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Matrix textures in unequilibrated ordinary chondrites
Earth and Planetary Science Letters, 35 (1977) 25-34 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
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[4]
MATRIX TEXTURES IN UNEQUILIBRATED ORDINARY CHONDRITES J.R. ASHWORTH Department o f Geological Sciences, University o f Aston in Birmingham, Gosta Green, Birmingham B4 7ET (Great Britain}
Received November 17, 1976 Revised version received February 8, 1977 High-voltage electron microscope observations are reported for specimens of the meteorites Hedjaz, Parnallee, Chainpur and Weston. Clastic matrix in Weston and Chainpur is distinguished from non-clastic material found in the other two specimens and in dark rims around chondrules in Chainpur. The variety of grain-size distributions and porosities found in this type of matrix is interpreted in terms of grain growth during the aggregation of the meteorite, and incipient solid-state recrystallization of aggregates (metamorphism). The formation of fine-grained non-clastic aggregates with low porosity was not accompanied by sufficient diffusion within the aggregates to equilibrate the mineral assemblages.
1. Introduction On average, about 10% by volume of a highly unequilibrated ordinary chondrite consists of particles whose sizes are < 1 0 0 / a m [1 ]. This material comprises the interstitial matrix between the coarser particles (chondrules, sensu lato). There has been much discussion of the nature of this matrix. One suggestion is that the fine grains were formed by comminution of chondrules, so that the matrix is essentially clastic (e.g. [2]). On the other hand, the matrix has been pictured as a condensate from a solar nebula: in the two-component model, devised to explain compositional features of chondrites, the low-temperature component that carries a full complement of several volatile elements is tentatively identified with matrix
[3]. The formation of equilibrated chondrites has been ascribed to metamorphism [4,5]. It has been proposed that this occurred after accretion of the meteorite parent bodies [5], or by various processes that may have operated during the aggregation of the rock, an episode that may not have been identical with incorporation into a parent body [ 6 - 8 ] . The matrix of some unequilibrated ordinary chondrites
would be expected to display incipient metamorphism, which may help to elucidate the nature of the process. This study of the fine-grained constituents is, in some aspects, intended to be complementary to the thorough account of textural features at particle sizes > 1 0 0 / l m recently presented by Dodd [1 ]. However, the present work is more restricted in scope, being based on observations in only four meteorites. The meteorite names and specimen numbers (British Museum, Natural History) are: Weston (BM 1920, 349), Chainpur (BM 1915, 86), Parnallee (BM 34792), and Hedjaz (BM 1925, 13). The observations were made by high-voltage transmission electron microscopy of material prepared by ion-beam thinning. The electron microscope specimens were taken from thinsections with dimensions ~1 cm, so only small parts of the meteorites were sampled. The range of grain sizes studied is that below about 10/am. The lower limit of the range is set by the virtual absence of particles smaller than ~ 1 0 nm. To avoid confusion with fine-grained material occurring within chondrules, interstitial areas just outside well-defined coarse objects were selected by optical microscopy during specimen preparation.
26 2. Observations Two distinct types of texture were found. These will be ca!led clastic and non-clastic; the former are simpler to interpret and will be described first. 2.1. Clastic textures
The gas-rich chondrite Weston has a clastic matrix, modified by mild shock-lithification [9]. Complex irradiation effects including shielded irradiation of rock fragments [10] suggest that the meteorite was formed in a parent-body regolith. The existence of a relation between grain-size and irradiation effects, similar to that in the lunar soil [11] tends to support this interpretation, as does mineralogical similarity between clastic grains and the minerals of the larger rock fragments [ 12]. The least shock-affected areas of my specimen have a matrix characterized by angular fragments of silicate minerals (Fig. la). These range widely in size, and there is insufficient of the finest grades to fill the interstices between larger grains. Thus the porosity is high. It is not practicable to measure porosity accurately from the micrographs, because of grain overlap in specimens of varying thickness. As a rough measure, it was found that the void areas due to intergranular porosity in the electron micrographs used for the grain-size study described below, with grain thicknesses ranging up to ~1/~m, range from 4 to 9% of the areas of the micrographs. Fragments of fine-grained polycrystalline material and glass tend to be less angular than monocrystalline fragments (cf. [9, fig. 2c1). Weston is regarded as a good example of a clastic matrix texture. The matrix of Chainpur was found to be similar. Chainpur is an interesting meteorite because of anomalies in its chemistry and mineralogy: it has a high content of volatile elements and very heterogeneous mineral compositions, suggesting that it aggregated at low temperature and is unmetamorphosed [13]. Thus it somewhat resembles carbonaceous chondrites. An aspect of its heterogeneity is the presence of material with apparently different cosmic ray exposure histories, suggesting relatively late accumulation under conditions transitional to those in which gas-rich meteorites formed [7]. Cosmic ray tracks were sought but not found in the present elec-
tron microscope study. Thus the matrix grains studied do not contain the high track densities (>107 mm -2) found in some grains in gas-rich meteorites [14]. The matrix of the specimen was found to consist predominantly of angular clasts (Fig. Ib). A few, more regularly shaped, small grains may be growth forms rather than fragments. Porosity is very conspicuous. Void space occupies 6 - 1 5 % of the micrographs used in the grain-size study. The material "festooning" the clasts in Fig. lb is amorphous, as shown by the absence of well-defined rings in electron diffraction patterns. Since the specimen was mounted on Lakeside cement for grinding and polishing, it is possible that some interstitial amorphous material was introduced during specimen preparation. However, the fact that the Chainpur specimen (which was not impregnated with epoxy resin) contains much more of this amorphous cement than Weston (which was impregnated), and the intimate penetration of this material among grains of size ~100 nm in Chainpur, suggest that some of the amorphous material is original. This would tend to ally Chainpur with the C III chondrite Allende, in which interstitial, amorphous or poorly crystalline cement is also suspected [ 15 ;16, p. 49]. However, further work is needed to establish the nature of the "glue", if any, in primitive meteorites. Allende appears to differ from Chainpur in containing a higher proportion of euhedral matrix grains, particularly of olivine [15,16]. 2.2. Dark-rimmed chondrules
The Chainpur specimen contains a second type of fine-grained material, which is non-clastic. This occurs in dark rims around chondrules. The dark colour in thin-section is due to a combination of fine grain-size and presence of opaque minerals. Similar features are also conspicuous in the Hedjaz and Parnallee specimens (Fig. 2), but only in Chainpur is their texture distinct from that of adjacent matrix. The dominant opaque mineral is troilite, which tends to be concentrated in the outermost parts, and to enclose a large number of small silicate grains. Silicates are predominant in the inner part of the rim. Wherever the junction between a fine-grained rim and a core of normal chondrule material could be examined in ion-beam thinned material, either optically or by electron microscopy,
27
Fig. 1. Electron micrographs illustrating textural features. Bright field, 1 MV. (a) Clastic matrix, Weston. (b) Clastic matrix, Chainpur. (c) and (d) Dark-rim material, Chainpur. (e) and (f) Parnallee matrix. The hole to the left of the large olivine grain in (f) was produced during specimen preparation.
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Fig. 2. A dark-rimmed object of irregular shape in Parnallee, as seen during specimen preparation. (a) Reflected light; the troilitic outer rim (white) contrasts with silicates (pale grey). Dark grey areas are due to pitting of the surface. (b) The edge of the object in transmitted light after ion-beam thinning. In the thin area near the ion-produced hole (bottom), fine transparent material is visible between the coarse core of chondrule material (C) and the opaque outer rim (R).
it appeared to be a sharp boundary. These dark-rimmed chondrules have some features in common with objects described from other meteorites. They may be compared with the "dark-zoned chondrules" described by Dodd and Van Schmus [ 17] in Sharps and Hallingeberg, but they lack the features described by those authors as indicating shock derivation from coarser material in situ. Large chondrule grains adjacent to the dark rims are not shock-modified, and grains in the rim show no deformation effects other than those due to late shock events affecting the specimen as a whole, which are seen in Hedjaz in particular [16]. Deposition o f dark rims on undeformed chondrules has been noted in the meteorite
Tieschitz, where the dark material is carbonaceous and is quite distinct from elastic debris [I 8]. It is not yet clear whether this "black matrix" in Tieschitz, or the superficially similar material in the carbonaceous chondrite Murchison [19, fig. 13a] is related to the material described here. In Hedjaz, silicate rim material is coarse enough for the mineralogy to be described. In the example studied, plagioclase is interstitial to granular olivine and pyroxene, as in adjacent matrix outside the rim. The outward changes in mineralogy, through fine silicate material to troilite-rich zones, indicate introduction of material chemically distinct from that of the central chondrule (cf. [17]). In the cases studied here, the chemical reactions do not appear to have been associated with deformation. In the Chainpur specimen, troilite tends to be patchily distributed in the dark rims rather than forming a distinct zone. Troilite also occurs as rims on metal grains, within the dark-rim material and also outside it (cf. [20, pp. 3 0 - 3 1 and fig. A66]). In one case the metal was identified by electron diffraction as kamacite. There are often outer zones or vein-like occurrences of goethite in the troilite. This is tentatively attributed to terrestrial alteration of the troilite (cf. [20, pp. 65--66]), and is distinct from the alteration of silicates found in some other meteorites [21,9]. Whereas the metal and troilite are well crystallized, the goethite forms cryptocrystalline aggregates with grain sizes ~ 100 nm. Intermediate alteration products of troilite were not observed. The dark rims in Chainpur serve as a starting point for the description of non-elastic matrix textures.
2.3. Non-elastic textures
Two examples of dark-rim material in Chainpur were studied in the electron microscope and found to be very similar. They are crystalline and consist mostly of very small, rather equidimensional grains, among which are embedded a few much larger grains having irregular shapes (Fig. 1c, in which the large grain is olivine). Porosity is much lower than in clastic matrix, but intergranular pores are present in some areas (Fig. ld). The matrix in the Parnallee and Hedjaz specimens
29 is also crystalline with low porosity. There is a tendency for a given area to have rather even grain-size, coarser than in the Chainpur dark rims but otherwise comparable in texture (Fig. le). Again, there are some grains of exceptionally large size (Fig. I f). The main minerals are olivine and calcium-bearing clinopyroxene, which occur as equidimensional grains, and interstitial plagioclase. Pyroxene compositions range widely (Fs I 1-38 and Wo2_28 in a small sample [22]). The Hedjaz specimen has generally coarser matrix than was found in Parnallee (cf. [23]). The average grainsize in the Hedjaz material is >1/am, and porosity was observed only in the form of cracks. In the Pamallee specimen, grain-size varies markedly among the matrix areas observed. Some relatively coarse areas have sharp boundaries, but grain-size and porosity were also found to vary gradually between some areas. In places there is intergranular porosity like that of the Chainpur dark rims, but with much coarser grain-sizes prevailing (0.5-2/am). There are usually no indications of deformation in the matrix, or in adjacent coarser grains, other than that due to late mild shock of the specimen as a whole. Some large clasts in Parnallee are heavily deformed, but probably attained this condition prior to incorporation in the meteorite [22].
contrast) were measured. Each sample comprised an area sufficient for approximately 100 grains to be measured. As is the usual practice in geological grain-size analysis, the measurements were converted to units: = -log2X
where X is in mm. They were then classified in intervals of 0.5 ~ units. Fig. 3 shows examples of these number-frequency distributions. Means and standard deviations for all the samples are given in Table 1. The clastic distributions have larger standard deviations, which means that they are less sharply peaked (cf. Fig. 3). The non-clastic distributions are individually more sharply peaked, with standard deviations in the range 0.9-1.3. However, the location Of the peak varies greatly among these samples (cf. Fig. 3). There is marked variation even between neighbouring areas in the same meteorite (Table 1, Parnallee). The peaks of the number-frequency distributions are at finer grain-sizes than would be those of the
20 WE STON I0
2. 4. Grain-size measurements I
Mention has been made of the wide range of grain sizes in a given clastic sample, and of variations between areas of non-clastic matrix. Some quantification of the particle sizes in a particular part of a specimen can be achieved by measuring the images in the photomicrographs. Since only 'the fine-grained part of the size distribution was observed, it is not possible to estimate volume percentage of grain-size, as was done by Dodd [1] through a correction procedure applied to optical data. Thus, the results tabulated here refer to the number of grains observed in certain size intervals. Since grain shape is variable, and often irregular, an arbitrary direction of measurement was used. The measured quantity, X, is the maximum intercept of the particle in the direction parallel to the short side of the photographic plate. In clastic matrix, clasts were measured rather than individual crystals within polycrystalline clasts. All grains whose boundaries were well defined (in good
DARK
20
RIM
IO ¢ oL
~o = u v
#.
to
2OIo
HEDJAZ
,~
,;
,~
,'-
,~--,,
,'o
~
;
Fig. 3. Grain-size histograms for a sample of clastic matrix (Weston) and three samples of non-elastic material.
30 r
TABLE 1 Grain-size characteristics of matrix samples Mean
Mean (um)
Standard deviation
Number of grains
2
11.18 10.89 11.21
0.432 0.520 0.424
1.55 1.70 1.35
100 100 100
Chainpur 14.42 dark rim 14.01 Hedjaz 9.13 Parnallee 11.90 11.52 12.04 10.40 9.93 12.72
0.047 0.061 1.813 0.250 0.343 0.238 0.741 1.046 0.152
1.24 1.32 1.18 1.10 0.93 1.05 0.91 0.92 1.11
100 100 83 104 100 100 80 100 100
Chainpur
I 0
Non-clastic
corresponding weight or volume frequencies, because large grains occupy more volume than do the same number of smaller ones. However, the observed peaks in the non-clastic samples definitely correspond to finer volume-frequency peaks than that of the chondrule population studied by Dodd [1 ]. To illustrate this point, the relative areal importance of the size in the plane of the section may be used as an approximate measure of their volumetric importance. This assumes that the percentage of grains of size X that are intersected by the section is proportional to X. Because large zelative areas are occupied by the relatively few large grains in the samples, it is necessary to pool the data for all the non-clastic distributions (867 grains in total) to reduce tbe effects o f random sampling errors. This can legimately be done, by reducing each distribution to the same mean, because they all have very similar standard deviations. The relative size is expressed as the difference from the mean, A¢, and grain numbers in the same interval of Atp are summed over all the samples. Then a measure of relative area is taken as:
A =x
A/
x
f.
n
where fn is the total grain frequency observed for the nth class, whose midpoint is AOn, and: X, = 2 -ao"
L
A
~
3
Clastic Weston
NON- C
4
/
~LOGNORMAL : ~0'-4'$ C.1'03
X ..I
CLASTIC
4
X
X
X
X
X
A¢ Fig. 4. Distinction between all elastic and all non-elastic observed grain-size distributions, using the estimated relative volumetric proportion, A. A~ is the difference in ~ units between the measured size of a particle and the mean value for the sample to which that particle belongs. The clastic distributions were pooled likewise for comparison (300 grains in total). Fig. 4 shows that the non-elastic distributions tend to peak within the range of observed grain-sizes (<10 tan). This confirms that non-clastic matrix areas represent fine modes superimposed on the coarse elastic distributions discussed by Dodd [1.]
3. Interpretation
3.1. Clastie grain-sizes Dodd [ 1] found that the size distributions of the coarser particles in ordinary chondrites are approximated by Rosin's Law, which can be expressed: Y= 1 - exp [-(x/k) n] where Y is the volume proportion that is finer than
31 size x: the parameter k is a measure of the average size, and n is a measure of the reciprocal of the dispersion of grain-sizes [24]. This leads to a volume-frequency distribution: r' -
d Y - n In 2
d~
- -
-
If
xn
exp [ - ( x / k )
n]
For small x: y' ~
- n In 2
tf
xn
Using A as a measure of Y', Rosin's Law should then give :
log2A = log2C - n • A¢ where the constant C depends on the proportionality between A and Y', as well as on k and n. Thus k is indeterminate but n should appear as the slope of a straight line when log2A is plotted against A¢ (Fig. 4). Martin and Mills [25] isolated chondrules sensu stricto (having almost spherical shapes) from two meteorites, one of these being Chainpur, and found sharply peaked size distributions which do not fit Rosin's Law. They found no such particles smaller than 400/am, a fact which sharply distinguishes the particle population they studied from the elastic distribution whose fine tail is considered here, which may probably be correlated with the rock fragments that predominate volumetrically in many chondrites [ 1]. If the observed elastic matrix is interpreted in terms of Rosin's Law, the distribution is "well-sorted" (n ~ 1.3). Neither the elastic nor the non-elastic set of samples (Fig. 4) could belong to l o g n o r m a l distributions with medians >100/am and o ~ 1 ; thus the observations agree with the indications in the work of Dodd [ 1] that the overall grain-size distributions in unequilibrated ordinary chondrites are strongly fineskewed rather than symmetrical (lognormal). In distributions which were simply lognormal, and had the observed parameters [1, table 2; 25, table 1], grain numbers would fall off much more strongly in the fine range observed in the present study. There would be less matrix and more porosity. Noting the small dispersions of the distributions (small o, large n) compared with other impact-elastic size distributions, Dodd [1] reasonably argues that the particles have physically undergone a sorting process. He favours an aerodynamic mechanism acting
during accretion, which introduces a difficulty: in its simplest form the theory predicts a sharp l o w e r limit to accretable particle-size, which is difficult to reconcile with fine-skewed size distributions. The u p p e r grades display the sharper decline in frequency, i.e., very large chondrules are rare. This is an inherent property of the Rosin's Law distribution, which presumably arises from a comminution process. Any subsequent sorting was not efficient enough to eliminate this feature; the parent body was able to accrete the observed fine matrix. This could perhaps be explained by some mechanism of adhesion of fine grains to the surfaces of accretable larger grains. Interstitial "glue", if present in Chainpur, may have a role in this process. Of course, Chainpur (anomalous) and Weston (gas-rich) may not be the best meteorites in which to seek evidence of the processes by which fine-skewed distributions were preserved in unequilibrated ordinary chondrites as a whole. It is possible that the commonest processes were those to be discussed below. 3.2. N o n - e l a s t i c m a t e r i a l
Grain-sizes as fine as those in the Chainpur dark rims can result from rapid crystallization of a melt or glass. It is possible that the rims formed partly be deposition of melt on to the chondrules (impact splashing?). However, dendritic and acicular grain morphologies, which are common in chondrule mesostasis believed to have originated as a melt or glass, were not seen in the dark rims. This suggests that the silicate portion of the dark rims formed either by exceptionally rapid cooling of a melt, or by a different process not involving a melt. The troiliterimming phenomenon suggests mobility of sulphide at a relatively late stage in the deposition of silicate, either as a troilitic liquid or through reactions involving an ambient gas phase. The texture of the porous fine-grained areas suggests grain growth from the gas or incipient grain growth following aggregation of dust. In any case, addition of material to the chondrule from the environment is indicated. The dark rims, then, are regarded as material accreted by the individual chondrules before their incorporation into the elastic matrix, which lacks effects of grain growth. The anomalously large grains within the dark rims are probably fragments of various chondrule minerals,
32 that were incorporated during this accretion. Material may have been accreted in the form of melt, or dust, or both. Grain growth is more advanced in the matrices of Parnallee and Hedjaz. The coarsest, non-porous products are regarded as low-grade stages of the metamorphic series which has been identified in ordinary chondrites as a whole [5]. This metamorphism was not associated with deformation. Sharply bounded, relatively coarse objects are presumably fragments of previously metamorphosed rocks (cf. [26]). In parts of the Parnallee specimen, grain growth has been relatively slight. Progressive recrystallization has begun to destroy the finest grades of clastic particles, producing integrated textures with more equilibrated grain morphologies, but some areas retain intergranular porosity and some non-porous areas are very finegrained. The low porosity, comparable with that of the Chainpur dark rims rather than clastic matrix, is interesting. Recrystallization of a clastic texture, of the type observed in Chainpur and Weston matrices, would only be accompanied by elimination of porosity if there was compaction of the bulk rock. This is possible during metamorphism within a parent body, where lithostatic pressures might reach ~1 kbar [5, p. 193]. However, it is difficult to see how a clastic matrix with very few fine grains could recrystallize to produce the fine non-clastic distributions. The alternative to metamorphism with compaction is filling of pores by growth of fine grains during aggregation. This could explain the gradational variations in grainsize, the coarsest patches having aggregated first and thus spent longer undergoing solid-state recrystalliza tion (metamorphism). The possibility of addition of fine crystalline material around chondrules is demonstrated by the Chainpur dark rims. The existence of dark troilitic rims in the Parnallee specimen shows that at least some of it went through a similar stage. The simplest interpretation of all the data is to propose that all the Parnallee matrix was incorporated under conditions where grain growth was proceeding, the final stages being represented by areas retaining some intergranular porosity. This interpretation amounts to saying that metamorphism occurred during aggregation. However, the metamorphism under discussion is very slight, and does not noticeable affect the chondrules or homogenize the mineral compositions: the arguments presented here do not neces-
sarily bear on the cause of the much more intense metamorphism inferred in other chondrites [5]. If grain growth during aggregation involved reaction with a gas phase while temperatures were falling, as seems likely from the troilite-rim effect and sporadic presence of porosity, it follows that condensation was also proceeding during aggregation. 3. 3. Bearing on cosmochemical theories
According to the interpretation just advanced, Chainpur aggregated at lower temperature than Parnallee. This accords with its high content of volatile elements. It also agrees with the order of "accretion temperature" inferred by Laul et al. [27] from volatile trace element contents, and the smaller variation of olivine compositions in Parnallee [28]. The estimated accretion temperatures [27] seem low, and the difference between them is small (428 K for Chainpur and 447 K for Parnallee), but these values depend on calculations in which condensation of the elements is modelled as a simple equilibrium process [29]. In this model, "dust continues to equilibrate with the four volatile metals and Fe 2÷. These 5 equilibria are frozen in ... by ... accretion" [30, p. 1323]. This equilibrated dust is elusive in the electron microscope. The clastic assemblages clearly did not equilibrate with their environment. Even in the slightly metamorphosed matrix of Parnallee or Hedjaz, the major element composition of pyroxene varies considerably [22]. The compositional distinction between analyzed samples of chondrule and matrix pyroxene suggests different environments of formation [22], and the interpretation presented above bears on the nature of the environment of matrix formation. Matrix pyroxene was not produced solely by fragmentation of chondrules, and it seems that pyroxene compositions were partly controlled by grain growth from a gas; but reactions among grains before and during aggregation were insufficient to homogenize the mineral assemblages between adjacent matrix areas. The simplest explanation is that conditions varied during grain growth and aggregation, and that grain compositions representing these different conditions were retained. This interpretation strongly resembles the gradual aggregation of "chunks" in the qualitative picture presented by Blander and Abdel-Gawad [6, pp. 712-713], though the metamorphism observed
33 in the present study is relatively slight, and the observations under discussion do not bear on the contention [6] that the reactions controlling major element condensation were metastable. Formation of "chunks" would overcome the difficulty of accreting small grains in the aerodynamic process of Dodd [1]; the unaggregated portion of the small grains would be lost, but those trapped in "chunks" could be accreted. Condensation under widely variable conditions would be consistent with a model in which condensation follows volatilization by impacts (cf. [26]). Insofar as clastic material retains its identity, condensation of volatile elements is presumably restricted to a fraction of the matrix volume. In these unequilibrated aggregates, simple partition behaviour of trace elements is unlikely. Small grainsize favours surface reactions [6]; trace elements may be trapped at energetically favourable defect sites over a range of temperatures higher than those for equilibrium condensation in perfect phases. There is some direct evidence for complex behaviour of a volatile trace element in the meteorites [31 ]. Thus, the two-component concept and the equilibrium calculations of Anders and colleagues, valuable as they are in modelling the histories of the chondrites, must be regarded as simplified approximations. Unequilibrated ordinary chondrites contain records of complicated histories, exemplified by the occurrence of two very different types of fine-grained material in Chainpur. There remains ample scope for the extraction of detailed information by further study of their chemistry.
4. Conclusions Clastic matrix, with relatively high porosity between angular fragments of widely ranging size, is distinct from the fine-grained consituents of the specimens of Hedjaz and Parnallee, and the Chainpur dark rims. Observed size distributions in samples of clastic matrix in Chainpur and Weston are consistent with a Rosin's Law distribution of the type found by Dodd [1 ] among coarser particles. Any sorting process which favoured incorporation of large particles was not efficient enough to remove the fine tail of the distribution.
The non-clastic samples show effects of grain growth. Prior to aggregation of Chainpur, very fine-grained material formed around some chondrules, producing dark rims. The coarsest of the non-clastic matrix observed is in Hedjaz, and is attributed to solid-state recrystallization (metamorphism). The main minerals at grain sizes less than about 10/am are the common silicates of ordinary chondrites: olivine, pyroxene, and plagioclase. Grain morphologies are simple. Similar areas with almost no porosity in Parnallee may sometimes constitute small rock fragments, but others grade into finer matrix. Recrystallization in the matrix of the Parnallee and Hedjaz specimens was not accompanied by diffusive equilibration over distances sufficient to produce uniform composition of matrix pyroxene grains. Crystalline matrix with intermediate grain-size and low porosity was observed in Parnallee. Its fine-grained nature suggests that this formed by addition of fine material to clastic grains during aggregation, rather than by metamorphism and compaction of clastic matrix. It is suggested that this matrix represents a more advanced stage of the processes that produce dark rims. In this interpretation, growth of fine grains at the expense of pore space, and incipient metamorphic recrystallization, are held to have been contemporaneous with aggregation.
Acknowledgements The observations were made while I was a NERC research fellow at the University of Essex, where I had the constant benefit of advice from Dr. D.J. Barber. I thank Dr. P.R. Swann (Imperial College, London) for the high-voltage electron microscope facilities, and Dr. R. Hutchison (British Museum, Natural History) for the specimens and for helpful criticism.
References 1 R.T. Dodd, Accretion of the ordinary chondrites, Earth Planet. Sci. Lett. 30 (1967) 281-291. 2 A.M. Reid and K. Fredriksson, Chondrules and chondntes, in: Researches in Geochemistry, 2, P.H. Abelson, ed. (Wiley, New York, N.Y., 1967) 170-203.
34 3 J.W. Larimer and E. Anders, Chemical fractionations in meteorites, II. Abundance patterns and their interpretation, Geochim. Cosmochim. Aeta 31 (1967) 1239-1270. 4 J.A. Wood, Metamorphism in chondrites, Geochim. Cosmochim. Acta 26 (1962) 7 3 9 - 7 4 9 . 5 R.T. Dodd, Metamorphism of the ordinary ehondrites: a review, Geochim. Cosmochim. Acta 33 (1969) 161-203. 6 M. Blander and M. Abdel-Gawad, The origin of meteorites and the constrained equilibrium theory, Geochim. Cosmochim. Acta 33 (1969) 7 0 1 - 7 1 6 . 7 P. Pellas, Irradiation history of grain aggregates in ordinary chondrites. Possible clues to the advanced stages of accretion, in: From Plasma to Planet, A. Elvius, ed. (Almqvist and Wiksells, Stockholm, 1972) 6 5 - 9 2 . 8 J.T. Wasson, Formation of ordinary chondrites, Rev. Geophys. Space Phys. 10 (1972) 7 1 1 - 7 5 9 . 9 J.R. Ashworth and D.J. Barber, Lithification of gas-rich meteorites, Earth Planet Sci. Lett. 30 (1976) 2 2 2 - 2 3 3 . 10 L. Sehultz, P. Signer, J.C. Lorin and P. Pellas, Complex irradiation history of the Weston chondrite, Earth Planet. Sci. Lett. 15 (1972) 4 0 3 - 4 1 0 . 11 L. Schultz, U. Frick and P. Signer, Noble gases in grain size fractions of Weston: a comparison with lunar soil data, Meteoritics 10 (1975) 483. 12 A.F. Noonan and J.A. Nelen, A petrographic and mineral chemistry study of the Weston, Connecticut, chondrite, Meteoritics 11 (1976) 111-130. 13 R.A. Schmitt, R.H. Smith and G.C. Goles, Chainpur-like chondrites: primitive precursors of ordinary chondrites?, Science 153 (1966) 6 4 4 - 6 4 7 . 14 D.J. Barber and P.B. Price, Solar flare particle tracks in lunar and meteoritic materials, Proc. 25th Anniv. Meet. EMAG Inst. Phys. (1971) 2 7 6 - 2 7 9 . 15 H.W. Green II, S.V. Radcliffe and A.H. Heuer, Allende meteorite: a high-voltage electron petrographic study, Science 172 (1971) 9 3 6 - 9 3 9 . 16 J.R. Ashworth and D.J. Barber, Electron petrography of shock-deformed olivine in stony meteorites, Earth Planet. Sci. Lett. 27 (1975) 4 3 - 5 0 . 17 R.T. Dodd and W.R. van Schmus, Dark-zoned chondrules, Chem. Erde 30 (1971) 5 9 - 6 9 .
18 M. Christophe Michel-L6vy, La matrice noire et blanche de la chondrite de Tieschitz (H3), Earth Planet. Sci. Lett. 30 (1976) 143-150. 19 L.H. Fuchs, E. Olsen and K.J. Jensen, Mineralogy, mineralchemistry and composition of the Murchison (C2) meteorite, Smithson. Contrib. Earth Sci. 10 (1973). 20 P. Ramdohr, The Opaque Minerals in Stony Meteorites (Elsevier, Amsterdam, 1973). 21 J.R. Ashworth and R. Hutchison, Water in non-carbonaceous stony meteorites, Nature 256 (1975) 714-715. 22 J.R. Ashworth and D.J. Barber, Electron microscopy of some stony meteorites, Philos. Trans. R. Soc. Lond. (in press). 23 J.R. Ashworth and D.J. Barber, Stony meteorites, in: Electron Microscopy in Mineralogy, H.-R. Wenk, ed. (Springer-Verlag, Berlin, 1976) 543-549. 24 W.C. Krumbein and F.W. Tisdel, Size distribution of source rocks of sediments, Am. J. Sci. 238 (1940) 296-305. 25 P.M. Martin and A.A. Mills, Size and shape of chondrules in the Bjurbfle and Chainpur meteorites, Earth Planet. Sei. Lett. 33 (1976) 239-248. 26 R.T. Dodd, The petrology of chondrules in the Sharps meteorite, Contrib. Mineral. Petrol. 31 (1971) 201-227. 27 J.C. Laul, R. Ganapathy, E. Anders and J.W. Morgan, Chemical fractionations in meteorites, VI. Accretion temperatures of H-, LL-, and E-chondrites from abundances of volatile trace elements, Geochim. Cosmochim. Acta 37 (1973) 329-357. 28 R.T. Dodd, W.R. van Schmus and D.M. Koffman, A survey of the unequilibrated ordinary chondrites, Geochim. Cosmochim. Acta 31 (1967) 921-951. 29 J.W. Larimer, Chemical fractionations in meteorites, VII. Cosmothermometry and cosmobarometry, Geochim. Cosmochim. Acta 37 (1973) 1603-1623. 30 E. Anders and J.W. Larimer, Validity of trace element cosmothermometer, Geochim. Cosmochim. Aeta 39 (1975) 1320-1324. 31 S. Jovanovic and G.W. Reed Jr., Pre- or post-accretional distribution of trace elements?, Meteoritics 8 (1973) 392-393.
25
[4]
MATRIX TEXTURES IN UNEQUILIBRATED ORDINARY CHONDRITES J.R. ASHWORTH Department o f Geological Sciences, University o f Aston in Birmingham, Gosta Green, Birmingham B4 7ET (Great Britain}
Received November 17, 1976 Revised version received February 8, 1977 High-voltage electron microscope observations are reported for specimens of the meteorites Hedjaz, Parnallee, Chainpur and Weston. Clastic matrix in Weston and Chainpur is distinguished from non-clastic material found in the other two specimens and in dark rims around chondrules in Chainpur. The variety of grain-size distributions and porosities found in this type of matrix is interpreted in terms of grain growth during the aggregation of the meteorite, and incipient solid-state recrystallization of aggregates (metamorphism). The formation of fine-grained non-clastic aggregates with low porosity was not accompanied by sufficient diffusion within the aggregates to equilibrate the mineral assemblages.
1. Introduction On average, about 10% by volume of a highly unequilibrated ordinary chondrite consists of particles whose sizes are < 1 0 0 / a m [1 ]. This material comprises the interstitial matrix between the coarser particles (chondrules, sensu lato). There has been much discussion of the nature of this matrix. One suggestion is that the fine grains were formed by comminution of chondrules, so that the matrix is essentially clastic (e.g. [2]). On the other hand, the matrix has been pictured as a condensate from a solar nebula: in the two-component model, devised to explain compositional features of chondrites, the low-temperature component that carries a full complement of several volatile elements is tentatively identified with matrix
[3]. The formation of equilibrated chondrites has been ascribed to metamorphism [4,5]. It has been proposed that this occurred after accretion of the meteorite parent bodies [5], or by various processes that may have operated during the aggregation of the rock, an episode that may not have been identical with incorporation into a parent body [ 6 - 8 ] . The matrix of some unequilibrated ordinary chondrites
would be expected to display incipient metamorphism, which may help to elucidate the nature of the process. This study of the fine-grained constituents is, in some aspects, intended to be complementary to the thorough account of textural features at particle sizes > 1 0 0 / l m recently presented by Dodd [1 ]. However, the present work is more restricted in scope, being based on observations in only four meteorites. The meteorite names and specimen numbers (British Museum, Natural History) are: Weston (BM 1920, 349), Chainpur (BM 1915, 86), Parnallee (BM 34792), and Hedjaz (BM 1925, 13). The observations were made by high-voltage transmission electron microscopy of material prepared by ion-beam thinning. The electron microscope specimens were taken from thinsections with dimensions ~1 cm, so only small parts of the meteorites were sampled. The range of grain sizes studied is that below about 10/am. The lower limit of the range is set by the virtual absence of particles smaller than ~ 1 0 nm. To avoid confusion with fine-grained material occurring within chondrules, interstitial areas just outside well-defined coarse objects were selected by optical microscopy during specimen preparation.
26 2. Observations Two distinct types of texture were found. These will be ca!led clastic and non-clastic; the former are simpler to interpret and will be described first. 2.1. Clastic textures
The gas-rich chondrite Weston has a clastic matrix, modified by mild shock-lithification [9]. Complex irradiation effects including shielded irradiation of rock fragments [10] suggest that the meteorite was formed in a parent-body regolith. The existence of a relation between grain-size and irradiation effects, similar to that in the lunar soil [11] tends to support this interpretation, as does mineralogical similarity between clastic grains and the minerals of the larger rock fragments [ 12]. The least shock-affected areas of my specimen have a matrix characterized by angular fragments of silicate minerals (Fig. la). These range widely in size, and there is insufficient of the finest grades to fill the interstices between larger grains. Thus the porosity is high. It is not practicable to measure porosity accurately from the micrographs, because of grain overlap in specimens of varying thickness. As a rough measure, it was found that the void areas due to intergranular porosity in the electron micrographs used for the grain-size study described below, with grain thicknesses ranging up to ~1/~m, range from 4 to 9% of the areas of the micrographs. Fragments of fine-grained polycrystalline material and glass tend to be less angular than monocrystalline fragments (cf. [9, fig. 2c1). Weston is regarded as a good example of a clastic matrix texture. The matrix of Chainpur was found to be similar. Chainpur is an interesting meteorite because of anomalies in its chemistry and mineralogy: it has a high content of volatile elements and very heterogeneous mineral compositions, suggesting that it aggregated at low temperature and is unmetamorphosed [13]. Thus it somewhat resembles carbonaceous chondrites. An aspect of its heterogeneity is the presence of material with apparently different cosmic ray exposure histories, suggesting relatively late accumulation under conditions transitional to those in which gas-rich meteorites formed [7]. Cosmic ray tracks were sought but not found in the present elec-
tron microscope study. Thus the matrix grains studied do not contain the high track densities (>107 mm -2) found in some grains in gas-rich meteorites [14]. The matrix of the specimen was found to consist predominantly of angular clasts (Fig. Ib). A few, more regularly shaped, small grains may be growth forms rather than fragments. Porosity is very conspicuous. Void space occupies 6 - 1 5 % of the micrographs used in the grain-size study. The material "festooning" the clasts in Fig. lb is amorphous, as shown by the absence of well-defined rings in electron diffraction patterns. Since the specimen was mounted on Lakeside cement for grinding and polishing, it is possible that some interstitial amorphous material was introduced during specimen preparation. However, the fact that the Chainpur specimen (which was not impregnated with epoxy resin) contains much more of this amorphous cement than Weston (which was impregnated), and the intimate penetration of this material among grains of size ~100 nm in Chainpur, suggest that some of the amorphous material is original. This would tend to ally Chainpur with the C III chondrite Allende, in which interstitial, amorphous or poorly crystalline cement is also suspected [ 15 ;16, p. 49]. However, further work is needed to establish the nature of the "glue", if any, in primitive meteorites. Allende appears to differ from Chainpur in containing a higher proportion of euhedral matrix grains, particularly of olivine [15,16]. 2.2. Dark-rimmed chondrules
The Chainpur specimen contains a second type of fine-grained material, which is non-clastic. This occurs in dark rims around chondrules. The dark colour in thin-section is due to a combination of fine grain-size and presence of opaque minerals. Similar features are also conspicuous in the Hedjaz and Parnallee specimens (Fig. 2), but only in Chainpur is their texture distinct from that of adjacent matrix. The dominant opaque mineral is troilite, which tends to be concentrated in the outermost parts, and to enclose a large number of small silicate grains. Silicates are predominant in the inner part of the rim. Wherever the junction between a fine-grained rim and a core of normal chondrule material could be examined in ion-beam thinned material, either optically or by electron microscopy,
27
Fig. 1. Electron micrographs illustrating textural features. Bright field, 1 MV. (a) Clastic matrix, Weston. (b) Clastic matrix, Chainpur. (c) and (d) Dark-rim material, Chainpur. (e) and (f) Parnallee matrix. The hole to the left of the large olivine grain in (f) was produced during specimen preparation.
28
Fig. 2. A dark-rimmed object of irregular shape in Parnallee, as seen during specimen preparation. (a) Reflected light; the troilitic outer rim (white) contrasts with silicates (pale grey). Dark grey areas are due to pitting of the surface. (b) The edge of the object in transmitted light after ion-beam thinning. In the thin area near the ion-produced hole (bottom), fine transparent material is visible between the coarse core of chondrule material (C) and the opaque outer rim (R).
it appeared to be a sharp boundary. These dark-rimmed chondrules have some features in common with objects described from other meteorites. They may be compared with the "dark-zoned chondrules" described by Dodd and Van Schmus [ 17] in Sharps and Hallingeberg, but they lack the features described by those authors as indicating shock derivation from coarser material in situ. Large chondrule grains adjacent to the dark rims are not shock-modified, and grains in the rim show no deformation effects other than those due to late shock events affecting the specimen as a whole, which are seen in Hedjaz in particular [16]. Deposition o f dark rims on undeformed chondrules has been noted in the meteorite
Tieschitz, where the dark material is carbonaceous and is quite distinct from elastic debris [I 8]. It is not yet clear whether this "black matrix" in Tieschitz, or the superficially similar material in the carbonaceous chondrite Murchison [19, fig. 13a] is related to the material described here. In Hedjaz, silicate rim material is coarse enough for the mineralogy to be described. In the example studied, plagioclase is interstitial to granular olivine and pyroxene, as in adjacent matrix outside the rim. The outward changes in mineralogy, through fine silicate material to troilite-rich zones, indicate introduction of material chemically distinct from that of the central chondrule (cf. [17]). In the cases studied here, the chemical reactions do not appear to have been associated with deformation. In the Chainpur specimen, troilite tends to be patchily distributed in the dark rims rather than forming a distinct zone. Troilite also occurs as rims on metal grains, within the dark-rim material and also outside it (cf. [20, pp. 3 0 - 3 1 and fig. A66]). In one case the metal was identified by electron diffraction as kamacite. There are often outer zones or vein-like occurrences of goethite in the troilite. This is tentatively attributed to terrestrial alteration of the troilite (cf. [20, pp. 65--66]), and is distinct from the alteration of silicates found in some other meteorites [21,9]. Whereas the metal and troilite are well crystallized, the goethite forms cryptocrystalline aggregates with grain sizes ~ 100 nm. Intermediate alteration products of troilite were not observed. The dark rims in Chainpur serve as a starting point for the description of non-elastic matrix textures.
2.3. Non-elastic textures
Two examples of dark-rim material in Chainpur were studied in the electron microscope and found to be very similar. They are crystalline and consist mostly of very small, rather equidimensional grains, among which are embedded a few much larger grains having irregular shapes (Fig. 1c, in which the large grain is olivine). Porosity is much lower than in clastic matrix, but intergranular pores are present in some areas (Fig. ld). The matrix in the Parnallee and Hedjaz specimens
29 is also crystalline with low porosity. There is a tendency for a given area to have rather even grain-size, coarser than in the Chainpur dark rims but otherwise comparable in texture (Fig. le). Again, there are some grains of exceptionally large size (Fig. I f). The main minerals are olivine and calcium-bearing clinopyroxene, which occur as equidimensional grains, and interstitial plagioclase. Pyroxene compositions range widely (Fs I 1-38 and Wo2_28 in a small sample [22]). The Hedjaz specimen has generally coarser matrix than was found in Parnallee (cf. [23]). The average grainsize in the Hedjaz material is >1/am, and porosity was observed only in the form of cracks. In the Pamallee specimen, grain-size varies markedly among the matrix areas observed. Some relatively coarse areas have sharp boundaries, but grain-size and porosity were also found to vary gradually between some areas. In places there is intergranular porosity like that of the Chainpur dark rims, but with much coarser grain-sizes prevailing (0.5-2/am). There are usually no indications of deformation in the matrix, or in adjacent coarser grains, other than that due to late mild shock of the specimen as a whole. Some large clasts in Parnallee are heavily deformed, but probably attained this condition prior to incorporation in the meteorite [22].
contrast) were measured. Each sample comprised an area sufficient for approximately 100 grains to be measured. As is the usual practice in geological grain-size analysis, the measurements were converted to units: = -log2X
where X is in mm. They were then classified in intervals of 0.5 ~ units. Fig. 3 shows examples of these number-frequency distributions. Means and standard deviations for all the samples are given in Table 1. The clastic distributions have larger standard deviations, which means that they are less sharply peaked (cf. Fig. 3). The non-clastic distributions are individually more sharply peaked, with standard deviations in the range 0.9-1.3. However, the location Of the peak varies greatly among these samples (cf. Fig. 3). There is marked variation even between neighbouring areas in the same meteorite (Table 1, Parnallee). The peaks of the number-frequency distributions are at finer grain-sizes than would be those of the
20 WE STON I0
2. 4. Grain-size measurements I
Mention has been made of the wide range of grain sizes in a given clastic sample, and of variations between areas of non-clastic matrix. Some quantification of the particle sizes in a particular part of a specimen can be achieved by measuring the images in the photomicrographs. Since only 'the fine-grained part of the size distribution was observed, it is not possible to estimate volume percentage of grain-size, as was done by Dodd [1] through a correction procedure applied to optical data. Thus, the results tabulated here refer to the number of grains observed in certain size intervals. Since grain shape is variable, and often irregular, an arbitrary direction of measurement was used. The measured quantity, X, is the maximum intercept of the particle in the direction parallel to the short side of the photographic plate. In clastic matrix, clasts were measured rather than individual crystals within polycrystalline clasts. All grains whose boundaries were well defined (in good
DARK
20
RIM
IO ¢ oL
~o = u v
#.
to
2OIo
HEDJAZ
,~
,;
,~
,'-
,~--,,
,'o
~
;
Fig. 3. Grain-size histograms for a sample of clastic matrix (Weston) and three samples of non-elastic material.
30 r
TABLE 1 Grain-size characteristics of matrix samples Mean
Mean (um)
Standard deviation
Number of grains
2
11.18 10.89 11.21
0.432 0.520 0.424
1.55 1.70 1.35
100 100 100
Chainpur 14.42 dark rim 14.01 Hedjaz 9.13 Parnallee 11.90 11.52 12.04 10.40 9.93 12.72
0.047 0.061 1.813 0.250 0.343 0.238 0.741 1.046 0.152
1.24 1.32 1.18 1.10 0.93 1.05 0.91 0.92 1.11
100 100 83 104 100 100 80 100 100
Chainpur
I 0
Non-clastic
corresponding weight or volume frequencies, because large grains occupy more volume than do the same number of smaller ones. However, the observed peaks in the non-clastic samples definitely correspond to finer volume-frequency peaks than that of the chondrule population studied by Dodd [1 ]. To illustrate this point, the relative areal importance of the size in the plane of the section may be used as an approximate measure of their volumetric importance. This assumes that the percentage of grains of size X that are intersected by the section is proportional to X. Because large zelative areas are occupied by the relatively few large grains in the samples, it is necessary to pool the data for all the non-clastic distributions (867 grains in total) to reduce tbe effects o f random sampling errors. This can legimately be done, by reducing each distribution to the same mean, because they all have very similar standard deviations. The relative size is expressed as the difference from the mean, A¢, and grain numbers in the same interval of Atp are summed over all the samples. Then a measure of relative area is taken as:
A =x
A/
x
f.
n
where fn is the total grain frequency observed for the nth class, whose midpoint is AOn, and: X, = 2 -ao"
L
A
~
3
Clastic Weston
NON- C
4
/
~LOGNORMAL : ~0'-4'$ C.1'03
X ..I
CLASTIC
4
X
X
X
X
X
A¢ Fig. 4. Distinction between all elastic and all non-elastic observed grain-size distributions, using the estimated relative volumetric proportion, A. A~ is the difference in ~ units between the measured size of a particle and the mean value for the sample to which that particle belongs. The clastic distributions were pooled likewise for comparison (300 grains in total). Fig. 4 shows that the non-elastic distributions tend to peak within the range of observed grain-sizes (<10 tan). This confirms that non-clastic matrix areas represent fine modes superimposed on the coarse elastic distributions discussed by Dodd [1.]
3. Interpretation
3.1. Clastie grain-sizes Dodd [ 1] found that the size distributions of the coarser particles in ordinary chondrites are approximated by Rosin's Law, which can be expressed: Y= 1 - exp [-(x/k) n] where Y is the volume proportion that is finer than
31 size x: the parameter k is a measure of the average size, and n is a measure of the reciprocal of the dispersion of grain-sizes [24]. This leads to a volume-frequency distribution: r' -
d Y - n In 2
d~
- -
-
If
xn
exp [ - ( x / k )
n]
For small x: y' ~
- n In 2
tf
xn
Using A as a measure of Y', Rosin's Law should then give :
log2A = log2C - n • A¢ where the constant C depends on the proportionality between A and Y', as well as on k and n. Thus k is indeterminate but n should appear as the slope of a straight line when log2A is plotted against A¢ (Fig. 4). Martin and Mills [25] isolated chondrules sensu stricto (having almost spherical shapes) from two meteorites, one of these being Chainpur, and found sharply peaked size distributions which do not fit Rosin's Law. They found no such particles smaller than 400/am, a fact which sharply distinguishes the particle population they studied from the elastic distribution whose fine tail is considered here, which may probably be correlated with the rock fragments that predominate volumetrically in many chondrites [ 1]. If the observed elastic matrix is interpreted in terms of Rosin's Law, the distribution is "well-sorted" (n ~ 1.3). Neither the elastic nor the non-elastic set of samples (Fig. 4) could belong to l o g n o r m a l distributions with medians >100/am and o ~ 1 ; thus the observations agree with the indications in the work of Dodd [ 1] that the overall grain-size distributions in unequilibrated ordinary chondrites are strongly fineskewed rather than symmetrical (lognormal). In distributions which were simply lognormal, and had the observed parameters [1, table 2; 25, table 1], grain numbers would fall off much more strongly in the fine range observed in the present study. There would be less matrix and more porosity. Noting the small dispersions of the distributions (small o, large n) compared with other impact-elastic size distributions, Dodd [1] reasonably argues that the particles have physically undergone a sorting process. He favours an aerodynamic mechanism acting
during accretion, which introduces a difficulty: in its simplest form the theory predicts a sharp l o w e r limit to accretable particle-size, which is difficult to reconcile with fine-skewed size distributions. The u p p e r grades display the sharper decline in frequency, i.e., very large chondrules are rare. This is an inherent property of the Rosin's Law distribution, which presumably arises from a comminution process. Any subsequent sorting was not efficient enough to eliminate this feature; the parent body was able to accrete the observed fine matrix. This could perhaps be explained by some mechanism of adhesion of fine grains to the surfaces of accretable larger grains. Interstitial "glue", if present in Chainpur, may have a role in this process. Of course, Chainpur (anomalous) and Weston (gas-rich) may not be the best meteorites in which to seek evidence of the processes by which fine-skewed distributions were preserved in unequilibrated ordinary chondrites as a whole. It is possible that the commonest processes were those to be discussed below. 3.2. N o n - e l a s t i c m a t e r i a l
Grain-sizes as fine as those in the Chainpur dark rims can result from rapid crystallization of a melt or glass. It is possible that the rims formed partly be deposition of melt on to the chondrules (impact splashing?). However, dendritic and acicular grain morphologies, which are common in chondrule mesostasis believed to have originated as a melt or glass, were not seen in the dark rims. This suggests that the silicate portion of the dark rims formed either by exceptionally rapid cooling of a melt, or by a different process not involving a melt. The troiliterimming phenomenon suggests mobility of sulphide at a relatively late stage in the deposition of silicate, either as a troilitic liquid or through reactions involving an ambient gas phase. The texture of the porous fine-grained areas suggests grain growth from the gas or incipient grain growth following aggregation of dust. In any case, addition of material to the chondrule from the environment is indicated. The dark rims, then, are regarded as material accreted by the individual chondrules before their incorporation into the elastic matrix, which lacks effects of grain growth. The anomalously large grains within the dark rims are probably fragments of various chondrule minerals,
32 that were incorporated during this accretion. Material may have been accreted in the form of melt, or dust, or both. Grain growth is more advanced in the matrices of Parnallee and Hedjaz. The coarsest, non-porous products are regarded as low-grade stages of the metamorphic series which has been identified in ordinary chondrites as a whole [5]. This metamorphism was not associated with deformation. Sharply bounded, relatively coarse objects are presumably fragments of previously metamorphosed rocks (cf. [26]). In parts of the Parnallee specimen, grain growth has been relatively slight. Progressive recrystallization has begun to destroy the finest grades of clastic particles, producing integrated textures with more equilibrated grain morphologies, but some areas retain intergranular porosity and some non-porous areas are very finegrained. The low porosity, comparable with that of the Chainpur dark rims rather than clastic matrix, is interesting. Recrystallization of a clastic texture, of the type observed in Chainpur and Weston matrices, would only be accompanied by elimination of porosity if there was compaction of the bulk rock. This is possible during metamorphism within a parent body, where lithostatic pressures might reach ~1 kbar [5, p. 193]. However, it is difficult to see how a clastic matrix with very few fine grains could recrystallize to produce the fine non-clastic distributions. The alternative to metamorphism with compaction is filling of pores by growth of fine grains during aggregation. This could explain the gradational variations in grainsize, the coarsest patches having aggregated first and thus spent longer undergoing solid-state recrystalliza tion (metamorphism). The possibility of addition of fine crystalline material around chondrules is demonstrated by the Chainpur dark rims. The existence of dark troilitic rims in the Parnallee specimen shows that at least some of it went through a similar stage. The simplest interpretation of all the data is to propose that all the Parnallee matrix was incorporated under conditions where grain growth was proceeding, the final stages being represented by areas retaining some intergranular porosity. This interpretation amounts to saying that metamorphism occurred during aggregation. However, the metamorphism under discussion is very slight, and does not noticeable affect the chondrules or homogenize the mineral compositions: the arguments presented here do not neces-
sarily bear on the cause of the much more intense metamorphism inferred in other chondrites [5]. If grain growth during aggregation involved reaction with a gas phase while temperatures were falling, as seems likely from the troilite-rim effect and sporadic presence of porosity, it follows that condensation was also proceeding during aggregation. 3. 3. Bearing on cosmochemical theories
According to the interpretation just advanced, Chainpur aggregated at lower temperature than Parnallee. This accords with its high content of volatile elements. It also agrees with the order of "accretion temperature" inferred by Laul et al. [27] from volatile trace element contents, and the smaller variation of olivine compositions in Parnallee [28]. The estimated accretion temperatures [27] seem low, and the difference between them is small (428 K for Chainpur and 447 K for Parnallee), but these values depend on calculations in which condensation of the elements is modelled as a simple equilibrium process [29]. In this model, "dust continues to equilibrate with the four volatile metals and Fe 2÷. These 5 equilibria are frozen in ... by ... accretion" [30, p. 1323]. This equilibrated dust is elusive in the electron microscope. The clastic assemblages clearly did not equilibrate with their environment. Even in the slightly metamorphosed matrix of Parnallee or Hedjaz, the major element composition of pyroxene varies considerably [22]. The compositional distinction between analyzed samples of chondrule and matrix pyroxene suggests different environments of formation [22], and the interpretation presented above bears on the nature of the environment of matrix formation. Matrix pyroxene was not produced solely by fragmentation of chondrules, and it seems that pyroxene compositions were partly controlled by grain growth from a gas; but reactions among grains before and during aggregation were insufficient to homogenize the mineral assemblages between adjacent matrix areas. The simplest explanation is that conditions varied during grain growth and aggregation, and that grain compositions representing these different conditions were retained. This interpretation strongly resembles the gradual aggregation of "chunks" in the qualitative picture presented by Blander and Abdel-Gawad [6, pp. 712-713], though the metamorphism observed
33 in the present study is relatively slight, and the observations under discussion do not bear on the contention [6] that the reactions controlling major element condensation were metastable. Formation of "chunks" would overcome the difficulty of accreting small grains in the aerodynamic process of Dodd [1]; the unaggregated portion of the small grains would be lost, but those trapped in "chunks" could be accreted. Condensation under widely variable conditions would be consistent with a model in which condensation follows volatilization by impacts (cf. [26]). Insofar as clastic material retains its identity, condensation of volatile elements is presumably restricted to a fraction of the matrix volume. In these unequilibrated aggregates, simple partition behaviour of trace elements is unlikely. Small grainsize favours surface reactions [6]; trace elements may be trapped at energetically favourable defect sites over a range of temperatures higher than those for equilibrium condensation in perfect phases. There is some direct evidence for complex behaviour of a volatile trace element in the meteorites [31 ]. Thus, the two-component concept and the equilibrium calculations of Anders and colleagues, valuable as they are in modelling the histories of the chondrites, must be regarded as simplified approximations. Unequilibrated ordinary chondrites contain records of complicated histories, exemplified by the occurrence of two very different types of fine-grained material in Chainpur. There remains ample scope for the extraction of detailed information by further study of their chemistry.
4. Conclusions Clastic matrix, with relatively high porosity between angular fragments of widely ranging size, is distinct from the fine-grained consituents of the specimens of Hedjaz and Parnallee, and the Chainpur dark rims. Observed size distributions in samples of clastic matrix in Chainpur and Weston are consistent with a Rosin's Law distribution of the type found by Dodd [1 ] among coarser particles. Any sorting process which favoured incorporation of large particles was not efficient enough to remove the fine tail of the distribution.
The non-clastic samples show effects of grain growth. Prior to aggregation of Chainpur, very fine-grained material formed around some chondrules, producing dark rims. The coarsest of the non-clastic matrix observed is in Hedjaz, and is attributed to solid-state recrystallization (metamorphism). The main minerals at grain sizes less than about 10/am are the common silicates of ordinary chondrites: olivine, pyroxene, and plagioclase. Grain morphologies are simple. Similar areas with almost no porosity in Parnallee may sometimes constitute small rock fragments, but others grade into finer matrix. Recrystallization in the matrix of the Parnallee and Hedjaz specimens was not accompanied by diffusive equilibration over distances sufficient to produce uniform composition of matrix pyroxene grains. Crystalline matrix with intermediate grain-size and low porosity was observed in Parnallee. Its fine-grained nature suggests that this formed by addition of fine material to clastic grains during aggregation, rather than by metamorphism and compaction of clastic matrix. It is suggested that this matrix represents a more advanced stage of the processes that produce dark rims. In this interpretation, growth of fine grains at the expense of pore space, and incipient metamorphic recrystallization, are held to have been contemporaneous with aggregation.
Acknowledgements The observations were made while I was a NERC research fellow at the University of Essex, where I had the constant benefit of advice from Dr. D.J. Barber. I thank Dr. P.R. Swann (Imperial College, London) for the high-voltage electron microscope facilities, and Dr. R. Hutchison (British Museum, Natural History) for the specimens and for helpful criticism.
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