Carbon isotope ratios and nitrogen abundances in relation to cathodoluminescence characteristics for some diamonds from the Kaapvaal Province, S. Africa

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

Secondary ion mass spectrometry (SIMS) techniques have been used to study the variation of C isotope ratio and N abundance within selected diamonds in relation to their crystal growth zones. The growth zones are seen in cathodoluminescence (CL), and include both octahedral and cuboid zones within typical diamonds of external octahedral morphology. Compositions were determined by use of a primary ¹³³ Cs (super +) ion beam and measurement of ¹² C (super -) , ¹³ C (super -) , and ¹² C ¹⁴ N (super -) secondary ions at high mass resolution on a Cameca ims-4f ion microprobe at Edinburgh University. In each of the diamonds, different growth zones have marked differences in N abundance, which are as great as 0-1400 ppm within one diamond. Changes of several hundred ppm N are common across both octahedral and cuboid growth zones, and appear sharp and abrupt at the boundaries of the growth zones. In general for the common blue CL, luminescence increases with N abundance. The changes in N abundance across fine scale ( approximately 100 mu m) growth zones show that the total N contents determined by IR spectroscopy may show great variations of abundance. In contrast, within detection limits, delta ¹³ C appears constant across many growth zone boundaries. Thus the factors controlling uptake of N from the fluid/melt reservoir in which natural diamonds grow often do not influence delta ¹³ C. No evidence of progressive variation or fractionation of C isotopes during growth was found. Some original variation in C isotope composition may have been eliminated by diffusion of C atoms subsequent to growth, because of the storage of natural diamonds over millions of years in the Earth's mantle at temperatures of 950-1250 degrees C. Such atomic mobility does not homogenize N distribution because of the proven tendency of N to form aggregates of atoms. A survey of experimental estimates of single atom (C) diffusion parameters, suggests that diffusion distances of approximately 100 mu m are likely at high temperatures ( approximately 1100 degrees C) over long time periods ( approximately 1.0 Ga). Therefore, with refinement of the diffusion parameters and measurements, the extent of C isotope homogenization in natural diamonds, as well as N aggregation state, might provide quantitative evidence of their time-temperature history under mantle conditions.

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