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Science 25 January 2008:
Vol. 319. no. 5862, pp. 451 - 453
DOI: 10.1126/science.1149606
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Reports

Elasticity of (Mg,Fe)O Through the Spin Transition of Iron in the Lower Mantle

J. C. Crowhurst,1* J. M. Brown,2 A. F. Goncharov,3 S. D. Jacobsen4

Changes in the electronic configuration of iron at high pressures toward a spin-paired state within host minerals ferropericlase and silicate perovskite may directly influence the seismic velocity structure of Earth's lower mantle. We measured the complete elastic tensor of ferropericlase, (Mg1–x,Fex)O (x = 0.06), through the spin transition of iron, whereupon the elastic moduli exhibited up to 25% softening over an extended pressure range from 40 to 60 gigapascals. These results are fully consistent with a simple thermodynamic description of the transition. Examination of previous compression data shows that the magnitude of softening increases with iron content up to at least x = 0.20. Although the spin transition in (Mg,Fe)O is too broad to produce an abrupt seismic discontinuity in the lower mantle, the transition will produce a correlated negative anomaly for both compressional and shear velocities that extends throughout most, if not all, of the lower mantle.

1 Chemistry, Materials, and Life Sciences Directorate, Lawrence Livermore National Laboratory (LLNL), Livermore, CA 94550, USA.
2 Earth and Space Sciences, University of Washington, Seattle, WA 98195, USA.
3 Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015, USA.
4 Department of Earth and Planetary Sciences, Northwestern University, Evanston, IL 60208, USA.

* To whom correspondence should be addressed. E-mail: crowhurst1{at}llnl.gov

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