Russian Journal of Earth Sciences Russian Journal of Earth Sciences Russian Journal of Earth Sciences 1681-1208 47078 ОРИГИНАЛЬНЫЕ СТАТЬИ ORIGINAL ARTICLES ОРИГИНАЛЬНЫЕ СТАТЬИ Helium isotopes in the ground fluids of the Baikal Rift and its surroundings: Contribution to continental rifting geodynamics Helium isotopes in the ground fluids of the Baikal Rift and its surroundings: Contribution to continental rifting geodynamics Polyak B G Polyak B G Geological Institute, Russian Academy of Sciences, Moscow, Russia ru Geological Institute, Russian Academy of Sciences, Moscow, Russia ru 5 1 45 66 10 11 2021 https://rjes.ru/en/nauka/article/47078/view

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.

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. Helium isotopes ground fluids Baikal Rift Zone rifting geodynamics.

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