Mathematics – Logic
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30..555n&link_type=abstract
Meteoritics, vol. 30, no. 5, page 555
Mathematics
Logic
Chondrites, H, Impact Melting, Krypton-Argon Dating, Meteorites, Allende, Yamato 790746, Rubidium-Strontium Systematics, Texture, Basaltic
Scientific paper
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.
Fujiwara Takanori
Kagami Hiroo
Kojima Hedeyasu
Misawa Kazuhiko
Nagao Keiichi
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