Seismic velocity gradients across the transition zone

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Presentation transcript:

Seismic velocity gradients across the transition zone CIDER-06 Group B Seismic velocity gradients across the transition zone

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

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

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

Phase diagrams

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)

Seismic velocity profiles (Vs) Piro Piclo PREM S&LB Weidner Piazzoni

Fit to global dataset of seismic P wave travel times (EHB catalog)

Fit to normal mode data: spheroidal modes

Fit to normal mode data: toroidal modes

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

Fit to global dataset of seismic P wave travel times (EHB catalog)

Test with 150K colder potential temperature Pyro Piclo PREM S&L 1600K S&L -150K S&L -150K

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.

Attenuation profiles