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
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
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.
Received 13 February 2003; published 18 February 2003.
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.
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