Abstract Among solar system materials there are variable degrees of depletion in moderately volatile elements (MVEs, such as Na, K, Rb, Cu, and Zn) relative to the proto-solar composition. Whether these depletions are due to nebular and/or parent-body (asteroidal or planetary) processes is still under debate. In order to help decipher the MVE abundances in early solar system materials, we conducted a systematic study of high-precision K stable isotopic compositions of a suite of whole-rock samples of well-characterized carbonaceous and ordinary chondrites. We analyzed 16 carbonaceous chondrites (CM1-2, CO3, CV3, CR2, CK4-5 and CH3) and 28 ordinary chondrites covering petrological types 3– 6 and chemical groups H, L, and LL. We observed significant K isotope (δ41K) variations (−1.54 to 0.70‰) among the carbonaceous and ordinary chondrites. In general, the two major chondrite groups are distinct: The K isotope compositions of carbonaceous chondrites are largely higher than the Bulk Silicate Earth (BSE) value, whereas ordinary chondrites show K isotope compositions that are typically lower than the BSE value. Neither carbonaceous nor ordinary chondrites show clear/resolvable correlations between K isotopes and chemical groups, petrological types, shock levels, cosmic-ray exposure ages, fall/find occurrence, or terrestrial weathering. Importantly, the lack of a clear trend between K isotopes and K content among chondrites indicates that the K isotope fractionations were decoupled from the relative elemental K depletions, which is inconsistent with a single-stage partial vaporization or condensation process to account for these MVE depletion patterns among chondrites. The range of K isotope variations in the carbonaceous chondrites in this study is consistent with a four-component (chondrule, refractory inclusion, matrix and water) mixing model that is able to explain the bulk elemental and isotopic compositions of the main carbonaceous chondrite groups, but requires a fractionation in K isotopic compositions in chondrules. We propose that the major control of the isotopic compositions of group averages is condensation and/or vaporization in pre-accretional (nebular) environments that is preserved in the compositional variation of chondrules. Parent-body processes, such as aqueous alteration, thermal metamorphism, and metasomatism, can mobilize K and affect the K isotopes in individual samples. In the case of the ordinary chondrites, the full range of K isotopic variations can only be explained by the combined effects of the size and relative abundances of chondrules, parent-body aqueous and thermal alteration, and possible sampling bias.

中文翻译：

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碳质和普通球粒陨石的钾同位素组成：对早期太阳系挥发性耗竭起源的影响

摘要 在太阳系材料中，相对于原太阳成分，中等挥发性元素（MVE，如 Na、K、Rb、Cu 和 Zn）存在不同程度的消耗。这些损耗是否是由星云和/或母体（小行星或行星）过程引起的，仍在争论中。为了帮助破译早期太阳系材料中的 MVE 丰度，我们对一组表征良好的碳质和普通球粒陨石的全岩样品的高精度 K 稳定同位素组成进行了系统研究。我们分析了 16 个碳质球粒陨石（CM1-2、CO3、CV3、CR2、CK4-5 和 CH3）和 28 个普通球粒陨石，涵盖岩石学类型 3-6 和化学组 H、L 和 LL。我们在碳质和普通球粒陨石中观察到显着的 K 同位素 (δ41K) 变化（-1.54 至 0.70‰）。一般来说，两个主要的球粒陨石群是不同的：碳质球粒陨石的 K 同位素组成大大高于块状硅酸盐 (BSE) 值，而普通球粒陨石显示的 K 同位素组成通常低于 BSE 值。碳质和普通球粒陨石都没有显示出 K 同位素与化学组、岩石学类型、冲击水平、宇宙射线暴露年龄、坠落/发现事件或陆地风化之间的明确/可解决的相关性。重要的是，球粒陨石中 K 同位素和 K 含量之间缺乏明确的趋势表明 K 同位素分馏与相对元素 K 消耗脱钩，这与解释这些 MVE 消耗的单级部分汽化或冷凝过程不一致球粒陨石之间的模式。本研究中碳质球粒陨石中 K 同位素变化的范围与四组分（球粒、耐火包裹体、基质和水）混合模型一致，该模型能够解释主要碳质球粒陨石群的整体元素和同位素组成，但需要对球粒中的 K 同位素组成进行分馏。我们提出，对群平均同位素组成的主要控制是在增生前（星云）环境中的凝结和/或汽化，这种环境保留在球粒的组成变化中。母体过程，例如水蚀变、热变质和交代作用，可以调动 K 并影响单个样品中的 K 同位素。在普通球粒陨石的情况下，