HR: 1340h
AN: V33A-1171    [Abstracts]
TI: Reduced Seismic Anomalies in the Lower Mantle: Role of the High-Spin to Low-Spin Electronic Transition
AU: * Brown, J M
EM: brown@ess.washington.edu
AF: University of Washington, Earth and Space Sciences Box 351310, Seattle, WA 98195, United States
AU: Crowhurst, J
EM: crowhurst1@llnl.gov
AF: Lawrence Livermore Laboratory, L-350 PO Box 808, Livermore, CA 94551, United States
AU: Goncharov, A
EM: goncharov@gl.ciw.edu
AF: Geophysical Laboratory, Carnegie Institution of Washington 5251 Broad Branch Road NW, Washington DC, 20015, United States
AU: Jacobsen, S D
EM: steven@earth.northwestern.edu
AF: Northwestern University, Department of Geological Sciences, Evanston, IL 60208, United States
AB: We determined the pressure dependence of the single crystal elastic constants of ferropericlase (6% iron) to a static pressure of 63 GPa. The velocities of the quasi-longitudinal and interfacial elastic wave were measured in the (100) crystal plane using impulsive stimulated light scattering. We observed anomalies in measured velocities over the pressure range of 40 to 60 GPa and associated them with the high-spin to low-spin (HSLS) transition in the iron cations of ferropericlase. We find that all three elastic elements (c₁₁, c₁₂, c₄₄) smoothly pass through minima with respect to an extrapolation of the pure high-spin state. This behavior is consistent with a macroscopic thermodynamic description for the HSLS transition that predicts no sharp changes in spin fractions and dependent physical properties. The associated enthalpy and volume changes scale correctly between these data and literature compression data obtained on a sample with higher iron concentration. A significant geophysical consequence of the observed behavior is that in the spin transition pressure regime, normal temperature derivatives of elasticity are reduced. Within Earth, a minimum in the temperature derivatives caused by the HSLS transition is predicted to occur at a depth of 1500 km. This correlates well with the depth where seismic tomographic results show minimum structure and model power. Thus, the HSLS transition may serve to mask velocity anomalies associated with lateral temperature differences.
DE: 3909 Elasticity and anelasticity
DE: 3924 High-pressure behavior
DE: 7208 Mantle (1212, 1213, 8124)
DE: 7270 Tomography (6982, 8180)
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting