Zincian chromite inclusions in diamonds: Possibility of deep recycling origin

Abstract

It was reported by Meyer and Boyd (1972) that zincian chromite, having 2 wt% to 3 wt% ZnO and high (>0.9) Cr# (= Cr/(Cr + Al) atomic ratio), appears as inclusions in diamonds obtained from kimberlite. Zincian chromite is characterized by a low (<0.03) Mg# [= Mg/(Mg + ferrous Fe + Zn + Mn + Ni + Co) atomic ratio] and an appreciable amount of MnO (0.4-0.5 wt%). The chemistry of zincian chromite is very different from commonly found chromite inclusions (magnesiochromite) in diamonds, which contain low amounts of ZnO (<0.1 wt%) and MnO (<0.1 wt%). Zincian and manganoan chromites are also commonly found in meteorites and in altered/ metamorphosed peridotites and related rocks. The chromites found in meteorites mostly have an intermediate Mg# (0.2-0.5) and a high Cr# (mostly >0.8); moreover, the major-element chemistry of these chromites is similar to that of the magnesiochromite inclusions in diamonds. Spinels obtained from altered rocks show a chemical range that comprises the zincian chromite inclusions found in diamonds. The origin of these inclusions possibly lies in deep recycling. They were initially formed at the Earth's surface in altered/metamorphosed peridotites; they then sank deep down into the mantle and were entrained to the surface again by kimberlite magmatism.