DIAMOND RESORPTION BY OXIDIZED INTERGRANULAR C–O–H FLUID

Experimental studies on modeling the diamond resorption processes during mantle metasomatism accompanied by oxidation process in solid-phase matrix in the presence of intergranular fluid have been carried out. The reaction conducted between diamond and periclase to form magnesite can be considered as prototype EMOD (enstatite-magnesite/olivine-diamond) or DCDD (dolomite-coesite/diopside-diamond) reactions. The experimental studies were conducted at a pressure of 6.3 GPa in the 1100–1400 °C temperature range under redox conditions corresponding to the WM (wüstite/magnesite) buffer. It was found that the reaction between diamond and periclase to form magnesite occurs only in the presence of 0.5–0.8 wt.% water at temperatures above 1200 °C. The morphology of diamond crystals partially dissolved by intergranular H2O-fluid at fO2 at the WM buffer level represents a typomorphic feature of diamond dissolution/resorption in water-containing carbonate and carbonate-silicate melts. The main microrelief elements of diamond dissolution forms are negatively orientated triangular etch pits on relict octahedral faces, shield-shaped or ditrigonal dissolution layers, and drop-shape hillocks. The obtained resorption rates at these P-T-fO2 parameters indicate that the absence of diamond in kimberlites or low-grade potentially diamondiferous kimberlite pipes might be caused by oxidizing metasomatic events in the lithospheric mantle in the regions of kimberlite emplacement.