ISOSEA · ISOLATED SEAMOUNTS AS WINDOWS INTO UPPER MANTLE GEOCHEMICAL HETEROGENEITY

ISOLATED SEAMOUNTS AS WINDOWS INTO UPPER MANTLE GEOCHEMICAL HETEROGENEITY

The goal of the ISOSEA project was to characterize upper mantle heterogeneities using the composition of non-hotspot isolated seamounts and seamount provinces as windows into the upper mantle. To achieve this, we compiled published data for non-hotspot related seamounts and studied additional samples obtained from International Ocean Discovery Program (IODP) and the GEOMAR archives to fully characterize the petrological, geochemical and melting processes associated with them. We used samples from seamounts near and around the Mid-Pacific Mountains as the main case study of the project and as a proxy to understand similar seamount provinces. Although studies have considered the Mid-Pacific Mountains a Large Igneous Province (generally believed to be formed by mantle plumes) there have been variable models proposed for their origin to date, including plume and non-plume models. A major goal of this study was to determine the origin of the Mid-Pacific Mountains and surrounding seamount structures, specifically to determine if they are hotspot related or not. We also look at the Emperor seamount chain and related seamounts to get an idea of how hotspot volcanoes and related seamounts look geochemically in order to compare with the seamounts around the Mid-Pacific Mountains. Isolated seamounts and seamount provinces that are unrelated to deep mantle upwellings (mantle plumes), sample upper mantle heterogeneities and thus provide a window into compositional variations in the upper mantle. Our working hypothesis is that enriched reservoirs exist in the upper mantle, and they are a result of the shallow recycling of younger oceanic crust and continental lithospheric materials (upper and/or lower crust and subcontinental lithospheric mantle). The upper mantle is generally considered to be geochemically depleted due to the extraction of continental crust and the formation of oceanic crust at mid-ocean ridges. As a result, it typically exhibits unradiogenic (low) Sr and Pb isotopic compositions, along with radiogenic (high) Nd and Hf isotopic signatures (Hofmann, 1997). Nevertheless, enriched domains within the upper mantle may differ from this simplified model in one or more of these isotopic systems. Compared to deep mantle enriched reservoirs, such as HIMU and EM endmembers (Homrighausen et al., 2018; Jackson et al., 2018; Willbold and Stracke, 2010), these upper mantle heterogeneities are expected to develop over shorter timescales, reflecting the relatively faster recycling processes operating in the upper mantle compared to those in the lower mantle (Hoernle et al., 2006; Sobolev et al., 2007). As mentioned, we focused on one of the most isolated structures found on the seafloor: the Mid-Pacific Mountains (MPM), a large structure that extends ~2200km in an E-W direction in the middle of the Pacific Ocean. This feature comprises seamounts, guyots, volcanic elongated ridges, and thicker plateaus that sit on top of seafloor with a varying range of ages according to the available magnetic lineations (Seton et al., 2020); from west to east seafloor ages decrease from ~155 Ma to ~110 Ma. A seamount province located to the SE of the MPM, lies between the structure and the continuation of the Line Island hotspot track (Davis et al., 2002; Pockalny et al., 2021), however, unlike the hotspot seamounts, this seamount province does not seem to have an age progressive track and their relationship with the MPM and the Line Islands remain unclear. In the past, a series of seagoing expeditions occurring in the 70’s and 90’s targeted several locations within and around the MPM, including expeditions DSDP 17-164, -165A, -166, -167, -169 & -170, DSDP32-313, and ODP 143-865A & 866A, which drilled boreholes on top of guyots, seamounts, plateaus, ridges, and normal ocean seafloor (Fig. 1). Even though the goals of each of the cores were variable (sedimentology, paleontology, paleoclimate reconstructions, for example), in all cases the drilling continued until reaching the basaltic bedrock, penetrating a few cm to m into the rocks. In most cases, limited geochemical analyses were performed on the basalts and very few included radiogenic isotope measurements or 40Ar/39Ar dating, yielding incomplete datasets. Thus, for the ISOSEA Project we requested samples from the IODP Gulf Coast Repository pertaining to the mentioned drilled cores (Fig. 1). In each case, we reviewed all the available report data, including photographs and descriptions of the cores to ensure that we were requesting the least altered material. From the ~60 samples received, 35 fulfill the criteria for fresh material (i.e. absence of secondary minerals, lack of filled vesicles or fractures, unaltered phenocrysts, dark grey to black groundmasses). Prior to this study, these unique samples from the non-hotspot-related seamounts and ridges had not been comprehensively analyzed, lacking both high-resolution geochemical and geochronological data acquired using modern analytical techniques. Through the ISOSEA project, we conducted state-of-the-art analyses that resulted in critical new insights into the origin, composition, and tectonic evolution of this isolated region. These results provide an essential contribution to our understanding of Pacific plate history and offer a robust geochemical framework for interpreting intraplate volcanism in this part of the ocean basin. References Davis, A.S., Gray, L.B., Clague, D.A., Hein, J.R., 2002. The Line Islands revisited: New 40Ar/39Ar geochronologic evidence for episodes of volcanism due to lithospheric extension. Geochemistry, Geophysics, Geosystems 3, 1–28. https://doi.org/10.1029/2001GC000190 Hoernle, K., White, J.D.L., van den Bogaard, P., Hauff, F., Coombs, D.S., Werner, R., Timm, C., Garbe-Schönberg, D., Reay, A., Cooper, A.F., 2006. Cenozoic intraplate volcanism on New Zealand: Upwelling induced by lithospheric removal. Earth and Planetary Science Letters 248, 350–367. https://doi.org/10.1016/j.epsl.2006.06.001 Hofmann, A.W., 1997. Mantle geochemistry: the message from oceanic volcanism. Nature 385, 219–229. Homrighausen, S., Hoernle, K., Hauff, F., Geldmacher, J., Wartho, J.-A., van den Bogaard, P., Garbe-Schönberg, D., 2018. Global distribution of the HIMU end member: Formation through Archean plume-lid tectonics. Earth-Science Reviews 182, 85–101. https://doi.org/10.1016/j.earscirev.2018.04.009 Jackson, M.G., Becker, T.W., Konter, J.G., 2018. Evidence for a deep mantle source for EM and HIMU domains from integrated geochemical and geophysical constraints. Earth and Planetary Science Letters 484, 154–167. Pockalny, R., Barth, G., Eakins, B., Kelley, K.A., Wertman, C., 2021. Multiple melt source origin of the Line Islands (Pacific Ocean). Geology 49, 1358–1362. https://doi.org/10.1130/G49306.1 Seton, M., Müller, R.D., Zahirovic, S., Williams, S., Wright, N.M., Cannon, J., Whittaker, J.M., Matthews, K.J., McGirr, R., 2020. A Global Data Set of Present-Day Oceanic Crustal Age and Seafloor Spreading Parameters. Geochemistry, Geophysics, Geosystems 21, e2020GC009214. https://doi.org/10.1029/2020GC009214 Sobolev, A.V., Hofmann, A.W., Kuzmin, D.V., Yaxley, G.M., Arndt, N.T., Chung, S.-L., Danyushevsky, L.V., Elliott, T., Frey, F.A., Garcia, M.O., Gurenko, A.A., Kamenetsky, V.S., Kerr, A.C., Krivolutskaya, N.A., Matvienkov, V.V., Nikogosian, I.K., Rocholl, A., Sigurdsson, I.A., Sushchevskaya, N.M., Teklay, M., 2007. The Amount of Recycled Crust in Sources of Mantle-Derived Melts. Science 316, 412–417. https://doi.org/10.1126/science Willbold, M., Stracke, A., 2010. Formation of enriched mantle components by recycling of upper and lower continental crust. Chemical Geology 276, 188–197. https://doi.org/10.1016/j.chemgeo.2010.06.005

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