氩同位素分馏的实验研究

尽管⁴⁰Ar和36Ar之间质量相差达1/10,但是受放射性成因⁴⁰Ar的影响,一般认为难以进行氩同位素分馏研究.本文通过自行设计的一套氩扩散迁移实验分析系统,对比研究了氩在不同的扩散介质条件下扩散迁移前后氩同位素组成变化情况,证实氩在致密的扩散介质条件下以分子流形式从一个储库向另一个储库的迁移过程中,由于36Ar和⁴⁰Ar的迁移速率不同,扩散后的⁴⁰Ar/36Ar值比扩散前的值要小,也即发生了氩同位素分馏.氩同位素分馏的特征表现为最初的一段时间内分馏程度逐步增强,在一定时间后,由于储库之间压力逐渐达到平衡,分馏程度逐步减小.研究自然界中存在的氩同位素分馏,不仅可以判别油气田中油气运移的方向、增强油气远景评价和地球化学勘探,而且对深源岩浆的起源、迁移等研究也可提供新的研究思路和途径.

EXPERIMENTAL STUDY ON ARGON ISOTOPIC FRACTIONATION

It was commonly thought difficult to study argon isotopic fractionation because of affection of radiogenic ⁴⁰Ar. But, the mass difference(Δm) between ⁴⁰Ar and ³⁶Ar is 4 and the fractional mass difference(Δm/m)is 1/10, which is larger than that of ¹³C and ¹²C(Δm/m=1/13), whose isotopic fractionation is common in nature. Fractionation may arise in the non equilibrium migration of material from one reservoir to another because of the migration rate difference between ⁴⁰Ar and ³⁶Ar. On basis of this, the authors design a set of experimental system to study the variation of argon isotopic ratio during diffusive migration. Experimental results reveal that the trend of decline of ⁴⁰Ar/³⁶Ar ratio value occurs when argon diffuses through compact path, that means, if argon diffuses compact path as molecular flow, argon isotopic fractionation can occur because of diffusive rate difference. There are two stages during this procession. At the first stage, degree of fractionation stepwise becomes larger; after this stage, degree of fractionation becomes less because two reservoirs approach to equilibrium. By means of argon isotopic fractionation, we can interpret the feature of argon isotope distribution and distinguish the direction of oil and gas migration, which can enhance oil and gas prospect evaluation and geochemical exploration. Even so, it may provide more important geological information such as origin of magma.