2007 GSA Denver Annual Meeting (2831 October 2007)
Paper No. 169-1
Presentation Time: 1:35 PM-1:55 PM

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

GRAHAM, D.W., College of Oceanic & Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, dgraham@coas.oregonstate.edu, REID, M.R., Department of Geology, Northern Arizona University, Flagstaff, AZ 86011, JORDAN, B.T., Department of Earth Sciences, University of South Dakota, Vermillion, SD 57069, GRUNDER, A.L., Department of Geosciences, Oregon State University, Corvallis, OR 97331, LEEMAN, W.P., Division of Earth Sciences, National Science Foundation, Arlington, VA 22230, and LUPTON, J.E., Pacific Marine Environmental Lab, National Oceanic & Atmospheric Administration, Hatfield Marine Science Center, Newport, OR 97365

Helium isotope variations in continental basalts provide valuable constraints on mantle source relationships. 3He/4He ratios of 7 to 9 RA (where RA 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 RA, 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 RA in the west to >19 RA 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 RA. Basalts from Newberry Volcano have 3He/4He = 7.6-8.3 RA and overlap the range for the Cascades arc (7.0-8.4 RA).  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.

2007 GSA Denver Annual Meeting (2831 October 2007)
General Information for this Meeting
Session No. 169
The Track of the Yellowstone Hot Spot II: What do Neotectonics, Climate Indicators, Volcanism, and Petrogenesis Reveal about Subsurface Processes?
Colorado Convention Center: 401/402
1:30 PM-5:30 PM, Tuesday, 30 October 2007

Geological Society of America Abstracts with Programs, Vol. 39, No. 6, p. 455

© Copyright 2007 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.