HR: 0830h
AN: S21E-0367    [PDF]
TI: The Pressure and Compositional Dependence of the Shear modulus of Magnesiowustite
AU: * Devine, J M
EM: jmdevine@midway.uchicago.edu
AF: Dept. of the Geophysical Sciences University of Chicago, 5734 S. Ellis Ave., Chicago, IL 60637 United States
AU: Heinz, D L
EM: heinz@uchicago.edu
AF: Dept. of the Geophysical Sciences University of Chicago, 5734 S. Ellis Ave., Chicago, IL 60637 United States
AU: Shen, G
EM: shen@cars.uchicago.edu
AF: Consortium for Advanced Radiation Sources University of Chicago, 5640 S. Ellis Ave., Chicago, IL 60637 United States
AB: One-dimensional seismic Earth models such as the PREM show discontinuous jumps of about 4.5% in compressional velocity, 6.3% in shear velocity, and 8.8% in density at 670km depth. At the pressure and temperature appropriate for this depth, (Mg,Fe)2SiO4 spinel (ringwoodite) dissociates into (Mg,Fe)SiO3 in the orthorhombic perovskite structure and magnesiowustite (Mg,Fe)O. All three phases are solid solutions between magnesium and iron end-members, and their properties are a function of pressure, temperature, and iron content. The discussion of whether or not the mantle is chemically stratified, involves matching compressional and shear velocities, and densities from seismic Earth models to mantle mineral assemblages. Magnesiowustite is presumed to be the second most abundant mineral in the lower mantle behind silicon-magnesium perovskite. It has no structural phase transitions throughout the silicate Earth. A series of experiments carried out at GeoSoilEnviroCARS Sector 13 of the Advanced Photon Source at the Argonne National Laboratory explored the shear properties of magnesiowustite as a function of pressure and iron content. Radial x-ray diffraction along with Singh's model for deviatoric strain was used to determine the full elastic tensor for (Mg25,Fe75)O, (Mg40,Fe60)O, (Mg60,Fe40)O, (Mg75,Fe25)O as a function of pressure to 50 GPa. The compositional dependence of the elastic tensor was determined from these combined experiments. The seismic equation of state is fitted to calculated bulk and shear moduli at room temperature. High temperature contributions to the elasticity of the end-member MgO are provided from ab initio calculations. The resulting compressional and shear velocities are extrapolated to the core-mantle boundary.
DE: 3909 Elasticity and anelasticity
DE: 3919 Equations of state
DE: 3924 High-pressure behavior
SC: Seismology [S]
MN: 2003 Fall Meeting