He, Ar and C isotopes in coated and polycrystalline diamonds

Helium, Ar and C isotopic measurements have been undertaken on different qualities of industrial grade and polycrystalline diamond from localities in Africa and Brazil. These include coated and cloudy diamonds which contain abundant volatile-rich micro-inclusions and framesites and carbonados which consist of aggregates of micron-sized diamond crystals interspersed with mineral inclusions. In an attempt to release selectively the primary trapped He from the diamonds, they were analysed using an in vacuo crushing technique. The He data are combined with Ar, K and U data obtained from 40Ar-39Ar stepped-heating studies of irradiated fractions of the diamonds and bulk carbon isotope data obtained from the crushed residues. Results indicate that coated and cloudy diamonds best preserve a mantle signature with He-3/He-4 values of 1-7 Ra and delta(13)C of -5 to -6 parts per thousand. Coated diamonds have uniformly high He and Ar concentrations of 10(-6)-10(-5) cm(3) g(-1). Framesites have lower He-3/He-4 values 0.1-3 Ra and bimodal delta(13)C values with modes at -6 and -20 parts per thousand. Carbonados show both the lowest delta(13)C values less than or equal to -25 parts per thousand and He-3/He-4 of < 0.1 Ra. U and K concentrations show a progressive increase from coats < framesites < carbonados. While it is tempting to attribute low delta(13)C, low He-3/He-4 and high U to the influence of subducted crustal material, it is likely that implantation of 4He by U, Th-rich minerals adjacent to the micro-diamonds is also a possible cause of the low He isotope ratios. (C) 1998 Elsevier Science B.V. All rights reserved.