72nd Annual Meeting of the Meteoritical Society, held July 13-18, 2009 in Nancy, France

Hidaka, Hiroshi

Yoneda, Shigekazu

Hiroshima University Institutional Repository

COSMIC-RAY EXPOSURE HISTORIES OF GAS-RICH BRECCIATED METEORITES.  H. Hidaka1 and S. Yoneda2. 1Department of  Earth and Planetary Systems Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan. E-mail: hi-daka@hiroshima-u.ac.jp. 2Department of Science and Engineer-ing, National Museum of Nature and Science, Tokyo 169-0073, Japan.  Introduction: Isotopic shifts in 150Sm/149Sm and 158Gd/157Gd caused by neutron capture effects are useful indicators to charac-terize the interaction of cosmic rays with planetary materials in space, because 149Sm and 157Gd have very large thermal neutron capture cross sections. A recent neutron capture study of the Sm and Gd isotopes of Pesyanoe also shows significant variations of neutron fluences in six different phases of a small meteorite frag-ment, starting with the early irradiation before compaction of the parent body [1].  In this study, Sm and Gd isotopic analyses of gas-rich brecciated meteorites other than aubrites were performed to find a possible contribution of the irradiation from the early Sun.   Samples and Experiments: Two samples of gas-rich mete-orites, NWA801 (CR2), and Kapoeta (howardite), and two brec-ciated meteorites, Zag (H3-5) and Cook011 (L3-5), were used in this study.  Zag consists of light-colored H6 clast and gray H3-4 matrix.  In addition to a whole rock sample (Zag #1), 200 mg of clast (Zag #2) and matrix (Zag #3) were individually separated from a main fragment, and used for the Sm and Gd isotopic study.  In the case of Kapoeta, a sequential acid-leaching experiment was carried out on the whole rock of Kapoeta, where 200 mg of the powdered sample was leached by 5 mL of 2 M HCl (Kapoeta #2), and aqua regia (Kapoeta #3) in succession.  The residue (Kapoeta #4) was finally decomposed by HF + HClO4 with heat.   Conventional ion exchange techniques using two column proce-dures were carried out to chemically separate Sm and Gd for in-dividual fractions [2].   A Micro-mass VG 54-30 thermal ionization mass spectrome-ter equipped with seven Faraday cup collectors was used for the isotopic measurements of Sm and Gd.   Results and Discussion: Isotopic shifts of Sm and Gd greater than the analytical uncertainties (2σ of the mean) were observed in Kapoeta #2, Kapoeta #3, Zag #1, Zag #2, NWA801 and Cook011, which correspond to the neutron fluences ranging from 0.76 x1015 to 9.0x1015 n cm-2.  Interestingly, the neutron fluences of the chemical separates from Kapoeta (Kapoeta #2, #3 and #4) show a large variation from <0.21x1015 ncm-2 (detection limit) to (9.0 ± 0.6) x1015 ncm-2, although that from the whole rock sample (Kapoeta #1) does not show clear evidence of neu-tron capture over the detection limit (<0.72 x1015 ncm-2).  The data suggest the existence of heterogeneous alien materials hav-ing much larger irradiation records than the matrix component in Kapoeta.   Zag also shows heterogeneous isotopic shifts on Sm and Gd caused by mixed irradiation records between irradiated and non-irradiated materials.  Our Sm and Gd isotopic data from Zag #2 and #3 are consistent with the CRE ages [3].   References: [1] Hidaka H. et al. 2006. Geochim. Cosmochim. Acta 70:3449-3456. [2] Hidaka H. and Yoneda S. 2007. Geochim. Cosmochim. Acta 71:1074-1086. [3] Whitby et al. 2000. Science 288: 1819-1821. 72nd Annual Meteoritical Society Meeting (2009) 5113.pdf

Cosmic-Ray Exposure Histories of Gas-rich Brecciated Meteorites

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Cosmic-Ray Exposure Histories of Gas-rich Brecciated Meteorites

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