Phlogopite K-Ca dating of Narayanpet kimberlites, south India: implications to the discordance between their Rb-Sr and Ar/Ar ages

Abstract

Coupled K-Ca and Rb-Sr dating of kimberlitic phlogopites, if analytically feasible, will provide ages based on two independent parent-daughter systems that are chemically similar but with very different decay constants. A concordance of the two ages increases the confidence in them and helps in resolving the serious discrepancies often seen between Rb-Sr and Ar/Ar ages of kimberlitic micas. We show for the first time that reliable K-Ca ages of phlogopites from Proterozoic kimberlites can be measured using the same mild acid-leaching technique as has been used for Rb-Sr dating. K-Ca model ages of phlogopite macrocrysts separated from two kimberlite intrusions from the Narayanpet kimberlite field, south India are close to 1100 Ma. These ages agree closely with Rb-Sr ages previously measured on the same kimberlites and those from the Wajrakarur field 250 km to the south. This agreement questions the validity of a single Ar/Ar age of 1400 Ma as representative of the entire Narayanpet field and interpretations of isotopically distinct source compositions for the two spatially separated groups of south Indian kimberlites. The radiogenic ⁴⁰Ca enrichment in phlogopites from one kimberlite is nearly 40% suggesting the feasibility of coupled K-Ca and Rb-Sr dating of much younger kimberlites using the same analytical protocol. Model or single-sample K-Ca ages calculated assuming an initial ⁴⁰Ca/⁴⁴Ca ratio of 47.153 can be quite robust, as this ratio has been found to be remarkably uniform in mantle and crustal rocks.