TY - JOUR
T1 - Mantle source provinces beneath the Northwestern USA delimited by helium isotopes in young basalts
AU - Graham, D. W.
AU - Reid, M. R.
AU - Jordan, B. T.
AU - Grunder, A. L.
AU - Leeman, W. P.
AU - Lupton, J. E.
N1 - Funding Information:
David Hilton provided unpublished results for the three helium isotope analyses, performed in the late Harmon Craig's lab, on samples collected by W.P. Leeman. We thank the following colleagues for discussion and constructive criticism over the years: Vic Camp, Rick Carlson, Bob Duncan, Barry Hanan, Bill Hart, Gene Humphreys, Jenda Johnson and Doug Pyle. Reviews by Mark Kurz and an anonymous reviewer improved the manuscript. The helium isotope analyses were supported by the Earth Sciences Division of NSF (EAR 97-25166 and CSEDI 05-51934), and by the NOAA/PMEL vents program. W. P. Leeman acknowledges the National Science Foundation for providing time devoted to this project.
PY - 2009/11/20
Y1 - 2009/11/20
N2 - We report new He, Nd and Sr isotope results for basalts from the northwestern United States. The new 3He/4He 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 3He/4He 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, 3He/4He ranges from ~ 11 RA in the west to > 19 RA 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 3He/4He 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 3He/4He in basalts along the SRP, the effect is not strong and it does not obscure the elevated 3He/4He mantle source signature. In contrast, young basalts from the HLP and the OP have 3He/4He values of 8.8-9.3 RA, 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 3He/4He (7.6-8.3 RA), within the range for other Cascades arc lavas (7.0-8.4 RA). Three alternative explanations are possible for the origin of the high 3He/4He signature along the SRP: (1) multi-component mixing of (a) magmas and/or CO2-rich fluids derived from plume mantle having high 3He/4He, (b) continental lithosphere having low 3He/4He, 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 3He 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.
AB - We report new He, Nd and Sr isotope results for basalts from the northwestern United States. The new 3He/4He 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 3He/4He 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, 3He/4He ranges from ~ 11 RA in the west to > 19 RA 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 3He/4He 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 3He/4He in basalts along the SRP, the effect is not strong and it does not obscure the elevated 3He/4He mantle source signature. In contrast, young basalts from the HLP and the OP have 3He/4He values of 8.8-9.3 RA, 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 3He/4He (7.6-8.3 RA), within the range for other Cascades arc lavas (7.0-8.4 RA). Three alternative explanations are possible for the origin of the high 3He/4He signature along the SRP: (1) multi-component mixing of (a) magmas and/or CO2-rich fluids derived from plume mantle having high 3He/4He, (b) continental lithosphere having low 3He/4He, 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 3He 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.
KW - High Lava Plains
KW - Snake River Plain
KW - Yellowstone
KW - helium isotopes
KW - mantle plume
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U2 - 10.1016/j.jvolgeores.2008.12.004
DO - 10.1016/j.jvolgeores.2008.12.004
M3 - Article
AN - SCOPUS:60949099597
SN - 0377-0273
VL - 188
SP - 128
EP - 140
JO - Journal of Volcanology and Geothermal Research
JF - Journal of Volcanology and Geothermal Research
IS - 1-3
ER -