Mathematics – Logic
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.559o&link_type=abstract
Meteoritics, vol. 30, no. 5, page 559
Mathematics
Logic
Chondrites, L, Neagari, Classification, Isotopes, Cosmogenic
Scientific paper
The Neagari meteorite fell on Feb. 18, 1995 at Neagari-machi, Nomi-gun, Ishikawa-ken, Japan (geographical coordinate: 36 degrees 26.9'N, 136 degrees 27.9'E). It was broken into several pieces when it hit a car upon falling. The largest piece weighing about 325 g and a small piece weighing 39 g were brought to the Kanazawa University for the measurements of gamma-rays emitted by cosmogenic nuclides only 2.7 days after the fall. Thereafter, the measurement was repeated several times. Other small pieces were used for petrographic, mineralogical and chemical studies. Noble gas mass spectrometry and AMS were also conducted. The Neagari meteorite shows a distinct, recrystallized structure under the microscopic observation of the thin section. Chondrules, 0.6 to 1.0 mm in diameter, are all present as relicts, buried in the well-recrystallized matrix. The chondrule-matrix boundaries are scarcely discernible in the granulated matrix. Olivine (Fa: 25.3 +/- 0.6 mole%) and orthopyroxene (Fs: 20.6 +/- 0.6 mole%) are the most abundant minerals both in matrix and in the chondrule relicts. Diopside is present as individual grains in the granular matrix. Interstitial feldspar crystal (Or(sub)6.3Ab (sub)88.0 An(sub)5.8) are common in the matrix and chondrule relicts, and often enclose minute pyroxene grains. Main opaque minerals are kamacite, taenite, troilite and chromite, and the metal phase is more abundant than the sulfide phase in the section. Both Fa and Fs values indicate that the Neagari meteorite is an L chondrite. The well-crystallized structure of the matrix, poorly defined outline of relict chondrules in the matrix, the prevalence of clear and well-developed plagioclase grains in the matrix and chondrule relicts and the absence of glass and monoclinic low-Ca pyroxene indicate the petrologic type to be 6. By the non-destructive gamma-ray measurement of the meteorite, eleven cosmogenic nuclides (^44mSc, ^52Mn, ^48V, ^51Cr, ^7Be, ^56Co, ^46Sc, ^57Co, ^54Mn, ^22Na, and ^26Al) have been identified and their contents were determined by using a mock sample having known amounts of natural radioactivities. Among these nuclides, ^44mSc has the shortest half life (2.44 d) and has also been measured in the Mihonoseki meteorite [1]. Cosmogenic components were also observed for ^3He, ^21Ne, and ^38Ar by noble gas mass spectrometry. Cosmic-ray exposure ages calculated from cosmogenic ^21Ne and ^38Ar contents coupled with production rates by Eugster [2] and Schultz et al. [3], respectively, seem to be consistent (about 45 Ma), but the age from ^3He is significantly lower. Considering the loss of radiogenic ^4He, the Neagari meteorite must have experienced a high temperature event in space. Cosmogenic radionuclides ^10Be, ^26Al, and ^36Cl were also measured in an aliquot (77 mg) of the Neagari meteorite using an AMS facility at Lawrence Livermore National Laboratory. The concentrations of these nuclides in conjunction with the noble gas data as well as data of elemental abundances provide better knowledge of the exposure history of this meteorite. References: [1] Shima M. et al. (1993) LPS XXIV, 1297-1298. [2] Eugster O. (1988) GCA, 52, 1649-1662. [3] Schultz L. et al. (1991) GCA, 55, 59-66.
Ebihara Mitsuru
Komura Kazuhisa
Miyamoto Yoshihisa
Nagao Keiichi
Nishiizumi Kuni
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