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

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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.