HR: 0800h
AN: T41C-1320    [Abstracts]
TI: Phase Separation During Deformation of a Two-Phase Rock
AU: * Larkin, L M
EM: lark0051@umn.edu
AF: University of Minnesota, Dept. of Geology and Geophysics 310 Pillsbury Dr. SE ste. 108, Minneapolis, MN 55455 United States
AU: Zimmerman, M E
EM: zimme030@umn.edu
AF: University of Minnesota, Dept. of Geology and Geophysics 310 Pillsbury Dr. SE ste. 108, Minneapolis, MN 55455 United States
AU: Kohlstedt, D L
EM: dlkohl@umn.edu
AF: University of Minnesota, Dept. of Geology and Geophysics 310 Pillsbury Dr. SE ste. 108, Minneapolis, MN 55455 United States
AB: Layering and banding is ubiquitous in deformed metamorphic rocks in the crust over a wide range of length scales (from mm to km); layering of olivine and pyroxene is also common in mantle rocks. While it is widely appreciated that deformation produces layering, implying phase separation, the physical mechanisms of the segregation process and the thermodynamic conditions at which it occurs are very poorly understood. To investigate the process of phase separation during deformation of two-phase rocks, we have performed a series of high-strain deformation experiments on aggregates of anorthite plus nickel. This combination of materials was selected because under our experimental conditions (i) both phases are solid, (ii) nickel is significantly less viscous than plagioclase, and (iii) the two phases do not react with one another. Samples were fabricated by hot-pressing mixtures of fine-grain (~3 μm) powders with either a 1:1, 6:1, or 19:1 ratio of anorthite to nickel. Cylinders were then deformed in torsion to shear strains of γ ~ 6 at a temperature of 1380 K and a confining pressure of 300 MPa in a gas-medium apparatus. Microstructures of both the starting and the deformed material were characterized using optical microscopy. Although deformation did not produce a significant change in the phase distribution in the sample composed of equal proportions of anorthite and nickel or the more anorthite rich sample, a marked phase separation developed during shear of the 6:1 material. Small clumps of anorthite that were present in the starting material elongated in the direction of shear, providing excellent strain markers. In contrast, nickel grains segregated into bands on the order of 100 μm in length and 10 μm in width, oriented antithetic to the shear direction and ~30° to the shear plane. This mechanical self-organization process is similar to that observed in large-strain shear experiments on partially molten rocks in which melt-rich bands form at an orientation similar to that of nickel in our study.
DE: 3630 Experimental mineralogy and petrology
DE: 3902 Creep and deformation
DE: 3939 Physical thermodynamics
DE: 3947 Surfaces and interfaces
SC: Tectonophysics [T]
MN: Fall Meeting 2005