Phlogopite phenocrystals with inclusions of chrome spinelides from non-diamond-bearing kimberlites: the case of An-195 body of the West Ukukit field

Abstract

The article presents the results of a microprobe study of the composition of phlogopite macrocrystals with inclusions of chrome spinelides from An-195 kimberlite body of the West Ukukit field; Yakut kimberlite province. The purpose of this study was to establish the genesis of such phlogopites: whether they are representatives of deep mantle matter or their formation is directly related to the formation of kimberlites composing An-195. It was found that the inclusions of chrome spinelides are confined only to chromic non–zonal phlogopites or to chromic sites of zonal macrocrystals (Cr2O3 0.75–1.55 wt.%; TiO2 2.67-3.84 wt.%; Mg# 83-88). Based on a comparative analysis of the studied phlogopites and chromespinelides with the composition of analogues from the parageneses of these minerals in various mantle xenoliths of ultrabasic rocks; eclogites and rocks of the MARID series; it was concluded that these phlogopites most likely represent a phenocrystal association of kimberlite minerals. They clearly differ from phlogopites of the MARID series rocks with equal Mg# and TiO2 in higher Al2O3 contents; from phlogopites of ultrabasic mantle rocks xenoliths – generally with lower magnesia content. Phlogopites from eclogites have a generally higher Cr2O3 content. Inclusions of high–chromium spinelides in phlogopite (Cr2O3 44-50 wt.%; TiO2 3-5.5 wt.%; Fe3+ < 0.35 f.e.) correspond in composition to spinelides from cataclysmic dunites and are products of their recrystallization under the influence of high-temperature mantle fluid. Inclusions of less chromic spinelides (Cr2O3 30-44 wt.%; TiO2 4-7.5 wt.%; Fe3+ > 0.35 fe) probably crystallized simultaneously with phlogopite macrocrystals and were captured by them during long-term co-growth.