RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 5, NO. 1, PAGES 45–66, doi:10.2205/2003ES000122, 2003

Helium isotopes in the ground fluids of the Baikal Rift and its surroundings: Contribution to continental rifting geodynamics

B. G. Polyak

Geological Institute, Russian Academy of Sciences, Moscow, Russia


[1]  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 times 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.

Received 13 February 2003; published 18 February 2003.

Keywords: Helium isotopes, ground fluids, Baikal Rift Zone, rifting geodynamics.

Citation: Polyak, B. G. (2003), Helium isotopes in the ground fluids of the Baikal Rift and its surroundings: Contribution to continental rifting geodynamics, Russ. J. Earth Sci., 5, No.1, 45-66, doi:10.2205/2003ES000122.

Copyright 2003 by the Russian Journal of Earth Sciences
Powered by TeXWeb (Win32, v.2.0).