The Asuka-87 and Asuka-88 collections of Antarctic meteorites: Preliminary examination with brief descriptions of some typical and unique-unusual specimens

Preliminary examinations were carried out of over 2400 pieces of the new Asuka meteorites after the initial processing at the NIPR. According to the initial processing, the Asuka-87 meteorite collection consists of one iron, one stony-iron, 9 achondrites, 3 carbonaceous meteorites and over 300 chondritic specimens. The Asuka-88 meteorites collection comprises 7 irons, 5 stony-irons, over 50 achondrites, 31 carbonaceous meteorites and over 2000 chondritic specimens. Through preliminary examinations with a polarizing microscope and electron microprobe, one mesosiderite, 2 ureilites, 2 diogenites (possibly paired) and 2 eucrites were identified in the Asuka-87 collection. In the Asuka-88 collection, one unbrecciated gabbro and one olivine-fassaite basalt were examined. These uniqueunusual specimens were determined to be a lunar mare gabbro and an angritetype achondrite. Diogenites, ureilites, and one fine-grained, one coarse-grained and one porphyritic eucrite were also newly identified in the Asuka-88 collection, together with a pallasite, a mesosiderite and CM, CV, CO chondrites. Some of the typical and unique-unusual specimens of the Asuka-87 and Asuka-88 collections are briefly described here

Angrite Asuka-881371: Preliminary examination of a unique meteorite in the Japanese collection of Antarctic meteorites

A new angrite-type achondrite was discovered at a new site of meteorite concentrations in Antarctica by the Japanese expedition party in December 1988. Antarctic meteorite Asuka-881371 (previously Asuka-9) is an angrite-type achondrite, which is a rounded individual stone, almost completely covered with a dull-black fusion crust. Pale green, relatively large porphyritic olivine crystals can be seen on the exposed face of the interior. Asuka-881371 is an unbrecciated, igneous rock with typically ophitic (doleritic) texture, consisting mainly of olivine, pyroxene (fassaite) and plagioclase with spinel. From its texture, mineral assemblages and chemical composition, Asuka-881371 is identified and classified as an angrite. However, based on several characteristics, especially bulk chemical composition, Asuka-881371 is different from Angra dos Reis, LEW86010 and LEW87051 angrites which are the three previously known angrites collected from non-Antarctic and Antarctic regions

Yamato-793592: The first enstatite achondrite (aubrite) in the Yamatometeorite collections

The first aubrite in the Yamato meteorite collections has been identified as a result of the recent initial processing at the National Institute of Polar Research (NIPR), Tokyo. Yamato-793592(Y-793592) is the only aubrite recovered so far among about 6000 classified specimens of the Yamato collections. Y-793592 is a friable small stone weighing 32g, with a white interior covered by a small amount of black fusion crust. It shows the typical brecciated texture and consists mostly of larger angular, white pyroxene (enstatite), less plagioclase and traces of olivine, with some opaque grains (blackish brown color). The constituent minerals of Y-793592 are almost pure Mg-pyroxene (enstatite), pure Mg-olivine (forsterite), sodic plagioclase, and small amounts of opaque phases such as Ni-poor metal, daubreelite, and troilites with highly variable compositions

Yamato-74063: Chondritic meteorite classified between E and H chondrite groups

Yamato-74063 (Y-74063) was found in Antarctica by the Japanese Antarctic Research Expedition in November 1974. Y-74063 is an almost complete, smoothly rounded stone weighing 35.4g covered with brownish-black fusion crust. The thin section shows that this meteorite has generally poorly traced chondritic texture and "chondrules" merge into the recrystallized matrix. Compositions of olivine and low-Ca pyroxene are homogeneous and average Fa_ and Fs_ respectively. These compositions strongly suggest that Y-74063 is not similar to all the previously known chondrites. Bulk analysis shows that the total iron content of Y-74063 is the lowest of the ordinary chondrite groups, and the abundance of troilite is much higher than those of all ordinary chondrites. Texture, bulk and mineral compositions of Y-74063 indicate that this meteorite is identified as chondrite and classified into a new type of chondrite group which is between E and H chondrite groups. Y-74063 is similar to Acapulco, ALH-77081 and ALH-78230 in mineral composition which occupies the intermediate site between the E and H chondrites. But the latter 3 chondrites have no evidence of chondrules in spite of "chondritic" texture, mineral assemblage and compositions. Bulk composition indicates that Y-74063 differs from Acapulco-type meteorites and all previously known chondrite groups. The presence of Y-74063 suggests that there is a great possibility of the existence of more unknown meteorite types in Antarctica, and non-Antarctic regions

Varieties of lunar meteorites recovered from Antarctica

By the 1988-1989 field season, more than 10 specimens of lunar meteorites have been recovered in Antarctica by the U. S. and Japanese Expeditions. The specimens from the Yamato Mountains, Allan Hills and MacAlpine Hills (Y-791197,Y-82192/193,Y-86032 [1], ALHA81005 [2], MAC88104/105) are all plagioclase (anorthite)-rich breccias from the lunar highlands. Y-793274 is a pyroxene-and plagioclase-rich breccia, and EET87521 [5] is basaltic clast-rich breccia : Both contain abundant components from the basaltic provinces (the maria) of the lunar crust. Asuka-31 and Y-793169 are unbrecciated, coarsegrained rocks consisting mainly of pyroxene and plagioclase (maskelynitized), together with ilmenite and troilite. The bulk compositions of Asuka-31 and Y-793169 are very similar to low-titanium and very low-titanium (VLT) lunar mare basalts. Oxygen isotope data strongly support the lunar origin of all the Antarctic lunar meteorites. The FeO/MnO ratios are consistent with the range of those of lunar pyroxenes, which are markedly different from those of basaltic achondrites. However, they contain a wide range of pyroxene compositions. The lunar meteorites have originated from several different places on the Moon surface, comprising single rock facies or monomict-polymict breccia facies. On the basis of lithology, texture, petrography, chemistry and mineral compositions, the lunar meteorites can be divided into 4 or more different types : namely anorthositic breccias (including 3-4 different facies), basaltic-anorthositic breccias, basaltic breccias, and unbrecciated diabase and gabbro. These types indicate that the samples might have originate from at least 7 different sites on the near-and far-side sites of the Moon. The different types of lunar meteorites strongly suggest that there are other unknown rock type (s) on the Moon, and that new meteorite types are to be expected in Antarctica

Yamato-8451: A newly identified pyroxene-bearing pallasite

Yamato-8451 is an unusual meteorite which is newly identified as a pyroxene-bearing pallasite. It consists mainly of Fe-Ni metal, olivine and pyroxene, and shows a porphyritic texture typical of most olivine, the same texture as that of all known pallasite meteorites. Yamato-8451 might have been classified as one of the common pallasites because of its exotic texture and mineral assemblage, a unique feature of pallasites, but it is highly unusual because it contains pyroxene. Most olivine and pyroxene crystals are well rounded to subrounded, but angular grains and grain aggregates are also recognized in the metallic host. Pyroxenes are mm-sized and consist of three different types in Ca content. These are : (1) exceptionally Ca-poor polysynthetic twinned orthopyroxene (under Wo 1.0), (2) Ca-poor non-polysynthetic twinned orthopyroxene (Wo 1.0-3.5) and (3) clinopyroxene (over Wo 40). The olivine composition is Fo 89.6 on average, the most magnesian known in all pallasites

40Ar-39Ar analyses of Y-74063 and ALH-78230: Consortium study onunique meteorites from Antarctica

As part of a consortium study on unique meteorites from Antarctica, ^Ar-^Ar analyses were performed for two meteorites Y-74063 and ALH-78230,which are related to lodranite and the Acapulco meteorite. Y-74063,93 shows an ^Ar-^Ar plateau age of 4556±53 Ma, containing about 10^cm^3STP/g trapped ^Ar. ALH-78230,55 also indicates an ^Ar-^Ar plateau age of 4531±23 Ma in the higher temperature fractions, but the lower temperature fractions suggest the occurrence of a later degassing event around 400-500 Ma. Trapped ^Ar is about 10^cm^3STP/g. These results indicate that the two meteorites Y-74063 and ALH-78230 experienced different thermal histories

Yamato-86720: A CM carbonaceous chondrite having experienced extensive aqueous alteration and thermal metamorphism

A petrographic and mineralogical study of Yamato-86720 (Y-86720) shows that it may be a CM carbonaceous chondrite that has experienced a considerably different alteration history from most CM chondrites. Y-86720 has an unusually high abundance of troilite (～9 vol%); most troilite occurs in submicron to micron grains dispersed throughout the meteorite. Ca-Mg carbonates and minor Fe-Ni metal, mostly taenite, are present, but magnetite and PCP are absent. This meteorite contains chondrules and aggregates that were completely replaced by optically translucent materials, presumably phyllosilicates. Thus Y-86720 was probably extensively affected by aqueous alteration; the degree of alteration may be the highest of the CM chondrites. Matrix consists largely of fine grains of Mg-Fe olivine, a nearly amorphous Si-Mg-Fe-rich material, and an Fe-rich material; the latter may be ferrihydrite. Phyllosilicates are rare. The textures suggest that the olivine and the Si-Mg-Fe-rich material were produced by alteration of phyllosilicates by heating. The replacement products of chondrules and aggregates show lower (Mg+Fe)/(Si+Al) ratios than serpentine and relatively high Na contents (up to 2.0wt% as Na_2O), suggesting the presence of not only serpentine but another type of phyllosilicate. However, the replacement products show consistently high analytical totals; thus they may have been dehydrated and partially altered to anhydrous phases. These results suggest that Y-86720 has experienced mild thermal metamorphism after the aqueous alteration; it appears to have been heated above 500℃ in a reduced condition. Most fine-grained troilite probably segregated from matrix during the metamorphic process. Many of the troilites are replaced by a ferrihydrite-like material; thus, Y-86720 may have been affected by additional mild aqueous alteration after the thermal metamorphism

The Asuka-87 and Asuka-88 collections of Antarctic meteorites: Search, discoveries, initial processing, and preliminary identification and classification

Over 2400 pieces were collected by the Asuka wintering party of the 29th Japanese Antarctic Research Expedition (JARE-29,1987-89) on the bare icefields around the S∅r Rondane Mountains in Queen Maud Land, East Antarctica. The Asuka party searched almost all of the bare icefields around the mountains in the 1987-88 and 1988-89 field seasons. About 110 pieces (Asuka-87) of meteorites were collected on the bare ice around Mt. Balchen in the eastern part of the mountains in January and February 1988. In the first reconnaissance, about 240 pieces of meteorites (also Asuka-87) were recovered from the Nansenisen Icefield in February and March 1988. The Asuka-88 meteorites, over 2100 pieces, were found during the systematic search of this icefield during November 1988 and January 1989. The specimens were named officially the Asuka (A)-87001 to A-87352,and A-880001 to A-882124,in order of discovery. The Asuka-87 and Asuka-88 meteorite collections were filed as the National Institute of Polar Research (NIPR) meteorites with details of date of find, weight, dimensions and comments. According to the initial processing, the Asuka-87 meteorites comprise one iron, one stony-iron, 9 achondrites, 3 carbonaceous chondrites and over 300 ordinary chondrites, the total weight being 120kg. The largest specimen in the Asuka-87 collections is an LL-group chondrite of about 46kg. The Asuka-88 meteorites comprise 7 irons, 5 stony-irons, over 50 achondrites, 31 carbonaceous chondrites and over 2000 ordinary chondirtes. The total weight is about 400kg. Two specimens in the Asuka-88 collection were tentatively identified as a very coarse-grained and unbrecciated gabbroic meteorite and an olivine-fassaite-plagioclase achondrite with crystalline texture