Experimental study of dissolution style of diamonds from volcaniclastic vs. hypabyssal kimberlite facies: the effect of melt composition on kimberlite eruption and geology

Kimberlites are the deepest and the most enigmatic magmas that reach the surface of the Earth. Their source, origin and even composition are a subject of debates. Kimberlites form hypabyssal sills and dykes but most often occur as explosion pipes, which comprise various volcaniclastic and magmatic units. Differences in the geological composition, shape and size of kimberlite pipes worldwide arise from the differences in the eruption processes and are the base for distinguishing three kimberlite classes. However, it is not clear if these differences result from the properties of the country rocks or from variable magma composition especially H2O : CO2 ratio.

During the ascent, kimberlites transport mantle fragments including diamonds to the surface and partially dissolve them. Previous studies have shown that dissolution features on diamond reflect the conditions in the host magma and especially presence and composition of fluid. Diamonds from volcaniclastic facies of different kimberlite classes all show very similar low-relief surface features indicating presence of fluid. Geometry of the trigonal etch pits on diamonds helps to deduce H2O:CO2 ratio of kimberlitic fluid. On the contrary, “corrosive” resorption styles of diamonds from hypabyssal kimberlite (HK) units are different between the three kimberlite classes allowing to examine differences in their crystallization conditions. This study aims to reproduce corrosive resorption of diamonds in controlled experiments in order to examine the composition of kimberlite magma in different kimberlite classes and its effects on magma emplacement.

Experiments were conducted in piston-cylinder apparatus at pressure 0.5 – 1 GPa and temperatures 1000 – 1200^oC using a range of volatile-undersaturated silicate and silico-carbonate melts. Experiments produced three specific resorption styles previously reported on natural diamonds from HK: (i) sharp pointy features common for diamonds from HK in class 3 kimberlites; (ii) corrosion sculptures common for diamonds from HK in class 1 kimberlite; (iii) deep channels – rare but prominent feature of natural diamonds. We compare our experimental results to the features of natural diamonds from HK units of class 1 kimberlites (Orapa kimberlite cluster, Botswana) and class 3 (Ekati Mine kimberlites, Canada) to compare magma composition and emplacement conditions of different kimberlite classes.