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

Geological Institute, Russian Academy of Sciences, Moscow, Russia

Abstract

[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 R_A (close to the MORB He), where R_A 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 R_A 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 R_A to 7.8 R_A, 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.