HIGH-BARIUM PHLOGOPITE FROM KIMBERLITE OF THE MALOKUONAPSKAYA PIPE (YAKUTIA): COMPOSITIONAL FEATURES AND 40AR/39AR DATING
D.V. Kuzmin 1*, A.A. Tomilenko 1, D.S. Yudin 1
Sobolev Institute of Geology and Mineralogy, SB RAS, Novosibirsk, Russia
Keywords: high-barium phlogopite, kimberlite, Malokuonapskaya pipe, 40Ar/39Ar age
Abstract
We present new data on high-barium phlogopites, first discovered in the kimberlite of the Malokuonapskaya pipe. The studied phlogopites can be divided into two groups: 1 – megacrysts up to 2 mm in size with a BaO content of up to 0.58 wt.%, characterized by an increase in the content of this component from the center to the edge (having direct zoning by BaO), and the 2nd group – zonal crystals up to 200 µm in size, containing minerals of the bulk mass of kimberlite, with a BaO concentration of up to 14.6 wt% in the central parts, regularly decreasing to 1 wt% towards the edges of the crystals. The formation of these two generations of micas corresponds to different, unrelated processes. Crystallization of megacrysts took place at the earliest stages of the evolution of the kimberlite melt. A slight increase in Ba concentration from the centers to the edges of phenocrysts may be a consequence of the evolution of kimberlite melt with its enrichment in incompatible elements. Phlogopites of the second group were formed at a later stage, together with the minerals of the groundmass. High barium contents in these phlogopites are possible with a light elements enrichment of the kimberlite melt. The source of these elements could have been the refertilized lithosphere under the influence of the Siberian traps.
40Ar/39Ar dating of phlogopite showed an age of 232.1+2.6 million years.
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