Russian Journal of Earth Sciences
Vol. 5, No. 1, February 2003
Helium isotopes in the ground fluids of the Baikal Rift
and its surroundings: Contribution to continental rifting geodynamics
B. G. Polyak
Abstract
The
3He/4He =
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=0.01RA (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 CH
4 -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.