The origin and composition of the large low shear-wave

Report
Determining the nature of the LLSVP
Post-AGU CIDER Workshop 2012
Maxim Ballmer, Jamie Barron, Rohan Kundargi, Curtis Williams, Rick Carlson,
Jasper Konter, Jackie Li, Sujoy Mukhopadhyay
Motivation
Why LLSVPs?
(1) “hidden” geochemical reservoir
(2) secular evolution of the Earth
Continental Crust
Upper Mantle / Lower Mantle
Early Enriched Reservoir = ???
= LLSVP ?
Lee et al. (2010)
Bulk Silicate Earth = ???
Labrosse et al. (2007)
What are the LLSVPs?
Seismic tomography shows two large low shear velocity regions
in the lowermost mantle: a degree 2 pattern centered on the
Pacific and Africa
Garnero et al. (2007)
Scenarios of LLSVP origin
Brandenburg and van Keken (2007)
Labrosse et al. (2007)
Tackley (2011)
-
MORB + harzburgite
Pyrolite
MORB alone
Ancient Fe-enriched
oceanic crust
- Magma Ocean Product <= team BMO
- Perovskite/Magnesiowüstite
crystallization
- Magma Ocean Cumulate
- KREEP-like
- Fe-rich differentiate
- Early Enriched Reservoir
- calculated from volumes of
LLSVPs and BSE-estimates
Plan of Attack
(1) Primitive
Reservoir
(2) Slab
Graveyard
(3) Hybrid
Scenario
Calculate physical properties
(Jackie Li’s code and/or BurnMan)
Density
Geodynamical test
Vp
Vs
Seismological test
Plan of Attack
(1) Primitive
Reservoir
(2) Slab
Graveyard
(3) Hybrid
Scenario
Calculate physical properties
(Jackie Li’s code and/or BurnMan)
Density
Geodynamical test
Vp
Vs
Seismological test
Plan of Attack
(1) Primitive
Reservoir
(2) Slab
Graveyard
(3) Hybrid
Scenario
Calculate physical properties
(Jackie Li’s code and/or BurnMan)
Density
Vp
Geodynamical test
Vs
Seismological test
calculate volumes
of LLSVPs from
seismic images
LLSVP volume - choosing contours
saw24b16
0.6%
contour
S40RTS
0.4%
contour
S362ANI
0.6%
contour
Choose contour so it
goes through region of
steep gradient at edge
of LLSVP
(compositional change
should be associated
with sharp gradient)
LLSVP volume – choosing height
From clustering analysis – all models show change in gradient for slow cluster
at approximately the same depth, around 2100km.
Lekic et al (2012)
LLSVP volume – prel. results
Calculate the volume within isosurface chosen for model, from CMB to 2100km depth
next step:
consider
structure that
is continuous
from CMB
only
Gyrs after solidification of basal reservoir
dense, radioactive material
Height above CMB
geodynamic test
760
Key ingredients:
2D models with a resolution of 96x192 elements
temperature [°C]
3800
-basal layer with intrinsic negative buoyancy
- and with excess radiogenic heat production
that decays over model time
Gyrs after solidification of basal reservoir
dense, radioactive material
Height above CMB
geodynamic test
760
Key ingredients:
2D models with a resolution of 96x192 elements
temperature [°C]
3800
-basal layer with intrinsic negative buoyancy
- and with excess radiogenic heat production
that decays over model time
- As soon as basal-layer material enter the
upper mantle, it looses its exceptional
properties (density,
)
Early Enriched Reservoir makes up 11% of the initial mantle
10
Model time: 4 Gyrs
10
Model time: 4 Gyrs
10
10
10
0.8
1.0
1.2
1.4
Δρ(basal layer) [g/cm3]
10
0.8
1.0
1.2
1.4
Δρ(basal layer) [g/cm3]
Initial heat production In basal layer [W/kg]
numerical parameter study
Early Enriched Reservoir makes up 11% of the initial mantle
10
Model time: 4 Gyrs
10
Model time: 4 Gyrs
10
10
10
0.8
1.0
1.2
1.4
Δρ(basal layer) [g/cm3]
10
0.8
1.0
1.2
1.4
Δρ(basal layer) [g/cm3]
Initial heat production In basal layer [W/kg]
numerical parameter study
trace-element concentrations and isotopic
ratios in Early Enriched Reservoir
…
Early Enriched Reservoir makes up 11% of the initial mantle
10
Model time: 4 Gyrs
10
Model time: 4 Gyrs
10
10
10
0.8
1.0
1.2
1.4
Δρ(basal layer) [g/cm3]
10
0.8
1.0
1.2
1.4
Δρ(basal layer) [g/cm3]
Initial heat production In basal layer [W/kg]
numerical parameter study
outlook
- good plan, but quite a lot of
work remains to be done
- rule out a couple specific
hypotheses for the
composition of the LLSVPs
- perhaps rule out a family of
hypotheses for their origin
(e.g. the slab graveyard
hypothesis)
- present results at EGU 2013
- semantics (LLSVP, BAR, MOP)
Compositional
Models
Compute
Physical
Properties
Geodynamic &
Seismic Test

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