ELASTICITY OF SOME MANTLE CRYSTAL STRUCTURES by HERBERT

The elasticity of high-pressure mantle phases may be characterized by crystal chemically similar compounds. Analogue compounds available as single crystals yield the most reliable elastic constant data. The single-crystal elastic constants of pleonaste and hercynite spinel, rutile-structure germanium dioxide, and spessartite-almandine garnet are studied, in most cases as a function of pressure and temperature. High-pressure phases of olivine, silica, and pyroxene occur in the spinel, rutile, and garnet structures, respectively.' The pressure derivative of the shear mode (C11 C12)/2 is slightly negative for pleonaste. Small shear moduli pressure derivatives are characteristic of the spinel structure and imply a low kinetic barrier to phase transformations and diffusion. The pleonaste and hercynite elastic constant data determine the compressional and shear velocities as a function of mean atomic weight for the spinel phase of olivine. The compressional velocity data indicate an increase in mean atomic weight in the transition zone while the shear velocity data indicate no increase. The bulk modulus pressure derivative, 3K/3P, of rutilestructure germanium dioxide is about 6.2. It is likely that aK/DP of stishovite is between 6 and 7. The seismic parameter 4 of stishovite is not well-determined at present and its use in a molar averaging scheme to deduce the iron and silicon content of the lower mantle contains inconsistencies. There is no strong evidence for enrichment of either component in the lower mantle. The elastic constant pressure derivatives of spessartitealmandine garnet are similar to almandine-pyrope. The elastic velocities of garnet-transformed pyroxenes may be estimated from an inverse proportionality relation between the seismic parameter @ and mean atomic weight M. The velocity change over the pyroxene -+ garnet transformation is then 20% for compressional waves and 10% for shear waves. Thesis Supervisor: Gene Simmons Title: Professor of Geophysics