The promise and potential pitfalls of acid leaching for Pb-Pb chronology

Kengo T.M. Ito, Yuki Hibiya, Yoshitaka Homma, Takashi Mikouchi, Tsuyoshi Iizuka

Chemical Geology
Volume 525, 20 October 2019, Pages 343-355



• We studied the mechanism by which acid leaching separates radiogenic and non-radiogenic components for Pb-Pb chronology.
• Textural changes during acid leaching were investigated for a range of minerals using the SEM technique.
• Acid leachates and residues of pyroxene and whole-rock fractions from three achondrites were analyzed for trace elements.
• The combined results revealed mineral dissolution behaviors and, by extension, Pb elution processes during acid leaching.
• We found that HF leaching can result in inaccurate age estimates for pyroxene grains having lamellar structures.”

“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 Pb-Pb 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 Pb-Pb 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) HNO3 and 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 HNO3 and 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 Pb-Pb 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.”