1. What are the Low-Velocity anomalies in the deep mantle? Bernhard Steinberger Center for Geodynamics, NGU, Trondheim, Norway

2. Mantle temperature and melting temperature profile Adiabatic temperature profile T(z): integrate dT/dz = T(z)  (z) g(z) / C p thermal expansivity gravity specific heat Melting temperature profile T m (Wang, 1999; Zerr and Boehler, 1994; Yamazaki and Karato, 2001) Mantle potential surface temperature ~1613 K, based on decompression melt studies of MORBs (White and McKenzie 1995, Iwamori et al. 1995) Temperature at CMB 4000 +- 600 K (Boehler, 1996) boundary Thermal boundary layer at base of mantle

4. Shear velocity anomalies in the deep mantle Kuo et al. (2000) D'' model

5. Castle et al. (2000) D'' model Large Low Shear Velocity Provinces in the deep mantle are robust features of all recent tomography models

6. smean (Becker and Boschi, 2000) model in lowermost mantle Steep gradients along the -1% contour

7. Large Low Shear Velocity Provinces in three dimensions: - 1% contour of smean model at different depths above CMB

8. Measuring size and weight of Large Low Velocity Anomalies African Pacific total Volume 8.4 (6.2)·10 9 km 3 5.8 (5.3)·10 9 km 3 14.2·10 9 km 3 (4.9·10 9 km 3 Wang and Wen, 2004) % of mantle 0.94% (0.69%) 0.65% (0.59%) 1.59% Mass 4.5 (3.4)·10 22 kg 3.1 (2.9)·10 22 kg 7.7·10 22 kg % of mantle 1.13% (0.84%) 0.79% (0.73%) 1.91% Area on CMB 1.6·10 7 km 2 1.6·10 7 km 2 3.2·10 7 km 2 (1.8·10 7 km 2 Wang and Wen, 2004) % of CMB 10.2% 10.6% 20.9% Max. height ~1800 (600) km ~1400 (600) km “Center of mass” (latitude, longitude, ave. elevation above CMB) bottom layer 17.0°S 13.6°E 11.4°S 164.3°W bottom 4 ~s 15.7°S 12.0°E 229km 10.9°S 162.4°W 192km 211km total 15.6°S 13.0°E 409km 11.0°S 162.9°W 239km 339km

9. P-wave models: Lower amplitude, different pattern pmean (Becker and Boschi, 2002) model in lowermost mantle

10. Jointly derived p and s wave model: SB10L18 (Masters et al., 2000) p-wave model in lowermost mantle

11. Jointly derived p and s wave model: SB10L18 (Masters et al., 2000) s-wave model in lowermost mantle

12. Jointly derived p and s wave model: SB10L18 (Masters et al., 2000) Bulk sound wave speed v c =(K s /  ) 1/2 in lowermost mantle

13. Masters et al. (2000) Anti-correlation of shear wave velocity and bulk sound velocity v c =(K s /  ) 1/2 in lowermost mantle

14. Density anomaly (degrees 2, 4, 6) determined directly using normal modes (Ishii and Tromp, 2004)

15. Wang and Wen (2004) – waveform modeling and travel time analysis “VLVP (Very Low Velocity Province) has rapidly varying thicknesses from 300 to 0 km, steeply dipping edges... structural and velocity features unambiguously indicate that the VLVP is compositionally distinct.”

16. Bimodal distribution

18. Frequency distribution of seismic velocity smean model (Becker and Boschi, 2002) Depth 2799 km (91 km above CMB)

19. Frequency distribution of seismic velocity smean model (continuous lines) Castle et al. (2000) (dotted) Kuo et al. (2000) (dashed)

20. chemically distinct D'' regions? Geodynamical argument: small CMB topography indicates chemically distinct D'' regions

21. CMB topography point constraints from PKKP CMB underside reflections (Garcia and Souriau, SEDI 1998)

22. CMB topography rms amplitude (Garcia and Souriau, 2000) CMB excess ellipticity 328 -- 346 m peak-to-valley (from geodetic constraints; Mathews et al., 2002).

23. Instantaneous flow computation Convert seismic velocity anomalies to density anomalies Thermal anomalies only: CMB topography rms amplitude about twice as large as observed CMB excess ellipticity several times as large as observed

24. chemically distinct D'' regions? Add chemical anomalies in lowermost ~ 300 km: Additional positive density anomaly wherever ~  v < -1%: CMB topography and excess ellipticity now matched

25. Laboratory Experiments: Davaille et al. (1999) Thermochemical domes should occur for density contrast less than about 1% Plumes are generated simultaneously Jellinek and Manga (2002) Dense layer determines location and dynamics of mantle plumes – causes them to become spatially fixed

26. Nakagawa and Tackley (2005) No thermochemical layer Thermochemical layer with intermediate density contrast Thermochemical layer with strong density contrast Nakagawa and Tackley (2005= Numerical Experiments:

27. With an intrinsic compositional viscosity increase, the compositionally distinct structures may acquire a rounded shape, more compatible with observations (McNamara & Zhong, 2005) With an intrinsic compositional viscosity increase, the compositionally distinct structures may acquire a rounded shape, more compatible with observations (McNamara & Zhong, 2005) Piles with sharp boundaries may occur if compositional density difference is depth-dependent (Tan and Gurnis, 2005) Piles with sharp boundaries may occur if compositional density difference is depth-dependent (Tan and Gurnis, 2005) Δρ 0 = 0.03; Δρ 0 = 0.02 Δρ 0 = 0.025 Figure from Tan & Gurnis (2005)

28. How high extend thermo-chemical piles? Contraints from uplift rates can be satisfied with mid-lower mantle beneath southern Africa 0.2% less dense and viscosity 10 22 Pas Lowest parts of African Anomaly may be anomalously dense, compatible with geologic constraints (Gurnis, Mitrovica et al., 2000)

29. Geochemical requirement for isolated mantle reservoir: Tolstikhin and Hofmann (2005) estimate minimum mass that could maintain present-day helium flux from Earth to atmosphere for 4.5 Ga to be 6.2·10 22 kg Recent measurements of contrasts in 142 Nd/ 144 Nd between terrestrial rocks and meteorites (Boyet and Carlson 2005, 2006) have made a new case for the existence of such a reservoir

30. Post-perovskite phase boundary: Sidorin, Gurnis, Helmberger (1999): Seismic triplication; infer spatially intermittent discontinuity – phase transition ~ 200 km above CMB, Clapeyron slope ~ 6 MPa/K

31. Tsuchiya et al. (2004) compute phase transition, again with large positive Clapeyron slope Experimental detection of phase transition by Murakami et al. (2004)

32. Correspondence between top and base of mantle: Continents LLSVPs oceanic lithosphere D'' material between LLSVPs subducted slabs mantle plumes negatively buoyant positively buoyant sinking rising cooling down heating up surface CMB subduction zones "Plume Generation Zones"

33. Plate tectonics: Oceanic lithosphere cools down at the surface and gradually becomes negatively buoyant. It moves towards subduction zones, mostly at the edges of chemically distinct and positively buoyant continents, where it sinks back into the mantle, in the form of subducted slabs. Dynamics of D'': D'' material outside LLSVPs heats up at the CMB and gradually becomes positively buoyant. It moves towards “Plume generation zones”, mostly at the edges of chemically distinct and negatively buoyant LLSVPs, where it rises back into the mantle, in the form of mantle plumes.

34. Platetektonikk: Havbunnslitosfære afkjøles på overflaten og graduelt blir tungere. Det beveger seg til subduksjonssoner, mest på render av kjemikalisk ulike og lettere kontinenter. Der synker det tilbake inn i mantelen, i form av subduserte plater. Dynamikk av D'': D'' material utenfor LLSVP'er varmes opp på Kjerne-Mantel-Grensen and graduelt blir lettere. Det beveger seg til “Varmesøyle frambringende soner”, mest på render av kjemikalisk ulike og tungere store lavhastighets områder. Der stiger det tilbake inn i mantelen, i form av varmesøyler.