HR: 13:40h
AN: MR33B-01    [Abstracts]
TI: Triclinic Elastic Constants for Albite: Implications for Shear Wave Splitting in the Lower Crust
AU: * Brown, J M
EM: brown@ess.washington.edu
AF: Dept. of Earth and Space Sciences, University of Washington, Seattle, WA 98195
AU: Abramson, E H
EM: evan@ess.washington.edu
AF: Dept. of Earth and Space Sciences, University of Washington, Seattle, WA 98195
AU: Angel, R J
EM: rangel@vt.edu
AF: Geosciences Department, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061
AB: The full set of elastic constants for plagioclase end-member phase albite (NaAlSi_3O_8) is reported for the first time. Velocities of surface acoustic waves (both Rayleigh and pseudo-surface waves) were measured using Impulsively Stimulated Light Scattering (ISLS) on polished surfaces having six different orientations (three normal to the Cartesian axes and three lying on diagonals). Data were inverted and results tested using several non-linear optimization techniques. Additional constraints, based on compliance moduli determined under hydrostatic compression, reduced covariance in the reported constants. The constants differ significantly from the previously reported pseudo-monoclinic constants that were based on velocity measurements on polysynthetic twinned crystal aggregates. Differences are consistent with systematic errors in the earlier study associated with sparse data and the presence of cracks and other imperfections. The constants were averaged for a hypothetical feldspar aggregate with axial lattice-preferred orientation (LPO) consistent with natural samples. With the albite a* and c axes forming a girdle and the b axis normal to the layering, compressional wave anisotropy is 25%. A maximum in shear wave splitting occurs at an angle of incidence of 60° (relative to the axial plane) where the two shear wave velocities differ by 28%. Little splitting is predicted for propagation within the axial plane. If LPO is present in the lower crust where feldspars are a dominant mineral phase, these results suggest that shear wave splitting could be highly variable as a function of propagation direction.
DE: 3909 Elasticity and anelasticity
DE: 7205 Continental crust (1219)
DE: 7220 Oceanic crust
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005