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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?.

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determining the nature of the llsvp

Determining the nature of the LLSVP

Post-AGU CIDER Workshop 2012

Maxim Ballmer, Jamie Barron, RohanKundargi, Curtis Williams, Rick Carlson,

Jasper Konter, Jackie Li, SujoyMukhopadhyay

motivation
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)

Labrosse et al. (2007)

Bulk Silicate Earth = ???

what are the llsvps
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)

slide4

Scenarios of LLSVP origin

Brandenburg and van Keken (2007)

Labrosse et al. (2007)

Tackley (2011)

  • 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
  • MORB + harzburgite
  • Pyrolite
  • MORB alone
  • Ancient Fe-enriched
  • oceanic crust
slide5

Plan of Attack

(1) Primitive Reservoir

(2) Slab Graveyard

(3) Hybrid Scenario

Team BMO

Mg/Si-group

Calculate physical properties

(Jackie Li’s code and/or BurnMan)

Density

Vp

Vs

Geodynamical test

Seismological test

slide6

Plan of Attack

(1) Primitive Reservoir

(2) Slab Graveyard

(3) Hybrid Scenario

Team BMO

Mg/Si-group

Calculate physical properties

(Jackie Li’s code and/or BurnMan)

Density

Vp

Vs

Geodynamical test

Seismological test

slide7

Plan of Attack

(1) Primitive Reservoir

(2) Slab Graveyard

(3) Hybrid Scenario

Team BMO

Mg/Si-group

Calculate physical properties

(Jackie Li’s code and/or BurnMan)

Density

Vp

Vs

Geodynamical test

Seismological test

calculate volumes

of LLSVPs from seismic images

llsvp volume choosing contours
LLSVP volume - choosing contours

saw24b16

0.6%

contour

S362ANI

0.6%

contour

S40RTS

0.4%

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
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
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

geodynamic test
geodynamic test

2D models with a resolution of 96x192 elements

Height above CMB

Gyrs after solidification of basal reservoir

dense, radio-active material

760 temperature [°C] 3800

Key ingredients:

-basal layer with intrinsic negative buoyancy

- and with excess radiogenic heat production

that decays over model time

geodynamic test1
geodynamic test

2D models with a resolution of 96x192 elements

Height above CMB

Gyrs after solidification of basal reservoir

dense, radio-active material

760 temperature [°C] 3800

Key ingredients:

-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, )

numerical parameter study
numerical parameter study

Early Enriched Reservoir makes up 11% of the initial mantle

Model time: 4 Gyrs

Model time: 4 Gyrs

10

10

10

Initial heat production In basal layer [W/kg]

10

10

10

0.8 1.0 1.2 1.4

0.8 1.0 1.2 1.4

Δρ(basal layer) [g/cm3]

Δρ(basal layer) [g/cm3]

numerical parameter study1
numerical parameter study

Early Enriched Reservoir makes up 11% of the initial mantle

Model time: 4 Gyrs

Model time: 4 Gyrs

10

10

10

Initial heat production In basal layer [W/kg]

10

10

10

0.8 1.0 1.2 1.4

0.8 1.0 1.2 1.4

Δρ(basal layer) [g/cm3]

Δρ(basal layer) [g/cm3]

numerical parameter study2
numerical parameter study

Early Enriched Reservoir makes up 11% of the initial mantle

Model time: 4 Gyrs

Model time: 4 Gyrs

10

10

10

Initial heat production In basal layer [W/kg]

10

10

10

0.8 1.0 1.2 1.4

0.8 1.0 1.2 1.4

Δρ(basal layer) [g/cm3]

Δρ(basal layer) [g/cm3]

outlook
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)

CompositionalModels

Compute Physical

Properties

Geodynamic & Seismic Test

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