predicting global perovskite to post-perovskite phase boundary
DESCRIPTION
Predicting Global Perovskite to Post-Perovskite Phase Boundary. Don Helmberger, Daoyuan Sun, Xiaodong Song, Steve Grand, Sidio Ni, and Mike Gurnis. D” region with velocity discontinuity. S-wave triplication suggests positive velocity discontinuity - PowerPoint PPT PresentationTRANSCRIPT
Predicting Global Perovskite to Post-Perovskite Phase Boundary
Don Helmberger, Daoyuan Sun, Xiaodong Song, Steve Grand, Sidio Ni, and Mike Gurnis
D” region with velocity discontinuity
S-wave triplication suggests positive velocity discontinuity
Strong beneath the circum-Pacific lower mantle fast velocity belt
Relate to phase boundary (Perovskite to Post-Perovskite)
(Grand, 2002)
D" beneath the Superplume region
?
Beneath Superplume
Phase boundary is very close to CMB
Chemical distinct
Need a valid model for Superplume for exploring D"
Zone P
Zone C
Zone M
Zone A
3D synthetics for middle mantle slab model
Synthetics for event A
Records of USArray for a South American event (20070721)
Depth-dep. Thermal Expansion (cont.)
If ch decreases with depth. total can become negative (unstable and rise) below the height of neutral buoyancy (HNB) but positive (stable and sink) below the HNB.
Metastable Superplume
Sharp boundary
Very low velocity zone along the edge
Small scale convection features inside the Superplume
Seismic validation of the Metastable Superplume (Sdiff, ScS and PcP)
Helmberger et al. [AGU Monograph, 2005]
Vertical boundaries: The lower mantle beneath S. AtlanticVertical boundaries: The lower mantle beneath S. Atlantic
3D effect for metastable Superplume model
Sd paths across the metastable Superplume
3D multipath detector example: African Superplume
D" beneath the African Superplume region
Blue circle: SKS pierce points at CMB
Green Circle: ScS bounce points at CMB
Model the D" beneath the Superplume region
III
D" beneath the African Superplume region
CM model
Hybrid model
Grand’s tomography model (2002) at the bottom mantle
Possible phase boundary discontinuity [Sidorin et al.,1999]
New phase boundary map
A
B C
Is the Metastable model suitable for large anomaly beneath Central Pacific?
Difference between the middle (A) and the edge (B,C) (without down-welling cold material) of the Superplume
Difference between the Superplume region and the cold slab region
The Metastable Superplume model including phase transition at the bottom
The Metastable Superplume model satisfies the seismological observations for the African Superplume
Phase boundary elevationA: 90 km under the African Metastable modelB: 100 - 145 km (He et al., 2006)C: 160 - 345 km (Lay et al., 2006)
Global map of the D"
Summary Huge Volume: 1000kmx1000kmx7000km; (Davaille,2000);
evidence 1 for chemical plume S:-3%; P 0~-0.5%,density +; evidence 2 for chemical
plume ; Sharp boundary (Ni et al, 2002; Ni and Helmberber
2003), evidence 3 for chemical plume The shape of the superplume correlates with Geoid and
Hotspots. Tomography and geoid modeling requires higher density
in the super plume. For some regions in the lower mantle, horizontal
gradients outweighs the vertical one. Thus some boundaries are more vertical than horizontal in the lower mantle.
= 6 MPa/K
Effect of γ on phase boundary
γ= 3 MPa/K, hph = 140 km γ= 9 MPa/K, hph = 75 km
Velocity discontinuity and Phase transition
1. Velocity Tomography model -> Non-adiabatic temperature perturbation
2. Determine the phase boundary with assuming Clapeyron slope (γ) and ambient phase transition elevation (hph)
3. Impose a velocity discontinuity (+1.5%) at the phase boundary
(Sidorin et al., 1999)
Data
SH wave, organized against azimuth