black ordinary chondrites: an analysis of abundance and ...abstract-blackordinary chondrite...

7
Meteoritics 26, 279-285 (1991) © Meteoritical Society, 1991. Printed in USA Black ordinary chondrites: An analysis of abundance and fall frequency DANIEL T. BRITT' AND CARLE M. PIETERS Department of Geological Sciences, Box 1846, Brown University, Providence, RI 02912, USA 'Current address: University of Arizona, Lunar and Planetary Lab, Tucson, AZ 85721, USA (Received 19 December 1990; accepted in revised/arm 9 August 1991) Abstract-Black ordinary chondrite meteorites sample the spectral effects of shock on ordinary chondrite material in the space environment. Since shock is an important regolith process, these meteorites may provide insight into the spectral properties of the regoliths on ordinary chondrite parent bodies. To determine how common black chondrites are in the meteorite collection and, by analogy, the frequency of shock-alteration in ordinary chondrites, several ofthe world's major meteorite collections were examined to identify black chondrites. Over 80% of all catalogued ordinary chondrites were examined and, using an optical definition, 61 black chondrites were identified. Black chondrites account for approximately 13.7% of ordinary chondrite falls. If the optically altered gas-rich ordinary chondrites are included, the proportion of falls that exhibit some form of altered spectral properties increases to 16.7%. This suggests that optical alteration of asteroidal material in the space environment is a relatively common process. INTRODUCTION METEORITES ARE INVALUABLE SOURCES OF INFORMATION on a whole range of solar system processes. Of particular interest to spectroscopists are processes that occur on the surfaces of solar system bodies. Remote sensing investigations using reflectance spectroscopy are limited to material on the upper few milli- meters to centimeters of a parent body's surface. These first few centimeters are the optically active zone which interacts with the incident light to produce diagnostic reflectance spectra. However, these same upper few centimeters are also the zone most exposed to the local environment. In the case of atmo- sphereless bodies like the Moon or the asteroids, this is the zone most exposed to the harsh conditions of the space environment. This upper zone is typically a regolith of fragmental material and it is exposed to a range of processes involving hard radiation, solar wind implantation, low-velocity accretion, high-velocity impact, and hard vacuum. Based on experience from the lunar regolith, regolith processes can produce significant optical al- teration in the surface material which can complicate the remote identification of surface mineralogy (Pieters, 1984). Impact pro- cesses are particularly important since impacts provide the en- ergy for a host of regolith processes such as heating, particle size comminution, lateral transport, plastic deformation, and shock darkening. Black ordinary chondrite meteorites are optically altered ordinary chondrites that have been blackened in the space environment by shock and heating from impacts (Hey- mann, 1967; Scott, 1982; Keil, 1982; Rubin, 1985; Ehlmann et al., 1988; Britt and Pieters, 1990). As such, they represent a source of information on several aspects of regolith and regolith- like (i.e., shock) processes. First, they illustrate the spectral ef- fects of shock on ordinary chondrite material in the space en- vironment. Although many black chondrites have probably never been part of the surface material of an asteroid, shock is a major regolith process and its effect on ordinary chondrite material is very important for understanding the spectral effects of regolith processes. Second, the numbers and distribution of black chon- drites in meteorite collections provide insight on the frequency of shock-blackening as an asteroidal process. As part of a spectral and petrographic study of low-reflectance ordinary chondrites, several major meteorite collections were surveyed to identify and catalogue black ordinary chondrites. The goals ofthis survey were to identify possible samples for the spectral study and to determine the abundance of black chondrites in the meteorite fall population. DATA COLLECTION Definitions and Characteristics Black and Normal Ordinary Chondrites The mineralogy of ordinary chondrites is dominated by sub- equal amounts of olivine and low-calcium orthopyroxene in chondrules, fragments of chondrules and clasts, with substantial amounts of free FeNi metal and troilite (Dodd, 1981). Most ordinary chondrites show some evidence of shock. Usually it is relatively mild shock evidenced by minor brecciation, fractur- ing, and veining or, more rarely, the heavy shock characteristic of black chondrites. The optical properties of most ordinary chondrites are dominated by the presence of large amounts of relatively bright olivine and orthopyroxene. Diffuse reflectance values, relative to a perfectly reflective diffuse Lambertion sur- face, range from 0.16 to 0.40 in visible wavelengths with higher metamorphic grade meteorites tending to be brighter (Gaffey, 1976). Laboratory reflectance spectra show strong Fe 2+-Fe3+ charge transfer absorptions in the U'V-visible wavelengths, prominent 1.0 /Lm Fe 2+ crystal field absorptions, and a moderate 2.0 /Lm Fe2+ crystal field absorption. All of these spectral features are characterized by excellent spectral contrast which makes mineralogical interpretations of ordinary chondrite spectra pre- cise and diagnostic (Cloutis et al., 1984). Black chondrites have been defined principally by their spec- tral (Gaffey, 1976; Britt and Pieters, 1989) and shock (Heymann, 1967) characteristics. Typically black chondrites are character- ized by pervasive petrographic shock features, low-gas retention ages, and low reflectance (Gaffey, 1976; Heymann, 1967; Britt and Pieters, 1989). As shown in Fig. 1, the spectral features of black chondrites are strongly subdued, lacking the high-contrast absorption features of normal ordinary chondrites. Visible re- flectances tend to be in the 0.6 to 0.15 range. These flat, subdued spectra are similar to the spectra of some carbonaceous chon- drites, principally because both meteorite types are dominated spectrally by opaque minerals (Gaffey, 1976; Britt and Pieters, 1990). However, regardless of their spectral similarity to car- 279

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Page 1: Black ordinary chondrites: An analysis of abundance and ...Abstract-Blackordinary chondrite meteorites sample the spectral effectsof shock on ordinary chondrite material in the space

Meteoritics 26, 279-285 (1991)© Meteoritical Society, 1991. Printed in USA

Black ordinary chondrites: An analysis of abundance and fall frequency

DANIEL T. BRITT' AND CARLE M. PIETERS

Department of Geological Sciences, Box 1846, Brown University, Providence, RI 02912, USA'Current address: University of Arizona, Lunar and Planetary Lab, Tucson, AZ 85721, USA

(Received 19 December 1990; accepted in revised/arm 9 August 1991)

Abstract-Black ordinary chondrite meteorites sample the spectral effects of shock on ordinary chondrite material in the spaceenvironment. Since shock is an important regolith process, these meteorites may provide insight into the spectral properties ofthe regoliths on ordinary chondrite parent bodies. To determine how common black chondrites are in the meteorite collectionand, by analogy, the frequency ofshock-alteration in ordinary chondrites, several ofthe world's major meteorite collections wereexamined to identify black chondrites. Over 80% of all catalogued ordinary chondrites were examined and, using an opticaldefinition, 61 black chondrites were identified. Black chondrites account for approximately 13.7% of ordinary chondrite falls. Ifthe optically altered gas-rich ordinary chondrites are included, the proportion of falls that exhibit some form of altered spectralproperties increases to 16.7%. This suggests that optical alteration of asteroidal material in the space environment is a relativelycommon process.

INTRODUCTION

METEORITES ARE INVALUABLE SOURCES OF INFORMATION on awhole range of solar system processes. Of particular interest tospectroscopists are processes that occur on the surfaces of solarsystem bodies. Remote sensing investigations using reflectancespectroscopy are limited to material on the upper few milli­meters to centimeters of a parent body's surface. These first fewcentimeters are the optically active zone which interacts withthe incident light to produce diagnostic reflectance spectra.However, these same upper few centimeters are also the zonemost exposed to the local environment. In the case of atmo­sphereless bodies like the Moon or the asteroids, this is the zonemost exposed to the harsh conditions of the space environment.This upper zone is typically a regolith of fragmental materialand it is exposed to a range of processes involving hard radiation,solar wind implantation, low-velocity accretion, high-velocityimpact, and hard vacuum. Based on experience from the lunarregolith, regolith processes can produce significant optical al­teration in the surface material which can complicate the remoteidentification of surface mineralogy (Pieters, 1984). Impact pro­cesses are particularly important since impacts provide the en­ergy for a host of regolith processes such as heating, particle sizecomminution, lateral transport, plastic deformation, and shockdarkening. Black ordinary chondrite meteorites are opticallyaltered ordinary chondrites that have been blackened in thespace environment by shock and heating from impacts (Hey­mann, 1967; Scott, 1982; Keil, 1982; Rubin, 1985; Ehlmann etal., 1988; Britt and Pieters, 1990). As such, they represent asource ofinformation on several aspects of regolith and regolith­like (i.e., shock) processes. First, they illustrate the spectral ef­fects of shock on ordinary chondrite material in the space en­vironment. Although many black chondrites have probably neverbeen part of the surface material of an asteroid, shock is a majorregolith process and its effect on ordinary chondrite material isvery important for understanding the spectral effects of regolithprocesses. Second, the numbers and distribution of black chon­drites in meteorite collections provide insight on the frequencyof shock-blackening as an asteroidal process. As part of a spectraland petrographic study of low-reflectance ordinary chondrites,several major meteorite collections were surveyed to identifyand catalogue black ordinary chondrites. The goals ofthis survey

were to identify possible samples for the spectral study and todetermine the abundance of black chondrites in the meteoritefall population.

DATA COLLECTION

Definitions and Characteristics

Black and Normal Ordinary Chondrites

The mineralogy of ordinary chondrites is dominated by sub­equal amounts of olivine and low-calcium orthopyroxene inchondrules, fragments of chondrules and clasts, with substantialamounts of free FeNi metal and troilite (Dodd, 1981). Mostordinary chondrites show some evidence of shock. Usually it isrelatively mild shock evidenced by minor brecciation, fractur­ing, and veining or, more rarely, the heavy shock characteristicof black chondrites. The optical properties of most ordinarychondrites are dominated by the presence of large amounts ofrelatively bright olivine and orthopyroxene. Diffuse reflectancevalues, relative to a perfectly reflective diffuse Lambertion sur­face, range from 0.16 to 0.40 in visible wavelengths with highermetamorphic grade meteorites tending to be brighter (Gaffey,1976). Laboratory reflectance spectra show strong Fe2+-Fe3+

charge transfer absorptions in the U'V-visible wavelengths,prominent 1.0 /Lm Fe2+ crystal field absorptions, and a moderate2.0 /Lm Fe2+ crystal fieldabsorption. All of these spectral featuresare characterized by excellent spectral contrast which makesmineralogical interpretations of ordinary chondrite spectra pre­cise and diagnostic (Cloutis et al., 1984).

Black chondrites have been defined principally by their spec­tral (Gaffey, 1976; Britt and Pieters, 1989)and shock (Heymann,1967) characteristics. Typically black chondrites are character­ized by pervasive petrographic shock features, low-gas retentionages, and low reflectance (Gaffey, 1976; Heymann, 1967; Brittand Pieters, 1989). As shown in Fig. 1, the spectral features ofblack chondrites are strongly subdued, lacking the high-contrastabsorption features of normal ordinary chondrites. Visible re­flectances tend to be in the 0.6 to 0.15 range. These flat, subduedspectra are similar to the spectra of some carbonaceous chon­drites, principally because both meteorite types are dominatedspectrally by opaque minerals (Gaffey, 1976; Britt and Pieters,1990). However, regardless of their spectral similarity to car-

279

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280 D. T. Britt and C. M. Pieters

BLACK AND NORMAl ORDINARY CHONDRllE METEORllES0.35

0.30

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0.05 Gaffey, 1976

Britt and Pieters, 1991

Manbhoom LL6

Homestead L5

Gruneburg H4

Farmington L5Gorlovka H3-4Paragould LL5

0.00 .........--'--'--'----'--'----'---l.-'--'--'-'----''---''......L.-'---'-.l..-L.......L......L.....>....-J

0.30 0.60 0.90 1.20 1.50 1.80 2.10 2.40Wavelength In Microns

FIo. I. Bidirectional reflectance spectra of normal and black ordi­nary chondrites. Normal ordinary chondrites are characterized by brightreflectances and strong spectral absorption features. Black ordinarychondrites show varying degrees of optical alteration in the form ofreduced reflectance and subdued absorption features. Spectral data fromGaffey (1976) and Britt and Pieters (1991).

bonaceous chondrites, black chondrites are well within the rangeof normal ordinary chondrite mineralogy and chemistry.

In our survey of meteorite collections for black chondrites,the primary focus was the optical characteristics of ordinarychondrite meteorites. Although black chondrites can be definedby their petrographic or gas-retention characteristics, it is logisti­cally impossible in a relatively short period of time to surveyhundreds of meteorites for these traits. To facilitate the iden­tification of black chondrites, a relatively simple definition wasdeveloped based on optical characteristics that could be easilyevaluated in hand sample. In the Gaffey (1976) study of me­teorite reflectance the 33 normal ordinary chondrites had re­flectances of greater than 0.16 and generally reflectances weremuch greater than the minimum value. The five black chon­drites in the study exhibited reflectances ranging from 0.125down to 0.067 (Gaffey, 1976). A maximum reflectance of 0.15for black chondrites was chosen because it was below the ob­served minimum value for normal ordinary chondrites, abovethe maximum value of known black chondrites, and at an opticalbreak point where objects began to appear "black" rather thangray. Another issue to address is how much of the meteoritehad to be black to qualify for the classification. Ordinary chon­drites are often highly heterogeneous in color with dark veins,dark spots, chondrules, light spots, and light/dark areas commonin individual meteorites. Several meteorites already recognizedas black chondrites, such as Paragould, have a structure ofstrongly contrasting light and dark areas. The light areas canhave reflectance values of as great as 0.40 and be in direct contactwith dark areas showing reflectances of 0.09 (Britt and Pieters,1991). To include these light/dark meteorites a minimum sizefor a dark area was set at 20% of the exposed surface of theavailable sample. This minimum was high enough to excludemost meteorites with minor veining, xenoliths, or "spots", andlow enough to include meteorites with observable areas of shock­darkening (Britt and Pieters, 1990). For the purposes of thisstudy, a black chondrite meteorite was defined as: "Any ordinarychondrite meteorite exhibiting distinctly low reflectance < 0.15

FlO. 2. Black ordinary chondrites. (a) Farmington (L5) on the leftand the normal ordinary chondrite Farmville (H4) on the right. Thestrong darkening shown in Farmington is characteristic of black chon­drites. (b) Paragould (LL5). This meteorite is an example of the secondtype of black chondrite, one with significant portions of light materialoften mixed intimately with the dark material.

in hand sample (type example is the L5 Farmington), or withmajor portions of the meteorite containing such low reflectancematerial (type example is the LL6 Paragould)." The type me­teorites Farmington and Paragould are shown in Figs. 2a and2b.

It is important to emphasize that the above definition is alimited one based solely on broad optical characteristics. It can­not be assumed that simply because a meteorite is opticallyblack that it is also highly shocked and has a low gas-retentionage. Most meteorites identified as black on optical grounds alsohave petrographic evidence of shock, but there are several no­table exceptions which will be discussed in later sections. Anadditional limitation on this optical definition is that two sub­groups of ordinary chondrites, the gas-rich and the unequili­brated ordinary chondrites, typically have optical characteristicssimilar to black chondrites. Both of these groups will be initiallyexcluded from the survey of black chondrites. However, theoptical alteration in gas-rich ordinary chondrites is probablycaused by shock processes similar to those that darken blackchondrites. The optical effects of this process has importantimplications for the study of the spectra of asteroidal regoliths.The characteristics of both gas-rich and unequilibrated chon­drites will be discussed in detail below.

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Black ordinary chondrites 281

DWALENI (H6): GAS-RICH ORDINARY CHONDRITE0.35

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Dwaleni (H6) light

~ Dwaleni (H6) dark

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FIG. 3. The gas-rich ordinary chondrite Dwaleni (H6). The darkmatrix is the only portion ofthe meteorite that contains solar wind gas­rich material that was probably part of the regolith soil of an ordinarychondrite parent body. These dark, gas-rich grains are intimately mixedwith light, non-gas-rich clast material.

Gas-Rich Ordinary Chondrites

Gas-rich ordinary chondrites contain areas of fine-graineddark matrix material which have concentrations ofgrains withhigh levels of solar-wind implanted gases and solar-flare tracks.For these gases to be implanted by the solar wind, the grainsmust have been in the top one millimeter of the parent body'ssurface material (Bischoff et al., 1983; McKay et al., 1989).These meteorites probably represent the re-lithified regolith soilof asteroids (Suess et al., 1964; Bischoff et al.. 1983). Typicallygas-rich chondrites are aggregations of relatively large clasts oflight normal ordinary chondrite material set in a matrix of fine­grained dark material (Keil, 1982). The matrix itselfis a complexmixture of gas-rich and non-gas-rich grains and small clastsintimately jumbled together and cemented by shock-inducedgrain-boundary melting (Bischoff et al., 1983; Bell and Keil,1988). The abundance of matrix material is highly variable andin extreme cases can consist of over half the meteorite's volumeor be confined to small branching veins that occupy much lessthan 5% of volume. The matrix itself often contains over 50volume% fine grained light clast material (Britt and Pieters,1991). As example of the structure of gas-rich ordinary chon-

0.00 L....----L--'---'---'-...L-.L...-l...-..I----L----'--'---'--'-.l..-..........--L----'---'-...L.-J...-J

0.30 0.60 0.90 1.20 1.50 1.80 2.10 2.40Wavelength In, Microns

FIG. 4. Bidirectional reflectance spectral of the dark and light por­tions of Dwaleni (H6). The dark gas-rich material shows significantalteration of its spectral properties relative to the light portion of themeteorite. Spectral data from Britt and Pieters (1991).

drites, a picture of the gas-rich ordinary chondrite Dwaleni (H6)is shown in Fig. 3. Spectrally, the light clasts are indistinguish­able from normal ordinary chondrites and the dark matrix ma­terial resembles a mix of normal and black chondrite material(Bell and Keil, 1988; Britt and Pieters, 1990). Gas-rich chon­drites are defined by their solar-wind implanted gas content.The accepted standard for a meteorite to be considered gas-richis if grains in their dark matrix show a concentration of 'He >3000 10-8 cc STP/g (Schultzand Kruse, 1989).With largeenoughconcentrations of gas-rich dark grains, the spectra of these areasbecomes very similar to the spectra of black chondrites. Shownin Fig. 4 are the spectra of the light and dark areas of the gas­rich chondrite Dwaleni. The modest content of dark grains (es­timated at 35 volume%) in the dark areas of Dwaleni producesan altered spectrum very similar to what would be produced bya mix of black and normal ordinary chondrite material (Brittand Pieters, 1990).

A survey of current literature identifies 39 ordinary chondritefalls as gas-rich (Schultz and Kruse, 1989; Bell and Keil, 1988).The completeness of any compilation of gas-rich chondrites,however, is limited by the completeness of the noble gas mea­surements. It is certainly possible that some of the meteoritesidentified as black will tum out to be solar gas-rich. The spectralsimilarities between gas-rich and black ordinary chondrites sug­gest that most gas-rich chondrite would meet the optical defi­nition of a black chondrite. Since the objective of this study isto identify black ordinary chondrites, those ordinary chondritesthat have already been identified as gas-rich will be treatedseparately so as to not bias the abundance and distributionstatistics for black chondrites.

Unequilibrated Ordinary Chondrites

Unlike most ordinary chondrites, the unequilibrated ordinarychondrites were subjected to very modest metamorphic tem­peratures and remained relatively unaltered from their accre­tional state. Their matrices tend to be rich in carbon mineralsand magnetite. Typically unequilibrated ordinary chondrites havewell-formed chondrules and clasts of chondrules in a dark fine-

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282 D. T. Britt and C. M. Pieters

FIG. 5b. Kodak 20-shade gray scale (KODAK publication Q-13)usedfor reflectance estimates in this study. The scale wascalibrated toabsolute reflectance at the densities corresponding to the A, M, and Bchips.

0.00Q~ MO MO vo

Wavelength In MicronsFIG. 5a. Bidirectional reflectance spectra of Kodak diffuse density

chipsA, M, and B.Thesechipsareadvertised to yieldrespectively 0.80,0.18,and 0.06average reflectance in thevisible wavelengths. Thesedataamplyconfirm the manufacturer's claims.

grained matrix that is often abundant enough to meet the opticaldefinition ofa black chondrite. Since this dark matrix probablyreflects early accretional processes rather than later optical al­teration, all ordinary chondrites classified as type 3 in the Cat­alogue of Meteorites (Graham et al., 1985) were excluded fromthis study so as to not bias the statistics on black chondrites.

Study Methods and Requirements

The objective ofthis study was to survey meteorite collectionsin order to identify and catalogue black ordinary chondrites. Tobe classified as a black chondrite a meteorite had to meet fivecriteria. First, the survey was restricted to observed falls becausefalls are much less likely to have undergone significant terrestrialalteration. Also, given the inherent bias of meteorite discoveryand collection, falls are more likely to represent a random se­lection of the flux ofordinary chondrites. Second, the study was

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KODAK GRAY SCAlE CAUBRATION

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Chip M (18% Reflectance) .....

Chip B (6%Reflectanc~

restricted to ordinary chondrites that had a chemical and pet­rographic classification listed in the Catalogue of Meteorites(Graham et al., 1985). This restricted the study to well-classifiedmeteorites and eliminated the possibility of including unequili­brated ordinary chondrites in the list of black chondrites. Thethird criterion was that the meteorite meet our optical definitionof a black chondrite: that its reflectance be less than or equal to15% in hand sample estimated from a cut or broken surface.

To provide a quick and convenient field measure of reflec­tance a Kodak 20-shade gray scale (KODAK publication Q-13)was calibrated in RELAB to absolute reflectance (Pieters, 1983).A meteorite sample would be compared with the calibrated greyscale to determine ifit was darker than the cut-off value of0.15.Shown in Fig. 5a are the spectra of Kodak diffuse density chipsthat correspond to 80%, 18%, and 6% reflectance. These chipsare Kodak density cards A, M, and B, respectively. The claimsof the manufacturer for reflectance and spectral uniformity invisible wavelengths were amply confirmed in the laboratoryspectra. The gray scale is shown in Fig. 5b. It is important toemphasize that this scale is a color density scale where densityis not linearly related to reflectance. This scale was very usefulfor relative reflectance judgements to determine ifa sample wasdarker or lighter than the cut-off value, but it was beyond thecapacity of the authors to make absolute reflectance measure­ments without the aid of a spectrometer. Relative reflectancejudgements were tested on a number ofsamples by first makinga relative reflectance judgement with the gray scale, then mea­suring the same sample with the RELAB spectrometer to de­termine if its absolute reflectance met the optical definition.

A fourth criterion was that the sample have sufficient massand internal exposure to make a reliable reflectance judgement.Usually this required a sample mass of greater than five grams.Since meteorites are often initially covered by a burnt fusioncrust, a fresh cut or broken surface over a substantial portionof the sample was required. Also all parts of an individual me­teorite are generally not available for study. A meteorite mayfall as hundreds offragments or may be fragmented by its own­ers. These fragments are usually widely dispersed in a numberofpublic and private collections. It would be a logistically hope­less task to examine all the fragments of most meteorites. Sincethe goal ofthis survey was to identify black chondrites for furtherstudy, classification decisions were limited to those fragmentsreasonably available for study in major collections. The fifthcriterion was that terrestrial weathering (rust) be minimized.The terrestrial weathering of ordinary chondrites converts theabundant FeNi metal to disseminated iron oxide, producingreduced reflectance, spectral alteration characteristic of iron ox­ide mineralogy, and a brown, rusty coloration. All samples wereexamined for evidence of terrestrial weathering and weatheredmeteorites were rejected. As previously discussed, all gas-richand unequilibrated ordinary chondrites were also excluded fromthe study.

Collections

Clearly for the best possible statistical results and the highestprobability of identifying new black chondrites, this survey wasdesigned to include as many ordinary chondrites as possible.As shown in Table 1 there are 1446 ordinary chondrites listedin the Catalogue ofMeteorites (Graham et al., 1985). Excludingthe 785 finds leaves 661 ordinary chondrite falls that were con-

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Black ordinary chondrites 283

TABLE I. Samples for Black Chondrite Survey TABLE 2. Black ordinary chondrite falls.

RESULTS AND DISCUSSION

Black Chondrites

A total of 61 ordinary chondrites were identified by this studyas black under our definition. Shown in Table 2 are the ordinarychondrite meteorites identified as "black". It is important toemphasize that these meteorites are "black" only under theoptical criteria established for this study. These criteria do notrequire the meteorites to be also highly shocked or show lowgas-retention ages. Probably most of these meteorites displaythe shock characteristics that are thought to be typical of blackchondrites, but there are some notable exceptions. Sena (H4)and Nadiabondi (H5) have been described as unshocked butdark meteorites, with friable textures reminiscent of accretion­ary breccias (Pellas 1989, pers. comm.). These meteorites arelisted as black chondrites under the optical criteria but they maynot have the shock history common to most other black chon­drites. However, for the purpose ofthis study it is the reflectanceproperties of the meteorite that determine its classification.

sidered for this study. Although most meteorites find their wayto major, usually governmental, meteorite collections where theycan be stored and subjected to rigorous scientific study, thecollection of meteorites is still a uniquely human enterprise andsubject to human foibles. A number of ordinary chondrites re­main unstudied and unclassified because they are held in privatecollections, sometimes as religious objects (Graham et al., 1985;McSween, 1987). A total of 52 falls, though identified as ordi­nary chondrites, are excluded because they have not been stud­ied sufficiently to classify both chemical and petrologic type. Anaddition 56 ordinary chondrites were excluded from the studybecause they are unequilibrated or gas-rich or both, leaving amanageable 553 possible meteorites for this survey.

A number ofclimatic, social-economic, and geo-political fac­tors influence the process of meteorite recovery and the con­centration of meteorites in major collections (McSween, 1987;Burke, 1986). Not surprisingly, samples of the vast majority ofmeteorites are in a relatively few collections maintained by pastor current global political powers. The collections ofthe HarvardUniversity Mineralogical Museum, the Smithsonian Institutionof Washington, the British Museum (Natural History) and theMuseum National d'Histoire Naturelle in Paris were surveyedfor this study. In addition the collection of the USSR Academyof Sciences in Moscow was surveyed by Dr. M. I. Pateav usingthe criterion established for this study. Since samples of thesame meteorite are often held by multiple institutions, relativereflectance judgements could be constantly cross-checked andconfirmed. In all 446 meteorites, representing 80.7% of totalordinary chondrite falls were checked for this study.

Collection codes: A: University ofMexico; L: British Museum (NaturalHistory); H: Harvard University Mineralogical Museum; M: USSRAcademy of Sciences; P: Museum National d'Histoire Naturalle; S:Smithsonian Institution.

Total ordinary chondrites listed*Minus findsMinus un-classified fallsMinus gas-rich and/or unequilibrated

Total "possible" for surveyTotal examinedPercentage

* In Graham et al.. 1985.

1446-785-52-56553446

80.65%

Meteorite Name

AkbarpurAnduraAtarraAthensBeddgelertBielokrynitschieBlanskoCanellasCastaliaCeresetoChandpurChantonnayChicoraDesuriDjati-PengilonEnsisheimErgheoFarmingtonGorlovkaHedeskogaHedjazJackalsfonteinJamkheirKaboKarkhKendletonKhetriKulakKunashakLimerickLuponnasMalakalMenowMoti-ka-naglaNadiabondiNakhon PathomOjuelos AltosOlmedilla de AlarconOrvinioPampangaParagouldParanaibaPervomaiskyRancho della PresaRose CitySallesSedikoySenaSeresSete LagoasSevrukovoShytalSiaveticSuchy DulSultanpurSupuheeTadjeraUdenUmbalaWaltersWittekrantz

Type

H4H6L4LL6H5H4H6H5H5H5L6L6LL6H6H6LL6L5L5H4-3H5L6-3L6H6H4L6L4H6L5L6H5H5-3L5H4H6H5L6L6H5L6L5LL5L6L6H5H5H6L6H4H4H4L5L6H4L6L6H6L5LL6LL5L6L5

Collection

L,S,HSSSSS,H,MSSS,HSL,PS,HSLSS,HSS,HM,SSS,H,PLLL,SL,SSLLS,M,HSPSL,S,HL,S,HPSSS,PS,PPSSS,MS,HSS,HLS,PS,L,HAM,SL,S,HLSLL,S,HS,HSLSL

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284 D. T. Britt and C. M. Pieters

TABLE 3. Black ordinary chondrite falls.

Meteorite type Checked Black Percentage

H4 39 9 23.08%H5 82 II 13.41%H6 53 9 16.98%TotalH 174 29 16.67%

L4 15 2 13.33%L5 40 8 20.00%L6 178 16 8.99%TotalL 233 26 11.l6%

LL4 3 0 0.00%LL5 II 2 18.18%LL6 25 4 16.00%Total LL 39 6 15.38%

Grand total 446 61 13.68%

The distribution of black chondrites between chemical groupsand petrographic types are summarized in Table 3. A total of61 black chondrites out of the 446 ordinary chondrite fallschecked gives an average fall abundance of black chondrites of13.7%. The standard deviation (c) for this distribution is ±4.5(Davis, 1986). The striking features of these data are the roughuniformity in the distribution between chemical groups andpetrographic types. All abundances in the petrographic andchemical types (with the exception of the LL4's which are af­flicted with a very low number of samples) are well within the2u value of ±9.0. In general H-chondrites, LL-chondrites, andthe L5's seem a little over-represented while the L6 chondritesseem underrepresented. The trends of the two largest sub-groups,L6 and H5, illustrate the differences in distributions. The H5chondrites have abundances of black chondrites very close tothe mean value for the whole population. However, the L6chondrites are by far the largest sub-group and have a blackchondrite abundance depleted relative to the overall mean. Thisresult is somewhat unexpected since the work of Heymann (1967)pointed to a major L-chondrite shock event at 520 ± 60 Ma.However, this shock event is most evident in the L5 chondrites,which have abundances of black chondrites that are substan­tially above the mean for the whole population. The L6's havemuch lower abundances of black chondrites and tend to beunder-represented in the L-chondrite shock event. Ofseven L6'sin the Heymann (1967) study, only two had gas-retention agesthat would put them in the 520 ± 60 Ma event. This wouldindicate that L6-chondrites were relatively rare in that shockevent and this rarity may be reflected in the fall abundance.Another possibility may be that the H-chondrites, with theirgreater free FeNi metal content, are more likely to become black­ened as a result of shock and thus have a higher overall abun­dance of black chondrites. In this case the lower abundance ofblack L6's may reflect the chemical differences between LandH-chondrites. Whatever the source of these meteorites, the sin­gle shock event does not seem to have significantly increasedthe population of black L-chondrites over levels common toother chemical groups.

It should be stressed that several of the ordinary chondritesubgroups have relatively small numbers of members due tothe rarity of these sub-groups in meteorite collection. The L4,LL4, LLS, and LL6 sub-groups are all populated by 25 or fewerexamined meteorites. These low population figures make their

abundances of black chondrites less reliable as indicators ofoverall population trends. However, even with low numbers,the abundances of black chondrites in these sub-groups (withthe exception of the LL4's) falls within approximately lu of theoverall population mean. The LL4 sub-group has only threeexamined members and none of those meteorites met the cri­teria for being a black chondrite. If we assume that the overallabundance of black chondrites is really 13.7% then the proba­bility of finding a black chondrite in any three ordinary chon­drites would be about 35%.To reach a 50%probability of findinga black chondrite under these conditions, five ordinary chon­drites would have to be examined. Given the small numbersinvolved, it is not surprising that black chondrites were notfound in the population of LL4 chondrites.

Gas-Rich Ordinary Chondrites

The spectral similarities between the dark portions of gas­rich ordinary chondrites and the black ordinary chondrites havebeen noted in the preceding sections. In order to isolate just theblack chondrite population, the known gas-rich chondrites werespecifically excluded from the compilation of black chondrites.However, most gas-rich chondrites would be classified as blackby the optical definition since they, by definition, contain sub­stantial proportions of blackened material (Britt and Pieters,

TABLE 4A. Gas-Rich ordinary chondrite falls.

Meteorite name Type Checked Black Collection

Benoni H6Bishunpur LLJ SBorodino H5 S,MBreitscheid H5Bremervorde H3 S,HCangas de Onis H5 S,H,PChainpur LLJ SDjermaia H SDubrovnik LJ-6Dwaleni H6 SElenovka L5 S,MFayetteville H4 SGutersloh H4 SHainaut H3-6 PHamelt LL4 SHeredia H5 L,PInnisfree LL5Ipiranga H6 SKilbourn H5 S,HKrymka LLJ S,H,MLeighton H5 LMafra LJ-4 SMalotas H5 SNgawi LLJ SNio H4Nullas H6 SOviedo L6 PPantar H5 SParnallee LLJ S,HPhum sambo H4 S,PPultusk H5 S,H,MRio Negro H4 SSaint-Severin LL6 SSharps H3 SSt Caprais-de Quinsa L6St Mesmin LL6 S,HTabor H5 S,HTysnes Island H4 S,HWeston H4 S,H

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Black ordinary chondrites 285

TABLE 4B. Gas-rich ordinary chondrite falls.

Meteorite type Checked Black Percentage

H3 2 2 100.00%H4 6 3 50.00%H5 9 4 44.44%H6 5 4 80.00%Total H 22 13 59.09%

L4 I 0 0.00%L5 I 0 0.00%L6 I I 100.00%Total L 3 I 33.33%

LL3 5 3 60.00%LL4 I 0 0.00%LL5 0 0 0.00%LL6 2 2 100.00%Total LL 8 5 62.50%

Grand total 33 19 57.58%

1990; Bell and Keil, 1988). These blackened and brecciatedgrains are extremely important since they contain the solar-windgases and represent samples of asteroidal regolith soil. Whileblackchondrites demonstrate the general spectral effectsof shockin the space environment, the dark portions of gas-rich chon­drites record not only shock, but all the other processes attendantto direct exposure to the space environment. In the course ofsurveying meteorite collections for black chondrites, relativereflectanceestimates were also collected for gas-rich chondrites.Shown in Table 4a is the compilation of gas-rich chondrites notincluded in the black chondrite study. Table 4b summarizes thestatistics of how many of the gas-rich chondrites could be con­sidered black under the optical definition of a black chondrite.In general, approximately 60 ± 8% of gas-rich ordinary chon­drites meet the criterion for being black. Interestingly, the vastmajority of gas-rich chondrites are H-chondrites and gas-richL-chondrites are very rare.

CONCLUSIONS

A survey of major meteorite collections shows approximately13.7% ± 4.5 of ordinary chondrite falls meet our optical defi­nition ofa black chondrite. This result indicates that black chon­drites are a numerically important sub-set of ordinary chon­drites. Given the uncertainties of the study, black chondritesare roughly equally distributed among the petrological andmetamorphic types. This implies that black chondrites are theresult of relatively common processes among ordinary chon­drites and not predominantly the product of a single event. Themajority of gas-rich ordinary chondrites also shows optical char­acteristics that would meet the definition of a black chondrite.Ifgas-rich ordinary chondrites are included in the analysis theproportion of ordinary chondrites that may have been subjectedto some sort of post-accretionary optical alteration increases to16.7%.

Acknowledgements-The authors wish to express thanks to C. A Francisof the Harvard University Mineralogical Museum, G. J. MacPhersonand T. Thomas ofthe Smithsonian Institution ofWashington, R. Hutch­ison and V. Jones of the British Museum (Natural History), P. Pellas

of the Museum National d'Histoire Naturelle, and M. I. Pateav of theUSSR Academy of Sciences for generously granting access to their re­spective collections and for the loan of meteorite samples for spectralanalysis. G. Wetherill, C. R. Chapman and an anonymous colleagueprovided helpful and constructive reviews. This research was supportedby NASA grant NAGW-28 and by a NASA Graduate Student ResearchFellowship to D. T. Britt. RELAB is supported as a NASA-multiuserfacility under grant NAGW-748.

Editorial handling: G. W. Wetherill.

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