Olivine dislocations

Extended defects interrupt the continuity of the crystal, generating crystal subgrains whose dimensions depend, in a complex fashion, on the density of extended defects per unit area. Table 4.1 gives examples of reported dislocation densities and subgrain dimensions in olivine crystals from the San Carlos perido-tite nodules (Australia). Assuming a mean dislocation density within 1.2 X 10 and 6 X 10 cm , Kirby and Wegner (1978) deduced that a directional strain pressure of 35 to 75 bar acted on the crystals prior to their transport to the surface by the enclosing lavas. 

Table 4J Dislocation densities (cm and mean subgrain dimensions (cm) in San Carlos olivines (from Kirby and Wegner, 1978). n.d. = not determined.

Kirby S. H. and Wegner M. W. (1978). Dislocation substructure of mantle-derived olivine as revealed by selective chemical etching and transmission electron microscopy. Phys. Chem. Minerals, 3 309-330. 

Figure 15-2. Decoration of dislocations in olivine by internal oxidation. Precipitates are Fe304 [by courtesy of D. Kohlstedt, University of Minnesota].

Figure 8.33. Arrays of dislocations forming low-angle tilt boundaries in olivine, (a) Dislocations in the two boundaries shown have the same Burgers vector and sign. (Courtesy J. D. Fitz Gerald.)...

Arrays of dislocations forming low-angle till boundaries in olivine are shown in Figure 8.33. The contrast of the dislocations in each of the two... 

The diffraaion effects directly attributable to periodic arrays of dislocations that form low-angle boundaries in olivine have been studied by Ricoult and Kohlstedt (1983). Figure 8.35 shows an edge-on tilt boundary approximately parallel to (100) and its associated SAD pattern. The extra... 

Figure 8-35. BF image of the dislocations fonning a (100) tilt boundary in olivine viewed edge-on, and the electron diffraction pattern from the boundary re on. Note the fine structure of the 605 and 705 reflections shown in the inserts. (From Ricoult and Kohlstedt 1983.)... 

The effects of very high stresses and strain-rates have been investigated in microhardness experiments. In these experiments, loads of 50-500 g (corresponding to stresses as high as 2 GPa) are exerted by a diamond or sapphire Vickers indenter for about 20 seconds at temperatures up to 1,(X)0°C. Clearly, steady-state flow is never achieved but such experiments have provided important information about the dislocations involved in the deformation of olivine, for example. 

More recently Mackwell, Kohlstedt, and Paterson (1985) studied the deformation of single crystals of San Carlos (Arizona) olivine deformed under hydrous conditions at 1,300 C, 300 MPa confining pressure, and 10 s strain-rate and found they were a factor of 1.5-2 weaker than those deformed in an anhydrous environment. TEM observations showed that specimens deformed under dry conditions, in an orientation such that the slip systems (001)[100] and (100)[001] would be activated, were characterized by a microstructure of generally curved dislocations and dislocation loops, but no organization into walls. The dislocation density was 10 -10 cm compared with an initial value of < 10 cm . Most of the dislocations and the loops lie approximately in the (010) plane because they are in contrast for g = 004, they probably have b = [001] dislocations with b = [010] and [100] would be out-of-contrast for this reflection. However, the slip system (010) [001] is not expected to be active. It is not clear, therefore, if these dislocations are actually involved in the deformation. The general geometry of the dislocation microstructure is not inconsistent with some climb mobility in fact, on the basis of the observations of Phakey et al. (1972), climb is certainly expected at 1,300°C. 

The pyroxenes are the most abundant minerals, after olivine, in perido-tites, which are the dominant constituents of the upper mantle. It is not surprising, therefore, that there has been considerable interest in the mechanical properties of the pyroxenes, and a review has recently been given by Doukhan et al. (1986). The orthorhombic pyroxenes deform by slip and by a shear transformation that produces monoclinic lamellae (one or a few unit cells thick) parallel to (100). Coe and Kirby (1975) and McLaren and Etheridge (1976) have shown that the shear transformation is achieved by the glide of partial dislocations of b = 0.83[001] in (100), which leave partial dislocations of b = 0.17[001] terminating the shear lamellae. The dominant slip system is (100) [001]. Recent TEM observations by van Duysen, Doukhan, and Doukhan (1985) suggest that the dislocations associated with this slip system may be dissociated into four partials and that the slip system (100) [010] may also be activated. These observations are discussed in Section 9.9.1. 

The previous sections have been mostly concerned with the dislocations and microstructures observed in single crystals deformed to various strains under known experimental conditions. In some minerals, notably quartz and olivine, the macroscopic deformational behavior, as revealed by the creep and stress-strain curves, can be understood in terms of the micro-structural evolution during deformation and, furthermore, certain quantifiable characteristics of the microstructure correlate with the imposed... 

Both types of microstructure found in olivine are indicative of a significant component of dislocation climb during deformation. The dissociated dislocations present in the low-temperature microstructure have not been reproduced in any experiments nor have they been found in other naturally deformed olivines. The climb-dissociation may affect the type... 

Figure 9.39. WBDF micrograph showing the termination of thickness fringes where a dislocation intersects the foil surfaces in olivine. In this micrograph, g = 062, and there are two terminating thickness fringes at each end of the dislocation. Hence, g-b = 2, which is consistent with b = [001].

Boland, J. N., McLaren, A. C., Hobbs, B. E. (1971). Dislocations associated with optical features in naturally-deformed olivine. Contrib. Mineral. Petrol., 30, 53-63. 

Durham, W. B., Goetze, C., Blake, B. (1977). Plastic flow of oriented single crystals of olivine. 2 Observations and interpretation of the dislocation structures. J. Geophys. Res., 82, 5755-70. 

Gaboriaud, R. J. (1986). Dislocations in olivine single crystals indented between 25 and 1100°C. Bull. Mineral., 109, 185-91. 

The primary mechanism for seismic anisotropy in the mantle is thought to be due to the lattice preferred orientation (LPO) of olivine and other dominant minerals. Numerical predictions of mantle flow-fields can be used to predict LPO as a result of dislocation mechanisms and hence the seismic anisotropy (e.g. Blackman et al. 1996 ... 

First, volatiles exert an important control on the physical properties of the mantle. For example, the presence of water reduces the strength of olivine aggregates and seriously alters the viscosity of the mantle. Experimental studies show that at 300 MPa, in the presence of water, the viscosity of olivine aggregates deformed in the dislocation creep regime is reduced by up to a factor of 140. Thus a wet mantle is a low viscosity mantle. Conversely a mantle that is dried out by partial melting will be stiffer and more refractory, as is the case for the lithospheric "lid" to modern oceanic mantle. Thus, if it is possible to estimate the volatile content of the mantle both now and in the Archaean, it will be possible to set some physical constraints on models of mantle evolution over time. 

Kohlstedt DL, Goetz C, Durham WB, VanderSande JB (1975) A new technique for decorating dislocations in olivine. Contrib Mineral Petrol 53 13-24... 

Olivine. In the olivine group of minerals (important orthorhombic silicates), the energies of the [100], [010], and [001] dislocations are all different. In fact, the energy of the [010] dislocation will be much greater since it has a much larger Burgers vector. 

What are the three shortest (crystallographically different) perfect-dislocation Burgers vectors in the following materials (i) olivine, (ii) wurzite, and (iii) YBCO. 

A tilt boundary in olivine lies on the (100) plane with a [001] rotation axis. The dislocations are all the same Burgers vector and are 100 nm apart. What is the rotation angle ... 

Dislocation-dissociation has also been an issue in olivine since the early studies of Poirier et al. [161,172]. Evidence for such dissociation, usingTEM, was first reported by Van der Sande and Kohlstedt [173], and more recently by Smith et al. [174] and Drury [34]. The dissociation [Eqs (18) and (19)] suggested by Poirier assumes that the oxygen ions can be considered as hard spheres ... 

Hydration-induced climb dissociation of dislocations in naturally deformed mantle olivine has been reported by Drury [171]. Here, the dissociation is pro-... 

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