Chemical heterogeneity in carbonado, an enigmatic polycrystalline diamond

Carbonade, a polycrystalline variety of diamond, is characterized by unusua l 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-1 3 from -1 to -10 parts per thousand) and fall near the lower extreme for ec logitic diamond (ranging from +3 to -34 parts per thousand). In combination with textural and inclusion data, these isotopic compositions have led sci entists to question whether carbonade originated in the mantle or in the cr ust. Previous studies of carbonade have revealed a bimodal grain size distr ibution that correlates with cathodoluminescence (CL) emissions. We believe that these textures result from a two-step growth process, and we have obt ained 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 t housand). Secondary ion mass spectrometry analyses also demonstrate that th is 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 suggest s that isotopic homogenization occurs extremely slowly, even under upper ma ntle 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 dynam ics of the growth process. (C) 2001 Elsevier Science B.V. All rights reserv ed.