Olivine deformation

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. 

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).

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. 

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... 

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... 

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. 

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

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. 

Nd-Sr isotopic studies in orogenic peridotites have emphasized their wide isotopic heterogeneity but also showed that the predominant fertile Uierzolite facies are actually fairly homogenous and characterized by depleted, MORB-type isotopic compositions. The low-alumina, deformed (olivine) websterites, that are chemically equilibrated with the peridotites (see Section 2.04.4.2.3), also show a limited range of isotopic variations and are generally indistinguishable... 

Harte B. (1977) Rock nomenclature with particular relation to deformation and recrystallisation textures in olivine-bearing xenoliths. J. Geology 85, 279-288. 

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... 

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