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Accretionary dark rims in unequilibrated chondrites
ICARUS 48, 460--472 (1981)
Accretionary Dark Rims in Unequilibrated Chondrites TRUDE V. V. KING 1 AND E L B E R T A. KING Department of Geology, University of Houston, itouston. Textls 770(.14 Received February 17, 1981; revised October 19, 1981 Textural and qualitative E D X investigations of dark-rimmed particles in six low petrologic type chondrites indicate that the rims accreted on host particles over a wide range of temperatures prior to initial accumulation and lithification of the meteorites in which the rimmed particles are now contained. Many dark rims are enriched in moderately volatile trace elements such as Na, CI, P, and K, relative to the host particles and matrix. The range of physical/chemical e n v i r o n m e n t s associated with hypervelocity impacts m a y have offered the setting for the formation of darkr i m m e d particles early in solar s y s t e m history. INTRODUCTION AND PREVIOUS WORK
The fine-grained dark material that surrounds some chondrules and lithic fragments in unequilibrated chondrites has been recognized for many years. However, only recently have attempts been made to investigate this material in detail (Reid and Fredriksson, 1967; Dodd and Van Schmus, 1971: Fuchs et al., 1973; Christophe Michel-Levy, 1976; Ashworth, 1977: Fruland et al., 1978; Wilkening et al., 1978: Allen et al., 1979: Nozette et al., 1979; Hutchison et a l . , 1979; Vander Velden, 1979). Observations confined to the Sharps and Hallingeberg chondrites led Dodd and Van Schmus (1971) to suggest rapid heating, perhaps by shock metamorphism of coarse material in situ, as a possible origin of "'dark zoned chondrules.'" In their description of the Murchison meteorite, Fuchs et al. (1973) illustrated a "'typical white inclusion surrounded by a narrow band of fine grained inclusion free matrix." In 1976, Christophe Michel-Levy suggested that the dark matrix material in Tieschitz was acquired as rims on particles before final accumulation into the meteorite. She found that the rims are carbon rich t Present address: Planetary Geosciences, Hawaii Institute o f G e o p h y s i c s , University o f Hawaii, Honolulu, Hawaii 96822.
compared to ordinary matrix material, but did not attempt to explain further the origin of the dark material. In high-voltage electron microscope observations of chondrites, Ashworth (1977) distinguished "non-clastic material in rims around chondrules in Chainpur." He attributed the dark color of the rims in thin section as due to the combination of fine grain size and the presence of opaque minerals. He determined that at least some dark rims have two parts, an inner silicate portion and an outer troilite-rich region. Ashworth stated that grain size as fine (mode approx 14.5 ~0) as those observed in the Chainpur dark rims can result from the rapid crystallization of a melt or glass, although many textural characteristics common to quenched melts are not present. He concluded that the rims were added to the chondrules before their incorporation into the meteorite and that they may have accreted from dust, melt, or both. A brief discussion of "dark zones" around dark lithic inclusions and their similarity to "dark halos" in the Allende meteorite was reported by Fruland et al. (1978). They suggested that these " z o n e s " could be formed by reaction of the inclusion with matrix at the time of accretion or at later times by post emplacement compaction and/or metamorphism. According to Fruland et al. (1978), a less probable alterna460
0019-1035/81 / 120460-13502.00/0 Copyright (~F 1981by AcademicPress. Inc All rights of reproduction in any form reserved
ACCRETIONARY DAR K RIMS tive for the origin of these dark zones "is an accretionary mantle that was accumulated prior to or during accumulation of the meteorite." Hutchison et ai. (1979), in their discussion of the Tieschitz (H3) meteorite, stated that "the meteorite comprises millimetre fragments and spherical chondrules of silicate, all of which have opaque rims." Wilkening et al. (1978) suggested that the "accretionary" rims surrounding chondrules in unequilibrated chondrites are represented by three types: (1) those that are characterized by an overall enrichment of sulfides, which are disseminated throughout very fine-grained material, and are free of larger grains; (2) rims relatively free of sulfides, appearing translucent in transmitted light and free of larger grains; (3) rims with massive sulfides, such as "canned" or armored chondrules in equilibrated chondrites. In subsequent work by the University of Arizona group, Allen et al. (1979, 1980) state that there are two types of rims: sulfide rich and sulfide poor. They conclude from their scanning electron microscope (SEM) observations that the rims appear to be coatings of fine-grained, relatively lowtemperature material accreted or deposited on individual chondrules before agglomeration, brecciation, and metamorphism. Nozette et al. (1979) and Allen et al. (1980) attribute the variations of texture and the sulfide grain sizes that they observed to slightly differing conditions of metamorphism during coalescence, sintering, cooling, or brecciation. They report that their overall study (University of Arizona group) found that the opaque chondrule rims are composed of variable amounts of finegrained silicates and sulfides; that rims are enhanced in moderately volatile elements; and that rims appear to have been deposited on preexisting "cold" chondrules. The present work further expands the knowledge of dark-rimmed inclusions by (1) showing textural relations between the dark rim material and the central particle and dark rim material and the matrix; (2)
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supplying additional mineralogical and chemical details of the dark rim material itself and the minerals included within it; and (3) a suggestion concerning the origin of dark rims. NOMENCLATURE AND EXPERIMENTAL METHODS The general nomenclature and classification of meteorites used in this work is taken from Wasson (1974). Chondrule terminology is from King and King (1978). In this paper, the definition of dark rims is based on textural evidence from the petrographic microscope in a combination of transmitted and reflected light. Rims are neither necessarily continuous nor uniform in thickness about any particle. In transmitted light, in standard thickness sections, the rims are opaque, contain only rare clasts, and commonly have mineral grains and/or lithic fragments aligned parallel to their outer edge in the matrix of the meteorite. In most cases, the contact of the dark rim and the particle it surrounds is well defined, but the outer margin of the dark rim commonly is gradational with the meteorite matrix. In reflected light, the rims commonly appear fine grained and contain reflective minerals, which have a wide range in abundance and grain size. Samples of six low petrologic type meteorites [Inman-L3 (Find), Bishunpur-L3 (Fall), Prairie Dog Creek-H3 (Find), Tieschitz-H3 (Fall), Murray-CM2 (Fall), and Ailende-CV3 (Fall)] were obtained from the collections of the British Museum, National Museum of Natural History, and American Museum of Natural History. Polished thin sections were prepared for those samples for which suitable thin sections for use in the petrographic microscope and scanning electron microscope were not already available. It is advantageous to work with polished thin sections rather than unshaped chips as the geometry of the included mineral phases is easily recognizable and the flat surface provides a relatively uniform plane for the incident electrons associated
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with the energy dispersive X-ray analysis unit. The thin sections were examined and photographed under the petrographic microscope in a combination of transmitted and reflected light. These photographs served as index photos for finding areas to be investigated with the SEM. The sections were ion etched on a Commonwealth Scientific Corporation Model VII ion-micro milling instrument (IMMI). This procedure is necessary for generating minor relief on the surface of the thin section such that textural relations can be readily observed with the SEM. A Jeolco JSM-U3 scanning electron microscope with an accelerating voltage of 25 kV and a Princeton Gamma Tech energy dispersive X-ray analysis unit (EDX), for qualitative elemental analysis and determination of mineral chemistries, were investigative tools used in this work. OBSERVATIONS AND ANALYSES Dark rims occur on both fluid drop and lithic chondrules and lithic clasts in the low petrologic types of ordinary and carbonaceous chondrites. The rims are neither necessarily continuous around the host particle nor of uniform thickness, and are not ubiquitous on panicles in any meteorite examined. In transmitted light, the dark rims mostly appear to be devoid of clasts but commonly have matrix mineral grains or lithic fragments aligned parallel to their outer margins. In reflected light the dark rims commonly appear to be finer grained than the central host particle or the surrounding matrix. In reflected light the rims are observed to contain at least one opaque mineral phase. In SEM images and elemental maps, the dark rim materials are obviously both texturally and chemically heterogeneous, whether in intra- or interspecimen comparisons. In spite of these textural differences it is possible to classify dark rims into two major categories, i.e., sulfide-rich and sulfide-poor dark rim materials as has been suggested previously by Allen et al. (1979, 1980).
Texturally, the dark rim material commonly appears to be aphanitic to amorphous at the scale of SEM images. However, rare fragments of mechanically included crystalline materials do occur within the very fine-grained dark rim material. The composition of the predominant component, the aphanitic to amorphous material in the dark rim, most commonly approximates a relatively iron-rich pyroxene, tentatively identified by EDX analyses. Also occurring as a dominant component of rims is a mixture of troilite and nickel-iron and/or a F e - N i - S - O phase similar to that described in Ramdohr (1973) and others. Recent work by Kerridge and Bunch (1979) and Bunch and Chang (1980) refers to this latter phase as "'poorly characterized phase (PCP)." Figure I shows sulfide-rich dark rim material in the Inman (L3) chondrite. In this image, the contact between the dark rim and the chondrule is defined by a decrease in the grain size in the dark rim material as compared to the interstitial glassy groundmass of the chondrule. At some areas around the circumference of the chondrule, the margin between the rim and chondrule appears to be texturally gradational, thus making it difficult or impossible to delineate an exact margin between the two regions. The fine-grained inner portion of the dark rim has an EDX elemental distribution pattern indicative of a mixture of a silicate (probably pyroxene), a sulfide (troilite), and nickel-iron. Individual amounts of the elements that constitute these minerals range from location to location; however, they are present in all areas of this rim. The outer margin of the dark rim in the Inman chondrite is enriched in troilite, hematite (terrestrial weathering product in keeping with the reddish color in transmitted light), and nickel-iron. In most sulfide-rich dark rims, the reflective minerals are located preferentially along the outer margin of the dark rim material as more massive accumulations; however, they also are present as disseminated grains within the amorphous to finely crystalline
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FIG. I. This scanning electron microscope image shows the typical textural relation between the chondrule and the rim, and the rim and the matrix in Inman inclusion I, which is a good example of a sulfide-rich dark rim. The boundary between the chondrule, that portion of the image that shows euhedral crystals in intersitital glass, and the dark rim is defined by a decrease in the grain size as compared with the interstitial groundmass of the chondrules. At some places around the circumference oftbe chondrule the two regions appear to be texturally as well as mineralogically gradational, making it difficult to determine an exact margin between the chondrule and the dark rim material. The boundary between the rim and the matrix commonly is well defined, as it is in this figure, owing to the concentration of highly reflective minerals near the exterior portion of the rim. Width of field of view is approximately 400 tzm. d a r k r i m m a t e r i a l a n d , in s o m e c a s e s , in small a m o u n t s a l o n g the i n t e r i o r m a r g i n o f the dark rim. T i e s c h i t z i n c l u s i o n 1 ( F i g . 2) e x e m p l i f i e s a s u l f i d e - p o o r d a r k rim. In this i m a g e , the finely c r y s t a l l i n e to a m o r p h o u s d a r k r i m m a t e r i a l h a s an E D X s p e c t r u m s i m i l a r to w h a t w o u l d be e x p e c t e d f r o m a r e l a t i v e l y iron-rich pyroxene. The elemental abund a n c e s o f Mg, F e , Si, a n d m i n o r Al v a r y f r o m o n e a n a l y s i s a r e a to a n o t h e r , but m a i n t a i n a p p r o x i m a t e l y the s a m e r a t i o s , a n d o n l y the m o d e r a t e l y v o l a t i l e t r a c e elem e n t s a p p e a r a n d d i s a p p e a r f r o m the s p e c t r a as t h e p o s i t i o n o f the b e a m is s h i f t e d . W i t h i n t h e d a r k r i m m a t e r i a l o c c u r the relat i v e l y v o l a t i l e e l e m e n t s N a , P, Ci, a n d K.
N o troilite w a s o b s e r v e d in the d a r k m a t e rial o f T i e s c h i t z ; h o w e v e r , a s m a l l a m o u n t of nickel-iron was detected. The bounda r i e s b e t w e e n the rim a n d the c h o n d r u l e and b e t w e e n the rim a n d the m a t r i x are well d e f i n e d in the S E M i m a g e s o f T i e s c h i t z inc l u s i o n 1. T h e d a r k rim m a t e r i a l is disting u i s h e d f r o m the m a t r i x a n d the c h o n d r u l e b y its o v e r a l l m u c h finer g r a i n size. T h e d a r k r i m m a t e r i a l a l s o h a s , in m o s t c a s e s , a w e l l - d e f i n e d m a r g i n w i t h the m a t r i x . In reg i o n s w h e r e the c o n t a c t is n o t well d e f i n e d , it is still d i s c e r n i b l e , o w i n g to the r e l a t i v e l y m o r e h o m o g e n e o u s t e x t u r e o f the p r e d o m i n a n t c o m p o n e n t o f the d a r k rim m a t e r i a l as c o m p a r e d w i t h t h a t o f the m a t r i x . It is n o t the c a s e , h o w e v e r , in all inclu-
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FIG. 2. SEM image (width of field of view 160 t~m) of Tieschitz inclusion 1 shows a typical relation between the chondrule and the rim and the matrix in a sulfide-poor dark rim. The dark rim material in this image is easily distinguished from the matrix and the chondrule by its overall much finer grain s i z e however, rarely within this amorphous to aphanitic material are crystals of other mineral fragments included as is shown in the upper right of this image.
sions studied that the margin between the dark rim and the matrix is well defined. In some isolated regions of a given inclusion rim, the margin may not be discernible. Inclusion 2 of Bishunpur (Fig. 3) is an example of the case in which the outermost margin of the dark rim is poorly defined or indistinguishable for most regions of the chondrule. Figure 4 shows a greater magnification SEM image of the predominant amorphous to aphanitic component of the dark rim material in Tieschitz inclusion I. Within this relatively texturally uninteresting material occur inclusions of mineral crystals and fragments that commonly have elemental distribution patterns similar or identical to the major mineral phases in the central particle that the dark rim is surrounding. In the ordinary chondrites and Allende, texturally and mineralogically distinct re-
gions occur near the nucleus/rim boundary and commonly have elemental distribution patterns indicative of whitlockite, melilite, or, rarely, a plagioclase feldspar. Whitlockite is present within the dark rim material of the inclusions studied in Inman and Prairie Dog Creek, and melilite was found in the dark rim materials in Bishunpur and AIlende. The only ordinary chondrite in which no mineral inclusions of composition different from the phases in the surrounding matrix or host particle were found was within the dark rims in Tieschitz. In the amorphous to aphanitic portion of the dark rim material of each of the two inclusions studied in Murray (CM2), abundant euhedral crystals are present (Fig. 5). These are the only relatively large euhedral crystals found within the dark rim material I Figs. 6 and 7). To date, the only occurrences of these crystals found within Mur-
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FIG. 3. SEM image of Bishunpur inclusion 2, showing the discontinuous dark rim around the chondrule (dark rim material in center portion of image). The rim is most easily recognized in areas that have a reflective mineral phase adjacent to the chondrule, or where the dark rim material is constricted, as in the upper left side of the image. The exterior margin of the rim is poorly defined or commonly not distinguishable. Width of field of view is approximately 1100t~m. ray are within the dark rims. It appears that these crystals are either monoclinic or orthorhombic, although from images it is not possible to determine the s y m m e t r y of these crystals unambiguously. E D X spectra indicate that the crystals are enriched in the elements sodium, sulfur, and o x y g e n relative to the predominant c o m p o n e n t o f the dark rim. It appears, from examination of SEM images, that the crystals have grown within the dark rim material and are not simply superposed on it. Although as yet we have neither complete chemical analyses o f the crystals nor X-ray diffraction data, we believe that this may be the first meteoritic o c c u r r e n c e of the sodium sulfate mineral thenardite, which commonly occurs as an evaporite mineral on the surface o f the Earth. We believe that these crystals formed on the meteorite parent b o d y and may be further evidence of
aqueous alteration inferred by others. The offset o f the crystals across cracks in the petrographic thin section and the appearance of the material penetrating into the dark rim material along the margin of the crack (Fig. 8) lead us to believe that these crystals cannot be an artifact o f sample preparation. In all the meteorites examined, the predominant amorphous to aphanitic component o f the dark rim material is mineralogically distinct, as well as texturally distinct, from the general meteorite matrix material that surrounds it. The predominant c o m p o n e n t of all the dark rims is a mixture of two or more mineral phases. From E D X spectra the amorphous to aphanitic phase, as has previously been mentioned, appears to be a mixture o f a silicate, a sulfide, and nickel-iron that ranges in mineral proportions from location
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FIG. 4. SEM image showing the contact of the silicate chondrale and the amorphous to aphanitic material in an embayed portion of the chondrule in Tieschitz inclusion I. Within this texturally uninteresting material occur inclusions of mineral crystals and fragments that commonly have elemental distribution patterns similar or identical to the major mineral phase that the dark rim is surrounding. This image also shows the silicate border around the circumference of the chondrule. This image is texturally representative of the dark rim material for virtually all the clark rims studied, with the exception of Allende, which has portions of its dark rim materials that appear ttexturally and mineralogically) similar to the comminuted crystals in its matrix. Width of field of view is 12/zm.
to location, or a silicate in addition to a F e N i - S - O phase as described by R a m d o h r (1973) and others. It is not known in what mineral phases the moderately volatile elements, such as Na, CI, P, and K, are present in the dark rim material, but it is believed that they are present in glass or as very fine alteration products or minerals on other grains. E D X spectra o f the matrix materials of the ordinary chondrites indicate that they are a mixture o f more than one phase and are lacking the concentration of moderately volatile trace elements that are present in the dark rim materials. CONCLUSIONS AND INTERPRETATIONS From textural relations between the host chondrule or lithic clast and the corre-
sponding dark rim material in each of the inclusions, it is obvious that the dark rim material was added to the particle subsequent to its formation and prior to its incorporation into the present meteorite. Furthermore, the dark rims are relatively enriched in moderately volatile elements as c o m p a r e d to the host chondrule or lithic clast or the enclosing meteorite matrix. We believe that the textures o b s e r v e d in the dark rim material and the distribution of the mineral phases within the dark rim material are the result o f the t e m p e r a t u r e differences b e t w e e n the host particle and the accreting dark rim material. The dark rim material that surrounds the chondrules and clasts investigated in this study have only the most basic characteristics in c o m m o n . Each inclusion is unique,
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FIG. 5. SEM image of the dark rim material and the euhedral crystals present within it as observed in two inclusions, in the Murray carbonaceous chondrite. These crystals are the only euhedral crystals observed in the dark rim material and appear to be either monoclinic or orthorhombic. Although we are not able to determine their symmetry unambiguously from images, these crystals are not similar to any phase associated with the particle that the dark rim is surrounding. Width of field of view is 36/~m. and only the most basic scenarios concerning their origins can be developed from their textural and mineralogical relations. The most probable p r i m a r y origin of dark rims is by hot particle accretion. It s e e m s obvious that some dark rim material was hotter than other dark rim material at the time o f accretion onto the host particle. The dark rim material that is relatively enriched in sulfides is likely to have been at a lower t e m p e r a t u r e when it was accreted onto the nucleus than dark rim material that is depleted in sulfides. It is impossible to determine whether or not the sulfide-poor dark rim material e v e r had any sulfides associated with it; h o w e v e r , in other parts o f the meteorite, which have generally the same bulk composition, sulfides are associated with the silicates, thus leading one to believe that the adhering particles are devoid of sulfides because of high temperature.
The dark material around Tieschitz inclusion 1 is believed to have had the greatest t e m p e r a t u r e of accretion of the inclusions examined. Within the dark rim there are no sulfides present: h o w e v e r , a small amount o f n i c k e l - i r o n is present. The sulfide-poor dark rim material probably accreted at t e m p e r a t u r e s lower than the liquidus t e m p e r a t u r e s o f the mafic silicates. Accreting dark rim material might or might not react with the nucleus depending on the t e m p e r a t u r e of the nucleus. During this accretionary process some nuclei were w a r m and plastic, capable of reacting with the adhering dark rim material, and other nuclei were cold, rigid, and incapable o f reacting with the accreting dark rim material. Inman inclusion l can be used to illustrate the scenario o f dark rim accretion on a w a r m and possibly hot and plastic chondrule. In reflected light and in SEM images,
FIcs. 6 and 7. SEM images of sodium sulfate crystals (thenardite?) in the dark rim material of the Murray (CM2) carbonaceous chondrite. Energy dispersive X-ray analysis of this crystal shows only sodium, sulfur, and oxygen in concentrations greater than the composition of the substrate. Magnification is 16,000 and 20,000x, respectively. 4614
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FIG. 8. SE M image (width of field of view 6.8/~m) of a group of white crystals present in the dark rim materials of Murray inclusion 1 and inclusion 2. This particular group is from Murray inclusion I and shows the offsetting of two crystals along a crack in the thin section. This leads the investigators to believe that the materials are not the result of an interaction between the meteorite and the atmosphere during or post sample processing, but are indigenous to the meteorite. the interstitial glass that is present b e t w e e n the relatively iron-rich crystals in the chondrule of I n m a n inclusion 1 a p p e a r s to have reacted with the dark rim material. In some portions of the chondrule the interstitial glass and dark rim material a p p e a r s to have a gradational contact. The compositions of the interstitial glass and the silicate portion o f the dark rim material are so similar that they might have been hot and plastic simultaneously. It should be noted that the exterior margin of the dark rim material is enriched in minerals such as troilite that are stable at lower t e m p e r a t u r e s . Figure 1 shows the contact o f the dark rim material and the chondrule. The interstitial glass in this inclusion a p p e a r s to have reacted with the dark rim material in such a way, probably by an exchange of elements across their mutual margin, that the dark rim material is more crystalline along its
interior margin. The crystalline texture grades outward into a more aphanitic and volatile-rich outer region. We believe that it was necessary for the relatively more massive chondrule to be the portion of I n m a n inclusion 1 that retained the heat necessary to allow the interaction b e t w e e n the chondrule and the rim to proceed, possibly aided b y the heat of crystallization of the central particle. It does not s e e m possible that the dark rim material had sufficient heat capacity to maintain a t e m p e r a t u r e that could bring about the observed interaction. If the t e m p e r a t u r e o f the relatively thin dark rim material remained sufficiently great to partially melt the interstitial glass in the chondrule, the temperature gradient between the inner and outer portions o f the rim probably would not be sufficient to allow the formation o f troilite along the outer margin. It is possible, how-
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FIG. 9. SEM image o f B i s h u n p u r inclusion I, showing the crack around the circumference o f the chondrule that separates the innermost boundary of the dark rim material from the host nucleus. This is a good e x a m p l e of dark rim accretion on a relatively cold previously existing chondrule. Width of field o f view is - 1100 p,m.
ever, that the troilite adhered to the silicate portion of the dark rim subsequent to rim formation. Several of the remaining dark-rimmed inclusions can serve as e x a m p l e s for the scenario that a chondrule or lithic clast can be relatively cold and have an accreted rim. Bishunpur inclusion 1, Figure 9, perhaps is the best example of dark rim accretion on a relatively cold previously existing chondrule. In this figure the dark rim material is mostly separated from the chondrule by a crack. It is possible to attribute this crack to temperature differences between the cold chondrule and the hot dark rim material. The accreting dark rim material was not hot enough to sinter to the chondrule and adhere. Thus, as the dark rim cooled, a crack developed b e t w e e n the chondrule and the dark rim material. The cooling of the dark
rim material was more rapid along the margin of the chondrule than the exterior portions o f the dark rim, thus resulting in the aphanitic texture in the interior portion o f the rim, and in gradual cooling on the margin, allowing for the formation of troilite. During sample preparation the authors have also noted that it is not u n c o m m o n for a fluid drop chondrule or a lithic fragment to fall out of the specimen leaving a distinct rind or rim intact in the matrix material, thus leading us to believe that this temperature relation was c o m m o n within unequilibrated meteorites. This point should be noted by those persons attempting to separate chondrules and clasts from the matrix for individual chemical analysis as it may well bias their data. In other inclusions the crack between the two portions of the inclusion is absent, but the general texture of the rims are corn-
ACCRETIONARY DARK RIMS pletely aphanitic and shows no textural evidence of any interaction between the dark rim material and the host particle. There are many observations of shock effects and other impact phenomena in chondrites. The observation of chondrules in a number of lunar breccia samples ( King et al., 1972; Kurat et al., 1972; Nelen et al., 1972; and others) plus the fact that both fluid drop and lithic chondrules (King and King, 1978) occur together in chondrites and lunar samples has now led a number of workers to suggest that at least some, if not many, meteoritic chondrules have been produced by impact processes (e.g., King et al., 1972). A previous interpretation of the origin of dark rims on chondrules by King and King (1980) suggested that some may have been formed in the plumes over impact craters "in a manner analogous to volcanic accretionary lapilli." However, at that time no accretionary lapilli from impact deposits had been described. This interpretation has recently received support from observations of Graup (1981), who finds excellent examples of accretionary lapilli in ejecta deposits from the Ries Crater, Germany. Thus, although there are a few studies that theoretically or experimentally demonstrate the physical conditions within the ejecta curtain or the plume over an impact crater, such a setting appears to provide the range of environments required to produce accretionary dark-rimmed chondrules and other particles. ACKNOWLEDGMENTS The investigators express appreciation to Brian Mason, National Museum of Natural History, Martin Prinz, American Museum of Natural History, and R. Hutchison, British Museum of Natural History for the loan of meteorite samples used in this work. Thin sections not borrowed from other sources were made in Northrop Services, Inc. thin section laboratory and by David S. Pettus. Portions of the work were computed while T.V.V.K. was a Visiting Scientist at the NASA, Johnson Space Center, in the scanning electron microscope laboratory of David S. McKay. A special thanks goes to two anonymous reviewers and to Georgann Nace for her tireless assistance. This
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work was financially supported in part by the University of Houston Geology Foundation and by NASA Grant NSG 7617.
REFERENCES ALLEN, J. S., NOZETTE, S., AND WILKENING, L. L. (1979). Chondrule rims: Composition and texture. Lunar Planet. Sci. 10, 27-29. [Abstract] ALLEN, J. S., NOZETTE, S., AND WILKENING, L. L. (1980). A study of chondrule rims and chondrule irradiation records in unequilibrated and ordinary chondrites. Geochim. Cosmochim. Acta 44, 11611175. ASHWORTH, J. R. (1977). Matrix textures in equilibrated ordinary chondrites. Earth Planet. Sci. Lett. 35, 25-34. BUNCH, T. E., AND CHANG, S. (1980). Carbonaceous chondrites. II. Carbonaceous chondrite phyllosilicates and light clement geochemistry as indicators to parent body processes and surface conditions. Geochim. Cosmochim. Acta 44, 1543-1577. CHRISTOPHE MICHEL-LEVV, M. (1976). La matrice noire et blanche de la chondrite de chondrite de Tieschitz (H3). Earth Planet. Sci. Lett. 30, 150. DODD, R. T., AND VAN SCHMUS, W. R. (1971). Darkzoned chondrules. Chem. Erde 30, 59-69. FRULAND, R. M., KING, E. A., AND McKAv, D. S. (1978). Allende dark inclusions. Proc. Lunar Planet. Sci. Conf. 9th: Geochim. Cosmochim. Acta Suppl. 10, I, 1305-1392. FUCHS, L. H., OLSEN, E., AND JENSEN, K. J. (1973). Mineralogy, mineral chemistry and composition of the Murchison (C2) meteorite. Smithson. Contrib. Earth Sci. 10, 39. GRAUP, G. (1981). Terrestrial chondrules, glass spherules and accretionary lapilli from the suevite, Ries Crater, Germany. Submitted for publication. HUTCmSON, R., BEVAN, A. W. R., AGRELL, S. O., AND ASHWORTH, J. R. (1979). Accretion temperature of the Tieschitz, H3, chondrite meteorite. Nature 280, !16-119. KERRIDGE, J. R., AND BUNCH, T. E. (1979). Aqueous activity on asteroids: Evidence from carbonaceous chondrites. In Asteroids (T. Gehrels, Ed.), pp. 745765. Univ. of Arizona Press, Tucson. KING, E. A., CARMAN, M. F., AND BUTLER, J. C. (1972). Chondrules in Apollo 14 samples: Implications for the origin ofchondrite meteorites. Science 175, 59-60. KING, T. V. V., AND gANG, E. A. (1978). Grain size and petrography of C2 and C3 carbonaceous chondrites. Meteoritics 13, I, 47-72. KING, T. V. V., AND KING, E. A. (1980). Accretionary dark rims in unequilibrated ordinary chondrites. Lunar Planet. Sci. 11, 2, 557-559. KURAT, G., KEIL, K., PRINZ, M., AND NEHRU, C. E. (1972). Chondrules of lunar origin. Proc. Lunar Sci.
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('raft: 3rd: (;eoehim. Cosmochim. Acta Suppl. 3, 1, 707-721. NELEN, J., NOONAN, A., AND FREDRIKSSON, K. (1972). Lunar glasses, breccias and chondrules. Proc. Lunar Sci. Conf. 3rd; (,eochim. ('o~'mochim. Acta Suppl. 3, I, 723-737. NOZETTE, S., ALLEN, J. S., A N D WILKENING, L. L. (1979). Chondrule rims and chondrule formation. Programs and Abstracts. 42nd Annual Meeting Meteoritical Society 74. RAMDOHR, P. (1973). The Opaque Minerals in Stony Meteorites. Akademie-Verlag, Berlin. REID, A. M., AND FREDmKSSON, K. (1967). Chondrules and chondrites. 1n Researches in Geochemis-
try (P. H. Abelson, Ed.). Vol. 2, pp. 170-203. Wiley, New York. RWH.~,Rr)SON, S. M. (1978). Vein formation in the C I carbonaceous chondrites. Meteoritic,s 13, 141-159. VANDER VELDEN, T. (1979). Mineralogical and petrological investigation of dark rims in low petrologic types of ordinary and carbonaceous chondrites. Master's thesis, University of Houston. WASSON, J. T. (1974). Meteorites. Springer-Verlag, Berlin. WILKENING, L. L., ALLEN, J. S., NOZETTL, S., AND SOEI.INGER, N. A. (1978). Chondrules: A study of dark rinds and nuclear tracks. Meteoritics 13, 669. [Abstract]
Accretionary Dark Rims in Unequilibrated Chondrites TRUDE V. V. KING 1 AND E L B E R T A. KING Department of Geology, University of Houston, itouston. Textls 770(.14 Received February 17, 1981; revised October 19, 1981 Textural and qualitative E D X investigations of dark-rimmed particles in six low petrologic type chondrites indicate that the rims accreted on host particles over a wide range of temperatures prior to initial accumulation and lithification of the meteorites in which the rimmed particles are now contained. Many dark rims are enriched in moderately volatile trace elements such as Na, CI, P, and K, relative to the host particles and matrix. The range of physical/chemical e n v i r o n m e n t s associated with hypervelocity impacts m a y have offered the setting for the formation of darkr i m m e d particles early in solar s y s t e m history. INTRODUCTION AND PREVIOUS WORK
The fine-grained dark material that surrounds some chondrules and lithic fragments in unequilibrated chondrites has been recognized for many years. However, only recently have attempts been made to investigate this material in detail (Reid and Fredriksson, 1967; Dodd and Van Schmus, 1971: Fuchs et al., 1973; Christophe Michel-Levy, 1976; Ashworth, 1977: Fruland et al., 1978; Wilkening et al., 1978: Allen et al., 1979: Nozette et al., 1979; Hutchison et a l . , 1979; Vander Velden, 1979). Observations confined to the Sharps and Hallingeberg chondrites led Dodd and Van Schmus (1971) to suggest rapid heating, perhaps by shock metamorphism of coarse material in situ, as a possible origin of "'dark zoned chondrules.'" In their description of the Murchison meteorite, Fuchs et al. (1973) illustrated a "'typical white inclusion surrounded by a narrow band of fine grained inclusion free matrix." In 1976, Christophe Michel-Levy suggested that the dark matrix material in Tieschitz was acquired as rims on particles before final accumulation into the meteorite. She found that the rims are carbon rich t Present address: Planetary Geosciences, Hawaii Institute o f G e o p h y s i c s , University o f Hawaii, Honolulu, Hawaii 96822.
compared to ordinary matrix material, but did not attempt to explain further the origin of the dark material. In high-voltage electron microscope observations of chondrites, Ashworth (1977) distinguished "non-clastic material in rims around chondrules in Chainpur." He attributed the dark color of the rims in thin section as due to the combination of fine grain size and the presence of opaque minerals. He determined that at least some dark rims have two parts, an inner silicate portion and an outer troilite-rich region. Ashworth stated that grain size as fine (mode approx 14.5 ~0) as those observed in the Chainpur dark rims can result from the rapid crystallization of a melt or glass, although many textural characteristics common to quenched melts are not present. He concluded that the rims were added to the chondrules before their incorporation into the meteorite and that they may have accreted from dust, melt, or both. A brief discussion of "dark zones" around dark lithic inclusions and their similarity to "dark halos" in the Allende meteorite was reported by Fruland et al. (1978). They suggested that these " z o n e s " could be formed by reaction of the inclusion with matrix at the time of accretion or at later times by post emplacement compaction and/or metamorphism. According to Fruland et al. (1978), a less probable alterna460
0019-1035/81 / 120460-13502.00/0 Copyright (~F 1981by AcademicPress. Inc All rights of reproduction in any form reserved
ACCRETIONARY DAR K RIMS tive for the origin of these dark zones "is an accretionary mantle that was accumulated prior to or during accumulation of the meteorite." Hutchison et ai. (1979), in their discussion of the Tieschitz (H3) meteorite, stated that "the meteorite comprises millimetre fragments and spherical chondrules of silicate, all of which have opaque rims." Wilkening et al. (1978) suggested that the "accretionary" rims surrounding chondrules in unequilibrated chondrites are represented by three types: (1) those that are characterized by an overall enrichment of sulfides, which are disseminated throughout very fine-grained material, and are free of larger grains; (2) rims relatively free of sulfides, appearing translucent in transmitted light and free of larger grains; (3) rims with massive sulfides, such as "canned" or armored chondrules in equilibrated chondrites. In subsequent work by the University of Arizona group, Allen et al. (1979, 1980) state that there are two types of rims: sulfide rich and sulfide poor. They conclude from their scanning electron microscope (SEM) observations that the rims appear to be coatings of fine-grained, relatively lowtemperature material accreted or deposited on individual chondrules before agglomeration, brecciation, and metamorphism. Nozette et al. (1979) and Allen et al. (1980) attribute the variations of texture and the sulfide grain sizes that they observed to slightly differing conditions of metamorphism during coalescence, sintering, cooling, or brecciation. They report that their overall study (University of Arizona group) found that the opaque chondrule rims are composed of variable amounts of finegrained silicates and sulfides; that rims are enhanced in moderately volatile elements; and that rims appear to have been deposited on preexisting "cold" chondrules. The present work further expands the knowledge of dark-rimmed inclusions by (1) showing textural relations between the dark rim material and the central particle and dark rim material and the matrix; (2)
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supplying additional mineralogical and chemical details of the dark rim material itself and the minerals included within it; and (3) a suggestion concerning the origin of dark rims. NOMENCLATURE AND EXPERIMENTAL METHODS The general nomenclature and classification of meteorites used in this work is taken from Wasson (1974). Chondrule terminology is from King and King (1978). In this paper, the definition of dark rims is based on textural evidence from the petrographic microscope in a combination of transmitted and reflected light. Rims are neither necessarily continuous nor uniform in thickness about any particle. In transmitted light, in standard thickness sections, the rims are opaque, contain only rare clasts, and commonly have mineral grains and/or lithic fragments aligned parallel to their outer edge in the matrix of the meteorite. In most cases, the contact of the dark rim and the particle it surrounds is well defined, but the outer margin of the dark rim commonly is gradational with the meteorite matrix. In reflected light, the rims commonly appear fine grained and contain reflective minerals, which have a wide range in abundance and grain size. Samples of six low petrologic type meteorites [Inman-L3 (Find), Bishunpur-L3 (Fall), Prairie Dog Creek-H3 (Find), Tieschitz-H3 (Fall), Murray-CM2 (Fall), and Ailende-CV3 (Fall)] were obtained from the collections of the British Museum, National Museum of Natural History, and American Museum of Natural History. Polished thin sections were prepared for those samples for which suitable thin sections for use in the petrographic microscope and scanning electron microscope were not already available. It is advantageous to work with polished thin sections rather than unshaped chips as the geometry of the included mineral phases is easily recognizable and the flat surface provides a relatively uniform plane for the incident electrons associated
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with the energy dispersive X-ray analysis unit. The thin sections were examined and photographed under the petrographic microscope in a combination of transmitted and reflected light. These photographs served as index photos for finding areas to be investigated with the SEM. The sections were ion etched on a Commonwealth Scientific Corporation Model VII ion-micro milling instrument (IMMI). This procedure is necessary for generating minor relief on the surface of the thin section such that textural relations can be readily observed with the SEM. A Jeolco JSM-U3 scanning electron microscope with an accelerating voltage of 25 kV and a Princeton Gamma Tech energy dispersive X-ray analysis unit (EDX), for qualitative elemental analysis and determination of mineral chemistries, were investigative tools used in this work. OBSERVATIONS AND ANALYSES Dark rims occur on both fluid drop and lithic chondrules and lithic clasts in the low petrologic types of ordinary and carbonaceous chondrites. The rims are neither necessarily continuous around the host particle nor of uniform thickness, and are not ubiquitous on panicles in any meteorite examined. In transmitted light, the dark rims mostly appear to be devoid of clasts but commonly have matrix mineral grains or lithic fragments aligned parallel to their outer margins. In reflected light the dark rims commonly appear to be finer grained than the central host particle or the surrounding matrix. In reflected light the rims are observed to contain at least one opaque mineral phase. In SEM images and elemental maps, the dark rim materials are obviously both texturally and chemically heterogeneous, whether in intra- or interspecimen comparisons. In spite of these textural differences it is possible to classify dark rims into two major categories, i.e., sulfide-rich and sulfide-poor dark rim materials as has been suggested previously by Allen et al. (1979, 1980).
Texturally, the dark rim material commonly appears to be aphanitic to amorphous at the scale of SEM images. However, rare fragments of mechanically included crystalline materials do occur within the very fine-grained dark rim material. The composition of the predominant component, the aphanitic to amorphous material in the dark rim, most commonly approximates a relatively iron-rich pyroxene, tentatively identified by EDX analyses. Also occurring as a dominant component of rims is a mixture of troilite and nickel-iron and/or a F e - N i - S - O phase similar to that described in Ramdohr (1973) and others. Recent work by Kerridge and Bunch (1979) and Bunch and Chang (1980) refers to this latter phase as "'poorly characterized phase (PCP)." Figure I shows sulfide-rich dark rim material in the Inman (L3) chondrite. In this image, the contact between the dark rim and the chondrule is defined by a decrease in the grain size in the dark rim material as compared to the interstitial glassy groundmass of the chondrule. At some areas around the circumference of the chondrule, the margin between the rim and chondrule appears to be texturally gradational, thus making it difficult or impossible to delineate an exact margin between the two regions. The fine-grained inner portion of the dark rim has an EDX elemental distribution pattern indicative of a mixture of a silicate (probably pyroxene), a sulfide (troilite), and nickel-iron. Individual amounts of the elements that constitute these minerals range from location to location; however, they are present in all areas of this rim. The outer margin of the dark rim in the Inman chondrite is enriched in troilite, hematite (terrestrial weathering product in keeping with the reddish color in transmitted light), and nickel-iron. In most sulfide-rich dark rims, the reflective minerals are located preferentially along the outer margin of the dark rim material as more massive accumulations; however, they also are present as disseminated grains within the amorphous to finely crystalline
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FIG. I. This scanning electron microscope image shows the typical textural relation between the chondrule and the rim, and the rim and the matrix in Inman inclusion I, which is a good example of a sulfide-rich dark rim. The boundary between the chondrule, that portion of the image that shows euhedral crystals in intersitital glass, and the dark rim is defined by a decrease in the grain size as compared with the interstitial groundmass of the chondrules. At some places around the circumference oftbe chondrule the two regions appear to be texturally as well as mineralogically gradational, making it difficult to determine an exact margin between the chondrule and the dark rim material. The boundary between the rim and the matrix commonly is well defined, as it is in this figure, owing to the concentration of highly reflective minerals near the exterior portion of the rim. Width of field of view is approximately 400 tzm. d a r k r i m m a t e r i a l a n d , in s o m e c a s e s , in small a m o u n t s a l o n g the i n t e r i o r m a r g i n o f the dark rim. T i e s c h i t z i n c l u s i o n 1 ( F i g . 2) e x e m p l i f i e s a s u l f i d e - p o o r d a r k rim. In this i m a g e , the finely c r y s t a l l i n e to a m o r p h o u s d a r k r i m m a t e r i a l h a s an E D X s p e c t r u m s i m i l a r to w h a t w o u l d be e x p e c t e d f r o m a r e l a t i v e l y iron-rich pyroxene. The elemental abund a n c e s o f Mg, F e , Si, a n d m i n o r Al v a r y f r o m o n e a n a l y s i s a r e a to a n o t h e r , but m a i n t a i n a p p r o x i m a t e l y the s a m e r a t i o s , a n d o n l y the m o d e r a t e l y v o l a t i l e t r a c e elem e n t s a p p e a r a n d d i s a p p e a r f r o m the s p e c t r a as t h e p o s i t i o n o f the b e a m is s h i f t e d . W i t h i n t h e d a r k r i m m a t e r i a l o c c u r the relat i v e l y v o l a t i l e e l e m e n t s N a , P, Ci, a n d K.
N o troilite w a s o b s e r v e d in the d a r k m a t e rial o f T i e s c h i t z ; h o w e v e r , a s m a l l a m o u n t of nickel-iron was detected. The bounda r i e s b e t w e e n the rim a n d the c h o n d r u l e and b e t w e e n the rim a n d the m a t r i x are well d e f i n e d in the S E M i m a g e s o f T i e s c h i t z inc l u s i o n 1. T h e d a r k rim m a t e r i a l is disting u i s h e d f r o m the m a t r i x a n d the c h o n d r u l e b y its o v e r a l l m u c h finer g r a i n size. T h e d a r k r i m m a t e r i a l a l s o h a s , in m o s t c a s e s , a w e l l - d e f i n e d m a r g i n w i t h the m a t r i x . In reg i o n s w h e r e the c o n t a c t is n o t well d e f i n e d , it is still d i s c e r n i b l e , o w i n g to the r e l a t i v e l y m o r e h o m o g e n e o u s t e x t u r e o f the p r e d o m i n a n t c o m p o n e n t o f the d a r k rim m a t e r i a l as c o m p a r e d w i t h t h a t o f the m a t r i x . It is n o t the c a s e , h o w e v e r , in all inclu-
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FIG. 2. SEM image (width of field of view 160 t~m) of Tieschitz inclusion 1 shows a typical relation between the chondrule and the rim and the matrix in a sulfide-poor dark rim. The dark rim material in this image is easily distinguished from the matrix and the chondrule by its overall much finer grain s i z e however, rarely within this amorphous to aphanitic material are crystals of other mineral fragments included as is shown in the upper right of this image.
sions studied that the margin between the dark rim and the matrix is well defined. In some isolated regions of a given inclusion rim, the margin may not be discernible. Inclusion 2 of Bishunpur (Fig. 3) is an example of the case in which the outermost margin of the dark rim is poorly defined or indistinguishable for most regions of the chondrule. Figure 4 shows a greater magnification SEM image of the predominant amorphous to aphanitic component of the dark rim material in Tieschitz inclusion I. Within this relatively texturally uninteresting material occur inclusions of mineral crystals and fragments that commonly have elemental distribution patterns similar or identical to the major mineral phases in the central particle that the dark rim is surrounding. In the ordinary chondrites and Allende, texturally and mineralogically distinct re-
gions occur near the nucleus/rim boundary and commonly have elemental distribution patterns indicative of whitlockite, melilite, or, rarely, a plagioclase feldspar. Whitlockite is present within the dark rim material of the inclusions studied in Inman and Prairie Dog Creek, and melilite was found in the dark rim materials in Bishunpur and AIlende. The only ordinary chondrite in which no mineral inclusions of composition different from the phases in the surrounding matrix or host particle were found was within the dark rims in Tieschitz. In the amorphous to aphanitic portion of the dark rim material of each of the two inclusions studied in Murray (CM2), abundant euhedral crystals are present (Fig. 5). These are the only relatively large euhedral crystals found within the dark rim material I Figs. 6 and 7). To date, the only occurrences of these crystals found within Mur-
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FIG. 3. SEM image of Bishunpur inclusion 2, showing the discontinuous dark rim around the chondrule (dark rim material in center portion of image). The rim is most easily recognized in areas that have a reflective mineral phase adjacent to the chondrule, or where the dark rim material is constricted, as in the upper left side of the image. The exterior margin of the rim is poorly defined or commonly not distinguishable. Width of field of view is approximately 1100t~m. ray are within the dark rims. It appears that these crystals are either monoclinic or orthorhombic, although from images it is not possible to determine the s y m m e t r y of these crystals unambiguously. E D X spectra indicate that the crystals are enriched in the elements sodium, sulfur, and o x y g e n relative to the predominant c o m p o n e n t o f the dark rim. It appears, from examination of SEM images, that the crystals have grown within the dark rim material and are not simply superposed on it. Although as yet we have neither complete chemical analyses o f the crystals nor X-ray diffraction data, we believe that this may be the first meteoritic o c c u r r e n c e of the sodium sulfate mineral thenardite, which commonly occurs as an evaporite mineral on the surface o f the Earth. We believe that these crystals formed on the meteorite parent b o d y and may be further evidence of
aqueous alteration inferred by others. The offset o f the crystals across cracks in the petrographic thin section and the appearance of the material penetrating into the dark rim material along the margin of the crack (Fig. 8) lead us to believe that these crystals cannot be an artifact o f sample preparation. In all the meteorites examined, the predominant amorphous to aphanitic component o f the dark rim material is mineralogically distinct, as well as texturally distinct, from the general meteorite matrix material that surrounds it. The predominant c o m p o n e n t of all the dark rims is a mixture of two or more mineral phases. From E D X spectra the amorphous to aphanitic phase, as has previously been mentioned, appears to be a mixture o f a silicate, a sulfide, and nickel-iron that ranges in mineral proportions from location
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FIG. 4. SEM image showing the contact of the silicate chondrale and the amorphous to aphanitic material in an embayed portion of the chondrule in Tieschitz inclusion I. Within this texturally uninteresting material occur inclusions of mineral crystals and fragments that commonly have elemental distribution patterns similar or identical to the major mineral phase that the dark rim is surrounding. This image also shows the silicate border around the circumference of the chondrule. This image is texturally representative of the dark rim material for virtually all the clark rims studied, with the exception of Allende, which has portions of its dark rim materials that appear ttexturally and mineralogically) similar to the comminuted crystals in its matrix. Width of field of view is 12/zm.
to location, or a silicate in addition to a F e N i - S - O phase as described by R a m d o h r (1973) and others. It is not known in what mineral phases the moderately volatile elements, such as Na, CI, P, and K, are present in the dark rim material, but it is believed that they are present in glass or as very fine alteration products or minerals on other grains. E D X spectra o f the matrix materials of the ordinary chondrites indicate that they are a mixture o f more than one phase and are lacking the concentration of moderately volatile trace elements that are present in the dark rim materials. CONCLUSIONS AND INTERPRETATIONS From textural relations between the host chondrule or lithic clast and the corre-
sponding dark rim material in each of the inclusions, it is obvious that the dark rim material was added to the particle subsequent to its formation and prior to its incorporation into the present meteorite. Furthermore, the dark rims are relatively enriched in moderately volatile elements as c o m p a r e d to the host chondrule or lithic clast or the enclosing meteorite matrix. We believe that the textures o b s e r v e d in the dark rim material and the distribution of the mineral phases within the dark rim material are the result o f the t e m p e r a t u r e differences b e t w e e n the host particle and the accreting dark rim material. The dark rim material that surrounds the chondrules and clasts investigated in this study have only the most basic characteristics in c o m m o n . Each inclusion is unique,
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FIG. 5. SEM image of the dark rim material and the euhedral crystals present within it as observed in two inclusions, in the Murray carbonaceous chondrite. These crystals are the only euhedral crystals observed in the dark rim material and appear to be either monoclinic or orthorhombic. Although we are not able to determine their symmetry unambiguously from images, these crystals are not similar to any phase associated with the particle that the dark rim is surrounding. Width of field of view is 36/~m. and only the most basic scenarios concerning their origins can be developed from their textural and mineralogical relations. The most probable p r i m a r y origin of dark rims is by hot particle accretion. It s e e m s obvious that some dark rim material was hotter than other dark rim material at the time o f accretion onto the host particle. The dark rim material that is relatively enriched in sulfides is likely to have been at a lower t e m p e r a t u r e when it was accreted onto the nucleus than dark rim material that is depleted in sulfides. It is impossible to determine whether or not the sulfide-poor dark rim material e v e r had any sulfides associated with it; h o w e v e r , in other parts o f the meteorite, which have generally the same bulk composition, sulfides are associated with the silicates, thus leading one to believe that the adhering particles are devoid of sulfides because of high temperature.
The dark material around Tieschitz inclusion 1 is believed to have had the greatest t e m p e r a t u r e of accretion of the inclusions examined. Within the dark rim there are no sulfides present: h o w e v e r , a small amount o f n i c k e l - i r o n is present. The sulfide-poor dark rim material probably accreted at t e m p e r a t u r e s lower than the liquidus t e m p e r a t u r e s o f the mafic silicates. Accreting dark rim material might or might not react with the nucleus depending on the t e m p e r a t u r e of the nucleus. During this accretionary process some nuclei were w a r m and plastic, capable of reacting with the adhering dark rim material, and other nuclei were cold, rigid, and incapable o f reacting with the accreting dark rim material. Inman inclusion l can be used to illustrate the scenario o f dark rim accretion on a w a r m and possibly hot and plastic chondrule. In reflected light and in SEM images,
FIcs. 6 and 7. SEM images of sodium sulfate crystals (thenardite?) in the dark rim material of the Murray (CM2) carbonaceous chondrite. Energy dispersive X-ray analysis of this crystal shows only sodium, sulfur, and oxygen in concentrations greater than the composition of the substrate. Magnification is 16,000 and 20,000x, respectively. 4614
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FIG. 8. SE M image (width of field of view 6.8/~m) of a group of white crystals present in the dark rim materials of Murray inclusion 1 and inclusion 2. This particular group is from Murray inclusion I and shows the offsetting of two crystals along a crack in the thin section. This leads the investigators to believe that the materials are not the result of an interaction between the meteorite and the atmosphere during or post sample processing, but are indigenous to the meteorite. the interstitial glass that is present b e t w e e n the relatively iron-rich crystals in the chondrule of I n m a n inclusion 1 a p p e a r s to have reacted with the dark rim material. In some portions of the chondrule the interstitial glass and dark rim material a p p e a r s to have a gradational contact. The compositions of the interstitial glass and the silicate portion o f the dark rim material are so similar that they might have been hot and plastic simultaneously. It should be noted that the exterior margin of the dark rim material is enriched in minerals such as troilite that are stable at lower t e m p e r a t u r e s . Figure 1 shows the contact o f the dark rim material and the chondrule. The interstitial glass in this inclusion a p p e a r s to have reacted with the dark rim material in such a way, probably by an exchange of elements across their mutual margin, that the dark rim material is more crystalline along its
interior margin. The crystalline texture grades outward into a more aphanitic and volatile-rich outer region. We believe that it was necessary for the relatively more massive chondrule to be the portion of I n m a n inclusion 1 that retained the heat necessary to allow the interaction b e t w e e n the chondrule and the rim to proceed, possibly aided b y the heat of crystallization of the central particle. It does not s e e m possible that the dark rim material had sufficient heat capacity to maintain a t e m p e r a t u r e that could bring about the observed interaction. If the t e m p e r a t u r e o f the relatively thin dark rim material remained sufficiently great to partially melt the interstitial glass in the chondrule, the temperature gradient between the inner and outer portions o f the rim probably would not be sufficient to allow the formation o f troilite along the outer margin. It is possible, how-
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FIG. 9. SEM image o f B i s h u n p u r inclusion I, showing the crack around the circumference o f the chondrule that separates the innermost boundary of the dark rim material from the host nucleus. This is a good e x a m p l e of dark rim accretion on a relatively cold previously existing chondrule. Width of field o f view is - 1100 p,m.
ever, that the troilite adhered to the silicate portion of the dark rim subsequent to rim formation. Several of the remaining dark-rimmed inclusions can serve as e x a m p l e s for the scenario that a chondrule or lithic clast can be relatively cold and have an accreted rim. Bishunpur inclusion 1, Figure 9, perhaps is the best example of dark rim accretion on a relatively cold previously existing chondrule. In this figure the dark rim material is mostly separated from the chondrule by a crack. It is possible to attribute this crack to temperature differences between the cold chondrule and the hot dark rim material. The accreting dark rim material was not hot enough to sinter to the chondrule and adhere. Thus, as the dark rim cooled, a crack developed b e t w e e n the chondrule and the dark rim material. The cooling of the dark
rim material was more rapid along the margin of the chondrule than the exterior portions o f the dark rim, thus resulting in the aphanitic texture in the interior portion o f the rim, and in gradual cooling on the margin, allowing for the formation of troilite. During sample preparation the authors have also noted that it is not u n c o m m o n for a fluid drop chondrule or a lithic fragment to fall out of the specimen leaving a distinct rind or rim intact in the matrix material, thus leading us to believe that this temperature relation was c o m m o n within unequilibrated meteorites. This point should be noted by those persons attempting to separate chondrules and clasts from the matrix for individual chemical analysis as it may well bias their data. In other inclusions the crack between the two portions of the inclusion is absent, but the general texture of the rims are corn-
ACCRETIONARY DARK RIMS pletely aphanitic and shows no textural evidence of any interaction between the dark rim material and the host particle. There are many observations of shock effects and other impact phenomena in chondrites. The observation of chondrules in a number of lunar breccia samples ( King et al., 1972; Kurat et al., 1972; Nelen et al., 1972; and others) plus the fact that both fluid drop and lithic chondrules (King and King, 1978) occur together in chondrites and lunar samples has now led a number of workers to suggest that at least some, if not many, meteoritic chondrules have been produced by impact processes (e.g., King et al., 1972). A previous interpretation of the origin of dark rims on chondrules by King and King (1980) suggested that some may have been formed in the plumes over impact craters "in a manner analogous to volcanic accretionary lapilli." However, at that time no accretionary lapilli from impact deposits had been described. This interpretation has recently received support from observations of Graup (1981), who finds excellent examples of accretionary lapilli in ejecta deposits from the Ries Crater, Germany. Thus, although there are a few studies that theoretically or experimentally demonstrate the physical conditions within the ejecta curtain or the plume over an impact crater, such a setting appears to provide the range of environments required to produce accretionary dark-rimmed chondrules and other particles. ACKNOWLEDGMENTS The investigators express appreciation to Brian Mason, National Museum of Natural History, Martin Prinz, American Museum of Natural History, and R. Hutchison, British Museum of Natural History for the loan of meteorite samples used in this work. Thin sections not borrowed from other sources were made in Northrop Services, Inc. thin section laboratory and by David S. Pettus. Portions of the work were computed while T.V.V.K. was a Visiting Scientist at the NASA, Johnson Space Center, in the scanning electron microscope laboratory of David S. McKay. A special thanks goes to two anonymous reviewers and to Georgann Nace for her tireless assistance. This
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work was financially supported in part by the University of Houston Geology Foundation and by NASA Grant NSG 7617.
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