A feasibility study of U−Pb and Pb−Pb dating of kimberlites using groundmass mineral fractions and whole-rock samples

Abstract

This paper describes partly successful attempts to determine emplacement ages of kimberlites by U−Pb and Pb−Pb methods, involving groundmass minerals with high U content (notably perovskite) and whole-rock kimberlite samples. U/Pb ratios in perovs-kite in the matrix of kimberlites can be two orders of magnitude larger than in the rest of the kimberlite material, and with simple mineral separation techniques moderate success was achieved in U−Pb dating of fresh samples of younger kimberlites (around 100 Ma). The differences in U/Pb ratios between kimberlite samples from different parts of the same pipe have also been found to be large enough, in some cases, to allow reasonably accurate U−Pb age determination. In older kimberlites the U−Pb ages obtained were mostly incompatible with geological constraints and results obtained by other methods. However, for such pipes use of Pb−Pb systematics yields realistic age limits in some cases.