The influence of thermochemical piles on topography at Earths coremantle boundary

1
The influence of thermochemical piles on
topography at Earths core-mantle boundary

• Teresa Mae Lassaka, Allen K. McNamaraa, and
  Shijie Zhongb
• AGU, December 13th, 2006
• aSchool of Earth and Space Exploration, Arizona
  State University, Tempe, AZ 85287-1404
• bDepartment of Physics, University of Colorado,
  Boulder, CO 80309-0309

NSF grant EAR-0510383 and EAR-0456356
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Motivation

• Large, low shear velocity anomalies

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4 Geodynamical Interpretations

• Isochemical superplumes
• Isochemical plume clusters
• Thermochemical superplumes
• Thermochemical piles

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CMB Topography

• Variety of CMB topography observations
• What causes CMB topography?

Sze and van der Hilst, 2003
Garcia and Souriau, 2003
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Goal

• Can observations of this topography act as a
  constraint on mantle models?
• How do thermochemical piles affect CMB
  topography?
• Hypothesis Thermochemical piles have similar
  buoyancy to slabs and, therefore, may not depress
  CMB topography

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Method

• Calculations 2D, finite element method in Citcom
  
• 3 temperature-dependent activation coefficients
  (A) leading to viscosity contrasts
• Dh 1, 103, 104
• Ra105 and 107
• Isochemical and thermochemical cases
• Stress at the CMB
• Convention stress (s), relief - stress
  (s), - relief

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Visual Layout
Buoyancy (B)
Temperature (T)
Composition (C)
Stress (s) or Topography (h)
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Case 1 Ra105, Dh1

• Isochemical
• Downwellings (-) topography
• Upwellings () topography

• Thermochemical
• Piles (-) topography

B
B
T
T
C
s
s
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Case 2 Ra105, Dh103

• Isochemical
• Downwellings (-) topography
• Upwellings () topography
• Stress magnitude decrease beneath upwellings

• Thermochemical
• Piles topography neither () nor (-)

B
B
T
T
C
s
s
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Case 3 Ra105, Dh104

• Thermochemical
• Downwellings Most (-) topography
• Piles Flat topography

• Isochemical
• Downwellings (-) topography
• Upwellings () topography
• Stress magnitude decrease beneath upwellings

B
B
T
T
C
s
s
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Case 4 Ra107, Dh103, 50x h660

• Thermochemical
• Downwellings Most (-) topography
• Piles Flat topography

• Isochemical
• Downwellings (-) topography
• Upwellings () topography

B
B
T
T
C
h
h
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Case 5 Ra107, Dh104, 50x h660

• Thermochemical
• Downwellings Most (-) topography
• Piles Flat topography
• Topography magnitude reduced

• Isochemical
• Downwellings (-) topography
• Upwellings () topography

B
B
T
T
C
h
h
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Case 5 evolution
Increasing time
Decreasing Dr
Composition
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Case 5 evolution
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Summary

• Isochemical
• (-) topography beneath slabs
• () topography beneath plumes
• Reduced topography beneath upwellings at higher
  Dh
• Thermochemical
• Temperature-dependence is key factor
• Dh1, (-) topography beneath piles
• Dh103 and 104, minimal to flat topography
  beneath piles
• Thermochemical piles reduce overall magnitude of
  CMB topography

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Conclusions

• How do thermochemical piles affect CMB
  topography?
• Reduce overall CMB topography magnitude
• Result in flat to minor relief CMB topography
• Can observations of this topography act as a
  constraint on mantle models?
• Isochemical mantle we expect topography to
  correlate with subduction
• Thermochemical piles model
• We expect topography associated with some
  subduction (not all)
• Expect relatively flat topography beneath pile
  regions
• Future Directions
• Other thermochemical mantle models
• 3D investigation