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]

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/gain effects, thus complicating data interpretation and reducing confidence in the reliability of the reported ages. In particular, it is suggested that the Pochampalle lamproite was emplaced <1400 Ma ago, rather than ca.1500 Ma, as suggested by Osborne et al. (2011). It is recommended that 40Ar/39Ar geochronology of kimberlite and lamproites include step-heating analyses of matrix phlogopite grains, with anomalous or contentious results verified using other dating methods.

► I comment on the article by Osborne et al (2011; Precambrian Res. 189, 91–103).
► 40Ar/39Ar results from two Indian kimberlites and a lamproite are critically evaluated.
► The common presence of extraneous argon in phlogopite xenocrysts/phenocrysts is highlighted.
► It is suggested that the Pochampalle lamproite is younger than ca. 1400 Ma.