The promise and potential pitfalls of acid leaching for Pb Pb chronology
The promise and potential pitfalls of acid leaching for Pb Pb chronology
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DOI：
10.1016/j.chemgeo.2019.07.035
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发表时间：
2019-10
期刊：
影响因子：
3.9
通讯作者：
K. Ito;Y. Hibiya;Y. Homma;T. Mikouchi;T. Iizuka
中科院分区：
文献类型：
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作者：
K. Ito;Y. Hibiya;Y. Homma;T. Mikouchi;T. Iizuka
酸浸通过分离样品中的放射性成因铅和非放射性成因铅，提高了年代学的精密度和准确度，因此在铅-铅年代学研究中得到了广泛的应用。然而，放射性成因铅分离的详细机制仍然不清楚。本文报道了三块非球粒陨石(D‘Orbigny，Ibitira和西北非洲6704)和一块石铁陨石(Brenham)中酸浸矿物的扫描电子显微镜观察。此外，我们还用电感耦合等离子体质谱对三个非球粒陨石的酸浸出液、辉石残留物和全岩馏分进行了化学分析。综合结果揭示了矿物对HNO3、HCl和HF酸处理的抵抗力，进而使我们能够探索酸浸对PbSingle Bond Pb年代学的前景和局限性。特别是，我们发现：(I)用稀酸洗涤可以有效地释放吸附在矿物表面的污染物陆地铅；(Ii)热的和更集中的(~6 M)HNO3和HCl可以将辉石与高度富含初始非放射性成因的铅的硫化物和斜长石分离；(Iii)钠长斜长石和辉石在HNO3和HCl处理过程中表现出有限的溶解，但被热的1 M HF逐步浸出，使得它们难以相互分离；以及(Iv)在具有外溶片层结构的单个辉石颗粒中，HF处理比低钙片层更有效地浸出高钙片层。在辉石片层的两阶段铅同位素演化模型的背景下，我们证明了辉石片层与稀HF的不均匀和不完全溶解会导致不准确的Pb单键铅年龄估算。这可能解释了以前观测到的陨石样品的分散的铅同位素数据，这些数据被作为高频处理期间放射性成因铅成分的同位素分馏的证据。
Acid leaching has been widely used in Pb–Pb chronology especially on meteorite samples, as it enhances the precision and accuracy of the chronology by separating radiogenic Pb from non-radiogenic Pb in the samples. Yet, the detailed mechanism of the radiogenic Pb separation remains elusive. Here we report scanning electron microscopy observations of acid-leached minerals from three achondrites that were previously analyzed for pyroxene Pbsingle bondPb dating (D'Orbigny, Ibitira and Northwest Africa 6704), and one stony‑iron meteorite (Brenham). Moreover, we present chemical analysis of acid leachates and residues of pyroxene and whole-rock fractions from the three achondrites using inductively coupled plasma mass spectrometry. The combined results reveal the resistance of minerals to acid treatments with HNO3, HCl, and HF and, in turn, allow us to explore the prospects and limitations of acid leaching for Pbsingle bondPb chronology. In particular, we show that (i) washing with dilute acids can efficiently liberate contaminant terrestrial Pb adsorbed on the mineral surface; (ii) hot and more concentrated (~6 M) HNO3and HCl can separate pyroxenes from sulfides and anorthitic plagioclase that are highly enriched in initial non-radiogenic Pb; (iii) albitic plagioclase and pyroxenes show limited dissolution during the HNO3and HCl treatments, but are progressively leached by hot 1 M HF, making it difficult to separate them from each other; and (iv) within single pyroxene grains having exsolved lamellar textures, high-Ca lamellae are more efficiently leached by the HF treatment than low-Ca ones. In the context of a two-stage Pb isotopic evolution model for pyroxene lamellae, we demonstrate that heterogeneous and incomplete dissolution of pyroxene lamellae with dilute HF can result in inaccurate Pbsingle bondPb age estimates. This may account for previously observed scattered Pb isotopic data for meteorite samples that were taken as evidence for isotope fractionation in a radiogenic Pb component during HF treatments.