Early solar system aqueous activity: K isotope evidence from Allende

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 parts per thousand, 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 (delta K-41: -0.30 to -0.25 parts per thousand). 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 (delta K-41: -0.87 parts per thousand to -0.24 parts per thousand). The matrix and bulks are nearly identical in K2O content and K isotope compositions (0.02-0.05 wt%; delta K-41: -0.62 to - 0.46 parts per thousand), 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 parts per thousand) 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 parts per thousand) 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.

Automated summary

The potassium isotopes can be used to distinguish primary and secondary processes in the solar nebula. Components in Allende carbonaceous chondrite, such as CAIs and chondrules, exhibit different K2O content and K isotope compositions, indicating alkali enrichment during metasomatism and aqueous processing. Most components in Allende have undergone aqueous alteration, as shown by their K isotopic compositions.