1. Cenozoic Basaltic Volcanism in the Pacific Northwest Richard W. Carlson DTM, Carnegie Institution of Washington William K. Hart Miami University Timothy L. Grove Massachusetts Institute of Technology EAR-CD

2. A Long-Lived Volcanic Margin Did not Start or Stop with the Flood Basalt Episode Miocene and YoungerEocene to Jurassic Modified from Geologic Map of the United States by Phillip King and Helen Beikman with HTML implementation by William Menke

3. What is Causing all this Volcanism? Colored Regions Show Volcanic Deposits Younger than 17 Million Years (Smith and Luedke, 1984) Diamond Craters Cascades Newberry Subduction?

4. What is Causing all this Volcanism? Colored Regions Show Volcanic Deposits Younger than 17 Million Years (Smith and Luedke, 1984) Yellowstone The Yellowstone Plume? Plume-like characteristics of SRP volcanism 1)Time progressive volcanism 2)Large volume volcanism 3)High 3 He/ 4 He 4)Topographic swell 5)Positive gravity anomaly 6)Enhanced seismicity 7)Seismically imaged plume conduit?

5. Compositional Variation in Space and Time Modified from Geologic Map of the United States by Phillip King and Helen Beikman with HTML implementation by William Menke Time Slices: 1) 0-14 Ma – Modern Cascades, HLP, SRP 2) 14-18 Ma – Flood basalt era 3) 19 – 195 Ma – Pre-flood basalt era

6. Compositional Variation in Space and Time – Rock Type HLP Data Compiled by NAVDAT

7. Compositional Variation in Space and Time – Mantle Input HLP Primitive Basalts 8%<MgO<12% Data Compiled by NAVDAT

8. Compositional Variation in Space and Time - Subduction Contribution Primitive Basalts 8%<MgO<12% HLP Data Compiled by NAVDAT

9. Open Compositional Variation - Lithosphere Input Open > 70% SiO 2 Filled < 56% SiO 2 HLP Precambrian North America CascadesCascades Data compiled by NAVDAT

10. Crustal Input Cannot be Neglected! Mantle Crust FC AFC Carlson et al., 1981 Grande Ronde Steens, Picture Gorge

11. What Causes the Lithophile Isotope Variation? 1)Subducted PNW offshore sediments 2)Archean lithosphere (mantle or crust?) Data sources: Hawaii, Juan de Fuca, Cascades, HLP - EarthChem; HAOT - Hart, Carlson; Newberry - Carlson, Grove, Donnelly-Nolan

12. The Subduction Contribution 87 Sr/ 86 Sr = 0.70317 187 Os/ 188 Os = 0.1518 87 Sr/ 86 Sr = 0.70382 187 Os/ 188 Os = 0.1922 87 Sr/ 86 Sr = 0.703751

13. Helium The Clearest (Only?) Chemical Signal of Deep Mantle Involvement Province-specific He isotope variation with SRP basalts outside the range typical of upper mantle (MORB, Arc) melts (Data and figures from Graham et al. JVGR 2009)

14. Both He and Sr (and Nd, Hf, Pb) Show Longitudinal Variation with a big Step at the PC border. To the East, Sr (and Nd, Hf, Pb) Move Towards More Lithospheric Compositions, While He Moves to More “Deep Mantle” Composition! How Does the “Plume” Know the Location of the Lithospheric Boundary?

15. Good Correspondence Between low Velocities and Sites of Recent HLP Volcanism, but only in Uppermost Mantle. SRP a Line of low Velocity to ~200 km N = Newberry, D = Diamond Crater, J = Jordan Crater

16. Conclusions: Flood basalt and HLP magmatism show strong regional isotopic and temporal compositional variation - lithosphere is playing an important role in modifying the composition of the melts. No geochemical signal of a deeper mantle “plume” signature, but a significant “subduction” compositional signature. Pb isotopes consistent with addition of fluids from PNW offshore sediments. Young volcanic centers underlain by low seismic velocities to depths of 100- 150 km. HLP volcanism is a shallow phenomena.

17. Conclusions: SRP basalts lack the subduction compositional signal and have high 3 He/ 4 He suggestive of a deeper mantle source. Low S-wave speeds continue deeper beneath SRP than HLP. High Ti and Ba contents suggest SRP basalts may be relatively lower degree melts and low Al and high Fe contents suggest deeper melting. Lithophile-element isotope systems (Sr, Nd, Pb) suggestive of a source in, or significant overprinting by, Archean lithospheric mantle.