differences between antarctic and non-antarctic meteorites ... · differences between antarctic and...

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Geuchimrca CI Cosmochimrca .4cia Vol. 55. pp. 3-18 Copyright 0 1991 Pergamon Press pk. Printed in USA 0016.7037/91/$3,00 + .OO Differences between Antarctic and non-Antarctic meteorites: An assessment CHRISTIAN KOEBERL' and WILLIAM A.CASSIDY~ ‘Institute of Geochemistry, University of Vienna, Dr.-Karl-Lueger-Ring 1, A-LO10 Vienna, Austria ‘Department of Geology and Planetary Science, University of Pittsburgh, Pittsburgh, PA 15260, USA Abstract-The discovery of a statistically significant number of meteorites in Antarctica over the past 20 years has posed many questions. One of the most intriguing suggestions that came up during the study of the Antarctic samples was that there might be a difference between the parent populations of Antarctic and non-Antarctic meteorites. This interpretation was put forward after the detection of a significant difference in the abundances of volatile and mobile trace elements in H, L, and C chondrites and achon- d&es. Other major differences include the occurrence of previously rare or unknown meteorites, different meteorite-type frequencies, petrographic characteristics, oxygen isotopic compositions, and smaller average masses. In addition, Antarctic meteorites have greater terrestrial residence ages and have collected on the ice shield over several lo5 years. The reality of numerous such differences has now been established beyond doubt; however, the main question regarding the cause of these differences remains. It seems that they have a wide variety of origins, ranging from pre-terrestrial traits to collection (recovery) effects and terrestrial weathering. Studies of terrestrial weathering have shown that, over the long time the meteorites spend in and on the ice, even subtle processes can produce substantial effects. For future investigations it will be important to pay more attention to the weathe~ng status of samples and to develop a more reliable and quantitative weathering index, e.g., based on infrared reflectance spectrometry or differential scanning calorimetry. Not ail differences between the Antarctic and non-Antarctic meteorite populations can be explained by weathering, pairing, or different collection procedures. Variable trace element abundances and distinct differences in the thermal history and thermoluminescence characte~stics have to be interpreted as being pre-terrestrial in origin. Such differences imply the existence of meteoroid streams, whose existence poses problems in the framework of our current knowledge of celestial mechanics. However, several independent studies support the existence of such meteoroid streams, thus being consistent with the suggestion of a time-variable influx of extraterrestrial material to Earth. The generally smaller average size of Antarctic meteorites may be the cause for the different meteorite-type frequency and the higher abundance of rare samples, because smaller meteorites may come from a slightly different parent population. In this paper we summarize the contributions in this series and provide a review of the current state of the question for the reality and cause of differences between Antarctic and non-Antarctic meteorites. 1. INTRODU~ION METEORITES ARE AMONG THE most important rocks on Earth; they allow us to look back in time to study the origin and evolution of our solar system. They are the oldest ma- terials available to us from our solar system and sample a large variety of parent bodies. They are especially valuable because they can be investigated in the laboratory with a complete set of sophisticated techniques. Up to about 20 years ago, a few thousand individual meteorites of different types were the only available samples of the meteorite population. Previously, these meteorites were found on all continents- only Antarctica was underrepresented. This changed rapidly with the discovery of meteorites on the so-called blue ice fields in Antarctica. Starting with a Jap- anese Antarctic expedition in 1969, an ever increasing num- ber of meteorites has been recovered from remote locations in Antarctica (LIPSCHUTZ and CASSIDY, 1986). These me- teorite-search expeditions have been supported in the United States by the National Science Foundation (Division of Polar Programs), in Japan by the National Institute of Polar Re- search, and by local agencies in other countries. To date, more than 12,000 individual specimens, representing an un- known number of falls, have been collected in Antarctica. Each one of these meteorites is a valuable research subject, and many are unique. The collection has proved to be of great value and it is of utmost impo~ance that the collection efforts be continued and expanded. A question important to many workers, in connection with the number of samples recovered from Antarctica, is: “How many of these samples are paired?” This problem is not easy to solve, but it has been estimated (e.g., SCOTT, 1984, 1989) that they represent about 2000-4000 distinct falls. This is a number that is (at this time) at least equal to, and probably larger than, the number of known non-Antarctic meteorite fails (which is on the order of 2500). Possibly because ofthese large totals, but perhaps for other reasons as well, the Antarctic collection contains many meteorite types that were either unknown or rare in the non-Antarctic meteorite collection. The most famous examples are the lunar meteorites (see be- low), but also include rare iron meteorites, shergottites, and other unusual achondrites and chondrites in the Antarctic collection. Antarctic meteorites are, however, also important because there are so many of them, providing material for statistical analysis. Gradually, within the past few years, it was found that there may be some differences between the Antarctic and the non-Antarctic meteorite popuiations- some subtle, some not so subtle. Such differences have been claimed to exist for a number of meteorite properties. DENNISON et al. (1986) and LIP- SCHULZ (1986) noted that, in a study of volatile and mobile

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Page 1: Differences between Antarctic and non-Antarctic meteorites ... · Differences between Antarctic and non-Antarctic meteorites: An assessment CHRISTIAN KOEBERL' and WILLIAM A.CASSIDY~

Geuchimrca CI Cosmochimrca .4cia Vol. 55. pp. 3-18 Copyright 0 1991 Pergamon Press pk. Printed in USA

0016.7037/91/$3,00 + .OO

Differences between Antarctic and non-Antarctic meteorites: An assessment

CHRISTIAN KOEBERL' and WILLIAM A.CASSIDY~ ‘Institute of Geochemistry, University of Vienna, Dr.-Karl-Lueger-Ring 1, A-LO10 Vienna, Austria

‘Department of Geology and Planetary Science, University of Pittsburgh, Pittsburgh, PA 15260, USA

Abstract-The discovery of a statistically significant number of meteorites in Antarctica over the past 20 years has posed many questions. One of the most intriguing suggestions that came up during the study of the Antarctic samples was that there might be a difference between the parent populations of Antarctic and non-Antarctic meteorites. This interpretation was put forward after the detection of a significant difference in the abundances of volatile and mobile trace elements in H, L, and C chondrites and achon- d&es. Other major differences include the occurrence of previously rare or unknown meteorites, different meteorite-type frequencies, petrographic characteristics, oxygen isotopic compositions, and smaller average masses. In addition, Antarctic meteorites have greater terrestrial residence ages and have collected on the ice shield over several lo5 years. The reality of numerous such differences has now been established beyond doubt; however, the main question regarding the cause of these differences remains. It seems that they have a wide variety of origins, ranging from pre-terrestrial traits to collection (recovery) effects and terrestrial weathering. Studies of terrestrial weathering have shown that, over the long time the meteorites spend in and on the ice, even subtle processes can produce substantial effects. For future investigations it will be important to pay more attention to the weathe~ng status of samples and to develop a more reliable and quantitative weathering index, e.g., based on infrared reflectance spectrometry or differential scanning calorimetry.

Not ail differences between the Antarctic and non-Antarctic meteorite populations can be explained by weathering, pairing, or different collection procedures. Variable trace element abundances and distinct differences in the thermal history and thermoluminescence characte~stics have to be interpreted as being pre-terrestrial in origin. Such differences imply the existence of meteoroid streams, whose existence poses problems in the framework of our current knowledge of celestial mechanics. However, several independent studies support the existence of such meteoroid streams, thus being consistent with the suggestion of a time-variable influx of extraterrestrial material to Earth. The generally smaller average size of Antarctic meteorites may be the cause for the different meteorite-type frequency and the higher abundance of rare samples, because smaller meteorites may come from a slightly different parent population. In this paper we summarize the contributions in this series and provide a review of the current state of the question for the reality and cause of differences between Antarctic and non-Antarctic meteorites.

1. INTRODU~ION METEORITES ARE AMONG THE most important rocks on Earth; they allow us to look back in time to study the origin and evolution of our solar system. They are the oldest ma- terials available to us from our solar system and sample a large variety of parent bodies. They are especially valuable because they can be investigated in the laboratory with a complete set of sophisticated techniques. Up to about 20 years ago, a few thousand individual meteorites of different types were the only available samples of the meteorite population. Previously, these meteorites were found on all continents- only Antarctica was underrepresented.

This changed rapidly with the discovery of meteorites on the so-called blue ice fields in Antarctica. Starting with a Jap- anese Antarctic expedition in 1969, an ever increasing num- ber of meteorites has been recovered from remote locations in Antarctica (LIPSCHUTZ and CASSIDY, 1986). These me- teorite-search expeditions have been supported in the United States by the National Science Foundation (Division of Polar Programs), in Japan by the National Institute of Polar Re- search, and by local agencies in other countries. To date, more than 12,000 individual specimens, representing an un- known number of falls, have been collected in Antarctica. Each one of these meteorites is a valuable research subject, and many are unique. The collection has proved to be of

great value and it is of utmost impo~ance that the collection efforts be continued and expanded.

A question important to many workers, in connection with the number of samples recovered from Antarctica, is: “How many of these samples are paired?” This problem is not easy to solve, but it has been estimated (e.g., SCOTT, 1984, 1989) that they represent about 2000-4000 distinct falls. This is a number that is (at this time) at least equal to, and probably larger than, the number of known non-Antarctic meteorite fails (which is on the order of 2500). Possibly because ofthese large totals, but perhaps for other reasons as well, the Antarctic collection contains many meteorite types that were either unknown or rare in the non-Antarctic meteorite collection. The most famous examples are the lunar meteorites (see be- low), but also include rare iron meteorites, shergottites, and other unusual achondrites and chondrites in the Antarctic collection. Antarctic meteorites are, however, also important because there are so many of them, providing material for statistical analysis. Gradually, within the past few years, it was found that there may be some differences between the Antarctic and the non-Antarctic meteorite popuiations- some subtle, some not so subtle.

Such differences have been claimed to exist for a number of meteorite properties. DENNISON et al. (1986) and LIP- SCHULZ (1986) noted that, in a study of volatile and mobile

Page 2: Differences between Antarctic and non-Antarctic meteorites ... · Differences between Antarctic and non-Antarctic meteorites: An assessment CHRISTIAN KOEBERL' and WILLIAM A.CASSIDY~

4 C. Koeberl and W. A. Cassidy

trace element abundances in H5 chondrites from Antarctic

and non-Antarctic populations, there are statistically signif- icant differences between the two populations. They further pointed out that the meteorite-type frequencies are also dif- ferent between Antarctic and non-Antarctic populations. For example, the ratio of H/L chondrite specimens in Antarctica was suggested to be three times the value for non-Antarctic falls (DENNISON et al., 1986). At about the same time, the high ratio of unusual or unclassified iron meteorites was no- ticed by CLARKE (1986). Subsequently, other chemical dif- ferences have been found in H, L, and C chondrites and eucrites (DENNISON and LIPSCHUTZ, 1987; KACZARAL et al., 1989; PAUL and LIPSCHUTZ, 1987, 1989, 1990).

In view of these differences, DENNISON et al. (1986) sug- gested that H chondrites and other meteorites found in Ant- arctica may represent a different population, on average, from that which exists today. Only a very short time ago the pos- tulated differences between Antarctic and non-Antarctic me- teorites were by no means generally accepted. One reason for this was the lack of extensive data sets, which is (slowly) changing. Another reason was the very suggestion that dif- ferent parent populations exist, which is not in accordance with our current understanding of celestial mechanics (e.g., WETHERILL, 1987, 1989). In addition, exposure age data (WEBER et al., 1988) did not support an obvious difference between Antarctic and non-Antarctic H chondrites. In view of the increasing interest in this subject, we thought that the time was ripe to hold a workshop dedicated to the question of differences between Antarctic and non-Antarctic mete- orites. This workshop was held at the University of Vienna in July 1989 (KOEBERL and CASSIDY, 1990; see also, WRIGHT and GRADY, 1989) and a number of the resulting papers are collected in this issue.

The important questions posed at the workshop were, “Are there any differences?“, “Which ones are significant?‘, and “What is their origin?’ Some differences seem to exist (e.g.. certain trace elements are higher in one set than in the other, and the iron meteorite population in Antarctica is different). The point is, what causes these effects? Two main reasons for the differences seem possible: (1) real differences in the meteorite parent populations and (2) effects resulting from differences in the terrestrial history (e.g., weathering) and/or collection strategies.

To clarify the reality and significance of differences between the two collections, the considerably different terrestrial his- tory of Antarctic meteorites must be considered. One of the most important is the much greater average terrestrial age of Antarctic (stony) meteorites (e.g., NISHIIZUMI et al., 1983, 1989: NISHIIZUMI, 1984, 1986). During their time on Earth, most Antarctic meteorites have remained buried in ice

(WHILLANS and CASSIDY, 1983; CASSIDY and WHILLANS, 1990) subjected to a conserving, but nevertheless extreme, climate. The effects of the Antarctic environment on metc-

orites are not known in detail, partly due to the long time scales involved. Some studies of weathering in Antarctica have been done before (e.g., BISWAS et al., 1980, 198 I; BULL and LIPSCHUTZ, 1982) and recently it was realized that weathering effects on the chemistry of Antarctic meteorites may be larger than previously thought (COODING, 1986; VELBEL, 1988). These effects must be taken into account in

a discussion of any differences. Furthermore, Antarctic me- teorites are much smaller than non-Antarctic meteorites; thus, mass and size-frequency effects may play a prominent role

(DENNISON et al., 1986). Recently, WASSON (1991) has at- tributed some differences to the fact that a smaller size dis-

tribution is sampled (see below). It is clear that at this time we cannot answer all questions

pertaining to differences between Antarctic and non-Antarctic meteorites. It is, however, important to raise the questions, and to provide an evaluation of the current state of the prob- lem together with directions for future research. This is at- tempted by the contributions in this issue. In the following

sections we want to provide a summary of these contributions and an overview of the current status of research on this

problem.

2. CHEMICAL COMPOSITIONAL DIFFERENCES

The first suggestion of possible differences between Ant-

arctic and non-Antarctic meteorites came from the study of volatile trace elements (DENNISON et al., 1986; LIPSCHUTZ, 1986). These authors carried out a comprehensive study of volatile and mobile element abundances (e.g., Au, In, Sb, Se, Cs, Zn, and Cd) in H5 chondrites. These elements cover a wide range of different geochemical properties and show dif- ferent geochemical behavior, and were thus well suited for showing any variations in genetic history, however small. It

was found that eight (Sb, Se, Rb, Bi, In, Tl, Zn, and Cd) of 13 elements show a statistically significant difference between Antarctic and non-Antarctic H5 chondrites (DENNISON et al., 1986). However, the significance of these differences was not immediately obvious. In the discussion that followed, two questions repeatedly emerged whenever the interpretation of these differences was addressed. These questions concerned the effect of weathering and how much pairing is present in the Antarctic meteorite collection. On the matter of weath- ering, there seems to be a consensus (see discussion in KOE- BERL and CASSIDY, 1990) that the present system of “weath- ering category” (ABC) is not sufficient for scientific studies

of weathering effects, although it constitutes a rough record of the condition of the specimens.

Subsequent studies have shown that there is a relationship between weathering index and chemical composition for H

chondrites. Samples of weathering type C are depleted in labile trace elements compared to weathering types A and B, probably by leaching processes (DENNISON and LIPSCHUTZ, 1987). However, the studies of DENNISON et al. (1986) spe- cifically excluded meteorites of weathering type C, using only interior samples. Taking this into account, DENNISON and

LIPSCHUTZ (1987) found that differences between the Ant- arctic and non-Antarctic populations, similar to the ones ob-

served for H5 chondrites, exist also for H4-H6 chondrites. These authors also found a different behavior for differently shocked meteorites-some elements (e.g., Co, Sb, Cs, and Tl) show higher abundances in shocked meteorites and in Victoria Land meteorites. This observation was interpreted by DENNISON and LIPSCHUTZ (1987) as being due to different average thermal histories (i.e., condensation temperatures), and fluctuation in the parent population flux over periods <I Ma.

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Assessment of differences and their causes 5

Other chemical differences have been shown to exist be- tween Antarctic and non-Antarctic eucrites (PAUL and LIP- SCHUTZ, 1987, 1990), with significantly higher abundances of the mobile elements Au, Bi, Cd, In, Rb, Se, Tl, and U being present in Antarctic eucrites. This may be in accordance with arguments by TAKEDA ( 199 1) who observed petrological differences between eucrites from the two populations. Sim- ilarly, mobile trace element abundances have been shown to be lower in Antarctic L chondrites by KACZARAL et al. (1989). It was argued by DENNISON and LIPSCHUTZ (1987) and PAUL and LIPSCHUTZ (1987) that, if weathering (leaching) is re- sponsible for any changes in trace element contents, there should be a depletion, not an enrichment, in Antarctic me- teorites (as was observed for meteorites of weathering type C). Only L chondrites show a depletion which might reflect weathering, but aside from chemical differences, the Antarctic population also shows distinctions in the shock facies (HAQ et al., 1988; KACZARAL et al., 1989). In a study of several Antarctic Cl and C2 chondrites, PAUL and LIPSCHUTZ (1989) observed siderophile and mobile element concentrations that are different from the non-Antarctic equivalents, and attrib- uted this to different thermal histories. They showed that the volatile element abundances in the Antarctic carbonaceous chondrites can be obtained by (artificially) heating Murchison samples at temperatures of 600-700°C thus indicating that (some) Antarctic Cl and C2 chondrites have had a different preterrestrial thermal history, in accordance with petrographic data.

The arguments of DENNISON et al. (1986) LIPSCHUTZ (1986) and DENNISON and LIPSCHUTZ (1987) were criticized on statistical grounds by CASHORE et al. (I 988) who con- tended that use of another statistical test (multivariate anal- ysis) would not show any significant differences (although Cashore and coworkers still find differences for a few ele- ments, which they attribute to weathering). This criticism was rebutted by SAMUELS and LIPSCHUTZ (1988) and led to the application of a variety of statistical procedures to test the significance of differences in trace element abundances (SAMUEL& 1990; LIPSCHUTZ and SAMUELS, 199 1). They have used multivariate statistical techniques (in particular, linear discriminant analysis and logistic regression) to study volatile and siderophile trace element contents in various populations of Antarctic and non-Antarctic H and L chondrites (LIP- SCHUTZ and SAMUELS, 1991). Employing their statistical tools, these authors found that for non-Antarctic L4-6 chon- drites, there is a clear distinction in trace element content between mildly and strongly shocked samples (i.e., the abun- dances of most volatile elements are lower in strongly shocked meteorites), while such a distinction is not found for Antarctic L4-6 chondrites. Furthermore, based on a larger data set, they confirmed the results of KACZARAL et al. (1989) that Antarctic L4-6 chondrites have generally lower mobile trace element abundances. This technique also confirms the pre- viously established differences between Antarctic and non- Antarctic H4-6 chondrites.

With these statistical studies (LIPSCHUTZ and SAMUELS, 199 1) it is now confirmed that a variety of chemical data display differences between Antarctic and non-Antarctic me- teorites. However, the dispute over the causes of these now established trace element variations is still going on (see dis-

cussion in KOEBERL and CASSIDY, 1990). It was argued by Lipschutz and coworkers (LIPSCHUTZ, 1986, 1990a,b; DEN- NISON et al., 1986; DENNISON and LIPSCHUTZ, 1987; PAUL and LIPSCHUTZ, 1987,1989, 1990; LIPXHUTZ and SAMUELS, 199 1) that these differences are not due to weathering (because then some elements should be depleted in Antarctic H chon- drites, and not enriched) but instead indicate that the source of meteorites reaching Earth has changed on average since the Antarctic meteorites fell (see below).

JAROSEWICH ( 1990) analyzed 119 ordinary chondrites (in- cluding 19 from Antarctica) for major and minor elements and found that chemical differences do not seem to be re- stricted to mobile trace elements. In his study, Jarosewich found differences between the composition of chondrites from Antarctica and non-Antarctic finds; the latter are lower in Fe, Na, and S and higher in water. For some elements, how- ever, non-Antarctic falls are intermediate between the pre- vious two groups; therefore, JAROSEWICH (1990) concluded that those compositional differences reflect only weathering and not parent bodies. He also found that there is no cor- relation between any alteration indicated by bulk chemistry and the weathering index ABC. The situation seems only slightly different for H, L, and LL chondrites.

The data for achondrites are not as abundant. As men- tioned before, PAUL and LIPSCHUTZ ( 1987, 1990) found in- dications for distinctions in the content of certain mobile elements; they did not, however, distinguish between eucrite types (monomict/polymict). SPITZ and BOYNTON (1988) an- alyzed Antarctic and non-Antarctic ureilites and found a suggestion of a difference between the two populations. This was recently supported by EBIHARA et al. ( 1990b), who found that the REE patterns for the magnesian ureilites (which are oniy found in Antarctica, see below) are different from other ureihte patterns. MITTLEFEHLDT and LINDSTROM ( 1990, I99 1) analyzed a number of Antarctic and non-Antarctic eucrites for REE contents. They showed that Antarctic eu- crites can be subdivided into two groups, one showing normal eucritic patterns and the other showing abnormal trace ele- ment abundances. The “abnormal” eucrites are depleted in REEs with respect to non-Antarctic eucrites (with LREEs lower than HREEs), have positive Eu anomalies, and mostly positive (but sometimes also negative) Ce anomalies. Positive Ce anomalies are virtually unknown in non-Antarctic eu- crites. This observation is interpreted as being caused by leaching of REEs during weathering, whereas much of the Eu remains fixed in plagioclase (MITTLEFEHLDT and LIND- STROM, 1990, 199 I ). Because partition coefficients for REE+3 are a smooth function of the ionic radii, a magmatic origin of the Ce anomaly can be excluded, thus leaving weathering as the more likely explanation (see below).

3. PETROLOGICAL AND MINERALOGICAL DIFFERENCES

Studies of petrological and mineralogical differences be- tween Antarctic and non-Antarctic meteorites are not as abundant as chemical data. No differences have been reported (or seem to exist) for ordinary chondrites. There are a number of (otherwise rare) Antarctic C4-6 carbonaceous chondrites (SCORE and LINDSTROM, 1990; YANAI et al., 1987) which differ from their non-Antarctic counterparts by relative

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6 C. Koeberl and W. A. Cassidy

abundance. Some differences have been reported for other

types of carbonaceous chondrites. Y-82 162 has been classified as a CI chondrite, but it shows a number of features that distinguish it from non-Antarctic CI chondrites: it has higher abundances of coarse phyllosilicates and pyrrhotite, but lacks sulfate or carbonate veins. This suggests that it was derived from a primary body that has experienced an aqueous alter- ation history different from non-Antarctic CI chondrites, and it may have been affected by mild thermal metamorphism

(TOMEOKA et al., 1989a; TOMEOKA and KOJIMA, 1990) which is also supported by chemical data (PAUL and LIP- SCHUTZ, 1989). Oxygen isotope data (CLAYTON and MA- YEDA, 1989; see below) show that this meteorite has a com- position similar to other Antarctic Us, but not identical to

non-Antarctic Cl’s. Similar observations regarding oxygen isotope data can be

made for Y-86720 and the Antarctic Belgica (B) 7904 group, which are petrologically classified as CM chondrites (TO- MEOKA and KOJIMA, 1990). Y-86720 is thought to have ex- perienced extensive (extraterrestrial) aqueous alteration and thermal metamorphism (TOMEOKA et al., 1989b. TOMEOKA and KOJIMA, 1990). and, together with Y-82162, B-7904, and Y-79332 1, shows mineralogical and petrographical properties that are not known from non-Antarctic carbo- naceous chondrites. Another unusual carbonaceous chon- drite, previously classified as C30, is LEW85332. Because of its unusual petrological properties, however, RUBIN and KALLEMEYN (1990) now designate it as “unique.” This and other rare meteorites are proof that in Antarctica we may be tapping a reservoir of meteorites that is wider in range than in the non-Antarctic collection.

One interesting type of chondrite that is found only in the Antarctic collections and that seems to have gone largely

unnoticed is the group of heavily shocked chondrites (but see MIYAMOTA et al., 1984, for a description of Y-790964 and volatile element analyses by KACZARAL and LIPSCHUTZ, in prep.). A number of meteorites, such as Y-790782, Y- 791775, or Y-793539, are ofa slag-like appearance and consist mainly of vesicular glassy material with voids of up to 1 cm in diameter. They have no apparent crystal structure, no fu- sion crust, and are probably shock-melted (see, e.g., Photos 102, 240, and 354 in YANAI et al., 1987, with description by CK). These meteorites certainly warrant closer attention. One interesting, and possibly related, example is Y-74 160, which recrystallized after partially melting at temperatures above 1090°C and, in composition, bridges the chondrite-achon- drite gap (TAKEDA et al., 1984).

Relatively abundant data are available for Antarctic HED achondrites and ureilites, mainly from the Yamato collection. TAKEDA ( 199 1) pointed out that the more than 50 Yamato diogenites are actually only 2 separate falls and that at least 9 of the 27 polymict eucrites from the Antarctic are paired. The 2 Yamato diogenite falls compare to 8 from the US Antarctic collection and 9 from the non-Antarctic collection. TAKEDA (199 1) also showed that at least the Yamato diog- enites are distinctly different from non-Antarctic ones, with the Y-75032 group showing unique shock textures and min- eral chemistries that fill the compositional gap between di- ogenites and cumulate eucrites. The Y-7401 3 group is also mineralogically different from other known diogenites.

TAKEDA ( 199 1) further pointed out that with the discovery of ureilites in Antarctica their number has almost tripled.

Some Antarctic ureilites are unlike any non-Antarctic urei- lites: magnesian ureilites and a&e-bearing ureilites are only found among the Antarctic samples. They also show a dif- ference in oxygen isotope composition (CLAYTON and MA- YEDA, 1990). TAKEDA (1991) also inferred some textural and chemical differences between Antarctic and non-Antarctic monomict eucrites, and that polymict eucrites seem more abundant in the Antarctic collection, but this may be related to incomplete pairing studies. On the other hand, no polymict eucrites (excluding howardite-like specimens) have been found in the non-Antarctic collection (TAKEDA, 1991). For any conclusive observations, however, it seems that we have to await the more complete investigation of the HEDs in the US Antarctic collection.

4. RELEVANCE OF ISOTOPIC, NOBLE GAS, AND THERMOLUMINESCENCE STUDIES

Oxygen isotope data on Antarctic and non-Antarctic me- teorites have been presented by CLAYTON and MAYEDA (1989, 1990). Even for oxygen there is some worry about

contamination, although in stony meteorites oxygen is not a trace constituent, but a major, rock-forming element. Ant- arctic water and ice are strongly depleted in heavy oxygen isotopes so that even l-2% of contamination from terrestrial

sources would cause a large anomaly (CLAYTON and MA- YEDA, 1990). The oxygen could be introduced from water

seeping into the meteorites, or by rust and other weathering products that involve extra-meteoritic oxygen. Correcting for these contaminations, CLAYTON and MAYEDA (1989, 1990) nevertheless found isotopic compositions in (some) Antarctic meteorites that are distinctly different from comparable non- Antarctic meteorites. Some of these isotopically anomalous meteorites seem to contain minor constituents that have en- tirely different oxygen isotope abundances. However, since these data points fall on the terrestrial fractionation line, they might be explained as being due to terrestrial weathering

products (F. BEGEMANN, in KOEBERL and CASSIDY, 1990).

The most prominent deviations from the non-Antarctic oxygen data are in the carbonaceous chondrite and ureilite groups. Large oxygen isotopic anomalies have been found in the magnesian ureilites which are only recovered from Ant-

arctica. For carbonaceous chondrites, there is a discrepancy between the petrologic classification and the chemical and

isotopic composition, as already mentioned above. Samples that are classified as CM chondrites have Cl chondrite oxygen isotopic abundances, and vice versa, suggesting the need for an expanded classification scheme (CLAYTON and MAYEDA, 1989). This behavior has not been known for non-Antarctic carbonaceous chondrites. On the other hand, C4-5 chondrites from Antarctica do not seem to differ from their non-Ant- arctic counterparts, and no difference in oxygen isotopes has yet been found for ordinary chondrites (CLAYTON and MA- YEDA, 1989, 1990).

The carbon isotope geochemistry of Antarctic and non- Antarctic chondrites has been studied by GRADY et al. (1989a.b. 1991) who found that, in the whole rock data, there is a difference between the two groups. They found that this difference is due to the presence of terrestrial weathering

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Assessment of differences and their causes 7

products, mainly hydrous carbonates. The reasoning behind this conclusion is that, if the weathering products are removed by acid treatment or combustion of the sample at SOO”C, the apparent distinction disappears. It seems to be very difficult to avoid such bicarbonates and hydrous carbonates, bearing in mind that MIYAMOTO (199 1) has shown from infrared reflectance measurements that they are present even in me- teorites that look unweathered (weathering index A). The weathering products in Antarctic achondrites seem to have a different nature than those in chondrites (GRADY et al., 199 1). This might simply be a reflection of mineralogical and textural differences between chondrites and achondrites.

A clear difference in the carbon isotopic composition seems to exist only for certain carbonaceous chondrites. The carbon abundance in these meteorites is too high to be easily com- promised by terrestrial weathering products. In whole rock analyses, no difference is evident, just a wide scatter in the data. Therefore, the isotopic signatures of individual carbon- bearing components were investigated by GRADY et al. (199 I), who showed that five of the ten Antarctic CM chon- drites they analyzed have higher carbonate carbon abun- dances than any non-Antarctic carbonaceous chondrites of any group-but the reason for this is not yet known. Data for stepped combustion of acid-resistant residues of CM chondrites above 1000°C (i.e., the most refractory carbon phases, which are uncontaminated by terrestrial products) show a distinction between the Antarctic and non-Antarctic populations, with the Antarctic chondrites being depleted in “C and having a lower combustion temperature of silicon carbide (GRADY et al., 1991). The depletion could be due to removal of 13C-rich material by aqueous processes, or to in- clusion of more isotopically light material in Antarctic car- bonaceous chondrites, while the reason for the different sil- icon-carbide combustion temperature is not yet known (GRADY et al., 1991).

Some differences also seem to exist for mercury isotopes. JOVANOVIC and REED ( 1987, 1990) have measured ‘96Hg and ‘02Hg isotopes by neutron activation analysis in a number of Antarctic achondrites. They reported that three of these samples showed anomalous Hg isotopic composition, while ten others have an apparently normal isotope ratio. Because of the small number of samples involved, not much emphasis regarding any differences between the two populations can be placed on these results.

The concentration of spallogenic noble gases is a property where neither contamination nor weathering should present a problem. WEBER et al. (1988) and SCHULTZ et al. (199 1) have studied the distribution of cosmic-ray exposure ages- as determined by noble gas analyses-of 31 H chondrites from the Allan Hills. Thirty of these chondrites were also analyzed by DENNISON and LIPSCHUTZ (1987). Their results are well suited to attack the question of pairing between in- dividual samples, but even similarities in cosmic-ray exposure ages and gas retention ages are not necessarily proof of pairing, nor are different gas contents indicative of different falls. This ambiguity is caused by the heterogeneity of stony meteorites, which is such that trapped noble gas concentrations in dif- ferent parts of the same meteorite can vary by up to several orders of magnitude (SCHULTZ et al., 1991). Using a number of petrological and isotopic criteria, however, SCHULTZ et al.

(199 1) claimed that the 3 1 H chondrites they analyzed rep- resent at least 17 independent falls.

Regarding the cosmic-ray exposure ages, 12 out of the 3 1 analyzed specimens (i.e., 39%; or 6 out of the 17 independent falls, i.e., 35%, if pairing is taken into account) have ages around 8 Ma (SCHULTZ et al., 1991). This compares well with the 45% of non-Antarctic H chondrites that have ages in a peak between 6 and 9 Ma which is superimposed on a wide distribution of ages reaching up to 80 Ma (SCHULTZ and KRUSE, 1989). This distribution has been interpreted as evidence for a major collision that produced these meteorites by disrupting their parent body about 8 Ma ago. L or LL chondrites do not show any prominent peak in their age dis- tribution (SCHULTZ and I&USE, 1989; SCHULTZ et al., 199 1). From the number-frequency distribution of their cosmic-ray exposure age data, SCHULTZ et al. (199 1) concluded that there is no evidence that Antarctic and non-Antarctic meteorites sample different parent-body populations. There is also no evidence that, if they sample the same population, the relative yields for individual components of this population are any different for the two sets. It has to be noted, however, that, although the data do not provide arguments supporting any differences, they are not proof that such differences cannot exist. SCHULTZ et al. (199 1) also compared the concentrations of the radiogenic nuclides 4He and @Ar in the two H chondrite populations and did not find convincing evidence for differ- ences (except for some difference in the 40Ar data, which they ascribed to inferior quality of earlier data).

Thermoluminescence properties of Antarctic and non- Antarctic H chondrites have been reported by HAQ et al. (1988) and were discussed by SEARS (1990). These authors found significant differences between the two populations re- garding their TL sensitivity and TL peak temperature/peak width relationships. Antarctic chondrites are skewed to lower TL sensitivities by a factor of about 3, which, based on acid- washing experiments, is thought to have been caused by weathering. These differences between the Antarctic and non- Antarctic data could not be explained in terms of petrographic type or weathering index ABC (HAQ et al., 1988). Non-Ant- arctic H chondrites show a statistically significant positive correlation between TL peak temperature and peak width, while such a correlation is completely absent in the data for Antarctic H chondrites which show a much narrower range of peak widths (SEARS, 1990). These differences cannot be attributed to weathering.

In a new study, SEARS et al. (199 1) used the noble gas concentration data of SCHULTZ et al. (199 1) to compare the cosmic-ray exposure ages with their TL peak temperature- peak width data. They found that for non-Antarctic H chon- drites, there is no relationship between TL peak shape and cosmic-ray exposure age, while the TL data for Antarctic H chondrites form two clusters. Both clusters have the same peak width ( 128-144), but all samples with ages > 20 Ma belong to the cluster with the lower peak temperatures (170- 188°C) while those with ages < 20 Ma fall in the high peak temperature (190-2 10°C) cluster (SEARS et al., 199 1). These results indicate strongly that Antarctic H chondrites with lower exposure ages (i.e., in the 8 Ma peak) had a thermal history that was different from Antarctic H chondrites with long exposure ages and also different from non-Antarctic H

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8 C. Koeberl and W. A. Cassidy

chondrites (SEARS et al., 1991). It is concluded (SEARS et al.,

199 I ) that the 8 Ma event may have produced two groups with different thermal histories, one of which is found only

among Antarctic samples. Cosmogenic radionuclides (e.g., “Be and 26A1) have been

measured in a number of Antarctic C2 chondrites by HER- PERS et al. (1990a). They found that these meteorites have extremely low cosmic-ray exposure ages (< 200,000 a), in- dicating that the radionuclide concentrations were not in sat- uration at the time of fall. HERPERS et al. (1990a) caution against pairing them all since the 26A1 and “Be activities in

these meteorites are so variable. In a related study, HERPERS et al. (1990b) reported the measurement of cosmogenic nu- elides in Antarctic and non-Antarctic euctites. Their results show that for non-Antarctic falls the cosmogenic nuclides are in saturation, while in Antarctic eucrites some “Be and most 26Al contents are lower than for the non-Antarctic group.

The lower 26Al contents may be attributed to the longer ter- restrial residence times of the Antarctic meteorites. On the other hand, because of the greater “Be half-life, this is unlikely to explain the lower “Be contents, but for any definitive con- clusions, more samples need to be analyzed. Similar conclu- sions have been obtained by AYLMER et al. (1990) for “Be

and 26Al in ureilites.

5. RELEVANCE OF TERRESTRIAL AGE DIFFERENCES

Terrestrial age is a property that everybody agrees is sig- nificantly different between Antarctic and non-Antarctic me-

teorites. Antarctic meteorites have, on average, much longer terrestrial residence times than most non-Antarctic meteorite falls. Under non-Antarctic conditions, meteorites (especially stony meteorites) are destroyed by erosion at a much faster rate than in Antarctica. BOECKL ( 1972) estimated the weath- ering half-life of a stony meteorite under normal continental conditions to be about 3600 years. Terrestrial age determi- nations of Antarctic meteorites, using cosmogenic radionu- elides (e.g., 14C, 36C1, and *‘Kr, with half-lives of 5730, 301,000, and 210,000 years, respectively) have shown that they have terrestrial ages of up to about 1 Ma (e.g., FREUNDEL et al.. 1986. NISHIIZUMI et al., 1983, 1989).

The majority of Antarctic meteorites have terrestrial ages between 10.000 and 20,000 years (see, e.g., NISHIIZUMI et al., 1989). It should be noted, however, that there are con- siderable terrestrial age differences between meteorites from different parts of Antarctica. Meteorites from the Allan Hills area usually have older terrestrial ages than, for example, meteorites from the Yamato Mountains, suggesting differ- ences in the accumulation process and the extent of the ac- cumulation zones (CASSIDY and WHILLANS, 1990). It would be interesting to find Antarctic meteorites with greater ter- restrial ages than presently known. The Antarctic ice sheet has been in existence for at least several million years, but most of the Antarctic meteorites have terrestrial ages of less than 0.5 Ma. Perhaps some day these old meteorite accu- mulations will be discovered somewhere in Antarctica.

The terrestrial age difference is one of the determining fac- tors for explaining other differences between Antarctic and non-Antarctic meteorites because it is argued that the Ant- arctic meteorites represent a much older meteorite population

than the modern falls. It is important to obtain terrestrial age data for as many Antarctic meteorites as possible, because

only meteorites with considerably greater terrestrial residence times should be used for discussing any differences. The age data for Antarctic meteorites shows that there are also a number of meteorites on each ice field that may have fallen recently (e.g., NISHIIZUMI et al., 1989) which should therefore not show any significant differences. On the other hand, the greater terrestrial residence time of Antarctic meteorites has also been used as one of the main arguments for a terrestrial cause of some chemical and petrological differences because of the long time available for weathering. GOODING (1986) and NISHIIZLJMI (1990) have not found any relation between weathering index ABC and the terrestrial age of Antarctic meteorites, but this may be due to the fact that long terrestrial residence times do not necessarily mean long exposure times

on the ice surface. NISHIIZUMI ( 1990) described measurements which ex-

plored the possibility that weathering affected terrestrial age measurements based on cosmogenic isotopes. Terrestrial age differences between collection sites and, in one case, from one end of a site to the other, have considerable implications for concentration mechanisms. However, metal and silicate separates yielded the same results, suggesting no redistribution of isotopes (but ‘“Be and *“Al, which are produced mainly in the silicate phases, have not yet been investigated in detail for weathering effects). This was found for meteorites of all types, so NISHIIZUMI (1990) concluded that the terrestrial age data are reliable. He suggested that the influx of different classes of meteorites relative to each other may change with time, if the measured frequency distribution of ages is real.

However, more terrestrial ages of meteorites are necessary to improve the statistical basis for this suggestion. NISHIIZUMI et al. (1989) and NISHIIZUMI (I 990) suggested that the ter- restrial age distribution can be seriously affected if there is significant pairing among the measured samples.

6. RARE METEORITES

A very interesting characteristic of the Antarctic meteorite population is the apparent overabundance of meteorites that are either rare or have not been found at all in the non- Antarctic meteorite population. Prior to the Antarctic me-

teorite discoveries, only eight ureilites were known; this number has now almost tripled. In addition, augite-bearing ureilites have not been found outside of Antarctica (see sum- mary by TAKEDA, 1991). With the discovery of Antarctic ureilites, the previously known compositional range of these meteorites has been extended, and it is now possible to dis- tinguish three ureilite groups. One of these groups, the so-

called magnesian ureilites (TAKEDA, 199 l), shows a distinct oxygen isotope composition (CLAYTON and MAYEDA, 1990) and is found only in Antarctica.

Lodran, an olivine-pyroxene stony-iron meteorite, which was previously unique, is no longer so. YANAI et al. (1987) list four samples (Y-74357, Y-75274, Y-791493, and Y-8002) which are classified as lodranites. In addition, YANAI and KOJIMA (1987) described a pyroxene-bearing pallasite (Y- 8451). The current catalog of the US Antarctic meteorite collection (SCORE and LINDSTROM, 1990) lists 30 aubrite

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Assessment of differences and their causes 9

specimens that represent three falls, and among the HED

achondrites, it has already been mentioned (see Chapter 3,

and TAKEDA, 1991) that certain polymict eucrites are only found in Antarctica and that the Antarctic HED achondrites extend the compositional ranges known for non-Antarctic HEDs. We have also already mentioned the group of the unusual glassy (shocked?) chondrites such as Y-790782. The discovery of two shergottites (ALHA77005, EETA79001) in

Antarctica has also been very important (a possible third specimen, ALHA 13 13, with 0.5 g weight, is listed by SCORE

and LINDSTROM, 1990). These meteorites belong to the group of SNC achondrites which are thought to have originated

from the planet Mars. Because previously only two shergot-

tites were known, the new Antarctic samples are of great importance, providing more data for the study of their origin.

The most exciting and unique discovery in Antarctica has

been the recognition of meteorites with a lunar origin. In 1982, an unusual-looking sample was collected in the Allan

Hills/Victoria Land, Antarctica, and was, after careful study identified as a rock from the Moon (MARVIN, 1983). Since then, several other samples have been found at different geo- graphical locations throughout Antarctica. At this time, eleven

lunar meteorites have been recovered from Antarctica. So far, seven samples from the Antarctic meteorite collection have been identified as lunar highland rocks (ALHA 8 1005,

Y-791 197, Y-82192, Y-82193, Y-86032, MAC88104, MAC88 105). It is interesting to note that the first lunar me- teorites found all originated from the lunar highlands and

are of anorthositic composition (see, e.g., WARREN and KAL- LEMEYN, 1986, 1987: TAKEDA et al., 1987, 1989: KOEBERL, 1988; KOEBERL et al., 1989, 199lb). One of these samples, Y-791 197, was shown to contain condensates from lunar

volcanic emanations (KACZARAL et al., 1986; KOEBERL and

KIESL, 1986). Only recently, EET87521 has been identified as the first

basaltic rock with lunar mare composition (WARREN and KALLEMEYN, 1989; DELANEY, 1989). Shortly afterwards,

another lunar basalt was identified in the Yamato meteorite collection and studied. This sample (Y-793274) is very small

(8.66 g), and detailed consortium studies (LINDSTROM and MARTINEZ, 1990; WARREN, 1990; KOEBERL et al., 199 la; LINDSTROM et al., 1991) demonstrated that it is a mixture of mare and highlands components in a ratio of about 2: 1.

It may have originated from an impact that occurred at a mare/highlands boundary. Furthermore, two other recently identified samples, Y-793 169 and Asuka-3 1, have been ten- tatively classified as mare cumulates (possibly containing a

VLT (very low Ti) basalt component; YANAI, 1990a,b; YANAI and KOJIMA, 1990). but no detailed analyses are available at this time.

Thus, there are seven samples from the highlands and four samples from mare regions. An important question arising is, “How many individual source regions are represented by the known lunar meteorites?” Pairing studies, using noble gas contents and cosmic-ray exposure histories (see, e.g., EUGSTER, 1988, 1989, 1990a,b) indicate that most probably four different impacts are responsible for the anorthositic highlands meteorites (1: ALHA-81005; 2: Y-791 197; 3: Y-

82 192/3, Y-86032; 4: MAC88 104/5). No detailed analyses are available of the mare meteorites so far, but they probably

represent four individual falls. Considering the abundance

of basalt on the lunar surface, mare meteorites therefore seem

overabundant in the current lunar meteorite collection. Lunar meteorites represent random samples of the lunar

crust, and therefore any new lunar meteorite contains a trove of information about the Moon. The study of the rare lunar meteorites contributes essential details toward a better un-

derstanding of the composition and history of the lunar sur- face (WARREN, 1990; LINDSTROM, 1990). In view of the

probable eight impacts that delivered the known lunar me- teorites over the past 100,000 years or so, it is interesting to

note that so far none have been found anywhere outside of Antarctica. This may have several reasons: they weather rap-

idly under non-Antarctic conditions: they are easy to confuse with terrestrial rocks; or they are generally too small to be

easily found. It was predicted by HUSS (1977) that many rare and unusual meteorites would be found in Antarctica. He

argued that unusual specimens are not normally recognized as meteorites in the non-Antarctic collection because of the

deviation of their properties from normal meteorites. while in Antarctica the collection is more complete. This might be the reason why some unusual meteorites are found in Ant-

arctica, but not elsewhere.

7. CAUSES AND INFLUENCES

7.1. Weathering and Terrestrial Contamination

The terrestrial age of Antarctic meteorites, which is, on

the average, much greater than that of non-Antarctic mete- orites (see above), presents a number of problems, of which the foremost might be weathering. A major uncertainty is the long timescale involved, during which otherwise subtle

or unnoticeable influences create, at the end, considerable effects. Many of the differences between Antarctic and non-

Antarctic meteorites that have been mentioned above (and about which there is unanimity) have been-depending on

the author-ascribed to the influence of weathering. Mobile trace element variations between the two populations, once acknowledged, have often been interpreted as being due to weathering (see, e.g., discussion in KOEBERL and CASSIDY,

1990). However, in this respect (and for other weathering discussions) it should not be forgotten that weathering cannot

work both ways, depleting trace elements in L chondrites, while enriching them in H chondrites and eucrites (LIP-

SCHLITZ and SAMUEL& 1991). On the other hand, it is now clear that weathering must have played an important role in Antarctic meteorite chemistry and mineralogy.

The importance of weathering has already been summa- rized by GOODING (1986). He pointed out that, in addition to being slower than in temperate climates, weathering in Antarctica may produce distinctly different products that are characteristic only of Antarctic meteorites. Important weath- ering processes start with abrasion of and crack-propagation in meteorites, where it is important to note that water is pres- ent even at temperatures well below the freezing point, in the form of unfrozen capillary water or as undercooled planar films on mineral surfaces. Therefore, weathering is proceeding slowly, and perhaps sometimes unnoticed, even during the time when the meteorites are buried in the ice. Our current

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10 C. Koeberl and W. A. Cassidy

ideas of meteorite accumulation (WHILLANS and CASSIDY,

1983; CASSIDY and WHILLANS, 1990) require that a large fraction of the meteorites spend most of their terrestrial “life” within the ice, leaving a lot of time for possible “cryogenic”

weathering. Once the meteorites are exposed on the ice surface, other

processes may become more important. DENNISON and LIP- SCHUTZ (1987) have shown that for the chondrite ALHA 82102, which was found emerging from the ice (GOW and CASSIDY, 1989) several mobile elements have lower abun- dances in the exposed portion of the meteorite (these were the same elements that are different between weathering types

A/B and C). In this respect, SCHULTZ (1990) reported the results of a very interesting experiment: during an Antarctic

expedition, he measured the temperature within a sample of Allende that was equipped with a temperature sensor and found that on days with sunshine and low windspeeds the

temperature within the meteorite reached up to + 5°C even though the outside temperature was well below the freezing

point. Such effects may be responsible for the formation of weathering products as described by GOODING ( 1986) VEL- BEL (1988) and VELBEL et al. (199 1). This is supported by the r4C analyses of JULL et al. (1988) who showed that the white efflorescences (mainly nesquehonite) on the H5 chon- drite LEW 85320 were formed within the past 40 years.

however, poorly understood. The carbon isotopic composi- tions (and total carbon content) seem to be most prone to the effects of contamination and/or weathering. GRADY et al. (199 1) in their study of carbon stable isotope compositions of Antarctic and non-Antarctic meteorites tried to avoid the influence of terrestrial contamination by stepped combustion of the samples. They found that the carbon isotopes of most Antarctic stony meteorites are severely contaminated from terrestrial weathering products (which is not, however, the explanation for differences observed in carbonaceous chon- d&es). A similar conclusion was reached by KARLSSON et al. (1990) who investigated the carbon and oxygen isotopic compositions of carbonates from weathered Antarctic chon- d&es. They found that there are indications for a slight in- crease in 6°C and a slight decrease in 6”O with terrestrial age, and that atmospheric CO* seems to have been the main

source for carbon in the carbonates.

One remaining problem mentioned above is the inade- quacy of the ABC weathering index, which is relatively sub-

jective. VELBEL and GOODING (1990) identified a number of processes that occur during weathering of stony meteorites, of which only 1 or 2 are represented in the weathering category (oxidation and hydration). Other processes may have been even more important, such as those (solution and chelation) that result in the appearance of evaporites on meteorite sur- faces. There does not seem to be any correlation between weathering category and the appearance of evaporites. Some major and trace elements are likely to be mobilized (leached) during weathering. VELBEL (1988) and VELBEL et al. (1991) noted that there is a connection between the internal chem- istry of meteorites and the occurrence of evaporites on their surface. Elements such as Rb, Co, I, and in some cases Ca, are depleted in the interior of evaporite-bearing meteorites (VELBEL, 1988), and exteriors and interiors of Antarctic chondrites are enriched in Cs (BISWAS et al., 1980; DENNISON and LIPSCHUTZ, 1987). It has to be shown, however, in a more systematic study, whether this holds true for all me- teorite types and whether other elements are equally affected. In a study of S and C contents of stony meteorites, HARTMETZ et al. (1989) found that S contents are slightly higher in Ant- arctic samples than in non-Antarctic meteorites, but that C contents are lower in Antarctic meteorites. They suggested that this is due to leaching of meteoritic C and formation of surficial carbonates. Carbon contents are lower because Ant- arctic samples are less contaminated. GRADY et al. (199 l), using data corrected for weathering products, showed that there was no difference in C content between Antarctic and non-Antarctic meteorites, and that isotopic data precluded the thesis that carbonate weathering products formed from meteoritic C (GRADY et al., 1989b).

The complexity of the weathering processes is not restricted to stony meteorites. BUCHWALD and CLARKE (1989) and BUCHWALD ( 1990) described six commonly found iron oxides and hydroxides which are weathering products, including ak-

aganeite (P-FeOOH), from iron meteorites as well as from stony meteorites. The mineral akaganeite is produced by a process catalyzed by Cl and occurs in the solid state even at sub-zero temperatures, unlike normal weathering with fluids. Although the structural relationships of the meteoritic com- ponents are initially preserved, the corrosion is already

changing the chemical composition. Chlorine is necessary for the formation of akaganeite, but there is a sufficient supply of chlorine in the Antarctic environment from marine

sources. There are also differences in major element compositions

that may show the influence of weathering. The data reported by JAROSEWKH (1990) for H-chondrites show that in Ant- arctic samples metallic iron is lower in concentration than in both non-Antarctic falls and finds, while water is consid- erably lower in non-Antarctic falls (- 0.4%) compared to Antarctic meteorites and non-Antarctic finds ( 1- 1 .S%). It is therefore conceivable that during their long residence time

in Antarctic ice the meteorites are hydrated and Fe is oxidized (the Fe0 content is highest for Antarctic samples). It is in-

teresting to note that this behavior is exactly mirrored by L and LL chondrites.

Another chemical difference that can be attributed to weathering is the presence of positive Ce anomalies in Ant- arctic eucrites (SHIMIZU et al., 1983; MITTLEFEHLDT and LINDSTROM, 1990, 199 1). To explain the (mostly) positive

Ce anomalies in Antarctic eucrites by weathering, a complex scenario seems to be involved (MITTLEFEHLDT and LIND- STROM, 199 I). The dark eucrites are warmed by solar energy, creating some liquid water which becomes acidic in the me- teorite. This acidic solution then reacts with the mesostasis phosphates (which contain most ofthe REEs, except Eu which is partitioned into plagioclase) by dissolving them, leaving a residual meteorite with positive Eu anomalies and low (and LREE/HREE fractionated) total REE. The Ce oxidized to the $4 state and fractionated relative to the +3 REEs, by stabilizing Ce +4 in the meteorite while the other REEs are

It was already pointed out by GOODING (1986) that weath- removed by the solution (MITTLEFEHLDT and LINDSTROM, ering may also result in isotopic fractionations, which are, 1991).

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Assessment of differences and their causes 11

However, the process may be more complicated than this. They also found that, even in interior chips from little- weathered stones, Ce can be either enriched or depleted with respect to non-Antarctic eucrites. A very interesting obser- vation was made by SHIMIZU et al. (1983) who analyzed dif- ferent portions of an Antarctic eucrite. They found that the inner part of the eucrite showed negative Ce anomalies, while the outer parts all showed positive Ce anomalies. This result, which is also confirmed in recent SIMS studies by HEAVILON and CROZAZ ( 1990) is a strong indication for a weathering- related origin of the positive Ce anomalies in Antarctic eu- c&es. Indeed, HEAVILON and CROZAZ (1990) found no cor- relation between Ce anomalies and the location of cracks or fractures, requiring very general REE mobilization.

Another significant difference between Antarctic and non- Antarctic eucrites is described by MITTLEFEHLDT and LIND- STROM ( I99 1): Se is consistently and considerably enriched in Antarctic eucrites. These enrichments are up to an order of magnitude above the range for non-Antarctic eucrites, and those samples (often the exterior parts) that are more severely

weathered have higher Se contents (although in at least one case the interior part contained more Se). There is no cor-

relation between Ce anomalies and Se enrichment (MITTLE- FEHLDT and LINDSTROM, 199 1). The high Se contents cannot be explained by mobilization of the element from eucrites that initially contained comparable abundances to non-Ant- arctic eucrites; Se is therefore likely to be terrestrial contam- ination. Volcanic dust, which is frequently found in the form of tephra layers in meteorite-bearing blue ice fields, is a likely source for this enrichment. KOEBERL (1989) has found that tephra layers from the Lewis Cliff ice fields are enriched in

Se, indicating that Se is obviously dissolved in the ice. This could also be a source for the (comparatively small) Se en-

richment found by DENNISON and LIPSCHUTZ (1987) in H chondrites; but the question remains as to why a Se enrich- ment is not found in other meteorite types. On the other hand, PAUL and LIPSCHUTZ (1990) suggested that the Se en- richment is due to volcanism on the eucrite parent body.

A very interesting form of terrestrial contamination was reported by DELISLE et al. (1989) from their studies of 42 meteorites that were found by the German GANOVEX IV expedition at the Frontier Mountain Range in North Victoria Land. Unexpectedly, most of the chondrites are enriched in uranium by factor of more than 300 compared to normal chondrites. This can only be explained by contamination from terrestrial sources; the meteorites were found in a mo-

raine consisting mostly of granite, from which uranium might have been leached and migrated into the meteorites (DELISLE et al., 1989).

Terrestrial contamination, similar to the possible intro- duction of Se from volcanic sources, was also invoked to explain the overabundances of iodine in Antarctic meteorites (DREIBUS and WXNKE, 1983; DREIBUS et al., 1986). By an- alyzing different parts of an Antarctic rock, HEUMANN et al. ( 1987) were able to show that the iodine abundance decreases from the surface to the interior. They therefore concluded that methyl iodide from the marine atmosphere was the source for this contamination. This conclusion was recently supported by the analysis of surface layers and interiors of Antarctic chondrites and iron meteorites, and Antarctic air,

by HEUMANN et al. ( 1990). EBIHARA et al. ( 1990a) carefully studied the iodine depth profile ofan Antarctic L6 chondrite, and they too found an enormous enrichment of iodine at the surface (and in a crack) but almost no variation in the Cl content.

Some progress has been made regarding a weathering in-

dicator that would be more precise than the ABC classification

that is now in use. MIYAMOTO (I 99 1) has performed a thor- ough and extensive study of infrared diffuse reflectance spectra

of Antarctic and non-Antarctic meteorites. He found that the integrated intensities of the absorption bands near 3 pm (which are caused by hydrous minerals) and 7.4 pm (probably

caused by hydrous carbonates) are useful indicators for the presence and degree of terrestrial weathering. MIYAMOTO (199 1) demonstrates that, even if weathering category A is reported, the meteorites may still show a large integrated in-

tensity in the 3 pm band and may therefore be weathered.

This method seems to be a more sensitive indicator as to the degree of freshness of a sample than the conventional ABC system but requires considerably more effort. MIYAMOTO ( I99 1) also found, from the 7.4 Km band, that hydrous car- bonates (produced by weathering) are ubiquitous in Antarctic ordinary chondrites. In a related study of a meteorite depth profile, EBIHARA et al. (1990a) showed that the integrated intensity of the 3 pm band is highest near the meteorite surface and lower in the interior. indicating a weathering gradient. A different approach to weathering studies was presented by GOODING (1990). He had success in using differential scan- ning calorimetry which is a sensitive indicator of both “rust”

(hydrous oxide) and hydrous carbonates.

7.2. Pairing

The identification of specimens that are paired, i.e., part of the same fall, is very important for most studies of Antarctic

meteorites. The existence of a large number of paired samples would reduce the statistically significant number ofAntarctic meteorites, and this could change the meteorite-type fre-

quency. It would also reduce the significance of terrestrial age distributions and other “different” parameters. Pairing

attempts to identify pieces of the same meteorite that were produced by fragmentation during atmospheric entry, impact

on the surface. or weathering in or on the ice sheet. The number of fragments per fall has been estimated for Antarctic

samples by various workers: the results vary somewhat. For example, SCOTT (1984) and ANNEXSTAD et al. (1986) suggest 2-6 fragments per fall, while GRAHAM and ANNEXSTAD ( 1989) and other workers arrive at 10 f 5 fragments per fall. These numbers are rather different-the latter estimate would indicate that only about 1000 individual falls are represented in the current Antarctic meteorite collection. However, GRAHAM and ANNEXSTAD (1989) arrived at their estimate in the following way: they assumed (from the iron:stony-iron meteorite ratio) that Antarctic meteorites have the same type frequency as non-Antarctic meteorites, and extrapolated the total number of falls from the number of iron meteorites found in Antarctica. This seems, however, a dangerous as- sumption because, as mentioned before (and discussed be- low), the type frequencies are most likely not identical. Nev- ertheless, pairing studies (e.g., SCOTT. 1984, 1989; SCHULTZ

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12 C. Koeberl and W. A. Cassidy

et al., 199 1) are tedious and time-consuming, and do not

always give unequivocal results.

7.3. Differences in Meteorite Populations

HARVEY and CASSIDY ( 1989) and CASSIDY ( 1990) studied differences in the distribution of Antarctic and non-Antarctic meteorites compared by mass, number, and type. The mass distributions of Antarctic finds and modern falls differ re- markably: the average mass of an Antarctic meteorite find is smaller by about a factor of 100 compared to that of a modem non-Antarctic meteorite fall. Furthermore, as mentioned above, the Antarctic collection includes types of meteorites absent from or rare among modern falls. Because of the pair- ing question, it is not known how many falls are represented in the Antarctic collection, and it is safer to compare the relative type abundances by mass rather than by numbers of

falls (HARVEY and CASSIDY, 1989). The total mass of non- Antarctic meteorites is orders of magnitude greater than the mass of the Antarctic meteorites, despite the fact that their numbers are comparable. However, large masses (showers) may easily alter the distribution; therefore, it is necessary to remove this effect by fitting the distribution frequency curves at intermediate masses and extrapolating to larger masses.

Such comparisons of mass distributions by meteorite type were presented by HARVEY and CASSIDY (1989) and CASSIDY

(1990). The raw data show an overabundance of ordinary chondrites and underabundances of achondrites and carbo- naceous chondrites in the Antarctic collections compared to modern falls (irons and stony-irons were excluded because of their small number). CASSIDY (1990) found that carbo- naceous chondrites and achondrites occur in the Antarctic and non-Antarctic (modern fall) collections in the same pro- portions relative to each other, but that the ratio to ordinary chondrites is different. He suggested that these differences could be explained if the abundance of ordinary chondrites arriving at Earth in the recent past were some 10% lower

than over the last 0.3 Ma (but see CASSIDY and HARVEY, 1991).

HARVEY and CASSIDY (1989) studied the mass distribu- tions among Antarctic and non-Antarctic meteorites by fitting the population distributions with log-normal curves. Log- normal distributions have a physical meaning: in sedimen- tology it is known that the particle-size distributions produced by mechanical breaking of rocks tend to follow some variety oflog-normal distribution (J. GOODING, pers. comm., 1990). The curves for the two populations differ, with the Antarctic population having a tail toward the larger sizes. HARVEY and CASSIDY (1989) also found an excess of small H chondrites in Allan Hills Main ice field which is not easy to explain, so that the ALHA meteorites are either not a good sample or comprise an unusually large number of paired samples. CAS- SlDY and HARVEY (199 1) reexamined the data and found that the Antarctic frequency distributions are not log-normal and thus different from their distribution model, which is based on modern falls. The reason for this could be the in-

fluence of field processes, such as wind winnowing (small rocks are blown away, and there may be a type dependency for this process), and fragmentation while in or on the ice. If these processes are taken into account (which was not done

by HARVEY and CASSIDY, 1989, and CASSIDY, 1990), the reconstructed mass frequency distribution curves show no

differences by type between the Antarctic and non-Antarctic

populations. Huss (1990), on the other hand, used a power law char-

acteristic of grinding processes to represent the pre-terrestrial number distribution. All meteorite populations deviate from the power law at low masses because of collection bias and post-fall terrestrial processes. HUSS ( 1990) suggested that the Roosevelt County, New Mexico, site, where the collection

efficiency may be similar to Antarctica, as a low-mass number density much like that of the Allan Hills and Yamato samples. All meteorite populations seem to deviate from power-law and log-normal distributions at high masses, probably because of the presence of showers. Assuming a normal infall rate, Huss ( 1990) found that direct infall can yield the meteorites

found on the Alan Hills Main ice field in 200,000 years, while the collection time for other Allan Hills areas is only 4000 to 16,000 years, and for Elephant Moraine 30,000 years.

A potentially important observation has been made by HLJSS (199 1). He examined the claim of DENNISON et al. (1986) and DENNISON and LIPSCHUTZ (1987) that Antarctic meteorites show an excess of H chondrites compared to non- Antarctic chondrites. HUSS ( 199 1) analyzed the abundance data for different Antarctic ice fields and found that the H/ L ratio is significantly higher (at the 99% confidence level)

only for the Allan Hills Main and Near Western ice fields, but in all other areas is similar to non-Antarctic falls or finds

or Roosevelt County finds. He furthermore found, by break- ing down the abundances and ratios by petrologic type and correcting for pairing, that the excess at the Allan Hills Main

and Near Western ice fields seems confined to H5 chondrites. The H5 to L ratio at these fields is nearly eight times that for

witnessed falls after pairing corrections. The distribution models can be explained by the presence of a recent H5 shower which has so far gone unrecognized. This would also explain the overabundance of H chondrites by HARVEY and

CASSIDY ( 1989). There is thus no apparent H chondrite excess in the Antarctic population, because it is confined to one ice field. HUSS ( 1990. 199 1) argued that differences in mass dis-

tribution and relative abundances of meteorite types between the two populations can thus be explained by terrestrial sam- pling or statistical effects.

The above-mentioned authors have only discussed stony meteorites and excluded iron meteorites because oftheir small numbers. However, iron meteorites in Antarctica differ from

iron meteorites in the non-Antarctic collection in a very sig- nificant way: in the Antarctic collection, a much higher frac- tion of anomalous or ungrouped iron meteorites is observed (CLARKE, 1986; WASSON et al., 1989). Among non-Antarctic irons, only 15% do not fall into any of the compositional groups, while in Antarctica 39% are ungrouped (WASSON, 1990). In a new analysis of the causes for the overabundance of ungrouped iron meteorites in Antarctica, WASSON (1990) argued that this cannot be attributed to any variations in terrestrial ages or latitudinal or stochastic effects, but is rather related to the much smaller average mass of Antarctic irons. He found that during collisions of meteorite parent bodies, smaller fragments are ejected at higher velocities and small meteoroids have undergone more subsequent orbital velocity

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Assessment of differences and their causes 13

changes than larger ones. He therefore concluded that the

number of possible (asteroidal) parent bodies is larger for

smaller samples than for higher-mass meteoroids. This con-

clusion can be of importance for explaining Antarctic/non- Antarctic differences for other meteorite types-smaller me- teorites (which are more easily found in Antarctica) might just sample a slightly different parent population.

7.4. Meteoroid Streams?

If a number of differences between the Antarctic and non- Antarctic populations cannot be explained by terrestrial weathering, incomplete statistics, collection bias, or the like,

only pre-terrestrial differences remain as an explanation. It has been pointed out by several authors (e.g., DENNISON et al., 1986; DENNISON and LIPSCHUTZ, 1987; LIPSCHUTZ,

1990b) that this requires the existence of meteoroid streams.

This was also recognized by WETHERILL (1986). In a discus- sion of dynamical time scales associated with meteorite im- pact, Wetherill (see G. HERZOG in KOEBERL and CASSIDY, 1990) found that any pre-terrestrial difference between the two populations implies a dramatic change in the source of

meteoroids on a time-scale of less than 0.1 Ma (the average

terrestrial age of Antarctic meteorites). Wetherill argued against the existence and persistence of meteoroid streams, because the available dynamical calculations showed that 2 Ma is a hard upper limit on the lifetime of a meteoroid stream and that 0.2 Ma is a more likely upper limit. Because of the

long orbital evolution times necessary to supply meteorites from their parent bodies in the asteroid belt to the Earth,

pulses and supply discontinuities would be smoothed and could not be detected on Earth. The cosmic-ray exposure ages of most meteorites are greater than 1 Ma and indicate

simple, one-stage exposure histories. In view of the long ex- posure ages, it seems unlikely that objects with exposure ages of, e.g., 8 Ma (SCHULTZ et al.. I99 1) could preserve the orbital elements of a stream.

However, the possible presence of meteoroid streams has

recently been supported by several independent studies. For example, OBERST and NAKAMURA (1987) and OBERST ( 1989) reported the detection of possible meteoroid swarms from data supplied by the Apollo seismic network on the Moon. In addition, OLSSON-STEELE (1988) inferred the existence of

meteoroid streams from radar orbit surveys. In an analysis ofevents (fireballs, meteors) observed by the Canadian MORP meteor camera-network and the US Prairie Network, HAL- LIDAY et al. (1990) showed that part of the events are pref- erably explained by four non-random groups of related ob- jects. These four groups may be streams of meteoroid frag- ments that just recently crossed the Earth’s orbit. HALLIDAY et al. (1990) concluded that, if there are four streams during the last 20 or so years, the number of probably active me- teoroid streams will be considerably larger. Such groups should account for part of the meteorites falling on the Earth at a given time, and could be responsible for a difference between the Antarctic and non-Antarctic populations.

The existence of meteoroid streams was also postulated by WOOD ( 1982) and DODD ( 1989). Both authors analyzed fall statistics of H chondrites and noted that they do not fall ran- domly throughout the year. WOOD (1982) studied the annual

fall statistics and found a 3 l-year periodicity in the H chon-

drite fall frequency, implying that most belong to meteoroid

streams. He furthermore found differences in the fall statistics

for H chondrites with short and long exposure ages, and in- terpreted the results as suggestive of a non-asteroidal source of at least some H chondrites.

The possibility of a change in the total flux rate of extra-

terrestrial matter falling on the Earth might have some influ- ence too. Micrometeorites found in the Antarctic ice might

be used for flux rate determinations. Yrou and RAISBECK ( 1990) studied micrometeorites from 120 kg of a Vostok ice core. They found 14 chondritic spherules, implying a terres-

trial influx of 1500 tons per year for particles with radii greater than 50 pm, a somewhat lower value than obtained by other means. The vertical distribution of the particles indicated a

possibility for a slightly greater flux in the recent past, but of course more detailed work (with more cores and particles)

will be necessary.

8. CONCLUSIONS AND RECOMMENDATIONS

One of the questions that has been discussed over the past years and that prompted the workshop can now, in our opin- ion, be answered confidently: “Yes, there are differences be- tween Antarctic and non-Antarctic meteorites.” It is only the

interpretation of the causes for these differences that is not, by any means, unanimous. There are several obvious and

undisputable differences in major and minor properties of the two collections. Major differences include different me-

teorite-type frequencies, such as the occurrence of previously rare or unique meteorites (e.g., lunar meteorites, ungrouped iron meteorites, and achondrites), meteorite textures, trace

element contents, or oxygen isotopic compositions. However, these differences are not indigenous to all Antarctic meteorites

but are mostly restricted to meteorites of certain types. Other differences between the two populations are more

obvious and are therefore rarely included in this discussion:

Antarctic meteorites have greater terrestrial ages, smaller typical masses (per individual sample), and are found in a

restricted geographical location on Earth. From a number of recent studies (e.g., WASSON, 1990; SEARS et al., 1991) it seems that these characteristics have a direct influence on all

other “real” differences, even if they are not necessarily genetic

properties of the individual meteorites. It should also be mentioned that neither the Antarctic meteorites nor the modem falls represent a homogeneous sample. The collection of modem falls, usually taken to represent the “non-Antarctic

meteorites,” is a snapshot in time, covering merely the past two centuries. This is short compared to the hundreds of thousands of years of “integration time” for the Antarctic samples.

We have to determine whether either of the two collections is a representative sample of the meteorite parent-body pop- ulation. It seems that the Antarctic collection might be the better sample because it was accumulated over a longer period of time. It has been argued that some of the meteorites that are rare or absent from the non-Antarctic population are found in Antarctica only because smaller meteorites are re- covered there. Why does the non-Antarctic meteorite collec- tion contain so few meteorites of these types, not even small

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14 C. Koeberl and W. A. Cassidy

ones? The recent work by WASSON ( 1990) indicates that the size difference between the Antarctic and non-Antarctic me-

teorites is, in fact, significant and may be the reason for a

number of the reported differences, simply because a slightly different population is sampled. Because of the increasing number of lunar meteorites found in the Antarctic collection we have to assume that they are part of the normal influx of extraterrestrial matter onto the Earth.

An important problem is the fact that the data are incom- plete. Adequate numbers of specific analyses have only been made on meteorites of selected types, but usually other types have not been studied in the same way. For example, some of the Antarctic meteorites seem to have different isotopic ratios, but others do not, and even different fractions and leachates of a single meteorite behave differently. It seems, for chemistry. petrology, and isotopic composition alike, that

the Antarctic meteorites expand the range of characteristics and compositions known before. This would support the no- tion that Antarctic meteorites are a more representative sam- ple ofthe parent population. In the future it will be necessary to study larger numbers of specimens from all available me-

teorite types for chemical, isotopic, petrologic, age, and cos- mogenic nuclide characteristics.

The fact that some Antarctic meteorites have properties that do not fit within the traditional classification scheme (e.g., some carbonaceous chondrites; see above) has been used to suggest a revision (and expansion) of this scheme. Achon- drites were found that have compositions that are interme- diate between the cumulate eucrites and the diogenites, and others extend the (non-Antarctic) compositional range (e.g., magnesian ureilites). Some Antarctic CI and CM meteorites may have experienced open-system thermal metamorphism on their parent bodies, which is not known from non-Ant-

arctic carbonaceous chondrites. As already mentioned, there are some factors that reduce

the “uniqueness” of certain Antarctic meteorites. One ofthem is the question of pairing: how many individual falls are rep- resented in the Antarctic collection? This question is easier to answer for iron meteorites and other rare samples (e.g., lunar meteorites) than for ordinary chondrites. However, we see from our experiences with lunar meteorites (EUGSTER, 1989) that there is no easy answer concerning pairing. In this respect it is important to measure the noble gas content, cos- mogenic nuclide abundances, petrological and chemical characteristics of as many meteorites as possible; but because of the natural spread of the data, there is often no absolute proof for or against pairing. New methods for measuring cos- mogenic radionuchdes and terrestrial ages (NISHIIZUMI et al., 1989; ENGLERT, 1990; REEDY, 1990) are needed to obtain more age data and age-distribution spectra. Pairing, however, explains only some of the differences; variations in textures, or in chemical and isotopic compositions, are still present even if the number of paired samples increases or decreases; only the relative number of specimens will increase or de- crease. Thus, as more specimens are found to be paired, the greater will become the proportion of unique and rare spec- imens.

Weathering was probably most often cited as the “un- known” factor that may be responsible for chemical and iso- topic differences. The terrestrial residence times of most Ant-

arctic meteorites are much longer than those of non-Antarctic samples (e.g., NISHIIZUMI et al., 1989). The unusual condi- tions in Antarctica (i.e., the meteorites are buried for a long

time, and then exposed to a very cold and dry climate) suggest that weathering in Antarctica promotes some processes which under non-Antarctic conditions are not of great importance. The time scale of the weathering process is not well known. We do not know exactly how long meteorites are buried in the ice, how long they are exposed, and whether climate changes over the past lo-100,000 years had any influence on this cycle. We do not know the long-term interaction be- tween the meteorites and solid ice. GOODING ( 1986), VELBEL (1988) and VELBEL et al. (199 1)

have shown that some weathering products (e.g., nesquehon- ite) have formed only very recently. Some meteorites continue to form efflorescences even while resting in storage. The time scale for the formation of evaporites is very short compared to the terrestrial age of the meteorites. Previously formed efflorescences may have been eroded, with unknown chemical effects. No obvious connection between the locality of me- teorite finds and the occurrence of evaporites on the mete- orites has been established. Some changes in chemical com- position may be ascribed to weathering, but no clear pattern emerges. Some elemental abundances may change, others do not; some types of meteorites show such an effect, others do not (e.g., Se enrichment only in eucrites?-MITTLEFEHLDT and LINDSTROM, 1991). One important problem is the weathering index. It is important to find an indicator showing whether meteorites are weathered or not. The “classical” ABC index system is not sufficient; differential scanning calorim- etry (GOODING, 1990) or the integrated intensity in the 3 pm

absorption band of the infrared reflectance spectra (MIYA- MOTO, 199 1) is a much more sensitive indicator of weathering.

Again, weathering is not (just like pairing) able to explain all differences between the two populations. It is, for example, difficult to explain effects that are probably caused by thermal metamorphism in this way. PAUL and LIP~CHUTZ (1989) have shown that the distribution of volatile elements in a number of Antarctic carbonaceous chondrites indicates thermal

metamorphism that is not known from related non-Antarctic samples. This conclusion was recently supported by HAQ et

al. ( 1988) and SEARS et al. ( 199 I), who studied the thermo- luminescence properties of Antarctic and non-Antarctic chondrites and found significant differences. They concluded that the populations had different thermal histories. Labo- ratory experiments may be performed in the future to study the chemical and TL effects.

More data, and more meteorites, are needed; future in- vestigations (along the lines of some of the points listed above) will have to be done to help clarify the open questions.

The following list of facts and recommendations is given to summarize the results and to stimulate further studies:

a) The existence of differences between Antarctic and non- Antarctic meteorites has been established.

b) Differences exist in terrestrial residence age, average size, certain major and trace element contents, textural char- acteristics, isotope ratios, meteorite-type frequency, ther- moluminescence, and others.

c) The differences have a wide variety of causes, ranging from

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Assessment of differences and their causes 15

4

incomplete knowledge of pairing to terrestrial weathering and pre-terrestrial differences. Recent studies of terrestrial weathering have shown that the process is much more complex, subtle, and important than previously assumed, and may therefore be invoked to explain a greater number of differences. In the future it will be important to study a large number of meteorites of all types with all available analytical methods; this is necessary to obtain pairing information and also to clearly define any differences. Some of the systematic differences (e.g., thermal history of carbonaceous chondrites, trace element enrichments in H chondrites, and depletions in L chondrites, oxygen isotopes, thermotuminescence) are highly unhkely to be due to terrestrial (post-fall) processes; they are more likely to be pre-terrestrial in origin. Pre-terrestrial differences imply the existence of meteoroid streams; these streams have previously been disputed on the basis of celestial mechanics, but new (and independent) research gives strong observational arguments (e.g., falf statistics, lunar seismic network, fireball camera-networks) for the existence of such streams (without, however, over- coming the difhcuhy of explaining them in terms of celes- tial mechanics).

h) Smaller (i.e., Antarctic) meteorites may sample a different meteoroid population than larger (i.e., non-Antarctic) meteorites; this can explain the overabundance of certain “rare” types among the Antarctic collection. In summary, Antarctic meteorites seem to represent a population that is slightly different on average from the non-Antarctic population and might even be more rep- resentative for the meteorite source. More Antarctic meteorites are needed to study this im- portant question; they will continue to provide invaluable information on the origin and evolution of our solar system.

Acknowledgments-We are grateful to all participants of the workshop and to all who contributed to this collection of papers that resulted from the workshop. We want to thank all sponsoring agencies, es- pecially the Lunar and Planetary Institute, Houston, USA, and the Institute ofGeochemistry, University of Vienna, Austria, for technical, logistical, and financial assistance. We are especially thankful to the following friends and colleagues who read the draft of this paper and provided comments and suggestions towards a revised version: J. Gooding, M. Grady, G. R. Huss, M. Lipschutz, D. MittlefehIdt. M. Miyamoto, R. Reedy, D. W. G. Sears, H. Takeda, and I. Wright. Any remaining errors and omissions are of course our own. We ap- preciate the cooperation of all authors and reviewers who kept this collection on time, and we want to thank G. Faure and his staff at GCA for their help with and interest in producing this collection of papers.

Editorial handii~g: G. Faure

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