He, Sr and Nd isotopes in xenoliths from Hawaii and other oceanic islands

Isotopic analyses on He (whole rock) and Sr, Nd (separated clinopyroxenes) are reported for ultramafic and gabbroic xenoliths from the Hawaiian Islands, Canary Islands, Kerguelen, Gough and Madeira. The He isotopic compositions in samples of varied petrologic type from Hawaii show a remarkably narrow range ( R/R a= 8.2–9.5 ), are within the previously reported range for Hawaiian xenoliths and are indistinguishable from the range of 3He/ 4He ratios seen in MORB. In contrast, Sr and Nd isotopic compositions show a significant range. Samples from the Canary Islands, Kerguelen, Gough and Madeira, considered as a group, show a similar trend with a restricted range in 3He/ 4He ( 5.2–7 × R a ) contrasted by Sr and Nd isotopic compositions extending over almost the entire range of oceanic basalts. The relationship between He and Sr Nd isotopes of ocean island xenoliths are strikingly similar to that observed for continental ultramafic xenoliths by Porcelli et al. (1986) [1]. These systematics suggest metasomatic introduction of lithophile (Sr and Nd) and volatile (He) elements into the lithosphere beneath ocean islands, overprinting depleted mantle isotopic characteristics. The addition of volatiles must be accomplished by a different process to that of lithophiles, probably due to selective transport, most likely migration of a lithophile-poor CO 2 H 2O fluid. Consideration of He Sr data for oceanic basalts suggests that similar fluid-related processes may operate prior to or during mantle melting. If so, the interpretation of He Sr relationships of oceanic basalts in terms of simple mixing between isotopically well-defined sources is likely to be invalid.