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Cenozoic Basaltic Volcanism in the Pacific Northwest

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. A Long-Lived Volcanic Margin

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Cenozoic Basaltic Volcanism in the Pacific Northwest

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  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 Younger Eocene 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? Subduction? Colored Regions Show Volcanic Deposits Younger than 17 Million Years (Smith and Luedke, 1984) Cascades Diamond Craters Newberry

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

  5. Compositional Variation in Space and Time 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 Modified from Geologic Map of the United States by Phillip King and Helen Beikman with HTML implementation by William Menke

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

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

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

  9. Compositional Variation - Lithosphere Input Open Cascades Open > 70% SiO2 Filled < 56% SiO2 Precambrian North America HLP Data compiled by NAVDAT

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

  11. What Causes the Lithophile Isotope Variation? • Subducted PNW offshore sediments • 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 87Sr/86Sr = 0.703751 87Sr/86Sr = 0.70317 187Os/188Os = 0.1518 87Sr/86Sr = 0.70382 187Os/188Os = 0.1922

  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 3He/4He 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.

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