热扩散驱动的元素分异和同位素分馏:一种不容忽视的硅酸盐成分分异机制

Thermodiffusion driven element and isotope fractionations: A remarkable differentiation mechanism in silicate systems

热扩散在地质过程中是否发挥重要作用一直存有争议。本文回顾了热扩散的研究历史和现状,重点总结了热扩散驱动的元素和同位素行为规律,并探讨了温度、硅酸盐组分、压力和氧逸度等因素对热扩散行为的影响。已有的研究表明,稳定热梯度下的硅酸盐热扩散效应类似于结晶分异或AFC过程,可以造成轻、重同位素分别在高温端和低温端富集,而主、微量元素的扩散方向则取决于两端化学势的高低和熔体中的电价平衡。从基性岩浆到酸性岩浆,熔体聚合度增大,黏度增加,热扩散速率明显降低,成网元素的热扩散效应减弱,变网元素则反之;水、氟、氯和硫化氢等挥发组分能增加熔体的非桥氧比例,降低熔体聚合度,因而能显著增强硅酸盐熔体中元素和同位素的热扩散效应。在此基础上,本文提出了当前硅酸盐体系热扩散研究中存在的五个亟需解决的问题,即:1)对不同硅酸盐体系的热扩散规律的研究还不够全面;2)对微量元素的热扩散行为认识不足;3)硅酸盐体系热扩散作用的影响因素及尺度还不够明确;4)热扩散作用的地质与地球化学关键识别标志有待确立;5)硅酸盐体系热扩散作用的理论模型有待建立。尽管硅酸盐体系热扩散的研究还存在诸多不足,但越来越多的证据表明,热扩散是地质过程中不容忽视的一种成分分异机制。这种机制会造成岩浆房或岩浆通道中的元素分异和同位素分馏,可能对于一些成分分异的岩石和矿床的形成具有重要的意义。

Whether thermodiffusion,a kind of diffusion driven by thermal gradient,plays an essential role during geological processes is a controversial issue.In this paper,we reviewed the research history of the thermodiffusion and major progresses in the field of geoscience,especially including a summary of the rules on major,trace element variations as well as isotope fractionations driven by thermodiffusion in the melted or partiallymolten silicate systems,and also discussed the controlling factors of the thermodiffusion,such as temperature,silicate component,pressure,and oxygen fugacity.Based on these results,we can see that thermodiffusion induced element variations in the silicate melts resemble the effect of crystal fractionation,and those in the partiallymolten silicate system are analogue to the results of assimilationfractional crystallization processes,while the light and heavy isotopes can be readily fractionated by the thermodiffusion into the hightemperature and lowtemperature ends,respectively.These effects could also be significantly enhanced in a volatilerich(e.g.,H2O,F,Cl,and H2S)and lowviscosity silicate magma.Basically,the general direction of elemental thermodiffusion is restricted by their chemical potentials at both ends and charge equilibrium in the system.Increasing evidence therefore suggests that thermodiffusion should be an essential differentiation mechanism in the magma systems,and might be important to the diagenesis and mineralization occurred under a standing temperature gradient.Though considerable efforts have been made to enhance our understanding of this mechanism,we finally raise five critical problems about the thermodiffusion in the silicate system that remained to be solved in the future.They include:1)the general rules of thermodiffusion in various silicate systems;2)the thermodiffusion behaviors of trace elements;3)the influence factors and their magnitude in particular;4)the key geological and geochemical identification features;and 5)the theoretical model of thermodiffusion in silicate systems.