The radial distribution of implanted and trapped 4He in single diamond crystals and implications for the origin of carbonado

We have investigated variations of (4)He content in the different zones of single natural diamond crystals using a step combustion technique. The results indicate that some diamonds may be strongly irradiated during their residence in the earth's crust, since concentrations of (4)He in the 30-mu m outer layer were found to be as high as 0.014 cm(3)/g. The observation leads to a conclusion that similarly high (4)He concentrations in carbonade diamonds do not necessary demand an exotic origin. Previous theoretical estimations of the magnitude of He implantation in diamonds, which use normal U-Th concentrations of ca. 30 ppm in the surrounding rocks, do not agree with experimentally-obtained results. The He concentrations actually found in the 'skins' (outer approximate to 30- mu m layer of the crystals) of the samples we studied, require concentrations of U and Th in the host rock from 160 ppm to 1000 ppm. The interiors of the samples have also been analysed and exhibited (4)He concentrations from 3 X 10(-7) cm(3)/g to 2 X 10(-5) cm(3)/g, indicating heterogeneity even within single diamond crystals and arguing for the changes in the growth environment. From (4)He zoning within a diamond from the Finsch kimberlite, a maximum (4)He diffusion coefficient was estimated to be approximate to 4 x 10(-21) cm(2)/s, lower than previous estimates, indicating that diamond may retain indigenous (4)He in the structure during its residence under mantle PT conditions. (C) 1998 Elsevier Science B.V. All rights reserved.