Cardoso, C. D.; Russier, E.; Marty, B.; Pik, R.; Bekaert, D. V.; Seltzer, A.; Broadley, M.W.; Byrne, D.; Rigaudier, T.; Géraud, Y.; Tarantola, A.; Hauville, B.
Chemical Geology, 2023, 634, 121588
Voir en ligne : https://doi.org/10.1016/j.chemgeo.2023.121588
Abstract :
Determining the source(s) of helium in regions of the crust where 3He anomalies occur can be challenging when no surface manifestations of magmatic activity or a clear active extension regime are present. This is the case of the Paris basin (France), where geothermal fluids and oils 3He/4He have been previously shown to range from 0.02 Ra up to 0.14 Ra (Marty et al., 1993; Pinti and Marty, 1995, 1998), whereas natural gases to the south of the Paris Basin (near the Massif central) show unambiguous evidence for mantle helium contribution, with 3He/4He up to 6.4 Ra (Br¨auer et al., 2017). However, higher 3He/4He than the crustal endmember (~ 0.01 Ra), as observed in the Paris Basin, could be a sign of either (i) a weak contribution of mantle He (enriched in primordial 3He) or
(ii) the contribution of 3He naturally produced by nuclear reactions of lithium, for which commercially exploitable concentrations have been reported in rare-metal-rich granites. One way to distinguish between these two possibilities is to combine He isotope systematics with other noble gas tracers of mantle inputs, such as xenon isotopes. Here, we report the isotope compositions of helium and xenon, measured by static and dynamic mass spectrometry, respectively, in gas samples collected in the Ni`evre County, between the Massif Central and the Paris Basin. 3He/4He ratios range between 0.16 and 0.22 Ra. Combined with previous studies, our data are consistent with a southward increase of He isotope ratios along major fault systems from the Paris Basin (~ 0.02 Ra) towards the Massif Central (6.4 Ra). This geographical trend, paired with high precision Xe isotopes exhibiting an excess of mantle 129Xe, shows a clear mantle input that is most likely restricted to the major fault systems in Central France. The occurrence of mantle-derived helium and xenon in Central France highlights the presence of cryptic magmatism in areas with no apparent volcanic manifestations.