The link between lithospheric scale deformations and deep fluid emanations: Inferences from the Southeastern Carpathians, Romania

Abstract

Understanding the formation, migration and emanation of deep CO₂, H₂O and noble gases (He–Ne) in deep-seated deformation settings is crucial to know the complex relationship between deep-originated fluids and lithospheric deformation. To gain a better insight into these phenomena, we studied the origin of H₂O, CO₂ and noble gases of gas-rich springs found in the Târgu Secuiesc Basin located in the southeasternmost part of the Carpathian-Pannonian region of Europe. This study area is one of the best natural examples to understand the connection between the deep sources of gas emanations and deep-seated deformation zones, providing an excellent analogue for regions with similar tectonic settings and fluid emanation properties. We studied the δ²H and δ¹⁸O stable isotopic ratios of the spring waters, and the δ¹³C, He and Ne stable isotopic ratio of the emanating CO₂-rich gases dissolved in the mineral spring waters in Covasna town and its vicinity. Based on the δ²H, δ¹³C, δ¹⁸O stable isotopic ratios, the spring waters and the majority of the gases are released through two consecutive fluid infiltration events. The preservation of the metamorphic signal of the upwelling H₂O is linked to the local groundwater flow and fault abundancy. Furthermore, the noble gas isotopic ratios show a high degree of atmospheric contamination in the dissolved water gasses that is most likely related to the local hydrogeology. Nevertheless, the elevated corrected helium stable isotopic ratios (R_c/Ra) of our filtered data suggest that part of the emanating gases have a potential upper mantle source component. Beneath the Southeastern Carpathians, mantle fluids can have multiple origin including the dehydration of the sinking slab hosting the Vrancea seismogenic zone, the local asthenospheric upwelling and the lithospheric mantle. The flux of the mantle fluids is enhanced by lithospheric scale deformation zones that also support the fluid inflow from the upper mantle into the lower crust. The upwelling CO₂–H₂O mantle fluids may induce the release of crustal fluids by shifting X(CO₂) composition of the pore fluid and, consequently, initiating decarbonisation and devolatilization metamorphic reactions as a result of carbonate and hydrous mineral destabilisation in the crust. Based on the p-T-X(CO₂) conditions of calc-silicates and the local low geotherm, we emphasise the importance of the upwelling fluids in the release and upward migration of further H₂O and CO₂ in the shallower lower and upper crust. We infer that migration of deep fluids may also play an important role in addition to temperature control on the generation of crustal fluids in deep-seated deformation zones.