Composition of olivines and spinel group minerals in aillikites from the Bushkanay dyke, South Siberian Craton: Insights into alkaline melt sources and evolution

Abstract

The potential sources and conditions that control the formation and evolution of alkaline melts are far from being fully understood. To address some of these fundamental questions, we have focused on the composition of olivines and spinel group minerals in aillikites from the Bushkanay dyke in the Siberian Craton. These ultramafic carbonate-rich lamprophyres contain 40–50 vol% fresh olivine macrocrysts (200–600 µm) within a groundmass consisting of phlogopite (60%), magnetite (15%), perovskite (10%), apatite (5%), calcite (3%), chromite (1%), clinopyroxene (up to 1%), barite (up to 1%), serpentine and chlorite (up to 4%). Two types of compositional zoning in olivine have been recognized: (1) Mg-rich cores consisting of a relatively high #Mg (86-89) and NiO (0.25–0.4 wt%); (2) Fe-rich cores with a relatively low #Mg (82-86) and NiO (0.10–0.25 wt%). Both types of olivine cores are igneous and crystallize from an ultramafic carbonate-rich melt. The Mg#-poor (Fe-rich) cores were derived from an early and more evolved aillikitic melt and were later entrained by a more primitive melt, responsible for the formation of the Mg#-rich (Mg-rich) cores. The spinels exhibit a more complex zoning with four major growth zones: (1) an aluminum chromite core; (2) an Fe-chromite and Cr-magnetite transitional zone; and (3) a Ti-magnetite rim. The most primitive spinels of Al-chromite compositions crystallized directly from undifferentiated pulse of the melt, while further growth involved a gradual decrease in Cr due to the crystallization of Cr-rich spinels, while Al remained the same due to the absence of early phlogopite. Our results on olivines and spinels suggest that the aillikites of the Bushkanay dyke were derived from a phlogopite-bearing carbonatized peridotite source. This idea is also supported by the presence of phlogopite, calcite, and amphibole as major phases in the chromite melt inclusions.