Intensive Variables in Primary Kimberlite Magmas (Lac de Gras, N.W.T., Canada) and Application for Diamond Preservation

Crystallization temperatures (T) and oxygen fugacity (fO2) are not well constrained for kimberlites. Knowledge of these intensive variables of kimberlite melt is important for understanding the origin and evolution of kimberlites and prediction of diamond preservation in the magma. Difficulties in interpreting the equilibrium mineral assemblages in kimberlites and the high degree of secondary alteration usually complicate use of mineral geothermometers and oxygen barometers. Some of Lac de Gras (N.W.T., Canada) kimberlites are extremely fresh and provide opportunity to apply mineral thermobarometers. The presence of numerous chromite inclusions in the rims of olivine phenocrysts allows application of the olivine-spinel thermometer and oxygen barometer to constrain T and fO2 of the melt. We performed T and fO2 calculations on samples from three kimberlite pipes - the Leslie, Aaron and Grizzly. The T obtained from olivine - chromite pairs for crystallization of olivine phenocryst rims are 1050° to 1100°C +/- 50°C (calculated at 1 GPa). Few olivine - chromite pairs from Leslie and Grizzly record higher temperatures of 1250° - 1350°C. The cores of olivine phenocrysts usually lack chromite inclusions and their crystallization T and fO2 could not be estimated. The fO2 recorded by coexisting olivine and chromite are +0.3 to 1.0 +/- 0.4 log units more oxidized than the nickel-nickel oxide (NNO) buffer. The established fO2 of kimberlites would require fO2 in their mantle source to be higher than that of cratonic mantle and oceanic lithosphere producing MORB's but comparable to the source of subduction-related magmas. The T and fO2 for the Lac de Gras kimberlites constrain the path of any mantle material entrained in these magmas in fO2-T-P space and provide limits on diamond destructive processes. Diamonds are not stable in kimberlite magma and are oxidized to CO2 or converted into graphite. The former process is more favorable for their preservation. The results of out thermo-barometric calculations show that at any pressure the Lac de Gras kimberlites were above the Graphite (Diamond)-CCO buffer. Diamonds entrained in these kimberlites were moved into stability field of CO2 without graphitization, favoring better preservation of diamonds.