HR: 1340h
AN: P33A-1005    [Abstracts]
TI: Significance of Sodalite and Nepheline in Chondrules
AU: * Takeshima, Y
EM: yuchin@rins.ous.ac.jp
AF: Grad. Sch., Okayama Univ.of Sci., 1-1 Ridai-cho, Okayama, 700-0005 Japan
AU: Gouzu, C
EM: gouzu@geohiruzen.co.jp
AF: Hiruzen Inst., 161-1 Sai, Okayama, 703-8248 Japan
AU: Hyodo, H
EM: hhyodo@rins.ous.ac.jp
AF: Res. Inst. Nat. Sci., Okayama Univ. of Sci., 1-1 Ridai-cho, Okayama, 700-0005 Japan
AU: Itaya, T
EM: itaya@rins.ous.ac.jp
AF: Res. Inst. Nat. Sci., Okayama Univ. of Sci., 1-1 Ridai-cho, Okayama, 700-0005 Japan
AB: The petrographic and mineralogic features of chondrules in the Allende meteorite were studied in view of obtaining geologically meaningful $^{40}$Ar/$^{39}$Ar ages. The $^{40}$Ar/$^{39}$Ar dating of terrestrial and extraterrestrial materials requires estimation of initial $^{40}$Ar component. In terrestrial materials, the amount of initial $^{40}$Ar is estimated from total $^{36}$Ar content. On the other hand, in extraterrestrial materials, total $^{36}$Ar in chondrules includes cosmogenic $^{36}$Ar, which is a decay product from cosmogenic $^{36}$Cl. If chondrite includes a Cl-bearing phase, the radiogenic $^{40}$Ar will be underestimated due to the cosmogenic $^{36}$Ar, yielding apparently younger $^{40}$Ar/$^{39}$Ar age. Moreover, parts of $^{35}$Cl and $^{37}$Cl may form HCl with hydrogen, and they will be detected as isobars of $^{36}$Ar and $^{38}$Ar which are indistinguishable in a mass spectrometer with resolution of less than 1000. The Allende chondrules consist mainly of prismatic and barred olivine (Fo85-99) with minor amounts of pyroxene (En52-98, Wo$<$48), plagioclase (An91-93), sulfides, metals and mesostasis. In this study, Cl-bearing phase in pinkish color occurs as an accessory mineral in the mesostasis. The compositional mapping by EMP confirmed not only the presence of nepheline without Cl, but also the mineral aggregates of a-few-microns-sized Cl-bearing phase closely associated with nepheline. EMP analyses were not successful to identify clearly the Cl-bearing phases due to their sizes. The chemical composition obtained by EMP suggests that they are mainly mixtures of sodalite and nepheline. Sodalite has been already reported as an alteration phase of chondrules in Allende meteorite. Therefore, it is most likely that the Cl-bearing phase is sodalite. Considering these results, bulk analyses in age dating of chondrules with Cl-bearing phases can be affected by the cosmogenic argons. Cl-free materials such as plagioclase and nepheline should be separated for $^{40}$Ar/$^{39}$Ar analysis. In-situ spot dating technique of plagioclase and nepheline using pulse-laser will be most powerful for such purpose. The application of the technique to nepheline may also be effective to reveal the time of secondary alteration in the chondrite.
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3662 Meteorites
SC: Planetary Sciences [P]
MN: 2004 AGU Fall Meeting