Mathematics – Logic
Scientific paper
Dec 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufm.p33a1005t&link_type=abstract
American Geophysical Union, Fall Meeting 2004, abstract #P33A-1005
Mathematics
Logic
3600 Mineralogy And Petrology (Replaces, 3662 Meteorites
Scientific paper
The petrographic and mineralogic features of chondrules in the Allende meteorite were studied in view of obtaining geologically meaningful 40Ar/39Ar ages. The 40Ar/39Ar dating of terrestrial and extraterrestrial materials requires estimation of initial 40Ar component. In terrestrial materials, the amount of initial 40Ar is estimated from total 36Ar content. On the other hand, in extraterrestrial materials, total 36Ar in chondrules includes cosmogenic 36Ar, which is a decay product from cosmogenic 36Cl. If chondrite includes a Cl-bearing phase, the radiogenic 40Ar will be underestimated due to the cosmogenic 36Ar, yielding apparently younger 40Ar/39Ar age. Moreover, parts of 35Cl and 37Cl may form HCl with hydrogen, and they will be detected as isobars of 36Ar and 38Ar 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 40Ar/39Ar 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.
Gouzu Chitaro
Hyodo Hironobu
Itaya Tetsumaru
Takeshima Yuusuke
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