Helium isotopes in Alpine regions

The isotopic composition of helium has been determined for a variety of groundwaters from springs and boreholes in the Alps and adjacent regions. The main Alpine block in France, Italy, Switzerland and Austria is characterized by a virtual absence of mantle-derived 3He in near-surface groundwaters. In contrast, adjacent areas where crust has been thinned by extension—to the west in the Rhone valley and Ardeche, to the north in the Rhine valley and to the east in the Vienna and Pannonian Basins—mantle-derived helium is clearly identified. Helium and undoubtedly other gases are introduced by melts whose generation is associated with crustal thinning. However, the surface distribution of mantle helium covers a much larger than that of associated surface volcanics. This probably results from the dispersion of helium following its solution in groundwaters. The Po and the Molasse Neogene sedimentary basins developing by loading on the south and the north sides of the Alps, respectively, have helium with typical crustal composition and a mantle component cannot be resolved. These results clearly indicate the control of tectonics on the distribution of helium isotopes in the Alpine regions.