Chemical heterogeneity in carbonado, an enigmatic polycrystalline diamond

Carbonade, a polycrystalline variety of diamond, is characterized by unusual carbon isotope compositions with bulk delta C-13 values clustered tightly between -23 and -30 parts per thousand (relative to PDB). These values are significantly lighter than harzburgitic diamond (with a range in delta C-13 from -1 to -10 parts per thousand) and fall near the lower extreme for eclogitic diamond (ranging from +3 to -34 parts per thousand). In combination with textural and inclusion data, these isotopic compositions have led scientists to question whether carbonade originated in the mantle or in the crust. Previous studies of carbonade have revealed a bimodal grain size distribution that correlates with cathodoluminescence (CL) emissions. We believe that these textures result from a two-step growth process, and we have obtained additional chemical evidence that supports the identification of two distinct crystal populations. Ion microprobe analyses of a Central African carbonade reveal a bimodal distribution of delta C-13 values of -24 and -26 parts per thousand (with an instrumental precision of +/- 0.29 parts per thousand). Secondary ion mass spectrometry analyses also demonstrate that this delta C-13 distribution coincides with variations in nitrogen abundance, and both of these chemical zonations correlate with CL emission signatures. A one-dimensional analysis of self-diffusion of carbon in diamond suggests that isotopic homogenization occurs extremely slowly, even under upper mantle conditions. Whereas the microscale distribution of carbon isotopes in carbonade does not constrain the temperature, pressure or time of carbonade formation, it does provide a geochemical signature that recorded the dynamics of the growth process. (C) 2001 Elsevier Science B.V. All rights reserved.