Reflection spectra of shocked ordinary chondrites and their relationship to asteroids

ICARUS

98, 43-53 (1992)

Reflection

Spectra of Shocked Ordinary Chondrites Relationship to Asteroids

and Their

KLAUS KEIL’ AND JEFFREY F. BELL Planetary Geosciences, Department of Geology and Geophysics, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, Hawaii 96822 AND D. T. BRITT Department of Geology, Brown University, Providence, Rhode Island 02912 Received January 27, 1992; revised March 30, 1992

among C asteroids. If this hypothesis is to be upheld, then recourse may have to be taken to the suggestion of Britt and Pieters (1991, Lunar Planet. Sci. XXII, 139-142) that the surfaces of ordinary chondrite parent asteroids appear spectrally similar to those of C asteroids because they are covered by a hypothetical, thin layer of fine-grained material similar to that present in the dark portions D 1%~Academic PM, of solar wind-bearing regolith breccias.

Although ordinary chondrites are the most common meteorites falling on Earth, reflectance spectra of only a few rare asteroids resemble those of powdered chondrites measured in the laboratory. Therefore, “space weathering” processess which may have altered the surfaces of ordinary chondrite asteroids so that their spectra resemble those of the abundant S asteroids have been suggested. Recently, Britt et al. (1989, Lunar Planet. Sci. Conf. 19th, 537-545; and 1989, LunarandPlunet. Sci. XX, 111-112) and Britt and Pieters (1989, Lunar Planet. Sci. XX, 109-110) measured spectra of “shock-blackened” ordinary chondrites which possess much lower reflectance and shallower absorption bands than those of “normal” ordinary chondrites and, in some cases, resemble those of carbonaceous chondrites and C asteroids. They therefore propose that surfaces of ordinary chondrite asteroids may have been shock-blackened by impact, and that these asteroids may be hidden among the C asteroids. We measured the spectral reflectance of a number of mineralogically well-characterized, shockblackened ordinary chondrites exhibiting four major types of black, shock-produced features: opaque melt veins (shock veins), melt pockets and irregular interconnected melt veins, melt dikes, and black chondrites, StBffler, Keil, and Scott (1991, Geochim. Cosmochim. Acta 55, 3845-3867.) We confirm that their spectra resemble those of C asteroids. However, the occurrence of these materials in impact crater basements and floors rather than on the surface, their low abundance in craters relative to brecciated and ejected material, and their low abundance among ordinary chondrite falls suggest that the surfaces of ordinary chondrite parent bodies are not likely to be covered by vast amounts of such shockblackened materials. Thus, these materials cannot be responsible for significant large-scale spectral alterations of the parent asteroids of ordinary chondrites, and they cannot be called upon in support of the hypothesis that ordinary chondrite asteroids are hidden ’ To whom correspondence

IIK.

INTRODUCTION Comparison of asteroid spectra with laboratory spectra of meteorites has proven to be a key technique for determining the mineralogical composition of asteroids (e.g., Gaffey et al. 1989). Among the discoveries made with this technique are the wide compositional variety among asteroids (e.g., Tholen 1984, Tedesco et al. 1989), the stratification of the asteroid belt into zones of similar composition (Gradie and Tedesco 1982), and the transition f rom strongly heated asteroids in the inner belt to primitive asteroids in the outer belt (Bell et al. 1989). Most work on asteroid composition has been based on the implicit assumption that the spectra of asteroids can be simulated by selecting a meteorite thought to represent the bedrock of the asteroid of interest, and pulverizing it to simulate the upper layer of the asteroid regolith which is optically visible to a telescope. In many cases this method can provide an adequate spectral match, allowing for small variations due to particle size differences and photometric geometry. However, there remain spectral classes of asteroids which have no meteoritic analogs and several classes of meteorites whose laboratory spectra did not

match those of any observed

should be addressed.

asteroids.

The case which

43 0019-1035192 $5.00 Copyright 0 1992 by Academic Press, Inc. All rights of reproduction in any form reserved.

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has attracted the most attention over the years is that of the ordinary chondrites. While these are the most common meteorites falling on Earth’s surface at present, only a few unusual asteroids have been suggested as possible spectral matches for them. The most popular recent candidate for an ordinary chondrite source body is the small Earth-crossing asteroid 1862 Apollo (McFadden er ul. 1985, Bell and Keil 1988). This asteroid has unique spectral features which caused Tholen (1984) to erect a new asteroid class “Q” of which 1862 Apollo is the only unambiguous member. However, many researchers find the apparent lack of any Q-type asteroids in the main asteroid belt a serious objection to this identification. It would be much more satisfying to identify ordinary chondrites with some very common main-belt asteroid type. The traditional candidates for this have been S-type asteroids. A variety of “space weathering” processes which might alter the regolith of an ordinary chondrite parent body to produce the highly reddened spectra observed in typical S asteroids have been proposed (e.g., Feierberg et al. 1982; McFadden 1983a,b; King e’t ul. 1984; Pieters 1984; McFadden and A’Hearn 1986). However, ordinary chondrite regolith breccia matrix material which is rich in implanted solar-wind gases does not show the characteristic red slope of S asteroids (Bell and Keil 1988). Since this material acquired solar gases during previous residence at the uppermost surface of a regolith, it must be representative of the optical surface of the parent body. Thus, the lack of spectral similarity of this material to S asteroids is strong evidence against the”space weathering” hypothesis in its “classic” form. Recently, spectra of “black” ordinary chondrites which possess much lower reflectance and shallower absorption bands than those of “normal“ ordinary chondrites were obtained (Britt et ml. 1989a, Britt and Pieters 1989). It should be noted that these authors define as black “any ordinary chondrite meteorite exhibiting distinctly low reflectance 50.15 in hand sample” (Britt and Pieters 1991a). This is a mixed group of samples: darkening of chondrites usually results from the presence of very finegrained, widely dispersed metallic Fe,Ni and troilite (unless terrestrial weathering is involved in the case of meteorite finds). It includes fragmental and regolith breccias and Keil 1963, Britt and Pieters (e.g., Fredriksson 1991b,c), which have generally experienced only relatively low shock levels (Stoffler et al. 1991). It also includes veined and “shocked-blackened” ordinary chondrites (e.g., Keil rt al. 1977), where localized melting has resulted from stress and temperature peaks in weakly to very strongly shocked chondrites (shock stages S3 to S6, Stoffler et al. 1991) resulting in local deviations from the equilibrium shock pressure due to differences in shock impedance (Stoffler et al. 1991). In some cases, the spectra obtained by Britt rf al. (1989a) and Britt and Pieters (1989,

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1991a) are similar to those of carbonaceous chondrites and C-type asteroids. They therefore proposed that the elusive ordinary chondrite parent bodies could be hiding among the C asteroid population, if their uppermost regoliths were composed mostly of shock-darkened material (Britt and Pieters 1987, 1991~; Britt et al. 1989b). While this hypothesis lacks some of the superficial charm of the classic ordinary chondrite = S-type space weathering hypothesis, in that it links ordinary chondrites with an asteroid class that already has an accepted meteoritic analog (carbonaceous chondrites of the CI and CM classes), it is much more reasonable because it appeals to clearly real asteroidal impact processes and is consistent with the existing meteorite spectral data. However, the major question regarding the validity of this hypothesis is whether, in fact, shock-darkened ordinary chondrite material can be expected to be sufficiently abundant on asteroidal surfaces to account for this effect. As shown below, based on comparisons to terrestrial impact craters, considerations of the fate of melt produced by large impacts on asteroids, and the abundance of black chondrites in meteorite collections, this does not seem to be the case. Thus, we suggest that black chondrites do not occur on ordinary chondrite asteroidal surfaces in sufficient abundance to cause telescopically obtained reflectance spectra of these objects to resemble C asteroids. If one is to maintain such a model, recourse would have to be taken to the suggestion of Britt and Pieters (1991b,c) that the dark portions of gas-rich regolith breccias are darkened by processes similar to those that darkened black chondrites. They suggest that the optical surfaces of the ordinary chondrite parent asteroids are made of very high proportions of this material, causing the spectra of these asteroids to resemble those of carbonaceous chondrites. We have obtained additional laboratory spectra of a variety of black veins in ordinary chondrites and of black ordinary chondrites to further test the hypothesis that shock blackening may be responsible for altering the surfaces of ordinary chondrite parent bodies so that they resemble. in their spectral properties, C-type asteroids. We have also carried out detailed mineralogical and petrographic studies of the specimens for which reflectance spectra were measured, in order to more precisely relate their reflectance properties to shockinduced alterations such as formation of shock veins and shock blackening. Since the occurrence of shock-veined and shock-blackened rocks at specific sites in impact craters has important implications for surface exposure and surface abundance of such materials, we also briefly discuss the origin and crater setting of these materials. Note that we restrict our work and discussion to solar gas-free shock-blackened ordinary chondrites, where the shock effects caused localized melting [i.e., opaque melt veins (shock veins), melt pockets and interconnected melt

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veins, melt dikes, and black chondrites in the sense of Stoffler et al. (1991)]; we do not discuss the role that the dark portions of solar gas-bearing regolith breccias, which have generally been exposed to relatively low shock pressures and, hence, little melting, may play in disguising surface properties of ordinary chondrites parent asteroids, as did Britt and Pieters (1991b,c). ORIGIN OF BLACK VEINS AND BLACK CHONDRITES

Black veins in chondrites (e.g., v. Reichenbach 1865) and black chondrites (e.g., Heymann 1967, Binns 1967) have been known for a long time and have even been used as criteria for classification in older chondrite classification schemes (e.g., Brezina 1904). Furthermore, a number of papers have been published describing in detail these features in specific chondrites (e.g., Keil et al. 1977, Dodd et al. 1982, Ehlmann et al. 1988), and experiments have been carried out to duplicate these in the laboratory (e.g., Fredriksson et al. 1963). Although Binns (1967) suggested that these features formed by heating alone, it is clear that they are the result of heating due to shock (e.g., Fredriksson et al. 1963, Heymann 1967, Keil et al. 1977, Dodd et al. 1982, Stoffler et al. 1991). The formation of black veins and black chondrites by shock involves shock-induced localized melting, largely as the result of localized stress and temperature concentrations and excursions at the interfaces of mineral grains of different shock impedance (Fig. 1C) (Stoffler et al. 1991). This results in the formation of polymineralic mixed melt, due to the simultaneous total melting and mixing of chondrite minerals such as olivine, pyroxene, plagioclase, and metallic Fe,Ni and troilite, and subsequent crystallization of immiscible silicate and metal/sulfide melts. These mixed melts occur in different textural settings and configurations and in four types, the first three of which are transitional (Stoffler et al. 1991). These are: (a) thin, opaque melt veins (shock veins), (b) melt pockets (Dodd and Jarosewich 1979, Hutson 1989) and interconnected irregular melt veins, (c) melt dikes, and (d) sulfide and metallic Fe,Ni injected into fractures in olivine and pyroxene (resulting in shock blackening and formation of black chondrites). Opaque melt veins (shock ueins) consist of mixed melt products of (rarely) glassy, mostly aphanitic, polycrystalline material that formed by in situ melting of the various chondrite constituents and contain immiscible metallic Fe,Ni-troilite droplets and stringers (Figs. 1B and 1C). They may be thin planar dikelets (Fig. 1A) or thicker curved veinlets (Figs. 2D and 2E)). They can form at relatively low equilibration shock pressures of 15-20 GPa in weakly shocked chondrites of shock stage S3 (Stoffler et al. 1991) and are produced by jetting (Kieffer 1977), which is caused by open fractures or pore space in combination with frictional melting by shearing. Melt veins are

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transitional to, and may thicken into, melt pockets and interconnected melt veins (Figs. 2A and 2B). These occur at somewhat higher equilibration shock pressures and consist mostly of submicroscopic intergrowths of silicate, sulfide, and metal grains, sometimes with the typical “fizzed” and eutectic metallic Fe,Ni-troilite intergrowths (Fig. ID); unmelted, shocked silicates are commonly intermingled with the melt. Melt dikes with a crystalline matrix and discordant boundaries and thicknesses on the order of centimeters to decimeters occur when abundant shock melt is produced. Commonly, melt dikes consist of opaque melt veins that have been intruded by shockproduced melt-breccia mixtures (Fig. 2C). This is due to a single impact event and reflects the time difference between the onset of the peak shock pulse, which forms the melt vein, and the completion of the pressure release phase when the compressed material turns into a state of dilatation, resulting in the formation of the intrusive melt-breccia dike (Stoffler et al. 1991). Troilite and metallic Fe,Ni injected into fractures in silicates result in the formation of black chondrites (Figs. IE, lF, and 2F); these fractures are commonly observed in close spatial relationship to zones of mixed melt. Melt veins, melt pockets and interconnected melt veins, melt dikes, and black chondrites all have in common the occurrence of finely disseminated metallic Fe,Ni-troilite assemblages. These trap photons and are responsible for the black color in hand specimens and the opaqueness in thin section of these shock-produced mixed-melt features. Melt veins (shock veins), interconnected melt veins, melt dikes, and shock-blackened chondrites are analogous to pseudotachylite dikes in terrestrial and lunar impact craters (e.g., Stoffler et al. 1988). Such rocks form as dikes in crater walls and crater basements and are not features of the surface of a regolith and, quantitatively, make up only a small portion of the total shocked material in an impact crater (Stoffler et al. 1991). It is therefore unlikely that entire surfaces of ordinary chondrite asteroids would be covered by black chondrite material produced by these major events. RESULTS

The spectra presented here were obtained at the Brown University reflection spectroscopy facility (RELAB). For convenience, we measured spectra of sawed or freshly broken surfaces. We emphasize that these spectra are not directly comparable to those of powdered meteorite samples or asteroids (they differ in the sense that their continua, particularly those of the brighter samples, are slightly bluer). However, comparisons within the data set (and particularly between spectra of shock-blackened areas and those relatively unaffected by shock melting on the same slab, e.g., the dark vein and light clasts in Gifu,

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FIG. 1. (A) The Mocs L6 chondrite, cut by a thin (0.5 mm) black melt vein (shock vein) (center, upper left to lower right). Specimen is from the Museum of Natural History, Gotha, Germany. Scale bar = 2 cm. (B) Photomicrograph of a melt vein (lower right to upper left, most of field of view) in the Nerft L6 chondrite. Vein consists of fine-granted, shock-melted metallic Fe,Ni and troilite (white) in minor glassy to fine-grained silicates. Note branching off of minute Fe,Ni-troilite veins (upper right and lower left). Silicates are gray, holes black. Plane-polarized, reflected light. Scale bar = 32 pm. (C) Photomicrograph of melt vein (upper right to lower left) in the Pultusk HXS3) chondrite regolith breccia, cutting through minerals of drastically different shock impedance (metallic Fe,Ni: silicates) and partly engulfing metallic Fe,Ni grain (white), suggesting that the movement of the melt was over very short distances. Silicates are grey. holes black. Plane-polarized, reflected light. Scale

SHOCKED ORDINARY CHONDRITES

Figs. 2A and 3) should and do reveal spectral differences due to formation of black veins and shock-blackened objects. The samples studied and their spectra are individually described below. They were selected to include examples of all four types of textural settings and geometries of polymineralic mixed shock melts, as defined by Stoffler et al. (1991). This includes examples of opaque melt veins (shock veins) in Taiban (Fig. 2E), of melt pockets and interconnected irregular melt veins in Gifu and La Criolla (Figs. 2A and 2B), of melt dikes in Sete Lagoas and Cavour (Figs. 2C and 2D), and of shock-blackened chondrites containing fractures filled with troilite and metallic Fe,Ni such as McKinney (Fig. 2F). Gifu (Full; July 24, 1909). We studied specimen C47.1 and polished thin section UNM997 from the University of New Mexico (UNM) Meteorite Collection (Scott et al. 1990). This L6 chondrite (Graham et al. 1985) belongs to shock stage S4 (moderately shocked; Stoffler et al. 1991). It consists of moderately shocked, light-colored, irregular to rounded fragments up to several centimeters in size that are surrounded and cut by black, opaque melt veins (shock veins) mostly less than a few millimeters in width, giving the rock a brecciated appearance (Fig. 2A). In some places, the veins broaden into interconnected irregular melt veins, one of which, as well as two light-colored clasts free of shock veins, was used for measurement of reflectance spectra (Figs. 2A, 3). The meteorite is very fresh and shows unaltered metallic Fe,Ni and troilite in hand specimen and thin section. The light clasts are highly recrystallized, with chondrule boundaries only vaguely discernible, thus confirming the type-6 classification. The veins are opaque in transmitted light and consist of stringers and globules of metallic Fe,Ni and troilite, as well as “fizzed” Fe,Ni-troilite intergrowths (Hentschel 1959, Scott 1982), embedded into melted and quenched silicates. Relict, unmelted silicate clasts of variable sizes are also distributed throughout the veins. The opacity of the veins is due to the presence of very fine-grained, shock-melted Fe,Ni-troilite assemblages. The sample exhibits two very different types of spectra (Fig. 3). The high-reflectance clasts have spectra very similar to those of powdered, normal ordinary chondrites, except for a slightly blue continuum slope which is a consequence of observing a cut surface instead of a loose powder. The dark, interconnected melt vein has a much lower reflectance and a greatly reduced spectral contrast; however, a faint hint of the l-pm absorption band remains. This is due to the presence of small, unmelted relict chondrite clasts within the vein. La Criolla (Fall; January 6, 1985). We studied specimen C273.2 and polished thin section UNM814 from the UNM collection. This L6 chondrite (Graham 1986) consists of light-colored, round to angular clasts, mostly of poikilitic texture, surrounded and cut by opaque melt veins and interconnected irregular melt veins (Fig. 2B). Relict, ghostlike chondrules are only rarely observed in the clasts, thus confirming the petrologic type 6 classification of the meteorite. One angular, very light clast about 10 mm across and several small ones have porphyritic textures. They consist mostly of euhedral olivine microphenocrysts em-

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bedded in a fine-grained, igneous matrix. They are depleted in metallic Fe,Ni and troilite compared to the main poikilitic clasts and are typical melt-rock clasts of impact origin (e.g., Keil 1982). Since the melt-rock clasts are cut by shock veins, it appears that La Criolla is a breccia which, after lithilication, experienced a later shock event that formed the veins. These veins are opaque in transmitted light and consist of shock-melted and quenched silicates, into which are embedded the typical stringers and extremely line-grained metallic Fe,Ni-troilite intergrowths, including those of the fizzed variety (Scott 1982). Relict silicate clasts occur in the veins, but they are finer grained than those in Gifu and often are nearly isotropic. The optical reflectance spectra of two light-colored areas mostly free of melt veins and of one interconnected melt vein were measured (Fig. 3). The spectra of the clasts in La Criolla are very similar to those in Gifu. However, the dark vein is considerably darker and the absorption bands are almost completely suppressed. This is probably due to the fact that this interconnected irregular melt vein has only small, highly shock-damaged light relict clasts intermingled with it. In Gifu, more light-colored, unmelted relict clasts are present in the veins. Sete Lagoas (Fall; December IS, 1908). We measured two specimens of this chondrite from the UNM collection representing adjacent cuts through the same specimen (Cl42.1; C142.2). Since both specimens and their spectra are essentially identical, we only present detailed data on the smaller sample (C142.2; Fig. 2C); however, we have included a spectrum of a vein from the larger specimen (Cl42.1) in Fig. 4. We also studied polished thin sections of UNM 84, 123, 124, 628, and 629, some of which were made from the same cut surfaces of these specimens. Sete Lagoas is a weakly shocked H4(S3) chondrite (Gomes and Keil 1977, 1980; Stoffler et al. 1991), has the typical light-dark structure of ordinary chondrite regolith breccias (Fredriksson and Keil 1963). and contains solar wind-implanted noble gases in its dark portions (Williams et a/. 1986). However, some specimens also display sizeable black veins and dark areas that are different from the typical light-dark structure and crosscut it. They are melt dikes in the sense of Stoffler et al. (1991) and formed after lithification of the regolith breccia in a later shock event unrelated to the formation of the light-dark structure of the regolith breccia. Macroscopically, these dikes have black, opaque melt vein borders and lighter interiors (Fig. 2C). Microscopically, the dikes consist of black borders made of the typical opaque melt veins (shock veins), whereas the interiors are clast-laden melt dike breccias consisting of a very fine-grained, crystalline, igneous matrix into which are embedded relatively unaltered mineral clasts and immiscible Fe,Ni-troilite intergrowths. However, the interior of the dike is not full of the minute stringers and globules of these phases, as are the typical opaque melt (shock) veins, which border the impact-melt breccia dike. Thus, only the bordering black melt veins are opaque in thin section in transmitted light, whereas the impact-melt breccia dike is transparent. Stoffler et al. (1991) proposed that these features formed in one shock event, with the opaque melt veins having formed first and the “intrusive” central melt dikes intruding along these conduits during a stage of dilatation, immediately after formation of the melt veins. The light clasts (Fig. 2C) are poikilitic in texture and contain “fizzed” Fe,Ni-troilite intergrowths, suggesting that they also were shocked. However, it is remarkable that the overall shock classification of this meteorite is only “weakly shocked” (shock stage S3), lending support to the proposal by Stoffler

bar = 125 Km. (D) Photomicrograph of the shock-blackened portion of the Chantonnay L6 chondrite, showing cracks in silicates (grey) filled with shock-melted metallic Fe,Ni (white) and troilite (tan). Fine-grained, “fizzed” Fe,Ni-troilite in lower right. Plane-polarized, reflected light. Scale bar = 100 Fm. (E) Photomicrograph of planar fractures in olivine (grey), filled with shock-melted metallic Fe,Ni-troilite intergrowths (white), in the black L4(S4) chondrite McKinney. Plane-polarized, reflected light. Scale bar = 32 pm. (F) Photomicrograph of melt pockets in the black L4(S4) chondrite McKinney, showing minute droplets of shock-melted metallic Fe,Ni (white)-troilite (light grey) intergrowths and fine veins. Plane-polarized, reflected light. Scale bar = 25 pm.

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A

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et al. (1991) that formation of melt veins and melt dikes is due to local excursions in the stress and temperature concentrations as a result of localized differences in shock impedance, rather than to high equilibration shock pressure. The spectrum of the “dark vein” (Fig. 3) is of an area covering both the opaque melt vein and the melt-breccia dike (Fig. 2C). The spectrum is very similar to those of other shock veins (e.g., La Criolla), but has somewhat higher reflectance, apparently due to the fact that a considerable portion of the dike is transparent impact-melt breccia full of light clasts, rather than an opaque melt vein. The impact-melt breccia dikes lack the fine stringers of opaques that occur in melt veins and serve as light traps and, thus. the spectrum of the impact-melt breccia dike is somewhat brighter than those of typical shock veins. The light clasts in Sete Lagoas, on the other hand, have considerably lower albedo than those in Gifu and La Criolla, although the spectra are otherwise similar (Fig. 3). This darkening of the clasts is due to the fact that they are relatively fine-grained and contain “fizzed” Fe.Ni-troilite intergrowths that serve as photon traps. Cavour (Find; before 1943). We obtained spectra of one area of the main mass and of a melt dike of specimen C21.2 from the UNM collection (Figs. 2D, 3). We also studied polished thin section UH36, which was made from another part of the meteorite but contains what appears to be the same melt dike that was measured. Cavour is classified as a brecciated H6 chondrite (Graham et ul. 1985) and was classified into shock stage S4 transitional to S.5 (D. Stoffler, personal communication, 1992). It is recrystallized, but relict chondrules are visible. We thus confirm the petrologic type 6 classification. Although large metallic Fe,Ni and troilite grains are generally rather fresh and unweathered, the macroscopically vein-free portions of the meteorite (the “light areas” in Fig. 3) contain abundant thin veins filled with iron oxide of terrestrial weathering origin. Relict metallic Fe,Ni and troilite grains suggest that these veins may be minute weathered melt veins. The melt dike branches into melt veins (shock veins), and many small and isolated melt pockets are present throughout the meteorite (D. Stoffler. personal communication, 1992). The melt dike and black veins are typical of melt veins in ordinary chondrites. Microscopically, they are generally opaque in transmitted light but contain variable amounts of relatively unaltered, large, transparent, relict silicate clasts. Metallic Fe.Ni-troilite intergrowths occur as minute globules and thin, long stringers. Some of these Fe,Ni-troilite assemblages are weathered to iron oxide of terrestrial origin, but abundant fresh opaques are dominant. In this meteorite, the reflectance differences between the dike and the rest of the rock (the “light area” in Figs. 2D and 3) are very small. In fact, the difference is negligible except near 0.6 pm, where the human eye is most sensitive. At other wavelengths, the distinction might be completely lost. Furthermore, the reflectance of the dike is relatively high and that of the “light area” relatively low. We suggest that the similarities in the spectra are due to the fact that the “light areas” were also affected by shock melting, as is evidenced by the occurrence of

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melt pockets throughout, and that the “dark vein” (melt dike) contains relict silicates. Spectra were measured of the main mass (la7’aiban (Find; 1934). beled “light area” in Fig. 3) and the widest area of the opaque melt vein (shock vein) that cuts through the center of the specimen from the collection at the University of Hawaii at Manoa. We also studied polished thin section UH3l from the University of Hawaii collection which was prepared from the specimen shown in Fig. 2E. Taiban has been classified as a black, veined LS chondrite (Graham er uI. 1985) and is very strongly shocked (shock stage S6; Stoffler rr al. 1991). Although a find, the meteorite is relatively unweathered, and generally only minor amounts of iron oxide of terrestrial origin are noted as thin veins and around metallic Fe,Ni grains. The melt vein is mostly opaque microscopically in transmitted light and contains minute globules and stringers of metallic Fe,Ni-troilite assemblages, intermingled with transparent but highly shock-altered and often mosaicized relict silicates. It should be noted that veins in sections of Taiban in the UNM collection contain the high-pressure phase ringwoodite. differences between the two litholoOnce again, spectral gies (“light area” and vein, Fig. 3) are almost undetectable and mostly confined to the human eye’s wavelength range. Since the peak near 0.6 ym is the brightest portion of the spectrum, it is the most resistant to being lowered in reflectance by the shock-blackening effects. Due to the coincidence that this is the visual range in human meteoriticists, the light-dark difference between the melt vein and its surroundings remains visible up to very high shock levels. even when the overall spectral distinctions are very small. The moderate reflectance of both the “light area” and the dark vein in Fig. 3 results from the fact that both lithologies are intermingled on a scale smaller than the size of the spectrometer beam. The weak absorption band at 0.6 pm is due to the presence of relict, microscopically transparent silicates, even in the vein. McKinney (Find; 1870). Spectra of one piece of this shock-blackened L4 chondrite (Graham et al. 1985) from the UH collection were measured (Figs. 2F and 3). The meteorite was assigned to shock stage S4 (D. Stoffler, personal communication, 1992). We also studied polished thin section UH29, prepared from the specimen shown in Fig. 2F. McKinney is a typical shock-blackened chondrite, whose dark color in hand specimen and thin section is due to the finely disseminated metallic Fe,Ni-troilite assemblages produced by shock melting. These assemblages frequently occur as fillings injected into shock-produced planar fractures in olivine (Fig. lE) and as droplets and stringers of shockmelted metallic Fe,Ni-troilite in melt areas (Fig. IF). Much of the original chondritic texture of the rock is preserved, but fine-grained material that is opaque in thin section in transmitted light is pervasive. The low reflectance of McKinney (Figs. 3 and 4) is the result of the ubiquitous presence of fine-grained opaques that serve as light traps. No absorption bands are noted due to the preponderance of impact-altered, opaque material.

FIG. 2. (A) The Gifu L6(S4) chondrite, specimen C47.1, University of New Mexico (UNM) Meteorite Collection, showing black melt veins (shock veins) and interconnected irregular melt veins outlining light clasts, which give the meteorite a brecciated appearance. Spectrum of the“large light clast” in Fig. 3 corresponds to the measured area C2MH22, that of the “small light clast”to C3MH22, and that of the“dark vein” to ClMH22. (B) The La Criolla L6 chondrite, specimen C273.2, UNM collection, showing black melt veins (shock veins) and interconnected irregular melt veins as in A. Spectrum of the “large light clast” in Fig. 3 corresponds to measured area C2MH23, that of the “small light clast” to C3MH23, and that of the “dark vein” to ClMH23. (C) The Sete Lagoas H4(S3) regolith breccia, specimen C142.2, UNM collection, showing black melt dikes. Spectrum of the “small light clast” in Fig. 3 corresponds to measured area C5MH21, that of “large light clast” to C6MH21, and that of “dark vein” to C4MH21. (D) The Cavour H6(S4-5) chondrite, specimen C21.2, UNM collection, showing black melt dike. Spectrum of the “light area” in Fig. 3 corresponds to measured area C2MH24, and that of the “dark vein” to ClMH24. (E) The Taiban L5(S6) chondrite from the University of Hawaii (UH) collection. Spectrum of the “dark vein” in Fig. 3 corresponds to region CIMH26, and that of the “light area” to C2MH26. (F) The McKinney L4(S4) black chondrite from the UH collection. The feature from the upper left to lower right is a saw mark. The spectrum in Fig. 3 was taken of area ClMH20.

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0.60

81..

1..

090

1.20

Wavelength

1

*

1.50

I

1.

1.60

*

1.

2.10

*

1

._

2.40

In Microns

1..,..,..1..1..I..I

0.00 0.30

0.60

0.90

1.20

Wavelength

1.50

1.60

In Microns

2.10

2.40

FIG. 3. Gifu: Spectra of the two light-colored clasts and the interconnected melt vein (labeled “dark vein”) shown in Fig. 2A. La Criolla: Spectra of two light-colored clasts and the interconnected melt vein shown in Fig. 2B. Sete Lagoas: Spectra of the two light-colored clasts and the melt dike shown in Fig. 2C. Cavour: Spectra of the melt dike (labeled “dark vein”) and the main mass of the meteorite (labeled “light area”) in Fig. 2D. Taiban: Spectra of the opaque melt vein (labeled “dark vein”) and the main mass of the meteorite (labeled “light area”) in Fig. 2E. McKinney: Spectrum of the shock-blackened chondrite shown in Fig. 2F.

In Fig. 4, the reflectance spectra of opaque melt veins (shock veins). interconnected melt veins, melt dike, and black chondrite are compared. These spectra are all very different from those of normal, ordinary chondrite material that has not been altered by shock melting. Common features include reduction of the reflectance to the 0.05-O. 10 level, great or total suppression of the silicate absorption bands near 1 and 2 pm,

and a residual absorption in the 0.3-0.6 pm region. As previously shown (Gaffey 1976, Britt et al. 1989a, Britt and Pieters 1989), this material spectrally resembles unshocked carbonaceous chondrite material. Due to the degree of variation in both the asteroids and the meteorites, it is probably not fruitful to compare individual examples of each set. but there certainly is enough overlap to allow very close matches.

SHOCKED 0.20

I

I, DARK

I

I,

8,

I,,

ORDINARY CHONDRITES

I,,

MATERIAL

,

9

I,

1

Taiben (Small SamDIe)

0.1 5

(Large Sample

8 5 2

0.1 0

;r : 0.05

0.00

0.30

0.60

0.90

1.20

1.50

Wavelength

1.60

2.1 0

2.40

In Microns

FIG. 4. Comparison of all optical reflectance spectra of the opaque melt veins, interconnected melt veins, melt dike, and black chondrite shown in Fig.3.

IMPLICATIONS

FOR ASTEROIDS

Are Ordinary-Chondrite Parent Bodies Hidden among the C-Type Asteroids? The C asteroids have been traditionally associated with the carbonaceous chondrites, specifically the CIl and CM2 hydrated carbonaceous chondrites (Johnson and Fanale 1973, Gaffey and McCord 1978). For some C asteroids, this hypothesis is strongly confirmed by the observation of an absorption band due to bound water in clay minerals (Lebofsky 1980); however, many others have no hydration band and clearly do not contain significant amounts of clay minerals. This subgroup of the C-types cannot be composed of any common type of chondritic material; their visual albedos and spectra are not similar to the anhydrous CO3 and CV3 chondrites, and they lack the deep 3.3~pm hydrated silicate absorption bands of CIl and CM2 chondrites. The usual assumption is that these asteroids are composed of material chemically similar to the CI or CM chondrites, but which escaped hydration during the asteroidal heating and melting period (Jones et al. 1990, Bell et al. 1989). Sometimes the imaginary meteorite classification “CM3” is used to describe this hypothetical material. Our results agree with previous suggestions that highly shocked (shock-melted) ordinary chondrite materials such as opaque melt veins (shock veins), melt pockets and irregular interconnected melt veins, melt dikes, and black chondrites (as defined by Stoffler et al. 1991) spectrally mimic some C-type asteroids. This is only relevant to interpretation of asteroid telescopic spectra if such materials cover a large fraction of an asteroid’s optical surface; there are several lines of evidence which suggest that they do not. Stoffler et al. (1991) note that melt rocks like those found in melt sheets in terrestrial impact craters are extremely rare among ordinary chondrites. Presumably, this

AND RELATION

TO ASTEROIDS

51

is because formation of such rocks requires high impact velocities, which results in the ejection of most of this material from the asteroid. They suggest that the largest high velocity impacts on small asteroids produce simple, bowl-shaped craters too small to form significant melt sheets. Thus, these authors conclude that asteroidal impact melts are “trapped” and preserved on asteroids and, hence, in chondritic meteorites only as opaque melt veins, melt pockets and interconnected melt veins, melt dikes, and black chondrites which are analogous to pseudotachylite and intrusive melt and fragmental breccia dikes occurring mostly in the crater floors and crater basements of terrestrial impact craters (e.g., Stoffler et al. 1988). They are of relatively low abundance in these craters, relative to the large amounts of brecciated and ejected material, and they are not surface features, as would be required if they were to have an effect on the optical properties of asteroid surfaces. We therefore conclude that these materials are not likely to play a major role in altering the surface reflectance properties of ordinary chondrite asteroids. This view is also supported by the relatively low abundance of these materials in the world’s meteorite collections. The meteorites studied in this paper were selected from the entire stock of ordinary chondrites in the UNM and UH collections, of which they make up about 5%. Britt and Pieters (199la) find that black chondrites make up about 15% of the chondrites they have studied, but their survey includes “any ordinary chondrite meteorite exhibiting distinctly low reflectance 50.15 in hand sample”; this undoubtedly includes chondrites that have not been shock-melted but are dark for other reasons. In view of all these considerations we conclude that the surfaces of ordinary chondrite parent bodies are not likely to be covered by vast amounts of shock-blackened material such as opaque melt veins, melt pockets and interconnected melt veins, melt dikes, and black chondrites. Thus, these materials cannot be responsible for significant largescale spectral alterations of the parent asteroids of ordinary chondrites, and they cannot be called upon in support of the hypothesis that ordinary chondrite asteroids are hidden among the C-type asteroids. If this hypothesis is to be upheld, then recourse may have to be taken to the recent suggestion of Britt and Pieters (199lb,c) that ordinary chondrite parent asteroid surfaces appear spectrally similar to those of C-type asteroids because they are totally covered by a hypothetical, thin layer of finegrained material similar to that present in the dark portions of solar wind-bearing regolith breccias. Are Class K Asteroids Parent Bodies?

Ordinary-Chondrite

Another group of asteroids has spectra similar to some of the shock-melted and shock-blackened material measured in this study. Bell et al. (1987) found that several

52

KEIL. BELL. AND BRITT

members of the Eos asteroid family have flat, featureless spectra in the l-2.5 pm range, even though their visible (0.3-l wrn) spectra are similar to those of S-type asteroids and they were almost ail classified S in the system of Tholen (1984). Bell (1988, 1989) proposed a provisional asteroid Class “K” to describe the unique properties of the Eos family, and Tedesco et ul. (1989) formally defined a similar class K in their taxonomic system. This unusual spectral signature is very similar to that of some of the shock-melted ordinary chondrite material in Fig. 3. The average albedo of Class K asteroids is about 0.09, also consistent with spectra of some of the shock-melted chondrites (Fig. 4). From a purely spectroscopic view. the K class is as likely to be composed of shock-melted ordinary chondrite material as any member of the C class. However, we do not believe that this is a reasonable interpretation. With rare exceptions, every asteroid in the Eos family shares the unusual Class K spectrum (Veeder ct al. 1992). This implies that the entire mass of the 120km parent body of the family would have to have been converted to black chondrite material, presumably during the impact which shattered it, to create the current asteroid family. Considering the low abundance of shockmelted and shock-blackened material among ordinary chondrites and the general paucity of asteroidal impact melts (e.g., Staffler et al. 1991), this is not a likely explanation. For the K-type asteroids, there is also a ready alternative interpretation. The spectrum of Eos, the bestobserved K asteroid, is almost identical to those of the CV and CO carbonaceous chondrites (Bell rt ~1. 1987, Bell 1988). The origin of these meteorites has always been a mystery, as they are too bright to match C-type asteroids. The discovery of the K class provides them with a reasonable asteroidal parent body. ACKNOWLEDGMENTS We thank H. Y. McSween and M. J. Gaffey for constructive reviews. This research was supported by NASA Grants NAG 9-454 (K. Keil), NAGW 712, and NAGW 802 (J. F. Bell). Acquisition of the spectra at the Brown University RELAB was supported by NASA Grant NAGW748, and D. T. Britt was supported by NASA Graduate Student Research Fellowship 90-137. This is Planetary Geosciences Publication No. 681 and School of Ocean and Earth Science and Technology Publication No. 2935.

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