Substantial quantities of terrigenous sediments are known to enter the mantle at subduction zones, but little is known about their fate in the mantle1. Subducted sediment may be entrained in buoyantly upwelling plumes and returned to the Earth’s surface at hotspots2,3,4,5, but the proportion of recycled sediment in the mantle is small, and clear examples of recycled sediment in hotspot lavas are rare6,7. Here we report remarkably enriched 87Sr/86Sr and 143Nd/144Nd isotope signatures in Samoan lavas from three dredge locations on the underwater flanks of Savai’i island, Western Samoa. The submarine Savai’i lavas represent the most extreme 87Sr/86Sr isotope compositions reported for ocean island basalts to date. The data are consistent with the presence of a recycled sediment component (with a composition similar to the upper continental crust) in the Samoan mantle. Trace-element data show affinities similar to those of the upper continental crust—including exceptionally low Ce/Pb and Nb/U ratios8—that complement the enriched 87Sr/86Sr and 143Nd/144Nd isotope signatures. The geochemical evidence from these Samoan lavas significantly redefines the composition of the EM2 (enriched mantle 2; ref. 9) mantle endmember, and points to the presence of an ancient recycled upper continental crust component in the Samoan mantle plume.
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We thank A. Hofmann and W. White for reviews. We thank N. Shimizu, R. Workman and R. Rudnick for discussions, and J. Natland for sharing unpublished data. This study was supported by National Science Foundation grants. We are grateful to the scientific party and ships crew of the R/V Kilo Moana for the success of the ALIA 2005 expedition (http://earthref.org/ERESE/projects/ALIA/).
Author Contributions M.G.J. performed most of the experimental work, developed the model and wrote the paper. S.R.H. and H.S. conceived the project, and were co-chiefs of the ALIA expedition. A.A.P.K. and J.K. were responsible for the cruise bathymetry, and A.A.P.K. greatly improved the figures. J.B. and M.K. provided analytical assistance and access to facilities. A.A.P.K., J.B., J.K. and J.A.R. helped with sample preparation. All authors participated in the discussion and interpretation of results, and commented on the manuscript.
Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests.
The file contains a Supplementary Discussion that gives a model of the Samoan EM2 source, including calculations and assumptions. The file also contains a Supplementary Table 1 and a Supplementary Table 2 that together provide the model parameters used to calculate the depleted Ta’u and EM2 Samoan sources, respectively. The file also contains a Supplementary Table 3 that provides the new Samoan geochemical data. Three supplementary figures also are included in the file: Supplementary Figure 1 shows new helium isotope data from the remarkably enriched Samoan lavas; Supplementary Figure 2 is an expanded view of main text Figure 1, and shows the composition of the Samoan lavas in the context of individual samples from the upper continental crust; Supplementary Figure 3 compares the spidergram of a model melt of the EM2 source with the spidergram of an isotopically-enriched Samoan EM2 lava. (PDF 2886 kb)
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Jackson, M., Hart, S., Koppers, A. et al. The return of subducted continental crust in Samoan lavas. Nature 448, 684–687 (2007). https://doi.org/10.1038/nature06048
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