锂同位素分馏机制讨论  

Discussion on Fractionation Mechanism of Lithium Isotopes

作　　者：汤艳杰[1] 张宏福[1] 英基丰[1] 

机构地区：[1]中国科学院地质与地球物理研究所岩石圈演化国家重点实验室,北京100029

出　　处：《地球科学（中国地质大学学报）》2009年第1期43-55,共13页Earth Science-Journal of China University of Geosciences

基　　金：中国科学院知识创新工程重要方向项目(No.KZCX-YW-103);国家自然科学基金项目(Nos.40534022,40773026)

摘　　要：作为一种新兴的稳定同位素示踪工具,锂同位素地球化学的研究近年来受到了国际地学界日益广泛的关注.其应用领域涵盖了从地表到地幔的流体与矿物之间的相互作用.在地表风化作用过程中,轻锂同位素(6Li)优先进入固体相,而7Li则进入流体相,因而地表风化作用淋滤出了岩石中的重锂,致使河水具有重的锂同位素组成,河水又将重锂同位素组分补给海洋,洋壳的低温蚀变作用使得海水的锂同位素组成进一步变重.在俯冲带,由于俯冲板片释放的流体具有重锂同位素组成的特征,它们上升并交代上覆的地幔楔和相邻的地幔,使得地幔楔的锂同位素组成变重.同时,深俯冲的板片由于脱水而具有较轻的锂同位素组成,它们在地幔中可能形成一个局部轻锂的地幔储源.影响地幔橄榄岩锂同位素分馏的因素主要有3个方面:温度、扩散机制以及外来熔体的反应.由于高温下地幔矿物之间的锂同位素分馏很小,而单纯的扩散分馏机制不能够很好的解释我国华北汉诺坝地区地幔橄榄岩中矿物之间的锂同位素分馏.因此,具有轻锂同位素组成的熔体与橄榄岩之间的反应是上述现象的一个合理解释.需要指出的是,在橄榄岩-熔体反应的过程中,锂同位素的扩散作用也对地幔矿物之间的同位素分馏有一定的贡献.As a burgeoning tracer of stable isotopes, lithium geochemistry has attracted increasing attention from the international geological community recently. Its application field covers the interactions between fluids and minerals from the surface to the mantle. During weathering, the lighter isotope ^6Li is preferentially retained in the solid phase while ^7Li goes into solution. Thus river waters have heavy Li isotopic compositions compared to the original bedrock. The heavy Li isotopic compositions of river water feed the oceans. Low-temperature alteration of oceanic crust then makes seawater heavier than river water. In a subduction zone, heavy Li of fluids escaping the slab at low temperature likely enrich the forearc mantle wedge and hydrate the adjacent mantle and make them isotopically heavier than before. Meanwhile, an isotopically light component of slab dehydrated is subducted deeply and may form a locally light-Li mantle reservoir. There are three main factors that can result Li isotopic fraetionation in mantle peridotites. They are temperature, diffusion-driven fractionation and mixing with exotic melts. At high temperatures of magmatic processes, Li isotopes do not show per mil-level mass fractionation, and simple diffusive process can not account for the Li isotopic fractionation between the minerals in peridotitic xenoliths from the Hannuoba area in the North China Craton, thus the reaction between peridotites and melts with light Li isotopic compositions maybe a reasonable explanation. It should point out that diffusion of Li isotopes during peridotite-melt reaction makes some contributions to the isotopic fractionation between the mantle minerals.

关 键 词：锂同位素 分馏机制 地球化学示踪工具 

分 类 号：P597[天文地球—地球化学]