CIDER06 Group B

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CIDER-06 Group B

• Seismic velocity gradients across the transition
  zone

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Participants

• Fabio Cammarano (UCB)
• Colleen Dalton (Harvard)
• Nico DeKoker (Michigan)
• Federica Marone (UCB)
• Antonio Piazzoni (Munich)
• Barbara Romanowicz (UCB)
• Yi Wang (SUNY)

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What have we tried to do?

• Compute elastic velocity profiles (Vp and Vs) in
  the upper mantle based on 3 different mineral
  physics based methods
• Compare two different compositional models
  pyrolite and piclogite
• Compare fits to seismic data (travel times and
  modes)
• Understand where differences are coming from and
  assess how fits to seismic data can be improved

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Compositional models(Ita and Stixrude 1992)
Pyrolite () MgO 49.33 SiO2 38.3 FeO
6.27 Al2O3 2.22 CaO 3.3
Piclogite () MgO 42.33 SiO2 41.94 FeO
6.29 Al2O3 1.78 CaO 8.67 Duffy and Anderson,
1989 Anderson and Bass, 1984
Ringwood, 1979
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Phase diagrams
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Computation of elastic properties from mineral
physics

• 1) Stixrude and Lithgow-Bertelloni (2005)
• Gibbs Free Energy Minimization (a)
• 2) Piazzoni et al. (2006 submitted to G-cubed)
• Gibbs Free Energy Minimization (b)
• 3) Weidner and Wang (1998, 2000)
• Use directly data from experiments
• All use the same temperature profile (1600K
  potential temperature)

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Seismic velocity profiles (Vs)
Piro Piclo
PREM SLB Weidner Piazzoni
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Fit to global dataset of seismic P wave travel
times (EHB catalog)
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Fit to normal mode data spheroidal modes
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Fit to normal mode data toroidal modes
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Conclusions

• Velocities obtained are too slow
• Colder temperature?
• Velocity gradient in the transition zone is
  better for piclogite than for pyrolite
• Effect of Ca content?
• Possible effect of water test using a homologous
  temperature model

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Fit to global dataset of seismic P wave travel
times (EHB catalog)
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Test with 150K colder potential temperature
Pyro Piclo
PREM
SL 1600K SL -150K
SL -150K
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Conclusions (2)

• What helps a combination of
• Colder temperatures
• Effect of convection on adiabats
• Stagnant slabs?
• Increase in Ca content (oceanic crust?)
• What can be improved (future)
• Elastic properties at high T,P need to be better
  known
• Understand better differences between
  thermodynamic models
• Effect of water through and on Q
• Need to include high resolution seismic
  constraints on velocity jumps at 400 and 660 km.

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Attenuation profiles