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

The collection of micrometeorites in the Yamato Meteorite Ice Field of Antarctica in 1998

In austral summer season of 1998,we collected micrometeorites at the Meteorite Ice Field around the Yamato Mountains, Antarctica. It was the first attempt to collect micrometeorites at inland bare ice in Antarctica and to use a filter with 10μm openings; it can capture particles that overlap in size with interplanetary dust particles collected in stratosphere. At the inland bare ice of the Antarctic Continent, an ancient flux of extraterrestrial dust is thought to be preserved in contrast with bare ice along the shore of the continent. For collecting micrometeorites from the bare ice, we used a tented sledge equipped with appliances for melting ice and filtering the melted water. We melted ∿ 36 tons of ice and obtained particles containing micrometeorites at 24 points in three areas of the Meteorite Ice Field

Sulfide textures of a unique CO3-chondrite (Y-82094) and its petrogenesis

The unique CO3-chondrite Y-82094 contains abundant FeNi metals. FeNi metal grains are often surrounded by troilite. More than one hundred troilite rimmed FeNi metals have been found in three polished thin sections of Y-82094 (CO3). These structures are frequently found in all CO3s. The troilite rim was formed by the reaction of FeNi metal in the core with S-rich gas. However, there are some differences in the metal-sulfide association between Y-82094 and other CO3s. In Y-82094,rims include two features that have not been observed in other CO3s : fizzed troilite and deviation of a reaction rim. In addition, FeNi metal cores include abundant large phosphates (∿10μm). In Y-82094,there is a large discrepancy of subtype classification between 3.0 obtained from variation in olivine composition, and 3.5 determined by thermoluminescence (TL). These features could be explained by shock metamorphism accompanying shock melting and subsequent rapid cooling. The shock effects were particularly effective for opaque mineral assemblages but not effective for silicates. Y-82094 has other unique features which cannot be explained by shock metamorphism; (1) mean chondrule size of Y-82094 being larger than those of CO3s irrespective of low petrologic subtype from olivine random analyses, and (2) unique bulk chemical composition. Thus Y-82094 is a unique CO3. This could mean that the formation and thermal history of Y-82094 is different from other CO3,implying that Y-82094 does not come from a different parent body from that of the other CO3s

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

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

On the relationship between troilite and/or magnetite rimmed FeNi metals and subtype in CO3 chondrites

A lot of troilite and/or magnetite rimmed FeNi metal grains have been found in 22 CO3 chondrites. The morphology of these grains is the most characteristic in opaque mineral assemblages in CO3s. These could be formed by reactions of FeNi metals with S-rich and/or O-rich gas. The number density of rimmed FeNi metals are correlated with subtype of CO3s. The grain size and the rim thickness of these grains are not significantly correlated with subtype. Magnetite is dominantly found in lower subtype (<3.2) and troilite is abundant but magnetite does not occur except Isna (3.6) and Ornans (3.3) in higher subtype (<3.2). In the subtype less than 3.2,troilite as inner rim and magnetite as outer rim could coexist for some rimmed FeNi metals (ALH-77307 and Y-81020). These textural variations were not formed by one series of thermal metamorphism but formed by (1) the differences of O/S conditions at the time of thermal metamorphism on the parent body, (2) oxidation from intermediate subtype to lower type and sulfidation from intermediate subtype to higher subtype, or (3) thermal metamorphism of rimmed FeNi metals especially in chondrules enclosed in mafic silicates at lower subtype formed in the solar nebula

ダイ44ジ ナンキョク チイキ カンソクタイ エットウ ホウコク 2003-2004

第44次南極地域観測隊越冬隊(第44次越冬隊)は,昭和基地とドームふじ観測拠点に別れて越冬した.越冬隊長小島秀康以下31名およびオブザーバー4名が昭和基地で越冬した.また,越冬副隊長大日方一夫以下7名がドームふじ観測拠点において越冬し,第VI期5か年計画の2年次の観測ならびに設営活動を行った. 昭和基地の運営は2003年2月1日から開始し,2004年1月31日をもって終了した.この間定常観測,モニタリング研究観測を継続して行うと共に,宙空系,気水圏系,地学系,生物・医学系のプロジェクト研究観測を実施した.また,設営関係では電力,上下水道,燃料,通信,食料,医療といった生活基盤の維持管理に加え,車両整備,機械設備工事,航空機の運用ならびに滑走路のメンテナンス,LANの運用,野外観測支援など多くの作業を行った. 2月中旬から4月末にかけて流失をくり返していたオングル海峡の海氷が極夜明けの8月はじめに流失した.そのため南方への野外行動は10月初めになった.しかし海氷が安定した10月から11月に地震,GPS観測,ペンギンセンサス等多くの沿岸調査旅行を実施した. 第44次隊では日本放送協会(NHK)のスタッフが同行者として越冬して南極の自然や隊の活動の映像を国内に送り続けた.The 44th Japanese Antarctic Research Expedition (JARE-44) wintering party conducted the VIth five-year JARE program from February 1st 2003 to January 31st 2004 at both Syowa and Dome Fuji Stations. Thirty-six members at Syowa Station and 8 members at Dome Fuji Station were engaged in the various scientific and logistic activities. Many observation programs in meteorology, upper atmospheric physics, atmospheric sciences and glaciology, geophysics and biology and medical science were carried out in addition to logistic activities such at Syowa Station. As sea ice in Ongul Strait was unstable until early August, the start of the field activities in the southern coastal area was delayed until early October. However, many field teams engaged in seismic, Global Positioning System (GPS) observations and a penguin census study made observations around the coastal area of east Lutzow-Holm Bay in October and November when sea ice was stable

Weathering of some Antarctic meteorites: Infrared spectroscopy

We studied the degree of weathering of some Antarctic meteorites using the integrated intensity of absorption bands near 3μm to enlarge the data base of the degree of weathering. There is no clear correlation between the integrated intensity and the degree of weathering on the A-B-C scale. A rough correlation can be seen between the integrated intensity of absorption bands near 3μm and the total amount of H_2O in a meteorite analyzed by a standard wet chemical analysis method. Some Yamato meteorites measured show weaker integrated intensities than the Allan Hills meteorites measured. Our method to determine the degree of weathering may be useful for choosing less weathered meteorites from among the Antarctic meteorite collection

Yamato-82162: A new kind of CI carbonaceous chondrite found in Antarctica

A petrographic and mineralogical study of the Yamato-82162 (Y-82162) meteorite shows that it may be the first CI carbonaceous chondrite that has ever been found in Antarctica. Y-82162 consists largely of fine-grained, phyllosilicaterich matrix and contains a large amount of Ni-bearing pyrrhotite and small amounts of framboidal, platy, spheroidal, and anhedral magnetite, Mg-Fe-rich carbonates, and Ca-phosphate. Isolated clusters of coarsely crystallized phyllosilicates are also present. The presence of these minerals supports the idea that Y-82162 is a CI carbonaceous chondrite. However, this meteorite shows several mineralogical features that apparently differ from non-Antarctic CI chondrites : it has much higher abundances of coarse phyllosilicates and pyrrhotite than non-Antarctic CI chondrites, and has no veins of sulfates and carbonates, suggesting that it was derived from different primary materials and has experienced a different aqueous alteration history from non-Antarctic CI chondrites. The Y-82162 matrix also contains abundant fine grains of olivine, which contrasts with non-Antarctic CI chondrites. The textures suggest that matrix phyllosilicates were dehydrated and altered to olivine by heating. Thus, this meteorite probably has been affected by mild thermal metamorphism. These results indicate that Y-82162 has experienced a distinct late history from the non-Antarctic CI chondrites, suggesting that it may have been derived from a different source from that of non-Antarctic CI chondrites

Origin of metal-troilite aggregates in six ordinary chondrites

Coarse metal-troilite aggregates several mms in sizes have been studied in six ordinary chondrites, Yamato (Y)-794006 (L4), Y-793211 (L6), Y-793213 (L6), Y-791629 (H4), Y-791686 (H5), and Y-791555 (H6). In each sample, textures of an aggregate and host meteorite show an evidence for the aggregate being solidified from metal-sulfide melt under a slow cooling condition. Metal and silicate texture in the host meteorite suggest a formation of the metal-sulfide melt by weak or moderate reheating by an impact process on the parent body. Bulk Fe-S compositions of the aggregates show variations in melting temperatures which correlate with petrologic grades of the host meteorites. These evidences suggest that the aggregates formed by impact melting on the "hot" parent bodies by weak or moderate reheating piled on the pre-impact temperatures during thermal metamorphism