THE RADIAL-DISTRIBUTION OF IMPLANTED AND TRAPPED HE-4 IN SINGLE DIAMOND CRYSTALS AND IMPLICATIONS FOR THE ORIGIN OF CARBONADO

We have investigated variations of He-4 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 du ring their residence in the earth's crust, since concentrations of He- 4 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 He-4 conce ntrations in carbonade diamonds do not necessary demand an exotic orig in. Previous theoretical estimations of the magnitude of He implantati on in diamonds, which use normal U-Th concentrations of ca. 30 ppm in the surrounding rocks, do not agree with experimentally-obtained resul ts. The He concentrations actually found in the 'skins' (outer approxi mate to 30- mu m layer of the crystals) of the samples we studied, req uire 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 exhibite d He-4 concentrations from 3 X 10(-7) cm(3)/g to 2 X 10(-5) cm(3)/g, i ndicating heterogeneity even within single diamond crystals and arguin g for the changes in the growth environment. From He-4 zoning within a diamond from the Finsch kimberlite, a maximum He-4 diffusion coeffici ent was estimated to be approximate to 4 x 10(-21) cm(2)/s, lower than previous estimates, indicating that diamond may retain indigenous He- 4 in the structure during its residence under mantle PT conditions. (C ) 1998 Elsevier Science B.V. All rights reserved.