HR: 1340h
AN: MR33A-0155    [Abstracts]
TI: Fabric transition in omphacite and its implications for seismic anisotropy
AU: * Zhang, J
EM: junfeng.zhang@email.ucr.edu
AF: IGPP and Dept. of Earth Sciences, University of California, Riverside, CA 92521 United States
AU: Green, H W
EM: harry.green@ucr.edu
AF: IGPP and Dept. of Earth Sciences, University of California, Riverside, CA 92521 United States
AB: It has been well documented in naturally deformed eclogites that there are two principle types of crystallographic fabrics in omphacite: the S-type and the L-type fabrics. There is strong evidence for dislocation creep for both types. However, two distinct hypotheses have been proposed to interpret the transition between the S-type and the L-type fabrics: a change of stress field from compression to constriction and a space group transformation from C2/c to P2/n in omphacite at 1023 K. We have reproduced in the laboratory S-type fabrics under axial compression and L-type fabrics under simple shear with synthetic omphacite specimens at a pressure of 3 GPa and temperatures of 1300-1500 K. We conclude that the transition between S-type and L-type omphacite fabric is associated with strain kinematics (shearing or flattening). Seismic anisotropy in eclogite is dominated by the crystallographic fabrics of omphacite since garnet is always randomly oriented in these rocks (both natural examples and our experiments. We show with experimentally deformed specimens that different fabrics induce different seismic anisotropy patterns in omphacite. All calculations were performed using the interactive programs provided by D. Mainprice (the Unicef Careware software package). The direction of highest P-wave velocity is subparallel to lineation and closely related to the maxima of c[001] axis of omphacite. The direction of slowest P-wave velocity is perpendicular to foliation and corresponds to maximum concentration of the b[010]-axis of omphacite. The S-wave anisotropy has complicated patterns with multiple concentrations of high anisotropy. Both the P-wave anisotropy and the S-wave anisotropy in omphacite aggregates are on the order of 2-3% in our experimentally deformed specimens, which are comparable to observations in natural eclogites.
DE: 3630 Experimental mineralogy and petrology
DE: 3902 Creep and deformation
DE: 3924 High-pressure behavior
DE: 5102 Acoustic properties
DE: 5120 Plasticity, diffusion, and creep
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005