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Mineralogical Magazine; December 1999; v. 63; no. 6; p. 857-878
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Extreme chemical variation in complex diamonds from George Creek, Colorado; a SIMS study of carbon isotope composition and nitrogen abundance

I. C. W. Fitzsimons, B. Harte, I. L. Chinn, J. J. Gurney, and W. R. Taylor

University of Edinburgh, Department of Geology and Geophysics, Edinburgh, United Kingdom

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 delta 13 C values can vary from 0 to 750 ppm and 0 to -20 per mil 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 mu m) for analysis by IR spectroscopy. delta 13 C 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 delta 13 C and N concentrations during the period of diamond growth, but the factors controlling N content during diamond growth did not control delta 13 C.

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