Inclusion of kokchetavite in a diamond crystal from Venezuela: an evidence of continental crust subduction

We have examined in detail diamond crystals from the Guanyamo placer deposit (Venezuela) with numerous mineral inclusions. Inclusions in the studied diamonds are represented by a typical set of eclogite-paragenesis indicator minerals: omphacite, garnet, kyanite, coesite and rutile. In addition to single mineral inclusions, polyphaser inclusions have also been diagnosed. The mineral associations of polyphase inclusions adjacent to each other within a distance of less than 100 micrometres within the same growth zone can vary significantly. For example, in one of the diamond crystals studied, polyphase inclusions are represented by the following associations: sanidine – dolomite – anatase and magnetite – rutile – dolomite – apatite – kokchetavite – graphite. It should be noted that this is the first discovery of kokchetavite as an inclusion in diamond crystals of kimberlite origin. Previously, this hexagonal polymorphic modification of KAlSi3O8 had only been diagnosed in minerals from continental crustal rocks formed under conditions of high or ultra-high mafic metamorphism. Thus, the presence of kokchetavite as inclusions in diamond crystals carried by kimberlites allows us to infer the subduction of continental crustal material to mantle depths and its important role in metasomatic alteration of upper mantle rocks.