New constraints on the HIMU mantle from neon and helium isotopic compositions of basalts from the Cook–Austral Islands Rita Parai a, ⁎, Sujoy Mukhopadhyay a , John C. Lassiter b a Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, MA 02138, USA b Department of Geological Sciences, University of Texas at Austin,1 University Station C1100, Austin, TX 78712, USA abstract article info Article history: Received 23 June 2008 Received in revised form 19 October 2008 Accepted 20 October 2008 Available online 2 December 2008 Editor: R.W. Carlson Keywords: helium neon isotopes HIMU mantle Cook–Austral Islands High 4 He/ 3 He ratios of 100 000 to 160 000 found at HIMU ocean islands (“high μ,” where μ is the U/Pb ratio) are usually attributed to the presence of recycled oceanic crust in the HIMU mantle source. However, significantly higher 4 He/ 3 He ratios are expected in recycled crust after residence in the mantle for periods greater than 1 Ga. In order to better understand the helium isotopic signatures in HIMU basalts, we have measured helium and neon isotopic compositions in a suite of geochemically well-characterized basalts from the Cook–Austral Islands. We observe 4 He/ 3 He ratios ranging from 56 000 to 141000, suggesting the involvement of mantle reservoirs both more and less radiogenic than the mantle source for mid-ocean ridge basalts (MORBs). In addition, we find that the neon isotopic compositions of HIMU lavas extend from the MORB range to compositions less nucleogenic than MORBs. The Cook-Austral HIMU He–Ne isotopic compositions, along with Sr, Nd, Pb, and Os isotopic compositions, indicate that in addition to recycled crust, a relatively undegassed mantle end-member (e.g., FOZO) is involved in the genesis of these basalts. The association of relatively undegassed mantle material with recycled crust provides an explanation for the close geographical association between HIMU lavas and EM (enriched mantle)-type lavas from this island chain: EM-type signatures represent a higher mixing proportion of relatively undegassed mantle material. Mixing between recycled material and relatively undegassed mantle material may be a natural result of entrainment processes and convective stirring in deep mantle. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Ocean island volcanism is attributed to the upwelling of mantle plumes from a boundary layer deep within the mantle. Consequently, ocean island basalts (OIBs) serve as chemical probes of the Earth's interior. Highly variable chemical and isotopic compositions observed in OIBs, relative to mid-ocean ridge basalts (MORBs), indicate the existence of distinct chemical reservoirs within the mantle (Zindler and Hart, 1986a; Hart et al., 1992; Farley et al., 1992; Graham, 2002; Porcelli and Ballentine, 2002). Helium and neon are powerful tracers of these mantle sources, and can potentially constrain the origin of chemical heterogeneities in the mantle. Basalts derived from mantle melting exhibit a large range of helium isotopic compositions. MORBs that are removed from the influence of hotspots exhibit relatively homogeneous 4 He/ 3 He ratios, ranging from 80 000 to 100 000 (Kurz et al., 1982; Graham et al., 1992b; Graham et al., 1996). The MORB helium isotopic composition reflects a balance between radiogenic production of 4 He and volatile depletion by degassing and melt extraction from the convective mantle along mid- ocean ridges. OIBs exhibit greater variation in helium isotopic ratios than MORBs, with 4 He/ 3 He ratios ranging from ~15000 to 200 000 (Kurz et al., 1983; Farley et al., 1992; Hilton et al., 1999; Kurz and Geist, 1999; Graham 2002; Stuart et al., 2003). Low 4 He/ 3 He ratios (b 80 000) measured at ocean islands such as Hawaii, Iceland, Galapagos, and Samoa are conventionally attributed to the sampling of a relatively undegassed mantle source with low time-integrated (U+Th)/ 3 He (Kurz et al., 1983; Farley et al., 1992; Hilton et al., 1999; Kurz and Geist, 1999; Moreira et al., 1999; Stuart et al., 2003). On the other hand, basalts from HIMU ocean islands (“high μ,” where μ is U/Pb) exhibit 4 He/ 3 He ratios higher than the canonical MORB range. For example, HIMU lavas from St. Helena, the Canary Islands, Tubuai, and Mangaia exhibit 4 He/ 3 He ratios that vary from 100000 to 160000 (Vance et al., 1989; Graham et al., 1992a; Hanyu and Kaneoka, 1997; Barfod et al., 1999; Hanyu et al., 1999; Hilton et al., 2000). Radiogenic Pb and Os isotopic compositions observed in HIMU basalts suggest that significant quantities of recycled oceanic crust are present in the HIMU source (Hofmann and White, 1982; Zindler et al., 1982; Palacz and Saunders, 1986; Hauri and Hart, 1993). The observed HIMU 4 He/ 3 He signature also indicates a component that is more radiogenic than the MORB mantle, and therefore appears to be consistent with the hypothesis that the HIMU signature derives from recycled oceanic crust. However, if HIMU lavas are largely derived from Earth and Planetary Science Letters 277 (2009) 253–261 ⁎ Corresponding author. Tel.: +1 617 384 9335; fax: +1 617495 8839. E-mail address: parai@fas.harvard.edu (R. Parai). 0012-821X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2008.10.014 Contents lists available at ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl