Abstract Lead isotope compositions were measured on both single and combined chondrules from the CV3 carbonaceous chondrite Allende with the goal of determining the range of Th/U implied by the radiogenic 208Pb*/206Pb* values. All samples were aggressively acid step-leached to separate radiogenic from primordial lead. It is found that apparent Th/U varies both between individual chondrules and between the different leaching fractions of each chondrule or group of chondrules. Specifically, the apparent Th/U ratio deviates from the planetary value (3.876), varying spectacularly from 0.65 to 14.6. Variations between leachates and residues disclose the existence of internal heterogeneities, while inter-chondrule variations reveal the presence of external heterogeneities. Three main explanations for the observed Th-U fractionation that are not mutually exclusive prevail: (1) uranium species, notably UO and UO2, coexisted in the nebular gas at high temperature, whereas Th existed exclusively as ThO2; (2) chondrules interacted with an exotic oxidized vapor; and (3) chondrules represent melt of dust of different origins, a hypothesis dictated by the evidence of internal heterogeneity. The extent to which the measured apparent Th/U variability is due to each of these particular processes is difficult to assess, but the existence of substantial Th/U heterogeneity, especially within, but also among, single (or pooled) chondrules from the same chondrite calls for caution when Pb-Pb linear arrays, or mixing lines, are assigned isochronous significance.

中文翻译：

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阿连德球粒中的 Th/U 变异性

摘要 在来自 CV3 碳质球粒陨石阿连德的单个和组合球粒上测量了铅同位素组成，目的是确定放射成因 208Pb*/206Pb* 值所暗示的 Th/U 范围。所有样品都经过强烈酸浸，以将放射性铅与原始铅分开。发现表观 Th/U 在单个球粒之间和每个球粒或球粒组的不同浸出分数之间变化。具体来说，表观 Th/U 比值偏离行星值 (3.876)，从 0.65 到 14.6 变化惊人。渗滤液和残留物之间的变化揭示了内部异质性的存在，而球粒间的变化揭示了外部异质性的存在。观察到的 Th-U 分馏的三种主要解释并不相互排斥：（1）铀物质，特别是 UO 和 UO2，在高温下共存于星云气体中，而 Th 仅以 ThO2 的形式存在；(2) 球粒与奇异的氧化蒸气相互作用；(3) 球粒代表不同来源尘埃的融化，这是由内部异质性证据决定的假设。由于这些特定过程中的每一个，测量的明显 Th/U 变异的程度难以评估，但存在大量 Th/U 异质性，尤其是在来自相同当 Pb-Pb 线性阵列或混合线被赋予等时意义时，球粒陨石需要谨慎。高温下共存于星云气体中，而Th仅以ThO2的形式存在；(2) 球粒与奇异的氧化蒸气相互作用；(3) 球粒代表不同来源尘埃的融化，这是由内部异质性证据决定的假设。由于这些特定过程中的每一个，测量的明显 Th/U 变异的程度难以评估，但存在大量 Th/U 异质性，尤其是在来自相同当 Pb-Pb 线性阵列或混合线被赋予等时意义时，球粒陨石需要谨慎。高温下共存于星云气体中，而Th仅以ThO2的形式存在；(2) 球粒与奇异的氧化蒸气相互作用；(3) 球粒代表不同来源尘埃的融化，这是由内部异质性证据决定的假设。由于这些特定过程中的每一个，测量的明显 Th/U 变异的程度难以评估，但存在大量 Th/U 异质性，尤其是在来自相同当 Pb-Pb 线性阵列或混合线被赋予等时意义时，球粒陨石需要谨慎。由内部异质性证据决定的假设。由于这些特定过程中的每一个，测量的明显 Th/U 变异的程度难以评估，但存在大量 Th/U 异质性，尤其是在来自相同当 Pb-Pb 线性阵列或混合线被赋予等时意义时，球粒陨石需要谨慎。由内部异质性证据决定的假设。由于这些特定过程中的每一个，测量的明显 Th/U 变异的程度难以评估，但存在大量 Th/U 异质性，尤其是在来自相同当 Pb-Pb 线性阵列或混合线被赋予等时意义时，球粒陨石需要谨慎。