Isotopes of the light noble gases in mineral waters in the eastern part of the Balkan peninsula, Bulgaria

Abstract

The concentration and isotopic composition for the light noble gases in spontaneous gas from mineral waters in the eastern part of Balkan peninsula are presented in this paper. The isotopic ratio 3He 4He in dissolved gases varies within the range 0.15–1.1 × 10 −6 and the concentration of helium in these gases is between 0.002 and 3.14 vol%. It is determined that 86.7–98.8, 0.4–9.2 and <1% of 4He is derived from the crust, mantle and atmosphere, respectively. The main source of 3He is the Earth's mantle. The isotopic composition of neon is nearly the same as that in the atmosphere. Several samples show a slight excess of light isotopes, probably due to the mass fractionation of atmospheric neon. The isotopic composition of argon is also quite near to that in the atmosphere. In three cases argon contains 18.1, 12.3 and 7.9% of the radiogenic component. The ( Ar N 2 ) a ratios as a rule vary between 0.012 (air) and 0.027 (dissolved air), so that N 2 in most of the samples is also assumed to be of atmospheric origin. The ratio ΣC 3He in many sites is as low as 10 7, but this fact cannot be used to reveal unambiguously the origin of C-containing gases.