罗布泊地下卤水中幔源稀有气体及其意义

The mantle source noble gas from underground brine of the Lop Nur in Xinjiang and its scientific significance

采集于塔里木盆地东部罗布泊地区的地下卤水中稀有气体具有高于大气的值,显示出有高3He/4He值源区流体的贡献。这揭示了该区域深部存在壳-幔流体相互作用,深部地幔流体可能沿活动构造断裂带(比如阿尔金断裂带)上涌。其卤水释出气的4He/20Ne比值在0.572～2.105,略高于大气值至7倍于大气值,这表明罗布泊的地下卤水对大气是比较开放的,即受到近地表流体或大气的混染。同时,采集于塔里木盆地北缘库车盆地(又叫库车坳陷)的2件样品具有远低于大气的R值,其4He/20Ne比值为585.3～84.8,是大气值的数百倍,说明该区地下卤水源自于构造稳定的深部地壳,近地表流体对卤水的改造不明显而没有强烈地改变其R值。样品的40Ar/36Ar比值均接近于大气值(295.5),这揭示了近地表流体参与了源区卤水的运移与改造,只是库车盆地地下卤水受到近地表流体改造的程度较低。如果扣除大气(来自浅层流体)的混染,那么罗布泊地下卤水的R值将会比实际检测到的值更高,这进一步揭示了地幔流体参与了罗布泊地下卤水的形成演化。中国西北地区具有地壳厚、盆地长期稳定的特点,因此地幔流体活动不易出露于地表,但地幔流体所携带的地幔物源和热源会通过断裂带而影响地壳流体的特征。

Gas samples whose R values （R=^3He/^4He） are higher than R values of the air were collected from underground brine in the Lop Nur within eastern Tarim Basin of Xinjiang. This shows the contribution of a fluid source with higher R values. It is also revealed that there exists the interaction between the crust fluid and the mantle fluid in the depth of the Lop Nut. The mande fluid may ascend through active fault belts （for example, the giant Altun fault belt）. Besides, the ^4He/^20Ne ratio of the gas released from the underground brine in the Lop Nur is between 0.572 （a little bit higher than that of the air） and 2.105 （ six times higher than the air ^4He/^20Ne ratio）, which indicates that the brine is relatively open to the atmosphere, i.e., it is contaminated by near-surface fluid or the air. For comparison, two gas samples were collected from Kuqa Basin （depression） on the northern margin of Tarim Basin, and the analytical result shows lower P, value than that of the air, and the ^4He/^20Ne ratio is 84.8 and 585.3 respectively （dozens of times to several hundred times higher than that of the air）. This shows that underground brine in Kuqa Basin comes from the stable deep crust, and its P, value hasn＇t been changed by near-surface fluid. What＇s more, the ^40Ar/^36Ar ratio is close to that of the air （295.5） in all noble gas samples, which indicates that the near-surface fluid more or less joins in the transfer and formation of deep original brine, but such effects in Kuqa Basin is not too obvious to change its R value. If the contamination from atmosphere （from the shallow fluid） had been deducted, the R value in underground brine in Lop Nur would have been much higher than the analytical result. This further demonstrates that the mantle source fluid plays a great role in the formation and evolution of underground brine in the Lop Nur. The crust in northwest China is very thick and stable, so mantle source fluid isn＇t apt to come out to the surface. However, the mantle source fluid can carry material and heat through fault belts, thus affecting the characteristics of the fluid in the crust.