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

Condition for the formation of the compound chondrules in the solar nebula

The conditions for compound chondrule formation described by J. L. GOODING and K. KEIL (Meteoritics, 16,17,1981) are reexamined using the criterion of gravitational instabilities (M. SEKIYA, Prog. Theor. Phys., 69,1116,1983). If a compound chondrule was formed by collisional sticking of a plastic particle with a solid or another plastic particle, the following conclusions are derived : (1) Chondrules were formed after the settling of dust particles had progressed and the number density of dust particles had increased by several orders of magnitude. (2) To reproduce the observed ratio of the compound chondrules to all the chondrules, collisional velocities of pre-chondrule particles must have been larger than about 1.6m s^, and the Weber number for the collision of molten pre-chondrule particles must have been larger than about 40,if we assume that the chondrule formation occurred in the asteroid region in the stage where dust particles were floating in the solar nebula. In this case, collisional sticking would be difficult. If sticking is impossible, then other formation mechanisms for the compound chondrules must be considered (e. g. J. T. WASSON et al., Geochim. Cosmochim. Acta, 59,1847,1995). (3) If chondrules were formed in the inner regions of the solar nebula, the lower limits for the relative velocities of the prechondrule particles would be reduced

Mineralogy of Antarctic micrometeorites recovered from the Dome Fuji Station

Mineralogy of six micrometeorites (MMs) was investigated by a TEM for electron petrography. These MMs were selected based on X-ray diffraction analysis by a Gandolfi X-ray camera, qualitative analysis by SEMEDS of the surfaces of the MMs, and based on textural observation and quantitative analysis of "polished" sections of the MMs by a SEM and an EPMA. We found one saponite-bearing MM F96CI024 among 83 MMs. It has textural and mineralogical characteristics similar to a saponite-bearing MM BI 91/3-108. From the mineralogical point of view, they are different from CI, CM, CR, and hydrated CV chondrites. F96CI020 and F96DI021 contain lepidocrocite, which was formed by terrestrial weathering. MMs are susceptible to terrestrial weathering even in snow (or ice). A magnesium-and iron-bearing mineral (probably (Mg, Fe) O) was found in the phyllosilicate-bearing MM and two pyroxene-rich MMs. In the latter MMs, the mineral coexists with pyroxene that was formed newly during atmospheric entry of the MMs

X-ray study of PCP from the Murchison CM carbonaceous chondrite

We have performed X-ray diffraction measurements and electron-microprobe analyses of PCP particles in the Murchison CM carbonaceous chondrite and determined the relative abundances of minerals in PCP. Type-I PCP, which is rich in Fe and S and occurs as rounded, massive forms, shows high abundance of tochilinite and variable amounts of cronstedtite and magnetite. Tochilinite shows very strong 002 reflection at 5.4A and very weak 001 reflection at 10.8A, indicating a higher Fe content in octahedral sites. Comparison of the observed X-ray diffraction pattern of type-I PCP with the calculated X-ray patterns of tochilinite in various compositions shows that Fe occupies more than 90% of the octahedral sites in tochilinite in PCP, which is quite different from terrestrial tochilinite. Type-II PCP, which occurs mainly in the matrix as fibrous needle clusters, is composed mainly of cronstedtite with a disordered stacking sequence along the c-axis. The other major component of type-II PCP differs between non-clastic and clastic portions. In the former, it is tochilinite, whereas in the latter, it is a mixed-layer mineral made of cronstedtite and tochilinite layers. Relative abundances of minerals in type-II PCP indicate that the proportion of the mixed-layer mineral increases with a decrease of tochilinite; thus the mixed-layer mineral is presumed to be a secondary alteration product of tochilinite. Therefore, the enrichment of the mixed-layer mineral in type-II PCP in clastic portion suggests that aqueous alteration in the clastic portion has advanced relative to that in the non-clastic portion and that dynamic processes such as mixing and fragmentation of the rocks might have assisted to activate the aqueous alteration reactions. Our experiments reveal that Murchison is a mixture of multiple lithic phases that have experienced different degrees of aqueous alteration and the alteration might have been enhanced by cataclastic effects probably induced by impacts on the meteorite parent body

Mineralogy and petrology of the CK chondrites Yamato-82104, Yamato-693 and a Carlisle Lakes-type chondrite Yamato-82002

We have studied the mineralogy and petrology of three metamorphosed chondrites, Yamato (Y)-82104,Y-693,and Y-82002,which were previously classified as C5,C4 and C5,respectively. The results indicate that Y-82104 and Y-693 should be included with the CK carbonaceous chondrites. Y-82104 and Y-693 are remarkably similar in mineralogy and texture (e.g., virtually identical compositions of olivines, Fa 29.2±0.4vs. Fa 29.4±0.7,respectively). Thus, they are probably paired. The compositional homogeneity of olivine and pyroxene and degree of recrystallization of olivine and plagioclase are consistent with Y-82104 and Y-693 being petrologic type 5. Y-82002 differs from most CK chondrites in mineralogy and petrology, but is similar to Carlisle Lakes-type chondrites which were recently proposed to be a new grouplet of chondrites (A. E. RUBIN and G. W. KALLEMEYN, Geochim. Cosmochim., 53,3035,1989). We believe that Y-82002 is a member of this grouplet. Y-82002 was previously classified as petrologic type 5,but the relatively inhomogeneous compositions of olivine (e. g., Fa 35.1±5.1 for chondrules) and low degree of recrystallization of olivine and plagioclase suggest that Y-82002 should be classified as petrologic type 3.8-3.9

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

Oxygen isotope study of Tsukuba chondrite, some HED meteorites and Allende chondrules

Laser fluorination technique coupled with stable isotope mass spectrometry using O_2 is successfully employed to determine the δ^O and δ^O values of different types of meteorites. The oxygen isotope data are reported for the recent fall Tsukuba meteorite confirming it to be an H-type ordinary chondrite. Three diogenites Tatahouine, Y-791199 and Y-791000 are analyzed for oxygen isotopic signatures. Y-791199 and Y-791000 are regarded as cumulate eucrites by many researchers on the basis of petrogenesis. The oxygen isotopes of Tatahouine and Y-791199 are consistent with the diogenite group data. Y-791000 is showing lighter oxygen isotopic composition which is unique among the other diogenites. It may indicate that the parental source for some of cumulate eucrites is different from the non-cumulate ones. The slope 1/2 trend of oxygen isotopes observed in various Allende chondrules (mostly small porphyritic type) signifies that aqueous alteration might be responsible for scattering of data along a nearly mass dependant trend. These chondrules are plotted on a δ^O-δ^O diagram with a slope of 0.52 having Δ^O=-4.24±0.27. The range of δ^O and δ^O data observed in these chondrules is from -7.26 to -0.19‰ and -7.98 to -4.04‰ respectively. The variability in the isotopic composition which caused the scatter on the terrestrial trend implies that on the parent body some low temperature hydrothermal alteration took place

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

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