The many unique characteristics of CB chondrites have resulted in the impact hypothesis becoming the favoured model for their formation. Here, we further investigate the formation mechanisms of CB chondrites by analysing the elemental and K isotope compositions of chondrules and bulk fractions from the CB_a chondrite Gujba. Similar to previous work, the refractory element ratios in the Gujba chondrules show evidence of a differentiated precursor, with the Nb/Ta, Zr/Hf, Sc/Th and Zr/Th ratios showing fractionation relative to other chondrites. In addition, the bulk fractions, and to a lesser extent the chondrules with attached matrix and metals, display significantly more refractory element fractionation and a large enrichment in light REEs. Based on EDS elemental mapping and comparisons with previous studies, the most likely source of this highly fractionated material appears to be the small amount of heterogeneously distributed interstitial fine-grained material within Gujba. These large refractory element fractionations (i.e., Nb/Ta, Zr/Hf, Sc/Th Zr/Th, and LREE/HREE) are best explained by a significant partial melting process such as crustal formation. Nevertheless, the mechanism of patrial melting cannot be conclusively determined with the data available here. The K isotopic compositions of the Gujba chondrules analyzed here range from −2.24‰ to −0.41‰ in δ⁴¹K, whereas the bulk analyses show δ⁴¹K values of −0.81‰ to −0.72‰. This range of chondrule K isotope compositions is significantly larger, and extends to much lighter compositions, compared to all other chondrites measured so far by bulk ICP-MS. In addition, the Gujba chondrules display a clear negative correlation of K isotopic composition with K concentration, with the chondrules showing the lightest K isotope compositions having the highest K concentrations. This distinctive correlation indicates that evaporation was likely the dominant process affecting the K isotopic variation observed in the Gujba chondrules. Nevertheless, the extremely light δ⁴¹K values seen in the most K-rich chondrules (which are lighter than any other early solar system material so far measured) indicate that incomplete condensation likely took place before evaporation. As such, we propose a two-stage model to explain the formation of chondrules in Gujba, with Stage 1 characterized by incomplete condensation of vaporized material with an average isotopic fractionation factor (α) of 0.9984 (when using the most K enriched chondrule to constrain the model), and Stage 2 representing partial evaporation in a vapor plume with an average α range of 0.9976 to 0.9990. Using these α values we calculate an approximate vapor saturation index value of 0.935 for condensation and between 0.903 and 0.960 for evaporation. This formation process requiring both condensation and evaporation for CB chondrules is consistent with an impact generated vapor plume and further expands our understanding of CB chondrite formation.

CB球粒陨石的许多独特特征导致撞击假说成为它们形成的首选模型。_{在这里，我们通过分析来自 CB a}的球粒和本体部分的元素和 K 同位素组成，进一步研究 CB 球粒陨石的形成机制。球粒陨石古杰巴。与之前的工作类似，Gujba 球粒陨石中的难熔元素比率显示出不同前体的证据，其中 Nb/Ta、Zr/Hf、Sc/Th 和 Zr/Th 比率显示出相对于其他球粒陨石的分馏。此外，主体部分，以及在较小程度上附着基质和金属的球粒，显示出明显更多的难熔元素分馏和轻稀土元素的大量富集。基于 EDS 元素映射和与先前研究的比较，这种高度分馏材料的最可能来源似乎是 Gujba 内少量不均匀分布的间隙细粒材料。这些大的难熔元素分馏（即 Nb/Ta、Zr/Hf、Sc/Th Zr/Th、和 LREE/HREE）最好用一个显着的部分熔融过程来解释，例如地壳形成。然而，不能用这里可用的数据最终确定父项融化的机制。此处分析的 Gujba 球粒的 K 同位素组成范围为 -2.24‰ 至 -0.41‰ δ⁴¹ K，而整体分析显示 δ ⁴¹ K 值为 -0.81‰ 至 -0.72‰。与迄今为止通过散装 ICP-MS 测量的所有其他球粒陨石相比，该球粒陨石 K 同位素组成范围明显更大，并扩展到更轻的成分。此外，Gujba 球粒显示出 K 同位素组成与 K 浓度的明显负相关，球粒显示最轻的 K 同位素组成具有最高的 K 浓度。这种独特的相关性表明蒸发可能是影响在 Gujba 球粒中观察到的 K 同位素变化的主要过程。然而，极轻的 δ ⁴¹在最富含 K 的球粒中看到的 K 值（比迄今为止测量的任何其他早期太阳系材料都轻）表明不完全冷凝可能发生在蒸发之前。因此，我们提出了一个两阶段模型来解释 Gujba 球粒的形成，阶段 1 的特征是汽化材料的不完全冷凝，平均同位素分馏因子 (α) 为 0.9984（当使用最富含 K 的球粒进行约束时）模型），第 2 阶段代表蒸汽羽流中的部分蒸发，平均 α 范围为 0.9976 至 0.9990。使用这些 α 值，我们计算出冷凝的近似蒸汽饱和指数值 0.935，蒸发的近似蒸汽饱和指数值介于 0.903 和 0.960 之间。