Geochemical characteristics of spherules from the Pivdenna kimberlite pipe, East Azov region (Ukraine): implications for their sources and origin

Abstract

The composition of spherules and particles of native metals from the Pivdenna kimberlite pipe, Ukraine, was studied using the SEM/EDS method. Three varieties of spherules have been distinguished: titanium-manganese-iron-silicate (TMIS) spherules, Ca-rich silicate spherules, and magnetite-wustite-iron (MW-I) spherules. TMIS spherules are composed of homogeneous glass, some having a native iron core. Large TMIS spherules may contain a crystalline phase with needle-like armalcolite. Ca-rich silicate spherules can be subdivided into two subtypes: calcium-silicate (CS) spherules where SiO2 and CaO are the dominant constituents, and calcium-iron-silicate (CIS) spherules with significant FeO content. CS spherules may contain a core consisting of native phases (Fe, Fe-Si, and Mn-Si-Fe). Native metal particles are represented by native Cu and native Zn. The spherule varieties from the Pivdenna pipe are similar to those from other kimberlite pipes in the world. We infer that the formation of spherules occurred in gas-melt streams, separately from the kimberlites, and propose a model for the formation of the most common variety of spherules (TMIS and MW-I varieties) in the region of the core-mantle boundary (CMB). First, a melt of the Fe-Ti-Mn-Si-O system was formed in ultra-low-velocity zones (ULVZ) as a result of thermochemical reactions (reduction) between the molten core and solid oxide-silicate rocks. The melt then migrates to shallower levels, where a decrease in temperature initiates oxidation with the formation of SiO2-TiO2-FeO-MnO-Fe0 melt, i.e. parent melt of TMIS and MW-I spherules. We interpret the formation of native metals in kimberlites as a result of the decomposition of nitrides, which came from the Earth’s core via intratelluric flows