Diversity of crystallization conditions of coherent kimberlites recorded in diamond surface textures: an experimental study

Abstract

Kimberlites transport diamonds from the roots of subcratonic lithosphere to the Earth’s surface. During the ascent, diamonds react with kimberlite magma and develop a wide range of dissolution features, the geometry of which reflects certain conditions in the host magma. Diamonds from various localities and types of volcanoclastic kimberlites (VK) show low-relief dissolution features, which, according to diamond dissolution experiments, are produced in the presence of C-O-H fluid. However, diamonds from coherent kimberlite (CK) units display distinct features, which not only vary between and within kimberlite units but also show a strong association with certain kimberlite localities. Here we test a hypothesis that these features are a product of diamond reaction with volatile-undersaturated kimberlite melt and examine how the melt composition and crystallization conditions (temperature and pressure) affect diamond dissolution. We conducted experiments in piston-cylinder apparatus at 0.5-2 GPa and 1050–1350 °C using synthetic analogues of evolved kimberlite melt and successfully produced the two diamond dissolution styles common for CK units: (i) fine facetted sharp hillocks and (ii) corrosion sculptures. We found that diamond resorption depends on temperature (T) and pressure (P) and not on the melt composition. Comparing diamonds from VK and CK units in Ekati Mine kimberlites in Canada revealed difference in diamond dissolution in VK and CK units. Our experimental results indicate that diamond resorption features typically reflect the conditions at ~ 15 km depth (0.5 GPa) before the final emplacement of kimberlite magma into a pipe or a dyke. Significant fluid exsolution below ~ 15 km is demonstrated by the presence of abundant C-O-H fluid in the part (pulse) of the magma column producing VK units and an absence of a fluid phase in the part (pulse) emplaced as CK. Our results indicate that dissolution of diamonds from various CK occur in a fluid-free kimberlite magma at 1050-1200^oC and 0.5-1 GPa. We apply our experimental results to diamonds from different CK units from Ekati Mine kimberlites and Orapa kimberlite cluster to gain insights to their conditions of formation.