HELIUM ISOTOPE COMPOSITION IN DIAMOND-BEA RING METAMORPHIC ROCKS OF THE NORTHERN KAZAKHSTAN

In view of the current debate concerning the origin of diamond in meta morphic rocks of northern Kazakhstan, the helium content and isotope c omposition in different size fractions of diamond from the metamorphic rocks of the Kokchetav Massif were studied. The diamond was extracted by the alkaline alloying technique. The anomalous isotope composition and content of helium were pointed out; the correlation between the h elium isotope composition and the grain size of diamond was demonstrat ed. The comparative analysis of the helium isotope composition in carb onade (Ubangi, central Africa) was carried out. Using results of therm al annealing and applying the graphic differentiation method, the dist inct difference in the kinetics of helium isotope emission from the ca rbonade and from the fine grained fractions of the Kokchetav diamonds were shown. The helium isotope composition was studied in diamond-bear ing carbonate-pyroxene and garnet-pyroxene rocks and in monomineral fr actions of garnet and pyroxene. Several phase-carriers of He-3 and He- 4 were established as well as the heterogeneous distribution of helium isotopes in the diamond-bearing rocks. With the aim to study regulari ties of helium isotope distribution in the diamond bearing rocks, the garnet-pyroxene rock underwent the successive acidic treatment. In the course of selective dissolution, the helium isotope composition of th e fractions obtained was analyzed. As was shown, the significant varia tion in the He-3/He-4 ratio in diamond-bearing rock is mainly caused b y the presence (or absence) of the He-4 phase-carrier and, to a lesser degree, by the presence (or absence) of several He-3 phases-carriers. Zircon was established as a He-4 phase-carrier. The hypothesis of He- 3 chemical fractionation during the process of migration and concentra tion of the radioactive precursor He-3 was proposed as a possible expl anation of the relative and absolute anomalous enrichment of microdiam onds in He-3.