ISOTOPES OF THE LIGHT NOBLE-GASES IN MINERAL WATERS IN THE EASTERN PART OF THE BALKAN PENINSULA, BULGARIA

The concentration and isotopic composition for the light noble gases i n spontaneous gas from mineral waters in the eastern part of Balkan pe ninsula are presented in this paper. The isotopic ratio He-3/He-4 in d issolved gases varies within the range 0.15-1.1 X 10(-6) and the conce ntration of helium in these gases is between 0.002 and 3.14 vol%. It i s determined that 86.7-98.8, 0.4-9.2, and <1% of He-4 is derived from the crust, mantle, and atmosphere, respectively. The main source of He -3 is the Earth's mantle. The isotopic composition of neon is nearly t he same as that in the atmosphere. Several samples show a slight exces s of light isotopes, probably due to the mass fractionation of atmosph eric neon. The isotopic composition of argon is also quite near to tha t in the atmosphere. In three cases argon contains 18.1, 12.3, and 7.9 % of the radiogenic component. The (Ar/N2)a ratios as a rule vary betw een 0.012 (air) and 0.027 (dissolved air), so that N2 in most of the s amples is also assumed to be of atmospheric origin. The ratio SIGMAC/H e-3 in many sites is as low as 10(7), but this fact cannot be used to reveal unambiguously the origin of C-containing gases.