华北地幔橄榄岩锂同位素研究进展

Progress in Li isotopes of mantle peridotites from the North China Craton

作为非传统稳定同位素的代表,Li同位素已被广泛应用于示踪与熔体和流体相关的地质过程。地幔橄榄岩高度不均一的Li含量和Li同位素组成特征是熔/流体与地幔橄榄岩相互作用过程中同位素分馏作用及温度下降过程中Li在矿物之间扩散分馏的结果,同位素扩散现象通常被矿物颗粒边部的组成特征所记录。由于Li在橄榄石中的扩散速度比在辉石中慢,所以大颗粒橄榄石核部的Li含量和同位素组成更能反映岩石圈地幔的组成,而辉石的组成特征更多地记录了后期的过程。大量地幔捕虏体的锂同位素组成特征研究表明,华北克拉通岩石圈地幔经历了蚀变洋壳在俯冲过程中、残留俯冲板片在深部地幔、以及上涌的软流圈所释放的不同组成的熔/流体的改造过程。由于改造作用的多阶段性和改造程度的不同,华北克拉通东部与中部地幔橄榄岩的组成特征具有系统性的差异。Li同位素与其他地球化学指标的联合示踪是未来地幔地球化学研究的发展趋势。

As a representative of untraditional stable isotopic systems,the lithium(Li)isotope has been widely used to trace a lot of geological processes related to melts and fluids,including earth surface and deep earth.The highly heterogeneous characteristic of Li concentrations and Li isotopic compositions of mantle-derived peridotite xenoliths is a combined result of isotopic fractionation during the interaction between melt/fluid and mantle peridotites and Li isotopic diffusive fractionation between mantle minerals during the xenolith cooling.The latter is usually recorded by compositional characteristics of the edge of mineral grains.Since Li diffusion is slower in olivine than in pyroxene,the Li concentration and isotopic composition in the core of large olivine grain can better reflect the primary composition of the lithospheric mantle.In contrast,pyroxene s compositional characteristics prefer recording the later processes.Many studies on lithium isotopic compositions of numerous mantle xenoliths demonstrate that the lithospheric mantle beneath the North China Craton has been modified by different melts/fluids released from the altered oceanic crust during subduction,the residual subducted plate in the deep mantle,and the upwelling asthenosphere.Due to the multiple stages and various degrees of modification,the compositional characteristics of mantle peridotites in the eastern and central parts of the North China Craton are systematically different.The joint tracing research of Li isotopes and other geochemical indices is the future development trend of mantle geochemistry.