Li同位素组成对太古宙海水相关的表生环境过程的初步限定  

Preliminary Definition of Li Isotope Compositions on Surficial Environmental Processes Associated with Archean Seawater

作　　者：付露露 肖益林[1] 张兴亮[2] 王洋洋[1] 谭东波 Fu Lulu;Xiao Yilin;Zhang Xingliang;Wang Yangyang;Tan Dongbo(CAS Key Laboratory of Crust-Mantle Materials and Environments,School of Earth and Space Sciences,University of Science and Technology of China,Hefei 230026,China;State Key Laboratory of Continental Dynamics,Shaanxi Key Laboratory of Early Life and Environment,Department of Geology,Northwest University,Xi’an 710069,China)

机构地区：[1]中国科学院壳幔物质与环境重点实验室,中国科学技术大学地球和空间科学学院,安徽合肥230026 [2]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,西北大学地质学系,陕西西安710069

出　　处：《地球科学》2021年第6期2073-2082,共10页Earth Science

基　　金：国家自然科学基金项目(No.41673031).

摘　　要：研究试图利用Li同位素地球化学对太古代海水相关的表生环境过程进行初步的限定.通过对来自南非Kaapvaal克拉通的海相碳酸盐岩样品进行Li同位素分析,发现在3.0~2.9 Ga期间形成的碳酸盐岩δ^(7) Li值为~+1‰,而在2.6~2.5 Ga期间,碳酸盐岩δ^(7) Li值达到7‰~10‰.经过反演计算得到对应时代的海水Li同位素组成分别为~+12‰和~+20‰,均明显低于现代海水值(~+31‰),但是2.6~2.5 Ga期间的海水δ^(7) Li值要比3.0~2.9 Ga时高出8‰.作为大陆硅酸岩风化的有效示踪剂,太古代海水较低的Li同位素组成表明当时的地表风化以源岩溶解为主,次生矿物形成极少,在3.0~2.5 Ga期间,海水整体温度下降以及次生矿物形成增加可能共同导致了海水δ^(7) Li值的升高.通过对太古代碳酸盐岩的Li同位素研究能够有效反演古海水的Li同位素组成,并为了解太古代表生风化过程对海水的影响提供了新的信息.In this study it attempts to use the Li isotope geochemistry to preliminarily limit the surficial environmental processes associated with Archean seawater.It performed Li isotope analysis on marine carbonate samples from the Kaapvaal craton in South Africa and finds that the carbonate shows light Li compositions of~+1‰during the period of 3.0-2.9 Ga,and increase of+7‰to+10‰during the period of 2.6-2.5 Ga.Through inversion calculation,the Li isotope compositions of seawater in the two periods are~+12‰and~+20‰,respectively,which are significantly lower than modern seawater(~+31‰).However,theδ^(7) Li value of seawater during 2.6-2.5 Ga is more than 8‰higher than that at 3.0-2.9 Ga.As an effective tracer for continental weathering of silicate rocks,Archean seawater shows relatively lowδ^(7) Li values,which indicates that the surficial weathering at that age was dominated by the dissolution of source rocks,and secondary minerals were rarely formed.During 3.0-2.5 Ga,the decrease in the overall temperature of the seawater and the increase in the formation of secondary minerals may jointly lead to an increase in the seawaterδ^(7) Li value during Late Archean.The study of Li isotopes of Archean carbonate can effectively invert the Li isotopic composition of paleoseawater,and provide new information for understanding the surficial environmental processes related to the Archean seawater.

关 键 词：太古代 碳酸盐 海水 Li同位素 地球化学 

分 类 号：P597[天文地球—地球化学]