HELIUM ISOTOPES IN THE GROUND FLUIDS OF THE BAIKAL RIFT AND ITS SURROUNDINGS: CONTRIBUTION TO CONTINENTAL RIFTING GEODYNAMICS

Abstract

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 <img src="latex008.gif" alt="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. -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.