CARBONADO - A COMPARISON BETWEEN BRAZILIA N AND UBANGUI SOURCES WITH OTHER FORMS OF MICROCRYSTALLINE DIAMOND BASED ON CARBON AND NITROGEN ISOTOPES

Carbonade is the unusual type of polycrystalline diamond, whose origin is still disputed. On the basis of previous studies several hypothese s for the formation of carbonade have been put forward [1-4]. However, no one explanation is completely satisfying in terms of previous obse rvations. The current work reports new data aimed at identifying links between the two populations of carbonados (Brazil and Central Africa) , we also report some results for other forms of polycrystalline diamo nd. Based on our data, carbonades from Ubangui are indistinguishable f rom those collected in Brazil in respect of delta(13)C, N abundance an d delta(15)N. Whereas Brazilian and Ubangui carbonades are similar the y can easily be discerned from framesites from Orapa and Jwaneng which have a higher N abundance and are enriched in delta(13)C and delta(15 )N isotopes. The polycrystalline aggregates of shock origin from the P opigai crater specimens are even more delta(13)C enriched were extreme ly low in nitrogen. The results are discussed with relation to various hypothesis for carbonade origin. The C and N isotopic composition of carbonade suggests some connection with a crustal reservoir of carbon because both delta(15)N and delta(13)C are in the range for organic ma terials. Given that some eclogitic type diamonds may have been produce d from crustal carbon sources, it may be difficult to distinguish diff erent processes where the crust is involved. Within our work we encoun ted several facts supporting relation of carbonade to the diamonds of mantle origin: one sample from Ubangui indicated delta(13)C of -5.8 pa rts per thousand, and delta(15)N of -5.0 parts per thousand; in one sa mple from Brazil reliable IR spectra indicated IaA type of nitrogen ag gregation; appearance of large crystallites (40-200 mu m in diameter) in some carbonade. Some specimens of carbonade have a very distinctly unequilibrated rim which could be explained by fast changing of growth conditions or partial transformations in the process which could be c aused by shock [5].