Olivine and Pyroxenes

19

Olivine and Pyroxenes

J.-C. C. MERCIER Institut de Physique du Globe & Université Paris VII Place Jussieu Paris, France

Forsterite-rich olivine is one of the most important minerals to study in order to understand the geodynamic processes and geophysical properties of the upper mantle. Indeed, common orthorhombic olivine constitutes approximately 10% of the earth's volume and 60% of this upper mantle which is an earth shell separated from the crust by the physicochemical discontinuity of Mohoro vicie and from the lower mantle by major phase transformations at a depth of approximately 400-km, where olivine becomes cubic ("spinel" structures) and natural coexisting pyroxenes (orthorhombic enstatite and monoclinic diopside) disappear through increasing solid solution in a hypersilicic garnet phase. 1. Intragranular F l o w and Related Microstructures 1.1. Deformation

Mechanisms

for

Olivine

Raleigh (1968)firstexperimentally investigated the complex nature of intragranular flow in olivine and the gross temperature and strain-rate dependence of the slip systems. Carter and Avé Lallemant (1970) extended this study to higher temperatures and also investigated the effect of pressure on the nature of the active slip systems. They observed that olivine experimentally deformed below 1000°C was generally characterized by kink bands and optically visible deformation lamellae due to photoelastic effects along individual slip planes of high edge-dislocation densities. Above the critical temperature of about 1000 ° C, these dislocations easily climbed out of the slip planes, lowering the density within these planes to values much too low to generate the long-range stresses needed for photoelastic effects. On the basis of optical studies, four different slip systems were then identified (Fig. la), one of which being interpreted as a "pencil glide" (0/c/}[100] characterized by a noncrystallographic slip plane in the zone [ 100]. However, this pencil glide was later PREFERRED ORIENTATION IN 407 DEFORMED METALS AND ROCKS: AN INTRODUCTION TO MODERN TEXTURE ANALYSIS

Copyright © 1985 by Academic Press, Inc. Allrightsof reproduction in any form reserved. ISBN 0-12-744020-8

J.-C. C. MERCIER

408 1400 1400

|\

~

slip

\\

Ί twinning • | inversion

1

1200 s. V

1000

^

600

X

jnoj tooi] 400

\

*^J

%. %* ^

800

1

1

J

(b)

\

1

■log «

\ \

1200

1000

1

1

5

4

— Tt-

^_Jwinning

_

\\%,

^ ^ s~

1

\ \

800

\V

600

^'°o

\

\ 400

" 14

200 Γ € = 10" /S/ l· € = 15%

(c)

0.5 1.0 Stress (GPa)

1

(d)

\

\ 5,

1

\\ \

, . 1. . . . 1i

. . . 1 .. i

0.5 1.0 Critical Stress (GPa)

1.5

Fig. l Deformation mechanisms for olivine and pyroxenes, (a) Olivine: slip systems and migration recrystallization grain size (Raleigh, 1968; Carter and Avé Lallemant, 1970). (b) Pyroxenes: slip versus twinning and inversion for monoclinic and orthorhombic structures, respectively (Raleigh et al, 1971; Avé Lallemant, 1978; Ross and Nielsen, 1978). (c) Stress dependence of transition between deformation processes. Based on the original steady-state experimental data. Slip systems refer to olivine. Polymorphic transformation slope from Coe (1970). (d) Critical stresses for slip and twinning in clinopyroxene single crystals and resulting behavior of pyroxene aggregates (Avé Lallemant, 1978; Tullis, 1980).

19. OLIVINE AND PYROXENES

409

recognized as caused by [100] screw dislocations cross-slipping on portions of lowindex oxygen-lattice planes (Poirier, 1975), that is, {Okl} actually composed of six discrete co-zonal potential glide planes, namely, (010), (010), (011), (001), (031), and (031), with an additional high-energy (021) glide observed only in single-crystal experiments and for a maximum resolved shear stress along (001)[100] (Darot and Gueguen, 1981). Transmission-electron-microscopy (TEM) studies of experimentally deformed olivines confirm the early results, though minor slip is found simultaneously on systems not recognizable optically, thereby yielding stepped subgrain boundaries. With increasing temperature, the dislocation pattern changes from dense tangles of edge and screw dislocations with b = [001] and [100] to helices, loops, and networks (recovery process) with b = [100] dominant. At high temperatures, such networks commonly consist of (100) walls of edge dislocations with [100] screw segments bowing out of these walls. Such microstructures are easily observed optically (Figs. 2a,b and c) through dislocation decoration techniques (900 °C heating in an open furnace Kohlstedt et al, 1976) possibly much alike the oxidation process of basalt olivines in nature. In the range of the experimental conditions, decreasing temperature (strain rate) or increasing pressure apparently lowers the transition between slip systems (Fig. la). According to reasonable geotherms and stress distributions (Mercier, 1980), (010)[100] slip should be dominant for upper-mantle conditions if linear extrapolations are warranted, and strain rates of less than 10~14 s"1 apply. Accordingly, the pencil glide (combined {0A:/}[100] system) reported for shallow spinel-facies peridotites generally results from emplacement-related late deformations, as is also the case for (001)[100] or {110}[100] systems identified for rare fades in massifs deformed under very high stresses and low-temperature conditions (Section 3.3). Moreover, from the experimental data, no slip system other than (010)[100] would even be expected to be dominant for transient deformations related to local and short-lived phenomena at depths greater than 130 km, since it would imply unrealistic strainrate conditions (> 10~5 s -1 )· Extrapolation to great depths is, however, not supported by evidence from the deepest natural samples (Section 3.4) and from the nature of the olivine-spinel transition (Poirier, 1981), that is, with increasing depth (1) deep porphyroclastic samples (170-210 km) from kimberlites show significant slip with [001] slip directions; (2) fluidal samples (210-230 km) from kimberlites have strong preferred orientations related to slip on a system not yet observed in experiments; (3) the martensitic olivine - spinel transition (~ 400 km) results from splitting of dislocations with [001 ] Burgers vectors gliding on ( 100), implying that the so-called low-temperature slip system is being activated at this depth. Actually, temperature-strain-rate diagrams (Fig. la) are but a convenient representation of the experimental observations: extrapolations are not warranted, as most data do not represent steady-state creep and as the nominal experimental strain rate applies only to the part of the sample at the highest temperature. From the limited steady-state data available, temperature-stress fields for slip systems could

410

(α)

J.-C. C. MERCIER

[010]

-[100]

|

50/xm

t

(f)

i

10 mm

i

Fig. 2 Deformation microstructures, (a) Free dislocations in olivine. San Carlos, Arizona, (b) Experimental subgrains in olivine. Mount Burnet dunite, cold-worked at 900°C, ε = 10~4 s - 1 to 3.15 GPa and subsequently annealed to 1000°C for 0.8 h. (c) Effect of recovery on free-dislocation densities.

411

19. OLIVINE AND PYROXENES

have complex relationships (Fig. lc) and further experimentation at high pressures is still critically needed for a real understanding of the processes active at depth. 1.2. Microstructural

Olivine

Piezometers

Above 1000°C and for common experimental strain rates (10~4-10~6 s_1)> dislocations produced during internal flow may rearrange into three-dimensional steadystate cell structures. Edge dislocations climb to form (001 ) walls, and screw dislocations cross-slip to form (001) boundaries, some of them being formed and destroyed at all times. This dynamic recovery, or polygonization, a softening process allowing steady-state creep to be achieved by compensation for work hardening, thus yields parallelepipedic stable cells or subgrains elongated parallel to [010] (Figs. 2b and c). During dynamic recovery, the excess dislocations needed to locally accomodate the strain gradient are taken up by subgrain boundaries. That is, the free-dislocation density (length per unit volume; Fig. 2a) within subgrains remains proportional at any time to the applied stress according to Ρΐ

= Α{ο(σ/μ)η

(1)

where μ is the shear modulus, b the Burgers vector, and At a constant with a theoretical value of 2 according to the dimension equation. For olivine, Kohlstedt and Weathers (1980) empirically showed (Fig. 3a) that ρ{=35.5σ1-5

(2)

However, free dislocations (Fig. 2a) are highly mobile and may only record maximum stress conditions related to emplacement. Furthermore, whenever significant recovery takes place (e.g., xenoliths in basalts), it may drastically reduce dislocation densities (Fig. 2c) to the point where relation (2) becomes useless for precise estimates of natural stresses. The average width of the subgrains (Fig. 2b) also depends on the flow stress according to the empirical relation ds = AsbWß)-m

(3)

where n should now have a theoretical value of 1 (from the dimension equation), though it is often significantly lower and close to 0.7. Relation (3) is tentatively explained by a linear decrease of the strain field associated with a subboundary, as a function of the distance to the latter, as is the case for dislocations. Raleigh and Kirby (1970) first made use of this relation by optically measuring subgrain sizes

San Carlos, Arizona, (d) Optical subgrains in olivine. Black Rock Summit, Nevada, (e) Rotation recrystallization in the mantle: (010) subboundaries predate (100) walls related to emplacement. Dreis Eifel, F. R. G. (f ) Tabular texture resulting from rotation recrystallization. Nunivak, Alaska, (g) Twin lamellae in clinopyroxene. Black Rock Summit, Nevada, (h) Exsolution lamellae in orthopyroxene. Thin planar lamellae may originally be clinoenstatite stabilized by Ca diffusion. San Quintin, Baja California, Mexico.

412

J.-C. C. MERCIER

E CO

-2

-I

Grain size

L• \*v

•Δ

E

-5

k\

\

·

îv

0.7h

— DEEP XENOLITHS

N.

[>

Migration

\ ·

Δ

V %>*£ <φ*

\ \ L \

Recrystallization processes

|

olivine (dry) enstatite (wet) diopside (wet)

V

log σ (GPa)

(b)

iog σ (GPa)

(a)

p

0

0.6)

^?

I h V\

1 - I (c)

1 .

X

^v

\ Ν^Λ 1

0

log cr(GPa)

<>

0.5 0.0

0.1 (d)

0.2

0.3

CT//X I 0 3

Fig. 3 Geopiezometers based on microstructures and textures, (a) Free-dislocation densities in olivine (Kohlstedt and Weathers, 1980). (b) Subgrain sizes in olivine (Ross et al, 1980; rectangle shows range of estimates by Durham et al, 1977). (c) Grain sizes for olivine and pyroxenes (Ross et al, 1980; Ross and Nielsen, 1978; Avé Lallemant, 1978). (d) Recrystallization processes for olivine in natural environments (Mercier, 1980). Dashed lines are mantle conditions inferred from xenolith suites and arrows are paths followed during emplacement.

413

19. OLIVINE AND PYROXENES

produced during creep experiments, but Green and Radcliffe (1972) subsequently found much smaller TEM subgrain sizes for the same samples, thus casting doubts about which parameter was relevant in Eq. (3). However, subgrain sizes inferred through the oxidation decoration technique (Kohlstedt et al, 1976) are virtually identical to TEM subgrain sizes and homogeneous at both crystal and sample scales. Therefore optical subgrains are now discarded (Fig. 2d) as not significant for inferring stresses (except for large tilt angles; Section 2.2; Figs. 2e and f ) as they depend on the state of anneal (e.g., undulatory extinction versus sharp kink bands). Neither strain nor strain rate has any effect on the decorated subgrain size, but it appears to be slightly sensitive to the presence of free water in the system (Ross et al, 1980; Fig. 3b): α(μτη) = 6.72 σ~62

(dry)

(4)

d(ßm) = 3.06 σ~·69

(wet)

(5)

and Though they are more stable than free dislocations, decorated subgrains are also significantly affected by recovery processes, as can be inferred from data for annealed samples (Mercier et al, 1977), which diverge significantly from relation (4), and from various unsuccessful attempts to get significant correlations between subgrain sizes and grain sizes for natural samples (Nicolas, 1978). Olivine microstructures as a whole are therefore inappropriate for estimating deep steady-state stresses, as they are too easily reequilibrated during emplacement, but they may yield useful estimates (though minimum values) for the maximum stresses related to late deformations.

1.3. Plastic Deformation

of Pyroxenes

and

Amphiboles

Because their structure is based on parallel chains of tetrahedra strongly bonded in pairs by octahedral cations, pyroxenes have only one potentially dominant slip system under most conditions, namely, (100)[001]. Minor slip systems involving [ 100] and [010] glide directions are also observed, but the mostsignificant behavior of these minerals is their ability to be sheared parallel to ( 100)[001 ] into a clinopyroxene correctly oriented for glide and usually represented by thin (100) lamellae. This mechanism, referred to as inversion for orthopyroxenes and twinning for clinopyroxenes, cannot by itself accomodate large deformations. It is less dependent on temperature than slip, being theoretically not thermally activated, though partial dislocations with [001] Burgers vectors aiding this coherent martensitic transformation are usually observed in TEM studies. Accordingly, activation energies in the twinning- inversion regime are about one-tenth of those in the slip regime for a given mineral species, and twinninglike mechanisms are dominant over slip at the lowest temperatures and highest strain rates or stresses (Figs, lb and c). As for orthopyroxene, inversion through a 13.3° shear of highly strained enstatite into clinoenstatite is exceptional for natural rocks and encountered only in shock

414

J.-C. C. MERCIER

deformation (meteorites) and in some ultramafic series deformed under high stresses (base of ophiolites). Raleigh and others ( 1971 ) experimentally defined the boundary for dry enstatite inversion and cross-checked the critical strain-rate-temperature conditions inferred with a theoretical boundary obtained by combining theflowlaws of enstatite in either regime as also done by Ross and Nielsen (1978) for wet conditions (Fig. lb). The temperature for inversion also increases sensibly with pressure. Although stress decreases with temperature along the inversion-slip boundary (Fig. 1 c), significant departures of the experimental data (average dT/da of 1.12 and 0.23 K MPa"1 for dry and wet enstatite, respectively; Fig. lc) from the boundary predicted for clinoenstatite as a low-temperature polymorph (1.77 K MPa - 1 ) show that this structure is but metastable in the absence of deviatoric stress. Above the transition temperatures, the dominant slip system is (100)[001] both in experiments and in nature, with only minor (010)[001] slip reported. However, the conditions are slightly different in nature as large strains (shear angle greater than 60°; elongations up to several hundred percent) are observed only in orthopyroxenes rich in (100) exsolution lamellae (Fig. 2h): orthopyroxene could thus be significantly weakened by a significant diffusion of octahedral cations such as Ca 2+ or Mg2+. Actually, in most cases, lamellae could be inverted-orthopyroxene (clinoenstatite) late structures stabilized, or even formation-enhanced, by Ca diffusion as the rocks slowly cooled down. For diopside, Raleigh and Talbot (1967) found that with increasing temperature and decreasing strain rate, the twin system changed from (001)[100] to (100)[001], the latter type being accompanied by ( 100)[001 ] slip (i.e., shear in the sense opposite to twinning) (Avé Lallemant, 1978; Fig. lb). Below the transition temperature, at a given strain rate, twinning may be achieved at much lower stresses than slip for crystals favorably oriented, but for a polycrystalline aggregate (Tullis, 1980), it requires stresses even higher than for slip, dropping suddenly near the transition temperature (Fig. 1 d). Above the latter, slip dominates and critical stresses are nearly the same for all conditions. The σ- Ttwinning boundary (projection of Tt = /{έ, σ, Τ}) has an average slope intermediate between wet and dry enstatite values (Fig. lc). Well-developed twinning is rarely observed in natural peridotites (Fig. 2g), and in such instances, it corresponds indeed to sudden deformations at extremely high stresses of at least several kilobars and high strain rates, as estimated from the behavior of coexisting olivine. At higher temperatures, both in experiments and in nature, the most common slip system is again (100)(001), but in dry experimental conditions, "multiple" slip may also occur when (100)[001] is activated, with slip along ( 110) directions in (001) or {110} planes (Avé Lallemant, 1978). Other systems do also occur when crystals are oriented for (100)[001] to be activated, but only ( 102)[201 ] has been clearly identified. As for enstatite, extensive slip of diopside in nature is limited to rocks for which appreciable cooling and phase reequilibration accompanied shearing, and twinning is limited to diopsidic clinopyroxenes free of exsolution lamellae (e.g., in some wehrlites). Slip could thus be more easily accommodated at host/lamellae interfaces. Amphiboles are very strong and rarely plastically deformed in rocks. But me-

415

19. OLIVINE AND PYROXENES

chanical twinning and slip ofCl/m clinoamphiboles (Rooney et al, 1975), have been qbservedin experiments and in some natural rocks with dominant twinning on (101)[101] involving complex translation and rotation of layers, chains and polyhedra. Slip occurs mainly on ( 100)[001 ] as expected from crystal structure, and other systems may become active only at dehydration temperatures.

2. Recrystallization Fabrics and Textures Recrystallization comprises any process modifying the grain sizes, shapes, or orientations, either after or during deformation (static and dynamic recrystallization, respectively). Furthermore, recrystallization does not mandatorily imply grainboundary migration and may encompass ultimate stages of recovery, beyond polygonization, whenever sufficient misorientation of the subgrains makes them behave as independent grains, though they retain some orientation characteristics of the former parent crystals. All four types of recrystallization have been recognized for olivine: (1) primary static recrystallization, whereby high-strain-energy deformed grains (paleoblasts) are being consumed by new dislocation-free grains (neoblasts) until all the original grains have disappeared; (2) secondary static recrystallization, whereby the coarsest strain-free neoblasts grow at the expense of the others to lower the grain-boundary energy (proportional to the grain-boundary surface per unit volume); (3) migration dynamic recrystallization, a steady-state process through which strained paleoblasts are continuously being replaced by neoblasts growing at their expense and, being deformed as they grow, which ultimately become the prey of still younger neoblast generations; and (4) rotation dynamic recrystallization, a grain-size-reducing process that makes independent grains out of subgrains through large rotations, with only minimal grain-boundary migration. 2.1. Experimental and Pyroxenes

Recrystallization

of Olivine

At the usual experimental strain rates, photoelastic deformation lamellae in olivine disappear at about 1000°C, and dynamic recrystallization (also referred to as "syntectonic" in the case of natural samples) resulting from nucleation and growth of neoblasts becomes important. Similarly, recrystallization of pyroxenes is significant only after high-temperature slip becomes dominant over twinning - inversion mechanisms. Although this recrystallization (Avé Lallemant and Carter, 1970) occurs dominantly at grain boundaries, new grains may also nucleate within the strained crystals (Figs. 4a and b). However, as recrystallization proceeds, these intragranular neoblasts are progressively consumed together with the original grains (Fig. 4c). It is difficult to experimentally define a limit for recrystallization in pressure temperature and strain-rate space as the smaller the neoblasts, the smaller the volume recrystallized within the time of the experiments (Fig. 4a), a phenomenon explained in terms of nucleation rates (e.g., rates at which are formed minute dislocation-free

5 mm

Fig. 4 Migration recrystallization olivine textures, (a) Lacelike texture: experimental, 900 MPa. T— 1100°C, t = 0.6 h. (b) Lacelike texture: natural, Black Rock Summit, Nevada, (c) Fine-grained homogranular texture, experimental, 520 MPa. T = 1000°C, t = 61 h. (d) Coarsest-grained experimental texture, 70 MPa. T= 1200°C, t = 14 h. (e) Granuloblastic texture, Kilbourne Hole, New Mexico. (f ) Granoblastic texture, Kilbourne Hole, New Mexico, (g) Fine-grained poikiloblastic texture, Borée, France, (h) Poikiloblastic texture (single crystal), Borée, France.

417

19. OLIVINE AND PYROXENES

domains with a potential for bulging out into the highly strained paleoblasts). Olivine grain sizes (Fig. la) based on Eqs. (7) and (11) combined with reasonable stresses for the upper mantle show that, for natural rocks, significant grain sizes will be achieved through recrystallization with (010)[ 100] as the only active slip system, at least as far as migration recrystallization is concerned. At the lowest experimental stresses, grain shape and sizes (0.3 mm) are quite similar to those of some natural rocks (Fig. 4d). In such conditions, full recrystallization of the sample is rapidly achieved, but the individual crystals never show optical evidence of internal deformation such as undulatory extinction or kink bands; even at large strains, densities of dislocations (observed through decoration or by TEM) always remain too low and subgrains never reach misorientations sufficient to provide such effects. Hence, the lack of optically visible deformation in natural crystals does not exclusively imply either late annealing or a magmatic origin, and on the opposite, such features incompatible with steady-state recrystallization may no longer be taken as evidence for a metamorphic origin (e.g., dunites from Hawaii; Jackson and Wright, 1970) as they merely represent a superimposed late deformation related to emplacement, under high-stress and/or low-temperature conditions. By analogy with primary recrystallization, the rapid migration of grain boundaries between low-dislocation-density neoblasts and high-dislocation-density (strained) paleoblasts results from a driving force that increases with the applied stress. However, in the case of dynamic recrystallization, the neoblasts also get strained as they grow (still without yielding optically visible deformation), and at constant stress, the driving force for growth of a given crystal decreases with time, whereas new stable nuclei are formed. Hence, there is a constant interaction between intragranular deformation and migration recrystallization: dynamically recrystallized grains yield a steady-state fabric characterized by a mean grain size dG, which depends on the applied stress through a relation dG = AGb{GlßT> (6) This is the same relation as for subgrains, but the exponent/?, which should again be equal to unity for the above equation to be dimensionally correct, is now empirically found to be in the range 0.8-1.2 (versus 0.7 for subgrains in olivine). This relation (σ in gigapascals, dG in micrometers) does not generally depend significantly on temperature (Ross et αί, 1980a; Ross and Nielsen, 1978; Avé Lallemant, 1978; Fig. 3c): dG = 7.36a-1·27 dG = 6.64<7-°·85 dG = 7.55ö—°·90

(olivine, damp/dry) (orthopyroxene, wet) (clinopyroxene, wet)

(7) (8) (9)

No significant difference in olivine grain size is observed between samples carefully dried before the experiments (Post, 1977) and damp conditions (.3% H 2 0 in the starting material). However, for olivine (as for quartz), a slight temperature dependence is observed if free water is released during the experiments through dehydration of the confining medium (talc), dG = 479σ"0·82 exp(- 5g/RT)

(olivine, wet)

(10)

418

J.-C. C. MERCIER Table 1 Values of a, n, and Q

Dunite dry damp wet Enstatitite dry wet Diopsidite wet Websterite dry wet

References

a

n

Q

11.6 11.5 5.2

3.0 3.8 2.1

527 415 226

Post (1977) Ross et ai (1980) Carter (1975)

6.7 6.2

2.4 2.8

293 271

Raleigh et al (1971) Ross and Nielsen (1978)

8.8

4.3

284

AvéLallemant(1978)

6.5 13.4

4.3 3.3

326 463

AvéLallemant(1978) AvéLallemant(1978)

The available steady-state flow laws for these minerals in the usual experimental conditions for dynamic recrystallization (hence, for polycrystals) are described by e=lOaGnexp(-Q/RT)

(11)

where the exponents a and n and the activation energy Q (kJ, at 1 GPa), have the values given in Table 1, and where pressure effects may eventually be taken into account by replacing Q by Q° + Pv, with v, the activation volume (13.4 kJ GPa" 1 for olivine; Ross et al, 1979).

2.2. Rotation

versus Migration Recrystallization

of Olivine

Alternatively, recrystallization may also be achieved through polygonization, at least for some materials, as has been well documented experimentally for NaCl (halite; Guillopé and Poirier, 1979). Such a recrystallization process has not yet been calibrated experimentally for olivine despite several attempts, as the starting material (e.g., Mount Burnet dunite) already has a dislocation microstructure (i.e., a relatively high strain energy). The latter promotes easy grain-boundary migration (annealinglike recrystallization under low stresses relative to those that produced the original microstructure), and even at low temperatures, the original microstructure makes rotations difficult to estimate. Thus, one would need first to fully recrystallize the starting material at zero stress to get a dislocation-free sample to which small strains may be imposed. However, this technique implies cold working of the sample prior to annealing for easier recrystallization and subsequent sample shortening is limited. Usual experimental dynamic recrystallization of olivine not only starts through grain-boundary migration as a result of stress annealing, but proceeds within the migration recrystallization field as can be ascertained through low-stress experi-

419

19. OLIVINE AND PYROXENES

ments. Even after full recrystallization has been achieved, the average internal strain of olivine neoblasts does not increase with time (Fig. 4d) as seen above, but individual crystals also remain isodimensional at any time even for large strains. Hence, significant migration must occur throughout the experiments, and the above calibrations apply to migration recrystallization exclusively. However, rotation recrystallization has long been observed in nature (Nicolas et al, 1971; Poirier and Nicolas, 1975). Ascribing discontinuities observed in grain-size variation as a function of estimated depths of origin of peridotite xenoliths to changes in recrystallization processes, Mercier (1980) provided an empirical calibration for rotation grain size, dG = 745(7. The change in recrystallization processes (Fig. 3d) is inferred independently through detailed fabric and textural studies (Section 3.1; Figs. 4e and f).

2.3. Static Recrystallization

in Experiments

and in Nature

Preliminary cold working of peridotite at 900 °C and at a strain rate of 10~6 s"1 provides sufficient strain to enhance fast recrystallization in static conditions during subsequent annealing ( 1200 -1400 ° C). Strain-free neoblasts grow at grain boundaries or from highly strained misoriented regions within paleoblasts, at some angle relative to the average orientation of the strained lattices. Olivine may eventually form tablet shaped crystals flattened parallel to (010) (Fig. 5a) and be slowed in growth by the formation of coherent boundaries for specific rotation angles (90° for the rotation axis 1(101) observed in many experiments). The driving force here is the internal strain energy of individual grains (dislocations), and the growth rate during such a primary recrystallization is given by 7 p = ^ p r[exp(-AG a /Är)][l - exp(-Ee/RT)]

(12)

where AGa is the activation energy for grain-boundary diffusion and Ee the strain energy empirically related to the maximum stress reached prior to annealing. For olivine (Mercier, 1978), Yp = 375 X 10 3 Γexp(- 168/£Γ)[1 - exp(- l2Sal33/RT)]

(13)

Primary recrystallization may be a very efficient recovery process in some natural conditions (annealed porphyroclastic textures from kimberlites, Section 3.2; Fig. 5c) and it yields olivine tablets that can be used as geotachymeters to infer velocities for kimberlite intrusion. As recrystallization proceeds, more and more neoblasts become surrounded by other strain-free crystals, in which case (Fig. 5b) the only driving force for further growth results from a reduction in surface energy. The growth rate for secondary recrystallization, or coarsening, is given by Yc = Acdö2r1/2

exp(-AG a /i?r)

(14)

Yc is very small and implies that such secondary static recrystallization is negligible for most natural peridotites. Only two exceptions are now known, both being

'#

(α)

500/xm

Fig. 5 Annealing-recrystallization olivine textures, (a) Primary recrystallization: olivine tablet growing at the expense of a strained porphyroclast; experimental; 1200°C, Ee = 2456 J, t = 0.4 h. (b) Secondary recrystallization (coarsening); experimental. Same sample as (a). Central part (7*~ 1250°C). (c) Annealing tablets in natural porphyroclastic peridotite. Kimberley, South Africa, (d) Mosaic texture (detail) resulting from coarsening; PHN-1611, Thaba Putsoa kimberlite, Lesotho, (e)-(h). Textures of peridotite xenoliths from Southern Africa kimberlites. In order of increasing depth, granoblastic (e), annealed porphyroclastic (f ), mosaic (g), andfluidal(h) textures.

421

19. OLIVINE AND PYROXENES

high-temperature peridotite xenoliths submitted to sudden high stresses immediately prior to fracturing and subsequent removal by the ascending magmas: (1) in rare alkali basalts or nephelinites (Eglazines, France; Salt Lake Crater, Hawaii), interpenetrative growth of strained crystals may occur, each crystal locally bulging out at dislocation walls (high strain energy) of neighboring crystals and (2) in some kimberlites (Thaba Putsoa, Lesotho), true secondary recrystallization (coarsening) occurs (as evidenced by inclusions in the olivine crystals; Mercier and Carter, 1975) after primary static recrystallization of olivine of the deepest samples is completed, as a result of the combination of high strain energies developed prior to removal of the xenoliths and the extremely high temperatures (> 1500°C) of the peridotites (Fig. 5d). 2.4. Deformation-Induced

Fabrics for Olivine and Chain

Silicates

The orientations of the neoblasts contained within paleoblasts are controlled entirely by the host-crystal orientation for both syntectonic and annealing (Avé Lallemant and Carter, 1970) recrystallization, the neoblast crystallographic axes being inclined at 20 ° to 40 ° to the host axes. Neoblasts at paleoblast boundaries, however, are related symmetrically to the axis of maximum applied stress and applied strain (compression tests) and seem to have little regard for host-crystal orientations. Accordingly, intragranular neoblasts being consumed together with their host, the resulting fabrics for fully recrystallized samples is directly related to the stress- strain directions. Similar preferred orientations are obtained experimentally either by slip on the high-temperature system (010)[100] (Nicolas et al, 1973; Fig. 6a) or by syntectonic recrystallization (Avé Lallemant and Carter, 1970; Fig. 6b). In the latter case, strain-free nuclei must have an active slip system correctly oriented to grow appreciably without accumulating too much strain energy. In axially symmetric deformations, [010] reorients parallel to the compression axis and [100] and [001] form a girdle normal to it, whereas in extrusion experiments, Avé Lallemant (1975) observed a [010] maximum parallel to the compression axis and a [ 100] maximum parallel to extension direction (Fig. 6c). This behavior corresponds to that of natural tectonites in which [100] reorients parallel to the mineral elongations. The common tectonite fabrics may therefore be interpreted as resulting from syntectonic recrystallization or plastic deformation, or both if those olivine crystals which were originally correctly oriented did not recrystallize, or grow at the expense of the others. Similar concentrations on pole figures may be observed for plastic deformation as for syntectonic recrystallization, but concentrations are strongly strain dependent in the former case. Pyroxenes are not only more resistant than olivine to plastic deformation, but also to polygonization and recrystallization. As observed for olivine, intragranular recrystallization of enstatite (George, 1975) is controlled by the host paleoblasts (misorientations of about 30 ° ), whereas enstatite powder recrystallized under stress shows a tendency for [100] axes to align parallel to εί/σί, with the [010] and [001] axes forming a girdle normal the [100] maximum, in the σ2-σ3 plane (Fig. 6d).

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(b)

(cH [ooi] 0,

î [010]

0oo] ol

[001]

Doo]

Fig. 6 Experimental preferred orientations developed from randomly oriented starting materials, (a) Olivine, deformed by slip, uniaxial compression, 1300°C, 1.4 GPa. ε = 58%, è = 10~4 s -1 , dry. Contours are 2 (dashed), 4,8 m.r.d. (Nicolas et al, 1973). (b) Olivine, migration recrystallization, uniaxial compression, 1100°C, 1.5 GPa, è = 10~6 s"1, wet. Avé Lallemant and Carter, 1970). Contours are 1 (dashed), 2,4,6,8 m.r.d. (c) Olivine, migration recrystallization, plane strain, 1100°C, 1.1 GPa, ε = 20%, έ = 10~6 s -1 , dry (Avé Lallemant, 1975). Contours are 1 (dashed) 2,4,6,8 m.r.d. (d) Enstatite, migration recrystallization, uniaxial compression at 1200 ° C, 10 GPa inverse polefiguresfor σ 1 at è = 10~4 (left) and 10"6 s"1 (right) (George, 1975). Contours are 0.5 (dashed), 1.0, 1.5 m.r.d.

19. OLIVINE AND PYROXENES

423

Natural high-temperature peridotite mylonites (xenoliths in the volcanics associated with the eastern extension of the North Pyrenean fault) yield additional information for the preferred orientation of amphiboles resulting from migration recrystallization. In these facies, textures, structures, and preferred orientations of olivine and pyroxenes (where present) are in full agreement with the experimental data presented above, and whenever diopside and enstatite are replaced by Cr-rich pargasite and anthophyllite, respectively, these unusual phases for the upper mantle recrystallize into finer-grained aggregates with a grain size similar to that of olivine. These dynamically recrystallized amphiboles have preferred orientations identical to that previously reported for enstatite, that is, [100] normal to the foliation (i.e., \\εί ) and [001] parallel to the lineation (ε3). Therefore, at high temperatures, all chain silicates behave in a similar way in either deformation regime, with preferred orientations related to the (100)[001] slip system. From the experimental data, the preferred orientations of olivine and enstatite in upper-mantle conditions should be such that [010], [001 ], and [ 100] olivine maxima correspond respectively to [ 100], [010], and [001 ] pyroxene maxima. However, this correlation is not always observed, which can be ascribed to the extreme difficulty, if not impossibility, for pyroxenes to recrystallize or flow under normal upper-mantle stress-strain-rate conditions when they are isolated within the much weaker olivine matrix. Accordingly, the enstatite fabric (and the same may apply to diopside), if significant, may be an inherited feature that survived from an unknown tectonic episode of the peridotite history; this original preferred orientation may have also been significantly weakened through bodily rotation of the pyroxene crystals. Asymmetric fabrics, commonly observed in massifs, but also in some xenoliths from basalts emplaced along lithospheric shear zones, are ascribed to strong plastic deformation in the simple shear regime (Section 3.3) on the basis of the elongated grain shapes oblique relative to the crystallographic axes. Such fabrics, illustrated later, do not represent steady-state flow in the upper mantle.

3. Textures of Natural Peridotites With the advent of the plate-tectonics theory and the growing interest in global geodynamics, geologists started interpreting many peridotites as fragments of the earth's interior brought up to the surface rather than mere aggregates of magmatic crystals. Such periodotites were composed of an olivine matrix (70 - 90%) with minor stronger phases, such as orthopyroxene, clinopyroxene, garnet, and/or spinel. Although isotope geochemistry and major element partitioning were soon to confirm these new interpretations, olivine textures were the first decisive arguments put forward in favor of such an origin. The pertinent textural nomenclature is quite esoteric as a result of parallel classifications proposed (e.g., Spry, 1969b; Mercier and Nicolas, 1975; Pike and Scharzman, 1977). Only a basic terminology will be used here, in accordance with Harte's (1977) and Bates and Jackson (1980).

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Natural olivine-rich rocks occur in two different types of environments, either as massifs, kilometer-sized masses of rocks brought to the surface by tectonic movements, or as xenoliths, up to 30 cm in diameter (with only a few exceptions), which are small fragments of deep rocks brought up through volcanic eruptions. The distinction between these two groups of rocks is important from a textural point of view because static annealing, which implies both high temperatures and no deviatoric stress, will always affect xenoliths on their way to the surface while surrounded by the magma, but virtually never the massifs which generally respond to increasing deviatoric stresses as the temperature decreases during emplacement. Thus in the case of xenoliths, the eventual effect of annealing recrystallization must be estimated first. Whereas syntectonic recrystallization yields neoblasts with significant dislocation densities, subgrains, and strong preferred orientations and paleoblast fragments often displaced relative to one another, annealing recrystallization should theoretically be characterized by neoblasts with virtually no dislocations or subgrains, randomized fabrics with neoblast orientations at about 20-30° from those of the paleoblasts, and paleoblast fragments with similar microstructures, which can be correlated across recrystallized zones, implying static consumption by the neoblast matrix. However, lacelike textures of this type may as well develop dynamically if high-stress conditions were suddenly reached with but a negligible finite strain (Fig. 4a and b). On the other hand, annealed textures may also be strained and crystals acquire dislocation microstructures at the ultimate stage of the eruption whenever explosive mechanisms are necessary, such as for the formation of kimberlite pipes. Furthermore, the effects of the high-stress deformation that occurred immediately prior to fracturation, magma intrusion, and xenolith sampling (i.e., before annealing) must also be estimated, especially for deep high-temperature fades prone to easy recrystallization. Under transient conditions, shortly after any stress jump, recrystallization-induced softening may yield a plastic instability (i.e., heterogeneous strain and ductile faulting), as observed in experiments (Post, 1977). In this case, individual samples may show textures ranging, at a given depth, from optically unstrained to completely recrystallized under high stresses. If no significant softening occurred (e.g., in the case of a more gradual stress increase), progressive textural changes observed throughout whole sample series may become critical indicators of the genetic processes involved. From a purely descriptive viewpoint, one may first distinguish homeoblastic textures for which the olivine grain sizes nearly approximate a unimodal distribution, from heteroblastictextures in which several generations of olivine crystals can usually be recognized. Of course, this distinction is applied only to nonannealed tectonites: in annealed textures resulting from static recrystallization, olivine grain sizes become of secondary importance relative to other textural criteria, as it is no longer univocally related to stress. With the recent development of quantitative methods in petrology, individual natural-rock samples may now be interpreted in terms of physical conditions (pressure, temperature, deviatoric stress) reached at different stages of their history, thereby yielding useful informations on the physical properties of the earth's interior (seismic, rheological, magnetic). Accordingly, the textures described here must only be considered as selected terms of continuous textural series, which,

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whether defined for a given massif or for xenoliths from a given volcanic vent, usually comprise samples belonging predominantly, but never exclusively, to one of the above three groups, with many of the xenolith series from alkali basalts dominated by homeoblastic textures, most of the massifs (whether intrusive alpine lherzolites or obducted ophiolitic harzburgites) dominated by heteroblastic textures, and many xenolith series from diamond-bearing kimberlites dominated by strongly annealed textures. Whereas xenolith series may be dominated by heteroblastic textures more characteristic of massifs, the massifs virtually never show any evidence for annealing recrystallization, in agreement with the stress-thermal history suggested by their environment. 3.1. Homeoblastic Series: Xenoliths from Extension-Zone Alkali Basalts

Continental

Two criteria, the grain size and grain shape of olivine, may be used to describe homeoblastic samples. Whereas such peridotites usually have equant or nearly equant xenomorphic olivines with only a slight flattening in some of the finestgrained samples (1:2; Fig. 4e), distinct textures with tablet-shaped crystals (Fig. 2f ) have been recognized in two regions of the world, Germany (both G. D. R. and F. R. G.; e.g., Eifel) and western Alaska (Nunivak Island). The difference between equant and tabular homeoblastic series may result from chemical impurities specific to the latter faciès as evidenced by an ubiquitous amphibole specific to these two regions. Grain sizes usually range from 1 to 12 mm (Fig. 4e and f), with the finest-grained granuloblastic textures (< 4 mm) limited to the shallowest lithospheric mantle (and granulite-facies crust) and the coarser granoblastic textures (> 4 mm) down to about 80 km. Secondary high-stress porphyroclastic textures characterized by small dynamically recrystallized neoblasts coexisting with large paleoblasts (or "porphyroclasts") are scarce and limited to the greatest depths sampled by the alkali basalts. Observed active slip systems are predominantly of the (010)[ 100] type. With the exception of the deepest samples, misorientation between adjacent grains is often small relative to the fabric scatter, with low-index crystallographic directions as rotation axes. This is taken as evidence that such samples deformed in the rotation recrystallization regime as independently confirmed by the systematic lack of later recrystallization related to magma intrusion (commonly observed for deeper samples) and by the discontinuity in the grain-size-depth correlation already ascribed to a change in recrystallization process (Mercier, 1980; Fig. 7). Direct evidence for significant subgrain rotation is also found in some tabular textures (Fig. 2e) as pure-tilt (100) dislocation walls (kink-band boundaries) developed during and after the late high-stress deformation related to magma intrusion, do not cut across complex (010) subboundaries eventually grading to grain boundaries. Hence, the latter must be preexisting microstructures formed in low-stress upper-mantle conditions, in agreement with the type of subboundaries needed to get the observed tablet-shaped crystals through rotation recrystallization.

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+

CONTINENTAL EXTENSION

50h

ZONES

G Kilbourne Hole, NM D San Carlos , AZ

4-

^τΞ^Ε

60| x Q.

70

80 0

2

4

_J_ 6

0LIVINE

8 GRAINSIZE

10

_L_ 12

14

16

(mm)

Fig. 7 Olivine grain size as a function of depth for Basin and Range xenoliths in alkali basalts (Mercier, 1980).

3.2. Poikiloblastic Basalts

Series: Xenoliths from Continental

Rift Alkali

This series is still poorly documented in the literature though its textural faciès have a worldwide distribution. Best illustrated by xenolith series from young alkali basalts of continental rift zones, these textures may also be represented in massifs (e.g., Vourinos ophiolitic complex, Greece; Ross et al., 1980) and in xenoliths from kimberlites (Reid et al, 1975) and from some oceanic basalts (De Paepe and Klerkx, 1971). This series may be regarded as a low-stress variant of the homeoblastic series. Thefinest-grainedfacies have granoblastic textures with a significant elongation of some olivines (1:1.5 to 2) defining a weak foliation and lineation. These textures show limited transitions toward the dominant poikiloblastic textures characterized by larger olivines (> 1 cm) with the other phases occurring as inclusions in these crystals (Fig. 4g). The metamorphic origin of such a texture, which resembles poikilitic (magmatic) ones, is evidenced by the nature and composition of the inclusions (typical phases for Iherzolites or harzburgites): such textures must therefore be ascribed to exaggerated growth of olivine under very low stresses. Pyroxenes subsequently acquire more or less rounded shapes so to reduce the total surface energy, and interphase equilibrium reactions become hindered by the impossibility of exchange through grain-boundary diffusion. In most samples, the poikiloblastic olivine crystals are so large (4-15 cm) that exact crystal sizes and shapes are difficult to ascertain,

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19. OLIVINE AND PYROXENES

inasmuch as misorientation across grain boundaries appears to be small. In rare fine-grained poikiloblastic samples (Fig. 4h), olivines have tablet shapes with well-developed straight (010) boundaries and sutured ends, with an elongation ratio over 1:5. Most of the tablets actually seem to derive from yet coarser crystals through rotation recrystallization with minimal migration and possibly reflect rotationmigration recrystallization cycles. The estimated stresses calculated from the observed grain sizes may thus even be overestimated and the exceedingly small actual stresses represented by such textures may only be the result of small (hundreds of meters to a few kilometers) solid-state diapirs rising along zones of lateral extension and general upwelling, as a result of thermal and/or gravitational instabilities. 3.3. Heteroblastic Series: Massifs and from Shear Zones

Xenoliths

The principal characteristic of such textural series is a relatively continuous increase in late strain and simultaneous reduction in grain size, from shallow cold faciès to deep ones originally at higher temperatures. The strain superimposed on original granoblastic textures may reach 1000% (according to the sheared enstatites), beyond which it can no longer be estimated. The rocks may eventually transform into pseudotachylites (e.g., Hare Bay, Newfoundland). At intermediate strains, porphyroclastic textures (Fig. 2h) usually present multiple grain size maxima and/or a wide grain-size distribution (depending on the σ-ε-t path) contrasting with the common bimodal distribution resulting from the high-stress deformations caused by intruding magmas. At the highest strains, the rock textures are variously referred to as sheared or mylonitic in a restricted sense excluding the textures described for xenoliths in kimberlites (Section 3.4). Strong cooling concurrent with shearing is common for this textural series (intrusive lherzolite massifs, obducted ophiolite complexes; Fig. 3d), but is not a necessary condition, as only limited cooling, if any, is observed for xenoliths from basalts intruded along lithospheric shear zones reactivated immediately prior to the eruption (North Pyrenean Fault, France; Californian fault system, San Quintin, Baja California). Initial preferred orientations (Fig. 8 a ), typical of the granoblastic textures become very strong as porphyroclastic ones form through extensive intragranular flow. Olivine (100)-pole point-maxima usually are at some significant angle (15-20°) to the well-defined foliation and lineation, which is ascribed to deformation by simple shear. Fabrics related unambiguously to [0A:/}[100] slip (rather than σχ ~ σ2) are uncommon, whereas sharp transition to (001 )[ 100] fabrics are well documented, the latter preferred orientation being characteristic of basal low-temperature mylonites in massifs (Mercier, 1977; Fig. 8c). Neoblasts may form through grain-boundary migration and/or through progressive misorientation of subgrains, with eventual disruption and scattering of paleoblast fragments. Both pyroxenes usually deform through ( 100)[001 ] slip (Fig. 8d), with simultaneous formation of exsolution lamellae attributed to syntectonic cooling, even in the case of the xenoliths (interpreted as fragments of small bodies rising diapirically along the shear zones, as did the Pyre-

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Fig. 8 Pole figures for peridotites from the Bay of Islands ophiolite complex, Newfoundland (Mercier, 1977). Foliation plane and lineation ( 1 ) are indicated, (a) Granoblastic harzburgite: high-temperature steady-state texture formed near a ridge, (b) - (d): Mylonitic harzburgite from the base of the massif. The preferred orientations were developed from (a) during obduction: olivine porphyroclasts (b), olivine neoblasts (c), and enstatite (d).

nean massifs). Accordingly, pyroxene fabrics are strong and well correlated with both foliation and lineation. 3.5. Annealed

Series: Xenoliths from

Kimberlites

The shallowest samples brought up by Southern African kimberlites strongly resemble the deepest ones from alkali basalts: coarse little-strained granoblastic tex-

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429

Fig. 9 Polefiguresfor peridotites from Southern African kimberlites (Mercier, 1977). Foliation and lineation ( 1 ) are indicated, (a) - (c): Olivines from annealed porphyroclastic peridotite: porphyroclasts (a), syntectonically recrystallized neoblasts (b), and annealed tablets (c) (Kimberley, South Africa), (d) Olivine fabric for a fluidal-textured periodotite. (Thaba Putsoa, Lesotho.)

tures are quite common and grade to porphyroclastic textures with increasing depth. However, the depths are now significantly greater (up to 120 km), in agreement with a regional thermal gradient lower than that characteristic of regions with alkali-basalt volcanism. Preferred orientations are variable, but generally weak. At still greater depths, as the relative amount of fine-grained dynamically recrystallized olivine increases, tablet-shaped olivines growing at the expense of the porphyroclasts are observed (Fig. 5c and f ). These tablets show well developed (010)

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faces and are usually terminated by low-index planar faces as seen in annealing experiments. Fabrics are typically strong (Fig. 9), even for annealing tablets, as a result of the relatively large strains achieved through internal slip of the porphyroclasts and despite combined [100] and [001] slip directions. Both the average size and the density of such crystals first increase with depth {annealed porphyroclastic textures), until the density becomes high enough to get the porphyroclasts fully recrystallized. Olivine then appears as afine-grainedmosaic with crystals formed in either dynamic or static conditions, or both, whereas the other phases imbedded in the softer olivine matrix remain unstrained {mosaic textures; Fig. 5g). In the deepest samples (>200 km), the high-equilibrium temperature (15001600°C) combined with the extremely high stresses, which ultimately broke peridotite blocks away from the mantle, yielded very high strain rates and strains. In such conditions, adjacent orthopyroxenes recrystallized into grains about 10 μπι across, and this fine-grained material is sheared into thin lamellae typical of the fluidal texture (Boullier and Nicolas, 1975; Fig. 5h). Whereas the present olivine grain size is related to subsequent coarsening (Fig. 5d), the observed preferred orientations (Fig. 9d) should provide clues about active slip mechanisms for the deepest regions of the earth from which samples are available. However, the strong double [010] and [ 100] maxima at 45 ° to the foliation have yet to be interpreted. Of all the textural types observed among xenoliths from kimberlites, only the shallow granoblastic samples represent primary mantle textures, though original grainsizes may be inferred for most porphyroclastic samples. Grain size data show that the deepest samples are representative of the lowermost rheological lithosphère, with strain rates close to 10~ 14 s _1 (Mercier, 1980). In addition, annealing textures suggest velocities of about 50 - 70 km h~~1 for kimberlite intrusion through the lithosphère, in agreement with constraints based on diamond stability (Mercier, 1978).