The role of recrystallization in the preferred orientation of olivine

Physics of the Earth and Planetary Interiors, 51 (1988) 107-122

107

Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

The role of recrystallization in the preferred orientation of olivine Shun-ichiro Karato Ocean Research Institute, University of Tokyo, Minamidai, Nakano, Tokyo (Japan) (Received September 12, 1986; revision accepted May 6, 1987)

Karato, S., 1988. The role of recrystallization in the preferred orientation of olivine. Phys. Earth Planet. Inter., 51: 107-122. The effects of recrystallization on the preferred orientation in olivine have been studied. The recrystallization mechanisms studied include grain growth, the deformation-induced grain boundary migration (DIGBM) and subgrain rotation (SGR). Grain boundary migration is involved in the first two, but not in SGR. Significant preferred orientation was found only in the latter two where dislocations are involved. The combined microstructural and fabric (preferred orientation) observations indicate that the grain boundary migration produces strong preferred orientation when the driving force is the dislocation energy but not when the grain boundary energy is the driving force. The nature of preferred orientation due to dislocation-related recrystallization (DIGBM or SGR) was found to depend on the mechanism of recrystallization. The DIGBM produces preferred orientation in which the low dislocation density grains with unfavourable orientation for easy slip dominate, thereby significantly altering the preferrred orientation formed by dislocation glide. In contrast, SGR does not essentially alter the deformation fabric, and therefore the fabric associated with SGR recrystallization is related to the geometry of flow. The present results demonstrate that the existence of seismic anisotropy in the upper mantle is strong evidence for the dislocation mechanism(s) of deformation, and suggest that care must be exercised in applying the results of laboratory fabric studies made for a particular mechanism of recrystallization to the Earth's interior where the dominant mechanism(s) of dynamic recrystallization can be different.

1. Introduction

In recent years, seismologists have made a significant contribution to geodynamics by revealing the anisotropic and the heterogeneous structures of the Earth (see e.g. Anderson and Dziewonski, 1984). The interpretation of such data to infer the dynamics of the Earth requires the application of materials science, but progress in this area has been slow, particularly regarding seismic anisotropy. This is due mainly to the experimental difficulties involved in duplicating the flow in the mantle. Hess (1964) first suggested that the preferred orientation of olivine may cause seismic anisotropy in the upper mantle. Two contrasting mechanisms may produce the preferred orientation. One is the lattice reorientation due to dislocation glide (and/or twinning) and the other is the 0031-9201/88/$03.50

© 1988 Elsevier Science Publishers B.V.

nucleation and/or growth of new grains (recrystallization). The former has been well studied (Nicolas et al., 1973; Lister et al., 1978) and kinematic origin (as opposed to control by stress) of the preferred orientation has been demonstrated (e.g., by Nicolas and Christensen, 1987). However, details regarding the role of secondary slip systems and/or some associated mechanisms are still uncertain (Lister, 1982; Urai et al., 1986). In contrast, the role of recrystallization is poorly understood despite its importance in the upper mantle (Mercier and Nicolas, 1975; Av~ Lallemant et al., 1980; Karato, 1984). Recent experimental studies suggest that various mechanisms of recrystallization may occur in the upper mantle (Guillop~ and Poirier, 1979; Karato et al., 1980, 1982, 1986; Karato, 1984). Two questions of geophysical importance are studied in this paper. The first is the role of deformation mechanisms in

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recrystallization. When dislocations are generated (in dislocation creep), they play a dominant role in the recrystallization (see e.g. Poirier, 1985). In the diffusion-creep regime where few dislocations are generated, the main driving forces are the grain boundary energy a n d / o r the elastic strain energy of the stressed solids. The latter energy is related to the orientation of crystals with respect to stress and will cause the preferred orientation (e.g. Paterson, 1973). However, the preferred orientation of minerals in the diffusion creep regime has not been studied except for the related work on naturally deformed peridotites (Boullier and Gueguen, 1975) and experimentally deformed calcites (Schmid et al., 1977). The other is the role of the deformation-induced grain boundary migration (DIGBM) relative to subgrain rotation (SGR) and lattice reorientation due to dislocation glide (Karato, 1987b). Recently, Karato et al. (1986) experimentally demonstrated the grain-size sensitive (diffusion) and insensitive (dislocation) creep regimes in olivine aggregates. This provides us with the first opportunity to study the preferred orientation of olivine in the diffusion creep regime. In addition, Karato et al. (1980, 1982) and Toriumu and Karato (1985) have clearly demonstrated the SGR mechanism of dynamic recrystallization in olivine. In this paper I present the results of microstructural and fabric (preferred orientation) studies of deformed olivines with special emphasis on the role of recrystallization. The geophysical implications of the results are discussed.

2. Samples and methods of observation

Both synthetic olivine aggregates (Karato et al., 1986) and recrystallized olivine single crystals (Karato et al., 1980, 1982) were used in this study. Both types of samples are Mg-rich natural olivines (approximately Fo90_92 ), the major impurity being NiO (see Karato et al., 1986). The deformation conditions are given in Table I. The deformation conditions were variable, particularly regarding the water content, total strain and temperature. These samples were selected for this study because they show, depending on the deformation condi-

TABLE 1 Samples and deformation conditions Sample Temperature Stress (K) (MPa)

Strain a Grain Stress (%) size (#m) exponent

Polycrystals b 4604 1573 4674 ¢ 1573 4814 1573

63 13.2 0 0 6.5--43 9.1

Single crystals c a-5 1823 b-14 1923

53 40

47 43

63 3--41d 9--45 d

-- 3 - -

1.3

- 3 - 3

a Strain is defined by (/o - l)/lo, where l o is the initial sample length and l is the final sample length. b All polycrystalline samples were hot-pressed (and deformed) at 300 MPa confining pressure in iron jackets with additional water (see Karato et al., 1986). c Hot-pressed at 300 MPa for 5 h. d Grain growth during the run. e All single crystals were deformed at 0.1 MPa in a H2//CO2 gas mixture. Samples a-5 and b-14 were deformed at [110]c and [101]c orientation respectively (for notation of orientation see Durham and Goetze, 1977). In sample a-5 the (010)[100] slip system was activated, while in sample b-14, the (001)[100] and (100)[001] slip systems were mainly activated.

tion, various aspects of recrystallization, including grain growth under stress, deformation-induced grain boundary migration (DIGBM) and subgrain rotation (SGR). Polycrystalline samples were hot-pressed (and deformed) at 300 MPa confining pressure using a gas medium high-temperature high-pressure apparatus. Deformation experiments were made in uniaxial compression (Karato et al., 1986) and the strain was approximately axisymmetric. Single crystals were also deformed in uniaxial compression at atmospheric pressure, but the strain was approximately plane-strain due to the plastic anisotropy of the single crystals (Karato et al., 1982). Thin sections were cut from the centre of the specimens and the microstructural and fabric studies were made by using an optical microscope, a scanning electron microscope (SEM) and a transmission electron microscope (TEM). The fabric studies reported in this paper are the results of U-stage measurements (the results of X-ray measurements will be reported in a separate paper). Dislocations were observed by the oxidation decoration method.

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Fig. 1. Optical photomicrographs of the samples (crossed polarizers). (a) Sample No. 4674, hot-pressed and undeformed sample; scale 100 /zm. (b) Sample No. 4814, deformed in grain-size sensitive, low-stress exponent regime; scale 100 /zm. (c) Sample No. 4604, deformed in grain-size insensitive, high-stress exponent regime; scale 100 ~m. (c) Sample b-14, recrystallized by subgrain rotation; scale 0.5 mm.

110

O

Fig. 2. Optical photomicrographs of olivine aggregates showing grain boundary migration (scale bars are all 10/~m). Arrows indicate the direction of grain boundary migration inferred from the shape of deformed bubbles. (a) Sample No. 4674; (b) sample No. 4814; (c) sample No. 4604; (d) sample No. 4604, dislocations are decorated by oxidation in air.

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Fig. 3. Microstructures of sample No. 4604, showing the recrystallizati on by grain hot mdary migration. (a) and (b) are optical micrographs, and (c) is a back-scattered electron image of a scanning elec:tron microscope of a decorated sample (Karato, 1987a). L and H denote, respectively, the low dislocation density grains with convex : boundaries and high dislocation density grains adjacent to L-grains having concave boundaries. The L-grains in (b) are enclosed by a H-grain, while L-grains in (a) and (c) are not.

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3. The microstructures

Figure 1 shows the optical photomicrographs of the studied samples (see also Toriumi and Karato, 1985; Karato et al., 1986). Sample No. 4674 is an (A) # 4 6 7 4 all

(B) ~#4814 all

t

grain

b-axes

a-axes

e-axes

t (C) # 4 6 0 4 all

1

b.,x.s

8-Bxes

t

t c.,x.s

grain

1

t

c-axes

grain

!

1

olivine aggregate hot-pressed with additional water. This sample, and also the other olivine aggregates studied in this paper, were hot-pressed with additional water and contain many waterfilled bubbles both inside the grains and on the

1

b-,x.s

t

c-,x.s

Fig. 4. Equal-area projections (on the lower hemisphere) of lattice preferred orientations. Arrows indicate the compression direction. Kamb's (1959) method is used. (a) Sample No. 4674, all grain. (b) Sample No. 4814, all grains. (c) Sample No. 4604, all grains. (d) Sample No. 4604, L-grains (low dislocation density grains, see Fig. 3). (e) Sample No. 4604, H-grains (high dislocation density grains being consumed by L-grains, see Fig. 3). (f) Sample No. 4604, the orientation of L-grains relative to contacting H-grains.

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(D) # 4 6 0 4 L-grain

1

1

t

a-ax.s

1

t

b-axes

t

o-axes

t

c-axes

(E) # 4 6 0 4 H-grain

t

t

a-axes

(F) Orientation

of

b-axes

L-grains Relative to H-grains

a.

a.

a.

a-axes

b-axes

c-axes

M

E+6o[zko)

Q

E+4o(±o)

A=O.083 E=8.3

E+2o(_o) E (±o)

l Fig. 4. (Continued),

]

E--2o(±o)

o=-2.8

114 grain boundaries. Although bubbles inside the grains are spherical or ellipsoidal in shape, bubbles on the grain boundaries are often asymmetrical, and turned out to be excellent markers to indicate the direction of grain boundary migration. Sample No. 4674 underwent significant grain growth at hydrostatic conditions. The grain boundaries show nearly equilibrium shape, but in many cases are curved. The bubbles on these boundaries are deformed (Fig. 2a) and show that these boundaries were moving toward the centre of the curvature. The TEM study showed that most of the grain boundaries of the studied samples were free from the secondary phase with a resolution down to - 5 nm (Karato et al., 1986). Sample No. 4814 is an olivine aggregate deformed in the grain-size sensitive, low-stress exponent regime (low-stress/small-grain-size regime), where diffusion creep is presumably the dominant deformation mechanism (see Karato et al., 1986). This sample has undergone significant graingrowth (from 9 to 45 /~m). The results of static grain-growth experiments clearly show that this grain growth occurred during deformation. Therefore, most of the specimen volume had been swept by moving boundaries during deformation. However, the microstructures of this sample are almost indistinguishable from those of the hot-pressed undeformed sample (No. 4674): grain boundaries are of nearly equilibrium configuration, but in cases where grain boundaries are curved, bubbles on the boundaries show evidence of grain boundary migration toward the centre of the curvature (Fig. 2b). Dislocation distribution is extremely heterogeneous, but, generally, dislocation density is low and few sub-boundaries are observed. Sample No. 4604 is an olivine aggregate deformed in a grain-size insensitive, high-stress exponent regime (high-stress/large-grain-size regime), where dislocation creep is the dominant deformation mechanism (Karato et al., 1986). The sample has undergone little grain growth. The grain boundaries have a serrated shape, indicating active heterogeneous grain boundary migration (Karato et al., 1986). However, the volume fraction of the sample swept by moving boundaries, as inferred from the volume fraction of low dislo-

cation density zones near grain boundaries, is very small (a few %). An estimation of grain boundary migration rates based on Karato (submitted) shows that the most of grain boundary migration occurred during deformation. The photomicrographs of this sample near the grain boundaries are shown in Fig. 2c,d (see also Fig. 3). It can be seen that, in contrast to sample Nos. 4674 and 4814, the grain boundaries of sample No. 4608 migrate away from their centre of curvature. The dislocation structure (Fig. 2d) clearly indicates that the grain boundaries were migrating toward the grains with higher dislocation densities (see also Toriumi, 1982; Karato, 1987b). In the grains with low dislocation densities (denoted as L in Fig. 3), no sub-boundaries are observed. Sub-boundaries are common in other higher dislocation density grains, but their misfit angles are generally small, - 1 °. Two types of geometrical relations between L-grains and contacting high dislocation density grains (denoted as H-grains in Fig. 3) are noted. One is shown in Fig. 3b in which L-grains are totally enclosed by Hgrains. The other, shown in Fig. 3a,c, is where L-grains are not enclosed by H grains. The former case consists of - 20% of the observed L-grains. Sample Nos. a-5 and b-14 are single crystals of olivine deformed and recrystallized by the subgrain rotation (SGR) mechanism (Karato et al., 1980, 1982; Toriumi and Karato, 1985). The grain boundaries (or sub-boundaries) are nearly straight, but occasionally, curved boundaries can be seen if the misfit angles exceed - 8 °. The dislocation distribution is more homogeneous than in sample No. 4604.

4. The preferred orientation The preferred lattice orientations are shown in Figs. 4 and 5. Figure 4 shows the preferred orientations of polycrystalline samples. The measurements on polycrystalline samples were made for 100 grains from the central portions of each thin section. For sample No. 4604, the measurement was made for all grains in a given region and also for two types of grains with characteristic microstructures; one is the low dislocation density grains (dislocation density less than half the aver-

115 a3

Fig. 5. Plots of lattice orientation in recrystallized olivine single crystals. (a) Sample No. a-5, (b) sample No. b-14. Triangles are c-axis orientation, squares a-axis, and circles b-axis. White thick arrows indicate the original orientations, and the black ones the theoretically expected final orientation assuming homogeneous deformation due to a single slip.

age value) surrounded by convex grain boundaries (hereafter referred to as E-grains, see Fig. 3) and the other is the high dislocation density grains adjacent to the L-grains that have concave grain boundaries at the interface (referred to as H-grains, see Fig. 3). The preferred orientation of these two types of grains (L- and H-grains) are considered to represent the effect of grain boundary migration, as discussed later. The L- and H-grains occupy only small volume fractions (a few %) of the sample and therefore the preferred orientation of all grains will be little affected by the grain boundary migration. The effect of SGR was not explicitly studied in this sample because of the small subgrain size ( - 10 /~m). However, the effect of SGR is believed to be negligible, because the misfit angles are small. For other polycrystalline samples, the measurements were made for 100 grains in the central portions of thin sections. The hot-pressed undeformed sample (No. 4674) shows very weak preferred orientation. The preferred orientations of all grains of the deformed samples (Nos. 4604 and 4814) are similar (a weak concentration of b-axes toward the compression

direction and weak girdles of a- and c-axes), although the sample deformed by dislocation creep (No. 4604) shows a somewhat stronger preferred orientation than the sample deformed by diffusion creep (No. 4814). In contrast to the small differences in the preferred orientation of all grains between the samples deformed by diffusion creep and dislocation creep, a significant difference in the preferred orientation can be seen between grains with different dislocation densities in a sample deformed by dislocation creep (No. 4604). Figure 4d shows that the low dislocation density grains (L-grains) have a significantly stronger preferred orientation than all grains. In addition, the L-grains show a slightly different preferred orientation: in addition to the strong concentration of b-axes toward the compression axis, a weak but significant concentration of the a-axes toward the compression axis is also seen. In contrast, the preferred orientation of Hgrains is similar to that of all grains and is much weaker than that of L-grains. Figure 5 shows the preferred orientation of sample Nos. a-5 and b-14 (see also Toriumi and Karato, 1985). In sample a-5, the c-axis orientation remains approximately in the original orientation, the a-axes rotate toward the maximum elongation direction (denoted as al) and the b-axes rotate toward the maximum shortening direction (a3). In sample b-14, a similar trend is observed if b- and c-axes are replaced by the c- and b-axes of sample a-5.

5. Discussion

This study has demonstrated a wide range of recrystallization mechanisms and their effects on the preferred orientation. In this section, the role of each recrystallization mechanism is discussed based on the microstructural and fabric studies of samples in which one of these recrystallization mechanisms was clearly demonstrated.

5.1. Mechan&ms of grain boundary migration The observed contrasting behaviour of grain boundary migration, one toward the centre of the

116 curvature (Fig. 2a,b) and the other away from the centre of curvature (Fig. 2c,d), indicates a fundamental difference in the mechanisms of grain boundary migration between the tWO cases. The grain boundary migration toward the centre of curvature was observed both in the sample deformed by diffusion creep (No. 4814) and in the undeformed sample (No. 4674). Therefore a mechanism that is not related to deformation must be responsible for this type of grain boundary migration. The observed geometry can be attributed to the grain boundary migration driven by grain boundary energy (Fig. 6a, see also Nicolas and Poirier, 1976). In this case, grain boundaries migrate so as to reduce the grain boundary energy. The main driving force acts at triple junctions, resulting in a migration toward the centre of curvature. The observed very weakly preferred orientation, despite significant grain boundary migration, (Fig. 4b) can be attributed to the fact that the driving force for grain boundary migra-

tion in this case (grain boundary energy) depends neither on the stress orientation nor the deformation geometry. In contrast, the sample deformed by dislocation creep shows grain boundary migration away from the centre of curvature. The dislocation structures show that the grain boundaries migrate toward the grains with higher dislocation densities, leaving nearly straight dislocations perpendicular to the moving boundaries (Fig. 2d). Therefore the difference in dislocation densities (bulk-free energy difference) is the driving force. In this case, triple junctions act as pinning points, resulting in a migration away from the centre of curvature (Fig. 6b, see also Nicolas and Poirier, 1976). This type of grain boundary migration may be called deformation-induced grain boundary migration (DIGBM). The low dislocation density grains surrounded by convex boundaries (L-grains in Fig. 3) are therefore the growing grains and the H-grains are the grains being consumed. Therefore, the L-grains may be referred to as the grains recrystallized by DIGBM.

GRAIN BOUNDARY MIGRA TION

5.2. Effect of deformation mechanisms (,4) driven by grain boundary energy pull

(B) driven by bulk free energy (e.g., dislocation energy)

pinned ~~ ill" T, /

. bu~b,bel _

I

pinned '

l

Fig. 6. Schematic illustrations to show the mechanisms of grain

boundary migration. Black arrows indicate the driving force and the white ones the direction of grain boundary migration. (a) Grain boundary migration driven by grain boundary energy. Grain boundaries migrate toward the centre of curvature. (b) Grain boundary migration driven by bulk-free energy difference. Grain boundaries migrate awav from the centre of curvature.

The relative importance of diffusion creep and dislocation creep in olivine has been studied by Karato et al. (1986). They have shown that diffusion creep is a possible mechanism of deformation in the upper mantle if deformation occurs at relatively low stress and small grain size. To assess the effect of deformation mechanisms on the preferred orientation in the upper mantle, the results of laboratory experiments must be extrapolated to lower stress and larger grain size, and to larger strains. Since the deformation due to diffusion creep has no rotational component that is required to form deformation-induced preferred orientation (e.g. van Houtte and Wagner, 1985), it is conceivable that the deformation by diffusion creep will not produce the preferred orientation. The key issue to be examined in the case of diffusion creep is the role of recrystallization (grain growth under stress). If the bulk-free energy of stressed solids (e.g. Paterson, 1973) or the dislocation energy dominates the driving force for grain boundary

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migration, a significant preferred orientation will develop because these driving forces depend on the orientation of crystals with respect to stress (or deformation geometry). If, however, the grain boundary energy dominates, little preferred orientation will result. Therefore the observed weak preferred orientation in diffusion creep, despite a significant grain growth under stress, implies that the grain boundary energy is more important than the bulk-free energy for the driving force of grain boundary migration at laboratory conditions. This result is quite consistent with the microstructural observation that suggests that the dominant driving force for grain boundary migration in sample No. 4814 is grain boundary energy. However, it is possible that diffusion creep in the upper mantle will occur at a much lower stress (less than - 1 MPa) and a larger grain size (up to 1 mm) than in the laboratory (Karato et al., 1986). To examine the effect of stress and grain size on the relative importance of strain energy and grain boundary energy in diffusion creep, I will assume that the strain energy (either elastic strain energy or dislocation energy) is proportional to o2//x and that the grain boundary energy is proportional to ~,/L (~; = grain boundary energy, L = grain size). Hence the relative importance of the two types of driving force can be expressed by a non-dimensional parameter, K = o~o2L/t~,/, where c~ is a non-dimensional constant. The observed predominance of grain boundary energy at the experimental condition indicates that K < 1 at that condition, yielding ~/~cy < 10-5 (MPa 2 /~m-a). Inserting the above mentioned values of stress and grain size for a possible diffusion creep in the upper mantle, one obtains K < 10 2. Therefore, the diffusion creep in the upper mantle will not produce a strong preferred orientation. The next point to consider is the extrapolation to larger strains. The laboratory experiments were only made to small strains (Table I). As a result, the overall preferred orientation is small and there is not much difference between diffusion creep and dislocation creep (Fig. 4b,c). However, a significant difference will be expected at larger strains where 'steady-state preferred orientation is developed. This is because the role of recrystallization

and deformation will be more significant at larger strains (Nicolas et al., 1973; Karato et al., 1980, 1982; Toriumi and Karato, 1985). Therefore the difference in the preferred orientation in recrystallized grains (Fig. 4b,d; note that almost all sample volumes of sample No. 4814 have been swept by moving boundaries and therefore the preferred orientation shown in Fig. 4b can be considered to be that for the recrystallized grains) will be much more important in the overall preferred orientation at larger strains. Further, at larger strains, the deformation by dislocation glide will produce a strong preferred orientation (Nicolas et al., 1973). However, deformation by diffusion creep will not produce a significant preferred orientation. Thus, at large strains, dislocation creep will produce strong preferred orientation either by deformation or by recrystallization, but diffusion creep will not produce a significant preferred orientation.

5.3. Effect of mechanisms of dynamic recrystallization The dislocation creep often results in dynamic recrystallization (e.g. Poirier, 1985). Depending on the importance of grain boundary migration relative to subgrain rotation, the recrystallization mechanisms may be classified into subgrain rotation (SGR) and deformation-induced grain boundary migration (DIGBM) (e.g. Poirier and Guillopr, 1979; Urai et al., 1986; Karato, 1987b). The present study has demonstrated that both produce a strong preferred orientation (Figs. 4d and 5). The recrystallization due to DIGBM was only in the incipient stage in the studied sample No. 4604 (this was also the case for other samples deformed by dislocation creep, by Karato et al. (1986)), and therefore the effects of DIGBM can only be studied to a limited extent. Nevertheless, an important insight into the role of DIGBM in preferred orientation can be obtained by combined microstructural and fabric observations. From the microstructural observations, it was concluded that the L-grains (see Fig. 3) consumed the H-grains (Fig. 3), the driving force being the dislocation energy. Therefore, the preferred orientation of L-grains may be considered to represent the effect of DIGBM.

118 It is noted, first of all, that the preferred orientation of L-grains is much stronger than that of all grains, which is presumably due to dislocation glide. This fact indicates an important role of DIGBM in producing the preferred orientation. An interesting question here is whether the preferred orientation of L-grains (growing grains) is determined mainly by the nature of L-grains themselves or whether it is significantly affected by the H-grains (consumed grains). To answer this question, the preferred orientation of L- and Hgrains are compared in Fig. 4d-f. It can be seen that the preferred orientation of L-grains is much stronger than that of H-grains, the preferred orientation of H-grains is not much different from that of all grains. Figure 4f shows the relative orientations of L-grains with respect to H-grains. Two trends can be observed. First, the L-grains tend to have similar orientations as H-grains. Second, in addition to the above trend, the L- and H-grains tend to have a- and c-axes in a common direction. The relative orientations of L- and H-grains do not appear to be clearly different between the Fig. 3a (c) type and Fig. 3b type grains. Two mechanisms may result in orientation relationships between L- and H-grains: genetic relationship between the two types of grains (L- and H-grains) a n d / o r the dependence of grain boundary migration rate on the relative orientation of the two types of grains. The similarity in orientation of L- and H-grains may be attributed to the SGR recrystallization, which results in a similarity in orientation of host and recrystallized grains (Hobbs, 1968; Bell and Etheridge, 1976; see also Av6 Lallemant and Carter, 1970). However, the observed relationship of a-axes parallel to c-axes cannot be attributed to SGR recrystallization. This observation may suggest a dependence of grain boundary migration rate on the relative orientation of two grains. In any case, the preferred orientation of H-grains (Fig. 4e) is much weaker than that of L-grains and therefore the preferred orientation of L-grains is mainly controlled by the nature of L-grains, although some effects of H-grains cannot be ruled out. The next question is why the L-gr/fins tend to have the preferred orientation shown in Fig. 4d.

This is an orientation in which the resolved shear stress (of applied stress) on the dominant slip system (010)[100] is small. Therefore, one may call this orientation hard orientation, although, in polycrystals, the interaction stress exerted by the surrounding grains will alter the stress at the scale of the grains. Two hypotheses may be envisaged to explain the observed preferred orientation. One is that the hard orientation grains tend to have low dislocation densities at the deformed, pre-recrystallization stage and therefore grow selectively. This hypothesis means that the resolved shear stress on the main slip system (the easiest slip system) at the hard orientation grains is low, implying that the local stress is dominated by the applied stress. This is in contrast to the Taylor model which predicts that the hard orientation grains will suffer relatively high local stress. The other is that among the low dislocation density grains (with various orientations), hard orientation grains remain undeformed and therefore keep growing, while soft orientation grains are easily deformed and stop growing (or their orientation is strongly affected by deformation-induced lattice reorientation). Azuma (1986) found that, in polycrystalline ice, the grains unfavourably oriented for easy slip tended to have small strains (and vice versa), which is consistent with the above idea. It is difficult to assess the relative importance of the two processes from the present results. It is possible that both contribute to the preferred orientation of L-grains. The preferred orientation associated with SGR (Fig. 5) is consistent with the dominant role of (010)[100] and (001)[100] slip systems in sample Nos. a-5 and b-14, respectively. The preferred orientation is essentially the same as the deformation-induced preferred orientation and is related to the deformation geometry: the slip direction rotates toward the maximum elongation and the slip plane normal toward the maximum shortening direction. However, more detailed characteristics of the preferred orientation show some difference. First, the c-axes remain exactly at the original orientation in sample No. a-5, but the b-axes in sample No. b-14 move significantly. This is presumably due to the operation of the easiest slip system (010)[100] in sample No. b-14 that was

119

promoted by the small misorientation of the original sample. The heterogeneous deformation due to SGR in this case leads to the operation of a new slip system ((010)[100]) and significantly alters the preferred orientation at large strain, although the fabric is still controlled by the geometry of deformation (Toriumi and Karato, 1985). ,~lso shown in Fig. 5 are the theoretically expected orientations of crystallographic axes due to deformation by the primary slip systems ((010)[100] in sample No. a-5, (001)[100] in sample No. b-14). The calculation assumes homogeneous deformation (Schmid and Boas, 1935). Figure 7 shows that the rotation of the lattice orientation is heterogeneous: the central part of the sample remains approximately in its original orientation, but the outer parts of the sample progressively rotate their lattice orientation. Figure 5 shows that this heterogeneity results in a significant scatter of the preferred orientation although the qualitative feature is not affected. In sample No. a-5, where only one slip system operates, the average of the scattered orientation agrees with the Schmid-Boas prediction. In sample No. b-14, however, the amount of the observed lattice rotation is considerably smaller than the theoretical prediction assuming a single slip. Apparently, as seen from Fig. 2d and 7, the secondary slip system ((100)[001]) sample b- 14 ,

,,

,,

-

~.,~,.~,X~

,~,

1 mm Fig. 7. Microstructure of sample No. b-14 with lattice orientations. The cross at the centre indicates the orientation of a- and c-axes at the core which remain approximately at the original orientation. Bars indicate the orientations of c-axes, b-axes remain approximately normal to the plane. Arrows indicate the compression direction.

also operates in this sample, which rotates the lattice to the opposite direction than the primary slip system. Also, the deformation due to dislocation climb (Durham and Goetze, 1977) might contribute to reducing the amount of the lattice rotation. In any case, it is concluded that SGR recrystallization does not essentially alter the deformation fabric, and therefore the fabric associated with SGR recrystallization is related to the deformation geometry.

5.4. Comparison with previous results The absence of a strong preferred orientation in materials deformed by diffusion creep or superplastic creep is well known in metallurgy, and the absence of a strong preferred orientation in deformed rocks has often been cited as evidence of diffusion creep or superplastic creep (e.g., Boullier and Gueguen, 1975). However, very few experimental studies have been conducted on the preferred orientation formed during diffusion creep or superplastic creep of rocks (Schmid et al., 1977). Virtually no studies have been made on the effect of grain growth under stress during diffusion creep or superplastic creep in rocks, which is a possible mechanism to form the preferred orientation. A clear demonstration of the absence of a strong preferred orientation due to grain growth during diffusion creep and the demonstration of a grain boundary migration mechanism appear to be the first indications in rocks. The role of dynamic recrystallization in preferred orientation has long been a matter of controversy (for a review, see Urai et al., 1986; Karato, 1987b). Av6 Lallemant and his co-workers (Av6 Lallemant and Carter, 1970; Av6 Lallemant, 1975; Kunze and Av6 Lallemant, 1981) argued that dynamic recrystallization resulted in the preferred orientation that is related to stress orientation rather than to deformation geometry. In these studies, however, the mechanisms of recrystallization and of grain boundary migration were not studied in detail. In contrast to Av6 Lallemant and Carter's (1970) and the present results, that emphasize an important role of dynamic recrystallization due to DIGBM, it is often argued that dynamic recrys-

120 tallization plays only a subsidiary role in preferred orientation (Lister and Price, 1978; Bouchez et al., 1983; Urai et al., 1986). Two types of observations appear to form the basis of this argument. Firstly, the majority of the preferred orientations of dynamically recrystallized rocks are indeed much the same as deformation fabrics (e.g. Bell and Etheridge, 1976; Lister and Price, 1978; Toriumi and Karato, 1985). It must be noted, however, that most of these studies were made on rocks where SGR recrystallization was dominant. Mercier (1985) reported a fabric of olivine in a totally recrystallized peridotite xenolith, which may be attributed to the effect of DIGBM (see Karato, 1987b). Secondly, it is often found that hard orientation grains are selectively consumed by recrystallization, and the recrystallization helps develop the deformation fabric in which soft orientation grains dominate (in simple shear) (see e.g. Urai et al., 1986 and references cited therein). This observation may appear to be inconsistent with the present results that suggest that the hard orientation grains dominate the recrystallization fabric due to DIGBM. However, the present results also show that the consumed grains (H-grains) have preferred orientation in hard orientation, although it is not very strong. The question is, whether the low dislocation density, growing grains (L-grains) can have a significant effect on the overall preferred orientation. The absence of strong effects of recrystallization on the overall preferred orientation found in these previous studies indicates that the consuming grains are deformed immediately or that the consuming grains have soft orientation grains (in contrast to the present results). Unfortunately, the preferred orientation of consuming grains was not selectively measured in the previous studies and it is not possible to clearly delineate the origin of this apparent discrepancy. However, one notes that both the orientation dependence of dislocation densities (which is related to the preferred orientation of consuming grains) and the relative kinetics of DIGBM and deformation will depend on the materials properties, and therefore various situations may arise in the preferred orientation development due to DIGBM. In particular, it appears quite possible that the consum-

ing grains occupy a significant volume fraction and have important effects on the overall preferred orientation if the DIGBM rate is significantly faster than the deformation rate (Karato, 1987b). However, this idea will remain hypothetical until well-defined experimental studies have been performed. To summarize, the present studies are not inconsistent with previous results concerning the effect of dynamic recrystallization, but rather, they suggest that most of the previous studies were made under the limited conditions where the effects of DIGBM were unimportant. The important role of DIGBM in the overall preferred orientation is a distinct possibility under certain conditions and the results by Av6 Lallemant and co-workers may be interpreted as being due to the effects of DIGBM (Karato, 1987b).

6. Summary and geophysical implications The present study has revealed some of the important aspects of the effects of recrystallization on preferred orientation in olivine. The importance of combined microstructural and fabric study has been demonstrated. In particular, the observations of dislocation structures and grain boundary microstructures provided an important clue as to the mechanisms of recrystallization. The results are summarized as follows: (1) the grain growth during diffusion creep produces little preferred orientation; (2) the grain boundary migration driven by dislocation energy (DIGBM) produces strong preferred orientation; (3) the recrystallization due to subgrain rotation (SGR) does not essentially alter the fabrics formed by dislocation glide, except for a randomizing effect. The above result (1) implies that the existence of seismic anisotropy in the Earth's upper mantle is strong evidence of the dislocation mechanism(s) of deformation. The application of the present results on the effects of dynamic (dislocation-related) recrystallization is not straightforward, especially in the case of DIGBM. Firstly, the DIGBM recrystallization

121

was only at the incipient stage in the studied sample. A significant volume fraction of the materials must be occupied by 'recrystallized' grains for the DIGBM to have significant effect on the overall preferred orientation. Karato (1987b) suggested that the nucleation and growth kinetics relative to deformation determine the importance of DIGBM recrystallization and that the DIGBM recrystallization is potentially important in the upper-mantle seismic anisotropy. However, experimental tests are needed to substantiate this suggestion. Secondly, the present results are based on experiments of uniaxial compression. However, the likely mode of deformation in the Earth's mantle contains a rotational component, such as simple shear (e.g., Nicolas and Christensen, 1987). It is well established that deformation fabrics depend on the mode of deformation since they are related to the rotational component of deformation (vorticity) (e.g. Lister et al., 1978; van Houtte and Wagner, 1985). In uniaxial compression, deformation and DIGBM recrystallization produce similar fabrics in which hard orientation grains dominate (the Taylor factor increases with progressive deformation). In simple shear, on the contrary, the deformation fabric is dominated by soft orientation grains (the Taylor factor decreases with progressive deformation), while the recrystallization fabric due to DIGBM will be dominated by hard orientation grains if the present result can be applied to simple shear. Therefore, Kunze and Av6 Lallemant's (1981) results, that the hard orientation grains dominate the preferred orientation in simple shear, may be attributed to the effect of DIGBM. However, the deformation geometry in Kunze and Av6 Lallemant's experiments was not clearly defined. Further experimental study in well-defined non-coaxial deformation is needed. Unfortunately, these limitations do not allow us to derive any definitive conclusions regarding the role of DIGBM recrystallization from this study. However, it is important to note that the present study suggests a fundamental difference in the underlying physics of the preferred orientation between DIGBM recrystallization and deformation (or SGR recrystallization). Therefore there is

a distinct possibility that the seismic anisotropy formed by DIGBM recrystallization is quite different from the anisotropy formed by dislocation glide (or SGR recrystallization). The mechanisms of dynamic recrystallization in the upper mantle are not well understood (e.g. Mercier, 1980; Karato, 1984; Av6 Lallemant, 1985). As to the mechanisms of dynamic recrystallization, care must be exercised in interpreting the seismic anisotropy of the deep upper mantle where dynamic recrystallization is likely to be significant.

Acknowledgements I thank Akio Fujimura for making the results of X-ray fabric analysis available prior to publication. The deformation experiments of olivine polycrystals were made at the Australian National University. I thank Mervyn Paterson for the use of the gas pressure deformation apparatus.

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