Evolution of the Victoria Pipe Ultramafic Alkaline Melt (Anabar Region, Siberian Craton): Melt Inclusion within Olivine and Groundmass Minerals

Abstract

The results of studying primary and secondary melt inclusions within olivine macrocrysts and groundmass minerals from monticellite–nepheline damtjernites of the Victoria pipe (Anabar diamondiferous province, Siberia craton) are presented. They were used to reconstruct the evolution of the ultramafic alkaline melt during the formation of the pipe. It is shown that the primary damtjernite melts were K–Na bearing carbonate–silicate in composition. Here, K and Na in the primary melt inclusions within olivine macrocrysts enter the composition of essentially silicate daughter phases. According to this, they are different in composition from similar inclusions in olivine from aillikites and kimberlites and emphasize the more alkaline character of the damtjernitic parental melts. At the subsequent stages of melt evolution during pipe formation, Na and K in the studied inclusions not only enter silicate daughter phases but can also form alkaline phosphates, carbonates, sulfates, and halides. This leads to the formation of alkaline carbonate and saline sulfate–phosphate–chloride–carbonate solutions. According to this, the evolution of the damtjernitic melt is similar to that of aillikite and carbonatite melts and can serve as a uniform mechanism of evolution of alkaline–ultramafic melts. The further reactionary interaction of the fluid phase with olivine led to the formation of monticellite and processes of degassing.