EAGER) project will test if it is possible to trace geochemically the inclusion of ancient marine carbonates, a common type of sedimentary rock formed in the ocean floor, in volcanic rocks currently erupting in oceanic islands such as Hawaii, Saint Helena and the Cook-Australs. The confirmation of such a crustal recycling process would have a number of significant implications that would advance the field of Earth Sciences. Marine carbonates have a distinctively high content of the radioactive parent U (Uranium), which decays to radiogenic daughter Pb (Lead) isotopes. This project will test this hypothesis using a new combination of geochemical proxies. Recycling of surface sediments via subduction zones into the source of oceanic volcanic rocks has been documented before. The project, however, will try to explore if variations of the stable isotopes of Ca (Calcium) and Mg (Magnesium) could be a good way of tracking this process for subducted carbonates. Finally, it would also help us better constrain the various chemical components in the Earth's mantle and further elucidate the evolution of our planet over time. These results will be accessible and disseminated through presentations in scientific meetings and publications in scientific journals. The unusually radiogenic Pb isotopic ratios of many, if not all, oceanic basalts have generated three major problems, or so-called Pb paradoxes, regarding their mantle sources. These sources must be characterized by 1) long time-integrated high U/Pb, 2) long time-integrated low Th/U and 3) constant Ce/Pb and Nb/U. Although some of the currently proposed solutions to individual Pb paradoxes are highly satisfactory, they are generally independent and at odds with each other. The Pb paradoxes, however, are inter-related and constitute a system of equations that should be solved all together, as the solution to each paradox must also be able explain the other paradoxes. This study hopes to test a hypothesis that recycled marine carbonates, with their high U/Pb and U/Th ratios, can flux-melt the subducted oceanic slab to produce some oceanic basalts. Melts containing mixtures of enriched and depleted mantle components (i.e., between recycled crust and the proposed FOZO and/or DMM mantle components) can generate the radiogenic Pb isotopic ratios and concomitant Pb paradoxes in oceanic basalts. This pilot project will verify the proposed marine carbonate recycling hypothesis through the analysis of a select set of extreme oceanic basalts using coupled Ca and Mg isotopic analysis combined with conventional geochemical methods. The results will have the potential to transform our understanding of mantle geodynamics including the formation of ocean island basalts, recycling of crustal materials, mantle convection, and history and evolution of the composition of the mantle.

这个早期概念探索研究拨款(AGERGE)项目将测试是否有可能从地球化学角度追踪在夏威夷、圣赫勒拿和南库克群岛等海洋岛屿目前喷发的火山岩中是否存在古代海相碳酸盐--一种在海底形成的常见类型的沉积岩。确认这一地壳再循环过程将产生一些重大影响，将推动地球科学领域的发展。海相碳酸盐具有非常高的放射性母体铀(铀)含量，它会衰变为放射性的子体铅(铅)同位素。这个项目将使用一种新的地球化学指标组合来检验这一假设。通过俯冲带将表层沉积物再循环成大洋火山岩的来源，以前就有过记载。然而，该项目将尝试探索钙(钙)和镁(镁)稳定同位素的变化是否可以很好地跟踪俯冲碳酸盐的这一过程。最后，它还将帮助我们更好地约束地幔中的各种化学成分，并进一步阐明我们的星球随时间的演变。这些成果将通过在科学会议上的陈述和在科学期刊上发表的出版物来获取和传播。许多大洋玄武岩(如果不是全部的话)异常的放射性成因铅同位素比率产生了关于其地幔来源的三个主要问题，即所谓的铅悖论。这些源的特征是：1)长时间积分高U/Pb，2)长时间积分低Th/U，3)Ce/Pb和Nb/U恒定。虽然目前提出的一些解决单个铅悖论的方法是非常令人满意的，但它们通常是独立的，相互矛盾的。然而，铅悖论是相互关联的，构成了一个应该一起解决的方程体系，因为每个悖论的解决方案也必须能够解释其他悖论。这项研究希望验证一种假设，即回收的海相碳酸盐具有高U/Pb和U/Th比值，可以熔融俯冲的洋板，产生一些大洋玄武岩。含有富集型和贫型地幔组分的熔体(即在再循环地壳和建议的FOZO和/或DMM地幔组分之间)可以在大洋玄武岩中产生放射性成因的铅同位素比值和伴生的铅悖论。这个试点项目将通过对一组极端的大洋玄武岩进行分析，结合常规的地球化学方法，利用钙和镁同位素分析来验证所提出的海相碳酸盐循环假说。这些结果将有可能改变我们对地幔地球动力学的理解，包括洋岛玄武岩的形成、地壳物质的再循环、地幔对流以及地幔成分的历史和演化。