Infrared spectral and carbon isotopic characteristics of micro- and macro-diamonds from the Panda kimberlite (Central Slave Craton, Canada) G.L. Melton a, ⁎, T. Stachel a , R.A. Stern a , J. Carlson b , J.W. Harris c a Canadian Centre for Isotopic Microanalysis, Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G 2E3, Canada b BHP Billiton, #1102 4920-52nd St., Yellowknife, NT X1A 3T1, Canada c School of Geographical and Earth Sciences, University of Glasgow, G12 8QQ, UK abstract article info Article history: Received 25 February 2013 Accepted 26 June 2013 Available online 3 July 2013 Keywords: Micro-diamond FTIR Nitrogen SIMS Carbon isotope Central Slave Craton One hundred and twenty-one micro-diamonds (b 1 mm) and 90 macro-diamonds (2.5 mm to 3.4 mm) 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 concentra- tions range from b 10 at. ppm to 1696 at. ppm (median = 805 at. ppm) and the median aggregation state is 23%B. Macro-diamonds range from b 10 at. ppm to 1260 at. ppm (median = 187 at. 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 resi- dence histories, both populations displaying bimodal residence temperature distributions with a gap between 1130 °C and 1160 °C (at 3.5 Ga residence). In addition, SIMS carbon isotopic analyses for the micro-diamonds were obtained: δ 13 C compositions range from -6.9‰ to + 1.8‰ (median = -4.3‰). CL imaging reveals distinct growth layers that in some samples differ by N 2‰, but mostly vary by b 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 δ 13 C from macro- to micro-diamonds and this shift documents distinct diamond forming fluids, fraction- ation 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 δ 13 C 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. © 2013 Elsevier B.V. All rights reserved. 1. Introduction The Panda kimberlite, Canada's first diamond mine, was mined from 1998 to 2003 as an open cut and from 2005 to 2010 as a sub-level retreat underground operation. The overall ore grade was approximately 1 carat per tonne. The Panda pipe is located on the Ekati Diamond Mine Property, 25 km north of Lac de Gras, Northwest Territories, Canada. The kimberlite has an age of 53.3 ± 0.6 Ma, based on Rb–Sr dating of macro- crystal phlogopite (Creaser et al., 2004). Diamond source lithologies, established through inclusion studies, are dominantly peridotitic with a small eclogitic proportion and rare sublithospheric sources (Stachel et al., 2003; Tappert et al., 2005). These previous studies provided data on inclusion-bearing, gem-quality (i.e. fully transparent) diamonds that measure between 2.5 mm and 3.4 mm. While most diamond studies form the Central Slave Craton focus on inclusion chemistry, much has been learned through carbon isotope and nitrogen data as well. Inclusion-based age determination studies observed that sulfide-bearing diamonds of both peridotitic and eclogitic paragenesis resided in cooler portions of the subcratonic lithospheric mantle (Aulbach et al., 2009; Westerlund et al., 2006). This observation is based on nitrogen concentration and aggregation characteristics that indicate silicate inclusions span a much broader range of residence temperatures (Westerlund et al., 2006). Discrepancies between nitrogen-based temperature estimates and silicate geothermometry have been attributed to cooling below the Slave Craton by 100 °C to 200 °C subsequent ancient (N 1 Ga) diamond formation or heating events (Donnelly et al., 2007; Stachel et al., 2003). Lithos 177 (2013) 110–119 ⁎ Corresponding author. E-mail address: gmelton@ualberta.ca (G.L. Melton). 0024-4937/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.lithos.2013.06.019 Contents lists available at ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos