Noble gases in the K-T boundary clay from Stevns Klint, Denmark

Helium and neon isotopes were investigated in two beds of the Cretaceous-Tertiary (K-T) boundary collected at Stevns Klint in Denmark. Observed enrichments in ^3He and ^Ne relative to atmospheric abundances suggest that the K-T boundary sediment contains spallogenic He and Ne. Although this is a favorable result for the asteroid impact hypothesis, we need more data on noble gas isotopes not only in the K-T boundary sediments but also in sedimentary rocks other than the K-T boundary

Solar-wind derived light noble gases in micrometeorites collected at the Dome Fuji Station: Characterization by stepped pyrolysis

Noble gases in micrometeorite-bearing particles were characterized by the total-melting and stepped-heating analyses, in order to determine average compositions of light noble gases of micrometeorites collected at the Dome Fuji Station. He and Ne are dominated by solar-wind derived noble gases and the concentrations are comparable to the highest ones detected so far in carbonaceous chondrites. Cosmogenic gases are a very minor component in micrometeorites, suggesting short exposure to solar and galactic cosmic rays. The high ratio of solar to cosmogenic gases suggests that the micrometeorites had been small particles in the interplanetary space to have large surface areas to be exposed to solar winds. The micrometeorites are supposed to have fallen on Antarctica in the recent fifty years with snow around the Dome Fuji Station (T. Nakamura et al., Antarct. Meteorite Res., 12,183,1999a), and hence they are particles generated in the modern solar system and came to the Earth after short periods of exposure to solar winds and galactic cosmic rays

Noble gases in Yamato-793274 and -86032 lunar meteorites

Isotopic abundances of noble gases are reported for whole rock samples, mineral and grain size separates prepared from lunar meteorites Yamato-793274 and -86032. Y-793274 is rich in trapped solar gases, whereas Y-86032 is depleted in them, resembling the Y-82192 and -82193 lunar meteorites. From measurements of radiogenic Ar, the K-Ar age of Y-86032 is calculated to be 3940±240 Ma, in agreement with the K-Ar and Ar-Ar ages for Y-82192 and -82193. From measurements of cosmogenic noble gases, the total duration of cosmic-ray exposure is calculated to be 510±140 and 11±1 Ma for Y-793274 and -86032,respectively. The exposure age of Y-86032 is in good agreement with that for Y-82192/3. The agreement in the K-Ar and exposure ages as well as in the trapped gas abundances supports the earlier result that Y-86032 is paired with Y-82192 and -82193. Y-793274 experienced most of exposures to cosmic-rays in the lunar regolith because the transit time from the moon to the earth has been reported to be very short, whereas it is supposed that Y-86032 experienced a large part of cosmic-ray exposure in the interplanetary space during the flight from the moon to the earth

Noble gas composition in unique meteorite Yamato-74063

Unique meteorite Yamato-74063 contains large amounts of trapped heavy noble gases whereas it is depleted in trapped He and Ne. The concentration of trapped ^Ar is comparable with that of E- and C-chondrites and ureilites. Trapped ^Xe is unusually abundant. Y-74063 contains a very high concentration of radiogenic ^Xe. Trapped ^Ne/^Ar is low and similar to that of ureilites. Trapped ^Ar/^Xe of 32±4 is lower than that of any meteorites ever reported. The trapped gases in Y-74063 are depleted in Ar relative to Xe. Planetary-type noble gases may be mixtures of an Ar-depleted component and the "sub-solar" or "Ar-rich" component isolated in E-chondrites. The cosmic-ray exposure age is 6.2±0.4 Ma. Gas-retention ages are calculated to be less than 3.7±0.2,and 4.8±0.4 Ga from radiogenic ^4He and ^Ar respectively. The K-Ar age older than the age of the solar system may attributed to chemical inhomogeneities resulting in an exceptionally high K concentration of the investigated sample. The gasretention ages, the large amounts of radiogenic ^Xe and the trapped noble gases indicate that the meteorite was a closed system for the noble gases since crystallization

40Ar-39Ar analyses of Yamato-75097 (L6) chondrite from Antarctica

^Ar-^Ar analyses were performed on the host phase of an H-clast bearing chondrite Yamato-75097 (L6). The result indicates a typical U-shaped age spectrum, suggesting that the serious degassing event occurred at about 490Ma or slightly younger age for this meteorite. Since the meteorite shows a sign of shock, this event might have been a collisional event. It is estimated that more than 99% of radiogenic ^Ar was degassed during this event. For this degassing, it requires prolonged time of at least a few hours even at 1200℃, and a longer time is required if the ambient temperature is lower and the diffusion mainly controls the degassing process

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

Cosmic-ray exposure age and heliocentric distance of the parent body of H chondrites Yamato-75029 and Tsukuba

Many small pieces of the H chondrites Yamato (Y-) 75029 and Tsukuba were investigated to characterize signatures of light noble gases. These meteorites contain large amounts of solar gases as well as cosmogenic ones. A simple regolith exposure model was developed in order to explain the correlations among solar ^Ne and ^Ar and cosmogenic ^Ne concentrations. Based on the regolith model, the parent body exposure ages, heliocentric distances, and space exposure ages of the two meteorites were calculated. The parent body exposure ages were more than 5.5Ma and 11.8Ma for Y-75029 and Tsukuba, respectively. The heliocentric distances were 2.2±^_ AU and 4.2±^_ AU for Y-75029 and Tsukuba, respectively. The space exposure age of Y-75029 was 5.2-5.8Ma, whereas that of Tsukuba was 8.1±0.6Ma. It has been suggested that the parent bodies of H chondrites are S-type asteroids with orbits that range from 2 to 3.5AU in the present solar system. On the other hand, the obtained heliocentric distances of Y-75029 and Tsukuba indicate locations of the parent bodies in the past when some parts of the meteorites were exposed to the sun. The heliocentric distances for Y-75029 is in a good agreement with current S-type asteroid distribution, while that for Tsukuba is at the upper tail of the distribution

Noble gas measurement in individual micrometeorites using laser gas-extraction system

All noble gases in 12 individual Antarctic micrometeorites, series F96C and F96D, taken from a water tank at the Dome Fuji Station, were measured by using Nd-YAG CW laser-extraction system. According to the results of light noble gases, the micrometeorites can be separated into two groups, "gas-rich" and "gas-poor". Samples in the gas-rich group have SEP-like ^3He/^4He ratios and Ne isotopic ratios between SEP- and Solar wind-Ne, which are consistent with previous reports on micrometeorites or IDPs. On the other hand, samples in the gas-poor group have very low ^3He/^4He ratios and ^Ne/^Ne ratios lower than the atmospheric values. The ^Ar/^Ar ratios are also separated into two groups, i. e., samples with lower ^Ar/^Ar belong to the gas-rich group. Cosmogenic noble gases were undetected in all the samples and cosmic-ray exposure ages may be shorter than 4.5Ma considering error limits of Ne data. Heavy noble gas elemental compositions are chondritic. There was no correlation between the concentration of volatile elements, sulfur, and that of He and Ne

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

Yamato-86789: A heated CM-like carbonaceous chondrite

We have studied the mineralogy and petrology of the Yamato (Y)-86789 meteorite and concluded that this meteorite is a new member of heated carbonaceous chondrites with CI-CM affinities. Y-86789 contains 14vol% of translucent chondrules which mainly consist of fibrous phyllosilicate-like materials. The high abundance of the phyllosilicate-like minerals in chondrules suggests severe aqueous alteration. Major parts of the matrix are also composed of the phyllosilicate-like materials. The phyllosilicate-like materials have compositions of a mixture of serpentine and saponite. However, the phyllosilicate-like materials show consistently high analytical totals; thus, they are dehydrated. X-ray diffraction measurements of the phyllosilicate-like materials reveal that they are mainly composed of olivine. Diffractions from serpentine and saponite are absent from the X-ray powder patterns. These results suggest that Y-86789 has experienced heating and the phyllosilicate minerals which were once present in Y-86789 were dehydrated to from anhydrous materials such as olivine. Y-86789 has textural, mineralogical, and compositional characteristics similar to those of Y-86720,which is one of the three unusual Antarctic carbonaceous chondrites with CI-CM affinities that show evidence of dehydration. Chondrules are completely replaced by the phyllosilicate-like materials and are surrounded by fine-grained rims. Some chondrules contain Ca-carbonates. There is an abundance of troilite in the matrix, but magnetite and PCP, which are common in CM chondrites, are absent. Some large troilite grains have a euhedral lath-like morphology. The bulk chemical composition of Y-86789 is also similar to that of Y-86720. On the basis of these similarities, we propose that Y-86789 and Y-86720 are paired meteorites