Extreme Chemical Variation in Complex Diamonds from George Creek, Colorado: A SIMS Study of Carbon Isotope Composition and Nitrogen Abundance

I. C. W. Fitzsimons1, *, B. Harte1, I. L. Chinn2, J. J. Gurney2 and W. R. Taylor3
1 Department of Geology and Geophysics, The University of Edinburgh, The Grant Institute, West Mains Road, Edinburgh EH9 3JW, UK
2 Department of Geological Sciences, University of Cape Town, Rondebosch 7700, South Africa
3 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
*Present address: School of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, Australia

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.

Keywords: carbon isotopes • cathodoluminescence • diamond • nitrogen concentration • SIMS

Mineralogical Magazine; December 1999 v. 63; no. 6; p. 857-878; DOI: 10.1180/002646199548970
© 1999, The Mineralogical Society
Mineralogical Society (www.minersoc.org)