Diamonds and their mineral inclusions from the Renard kimberlites in Quebec

Mineral inclusions in diamonds from the Renard kimberlites, on the eastern Superior Craton, document an almost exclusively peridotitic inclusion suite with an unusually high abundance of SiO ₂ phases (coesite and presumably retrograde quartz). Whilst coesite is often associated with eclogitic diamond sources, there is an almost complete absence of unequivocally eclogitic mineral inclusions at Renard. Consequently, the presence of abundant SiO ₂ inclusion phases is likely caused by influx of CO ₂ within localised regions of the lithospheric mantle with very high fluid/rock ratios. This caused a progression of carbonation reactions within the host peridotite, likely along veins, locally consuming all olivine, orthopyroxene, and clinopyroxene present and producing free SiO ₂.Nitrogen contents and aggregation states of the diamonds indicate normal mantle residence temperatures, predominantly between 1100 and 1200°C. Co-variations of carbon isotopic composition and nitrogen content suggest formation from both reducing (CH ₄) and oxidising (CO ₃ ^2- or CO ₂ bearing) melts/fluids. The observation of single diamonds with non-isothermal co-variations of nitrogen content and aggregation state, or containing disequilibrium inclusion parageneses (e.g., variations in olivine Mg# up to 0.5 within single diamonds), indicates that Renard diamonds grew during a number of precipitation events within a physically and chemically changing environment.