The 3 He/ 4 He = R ratio was studied in underground fluids from 104 sites from the Baikal Rift Zone BRZ and adjacent areas in Russia and Mongolia. The R -values vary in a wide range from R=0.01R A crustal radiogenic He to 7.8 RA close to the MORB He, where RA is the atmospheric 3He/4He=1.4 10-6. The lowest R values distinguish CH4 -rich gases. More diverse R values were measured in N2 - and CO2 -rich fluids, and the latter show the highest R. The N2 /Ar ratios for the N2 -rich gases are close to atmospheric values. The fN2 /fNe ratio value in CO2 - rich fluids indicates the excess non-atmospheric nitrogen. The comparison of the R values with He concentrations and predominate components of a fluid gas phase shows, that this phase is formed under the effects of solubility-controlled fractionation in gas-water system and gain/loss of chemically active gases within the crust. Gases of the pre-Riphean Siberian Platform have an average R = 0.026 RA which is close to the "canonical'' radiogenic crustal value. The distribution of the R values across the BRZ strike indicates a discharge of heat-mass flux from the mantle not only inside the BRZ as such, but much further to the east. The spectrum of R -values in the BRZ fluids is very wide: from 0.035 RA to 7.8 RA, but there is a clear tendency to lowering of R -values at both sides with the distance from the Tunka depression considered as a "center of rifting''. This trend correlates with both the heat flow density and the sizes of the rift depressions and demonstrates decreasing mantle-derived heat-mass flux to the margins of the rift zone. The comparison of BRZ data with those for other active continental rifts and mid-oceanic ridges suggests that the mechanisms of mantle-crust interaction during oceanic spreading and continental rifting are radically different.
Helium isotopes, ground fluids, Baikal Rift Zone, rifting geodynamics.
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