Petrogenesis of juvenile pelletal lapilli in ultramafic lamprophyres: A new type of “melt inclusions”

Abstract The Chadobets alkaline-carbonatite complex (Siberian craton) is a natural laboratory for all varieties of ultramafic lamprophyres, including damtjernites formed by fluid-explosion mechanisms, which contain a large number of pelletal lapilli. Data obtained from comprehensive mineralogical, structural and chemical studies of these pelletal lapilli show strong similarity with the main magmatic mineral assemblage of damtjernites, suggesting a juvenile composition for them. The composition of phlogopite, carbonate and fluorapatite in pelletal lapilli is mostly constrained toward primitive compositions (beginning) of mineral crystallization trends in ultramafic lamprophyres. According to the petrographic and mineralogical features found in pelletal lapilli from damtjernites, these can be divided into three types based on the conditions and depth they were formed at. Estimated late magmatic temperatures of pelletal lapilli mica and fluorapatite formation vary from 815–990°C. The distinct composition and genesis of juvenile pelletal lapilli allows us to propose them as a new type of melt inclusions in ultramafic lamprophyres.