Ekaterina Ivanova

The main techniques used for gas extraction from solid geological samples for geochronological studies are their thermal destruction either in an isothermal mode, for example in a double vacuum furnace, or under the action of laser radiation. Approaches based on the use of laser technologies are more express and have a low-background. The article describes an original high-vacuum experimental setup based on an ytterbium fiber optic laser for gas extraction from milligram samples of rocks and minerals in a scanning mode. In the proposed configuration, a static vacuum of 10^–8 Torr can be achieved in the high vacuum laser cell. Heating of individual mineral grains to a temperature of over 1800°C is ensured. Using the example of the international standard for (U,Th)–He dating—Durango apatite—it is demonstrated that the laser installation ensures complete extraction of radiogenic helium from the mineral and can be used for isotope-geochronological studies.

We report on the application of the U-Th-He method for the direct dating of pyrite from the alteration halo of the Novogodnee-Monto Au-Fe-skarn deposit, Polar Urals. The deposit is genetically related to the formation of volcanogenic complexes of the Ural Paleozoic belt. A modification of the original methodology for measuring U, Th and He isotopes in a single grain allowed us to determine a U-Th-He age of 382 ± 8 Ma (2σ) based on six pyrite samples from the altered rocks of the deposit (U mass fraction ~0.2 mg/kg; Th/U ~3.5; ⁴He specific volume ~10⁻⁵ cm³·STP·g⁻¹). This age is consistent with estimates of the age of ore formation and coeval with the end of the period of island arc magmatic activity. Our results indicate that U-Th-He dating for pyrite samples of ~1 mg in weight from the hydrothermal-metasomatic halo of ore bodies is possible, providing a crucial next step in the development of U-Th-He pyrite geochronology.