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
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 smal
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