摘要
锂同位素示踪是近几年发展起来的一门新兴的稳定同位素地球化学方法。锂有两个稳定同位素 :6Li和7Li。自然界锂同位素的组成变化很大 ,其δ6Li值变化幅度超过 6 0‰。现代大洋水的δ6Li值为 - 31.0‰。洋中脊玄武岩 (MORB)的δ6Li值为 - 4 .7‰～ - 3.4‰。由于锂同位素存在大的分馏和不同地质体中存在截然不同的δ6Li值 ,因此锂同位素地质应用前景十分广泛。目前 ,锂同位素在研究星云形成过程和宇宙事件、洋壳蚀变和海底热液活动、壳 -幔物质循环和板块俯冲作用过程。
Lithium has two stable isotopes ( 6Li and 7Li) with nature abundance of 7.52% and 92.48% , respectively. Due to the large mass difference between the two lithium isotopes and low Li contents in geological samples, Li isotope measurements have met large difficulty in the past several decades. High precision Li isotope analytical methods are only developed recently, and three major methods are currently used: (1) TIMS method with analytical precision of <0.3‰; (2) MC ICP MS method with analytical precision of ～1.1‰; (3) Ion probe method with analytical precision of 1.0～3.4‰. The first two methods need complicated and time consuming column chemistry to purify the samples, but can achieve higher precision than the ion probe method that has been used mainly in meteorite study. Large fractionation of Li isotopes occurs in nature with δ 6Li variation more than 60‰. Modern oceanic water has a rather constant δ 6Li value of -31.0‰, whereas the river water shows a large δ 6Li variation of -32.2～-6.0‰. Modern submarine hydrothermal fluids display a δ 6Li variation of -11.5～-2.6‰. Meteorites have δ 6Li values of -10.0±10‰. Marine carbonates show large δ 6Li variations from -40～ +12‰ . In contrast, marine clastic sediments display a small δ 6Li variation of -14.7～ -0.9‰ . Mid ocean ridge basalts (MORB) show very small δ 6Li variations of -4.7～ -3.4‰, whereas OIB have δ 6Li values of -6.6～-4.7‰, and BABB have slightly higher δ 6Li values of -2.9～ -0.7‰ . Granites have positive δ 6Li values of 0.3～ 1.5‰. The large δ 6Li variations in nature have made Li isotopic composition a very sensitive geochemical tracer with its major applications in the following fields: (1) cosmochemistry and star formation processes; (2) modern submarine hydrothermal activity and alteration of oceanic crusts; (3) crust mantle evolution and subduction related processes; and (4) origin and evolution of brines.
出处
《高校地质学报》
CAS
CSCD
2001年第4期390-398,共9页
Geological Journal of China Universities
基金
国家杰出青年科学基金资助项目 (编号 :4992 5 30 6 )
关键词
锂同位素
测量方法
陨石
火成岩
沉积岩
水圈
地质应用
稳定同位素
lithium isotopes
analytical methods
meteorite
igneous rocks
sediments
hydrosphere
geological applications.
