HIGH‐MG OLIVINE IN THE UNALTERED KIMBERLITES OF UDACHNAYA-EAST PIPE (SIBIRIAN CRATON): OCCURRENCE, COMPOSITION AND ORIGIN
A.A. Tarasov1, A.V. Golovin1
1Sobolev Institute of Geology and Mineralogy Siberian Branch Russian Academy of Sciences, Novosibirsk, Russia
Keywords: high-Mg olivine, kimberlites, Udachnaya-East pipe, melt inclusions, rinds, alkali-carbonate melts, chloride melts, oxygen fugacity, olivine zoning, kimberlite magma evolution.
Abstract
The study of the composition and crystallization conditions of olivine in kimberlites is of great importance for understanding the processes of their petrogenesis and predicting diamond potential. The aim of this study is to investigate the origin of the least studied generation of this mineral – late high-magnesium olivine. The material for the study consisted of samples of unaltered kimberlites from the Udachnaya-East pipe, where all generations of olivine are preserved. Results from scanning electron microscopy and microprobe analysis showed that high-Mg olivine exhibits the following variations in chemical composition: Mg# (Mg/(Mg+Fe2+)×100, mol. %) 93.3-98.7, 0.01-0.05 wt. % NiO, 0.12-1.88 wt. % CaO, 0.18-0.94 wt. % MnO. This olivine forms a paragenetic association with late magmatic minerals of kimberlites: magnetite, perovskite, apatite, monticellite, sodalite, phlogopite, djerfisherite, and calcite. The following forms of high-Mg olivine were identified: individual grains (the first finding in kimberlites), rinds, daughter phases within melt inclusions, phases in fractures earlier olivine generations, and in the interstices of microxenoliths. It was found that its crystallization occurred from alkaline-carbonate-chloride melts. The temperatures and oxygen fugacity values of the crystallization of high-Mg olivine can be estimated semi-quantitatively at 670-780 °C and +3.6 - +7.4 log. units ΔQFM.. The obtained data indicate that such olivine crystallized from evolved kimberlitic melts, which contradicts previously proposed models suggesting the formation of this olivine from fluids or during the serpentinization of kimberlites.
|
