不同类型黏土矿物吸附作用对镁同位素分馏的影响  

Influence of different clays-adsorption on magnesium isotopic fractionation

作　　者：郭元强 黄康俊 赵燕[1] 王琦[1] 路雅雯 马龙[1] GUO Yuanqiang;HUANG Kangjun;ZHAO Yan;WANG Qi;LU Yawen;MA Long(Department of Geology,Shaanxi Key Laboratory of Early Life and Environment,State Key Laboratory of Continental Dynamics,Northwest University,Xi’an 710069,Shaanxi,China)

机构地区：[1]西北大学,大陆动力学国家重点实验室,陕西省早期生命与环境重点实验室地质学系,陕西西安710069

出　　处：《地球化学》2024年第4期525-535,共11页Geochimica

基　　金：第四纪国家重点实验室开放基金项目(SKLLQG2116);国家自然科学基金项目(41973008)联合资助

摘　　要：明确表生过程中Mg同位素分馏行为是示踪全球Mg循环和大陆风化过程的基础。黏土矿物吸附–解吸附是影响Mg同位素分馏的重要机制,然而这一过程中Mg同位素的分馏行为存在较大争议,尤其是黏土矿物类型和其他阳离子的影响还未有效评估。本研究选择高岭石、伊利石和蒙脱石3种典型的黏土矿物,在解吸附矿物初始吸附的Mg元素之后,加入不同含量的MgCl_(2)溶液和含有其他阳离子的河水中进行吸附–解吸附实验,并测试吸附前、后溶液和吸附组分中的元素含量和Mg同位素组成。结果表明,在相同含量的MgCl_(2)溶液中,蒙脱石对Mg的吸附能力明显高于伊利石和高岭石,这是由于蒙脱石存在层间域吸附。当吸附达到饱和状态时,黏土矿物对Mg的吸附量将不受溶液中Mg含量影响。当有其他阳离子(Na^(+)、K^(+)、Ca^(2+)等)存在时,交换作用会明显降低蒙脱石对Mg的吸附能力,但对高岭石和伊利石影响不大,这是因为交换作用主要发生在层间域。此外,在吸附–解吸附过程中,Mg同位素的分馏不明显(Δ^(26)Mg≈0),主要是由于黏土矿物吸附过程中受平衡分馏影响。Clarifying the behavior of magnesium(Mg)isotopes during surface processes is the basis of tracing the global Mg cycle and continental weathering.However,the behavior of Mg isotopes during clay adsorption-desorption,an important mechanism that controls Mg isotope fractionation,remains highly debated.Particularly,the roles of clay mineral types and other cations are not well understood.In this context,three typical clay minerals(i.e.,kaolinite,illite,and montmorillonite)were used to conduct adsorption-desorption experiments in MgCl_(2)solutions and river water after the original Mg in the clays was desorbed.The elemental and Mg isotopic compositions of the solutions before and after adsorption were then determined.Results show that montmorillonite adsorbs more Mg than kaolinite and illite from MgCl_(2)solutions of the same concentration,which can be attributed to interlayer space adsorption in montmorillonite.The amount of adsorbed Mg was not affected by the Mg concentration of the solution when the clay-adsorption reached saturation,whereas the presence of other cations(Na^(+),K^(+),Ca^(2+))in the solution decreased the amount of adsorbed Mg in montmorillonite but not in kaolinite and illite due to interlayer space adsorption.The Mg isotope fractionation(Δ^(26)Mg≈0)during adsorption and desorption was not obvious,which was mainly due to the effect of equilibrium fractionation during the adsorption of clay minerals.

关 键 词：MG同位素 黏土矿物 吸附–解吸附 平衡分馏 

分 类 号：P594[天文地球—地球化学] P599[天文地球—地质学]