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Mantle Source Provinces Beneath the Northwestern US

Mantle Source Figure 4

Click image to enlarge. Map of 3He/4He in the western U.S. Geologic/physiographic provinces are outlined. Locality abbreviations are CRB=Columbia River Basalts, HLP=High Lava Plains, YSRP=Yellowstone-Snake River Plain, OP=Owyhee Plateau, GV=Gem Valley, ML=Medicine Lake. The dashed line for 87Sr/86Sr = 0.706 outlines the known edge of the ancient North American craton. The helium variations reveal three distinct tectono-magmatic provinces in the northwestern US; 1) the Cascades volcanic arc including Newberry volcano, 2) the High Lava Plains and Owyhee Plateau, and 3) the Yellowstone-Snake River Plain system.

3He/4He ratios in olivine phenocrysts from basaltic lavas from Yellowstone, the eastern Snake River Plain (SRP), the Oregon High Lava Plains (HLP), and the high Cascades delineate distinct volcanic sources beneath each of these provinces. The elevated 3He/4He ratios for Yellowstone and the Snake River Plain are unique in the western US (see figure), and are consistent with the presence of deep mantle plume beneath that region, as suggested by seismic tomography. In contrast, the High Lava Plains source is the shallow upper mantle based on the similarity of those lavas to mid-ocean ridge basalts. The Cascades (including Newberry Volcano, previously suggested to be derived from a deep mantle hotspot) also have compositions dominated by a shallow upper mantle source, but with some influence from the subducting Juan de Fuca Plate.

References

Graham, D. W., M. R. Reid, B. T. Jordan, A. L. Grunder, W. P. Leeman, J. E. Lupton (2009) Mantle source provinces beneath the northwestern USA delimited by helium isotopes in young basalts. J. Volc. Geotherm. Res. 188, 128-140. doi:10.1016/ j.volgeores. 2008.12.004. PDF

Projects

Mid-ocean Ridges and Upper Mantle Processes

Mantle Plumes and Geodynamics of Earth's Interior