Mobility of radiogenic isotopes 4He and 3He and their retention in a mineral (using amphibole as an example)

Studies of ⁴⁰Ar and ⁴He mobility in minerals (performed for isotope geochronometry problems) generally assumed that the migration parameters, obtained for the atoms preserved in a mineral, described the mobility of all atoms that were produced in the mineral, including those that were lost in the past. To test this assumption, an analysis of the U-Th-Li-⁴He-³He isotope system in amphiboles, separated from alkaline granites of the Ponoi massif, Kola Peninsula, was performed. The retention of He isotopes was determined, i.e., the ratio of the amount of radiogenic He remaining in a mineral to its amount, which should have been accumulated since the formation (metamorphism) of the mineral. It turned out that 36% of ³He and only 14% of ⁴He were preserved in the mineral. The results of experiments on ³He and ⁴He migration from amphibole grains during their step wise heating in vacuum were successfully approximated by a diffusion model. However, the parameters, obtained during the simulations, did not allow reproducing the above retentions of He isotopes. The mechanisms of ³He and ⁴He migration in the past, which have led to such different retentions, differ from the diffusion mechanism, which adequately describes migration of the atoms remaining in the mineral.