Resolving lithospheric and sub-lithospheric contributions to helium isotope variations in basalts from the southwestern US

The ³He/⁴He ratios of continental basalts and their peridotitic xenoliths delimit the (U + Th)/³He evolution of continental lithospheric mantle, but their interpretation is complicated by the potential for differential mobility of He with respect to lithophile elements. In this study we report ³He/⁴He ratios of 5.5-7.7 R_A for olivines separated from young basalts in the southwestern US. Helium isotope ratios are lowest in samples that exhibit Th and Pb isotope evidence for derivation from an enriched source in the lithospheric mantle that has remained unmodified since ∼ 1.7 Ga. There is a strong, positive correlation between ³He/⁴He and ∈_Nd in southwestern US lavas, suggesting a time-integrated affinity between light rare earth- and (U + Th)/³He-enrichment in the underlying Proterozoic lithospheric mantle. This relationship further suggests that the Proterozoic lithosphere mantle in the southwestern US is probably not a highly degassed reservoir which is contaminated by He derived from the underlying asthenosphere. Instead, ³He/⁴He ratios of 5.7 ± 0.3 R_A in the most enriched source regions suggest that their (U + Th)/³He is only slightly elevated compared to the depleted upper mantle, and that processes which enrich the lithospheric mantle in large-ion lithophile elements also enrich it in He.