Diagnostics of genetic helium isotope ratio 3He/4He on example of minerals from melilite-olivine-calcite pegmatite in olivinites of the Kovdor massif (Kola peninsula)

The isotopic composition of helium indicates mineral-forming environment. With reference to the study of helium isotopes retaining in minerals the following minerals from pegmatite of the Kovdor massif were studied: olivine, diopside, magnetite and apatite. Extraction of helium was carried out both by crushing of samples for helium release from fluid micro-inclusions and by melting, when the whole helium (captured and accumulated in situ ) was released. The content and isotopic composition of helium was measured on a mass spectrometer MI 1201. The 3He maximum concentration was reached in apatite as it finalized the crystallization process, but the initial helium isotope composition in apatite was obscured by radiogenic 4He, generated by the decay of U and Th. The presence of mantle helium in secondary minerals (magnetite and diopside) indicates that the late magmatic fluids can also keep mantle isotopic composition of helium, and suggests the possibility of inheritance effect for helium isotopic ratio during the replacement of olivine by diopside.