Ce isotope measurements reveal the role of recycled sediments and the origin of the temporal variability of the Hawaiian plume source

夏威夷地幔柱是最突出的地幔柱。它的地球化学和地球物理研究使人们对地幔柱如何工作、地幔非均质性的起源以及地表和深层地幔之间的地球化学循环有了新的认识。然而，尽管有大量的研究，关于在喷发熔岩中观察到的化学变化的起源存在相互矛盾的模型。因此，需要新的分析工具来进一步确定夏威夷岩浆源的组成。本文拟对夏威夷瓦胡岛Koolau火山盾构期和夏威夷niihao火山盾构期到恢复期过渡时期的岩浆进行ce同位素比值分析。与其他夏威夷熔岩相比，库劳的Makapu 'u阶段拉斑岩和Niihau复兴熔岩在给定Nd同位素比率下具有更高的Hf。在这两种情况下，都援引了其来源中的再生碳酸盐或远洋沉积物成分。然而，其他模型主张地幔内部的融化和交代作用，而不需要再循环的沉积物。沉积物中La/Ce和Lu/Hf相对于Sm/Nd的亲子比值范围较大，使得Ce-Nd-Hf同位素比值组合成为一种特别敏感的工具，不仅可以追踪这些熔岩的存在，还可以追踪这些熔岩来源中的沉积物类型。作为正在进行的项目的一部分，新开发的高精度Ce同位素分析因此提供了一个明确的测试，以确定在夏威夷火山和地幔衍生熔体的来源中，上层沉积物与内部地幔分化过程（即熔融、交代）的作用。具体而言，我们将通过将新的ce同位素数据与现有的Hf-Nd-Sr-Pb同位素比值以及所选样品的主要元素和微量元素数据相结合，测试Koolau和Niihau熔岩源中是否存在回收碳酸盐或海洋沉积物。获得的数据还将有助于确定Ce-Nd同位素地幔阵列相对于球粒体参考点的位置，这对硅酸盐土的总体组成和控制组成演化的过程具有关键意义。该项目是与美国南卡罗莱纳大学Michael Bizimis教授合作完成的。比齐米斯教授手上有这些样品，已经生成了许多现有数据，并将为Ce同位素分析和新数据集的解释做出贡献。

The Hawaiian plume is the most prominent mantle plume. Its geochemical and geophysical study led to novel insights about how plumes work, the origin of mantle heterogeneity, and the geochemical cycling between the surface and deep mantle. Despite the large number of studies however, conflicting models exist about the origin of the chemical variability observed in the eruptive lavas. Therefore, new analytical tools are needed to further constrain the composition of the Hawaiian magma source. Here, we propose to analyze Ce-isotope ratios of well-characterized lavas from the shield stage of the Koolau volcano (Oahu, Hawaii) and lavas from the shield to rejuvenated stage transition in Niihau, Hawaii. The Makapu’u stage tholeiites of Koolau, and the Niihau rejuvenated lavas have higher Hf for given Nd isotope ratios compared to other Hawaiian lavas. In both cases, a recycled carbonate or pelagic sediment component in their source has been invoked. Yet, other models advocated melting and metasomatism within the mantle, without the need of recycled sediments. The large range of the parent-daughter ratios La/Ce and Lu/Hf relative to Sm/Nd in sediments make the combination of Ce-Nd-Hf isotope ratios a particularly sensitive tool to trace not only the presence, but also the type of sediments in the source of these lavas. Newly developed high-precision Ce isotope analyses, as part of the ongoing project, thus provide an explicit test for the role of pelagic sediments vs. internal mantle differentiation processes (i.e., melting, metasomatism) in the source of Hawaiian volcanics, and mantle-derived melts in general. Specifically, we will test whether recycled carbonates or pelagic sediments are present in the source of the Koolau and Niihau lavas, by combining new Ce-isotope data with existing Hf-Nd-Sr-Pb isotope ratios, and major and trace element data on the selected samples. The acquired data will also contribute to establish the position of the Ce-Nd isotope mantle array relative to the chondritic reference point, which has key implications for the bulk composition and the processes that govern the compositional evolution of the silicate earth. The project is a collaborative effort with Prof. Michael Bizimis, University of South Carolina, USA. Prof. Bizimis has the samples in hand, has generated much of the existing data, and will contribute to the Ce isotope analyses and the interpretation of the new data set.