Dissolution forms in Lac de Gras diamonds and their relationship to the temperature and redox state of kimberlite magma

Abstract

The degree and character of diamond dissolution were compared to crystallization temperatures (T) and oxygen fugacities (fO2) estimated from chromite inclusions in olivine phenocrysts in several kimberlites from Lac de Gras, Northwest Territories, Canada. The T and fO2 values calculated at an assumed pressure of 1 GPa are in the range of 970–1,140±50°C and 2.8–4.4 log fO2units below the nickel–nickel oxide (NNO) buffer. The T and fO2 vary between kimberlites from northwest and southeastern clusters within 150°C and 1 log unit, respectively. A detailed description of morphological forms and surface dissolution features for diamond parcels from the Panda, Beartooth, Grizzly, Misery and Jay kimberlites (>7000 stones) show that an increase in diamond resorption in the kimberlites corresponds to increase in T. The development of various surface dissolution pits and structures correlates with higher fO2 of kimberlites and therefore mainly happens in the magma. The two processes of diamond dissolution, volume resorption and surface etching, do not show a strong correlation with each other, since some of the resorption occurs in the mantle. We suggest that the fO2 of the kimberlite magma plays an important role in both the processes. The proportion of plastically deformed brown diamonds does not correlate with the degree of volume resorption, but does correlate with the development of surface forms. The diamond grade is higher in kimberlites with lower fO2, confirming that conditions of kimberlite crystallization can have notable effect on diamond dissolution.