Weisberg et al.: Systematics and Evaluation of Meteorite Classification 19 Systematics and Evaluation of Meteorite Classification Michael K. Weisberg Kingsborough Community College of the City University of New York and American Museum of Natural History Timothy J. McCoy Smithsonian Institution Alexander N. Krot University of Hawai‘i at Manoa Classification of meteorites is largely based on their mineralogical and petrographic charac- teristics and their whole-rock chemical and O-isotopic compositions. According to the currently used classification scheme, meteorites are divided into chondrites, primitive achondrites, and achondrites. There are 15 chondrite groups, including 8 carbonaceous (CI, CM, CO, CV, CK, CR, CH, CB), 3 ordinary (H, L, LL), 2 enstatite (EH, EL), and R and K chondrites. Several chondrites cannot be assigned to the existing groups and may represent the first members of new groups. Some groups are subdivided into subgroups, which may have resulted from aster- oidal processing of a single group of meteorites. Each chondrite group is considered to have sampled a separate parent body. Some chondrite groups and ungrouped chondrites show chemi- cal and mineralogical similarities and are grouped together into clans. The significance of this higher order of classification remains poorly understood. The primitive achondrites include ureil- ites, acapulcoites, lodranites, winonaites, and silicate inclusions in IAB and IIICD irons and probably represent recrystallization or residues from a low-degree partial melting of chondritic materials. The genetic relationship between primitive achondrites and the existing groups of chondritic meteorites remains controversial. Achondrites resulted from a high degree of melt- ing of chondrites and include asteroidal (angrites, aubrites, howardites-diogenites-eucrites, mesosiderites, 3 groups of pallasites, 15 groups of irons plus many ungrouped irons) and plane- tary (martian, lunar) meteorites. 1. INTRODUCTION on the number of planetary bodies (asteroids) represented in the meteorite record. Meteorite classification is the basic framework from A variety of characteristics are used to classify meteor- which meteoriticists and cosmochemists work and commu- ites (e.g., mineralogy, petrology, whole-rock chemistry, and nicate. It is a process designed to group similar meteorites O isotopes), but the groupings based on these data are not together and evolves with the collection of new data, dis- always consistent with each other. Additionally, a number covery of new types of meteorites, and new ideas about re- of meteorites do not fit into the existing groups. In this chap- lationships between meteorites. Although classification does ter, we present the basic systematics of meteorite classifi- not necessarily need to imply genetic relationship, it has cation and describe meteorite classes, clans, groups, and provoked questions about the processes that resulted in the subgroups to illustrate the diversity of solar system materi- similarities and differences among the meteorite groups and als and the methods used to classify them. We also provide individual members within groups. Additionally, the rela- a critical evaluation of current meteorite classification. The tionship between primitive (chondrites) and differentiated reader is also directed to a number of contemporaneous (achondrites) meteorites and the parent bodies from which writings on meteorite classification and properties that pro- meteorites are derived is an important issue in meteorite vide additional detail to the information and discussion study. Each chondrite group, for example, is often inter- offered here (e.g., Brearley and Jones, 1998; Mittlefehldt et preted to represent materials from a single parent body. Dif- al., 1998; Krot et al., 2004; Mittlefehldt, 2004). ferences in the characteristics among the chondrite groups have provided information on the types of primitive mate- 2. HISTORICAL NOTE rials that were available during planetary accretion and the systematic geochemical trends of planetary bodies with their The current classification scheme for meteorites had its increasing distance from the Sun (e.g., Wasson, 1985). The beginnings in the 1860s with G. Rose’s classification of the wide variety of meteorite types has given rise to thoughts meteorite collection at the University Museum of Berlin and 19 20 Meteorites and the Early Solar System II Maskelyne’s classification of the British Museum collec- asteroids (and possibly comets) that did not experience plan- tion. Rose was the first to split stones into chondrites and etary differentiation. The achondrites are igneous rock nonchondrites. Maskelyne classified meteorites into sider- (melts, partial melts, melt residues) or breccias of igneous ites (irons), siderolites (stony irons), and aerolites (stones). rock fragments from differentiated asteroids and planetary This was followed by Tshermak’s modification of the Rose bodies (Mars, Moon). Some meteorites have achondritic classification in 1883 and Brezina’s modification of the (igneous or recrystallized) textures (Plates 1 and 2), but re- Tshermak-Rose classification scheme (see Prior, 1920). The tain a primitive chemical affinity to their chondritic precur- first classification of meteorites based on chemical compo- sors. They are referred to as primitive achondrites because sition was by Farrington (1907), who chemically analyzed they are nonchondritic, but are thought to be closer to their irons. Based on these earlier schemes, Prior (1920) devel- primitive chondritic parent than other achondrites. oped a comprehensive meteorite classification scheme, in- A number of variations on classification have been pre- troducing terms like mesosiderite and lodranite, that became sented over the years. Like any natural history discipline, the backbone of the current meteorite classification. Prior’s meteorite taxonomy is an attempt to group objects of simi- system was later modified by Mason (1967). lar type. At the lowest level (e.g., objects so similar that they The meteorite classification system that is currently used must have originated from common materials, processes, and discussed in this chapter is broadly based on Prior and locations), general agreement exists as to groupings of (1920) and Mason (1967) and its modifications over the last meteorites. Disagreements center on higher-order systems four decades by a number of modern studies of the petro- that attempt to relate disparate materials through common logic, bulk chemical, and O-isotopic characteristics of me- processes (e.g., melted vs. unmelted), nebular reservoirs teorites, infused with continual discovery of new meteorite (e.g., O-isotopic composition or oxidation state), or parent samples. bodies. The terms used for meteorite classification have evolved over time but they have not always been rigorously 3. CLASSIFICATION METHODS and consistently applied. Furthermore, the classification of chondrites and achondrites evolved independently, produc- 3.1. Terminology and Nomenclature ing terminology for classification of achondrites and primi- tive achondrites that differs from that of chondrites. Wasson Meteorites are classified in a variety of ways to com- (1985) divided meteorites into chondrites and differentiated municate their different attributes, which have important meteorites, and subdivided the differentiated meteorites into implications. Meteorites are considered falls if they can be achondrites, stony irons, and stones. Krot et al. (2004) re- associated with an observed fall event and finds if they cently divided meteorites into chondrites and nonchondrites cannot be connected to a recorded fall event. This is an im- and subdivided the nonchondrites into achondrites, primi- portant distinction because finds, depending on the time tive achondrites, stony irons, and irons. Kallemeyn et al. they spent on Earth, are more prone to chemical interac- (1996) recognized problems in dividing chondrites into tion with the terrestrial environment. Therefore, the chemi- classes and in distinguishing between class and clan affilia- cal trends they record need to be carefully considered. Most tions in chondrites. They divided chondrites into two major of the meteorites in our collections are finds and many of classes, which they called carbonaceous and noncarbona- these are materials recovered from private and nationally ceous “super clans.” The noncarbonaceous super clan in- organized search efforts in hot (Africa, Australia) and cold cluded the O, R, and E chondrites. They then divided the (Antarctica) deserts. super clans into clans. Meteorites are given names based on the location in The hierarchy of terms for chondrite classification used which they are found (e.g., the Allende meteorite is a fall in this chapter include, in order, class, clan, group, and from Pueblito de Allende, Mexico). Meteorites from desert subgroup. A chondrite class consists of two or more groups regions, such as Antarctica, are given names and numbers sharing primary whole-rock chemical and O-isotopic prop- [e.g., Allan Hills (ALH) 84001 is a meteorite find that was erties (e.g., Hutchison, 2004). Chondrites within a class collected in Allan Hills, Antarctica, during the 1984–1985 have broadly similar refractory lithophile-element abun- field season and was the first meteorite described in that dances that are enriched, depleted, or equal to solar abun- field season]. dances (Fig. 3a,b), with CI chondrites defining our best The terms stones, irons, and stony