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

transcript

Elasticity of Ferro-Periclase Through

the High Spin - Low Spin Transition

J. Michael Brown - University of WashingtonJonathan Crowhurst - Lawrence Livernmore Lab.

Alexander Goncharov - Geophysical Lab.Steven Jacobsen - Northwestern University

J. Michael Brown - University of WashingtonJonathan Crowhurst - Lawrence Livernmore Lab.

Alexander Goncharov - Geophysical Lab.Steven Jacobsen - Northwestern University

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?

Summary(Three Major Topics)

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

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

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

Less structure in lower mantle

A possible connection to the high-spin low-spin transition

“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

Physics of the High spin to Low spin

Transition

Physics of the High spin to Low spin

Transition

High spin - low spin iron

Transition is Intrinsically non-1st order Readily described by robust macroscopic thermodynamics

Characterized by H = E + PV Associated with anomalies in physical properties

Transition is Intrinsically non-1st order Readily described by robust macroscopic thermodynamics

Characterized by H = E + PV Associated with anomalies in physical properties

Truly exciting both in terms of High pressure physics and chemistry

Understanding Earth’s mantle

But - some re-appraisals are needed

Truly exciting both in terms of High pressure physics and chemistry

Understanding Earth’s mantle

But - some re-appraisals are needed

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

QuickTime™ and aTIFF (LZW) decompressor

Clapyron Slope

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

Fine PrintFine 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

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

Macroscopic ThermodynamicsMacroscopic 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

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

1+ m(2S +1)eΔH

• m = degeneracy (3)• S = Spin state (2)

• H = E + PV

Tsuchiya et al 2006

also: Slichter and Drickamer 1972, Gütlich et al 1979

Theory vs Experiment?Theory vs Experiment?

New Experimental DataNew Experimental Data

Impulsive Stimulated Light Scattering

1064 nm

PROBESIGNAL

(Mg,Fe)O 5.6% Fe(100) surface

RubyRuby

50 microns

Rhenium Gasket

QuickTime™ and a decompressor

Extension to High Temperature?

Intrinsic Spin Transition Total

Predicted Seismic Structure

SUMMARYSUMMARY 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

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

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

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