Abstract Chondrules in chondritic meteorites are unique witnesses of nebular and asteroidal processes that preceded large-scale planetary accretion. Together with refractory calcium-aluminium-rich inclusions (CAIs), they are the sources of our knowledge of the initial evolution of the early Solar System. We have investigated a single very large (>10 mm in longer dimension) chondrule, hereafter, the mega-chondrule A25-2, extracted from the Allende CV3 chondrite. We characterised texture, mineralogy and mineral chemistry of this chondrule, and studied its Al-Mg, U-Pb and U-isotope systematics. We also studied the distribution of U, Th and Pb, and measured Pb isotopic composition in individual minerals of A25-2 by secondary ion mass-spectrometry (SIMS). The main difficulty in absolute age determination was the presence of pervasive and resilient non-radiogenic Pb. In the search for the best way to separate radiogenic Pb from non-radiogenic Pb components of terrestrial and asteroidal origins, we used various protocols of multi-step leaching and assessed their efficiency in generating data suitable for the construction of an isochron. Testing the data filtering procedure led us to explore the behaviour of the stepwise leaching method in the presence of pervasive and resilient non-radiogenic Pb. The model age patterns observed in the final HF partial dissolution steps were probably induced by isotopic fractionation. Although step leaching did not yield fractions with highly radiogenic Pb, a Pb-Pb isochron age, corrected for measured 238U/235U was obtained by: (1) data filtering process based on strict analytical and geochemical criteria to include in the Pb-Pb isochron only leaching steps free from terrestrial contamination and (2) arithmetically recombined analyses to cancel the effects of leaching-induced isotopic fractionation. This extensive data processing yielded the age of 4568.5 ± 3.0 Ma, which we consider reliable within its uncertainty limits, although it is not as precise as, and more model dependent than, the age that could have been obtained if Pb isotopic compositions were more radiogenic. The 238U/235U ratio of the mega-chondrule is 137.764 ± 0.016, which is similar to the ratios obtained from single chondrules yet slightly different from small pooled Allende chondrules. The initial 27Al/26Al ratio inferred from internal isochron obtained from SIMS Al-Mg isotope measurements is (5.4 ± 6.5) × 10–6, which corresponds to 4565.0 + 0.8/−∞ Ma, assuming homogeneous distribution of 26Al throughout the protoplanetary disk at the canonical level (∼5.2 × 10−5). This age is 3.5 ± 3.1 Ma younger than the Pb-isotopic age. Calculation of 26Al-26Mg age assuming initial (27Al/26Al)0 of (1.36 ± 0.72) × 10–5 in the CV chondrule-forming region yields the age of 4566.4 + 0.8/−∞, which is consistent with the Pb-isotopic age.

中文翻译：

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探索逐步溶解去除球粒 U-Pb 定年中顽固的非放射性铅的效率

摘要 球粒陨石中的球粒是大尺度行星吸积之前星云和小行星过程的独特见证。连同难熔的富含钙铝的内含物 (CAI)，它们是我们了解早期太阳系初始演化的知识来源。我们研究了一个非常大的球粒（长度大于 10 毫米），以下称为巨型球粒 A25-2，从阿连德 CV3 球粒陨石中提取。我们对该球粒的质地、矿物学和矿物化学进行了表征，并研究了其 Al-Mg、U-Pb 和 U-同位素系统学。我们还研究了 U、Th 和 Pb 的分布，并通过二次离子质谱法 (SIMS) 测量了 A25-2 单个矿物中的 Pb 同位素组成。确定绝对年龄的主要困难是普遍存在且具有弹性的非放射性铅。在寻找将放射性铅与地球和小行星来源的非放射性铅成分分离的最佳方法时，我们使用了各种多步浸出方案，并评估了它们在生成适合构建等时线的数据方面的效率。测试数据过滤程序使我们探索了在普遍存在且有弹性的非放射性铅存在的情况下逐步浸出方法的行为。在最终 HF 部分溶解步骤中观察到的模型年龄模式可能是由同位素分馏引起的。虽然分步浸出没有产生具有高放射性铅的部分，但通过测量 238U/235U 校正的 Pb-Pb 等时线年龄是通过以下方式获得的：(1) 基于严格的分析和地球化学标准的数据过滤过程，在 Pb-Pb 等时线中仅包括不受陆地污染的浸出步骤；(2) 算术重组分析以消除浸出引起的同位素分馏的影响。这种广泛的数据处理产生了 4568.5 ± 3.0 Ma 的年龄，我们认为它在其不确定性范围内是可靠的，尽管它不如 Pb 同位素组成更具放射性的情况下可以获得的年龄那么精确，并且更依赖于模型. 巨型球粒的 238U/235U 比率为 137.764 ± 0.016，这与从单个球粒获得的比率相似，但与小汇集的阿连德球粒略有不同。从 SIMS Al-Mg 同位素测量获得的内部等时线推断的初始 27Al/26Al 比率为 (5.4 ± 6. 5) × 10–6，对应于 4565.0 + 0.8/−∞ Ma，假设 26Al 在标准水平 (∼5.2 × 10−5) 整个原行星盘中均匀分布。这个年龄比铅同位素年龄小 3.5 ± 3.1 Ma。假设初始 (27Al/26Al)0 为 (1.36 ± 0.72) × 10–5 在 CV 球粒形成区，计算 26Al-26Mg 年龄得出 4566.4 + 0.8/−∞ 年龄，这与 Pb 同位素一致年龄。