Intra-crystal co-variations of carbon isotopes and nitrogen contents in diamond from three North American cratons

Abstract

Eighteen diamond samples from the A154 South kimberlite pipe (Diavik Mine), Slave Craton, Northwest Territories (Canada); sixteen diamond samples from the Lynx kimberlite dyke, Superior Craton, Quebec (Canada) and twelve diamond samples from the Kelsey Lake kimberlite pipe, Wyoming Craton, Colorado (USA), were cut through the core-zones, polished, imaged by cathodoluminescence (CL), and analyzed by secondary ion mass spectrometry (SIMS) for carbon isotope composition and nitrogen abundance. Twenty Kelsey Lake diamond plates, including the twelve crystals analyzed by SIMS, were analyzed by Fourier transform infrared spectrometry (FTIR) for nitrogen concentration and aggregation state.Diamond samples from Diavik and Kelsey Lake have average δ13CPDB and nitrogen contents (atomic ppm) similar to those found by earlier studies: averaging between −3.9‰ and 486ppm, and −7‰ and 308ppm, respectively. Samples from the Lynx dyke, investigated for the first time, are substantially different, having δ13C=−1.2‰ and nitrogen content=32ppm (averages). All three localities have examples of significant variations in nitrogen content (>100ppm) within single stones. Carbon isotope variation within individual stones is relatively minor (<2‰). In terms of nitrogen aggregation, samples from the Kelsey Lake kimberlite are dominated by zones of Type IaA, but mixed-type and Type IaB (less common) stones also occur. For the majority of samples, overall intra-diamond zonations of nitrogen abundances and carbon isotope ratios are not in agreement with modeled trends for single-event Rayleigh fractionation of diamond from fluid under nitrogen-compatible conditions at 1100°C. The involvement of fluids from subducted crustal reservoirs with exceptionally light, and in the case of Lynx samples, exceptionally heavy δ13CPDB values is necessary to explain the observed growth histories of all the samples studied here.