Toward the classification of chondrites using the powder X-ray diffraction method, a preliminary report

第6回極域科学シンポジウム[OA] 南極隕石11月16日（月）　国立国語研究所 2階 講

Lattice strain analyses for the L6 ordinary chondrites by the X-ray diffraction method

The Tenth Symposium on Polar Science/Poster presentations: [OA] Antarctic meteorites, Wed. 4 Dec. / Entrance Hall (1st floor), National Institute of Polar Researc

Sulfidation of metallic iron in the primordial solar nebura

The phase diagram of the Fe-S-H system was constructed based on thermochemical calculations at low total pressure (p(total)=p(H_2)=10^ and 10^ atm). Under these conditions both metallic iron and troilite, and troilite and pyrite are in reaction relation over a wide range of the phase diagram. The most important characteristic of the phase diagram is that the peritectic line is highly proximate to the H-S tie line. Based on the reaction relation the rate of the reaction in the solar gas was studied by applying the experimental data of the FeS formation reaction in the field of corrosion science of metals. The rate is controlled by diffusion in a FeS layer and the parabolic rate constant, k, is approximately given by k (cm^2/s)=0.3 exp (-3.5×10^4(cal/mol)/RT) in the solar nebula condition. The reaction degree was estimated based on various conditions of the cooling solar nebula : (1) contant p (S_2), (2) decreasing p (S_2) and (3) a certain saturation temperature above 0K, when the grain size of metallic iron was controlled by homogeneous nucleation theory

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

Antarctic micrometeorite collection at a bare ice region near Syowa Station by JARE-41 in 2000

We collected Antarctic Micrometeorites (AMMs) at a bare ice field near Syowa Station in the austral fall and spring seasons of 2000. The facility for the AMMs collection introduced by the 39th Japanese Antarctic Research Expedition (JARE-39) was improved to increase the filtering rate of the melt water. The filtering rate became 4-5 times quicker than the previous system, ∿1000 liter/hour, by the addition of a new water pump in parallel with the previous pump. About 50 tons of melt water were formed, of which about 40 tons were filtered using this new system, and 18 holes were made in the bare ice region in 23 days. We obtained particles in which abundant micrometeorites should be included

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

Search for Antarctic meteorites in the bare ice field around the Yamato Mountains by JARE-41

The wintering party (November 14,1999-March 28,2001) of the 41st Japanese Antarctic Research Expedition (JARE-41) conducted a meteorite search in the bare ice field around the Yamato Mountains. The period of the travel was 89 days. Departure from Syowa Station was on October 27,2000,entrance into the bare ice field on November 17,2000,leaving the bare ice field on January 10,2001,and arrival at Syowa Station on January 23,2001. The 6 men party (field leader : Y. Shimoda, and sub-leader : N. Imae) of JARE-41 collected about 3500 meteorites. The total sample weight was about 196kg. The average meteorite weight was 55g. The most frequent weight was in the bin of 3.2-10g of the weight histogram. The heaviest meteorite collected on the present expedition was an iron meteorite of 50.5kg. This is also the heaviest meteorite among the Yamato bare ice region since the first expedition in 1969 (JARE-10). The iron meteorite was found on the northwest region of JARE IV Nunataks. Compared with other areas, relatively large meteorites were found here. The number of meteorites on the bare ice region around Minami-Yamato Nunataks, which is the most famous meteorite concentration area, was large but the weight of each meteorite was lighter compared with other areas. Sublimation rate, which must be one of the most important factors for the concentration of meteorites, was measured at the near YM175 (71°44.4′S, 35°54.7′E, 2138m) using two independent methods. Both the stake method and the empirical formula method gave nearly comparable values in average; 50-80 and 130.8mg cm^d^, respectively, which are larger than that at Mizuho Station. Sublimation using the stake method was measured at 8 points. Precise GPS measurements for 20 days at YM175 showed that the bare ice is moving 3cm to the west and 2cm upward. This suggests that the upward movement is nearly balanced with the sublimation of ice. However, in order to consider the annual balance, longer measurements are needed

An attempt to reproduce petrographic features of mesosiderites

The Tenth Symposium on Polar Science/Poster presentations: [OA] Antarctic meteorites, Wed. 4 Dec. / Entrance Hall (1st floor), National Institute of Polar Researc

Noble gas signatures of Antarctic nakhlites, Yamato (Y) 000593, Y000749, and Y000802

We have measured noble gases in three nakhlites from Antarctica, Yamato (Y) 000593, Y000749, and Y000802, by step-heating and total-melting methods.The trapped ^Ar/^Kr/^Xe ratios determined for the bulk samples are around 80/3/1, identical to those of Nakhla. The Yamato nakhlites also release noble gases showing high ^Xe/^Xe (up to 1.486) and low ^Kr/^Xe (～1.5) at 1000 and 1300 ℃, which is one of the most characteristic signatures of nakhlites. The low ^Kr/^Xe, as compared to that of the Mars atmosphere, suggests the presence of a fractionated Martian atmosphere.Cosmic-ray exposure ages based on cosmogenic ^Ne are 11.7, 11.9, and 13.0 Ma for Y000593, Y000749, and Y000802, respectively. This supports the pairing based on the mineralogical and petrographical similarities and the location of the finds. The average of the ^Ne exposure ages is 12.05±0.69 Ma. We also calculated an apparent ^Kr-Kr age as 11.8±1.0 Ma from cosmic-ray produced radioactive ^Kr and stable Kr isotopes from Y000593. The coincidence with the ^Ne exposure age indicates a short terrestrial age (Ne exposure age and terrestrial age, is 12.1±0.7 Ma. Calculated K-Ar gas retention age for the Yamato nakhlites is 1.24±0.22 Ga. The ejection time and gas retention age are close to those of non-Antarctic nakhlites and Chassigny. This suggests that the Yamato nakhlites were ejected from Mars together with other nakhlites and Chassigny. Xenon isotopes are mixtures of Chassigny Xe, fission Xe, and the Mars atmosphere. High-temperature fractions (1000-1750℃) are enriched in the Mars atmosphere and fission Xe components, compared to lower temperature fractions. There are similarities in Xe isotopes between Y000749 and Y000802 showing excesses in ^Xe and ^Xe, whereas Y000593 has only small excesses. The release pattern of ^Xe for Y000593 is also dif