SINGLE MINERAL PB-PB DATING

A new stepwise leaching procedure applied to Ca-bearing silicates enab les the selective recovery of radiogenic and common Pb from a mineral, and thus makes single-phase Pb-Pb dating possible. This permits indep endent dating of rock-forming minerals within a paragenetic sequence a nd the direct combination of age data with petrological results. Furth ermore, the stepwise leaching technique has the advantage over multi-p hase Pb-Pb dating in that it does not require initial Pb isotope equil ibrium among the phases, but may be used to test it. The presence of m icroscopic solid inclusions, which complicate dating of single phases if they are not in equilibrium with the host, can be traced via the be haviour 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-20 4 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 se lf-consistent P-T-t modelling.