HELIUM ISOTOPIC DATA FOR THE GROUND WATERS IN THE BAIKAL RIFT-ZONE

The paper considers regularities of distribution and concentration of total helium and its isotopes in the solute gas in the groundwaters of the Baikal rift zone. The He-3/He-4 ratios obtained for 31 sites showed geographical variability throughout the Baikal rift zone, a variation according to types of groundwaters (methane, carbon dioxide or nitric hydrotherms, and cold fresh waters) and certain relationship with total concentration of helium. In the south-western termination of the Baikal rift zone, high He-3/He-4 ratios align along the rift axis in an area where mantle-born helium is brought by ascending flows in hot and cold springs, with its maximum percentage (above 50%) within the Tunka rift basin which is one of few volcanic provinces in the rift. The highest contributions of mantle-born helium are attributed to methane and carbon dioxide hydrotherms, and to cold fresh-water springs at the boundaries of the high He-3/He-4 area. The groundwaters throughout the rest of the Baikal rift zone contain mainly radiogenic (crust-born) helium where the mantle component contributes under 10%, that is consistent with the hypothesis of the infiltrational origin of deep groundwaters. Outside the high He-3/He-4 area the isotopic ratio is higher in sulphate nitric hydrotherms than in those of carbonate chemistry. The He-3/He-4 ratios decrease at the boundaries of the rift zone with the Siberian platform and the Trans-Baikal fold area. The concentrations of mantle-born helium in the Baikal rift zone was compared to that in the Eger rift (West Europe).