A Find of Coesite in Diamond-Bearing Kyanite Eclogite from the Udachnaya Kimberlite Pipe, Siberian Craton

A find of coesite in a kyanite graphite–diamond-bearing eclogite xenolith from the Udachnaya–Vostochnaya kimberlite pipe is described in this paper. The coesite relics were found in intensely fractured garnet indicating some influence of the kimberlite melt, which is supported by the typical secondary mineral assemblage around this inclusion. These data indicate that shallower diamond-free coesite-grade rocks (2.7 GPa) underwent metamorphism distinct from diamond-bearing coesite eclogites (≥4 GPa). The metasomatic alteration of rock as a result of the C–O–H fluid-rock interaction during diamond crystallization may be another possible reason for the absence of coesite in diamond-bearing xenoliths.