(Table 1) Isotopic and geochemical parameters of gas phase from underground fluids of the Baikal rift and its framing

Results of studying isotopic composition of helium in underground fluids of the Baikal-Mongolian region during the last quarter of XX century are summarized. Determinations of 3He/4He ratio in 139 samples of gas phase from fluids, collected at 104 points of the Baikal rift zone and adjacent structures are given. 3He/4He values lie within the range from 1x10**-8 (typical for crustal radiogenic helium) to 1.1x10**-5 (close to typical MORB reservoir). Repeated sampling in some points during more than 20 years showed stability of helium isotopic composition in time in each of them at any level of 3He/4He values. There is no systematic differences of 3He/4He in samples from surface water sources and deeper intervals of boreholes in the same areas. Universal relationship between isotopic composition of helium and general composition of gas phase is absent either, but the minimum 3He/4He values occurred in methane gas of hydrocarbon deposits, whereas in nitrogen and carbon dioxide gases of helium composition varied (in the latter maximum 3He/4He values have been measured). According to N2/Ar_atm ratio nitrogen gases are atmospheric. In carbonic gas fN2/fNe ratio indicates presence of excessive (non-atmogenic) nitrogen, but the attitude CO2/3He differs from one in MORB. Comparison of helium isotopic composition with its concentration and composition of the main components of gas phase from fluids shows that it is formed under influence of fractionation of components with different solubility in the gas-water system and generation/consumption of reactive gases in the crust. Structural and tectonic elements of the region differ from the spectrum of 3He/4He values. At the pre-Riphean Siberian Platform the mean 3He/4He = (3.6+/-0.9)x10**- 8 is very close to radiogenic one. In the Paleozoic crust of Khangay 3He/4He = (16.3+/-4.6)x10**-8, and the most probable estimate is (12.3+/-2.9)x10**-8. In structures of the eastern flank of the Baikal rift zone (Khentei, Dauria) affected by the Mz-Kz activization 3He/4He values range from 4.4x10**-8 to 2.14x10**-6 (average 0.94x10**-6). Distribution of 3He/4He values across the strike of the Baikal rift zone indicates advective heat transfer from the mantle not only in the rift zone, but also much further to the east. In fluids of the Baikal rift zone range of 3He/4He values is the widest: from 4x10**-8 to 1.1x10**-5. Their variations along the strike of the rift zone are clearly patterned, namely, decrease of 3He/4He values in both directions from the Tunka depression. Accompanied by decrease in density of conductive heat flow and in size of rift basins, this trend indicates decrease in intensity of advective heat transfer from the mantle to peripheral segments of the rift zone. Comparing this trend with data on other continental rift zones and mid-ocean ridges leads to the conclusion about fundamental differences in mechanisms of interaction between the crust and the mantle in these environments.

本文总结了20世纪后叶贝加尔-蒙古地区地下流体中氦同位素组成的研究成果。文中给出了在贝加尔裂谷带及邻区构造104个采集点获取的139份流体气相样品的³He/⁴He比值测定结果，该比值范围为1×10⁻⁸（地壳放射成因氦的典型比值）至1.1×10⁻⁵（接近典型洋中脊玄武岩（Mid-Ocean Ridge Basalt，MORB）储库特征）。对部分点位超过20年的重复采样结果显示，无论³He/⁴He比值处于何种水平，各点位的氦同位素组成均未随时间发生显著变化。同一区域内，地表水来源样品与钻孔深部层段样品的³He/⁴He比值无系统性差异。氦同位素组成与气相整体组成之间亦不存在普遍关联；但烃类矿床的甲烷气中³He/⁴He比值最低，而氮气和二氧化碳气的氦同位素组成存在差异（其中二氧化碳气中测得最高的³He/⁴He比值）。根据N₂/Ar_大气比值判断，该区域氮气气体均为大气来源。碳酸气中的fN₂/fNe比值表明存在过剩（非大气成因）氮气，但其CO₂/³He比值与洋中脊玄武岩储库中的比值存在显著差异。将氦同位素组成与流体中氦浓度及气相主要组分组成进行对比后可知，该同位素组成受气-水体系中不同组分的分馏作用，以及地壳内反应性气体的生成与消耗过程共同影响。不同构造-大地构造单元的³He/⁴He比值分布范围存在显著差异：在前里菲纪西伯利亚地台区，³He/⁴He的平均值为(3.6±0.9)×10⁻⁸，与地壳放射成因氦的比值极为接近；在杭爱地区的古生代地壳中，³He/⁴He平均值为(16.3±4.6)×10⁻⁸，最可靠的估算值为(12.3±2.9)×10⁻⁸；受中生代-新生代构造活化作用影响的贝加尔裂谷带东翼构造区（肯特山、达乌里亚），其³He/⁴He比值范围为4.4×10⁻⁸至2.14×10⁻⁶，平均值为0.94×10⁻⁶。沿贝加尔裂谷带走向的³He/⁴He比值分布特征表明，地幔对流热传输不仅存在于裂谷带内部，还延伸至东侧更远的区域。贝加尔裂谷带流体中的³He/⁴He比值范围最广，为4×10⁻⁸至1.1×10⁻⁵；该比值沿裂谷带走向的变化具有明显规律：从通卡凹陷向两侧，³He/⁴He比值均逐渐降低。这一变化趋势伴随传导热流密度与裂谷盆地规模的同步减小，表明地幔向裂谷带外围区段的对流热传输强度逐渐减弱。将该趋势与其他大陆裂谷带及洋中脊的相关数据进行对比后可知，上述两类环境下地壳与地幔的相互作用机制存在根本性差异。