Cosmic-ray Exposure Histories of Gas-rich Brecciated Meteorites

Introduction: Isotopic shifts in 150 Sm/ 149 Sm and 158 Gd/ 157 Gd caused by neutron capture effects are useful indicators to characterize the interaction of cosmic rays with planetary materials in space, because 149 Sm and 157 Gd 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 fragment , 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 meteorites , 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 + HClO 4 with heat. Conventional ion exchange techniques using two column procedures were carried out to chemically separate Sm and Gd for individual fractions [2]. A Micro-mass VG 54-30 thermal ionization mass spectrometer 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 x10 15 to 9.0x10 15 n cm-2. Interestingly, the neutron fluences of the chemical separates from Kapoeta (Kapoeta #2, #3 and #4) show a large variation from <0.21x10 15 ncm-2 (detection limit) to (9.0 ± 0.6) x10 15 ncm-2 , although that from the whole rock sample (Kapoeta #1) does not show clear evidence of neutron capture over the detection limit (<0.72 x10 15 ncm