Extreme chemical variation in complex diamonds from George Creek, Colorado: a SIMS study of carbon isotope composition and nitrogen abundance

Abstract

Abstract Diamonds from George Creek, Colorado, preserve complex intergrowth textures between two major growth generations: homogeneous diamond with yellow-buff cathodoluminescence (CL); and diamond with blue-green CL and local growth zonation. Secondary-ion mass spectrometry (SIMS) has revealed large variations in N concentration and C isotope composition within these diamonds. Even within single stones, N contents and δ13C values can vary from 0 to 750 ppm and 0 to −20‰ respectively. These variations are similar to those recorded elsewhere for entire diamond suites. The CL characteristics correlate directly with N: diamond with yellow-buff CL has uniform N contents, whereas the zoned diamond has bright blue CL bands with high N (50–750 ppm) and dark blue or green CL bands with low N (0–20 ppm). These bands are too narrow (10–5 µm) for analysis by IR spectroscopy. δ13C also varies between the two growth generations in any one diamond plate, and to a lesser extent within these generations, but shows no consistent correlations with either CL or N. The George Creek stones preserve evidence of extreme temporal and/or spatial variations in both δ13C and N concentrations during the period of diamond growth, but the factors controlling N content during diamond growth did not control δ13C.