seismological tests (and implications) of post-perovskite presence in the deep mantle in...
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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
In collaboration withEd Garnero, Alex HutkoJohn Hernlund
In collaboration withEd Garnero, Alex HutkoJohn Hernlund
Thorne LayUniversity of California, Santa CruzDepartment of Earth and PlanetarySciences
University of California, Santa CruzDepartment of Earth and PlanetarySciences
Lay, Garnero and Williams, PEPI, 2004
Is the D” S-wave velocitydiscontinuity first observedby Lay and Helmberger (1983) caused by the Perovskite-to-Post-perovskite phase changediscovered 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
MgSiO3
Pv-pPv
Lay and Garnero, AGU Monograph, 2007
Synthetic predictions for P-wave models
Model PWDK Model WOOKWeber and Davis [1993] Wookey et al. [2005]
Pv-pPvphase change
model
PdP ?
Pv-pPv predicts littleP wave reflectivityWookey et al. 2005 Wentzcovich et al. 2006
P-wave models arenot easily reconciledwith predictions of thePv-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 andModels
Pv-pPv
Some S-wave models can be reconciledwith predictions of the Pv-pPv transition, and the predicted effects are observable. But some models arenot 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, CentralAmerica, Cocos Plate
High S velocity, slightlyHigh P velocity
A dozen Swave studiesindicate wide-spread Sdiscontinuity200-300 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 complexityaffecting the phase boundary. Seismic migration isUsed to image 3D structure.
Hutko, Lay, Garnero and Revenaugh, Nature, 2006
Rapid changes in depth of reflector appear to exist; needsspecial circumstances to account for by phase boundary.
Kito, Rost, Thomas, Garnero, GJI, 2007
Step in S velocity supported.P velocity decreases (?), maybe 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 haveDistinctive Vs/Vp Behavior
Shear velocitiesanti-correlatedwith bulk soundvelocities.
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
Lay, Hernlund, Garnero and Thorne, Science, 2006
Paired velocitydiscontinuitiesmay be due todouble-crossingof the phaseboundary in lowermost thermalboundary layer,as suggested by Hernlund,Thomas, andTackley, 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/m2
Lay, Hernlund, Garnero and Thorne, Science, 2006
So, we think pPv could exist both in cold down-welling regionsand 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-perovskiteIn 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-scaleLowermost mantle heterogeneity
Large-scaleLowermost mantle heterogeneity
Lay, Garnero, Williams, PEPI, 2004
Mapping Tomographic Shear Velocity VariationsInto a Post-perovskite Layer
Helmberger, Lay, Ni and Gurnis, PNAS, 2005
Garnero, Lay and McNamara, GSA Monograph, in press, 2007
Cool down-wellings onedge of LLSVP maybe where a post-perovskitelens can form. It will not be found in everyregion then.
Phase Change Could Occur Twice in Thermal Boundary Layer
But, if is Hard to Observe
Flores and Lay, GRL, 2005