EAGER: Collaborative Research: Testing the marine carbonate recycling hypothesis

这个早期概念探索研究基金（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（一种形成于海底的常见沉积岩）在夏威夷、圣赫勒拿岛和库克-奥斯特拉斯群岛等海洋岛屿上喷发的火山岩中的包裹物。这种地壳再循环过程的证实将产生若干重大影响，推动地球科学领域的发展。海洋碳酸盐具有特别高的放射性母体U（铀）含量，其衰变为放射性子同位素Pb（铅）。该项目将使用一种新的地球化学代理组合来验证这一假设。地表沉积物通过俯冲带进入海洋火山岩源的再循环在以前就有记录。然而，该项目将尝试探索Ca（钙）和Mg（镁）稳定同位素的变化是否可以作为追踪俯冲碳酸盐这一过程的好方法。最后，它还将帮助我们更好地约束地幔中的各种化学成分，并进一步阐明地球随时间的演变。这些成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。许多（如果不是全部的话）海洋玄武岩的不同寻常的放射性成因铅同位素比率产生了三个主要问题，即所谓的铅悖论，与它们的地幔来源有关。这些源必须具有1)长时间积分高U/Pb， 2)长时间积分低Th/U和3)恒定的Ce/Pb和Nb/U的特征。虽然目前对个别Pb悖论提出的一些解决方案非常令人满意，但它们通常是独立的，相互矛盾的。然而，Pb悖论是相互关联的，并构成了一个应该一起解决的方程系统，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岩具有较高的U/Pb和U/Th比值，可以熔解俯冲的海洋板块，从而产生一些海洋玄武岩。熔体中含有富集和贫化地幔组分的混合物（即在再循环地壳与所提出的FOZO和/或DMM地幔组分之间）可以产生海洋玄武岩中的放射性成因铅同位素比率和伴随的铅悖论。该试点项目将通过对一组精选的极端海洋玄武岩进行分析，并结合常规地球化学方法，使用耦合Ca和Mg同位素分析来验证所提出的海洋碳酸盐循环假设。这些结果将有可能改变我们对地幔地球动力学的理解，包括海洋岛屿玄武岩的形成、地壳物质的再循环、地幔对流以及地幔成分的历史和进化。

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.