Antarctic primitive achondrites Yamato-74025, -75300, and -75305:Their mineralogy, thermal history and the relevance to winonaite

Three Antarctic primitive achondrites, Yamato (Y)-74025,-75300,and -75305 were mineralogically and chemically studied. They consist of anhedral to subhedral silicate and opaque minerals. The major constituent minerals are typical of equilibrated ordinary chondrites. However, they do not have any relic of chondrule, and the presence of various accessory minerals, such as K-feldspar, schreibersite, daubreelite, phosphate, Nb-bearing rutile, and magnesiochromite, characterizes these meteorites. Y-75305 has a composite grain containing Cu, Mn, and S, probably consisting of alabandite, an unknown Mn-bearing Cu-sulfide, and digenite. Y-74025 has a REE pattern typical of chondrite. Siderophile elements in Y-74025 are depleted relative to Cl chondrites, which is consistent with poor abundance of Fe-Ni metal in Y-74025. Holocrystalline texture, homogeneous mineral compositions, and high equilibration temperatures for pyroxenes, suggest that these primitive achondrites experienced high-temperature metamorphism. Mineralogical and chemical characteristics suggest that they resemble Winona-like meteorites (winonaites). The compositions of pyroxene and olivine, and accessory minerals suggest that winonaites formed under an intermediate redox condition between E-chondrites and Acapulco-like primitive achondrites. The abundance of troilite and Fe-Ni metal varies widely. The metal-sulfide fractions of winonaites probably melted and fractionated, although silicate fractions of winonaites do not have any evidence for melting

Recent progress in Tephra Study

Significant progresses in recent studies on tephra are reviewed. In relation to volcanological aspects of tephra, recent works on mechanism of vesiculations and disruption of magma, formation of eruption column, transportation of tephra, recognition of types and magnitude on eruptions, and roles of interaction between external water and magma are discussed. The progresses are mainly based on the intensive researches on the recent eruptions observed by volcanologists such eruptions as the 1980 St. Helens, the 1977 Usu and so on. Attempts on quantitative understanding of eruption phenomena such as total eruption volume, initial population of grain-size, and dispersal/fragmentation values of tephra are discussed. Recent studies on phreatomagmatic eruptions on the basis of the experimental researches of magma/water interactions and field observations are remarkable progresses in volcanological studies. In relation to tephrochronological aspects, recent works on identification techniques of tephra layers, discovery of wide-spread tephras, dating methods of tephras, and applications of tephrochronology are summarized. Identification techniques of tephra layers based on determinations of refractive indices and major, minor and trace element compositions of glass and/or minerals were highly advanced in the last 10 to 20 years. Those are effective tools for identification of wide-spread tephras. Wide-spread tephras such as AT ash from Aira Caldera, K-Ah ash from Kikai Caldera, B-Tm ash from Baegdusan Volcano and others were recognized in Japanese Islands and also in deep-sea sediments around Japan. They were dated by radiometric and stratigraphic dating methods. Radiometric dating methods applicable to the late Quaternary tephras are ^C, Fisson Track, Ionium, K-Ar, Thermoluminescence, and Electron Spin Resonance methods. Because the dates of tephras were mostly obtained by ^C dating, the radiometric ages of tephras over 40000 years are lacking so far. The accurate radiometric ages of tephras for this range are strongly required for further developments of tephra studies. It is stressed that tephra studies will play great roles on establishment of eruption histories of poligenetic volcanoes, activity aspects of one cycle eruptions, life time of magma and zoning of magma chamber

Preliminary examination of the Yamato-86032 lunar meteorite: II. Major and trace element chemistry

The chemical composition of the new lunar meteorite Yamato-86032 has been studied by several laboratories in a consortium study. A preliminary report on the first analytical results from seven laboratories is given in this paper. The meteorite, which is the largest lunar meteorite recovered so far, is more heavily shocked than the other five lunar meteorites, which makes it difficult to classify the rock exactly. Although it may be classified as an anorthositic breccia the trace element composition of Y-86032 is somewhat different from the composition of the other known lunar meteorites. The major element chemistry of Y-86032 is similar to the other lunar meteorites, except for the iron content, which is lower by a factor of about 1. 4. Since the magnesium abundance is nearly identical there is a disparity in the mg ratio. The REE abundances in Y-86032 are very low and comparable to Y-82192/3. There is no evidence of any KREEP component. The abundances of several lithophile and incompatible elements are lower in Y-86032 than in the other lunar meteorites. The siderophile element contents are low and vary between individual chips. Sc, Cr, Mn, and Co have significantly lower abundances than in Y-82192/3. The chemical investigations demonstrate that Y-86032 is a new and important sample from the lunar highlands

A consortium study of Antarctic micrometeorites recovered from the Dome Fuji Station

Deposits in the water tank at the Dome Fuji Station were collected by the 37th Japanese Antarctic Research Expedition team in 1996. We recovered 233 micrometeorites from the deposits. A consortium study was started in late 1998 to investigate mineralogy, petrology, bulk chemistry, and isotopic compositions of the micrometeorites. This is the first case of an organized study of micrometeorites in Japan, in order to establish the methods to investigate micrometeorites routinely. Consortium results on mineralogy, petrology, minor and trace element compositions, isotopic compositions of noble gases of the micrometeorites are reported in this volume. We also found a sequence of mineralogical and compositional changes of micrometeorites experienced from frictional heating during atmospheric entry. INAA and ion probe studies are now in progress

Antarctic micrometeorites collected at the Dome Fuji Station

Antarctic micrometeorites (AMMs) were found among the precipitated fine particles recovered from a water tank in the Dome Fuji Station. These AMMs had been contained in the recent fallen snow around the station. Initial processing of the precipitated particles revealed that they were dominated by natural and artificial terrestrial materials, thus a series of processes were developed to separate AMMs from terrestrial particles. The recovery rate of AMMs by the processes was approximately 45% in weight, which was determined from a weight ratio of recovered/accreted AMMs. The micro-morphology and major-element concentration of the recovered AMMs were characterized. They appear to have been heated upon atmospheric entry to varying temperatures and can be classified into two major types based on the degree of heating : (1) fine-grained, irregular-shaped, partial-melted micrometeorites with chondritic composition, and (2) total-melted spherical micrometeorites with chondritic composition except for volatile elements. A digital catalog for the AMMs identified in this study was established on the web site [URL : http : //dust. cc. gakushuin. ac. jp/], in which optical characteristics, high-resolution images, and chemical compositions of individual AMMs are presented. The AMMs listed in the catalog are the first Japanese collection of extraterrestrial dust. The criterion and techniques developed for the selection and initial analysis of AMMs are applicable for the dust samples that are being collected by the 39th Japanese Antarctic Research Expedition team

General characterization of Antarctic micrometeorites collected by the 39th Japanese Antarctic Research Expedition: Consortium studies of JARE AMMs (III)

From November 1998 to January 1999,the 39th Japanese Antarctic Research Expedition (JARE-39) undertook Japanese first large-scale collection of Antarctic micrometeorites (AMMs), with sizes larger than 10μm, at the Meteorite Ice Field around the Yamato Mountains in Antarctica (at three different locations, for a total of 24 collection sites). The number of collected AMMs larger than 40μm is estimated to be about 5000. Here we present the general characterization (i.e., micro-morphology and surface chemical composition using SEM/EDS) of ∿810 AMMs chosen from 5 of the 24 sites. Additionally, the mineral composition of 61 out of 810 AMMs was determined by Synchrotron X-ray radiation. Preliminary results on mineralogical and chemical compositions show similarities with that of previous studies, even though a pronounced alteration of some AMMs is noticed. A correlation is found between the Mg/Si ratio at the sample\u27s surfaces of unmelted AMMs and the age of snow/ice in which the AMMs are embedded

Estimation the source of tephra in Antarctic ice

第3回極域科学シンポジウム/第35回極域気水圏シンポジウム 11月29日（木） 国立国語研究所 ２階ロビ

Chemical compositions of the ALH-77302 polymict eucrite

The abundances of 16 major, minor, and trace elements (Fe, Na, Cr, Co, Sr, Ba, Sc, La, Nd, Sm, Eu, Tb, Yb, Lu, Hf and Ta) in two matrices, one fusion crust, 10 igneous clast samples of ALH-77302 polymict eucrite have been determined by instrumental neutron activation analysis. The abundances of 14 minor and trace elements (K, Rb, Sr, Ba, La, Ce, Nd, Sm, Eu, Gd, Dy, Er, Yb and Lu) in a matrix sample of ALH-77302 have been also determined by mass spectrometric isotope dilution method. The chondritic normalized REE patterns of the matrices, fusion crust, and igneous clasts are similar to those of known non-cumulative eucrites. The La abundances in the clasts range from the lowest La abundance in the previously reported non-cumulative eucrites to the highest one. The chemical features of the ALH-77302 polymict eucrite have been discussed in more detail

Chemistry of Yamato-791197 Antarctic meteorite: Evidence for its lunar highland origin

We report chemical data for 33 major, minor and trace elements in one clast, two matrices and two bulk samples of the Yamato-791197 meteorite by instrumental neutron activation analysis (INAA). Based on the well-established characteristic lunar and meteoritic ratios of FeO/MnO, (Cr)_2O_3/V and K/La, and the large-ion lithophile (LIL) element patterns, the Y-791197 meteorite is undoubtedly an anorthositic gabbro breccia of lunar highland origin. The similarity of chemical compositions of the Y-791197 and ALHA81005 meteorites suggests that both meteorites may have been ejected from the same lunar region. Based on the overall chemical abundances and the very low K_2O content, it is suggested that both meteorites originated from the far side of the moon. The similar Ir/Ni/Au ratios of the Y-791197 and ALHA81005 meteorites suggest that they may be related to one and the same impact event