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Thermal and shock metamorphism of the Tenham chondrite: A TEM examination
Geochimica
et Cosmochimica
Pergamon
Acta, Vol. 59, No. 9, pp. 1835- 1845, 1995 Copyright 0 1995 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/95 $9.50 + .OO
0016-7037(95)00086-O
Thermal and shock metamorphism of the Tenham chondrite: A TEM examination FALKO LANCENHORST,* PASCAL JOREAU, Laboratoire de Structure et Propi&% de I’Etat Solide,
59655 Villeneuve (Received
September
and
JEAN CLAUDE DOUKHAN
UniversitC des Sciences d’Ascq Cedex, France
12, 1994; accepted
et Technologies
de Lille,
in revised form January 28, 1995)
Abstract-During the early episode of the solar system, the L6 chondrite Tenham has been affected by intense thermal metamorphism. Microanalytical data reveal homogeneous compositions of olivine (FoTSFaZ5),enstatite (En79Fs19W02), and diopside (En4,Fs8Wo& Using these data, empirical pyroxene thermometers yield temperature estimates for this thermal metamorphism, ranging from 810 to 870°C. Due to the presence of thin shock veins, which contain the high-pressure phases majorite and ringwoodite, the L6 chondrite Tenham is an instructive example for strong shock metamorphism. In contrast to previous transmission electron microscopy (TEM) studies, which concentrated on these shock veins, we also systematically characterized the shock signature of the silicates occurring in the bulk of Tenham. Plagioclase is either pervaded by thin (200 nm), amorphous lamellae, so-called planar deformation features (’‘PDFs”), or it is transformed to maskelynite, a diaplectic glass of feldspar composition. In olivine, shock deformation has caused the formation of irregular and planar fractures and the activation of numerous (2 X lOI m-‘) c dislocations in the glide planes (100) and ( 110); energetically favorable but less mobile a dislocations are totally absent. Fracturing in olivine is interpreted as the cause of dislocation formation. A low dislocation density (
1) Thermal metamorphism is expressed by blurring of chondrule outlines, recrystallization, absence of glass, formation of plagioclase, and homogenization of mineral compositions, leading to a loss of the primary nebular signature. According to van Schmus and Wood (1967), Tenham was consequently classified as a type 6 ordinary chondrite.
2) Indicators for shock metamorphism are pseudotachylitelike shock veins, which randomly pervade Tenham and contain ringwoodite and majorite, the respective y highpressure polymorphs of olivine and pyroxene (Binns et al., 1969; Binns 1970; Price et al., 1979; Putnis and Price, 1979; Madon and Poirier, 1980, 1983). These black veins probably represent quenched melts resulting from localized temperature excursions during shock compression (StGffler et al., 1988). Ringwoodite and majorite are considered either to have crystallized from such high-pressure melts or to have formed by solid-state transformation of mineral grains incorporated in the shock veins (Price et al., 1979; Madon and Poirier, 1983). According to the new and revised shock classification of ordinary chondrites (Stijffler et al., 1991), the presence of shock veins and ringwoodite indicates shock stage S6 with pressures of at least 50 GPa.
* Present address: Institut kunde der Humboldt-Universitl 10115 Berlin, Germany.
Meteorites like Tenham are generally considered as ideal rocks for the recognition and characterization of shock effects, because they have been affected by at least one relatively
The Tenham meteorite fell in 1879 near Tenham Station (25”44’S, 142”57’E) located in South Gregory, Queensland, Australia. It belongs to the L6 group of ordinary chondrites. Chemical and petrological data, and transmission electron microscope (TEM) observations have revealed that the Tenham meteorite has experienced a high degree of (1) thermal and (2) shock metamorphism (Spencer, 1937; Mason, 1973; Binns, 1967; Binns et al., 1969; Putnis and Price, 1979; Madon and Poirier, 1980):
fiir Mineralogie, Museum fiir Naturzu Berlin, InvalidenstraJ3e 43, D-
1835
1836
large-scale
F. Langenhorst, impact,
capable
of ejecting
them
from
P. Joreau.
their parent
In spite of this, it is surprising that only two systematic optical (Dodd and Jarosevich, 1979; Stiiffler et al., 1991) and a few TEM investigations (e.g., Ashworth, 1980a,b) of the shock signature of ordinary chondrites have been performed. Especially more TEM work is required for extending our knowledge about shock metamorphism of ordinary chondrites, because the nature and microstructure of shock effects can often not be fully elucidated with the limited resolution of an optical microscope. Previous TEM investigations on Tenham have only concentrated on the formation of ringwoodite and majorite in shock veins (Putnis and Price, 1979; Price et al., 1979; Madon and Poirier, 1980, 1983). In the present paper, we report on a detailed and systematic TEM investigation of the shock effects in silicate phases (olivine, pyroxenes, and plagioclase) occurring outside the shock veins. The major goal of our study was to characterize the observed shock effects and to infer the shock conditions which generated them. In addition, new microchemical data provide information about the temperatures of early metamorphism, which caused equilibration of mineral compositions. bodies.
EXPERIMENTAL
and J. C. Doukhan
TECHNIQUES
Doubly polished thin sections, 20-pm-thick, were prepared for both optical and TEM investigations. For TEM work, several discs of 3 mm diameter were drilled from one petrographic thin section, glued on copper grids, and finally thinned to perforation by argon ion-beam bombardment at 4.5 kV and an incidence angle of 14”. They were then coated on their bottom face with a thin carbon layer (~30 nm). These thin foils were observed at 300 kV in a Philips CM30 scanning transmission electron microscope (STEM). Microanalyses were carried out either in scanning or nanoprobe mode with a TRACOR energy dispersive spectrometer equipped with a Ge detector. The detector is separated from the TEM column by an ultra-thin window of organic polymer (= 100 nm thick). This attachment pcrmits the detection of light elements (2 2 5). especially of oxygen. As volatile cations are known to be highly mobile under the electron beam, we used a liquid nitrogen cooled Be holder for microanalyses of piagioclase. In this case the beam intensities were furthermore reduced to moderate values (electronic density = 10’ Am-*) by fast scanning over rectangular areas (0.2-0.5 pm2 in size). In order to obtain the precise mineralogical compositions, it is essential (1) to know the exact kws, factors and (2) to perform an absorption correction. (1) The kNs, factors, which represent the sensitivity of the analysing system for the element X relative to the reference element (Si in the present case) for extremely thin samples (no absorption correction), were experimentally determined by the so-called “parameterless correction method (PCM)” (Van Cappellen, 1990). For instance, pure synthetic quartz (SiOZ) was used as a standard for the determination of the ko,s, factor. A series of reference X-ray spectra was recorded under similar conditions as for the samples (same take-off angle and dead times of 25-30%) and with constant beam intensity on variably thick areas of the quartz standard. The net number of counts per unit time ([Si] + [01/t), which increases monotonously with increasing sample thickness, is correlated with the raw (no absorption correction) concentration of oxygen ([O]/[Si] + [O]), as shown in Fig. 1.This correlation is empirically fitted by simple polynomial regression. A first degree function Cy = 63.679 - 0.0575~) or a second degree function (r = 63.851 - 0.0595x - O.OCKlOO533~) yield good fits both with a correlation coefficient R of 0.997. Both functions extrapolated to zero thickness yield practically the same &, factor of 1.04with an uncertainty < 1%. The fits for other elements X are less precise. The uncertainties in the k,,,, factors are estimated to be = 1% for Mg, Fe, and Ca and 2-3% for Na, Al, Mn, Ni, and Cr.
@
70
-
correctvalue
s
0”
60
‘7
50 U
40
5
0
100
J
200
300
400
[Si]+[O]/t (counts/s) FIG. 1. Experimental determination of the b,s, factor. Twenty-two spectra were recorded on a quartz standard in areas of different thicknesses. [0] and [Si] are the net counts for oxygen and silicon, respectively. The raw concentration of oxygen ([O]/[Si] + [0]) plotted against the total net counts per unit time ([Si] + [01/r) defines a regression line, which is fitted by a polynomial function and extrapolated to zero thickness. The ko/si factor is the ratio between the correct (2/3) and the extrapolated value.
(2) The absorption correction requires the precise knowledge of the sample thickness. In crystals with small lattice parameters, the thickness can be. precisely determined by the convergent beam technique. This technique is, however, not adapted toesilicates like olivine and pyroxene with large lattice parameters (>5 A). Alternatively, the thickness can also be estimated by the contamination spot method, but this leads to values with uncertainties of at least 10%. Therefore, we have used a recently described correction procedure (Van Cappollen and Doukhan, 1994). which is simply based on the principle of electroneutrality, i.e., the sum of all cations and anions (oxygen) times their respective valence states must balance. This correction method exploits that (1) the ultra-thin window and the high resolution of the Ge detector allow the detection of light elements and an accurate spectrum deconvoiution of light element K-lines and intermediate element L-lines and that (2) the K-line of oxygen suffers significant absorption and is thus very sensitive to the sample thickness. According to the classical absorption correction, the concentrations of cations and anions (oxygen) were calculated for various thicknesses until electroneutrality is achieved. Finally the quality of analyses was checked by calculating the structural formulae. For instance, the chromite analysis yields a structural formula of (Mgu 24Fe&)(CrI 47A10.21 Fe&Sio.l IT41.0&.00 (Table 1). which agrees well with the general spine1 formula (ABZ04). Most of the analyses presented in Table 1 have been recorded with a200 seconds live time; this corresponds to 30000 to 50000 net counts for the major elements. Taking into account this counting statistics and the uncertainties in the kWs, factors, the precision on the concentrations of major elements is estimated to be 3-5%.
MINERALOGY The investigated thin sections of the Tenham chondrite contain the following minerals (listed according to decreasing abundance): olivine, Ca-poor pyroxene, Ca-rich pyroxene, plagioclase, Fe-Ni alloy, troilite, and subordinate apatite, chromite, ringwoodite, and major&e. With exception of FeNi alloy, troilite, and apatite, all other phases were quantitatively analysed. Results of these measurements are given in Table 1 and Fig. 2. Plagioclase appears as a fine-grained (grain size < 100 ,um) and minor (<5 vol.%) constituent in matrix and chondrules which are still recognizable despite recrystallization. Because of extreme mobility of Na and K cations under the electron beam, even at low temperature in the cold Be holder, the com-
1837
Metamorphism of the Tenham meteorite
Di I-
position of plagioclase could not be determined precisely and is, thus, not listed in Table 1. The WA1 ratios yield, however, rough estimates of anorthite (An) contents, which range from An,r to An,,,. These oligoclase compositions are in relatively good agreement with other data for L6 chondrites (van Schmus and Ribbe, 1968). The scatter in An contents reflects our analytical uncertainties and, hence, cannot be unequivocally ascribed to heterogeneity. Olivine and pyroxenes are more resistant to beam damage and their analyses yield stoichiometric formulae. The mean composition of olivine, the most abundant mineral in Tenham (ca. 40 vol%), is Fo,5.3Fa24.7 (chrysolith); the relatively low standard deviations in Fo and Fa contents (0 = 0.9%) indicate chemical equilibration (Table 1). The chemical composition of pyroxenes is shown in Fig. 2. According to the revised nomenclature scheme for pyroxenes (Morimoto, 1988), the compositions of Ca-rich and Capoor pyroxenes are typical of diopside and enstatite, respectively (cf. Table 1 for average compositions). Like oligoclase, diopside was mainly found in finely recrystallized chondrules with grain sizes < 200 pm, whereas enstatite, the second most abundant mineral (ca. 25 ~01%) in Tenham, occurs as medium- to coarse-grained constituent in chondrules and, subsidiary, in the matrix. Both pyroxenes are homogeneous in composition (cf. a-values in Table 1) and probably equilibrated together with olivine and plagioclase. Some enstatites contain thin (30-80 nm) platelets of chromite crystals oriented in an epitactic relationship parallel to the (100) plane of the host pyroxene (Fig. 3). Microanalyses of chromites reveal that appreciable amounts of Fe*+ in tetrahedral (T) and Cr in
Pv diopside
En
majorite enstatite ,,wQ k \/
\/
20
40
mole % FeSi03
FIG. 2. Composition of Tenham diopsides, enstatites, and majorites plotted in the Mg-rich half of the diopside (Di)-hedenbergite-enstatite (En)-ferrosilite quadrilateral. Average compositions are given in Table 1.
octahedral (0) sites are replaced by Mg (T), and Al, Fe3+, Ti, and Si (0) (see Table 1). In the thin shock vein examined in this TEM study, the high-pressure phases major&e and ringwoodite occur as finegrained (grain sizes < 2-3 pm) polycrystalline aggregates
TABLE 1. Average chemical compositions (in wt. % of elements) and structural formulae of
olivine, pyroxenes, chromite, and the high-pressure phases ringwoodite and majorite based on about 10 analyses for each mineral. Data were obtained by energy-dispersive analysis with an ultra-thin window, which allows the detection of oxygen (see text for details). The Fo- and Facontents of olivine and ringwoodite, as well as the En-, Fs-, and Wo-contents of pyroxenes are given together with standard deviations (0). Olivine Ringwoorlite wt. % Si 17.6 17.8 Ti n.d. n.d. Al 0.45 0.07 Cr n.d. nd. Fe 17.5 17.8 Mn 0.39 0.17 Mg 23.1 22.9 Ca nd. 0.05 Na nd. 0.27 0 41.0 40.9 structural formulae Si 0.98 0.99 Ti nd. n.d. Al 0.03 co.01 Cr n.d. n.d. Fe3+ nd. n.d. FeZ+ 0.49 0.50 Mn 0.01
+
Diopside 25.0 0.33 0.33 0.57 4.0 0.15 9.9 15.7 0.60 43.5
Enstatite 25.8 0.09 0.24 0.05 10.1 0.41 17.7 0.51 0.46 44.7
Majorite 24.3 0.05 2.8 0.40 6.3 0.24 18.6 1.2 0.70 45.4
Chromite 1.5 1.9 2.8 36.8 23.5 nd. 2.9 nd. n.d. 30.7
1.96 0.02 0.03
1.97 co.01 0.02 co.01 0.02 0.37 0.02 1.56 0.03 0.04 5.98 78.8 i0.7
1.82 co.01 0.22 0.02 n.d. 0.24 0.01 1.61 0.07 0.06 5.96
0.11 0.08 0.21 1.47 0.12 0.76 nd. 0.24 n.d. n.d. 4.00
84.3 f1.9
19.6 3~0.7
12.3 ft.5
1.6 iO.6
3.4 M.5
0.02
n.d. 0.16 co.01 0.89 0.87 0.06 6.00 46.7 il.0 a.2 fl.1
45.1 il.4
1838
F. Langenhorst,
P. Joreau,
and J. C. Doukhan
majorite suggest a rapid crystallization from a shock-produced high-pressure melt, which was composed of at least three components, namely enstatite, the chromite inclusions, and a small fraction of a Ca-rich component (plagioclase or diopside). This mixture can, however, not account for the distinct change in the Fe content from 10 wt% in enstatite to 6 wt% in majorite (Table 1). Since we have not detected a second generation of Fe-rich majorites or pyroxenes, it cannot be excluded that the (Fe,Ni)-oxides or -hydroxides surrounding the majorite grains (Fig. 4a) represent, eventually, weathering products of pure Fe, which also crystallized from the shock-produced melt. The absence of such Fe-rich phases around the ringwoodite grains (Fig. 4b), which are of identical composition as equilibrated olivine, is an argument for this hypothesis.
FIG. 3. HRTEM image of a chromite platelet (Chr) occurring as inclusion in orthoenstatite (En). The lattice fringes in orthoenstatite are parallel (100). Fringes in Chromite are absent, because zone axes are inclined to the electron beam.
(Fig. 4). Majorite is generally surrounded by (Fe, Ni)-oxides or -hydroxides (Fig. 4a), as revealed by microanalytical Xray spectra. In contrast to the other silicates, which were analysed at various places of the thin section, analyses of majorite and ringwoodite were carried out on several neighbouring grains, which are only about 5 10 pm away from each other. Even at this small scale the chemical compositions of the high-pressure phases are less homogeneous than those of the equilibrated mineral phases (cf. u-values in Table 1). Apart from this, the composition of majorite differs systematically from that of enstatite (Fig. 2). A distinct enrichment in Al, Cr, Mg, Ca, and Na, and a significant depletion in Si, Mn, and especially in Fe could be detected for majorite (Table 1). It is unlikely to produce such a change in chemistry by a solid-state transition from enstatite to majorite. On the contrary, the different composition and the small grain sizes of
DEFECT STRUCTURES IN SILICATES Plagioclase
(Oligoclase)
In the thin section investigated with TEM, the plagioclase microstructure is dominated by optically resolvable sets of thin, parallel lamellae, which, according to an international agreement, are termed planar deformation features (‘‘PDFs,” Grieve et al., 1990). Similar shock-induced defect structures have been already detected in numerous plagioclase-bearing meteorites, terrestrial impact rocks, and experimentally shocked single-crystal plagioclases (Stoffler, 1972, and references therein; Ostertag, 1983; Stijffler et al., 1986). Up to four crossing sets of PDFs traversing entire Tenham plagioclase grains can be distinguished by optical microscopy (Fig. 5a). Their birefringence seems to be slightly reduced due to the presence of PDFs. TEM observations reveal a PDF thickness of about 0.2 to 0.3 pm with a regular interlamellar spacing of 0.5 pm (Fig. 5b). Diffuse scattering rings in selected area electron diffraction (SAED) patterns and lack of contrast changes in tilt experiments prove the amorphous nature of PDFs. Within the error limits of TEM microanalyses,
FIG. 4. TEM bright field images of minute (grain size < 2-3 pm) high-pressure phases occurring in a thin shock vein. (a) Majorite is defect-free and surrounded by alteration products of Fe-Ni alloy. (b) Ringwoodite shows numerous stacking faults parallel to ( 1101 planes.
Metamorphism
of the Tenham
meteorite
1839
FIG. 5. (a) Photomicrograph of oligoclase displaying several sets of planar deformation features (PDFs); parallel nicols. (b) TEM dark field image of amorphous PDFs in oligoclase seen egde-on. The diffuse scattering in the SAED pattern (inset) reveals the amorphous nature of PDFs. The diffraction spots stem from crystalline areas between the PDFs.
the chemical talline
composition
of PDFs is identical
to that of crys-
areas.
Adjacent to the shock veins and in other thin sections, which have been only optically examined, plagioclases are totally converted to isotropic maskelynite. Price et al. (1979) reported the same trend.
Olivine Optical inspection revealed two shock effects in olivine grains: (1) mosaicism and (2) a consistently high density of planar and, subordinate, of irregular fractures (Fig. 6a). As known from observations on olivine-bearing lunar and terrestrial impact rocks, both effects are characteristic for shock compression of olivine (StGffler, 1972; Snee and Ahrens, 1975; Lally et al., 1976). The presence or absence of mosaicism and the orientation of planar fractures have also been pressure-calibrated through shock experiments (Carter et al., 1968; Miiller and Homemann, 1969; Snee and Ahrens, 1975; Reimold and StGffler, 1978; Bauer, 1979; Jeanloz, 1980) and are included as shock pressure indicators in accepted shock classifications (Dodd and Jarosevich, 1979; StGffler et al., 1991). Mosaicism in olivine from Tenham, which can be recognized as a slightly mottled extinction behavior under crossed nicols, varies from weak to strong. Fractures in Tenham olivines occur as either open fissures or are partially filled with red to brownish coloured material. At the TEM scale, such fractures are also recognizable (Fig. 6b), and microanalytical X-ray spectra recorded on their fillings show only peaks of iron, nickel, and oxygen, indicating that fractures contain (Fe,Ni)-oxides or -hydroxides (alteration products). TEM observations reveal, furthermore, the occurrence of planar and closed fissures (Fig. 6~). Attempts to detect material inside of these features by high resolution transmission electron microscopy (HRTEM) and diffraction techniques failed, however. Therefore, it is assumed that these closed fissures, like the open and filled ones, represent planar fractures. Ashworth and Barber (1975) described similar fea-
tures as “healed cracks.” Standard stereographic analyses carried out on Tenham olivines show that planar fractures are predominantly oriented parallel to the low index planes (loo), (OlO), (OOl), (130), and (1 IO), which belong to pinacoidal and prismatic forms. Bipyramidal orientations, such as (11 l), were only rarely identified (Fig. 6~). Besides planar fractures, the most remarkable effect observed in olivine is the formation of numerous screw dislocations with Burgers vector c (i.e., [OOl]; Fig. 6d). Some of these screw dislocations turn at their tips to edge character enabling one to determine their glide planes. The glide planes are, in agreement with those generally known for olivine (Poirier, 1975), (loo), { 1 lo], and others parallel to (hkO}. The high dislocation density of up to 2 x lOI mm2 determined on our samples confirms previous data of Madon and Poirier (1983). Practically no dislocations with Burgers vector a (i.e., [ 1001) have been detected. Therefore, shock-deformed olivine exhibits a defect microstructure similar to that induced by conventional deformation at moderate temperature. Only c dislocations are activated in (hkO] planes (the so-called pencil glide), even under very high shock stresses. It is furthermore noteworthy that relatively long segments of c edge dislocations have been observed at the tips of planar fractures, as shown in Fig. 6c. With increasing distance to the tips, c dislocations change to screw character which is plausibly attributable to greater velocity of edge components during shock deformation. In addition, the presence of dislocations at the tips of planar fractures suggests that fracturing might be the cause for the formation of dislocations. Diopside By far, the greatest diversity of shock defects was generated in diopside (Figs. 7 and 8). We detected (1) mechanical twinning, (2) a large dislocation density, and (3) PDFs. (1) Mechanical twins were identified by SAED patterns and HRTEM images. They occur parallel to (001) and (100) (Fig. 7a). (100) twins, which are only a few lattice repeats
F. Langenhorst,
1840
P. Joreau,
and J. C. Doukhan
FIG. 6. (a) Photomicrograph of planar and irregular fractures in olivine; crossed nicols. (b) Dark held image of an olivine grain with three fractures and numerous c screw dislocations; g = [004]. The fractures are filled with alteration products of Fe-Ni alloy (hydroxides or oxides). (c) Dark field image of closed, planar fractures in olivine oriented parallel to (111); g = [004]. (d) Dark field image of olivine showing the large density of c screw dislocations. Their glide systems are (lOO)[OOl] and ( 1 lO][OOl]; g = 10041
thinner than (001) twins (70only the (001) twins could be optically recognized by their alternating extinction behavior under crossed nicols (Fig. 7b). In the (OlO)-plane the angle between (001) twin individuals is about 74” (Fig. 7c,d), in good accordance with crystallographic predictions and previous TEM observations (Kirby and Christie, 1977). When twins are inclined to the electron beam, characteristic fringe patterns are detected in two-beam conditions. Because (100) twins are very thin, each fringe pattern in Fig. 7e represents only one twin individual. The fringe patterns of (100) twins are underlined by a number of concentric partial dislocations, whereas such dislocations are totally absent in (001) twin boundaries. This suggests a different formation mechanism for the two types of twins. With decreasing distance to shock veins, the number and thickness of (100) twins increases distinctly. For example, twins adjacent to veins can be one order of magnitude thicker (40 nm) than those occurring several millimeters away from veins. This trend indicates that formation and development of (100) twins is enhanced in the vicinity of shock veins where the shock temperature was higher. The (001) and (100) twinning operations in diopside have been produced in static and dynamic deformation experiments (~5
nm)
wide,
are markedly
260 nm). Therefore,
(Homemann and Mtlller, 1971; Kirby and Christie, 1977; Miiller, 1993; Leroux et al., 1994). In contrast to (100) twins, which have also been observed in tectonically deformed diopside (Skrotzki, 1994) (001) twins are only known from shocked lunar rocks or meteorites (Nord and McGee, 1979; Ashworth, 1980a,b) and, hence, have to be regarded as a diagnostic shock indicator. In static experiments, both twin types develop by gliding of partial dislocations at strain rates of 10. 4 s ‘, which are several orders of magnitude lower than strain rates typical for shock deformation (in the order of IO6 to 10’ s-l). As far as partial dislocations are absent in dynamically produced (001) twins, their formation mechanism could be a rapid kinking process, instead of dislocation gliding. (2) Numerous dislocation bands were predominantly activated in the glide systems (lOO)[OOl] and, to a minor extent, in ( 1 lO][OOl]. The dislocation density varies with the distance to shock veins. Several millimeters away from shock veins, the density is about 1 x 10“’ mm2 (Fig. 7f), but this value can decrease by nearly one order of magnitude in the vicinity of shock veins. As already discussed for the (100) twins, this also suggests a change of the deformation mode and the shock conditions with decreasing distance to shock veins. (3) To our knowledge, we report here on the lirst observation of straight and narrow, amorphous lamellae in naturally
Metamorphism
of the Tenham
meteorite
FIG. 7. (a) TEM bright field image of a diopside grain observed near a shock vein. The diopside grain displays many twins parallel to (001) and (100). (b) Photomicrograph of diopside twinned parallel to (001); crossed nicols. (c) HRTEM image of (001) twins in diopside. The lattice fringes are parallel to (100). (d) SAED pattern recorded on (001) twins; the reciprocal directions [ IOO]: and [ 1CJO]$ indicate the different orientation of twin individuals. (e) Dark field image of thin (100) twins in diopside. Each fringe system represents an inclined (100) twin. Several partial dislocations are visible in the twin boundaries; g = [22i]. (f) Dark field image of dislocation bands in diopside with the glide configuration (lOO)[OOl]. Dislocation bands are deflected by (001) twins.
1841
1842
F. Langenhorst,
P. Joreau,
and J. C. Doukhan
FIG. 8. (a) Dark field image of diopside with two PDF sets parallel to (221) and (221); g = [002]. Intersecting PDFs displace each other (see arrow) and, additionally, are deflected by (001) twins. Inset shows a SAED pattern with diffuse scattering rings, which indicate the amorphous nature of PDFs. (b) Dark field image of (001) twins. dislocation bands, and PDFs in diopside; g = [002]. PDFs are oriented parallel to (221). (c) HRTJZM image of an amorphous PDF parallel to (221). (d) HRTEM image of a (001) twin boundary displaced by an amorphous PDF, which is parallel to (221).
shocked
diopside
(Fig.
8a,b).
Their
amorphous
nature
is evi-
in SAED patterns and HRTEM images (Fig. 8a,c,d). To date, such features were only detected and characterized in experimentally shocked diopside (Leroux et al., 1994). These observations together indicate that the occurrence of amorphous lamellae in diopside is typical of shock damage. Due to their similarity to PDFs in quartz, it is reasonable to use the same term for the amorphous lamellae in diopside. In contrast to diopside experimentally dent
from
diffuse
scattering
rings
shocked at 45 GPa (Leroux et al., 1994), PDFs do not pervade entirely Tenham diopsides and are even absent in some grains. Standard stereographic analysis revealed that_- PDFs are predominantly oriented parallel to the planes (221), (221), and (221), which belong to the two monoclinic prisms (221) and (22i). In experimentally shocked diopside, the predominant orientation of PDFs was found to be (33i), but subordinate the { 22TJ orientation was also detected (Leroux et al., 1994). PDFs in Tenham diopsides appear often in the form of ap-
Metamorphism of the Tenham meteorite parent pairs with variable interlamellar spacings ranging from 0.1 to 0.9 pm (Fig. 8a); their thickness is in the order of 50 nm. When PDFs intersect twins, PDFs are deflected and twins are displaced (Fig. 8a,b,d). On the one hand, this observation indicates the shearing character of PDF formation and, on the other hand, it suggests a simultaneous formation of PDFs and twins. At the intersection of two sets of PDFs, both are displaced (Fig. 8a).
Enstatite At the optical scale, equilibrated enstatite shows intensive, internal fracturing, predominantly parallel to (010) and (001) but other orientations and irregular fractures were also observed. Like in olivine, these fractures are filled with alteration products of iron (Fe-hydroxides and -oxides). Two further defects are visible at the TEM scale: dislocations and clinoenstatite lamellae. Most of the dislocations occur in the glide plane (100). The total dislocation density never exceeds 10” mm’, a value appreciably lower than the dislocation densities observed in neighbouring olivine and diopside grains. Clinoenstatite lamellae parallel to (100) are also present, and this strongly suggests that [OOl](lOO) slip was activated with dislocation splitting. In HRTEM images, clinoenstatite is recognized by its (100) spacing of 9 A, which is exactly half of the spacing in orthoenstatite (18 A). Clinoenstatite lamellae commonly extend over up to twenty lattice repeats, which equals 18 nm (Fig. 9a). SAED pattern of regions with high clinoenstatite concentrations show typical streaking of the diffraction spots along the reciprocal a* direction (Fig. 9b). At least two mechanisms are known to cause the ortho-/ clino-enstatite inversion: shear stresses > 70 MPa on (100) in the [OOl] direction (Coe and Kirby, 1975; J. C. Doukhan, unpub. data) and rapid cooling from the protoenstatite stability field (Smith, 1974). In fact, both mechanisms could be identical, because rapid cooling must induce deviatoric elastic media. The enormous deviatoric stresses in anisotropic
I843
stresses accompanying shock compression certainly overcome the moderate threshold stress of inversion, and it is thus not surprising that clinoenstatite lamellae are detected in orthoenstatite from the strongly shocked Tenham meteorite. Buseek et al. (1980) suggested that the width of clinoenstatite lamellae (more precisely, the number-odd or even-of 9A lattice repeats) and the presence or absence of twinning in clinoenstatite are useful criteria to distinguish shear-induced clinoenstatite lamellae from those generated by cooling from the protoenstatite field. We precisely measured the number of elementary lamellae in Tenham enstatite on HRTEM images and found even multiples of 18 A,, suggesting a mechanical (shock) origin but twinning could not be unequivocally confirmed on SAED patterns. Since all other silicates in Tenham show strong shock damage, it is, however, likely that the ortho-/clino-enstatite inversion is also shock induced. High-Pressure
Phases
Majorite, the high-pressure phase of enstatite, is absolutely defect-free (Fig. 4a), which indicates a formation during the shock event. Ringwoodite, the high-pressure polymorph of olivine, shows stacking faults parallel to (110) planes (Fig. 4b). These stacking faults have been already described in previous studies and either a martensitic solid-state transformation mechanism or nucleation from a high-pressure melt have been suggested for their formation (Putnis and Price, 1979; Madon and Poirier, 1983). The small grain sizes of the equigranular ringwoodite crystallites (< 1 pm) and their slightly heterogeneous composition are more compatible with the latter mechanism. The stacking faults might, therefore, represent growth defects. DISCUSSION Conditions
of Thermal Metamorphism
The absence of water and low pressures McSween et al., 1988) and stresses prevailing
(<2.5 GPa, on chondrite
FIG. 9. (a) HRTEM image of orthoenstatite wit! clinoenstatite lamellae. Lattice fringes are parallel to (100). 9 A lattice repeats correspond to clinoenstatite and 18 A repeats to orthoenstatite. (b) SAED pattern of an area with a high density of clinoenstatite lamellae. Notice the streaking in the [lOO]* direction.
1844
F. Langenhorst,
P. Joreau,
parent bodies have the consequence that pressure-temperature characteristic mineral assemblages, as known for terrestrial metamorphic rocks, are lacking in ordinary chondrites. Therefore, the temperature conditions of metamorphism can only be estimated on the basis of mineral compositions. Our microanalyses have shown that the compositions of the equilibrated mineral phases outside the shock veins (i.e., exclusive ringwoodite and majorite) are homogeneous and unaffected by the impact event. The low minor-element (e.g., of Al, Ti, Na) contents and the absence of exsolutions in pyroxenes indicate that the Tenham chondrite is apparently an appropriate candidate for the application of the empirical two-pyroxene thermometer of Lindsley (1983). According to the special projection method proposed by Lindsley (1983), average metamorphic temperatures of 810°C for enstatite and 870°C for diopside have been graphically determined. Scattering of temperatures calculated for individual analyses is in the order of ?lOO”C. The more recent thermometer of Brey and Kohler (1990), which is based on the Ca partitioning between coexisting orthopyroxenes and clinopyroxenes, yields an average temperature of 820 2 15°C thus better constraining the enstatite temperatures than the method of Lindsley (1983). These temperatures and the mean compositions of equilibrated enstatite (Fs& and olivine (Fa& are consistent with the classification of Tenham as L6 chondrite (Sears and Dodd, 1988; McSween et al., 1988). It is generally accepted that ordinary chondrites have experienced this thermal metamorphism during the early solar period, i.e., around 4.0 to 4.5 Ga ago (Turner, 1988). However, the impact event on Tenham’s parent body occurred distinctly later, because nearly all exposure ages of L6 chondrites are below 50 m.y. (Caffee et al., 1988). Shock Conditions As revealed by our TEM examination, almost all defect structures observed in the silicates of Tenham are related to shock metamorphism. The results of this study substantiate that there are some diagnostic shock defects, such as c screw dislocations in olivine, as well as (100) twins and PDFs in diopside, which are only visible with the TEM. In analogy to quartz (Langenhorst and Deutsch, 1994), the PDF orientations in diopside could eventually be a potential shock barometer, but they are not yet calibrated by TEM studies. Optically resolvable shock effects have already been calibrated through shock recovery experiments on single crystal and polycrystalline olivine, pyroxene, and plagioclase, as well as on rocks composed of these constituents (Carter et al., 1968; Mtiller and Homemann, 1969; Homemann and Mtiller, 1971; Snee and Ahrens, 1975; Reimold and Stoffler, 1978; Bauer, 1979; Ostertag, 1983; Huffman et al., 1993). These experimental data are used as a basis for shock classifications (Dodd and Jarosevich, 1979; Stoffler et al., 1991). Pyroxene is excluded in these classifications because it optically shows no significant changes in shock features over a wide range of pressures. Therefore, we must rely heavily on data for olivine and plagioclase to infer the shock pressure in the bulk of Tenham. In the thin section where high-pressure phases were detected with TEM, plagioclase contains PDF sets of different orientations, and its birefringence is slightly reduced. Accord-
and J. C. Doukhan
ing to Ostertag (1983) and Langenhorst (1989), this shock signature indicates pressures between 25 and 30 GPa. In other thin sections of Tenham, plagioclase is, however, totally transformed to isotropic maskelynite, which suggests pressures up to 45 GPa (Stoffler et al., 1988, 1991). This heterogeneous distribution of shock pressure is also reflected in the shock signature of olivine. Olivine coexisting with PDF-bearing plagioclase shows weak mosaicism, whereas strong mosaicism is typical for olivine occurring together with maskelynite. In summary, the olivine and plagioclase grains in our Tenham specimens record shock pressures varying from shock stage S4 to S5 (Stoffler et al., 1991). The temperature excursions in the melt veins, which contain majorite and ringwoodite, are principally believed to result from localized peak pressures exceeding at least 50 GPa (Stoffler et al., 1988, 1991). Apart from the fact that such pressure excursions are not recorded in our samples, it is hard to imagine why such pressure deviations could be organized in the form of veins. Alternatively, the high concentration of shear-induced (100) twins and PDFs in diopsides, which occur in the vicinity of shock veins, indicates that a heterogeneous distribution of other physical parameters, such as strain and strain rate, could also account for localized melting. Since majorite and ringwoodite are known to form in static experiments at pressures below 20 GPa (Presnall and Gasparik 1990; Rubie and Brearley 1994) their nucleation from a shock-produced melt vein might be already possible at overpressures of about 30 to 40 GPa (i.e., in shock stage S5 or even S4), if shock compression lasts long enough. Acknowledgments-We are gratefully indepted to D. StSffler and C. Lingemann for fruitful discussions and for providing thin sections of the Tenham meteorite. One of us (F.L.) thanks the Minis&e de la Recherche et de la Technologie de France for receiving a research grant. We acknowledge also the financial support of Il%U (Action Sp&ifique Sciences de I’Univers 1994/95). A. Therriault, W. U. Reimold, and L. Keller provided valuable and constructive reviews. Editorial
handling: C. Koeberl
REFERENCES Ashworth J. R. (1980a) Deformation mechanisms in mildly shocked chondritic diopside. Meteoritics 15, 105-l 15. Ashworth J. R. (1980b) Chondrite thermal histories: Clues from electron microscopy of orthopyroxene. Earth Planer. Sci. Let?. 46, 167-177. Ashworth J. R. and Barber D. J. (1975) Electron petrography of shock-deformed olivine in stony meteorites. Earth Planer. Sci. LQtt. 21,43-50. Bauer J. F. (1979) Experimental shock metamorphism of mono- and polycrystalline olivine: A comparative study. Proc. 10th Lunar. Sci. Con$, 2573-2596. Binns R. A. (1967) Structure and evolution of non-carbonaceous chondritic meteorites. Earth Planet. Sci. Leti. 2, 23-28. Binns R. A. (1970) (Mg,Fe),SiO, spine1 in a meteorite. Phys. Earrh Planet. Intl. 3, 156-160. Binns R. A., Davis R. J., and Reed S. J. B. (1969) Ringwoodite, natural (Mg, Fe),SiO, spine1 in the Tenham meteorite. Narure 221, 943-944. Brey G. P. and Kiihler T. (1990) Geothermobarometry in four-phase Iherzolites. II. New thermobarometers, and practical assessment of existing thermobarometers. .l Petrol. 31, 1353- 1378. Buseck P. R., Nord G. L., Jr., and Veblen D. R. (1980) Subsolidus phenomena in pyroxenes. In Reviews in Mineralogy-Pyr[~xenes (ed. C.T. Prewitt), pp. 117-211. Mineral. Sot. Amer.
Metamorphism
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Caffee M. W., Goswami J. N., Hohenberg C. M., Marti K., and Reedy R. C. (1988) Irradiation records in meteorites. In Mereorites and the Early Solar System (ed. J.F. Kerridge and M.S. Matthews), pp. 205-245. Univ. Arizona Press. Carter N. L., Raleigh C. B., and De Carli P. S. (1968) Deformation of olivine in stony meteorites. J. Geophys. Rex 73, 5439-5461. Coe R. S. and Kirby S. H. (1975) The orthoenstatite to clinoenstatite transformation by shearing and reversion by annealing: MechaContrib. Mineral. Petrol. 52,29nism and potential applications. 55. Dodd R. T. and Jarosevich E. (1979)Incipient melting in and shock classification of L-group chondrites. Earth Planet. Sci. Lett. 44, 335-340. Grieve R. A. F., Sharpton V. L., and Stoffler D. (1990) Shocked minerals and the K/T controversy. Eos 71, 1792- 1793. Homemann U. and Mtlller W. F. (1971) Shock-induced deformation twins in clinopyroxene. Neves Jahrb. Mineral. Mh. 6, 247-256. Huffman A. R., Brown J. M., Carter N. L., and Reimold W. U. (1993) The microstructural response of quartz and feldspar under shock loading at variable temperatures. J. Geophys Res. 98, 2217 l22197. Jeanloz R. (1980) Shock effects in olivine and implications for Hugoniot data. J. Geophys. Res. 85, 3 I63 -3 176. Kirby S. H. and Christie J. M. (1977) Mechanical twinning in diopside Ca(Mg,Fe)Si206: Structural mechanism and associated crystal defects. Phys. Chem. Minerals 1, 137-163. Lally J. S., Christie J. M., Nord G. L., and Heuer A. H. (1976) Deformation, recovery and recrystallization of lunar dunite 72417. Proc. 7th Lunar. Sci. Co& 1845- 1863. Langenhorst F. (I 989) Experimentally shocked plagioclase: changes of refractive indices and optic axial angle in the lo-30 GPa range. Meteoritics 24, 291. Langenhorst F. and Deutsch A. (1994) Shock experiments on preheated a- and P-quartz. I. Optical and density data. Earth Planet. Sci. L&t. 125, 407-420. Leroux H., Doukhan J. C., and Langenhorst F. (1994) Microstructural defects in experimentally shocked diopside: A TEM characterization Phys. Chem. Min. 20, 521-530. Lindsley D. H. (1983) Pyroxene thermometry. Amer. Mineral. 68, 447-493. Madon M. and Poirier J. P. (1980) Dislocations in spine1 and garnet high-pressure polymorphs of olivine and pyroxene: Implications for mantle rheology. Science 207,66-68. Madon M. and Poirier J. P. (I 983) Transmission electron microscope observation of (Y, p and y (Mg,Fe),SiO, in shocked meteorites: Planar defects and polymorphic transitions. Phys. Earth Planet. Intl. 33, 3 I -44. Mason B. (1973) Hammond Downs, a new chrondrite from the Tenham area, Queensland, Australia. Meteoritics 8, l-7. McSween H. Y., Sears D. W. G., and Dodd R. T. (1988) Thermal metamorphism. In Meteorites and the Early Solar System (ed. J. F. Kerridge and M. S. Matthews), pp. 102- 113. Univ. Arizona Press. Morimoto N. (1988) Nomenclature of pyroxenes. Mineral. Mug. 52, 535-550. Mfiller W. F. (1993) Thermal and deformation history of the Shergotty meteorite deduced from clinopyroxene microstructure. Geochim. Cosmochim. Acta 57,43 I l-4322. Mtiller W. F. and Homemann U. (1969) Shock-induced planar deformation structures in experimentally shock-loaded olivines and in olivines from chondritic meteorites. Earth Planet. Sci. Lett. 7, 251-264.
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Nord G. L. and McGee J. J. (1979) Thermal and mechanical history of granulated norite and pyroxene anorthosite clasts in breccia 73255. Proc. 10th Lunar Planet. Sci. Con$, 817-832. Ostertag R. (1983) Shock experiments on feldspar crystals. J. Geophys. Res. 88, B364-B376. Poirier J. P. (1975) On the slip systems of olivine. L Geophys. Res. 80,4059-4061. Presnell D. C. and Gasparik T. (I 990) Melting of enstatite (MgSiOl) from 10 to 16.5 GPa and the forsterite (Mg,SiO,)-majorite (MgSiO?) eutectic at 16.5 GPa: implications for the origin of the Mantle. J. Geophys. Res. 95, 15771- 15777. Price G. D., Putnis A., and Agrell S. 0. (1979) Electron petrography of shock-produced veins in the Tenham chondrite. Contrib. Mineral. Petrol. 71, 21 l-218. Putnis A. and Price G. D. (1979) High-pressure (Mg,Fe)*Si04 phases in the Tenham chondritic meteorite. Nature 280, 217-218. Reimold W. U. and Stoffler D. (1978) Experimental shock metamorphism of dunite. Proc. 9th Lunar Phnet. Sci. Conf, 28052824. Rubie D. C. and Brearley A. J. (1994) Phase transitions between p and y (Mg,Fe)$i04 in the Earth’s mantle: mechanisms and rheological implications. Science 264, l445- 1448. Sears D. W. G. and Dodd R. T. (1988) Overview and classification of Meteorites. In Meteorites and the Early Solar System (ed. J. F. Kerridge and M. S. Matthews), pp. 3-31. Univ. Arizona Press. Skrotzki W. (1994) Defect structure and deformation mechanisms in naturally deformed augite and enstatite. Tectonophysics 229, 4368. Smith J. R. (1974) Experimental study on the polymorphism of enstatite. Amer. Mineral. 59, 345-352. Snee L. W. and Ahrens T. J. (1975) Shock-induced deformation features in terrestrial peridot and lunar dunite. Proc. 6th Lunar. Sci. Conj, 833-842. Spencer L. J. (1937) The Tenham meteoritic shower. Mineral. Mug. 24,437-452. Stoffler D. (1972) Deformation and transformation of rock-forming minerals by natural and experimental shock processes. I. Behavior of minerals under shock compression. Fortschr. Mineral. 49, 50113. Stoffler D., Ostertag R., Jammes C., and Pfannschmidt G. (1986) Shock metamorphism and petrography of the Shergotty achondrite. Geochim. Cosmochim. Acta 50,889-903. Stoffler D., Bischoff A., Buchwald V., and Rubin A. E. (1988) Shock effects in meteorites. In Meteorites and the Early Solar System (ed. J. F. Kerridge and M. S. Matthews), pp. 165-202. Univ. Arizona Press. Stoffler D., Keil K., and Scott R. D. (1991) Shock metamorphism of ordinary chondrites. Geochim. Cosmochim. Acta 55, 3845-3867. Turner G. (1988) Dating of secondary events. In Meteorites and the Early Solar System (ed. J. F. Kerridge and M. S. Matthews), pp. 276-288. Univ. Arizona Press. van Cappellen E. (I 990) The parameterless correction method in Xray microanalysis. Microsc. Microanal. Microstruct. 1, l-22. van Cappellen E. and Doukhan J. C. (1994) Quantitative X-ray microanalysis of ionic compounds. Ultramicroscopy 53, 343-349. van Schmus W. R. and Ribbe P. H. (1968) The composition and structural state of feldspar from chondritic meteorites. Geochim. Cosmochim. Acta 32, l327- 1342. van Schmus W. R. and Wood J. A. (1967) A chemical-petrological classification for the chondritic meteorites. Geochim. Cosmochim. Acta 31, 747-765.
et Cosmochimica
Pergamon
Acta, Vol. 59, No. 9, pp. 1835- 1845, 1995 Copyright 0 1995 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/95 $9.50 + .OO
0016-7037(95)00086-O
Thermal and shock metamorphism of the Tenham chondrite: A TEM examination FALKO LANCENHORST,* PASCAL JOREAU, Laboratoire de Structure et Propi&% de I’Etat Solide,
59655 Villeneuve (Received
September
and
JEAN CLAUDE DOUKHAN
UniversitC des Sciences d’Ascq Cedex, France
12, 1994; accepted
et Technologies
de Lille,
in revised form January 28, 1995)
Abstract-During the early episode of the solar system, the L6 chondrite Tenham has been affected by intense thermal metamorphism. Microanalytical data reveal homogeneous compositions of olivine (FoTSFaZ5),enstatite (En79Fs19W02), and diopside (En4,Fs8Wo& Using these data, empirical pyroxene thermometers yield temperature estimates for this thermal metamorphism, ranging from 810 to 870°C. Due to the presence of thin shock veins, which contain the high-pressure phases majorite and ringwoodite, the L6 chondrite Tenham is an instructive example for strong shock metamorphism. In contrast to previous transmission electron microscopy (TEM) studies, which concentrated on these shock veins, we also systematically characterized the shock signature of the silicates occurring in the bulk of Tenham. Plagioclase is either pervaded by thin (200 nm), amorphous lamellae, so-called planar deformation features (’‘PDFs”), or it is transformed to maskelynite, a diaplectic glass of feldspar composition. In olivine, shock deformation has caused the formation of irregular and planar fractures and the activation of numerous (2 X lOI m-‘) c dislocations in the glide planes (100) and ( 110); energetically favorable but less mobile a dislocations are totally absent. Fracturing in olivine is interpreted as the cause of dislocation formation. A low dislocation density (
1) Thermal metamorphism is expressed by blurring of chondrule outlines, recrystallization, absence of glass, formation of plagioclase, and homogenization of mineral compositions, leading to a loss of the primary nebular signature. According to van Schmus and Wood (1967), Tenham was consequently classified as a type 6 ordinary chondrite.
2) Indicators for shock metamorphism are pseudotachylitelike shock veins, which randomly pervade Tenham and contain ringwoodite and majorite, the respective y highpressure polymorphs of olivine and pyroxene (Binns et al., 1969; Binns 1970; Price et al., 1979; Putnis and Price, 1979; Madon and Poirier, 1980, 1983). These black veins probably represent quenched melts resulting from localized temperature excursions during shock compression (StGffler et al., 1988). Ringwoodite and majorite are considered either to have crystallized from such high-pressure melts or to have formed by solid-state transformation of mineral grains incorporated in the shock veins (Price et al., 1979; Madon and Poirier, 1983). According to the new and revised shock classification of ordinary chondrites (Stijffler et al., 1991), the presence of shock veins and ringwoodite indicates shock stage S6 with pressures of at least 50 GPa.
* Present address: Institut kunde der Humboldt-Universitl 10115 Berlin, Germany.
Meteorites like Tenham are generally considered as ideal rocks for the recognition and characterization of shock effects, because they have been affected by at least one relatively
The Tenham meteorite fell in 1879 near Tenham Station (25”44’S, 142”57’E) located in South Gregory, Queensland, Australia. It belongs to the L6 group of ordinary chondrites. Chemical and petrological data, and transmission electron microscope (TEM) observations have revealed that the Tenham meteorite has experienced a high degree of (1) thermal and (2) shock metamorphism (Spencer, 1937; Mason, 1973; Binns, 1967; Binns et al., 1969; Putnis and Price, 1979; Madon and Poirier, 1980):
fiir Mineralogie, Museum fiir Naturzu Berlin, InvalidenstraJ3e 43, D-
1835
1836
large-scale
F. Langenhorst, impact,
capable
of ejecting
them
from
P. Joreau.
their parent
In spite of this, it is surprising that only two systematic optical (Dodd and Jarosevich, 1979; Stiiffler et al., 1991) and a few TEM investigations (e.g., Ashworth, 1980a,b) of the shock signature of ordinary chondrites have been performed. Especially more TEM work is required for extending our knowledge about shock metamorphism of ordinary chondrites, because the nature and microstructure of shock effects can often not be fully elucidated with the limited resolution of an optical microscope. Previous TEM investigations on Tenham have only concentrated on the formation of ringwoodite and majorite in shock veins (Putnis and Price, 1979; Price et al., 1979; Madon and Poirier, 1980, 1983). In the present paper, we report on a detailed and systematic TEM investigation of the shock effects in silicate phases (olivine, pyroxenes, and plagioclase) occurring outside the shock veins. The major goal of our study was to characterize the observed shock effects and to infer the shock conditions which generated them. In addition, new microchemical data provide information about the temperatures of early metamorphism, which caused equilibration of mineral compositions. bodies.
EXPERIMENTAL
and J. C. Doukhan
TECHNIQUES
Doubly polished thin sections, 20-pm-thick, were prepared for both optical and TEM investigations. For TEM work, several discs of 3 mm diameter were drilled from one petrographic thin section, glued on copper grids, and finally thinned to perforation by argon ion-beam bombardment at 4.5 kV and an incidence angle of 14”. They were then coated on their bottom face with a thin carbon layer (~30 nm). These thin foils were observed at 300 kV in a Philips CM30 scanning transmission electron microscope (STEM). Microanalyses were carried out either in scanning or nanoprobe mode with a TRACOR energy dispersive spectrometer equipped with a Ge detector. The detector is separated from the TEM column by an ultra-thin window of organic polymer (= 100 nm thick). This attachment pcrmits the detection of light elements (2 2 5). especially of oxygen. As volatile cations are known to be highly mobile under the electron beam, we used a liquid nitrogen cooled Be holder for microanalyses of piagioclase. In this case the beam intensities were furthermore reduced to moderate values (electronic density = 10’ Am-*) by fast scanning over rectangular areas (0.2-0.5 pm2 in size). In order to obtain the precise mineralogical compositions, it is essential (1) to know the exact kws, factors and (2) to perform an absorption correction. (1) The kNs, factors, which represent the sensitivity of the analysing system for the element X relative to the reference element (Si in the present case) for extremely thin samples (no absorption correction), were experimentally determined by the so-called “parameterless correction method (PCM)” (Van Cappellen, 1990). For instance, pure synthetic quartz (SiOZ) was used as a standard for the determination of the ko,s, factor. A series of reference X-ray spectra was recorded under similar conditions as for the samples (same take-off angle and dead times of 25-30%) and with constant beam intensity on variably thick areas of the quartz standard. The net number of counts per unit time ([Si] + [01/t), which increases monotonously with increasing sample thickness, is correlated with the raw (no absorption correction) concentration of oxygen ([O]/[Si] + [O]), as shown in Fig. 1.This correlation is empirically fitted by simple polynomial regression. A first degree function Cy = 63.679 - 0.0575~) or a second degree function (r = 63.851 - 0.0595x - O.OCKlOO533~) yield good fits both with a correlation coefficient R of 0.997. Both functions extrapolated to zero thickness yield practically the same &, factor of 1.04with an uncertainty < 1%. The fits for other elements X are less precise. The uncertainties in the k,,,, factors are estimated to be = 1% for Mg, Fe, and Ca and 2-3% for Na, Al, Mn, Ni, and Cr.
@
70
-
correctvalue
s
0”
60
‘7
50 U
40
5
0
100
J
200
300
400
[Si]+[O]/t (counts/s) FIG. 1. Experimental determination of the b,s, factor. Twenty-two spectra were recorded on a quartz standard in areas of different thicknesses. [0] and [Si] are the net counts for oxygen and silicon, respectively. The raw concentration of oxygen ([O]/[Si] + [0]) plotted against the total net counts per unit time ([Si] + [01/r) defines a regression line, which is fitted by a polynomial function and extrapolated to zero thickness. The ko/si factor is the ratio between the correct (2/3) and the extrapolated value.
(2) The absorption correction requires the precise knowledge of the sample thickness. In crystals with small lattice parameters, the thickness can be. precisely determined by the convergent beam technique. This technique is, however, not adapted toesilicates like olivine and pyroxene with large lattice parameters (>5 A). Alternatively, the thickness can also be estimated by the contamination spot method, but this leads to values with uncertainties of at least 10%. Therefore, we have used a recently described correction procedure (Van Cappollen and Doukhan, 1994). which is simply based on the principle of electroneutrality, i.e., the sum of all cations and anions (oxygen) times their respective valence states must balance. This correction method exploits that (1) the ultra-thin window and the high resolution of the Ge detector allow the detection of light elements and an accurate spectrum deconvoiution of light element K-lines and intermediate element L-lines and that (2) the K-line of oxygen suffers significant absorption and is thus very sensitive to the sample thickness. According to the classical absorption correction, the concentrations of cations and anions (oxygen) were calculated for various thicknesses until electroneutrality is achieved. Finally the quality of analyses was checked by calculating the structural formulae. For instance, the chromite analysis yields a structural formula of (Mgu 24Fe&)(CrI 47A10.21 Fe&Sio.l IT41.0&.00 (Table 1). which agrees well with the general spine1 formula (ABZ04). Most of the analyses presented in Table 1 have been recorded with a200 seconds live time; this corresponds to 30000 to 50000 net counts for the major elements. Taking into account this counting statistics and the uncertainties in the kWs, factors, the precision on the concentrations of major elements is estimated to be 3-5%.
MINERALOGY The investigated thin sections of the Tenham chondrite contain the following minerals (listed according to decreasing abundance): olivine, Ca-poor pyroxene, Ca-rich pyroxene, plagioclase, Fe-Ni alloy, troilite, and subordinate apatite, chromite, ringwoodite, and major&e. With exception of FeNi alloy, troilite, and apatite, all other phases were quantitatively analysed. Results of these measurements are given in Table 1 and Fig. 2. Plagioclase appears as a fine-grained (grain size < 100 ,um) and minor (<5 vol.%) constituent in matrix and chondrules which are still recognizable despite recrystallization. Because of extreme mobility of Na and K cations under the electron beam, even at low temperature in the cold Be holder, the com-
1837
Metamorphism of the Tenham meteorite
Di I-
position of plagioclase could not be determined precisely and is, thus, not listed in Table 1. The WA1 ratios yield, however, rough estimates of anorthite (An) contents, which range from An,r to An,,,. These oligoclase compositions are in relatively good agreement with other data for L6 chondrites (van Schmus and Ribbe, 1968). The scatter in An contents reflects our analytical uncertainties and, hence, cannot be unequivocally ascribed to heterogeneity. Olivine and pyroxenes are more resistant to beam damage and their analyses yield stoichiometric formulae. The mean composition of olivine, the most abundant mineral in Tenham (ca. 40 vol%), is Fo,5.3Fa24.7 (chrysolith); the relatively low standard deviations in Fo and Fa contents (0 = 0.9%) indicate chemical equilibration (Table 1). The chemical composition of pyroxenes is shown in Fig. 2. According to the revised nomenclature scheme for pyroxenes (Morimoto, 1988), the compositions of Ca-rich and Capoor pyroxenes are typical of diopside and enstatite, respectively (cf. Table 1 for average compositions). Like oligoclase, diopside was mainly found in finely recrystallized chondrules with grain sizes < 200 pm, whereas enstatite, the second most abundant mineral (ca. 25 ~01%) in Tenham, occurs as medium- to coarse-grained constituent in chondrules and, subsidiary, in the matrix. Both pyroxenes are homogeneous in composition (cf. a-values in Table 1) and probably equilibrated together with olivine and plagioclase. Some enstatites contain thin (30-80 nm) platelets of chromite crystals oriented in an epitactic relationship parallel to the (100) plane of the host pyroxene (Fig. 3). Microanalyses of chromites reveal that appreciable amounts of Fe*+ in tetrahedral (T) and Cr in
Pv diopside
En
majorite enstatite ,,wQ k \/
\/
20
40
mole % FeSi03
FIG. 2. Composition of Tenham diopsides, enstatites, and majorites plotted in the Mg-rich half of the diopside (Di)-hedenbergite-enstatite (En)-ferrosilite quadrilateral. Average compositions are given in Table 1.
octahedral (0) sites are replaced by Mg (T), and Al, Fe3+, Ti, and Si (0) (see Table 1). In the thin shock vein examined in this TEM study, the high-pressure phases major&e and ringwoodite occur as finegrained (grain sizes < 2-3 pm) polycrystalline aggregates
TABLE 1. Average chemical compositions (in wt. % of elements) and structural formulae of
olivine, pyroxenes, chromite, and the high-pressure phases ringwoodite and majorite based on about 10 analyses for each mineral. Data were obtained by energy-dispersive analysis with an ultra-thin window, which allows the detection of oxygen (see text for details). The Fo- and Facontents of olivine and ringwoodite, as well as the En-, Fs-, and Wo-contents of pyroxenes are given together with standard deviations (0). Olivine Ringwoorlite wt. % Si 17.6 17.8 Ti n.d. n.d. Al 0.45 0.07 Cr n.d. nd. Fe 17.5 17.8 Mn 0.39 0.17 Mg 23.1 22.9 Ca nd. 0.05 Na nd. 0.27 0 41.0 40.9 structural formulae Si 0.98 0.99 Ti nd. n.d. Al 0.03 co.01 Cr n.d. n.d. Fe3+ nd. n.d. FeZ+ 0.49 0.50 Mn 0.01
+
Diopside 25.0 0.33 0.33 0.57 4.0 0.15 9.9 15.7 0.60 43.5
Enstatite 25.8 0.09 0.24 0.05 10.1 0.41 17.7 0.51 0.46 44.7
Majorite 24.3 0.05 2.8 0.40 6.3 0.24 18.6 1.2 0.70 45.4
Chromite 1.5 1.9 2.8 36.8 23.5 nd. 2.9 nd. n.d. 30.7
1.96 0.02 0.03
1.97 co.01 0.02 co.01 0.02 0.37 0.02 1.56 0.03 0.04 5.98 78.8 i0.7
1.82 co.01 0.22 0.02 n.d. 0.24 0.01 1.61 0.07 0.06 5.96
0.11 0.08 0.21 1.47 0.12 0.76 nd. 0.24 n.d. n.d. 4.00
84.3 f1.9
19.6 3~0.7
12.3 ft.5
1.6 iO.6
3.4 M.5
0.02
n.d. 0.16 co.01 0.89 0.87 0.06 6.00 46.7 il.0 a.2 fl.1
45.1 il.4
1838
F. Langenhorst,
P. Joreau,
and J. C. Doukhan
majorite suggest a rapid crystallization from a shock-produced high-pressure melt, which was composed of at least three components, namely enstatite, the chromite inclusions, and a small fraction of a Ca-rich component (plagioclase or diopside). This mixture can, however, not account for the distinct change in the Fe content from 10 wt% in enstatite to 6 wt% in majorite (Table 1). Since we have not detected a second generation of Fe-rich majorites or pyroxenes, it cannot be excluded that the (Fe,Ni)-oxides or -hydroxides surrounding the majorite grains (Fig. 4a) represent, eventually, weathering products of pure Fe, which also crystallized from the shock-produced melt. The absence of such Fe-rich phases around the ringwoodite grains (Fig. 4b), which are of identical composition as equilibrated olivine, is an argument for this hypothesis.
FIG. 3. HRTEM image of a chromite platelet (Chr) occurring as inclusion in orthoenstatite (En). The lattice fringes in orthoenstatite are parallel (100). Fringes in Chromite are absent, because zone axes are inclined to the electron beam.
(Fig. 4). Majorite is generally surrounded by (Fe, Ni)-oxides or -hydroxides (Fig. 4a), as revealed by microanalytical Xray spectra. In contrast to the other silicates, which were analysed at various places of the thin section, analyses of majorite and ringwoodite were carried out on several neighbouring grains, which are only about 5 10 pm away from each other. Even at this small scale the chemical compositions of the high-pressure phases are less homogeneous than those of the equilibrated mineral phases (cf. u-values in Table 1). Apart from this, the composition of majorite differs systematically from that of enstatite (Fig. 2). A distinct enrichment in Al, Cr, Mg, Ca, and Na, and a significant depletion in Si, Mn, and especially in Fe could be detected for majorite (Table 1). It is unlikely to produce such a change in chemistry by a solid-state transition from enstatite to majorite. On the contrary, the different composition and the small grain sizes of
DEFECT STRUCTURES IN SILICATES Plagioclase
(Oligoclase)
In the thin section investigated with TEM, the plagioclase microstructure is dominated by optically resolvable sets of thin, parallel lamellae, which, according to an international agreement, are termed planar deformation features (‘‘PDFs,” Grieve et al., 1990). Similar shock-induced defect structures have been already detected in numerous plagioclase-bearing meteorites, terrestrial impact rocks, and experimentally shocked single-crystal plagioclases (Stoffler, 1972, and references therein; Ostertag, 1983; Stijffler et al., 1986). Up to four crossing sets of PDFs traversing entire Tenham plagioclase grains can be distinguished by optical microscopy (Fig. 5a). Their birefringence seems to be slightly reduced due to the presence of PDFs. TEM observations reveal a PDF thickness of about 0.2 to 0.3 pm with a regular interlamellar spacing of 0.5 pm (Fig. 5b). Diffuse scattering rings in selected area electron diffraction (SAED) patterns and lack of contrast changes in tilt experiments prove the amorphous nature of PDFs. Within the error limits of TEM microanalyses,
FIG. 4. TEM bright field images of minute (grain size < 2-3 pm) high-pressure phases occurring in a thin shock vein. (a) Majorite is defect-free and surrounded by alteration products of Fe-Ni alloy. (b) Ringwoodite shows numerous stacking faults parallel to ( 1101 planes.
Metamorphism
of the Tenham
meteorite
1839
FIG. 5. (a) Photomicrograph of oligoclase displaying several sets of planar deformation features (PDFs); parallel nicols. (b) TEM dark field image of amorphous PDFs in oligoclase seen egde-on. The diffuse scattering in the SAED pattern (inset) reveals the amorphous nature of PDFs. The diffraction spots stem from crystalline areas between the PDFs.
the chemical talline
composition
of PDFs is identical
to that of crys-
areas.
Adjacent to the shock veins and in other thin sections, which have been only optically examined, plagioclases are totally converted to isotropic maskelynite. Price et al. (1979) reported the same trend.
Olivine Optical inspection revealed two shock effects in olivine grains: (1) mosaicism and (2) a consistently high density of planar and, subordinate, of irregular fractures (Fig. 6a). As known from observations on olivine-bearing lunar and terrestrial impact rocks, both effects are characteristic for shock compression of olivine (StGffler, 1972; Snee and Ahrens, 1975; Lally et al., 1976). The presence or absence of mosaicism and the orientation of planar fractures have also been pressure-calibrated through shock experiments (Carter et al., 1968; Miiller and Homemann, 1969; Snee and Ahrens, 1975; Reimold and StGffler, 1978; Bauer, 1979; Jeanloz, 1980) and are included as shock pressure indicators in accepted shock classifications (Dodd and Jarosevich, 1979; StGffler et al., 1991). Mosaicism in olivine from Tenham, which can be recognized as a slightly mottled extinction behavior under crossed nicols, varies from weak to strong. Fractures in Tenham olivines occur as either open fissures or are partially filled with red to brownish coloured material. At the TEM scale, such fractures are also recognizable (Fig. 6b), and microanalytical X-ray spectra recorded on their fillings show only peaks of iron, nickel, and oxygen, indicating that fractures contain (Fe,Ni)-oxides or -hydroxides (alteration products). TEM observations reveal, furthermore, the occurrence of planar and closed fissures (Fig. 6~). Attempts to detect material inside of these features by high resolution transmission electron microscopy (HRTEM) and diffraction techniques failed, however. Therefore, it is assumed that these closed fissures, like the open and filled ones, represent planar fractures. Ashworth and Barber (1975) described similar fea-
tures as “healed cracks.” Standard stereographic analyses carried out on Tenham olivines show that planar fractures are predominantly oriented parallel to the low index planes (loo), (OlO), (OOl), (130), and (1 IO), which belong to pinacoidal and prismatic forms. Bipyramidal orientations, such as (11 l), were only rarely identified (Fig. 6~). Besides planar fractures, the most remarkable effect observed in olivine is the formation of numerous screw dislocations with Burgers vector c (i.e., [OOl]; Fig. 6d). Some of these screw dislocations turn at their tips to edge character enabling one to determine their glide planes. The glide planes are, in agreement with those generally known for olivine (Poirier, 1975), (loo), { 1 lo], and others parallel to (hkO}. The high dislocation density of up to 2 x lOI mm2 determined on our samples confirms previous data of Madon and Poirier (1983). Practically no dislocations with Burgers vector a (i.e., [ 1001) have been detected. Therefore, shock-deformed olivine exhibits a defect microstructure similar to that induced by conventional deformation at moderate temperature. Only c dislocations are activated in (hkO] planes (the so-called pencil glide), even under very high shock stresses. It is furthermore noteworthy that relatively long segments of c edge dislocations have been observed at the tips of planar fractures, as shown in Fig. 6c. With increasing distance to the tips, c dislocations change to screw character which is plausibly attributable to greater velocity of edge components during shock deformation. In addition, the presence of dislocations at the tips of planar fractures suggests that fracturing might be the cause for the formation of dislocations. Diopside By far, the greatest diversity of shock defects was generated in diopside (Figs. 7 and 8). We detected (1) mechanical twinning, (2) a large dislocation density, and (3) PDFs. (1) Mechanical twins were identified by SAED patterns and HRTEM images. They occur parallel to (001) and (100) (Fig. 7a). (100) twins, which are only a few lattice repeats
F. Langenhorst,
1840
P. Joreau,
and J. C. Doukhan
FIG. 6. (a) Photomicrograph of planar and irregular fractures in olivine; crossed nicols. (b) Dark held image of an olivine grain with three fractures and numerous c screw dislocations; g = [004]. The fractures are filled with alteration products of Fe-Ni alloy (hydroxides or oxides). (c) Dark field image of closed, planar fractures in olivine oriented parallel to (111); g = [004]. (d) Dark field image of olivine showing the large density of c screw dislocations. Their glide systems are (lOO)[OOl] and ( 1 lO][OOl]; g = 10041
thinner than (001) twins (70only the (001) twins could be optically recognized by their alternating extinction behavior under crossed nicols (Fig. 7b). In the (OlO)-plane the angle between (001) twin individuals is about 74” (Fig. 7c,d), in good accordance with crystallographic predictions and previous TEM observations (Kirby and Christie, 1977). When twins are inclined to the electron beam, characteristic fringe patterns are detected in two-beam conditions. Because (100) twins are very thin, each fringe pattern in Fig. 7e represents only one twin individual. The fringe patterns of (100) twins are underlined by a number of concentric partial dislocations, whereas such dislocations are totally absent in (001) twin boundaries. This suggests a different formation mechanism for the two types of twins. With decreasing distance to shock veins, the number and thickness of (100) twins increases distinctly. For example, twins adjacent to veins can be one order of magnitude thicker (40 nm) than those occurring several millimeters away from veins. This trend indicates that formation and development of (100) twins is enhanced in the vicinity of shock veins where the shock temperature was higher. The (001) and (100) twinning operations in diopside have been produced in static and dynamic deformation experiments (~5
nm)
wide,
are markedly
260 nm). Therefore,
(Homemann and Mtlller, 1971; Kirby and Christie, 1977; Miiller, 1993; Leroux et al., 1994). In contrast to (100) twins, which have also been observed in tectonically deformed diopside (Skrotzki, 1994) (001) twins are only known from shocked lunar rocks or meteorites (Nord and McGee, 1979; Ashworth, 1980a,b) and, hence, have to be regarded as a diagnostic shock indicator. In static experiments, both twin types develop by gliding of partial dislocations at strain rates of 10. 4 s ‘, which are several orders of magnitude lower than strain rates typical for shock deformation (in the order of IO6 to 10’ s-l). As far as partial dislocations are absent in dynamically produced (001) twins, their formation mechanism could be a rapid kinking process, instead of dislocation gliding. (2) Numerous dislocation bands were predominantly activated in the glide systems (lOO)[OOl] and, to a minor extent, in ( 1 lO][OOl]. The dislocation density varies with the distance to shock veins. Several millimeters away from shock veins, the density is about 1 x 10“’ mm2 (Fig. 7f), but this value can decrease by nearly one order of magnitude in the vicinity of shock veins. As already discussed for the (100) twins, this also suggests a change of the deformation mode and the shock conditions with decreasing distance to shock veins. (3) To our knowledge, we report here on the lirst observation of straight and narrow, amorphous lamellae in naturally
Metamorphism
of the Tenham
meteorite
FIG. 7. (a) TEM bright field image of a diopside grain observed near a shock vein. The diopside grain displays many twins parallel to (001) and (100). (b) Photomicrograph of diopside twinned parallel to (001); crossed nicols. (c) HRTEM image of (001) twins in diopside. The lattice fringes are parallel to (100). (d) SAED pattern recorded on (001) twins; the reciprocal directions [ IOO]: and [ 1CJO]$ indicate the different orientation of twin individuals. (e) Dark field image of thin (100) twins in diopside. Each fringe system represents an inclined (100) twin. Several partial dislocations are visible in the twin boundaries; g = [22i]. (f) Dark field image of dislocation bands in diopside with the glide configuration (lOO)[OOl]. Dislocation bands are deflected by (001) twins.
1841
1842
F. Langenhorst,
P. Joreau,
and J. C. Doukhan
FIG. 8. (a) Dark field image of diopside with two PDF sets parallel to (221) and (221); g = [002]. Intersecting PDFs displace each other (see arrow) and, additionally, are deflected by (001) twins. Inset shows a SAED pattern with diffuse scattering rings, which indicate the amorphous nature of PDFs. (b) Dark field image of (001) twins. dislocation bands, and PDFs in diopside; g = [002]. PDFs are oriented parallel to (221). (c) HRTJZM image of an amorphous PDF parallel to (221). (d) HRTEM image of a (001) twin boundary displaced by an amorphous PDF, which is parallel to (221).
shocked
diopside
(Fig.
8a,b).
Their
amorphous
nature
is evi-
in SAED patterns and HRTEM images (Fig. 8a,c,d). To date, such features were only detected and characterized in experimentally shocked diopside (Leroux et al., 1994). These observations together indicate that the occurrence of amorphous lamellae in diopside is typical of shock damage. Due to their similarity to PDFs in quartz, it is reasonable to use the same term for the amorphous lamellae in diopside. In contrast to diopside experimentally dent
from
diffuse
scattering
rings
shocked at 45 GPa (Leroux et al., 1994), PDFs do not pervade entirely Tenham diopsides and are even absent in some grains. Standard stereographic analysis revealed that_- PDFs are predominantly oriented parallel to the planes (221), (221), and (221), which belong to the two monoclinic prisms (221) and (22i). In experimentally shocked diopside, the predominant orientation of PDFs was found to be (33i), but subordinate the { 22TJ orientation was also detected (Leroux et al., 1994). PDFs in Tenham diopsides appear often in the form of ap-
Metamorphism of the Tenham meteorite parent pairs with variable interlamellar spacings ranging from 0.1 to 0.9 pm (Fig. 8a); their thickness is in the order of 50 nm. When PDFs intersect twins, PDFs are deflected and twins are displaced (Fig. 8a,b,d). On the one hand, this observation indicates the shearing character of PDF formation and, on the other hand, it suggests a simultaneous formation of PDFs and twins. At the intersection of two sets of PDFs, both are displaced (Fig. 8a).
Enstatite At the optical scale, equilibrated enstatite shows intensive, internal fracturing, predominantly parallel to (010) and (001) but other orientations and irregular fractures were also observed. Like in olivine, these fractures are filled with alteration products of iron (Fe-hydroxides and -oxides). Two further defects are visible at the TEM scale: dislocations and clinoenstatite lamellae. Most of the dislocations occur in the glide plane (100). The total dislocation density never exceeds 10” mm’, a value appreciably lower than the dislocation densities observed in neighbouring olivine and diopside grains. Clinoenstatite lamellae parallel to (100) are also present, and this strongly suggests that [OOl](lOO) slip was activated with dislocation splitting. In HRTEM images, clinoenstatite is recognized by its (100) spacing of 9 A, which is exactly half of the spacing in orthoenstatite (18 A). Clinoenstatite lamellae commonly extend over up to twenty lattice repeats, which equals 18 nm (Fig. 9a). SAED pattern of regions with high clinoenstatite concentrations show typical streaking of the diffraction spots along the reciprocal a* direction (Fig. 9b). At least two mechanisms are known to cause the ortho-/ clino-enstatite inversion: shear stresses > 70 MPa on (100) in the [OOl] direction (Coe and Kirby, 1975; J. C. Doukhan, unpub. data) and rapid cooling from the protoenstatite stability field (Smith, 1974). In fact, both mechanisms could be identical, because rapid cooling must induce deviatoric elastic media. The enormous deviatoric stresses in anisotropic
I843
stresses accompanying shock compression certainly overcome the moderate threshold stress of inversion, and it is thus not surprising that clinoenstatite lamellae are detected in orthoenstatite from the strongly shocked Tenham meteorite. Buseek et al. (1980) suggested that the width of clinoenstatite lamellae (more precisely, the number-odd or even-of 9A lattice repeats) and the presence or absence of twinning in clinoenstatite are useful criteria to distinguish shear-induced clinoenstatite lamellae from those generated by cooling from the protoenstatite field. We precisely measured the number of elementary lamellae in Tenham enstatite on HRTEM images and found even multiples of 18 A,, suggesting a mechanical (shock) origin but twinning could not be unequivocally confirmed on SAED patterns. Since all other silicates in Tenham show strong shock damage, it is, however, likely that the ortho-/clino-enstatite inversion is also shock induced. High-Pressure
Phases
Majorite, the high-pressure phase of enstatite, is absolutely defect-free (Fig. 4a), which indicates a formation during the shock event. Ringwoodite, the high-pressure polymorph of olivine, shows stacking faults parallel to (110) planes (Fig. 4b). These stacking faults have been already described in previous studies and either a martensitic solid-state transformation mechanism or nucleation from a high-pressure melt have been suggested for their formation (Putnis and Price, 1979; Madon and Poirier, 1983). The small grain sizes of the equigranular ringwoodite crystallites (< 1 pm) and their slightly heterogeneous composition are more compatible with the latter mechanism. The stacking faults might, therefore, represent growth defects. DISCUSSION Conditions
of Thermal Metamorphism
The absence of water and low pressures McSween et al., 1988) and stresses prevailing
(<2.5 GPa, on chondrite
FIG. 9. (a) HRTEM image of orthoenstatite wit! clinoenstatite lamellae. Lattice fringes are parallel to (100). 9 A lattice repeats correspond to clinoenstatite and 18 A repeats to orthoenstatite. (b) SAED pattern of an area with a high density of clinoenstatite lamellae. Notice the streaking in the [lOO]* direction.
1844
F. Langenhorst,
P. Joreau,
parent bodies have the consequence that pressure-temperature characteristic mineral assemblages, as known for terrestrial metamorphic rocks, are lacking in ordinary chondrites. Therefore, the temperature conditions of metamorphism can only be estimated on the basis of mineral compositions. Our microanalyses have shown that the compositions of the equilibrated mineral phases outside the shock veins (i.e., exclusive ringwoodite and majorite) are homogeneous and unaffected by the impact event. The low minor-element (e.g., of Al, Ti, Na) contents and the absence of exsolutions in pyroxenes indicate that the Tenham chondrite is apparently an appropriate candidate for the application of the empirical two-pyroxene thermometer of Lindsley (1983). According to the special projection method proposed by Lindsley (1983), average metamorphic temperatures of 810°C for enstatite and 870°C for diopside have been graphically determined. Scattering of temperatures calculated for individual analyses is in the order of ?lOO”C. The more recent thermometer of Brey and Kohler (1990), which is based on the Ca partitioning between coexisting orthopyroxenes and clinopyroxenes, yields an average temperature of 820 2 15°C thus better constraining the enstatite temperatures than the method of Lindsley (1983). These temperatures and the mean compositions of equilibrated enstatite (Fs& and olivine (Fa& are consistent with the classification of Tenham as L6 chondrite (Sears and Dodd, 1988; McSween et al., 1988). It is generally accepted that ordinary chondrites have experienced this thermal metamorphism during the early solar period, i.e., around 4.0 to 4.5 Ga ago (Turner, 1988). However, the impact event on Tenham’s parent body occurred distinctly later, because nearly all exposure ages of L6 chondrites are below 50 m.y. (Caffee et al., 1988). Shock Conditions As revealed by our TEM examination, almost all defect structures observed in the silicates of Tenham are related to shock metamorphism. The results of this study substantiate that there are some diagnostic shock defects, such as c screw dislocations in olivine, as well as (100) twins and PDFs in diopside, which are only visible with the TEM. In analogy to quartz (Langenhorst and Deutsch, 1994), the PDF orientations in diopside could eventually be a potential shock barometer, but they are not yet calibrated by TEM studies. Optically resolvable shock effects have already been calibrated through shock recovery experiments on single crystal and polycrystalline olivine, pyroxene, and plagioclase, as well as on rocks composed of these constituents (Carter et al., 1968; Mtiller and Homemann, 1969; Homemann and Mtiller, 1971; Snee and Ahrens, 1975; Reimold and Stoffler, 1978; Bauer, 1979; Ostertag, 1983; Huffman et al., 1993). These experimental data are used as a basis for shock classifications (Dodd and Jarosevich, 1979; Stoffler et al., 1991). Pyroxene is excluded in these classifications because it optically shows no significant changes in shock features over a wide range of pressures. Therefore, we must rely heavily on data for olivine and plagioclase to infer the shock pressure in the bulk of Tenham. In the thin section where high-pressure phases were detected with TEM, plagioclase contains PDF sets of different orientations, and its birefringence is slightly reduced. Accord-
and J. C. Doukhan
ing to Ostertag (1983) and Langenhorst (1989), this shock signature indicates pressures between 25 and 30 GPa. In other thin sections of Tenham, plagioclase is, however, totally transformed to isotropic maskelynite, which suggests pressures up to 45 GPa (Stoffler et al., 1988, 1991). This heterogeneous distribution of shock pressure is also reflected in the shock signature of olivine. Olivine coexisting with PDF-bearing plagioclase shows weak mosaicism, whereas strong mosaicism is typical for olivine occurring together with maskelynite. In summary, the olivine and plagioclase grains in our Tenham specimens record shock pressures varying from shock stage S4 to S5 (Stoffler et al., 1991). The temperature excursions in the melt veins, which contain majorite and ringwoodite, are principally believed to result from localized peak pressures exceeding at least 50 GPa (Stoffler et al., 1988, 1991). Apart from the fact that such pressure excursions are not recorded in our samples, it is hard to imagine why such pressure deviations could be organized in the form of veins. Alternatively, the high concentration of shear-induced (100) twins and PDFs in diopsides, which occur in the vicinity of shock veins, indicates that a heterogeneous distribution of other physical parameters, such as strain and strain rate, could also account for localized melting. Since majorite and ringwoodite are known to form in static experiments at pressures below 20 GPa (Presnall and Gasparik 1990; Rubie and Brearley 1994) their nucleation from a shock-produced melt vein might be already possible at overpressures of about 30 to 40 GPa (i.e., in shock stage S5 or even S4), if shock compression lasts long enough. Acknowledgments-We are gratefully indepted to D. StSffler and C. Lingemann for fruitful discussions and for providing thin sections of the Tenham meteorite. One of us (F.L.) thanks the Minis&e de la Recherche et de la Technologie de France for receiving a research grant. We acknowledge also the financial support of Il%U (Action Sp&ifique Sciences de I’Univers 1994/95). A. Therriault, W. U. Reimold, and L. Keller provided valuable and constructive reviews. Editorial
handling: C. Koeberl
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Metamorphism
of the Tenham
Caffee M. W., Goswami J. N., Hohenberg C. M., Marti K., and Reedy R. C. (1988) Irradiation records in meteorites. In Mereorites and the Early Solar System (ed. J.F. Kerridge and M.S. Matthews), pp. 205-245. Univ. Arizona Press. Carter N. L., Raleigh C. B., and De Carli P. S. (1968) Deformation of olivine in stony meteorites. J. Geophys. Rex 73, 5439-5461. Coe R. S. and Kirby S. H. (1975) The orthoenstatite to clinoenstatite transformation by shearing and reversion by annealing: MechaContrib. Mineral. Petrol. 52,29nism and potential applications. 55. Dodd R. T. and Jarosevich E. (1979)Incipient melting in and shock classification of L-group chondrites. Earth Planet. Sci. Lett. 44, 335-340. Grieve R. A. F., Sharpton V. L., and Stoffler D. (1990) Shocked minerals and the K/T controversy. Eos 71, 1792- 1793. Homemann U. and Mtlller W. F. (1971) Shock-induced deformation twins in clinopyroxene. Neves Jahrb. Mineral. Mh. 6, 247-256. Huffman A. R., Brown J. M., Carter N. L., and Reimold W. U. (1993) The microstructural response of quartz and feldspar under shock loading at variable temperatures. J. Geophys Res. 98, 2217 l22197. Jeanloz R. (1980) Shock effects in olivine and implications for Hugoniot data. J. Geophys. Res. 85, 3 I63 -3 176. Kirby S. H. and Christie J. M. (1977) Mechanical twinning in diopside Ca(Mg,Fe)Si206: Structural mechanism and associated crystal defects. Phys. Chem. Minerals 1, 137-163. Lally J. S., Christie J. M., Nord G. L., and Heuer A. H. (1976) Deformation, recovery and recrystallization of lunar dunite 72417. Proc. 7th Lunar. Sci. Co& 1845- 1863. Langenhorst F. (I 989) Experimentally shocked plagioclase: changes of refractive indices and optic axial angle in the lo-30 GPa range. Meteoritics 24, 291. Langenhorst F. and Deutsch A. (1994) Shock experiments on preheated a- and P-quartz. I. Optical and density data. Earth Planet. Sci. L&t. 125, 407-420. Leroux H., Doukhan J. C., and Langenhorst F. (1994) Microstructural defects in experimentally shocked diopside: A TEM characterization Phys. Chem. Min. 20, 521-530. Lindsley D. H. (1983) Pyroxene thermometry. Amer. Mineral. 68, 447-493. Madon M. and Poirier J. P. (1980) Dislocations in spine1 and garnet high-pressure polymorphs of olivine and pyroxene: Implications for mantle rheology. Science 207,66-68. Madon M. and Poirier J. P. (I 983) Transmission electron microscope observation of (Y, p and y (Mg,Fe),SiO, in shocked meteorites: Planar defects and polymorphic transitions. Phys. Earth Planet. Intl. 33, 3 I -44. Mason B. (1973) Hammond Downs, a new chrondrite from the Tenham area, Queensland, Australia. Meteoritics 8, l-7. McSween H. Y., Sears D. W. G., and Dodd R. T. (1988) Thermal metamorphism. In Meteorites and the Early Solar System (ed. J. F. Kerridge and M. S. Matthews), pp. 102- 113. Univ. Arizona Press. Morimoto N. (1988) Nomenclature of pyroxenes. Mineral. Mug. 52, 535-550. Mfiller W. F. (1993) Thermal and deformation history of the Shergotty meteorite deduced from clinopyroxene microstructure. Geochim. Cosmochim. Acta 57,43 I l-4322. Mtiller W. F. and Homemann U. (1969) Shock-induced planar deformation structures in experimentally shock-loaded olivines and in olivines from chondritic meteorites. Earth Planet. Sci. Lett. 7, 251-264.
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