Determination of Silicon Isotopic Compositions of Rock and Soil Reference Materials by MC-ICP-MS
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摘要:
随着分析方法的进步和分析精度的提高，硅(Si)同位素被越来越多地应用于地球化学、宇宙化学和环境化学研究中，可以用于示踪壳幔物质循环、俯冲流体的来源, 以及制约月球和地外天体的起源与演化等。为确保不同类型样品中硅同位素测量的准确性和不同实验室间数据可以进行对比，需标定一系列标准物质的硅同位素组成。前人工作中已经标定一系列来自美国地质调查局(USGS)的标准物质的硅同位素，为硅同位素的研究奠定了坚实的基础。但由于这些标准物质已经售罄，今后继续开展硅同位素研究面临无标样可用的境况。为了能持续性地用高精度硅同位素数据对相关领域研究提供支持，急需对新的标准物质进行高精度的硅同位素的测量。本文采用氢氧化钠碱熔法消解样品，经化学纯化后，利用多接收电感耦合等离子体质谱法精确测量了30个国家标准物质的硅同位素组成，δ30Si值测试精度优于0.08‰。这些标准物质包括11个火成岩、2个变质岩、2个沉积岩、6个河流和海洋沉积物以及9个土壤，SiO2含量范围为32.69%~90.36%，覆盖了大部分自然样品的变化范围。在这些标准物质中，河流沉积物GBW07310具有最高的δ30Si值，为0.85‰±0.01‰，而受高度风化作用影响的黄红色土壤GBW07405和砖红壤GBW07407具有较低的δ30Si值，硅同位素组成分别为-0.68‰±0.03‰和-1.82‰±0.03‰，其余大部分标准物质的δ30Si值变化范围为-0.42‰~-0.07‰。本文对这些国家标准物质硅同位素组成的精确标定，丰富了硅同位素研究的标准样品数据库，为全球不同实验室的硅同位素测试提供了基础数据，为后续在多种领域开展硅同位素研究打下坚实的基础。
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关键词:
- 硅同位素 /
- 国家标准物质 /
- 碱熔法 /
- 多接收电感耦合等离子体质谱法 /
- 高精度测试方法
Abstract:BACKGROUND With the analytical technique development, the precision of Si isotopes analysis increases rapidly.Silicon isotopes are widely used in geochemistry, cosmochemistry, environmental chemistry and so on, and can be used to trace the circulation of crust-mantle material, the source of subducting fluid, and constrain the origin and evolution of the moon and extraterrestrial materials.To compare the precision and accuracy of Si isotope analysis results in different laboratories, it is necessary to analyze Si isotopes of reference materials with published Si isotope data.As generally used USGS reference materials are currently unavailable, it is important to report Si isotopes of new reference materials.
OBJECTIVES In order to continuously conduct research in various fields with high-precision silicon isotope data, by providing a supply of new reference materials.Silicon isotopes of 30 GBW reference materials with different compositions, including 11 igneous rocks, 2 sedimentary rocks, 2 metamorphic rocks, 6 river/marine sediments and 9 soils, were analyzed.The SiO2 content of these reference materials ranged from 32.69% to 90.36%, covering the variation range of most natural samples.
METHODS Alkali fusion method was used for sample digestion.Approximately 3-5mg of sample powder and 200mg of powdery NaOH were weighed in a 10mL silver crucible and heated.The Si purification was obtained using cation exchange resin AG50W-X12.6mol/L HNO3 and ultrapure water were used to clean the resin before sample loading.Silicon isotopes were measured by multi-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS, Neptune Plus) at the laboratory in the University of Science and Technology of China (USTC), and the instrument mass bias was corrected by standard-sample-standard method, with a bracketing standard of NBS-28.The long-term reproducibility (over two years) of Si isotope analysis of one in-house standard (USTC-Si) and one international rock reference material (BHVO-2) were represented, with the δ30Si values of USTC-Si and BHVO-2 of-0.07‰±0.06‰(n=117, 2SD) and-0.29‰±0.06‰(n=320, 2SD), respectively.During Si isotope analysis, two rock reference materials BHVO-2 and AGV-2 were also analyzed to ensure the precision and accuracy of the data.The Si isotopic compositions of BHVO-2 and AGV-2 were consistent with the reported data in the previous literature (Figure 3), demonstrating the reliability of this measurement.
RESULTS Except one sediment and two soil samples, the δ30Si values of most reference materials analyzed in this study range from-0.42‰ to-0.07‰, within the range of upper continental crust.The drainage sediment GBW07310 has the highest δ30Si value (0.85‰±0.01‰), while the yellow-red soil GBW07405 and the latosol GBW07407 have the lowest δ30Si values of-0.68‰±0.03‰ and-1.82‰±0.03‰, respectively.
CONCLUSIONS The high-precision Si isotope data of 30 GBW reference materials helps replenish the database for Si isotope analysis.The Si isotope data of these standard materials show that the river sediment GBW07310 has a very high δ30Si value of-1.82‰±0.03‰, indicating that it may be formed by dissolved silicon precipitation, which are enriched in heavy Si isotopes; while highly weathered yellow-red soil GBW07405 and the latosol GBW07407 have the lowest δ30Si values of-0.68‰±0.03‰ and-1.82‰±0.03‰, respectively, indicating that the weathering and desiliconization process may lead to the loss of heavy Si isotopes.The δ30Si values of most remaining reference materials analyzed in this study vary from-0.42‰ to-0.07‰, within the variation range of the upper continental crust.There is no obvious correlation between δ30Si values and SiO2 contents of the 11 igneous rock reference materials, revealing that their Si isotopes were not controlled by partial melting or mineral crystallization processes, and there may be other processes which would affect the Si isotopic composition of these standards.In the case that generally used USGS reference materials have been sold out, these high-precision Si isotope data of GBW reference materials will supplement basic data for Si isotope testing in different laboratories and lay solid isotope research in various fields.
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表 1 硅的化学纯化流程
Table 1. Chemical purification method of silicon
淋洗液 使用体积
(mL)实验目的 6mol/L硝酸 3 清洗树脂 6mol/L硝酸 3 清洗树脂 6mol/L硝酸 3 清洗树脂 二次超纯水 2 清洗树脂，调节pH值 二次超纯水 3 清洗树脂，调节pH值 二次超纯水 3 检验滤出液pH值为中性 样品母液 1 上样接硅 二次超纯水 3 接硅 二次超纯水 3 接硅 
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表 2 MC-ICP-MS仪器测试Si同位素的主要工作条件
Table 2. Operating conditions for Si isotopic determination of MC-ICP-MS instrument
工作参数 实验条件 工作参数 实验条件 射频功率 1200W 锥 H截取锥和Ni Jet样品锥 冷却气流速 ~16L/min 雾化器 PFA雾化器(50μL/min) 辅助气流速 ~0.8L/min 雾室 双路气旋式石英雾室 样品气流速 ~1.0L/min 杯结构 L3 (28Si)，C (29Si)，H3 (30Si) 灵敏度 28Si为~5V/(μg/g) 分辨率 高分辨(>6000)
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表 3 国际标准物质的硅同位素组成测量结果
Table 3. Silicon isotopic compositions of reference materials with different laboratories and our data
标准物质
编号δ29Si
(‰)2SD
(‰)δ30Si
(‰)2SD
(‰)n 来源文献 AGV-2
(安山岩)-0.08 0.04 -0.19 0.07 9 本文研究 -0.10 0.03 -0.21 0.07 11 Savage等[23](2011) -0.07 0.05 -0.15 0.06 6 Zambardi等[24](2011) -0.09 0.06 -0.21 0.07 3 Yu等[25](2018) BHVO-2
(玄武岩)-0.14 0.05 -0.29 0.05 27 本文研究 -0.14 0.05 -0.27 0.10 192 Savage等[12](2010) -0.14 0.05 -0.27 0.08 42 Zambardi等[24](2011) -0.15 0.03 -0.30 0.05 24 Yu等[25](2018) 注：2SD为一份溶液测量n次的标准偏差的2倍。
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表 4 30个国家标准物质(GBW)的硅同位素组成
Table 4. Silicon isotopic composition of thirty Chinese geological reference materials (GBW)
样品类型 标准物质编号 岩石类型 δ29Si
(‰)2SD
(‰)δ30Si
(‰)2SD
(‰)n SiO2含量
(%)火成岩序列 GBW07101 超基性岩(Ultramafic) -0.16 0.08 -0.37 0.06 3 34.34 GBW07102 超基性岩(Ultramafic) -0.11 0.09 -0.29 0.03 3 37.75 GBW07103 花岗岩(Granite) -0.07 0.03 -0.23 0.06 3 72.83 GBW07104 安山岩(Andesite) -0.05 0.04 -0.15 0.05 3 60.62 GBW07105 玄武岩(Basalt) -0.13 0.05 -0.20 0.06 3 44.64 GBW07109 霓石正长岩(Ijolite syenite) -0.10 0.03 -0.29 0.02 3 54.48 GBW07110 粗面安山岩(Trachyte andesite) -0.01 0.09 -0.07 0.01 3 63.06 GBW07111 花岗闪长岩(Granodiorite) -0.15 0.04 -0.31 0.06 3 59.68 GBW07112 辉长岩(Gabbro) -0.09 0.01 -0.19 0.04 3 35.69 GBW07113 流纹岩(Rhyolite) -0.09 0.05 -0.21 0.08 3 72.78 GBW07113R 流纹岩(Rhyolite) -0.07 0.08 -0.18 0.04 3 72.78 GBW07123 辉绿岩(Diabase) -0.17 0.04 -0.28 0.06 3 49.88 变质岩序列 GBW07121 花岗片麻岩(Granite gneiss) -0.07 0.07 -0.19 0.06 3 66.27 GBW07122 角闪岩(Amphibolite) -0.15 0.01 -0.27 0.05 3 49.62 沉积岩序列 GBW07106 石英砂岩(Quartz sandstone) -0.15 0.01 -0.27 0.05 3 90.36 GBW07107 页岩(Shale) -0.08 0.02 -0.16 0.04 3 59.23 河水/海洋沉积物系列 GBW07301a 河流沉积物(Stream sediment) -0.12 0.01 -0.20 0.04 3 59.07 GBW07301aR 河流沉积物(Stream sediment) -0.11 0.01 -0.18 0.02 3 59.07 GBW07309 河流沉积物(Stream sediment) -0.12 0.12 -0.22 0.06 3 64.89 GBW07310 排水沉积物(Drainage sediment) 0.47 0.07 0.85 0.01 3 88.89 GBW07312 河流沉积物(Stream sediment) -0.12 0.11 -0.22 0.02 3 77.29 GBW07314 近岸海洋沉积物(Offshore marine sediment) -0.11 0.08 -0.29 0.05 3 61.91 GBW07333 海洋沉积物(Marine sediment) -0.21 0.10 -0.42 0.06 3 54.00 土壤序列 GBW07402 栗色土壤(Chestnut soil) -0.07 0.06 -0.18 0.05 3 73.35 GBW07405 黄红色土壤(Yellow-red soil) -0.33 0.04 -0.68 0.03 3 52.57 GBW07407 砖红壤(Latosol) -0.95 0.01 -1.82 0.03 3 32.69 GBW07408 黄土(Loess) -0.09 0.02 -0.19 0.03 3 58.61 GBW07423 湖成沉积土壤(Lacustrine deposit) -0.14 0.02 -0.29 0.04 3 61.69 GBW07425 土壤(Soil) -0.12 0.05 -0.23 0.05 3 69.42 GBW07426 来自上覆地区的土壤(Soil from overburden region) -0.06 0.03 -0.22 0.07 3 60.01 GBW07427 土壤(Soil) -0.08 0.02 -0.22 0.05 3 64.88 GBW07446 砂壤(Sandy soil) -0.08 0.07 -0.14 0.04 3 78.30 注：GBW07113R和GBW07113、GBW07301a和GBW07301aR是单独称样的一对重复样。
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