The inclusions of SiO2 in sublithospheric diamonds

Abstract

The specific features of the mineralogy of SiO2 inclusions in sublithospheric diamonds are described in this study. Such diamonds are characterized by a complex growth history with stages of growth and dissolution and postgrowth processes of deformation and crushing. The nitrogen content in all studied crystals does not exceed 71 ppm and nitrogen is detected only as B-defects. The carbon isotope composition of diamonds varies widely from -26.5 to -6.7 ‰ of δ13С. SiO2 inclusions associate with omphacitic clinopyroxenes, majoritic garnets, CaSiO3, jeffbenite and ferropericlase. All SiO2 inclusions are coesite, which is often accompanied by micro-blocks of kyanite. These phases are suggested to represent the product of the retrograde transformation of the primary Al-stishovite. Significant internal stresses in the inclusions and deformations around them can be evidence of thise phase transformation. The heavier oxygen isotope composition of SiO2 inclusions in sublithospheric diamonds (up to 12.9 δ18O) indicates the crustal origin of their protoliths. The observed anti-correlation of δ18O of SiO2 inclusions and δ13C of their host diamonds reflects the processes of interaction of slab-derived melts with reduced mantle rocks at depths above 270 km.