Origin of chondrule rims and interchondrule matrices in unequilibrated ordinary chondrites

Earth and Planetary Science Letters, 95 (1989) 187-207 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

187

[1]

Origin of chondrule rims and interchondrule matrices in unequilibrated ordinary chondrites C.M.O. A l e x a n d e r 1,., R. H u t c h i s o n 2 , . . a n d D.J. B a r b e r 3 t Physics Department, University of Essex, Colchester C04 3SQ, and Earth Sciences Department, The Open University, Milton Keynes MK7 6AA (U.K.) : Mineralogy Department, British Museum (Natural History), Cromwell Road, London S W 7 5BD (U.K.) 3 Physics Department, University of Essex, Colchester C04 3SQ (U.K.) Received April 19, 1989; revised version accepted August 8, 1989 Optical microscopy, electron probe microanalysis, scanning electron microscopy and analytical transmission electron microscopy were used to study the interchondrule matrix and opaque chondrule rims in Bishunpur, Krymka, Semarkona, Chainpur (all LL3), Tieschitz and Sharps (both H3). The size-distribution of grains in matrix in Bishunpur obeys a power law. Rims around chondrules tend to be finer grained than matrix and may be layered. All rims and matrices studied contain clasts of forsteritic olivine and low-Ca pyroxene. In Bishunpur the clasts may be cemented by amorphous "glue", rich in normative albite, and partly altered to smectite. Rims in Krymka, Sharps, Chainpur and Tieschitz have a densely packed olivine groundmass with a grain-size of about 0.1 ~m. In Krymka, groundmass olivine is Fo20_30 and forms interlocking dendrites. Various Fe oxides are present in rims and matrices. Grain-size distribution and chemical data indicate that clastic olivines and pyroxenes are derived from chondrules. From analyses, m a t r i c e s / r i m s in the LL-group apparently are enriched in a feldspathic component. Tieschitz rims are slightly enriched, but here the feldspathic component is present as white matrix. Sharps rims are not enriched in a feldspathic component. Opaque interchondrule matrices and chondrule rims probably formed from the products of fragmentation of chondrules, partly induced by contraction of protopyroxene on inversion to clinopyroxene. Fragmented silica-rich chondrule mesostases reacted with Fe oxides and Na to form the groundmass of fayalitic olivine and feldspathic "glue". A low-temperature, nebular or pre-solar, component is limited to 3 vol.% of each meteorite, so equilibrium condensate was not the carrier of volatiles such as TI and Bi.

1. Introduction

material in all ordinary chondrites exceeds 85%

The two-component theory of the composition of chondrites has been with us for over 20 years [1,2]. In this theory, refractory elements such as Ca, A1 and Ir were contributed by a volatile-free high-temperature component comprising silicate chondrules and their debris together with melted metal-sulfide. The volatiles were contributed by low-temperature, primitive dust that did not melt before, during or after accretion. Some 25 wt.% of the dust are required in the unequilibrated ordinary chondrites (UOCs) [3]. More recent evidence, however, indicates that the two-component model is an oversimplification and that high-temperature

In the UOCs, fine-grained material that may not have been melted occurs in rims around silicate chondrules and clasts, metal-sulfide objects, and fragments thereof. Additionally, the space between chondrules, clasts, etc., may be filled with fine-grained matrix; such material has also been observed within silicate chondrules [4]. This last observation indicates that m a t r i x / r i m material existed when chondrules formed, so the precursors of chondrules may be present as rims and matrices. The fine, often sub-micrometer, grain-size of rims and matrices renders them opaque in thinsection and presents problems in establishing textural, mineralogical and chemical relationships within them. At first, the techniques capable of resolving the grains in matrices and rims were incapable of providing chemical data (e.g. [5,6]), but this deficiency was remedied by the addition

[2].

* Now at: The McDonnell Center for the Space Sciences, Campus Box 1105, Washington University, St Louis, M O 63130, U.S.A. * * To whom correspondence should be addressed. 0012-821X/89/$03.50

© 1989 Elsevier Science Publishers B.V.

188 of energy dispersive X-ray analysis to scanning electron microscopes (e.g. [7]) and to transmission electron microscopes (e.g. [8]). Although technical advances have been made (see the review in [2]) and we should now be capable of identifying phases in the finest grained material, this has not yet been done. Anders [9, p. 290] suggests that "it is quite conceivable that matrix of primitive meteorites is only lightly reprocessed interstellar dust" and, further, that "This is a testable proposition that should be checked experimentally". We present new observations aimed at testing such hypotheses. We studied the fine-grained rims and interchondrule matrices of six UOCs. The instruments used include the optical microscope, scanning electron microscope (SEM), electron probe microanalyser (EPMA) and analytical transmission electron microscope (ATEM). The A T E M provides textural, structural mineralogical, and chemical data, all with submicrometer resolution on a single sample. It is mainly limited by the small area that may be studied in each preparation, often less t h a n 10 4 /~m 2. First we made optical, SEM and E P M A observations at low resolution on demountable polished thin-sections [10], then selected parts for ion-beam thinning for the ATEM. 2. Selection of meteorites for study

In their study of chondrule rims and interchondrule matrices in UOCs, Huss et al. [11] observed that all have been affected to some extent by metamorphism. To identify the chondrites with the most pristine rims and matrices, we chose only observed falls that had been classified by Sears et al. [12] as type 3.0 to 3.6, based on thermoluminescence (Table 1). The six chosen are the least metamorphosed of the suite studied by Huss et al. [11]. During our work, however, it transpired that Semarkona, LL3.0, and Bishunpur, LL3.1, are partially hydrothermally altered [13], so the most pristine UOCs probably belong to type 3.1 or 3.2. Four of the meteorites are LL-group, Sharps and Tieschitz being H-group [2]. 3. Definition

Here we take the "traditional view" that matrix includes any material located between chondrules

TABLE 1 Meteorites studied

Name Bishunpur Chainpur Krymka Semarkona Sharps Tieschitz

Class LL3.1 LL3.4 LL3.0 LL3.0 H3.4 H3.6

Spec. No. 80339 1915,86 1956,325 1805 640 1975,M.11

Source BM(NH) BM(NH) BM(NH) SI SI BM(NH)

Classification: Chemical group follows Scott et al. [2]; petrologic sub-type after Sears et al. [12]. BM(NH): British Museum (Natural History); SI: Smithsonian Institution.

and clasts that apparently does not form a coating on them [2]. Matrix does not seem to be a discrete component of UOCs, but is part of a continuum from chondrules, through identifiable fragments of chondrules, to aggregates of mineral grains with sizes less than 1 /~m, to rare amorphous material (Fig. la, b). Scott et al. [4] recognized " m a t r i x lumps" in most U O C s and argued that the lumps were free-floating before accretion. We found only one lump, in Bishunpur, that proved to be identical to the matrix, so is not discussed separately. Ashworth [6] recognized the rims around chondrules and clasts in the UOCs as petrologically distinct from matrix. Rims are defined as fine-grained material adhering to larger identifiable objects. Our observations, however, are based on two-dimensional surfaces so we may have misidentified some material as matrix that, in the third dimension, adheres to chondrules or clasts. Some rims are texturally similar to matrix, but others are layered and are texturally different. 4. Matrix in Bishunpur

A m o n g the chondrites studied, Bishunpur has the highest proportion of fine-grained opaque material and has a well developed interchondrule matrix (Fig. la). The relationships between rims, matrix and chondrules are most clearly shown in this meteorite, so most of our textural study was performed on it. M a n y features, however, are c o m m o n to all six UOCs. We address two major questions: (1) Could all of the matrix have been produced by the fragmentation of pre-existing solids? Alternatively: (2) Is there textural evidence that the matrix formed

Fig. 1. Images of Bishunpur at different magnification. (a) Transmitted light micrograph, showing chondrules and chondrule fragments in matrix (black). Width of field 1.5 mm. (b) Scanning electron micrograph, clastic matrix. Metal a n d / o r sulfide, white; silicates gray. Note the angular chondrule fragment with skeletal crystals. (c) Scanning electron micrograph. Matrix-like rim, showing four layers. Embayments in the chondrule, top right, contain large (30 /xm) olivine fragments in fine-grained diopsidic and feldspathic material (layer 1). Layer 2 is thin, and is followed by two thick layers (3, 4) each comprising grains ( < 10/~m) of olivine and low-Ca pyroxene in a very fine-grained groundmass. Note the delicately textured metal-sulfide object (ms) with silicate rim; the layering (arrowed) adjacent to the central metal object may have been produced by condensation or by reaction with a fluid. (d) Transmission electron micrograph, matrix comprising clastic silicate mineral fragments (dark) in amorphous material rich in normative albite (light gray, arrowed). Ion-beam thinned sample, with holes, center, top.

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0.5/~m in diameter. The remaining 34% comprises voids, amorphous material and grains less than 0.5 /~m across. Huss et al. [11], found that Bishunpur contains 3.1 and 10.8 vol.% of recrystallized matrix and opaque matrix, respectively. Our data indicate that pores, grains less than 0.5 /~m and amorphous material together account for less than 5 vol.% of the bulk meteorite, although matrix accounts for 14%.

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from the products of condensation or by the reaction of chondrules with a gas? In an attempt to find answers, the size-distribution of grains in Bishunpur matrix was detern'fined. A photomosaic of SEM micrographs of a typical silicate-rich area (1.6 x 105 ~m 2) of matrix was prepared, and the sizes of the grains were measured. Fragments of chondrules and clasts were treated as single grains. Because the third dimension is unknown the apparent grain-size is probably less than the real maximum. To obtain the true size-distribution the grains were sorted into a series of regular size-intervals and corrected for random sectioning [14]. Over 1000 grains were counted. Both silicate and metal/sulfide size-distributions obey a power law over ranges from 100 to 0.5/~m (Fig. 2). Poor sampling may be partly responsible for the deviations at the extremes of grain-size. For the larger particles this is because the area studied was small, only 400 /~m across; for the smaller grains the constraint was the resolution of the SEM. The size-distributions for silicate and metal/sulfide are essentially parallel, indicating that both obey the same power law, the metal/sulfide grains being shifted to smaller grain-sizes. A similar relationship exists in ordinary chondrites for grains larger than 100 /~m [15]. The larger particles include chondrule fragments so there probably is a single population of "grains" from fragmented chondrules to fragmented crystals. The area of matrix comprises 59 vol.% silicate particles and 7% metal/sulfide grains greater than

Rims composed of fine-grained silicates, sulfide and Fe,Ni metal surround all types of chondrules and clasts, including metal-sulfide-rich types. Rare compound chondrules, however, lack rims on their surfaces of contact, so these chondrules coalesced before they had acquired rims. Some rims were deformed with the chondrules they enclose, indicating that these rims were accreted while the chondrules were hot and plastic [16,17]. Other rims were acquired by fragments of chondrules and lithic clasts which had undergone brittle fracture, so were cold at the time [16]. The process of rim formation, therefore, need not have been associated with chondrule formation or with any single heating/cooling episode. Chondrules and clasts are often only partially surrounded by a rim. Indeed, the only vestige of a rim is sometimes preserved in embayments in the margin of a chondrule or clast which presumably afforded some protection against abrasion. Layering within silicate-rich rims is common, indicating that the central object underwent more than one episode of rim accretion and erosion. It has previously been widely noted that clast chondrules were rounded by some abrasive process (e.g. [15]), which may also have affected rims. From textural observation, mainly on Bishunpur, we distinguish two extreme types of silicaterich rim: (1) Very fine-grained rims with few, or no, resolvable grains, even in the SEM. This is the dominant type in most UOCs. These rims tend to be thin, 10-30 /Lm in width, and are normally associated with both silicate and metal-sulfide droplet chondrules. They often exhibit layering in both silicates and metal/sulfide (e.g. the m e t a l / sulfide object, Fig. lc). Some delicately leaved Fe-S-rich layers may have formed by reaction or

191

vapor deposition rather than by the accretion of particles [18]. (2) Rims containing coarser grains than the first. Grains may be up to 5 ~tm in size and are mostly magnesian olivine or low-Ca pyroxene. Such rims resemble finer-grained areas in the interchondrule matrix. Matrix-like rims are generally thick, up to 50/zm or more, usually surround clast chondrules, and their silicate dominated component is often layered (Fig. lc). Each layer in the rim may have formed from materials, or under conditions, different from those that produced its neighbors. In addition to the common silicate-rich rims just described, others are dominated by metal and sulfide. In some there is evidence that sulfide and metal were molten when they coated their host chondrules [6,8,19]. This is suggested by the "frothy" appearance of some sulfide-rich rims containing many small silicate particles, which tend to be concentrated towards the outer edge. In one instance several metal-sulfide spherules appear to have been trapped during expulsion from a silicate-rich chondrule. Veins from the spherules seem to penetrate the silicate rim, which must have been accreted by the chondrule before the veins were emplaced. Massive metal-sulfide rims seldom, if ever, occur around clast chondrules or lithic clasts, so these rims may have formed from the metal and sulfide expelled from molten

chondrule interiors. We note that some sulfide and metal are always disseminated throughout silicaterich rims so they must have been available, with silicate, during rim formation. 6. Pyroxene and olivine compositions in matrices and chondrule rims From the size-distribution, Bishunpur matrix comprises a single population of silicate particles which includes composite grains (chondrule fragments) and discrete crystals. The latter, however, may have come from more than one source. To test this possibility, the sizes and compositions of olivine and low-Ca pyroxene grains in r i m s / m a t r i x of each meteorite were determined to see if sizerelated compositional discontinuities occur. If present, they would indicate that several sources were involved. The sizes of grains greater than 2/~m in radius were obtained from electron images (SEM), then the grains were analysed by wavelength dispersive microprobe. Small grains were analyzed with a focussed beam, large grains with a rastered beam. Standard measurements with the appropriate type of beam were made. Use of the ATEM extended the size-range to 0.1 ~tm. However, because of the rarity of uniformly thin grains with radii greater than 0.5 /~m in preparations for the ATEM, we lack data for 0 . 5 - 2 / z m grains. The mean composi-

TABLE 2 Mean compositions, low-Ca pyroxenes in unequilibrated ordinary chondrites 1

2

No.

30

SiO 2 TiO 2 A1203 Cr203 FeO MnO MgO CaO Na20

57.61 +- 2.06 0.05 +- 0.03 0.79+_0.71 0.73 +_0.25 8.61 +_6.38 0.39 +_0.23 31.14 + 5.02 0.73 +- 0.60 0.065:0.17

Sum

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57.32 0.01 0.67 0.70 7.37 0.41 32.63 0.76 0.14

56.83 0.07 0.56 0.68 8.23 0.34 32.42 0.83 0.03

56.02 0.72 0.78 8.25 0.38 32.70 1.02 0.20

55.8 0.6 0.3 10.0 0.3 32.2 0.5 -

55.78 0.03 0.83 0.64 10.36 0.38 31.92 1.08 0.02

55.8 0.3 10.1 0.4 32.9 0.3 -

99.99

100.07

99.7

101.04

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10 16 56.09 0.05 0.06 0.73 9.01 0.41 31.80 1.09 0.22

100.0

5 55.8 0.3 0.2 10.6 0.8 32.1 0.3 100.1

1 = mean composition, with one standard deviation, of EPMA analyses of 30 pyroxenes in Bishunpur chondrules; 2 = mean composition, pyroxenes in chondrules in UOCs [20]; 3 = mean composition, pyroxenes in matrix and rims, Bishunpur, by EPMA; 4 = mean composition, pyroxenes in rims, Sharps, by EPMA; 5, 6 = mean composition, pyroxenes in rims and matrix, Chainpur, by EPMA and ATEM, respectively; 7, 8 = Krymka, as 5 and 6; 9, 10 = Tieschitz, as 5 and 6.

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tions and compositional ranges of low-Ca pyroxenes and olivines in Bishunpur chondrules were determined for comparison with those in matrices and rims (Table 2; Figs. 3 and 4). There is little difference between EPMA and A T E M results (cf. Table 2 nos 5, 7, 9 with 6, 8, 10). The compositions of low-Ca pyroxenes in the matrices/rims of the 6 UOCs lie almost entirely within the one standard deviation range of chondrule pyroxenes and are unrelated to grainsize (Fig. 3 and [2]). Thus the pyroxenes appear to comprise a single population. Apparent differences in the minor elements result from a higher detection limit and higher statistical error in the A T E M compared with the EPMA. The results for the olivines are more ambiguous (Fig. 4). Various authors [7,11] found that the matrices of the least equilibrated UOCs contain olivines ranging from Folo o to Foso or less. Our data show similar ranges. In Bishunpur, for example, large olivines are mostly within the chondrule range (Fig. 4), but with decreasing size they tend to be more Fe-rich, Fo65_70, and close to the mean M g / ( M g + Fe) atomic ratio of bulk matrix, 0.67. More obvious trends are exhibited by Chainpur and Sharps. With decreasing size the olivines extend beyond the range in chondrules and attain M g / ( M g + Fe)

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Fig. 5. Ca contents (atomic per cent) vs. size in matrix a n d / o r rim olivines of Fig. 4. Squares: Bishunpur; small circles: Chainpur; dots: Tieschitz; triangles: Sharps; large circles: Krymka. EPMA analyses. For grains with radii between 2 and 100 # m there is no significant relationship between Ca content and size. Visually there appears to be a negative correlation between Ca content and grain-size for Bishunpur olivines, and a positive correlation for those of Krymka, but the correlation coefficients are only 0.57 and 0.66, respectively. Most matrix or rim olivines have Ca contents within one standard deviation (dashed lines) of the mean (solid line) of ohvines from Bishunpur chondrules and there is no hiatus within the population.

ratios close to those of their respective bulk matrices. We note that the most Fe-rich compositions in Chainpur and Tieschitz are not found in the smallest grains but in those with radii from 4 to 10 ffm. Few minor element data are available for sub-micrometer, non-clastic g r o u n d m a s s olivines. Semarkona lacks the more Fe-rich olivine grains in its matrix and rims. This, we argue, has been caused by the action of hydrothermal fluids [21]. Our data indicate that in each UOC, matrix and rim olivines belong to a single population which comprises grains with compositions consistent with a chondrule origin. This interpretation is supported by the Cr and Ca contents (Fig. 5) which are independent of size and cover the range of chondrule olivines. Chromium and Ca seem to be less susceptible to exchange than Mg-Fe(Mn). Krymka, Sharps, Chainpur and Tieschitz rims have a new generation of more Fe-rich groundmass olivine that is absent in Semarkona and Bishunpur. This olivine g r o u n d • a s s may occur as overgrowths on larger, angular olivine grains with which it has partially reacted.

F r o m A T E M observations, mineral fragments greater than 0.2 /~m across occur in the interchondrule matrices and rims around chondrules and clasts in all six meteorites studied. The fragments are mostly of olivine, with lesser amounts of low-Ca pyroxene; diopside, apatite and chromite are also present. The low-Ca pyroxene occurs as the twinned monoclinic polymorph [2] typical of chondrules and interpreted as inverted quenched protopyroxene [22]. In Bishunpur the mineral fragments are cemented by amorphous "glue" (Fig. ld), rich in normative albite. The amorphous material is partly replaced by smectite, and a smectite groundmass dominates Semarkona rims and matrix [13,21]. In the other four UOCs the mineral fragments are set in an olivine groundmass with a grain-size generally less than 0.1 /~m and which varies texturally and chemically from one meteorite to another. The groundmass usually contains some amorphous, or semi-amorphous, feldspar-rich material in the interstices. Several oxides of Fe are present in the rims a n d / o r matrices of all six meteorites, being most abundant in Krymka. Here, fibrous magnetite often occurs in veins that are not confined to chondrule rims (Fig. 6a), hematite is rarely present and the rims contain little or no fine-grained metal [23]. Some magnetite in K r y m k a formed by the oxidation of sulfide (Fig. 6a). Magnetite occurs in Sharps and Tieschitz. The oxides have Ni contents below 5 wt.%, except for two that may be wi~stite and have F e / N i ratios close to unity. The olivine groundmass in the opaque chondrule rims appears to form a textural sequence from interlocking dendrites in Krymka, through loosely packed grains in Sharps and Chainpur to a more densely packed granular texture in Tieschitz. The textural sequence is accompanied by increasing homogenisation between groundmass and angular olivines.

7.1. Krymka The groundmass is very densely packed (Fig. 6b). The olivines occur as numerous subparallel, elongate crystallites m a n y of which approximate to a single crystallographic orientation indicating that they comprise one dendritic crystal. Crystallites often appear to have a " v e i n " running down

Fig. 6. Krymka, transmission electron micrographs of ion beam thinned samples. (a) Vein of magnetite (Mag.) in troilite (Tr.). Note the often diffuse boundary between the phases (e.g. center right), indicating that the troilite was being oxidized. (b) Two sets of interlocking olivine dendrites in a chondrule rim. Note the paucity of light colored mottled feldspathic material between the olivine crystallites. (c) Angular, clastic olivine fragment with overgrowth of dendritic olivine.

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Fig. 7. Sharps and Chainpur, transmission electron micrographs of ion beam thinned samples. (a) Chondrule rim, Sharps, with coarse olivine fragments set in an olivine groundmass. (b) Sharps rim with groundmass of olivines, some of which have fused. Note the semi-amorphous, mottled material between the olivines. The mottled material is rich in normative albite and often fills embayments in olivine, suggesting that reaction was occurring. A mottled appearance is often indicative of the presence of Fe oxides. (c) Chainpur rim, with olivine groundmass. (d) Chainpur matrix with olivine-rich groundmass partially overgrowing a clastic olivine, lower left. Note the suggestion of a preferred orientation, top right.

0.5Fro

196

their middle, again indicative of dendritic growth. At junctions between regions the dendrites interlock (Fig. 6b). There is little intervening amorphous feldspathic material; indeed, rims around m e t a l / s u l f i d e objects often comprise monomineralic olivine. Mineral fragments are always present, the olivines often having overgrowths morphologically similar to the groundmass (Fig. 6c). Angular grains and their overgrowths have compositions in the range Fo30-Fo20, identical to the groundmass. Fragments of low-Ca pyroxene, if present, lack olivine overgrowths. Some olivines, especially in densely packed areas, contain numerous voids, often with a preferred orientation. Rare "crystals" with many voids have the chemical composition of fayalitic olivine, but selected area diffraction (SAD) indicates that they do not have the structure of olivine. Some olivines have cavities partly filled by material that was trapped during crystal growth. This suggests that the voids may be artifacts produced by selective erosion of the filling by ion-beam thinning. 7.2. Sharps Rims contain olivine fragments, greater than 0.2 /~m in size, set in a groundmass of olivine grains (Fig. 7a) less densely packed than in Krymka or Tieschitz, but they are sometimes fused (Fig. 7b) as in Tieschitz. All the olivines analyzed by ATEM are Fos0. The groundmass contains semiamorphous interstitial material, rich in A1 and Na, which has a mottled appearance, perhaps due to the presence of Fe oxide. Olivines in the groundmass are often embayed where they make contact with the semiamorphous material, indicative of in-situ reaction (Fig. 7b). 7.3. Chainpur Our brief study indicates that chondrule rims have a groundmass of densely packed olivines (Fig. 7c) close to F%0 and sometimes with an elongate morphology as in Krymka and Tieschitz. Chainpur rims contain larger olivine fragments, often with overgrowths of "non-clastic" groundmass olivine [6]; twinned monoclinic, low-Ca pyroxene fragments occur. A well developed, highly porous, clastic matrix is present [6], the single area examined having an olivine groundmass (Fig. 7d), like the rims.

7.4. Tieschitz All chondrules and clasts are coated by dark rims with a densely packed olivine groundmass. In some cases crystallites have an elongate morphology (Fig. 8a), like that in Krymka. More typically the groundmass olivines are about 0.1 /~m across and equant, with the partial development of 120 ° triple grain boundaries (Fig. 8b), where amorphous material tends to be concentrated. Fragmentary olivine and twinned, monoclinic low-Ca pyroxene grains are common. Most olivine fragments have overgrowths of groundmass olivine (Fig. 8b) that are not so obvious as in K.rymka. Analyses by A T E M indicate that groundmass olivines range from Fo53 to Fos0, less Fe-rich than some analyzed by EPMA. 7.5. White m a t r i x - - Tieschitz White matrix is a component that often fills channels a few tens of micrometers wide between chondrules a n d / o r clasts, so is usually in contact with dark rims [5,16]. Ashworth [8], from TEM observations, reported the occurrence of plagioclase and nepheline mainly as clastic grains. Hutchison [24] noted the presence of areas texturally resembling the products of rapid crystallization from a melt, and chemical effects that he interpreted as the result of reaction at elevated temperature. During the present work a grain 60 ~m long was found to have the chemical composition of nepheline (EPMA analysis). However, in the A T E M neither nepheline nor plagioclase was identified. Most white matrix proved to be an intergrowth of two phases (Fig. 8c) in regular layers that may be continuous over several micrometers. The diffracting power of the more abundant phase is variable, which may indicate differing degrees of crystallinity. Many layers have similar crystallographic orientations and they often exhibit multiple twinning (Fig. 8d). The twinned phase is somewhat sensitive to the electron beam which rapidly produces voids, probably because of the volatilization of Na, which is present as 10-12 wt.% N a 2 0 (Table 3, nos. 2 and 3). Alternatively the loss of some other volatile, such as water or CO 2, may be the cause. The phase is chemically similar to albite except for a deficiency in S i O 2 and the presence of significant MgO and FeO; furthermore, diffraction data eliminate albite as a possibility. In one

C

0.5pm

Fig. 8. Tieschitz, transmission electron micrographs of ion beam thinned samples of opaque rims and white matrix. (a) Chondrule rim with densely packed groundmass of olivine with an elongate morphology, similar to that in Krymka (Fig. 6b). (b) Chondrule rim with groundmass of equant olivine grains, some overgrowing a clastic olivine, center to top left. Intergranular, plagioclase-rich material is arrowed. (c) White matrix comprising a layered intergrowth of two phases (see text). (d) White matrix illustrating multiple twinning in the thicker layers that shows them to have a preferred orientation.

1.0pro

0.4pm

198 TABLE 3 Analyses of white matrix, Tieschitz and Sharps

SiO 2 AI203 FeO MgO CaO Na20 K 20 S

1

2

3

4

5

6

7

8

58.7 19.1 5.0 3.5 4.0 9.3 0.4 -

59.0 20.2 3.7 1.8 2.5 12.2 0.4 0.2

58.6 18.5 10.0 1.0 1.3 10.2 0.3 0.2

43.1 30.7 7.8 2.6 0.5 11.0 3.9 0.3

58.5 15.3 10.2 2.4 7.4 5.1 0.0 1.1

67.3 19.0 2.4 4.1 2.1 4.8 0.0 0.2

63.8 21.0 1.4 2.4 6.8 4.1 0.2 0.2

59.9 9.7 3.3 9.9 13.0 3.8 0.1 0.2

1 = bulk analysis, Tieschitz white matrix [13], calculated to 100%. 2-8 = ATEM analyses: 2, 3 = coarse, poorly diffracting layers, Tieschitz; 4 = Al-rich area, Tieschitz; 5 = fine, strongly diffracting layer, Tieschitz; 6, 7 = coarse layers, Sharps white matrix; 8 = thin, discontinuous layer, Sharps.

area small grains of a more Al-rich phase are chemically like nepheline (Table 3). The grains yielded diffraction data similar to the dominant layer phase, but the d-spacings are not those of nepheline. The similarity in lattice spacings of the feldsparqike and (Al-rich) nepheline-like phases suggests that they may be varieties of the same mineral. The thinner bands (Fig. 8c) are less beam sensitive and diffract more strongly than the twinned layers, but the small size of the former makes diffraction measurement difficult. They contain more FeO and CaO (Table 3 no. 5), suggestive of pyroxene or amphibole, the high S content favoring amphibole. The d-spacings obtained do not fit either group of minerals, but some larger definitive spacings may be missing from the electron diffraction data. 7. 6. White matrix--Sharps In Sharps, white matrix was not found in the interchondrule matrix. However, within the margins of two chondrules we discovered an intergrowth similar to that described above, although with layering less pronounced than in the white matrix of Tieschitz. In one area in Sharps multiple twinning is developed parallel to the layering, in contrast to Tieschitz. Diffraction patterns from this area show some streaking, probably due to the presence of defects such as stacking faults. The d-spacings for Sharps "white matrix" are similar to those in Tieschitz, but the materials may be chemically distinct (Table 3), that in Sharps having less Na 20 but higher SiO2, MgO and CaO. It is, however, possible that the lower Na 2O content

in Sharps is an artifact of loss during SAD measurements before analysis. Diffraction data were not obtained from the finer layers in Sharps "white matrix", which have distinctly higher MgO and CaO and lower FeO and A1203 than the equivalent phase in Tieschitz (Table 3). They may be pyroxene, but a small S content indicates that they may contain sulfate. 8. Bulk compositions of matrices and chondrule rims Bulk chemical compositions of rims a n d / o r matrix in each of the UOCs were determined. Each analysis in Table 4 is the mean of a series, by EPMA, using a rastered beam to inhibit migration of Na and K. Standard counts were taken in the same way, but this proved unnecessary. Huss et al. [11] and Matsunami [25] surveyed the bulk chemistry of rims and matrices in UOCs, including those discussed here. Their data are similar to ours. Most analyses sum to about 90 wt.% or less, but neither carbon [25] nor water contents (apart from Semarkona [13]) can account for the low totals. We are therefore forced to conclude that porosity is responsible, but this is not apparent even in the ATEM, except for Chainpur matrix, analyses of which total only 77%. This discrepancy may be the result of sample bias, in our A T E M study, against porous areas which tend to disintegrate. Discrepancies exist between the rim and matrix compositions of Huss et al. [11], Matsunami [25] and this study. These are most marked for Na, K and A1. For example, Matsunami's [25] figures for

199 TABLE 4 Mean chemical compositions of matrices and rims 1M

1R

2M

2R

3R

4M

5R

6R

n

24

25

16

8

9

6

6

19

SiO 2 A1203 Cr203 FeO MnO MgO CaO Na 2° K20

52.4 5.6 0.5 16.5 0.3 18.7 2.2 3.1 0.6

46.7 4.0 0.4 27.8 0.4 16.2 2.2 2.0 0.3

41.7 5.2 0.3 30.3 0.4 17.0 2.2 2.4 0.4

39.8 4.2 0.4 33.5 0.3 15.4 3.1 2.3 0.3

38.7 4.0 0.2 41.7 0.4 12.0 1.4 1.4 0.3

46.8 4.1 0.4 29.2 0.3 14.4 0.9 3.2 0.8

37.8 2.4 0.4 27.3 0.5 23.2 1.8 1.0 0.3

39.2 3.2 0.4 32.6 0.5 20.3 1.7 1.8 0.3

FeS Fe m

3.4 6.1

3.6 5.2

3.4 2.1

4.9 1.8

0.6 4.5

3.2 2.0

1.5 6.3

0.0 0.0

Sum

92.0

90.3

77.5

88.5

82.2

86.4

83.4

91.1

rng

66.9

51.0

50.0

45.0

33.9

46.6

60.3

52.6

n = Number of areas analyzed. M = matrix; R = rims. Mean compositions determined by wavelength dispersive EPMA; rastered beam, accelerating voltage 20 kV; beam current 2.50 × 10 8 A. Silicate recalculated to 100 wt.%. All S calculated as FeS. Fem (Fe metal) calculated by assuming F e / N i ratio is that in the bulk [26,27]. " S u m " refers to raw data, but includes FeS and Fem. 1 = Bishunpur; 2 = Chainpur; 3 = Krymka; 4 = Semarkona; 5 = Sharps; 6 = Tieschitz. Analyst: C.M.O'D. Alexander, except 4, from [16, table 2, no. 7, mean of 6 10-/~m "spots"].

N a 2 0 and K 2 0 abundances in Krymka rims and matrix are significantly lower than those for matrix of Huss et al. [11], or for rims in this work (Table 5). Our data generally correspond closely to those of Huss et al. [11], but the differences between the three sets of data are not systematic. Selective bias in the areas analysed, heterogeneity TABLE 5 Krymka rims and matrix

SiO 2 A1203 CrzO 3 FeO MnO MgO CaO Na20 K20

AR

* HM

* MM

* MR

38.7 4.0 0.2 41.7 0.4 12.0 1.4 1.4 0.3

37.7 3.0 0.3 42.8 0.4 13.0 1.2 0.9 0.4

35.4 2.8 0.3 46.4 0.5 13.1 0.8 0.2 0.1

38.3 3.8 0.3 39.2 0.5 15.1 1.8 0.5 0.1

AR = rims, this work; HM = Huss et al. [11], matrix; MM = matrix and MR = rims, Matsunami [25]. Silicate recalculated to 100 wt.%. * TiO 2 and P205 contents omitted.

within the matrix of each meteorite [4], or technical differences may have been responsible. Each data set should, however, be internally consistent, so variations within each should be real. One feature c o m m o n to all three studies is that matrices and rims of all the UOCs have mg numbers (rag = 100 M g / ( M g + Fe + Mn) atomic) (Table 4) lower than those of the bulk meteorites, which lie in the restricted range of 78.5 (Bishunpur) to 69.5 (Krymka) (Bulk analyses from [26], except Krymka, from [27]). This confirms the mineralogical evidence that rims and matrices are the most oxidized component of the meteorites. In Bishunpur and Chainpur, rims are more oxidized than the co-existing matrix, indicating that the finest-grained material is the most highly oxidised. The lowest mg number (33.7) was determined in K r y m k a rims which are mostly olivine with m g = 20 (Fa80). Chondrule rims in the two H3 chondrites have mg numbers within the range of rims and matrices in the LL-group UOCs. Thus, both H3s have bulk silicates and chondrule rims with the same degree of oxidation as the LL3s [2]. In other respects, however, Sharps and Tieschitz

200

rims are chemically different from those in the LL3s. Our analyses indicate that matrices and rims in the LL3s have significantly more A1203 than Sharps and Tieschitz rims (Table 4) or the respective bulk meteorite (Table 6). This is consistent with the data of Huss et al. [11], (Wlotzka [28] and Scott et al. [4]. In contrast, the rims in the H3s are not significantly enriched in A1203 relative to the bulk [28]. All rims and matrices are enriched in K 2O and N a 20 relative to the corresponding bulk meteorite, except for Sharps, where the rims are depleted in Na2 O. There is no systematic variation in the distribution of CaO within the U O C s (Table 6). Scott et al. [4] emphasised that bulk UOCs have uniform lithophile element contents, but that Mg/Si, N a / S i and A1/Si ratios of matrices are fractionated relative to the bulk (their fig. 16), a conclusion supported by us. This seems in conflict with our suggestion that rims and matrices were derived from chondrules, but the apparent contradiction may easily be reconciled. Variation in lithophile element ratios between the matrices of different UOCs may be a function of grain-size. In analysing fine-grained matrix there may be a bias against the inclusion of resolvable (olivine and pyroxene) grains in the areas selected. Thus the trend in matrices from Al-rich to Mg-rich [4, fig. 16] may reflect an increasing abundance of femic mineral clasts in the areas analyzed. This was tested and confirmed for Bishunpur matrix (Fig. 9). The M g / S i and A1/Si ratios of the whole area analysed are close to those of bulk ordinary chondrites. Large areas of matrix have bulk silicate compositions representative of the meteorites as a whole, but areas of matrix selected for their fine grain-size have more of the feldspathic component.

2.2 '~ t d e s o .

2.0

~',

1.8 -

~ '.

1.6 1.4 •~- 1.2 .~

1.0

H,L.LL

,,-~ 0.g

~

0.6

0.4

\~ ~

0.2

,

Px. h..._ ............

0.0

....

, ....

, ....

, ....

, ....

, ....

0.0 0.2 0.4 0.6 0.8 10

, ....

", ' ~,

,,,m,

01. ....

1.2 1.4 1.6 1.8

(Mg/Si)/CI Fig. 9. AI/Si and M g / S i atomic ratios, CI normalized, of one area of Bishunpur matrix by EPMA. The area (100 p~m× 85 #m) is apparently devoid of chondrule fragments. Spot is the mean of six spot analyses, each with a 2 /~m radius of excitation, in a selected very fine-grained central area. Areas 20 ptmX15 /tm (rastered beam) were analyzed in a spiral away from the central area. The small dots are means after 2, 4, 6, etc., analyses until the whole area is represented. As the area analyzed increases, the A I / S i and M g / S i ratios tend to those in bulk H, L and LL chondrites (from [15, p. 20]). Mesostasis, olivine and pyroxene points are means from Bishunpur chondrules.

Alexander et al. [29] showed that N a and K are distributed between two components in Bishunpur rims and matrix. One end-member component has no K and about 1 atomic per cent Na, whereas the other is Na-rich, with K / N a (atomic) about 0.15. The K / N a ratios of the amorphous "glue" in Bishunpur matrix and rims (Table 7) are 0.12 and 0.19, respectively, so this material is well endowed with the Na-rich component. The mesostases of Bishunpur chondrules lie on the same trend to K and N a enrichment as rims and matrix [29]. However, almost half the matrix areas analysed are peralkaline ((Na + K) > A1

TABLE 6 Si-normalized feldspathic elements relative to bulk

AI Ca Na K

1M

1R

1H

2M

2R

2H

3R

3H

4M

4H

5R

5H

6R

6H

1.9 0.9 2.5 4.6

1.6 1.0 1.8 2.6

1.6 0.8 1.9 6.2

1.9 1.4 3.1 3.1

1.6 2.0 3.1 2.5

1.6 1.4 2.5 3.6

1.9 0.8 1.8 3.5

1.5 0.7 1.2 4.8

1.6 0.4 2.9 7.0

2.0 0.5 2.4 6.8

1.2 1.0 1.2 4.0

1.0 0.8 0.6 2.6

1.3 1.0 1.8 6.5

1.3 1.0 1.4 6.3

R a t i o s o f S i - n o r m a l i z e d a t o ~ c a b u n d a n c e , from T ~ l e 6 , t o S i - n o r m a l i z e d a b u n d a n c e f r o m b u l k a n a l y s i s . H r e f e r s t o H u s s e t a l . [ l l ] . Bulkanalyses: M = m a t f i x ; R = f i m s , l = B i s h u n p u r ; 2 = C h a i n p u r ; 3 = K r y m k a ; 4 = S e m a r k o n a ; 5 = S h a ~ s ; 6 = T i e s c N t z .

201 TABLE 7 Analyses and norms of chondrule mesostases and matrix "glue" in Bishunpur Bishunpur

SiO 2 TiO 2 A1203 CrzO3 FeO MnO MgO CaO Na20 KzO

Semarkona

"Glue"

min.

mean

max.

min.

mean

max.

64.1 0.24 7.35 0.00 2.08 0.16 0.65 0.89 1.50 0.60

69.2 0.40 13.2 0.23 6.57 0.29 4.55 2.08 2.37 0.83

72.2 0.46 17.6 0.58 11.4 0.49 11.6 3.33 3.67 0.98

64.5 0.40 9.60 0.00 3.38 0.14 0.40 0.35 0.47 0.36

70.2 0.49 14.7 0.07 6.69 0.35 1.56 2.97 1.61 0.71

79.1 0.61 19.7 0.23 12.4 0.90 4.01 5.83 4.11 0.97

1

2

56.8 6.0

58.1 10.6

10.2

10.2

19.6 2.0 4.2 1.2

11.2 2.9 5.9 1.0 99.9

99.72

99.35

100.0

Quartz Albite Orthoclase Anorthite Corundum Acmite Diopside Hypersthene Olivine Chromite Ilmenite

34.0 21.5 5.0 10.5 5.2

42.5 15.0 4.5 15.5 6.6

24.0 7.0 -

mg

Sum Molecular norms

-

-

-

-

22.8

15.2

-

-

0.3 0.6

0.0 0.6

54.2

28.6

50.0 6.0 -

9 . 6

1 . 2

7.6 33.6 18.3

11.2 10.8 21.3 -

77.4

66.1

Mean and range, mesostases in 7 chondrules of 18 from Bishunpur; in 19 chondrules of 41 from Semarkona. EPMA with focussed beam. Analysts: C.M.O'D. Alexander and R. Hutchison. Extremes of the range of composition of amorphous "glue". ATEM analyses by C.M.O'D. Alexander. Chondrule mesostases are peraluminous, with normative corundum, whereas the "glue" is peralkaline with normative acmite (NaFe 3+ Si 206).

(atomic)), with excess Na after the "formation" of normative orthoclase and albite. This is true also for the "glue" (Table 7). In contrast to rims, matrix and "glue", many chondrules in Bishunpur and Semarkona have mesostases that are peraluminous, with atomic A1 > (Na + K + 2Ca). Bishunpur matrix and rims have excess alkalis relative to A1, whereas many chondrule mesostases are alkali (and Ca) deficient (Table 7). It is therefore probable that in the LL3s the alkalis were partially redistributed from chondrules into matrix and rims, which is consistent with the acquisition by matrix of volatiles lost during chondrule formation (4). 9. Origin of rims and matrices Most earlier theories of the origin of matrices and rims were of necessity based on chemical data

(e.g. in [2,4]). Huss et al. [11] and Nagahara [7], however, were able to identify the mineral constituents and their textural relationships down to the micrometer size-range. The former authors (p. 46) concluded that "before metamorphism" the opaque matrix of UOCs (being rich in Fe olivine) has a "grain size, mineralogy and composition consistent with the characteristics predicted by Latimer and Anders (1967, 1970) for a low temperature condensate from the solar nebula". Nagahara [7, p. 2590] more cautiously states that a nebular origin "is not clear". The evidence from our ATEM observations is less ambiguous and argues against a nebular origin. Ashworth [6] established that in Chainpur (and Weston, H4) the size-distribution of grains in "clastic matrix" obeys a power law. This also applies to Bishunpur matrix, whose particles range from chondrule fragments to angular olivine and

202 monoclinic low-Ca pyroxene grains with compositions consistent with their derivation from fragmented chondrules. "Large", angular fragments of mainly olivine and pyroxene crystals occur in the rims of Krymka, Sharps, Chainpur and Tieschitz chondrules. The compositional ranges of these grains indicate that they too are from fragmented chondrules [6]. The more Fe-rich groundmass olivines probably belong to a new generation of "non-clastic" matrix with a log-normal size-distribution [6]. Thus, the matrices and rims of the UOCs probably formed from the break-up of chondrules, superimposed on which was a later period of olivine grain-growth, the products of which are most readily observed in the rims. Fragmentation of chondrules would have released the other constituents and not only olivine and pyroxene; mesostases rich in feldspathic elements must have been redistributed. By analogy with the Moon [30,31], if chondrules and their products accreted as a regolith, the finest-grained fraction, in matrices and rims, would be most enriched in the feldspathic component. This is supported by our observations. Sharps is an exception, but Tieschitz is not, for although its rims are barely enriched in feldspathic component, this is present in white matrix. We now: (1) Argue that the chondrules most likely to have fragmented have quartz-normative mesostases. (2) Present chemical analyses of such mesostases from Bishunpur and Semarkona chondrules. (3) Show that reaction of mesostases with FeO and Na2 O could have produced fayalitic (groundmass) olivine and a sodic plagioclase component, as observed.

9.1. Spontaneous fragmentation of chondrules According to Dodd [15, p. 116], "Droplets typically comprise about 25 volume per cent of the chondrules in type 3 ordinary chondrites", which implies that 75% were fragmented or abraded. Both processes produce many small fragments, so unless some mechanism caused their removal, they ought to be present in the ordinary chondrites. This is consistent with the ubiquitous distribution of clastic grains throughout rims and matrices. Chondrule liquids oversaturated in SiO 2 have olivine a n d / o r low-Ca pyroxene as liquidus

phases. Quenching such liquids produces protopyroxene when the mg number is greater than about 75 [32]. Protopyroxene can persist metastably to room temperature, but is prone to rapid inversion to the monoclinic polymorph, accompanied by contraction of almost 3% along the c axis [33]. (No similar phase transformation occurs during the cooling of olivine.) In the laboratory this phase transformation often causes experimental charges (G.M. Biggar, private communication) or synthetic chondrules (G. Lofgren, private communication) to crumble spontaneously and chondrules rich in quenched pyroxene would have had similar behavior. This argument implies that Mg- and pyroxenerich chondrules are underrepresented in chondrites relative to their abundance during chondrule formation. Conversely, the products of their fragmentation should be overrepresented among chondrule rims and interchondrule matrices. These products would have included fragmented forsteritic olivine, low-Ca pyroxene and microcrystalline to glassy mesostases rich in SiO 2.

9.2. Composition of chondrule mesostases--Bishunpur and Semarkona The constituents of chondrules separated from Bishunpur and Semarkona were analysed by EPMA. The chondrules were selected at random, mainly on the basis of sphericity. Seven chondrules from Bishunpur (all with low-Ca pyroxene) and 19 from Semarkona (14 with low-Ca pyroxene) have mesostases with over 64 wt.% SiO 2 (Table 7). The norms of the mesostases have considerable quartz and significant corundum (Table 7). The most apparent difference is in mg number, mesostases in Bishunpur chondrules having more MgO relative to FeO than in Semarkona. As argued above, corundum (excess A1203) in the norms is probably the result of Na loss by volatilization. Our interpretation of the presence of normative corundum as the result of secondary processes implies that chondrules formed from a feldspathic precursor. This is supported by Hutchison et al. [34], who found that the mesostases of most Tieschitz chondrules are albitenormative, and by Wlotzka, who found no deficiency in Na in mean mesostases of Tieschitz and Sharps chondrules [28]. The lack of significant

203 Na enrichment in Tieschitz and Sharps rims is consistent with these observations.

9.3. Origin of fayalitic groundmass olivine The origin of the fayalitic olivine is a problem that was recently addressed by Nagahara [7] and Nagahara and Kushiro [35]. Fayalitic olivine could not have formed by the reaction of FeO with enstatite because Fas0 is the most Fe-rich composition thus attainable, although more Fe-rich compositions could have formed by reaction with Fe-bearing low-Ca pyroxenes. However, we have found no evidence for the preferential destruction of such pyroxenes in matrices and rims. Nagahara [7, fig. lc] did find angular enstatite fragments rimmed by fayalitic olivine (Fo66_57) in "interlocking matrix" in Chainpur. We argue that such textural intergrowths formed after accretion but even here, Fe enrichment has not reached Fos0. In each of Krymka, Sharps, Chainpur and Tieschitz, groundmass olivines are texturally uniform, indicating that during their growth, conditions were uniform throughout the precursor material of each, although rims were originally acquired when chondrules and clasts were at different temperatures (e.g. [16]). The crystallization of interlocking olivine dendrites, as in Krymka rims, seems impossible without the prior juxtaposition of chondrules and clasts as a substrate. Overgrowths of groundmass olivine on clastic cores and the presence of low-Ca pyroxene fragments also suggest that the groundmass formed after accretion. A nebular origin of the fayalitic olivines thus seems to be precluded on textural and compositional grounds. If, as the textural and mineralogical evidence suggests, the groundmass crystallised in situ, from what could it have formed? With the exception of the amorphous "glue" in Bishunpur, the matrices and rims in the UOCs studied do not have a component that can readily be equated with chondrule mesostases, which ought to be represented. The absence of identifiable mesostasis, as "dust" or droplets, could be the result of its having reacted to form the groundmass. This implies that the "glue" and its included clastic grains represent the most primitive type of matrix and rim. We assume that the mesostasis component in rims received Na from hot chondrules and that

abundant FeO was available from the oxidation of metal and sulfide ([18,23]; this work). The products of reaction of mesostases with Na and FeO can be calculated by apportioning Na and excess SiO 2 to normative corundum (Table 7) to " f o r m " albite, and FeO to the remaining SiO 2 and hypersthene to " f o r m " olivine. The recalculated norms indicate that Bishunpur mesostases could have yielded 58% (molecular) olivine, FOll , and 42% feldspar, AbvsOr8An17. Semarkona mesostases could have produced 56% olivine, Fo 4 and 44% feldspar, Ab76Or6An17. In reality, partial equilibration between "glue" and forsteritic olivine clasts would have rendered newly crystallised groundmass olivine more abundant and more magnesian than the calculated values. The normative plagioclase is close to the mean modal composition in equilibrated ordinary chondrites, Ab82Or6Ana2 [36]. This result was unexpected because our calculations aimed to model the formation of fayalitic olivine, not plagioclase. Thus, reaction of disaggregated, peraluminous, Si-rich chondrule mesostases with peralkaline recondensed material and excess FeO could have produced fayalitic olivine and sodic plagioclase "glue" in roughly the proportions observed. In the matrices and chondrule rims of UOCs this groundmass component is, of course, additional to clastic olivine, low-Ca pyroxene, sulfide and unreacted Fe oxides. Redistribution of the chemical elements and crystallization during cooling after hot accretion [16] or thermal metamorphism after cold accretion [3] could have produced the mineral assemblages of types 4 - 6 ordinary chondrites. The amorphous "glue" in Bishunpur may represent a stage in the evolution of mesostasis material, in rims and matrix, before the nucleation of groundmass olivine or abundant smectite. The "glue" is olivine and hypersthene normative and has high mg numbers compared with chondrule mesostases (Table 7). Thus, if derived from Si-rich mesostases, it must have been enriched in FeO and MgO. The Fe content of the precursor of the "glue" would have been buffered by Fe, from Fe-oxides, which would have been exchanged for Mg from ubiquitous forsteritic olivine. The high MgO content of the "glue" relative to chondrule mesostases can therefore be explained. Water may have played some part in this process. In Semarkona, where water was more abun-

204 dant, smectite formed in preference to fayalitic olivine. Progressively higher temperatures and drier conditions presumably led to the growth of fayalitic olivine dendrites, as in Krymka, followed by recrystallization and partial equilibration with clastic olivine in the sequence to Tieschitz, and beyond. This scenario is supported by our observation that in Chainpur and Tieschitz the most fayalitic olivines have radii from 4 to 10/zm (Fig. 4). Presumably these larger grains are the relics of fayalitic groundmasses with a coarser grain-size than in Krymka. 10. Discussion

One possible nebular scenario might include condensation, sintering and intergrowth of fayalitic olivines to enclose free-floating fragments of forsteritic olivine and low-Ca pyroxene. This would, however, demand that micrometer-sized clastic grains remained unreacted with ambient gas during cooling on a nebular scale. The problem is amplified when the pyroxenes are considered, because they probably are inverted protopyroxene from (chondrule?) melts. Such small grains would transform to the ortho form in less than one week at 8 0 0 ° C [37]. Also against an origin from nebular condensate is the fact that the groundmass does not have an unfractionated chondritic chemical composition. Bishunpur matrix comprises 59 vol.% silicate particles, 7% metal/sulfide and 34% voids and material below 0.5 /~m in grain-size. F r o m these data we now estimate approximate limits to the volume of material not of chondrule origin that may be present in Bishunpur matrix. Porphyritic chondrules contain some 65% of optically resolvable crystals and 35% mesostasis (C.M.O. Alexander, unpublished). The latter figure is an upper limit because we have neglected crystallites resolvable in the SEM. The abundance of "true", microcrystalline or glassy mesostasis must be less than 35%. Thus, the particles of chondrule origin should be accompanied by up to 30% mesostasis material and 7% metal/sulfide, accounting for 96% of the volume. If, however, mesostasis is only half as abundant as our estimate, then some 80% of the matrix would be of chondrule origin. This leaves only 4-20% for voids and material that may not have been derived from the fragmentation of

chondrules. We recall that matrix constitutes 14% of bulk Bishunpur, so limiting the nebular or pre-solar component in the bulk to 0.6-2.8 vol.%. Although this estimate is based on limited data, it is supported by our observations that the fraction of matrix with grain-size less than 0.5 /~m does include mineral clasts. We note that Krymka, Chainpur, Sharps and Tieschitz rims comprise clastic grains in a fine-grained groundmass of fayalitic olivine. These meteorites have " m a t r i x " contents similar to Bishunpur [11], so they must have an even smaller proportion of low-temperature material. Although we have been unable to identify it, the UOCs do have a low temperature, interstellar component [38,39]; matrices and rims were not derived exclusively from disrupted chondrules. We recall that layering in some rims may be the result of condensation from, or reaction with, a S-rich vapor. In addition, Clayton et al. [40] found that in H-group chondrites, where numerous chondrules are less than 300/zm, there is enrichment in 160. If the enrichment were the result of isotopic exchange with a (nebular?) gas, then matrices and rims in U O C s ought to be even more enriched and the presence of an extraneous component is implied. This component could have been present in a reactive carrier comprising 1 - 3 vol.% of the matrix. The reality is more complex, however, because Semarkona matrix has oxygen isotopic ratios depleted in 160 relative to chondrules [41], but Allan Hills 76004 (LL3) is enriched, as expected [42]. We stress that isotopic inhomogeneity in oxygen among chondrules and matrices is a problem for any theory in which they are genetically related. We suggest that the lithophile elements in all ordinary chondrites were contributed by a single precursor population of chondrules. Different proportions of this population became disrupted before or during the accretion of different UOCs, to produce different proportions of r i m / m a t r i x relative to chondrules and clasts. Chondrule formation, disruption and r i m / m a t r i x formation were probably repetitive and overlapping processes. Accretion, however, must have been complete, with little size fractionation of silicate dust from larger chondrules or clasts. Thus the ordinary chondrites must represent the complete recombination of pre-existing silicate material after one or

205 more chondrule-forming events, otherwise how may we account for the essentially uniform lithophile element abundances? We point out that if pyroxene-rich chondrules preferentially fragmented, the remaining olivine-rich chondrules should have a higher M g / S i ratio than the bulk. This holds for Tieschitz, the average M g / S i ratio of each of Wlotzka's groups of chondrules being greater than that of the bulk chondrite [28]. The metal-silicate fractionation between the ordinary chondrite groups poses a problem for our hypothesis, some incomplete accretion of metal being required if a single precursor gave rise to all three groups. 11. Conclusions

We surveyed six UOCs from the millimeter to the sub-micrometer range. Opaque matrices are dominated by clastic material. In Bishunpur its size distribution obeys a power law, as in Chainpur and Weston [6]. Matrices generally are composed of clastic olivines and pyroxenes, similar in composition to those in chondrules, plus feldspathic material and perhaps groundmass, fayalitic olivine. Opaque rims are finer-grained than matrix except for some "matrix-like" rims. Rims are often layered; some layering may be the result of reaction with a fluid, or fluids. Rims always contain some clastic grains, usually cemented by a nonclastic groundmass dominated by fayalitic olivine and a feldspathic component. Rims and matrices contain Fe oxide minerals and are more oxidized than co-existing silicate and metal/sulfide chondrules. In Semarkona, matrix and rims lack fayalitic olivine but have abundant smectite [13,21]. White matrix in Tieschitz (rarely in Sharps chondrules) is largely an intergrowth of two NaAl-rich minerals. In Tieschitz the feldspathic and fayalitic components may have been separated into matrix, and rims, respectively. In the LL-group, rims and matrices are enriched in feldspathic constituents relative to the bulk meteorites, but "enrichment" can result from selection bias against the inclusion of clastic grains in the areas analyzed. Rims and matrices are composed of fragmented chondrules. Those most likely to fragment have high mg numbers, are rich in normative quartz,

and precipitated protopyroxene that inverted to clinopyroxene. The clinopyroxene has a basal {001} parting which would prodtice fragments of rectangular cross-section and multiple twinning parallel to {100}, normal to the elongation. Such clastic pyroxenes are common in rims and matrices of UOCs [2]. We have failed to identify interstellar, presolar or nebular material of high- or low-temperature origin. Its abundance is limited to 3 vol.% in each bulk meteorite, apart from Semarkona, where hydrothermal alteration has erased the evidence. Equilibrium condensation from a gas of solar composition [3] cannot account for the abundances of volatile elements such as TI and Bi. The presence of 1-3% of reactive material rich in volatiles and in 160 must be invoked. Such " m y s terite" [43] may have been composed of phyllosilicate, carbonate, fine-grained metal and sulfide. In the UOCs it presumably reacted with high-temperature material to leave only its signature of enhanced volatile element and 160 contents. Alternatively, the ordinary chondrites underwent open system behavior after accretion. We believe that we have produced a self-consistent theory of the origin of matrices and rims based on direct mineralogical, textural and chemical observations, the interpretation of which is supported by laboratory experiment. Acknowledgements

This work was partly supported by the Natural Environment Research Council of the U.K. We thank Roy Clarke and the Smithsonian Institution for samples of Semarkona and Sharps, and two referees, various colleagues including Monica Grady, Andrew Graham, Paul Henderson and Ian Wright, for discussion and comment. References

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