Elasticity of Ferro-Periclase Through the High Spin
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Transcript Elasticity of Ferro-Periclase Through the High Spin
Elasticity of FerroPericlase 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)
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)
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?
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
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
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 DH = DE + PDV
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
Qu i c k T i m e ™ a n d a
T I F F (L Z W ) d e c o m p re s s o r
a re n e e d e d to s e e t h i s p i c t u re .
Qu i c k T i m e ™ a n d a
T I F F (L Z W ) d e c o m p re s s o r
a re n e e d e d to s e e t h i s p i c t u re .
Clapyron Slope
dP DV
dT DS
Clapyron Slope
dP DV
dT DS
Low-spin iron is an
“additional chemical
component in the
mantle”
Low-spin iron is an
“additional chemical
component in the
mantle”
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
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
Tsuchiya et al 2006
also: Slichter and Drickamer 1972, Gütlich et al 1979
1
n
1 m(2S 1)e
• m = degeneracy (3)
• S = Spin state (2)
• DH
= DE + PDV
DH
kTx
Theory vs Experiment?
New Experimental Data
Impulsive Stimulated Light
Scattering
1064nm
SIGNAL
PROBE
1064nm
Rhenium Gasket
(Mg,Fe)O
5.6% Fe
(100) surface
Argon
Ruby
50 microns
QuickTime™ and a
decompressor
are needed to see this picture.
Extension to High
Temperature?
Predicted Seismic Structure
Intrinsic
Spin Transition
Total
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