Noble gases in hosts and inclusions from Yamato-75097 (L6), -793241 (L6) and-794046 (H5)

Noble gas analyses for the inclusions and hosts from Yamato (Y)-75097,Y-793241 and Y-794046 have been performed. The concordant exposure ages as well as gas retention ages are observed between the host and the inclusion of each meteorite. These indicate that both experienced the same heating event and exposure history. High ^Xe/^Xe ratios (≅50) and neutron induced ^Kr, ^Kr and ^Xe are due to very low concentrations of trapped noble gases in the inclusions from Y-75097 and Y-793241. This indicates a close genetic relationship and an early crystallization when ^I was still alive. The inclusion of Y-794046,however, has a noble gas signature which is quite different from the other ones, suggesting a different origin. Negligible amounts of radiogenic ^Xe and fissiogenic ^Xe suggest a late formation

Noble gases and 81Kr-Kr exposure ages of non-Antarctic ordinarychondrites: An attempt to measure terrestrial ages of Antarctic meteorites

Cosmogenic ^Kr as well as other noble gases of three ordinary chondrites Long Island (L6), Densmore (1950) (H6), and Gladstone (stone) (H6) have been measured with the mass spectrometer newly installed in 1989 to examine the analytical accuracy using the production rates of cosmogenic noble gases proposed by EUGSTER (Geochim. Cosmochim. Acta., 52,1649,1988). For Long Island and Densmore, the ^Kr-Kr ages are 16±2Ma and 7.7±0.5Ma, respectively, and are in agreement with the ages by ^Ne and ^Kr, which indicates the validity of both ^Kr analysis and the production rates P_ and P_ used in this work. Cosmogenic ^3He contents in these meteorites are lower than those expected from the production rate P_3,which might be caused by partial loss of He. The ages T_ based on ^Ar and P_ were shorter than the ages T_ and T_. This may be due to the unreasonably high ^Ar production rates calculated for these meteorites, whose low cosmogenic ^Ne/^Ne and ^Kr/^Kr ratios indicate heavy shielding against cosmicray irradiation. The constant production rate of cosmogenic ^Ar indicates the better internal concordance than that as a function of shielding depth proposed by EUGSTER. Gladstone is a gas-rich meteorite, for which the abundant solar noble gases made it difficult to estimate the cosmogenic noble gas concentrations and the resulted ages have large uncertainties

Noble gases of Yamato 980459 shergottite

Isotopic ratios and concentrations of noble gases were determined for the Yamato (Y) 980459 olivine-phyric shergottite with a stepped heating extraction method. Trapped noble gas concentrations are low, and especially He and Ne are dominated by cosmogenic nuclides. Heavy noble gases, Ar, Kr and Xe, in the high temperature fractions (1000-1750°C ) show the martian atmospheric signatures: 40Ar36Ar and 129Xe132Xe ratios corrected for cosmogenic gases are >1000 and >1.4, respectively, and the data points plot along the mixing line between the Mars atmosphere and Chassigny in the system of 129Xe132Xe vs. 84Kr132Xe. Contribution of elementally fractionated Earth\u27s atmospheric noble gases is significant in the low temperature fractions (400-800°C ), which has been frequently reported for meteorites from hot deserts. Cosmic-ray exposure ages calculated based on cosmogenic 3He, 21Ne and 38Ar are 1.6, 2.5 and 2.1Ma, respectively. Considering partial loss of He from the meteorite, the exposure age of Y980459 would be around 2.1-2.5Ma. Though the terrestrial age of this meteorite has not been reported so far, the ages reported for Antarctic SNCs are 0.29Ma. Hence, the ejection age for Y980459 could be in the range of 2.1-2.8Ma, which is comparable to those of some basaltic shergottites, but different from other olivine-phyric shergottites ranging mostly 0.7-1.2Ma. Isotopic ratios of Kr show excesses in 80Kr and 82Kr, with 82Kr80Kr of 0.375, which results from epithermal neutron captures on 79Br and 81Br. The minimum pre-atmospheric size of Y980459 was calculated as 27cm in radius, based on the 21Ne exposure age and the n-capture 80Kr and Br concentrations of 3.0×1013cm3STPg and 0.205ppm, respectively. The calculated K-Ar age is 0.99Ga from the total 40Ar and reported K concentration of 157ppm. The age, however, gives an upper limit for the crystallization age of this meteorite, because of possible contribution of martian atmospheric Ar, as well as the terrestrial atmosphere

Noble gases and 81Kr-terrestrial age of Asuka-881757 lunar meteorite

Noble gas compositions of the Asuka-881757 lunar meteorite were determined using 10.2 and 395.3mg specimens. This meteorite has virtually no solar type noble gases. Small amounts of trapped Ar, Kr and Xe have relative abundances similar to those of atmospheric and planetary noble gases. K-Ar age is 3.75±0.35Ga, which agrees with the ages by other methods such as U-Th-Pb, Rb-Sr and Ar-Ar. Cosmogenic isotope concentrations are the lowest among the lunar meteorites reported until now. These isotopes were produced in space by 4π geometry irradiation. The history of A-881757 based on the cosmic-ray produced stable isotopes and ^Kr is as follows : It has been shielded from the cosmic ray irradiation until it was ejected from the moon 1.35±0.17Ma ago and fell on the earth 0.06±0.04Ma ago after a moon-earth transit time of 1.29±0.13Ma. These results require an ejection event for A-881757 in addition to those responsible for the other lunar meteorites

40Ar-39Ar analyses of a lumar meteorite (Yamato-86032) and a few LL3 and LL4 chondrites from Antarctica

^Ar-^Ar analyses were performed for a lunar meteorite (Yamato-86032), two LL3 (Y-790448,Allan Hills-764) and one LL4 (Y-74442) chondrites from Antarctica. The lunar meteorite Y-86032 shows a scattered ^Ar-^Ar age spectrum with anomalously high ages in the higher temperature fractions, indicating a shock effect on the analyzed sample. Among the three LL chondrites analyzed, the sample Y-790448 indicates a plateau ^Ar-^Ar age of 4521±28Ma in the lower temperature fractions (600-800℃). The other two LL chondrites show inverse staircase spectra in the higher temperature fractions. In the LL chondrite group, un-equilibrated chondrites seem to show older ^Ar-^Ar ages compared with equilibrated ones, which may be related to the differences in the thermal history of each portion of their parent body

Noble gases in solar-gas-rich and solar-gas-free polymict breccias

Polymict breccias are useful for research of solar activity because some preserve plenty of solar noble gases implanted during asteroidal formation processes. In this study, noble gas compositions of Antarctic and non-Antarctic polymict breccias were determined using laser gas-extraction and stepwise pyrolysis techniques. Of the polymict breccias measured in this work, 5 of 18 were identified as gas-rich meteorites (regolith breccias) and 4 of those 5 are H chondrites. The high population of gas-rich H chondrites compared with L and LL chondrites was presumably related to the depth of regolith formed on each parent body. It is notable that the major part of polymict breccias did not have solar noble gases. Noble gas analyses with stepwise heating method were done for 11 polymict breccias. Gas-rich meteorites have high concentrations of solar-derived He and Ne, which were released at relatively low temperature steps. Cosmogenic nuclides were comparatively dominant at high temperature steps. Five sources determined the observed Ar isotopic compositions. The components were: atmospheric, radiogenic, solar wind (SW), solar energetic particles (SEP), and cosmogenic. In contrast, Ne isotopic compositions of most regolith breccias can be explained simply by three-component mixing, such as SW, SEP, and cosmogenic. Indications of primordial trapped components were observed only in Willard (b), in which carbonaceous chondrite clasts were discovered previously. Cosmic-ray exposure ages were calculated from excess ^3He, ^(21)Ne, and ^(38)Ar. Regolith breccias did not have systematically longer ages than gas-poor samples, indicating that the parent body exposure ages of the meteorites on the order of tens of millions of years at most

Noble gas and chronological study of Asuka eucrites: A-880761 and A-881388 are paired, but A-880702 is not

Noble gas isotopic and elemental compositions of three Antarctic eucrites Asuka-880702, Asuka-880761 and Asuka-881388 have been measured by two steps of heating temperatures (400 and 1750°C ). Cosmic-ray exposure ages calculated from cosmogenic ^(21)Ne were 20.5, 32.3 and 29.7m.y. for A-880702, A-880761 and A-881388, respectively. Combining the exposure ages with ^(81)Kr-Kr apparent exposure ages, terrestrial ages of 0.20, 0.22 and 0.23m.y. were derived for A-880702, A-880761 and A-881388, respectively. ^(244)Pu-Xe ages based on fissiogenic Xe isotopes showed that the Asuka eucrites began Xe retention at around the crystallization age of Angra dos Reis. Among the Asuka eucrites, A-880702 is about 20m.y. older than the others. From the noble gas and age data described above, we conclude that A-880761 and A-881388 are paired, but A-880702 is not

On low noble gas concentrations in Antarctic micrometeorites collected from Kuwagata Nunatak in the Yamato Meteorite Ice Field

Noble gas concentrations and isotopic compositions were determined on Antarctic Micrometeorites (AMMs) collected in Kuwagata Nunatak in the Yamato Meteorite Ice Field around the Yamato Mountains in 1998. Noble gas concentrations in Kuwagata AMMs are lower than those of previously measured Dome Fuji AMMs. The heavy noble gas composition is chondritic and shows no atmospheric contamination. In order to clarify the differences of noble gas concentrations between Kuwagata and Dome Fuji AMMs, statistic calculations were done. The differences in ^4He, ^Ar, ^Kr, and ^Xe concentrations between the two types of AMMs are significant in both F-test and T-test. On the other hand, ^Ar is not significant in the F-test, and ^Ne is not significant in both tests. These results revealed a systematic decrease of noble gas concentrations in Kuwagata AMMs compared to those of the Dome Fuji AMMs. Since jarosite-a by-product mineral from aqueous alteration of sulfide minerals-was detected in Kuwagata samples, He loss may be due to aqueous alteration in Antarctic glacial ice. A contamination by atmospheric noble gases might be avoided by sample preparation using acetone to detach AMMs from stainless steal filters

Mid-infrared transmission spectra of individual Antarctic micrometeorites and carbonaceous chondrites

Mid-infrared absorption spectra were obtained for 6 carbonaceous chondrites and 6 Antarctic micrometeorites (AMMs) with transmission light using an IR microscope. Obvious absorption bands caused by O-H stretching vibration at 3400cm^ and H-O-H bending vibration at 1640cm^ were detected for CI, CM2,CR2,and CO3 chondrites. All the chondrite samples had Si-O stretching band at around 1000cm^. Allende (CV3) exhibited only Si-O stretching band at near 900cm^ and no O-H bands. Infrared absorption spectra for AMMs showed no O-H stretching vibration reflecting their dry condition, except for F97AC019 and F97AC017 with weak O-H stretching band. Heating experiments on the grains of Murchison and Orgueil with sizes as small as AMMs (∿100μm) were carried out in order to clarify the dehydration effect of micrometeorite during the atmospheric entry. Heating times were 30 and 60s at temperatures every 100℃ from 500℃ to 1000℃. At relatively lower temperature condition (≤700℃), water in carbonaceous chondrites was not depleted. O-H stretching vibration at 3400cm^ was, however, vanished at higher temperature (≥800℃). This might show that AMMs were heated to 800℃ or more during atmospheric entry if our hearting duration is reasonable. Noble gas measurement using laser heating was performed for individual AMMs after the infrared microscopic analyses. Solar-He was detected for 4 samples. F97AC019 with very high concentration of helium (8.1×10^cm^3 STP/g) preserve solar-Ne and primordial trapped argon component. We have discovered an extraordinary AMM, F97AC021,with highly concentrated organic matters. Intense C-H stretching vibration at 2900cm^ and complex absorption pattern appeared in the range from 1400cm^ to 1800cm^ were detected. The AMM might have a potential to have plenty of extraterrestrial organic compounds

Where are noble gases trapped in Yamato-74063 (unique) ?

Yamato-74063,a primitive achondrite with affinities to Acapulco, has been analyzed for noble gas isotopes by laser ablation mass spectrometry. Some grains released huge amounts of trapped Xe as well as trapped Ar and Kr. Among them, an orthopyroxene grain containing abundant inclusions of tiny metal spherules released equally large amounts of Xe as found for carbon residues of ureilites and carbonaceous chondrites, with ^Ar/^Xe=11. In contrast, other grains including silicates, Fe-Ni metal and troilite released negligible amounts of trapped gases. Mineralogical studies of polished thin sections, prepared from the same chip as was analyzed for noble gases, indicate that the silicate grains containing the tiny metal inclusions also contain abundant tiny voids, one to several μm across. Originally these voids must have been filled with gases or/and fluids. We propose bubbles as a candidate of hosts that trap large amounts of noble gases in the silicate. With data on noble gas behavior in bubbles, although available only for andesite melt, the extremely low ^Ar/^Xe ratio in the silicate phase of Y-74063 can be understood in terms of the bubble hypothesis