In this study we test the possibility that radiogenic 207Pb/206Pb ratios (207Pb*/206Pb*) in meteorites can be fractionated during partial dissolution, and explore the consequences of this fractionation for Pb-isotope chronology of meteorites. We report the results of experiments tailored to detect Pb-isotope fractionation, induced by partial dissolution through acid leaching, in plutonic angrite Northwest Africa (NWA) 4801 and ungrouped achondrites NWA 10132 and Erg Chech (EC) 002. We also re-examine previously published U-Pb data for other achondrites and for Ca-Al-rich refractory inclusions (CAIs), to seek evidence of such fractionation. We observe that, in primitive achondrite NWA 10132, differences in 207Pb*/206Pb* ratios, corresponding to the age bias of ca. 1–2 Ma, exist between the 0.5 M hydrofluoric acid leachates of pyroxene or crushed rock, and the residues after such leaching. In angrite NWA 4801, similar acid treatment of pyroxene separates did not cause a resolvable age bias. In EC 002, three steps of partial dissolution in 0.2 M – 5 M HF caused irregular 207Pb*/206Pb* fractionation between leaching steps, and generally higher 207Pb*/206Pb* ratios in the residues than in HF leachates. These age biases were observed in leaching pairs with highly radiogenic Pb, and cannot be explained by mixing between radiogenic Pb, primordial Pb, and Pb introduced by terrestrial contamination. Instead, the observed isotope fractionation is attributed to the combined effects of the size difference between α-recoil tracks in the decay chains of 238U and 235U, and exsolution of primary pigeonite, leading to the formation of a lamellar structure consisting of augite and low-Ca pyroxene by either slow-cooling or subsequent metamorphic reactions. Where extensive acid leaching intended for removal of non-radiogenic Pb causes fractionation of radiogenic Pb isotopes, its detrimental effect can be reversed by performing a numeric recombination of partial leachate and residue data. Currently, it is unclear how common leaching-induced isotopic fractionation is in Pb-isotopic chronology to meteoritic materials. Acid leaching is an essential step for removal of non-radiogenic Pb in the precise Pb-isotopic dating of meteorites, which currently does not have viable alternatives. However, it is important to be aware of its possible side effects, and to continue search for new non-radiogenic Pb removal techniques that do not cause radiogenic 207Pb* and 206Pb* fractionation.

中文翻译：

---

酸浸诱导的陨石中放射性 Pb 同位素及其成分的分馏


在这项研究中，我们测试了陨石中放射性 207Pb/206Pb 比率 （207Pb*/206Pb*） 在部分溶解过程中被分馏的可能性，并探讨了这种分馏对陨石 Pb-同位素年代学的影响。我们报告了在深成岩西北非洲 （NWA） 4801 和未分组的软骨石 NWA 10132 和 Erg Chech （EC） 002 中检测通过酸浸出部分溶解诱导的 Pb-同位素分馏的实验结果。我们还重新检查了以前发表的其他软骨陨石和富含 Ca-Al 的难熔内含物 （CAI） 的 U-Pb 数据，以寻找这种分馏的证据。我们观察到，在原始软骨矿NWA 10132中，207Pb* / 206Pb*比率的差异，对应于约1-2 马的年龄偏差，在辉石或碎石的0.5 M氢氟酸浸出液与浸出后的残留物之间存在差异。在 angrite NWA 4801 中，辉石分离物的类似酸处理不会导致可分辨年龄偏倚。在 EC 002 中，在 0.2 M – 5 M HF 中部分溶解的三个步骤导致浸出步骤之间的 207Pb*/206Pb* 分馏不规则，并且残留物中的 207Pb*/206Pb* 比率通常高于 HF 渗滤液。这些年龄偏差是在具有高放射性 Pb 的浸出对中观察到的，不能用放射性 Pb、原始 Pb 和陆地污染引入的 Pb 之间的混合来解释。相反，观察到的同位素分馏归因于 238U 和 235U 衰变链中α反冲轨迹之间的尺寸差异以及原生鸽子岩的溶解的综合效应，导致通过缓慢冷却或随后的变质反应形成由奥氏石和低 Ca 辉石组成的层状结构。 当用于去除非放射性 Pb 的广泛酸浸出导致放射性 Pb 同位素分馏时，可以通过对部分渗滤液和残留数据进行数字重组来逆转其不利影响。目前，尚不清楚浸出诱导的同位素分馏在陨石材料的 Pr 同位素年代学中有多常见。酸浸是在陨石的精确 Pb 同位素测年中去除非放射性 Pb 的重要步骤，目前没有可行的替代方案。然而，重要的是要了解其可能的副作用，并继续寻找不会导致放射性 207Pb* 和 206Pb* 分馏的新型非放射性 Pb 去除技术。