Mantle source provinces beneath the Northwestern USA delimited by helium isotopes in young basalts

We report new He, Nd and Sr isotope results for basalts from the northwestern United States. The new He-3/He-4 results for olivine phenocrysts in basalts from the eastern Snake River Plain (SRP), the Owyhee Plateau (OP) and the Oregon High Lava Plains (HLP), together with published He isotope data for Yellowstone and the Cascades volcanic arc, delineate distinct mantle sources for each of these sub-provinces. All basalts from the eastern SRP (8 Quaternary localities plus 1 Miocene locality) have He-3/He-4 ratios higher than observed in normal mid-ocean ridge basalts, but overlapping with ranges observed in hotspot-related oceanic islands. For a lateral distance of some 400 km along the SRP, He-3/He-4 ranges from similar to 11 R-A in the west to >19 R-A adjacent to Yellowstone. Such high ratios have not been observed elsewhere in the western U.S., and are consistent with the presence of a mantle plume. The lateral gradient in He-3/He-4 suggests that the proportion of plume-derived He decreases westward, but this interpretation is complicated by possible addition of crustal helium during open-system crystal fractionation in some SRP basaltic magmas. Although crustal contamination may modulate He-3/He-4 in basalts along the SRP, the effect is not strong and it does not obscure the elevated He-3/He-4 mantle source signature. In contrast, young basalts from the HLP and the OP have He-3/He-4 values of 8.8-9.3 R-A, within the range for mid-ocean ridge basalts; these data reflect a shallow asthenospheric source with no discernible influence from the Yellowstone hotspot. Basalts from Newberry volcano have slightly lower He-3/He-4 (7.6-8.3 R-A), within the range for other Cascades arc lavas (7.0-8.4 R-A). Three alternative explanations are possible for the origin of the high He-3/He-4 signature along the SRP: (1) multi-component mixing of (a) magmas and/or CO2-rich fluids derived from plume mantle having high He-3/He-4, (b) continental lithosphere having low He-3/He-4, and (c) shallow asthenospheric mantle (MORB source); (2) a mantle plume beneath Yellowstone that has an unusual combination of He, Nd and Sr isotope characteristics: or (3) a continental lithospheric mantle that experienced ancient enrichment of He-3 relative to (U + Th). The isotope relations between He-Nd and He-Sr, along with other considerations, generally favor the first explanation, but the other possibilities cannot be ruled out at the present time. (C) 2008 Elsevier B.V. All rights reserved.