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 3He/4He values of 1–7 Ra and δ13C of −5 to −6‰. Coated diamonds have uniformly high He and Ar concentrations of 10−6–10−5 cm3 g−1. Framesites have lower 3He/4He values 0.1–3 Ra and bimodal δ13C values with modes at −6 and −20‰. Carbonados show both the lowest δ13C values ≤−25‰ and 3He/4He of <0.1 Ra. U and K concentrations show a progressive increase from coats<framesites<carbonados. While it is tempting to attribute low δ13C, low 3He/4He 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.