Deep-sourced fluids released in central-western Europe: new evidences of active degassing in Serbia region

Abstract

Identification of transfer of mantle-derived fluids (e.g. CO2, N2, noble gases) in continental regions is critical for developing exploration strategies of natural resources (i.e., hydrocarbons, ore deposits, heat sources) and also to investigate the processes that control the deep and shallow evolution of Earth such as subduction, volcanism, natural degassing vs. active tectonic and earthquakes (e.g., O’Nions & Oxburgh, 1988; Caracausi et al., 2013; Labidi et al., 2020). Recently the outgassing of deep-mantle volatiles has been verified in different crustal segments of central-western Europe, but the geological and tectonic controls on migration of these deep fluids remain to be fully understood. To figure out the source of these volatiles can furnish new elements to the understanding of the complex regional geodynamic evolution and for the exploitation of natural resources. Furthermore, that region is also characterized by an high heat flow (< 150 mW/m2) that highlights an high geothermal energy potential that is coupled to seismicity trough the overall crust (Horwarth et al., 2015). Here, we report on the results of an extensive geochemical survey of gas manifestations in the Serbian part of the Vardar zone, a mega-suture zone between Eurasia and Africa plates. Based on their chemical compositions (C, He, Ne, Ar), the studied gases are clustered into three distinct groups (CO2-, N2- and CH4-dominated). The He isotope ratios identify a weak (<20 %) but persistent regional mantle-derived component, with the lowest 3He/4He values reflecting radiogenic He production from nearby granitoid intrusions. The combined analysis of He, C(CO2) abundances and isotopic composition demonstrates which there are two main processes that control the chemistry of the outgassing volatiles:1) mixing between crustal and mantle-derived volatiles and 2) the partial dissolution of volatiles in groundwater. Finally, we estimate the mantle He flux at 9.0 x 109 atoms m-2 s-1, or up to 2 orders of magnitude higher than the typical fluxes in stable continental areas, suggesting a tectonic control of fluid migration through the crust.