Noble gases in angrites Northwest Africa 1296, 2999/4931, 4590, and 4801: Evolution history inferred from noble gas signatures |
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| Authors: | Daisuke Nakashima Keisuke Nagao Anthony J Irving |
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| Institution: | 1. Department of Earth and Planetary Material Sciences, Faculty of Science, Tohoku University, Aoba, Sendai, Miyagi, 980-8578 Japan;2. Geochemical Research Center, Graduate School of Science, University of Tokyo, Hongo, Bunkyo, Tokyo, 113-0033 Japan
Division of Polar Earth-System Sciences, Korea Polar Research Institute, Incheon, 21990 Korea;3. Department of Earth & Space Sciences, University of Washington, Seattle, Washington, 98915 USA |
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| Abstract: | Noble gases in the five angrites Northwest Africa (NWA) 1296, 2999, 4590, 4801, and 4931 were analyzed with total melting and stepwise heating methods. The noble gases consist of in situ components: spallogenic, radiogenic, nucleogenic, and fission. Cosmic-ray exposure ages of the angrites (including literature data) spread uniformly from <0.2 to 56 Ma, and coarse-grained angrites have longer exposure ages than fine-grained angrites. It is implied that the parent bodies from which the two subgroups of angrites were ejected are different and have distinct orbital elements. The 244Pu-136Xe relative ages of the angrites obtained by using 244Pu/150Nd ratios are as old as that of Angra dos Reis, reflecting their early formation. On the other hand, another method to obtain 244Pu-136Xe relative ages, using fission 136Xe, spallogenic 126Xe, and Ba/REE ratios, yields systematically older 244Pu-136Xe ages than those obtained by using 244Pu/150Nd ratios, which is explained by apparently high Ba/REE ratios caused by Ba contamination during terrestrial weathering. The 244Pu/238U ratio at 4.56 Ga of angrites is estimated as 0.0061 ± 0.0028, which is consistent with those for chondrules, chondrites, achondrites, and a terrestrial zircon. It is suggested that initial 244Pu/238U ratio has been spatially homogeneous at least in the inner part of the early solar system. |
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