Comparisons of Antarctic and non-Antarctic achondrites and possible origin of the differences

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Comparisons of Antarctic and non-Antarctic achondrites and possible origin of the differences* HIROSHI TAKEDA Mineralogical Institute, Faculty of Science, University of Tokyo, Hongo, Tokyo 113,Japan (Received February 14, 1990; accepted in revised,form September

1I, 1990)

Abstract-In order to discuss differences between Antarctic and non-Antarctic achondrites, we investigated pairing of mainly the Yamato achondrites by mineralogical techniques. In addition to five paired specimens of the Y74159-type polymict eucrites, three new groups distinct from the Y74159-type have been recognized. One group (Y791960) of howarditic polymict eucrites is similar to those of non-Antarctic origin. Some Antarctic monomict eucrites differ in texture and chemistry from the non-Antarctic specimens. More than 50 diogenite specimens recovered from the Yamato bare ice field can be grouped into two falls of separate locations, compared with 8 from the USA collection and 9 from the non-Antarctic groups. Thirty Antarctic ureilites are separate falls (2 pairings), and their individual masses are small. These observations suggest that discussion of differences between the Antarctic and non-Antarctic achondrites largely depends on detection of pairings of the Antarctic specimens. The pairings of the specimens from Victoria Land have not been as well characterized as those of the Yamato Mountains. Comparisons with non-Antarctic meteorites show that the Antarctic HEDs and ureilites differ from non-Antarctic specimens by the following criteria: 1) Polymict eucrites excluding those with howarditic affinity have not been found in the non-Antarctic collections. 2) Magnesian ureilites and augite-bearing ureilites were found only in Antarctica. The differences may not be attributed to just one factor. The above evidence of two unique diogenite falls preserved on the restricted ice fields suggests that the Yamato diogenites may represent falls on restricted areas in the distant past. The discovery of many small unique eucrites and ureilites suggests that within the large number of small specimens on a bare ice field there is a higher chance of finding unique varieties. INTRODUCTION

specimens, and seven groups of falls have been recognized and were compared with the non-Antarctic specimens by TAKEDA et al. (1983a,b), DELANEY et al. (1984), and TAKEDA ( 1987a). Statistics of the Victoria Land achondrites were given in the ANTARCTIC METEORITE WORKING GROUP (1990), but the pairing of these specimens has not been investigated in the same detail as for the Yamato collection; therefore, we will discuss mainly the Yamato collection. At the second Antarctic Meteorite Workshop in Mainz, the following possibility for the difference was proposed (TAKEDA, 1986). There is evidence to support the idea that meteorites on a specific ice field represent a fall in the local area at a certain time in the past. The distribution of achondrites reaching the earth might have changed with time, so that the Antarctic collection represents an average over a much longer time interval or during a specific period in the distant past. However, it is important to study pairing problems before we discuss such a possibility. Old terrestrial ages of Antarctic meteorites (SCHULTZ, 1986; NISHIIZUMI, 1986) and differences between the Yamato and Victoria Land collections (TAKEDA et al., 1983b; TAKEDA, 1987a) have been considered as evidence for this hypothesis, but problems of celestial mechanics and statistics have to be solved before proposing such a possibility. Detailed studies of non-Antarctic HED (Howardites, Eucrites, Diogenites) achondrites have been reported by MASON et al. (1979). A mineralogical comparison of Antarctic and

ALREADY AT THE beginning of our Antarctic meteorite research, it was recognized that textures and mineral assemblages of Antarctic achondrites are different from those of non-Antarctic specimens. For example, the first Antarctic diogenite, Y6902, showed a recrystallized granoblastic texture uncommon in diogenites (TAKEDA et al., 1981; TAKEDA, 1987a); the first eucrite, Y74159, was a polymict eucrite (TAKEDA et al., 1978); and the first ureilite, Y74659, was very magnesian (TAKEDA, 1987b; 1989). The number of such

unusual achondrites increased as the total number of the collection increased (YANAI and KOJIMA, 1987). As the total number of specimens increases, the probability of finding rare, unique, or unknown types is increased too. However, the true number of falls will be reduced, as most of them are paired falls, which are difficult to identify. The number of unique Antarctic specimens within a known class appeared to be greater than expected from statistics of the non-Antarctic collections (TAKEDA, 1987a), but they are not so unique any more. The unusual (compared to the non-Antarctic specimens) Yamato diogenites were found to represent two falls (TAKEDA et al., 1981; TAKEDA and MORI, 1985). Polymict eucrites have been identified as paired

* This paper is part of a series which developed out of a workshop held in Vienna, July 1989. 35

H. Takeda

36

non-Antarctic HED achondrites at an early stage showed that many non-Antarctic eucrites have almost identical chemical and mineralogical compositions (MASON et al., 1979; TAKEDA et al., 1983a). As the total number of the Antarctic meteorites increases, the number of eucrites in these groups increased gradually. DELANEY et al. (1984) suggested that six non-Antarctic breccias, which were previously classified as howardites, show close similarities to polymict eucrites. If this group (howarditic polymict eucrites) is distinct from many polymict eucrites found in Antarctica, we can conclude that polymict eucrites, which are abundant in Antarctica, have not been found in the non-Antarctic collection. We investigated possible howarditic polymict eucrites from the Yamato collection in this study and found that such a group is indeed distinct from major Antarctic polymict eucrites which are absent from the non-Antarctic collection. The nomenclatures, which were used in previous studies in this field, are summarized in Table 1. Some newly found unbrecciated and monomict eucrites have also been studied to show that they are mineralogically not entirely identical to the nonAntarctic counter-parts. Lunar meteorites are found only in Antarctica, and they differ from Apollo lunar samples (e.g., WARREN et al., 1983; Table

1.

KOEBERL et al., 1989). Among 11 lunar meteorites (1: Y82192, Y82193, Y86032 (3 specimens), 2: MAC88104/5 (2), 3: ALH8 1005, 4: EET87521, 5: Y793274, 6: Asuka 3 1, 7: Y793169, 8: Y791197), two pairings (reducing the number of individual falls to 8) have been proposed from noble gas abundances by EIJGSTER et al. (1989) and mineralogical studies by TAKEDA et al. (1989). The lunar meteorites are good examples for the case where meteorites from different bare ice areas sample different regions of their parent body, in this case the Moon. On the basis of these data we examine possibilities to explain the differences between Antarctic and non-Antarctic achondrites. Because the discussion is mainly based on the texture and chemistry of minerals, no statistical treatment has been attempted. These mineralogical features are related to crystallization and cooling histories and, hence, locations within the parent body or bodies. SAMPLES

AND EXPERIMENTAL

Yamato 791195, 791438, 791439, 791960, 792769, and 82037 have been selected for comparison with non-Antarctic meteorites. They have some similarity with non-Antarctic HED achondrites, but show characteristic features not previously known. Polished thin sec-

Explanations for Subclasses of the HED Achondrites

1. Diogenites monomict or unbrecciated Ca-poor achondrites with major orthopyroxene. 2. Eucrites pyroxene-plagioclase achondrites with less than 10% (modal) orthopyroxene. A.

non-Cumulate Eucrites a.

Ordinary (0-) Eucrites (monomict or unbrecciated) Basaltic ones with homogeneous host phase and fine exsolution lammellae.

i)

Main Group Eucrites (Juvinas-type). following sub-t&es.

0-eucrites excluding the

This group has almost identical chemical and

mineralogical composition having excess Si02. ii) Stannern and Nuevo Laredo-types. 0-eucrites with a specific chemical trend.* b.

Surface Eucrites (Pasamonte-type or lava-like eucrite) Basaltic pyroxenes with ones including extensive chemical zoning.

B.

Cumulate Eucrites coarse-grained eucrites with inverted pigeonite. (Magnesia" Eucrites: Eucrites with Mg/Fe of pyroxene greater than nearly one, more magnesia" than Moore Co. including non-cumulate eucrites) i)

Binda-type with decomposed low-Ca clinopyroxene and with blebby augite inclusions.

ii) Moore Co-type with inverted pigeonites with coarse (001) exsolution lamellae. C.

TECHNIQUES

Polymict Eucrites Polymict breccias of the above eucrites.

3. Howardites Polymict breccias of HED's with orthopyroxene more than 10% (modal)

References: Mason et al. (1979). Delaney et al. (1984) and *Basaltic Volcanism Study Project (1981)

Antarctic and non-Antarctic achondrites tions were supplied by the National Institute of Polar Research (NIPR) and have been examined by optical microscope and electron microprobe. Physical descriptions of these meteorites are given in YANAI and KOJIMA (1987). Samples of Nagaria and Medanitos from the British Museum (Natural History) were also studied for comparison with magnesian eucrites. Chemical analyses were carried out using two electron microprobes, JEOL 733 Mark II at the Geological Institute and JEOL 840A at the Mineralogical Institute, University of Tokyo. Chemical zoning and exsolution phases in pyroxenes were examined by JEOL 840A with the Kevex Super 8000 System which has the capability of chemically mapping the distribution of 15 elements. Quantitative chemical analyses were made with a JEOL 733 Mark II by employing the method used by NAKAMURAand KUSHIRO(1970). Pyroxene crystals separated from these meteorites were examined with an X-ray precession camera to identify pyroxene structure types and the presence of exsolved phases, using Zr-filtered MoKoc radiation.

NON-ANTARCTIC

CP

Fe

4 CO

ANTARCTIC 01,

n

t

r,Hd

HED ACHONDRITES Monomict and Unbrecciated Eucrites This type of eucrite is common in the non-Antarctic collection. The term “ordinary eucrite” used in this paper is explained in Table 1. It is an ophitic or subophitic basalt, in which pigeonites show the homogeneous chemistry of the host phases of pyroxene (pigeonite) and exhibit a fine planar exsolution texture of augite on (001). Most non-Antarctic monomict eucrites, with the exception of Pasamonte, are ordinary eucrites (TAKEDA et al., 1978). Pyroxene compositions of non-Antarctic eucrites are compiled in Fig. 1 to show the characteristics of some of the subclasses in comparison with the Antarctic samples. Our recent study of the Antarctic monomict eucrites Y791186, Y792510 (TAKEDA and GRAHAM, 1990), Y791195, Y791438 (SAW et al., 1990), and Y82037 indicates that their mineralogy is similar to that of non-Antarctic ordinary eucrites, but that they differ texturally or chemically. Pyroxene compositions of Y79 1195 and Y79 1438 plot outside the range of the non-Antarctic ordinary eucrites (Fig. 1). Most non-Antarctic ordinary eucrites are products of thermal annealing (NYQUISTet al., 1986). Y82037 is an ordinary eutrite (Fig. l), but does not show the clouding of pyroxenes (tiny inclusions of ilmenite and chromite) common in this type (HARLOW and KLIMENTIDIS, 1980). The Y79 1186 and Y7925 10 pyroxenes were almost completely homogenized (Fig. l), and a fraction of pigeonite is inverted to orthopyroxene. The exposure ages obtained by NAGAOand OGATA (1989) are in agreement with the pairing of the two eucrites (Y791186 and Y792510). Y79 1195 is an unbrecciated eucrite chemically rather similar to ordinary eucrites (TAKEDA et al., 1988a), but it is more Mg-rich than the most Mg-rich non-Antarctic ordinary eucrites (Fig. 1). Our new observations show that the coarse augite exsolution texture in it is suggestive of slower cooling rates than for most ordinary eucrites. Y791195 has augite lamellae on (001) that are about 10 pm thick at 20 to 40 Frn spacing of the host pigeonite. Y791195 shows a granular to microgabbroic texture (Fig. 2). Short prismatic forms of plagioclase are rare, and parts of the crystalline texture are disturbed and recrystallized. The homogeneous plagioclase composition indicates slower growth. This eucrite fills a compositional gap between non-Antarctic cumulate eucrites and ordinary eucrites in the pyroxene quadrilateral (Fig. 1) in that Y79 1195 is more magnesian than most non-Antarctic

Fe

Mg

FIG. 1. Pyroxene compositions of non-Antarctic and Antarctic eucrites for some eucrite subclasses. Data after MASONet al. (1979) except for Pomozdino (Pz) (WARRENet al., 1990) and Nagaria (Ng) (this study). Cumulate eucrites include Binda (Bi), Moama (Ma), Se& de Mag&(Sm), Nagaria (Ng), Moore County (MC),and ordinary eucrites: Millbillillie (Ml), Ibitira (I), Juvinas (Jv), Cachari (C), Haraiya (H), Peramiho (Pr), Jonzac (J), Emmaville (E), Lakangaon (L), Nuevo Laredo (NI). Antarctic pyroxenes are represented by the following meteorites: Data for I: Y791199, 2: Y791200, 3: Y791201, 9: Y791439 are those of large clasts; and 5: Y791195, 6: Y791186, 7: Y82037, 8: Y791438, 10: Y792510 are individual eucrites. Open circles: bulk compositions; solid circles: host; and triangles: exsolved augites.

ordinary common

eucrites, and exsolution cumulate eucrites.

lamellae

are thinner

than

Magnesian Eucrites As defined in Table 1, non-Antarctic magnesian eucrites have been called cumulate eucrites. Although Nagaria and Medanitos are known as cumulate eucrites, their growth textures and pyroxene exsolution textures are different from those of other cumulate eucrites (Fig. 2) as explained below. The short prismatic, medium-grained growth textures of these two do not show evidence of a settling-type cumulate texture, and the widths of exsolution lamellae of augites are thinner (ca. 10 microns) than for true cumulate eucrites such as Serrl de Magt. Cumulate eucrites generally have unzoned plagioclase. Minor zoning of plagioclase crystals in Nagaria suggests faster growth than in cumulates. Medanitos is a brecciated monomict eucrite, but pyroxene textures vary to an extent that suggests a slight polymict nature (Fig. 2). The amount of plagioclase is less than in cumulate eucrites, and the precursor of Medanitos may have cooled faster, although the REE data are those of a cumulate. Nagaria is a unbrecciated eucrite with a texture similar to but coarser than Y79 1195.

H. Takeda

38

FIG. 2. Photomicrographs of(a) Y791 195, (b) magnesian eucrite. Y791438. (c) Nagaria, and (d) Medanitos (crossed nicols). Width is 3.3 mm.

Yamato 79 1438

This eucrite has a pyroxene chemistry similar to that of magnesian cumulate eucrites (Fig. l), but it shows fine exsolution textures similar to ordinary eucrites. Y79 1438, a rare unbrecciated eucrite found in Antarctica, has a texture slightly disturbed by shock (Fig. 2) which is intermediate between subophitic and equant. Lath shapes of plagioclase can be recognized in it. The modal abundance of plagioclase (20 ~01%) is less than that of pyroxene. Modally, Y79 1438 looks like a Binda or Y75032-type, but exsolution is not alike. Pigeonites show fine exsolution textures comparable to those of ordinary eucrites such as Juvinas, and the augite lamellae are barely resolvable by the electron microprobe. This fine texture is different from non-Antarctic cumulate eucrites of this composition. The bulk chemical composition of pigeonite is CasMg,,,Fe4> (Fig. l), i.e., more Mg-rich than pigeonite of Moore County and comparable to Serra de MagC and Nagaria (TAKEDA et al., 1988a). If we emphasize the ordinary eucrite-like texture of Y791438, the compositions of ordinary eucrites extend more to the magnesian side than the non-Antarctic ordinary eucrites. The An content of plagioclase ranges from 90 to 95, and we believe that it represents the original igneous zoning formed during rapid crystal growth. The detailed mineralogy and chemistry are given in SAIKI et al. (1990).

If we consider that its chemistry is interesting, Y791438 was unusually rapidly cooled compared to the non-Antarctic group. The exsolution and inversion textures of pyroxenes in Nagaria, Medanitos, and Y791438 share some common features. They do not show the characteristic textures of inverted pigeonite as in Serra de MagC (TAKEDA et al., 1983b; HARLOW et al., 1979). Blebby augites decomposed from the host pigeonites after exsolving thick exsolution lamellae on (001) have not been observed. The width of the exsolution lamellae in Moore County is about 50 pm. Lamellae of Nagaria have a thickness of 15 to 20 Frn at 60 to 80 pm intervals. The finer exsolution lamellae in Medanitos are similar to that in Y79 1438. The width of the coarser exsolution lamellae in Y791438 is 1.2 pm at 7.5 pm intervals. Chemical compositions of host and of lamellae pairs of Antarctic eucrites are compared in Fig. I with non-Antarctic ones after MASON et al. (1979). Between the diogenite and ordinary eucrite regions in the pyroxene quadrilateral, there have been two compositional gaps at both sides of the cumulate eucrite region. Many clasts found in Antarctic HEDs and individual eucrites, such as Y791438 and Y791195, fill this gap. However, new data of the three non-Antarctic magnesian eucrites Pomozdino (WARREN et al., 1990) Medanitos, and Nagaria also plot into this region. These three eucrites are similar to Y791438 in pyroxene composition. Inversion to orthopyroxene is detected only in Nagaria; in

Antarctic and non-Antarctic achondrites

tasis-rich basaltic clast with extensively zoned pyroxene as in Y75011,84 (NYQUIST et al., 1986), olivine veinlets in a pyroxene crystal, presence of shock melt in matrices, and of Na-rich plagioclase; Nd/Sm isotopic ratios, Rare Earth Element (REE) abundance patterns, terrestrial and cosmic-ray exposure ages (NAGAOand OGATA, 1989). Negative terrestrial ages in Table 2 indicate young ages (within errors from the present time). Different letters of pairing for the same group suggest that other criteria lead to different pairings. It was found that REE patterns are sensitive to sample location. The number of identified distinct falls of Yamato polymict eucrites (minimum 5) has increased from 2 as proposed in 1986 (TAKEDA, 1986). Judging from our studies, we identify the following pairing (TAKEDA, 1987a)-A (Y74159-type): Y74159, Y74450, Y75011, Y75015, Y790007; B (Y790266type): Y790260, Y790266, Y790 122(?); C: Y79 1960, Y79 1962; and D: Y792769. Samples of group A and B come from different ice fields, 32 km apart from group C and D (TAKEDA and YANAI, 1982; NAGAO and OGATA, 1989). At least two howardite falls have been recognized (TAKEDA et al., 1984). The pairing of polymict eucrites in the USA Antarctic meteorite collection as proposed by DELANEYet al. (1984) includes four groups. This number increased to six (one howarditic group) in a more recent compilation in the ANTARCTICMETEORITEWORKING GROUP (1990). The number of non-Antarctic polymict eucrites listed by DELANEYet al. ( 1984) is five, but they are classified as howardites according

others, some grains are only partly inverted. In the case of magnesian eucrites, new data for both Antarctic and nonAntarctic specimens are filling the previously controversial compositional gap (MASON et al., 1979). Polymict Eucrites

The polymict eucrites are fragmental breccias of eucritic compositions produced by impacts, which destroyed, mixed, and excavated dominantly eucritic terrains of different chemistries and thermal histories (DELANEY et al., 1984; TAKEDA et al., 1978). They do not contain solar wind gases as do lunar regolith breccias and howardites. The definitions of the HED achondrite subclasses are summarized in Table 1. Discoveries of large numbers of polymict eucrites in both Yamato and Victoria Land raised the question of why these meteorites are more abundant in Antarctica than elsewhere. Their mineralogy suggested that many of them are paired (TAKEDAet al., 1983b). Subsequently, exposure ages and terrestrial ages (SCHULTZ, 1986; NAGAO and OGATA, 1989) confirmed numerous pairings. Table 2 presents proposed pairings of the Yamato eucrites and howardites. In this Table, the proposed pairing is represented by a letter (A to I) in parentheses. Criteria used to identify pairing include mineralogical characteristics (DELANEYet al., 1984) such as presence of pyroxene with particular textures (e.g., Binda-type, Moore County-type inverted pigeonite), presence of mesos-

Table 2.

Samples

Texture class

39

Pairings of Antarctic Yamato HED Achondrites

Mineralogical

Criteria for Pairings Nd/Sm REE Exp. age Ma

characteristics

Terr. age

Ma

-Y74159

BD BD BD BD BD

Y74450 Y75011 Y75015 Y790007 Y790020

PE PE PE PE PE PE

Y790122 Y790260 Y7902bb

PE PE PE

Y791960 Y791962 Y792769

PE PE PE

DI DI

Y791186 Y792510

mE nl?

MC MC

Y7308 Y790727 Y791208 Y791492

PH PH PH PH

PC PC PC PC PC

OV OV OV

SM SM

OV

(A) (A) (A) (A)

Na Na Na

(B) N.?l (B)

(1) (A) (A) (A) (H) (H)

72 73.0 (A)

-0.041 (A) -0.032 (A)

73.3 (A)

-0.029 (A)

(1) (B) (B)

24.4 (?) 21.6 (B) 21.9 (B)

0.110 (?) 0.140 (B) 0.150 (B)

CC) CC) (‘J) Na Na

(E) (E) (F) (G) (G) (G)

(El

6.81(C)

0.27

(C)

3.73(D)

0.35

(D)

13.33(E) 11.65(E)

0.24 0.23

(E) (E)

12.7

0.150

Letters in parentheses (A to I) at the last colum for each crlterio" represent possible paired groups. Different Letters for the same meteorite indicate another porposed pairing by a different criterion. p: polymict breccia, m: monomict, E: eucrite, H: howardite. Mineralogical criteria used to identify pairing include the following. BD: Binda-type Inverted Pigeonite present. DI: minor diogenitic pyroxene present, MC: Moore Co,-type pyroxene present, PC: zoned pyroxene clast as in Y75011,84, OV: olivine (fayalite) veinlets in pyroxne, SM: shock melt, Na: Na-rich plagioclase present. Isotope ratio (Nd/Sm ratio) after Wooden et al. (1983), Terrestrial age (Terr. age) and cosmic ray exposure age (Exp. age) after Schultz (1986) and Nagao and Ogata (1989). Rare Earth Element (REE) distribution pattern after F'ukuokaand Ikeda (1983).

H. Takeda

40 to the criteria

of MASON et al. (1979). The non-Antarctic polymict eucrites are similar to howarditic polymict eucrites (e.g., Y79 1960 and ALH78006) which will be described later. Detailed studies of the Y74159-type polymict eucrites (Table 2) and comparisons with the Victoria Land collection have been given by TAKEDA et al. (1983b). DELANEY et al. (1984) and WOODEN et al. ( 1983). Some data on the Yamato Y-79 collection are given by TAKEDA and YANAI ( 1982). Y74159-type eucrites are characterized by the presence of mesostasis-rich basaltic clasts (e.g., Y7501 1.84) and inverted pigeonite similar to Binda. The composition of pyroxene fragments in the Y74159-type eucrites varies widely in the pyroxene quadrilateral (Fig. 3). Some reflect igneous zoning trends typical of the basaltic pyroxenes as in Y75011,84. but part of the distribution represents effects of subsolidus exsolution. The zoning trends of basaltic pyroxenes in the Victoria Land and the non-Antarctic specimens are not as extensive as in this type. The mineralogy of Y790266-type polymict eucrites does not show the unique features exhibited by the Y74 159-type, because the range of zoning is smaller than for the Y74 159type (Fig. 3). Y790266 is a clast-rich eucrite with pigeonites (Fig. 4a) partly homogenized by subsolidus reheating. A thin section of Y790 122 that we studied was so rich in clasts that the mineralogy of the matrix portion between the clasts is not well characterized (TAKEDA and YANAI, 1982). TAKEDA and YANAI (1982) have shown that Y 790266 is distinct from the Y74 159-type, but its affinity to Y790260 is not clarified, because the matrix in Y790260 is glassy which is not observed in the Y74 159-type. This glassy appearance is similar to that

of Y792769, but Y792769 is fine grained and clast poor (Fig. 4b). The pyroxene quadrilaterals of these meteorites are given in Fig. 3. The Y790266-type pyroxenes show smaller compositional ranges, indicating partial homogenization. Figure 3 shows that there are three chemical pyroxene trends (represented by Y74159, Y790266, and Y792769) that are distinct and that they belong to three paired groups. Pyroxene compositions of the USA Antarctic polymict eucrites have been summarized by DELANEY et al. (1984). Allan Hills samples do not contain Binda-type components. Yamato 792769 This eucrite shows a compact fine-grained texture with numerous dark veins, visible even on a large cut surface. Lithic clasts are rare, but one light colored coarser-grained lithic clast 2 cm in diameter is observed along one edge of the specimen. The thin section of part of this clast (BSl) shows subophitic textures of pigeonite and plagioclase, but their chemistry is different from those of the Y74159-type (AOYAMA et al., 1987) as given below. Matrix portions of Y792769 show a fine-grained sintered texture lacking very fine fragments (Fig. 4b). In some areas, very fine laths of plagioclase can be seen in the dark brown pyroxene-rich matrices. Pyroxene compositions (Fig. 3) are similar to that ofthe most Fe-rich clasts (e.g., BSl), but exhibit a more calcic trend than BSl The matrix consists of aggregates of pigeonite and plagioclase ranging from several microns to a few tens of microns in size. The matrix crystal is

Yl92769

Y74159-type

Matrices

(1” I

Dl

Cllfi

n

Hd

Fe

Mg

“g

Y192769 Clasts

Y790266-type

01

Ca J

A

Hd

p.:r-"

Mg

Fe

Mg

FIG. 3. Chemical compositions of pyroxene fragments from the three types of Yamato polymict eucrites (Y74 159, Y790266, Y792769) shown in pyroxene quadrilaterals. Small dots are individual analyses; open circles for the Y74 159type are bulk pyroxene compositions obtained by broad beam electron microprobe analysis. Different symbols of Y792769 clasts stand for different clasts.

Fe

Antarctic

FIG. 4. Photomicrographs (crossed nicols); cumulate

and non-Antarctic

achondrites

41

of the Yamato polymict eucrites. Width is 3.3 mm. (a) Y790266; (b) Y792769: (c) Y79 1960 eucrite (Eucl) is at the bottom: ordinary eucrite clast (Euc3) is at top in the middle: and (d)

Y791960, Y75032-type diagonally elongated two clasts

well sintered, with low porosity and well-developed equilibrated grain boundaries (AOYAMA et al., 1987). These features are distinct from the matrices of the Y74 159-type eucrites. Microprobe analyses of pyroxenes in BS 1 (Y792769) show a homogeneous Fe and Mg distribution (Fig. 3) and the data plot along a line between Ca45Mg27.5Fe67.h and Ca42.7Mg24.RFe32.5(solid triangles in Fig. 3). This is one of the most Fe-rich eucrites. The lath-shaped plagioclase crystals exhibit chemical zoning with An ranging from 9 1 to 75. Other types of hthic and mineral clasts are present, but their abundances are lower than those in common polymict eucrites such as Y74 159-type. The Y792769 pyroxenes in these different clasts also show a homogenized trend compared to ordinary eucrites (DELANEY et al., 1984) but the mg numbers (=[ 100 X Mg/(Mg + Fe)]) differ from one clast to another (Fig. 3). One coarse-grained clast has pyroxene compositions around CalOM&FeSO. Pale colored pyroxene fragments show the most Mg-rich composition, CaSMg,,,Fejd (open circles in Fig. 3). The observation suggests that this is a mixture of homogenized clasts of different types. The Y792769 polymict eucrite is distinct from most Yamato polymict eucrites (e.g., the Y74159-type; YANAI and KOJIMA, 1987) in that mesostasis-rich basaltic clasts (e.g.,

Y75011,84) are absent. Instead, ordinary euctite (Table 1) clasts with differently homogenized pyroxene trends are abundant, and fine fragments of the matrix are well-sintered, forming a compact matrix with few clasts. This eucrite is polymict, but the clast types and chemical ranges are limited. Howarditic

Polymict

Eucrites

The review paper of DELANEY et al. (1984) mentioned that six non-Antarctic HEDs (Bialystok, Brient, Jodzie, Macibini, Marvern, Nobleborough, Petersburg), one Allan Hills (ALH78006) and two Elephant Moraine meteorites (79005/ 4) have some affinities to howardites. Most Yamato polymict eucrites of the Y74 159-type have similar modal abundances of minerals, but Y791960 (DELANEY et al., 1984) contains significantly more low-Ca, Mg-rich pyroxene. Y790007 contains few such pyroxenes but is believed to be a pyroxenerich sample of the Y 74 159-group rather than a separate meteorite type (NAGAO and OGATA, 1989). Yamato 791960 Y79 1960 is also reported to be a polymict eucrite (YANAI and KOJIMA, 1987). but it is the only Yamato specimen con-

42

H. Takeda

taining easily detectable diogenitic pyroxene; it contains more than 2 ~01% of diogenitic orthopyroxene. Y79 1962 is reported to be similar to Y79 1960 (YANAI and KOJIMA, 1987; NAGAO and OGATA, 1989). Since Y79 1960 was included previously with a paired fall of the Y74 159-type (DELANEY et al., 1984), we investigated a new thin section (9 l-2) of Y791960 to see if it is paired with the Y74 159-type, or if it is a separate fall similar to non-Antarctic polymict eucrites. Y79 1960,9 l-2 is a polymict pyroxene-plagioclase breccia, similar to Y74 159, with numerous mafic lithic clasts in addition to mineral clasts. However, ,9 l-2 differs in texture from most other Yamato polymict eucrites of the Y74 159-type in that it does not contain large mesostasis-rich basaltic clasts (e.g., Y75011,84) that are typical of the Y74159-type (TAKEDA et al., 1983b). The common eucrite clasts in Y79 1960 are granulitic (Fig. 4c) and are similar to some ordinary eucrite clasts found in howardites (e.g., Y7308; IKEDA and TAKEDA, 1985). MagneSian orthopyroxenes are not common, but are more abundant (2 to 7 ~01%) than in other Yamato polymict eucrites and similar to non-Antarctic polymict eucrites. The largest magnesian (diogenitic) pyroxene found in the PTS is 0.66 X 0.48 mm in size. Another interesting pyroxene-rich clast is similar to the Y75032-type achondrites (TAKEDA and MORI, 1985). Three large cumulate eucritic clasts were found, and one of them is the largest cumulate eucrite clast found in any polymict eucrite (Fig. 4~). These features are rather similar to howardites. The pyroxene compositions in Y79 1960 (Fig. 5) differ from those of Y74159-type polymict eucrites and howardites (DELANEY et al., 1984). Noticeable differences are an absence of chemically zoned pyroxenes of the Y75011,84 type and the small abundance of more Mg-rich pyroxene than in diogenites. Modes of Yamato and non-Antarctic polymict eucrites show that Y791960 has a higher modal abundance of low calcium (52% Ca) orthopyroxene. More than half (55 ~01%) of the pyroxenes are identical to those known from ordinary eucrites (See Table 1), and 35% of them are similar to cumulate eucrites such as Binda, Moama, and Moore County (TAKEDA et al., 1983b). The composition of pyroxene fragments larger than about 100 pm is similar to that of the whole rock except that the former contain more magnesian diogenitic pyroxene. The diogenitic magnesian pyroxenes range from mg number 7077 and represent a pyroxene component not found in other Yamato polymict eucrites. The most Mg-rich pyroxenes of Y79 1960 extend beyond the range of diogenite pyroxenes but not to mg number 85, the most Mg-rich pyroxenes found in howardites. The modal abundance of the diogenitic pyroxene is about 2% as measured by DELANEY et al. (1984). Two pyroxene-rich clasts of the Y75032-type (Fig. 4d) contain very small amounts of plagioclase at the interstices of pyroxene grains. Pyroxene crystals show the exsolution texture of primary orthopyroxene with thin augite lamellae parallel to ( 100). This texture and the pyroxene compositions (Fig. 5) (which are distinctly more Fe- and Ca-rich than in normal diogenites) indicate that the clast is an orthopyroxenite similar to those found in Y75032 and Y791199 (TAKEDA and MORI, 1985). The importance of this observation

Y791960 Matrices DI

Ca n

t

Mg

n

Fe

Y791960 [lasts Di,

Ca

t

Qg

Mg

Fe

FIG. 5. Pyroxene compositions of the howarditic polymict eucrite Y79 1960. Open circles: bulk composition; triangles: exsolved augite; large solid circles: host; small solid circles: individual analyses of the pyroxene fragments in matrices; and squares: diogenitic orthopyroxene. Y79 1960 clasts: compositions of the crystalline clasts. Tie lines connect exsolved augite and bulk and host compositions of pigeonites of the Eucl and Euc3 clasts. Open circles with dots: Y75032-type.

is that Y791960 samples a group of HEDs not known from the non-Antarctic collections. The largest cumulate eucrite clast (Eucl) shows an exsolution texture almost identical to that of the cumulate eucrite, Serra de Mage. Blebby augite of both (100) and (001) orientations has been recognized (Fig. 4~). The pyroxene composition of Eucl is shown in Fig. 5. Clast Eucl is an intermediate cumulate between Moore County and Serra de Mage. The ordinary eucrite clast in Y791960 (Euc3) has a texture similar to those ofgranulitic clasts found in howardites (IKEDA and TAKEDA, 1985). The clast Euc3 contains minerals of uniform composition and pigeonite-augite coexisting grains. The plagioclase compositions of Y791960 are similar to those of the Y74 159-type. Matrix plagioclase ranges from An 89 to 96. The plagioclase compositions of three lithic clasts are: Y75032-type diogenite (An 93 to 96) cumulate eucrite (Eucl 90 to 93), and ordinary eucrite (Euc3: An 90 to 92). The An contents of plagioclase plotted against the mg number of the coexisting pyroxenes suggest a differentiation trend of the crust of their parent body (Fig. 6). Assuming that the three clasts came from the same pluton, the Y75032-type diogenitic clast, the cumulate eucrite clast (Eucl), and the ordinary eucrite clast (Euc3) follow the differentiation trend of known non-Antarctic HEDs and howardites (IKEDA and TAKEDA, 1985). This suggests that Y79 1960 sampled the same differentiated products as did non-Antarctic HEDs and

43

Antarctic and non-Antarctic achondrites the howardite fields. The bulk chemistries vern are within the howardite field.

of Jodzie and Mal-

Diogenites

l

Y191439 40

50 MgxlOO/(Mg+Fe)

60

70

About 36 diogenites with granoblastic textures represented by Y 740 13 were found by TAKEDA et al. ( I98 1) to be pieces of the same fall. Another group represented by Y75032 (I 5 in total) shows unique heavily shocked textures with glassy matrix and more Ca-rich chemistries than common diogenites. The Y75032 group and Binda occupies the compositional gap between non-Antarctic diogenites and cumulate eucrites (Fig. 7) and shows intermediate mineralogical characteristics consistent with a fractional crystallization trend (TAKEDA and MORI, 1985).

Atomtc %

FIG. 6. An mot% = CaXlOO/(Ca+Na+K) vs. mg number = MgX lOO/(Mg+Fe) of plagioclases and pyroxenes. Triangle: clasts in Y79 1960. Differentiation trends of non-Antarctic HEDs (cEuc to Euc and Dio) and the Y75032-type meteorites are also given.

howardites, but that their relative abundances type are different.

in each HED

Howardites In 1983, Y7308 was the only howardite known in Antarctica, but the number of howardites increased after the discovery of Y790727, Y791208. Y791492 (TAKEDA et al., 1984), and many others (32 in total for Y-79), and Y82052 and Y82091, which are distinct from Y7308 (YANAI and KOJIMA, 1987). By 1989, the USA collection contained 17 howardites, for which three paired groups, EET79006, EET87503, and LEW85300 have been suggested (ANTARCTIC METEORITE WORKING GROUP, 1990). Because howardites are breccias of common component materials, it is difficult to identify the differences between Antarctic and non-Antarctic members. Only the relative amounts of the components are expected to vary between the individual specimens. More systematic studies will be required to establish their pairing and abundances. Yamato howardites are not much different from the nonAntarctic specimens, except for Y7308, which contains more magnesian diogenitic components than common howardites. In the Workshop Summary on Antarctic Meteorites in 1985, ANNEXSTAD et al. (1985) used a plot to show the Ca content as a function of the Mg concentration of the HED group and mentioned that the Antarctic meteorites from Elephant Moraine (EET79004, 79005, and 79011) and the Yamato Mountains (Y75032) provide links between these three groups. Y75032 is transitional between diogenites and howardites, and the EET79 polymict eucrites are compositionally intermediate between the eucrite and howardite fields. Y7308 plots into the gap between diogenites and howardites in the CaO vs. MgO diagram (Fig. 7). A eucrite-rich howardite subclass (howarditic polymict eucrites) corresponds to Y79 1960, but there are six such meteorites in the non-Antarctic collection. Their compositions vary widely from the eucrite to

Polymict

Diogenites-Cumulate

Eucrites

Y791073, Y791200, Y791201, and Y791439 belong to the Y75032-type, but contain Mg-rich cumulate eucrite in addition to Fe-rich diogenite components. Y79 1439 is polymitt and contains rare Fe-rich ordinary eucrite clasts with mg = 35. One characteristic of Y791439 is that cumulate eucrite clasts as Fe-rich as Serr$ de Ma& (SM) and Moore County (MC) with larger modal abundances of plagioclase are abundant, and pyroxene fragments from such clasts are also present. The exsolution textures of inverted pigeonites and the chemistry of these clasts are similar to SM. Pyroxene compositions in Y791439 fall in a limited range of mg number 70 to 50. The chemical variation of pyroxenes is wider than that of the diogenite-rich members of the monomict Y75032 group (e.g., Y75032. Y791000, Y791 199, Y 79 1422, Y 79 1466). The Mg-rich end is an Fe-rich diogenitelike pyroxene (CazMh,Fejl) common in the Y75032 group, but the range extends more to the Fe-rich side. Y791439 contains pyroxene clasts (Ca, IMg53Fe36) texturally like that in MC (Fig. I) and small fragments of ordinary eucrites such as Juvinas. The compositions of all plagioclase fragments fall between An 93 to 87 mol%. This range is smaller than that

I

10 -

E

r 0 8

5-

,U 0

5

10

15 MgO

20

25

3(

wt.%

FIG. 7. CaO-MgO-plot (wt%) for eucrites (solid circles), cumulate eucrites (circles with dot), howardites (solid triangles), and diogenites (solid squares) of non-Antarctic collections. Antarctic meteorites provide links between the members of these families. Y75032-type (open squares), Antarctic howardites (open triangles) and howarditic polymict eucrites (half-filled circles), and Antarctic polymict eucrites (open circles) are compared. Shergottite (hexagon).

44

H. Takeda

of the monomict Y75032 group. Chromite crystals are present in pyroxenes and show intermediate to cumulate eucrite trends. The An vs. mg diagram (Fig. 6) shows that the clasts plot along the cumulate eucrite (mg 63.5, An 91.5) to ordinary eucrite (mg 48.7, An 88.0) crystallization trend. This trend contrasts that of the monomict Y75032 group, especially of Y79 1466, which shows An-variation with a nearly constant mg number. Y79 1439 sampled more cumulate eucrite components (of the MC-type), but the Binda-type are also present. Y791439 is more polymict than most of the Y75032 group, because they contain more Fe-rich cumulate eucrites, but its variety of lithic components is more limited, and large lithic clasts are more abundant than in normal howardites. The association of many clasts of only a few types in the Y75032 group suggests that it sampled three or four layers of the layered parent body crust that was produced by crystal fractionation, but did not sample any deeper Mg-rich diogenite source regions. It suggests that the Y75032-group specimens are paired, polymict, and came from a restricted region within the HED body. Both Yamato diogenite types differ from non-Antarctic diogenites. This biased statistics is difficult to interpret, but one possibility is that the Yamato meteorite collection represent meteorite falls within small regions in Antarctica. UREILITES

AND

OTHER

ACHONDRITES

Ureilites Discoveries of Antarctic samples have almost tripled the number of known ureilites. However, most are of small size. Augite- or orthopyroxene-bearing ureilites found in Antarctica fall outside the original definition of a ureilite (TAKEDA, 1987b, 1989; TAKEDA et al., 1988b). All non-Antarctic ureilites described by BERKLEY et al. (1980) contain pigeonite as a major phase, and their compositions vary within a limited range as shown in Fig. 8. Orthopyroxenes in Y791538 and LEW85440 are more magnesian, and augites in Y74 130 and

Hd

kza ..::::~,: ::::::::

Urehte

Pyroxenes

Ca

%I

a 008 0130

0’

0b

”

”

10

20

\

30

-Fe

LO

FIG. 8. Chemical compositions of ureilite pyroxenes plotted in an enlarged portion of the pyroxene quadrilateral. Triangles: magnesian, high I60 group; solid circles: low I60 group; open circles with abbreviated letters: non-Antarctic ones (BERKLEYet al., 1980), including Dingo Pup Donga (DPD), NorthHaig (NH), Novo-Urei (NU), Kenna (K), Haverii (H), Dyalpur (D), and Goalpara (G). The last three digits of the specimen numbers of the Antarctic ureilites are given (TAKEDA et al., 1988b).

5 1 non-Antarctic

s % 5

Antarctic

z” 0 0

5

IO Fe/(Mg+Fe)

15

20 25 Atomic %

FIG. 9. Histogram of Fa contents of core compositions in olivine of Antarctic (TAKEDA et al., 1988b) and non-Antarctic ureilites (BERKLEYet al., 1980). Shaded areas representthe magnesian subgroup.

MET78008 are more Ca- and Fe-rich than the non-Antarctic ureilites. They are all believed to be from separate falls except for ALH82106/130 (TAKEDA et al., 1988b) and EET8751 l/ 523 (ANTARCTIC METEORITE WORKING GROUP, 1990). This situation is in contrast to the Antarctic HED achondrites, where many are pieces of the same fall. The range of chemical compositions of olivines and pyroxenes from the Antarctic ureilites extends towards more Mg-rich, Fe-rich, and Ca-rich components. The range of Fa contents of core olivines extended from 14-22 atomic % to 3-24 atomic % (Fig. 9). It should be noted that large oxygen isotope anomalies were found only in Antarctic ureilites of the magnesian subgroup (CLAYTON and MAYEDA, 1988). An explanation for the scattering of ureilites along an I60 mixing line rather than the fractionation line is given by TAKEDA ( 1989), who attributed it to the anomaly left in refractory magnesian silicates inherited from the carbonaceous chondrite-like materials by a planetesimal-scale collision. The wider range of the mg numbers in pyroxene and correlation with the oxygen isotope subgroups of CLAYTON and MAYEDA (1988) enabled us to classify systematic variations of pyroxene chemistry (TAKEDA, 1989). Three subgroups were identified, with the magnesian subgroup found only in Antarctica. The grouping of ureilites on the basis of both oxygen isotopes and major element chemistry could indicate that these three groups came from different parent bodies. However, this scenario is not likely since the polymict ureilites (WARREN and KALLEMEYN, 1989) contain components of all three groups. A meteorite impact or impacts produce breccias containing different portions of the parent body. The isotopic groups of each component in the ureilite breccias have not been confirmed, but it can be assumed that polymict ureilite regoliths developed on the same parent body and that the three groups represent different locations within one parent body. The oxygen isotope variations can be explained by a mechanism of planetesimal-scale collision discussed above. Aubrites and SNC A comprehensive listing of USA Antarctic meteorites sorted by petrologic type (ANTARCTIC METEORITEWORKING

Antarcticand non-Antarcticachondrites

GROUP, 1990) lists 30 aubrite specimens, which are grouped into three paired falls. No aubrite have been described from the Yamato collection (YANAI and KOJIMA, 1987). Two SNCs, ALH77005 and EET79001, have been given in the above listing. The number of samples in these groups is too small to discuss any difference to non-Antarctic collections. DISCUSSION To obtain a better understanding of the differences between Antarctic and non-Antarctic meteorites, their sources, and their relation to asteroids, the pairing information on several Antarctic achondrites studied in this work will help to provide information on the parent body. The first comparisons of Yamato and Victoria Land eucrites were given by TAKEDA et al. ( 1983b) and DELANEY et al. ( 1984). MASON et al. ( 1979) provided chemical analyses for 20 non-Antarctic eucrites and howardites and brief accounts of these meteorites. Mineralogical comparison of Antarctic and non-Antarctic HED achondrites indicated that their components may be classified by the degree of homogenization of the pyroxene (TAKEDA et al., 1983a). Most non-Antarctic monomict eucrites are the product of thermal metamorphism. NYQUIST et al. (1986) and TAKEDA and GRAHAM ( 199 1) proposed a model to produce non-Antarctic monomict eucrites and polymict eucrites on their parent body by impacts. A comprehensive listing of USA Antarctic meteorites sorted by petrologic type has been given by the ANTARCTIC METEORITE WORKING GROUP ( 1990).

Some characteristic features of particular Antarctic achondrites have been pointed out in the chapter on each achondrite class. The current state of some characteristics of the Antarctic (mainly Yamato) achondrites on the basis of pairing (Table 2) can be summarized as follows:

1) The number of polymict

2)

3)

4)

5) 6) 7)

eucrites recovered in an early stage of the search for Antarctic meteorites was high, but discoveries of many howardites in recent years and recognition of pairing among polymict eucrites have changed the statistics. Polymict eucrites excluding those with a similarity to howardites (e.g., Y791960, Macibini etc.) have not been found in the non-Antarctic collections. Textures, mineral chemistry, and cooling rates of pyroxenes in some Antarctic monomict or unbrecciated eucrites are different from those of the non-Antarctic ones. They fill the gap or extend the ranges known in the non-Antarctic members. In comparison with 8 diogenites in the USA collection and 9 in non-Antarctic group, the more than 50 diogenite specimens in the Yamato collection can be grouped into only two fails of separate locations. Among 30 Antarctic ureilites, in which small sized specimens are common, only 2 pairings have been recognized. Magnesian ureilites and augite-bearing ureilites were found only in Antarctica. About 30 aubrites were grouped into three groups.

In spite of the chemical and textural differences between Antarctic and non-Antarctic HED achondrites, their mineral and clast components are similar. The components can be

45

arranged in the order of mg number and exsolution and inversion textures. The difference between polymict eucrites and howardites is artificial as both represent portions of a continuous spectra of breccia. Although some Antarctic eucrites are unusual with respect to their pyroxene chemistry or exsolution and inversion textures, such variaties just fill the gap between the non-Antarctic collections or extend the known ranges. The differences are mainly in proportions of the components and in brecciation, shock, and homogenization textures. These facts suggest that there was geologic activity on the parent body which allowed a period of development of different kinds of regolith and fragmental breccias, and that the different type of breccias at different locations were excavated or fragmented and came to Earth at different times. It should be noted that polymict eucrites have older terrestrial ages than non-Antarctic eucrites, but not the oldest among the Antarctic HED achondrites (Table 2; SCHULTZ, 1986; NAGAOand OGATA, 1989). The Y790266 and Y792769 polymict eucrites show chemical trends and lithic clast-types less variable than the Y74159-type (Fig. 3). Although these partly homogenized eucrites are polymict, they have some features similar to monomict eucrites. NYQUIST et al. (1986) and TAKEDA and GRAHAM (1991) proposed a model for formation of monomict eucrites from surface lava-like eucrites. The model suggests that they may be derived from near the floors or walls of craters. In such regions, some clasts could be thermally metamorphosed by the residual heat of the impact, and the matrix was sintered and annealed in a thick hot ejecta blanket while Mg-rich pyroxene cores are partly preserved. The Y790266 and Y792769 groups may be in an intermediate stage from polymict eucrites towards monomict eucrites. The clast types and their compositional trends in Y79 1960 revealed that they have a howarditic character. However, the bulk composition as plotted in a CaO vs. MgO diagram (Fig. 7) is still close to the eucrite region. Another possibility is that their CaO content is just lower than those of eucrites, in agreement with the presence of detectable diogenitic components. If the nomenclature of DELANEY et al. (1984) is followed, then Y79 1960, ALH78006, and the non-Antarctic specimens, Bialystok, Macibini, and Nobleborough are polymict eucrites. However, the texture, clast types, and the distribution of pyroxene compositions are so similar to howardites that it Seems difficult to equate Y79 1960 with the Y74 I59-type. Since they were previously classified as howardites (DELANEY et al., 1984), there is no true polymict eucrite (such as Y74159and ALH79006-types) in the non-Antarctic collection. The parent material of Y791960 should be the same as for the other Yamato polymict eucrites, because the amount of the diogenitic components is expected to vary from one location to the other in the parental body. However, the absence of lava-like eucrites with zoned pyroxene is an important difference between polymict eucrites and the howarditic groups. It should be noted that the differences between Antarctic and non-Antarctic achondrites result from different factors. The large numbers of smaller specimens may account for higher abundances of polymict eucrites, ureilites, and primitive achondrites as suggested previously (TAKEDA, 1986). The presence of only two distinct diogenite showers among

46

H. Takeda

the Yamato collection indicates that they represent two falls at specific times. The fact that specimens distribute close together and that large masses are located at the end (See Fig. 2 of TAKEDA et al., 198 1) suggests that the Y740 13-type diogenite (A group) represent the remnant of a strewn field on a snow field that was modified by glacial movement. The location of the Y740 13 group is about 45 km S of the field of the Y75032-type (B group) as shown on the meteorite location map (TAKEDA and YANAI, 1982). Two tight groupings at different locations indicate that two falls occurred at different locations and time intervals. Four groups of polymict eucrites from the Yamato meteorite ice field have been found from a geographically limited area. The Y74 159-type eucrites (Eucrite A) were collected about 36 km SW from the location of Eucrite B, which includes the Y-79 eucrites with higher sample numbers (TAKEDA and YANAI, 1982). These four groups show different terrestrial ages (Table 2). Three groups of polymict eucrites have been found at the Allan Hills as well as three groups at the Elephant Moraine, but no detailed descriptions are available. Because it is expected that impacts on or collisions of parent bodies may yield fragments from different orbits, we cannot exclude the possibility that a few Antarctic meteorites provide us with samples from parent body portions not known from the contemporary non-Antarctic meteorites. If so, such samples may help us to reconstruct the parent bodies for genetically related meteorites. To obtain a better understanding of the parent sources and their relation to asteroids, the synthesis of the parent bodies of Antarctic achondrites meteorites should be continued, since not all samples have yet been investigated in detail. We hope to incorporate data from the Victoria Land samples in order to obtain information on all meteorites. In the case of the HED achondrites, because of the signatures on crystallization and cooling histories present in the pyroxenes, it has been relatively easy to locate their relative burial depths on the parent body. Although many features of the HED samples are also present in non-Antarctic samples, some Antarctic meteorites with intermediate features can be used to relate the two apparently different groups. Ureilites show characteristic oxygen isotope abundances related to the mg numbers. This information was essential in order to propose an origin of ureilites by planetesimal-scale collisions (TAKEDA, 1989). The discovery of the Vesta-like surface materials on nearEarth asteroids, 19 15 Quetzalcoatl (MCFADDEN et al., 1982), 1980PA, 1985D02, and 3551 (1983RD) (CRUIKSHANK et al., 1989) suggests that parts of these different bodies can actually fall on Earth. It has been pointed out that the time scale of evolution of asteroid orbits is longer than the oldest terrestrial ages of the Antarctic meteorites (WETHERILL, 1989). However, the possibility that small-sized fragments from near-Earth asteroids will be subjected to larger gravitational effects might help US to explain the differences between Antarctic and non-Antarctic meteorites. In summary: 1) Polymict eucrites (excluding howarditic varieties) not been found in non-Antarctic collections.

have

2) Magnesian ureilites and at&e-bearing 3) 4)

5)

6)

ureilites were found only in Antarctica. The differences may not be attributed to just one factor. Evidence of two unique Yamato diogenite falls on two ice fields in comparison with 8 separate falls in the USA collection indicates that these local variations are difficult to be treated by statistics. The discovery of many small unusual eucrites and ureilites suggests that the large collection of small specimens from the ice fields may also contain more unusual varieties. In any event, Antarctic achondrites provide us with useful information on their parent bodies.

Acknowledgments-The author is indebted to the National Institute of Polar Research, the Meteorite Working Group and the British Museum (Natural History) for meteorite samples; Drs. C. Koeberl, J. S. Delaney, and D. W. Mittlefehldt for critical reading of the manuscript; Profs. W. K. Hartmann, M. Lipschutz, M. Masuda, Drs. K. Nagao, P. H. Warren, K. Yanai, K. Tomeoka, A. L. Graham, H. Mori, H. Shimizu, H. Kojima, and Miss A. Ogata for discussion. Some Yamato eucrites assigned to Dr. T. Tagai have been studied with the help of Dr. H. Mori, Mr. T. Aoyama, Mr. K. Saiki, and Mrs. H. Oaata. We thank Mr. 0. Tachikawa. Mr. H. Yoshida. Mrs. M. Hatan;, and Mrs. K. Hashimoto for their technical assistance. This work was supported in part by the Grant-in-Aid for Scientific Research of the Japanese Ministry of Education, Science, and Culture and by the Mitsubishi Science Foundation. Editorial handling: C. Koeberl REFERENCES ANNEXSTADJ. O., SCHULTZL., and W~~NKEH. (1985) Workshop Summary. In International Workshop on Antarctic Meteorites (eds. J. 0. ANNEXSTADet al.); LPI Tech. Rept. 86-01, pp. 1I-18. Lunar and Planet. Institute, Houston. ANTARCTICMETEORITEWORKINGGROUP (1990) Comprehensive listing of meteorites sorted by petrologic type. Antarctic Meteor. Newslett. 13, No. I, pp. 84-86. AOYAMAT., TAKEDAH., and MORI H. (1987) The geologic setting of the partly homogenized Yamato 792769 polymict eucrite on the HED parent body. Meteoritics 22, 3 17-3 18. BASALTICVOLCANISMSTUDYPROJECTS(1981) Basaltic Volcanism on the Terrestrial Planets. Pergamon Press. BERKLEYJ. L., TAYLORG. J., &IL K., HARLOWG. E., and PRINZ M. (1980) The nature and origin of ureilites. Geochim. Cosmochim. Acta44, 1579-1597. CLAYTONR. N. and MAYEDAT. K. (1988) Formation of ureilites by nebular processes. Geochim. Cosmochim. Acta 52, 13 13-I 3 18. CRUIKSHANKD. P., HARTMANNW. K., THOLEND. J., BELLJ. F., and BROWN R. H. (1989) Basaltic achondrites: Discovery of source asteroids (abstr.). Meteoritics 24, 260. DELANEYJ. S., PRINZ M., and TAKEDAH. (1984) The polymict eucrites. Proc. 15th Lunar Planet. Sci. Corzf: J. Geophys. Rex 89, C25 l-C288. EUGSTERO., NIEDERMANNS., BURGER M., KRAHENBUHLU., WEBERH., CLAYTONR. N., and MAYEDAT. K. (I 989) Preliminary report on the Yamato-86032 lunar meteorite; III Ages, noble gas isotopes, oxygen isotopes and chemical abundances. Proc. NIPR Symp. Antarctic Meteorites 2, 25-35. FUKUOKAT. and IKEDAK. (1983) Chemical compositions of 5 Yamato polymict eucrites. Abstr. 8th Symp. Antarctic Meteorites, p, 49. NIPR, Tokyo. HARLOWG. E. and KLIMENTIDISR. (1980) Clouding of pyroxenes and plagioclase in eucrites: Implication for post-crystallization processing. Proc. I Ith Lunar Planet. Sci. Conj. I I3 I - 1143. HARLOWG. E., NEHRU C. E., PRINZ M., TAYLORG. J., and I&IL K. (1979) Pyroxenes in Serrl de Magt: Cooling history in comparison with Moama and Moore County. Earth Planet. Sci. Lett. 43, 173-181.

Antarctic and non-Antarctic achondrites H. ( 1985) A model for the origin of basaltic achondrites based on the Yamato 7308 howardites. Proc. 15th

IKEDA Y. and TAKEDA

Lunar Planet. Sci. Conf,

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KOEBERLC., WARRENP. H., LINDSTROMM. M., SPETTELB., and FUKUOKA T. (1989) Preliminary examination on the Yamato86032 lunar meteorites: II, Major and trace element chemistry. Proc. NIPR Symp. Antarctic Meteorites

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MASONB., JAROSEWICHE., and NELENJ. A. (1979) The pyroxeneplagioclase achondrites. Smithsonian Contrib. Earth Sci. 22, 2145.

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