HR: 17:15h
AN: S22E-06    [PDF]
TI: Melt Segregation \& LPO in Anorthite-Basalt Deformed in Torsion
AU: * Zimmerman, M E
EM: zimme030@umn.edu
AF: Dept Geology \& Geophysics, Univ of Minnesota, 310 Pillsbury Dr, Minneapolis, MN 55455 United States
AU: Kohlstedt, D L
EM: dlkohl@umn.edu
AF: Dept Geology \& Geophysics, Univ of Minnesota, 310 Pillsbury Dr, Minneapolis, MN 55455 United States
AB: Deformation in the middle and lower crust is in large part controlled by the rheology of feldspar. Seismic studies have shown that the middle crust of orogenic belts is partially molten. Structural studies of mylonites and migmatites from these terrains record large strain deformation. Therefore, we performed torsional shear deformation experiments on fine grained (10 $\mu$m) samples of Beaver Bay anorthite (An$_{70}$) $\pm$ 10 vol% basalt to shear strains $\gamma$ = 2-6 to investigate the development of lattice preferred orientation (LPO) and melt segregation at large shear strains. We performed experiments in a gas medium apparatus equipped with an internal torque cell at T = 1450 K, P = 300 MPa, and constant twist rate. Melt segregated in the An$_{70}$ + basalt samples into melt-rich bands oriented at $\sim$20$\deg$ to the shear plane and antithetic to the shear direction. The spacing between bands is $\sim$0.5 mm. Distortion of the iron jacket demonstrates that strain localized in the melt-rich bands. We determined the LPO of An$_{70}$ with scanning electron microscopy using electron back scatter diffraction (SEM-EBSD). In patterns from an An$_{70}$+ basalt sample deformed to $\gamma \approx$ 2.5, (001) planes are aligned subparallel to the shear plane and [100] axes are concentrated close to the shear direction. Both the (001) and the [100] are rotated counter clockwise from the shear direction by 20-25$\deg$. The formation of melt-rich bands is consistent with results from simple shear experiments on olivine + chromite + basalt and olivine + FeS $\pm$ basalt, as well as An$_{70}$ + basalt and indicates that deformation can drive melt segregation. Deformation drives the self organization of melt-rich bands and decreases the effective viscosity of the rock. The LPO is consistent with results from experiments on albite in shear and anorthite in compression and compatible with slip dominantly on (001) with [100] as the slip direction. A similar back rotation, attributed to partitioning of the strain between melt-rich and melt-depleted regions, occurred in partially molten olivine rich samples deformed in simple shear. The formation of melt-rich bands and the development of LPO in plagioclase during high strain shear experiments will lead to significant shear wave splitting and seismic anisotropy in the continental lithosphere.
DE: 5120 Plasticity, diffusion, and creep
DE: 5139 Transport properties
DE: 5144 Wave attenuation
DE: 7205 Continental crust (1242)
DE: 7218 Lithosphere and upper mantle
SC: Seismology [S]
MN: 2003 Fall Meeting