MANTLE SOURCE PROVINCES BENEATH THE PACIFIC NORTHWEST REVEALED BY HELIUM ISOTOPE VARIATIONS IN BASALTIC LAVAS

Helium isotope variations in continental basalts provide valuable constraints on mantle source relationships. ³He/⁴He ratios of 7 to 9 R_A (where R_A is the air ratio) are typical of mid-ocean ridge basalts and indicate a source within the convecting upper mantle. Lower (more radiogenic) ³He/⁴He reflects additional contributions from the lithospheric mantle or continental crust. Elevated ³He/⁴He, above 10 R_A, indicates derivation from a region with lower time-integrated (U+Th)/³He, usually taken to be a deep mantle (thermochemical plume) source.

Helium isotope results for Quaternary basalts from the eastern Snake River Plain (SRP), the Owyhee Plateau and the Oregon High Lava Plains (HLP), along with published data for Yellowstone and the Cascades volcanic arc, provide insight to the different mantle sources beneath these regions. The westward-migrating silicic volcanism of the HLP enigmatically mirrors the eastward progression of volcanism along the SRP, and is sometimes taken as evidence against the YSRP system as a hotspot track produced by a mantle plume currently located beneath Yellowstone. However, all basalts from the eastern SRP have high ³He/⁴He. Furthermore, they display a gradient over a lateral distance of  400 km,  from >13 R_A in the west to >19 R_A adjacent to Yellowstone. Such high ratios are absent elsewhere in the western US. The high ³He/⁴He ratios along the SRP indicate the presence of mantle plume-derived material, either in the shallow asthenosphere or emplaced within the continental lithosphere downstream from Yellowstone. The gradient suggests that the proportion of Yellowstone plume material decreases westward. In contrast, basalts from the HLP and the Owyhee Plateau have lower ³He/⁴He and a narrow range of 8.8-9.3 R_A. Basalts from Newberry Volcano have ³He/⁴He = 7.6-8.3 R_A and overlap the range for the Cascades arc (7.0-8.4 R_A).  Basalts from the HLP therefore have ³He/⁴He similar to mid-ocean ridge basalts, reflecting a shallow asthenospheric source with no discernible influence from the Yellowstone hotspot. Collectively, these ³He/⁴He variations reveal three distinct tectono-magmatic provinces in the northwestern US; 1) the Cascades volcanic arc, 2) the High Lava Plains-Owyhee Plateau, and 3) the Yellowstone-Snake River Plain system.