镁同位素地球化学研究新进展及其应用

Advances and application in magnesium isotope geochemistry

作为一种新兴的地质示踪剂,Mg同位素正受到国际地学界日益广泛的关注。Mg同位素地球化学研究已取得了巨大的进展,近期研究工作主要包括两个方面。首先,调查了地球各主要储库和陨石的Mg同位素组成特征,结果表明陨石和地球地幔具有均一并且相似的Mg同位素组成,平均δ26Mg值分别为-0.28±0.06‰和-0.25±0.07‰;相反,上地壳和水圈的Mg同位素组成很不均一,δ26Mg值变化范围分别为-4.84‰～+0.92‰和-2.93‰～+1.13‰。其次,对一些地质和物理化学过程中Mg同位素的分馏行为进行研究,结果表明:(1)地表风化作用可以造成大的Mg同位素分馏,导致重Mg同位素残留在风化产物中而轻Mg同位素进入水圈;(2)岩浆分异过程中Mg同位素平衡分馏很小;(3)高温化学扩散和热扩散过程中Mg同位素会发生显著的动力学分馏。基于这些研究成果,Mg同位素体系已经被初步应用于示踪早期地球形成和壳内物质再循环等过程,并有望在不久的将来应用于示踪大陆地壳的化学演化和地质温度计等研究领域。

As new geological tracers,Mg isotopes attract more and more attentions from international geologists.Great achievements have been made in the Mg isotope geochemistry.Recent studies have been focusing on two aspects.First,Mg isotopic compositions of the major terrestrial reservoirs and chondrites have been investigated.Chondrites and the mantle have homogeneous and similar Mg isotopic compositions,with the average δ26Mg values of-0.28±0.06‰ and-0.25±0.07‰,respectively.By contrast,the upper continental crust and hydrosphere are highly heterogeneous,with δ26Mg values varying from-4.84‰ to ＋0.92‰ and from-2.93‰ to ＋1.13‰,respectively.Second,studies on behaviors of Mg isotopes during geological and physicochemical processes suggest that：（1）large Mg isotope fractionation occurs during continental weathering with heavy Mg isotopes retained in the weathered products and light Mg isotopes released into the hydrosphere;（2）equilibrium Mg isotope fractionation during magma differentiation is limited;（3）significant kinetic Mg isotope fractionation occurs during chemical and thermal diffusion at high temperatures.Based on these studies,Mg isotopes have been used to trace accretion of the Earth and recycling of crustal materials,and,in the near future,may potentially be applied to tracing chemical evolution of the continental crust and be used as geological thermometers.