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Noble gases in angrites Northwest Africa 1296, 2999/4931, 4590, and 4801: Evolution history inferred from noble gas signatures
Author(s) -
Nakashima Daisuke,
Nagao Keisuke,
Irving Anthony J.
Publication year - 2018
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/maps.13039
Subject(s) - noble gas , radiogenic nuclide , achondrite , chondrite , geology , zircon , geochemistry , meteorite , astrobiology , physics , atomic physics , mantle (geology)
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 244 Pu‐ 136 Xe relative ages of the angrites obtained by using 244 Pu/ 150 Nd ratios are as old as that of Angra dos Reis, reflecting their early formation. On the other hand, another method to obtain 244 Pu‐ 136 Xe relative ages, using fission 136 Xe, spallogenic 126 Xe, and Ba/ REE ratios, yields systematically older 244 Pu‐ 136 Xe ages than those obtained by using 244 Pu/ 150 Nd ratios, which is explained by apparently high Ba/ REE ratios caused by Ba contamination during terrestrial weathering. The 244 Pu/ 238 U 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 244 Pu/ 238 U ratio has been spatially homogeneous at least in the inner part of the early solar system.