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

Abstract

We have investigated variations of 4He 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 4He in the 30-μm outer layer were found to be as high as 0.014 cm3/g. The observation leads to a conclusion that similarly high 4He concentrations in carbonado 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 ~30- μm 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 4He concentrations from 3x10-7 cm3/g to 2x10-5 cm3/g, indicating heterogeneity even within single diamond crystals and arguing for the changes in the growth environment. From 4He zoning within a diamond from the Finsch kimberlite, a maximum 4He diffusion coefficient was estimated to be ~4x10-21 cm2/s, lower than previous estimates, indicating that diamond may retain indigenous 4He in the structure during its residence under mantle PT conditions.