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.

中文翻译：

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太阳系早期水活动：阿连德的钾同位素证据

碱金属元素K具有中等挥发性，并且可以流动。因此，它既会受到太阳星云中的主要过程（蒸发和再凝结）又会受到母体中的次要过程（热和水的变化）的影响。由于这些主要和次要过程将诱导不同的同位素分馏，因此K同位素可能成为区分它们的潜在示踪剂。使用最近开发的精度更高的方法（0.05‰，95％置信区间），我们系统地测量了碳质中软骨成分（18个软骨，3个CAI，2个基质和5个体积）的钾同位素组成和主要/痕量元素组成。陨石坠落阿连德。在这项研究中分析的所有组件中，CAI，₂ O含量（平均0.53重量％）和具有最富集重同位素ķ同位素组合物（δ ⁴¹ K：-0.30 -0.25‰）。这样的观察结果与先前的岩石学研究一致，后者表明阿连德的CAI在交代过程中经历了碱富集。相反，球粒含有较低K i ₂ O含量（0.003-0.17重量％）和一般较轻ķ同位素组合物（δ ⁴¹ K：-0.87‰至-0.24‰）。矩阵和块材是以K几乎相同的₂ O含量和K同位素组成（0.02-0.05％（重量）;δ ⁴¹K：-0.62至-0.46‰），正如预期的那样，正好在CAI和软骨的中间。这有力地表明，阿连德的大多数软骨成分经历了水相变化，它们的K同位素组成是阿连德母体加工的产物，而不是主要的神经突状特征。尽管如此，我们建议在阿连德组分中观察到的小K同位素分离（<1‰）可能与在星云状环境中发生的K同位素分离的总体范围相似。此外，本研究中在阿连德成分中发现的K同位素组成与普通球粒陨石中成分的MC-ICP-MS分析一致，这也表明不存在大的（10‰）同位素分馏。这不是预期的，因为在星云条件下的蒸发实验表明应该有较大的K同位素分馏。然而，可能的星状过程（如软骨形成时与周围气体回交换）可以解释这种缺乏大的K同位素变化的现象。