High-temperature creep of olivine single crystals, 2. dislocation structures

Abstract

Dislocation structures and densities in experimentally deformed single crystals of San Carlos olivine were examined using the oxidation-decoration technique. The high-temperature deformation experiments were conducted at various temperatures (T), differential stresses (σ), and oxygen fugacities ($\text{ƒ}{}_{\mathrm{o}}{}_2$); the samples were buffered against either orthopyroxene (opx) or magnesiowustite (mw) solid-state powders and compressed along one of the three 45° orientations.

In samples that were deformed and subsequently quenched under load, seven distinct dislocation structures were observed. (a) For samples compressed parallel to [110]_c at 16 <σ < 50 MPa and , two different dislocation arrangements were identified. For opx-buffered samples at 1400 < T < 1475°C, the dislocation structure was composed of curved screw segments that had usually cross-slipped and straight edge segments of comparable length. At 1300 < T < 1350°C for opx-buffered samples and at 1350 < T < 1500°C for mw-buffered samples, the dislocation structure is dominated by arrays of very long edge or mixed dislocations with only a few screw dislocations, (b) For both opx-buffered and mw-buffered samples compressed parallel to [101]_c at 1340 < T < 1400°C and 30 < σ <60 MPa, the dislocation configurations are characterized by zigzag near-edge dislocations at and by straight, cross-slipped screw dislocations plus pinned edge dislocations at . (c) For samples compressed parallel to [011]_c at T = 1400°C and 70 <σ < 150MPa, the primary dislocations change from long, straight edge dislocations (for and opx buffer) to a combination of straight edge dislocations and polygon-shaped half loops (for and mw buffer), to gently curved near-edge dislocations (for for both solid-state buffers). Abundant long, straight screw dislocations were present in each case for the [011]_c samples. All of the above variations in dislocation structure are paralleled by changes in the measured power-law equation that describes the behavior of creep mechanisms of olivine. Thus, the different dislocation structures are associated with different rate-controlling creep mechanisms. For all of the dislocation structures, the dislocation density (ρ) increases with differential stress according to the relation ρ ∞ σ^1.4.

Dislocation structures in samples that were deformed and then statically annealed have also been studied. For both [110]_c and [011]_c samples deformed over a wide range of experimental conditions, the dislocation structures on the slip planes generally consist of short, curved edge or mixed dislocation segments. In [101]₁ samples deformed at low $\text{ƒ}{}_{\mathrm{o}}{}_2$, 50 MPa and 1400°C, the dislocation structure was much less altered by annealing than the dislocation structures in [110]_c and [011]_c samples. The density of dislocations decreased much less for these [101]_c samples than for the [110]_c and [011]_c samples. Due to climb of the edge segments, the straight screw segments found in quenched samples became curved and many low-angle (100) tilt boundaries, connected by straight [100] screw dislocations, developed during the annealing process.