A new method of isotope geochronology was proposed for dating native platinum minerals on the basis of the α-decay of the natural isotope ¹⁹⁰Pt. The analysis of the thermal desorption of helium in the crystal lattice of native metals, including platinum, allowed us to predict a very high thermal stability (retentivity) of radiogenic ⁴He in native platinum minerals up to their melting temperatures. In order to validate the proposed ¹⁹⁰Pt-⁴He method, direct isotopic dating was performed for isoferroplatinum from the Galmoenan dunite-clinopyroxenite and Kondyor alkaline ultramafic massifs. The results of dating obtained by this method for primary ore platinum from the Galmoenan Massif (70 ± 5 Ma) are consistent with geological observations and mean Sm-Nd and Rb-Sr isotopic age estimates. The ¹⁹⁰Pt-⁴He age obtained for placer isoferroplatinum from the Kondyor Massif (112 ± 7 Ma) also agrees with geological observations and is close to the K-Ar and Rb-Sr ages of koswites (phlogopite-magnetite pyroxenites, gabbros, nepheline syenites, and metasomatic rocks after dunites). Our experimental data demonstrated that the ¹⁹⁰Pt-⁴He method is a promising tool for dating native platinum minerals.

It was shown that the behavior of ⁴He 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 ∼ 10¹³ 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 ⁴He 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.

We report on the application of the U-Th-He method for the direct dating of pyrite and provide an original methodological approach for measurement of U, Th and He in single grains without loss of parent nuclides during thermal extraction of He. The U-Th-He age of ten samples of high-crystalline stoichiometric pyrite from unoxidized massive ores of the Uzelga volcanogenic massive sulfide (VMS) deposit, South Urals, is 382 ± 12 Ma (2σ) (U concentrations ~1–5 ppm; ⁴He ~10⁻⁴ cm³ STP g⁻¹). This age is consistent with independent (biostratigraphic) estimations of the age of ore formation (ca, 389–380 Ma) and is remarkably older than the probable age of the regional prehnite-pumpellyite facies metamorphism (~340–345 Ma). Our results indicate that the U-Th-He dating of ~1 mg weight pyrite sample is possible and open new perspectives for the dating of ore deposits. The relative simplicity of U-Th-He dating in comparison with other geochronological methods makes this approach interesting for further application.

The migration of helium from the crystal lattices of sulfides (pyrite, pyrrhotite, chalcopyrite, bornite, and sphalerite) and sulfosalts (tennantite and tetrahedrite) was studied. It was...