A feasibility study of uranium-lead ilmenite geochronology, Monastery kimberlite, South Africa.

摘要

The Monastery kimberlite, South Africa, is considered to be a Group 1 kimberlite with an emplacement age of 90 Ma. This study demonstrates that multiple isotopic techniques can be applied to a single kimberlite and yield precise and consistent emplacement ages: Rb-Sr phlogopite age of 89.9 ± 3.1 Ma, U-Pb perovskite age of 91.0 ± 1.0 Ma, U-Pb mantle zircon age of 89.9 ± 3.5 Ma.; The results of a U-Pb ilmenite geochronology study are highly encouraging. There is a considerable variation in parent/daughter ratios (39.8–311.4) between ilmenite megacrysts and macrocrysts and even within a single megacryst (39.8–115.5). These variations are interpreted to be of primary origin. The U-Pb ilmenite age obtained for the Monastery kimberlite in this study is 95 ± 14 Ma, in good agreement with the age of kimberlite emplacement determined by other radiometric techniques both in this study and in previous studies. Although further refinement of the technique is needed, this study demonstrates for the first time the feasibility of U-Pb geochronology of kimberlitic ilmenite.