HR: 15:10h
AN: T43D-07    [Abstracts]
TI: Stress-Driven Melt Segregation and Shear Localization in Partially Molten Aggregates: Experiments in Torsion
AU: * King, D S
EM: king0314@umn.edu
AF: University of Minnesota, Department of Geology and Geophysics, 310 Pillsbury Dr. SE, Minneapolis, MN 55455,
AU: Kohlstedt, D L
EM: dkohl@umn.edu
AF: University of Minnesota, Department of Geology and Geophysics, 310 Pillsbury Dr. SE, Minneapolis, MN 55455,
AU: Zimmerman, M E
EM: zimme030@umn.edu
AF: University of Minnesota, Department of Geology and Geophysics, 310 Pillsbury Dr. SE, Minneapolis, MN 55455,
AB: Simple shear experiments of partially molten aggregates have demonstrated that shear induces organized patterns of melt distribution and strain localization [1,2]. New torsion experiments on partially molten aggregates of olivine + chromite + 4 vol.% mid-ocean ridge basalt provide additional insights into the interactions between deformation and melt segregation. Samples were deformed at constant strain rates ranging from 3x10^-5 to 3x10^-4 /s, corresponding to shear stresses of ~60 and ~100 MPa, respectively. When samples are sheared, melt segregates into distinct melt-rich bands oriented ~15° antithetic to the shear direction. The melt fraction in the bands ranges from 0.1 to 0.2. In samples deformed at higher stress, bands are narrower and more closely spaced. At a shear stress of ~60 MPa bands are ~21 μm wide and spaced ~104 μm apart. At a shear stress of ~100 MPa bands are ~15μm wide and spaced ~71 μm apart. Melt segregation occurs in both the dislocation creep and diffusion creep regimes. Our experiments demonstrate that melt-rich bands form at a strain of ~100% regardless of the stress at which the sample is deformed. Near this strain threshold, the behavior of the sample changes from strain hardening to strain softening. This observation indicates that, in addition to providing high-permeability pathways through which melt can travel, melt bands also become zones of localized deformation. Strain is partitioned and localized into melt-rich bands because of a viscosity reduction in regions of elevated melt fraction. This process of stress driven melt segregation has implications for melt transport in many geological settings, including beneath mid-ocean ridges, and for the formation of shear zones in partially molten rocks. [1] Zimmerman, M. et al., Geophys. Res. Lett., 26, 1999. [2]Holtzman, B. et al., Geochem. Geophys. Geosyst., 4, 2003.
DE: 3902 Creep and deformation
DE: 4435 Emergent phenomena
DE: 8030 Microstructures
DE: 8160 Rheology: general (1236, 8032)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting