Abstract
The occurrence of hydrocarbon gases (HCG) in unusually high concentrations for magmatic complexes, in the Lovozero and some other alkaline massifs, is of both geochemical and practical interest. The nature of these gases, despite the long history of research, remains the subject of debate. As an approach to solving this problem, we studied the coupled distribution of occluded HCG and the recognized tracers of various geological processes, such as helium and argon isotopes. The extraction of the gas components trapped in fluid micro-inclusions was carried out by the mechanical crushing of rock and mineral samples. A positive correlation was found between the 3He/4He and CH4/C2H6 ratios, whereas a negative correlation of the latter was found with the 36Ar concentration, which in turn was directly related, in varying degrees, to the content of HCG and most strongly with pentanes. Conjugacy of the processes of the heavier gaseous hydrocarbons, a loss of the deep component of the fluid phase and dilution of it with the atmogenic component was established. In the absence of a correlation between CH4 and 3He, the value of the CH4/3He ratio in the Lovozero gas substantially exceeded the estimates of it in gases of a mantle origin, and mainly corresponded to the crustal values. However, in some samples, a small fraction of mantle methane was allowed. The peculiarities of the relationships between hydrocarbon gases and the isotopes of noble gases indicate a sequential process of abiogenic generation and transformation of HCG at the magmatic and post-magmatic stages during the formation of the Lovozero massif. The obtained results confirm the usefulness of this approach in solving the origin of reduced gases in alkaline igneous systems.
Highlights
A notable feature of some nepheline syenite and foidolite massifs, primarily in the world’s largest Khibiny and Lovozero alkaline plutons and the related huge deposits of phosphorus and rare elements, as well as in Ilimaussaq in Greenland, is a concentration of reduced hydrogen-hydrocarbon that is unusually high for igneous rocks, with a predominance of methane gases [1,2,3,4,5,6]
When the distribution of the isotopes of noble gases was separately studied in earlier research, as well as methane and its homologues in Lovozero rocks, attention was drawn to the similar peculiar properties of the variations in the vertical section of the massif in the average 3 He/4 He and CH4 /C2 H6 ratios [22,23,70]
The second ratio reflects a decrease in the temperature range of the hydrocarbon gases (HCG) generation, post-magmatic changes in the minerals and the formation of fluid inclusions [14,23,66,70]
Summary
A notable feature of some nepheline syenite and foidolite massifs, primarily in the world’s largest Khibiny and Lovozero alkaline plutons and the related huge deposits of phosphorus and rare elements, as well as in Ilimaussaq in Greenland, is a concentration of reduced hydrogen-hydrocarbon that is unusually high for igneous rocks, with a predominance of methane gases [1,2,3,4,5,6]. The applied value is due to the release of hazardous natural combustible and explosive gases into the atmosphere of the underground mines working on mineral deposits associated with nepheline-syenite massifs [1,4,6,9]. We used an approach that was not previously employed to understand some aspects of the origin of the reduced fluid in nepheline-syenite magmatic complexes, as well as for studying the coupled distribution of occluded HCG and helium and argon isotopes in the rocks of the Lovozero massif. The results of these studies are presented in this article
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