Shock effects of the Leoville CV carbonaceous chondrite: a transmission electron microscope study

Earth and Planetary Science Letters, 114 (1992) 159 170

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Elsevier Science Publishers B.V., A m s t e r d a m

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Shock effects of the Leoville CV carbonaceous chondrite: a transmission electron microscope study Tomoki Nakamura, Kazushige Tomeoka * and Hiroshi Takeda Mineralogical Institute, Faculty of Science, Unit~ersityof Tokyo, Hongo, Tokyo 113, Japan Received April 3, 1992; revision accepted October 6, 1992

ABSTRACT Leoville is a CV carbonaceous chondrite with a strong preferred orientation of the elongated chondrules and inclusions. The texture probably resulted from deformation. Olivine grains in most chondrules show undulatory extinction and fine planar fractures. Transmission electron microscope observations reveal that most olivine grains in the matrix exhibit high densities of micro-cracks and dislocations with Burgers vector b = [001]; the densities of the dislocations are comparable to those in chondrule olivines. Olivine in the matrix also forms aggregates in places, which comprise extremely small, rounded to sub-rounded grains and glassy material, closely resembling the recrystallized olivine commonly seen in shocked ordinary chondrites. Enstatite in the matrix has numerous dislocations and lamellae that are caused by (100) stacking faults. These micro-textures are characteristic of deformation at high strain rates and are very similar to those in shocked ordinary chondrites and experimentally shocked materials. Therefore, this suggests that the chondrules and matrix in the Leoville experienced shock, probably after accretion to the meteorite parent body. Comparison of the textures to those in experimentally shocked materials suggests that the shock pressures experienced by Leoville were in the range 5 - 2 0 Gpa. We believe that multiple impacts by such relatively mild shock pressures compacted the Leoville meteorite and caused the deformation of chondrules and the foliation.

Introduction

Carbonaceous chondrites are among the most primitive rock samples in the solar system and are regarded as having once been part of the early planetesimals. It is widely believed that the planetesimals accumulated through mutual collisions and grew to form protoplanets and eventually terrestrial-type planets [1]. In that process, some planetesimals may also have been broken up into smaller pieces by collisions. Therefore, it is expected that some effects of such a dynamic process would remain in the carbonaceous chondrites. However, up to now, the effects on the carbonaceous chondrites caused by the dynamic processes have been poorly understood.

Correspondence to: K. Tomeoka, Mineralogical Institute, Faculty of Science, University of Tokyo, Hongo, Tokyo 113, Japan. * Present address: Department of Earth Sciences, Kobe University, Nada, Kobe 657, Japan.

Leoville is a unique CV carbonaceous chondrite. It shows foliation defined by the alignment of flattened chondrules and inclusions with high aspect ratios [e.g., 2,3]. Olivines in chondrules commonly exhibit undulatory extinction [4]. Previous TEM studies showed that the olivines in chondrules contain high densities of screw dislocations, with Burgers vector b = [001] [5,6]. These features suggest deformation. Leoville contains CM chondrite-like xenoliths which display a strong preferred orientation of chondrules and inclusions parallel to the Leoville host, suggesting that the deformation took place after accretion [7]. However, whether the deformation resulted from shock-induced pressure [8] or static compaction due to overburden [3] remains inconclusive. Previous mineralogical studies of Leoville mostly focused on relatively coarse-grained materials, such as chondrules and inclusions, and the detailed mineralogy of matrix is still poorly known. In the hope of obtaining more detailed informa-

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tion about the deformation, we studied olivine and pyroxcne in the matrix and chondrules in the Leoville CV chondrite using scanning and transmission electron microscopy (SEM and TEM). Our study reveals that most of the olivine and pyroxene in the matrix and chondrules exhibit a variety of the complex features characteristic of shock-induced deformation. Such evidence of extensive shock effects has not been previously found in any other carbonaceous chondrites. Sample and methods A polishcd thin section of Leoville was studied using an optical microscope, a scanning electron microscope ( J E O L JSM 840), equipped with an energy-dispersive X-ray spectrometer (EDS), and an electron-microprobe analyzer ( J E O L 8600 Superprobe), equipped with wavelength-dispersive X-ray spectrometers (WDS). EDS analyses were obtained at 15 kV and 3 nA, with a focused beam ~ 2 p.m in diameter. WDS analyses were obtained at 15 kV and 12 nA, with a focused b e a m

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~ 3 ~ m in diameter. After petrographic characterization, six areas of matrix and two areas of chondrules were extracted from the thin section, mounted on T E M grids, and thinned by argon ion bombardment. Imaging and electron diffraction were performed with a J E O L JEM 100CX transmission electron microscope (TEM), operated at 100 kV and a Hitachi H-600 TEM, operated at 100 kV. Structural identification was based on high resolution imaging and selected area electron diffraction. Qualitative chemical analyses were performed using EDS with the Hitachi H-600 TEM. The electron beam was usually focused to 200 in diameter. Results Leoville contains chondrules and inclusions in a highly compacted matrix. Most chondrules and inclusions in our thin section arc highly elliptical, with axial ratios (long axis/short axis) from 1.4 to 2.7 and show a strong preferred orientation. An

Fig. 1. Back-scattered electron (BSE) image of matrix and chondrules in Leoville. Light grey portion from upper left to lower right is matrix. The orientation of elongated chondrules is roughly in the horizontal direction in this photo. Note that kamacite, troilite and pentlandite (bright grains) in the matrix are strongly elongated along the surfaces of chondrules.

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Fig. 2. (a) BSE image of the matrix of the Allende CV chondrite. The matrix is mainly composed of a loosely packed aggregate of Fe-rich olivine grains which occur in subhedral to euhedral plates. The bright grains are troilite. (b) BSE image of the matrix of the Leoville CV chondrite on the same scale as in (a). Note that the Leovil!e matrix is strongly compacted compared to the Allende matrix.

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Fig. 3. TEM images of the coarse olivine grains in the Leoville matrix. (a) Screw dislocations. (b) Sub-parallel cracks from lower left to upper right. Most dislocations seen in this photo are bent in various directions (indicated by arrows). (c) A portion of an intensely strained olivine grain, where the density of dislocations is much higher than those in other olivine grains.

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Fig. 3 (continued). average value for axial ratios of chondrules in our section is 1.9, which is almost equivalent to the average value of the aspect ratios of the strain ellipses measured for the whole meteorite by Cain et al. [3]. Therefore, our thin section is probably nearly parallel to the deformation axis of chondrules.

Matrix The matrix of Leoville consists mainly of Ferich olivine (Fo40_55) and minor enstatite, kamacite, troilite, Ca-rich clinopyroxene, magnetite and pentlandite (these minerals are roughly in the order of relative abundance). In SEM images it is obvious that these minerals are strongly compacted along with chondrules (Fig. 1). Sulfides and metals show especially notable elongation. The matrices of CV chondrites are generally composed of relatively loosely compacted aggregates of tiny, discrete grains ( < 0.1-10 ~xm in diameter) of mainly olivine; for example, an SEM image of the Allende matrix is shown in Fig. 2a. In contrast, the matrix of Leoville is so strongly compacted that each olivine grain can hardly be

distinguished in SEM images (Fig. 2b; Fig. 2a and b are at the same magnification). T E M observations reveal that olivine in the matrix occurs mainly in two different forms: (1) relatively coarse grains ranging in diameter from 0.1 to 2 Ixm; and (2) aggregates of extremely small, rounded to sub-rounded grains, eacho grain ranging in diameter from 100 to 1000 A. The coarse olivine grains exhibit a variety of complex features such as cracks and dislocations. The dislocations have Burgers vector b = [001] and have long segments in the [001] screw orientations (Fig. 3a). The dislocations commonly bend in various directions (Fig. 3b). The densities of the dislocations range from 3 × 10 '~ cm 2 to 1 × 10 l° cm 2, and are higher in places (Fig. 3c). The densities are comparable to those reported from chondrules in Leoville [5,6]. Such high densities of dislocations of this type are known to be characteristic of deformation at low temperatures and high strain rates [9]. The extremely small olivine grains in aggregates occur in random orientations and have amorphous material, probably glass, in their interstices (Fig. 4a). These olivine grains closely

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Fig. 4. (a) TEM image of a portion of an aggregate of small olivine grains in the Leoville matrix. A few olivine grains having high densities of dislocations (indicated by arrows) occur together with dislocation-free olivine grains. (b) A portion of a coarse olivine grain (upper right) surrounded by an aggregate of small olivine grains. (c) A portion of a boundary between an enstatite grain and aggregates of small olivine grains.

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Fig. 4 (continued). resemble the "fine-grained olivine" reported from shocked ordinary chondrites by Ashworth [10] (cf. figs. 7b and c in [10]). He interpreted the "finegrained olivine" as having resulted from recrystallization during shock deformation. Most of the small olivine grains in the Leoville matrix also contain [001] screw dislocations, but their densities are generally much lower than those in the coarse olivine grains, suggesting that annealing occurred during recrystallization. However, core regions of some aggregates include small olivine grains whose dislocation densities are comparable to those in the coarse olivine grains (Fig. 4a); thus, the annealing occurred, if at all, only in the peripheries of the aggregates. The boundaries between the olivine aggregates and the coarse olivine grains are commonly irregular (Fig. 4b), suggesting that the olivine aggregates recrystallized at the expense of the coarse olivine grains. On the other hand, the boundaries between the olivine aggregates and the pyroxene grains are commonly sharp (Fig. 4c), suggesting that there is no genetic relationship between them.

Enstatite in the matrix also has distinctive micro-textures, suggesting shock deformation. Most enstatites consist of unit cell-scale intergrowths of ortho-enstatite (OREN) and clino-enstatite (CLEN) slabs, and show streaking parallel to a* in their electron diffraction patterns (Fig. 5a). These two types of pyroxene slabs are distributed unevenly within and between grains, suggesting inhomogeneous deformation [10,11]. They also commonly contain partial dislocations, unit dislocations and micro-cracks, exhibiting extraordinarily complex strain contrast (Fig. 5a and b). Enstatite consisting of such an intimate mixture of OREN and CLEN has been interpreted as being produced by two different processes: quenching of the high temperature polymorph (proto-enstatite) from > 1000°C [12], and stressinduced transformation [13], although shock is not necessarily required in the latter case. Enstatite formed by the former process contains polysynthetic twinning within its CLEN fields [ 12]; whereas enstatite formed by the latter process is generally untwinned [11,14]; i.e., most of CLEN

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occur in the same crystallographic orientation [14]. Electron diffraction patterns from the enstatites in Leoville including both O R E N and C L E N show two distinct C L E N maxima whose intensities are different, indicating that the C L E N occurs predominantly in the same orientation. Thus, they are consistent with stress-induced transformation. Chondrules

The chondrules of Leoville are mostly of porphyritic type and consist mainly of Mg-rich olivine

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Mg-rich pyroxene with little Fe (Fs o s) and Ca (Wo0_5), troilite, kamacite, pentlandite, taenite and mesostasis glass. There is an apparent tendency for chondrules containing larger amounts of troilite, kamacite, pentlandite and taenite to have higher axial ratios; suggesting that the metals and sulfides were more ductile during deformation. Olivine and pyroxene grains in chondrules show extremely fine fractures under the optical microscope (Fig. 6): fracture domains range in size from 10 to 50 #,m. Most of the chondrules display undulatory extinction. This optical characteristic is consistent with that re(F°9o-100),

Fig. 5. T E M images of enstatite in the Leoville matrix. (a) N u m e r o u s (100) stacking faults, abundant partial dislocations and unit dislocations (the most pronounced one is indicated by an arrow) are observed. Viewed almost parallel to the (100) lamellae boundaries. Inset shows accompanying electron diffraction pattern showing streaking parallel to a*. Unequal intensities of (2.02) CLEN maxima (indicated by small arrows) suggest that most of CREN occur in the same orientation. (b) Strongly deformed enstatite showing subgrain boundaries (indicated by arrows).

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Fig. 5 ( c o n t i n u e d ) .

ported by Scott et al. [4]. In addition, we found that relatively large olivine grains commonly show planar fractures with spacings of 10-30 ~m oblique to the deformation axis (Fig. 6); the fractures in different chondrules occur in roughly similar orientations. We measured crystallographic orientations of the planar fractures and confirmed that they are oblique to {100}, {010} and {001}. Our TEM observations show that the olivine and pyroxene in chondrules exhibit deformation features similar to those in matrix. Discussion

Early TEM studies of olivine in Leoville chondrules [5,6] showed a high density of dislocations and suggested that they were caused by shock deformation. In contrast, more recent work by Cain et al. [3] has suggested that the olivines in chondrules were deformed by a diffusional flow mechanism, based on their failure to observe thermally decorated or chemically etched disloca-

tions by an optical microscope. Since diffusional flow requires relatively low stress levels, they argued against shock as the explanation for the deformation of olivine-bearing chondrules. However, our present study reveals much mineralogical evidence that both matrix and chondrules in the Leoville chondrite have been extensively affected by deformation probably induced by shock. The high densities of dislocations and microcracks in olivines, and the heterogeneous distributions of OREN-CLEN intergrowths and dislocations in pyroxenes in the Leoville matrix and chondrules are characteristic of deformation at low temperatures and high strain rates [9,11]. These micro-textures are exactly like those found in shocked ordinary chondrites [e.g., 9-11] and experimentally shocked materials [15]. Olivines in both matrix and chondrules exhibit dislocations with similar high densities, suggesting that the shock deformation occurred in situ. In addition to the undulatory extinction, we found that olivine grains in many chondrules show char-

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acteristic planar fractures oblique to the deformation axis (Fig. 6). The experimental study of Miiller and Horneman [16] and Reimold and St6fflcr [17] showed that similar fractures were produced in olivine by shock pressures from 5 to 59 Gpa. The planar fractures in different chondrules in Leoville occur in roughly similar orientations, supporting the idea that the shock deformation took place in situ, after accretion to the meteorite parent body. Based on the results from the present study and the previous studies of naturally and experimentally shocked materials [18], we can make an approximate estimation about the apparent shock pressure ranges experienced by Leoville. St6ffier et al. [19,20] recently proposed a new shock classification of chondrites on the basis of characteristic shock effects in olivine and plagioclase. Based on their classification, Leoville can be placed between shock stages 2 and 3, because of the presence of undulatory extinction and planar fractures in olivine and the absence of melt products such as melt veins and melt pockets. The

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shock stages are estimated to be in the pressure range of 5-20 Gpa. St6ffler et al. [18] mentioned that porous material, like carbonaceous chondrites, reacts by total melting at shock pressures as low as 30 Gpa. Therefore, shock pressures experienced by Leoville should definitely be much lower than 30 Gpa. This relatively mild pressure range obviously did not cause extensive thermal metamorphism in Leoville, because Leoville shows neither depletions of volatile elements relative to other CV chondrites [7] nor evidence of extensive melting. However, Leoville, especially its matrix, could have been affected by local, mild heating caused by impact. The matrix of Leoville is composed mostly of fine grains of olivine; it must have been much more porous than chondrules before compaction, like the matrices of most other CV chondrites. It is known that the shock effect on porous and multiphase materials is extremely heterogeneous and the amount of post-shock heat strongly increases with increasing porosity [e.g., 18,21]. St6ffler et a[. [18] noted that incipient melting in

Fig. 6. Plane-polarized transmitted light micrograph of olivine in a chondrule exhibiting planar fractures parallel to {hkl} (bipyramids).

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porous material is apparent at pressures as low as ~ 5 Gpa, whereas melting in nonporous material starts only at shock pressures above ~ 30 Gpa. Therefore, if Leoville was indeed affected by shock pressures of ~ 5 Gpa or higher, it is presumed that the matrix of Leoville was heated locally on an extremely fine scale by shock. In this regard, especially significant is the presence of the aggregates of extremely small, rounded to sub-rounded olivine grains in the Leoville matrix (Fig. 4a). They closely resemble the "fine-grained olivine" found in shocked ordinary chondrites [10]. Ashworth [10] interpreted the "fine-grained olivine" as being produced by shock heating; a mechanism for the formation of such olivine is either crystallization from an amorphous phase (probably a melt) or solid-to-solid recrystallization of a deformed crystal to a new aggregate of low-energy (unstrained) grains. The olivine aggregates in the Leoville matrix retain grains which have dislocations, especially in their core regions; suggesting that the solid-to-solid recrystallization mechanism is more appropriate for Leoville. Many olivine grains in the Leoville matrix show bent dislocations (Fig. 3b); such bending of dislocations can be probably ascribed to recovery by post-shock heating [22]. Numerous unit dislocations in the matrix enstatite may also have resulted from the reversion from C L E N to O R E N by re-heating [23]. There remains an important question to be addressed: can the chondrule foliation in Leoville can be ascribed to the post-accretionary shock events or not? We believe that, if the foliation simply resulted from compaction due to overburden in the meteorite parent body, such a strong foliation as seen in Leoville should be more common in other carbonaceous chondrites. There is no evidence from other carbonaceous chondrites of such pervasive shock effects as those exhibited by Leoville. Based on these observations and evidence, we suggest that compaction due to impact-induced shock pressure played an important role in causing the deformation in Leoville. However, assuming that chondrules were originally nearly spherical (as they are in most other chondrites), the pressures estimated by the textures of olivine and pyroxene in Leoville ( < 20 GPa) were apparently too low to deform the chondrules to such high aspect ratios as those seen in Leoville

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chondrules. There are many ordinary chondrites that were much more heavily shocked than Leoville, but none show such strong chondrule foliation [24] and many show no foliation at all. A possible explanation for the discrepancy might lie in the difference in texture and porosity between carbonaceous chondrites and ordinary chondrites. We also believe that multiple shock events are a key to the question. Carbonaceous chondrites consist mostly of relatively large, solid objects (chondrules) and fine-grained, porous matrix, whereas ordinary chondrites of petrologic types higher than 4 are much less porous, as a whole, and homogeneous in terms of porosity. It is conceivable that preferred orientation is more prominent if the more porous material is compacted in some specific direction and thus the degree of compaction (i.e., the degree of preferred orientation) is probably related to porosity before compaction. We envision that repeated impacts by relatively mild shock pressures could have gradually compacted the Leoville chondrite; the effect may have been somewhat similar to compaction due to overburden. Simultaneously, repeated impacts could have produced finely divided domains in olivine and pyroxene in chondrules. Once olivine and pyroxene in chondrules were pulverized, they may have behaved like a mosaic, rather than a single crystal, during successive shock events, thus possibly facilitating the chondrule flattening. Another possibility that can be considered, although less likely in our view, is that chondrules in Leoville were still fairly hot and plastic when they were shocked. Miiller and Wlozka [6] found that the characteristics of shock effects are lacking in the minerals of Ca-Al-rich inclusions (CAIs) in Leoville. More recently, Caillet et al. [25] reported that an unusual CAI, highly flattened and ~ 3 cm in maximum dimension, from Leoville showed evidence of melting of the adjacent matrix, and suggested that the CAI was hot (partially molten) when it accreted to the meteorite. These observations suggest that shock compression in Leoville occurred in a short period shortly after accretion, before CAIs had sufficiently cooled. In such an early period chondrules may have also been fairly hot. The hot and plastic chondrules could have been deformed easily by compaction due to shock pressure. However, if

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t h a t was really the case, c h o n d r u l e s in Leoville s h o u l d show m o r e recovery o f the shock t e x t u r e s t h a n t h o s e o b s e r v e d in o u r T E M study.

Acknowledgements We would like to thank Dr. C.B. Moore (Arizona State University) for providing us with the meteorite sample, Drs. K. Fujino and H. Mori for helpful discussions and Messrs. O. Tachikawa and H. Yoshida for assistance with the microprobe analysis. We also thank Dr. H.Y. McSween and two anonymous reviewers for helpful and constructive reviews. This study was supported by the Grant-in-Aid of the Ministry of Education, Science and Culture, Japan (No. 02640622 to KT and No. 63400003 to HT). References 1 C. t tayashi, K. Nakazawa, and Y. Nakagawa, Formation of the solar system, in: Protostars and planets 11, D.C. Black and M.S. Matthews, eds., pp. 11110-1154, Univ. Arizona Press, Tuscon, Ariz., 1985. 2 P.M. Martin, A.A. Mills, and E. Walker, Preferential orientation in four (73 chondritic meteorites, Nature 252, 37 38, 1975. 3 P.M. Cain, H.Y. McSween, and N.B. Woodward, Structural deformation of the Leoville chondrite, Earth Planet. Sci. Len. 77, 165-176, 1986. 4 E.R.D. Scott, K. Keil, and D. St6ffler, Shock metamorphism of carbonaceous chondrites, Lunar Planet. Sci. Conf. ~'~ 1207-1208. 1991. 5 M.C. Michel-Levy, M. Madon, and C. Willaime, Optical and T E M studies of some Lcovillc textural features, Meteoritics 14, 366, 1979. 6 W.F. Mfiller and F. Wlotzka, Mineralogical study of the Leoville meteorite (CV3): macroscopic texture and transmission electron microscopic observations, Lunar Planet. Sci. Conf. 13, 558-559, 1982. 7 A. K.acher, K. Kcil, G.W. Kallemeyn, J.T. Wasson, R.N. Clayton, and G.I. ttuss, The Leoville (CV3) accretionary breccia, in: Proc. 16th Lunar Planetary Science Conf., J. Geophys. Res., 90. 123 135. 1985. 8 J.T. Wasson, Meteorites, pp. 164 165, Freeman, New York, 1985. 9 J.R. Ashworth and D.J. Barber, Electron petrography of shock-deformed olivine in stoney meteorites, Earth Planet. Sci. Letl. 27, 43-511, 1975.

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