Helium isotopes in contact zones of alkaline intrusions of different size: A case study of the alkaline-ultrabasic Ozernaya Varaka intrusive and the Lovozero massif of nepheline syenites (Kola Peninsula)

The isotopes of helium were studied in the contact zones with the country rocks of the intrusions of different size (the alkaline-ultrabasic Ozernaya Varaka massif and the Lovozero massif of nepheline syenites). In case of medium-sized Ozernaya Varaka intrusion, helium extracted by powdering from igneous rocks of the intrusion and from the country fenites showed the same isotope composition exceeding the average crustal value by more than an order of magnitude. This is caused by the relatively rapid crystallization of the intruded small volume of the melt accompanied by the separation of igneous fluids and their permeation into the enclosing rocks. The distribution of helium isotopes at the contact of the larger Lovozero massif with gneisses is characterized by low—nearly crustal—3He/4He ratios in igneous rocks (nepheline syenites of the endocontact and by increased values (to 110 × 10−8) in fenites of the exocontact. It is suggested that the increased 3He/4He ratios registered in fenites correspond to the beginning of the intrusion process and that the low values in nepheline syenites are related to the completion of crystallization of the rocks of a differentiated complex.