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