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

Lay, Garnero and Williams, PEPI, 2004 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 et al., EOS, 2005

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

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

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? Lay and Garnero, AGU Monograph, in press, 2007 P-Wave Predictions and Models Pv-pPv

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? Lay and Garnero, AGU Monograph, in press, 2007 S-Wave Predictions and Models Pv-pPv

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

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

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

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

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

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

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

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

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

Lay, Hernlund, Garnero and Thorne, Science, 2006 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

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 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/m 2

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

Lay et al., Science, 2006

Lay and Garnero, AGU Monograph, in press, 2007

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.

Lay and Garnero, AGU Monograph, 2007

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

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

Garnero, Lay and McNamara, GSA Monograph, in press, 2007 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.

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