HR: 0800h
AN: T41B-0575    [Abstracts]
TI: Strain weakening versus strain localization in olivine aggregates
AU: * Holyoke, C W
EM: holyoke@geo.tamu.edu
AF: Texas A&M University, Dept. of Geology and Geophysics, TAMU Halbouty Building College Station, TX, College Station, TX 77843, United States
AB: Evidence of strain localization in olivine aggregates has been observed in both obducted slices of upper mantle and in high stress experiments (Post, 1977). Most experimental deformation studies of olivine aggregates have been done in gas apparatus at low P (<300 MPa) and thus at low flow stress; both axial compression and shear experiments involving climb-accommodated dislocation creep show steady state flow, with little if any strain weakening. However, experimental studies of quartz and feldspar at high P have shown the existence of a low T, high stress dislocation creep regime in which climb is very limited and creep is accommodated by bulging recrystallization; in this regime extreme strain weakening and/or localization occur. A series of deformation experiments were performed natural and synthetic aggregates using a modified molten salt assembly in a Griggs apparatus at a P of 1.5 GPa, T of 950 to 1200°C, and strain rates of 5x10^-5/s to 10^-6/s, in both axial compression and general shear. The starting material was either Balsam Gap dunite (d~500 μm) or synthetic aggregates hot pressed from San Carlos olivine powders (~20 μm). Prior to weld sealing in Ni and outer Pt, each sample was dried for 24 hrs at 900°C in a CO/CO₂ atmosphere. Shear experiments were performed using three types of shear pistons: Balsam Gap dunite, alumina and cores from single crystals of San Carlos olivine oriented with (010) perpendicular to ƒã1. The peak stresses of experiments ranged from 1380 MPa to 250 MPa and all experiments underwent some degree of strain weakening. Deformation mechanisms observed in samples from axial compression experiments or shear experiments with alumina or San Carlos single crystal shear pistons can be characterized by three behavioral regimes. Microstructures in these samples were homogeneous and no strain was accommodated by the shear pistons. Samples from experiments deformed the highest stresses contain very high dislocation densities, cracks and few recrystallized grains, indicating the dominant deformation mechanism is semi-brittle flow. The samples from intermediate peak stress experiments (350-780 MPa) contained porphyroclasts with rims of recrystallized grains (2-5 μm) and the porphyroclasts have high densities of tangled dislocations and serrated grain boundaries. The fine grains at the edges of porphyroclasts have few to no dislocations. These microstructures indicate the intermediate samples are deforming by recrystallization accommodated dislocation creep. At the lowest stresses, the remaining porphyroclasts contain subgrains, have very low dislocation densities and the recrystallized grains are considerably larger (10-25 μm), indicating deformation by climb-accommodated dislocation creep. In the experiments which had Balsam Gap shear pistons with a fine-grained (20 μm) layer of San Carlos powders, at high stresses (550 MPa) recrystallization accommodated dislocation was the dominant deformation mechanism creep and strain was localized within the fine grained layer. However, at low stresses (250 MPa), both the olivine in the shear pistons and fine-grained layer deformed by climb-accommodated dislocation creep and strain was homogeneous throughout the sample. These results indicate that all olivine aggregates undergo some degree of strain weakening during shear deformation, but strain localization only occurs in olivine aggregates deforming by recrystallization-accommodated dislocation creep when a fine-grained zone exists prior to deformation. Therefore, heterogeneities in the grain size or different deformation mechanisms may be required to form shear zones in the mantle.
DE: 3902 Creep and deformation
DE: 8012 High strain deformation zones
DE: 8033 Rheology: mantle (8162)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting