δ 13 C, CO 2 ∕ 3 He and 3 He ∕ 4 He ratios reveal the presence of mantle gas in the CO 2 -rich groundwaters of the Ardennes massif (Spa, Belgium)

International audience ; Although natural CO 2 -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, CO 2 content and carbon isotopic composition. Among them, 13 samples were also analyzed for 3 He /4 He and 4 He /20 Ne ratios. The combination of δ 13 C (between ca. −9 ‰ VPDB 1 and −2 ‰ VPDB), CO 2 /3 He ratio (between 1.9×10 8 and 2.9×10 9 )... Mehr ...