Inclusion of Kokchetavite in a Diamond Crystal from Venezuela: Evidence of Subduction of the Continental Crust

Abstract

Diamond crystals with numerous mineral inclusions from the Guanamo placer deposit (Venezuela) were studied in detail. The inclusions in the studied diamonds are represented by a set of minerals typical of eclogite paragenesis indicators—omphacite, garnet, kyanite, coesite and rutile. Besides single mineral inclusions, multiphase inclusions were also identified. Mineral associations of multiphase inclusions adjacent within less than 100 microns of each other within the same growth zone can vary considerably. For example, in one of the diamond crystals examined, multiphase inclusions are represented by the following associations: sanidine-dolomite-anatase and magnetite-rutile-dolomite-apatite-kokchetavite-graphite. Note that this is the first discovery of kokchetavite as an inclusion in kimberlite diamond crystals. Until now, this hexagonal polymorphic modification of KAlSi₃O₈ has only been diagnosed in minerals from rocks of the continental crust that were formed under conditions of high or ultra-high pressure metamorphism. Thus, the subduction of continental crustal material into the mantle and its important role in the metasomatic alteration of upper mantle rocks in Venezuela can be inferred from the presence of kokchetavite as inclusions in kimberlite diamond crystals.