Deformation of olivine

The dislocation microstructures that develop during creep with stresses in the range 15 a 110 MPa at temperatures from 1,400 to 1,600°C were studied in detail by Darot and Gueguen (1981) on the optical microscope scale using the dislocation decoration technique as modified by Jaoul et al. (1979). However, TEM was used to determine Burgers vectors by application of the g-b = 0 and g-bxu = 0 invisibility criteria (Section 5.6). 

The observed mircrostructures and rheological behavior are consistent with the suggestion by Durham, Goetze, and Blake (1977) that the dislocations glide essentially unhindered by other dislocations and that deformation is limited by the number of active dislocation sources. 

2 Deformation induced by indentation. The deformation induced in single crystals of San Carlos (Arizona, USA) olivine around the in-denter in a Vickers microhardness test has been studied by Gaboriaud 

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. 

Mackwell et al. (1985) found that when specimens that had been deformed under anhydrous conditions were subsequently further deformed under wet conditions, there was a significant change in microstructure. TEM observations revealed enhanced formation of dislocation walls, despite the reduced stress levels. This observation was interpreted as due to enhanced dislocation climb under wet conditions. However, the two walls illustrated by Mackwell et al. (1985) could be interpreted as healed or partly healed fractures. One wall consists of a very irregular network of dislocations with many bubbles, particularly at dislocation intersections. 

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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. 

Kashima, K., Sunagawa, I., Sumino, K. (1983). Plastic deformation of olivine single crystals. Science Reports of the Tohoku University, series III, 15,281-407. 

Mei S. and Kohlstedt D. L. (2(XX)) Influence of water on plastie deformation of olivine aggregates 1. Diffusion creep regime. J. Geophys. Res. 105, 21457-21469. 

Buiskool Toxopeus J. M. A. and Boland X M. (1976). Several types of natural deformation in olivine An electron microscope study. Tectonophysics, 32 209-233. 

Figure 9. NE measured while scanning a QMS across masses 0 to 110 once every 80 ms during deformation and fracture of olivine from San Carlos, Arizona, USA. Fracture occurred during the second scan, at the time marked by the arrow. NE at some masses saturated the QMS electrometer during the remaining scans. (Reproduced with permission from Ref. 14. Copyright 1986 Kluwer Academic Publishers).

Green H. W. and Guegen Y. (1983) Deformation of peridotite in the mantle and extraction by kimberlite a case history documented by fluid and solid precipitates in olivine. 

Anisotropy in the upper mantle is primarily attributed to the preferred alignment of olivine and, to a lesser extent, the alignment of other upper-mantle minerals. Evidence comes from measurements on mantle xenoliths (Mainprice Silver 1993 Ben Ismail Mainprice 1998) and ophiolites (Nicolas Christensen 1987) and laboratory deformation studies (Zhang Karato 1995). These measurements help guide the interpretation of seismic observations. It is conventionally assumed that the anisotropy is c.4-5% in magnitude and has hexagonal symmetry with a horizontal synunetry axis (azimuthal anisotropy) (Mainprice Silver 1993). Numerical simulations of the lattice preferred orientation... 

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. 

High-temperature creep deformation of synthetic forsterite crystals, as studied by Darot and Gueguen [162], occurred by (010) [100] sUp, which is apparently the preferred slip system. This same slip system was detected around Vickers indents produced at temperatures of 600 °C and above by Gaboriaud et al. (163, 164], who used natural olivine single crystals from San Carlos, Arizona (110) [001] slip was also activated. There have been many other extensive deformation studies of the olivine minerals (e.g., see Kohlstedt and Ricoult [165] and Poumellec and Jaoul [166]). 

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

Ashworth, and Hutchison, 1975 [11] made electron microscopic observations of the hydrous alteration products of olivine in an achondrite and in an ordinary chondrite. Their conclusion was that the Nakhla achondrite, and possibly the Weston chondrite, contain water of extraterrestrial origin which was mobilized by mild shock deformation. Carbonaceous chondrites are believed to be unaltered material left over from the formation of the solar system. They contain substantial amounts of reduced carbon and of water in the form of hydroxyl ions. The oxidation state of iron in some carbonaceous chondrites has been determined by means of Moess-bauer spectroscopy, and it is demonstrated that there is a correlation between the oxidation state of iron and the content of water and reduced carbon in the meteorites (Roy-Poulsen et al., 1981 [284]). 

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... 

Phakey, P. P., Dollinger, G., Christie, J. M. (1972). Transmission electron microscopy of experimentally deformed olivine crystals. In Flow and Fracture of Rocks, Geophysical Monograph 16, edited by H. C. Heard, I. Y. Borg, N. L. Carter, C. B. Raleigh, pp. 117-38. Washington, DC American Geophysical Union. 

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