δ¹³C, CO₂ ∕ ³He and ³He ∕ ⁴He ratios reveal the presence of mantle gas in the CO₂-rich groundwaters of the Ardennes massif (Spa, Belgium)

Agathe Defourny (1) , Pierre-Henri Blard (2) , Laurent Zimmermann (2) , Patrick Jobé , Arnaud Collignon , Frédéric Nguyen (1) , Alain Dassargues (1)

1 Urban and Environmental Engineering [Liège]
2 CRPG - Centre de Recherches Pétrographiques et Géochimiques

Although natural CO₂-rich groundwaters of eastern Belgium have been known for centuries, the exact origin of their gas is still unclear. This paper presents the results of a sampling campaign in Belgium (Spa, Stoumont, Malmedy): 30 samples of both carbogaseous and non-carbogaseous groundwaters were analyzed for major elements, CO₂ content and carbon isotopic composition. Among them, 13 samples were also analyzed for ³He^/4He and ⁴He^/20Ne ratios. The combination of δ¹³C (between ca. −9 ‰ VPDB¹ and −2 ‰ VPDB), CO₂^/3He ratio (between 1.9×10⁸ and 2.9×10⁹) and ³He^/4He (between 0.92 and 2.70 Ra) shows with a high level of confidence that the CO₂ in the carbogaseous groundwater of Spa and Bru has a mantle origin. It can likely be attributed to the degassing of mantle from the neighboring Eifel volcanic fields, located at a distance of 100 km eastwards.

The identity and nature of the deep-rooted fractures that act as CO₂ transport pathways to the surface are still to be clarified, but several major thrust faults exist in the Rhenish Massif and could connect the Eifel volcanic fields with the studied area.

Domains: Sciences of the Universe [physics]; Hydrology

Agathe Defourny, Pierre-Henri Blard, Laurent Zimmermann, Patrick Jobé, Arnaud Collignon, et al.. δ¹³C, CO₂ ∕ ³He and ³He ∕ ⁴He ratios reveal the presence of mantle gas in the CO₂-rich groundwaters of the Ardennes massif (Spa, Belgium). Hydrology and Earth System Sciences, 2022, 26, pp.2637-2648. ⟨10.5194/hess-26-2637-2022⟩. ⟨insu-03712868⟩