新疆罗布泊盐湖卤水的氚同位素特征及其地质意义

Characteristics and origin of tritium in the potassium-rich brine in Lop Nur, Xinjiang

新疆罗布泊罗北凹地盐湖第四系上部盐层中蕴藏丰富的卤水,卤水中富含钾(KCl平均品位1.40%),潜卤层上部的卤水中检测到较高含量的氚。本文研究罗布泊富钾卤水氚的分布特征,由活塞模型和全混模型算得罗北凹地表层卤水的平均年龄为36—80a。卤水中含有大量的核爆氚,表明自1952年以来通过干盐湖发生过大气降水的补给。含氚的现代降水经由溶蚀孔隙十分发育的包气带盐壳渗入地下后,与浅层卤水均匀混合,由于卤水的蒸发作用缓慢,蒸发过程中氚同位素发生分馏,“轻水”蒸发,氚在残余水体(潜卤水)中富集。潜卤水氚含量的平面分布特征主要反映出以蒸发排泄为主的盐湖地下水流网特征。

Lop Nur is a playa located in the east of Tarim basin, Xinjiang, China. A plenty of brine is reserved in the upper evaporates of the upper-middle Pleistocene strata in Luobei depression, Lop Nur. The brines have higher salinity and are rich in potassium, with average 1.40% tenor of KCl. And high percentage of tritium in the upper ground water is measured, with average content of 40.6 TU. Laterally the tritium content variation is: higher in periphery and lower in center; while vertically the tritium content becomes lower with the burial depth increasing. With the use of piston model and completely-mixing model, the age of the upper brine is calculated as 36—80 a. Therefore, brine contained large portion of nuclear-bombing tritium, which verified that since 1952, atmospheric water had precipitated and seeped into the playa of this area. The tritium-bearing modern precipitated water is imbibed into sediments through the aeration zone, in which karst is developed intensively, and tritium mixed with shallow brine uniformly. Owing to the slow evaporation of brine, fractional distillation happened, and “lighter water” with less tritium evaporated. So most of tritium has been rich in relic brine. The characteristics of lateral tritium distribution are believed to be the results of groundwater flow pattern in a salt lake basin.