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High-pressure phase transformations of olivine within the Earth's upper mantle cause large changes in physical properties such as density, elasticity, and rheology. These changes are especially significant in subduction zones where large lateral variations in temperature are expected to control the depth at which transformations occur. Because high stresses are likely to occur in subducting lithosphere, it is important to investigate deformation of mantle phases and the effects of stress on transformation mechanisms under subduction zone conditions.

To investigate deformation and transformations of olivine at high pressures, we have carried out a series of experiments using a multianvil press as a high-pressure deformation apparatus. The deformation assembly is a simplified version of that used by Bussod et al. (Annual Reports 1992, 1993) for the deformation of olivine single crystals. Our starting material consisted of powdered San Carlos olivine that was hot pressed at 11 GPa and 1250 °C for 2h and then deformed at temperatures and pressures ranging from 600 to 1000 °C and 15 to 18 GPa, respectively. Samples were characterized with transmission electron microscopy to determine the deformation mechanisms and dislocation microstructures as well as transformation mechanisms in partially transformed samples.

Because little is known about the mechanical properties and deformation mechanisms of olivine at high pressures (stress-strain data are not available from multianvil experiments) it is important to characterize the defect microstructures in the deformed olivine. In a sample deformed at 11 GPa and 1250 °C for 2h, 90 % of the grains are defect free while the remaining 10 % contain dislocations with free-dislocation densities up to 5x10¹² m^-2. The free dislocations consist of a [100] edge dislocations and few c [001] screw dislocations. The a edge dislocations have straight segments (L = [001]) as well as curved segments, and in some cases occur as loops. We also detected well-organized pure tilt boundaries (Fig. 3.1-3) parallel to (001). These features indicate recovery and are consistent with a high temperature deformation mechanism controlled by dislocation climb. In a sample deformed at the same conditions and then further deformed at 18 GPa and 600 °C for 30 min, we observe grains with highly heterogeneous dislocation densities varying from 10¹¹ to 5x10¹³ m^-2, depending on the crystallographic orientation of each grain relative to the principal stress. The deformed grains contain mostly straight c screw dislocations with a minor amount of straight a-edge dislocations (Fig. 3.1-4). These latter dislocations could be remnants from the hot pressing stage. This morphology of straight screw c dislocations suggests: (i) that the edge segments of c dislocations move very rapidly during deformation at 18 GPa and 600 °C; and (ii) lattice friction constrains c dislocations to remain in Peierls valleys. The defect microstructures of these two samples are very similar to those found in silicate and in germanate olivine deformed at similar temperatures but at much lower pressures (P < 3 GPa).
 

Fig. 3.1-3: Weak-beam TEM image of deformed olivine. Most of the free dislocations are a edge dislocations while some are c screw dislocations. A pure tilt subgrain boundary parallel to (001) can be observed at the center of this micrograph. The density of free dislocations is 2x1012 m-2.

One sample deformed at 15 GPa and 1000 °C for 30 minutes was partially transformed into ß-phase. Optically, we observed elongate grains of ß-phase that nucleated at olivine grain boundaries and grew parallel to the principal compressive stress direction. The remnant olivines are elongated perpendicular to the principal compressive stress as a result of the preferred growth direction of ß-phase (Fig. 3.1-5). This observation suggests that transformation under stress results in grain-shape anisotropy and possibly preferred orientations of ß-phase controlled by stress. Preliminary TEM investigations show that the relict olivine grains contain a high density of mostly c dislocations ( = 7 x 10¹³ m^-2) while the ß-phase grains contain few dislocations ( = 3 x 10¹² m^-2) as well as stacking faults lying in (010) planes. From these results, we can infer that ß-phase is stronger than olivine.
 

Fig. 3.1-4: Weak-beam TEM image of straight c screw dislocations. The dislocation density is 3 x 1013 m-2.

Fig. 3.1-5: Optical micrograph showing two partially transformed olivine grains elongated perpendicular to the principal compressive stress direction. Elongate ß-phase grains that nucleated at olivine gain boundaries grew inwards parallel to the principal compressive stress.