Large Low Shear Velocity Provinces

in the lowermost mantle,

and Plume Generation Zones at their margins

Bernhard Steinberger

Collaborators: Kevin Burke (University of Houston), Trond Torsvik, Mark Smethurst (NGU), Thorsten Becker (University of Southern California)

Where do mantle plumes originate from?

Is there a chemically distinct reservoir in the mantle, and if so, where and how large is it?

Richards, Duncan, Courtillot (1989):

Flood basalts and hotspot tracks:Plume headsand tails

Montelli et al. (2006)

Mantle tomographybeneath Hawaii

Large Igneous Provinces (LIPs) could bemore representative of deep mantle plumes

Reconstructed LIP eruption sites (circles) and hotspots (crosses) plotted on smean tomography model in lowermost mantle.Blobs with other color scale for steep gradients

Plume heads from the edges (-1% contour)of Large Low Shear Velocity Provinces (LLSVPs)Steep gradients along the same contour

Masters et al. (2000)Anti-correlation ofshear wave velocity and bulk sound velocity v

c=(K

s/)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)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.”

Further evidence (more quantitative) that those proposed chemically distinct bodies actually existfor what they look likethat the coincidence of reconstructed LIPs and LLSVP boundaries is extremely unlikely to result by pure chance

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)

1 % contours of smean tomography model -contours of chemically distinct piles?

Reconstructed LIPs and hotspots with Kuo et al. (2000) D'' model

DON

TUZOJASON

GILL

Reconstructed LIPs and hotspots with Castle et al. (2000) D'' model

Scoresheet

TUZO JASON (African) (Pacific) total

Volume 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

Reconstructed LIPs (here: global moving hotspot reference frame)

close to +13 m contour of geoid

Similar shape of LLSVPsand geoid highsaway from subduction zones

Correlation of LIPs (paleomagnetic frame)with tomography (thick lines) and gradientsCalculations done by Thorsten Becker

Belts with 5° half width on either side of -0.96% contour occupy 23.5% of CMB18 out of 24 reconstructed LIPs within beltsProbability for 18 out of 24 or more randomly chosen points to lie within 23.5% area is ~1/7,000,000

Probability for number of LIPs found within a certain distance of LLSVP/LSVP margins or higher number to occur for random points

Dashed lines:In situ LIPs

Continuous lines:reconstructed LIPs(paleomagnetic reference frame)

LLSVP/LSVP marginsinferred fromsmean lowermost layerCastle et al. (2000)Kuo et al. (2000)

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.

Q: Where do mantle plumes originate from?

Q: Is there a chemically distinct reservoir in the mantle, and if so, where and how large is it?

Q: Where do mantle plumes originate from?A: From the lowermost mantle (at least those causing LIP eruptions), more specifically from the Plume Generation Zones at the edges of Large Low Shear Velocity Provinces (LLSVPs)Q: Is there a chemically distinct reservoir in the mantle, and if so, where and how large is it?

Q: Where do mantle plumes originate from?A: From the lowermost mantle (at least those causing LIP eruptions), more specifically from the Plume Generation Zones at the edges of Large Low Shear Velocity Provinces (LLSVPs)Q: Is there a chemically distinct reservoir in the mantle, and if so, where and how large is it?A: Yes, the African and Pacific LLSVPs. Each of them contains about 1 % of mantle mass.