Elasticity of Ferro-Periclase Through the High Spin

Report
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

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