硅同位素动力学分馏的实验研究及地质应用

EXPERIMENTAL STTUDY OF SILICON ISOTPE DYNAMIC FRACTIONATION AND ITS GEOLOGICAL APPLICATION

由于硅在自然界中没有化学价态变化，主要呈硅氧四面体形式存在，因此硅同位素的热力学分馏很小，由此产生的硅同位素变化不大。溶液中ＳｉＯ＿２沉淀过程的同位素动力学分馏是引起自然界硅同位素明显变化的主要原因。本文实验测定了溶液中ＳｉＯ＿２沉淀过程的硅同位素动力学分馏系数α；并结合所测定的硅同位素动力学分馏系数对现代海底黑烟囱、太古代条带状磁铁石英岩、风化成因粘土矿物的δ￣（３０）Ｓｉ值明显偏低；浅海碳酸盐台地中硅质岩的δ￣（３０）Ｓｉ值明显偏高的现象做了理论上的解释。

Silicon has no valence change in nature and is present mainly as tentra-valent cations in silicon-oxygen tetrahedra,showing very small silicon isotopeequilibrium fractionation. Dynamic fractionation is the major cause for obviouschange in silicon isotope composition. One of the most effective processes thatcause silicon isotope dynamic fractionation is the precipitation of silica gelfrom the solution, In our experimental study,a silica acid solution(H_4SiO_4)was prepared at temperature(16～18℃)by reactition of 4 mol/1 HCl withsodium-silicate(Na_2SiO_3).The solution was unstable,and a polyreaction auto-matically proceeded,resulting in silica gel precipitation. Silicon isotope compo-sitions of the precipitates and the solution were determined,and the precipi-tated/dissolved silicon isotope dynamic fractionation factors(a)were in therange of 0.9990～0.9996.The experiment shows that silicon isotope dynamicfractionation is controlled both by temperature and by precipitation speed.The faster the speed of silicon precipitation,the smaller the dynamic fractio-nation factor between the precipited and the dissolved phases.The experimental results have been used to account theoretically for siliconisotope variation of some natural occurrences, of which modern hot springsinters, siliceous rocks from black smokers on seafloor, weathering clay mineralsand Precambrian exhalative banded magnetite-quartzite show large negativeδ￣(30)Si values,whereas siliceous rocks frorn shal low-sea carbonate rock sequence,on the other hand,possess obvious high δ￣(30)Si values.