镁同位素地球化学研究进展

Recent advances in research on magnesium isotope geochemistry

镁有3个稳定同位素:24Mg、25Mg和26Mg。已有数据表明,自然界Mg同位素组成的变化在陨石中较大,δ26Mg值在-0.71‰~11.92‰之间(相对于DSM3标准),而地球岩石中Mg同位素组成的变化较小,如地幔橄榄岩的δ26Mg变化范围为-3.01‰~1.03‰,沉积碳酸盐岩的δ26Mg变化范围为-4.84‰^-1.09‰,黄土的δ26Mg为-0.60‰。目前,大多数实验室均采用MC-ICP-MS方法来获得高精度的Mg同位素数据。原有Mg同位素国际标准SRM980由于具有不均一性应该废弃,应使用新的DSM3标准。Mg同位素地球化学的研究主要集中在太阳系星云形成过程、记录亏损地幔和地幔交代作用、揭示地质历史时期海水的Mg同位素组成演化、估算大陆风化通量等方面。我国开展的Mg同位素研究还非常少,且尚处于建立Mg同位素分析方法的研究阶段。随着技术的不断发展以及Mg同位素分馏机理的深入研究,Mg同位素的地质应用前景将日趋广泛。

Magnesium has three stable isotopes： ^24Mg, ^25Mg and ^26Mg. Availabledata show that the largest Mg-isotopic variation occurs in meteorites, with δ^26Mg values of - 0.71‰-11.92‰ （relative to DSM3 standard）, whereas the terrestrial rocks show smaller Mg-isotopic variation. For example, mantle rocks show a δ^26Mg variation of - 3.01‰ -- 1.03‰ ; sedimentary carbonates display a δ^26Mg variation from - 4.84‰ to - 1.09‰ ; and loess has a δ^26Mg value of - 0.60‰. At present, many laboratories use MC-ICPMS for the precise measurement of Mg-isotopic composition. Because of the isotopic heterogeneity of the standard SRM980 produced by NIST, the new standard DSM3 has been recommended as the reference material to report Mg-isotopic compositions. At present the researeh on magnesium isotope geochemistry is focusing on the following aspects： ① cosmochemistry and star formation processes; ② tracing mantle metasomatism; ③ variation of seawater δ^26Mg value through geologic history; ④ estimates of the Mg flux during continental weathering. Study on Mg-isotope geochemistry in China is just starting and the application of Mg isotopes to solving various geological problems will be enhanced in the near future both at home and abroad.