N and C isotropic composition of different varieties of terrestrial diamonds and carbonado

Abstract

During the course of this research an instrument utilising combustion as a means of gas extraction capable of N, C, Ar and He analysis has been developed and the entire analytical procedure has been automated. N and C isotopic analysis has been performed on eclogitic and peridotitic diamonds (mainly from Yakutian kimberlites and the Roberts Victor kimberlite pipe). Diamonds with unusual (light and heavy relative to the peak of δ13C distribution of mantle diamonds) carbon isotopic signatures were considered for the research so that diamonds in the range of δ13C from -30‰to +2.8‰ were characterised for δ15N. The results together with data obtained previously by the others define fields for eclogitic and peridotitic diamonds on a plot of δ15N vs. δ13C. The model of mantle nitrogen and carbon evolution is discussed. A comprehensive comparison between carbonado and other known forms of microcrystalline diamond (framesites and shock diamonds) has been made for a number of parameters: N and C isotopic composition and N content; 4He content; morphology of the inner structure of diamond aggregates. It can be concluded that carbonado is generally similar to frame sites and all facts known about carbonado can be explained on the grounds of common mantle origin involving subducted carbon and nitrogen. Since extremely high 4He concentrations are encountered in carbonado this parameter is considered to be the most singular feature of this diamond variety and the radial distribution of 4He in single diamond crystals has been studied. A 4He content comparable with that in carbonado was found in the 30 µm skin of diamond crystals (up to 1.4 x 10-2 ) suggesting that carbonado could acquire high 4He concentration in the same geological processes as single diamond crystals and making carbonado indistinguishable from mantle diamonds in terms of He content. An additional result of the investigation is that the maximum of 4He diffusion coefficient for diamond at mantle P,T conditions can be estimated (≈4 x 10-21 ) from the 4He zoning identified in the interior of a diamond crystal from the Finsch kimberlite. A number of diamonds of impact origin from Popigai crater and Ebeliakh river placer deposits were studied for N, C and Ar isotopic compositions. It was concluded that diamonds from these two localities have resulted from separate impact events and that diamond aggregates studied are most probably consist of a mixture of different Ar and N carriers (e.g. two types of diamond grains)