Comment on "New Ar-Ar ages of southern Indian kimberlites and a lamproite and their geochemical evolution" by Osborne et al. [Precambrian Res. 189 (2011) 91-103]

Abstract

Osborne et al. (2011; Precambrian Res. 189, 91-103) report new 40Ar/39Ar age results for two kimberlites (Muligiripalle pipe 5 and Tummatapalle pipe 13) and one lamproite (Pochampalle), from the Archaean Dharwar craton in southern India. Previous studies indicate that kimberlites and related rocks are highly susceptible to extraneous argon contamination, leading to anomalously old ages; although reliable 40Ar/39Ar ages can be obtained from step-heating analyses of groundmass phlogopite grains. Osborne et al. (2011) carried out single-step 40Ar/39Ar fusion analyses on phlogopite phenocrysts and xenocrysts. This approach is not favoured, because it does not allow identification of argon loss/g..