Intensive Variables in Kimberlite Magmas, Lac de Gras, Canada and Implications for Diamond Survival

Abstract

Abstract Crystallization temperatures (T) and oxygen fugacities (fO2) of kimberlite magma are estimated from oxides included in olivine phenocrysts from the Leslie, Aaron, Grizzly and Torrie kimberlite pipes in the central Slave Province, Canada. Crystallization temperatures recorded by olivine–chromite pairs at an assumed pressure of 1·0 GPa are 1030–1170°C ± 50°C, with a mean of ∼1080°C. At these temperatures, the fO2 of coexisting olivine and chromite is 2–3 log units less oxidized than the nickel–nickel oxide (NNO) buffer at a silica activity limited by the presence of monticellite. Mass balance of olivine, bulk-rock and liquid compositions in equilibrium with olivine phenocryst rims suggests that these kimberlites represent crystallization from a magma with 11–28 mol % of liquid, 10 mol % of earlier precipitated olivine phenocrysts and 62–79 mol % of mantle xenocryst olivine. The calculated T–fO2 values indicate that diamonds entrained in the Lac de Gras kimberlites were probably transported to the surface within the stability field of graphite but close to the graphite–CO2 boundary.