Noble gases were extracted in steps from grain size fractions of microdiamonds ( < 100 μm) from the Kokchetav Massif, Northern Kazakhstan, by pyrolysis and combustion. The concentration of 4He in the diamonds proper (liberated by combustion) shows a 1 r dependence on grain size. For grain diameters > 15 μm the concentration also decreases with the combustion step. Both results are clear evidence that 4He has been implanted into the diamonds from α-decaying elements in the surrounding matrix. The saturation concentration of 4He(5.6 × 10-4> cm3 STP/g) is among the very highest observed in any terrestrial diamonds. Fission xenon from the spontaneous fission of 238U accompanies the radiogenic 4He; the 136Xef 4He ratio of (2.5 ± 0.3) × 10-9 agrees well with the production ratio of 2.3 × 10-9 expected in a reservoir where Th U ∼ 3.3. Radiogenic 40Ar is predominantly ( > 90%) set free upon combustion; it also resides in the diamonds and appears to have been incorporated into the diamonds upon their formation. 3He, on the other hand is mainly released during pyrolysis and hence is apparently carried by 'contaminants'. The concentration in the diamonds proper is of the order of 4 × 10-12 cm3 STP/g, with a 3He 4He ratio of 1 × 10-8. Excess 21Ne, similarly, appears to be present in contaminants as well as in diamonds proper. These two nuclides in the contaminants must have a nucleogenic origin, but it is difficult to explain their high concentrations.
ASJC Scopus subject areas
- Geochemistry and Petrology
- Earth and Planetary Sciences (miscellaneous)
- Space and Planetary Science