Barium isotopic compositions of ordinary chondrites

第6回極域科学シンポジウム[OA] 南極隕石11月17日（火）　国立国語研究所 2階 講

Cosmic-Ray Exposure Histories of Gas-rich Brecciated Meteorites

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

Regolith history of the aubritic meteorite parent body revealed by neutron capture effects on Sm and Gd isotopes

Enstatite achondrites (aubrites) when compared to other stone meteorites have unusually long cosmic-ray exposure (CRE) ages. We report here the 150Sm/149Sm and 158Gd/157Gd ratios in six different structural phases, i.e., light and dark (shocked) grains and in matrix materials of Pesyanoe, in three different fragments from Pena Blanca Spring, and in one from Norton County, Shallowater, and Khor Temiki, to investigate the regolith history on the aubrite parent body. The results from phases components of Pesyanoe confirm earlier reported evidence for regolith irradiation of several aubrites. The inferred neutron fluences for six Pesyanoe separates vary between (2.13 and 2.82) × 1016 n cm-2. The fluences also significantly exceed those expected from cosmic-ray irradiation during transit to Earth and approach those observed in the lunar regolith. These observations confirm that the brecciated Pesyanoe meteorite, which contains solar wind (SW) gases only in dark phases, was processed in a regolith and that structural phases were differentially irradiated before compaction. On the other hand, in some aubrites (Mt. Egerton, Shallowater, Pena Blanca Spring, Norton County) neutron capture effects may entirely be due to space irradiation

Isotopic evidence of non-thermalized neutron irradiation in solar-gas-rich meteorites: Possibility of the interaction with solar neutrons and activity from the early Sun

Sm-150/Sm-149 and Gd-158/Gd-157 isotopic shifts in extraterrestrial materials provide evidence of the occurrence of neutron capture reactions caused by the interaction of cosmic rays with the surface of the materials. The Sm and Gd isotopic shifts of chemical leachates from the Kapoeta meteorite, known as a solar-gas-rich meteorite, show a clear neutron capture record corresponding to the neutron fluence up to 9.0 x 10(15) n cm(-2). The intensities of the neutron fluences of two fractions from Kapoeta significantly exceed what is expected from the time for cosmic ray irradiation during the transit to Earth. Furthermore, the combination of Sm and Gd isotopic shifts, defined as epsilon(Sm)/epsilon(Gd) to understand the neutron energy spectrum, shows a different neutron irradiation environment for certain components of Kapoeta (epsilon(Sm)/epsilon(Gd) = 1.9 +/- 0.5, and 1.9 +/- 0.7) from those of other meteorites and lunar samples (epsilon(Sm)/epsilon(Gd) = 0.4-0.9). The isotopic data of Sm and Gd from Kapoeta suggest the irradiation of non-thermalized neutrons with the energies over 1 eV from the early Sun

Sm and Gd isotopic shifts of Apollo 16 and 17 drill stem samples and their implications for regolith history

The isotopic compositions of Sm and Gd in lunar regolith samples from the Apollo 16 and 17 deep drill stems showed clear isotopic shifts in 150Sm /149Sm (ε= +124 to +191 for A-16, and +37 to +111 for A-17) and 158Gd/157Gd (ε= +107 to +169 for A-16, and +31 to +84 for A-17) corresponding to neutron fluences of (5.68 to 9.03)x1016 ncm-2 for A-16 and (1.85 to 5.04)x1016 ncm-2 for A-17. The depth profiles of neutron fluences suggest that the regoliths at both sites were due to incomplete mixing of three different slabs which experienced individual two-stage irradiation before and after deposition of the upper slabs. The variations in REE compositions provide chemical evidence for incompletely vertical mixing of regoliths especially at upper layers of the two sites. The thermal neutron energy index estimated from the combination of Sm and Gd isotopic shifts, defined as εSm/εGd, shows a small variation (0.61-0.64) in the A- 16 core except for the surface layer. On the other hand, a large variation in εSm/εGd = 0.67 to 0.83 in the A-17 core may result from complicated history such as two-stage irradiation and incomplete mixing during the gardening processes. Isotopic enrichments of 152Gd and 154Gd correlated with Eu/Gd elemental abundances and neutron fluences were also observed in almost all of fifteen samples, showing evidence of neutron capture from 151Eu and 153Eu, respectively