Preliminary Definition of Li Isotope Compositions on Surficial Environmental Processes Associated with Archean Seawater
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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δ7Li值为~+1‰，而在2.6~2.5 Ga期间，碳酸盐岩δ7Li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δ7Li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δ7Li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.Abstract: 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, the δ7Li 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 low δ7Li 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 seawater δ7Li 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.
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Key words:
- Archean /
- carbonate /
- seawater /
- Li isotope /
- geochemistry
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图 1 Kaapvaal克拉通简图与样品位置(改自Frimmel et al., 2005)
Fig. 1. Simplified map of Kaapvaal craton and sample regions (modified from Frimmel et al., 2005)
图 2 样品照片及采样示意图
a.Nsuze-1（3.0~2.9 Ga）来自Pongola Supergroup；b.Transvaal-1（2.6~2.5 Ga）来自Transvaal Supergroup；c.Transvaal-2（2.6~2.5 Ga）来自Transvaal Supergroup
Fig. 2. Photos of carbonate samples and sampling position
图 3 不同温度下海水与碳酸盐岩间的分馏值(a)和太古代海水的Li同位素组成(b)
Fig. 3. Fractionation values between seawater and carbonate at different temperatures (a), Li isotope composition of Archean seawater (b)
图 4 现代海水和太古代海水的Li同位素组成模型
Fig. 4. Li isotope composition models of modern and archean seawater
图 5 海水δ7Li值的变化趋势
据Misra and Froelich（2012）；von Strandmann et al.(2013, 2017)；Sun et al.（2018）
Fig. 5. Variation trend of δ7Li in seawater
表 1 碳酸盐中Li同位素和元素含量分析结果
Table 1. Analysis results of Li isotope and elemental contents in carbonate
样品号 Li/Ca (μmol/mol) Ca/Mg (mol/mol) Fe/Ca (mmol/mol) Mn/Ca (mmol/mol) Al/(Ca+Mg) (μmol/mol) Li含量(10-6) δ7Li (‰) 2SD Nsuze-1 Ns-1 2.54 2.04 73.99 11.14 0.93 0.6 1.1 0.1 Ns-2 3.19 1.79 88.16 11.77 1.74 0.8 0.4 0.3 Ns-3 2.93 1.85 74.66 9.82 1.01 0.7 0.9 0.1 Ns-4 2.79 1.88 73.25 9.48 1.10 0.7 0.9 0.0 Ns-4’ 2.92 1.87 74.28 9.59 1.09 0.7 1.1 0.1 Ns-5 2.00 1.93 70.88 9.38 0.72 0.5 0.6 0.2 Ns-6 2.03 1.97 60.08 8.01 0.52 0.5 1.0 0.0 Ns-7 2.08 1.96 64.99 8.78 0.71 0.5 0.5 0.0 Ns-8 4.19 1.89 79.31 10.69 2.27 1.0 1.5 0.1 Ns-9 2.86 1.87 81.17 11.42 1.30 0.8 0.8 0.3 Transvaal-1 Tr-1-1 1.14 1.89 34.51 33.68 0.08 0.3 7.3 0.1 Tr-1-2 1.23 1.95 30.07 33.13 0.14 0.4 7.7 0.0 Tr-1-3 1.30 1.96 32.25 33.56 0.12 0.3 6.7 0.1 Tr-1-4 1.13 1.90 24.49 32.52 0.19 0.4 7.3 0.6 Tr-1-4 1.67 1.86 23.72 32.25 0.05 0.4 7.4 0.2 Tr-1-5 1.03 1.91 33.40 34.37 0.02 0.3 7.3 0.2 Tr-1-6 1.35 1.88 32.96 34.81 0.05 0.4 7.0 0.3 Tr-1-7 1.24 1.88 34.02 34.56 0.02 0.4 6.9 0.1 Tr-1-8 1.46 1.91 31.95 34.56 0.06 0.4 7.0 0.0 Tr-1-9 1.42 1.98 50.46 35.93 0.09 0.4 6.6 0.3 Tr-1-10 1.32 1.86 32.40 33.44 0.07 0.4 7.0 0.2 Transvaal-2 Tr-2-1 2.40 1.91 35.92 44.03 0.18 0.6 10.2 0.2 Tr-2-2 2.82 1.90 31.17 42.10 0.11 0.7 10.0 0.1 Tr-2-3 2.29 1.86 33.40 42.81 0.09 0.7 9.1 0.1 Tr-2-4 3.23 1.92 28.51 41.78 0.08 0.8 9.5 0.0 Tr-2-4 3.45 1.94 28.52 41.46 0.09 1.0 9.2 0.2 Tr-2-5 3.18 1.94 45.53 49.54 0.22 0.9 9.8 0.0 
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