EAGER: Collaborative Research: Testing the marine carbonate recycling hypothesis

这个早期概念的探索性研究补助金（EAGER）项目将测试是否有可能在地球化学上追踪古代海洋碳酸盐岩的内含物，这是一种常见的海底沉积岩，目前在夏威夷，圣赫勒拿和库克-南方群岛等海洋岛屿爆发的火山岩。这种地壳再循环过程的确认将产生若干重大影响，推动地球科学领域的发展。海洋碳酸盐中放射性母铀（铀）含量特别高，衰变为放射性子铅（铅）同位素。本项目将使用一种新的地球化学代用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到海洋火山岩的来源之前已有文献记载。然而，该项目将试图探索Ca（钙）和Mg（镁）的稳定同位素的变化是否可以成为追踪俯冲碳酸盐岩这一过程的好方法。最后，它还将帮助我们更好地限制地球地幔中的各种化学成分，并进一步阐明我们星球随时间的演变。这些结果将通过在科学会议上的介绍和在科学期刊上的出版物提供和传播。异常放射成因铅同位素比值的许多，如果不是所有的，大洋玄武岩产生了三个主要问题，或所谓的铅悖论，关于他们的地幔来源。这些源区的特征是：（1）长时间积分的高U/Pb，（2）长时间积分的低Th/U，（3）恒定的Ce/Pb和Nb/U。虽然目前提出的一些解决方案，个别铅悖论是非常令人满意的，他们通常是独立的，并在相互矛盾。然而，铅悖论是相互关联的，并构成了一个方程组，应该一起解决，因为每个悖论的解决方案也必须能够解释其他悖论。本研究希望验证一个假设，即回收的海洋碳酸盐，具有高U/Pb和U/Th比值，可以熔融俯冲洋板，产生一些大洋玄武岩。含有富集和贫化地幔成分混合物的熔体（即，再循环地壳与FOZO和/或DMM地幔成分之间的相互作用）可以产生大洋玄武岩中放射成因Pb同位素比值和伴随的Pb反常现象。这一试点项目将利用钙镁同位素分析结合常规地球化学方法，对一组选定的极端大洋玄武岩进行分析，从而验证拟议的海洋碳酸盐再循环假说。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。

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.