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.