Noble gases in angrites Northwest Africa 1296, 2999/4931, 4590, and 4801: Evolution history inferred from noble gas signatures

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.

Primordial heavy noble gases in the pristine Paris carbonaceous chondrite

“…) is taken (in agreement with the recently determined value of 0.0061 ± 0.0028 for angrites; Nakashima et al. ), we observe that the Xe isotope corrections for contributions from 238 U and 244 Pu spontaneous fission are negligible (white symbols versus gray ones, Fig. in supporting information).…”

“…Conversely, a high initial 244 Pu/ 238 U might require late inputs of freshly synthesized material in the early solar system. Here, if the widely adopted chondritic 244 Pu/ 238 U of 0.0068 AE 0.0010 (Hudson et al 1989) is taken (in agreement with the recently determined value of 0.0061 AE 0.0028 for angrites; Nakashima et al 2018), we observe that the Xe isotope corrections for contributions from 238 U and 244 Pu spontaneous fission are negligible (white symbols versus gray ones, Fig. S4 in supporting information).…”

Potassium isotope heterogeneity in the early Solar System controlled by extensive evaporation and partial recondensation

“…Angrites are randomly sampled fragments of the APB over a wide time window of ~ 56 Myr 32 , and they likely originated from various locations within the APB. The binary mixing line between δ 41 K and U/K in angrites (Fig.…”

“…2) 19 . Volcanic angrites also have smaller cosmic ray exposure (CRE) ages 32 , hence are less affected by the cosmogenic production of 41 K from 41 Ca. The selected samples encompass the compositional range documented for volcanic angrites (Supplementary Tables 1 and 2 and Supplementary Fig.…”

Cosmic‐ray exposure age and heliocentric distance of the parent body of the Rumuruti chondrite PRE 95410

“…The isotopes 3 He, 21 Ne, and 38 Ar are generally used for discussions of cosmogenic noble gases (e.g., Nakashima et al 2018) because they have relatively high cosmogenic/trapped gas ratios compared to other isotopes. However, 3 He is easily disturbed by diffusive loss (Schultz et al 2005) and 38 Ar is potentially lost by terrestrial weathering (Okazaki et al 2000;Patzer and Schultz 2001).…”