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Elasticity of Ferro-Periclase Through the High Spin - Low Spin Transition

Elasticity of Ferro-Periclase Through the High Spin - Low Spin Transition. J. Michael Brown - University of Washington Jonathan Crowhurst - Lawrence Livernmore Lab. Alexander Goncharov - Geophysical Lab. Steven Jacobsen - Northwestern University. Summary (Three Major Topics).

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Elasticity of Ferro-Periclase Through the High Spin - Low Spin Transition

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  1. Elasticity of Ferro-Periclase Through the High Spin - Low Spin Transition J. Michael Brown - University of Washington Jonathan Crowhurst - Lawrence Livernmore Lab. Alexander Goncharov - Geophysical Lab. Steven Jacobsen - Northwestern University

  2. Summary(Three Major Topics) • Mantle Tomography: Why are slabs hard to image in the lower mantle? • Do not penetrate? • Off-setting chemical and thermal effects? • High spin - low spin transition?

  3. Summary(Three Major Topics) • Mantle Tomography: Why are slabs hard to image in the lower mantle? • Do not penetrate? • Off-setting chemical and thermal effects? • High spin - low spin transition?

  4. Physics of the High spin low spin transition • Outstanding experimental data • Robust macroscopic thermodynamic theory

  5. New measurements of sound velocities through the HS-LS transition • Some experimental details • All elastic constants determined to 63 GPa • Help validate the macroscopic thermodynamic description • Support idea that thermal anomalies have small velocity perturbations in lower mantle

  6. Less structure in lower mantle “Using the best mineral physics data, slabs should be visible in seismic images of the mid lower mantle - that they are not seen is somewhat surprising” Guy Masters 2006 AGU meeting A possible connection to the high-spin low-spin transition

  7. Physics of the High spin to Low spin Transition

  8. High spin - low spin iron • Transition is • Intrinsically non-1st order • Readily described by robust macroscopic thermodynamics • Characterized by DH = DE + PDV • Associated with anomalies in physical properties

  9. Truly exciting both in terms of • High pressure physics and chemistry • Understanding Earth’s mantle But - some re-appraisals are needed

  10. Clapyron Slope

  11. Clapyron Slope

  12. Low-spin iron is an “additional chemical component in the mantle”

  13. Low-spin iron is an “additional chemical component in the mantle”

  14. Fine Print • Focus on (Mg,Fe)O - • similar behavior for Perovsikte? • LS iron has smaller “ionic radius” • D-orbitals directed where oxygen is not • Iron sites are non-interacting • Properties in proportion to iron concentration • Little difference in EOS of HS and LS iron • “Softening” expected in transition region • Increment of pressure causes “normal” strain plus additional strain with HS to LS transition • If spin flip is “fast” compared to acoustic frequency, velocities can decrease

  15. Macroscopic Thermodynamics • Gibbs energy: G(P,T,n,x) • n is low spin occupation (0 to 1) • x is fraction of sites occupied by Fe (0 to 1) • G = Glattice + Gvibration + Gmagnetic + G mixing • Minimize G with respect to n

  16. Tsuchiya et al 2006 also: Slichter and Drickamer 1972, Gütlich et al 1979 • m = degeneracy (3) • S = Spin state (2) • H = E + PV

  17. Theory vs Experiment?

  18. New Experimental Data

  19. Impulsive Stimulated Light Scattering

  20. (Mg,Fe)O 5.6% Fe(100) surface Rhenium Gasket Argon Ruby 50 microns

  21. Extension to High Temperature?

  22. Predicted Seismic Structure Spin Transition Total Intrinsic

  23. SUMMARY • Large anomalies in Vp and Vs for HSLS transition • Macroscopic thermodynamic description works • Tested vs pressure and composition • High temperature test is needed • Mantle velocity anomalies may be suppressed - dV/dTHSLS > 0 • Explanation for lack of mid-mantle tomographic structure? • Perovskite is presumed to have analogous behavior

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