The effect of oxygen fugacity on diamond resorption in ascending kimberlite melt

Abstract

When transported by magmas to the Earth's surface, diamond crystals underwent resorption, the intensity of which significantly differed in various kimberlite pipes. We experimentally simulated diamond resorption at different oxygen fugacities (fO2) in ascending kimberlite magma enriched in CO2 and H2O. The experiments were carried out using specially prepared unaltered Group I kimberlite from the Udachnaya East pipe (Yakutia) and model carbonatite at 3.0 GPa, 1200–1400 °C, and fO2 in a range of NNO–2 to NNO + 3.2 log units (where NNO is Ni-NiO buffer). Over the investigated range of conditions, resorption of octahedral diamond crystals is found to occur according to a single scenario. Negative trigons and shield-shaped laminae develop on the {111} faces and crystal edges are truncated by the surfaces of tetrahexahedroids. The rate of diamond resorption increases in all studied systems as fO2 and temperature are raised. In this case, water-enriched melts are the most aggressive media in the investigated T–fO2 interval. Among the most oxidized high-temperature melts, it is carbonatite melts depleted in SiO2 that provide the maximum rate of diamond resorption. Furthermore, the rates of diamond resorption we obtained are an order of magnitude higher than those previously measured in silicate melts containing CO2 and H2O, at fO2 values from the NNO buffer to NNO-2. Therefore, high oxygen fugacity, a temperature of ~1400 °C, and essentially carbonate composition of water-containing magma could provide a high intensity of diamond resorption at the mantle stage of magma ascent to the surface. Apparently, this process primarily influenced the formation of the appearance and preservation of natural diamond crystals in kimberlite pipes.