Degassing of deep fluids in the Pannonian basin and adjacent areas

Abstract

The Pannonian Basin (PB), in Central-Eastern Europe, is a continental area characterized by widespread presence of natural resources, high heat fluxes and outgassing of deep-sourced fluids (i.e. mantle-magma and/or crustal-derived). Moreover, the region is interested by ascent of the asthenosphere and a thin lithosphere (≈75 km). Here, we review 40 years of geochemical studies on natural gas emissions in the PB system and nearby areas providing the first comprehensive geochemical characterization of gas manifestations for the Croatian segment of PB. We use stable isotope (δ¹³C_CO2) geochemistry, noble gases data, and CHe systematics to reconcile geochemical information with geophysical and geodynamic models at regional scale, and hence to characterize (i) the source/s of fluids outgassing at the surface and (ii) the main processes occurring during their storage in, and transit through, the crust.

The chemical composition of the emitted fluids is very heterogeneous in the PB. We identify three distinct gas types (CO₂-dominated, N₂-dominated, and CH₄-dominated) that are variably distributed in different sectors of PB. The He isotopic composition range from 0.07 to 6.32Ra (Ra is the air He isotopic signature), suggesting the coexistence of crustal and mantle components in the area. Furthermore, the same components also occur in the Croatian PB, where the He isotopic ratios range from 0.02 Ra to 2.21 Ra. The groundwater circulation in the PB implies an addition of atmospheric-derived noble gas component to the deep fluids (mantle vs crust-derived). The volumetric gas/water ratios (Vg/Vl) are highly variable (0.002 to 66) with the highest values in N₂-dominated samples, and correlate with atmospheric-derived ²⁰Ne concentration, pointing to shallow gas origin for these samples (relative to CO₂ and CH₄-dominated samples). The CHe systematics, coupled with the δ¹³C of CO₂, indicates extensive chemical and isotopic fractionation due to partial dissolution of gas in water in the shallow crustal layers and consequent CO₂ trapping in deep aquifers and/or in precipitating carbonates. In addition, methanogenesis could work as an additional potential CO₂ sink in the crust. The mantle-derived He flux, on a regional scale, is estimated at 1.7 × 10¹⁰ to 1.7 × 10¹² atoms m⁻² s⁻¹, one order of magnitude greater than found by O'Nions and Oxburgh (1988), and similar to what found in other tectonically active regions. The mantle-related CO₂ flux computed using CO₂/³He ratios and the mantle He fluxes, range between 10³ and 10⁵ mol·km⁻²·year⁻¹. Despite representing a rough estimation, these values are in the range of the CO₂ fluxes in active and quiescent worldwide volcanic systems. We propose the transfer of mantle-derived volatiles to occurs through lithospheric faults in the PB and adjacent regions, although the presence of magmatic intrusions in crustal layers is an additional contributing factor.