K. Yanai’s research while affiliated with Kobe University and other places

40Ar-39Ar and noble gas analyses were performed for a H-clast in a shocked L-chondrite (Yamato 75097) from Antarctica in order to reveal the shock effect on the degassing state of noble gases and their origin. 40Ar-39Ar analyses have revealed that the H-clast show a 40Ar-39Ar age spectrum with a U-shaped pattern indicating a degassing event about 490Ma, which accords with that estimated from the 40Ar-39Ar age of the host meteorite. However, the amount of inherited 40Ar in the higher temperature fractions in the clast are much larger than that of the host rock, reflecting the different degree of shock effect on the degassing state of noble gases between the clast and the host. The clast contains mostly cosmogenic He and Ne, but retain a portion of Ar and probably some amounts of trapped Kr and Xe. A very high 129Xe/132Xe ratio (30.3±2.6) observed in the clast indicates a high I/Xe ratio when the clast was formed; a relatively low Xe content might be a main reason to cause such a high I/Xe ratio. About 13ppb of I is required to explain the observed ratio with the conditions that the observed amount of Xe represents the original Xe content and the 129I/127I was 10-4 at the end of nucleosynthesis. Such a high 129Xe/132Xe ratio has not been reported for the host meteorite.

Extensive K-Ar age determinations have been carried out for shocked chondrites [1]. However the young shock ages obtained by the less susceptible clocks such as Rb-Sr isotopes are still rarely reported for chondritic meteorites [2,3], particulary only for H-chondrites [4]. In order to search for ages, chemical and petrological characteristics of shocked H-chondrites, we have carried out analyses of Rb-Sr, K-Ar and other rare gass isotopes along with isotope dilution analyses of K, Rb, Sr, Ba, Ca, Mg, and Fe for impact melted Antarctic H-chondreite, Y-790746. The meteorite contains shock veins (widths: 5-10mm) formed from melts. One intensely melted portion was sampled for the present study. The observations by an optical microscope and a scanning electron microscope indicate that the specimen has basaltic texture consisting of relic olivine grains with several hundred micrometer sizes, cryptocrystalline subhedral olivines and pyroxenes associated with glassy groundmass. Relic olivines with undulatory extinction and planar fractures occasionally carry several micrometer sized Fe-Ni deposits and shock veins. Recrystallized olivines and pyroxenes are mostly zoned by Fe-rich overgrowth rims. Ca-rich pyroxenes also carry Ca depleted overgrowth rim materials. Numerous Fe-Ni metal droplets (several ten to hundred micrometer sizes) associated with troilites also exist in interstitial areas of mineral grains. No plagioclase nor maskerynite grains were identified but instead abundant glassy materials exist in the interstitial areas. Abundances of alkali metals (K, Rb) and alkaline earths (Ba, Sr) in the whole-rock are almost H-chondritic (x1.02, x1.41, x1.19, x1.1; relative to average H-chondrites). The major element abundances normalized to average H-chondrites are low in Mg (x0.84), Fe (x0.70), and Ca (x0.87), indicating that Y-790746 is depleted in mafic components. These results suggest that Fe-Ni metal, (possibly, olivines and pyroxenes) was removed from the source during impact-melting event on the parent body. For the purpose of internal Rb-Sr isochron age determination, bulk sample was crushed, seived and subjected to mineral separation with a Franz isodynamic separator and 6 mineral separate fractions (coarse grain, medium grain, fine grain, further mineral separates) were obtained. Three specimens were analyzed for rare gasses, and all fractions were analyzed for Rb-Sr isotopes. The K-Ar age of 2.66 +/- 0.20 Ga was obtained for the whole rock. This age is substantially in agreement with those of two mineral separates. The results of Rb-Sr analyses indicate that the largest spread of the horizontal axis (^87Rb/^86Sr ratio) is only about 10%, so the precise age was not obtained. Data point of whole-rock is plotted on the left side of the 4.55 Ga ALL reference isochron [5]. On the other hand, data points of most mineral separates are plotted on the right side of the referene line. The tie line between whole rock and mineral separates corresponds to 288 +/- 190 Ma which have no age meaning. Therfore, the Rb-Sr system was perturbed by a late event. Further detailed Rb-Sr isotopic analyses are now in progress. From the chemical and petrological features, it is suggested that a large scale impact melting event underwent on the H-chondrite parent body at significant later time and Y-790746 may have been derived from a melt-sheet of an impact crater. References: [1] Bogard D. D. (1995) Meteoritics, 30, 244-268. [2] Nakamura N. and Okano O. (1985) Nature, 315, 563-566. [3] Nakamura N. et al. (1990) Nature, 345, 51-52. [4] Fujimaki H. et al. (1994) Proc. NIPR Symp. Antarct. Meteorites, 7, 387-388. [5] Gray C. M. et al. (1973) Icarus, 20, 213-239.

The UThPb and SmNd isotopic systematics of mineral separates from the Ca-rich ureilite MET 78008 are reported. Extensive acid leaching procedures were used to eliminate terrestrial contamination because the amounts of U, Th, Pb, Sm, and Nd are extremely low in the meteorite. The three residues without terrestrial Pb contamination yield a PbPb age of 4.563 ± 0.021 Ga. On a UPb concordia diagram, the residue data corrected for an initial lead isotopic composition calculated with μ = 11.7 for 3 Ma from starting Canõn Diablo troilite Pb in indicate a 2O7Pb∗/206Pb∗ model age of 4.563 Ga. The SmNd isotopic data are scattered around the 4.56-Ga chondritic isochron. However, the SmNd model age for handpicked augite is 4.573 ± 0.029 Ga, assuming an initial CHUR composition. Two separates show a positive 142Nd anomaly similar to those found in other meteorites of the same age (4.5–4.6 Ga). These SmNd isotopic results support the PbPb and UPb ages of this meteorite. Therefore, it is very likely that the formation age of ureilites is consistent with the so-called “4.56-Ga canonical age of meteorites.” It is also possible that extensive igneous processes for generating the ultramatic mineralogy of ureilites took place in the parent body within a few million years after formation of the solar nebula.

A petrographic and electron probe microanalysis (EPMA) study revealed the presence of different populations of perovskites and hibonites in Ca-Al rich inclusions (CAIs) from the Yamato-791601 CV3 chondrite. Some of them are probably of extraneous origin. This CAI (YM1) is very large and has a core-mantle structure. The core consists mainly of loosely packed coarse spinels with anorthite, fassaite, diopside and grossular in their interstices. The mantle consists of melilite with minor populations of spinel, hibonite and perovskite. A few euhedral grains of corundum, several micrometers in size, are found in the melilite mantle. The narrow rims are developed as successive layers on the outermost mantle. In thin section YM1 is quadrilateral in shape and lacks mantle and rim in both elongated sides. It seems that the spinel core lies between the two mantles. The mineral assemblage resembles type A CAI (CTA); however, the abundance of spinel and melilite and the textural features differ from type A. Three distinct populations of hibonites were encountered in the rim sequence, in spinel framboids, and in captured multiphase fragments. Hibonites in captured fragments are rich in V2O3. In contrast, hibonites in spinel framboids are depleted in V2O3. These hibonites display a positive correlation between V2O3 and TiO2 contents. This result suggests that the hibonite populations originated from different sources. There are three categories of perovskites: grains in the rim sequence, as a composite grain, and subhedral grains in the melilite mantle. Perovskites forming the innermost rim with spinel and hibonite are heterogeneously distributed. The composite grain is amoeboid in shape and consists of a bright and a dark portion on BEI image. Perovskites in the rims show a negative relationship between Y2O3 and ZrO2 and mainly belong to type 1.