Helium and neon isotopes in phenocrysts from Samoan lavas: Evidence for heterogeneity in the terrestrial high 3He/4He mantle

We report the first neon isotope measurements on phenocrysts from subaerial and submarine Samoan lavas. These new data complement existing Ne-isotope data from Samoan peridotite xenoliths (Poreda, R.J., Farley, K.A.,1992. Rare gases in Samoan xenoliths. Earth Planet Sci. Lett. 113, 689 129-144). Neon isotopes were measured in magmatic phenocrysts separated from nine shield-stage lavas from 5 Samoan volcanoes, including Ofu, Ta'u, Vailulu'u, Malumalu and Savai'i Phenocrysts from subaerial Samoan lavas have higher Ne-20/Ne-22 than submarine phenocrysts, which may suggest different atmospheric contamination mechanisms for these two different eruptive environments. Olivine phenocrysts from a subaerial Ta'u lava exhibit the highest Ne-20/Ne-22 (11.33 +/- 0.08, 1 sigma) value in this study. Two subaerial Samoan samples from Ofu Island with high He-3/He-4 ratios (Ofu-04-06 and Ofu-04-03; 34 and 24 Ra [ratio to atmosphere], respectively) also have elevated Ne-20/Ne-22 ratios relative to air, and in a three-isotope neon plot the Ofu samples are indistinguishable from measurements made on high He-3/He-4 lavas from Hawaii, Iceland and Galapagos. The similarity of helium and neon isotopes in Ofu and Hawaii-Iceland-Galapagos is remarkable given the distinctions in Sr-87/Sr-86 (0.70458 for Ofu vs. 0.70329-0.70368 for Hawaii-Iceland-Galapagos) and Nd-143/Nd-144 (Ofu's 0.51283 vs. 0.51294-0.51297 for Hawaii-Iceland-Galapagos) in high He-3/He-4 lavas from these localities. Thus, the occurrence of similar helium and neon isotope compositions in lavas from these 4 hotspots does not appear to be linked to their variable Sr and Nd isotope compositions, and demonstrates a complex relationship between the isotopes of noble gases and the more refractory radiogenic isotopes. Additionally, we observe strongly nucleogenic neon isotopes in one the most isotopically-enriched (Sr-87/Sr-86>0.718) Samoan lavas. This observation is consistent with the presence of recycled, continentally-derived marine sediment in the mantle source of the most enriched lavas from Samoa. (C) 2009 Elsevier B.V. All rights reserved.