科研之友
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摘要
We report new He, Nd and Sr isotope results for basalts from the northwestern United States. The new (3)He/(4)He 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 (3)He/(4)He 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, (3)He/(4)He 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 (3)He/(4)He 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 (3)He/(4)He in basalts along the SRP, the effect is not strong and it does not obscure the elevated (3)He/(4)He mantle source signature. In contrast, young basalts from the HLP and the OP have (3)He/(4)He 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 (3)He/(4)He (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 (3)He/(4)He signature along the SRP: (1) multi-component mixing of (a) magmas and/or CO(2)-rich fluids derived from plume mantle having high (3)He/(4)He, (b) continental lithosphere having low (3)He/(4)He, 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 (3)He 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.
- 出版日期2009-11-20
- 单位国家自然科学基金委员会
