A geochemical traverse across the Eastern Carpathians (Romania): constraints on the origin and evolution of the mineral water and gas discharges

Abstract

The inner sector of the Eastern Carpathians displays a large number of Na–HCO 3, CO 2-rich, meteoric-originated cold springs (soda springs) and bore wells, as well as dry mofettes. They border the southern part of the Pliocene–Quaternary Calimani–Gurghiu–Harghita (CGH) calc-alkaline volcanic chain. Both volcanic rocks and CO 2-rich emissions are situated between the eastern part of the Transylvanian Basin and the main east Carpathian Range, where active compression tectonics caused diapiric intrusions of Miocene halite deposits and associated saline, CO 2-rich waters along active faults. The regional patterns of the distribution of CO 2 in spring waters (as calculated pCO 2) and the distribution pattern of the 3He/ 4He ratio in the free gas phases (up to 4.5 R m/ R a) show their maximum values in coincidence with both the maximum heat-flow measurements and the more recent volcanic edifices. Moving towards the eastern external foredeep areas, where oil fields and associated brines are present, natural gas emissions become CH 4-dominated. Such a change in the composition of gas emissions at surface is also recorded by the 3He/ 4He ratios that, in this area, assume ‘typical’ crustal values ( R m/ R a=0.02). In spite of the fact that thermal springs are rare in the Harghita volcanic area and that equilibrium temperature estimates based on geothermometric techniques on gas and liquid phases at surface do not suggest the presence of shallow active hydrothermal systems, a large circulation of fluids (gases) is likely triggered by the presence of mantle magmas stored inside the crust. If total 3He comes from the mantle or from the degassing of magmas stored in the crust, CO 2 might be associated to both volcanic degassing and thermometamorphism of recently subducted limestones.