IMPLANTED RADIOGENIC AND OTHER NOBLE-GASES IN CRUSTAL DIAMONDS FROM NORTHERN KAZAKHSTAN

Noble gases were extracted in steps from grain size fractions of micro diamonds (<100 mu m) from the Kokchetav Massif, Northern Kazakhstan, b y pyrolysis and combustion. The concentration of He-4 in the diamonds proper (liberated by combustion) shows a 1/r dependence on grain size. For grain diameters >15 mu m the concentration also decreases with th e combustion step. Both results are clear evidence that He-4 has been implanted into the diamonds from alpha-decaying elements in the surrou nding matrix. The saturation concentration of He-4 (5.6 x 10(-4) cm(3) STP/g) is among the very highest observed in any terrestrial diamonds . Fission xenon from the spontaneous fission of U-238 accompanies the radiogenic He-4; the Xe-136(f)/He-4 ratio of (2.5+/-0.3) x 10(-9) agre es well with the production ratio of 2.3 x 10(-9) expected in a reserv oir where Th/U similar to 3.3. Radiogenic Ar-40 is predominantly (>90% ) set free upon combustion; it also resides in the diamonds and appear s to have been incorporated into the diamonds upon their formation. He -3, on the other hand is mainly released during pyrolysis and hence is apparently carried by 'contaminants'. The concentration in the diamon ds proper is of the order of 4 x 10(-12) cm(3) STP/g, with a He-3/He-4 ratio of 1 x 10(-8). Excess Ne-21, similarly, appears to be present i n contaminants as well as in diamonds proper. These two nuclides in th e contaminants must have a nucleogenic origin, but it is difficult to explain their high concentrations.