We analyzed the highly siderophile element (HSE) contents and bulk Ge isotopic compositions of large metal grains in the CB chondrites Bencubbin (CB_a), Gujba (CB_a), and HaH 237 (CB_b). Our results suggest that the large grains were formed by the aggregation of smaller condensed grains, and the two Benccubinite groups are distinguishable based on their bulk metal δ^74/70Ge mass-dependent isotopic values of 0.99 ± 0.30‰ (CB_a) and −0.65 ± 0.10‰ (CB_b). Based on our observations of these three samples, the isotopic compositions of metal in CB_a chondrites are best explained by condensation at slow cooling rates in the center of an impact plume, whereas the metal in CB_b chondrites formed under fast cooling rates along the plume edges. We also analyzed the Ge contents and isotopic compositions of the core, intermediate, and rim fractions of two Gujba metal grains, which were separated by sequential digestion. These results show a gradual decrease in δ^74/70Ge and [Ge] from core to rim. We suggest that these δ⁷⁴Ge zonations result from near-equilibrium condensation and evaporation processes in a heterogeneous plume. We propose a model for their formation in which (1) small grains (to become grain cores) condensed at equilibrium; (2) these grains were transported to a warmer region of the plume where they reached temperatures lower than that of Fe-Ni condensation, but high enough for the rapid evaporation of Ge; (3) Ge evaporation followed by slow cooling enriched the grains in heavy Ge isotopes and the surrounding gas in light Ge isotopes; and (4) equilibrium recondensation of metal from the gas and around the small grains formed the light Ge isotopic zonations observed in grain rims.

中文翻译：

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CB 球粒陨石形成过程中的冷凝和蒸发过程：Ge 同位素和高亲铁元素丰度的见解

我们分析了大的金属晶粒的高度亲铁元件（HSE）内容和块体Ge同位素组成在CB陨石本卡宾（CB_一个），Gujba（CB_一个），和HAH 237（CB _b）。我们的结果表明大晶粒是由较小的凝聚晶粒的聚集形成的，并且基于它们的块状金属 δ ^74/70 Ge 质量相关同位素值 0.99 ± 0.30‰ (CB _a ) 和 - 0.65 ± 0.10‰ (CB _b )。根据我们对这三个样品的观察，CB _a中金属的同位素组成球粒陨石的最好解释是在撞击羽流的中心以缓慢的冷却速度凝结，而 CB _b球粒陨石中的金属在沿着羽流边缘的快速冷却速度下形成。我们还分析了通过顺序消化分离的两种 Gujba 金属颗粒的核心、中间和边缘部分的 Ge 含量和同位素组成。这些结果显示δ ^74/70 Ge 和[Ge] 从核心到边缘逐渐减少。我们建议这些 δ ⁷⁴Ge 分带是由非均质羽流中近乎平衡的冷凝和蒸发过程引起的。我们提出了一种形成它们的模型，其中 (1) 小颗粒（成为颗粒核）在平衡时凝聚；(2) 这些颗粒被输送到羽流的较温暖区域，在那里它们的温度低于 Fe-Ni 冷凝的温度，但足以使 Ge 快速蒸发；(3) 锗蒸发后缓慢冷却使晶粒富集重锗同位素，周围气体富集轻锗同位素；(4) 气体和小晶粒周围金属的平衡再凝聚形成了在晶粒边缘观察到的轻Ge同位素带。