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

Abstract：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 Nur. The mantle 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 R 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 R 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.