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Shun-ichiro Karato Yale University Department of Geology & Geophysics New Haven, CT

Water distribution in the Earth ’ s mantle Inferred from Electrical Conductivity implications for the global water cycle. Shun-ichiro Karato Yale University Department of Geology & Geophysics New Haven, CT. Conclusions.

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Shun-ichiro Karato Yale University Department of Geology & Geophysics New Haven, CT

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  1. Water distribution in the Earth’s mantle Inferred from Electrical Conductivityimplications for the global water cycle Shun-ichiro Karato Yale University Department of Geology & Geophysics New Haven, CT

  2. Conclusions • Electrical conductivity is a useful sensor for the water content in the mantle. • Water content is both radially and laterally heterogeneous. • A large contrast in water content between the upper mantle and the transition zone suggests partial melting at ~410-km.  Most of the upper mantle is partially melted (melt fraction is small and does not affect properties except for seismic wave velocities in the deep upper mantle).  Partial melting at 410-km stabilizes the ocean mass.

  3. How to infer the distribution of water from geophysical observations? X X ? X * * X X *: mostly for the upper mantle Properties involving thermally activated processes are sensitive to water content. Lab studies are more complete for electrical conductivity than for Q and LPO.

  4. seismic wave velocity versus water content Seismic velocities are insensitive to water content.

  5. Influence of water on seismic discontinuities oli wad wad oli Topography of discontinuities is insensitive to water content (at high T).

  6. electrical conductivity from geophysical studies Kelbert et al. (2009) Tarits et al. (2004) Ichiki et al. (2006) Baba et al. (2010)

  7. olivine, orthopyroxene, garnet, wadsleyite, ringwoodite wadsleyite Dai and Karato (2009b)

  8. Sensitivity of electrical conductivity to T, Cw, fO2, Mg#  Electrical conductivityis sensitive to Cw, but not to other parameters.

  9. Testing the model for the upper mantle pyrolite (olivine+opx+pyrope), SIMS water calibration [Dai and Karato (2009)]

  10. Electrical conductivity and water in the mantle Geophysical model Mineral physics model

  11. X Water content is layered (+ lateral heterogeneity)  Partial melting at ~ 410-km

  12. What happens after 410-km melting? Most of the upper mantle is partially melted (with a small melt fraction). a thick low velocity layer (due to complete wetting)

  13. thick low velocity regions above the 410-km (Tauzin et al. 2010)

  14. No mid-mantle melting With mid-mantle melting  410-km partial melting stabilizes the ocean mass.

  15. conclusions • Water content (Cw) in the transition zone/upper mantle can be mapped from electrical conductivity observations. • Mantle water content is layered. • ~0.01 wt% for the upper mantle, ~0.1 wt% for the transition zone • partial melting at 410-km • a majority of the upper mantle is partially melted. • a thick low velocity layer above 410-km • Ocean mass is buffered by partial melting at 410-km • Need for experimental studies on lower mantle minerals • Need for geophysical observations for the lower mantle

  16. Dixon et al. (2002) Ito et al. (1983) MORB source region (asthenosphere): well constrained (~0.01 wt%) OIB source regions: water-rich (FOZO) (~0.1 wt%) How are they distributed? localized? global (layered)?

  17. Influence of element partitioning H Fe wadsleyite

  18. Water-temperature distribution from VP,S and MTZ thickness Meier et al. (2009) puzzling results <-- due to insensitivity of seismological properties to water content? <-- radial heterogeneity in water content? <-- influence of kinetics on phase boundary topography?

  19. Water may affect seismological observations • T-effect and water-effect on seismic wave velocities • T-effect and water-effect on the phase boundary h V

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