What are the Low-Velocity anomalies

in the deep mantle?

Bernhard Steinberger

Center for Geodynamics, NGU, Trondheim, Norway

Mantle temperature and melting temperature profileAdiabatic temperature profile

T(z): integrate dT/dz = T(z) (z) g(z) / Cp

thermal expansivity gravity specific heat

Melting temperatureprofile Tm (Wang, 1999; Zerr and Boehler, 1994;Yamazaki and Karato, 2001)

Mantle potential surface temperature ~1613 K, based on decompressionmelt studies of MORBs (White and McKenzie 1995, Iwamori et al. 1995)

Temperature at CMB4000 +- 600 K (Boehler, 1996)

Thermal boundaryboundary layer at base of mantle

Shear velocity anomalies in the deep mantle

Kuo et al. (2000) D'' model

Castle et al. (2000) D'' model

Large Low Shear Velocity Provinces in the deep mantle are robust features of all recent

tomography models

smean (Becker and Boschi, 2000) model in lowermost mantle

Steep gradients along the -1% contour

Large Low Shear Velocity Provinces in three dimensions: -1% contour of smean model at

different depths above CMB

Measuring size and weight of Large Low Velocity Anomalies African Pacific totalVolume 8.4 (6.2)·109 km3 5.8 (5.3)·109 km3

14.2·109 km3

(4.9·109 km3 Wang and Wen, 2004)

% of mantle 0.94% (0.69%) 0.65% (0.59%) 1.59%

Mass 4.5 (3.4)·1022 kg 3.1 (2.9)·1022 kg

7.7·1022 kg

% of mantle 1.13% (0.84%) 0.79% (0.73%) 1.91%

Area on CMB 1.6·107 km2 1.6·107 km2

3.2·107 km2

(1.8·107 km2 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°Wbottom 4 ~s 15.7°S 12.0°E 229km 10.9°S 162.4°W 192km 211kmtotal 15.6°S 13.0°E 409km 11.0°S 162.9°W 239km 339km

P-wave models: Lower amplitude, different patternpmean (Becker and Boschi, 2002) model in lowermost mantle

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

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

Jointly derived p and s wave model: SB10L18 (Masters et al., 2000) Bulk sound wave speed vc=(Ks/)1/2 in lowermost

mantle

Masters et al. (2000)Anti-correlation ofshear wave velocity and bulk sound velocity vc=(Ks/)1/2

in lowermost mantle

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

Wang and Wen (2004) – Wang and Wen (2004) – waveform modeling and travel time analysiswaveform 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.”

Bimodal distribution

Frequency distribution of seismic velocitysmean model (Becker and Boschi, 2002)Depth 2799 km (91 km above CMB)

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

chemically distinct D''regions?

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

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

CMB topography rms amplitude (Garcia and Souriau, 2000)

CMB excess ellipticity328 -- 346 m peak-to-valley (from geodetic constraints; Mathews et al., 2002).

Instantaneous flow computationConvert seismic velocity anomaliesto density anomaliesThermal anomalies only:CMB topography rms amplitude about twice as large as observedCMB excess ellipticity several times as large as observed

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

Laboratory Experiments:Laboratory Experiments:Davaille et al. (1999)Davaille et al. (1999)Thermochemical domes should occur for density contrast less than about 1%Thermochemical domes should occur for density contrast less than about 1%Plumes are generated simultaneouslyPlumes are generated simultaneously

Jellinek and Manga (2002)Jellinek and Manga (2002)Dense layer determines location Dense layer determines location and dynamics of mantle plumes – and dynamics of mantle plumes – causes them to become spatially causes them to become spatially fixedfixed

Nakagawa and Tackley (2005)

No thermochemical layer

Thermochemical layer with intermediate density contrast

Thermochemical layer with strong density contrast

Nakagawa and Tackley (2005= Nakagawa and Tackley (2005=

NumericalNumerical Experiments: Experiments:

With an intrinsic compositional viscosity increase, the compositionally distinct With an intrinsic compositional viscosity increase, the compositionally distinct structures may acquire a rounded shape, more compatible with observations structures may acquire a rounded shape, more compatible with observations (McNamara & Zhong, 2005)(McNamara & Zhong, 2005)Piles with sharp boundaries may occur if compositional density difference is depth-Piles with sharp boundaries may occur if compositional density difference is depth-dependent (Tan and Gurnis, 2005)dependent (Tan and Gurnis, 2005)

Δρ0 = 0.03;

Δρ0 = 0.02

Δρ0 = 0.025

Figure from Tan & Gurnis Figure from Tan & Gurnis (2005)(2005)

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 1022Pas Lowest parts of African Anomaly may be anomalously dense,compatible with geologic constraints (Gurnis, Mitrovica et al., 2000)

Geochemical requirement for isolated mantle Geochemical requirement for isolated mantle reservoir: 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 6.2·1022 kgRecent measurements of contrasts in 142 Nd/

144Nd between terrestrial rocks and meteorites (Boyet and Carlson 2005, 2006) have made a new case for the existence of such a reservoir

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

Tsuchiya et al. (2004) compute phase transition, again Tsuchiya et al. (2004) compute phase transition, again with large positive Clapeyron slopewith large positive Clapeyron slope

Experimental detection of phase transition by Murakami et al. (2004)Experimental detection of phase transition by Murakami et al. (2004)

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"

Plate tectonics:Oceanic lithospherecools downat the surfaceand gradually becomesnegatively buoyant.It moves towardssubduction zones,mostly at the edges ofchemically distinct andpositively buoyantcontinents,where it sinks back into the mantle,in the form ofsubducted slabs.

Dynamics of D'':D'' material outside LLSVPsheats upat the CMBand gradually becomespositively buoyant.It moves towards“Plume generation zones”,mostly at the edges ofchemically distinct andnegatively buoyantLLSVPs,where it rises back into the mantle,in the form ofmantle plumes.

Platetektonikk:Havbunnslitosfæreafkjølespå overflatenog graduelt blirtungere.Det beveger seg tilsubduksjonssoner,mest på render avkjemikalisk ulike ogletterekontinenter.Der synker dettilbake inn i mantelen,i form avsubduserte plater.

Dynamikk av D'':D'' material utenfor LLSVP'ervarmes opppå Kjerne-Mantel-Grensenand graduelt blirlettere.Det beveger seg til“Varmesøyle frambringende soner”,mest på render avkjemikalisk ulike ogtungerestore lavhastighets områder.Der stiger dettilbake inn i mantelen,i form avvarmesøyler.