Geothermochronology based on noble gases: III. Migration of radiogenic He in the crystal structure of native metals with applications to their isotopic dating

a It was shown that the behavior of 4 He in native and technical metals is very similar owing to the symmetric and stable electron shells of its atoms, which cannot gain electrons from other atoms or donate their own electrons to metal atoms in a crystal lattice. Therefore, they rapidly migrate toward grain boundaries and dislocations, where they are released as vesicles or He clusters. It was found that the thermal desorption of radiogenic He occurring in the crystal lattice of native metals as gas clusters requires activation energies of 100 and even 180 kcal/mol up to the attainment of the melting temperature of the metal. The frequency factor is several orders of magnitude higher than the limiting value k 0 ∼ 10 13 s −1 for the migration of single atoms in the crystal lattice. Near the melting temperature and tens-hundreds degrees above it, the character of the thermal desorption of radiogenic 4 He changes fundamentally. The migration is strongly accelerated, and sharp narrow peaks appear on the kinetic curves of thermal desorption. A similar phenomenon was observed during the annealing of technical metals and is known as the burst-effect. The destruction of the crystal structure results in the disappearance of helium clusters (vesicles). At the very high temperature, He migrates as individual atoms relatively rapidly from the melt. The activation energy for He thermal desorption and the pre-exponential frequency factor acquire values characteristic of ordinary migration. Such peculiarities of radiogenic He provide unique opportunities for its preservation in the structure of gold and other native metals below their melting temperatures. The rapid advances of (U-Th)/He geochronology is still hampered by the experimentally established extremely heterogeneous distribution of U, He, and, probably, Th in the structure of gold and other natural metals. This difficulty can be circumvented by the development of a method for the determination of the contents of all the mentioned chemical elements in a single aliquot from each sample.