HIGH-Mg OLIVINE IN UNALTERED KIMBERLITES FROM THE UDACHNAYA-EAST PIPE ( <i>Siberian craton</i> ): MODES OF OCCURRENCE, COMPOSITION, AND ORIGIN

Studying the composition and crystallization conditions of olivine in kimberlites is crucial for understanding their petrogenesis and assessing their diamond potential. Our investigation focuses on the origin of the least studied generation of this mineral: late high-Mg olivine. The research material comprises samples of unaltered kimberlites from the Udachnaya-East pipe, where all olivine generations are fully preserved. Scanning electron microscopy and microprobe analysis reveal that high-Mg olivine exhibits the following compositional variations: Mg# (Mg/(Mg+Fe2+)×100, mol.%) 93.3–98.7, 0.01–0.05 wt.% NiO, 0.12–1.88 wt.% CaO, and 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. High-Mg olivine occurs as individual grains (the first such finding in kimberlites), rinds, daughter phases within melt inclusions, phases in embayments and fractures within earlier olivine generations, and phases in the interstices of microxenoliths. Its crystallization is interpreted to have occurred from alkaline-carbonate-chloride melts. The temperatures and oxygen fugacity values for the crystallization of high-Mg olivine are semi-quantitatively estimated at 670–780 °C and +3.6 to +7.4 lg units ΔQFM. These findings indicate that such olivine crystallized from evolved kimberlitic melts, which contradicts earlier models proposing its formation from fluids or during kimberlite serpentinization.