Single mineral Pbsingle bondPb dating

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Abstract

A new stepwise leaching procedure applied to Ca-bearing silicates enables the selective recovery of radiogenic and common Pb from a mineral, and thus makes single-phase Pbsingle bondPb dating possible. This permits independent dating of rock-forming minerals within a paragenetic sequence and the direct combination of age data with petrological results. Furthermore, the stepwise leaching technique has the advantage over multi-phase Pbsingle bondPb dating in that it does not require initial Pb isotope equilibrium among the phases, but may be used to test it. The presence of microscopic solid inclusions, which complicate dating of single phases if they are not in equilibrium with the host, can be traced via the behaviour of the 208Pb/204Pb ratio through the various leaching steps. This constitutes a major advantage over conventional bulk-phase Pbsingle bondPb dating. In addition, the precision of an isochron age can be improved relative to that of the bulk mineral approach, since the 207Pb/204Pb and 206Pb/204Pb data spread is significantly enhanced. Therefore, the technique may both be used to date younger phases and/or minerals with less favourable U/Pb ratios and to extend the applicability of self-consistent P-T-t modelling.

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    Citation Excerpt :

    The separated chondrules were processed for Pb isotopic analysis under clean laboratory conditions at the Ecole Normale Supérieure in Lyon (ENS Lyon). Because aggressive acid step leaching is known to achieve efficient separation of radiogenic and initial Pb (Frei and Kamber, 1995), after weighing each individual chondrule, or pool of multiple chondrules, into new, pre-cleaned Savillex beakers, they were leached successively in double-distilled hot 1 M HBr, hot 6 M HCl, and hot concentrated HNO3, each acid a single 1 hour leach session at 120 °C interspersed with ultrasonication steps and clean (18.2 MΩ cm−1) water rinses, then dissolved in a 3:1:0.5 mixture of double-distilled concentrated HF:HNO3:HClO4. All residue and leachate fractions were run individually through 0.25 ml anion-exchange chromatographic columns to separate Pb (collected with double-distilled 6 M HCl after elution of the sample matrix with double-distilled 1 M HBr), which was analyzed for its isotopic composition by MC-ICP-MS, also at ENS Lyon.

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