SiO2 Inclusions in Sublithospheric Diamonds

Abstract

The paper describes mineralogical characteristics of SiO₂ inclusions in sublithospheric diamonds, which typically have complicated growth histories showing alternating episodes of growth, dissolution, and postgrowth deformation and crushing processes. Nitrogen contents in all of the crystals do not exceed 71 ppm, and nitrogen is detected exclusively as B-defects. The carbon isotope composition of the diamonds varies from δ¹³С = –26.5 to –6.7‰. The SiO₂ inclusions occur in association with omphacitic clinopyroxenes, majoritic garnets, CaSiO₃, jeffbenite, and ferropericlase. All SiO₂ inclusions are coesite, which is often associated with micro-blocks of kyanite in the same inclusions. It was suggested that these phases have been produced by the retrograde dissolution of primary Al-stishovite, which is also evidenced by the significant internal stresses in the inclusions and by deformations around them. The oxygen isotope composition of SiO₂ inclusions in sublithospheric diamonds (δ¹⁸O up to 12.9‰) indicates a crustal origin of the protoliths. The negative correlation between the δ¹⁸O of the SiO₂ inclusions and the δ¹³C of their host diamonds reflects interaction processes between slab-derived melts and reduced mantle rocks at depths greater than 270 km.