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Étude isotopique de l'hélium atmosphérique à haute précision : applications environnementales et volcaniques

Abstract : The helium isotopic composition of air (RA = 3He/4He = 1.39 × 10-6) is related to the natural and anthropogenic fluxes of 3He and 4He from the Earth (and from space for 3He). Industrial activities emit gases enriched in radiogenic 4He (exploitation of fossil fuels) and in 3He (nuclear activities). It has been proposed that the 3He/4He ratio in air has varied over time and also spatially because of these helium fluxes. Such variations could allow the use of this ratio as a new tracer of anthropogenic and/or natural pollutants. To verify these possibilities, we undertook high-precision measurements of the composition of atmospheric helium (2-6‰, 2σ) at the Centre de Recherches Pétrographiques et Géochimiques (University of Lorraine, Centre National de Recherche Scientique, Vandoeuvre-lès-Nancy, France). We performed sample-standard bracketing analyses with a double collector Helix SFT Mass Spectrometer. During each of these analyses, several individual analyses of an air sample are conducted in alternation with an air standard. We found no evidence of temporal variation of the 3He/4He ratio in French air trapped in petanque balls (1965, 1990, 2010, 2013), metallic tanks (2010, 2016) and in a carburettor (1910). Including the measurements of Mabry et al. (2015) for the Cape Grim air archive (Tasmania, Australia), we obtained a trend of -0.05 ± 2.46‰ over 106 years. At the global level, we only detected two air samples out of sixteen (Dôme C-Antarctica, Tokyo-Japan) whose compositions are statistically distinct, marked by 3He excesses of 2.0 ± 1.4‰, and 1.7 ± 1.5‰, respectively (95% confidence interval). These excesses could be related to local and regional helium fluxes input such as: (i) auroral precipitation in Antarctica; (ii) volcanoes and/or nuclear releases (Fukushima Daï-Ichi) in Japan. In the Afar region (Ethiopia), we detected 3He excesses of ~1% above the lava lake of the Erta Ale volcano, which can be used to trace reactive gases (e.g. CO2) released simultaneously. These excesses are quickly affected by air mixing with the regional air of Afar. The diffusive soil fluxes of 3He (via increase of the 3He/4He ratio) and CO2 have been estimated from the accumulation of gases released from the soil of the Erta Ale crater in an inverted chamber placed on its surface. Overall, this thesis supports the use of RA as an inter-laboratory standard. In some areas (Dôme C, Japon), a more thorough investigation should be carried out over time to constrain the sources that may cause small helium isotopic variations in air
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Christine Boucher. Étude isotopique de l'hélium atmosphérique à haute précision : applications environnementales et volcaniques. Géochimie. Université de Lorraine, 2017. Français. ⟨NNT : 2017LORR0194⟩. ⟨tel-01906014⟩



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