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Shun-ichiro Karato Yale University New Haven, CT, USA

Geophysical Anomalies in the Central Pacific Upper Mantle Implications for Water Transport by a Plume. Shun-ichiro Karato Yale University New Haven, CT, USA. Outline. Anomalies in the central Pacific upper mantle Seismic anisotropy Electrical conductivity Some mineral physics background

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Shun-ichiro Karato Yale University New Haven, CT, USA

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  1. Geophysical Anomalies in the Central Pacific Upper MantleImplications for Water Transport by a Plume Shun-ichiro Karato Yale University New Haven, CT, USA VLab-workshop

  2. Outline • Anomalies in the central Pacific upper mantle • Seismic anisotropy • Electrical conductivity • Some mineral physicsbackground • Seismic anisotropy and water • Electrical conductivity and water • Conclusions • Plume-asthenosphere interaction VLab-workshop

  3. Upper mantle in the central Pacifichas unusually strong VSH>VSV anisotropy. Ekström and Dziewonski (1998) VLab-workshop

  4. Upper mantle in the central Pacific has unusually low conductivity. Karato (2007) Electrical conductivity, S/m VLab-workshop

  5. Cause for anomalies How do various laterally varying parameters affect seismic anisotropy and electrical conductivity? Mineral physics of anisotropy and conductivity Temperature X Major element chemistry X Partial melting X O Water (hydrogen) O VLab-workshop

  6. Seismic anisotropy = elastic anisotropy of minerals + LPO (lattice-preferred orientation) Temperature or major element chemistry (e.g., Fe/Mg) does not change the elastic anisotropy so much. --> A change in LPO should be a cause. VLab-workshop

  7. LPO changes with physical/chemical conditions. • LPO is determined by the dominant slip system(s). • Conventional interpretation of anisotropy assumes one type of slip system ([100](010)). • If the dominant slip system changes, LPO will change (fabric transition), then the nature of seismic anisotropy will change. • How could the dominant slip system change with physical/chemical conditions? olivine VLab-workshop

  8. LPO is controlled by the relative strength of slip systems. Deformation with b = [001] slip systems is more enhanced by water than deformation with b = [100] slip systems. Could fabric transition occur at higher water content? [Karato (1995)] -->simple shear deformation experiments (change in the slip direction) at higher water fugacity (0.3 GPa-->15 GPa) VLab-workshop

  9. Olivine LPO Conventional interpretation of seismic anisotropy is based on the limited observations and assumed A-type fabric. But lab studies in my group have shown that other LPOs dominate under different conditions. Karato et al. (2007) VLab-workshop

  10. Influence of water content and stress on deformation fabrics of olivine (at T~1500-1600 K (asthenospheric temperature)) Seismic anisotropy in the asthenosphere, plume roots is likely caused by A-, or E- or C-type olivine fabrics depending on the water content. E- or C-type fabric.In the typical asthenosphere, dominant fabric is likely E-type. VLab-workshop

  11. A-type olivine fabric causes strong VSH>VSV anisotropy. E-type olivine fabric causes weak VSH>VSV anisotropy. c E-type b a b A-type c a Karato (2007) VLab-workshop

  12. Influence of water (hydrogen) on electrical conductivity in olivine • A large amount of hydrogen can be dissolved in olivine. • Hydrogen diffusion is fast. • Hydrogen may enhance conductivity?(Karato, 1990) Electrical conductivity, S/m Wang et al. (2006) Water content VLab-workshop

  13. Conductivity in “normal” asthenosphere can be explained by a typical water content (~0.01 wt%).Conductivity in the central Pacific corresponds to “dry” olivine. VLab-workshop

  14. Anomalies can be attributed to the dry asthenosphere in the central Pacific. Why is the asthenosphere of the central Pacific dry? What is the role of the Hawaii plume on modifying the properties of the central Pacific asthenosphere? VLab-workshop

  15. Roles of a plume to modify the composition of the asthenosphere • Plume: enriched (undepleted) = more water • direct mixing does not explain “dry” asthenosphere. • plume=wet + hot-> deep melting-> depleted materials VLab-workshop

  16. deep melting in a plume (from Hirschmann (2006)) VLab-workshop

  17. In a plume column, melting occurs in the deep asthenosphere, providing “depleted” (dry) materials to the asthenosphere. VLab-workshop

  18. A plume will feed “depleted (dry)” materials to the asthenosphere due to deep melting --> cause for geophysical anomalies in the central Pacific? Karato et al. (2007) VLab-workshop

  19. Conclusions • Both seismic anisotropy and electrical conductivity in the central Pacific are anomalous. • These anomalies can be attributed to a low water (hydrogen) content in the asthenosphere in this region. • A plume supplies “depleted (dry)” materials to the asthenosphere due to deep melting. VLab-workshop

  20. A three-dimensional fabric diagram of olivine Karato et al. (2007) B A Couvy et al. (2004) C C E E VLab-workshop

  21. At low stress and high T, A-, E- or C-type olivine fabrics will be important. VLab-workshop

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