About 300 zircon grains have been recovered from the kimberlite concentrate of the Nyurbinskaya pipe with the aim of finding mantle zircons that would allow estimation of the age of kimberlite emplacement. We evaluated the kimberlitic or crustal origin of individual zircon grains using a two-step approach. The first step included the preparation of mounts, collecting cathodoluminescense images of the zircons, electron microprobe analysis of major elements and LAM-ICPMS analysis of trace elements. These data identified zircons with low content of U (<50ppm) and Y (1-150ppm) and low Nb/Ta (<3) ratio of probable mantle (kimberlitic) origin and zircons of crustal origin with increased Y and U content. The second step involved U-Pb dating and Hf-isotope analysis. The largest population, consisting of crustally-derived zircons, defines a U-Pb age peak at ca 2700 Ma, probably representing the Archean age of the lower crust below the Nakynsky kimberlite. This main population shows a range of epsilon Hf (t) values of 3.2 to -14.6 and T (sub DM) crustal model ages between 3.6 and 2.7 Ga, and suggests that at least 70% of the deep crust was formed from 3.2-2.8 Ga ago. The minor age population represented by the kimberlite zircons of the Nurbinskaya pipe gave a weighted mean (super 206) Pb/ (super 238) U age of 381+ or -7 Ma, which is considered as the best estimate for the kimberlite emplacement. The zircons of this younger age population have a narrow range of epsilon Hf(t) values of -33.5 to -34.5 and T (sub DM) crustal model age of 3.2 Ga.
|Number of pages||1|
|Publication status||Published - 2011|
|Event||Goldschmidt Conference (21st : 2011) - Prague, Czech Republic|
Duration: 14 Aug 2011 → 19 Aug 2011