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Seismological Tests (and Implications) of Post-Perovskite Presence in the Deep Mantle

Seismological Tests (and Implications) of Post-Perovskite Presence in the Deep Mantle. Thorne Lay. University of California, Santa Cruz Department of Earth and Planetary Sciences. In collaboration with Ed Garnero, Alex Hutko John Hernlund. Lay et al., EOS , 2005.

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Seismological Tests (and Implications) of Post-Perovskite Presence in the Deep Mantle

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  1. Seismological Tests (and Implications) of Post-Perovskite Presence in the Deep Mantle Thorne Lay University of California, Santa Cruz Department of Earth and Planetary Sciences In collaboration with Ed Garnero, Alex Hutko John Hernlund

  2. Lay et al., EOS, 2005 Is the D” S-wave velocity discontinuity first observed by Lay and Helmberger (1983) caused by the Perovskite-to-Post-perovskite phase change discovered in 2004? Lay, Garnero and Williams, PEPI, 2004

  3. A Diversity of P- and S-Velocity Structures in D” is Observed MgSiO3 Pv-pPv Lay and Garnero, AGU Monograph, in press, 2007

  4. Synthetic predictions for P-wave models Model PWDK Model WOOK Weber and Davis [1993] Wookey et al. [2005] Pv-pPv phase change model Pv-pPv predicts little P wave reflectivity Wookey et al. 2005 Wentzcovich et al. 2006 PdP ? Lay and Garnero, AGU Monograph, 2007

  5. P-Wave Predictions and Models P-wave models are not easily reconciled with predictions of the Pv-pPv transition, but the predicted effects are small. How to explain large P- discontinuties in some regions? Anisotropy? Effect of Fe, Al? Pv-pPv Lay and Garnero, AGU Monograph, in press, 2007

  6. S-Wave Predictions and Models Some S-wave models can be reconciled with predictions of the Pv-pPv transition, and the predicted effects are observable. But some models are not consistent. How to explain S- velocity decreases? Fe, Al, anisotropy? Pv-pPv Lay and Garnero, AGU Monograph, in press, 2007

  7. Most Studied Region: Caribbean, Central America, Cocos Plate A dozen S wave studies indicate wide- spread S discontinuity 200-300 km above the CMB High S velocity, slightly High P velocity

  8. Complexity of D” reflector observed beneath the Cocos Plate. Lay, Garnero, Russell, GRL, 2004

  9. Complexity of reflectors may imply thermal complexity affecting the phase boundary. Seismic migration is Used to image 3D structure. Thomas, Garnero, Lay, JGR, 2004

  10. Rapid changes in depth of reflector appear to exist; needs special circumstances to account for by phase boundary. Hutko, Lay, Garnero and Revenaugh, Nature, 2006

  11. Step in S velocity supported. P velocity decreases (?), may be two reflectors for P (?), near 340 km and 190 km above the CMB. Kito, Rost, Thomas, Garnero, GJI, 2007

  12. P velocity structure investigation with very large data set. Hutko, Lay, Revenaugh, Garnero, in prep., 2007

  13. Can we determine sharpness of the reflectors? S increase Hutko, Lay, Revenaugh, Garnero, in prep., 2007

  14. Are the P velocity decreases really there? Maybe, but <0.5%. S increase Hutko, Lay, Revenaugh, Garnero, in prep., 2007

  15. LLSVPs have Distinctive Vs/Vp Behavior Shear velocities anti-correlated with bulk sound velocities. Lay, Garnero, Williams, PEPI, 2004

  16. Large low shear velocity provinces (LLSVPs) are usually thought to be warm; hence no post-perovskite, or only in a thin layer. But, this appears to be wrong. S-wave discontinuities are seen where they “shouldn’t be”. Lay, Hernlund, Garnero and Thorne, Science, 2006

  17. Lay, Hernlund, Garnero and Thorne, Science, 2006

  18. Paired velocity discontinuities may be due to double-crossing of the phase boundary in lowermost thermal boundary layer, as suggested by Hernlund, Thomas, and Tackley, Nature, 2005 Lay, Hernlund, Garnero and Thorne, Science, 2006

  19. The local CMB heat flux can be estimated; depends on the Clapeyron slope of Pv-pPv and on excess temperature above the phase boundary at the CMB, along with thermal conductivity (poorly constrained). My preference ~85 mW/m2 Lay, Hernlund, Garnero and Thorne, Science, 2006

  20. So, we think pPv could exist both in cold down-welling regions and in cool down-wellings inside large chemical piles. Lay, Hernlund, Garnero and Thorne, Science, 2006

  21. Lay et al., Science, 2006

  22. Lay and Garnero, AGU Monograph, in press, 2007

  23. So, is there post-perovskite In the lower mantle? It seems like the best working hypothesis we have to explain shear velocity discontinuities. It may account for a significant portion of large-scale heterogeneity patterns, reducing inferred thermal and chemical variations But, further testing is warranted. All interpretations subject to future revision.

  24. Lay and Garnero, AGU Monograph, 2007

  25. Large-scale Lowermost mantle heterogeneity Lay, Garnero, Williams, PEPI, 2004

  26. Mapping Tomographic Shear Velocity Variations Into a Post-perovskite Layer Helmberger, Lay, Ni and Gurnis, PNAS, 2005

  27. Cool down- wellings on edge of LLSVP may be where a post-perovskite lens can form. It will not be found in every region then. Garnero, Lay and McNamara, GSA Monograph, in press, 2007

  28. Phase Change Could Occur Twice in Thermal Boundary Layer But, if is Hard to Observe Flores and Lay, GRL, 2005

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