Infrared spectral and carbon isotopic characteristics of micro- and macro-diamonds from the Panda kimberlite (Central Slave Craton, Canada)

One hundred and twenty-one micro-diamonds (<1mm) and 90 macro-diamonds (2.5mm to 3.4mm) from the Panda kimberlite (Ekati mine, Central Slave Craton, Canada) were analyzed for nitrogen content, nitrogen aggregation state (%B) and platelet and hydrogen peak areas (cm^-2). Micro-diamond nitrogen concentrations range from <10at.ppm to 1696at.ppm (median=805at.ppm) and the median aggregation state is 23%B. Macro-diamonds range from <10at.ppm to 1260at.ppm (median=187at.ppm) nitrogen and have a median nitrogen aggregation of 26%B. Platelet and hydrogen peaks were observed in 37% and 79% of the micro-diamonds and 79% and 56% of the macro-diamonds, respectively. Nitrogen based time averaged residence temperatures indicate that micro- and macro-diamonds experienced similar thermal mantle residence histories, both populations displaying bimodal residence temperature distributions with a gap between 1130°C and 1160°C (at 3.5Ga residence).In addition, SIMS carbon isotopic analyses for the micro-diamonds were obtained: δ¹³C compositions range from -6.9‰ to +1.8‰ (median=-4.3‰). CL imaging reveals distinct growth layers that in some samples differ by >2‰, but mostly vary by <0.5‰. Comparison of only the "gem-quality" samples (n=49 micro- and 90 macro-diamonds) between the two diamond sets, indicates a statistically significant shift of +1.3‰ in average δ¹³C from macro- to micro-diamonds and this shift documents distinct diamond forming fluids, fractionation process or growth histories. A broad transition to heavier isotopic values is also observed in connection to decreasing mantle residence temperatures.The bimodal mantle residence temperature distribution may coincide with the transition from highly depleted shallow to more fertile deep lithospheric mantle observed beneath the Central Slave Craton. The increase in δ¹³C with decreasing residence temperature (proxy for decreasing depth) is interpreted to reflect diamond formation from a carbonate-bearing metasomatic fluid/melt that isotopically evolves as it percolates upward through the lithosphere.