Experimental deformation of a synthetic dunite at high temperature and pressure. II. Transmission electron microscopy |
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| Authors: | David H Zeuch HW Green II |
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| Institution: | Department of Geology, University of california, Davis, CA 95616, U.S.A. |
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| Abstract: | We have performed detailed transmission electron microscope on most of the deformed synthetic dunite specimens prepared in the study by Zeuch and Green (1984). We have identified three basic types of sub-boundaries, simple tilt walls in (100) and (001). composed by b = 100] and b = 001] edge dislocations, respectively, and twist boundaries in (010) composed of b = 100] and b = 001] screws. We have also observed more complex, asymmetric lilt boundaries in (100) and (001). Like the (010) twist boundaries, these asymmetric tilt walls are common only at the highest temperatures and lowest strain rates. Subgrain development is extensive at the higher temperatures and lower strain rates, and subgrains are composed of the above-mentioned three types of sub-boundaries; edge components in (100) and (001) ire “knitted” to screw components in (010) as described by Kirby and Wegner (1978) for naturally deformed olivine. In many areas of the samples which we studied, subgrain development is not observed, but parallel arrays of tilt boundaries of one type or the other are present. At higher temperatures and lower strain rates. “(100) organization” (Durham et al., 1977) is common; this structure consists of parallel arrays of (100) tilt boundaries with b = 100] screws connecting the sub-boundaries. At lower temperatures we have observed an analogous arrangement of (001) sub-boundaries and b = 001] screws, which we refer to as “(001) organization”. Under all experimental conditions, dislocations with b = 100] and b = 001] are present in approximately equal numbers. However, the two types of dislocations also have distinctly different geometries under all test conditions. We suggest that the transition from slip parallel to 001] to slip parallel to 100] with increasing temperature, which has been reported in earlier studies may also depend upon water content. The substructures which we observe are virtually identical to those seen in many naturally deformed peridolites. and we conclude that the mechanisms involved in both natural and laboratory deformation of olivine polycrystals are similar. On the other hand, the substructures reported here are very different from those observed in experimentally deformed olivine single crystals. It seems likely that these substructural differences reflect fundamental differences in the behavior oh single crystals and polycrystals. which are in turn reflected in different measured creep strengths. |
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