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

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.