Abstract:
Noble gases were extracted in steps from grain size fractions of microdiamonds (less than 100 microns) from the Kokchetav Massif, Northern Kazakhstan, by 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 greater than 15 microns the concentration also decreases with the combustion step. Both results are clear evidence that He-4 has been implanted into the diamonds from alpha-decaying elements in the surrounding matrix. The saturation concentration of He-4. (5.6 x 10(exp -4) cu cm 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 (136)Xe(sub f)/He-4 ratio of (2.5 +/- 0.3) x 10(exp -9) agrees well with the production ratio of 2.3 x 10(exp -9) expected in a reservoir where Th/U about 3.3. Radiogenic Ar-40 is predominantly (greater than 90%) set free upon combustion; it also resides in the diamonds and appears 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 diamonds proper is of the order of 4 x 10(exp -12) cu cm STP/g, with a He-3/He-4 ratio of 1 x 10(exp -8). Excess Ne-21, 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.