HR: 17:15h
AN: MR54A-06    [Abstracts]
TI: PartialLy Shock-Transformed Olivine in Shocked Chondrites: Mechanisms of Solid-State Transformation
AU: * Sharp, T G
EM: tom.sharp@asu.edu
AF: School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404, United States
AU: Xie, Z
EM: Zhidong.Xie@asu.edu
AF: School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404, United States
AB: High-pressure minerals, produced by shock meta-morphism, are common in and around melt veins in highly shocked chondrites. These minerals either crys-tallized from silicate melt in the shock-vein or formed by solid- state transformation of host-rock fragments entrained in the melt or along shock-vein margins. Olivine- ringwoodite transformation kinetics can be used to constrain shock duration if one knows P-T conditions and transformation mechanisms. Here we examine the solid-state transformation of olivine to ringwoodite and the formation of ringwoodite lamellae in Tenham. Partially transformed olivines show a variety of ringwoodite textures. Some have granular textures whereas others have straight or curved ringwoodite lamellae, made up of distinct (1 to 2 ?m) crystals. Many of these polycrystalline ringwoodite lamellae occur in pairs. Where these paired lamellae cross the are offset, suggesting that the lamellae are associated with shearing. Electron diffraction reveals that the ringwoodites in the polycrystalline lamellae, occur in roughly the same crystallographic orientation, defining a lattice-preferred orientation. TEM also shows that the remnant olivine is highly deformed, with high densities of complex dislocations. This olivine has a poorly organized sub-grain structure that grades into polycrystalline olivine. The nearby untransformed olivine is also highly de-formed, but less than the partially transformed olivine. TEM images of complex dislocation and sub-grain microstructures suggests that the transformation of olivine to ringwoodite involves extensive deformation. High densities of dislocations provide potential sites for heterogeneous nucleation of ringwoodite and may enhance Fe-Mg inter-diffusion. The differential stress at the initial stage of the shock results in high strains and local heating. The paired ringwoodite lamellae in olivine appear to result from shearing and possibly shear heating, where nucleation occurs on both sides of a shear band.
DE: 3625 Petrography, microstructures, and textures
DE: 3662 Meteorite mineralogy and petrology (1028, 6240)
DE: 3902 Creep and deformation
SC: Mineral and Rock Physics [MR]
MN: 2007 Fall Meeting