LETTERS The return of subducted continental crust in Samoan lavas Matthew G. Jackson 1 , Stanley R. Hart 2 , Anthony A. P. Koppers 3,4 , Hubert Staudigel 3 , Jasper Konter 3 , Jerzy Blusztajn 2 , Mark Kurz 2 & Jamie A. Russell 3 Substantial quantities of terrigenous sediments are known to enter the mantle at subduction zones, but little is known about their fate in the mantle 1 . Subducted sediment may be entrained in buoyantly upwelling plumes and returned to the Earth’s surface at hotspots 2–5 , but the proportion of recycled sediment in the mantle is small, and clear examples of recycled sediment in hotspot lavas are rare 6,7 . Here we report remarkably enriched 87 Sr/ 86 Sr and 143 Nd/ 144 Nd 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 87 Sr/ 86 Sr 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 ratios 8 —that complement the enriched 87 Sr/ 86 Sr and 143 Nd/ 144 Nd isotope sig- natures. The geochemical evidence from these Samoan lavas sig- nificantly 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. The Earth’s mantle, as sampled by ocean island basalts erupted at hotspots, is chemically and isotopically heterogeneous. However, the origin of the geochemical heterogeneity of the mantle is not well understood. One model for the geochemical evolution of the mantle assumes that much of the chemical diversity is a result of subduction, a tectonic process that introduces enriched oceanic crust and com- positionally heterogeneous sediment into a largely primitive (or slightly depleted) mantle 5,10,11 . Following subduction, these surface materials mix with a peridotitic mantle, thus imprinting their enriched chemical and isotopic signatures on its various domains. A number of isotopically distinct geochemical reservoirs, as sampled by ocean island basalts, have resulted from this process. The isotopic endmembers are often referred to as HIMU (high m 5 238 U/ 204 Pb), EM1 (enriched mantle 1) and EM2 (enriched mantle 2) and DMM (depleted mid-ocean-ridge basalt mantle) 9 . Although the most radio- genic Pb isotope ratios observed in the HIMU component have been proposed to result from a contribution of recycled oceanic crust 9,12 , most models for the creation of the EM1 and EM2 mantle reservoirs invoke a small portion of lithologically distinct sediments that have been recycled into the mantle 9,13 . The volcanically active Samoan islands and seamounts define a hotspot track with a classical EM2 pedigree 7,14,15 . The first high- precision 87 Sr/ 86 Sr and 143 Nd/ 144 Nd measurements from Samoan lavas were interpreted as evidence of sediment recycling 5 . Recently, however, the proposed recycled sediment origin of the enriched Samoan basalts has been questioned (see Supplementary Discussion), and an alternative model favouring source enrichment by metasomatic processes was proposed 7 . The extreme isotopic and chemical enrichment in the new Samoan EM2 lavas exhibit distinctly continental fingerprints, and argue for a role for a component similar to ancient recycled upper continental crust (UCC) in the Samoan plume (see Supplementary Discussion for the ALIA 2005 cruise dredge locations and geochemical data). The most isotopically enriched Samoan whole-rock 87 Sr/ 86 Sr sig- nature (0.720469, Mg# 5 57.2) is recorded in a trachyandesite, dredge sample D115-21, which was taken from the southwestern flank of Savai’i. Clinopyroxene mineral separates from the same sample yielded an even higher 87 Sr/ 86 Sr ratio (0.721630). A trachy- basalt (D115-18) hosts the second-most-enriched 87 Sr/ 86 Sr (0.718592, Mg# 5 58.7), and clinopyroxene mineral separates from the sample also gave more enriched ratios (0.720232–0.720830). Six other lavas recovered in the same dredge also exhibit enriched 87 Sr/ 86 Sr ratios (0.708175–0.716394, Mg# 5 52.0–65.1). Dredge D118, located on the far western end of the Savai’i lineament, con- tained an alkali basalt with enriched 87 Sr/ 86 Sr (0.710337, measured on fresh clinopyroxene). Dredge D128, taken on the northeastern flanks of Savai’i, yielded a transitional basalt with a high 87 Sr/ 86 Sr ratio (0.712500, Mg# 5 70.5) and several other basalts with less enriched 87 Sr/ 86 Sr (0.706397–0.708170, Mg# 5 61.2–63.9). Dredge D114, taken on the southwestern flanks of Savai’i, provided younger shield basalts of transitional chemistry and normal 87 Sr/ 86 Sr (0.705422–0.705435, Mg# 5 67.2 and 76.3). The 87 Sr/ 86 Sr isotopes in the basalts from all three ultra-enriched sampling localitites are complemented by enriched (low) 143 Nd/ 144 Nd and the lowest 3 He/ 4 He ratios (4.31–4.93 Ra, or ratio to atmosphere) observed in Samoan basalts. Together, the new data extend the Samoan isotope array to a region outside the global ocean island basalt field (Fig. 1). Highly enriched EM2 signatures have previously been observed only in metasomatized xenoliths from Savai’i ( 87 Sr/ 86 Sr up to 0.712838; ref. 16), and the Samoan EM2 basalts provide the first evidence that the enriched component hosted in these xenoliths also occurs as erupted basalts. The enriched 87 Sr/ 86 Sr and 143 Nd/ 144 Nd isotope ratios, coupled with the low 3 He/ 4 He, are consistent with a recycled UCC component in the man- tle source of the Samoan EM2 basalts. The UCC reservoir exhibits several diagnostic trace-element char- acteristics that can be useful for detecting its presence in Samoan EM2 lavas. Compared to ocean island basalt and mid-ocean-ridge basalt lavas, UCC displays exceptional depletion in Nb (and Ta), Ti and Eu, and enrichment in Pb (Fig. 2). Samoan basalts have 1 Massachusetts Institute of Technology, Woods Hole Oceanographic Institution Joint Program, 2 Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543-1525, USA. 3 Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California 92093-0225, USA. 4 College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331-5503, USA. Vol 448 | 9 August 2007 | doi:10.1038/nature06048 684 Nature ©2007 Publishing Group