Laser-probe 40Ar/39Ar analyses of phenocrystal and xenolith-derived phlogopites have been used to evaluate the method for dating kimberlite emplacement ages and to investigate some of the problems inherent in previous K-Ar and incremental heating 40Ar/39Ar analyses of kimberlitic phlogopite. Individual laser 40Ar/39Ar spot ages of microphenocrystal phlogopites for two Siberian kimberlites, Udachnaya and Mir, vary from an error of the likely emplacement ages to 10% and higher. The mean age of microphenocrysts from another, previously undated pipe, Leningrad, is within the range for other pipes in the same field although the error is high (385±22 Ma). Phlogopite from the Obnazhennaya pipe produced 40Ar/39Ar ages substantially older (416±36 Ma) than the poorly constrained but geologically more reasonable age of 135 Ma. The resolution of the laser probe technique reveals substantial heterogeneities in the spatial distribution of 40Ar within the kimberlite microphenocrysts. The minimum ages for individual laser extractions appear to give the closest approximation to the pipe emplacement age. A macrocrystic mica from the Udachnaya kimberlite contains considerably more "excess" 40Ar in the center of the grain, giving ages >60% higher than the U-Pb perovskite emplacement age of 353-367 Ma. Phlogopites from southern African and Siberian mantle xenoliths in kimberlites show intracrystalline 40Ar variations indicating incomplete Ar loss during kimberlite ascent. Some Ar loss profiles approximate to those theoretically predicted for volume diffusion while others show profiles indicating more complex processes. The much lower abundance of radiogenic 40Ar in phlogopite microphenocrysts compared to macrocrystal and xenolith derived micas suggest that this "excess" of Ar, causing the anomalously old K-Ar ages and incremental heating 40Ar/39Ar spectra, is probably incorporated within the mantle and progressively degassed during kimberlite ascent.
|Number of pages||12|
|Journal||Geologiya i Geofizika|
|Publication status||Published - 1 Dec 1997|