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

夏威夷热柱是最突出的地幔热柱。它的地球化学和地球物理研究导致了对地柱如何工作、地幔不均质性的起源以及地幔表层和深层之间的地球化学循环的新见解。然而，尽管有大量的研究，但关于喷发熔岩中观察到的化学变异性的起源，仍然存在相互矛盾的模型。因此，需要新的分析工具来进一步限制夏威夷岩浆源的成分。在这里，我们建议分析来自库劳火山(夏威夷瓦胡岛)屏蔽期的特征良好的熔岩和夏威夷尼豪岛从屏蔽期到复兴期过渡的熔岩的Ce同位素比值。与其他夏威夷熔岩相比，库劳Makapu‘u期拉斑玄武岩和Nihau复活熔岩具有更高的Hf。在这两种情况下，都援引了来源中的循环碳酸盐或远洋沉积物成分。然而，其他模型主张地幔内的熔融和交代作用，而不需要回收的沉积物。沉积物中La/Ce和Lu/Hf的母子比相对于Sm/Nd具有很大的范围，这使得Ce-Nd-Hf同位素比值组合成为追踪这些熔岩来源中沉积物类型的特别敏感的工具。因此，作为正在进行的项目的一部分，新开发的高精度Ce同位素分析提供了对夏威夷火山岩来源和地幔内部分异过程(即熔融、交代)的作用的明确测试，以及一般地幔熔体的作用。具体地说，我们将通过将新的Ce同位素数据与现有的Hf-Ndsr-Pb同位素比率以及所选样品的常量和微量元素数据相结合，来测试Koolau和Nihau熔岩的源头是否存在循环碳酸盐或远洋沉积物。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陨石参考点的位置，这对硅酸盐地球的块体成分和控制成分演化的过程具有关键意义。该项目是与美国南卡罗来纳大学的Michael Bizimis教授合作完成的。比齐米斯教授手头有样品，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.