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 20Ne/22Ne 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 20Ne/22Ne (11.33±0.08, 1σ) value in this study. Two subaerial Samoan samples from Ofu Island with high 3He/4He ratios (Ofu-04-06 and Ofu-04-03; 34 and 24 Ra [ratio to atmosphere], respectively) also have elevated 20Ne/22Ne ratios relative to air, and in a three-isotope neon plot the Ofu samples are indistinguishable from measurements made on high 3He/4He 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 87Sr/86Sr (0.70458 for Ofu vs. 0.70329–0.70368 for Hawaii–Iceland–Galapagos) and 143Nd/144Nd (Ofu's 0.51283 vs. 0.51294–0.51297 for Hawaii–Iceland–Galapagos) in high 3He/4He 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 (87Sr/86Sr > 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.