Reactionary interaction of upper mantle minerals with SiC at 6 GPa and 1100–1500 °C: Implication for moissanite appearance in ophiolitic peridotite and other uncommon natural environments

Abstract

Experiments were carried out on the interaction of mantle rocks and minerals with SiC at 6 GPa and 1100–1500 °C. Fine powders of natural peridotites and eclogites from kimberlites with the addition of 10 wt% SiC, as well as monomineralic powders and crystals of chromite, olivine, and garnet, were used as starting materials. The experiments were carried out on a 1500-ton multianvil press. In all experiments, the reduction of silicates and chromite was observed with an increase in the magnesium number (Mg#=100 Mg/(Mg + Fe)) of minerals to 97–100 % and the precipitation of metallic phases − alloys in the Fe-Ni and Fe-Si-Cr systems. The most pronounced reactions occurred at 1500 °C. Equilibrium was not reached in all the studied systems even after 72 h of annealing at 1300 °C. Instead, reaction rims were formed on mineral grains and finely dispersed iron alloys were precipitated throughout the volume of crystals. The reaction between SiC and chromite was fastest. The Mg# of chromite changed from 57 (in the initial chromite) to 80–94 %. It was shown that at 6 GPa, SiC reacted quickly with minerals and mantle rocks to form metal phases, high-magnesian silicates, and chromite. This means that they cannot be in equilibrium, as was previously stated for ophiolite peridotite and chromitite, and volcanic rocks such as basalts and kimberlites. The kinetic relationships indicate that a few millimeter grains of SiC can survive in the fluid-free mantle during subduction-exhumation only in cold environments below 700–800 °C. We believe that SiC in most natural rocks is related to anthropogenic contamination. In rare cases, which should be carefully documented, moissanite may have a natural terrestrial origin related to very localized events of formation of highly reduced hydrogen-bearing fluids.