Early solar system aqueous activity: K isotope evidence from Allende

1,2Yun Jiang,3Piers Koefoed,3Olga Pravdivtseva,3,4Heng Chen,2,5Chun‐Hui Li,2,5Fang Huang,2,5Li‐Ping Qin,2,5Jia Liu,3Kun Wang
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13588]
1CAS Key Laboratory of Planetary Sciences, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing, 210008 China
2CAS Center for Excellence in Comparative Planetology, Hefei, China
3Department of Earth and Planetary Sciences, McDonnell Center for the Space Sciences, Washington University in St. Louis, One Brookings Drive, St. Louis, Missouri, 63130 USA
4Lamont‐Doherty Earth Observatory, Columbia University, Palisades, New York, 10964 USA
5CAS Key Laboratory of Crust‐Mantle Materials and Environments, School of Earth and Space Sciences,
University of Science and Technology of China, Hefei, Anhui, 230026 China
Published by arrangement with John Wiley & Sons

The alkali element K is moderately volatile and fluid mobile; thus, it can be influenced by both primary processes (evaporation and recondensation) in the solar nebula and secondary processes (thermal and aqueous alteration) in the parent body. Since these primary and secondary processes would induce different isotopic fractionations, K isotopes could become a potential tracer to distinguish them. Using recently developed methods with improved precision (0.05‰, 95% confidence interval), we systematically measured the K isotopic compositions and major/trace elemental compositions of chondritic components (18 chondrules, 3 CAIs, 2 matrices, and 5 bulks) in the carbonaceous chondrite fall Allende. Among all the components analyzed in this study, CAIs, which formed initially under high‐temperature conditions in the solar nebula and were dominated by nominally K‐free refractory minerals, have the highest K2O content (average 0.53 wt%) and have K isotope compositions most enriched in heavy isotopes (δ41K: −0.30 to −0.25‰). Such an observation is consistent with previous petrologic studies that show CAIs in Allende have undergone alkali enrichment during metasomatism. In contrast, chondrules contain lower K2O content (0.003–0.17 wt%) and generally lighter K isotope compositions (δ41K: −0.87‰ to −0.24‰). The matrix and bulks are nearly identical in K2O content and K isotope compositions (0.02–0.05 wt%; δ41K: −0.62 to − 0.46‰), which are, as expected, right in the middle of CAIs and chondrules. This strongly indicates that most of the chondritic components of Allende suffered aqueous alteration and their K isotopic compositions are the ramification of Allende parent‐body processing instead of primary nebular signatures. Nevertheless, we propose the small K isotope fractionations observed (< 1‰) among Allende components are likely similar to the overall range of K isotopic fractionation that occurred in nebular environment. Furthermore, the K isotope compositions seen in the components of Allende in this study are consistent with MC‐ICP‐MS analyses of the components in ordinary chondrites, which also show an absence of large (10‰) isotope fractionations. This is not expected as evaporation experiments in nebular conditions suggest there should be large K isotopic fractionations. Nevertheless, possible nebular processes such as chondrules back exchanging with ambient gas when they formed could explain this lack of large K isotopic variation.


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