Xeno-autolithic nodules from kimberlites of the “Nadiia” pipe (Eastern Azov region, Ukraine)

Abstract

Xeno-autolithic nodules from kimberlites of the Nadiia pipe (the Eastern Azov region, Ukraine) have been investigated. Based on petrographic and petrochemical criteria it was concluded that the analyzed nodules were formed according to the principle of the “snowballs” formation in the apical parts of the penetrating magma column at various pressures and temperatures. Nodules originated around xenocrysts, xenoliths and viscous magmatic clots by overgrowing them with orbicular shells under conditions of a mobile medium and flotation gas-liquid differentiation. The nodule shells are represented by porphyric kimberlite, composed of porphyric pseudomorphs of diaphtoritic minerals after olivine, clinopyroxene, phlogopite, biotite and ilmenite, and a groundmass consisted of micron-sized crystallites of calcite, barite, serpentine, perovskite, phlogopite, amphibole, chlorite, formed by crystallization of kimberlite glass of heterogeneous composition. The groundmass also contains newly formed feldspar and quartz. Accessory minerals include zircon, apatite, and rutile. Ore minerals are represented by magnetite, titanomagnetite, ilmenite, nickel and copper sulfides. Due to the assimilation of carbonate rocks by the melt, the composition of the shells, in the direction from the center to the edge of the nodules, changes from serpentine, through chlorite-serpentine to silicate-carbonate. The initial portions of the melt with olivine and phlogopite megacrysts were successively transformed into a homogeneous one. The estimated depth of kimberlite magma generation did not exceed 150 km at the initial crystallization temperatures of the silicate component of 1300-900 °C. Vitrification of the nodules groundmass in the crater zone of diatreme, estimated using monomineral phlogopite geothermometer, occurred at temperatures of 650-550 °C. The absence of diamond paragenesis minerals in nodules, in combination with other mineralogical, petrological and petrochemical criteria indicates that kimberlites of the «Nadiia» pipe are diamond barren.