Spherules in kimberlites: The products of an unknown extreme type of igneous activity of the Earth

Small spherical mineral formations called spherules are encountered in kimberlites. Spherules of kimberlite origin are a poorly studied phenomenon, and their nature and relationship to kimberlites remain an open question. In this study, the authors present the textural, mineralogical, and geochemical characteristics of spherules recovered from 9 kimberlite-lamproite structures in the Ukrainian Shield, Arkhangelsk, and Yakutia diamond provinces. Chemical compositions were studied by energy-dispersive spectrometer (EDS) equipped in the SEM. The studied suite of spherules can be distinctly divided into three varieties: (1) titanium-manganese-iron-silicate (TMIS) spherules consisting of TiO2–MnO–FeO–SiO2 glass; (2) calcium-silicate (CS) spherules consisting of SiO2–CaO glass; (3) magnetite-wüstite-iron spherules (MW-I) made up of microcrystalline magnetite-wüstite aggregate. All spherule varieties may include a core composed of native iron. Observations have shown that spherules commonly occur in association with other highly reduced mineral formations including particles of native Pb, Sn, Sb, Cu, Zn, Au, and their alloys; Ti3+-bearing corundum with inclusions of Fe, Si–Fe, Si–Ti–Fe, TiN, and TiC phases; and individual grains of oxygen-free minerals such as qusongite (WC), moissanite (SiC), and diamond (C). According to the proposed model, spherules and the associated highly reduced particles are considered to be formed in gas-melt streams sourced from deep mantle reservoirs. The parental spherule melts are formed in the lowermost mantle, the D" layer, as a result of thermochemical redox reactions, and then migrate to the upper mantle level with rising plumes. The possibility of using spherules as powerful mineral indicators in the search for kimberlite-hosted and placer diamond deposits has been considered.