20-EPN2-012: Discovering the origin of dissolved gases in CO2-rich mineral groundwaters from Aquae Spadanae (Spa, eastern Belgium).
Visit by Agathe Defourny, University of Liège (Belgium), to TA2.10 Stable, Rare Gas and Radiogenic Isotope Facility at CRPG (France).
Dates of visit: 21 June – 02 July 2021
Report Summary: The visit at CRPG aimed at better assessing the origin of dissolved CO2 found in naturally sparkling groundwater springs from the east of Belgium. Previous analysis on δ13C had shown that the carbon could be either from mantellic or sedimentary (dissolved carbonates) origin, but a clear distinction between both could not be made. The goal of the stay at CRPG was then to focus on the analysis on other dissolved gases, in particular He and Ne. The combination of their isotopic signature, together with the isotopic composition of carbon is a powerful tool to highlight degassing from either crustal or mantel origin.
The results were really clear. The majority of the 4He/20Ne ratios stands between 50 and 500, indicating that more than 99% of the helium is not atmospheric and result from a mixture of crustal and mantellic gaz. Moreover, the ratio between CO2/3He (~109) versus δ13C (from -8 to -2 ‰) clearly shows that the dissolved CO2 in theses springs is from mantellic origin.
A few samples from non-carbogazeous springs from the same area were also collected and analysed and present a very different signature, with more negative δ13C values, and lower 4He/20Ne ratios. The measured value could be compared to different samples from the literature, particularly gas samples from the Eifel volcanic fields, at the border with Germany, showing very similar signatures. We can hence conclude with a high confidence level that the gases dissolved in the naturally sparkling spring from eastern Belgium come from the degassing of the Eifel mantellic plume, at a distance of about 100 km.