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

Secondary ion mass spectrometry (SIMS) techniques have been used to study t he variation of C isotope ratio and N abundance within selected diamonds in relation to their crystal growth zones. The growth zones are seen in catho doluminescence (CL), and include both octahedral and cuboid zones within ty pical diamonds of external octahedral morphology. Compositions were determi ned by use of a primary Cs-133(+) ion beam and measurement of C-12-, C-13-, and (CN-)-C-12-N-14 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 o f 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 (similar to 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 dia monds grow often do not influence delta(13)C. No evidence of progressive va riation or fractionation of C isotopes during growth was found. Some original variation in C isotope composition may have been eliminated b y diffusion of C atoms subsequent to growth, because of the storage of natu ral diamonds over millions of years in the Earth's mantle at temperatures o f 950-1250 degrees C. Such atomic mobility does not homogenize N distributi on because of the proven tendency of N to form aggregates of atoms. A surve y of experimental estimates of single atom (C) diffusion parameters, sugges ts that diffusion distances of similar to 100 mu m are likely at high tempe ratures (similar to 1100 degrees C) over long time periods (similar to 1.0 Ga). Therefore, with refinement of the diffusion parameters and measurement s, the extent of C isotope homogenization in natural diamonds, as well as N aggregation state, might provide quantitative evidence of their time-tempe rature history under mantle conditions.