RESOLVING LITHOSPHERIC AND SUB-LITHOSPHERIC CONTRIBUTIONS TO HELIUM ISOTOPE VARIATIONS IN BASALTS FROM THE SOUTHWESTERN US

The He-3/He-4 ratios of continental basalts and their peridotitic xeno liths delimit the (U+Th)/He-3 evolution of continental lithospheric ma ntle, but their interpretation is complicated by the potential for dif ferential mobility of He with respect to lithophile elements. In this study we report He-3/He-4 ratios of 5.5-7.7 R(A) for olivines separate d from young basalts in the southwestern US. Helium isotope ratios are lowest in samples that exhibit Th and Pb isotope evidence for derivat ion from an enriched source in the lithospheric mantle that has remain ed unmodified since similar to 1.7 Ga. There is a strong, positive cor relation between He-3/He-4 and epsilon(Nd) in southwestern US lavas, s uggesting a time-integrated affinity between light rare earth- and (UTh)/He-3-enrichment in the underlying Proterozoic lithospheric mantle. This relationship further suggests that the Proterozoic lithosphere m antle in the southwestern US is probably not a highly degassed reservo ir which is contaminated by He derived from the underlying asthenosphe re. Instead, He-3/He-4 ratios of 5.7+/-0.3 R(A) in the most enriched s ource regions suggest that their (U+Th)/He-3 is only slightly elevated compared to the depleted upper mantle, and that processes which enric h the lithospheric mantle in large-ion lithophile elements also enrich it in He.