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

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• 作者：David Phillips ; ^{dphillip@unimelb.edu.au}
• 关键词：Geochronology ; 40Ar/39Ar ; Kimberlite ; Lamproite ; Phlogopite
• 刊名：Precambrian Research
• 出版年：2012
• 期刊代码：75_03019268
• 类别：ear
• 出版时间：July, 2012
• 卷：208-211
• 期：Complete
• 页码：49-52
• 文件大小：315 K

摘要

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. 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 . 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.