Relationships between Noble Gases and Indicators of Geochemical Enrichment in Carbonatite Metasomatized Xenoliths from Samoa

Rejuvenated lavas from oceanic hotspots can host peridotite mantle xenoliths that provide windows into the composition and history of the upper mantle. Peridotite xenoliths hosted in Samoan rejuvenated lavas from the Island of Savaiíi exhibit evidence for variable enrichment by carbonatitic melts. Some xenoliths are trace element depleted, while others host incompatible element budgets on par with enriched alkali basalts (Hauri and Hart, 1994). The most incompatible element enriched xenoliths have 87Sr/86Sr ratios (up to 0.7128) that are the highest observed in the oceanic mantle. Poreda and Farley (1992) examined the noble gas isotopic compositions of a different suite of xenoliths from the same locality. They identified a narrow range of 3He/4He ratios (8.6 to 12.04 Ra), while the neon isotopic compositions spanned an enormous range, from the Loihi-Kileaua line (unradiogenic) to the MORB line. The xenoliths with the highest 3He/4He have neon isotopes that are most unradiogenic, while samples with lower 3He/4He have MORB-like neon isotopic compositions. It is not known which xenoliths in this suite have experienced carbonatite metasomatism, as lithophile element concentrations were not reported by Poreda and Farley (1992), and the noble gas signature associated with carbonatite metasomatism beneath Samoa is unknown. We report new helium measurements on olivine and orthopyroxene separates from 14 Savaii peridotite xenoliths that were previously characterized by Hauri and Hart (1994) and Hauri et al (1993). All samples are extremely fresh. Their 3He/4He ratios span 11.6 to 12.41 Ra, at the high end of the range reported by Poreda and Farley (1992). We also report whole-rock trace element abundances on these xenoliths (trace elements on individual clinopyroxenes from each sample were reported by Hauri and Hart and Hauri et al (1993)). The xenoliths that exhibit trace element evidence for enrichment from carbonatite fluids (low Nb/U, and high La/Nd and Nb/Ta) have the lowest 3He/4He. By contrast, the most trace element depleted xenoliths have higher 3He/4He. These trace element correlations suggest that the carbonatitic fluid responsible for the enrichment has lower 3He/4He. Based on the known correlations between helium and neon, this would suggest that the lower 3He/4He is also associated with more radiogenic neon isotopes , and that the trace element depleted xenoliths should have unradiogenic neon. The samples with the highest 4He concentrations (approximately 1.46E-7 to 7.5E-7 g/cc) will be further analyzed for Ne and Ar isotopic compositions.