SINGLE MINERAL PB-PB DATING

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 Pb-Pb 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 Pb-Pb 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 Pb-208/Pb-204 ratio through the various leaching steps. This constitutes a major advantage over conventional bulk-phase Pb-Pb dating. In addition, the precision of an isochron age can be improved relative to that of the bulk mineral approach, since the Pb-207/Pb-204 and Pb-206/Pb-204 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.