A Review of Research Progress on the Analytical Method of Large-n Detrital Zircon U-Pb Geochronology
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摘要:
碎屑锆石U-Pb年代学是识别沉积物来源和确定地层最大沉积年龄的重要工具。利用激光剥蚀电感耦合等离子体质谱(LA-ICP-MS)进行物源分析的碎屑锆石测试数量为60~120颗，在这个范围内，年龄组分通常不能从样本中识别出来。近年来，为提高物源分析的可靠性，LA-ICP-MS测试要求有更多数量的锆石颗粒(n≥300)，甚至大于1000颗的大样本量(large-n)试验。大样本量碎屑锆石U-Pb年代学的出现对数据测试方法、处理和评估都提出了挑战。本文通过对国内外大样本量文献进行梳理，总结了大样本量碎屑锆石U-Pb年代学在测试方法、数据处理以及数据评估方面的进展。首先，单颗粒测试需要对U、Pb同位素信号进行快速获取，这可以通过改进气溶胶传输效率实现，“峰值”信号模式代替“平顶”信号接收也可实现快速测试。其次，大样本量产生的数据，需要高效的数据处理协议和强大的软件(如Iolite)进行处理，以减少实验室间比较的误差；针对U-Pb数据处理流程，介绍了同位素分馏校正以及不确定度传播等方面的方法优化；此外，还引入了累积计数法和线性回归校正法两种处理方法专门处理“峰形”信号。在数据评估方面，新的U-Pb和Pb-Pb年龄不谐和度计算方法的提出，如采用Aitchison谐和距离，使数据过滤更加合理。基于上述新进展，对仪器和处理软件的选取进行了讨论，并对未来大样本量碎屑锆石U-Pb年代学分析的自动化、规范化提出了展望。基于已有研究，未来大样本量碎屑锆石U-Pb年代学的发展具有广阔前景，在物源示踪及确定地层年代等研究中将发挥更大的作用。
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关键词:
- 碎屑锆石U-Pb年代学 /
- 大样本量 /
- 激光剥蚀电感耦合等离子体质谱法 /
- 快速U-Pb定年 /
- 不谐和度
要点(1) 提升气溶胶传输效率和引入“峰形”信号接收可大大缩短LA-ICP-MS测试时间。
(2) 规范的LA-ICP-MS数据处理流程和适合的数据处理方法是保障年龄数据准确和精确的关键。
(3) 谐和年龄和Aitchison谐和距离不谐和度过滤能提高U-Pb年代学数据的可靠性。
HIGHLIGHTS(1) Improving ablation efficiency and using "peak" signal reception mode can greatly reduce LA-ICP-MS measurement time.
(2) A standard data processing flow and suitable data processing methods are key to the accuracy and precision of data.
(3) Concordia age and the discordance filter of Aitchison concordia distance can improve the reliability of U-Pb chronological data.
Abstract:BACKGROUNDDetrital zircon U-Pb geochronology is an important tool for identifying sedimentary provenance and determining the maximum depositional age. The numbers of grains for detrital zircon provenance investigations using laser-ablation inductively coupled-plasma mass spectrometer (LA-ICP-MS) typically range from 60 to 120. In this range, age components are commonly not identified from the sample aliquot. In order to improve the reliability of provenance investigation, analysis of more grains (n≥300) or even the large-n aliquot with more than 1000 grains (n > 1000) are required. The emergence of large-n detrital zircon U-Pb geochronology is challenging the methods of data measurement, reduction and evaluation.
OBJECTIVESTo summarize the progress of measurement, data reduction and data evaluation of large-n detrital zircon U-Pb geochronology.
METHODSBy summarizing the method innovation of domestic and foreign literature.
RESULTSFirstly, each measurement requires rapid acquisition of U and Pb isotope signals, which can be conducted by improving the transmission efficiency of aerosol. The "flat" signal acquisition time can be shortened or transformed to a "peak" signal mode for rapid measurement. Secondly, large-n data require efficient data reduction protocol or powerful software (e.g. iolite) to improve visualization and reduce the variability between inter-laboratory comparisons. For U-Pb data processing flow, several optimized methods are introduced for fractionation correction and propagating uncertainty. In addition, total integrated counts and linear regression correction are introduced to specially process "peak" signals. Thirdly, the new calculation method of U-Pb and Pb-Pb age discordance, such as using Aitchison concordia distance, makes data filtering more reasonable. Based on recent research progress, the future of automation and standardization of large-n detrital zircon U-Pb geochronology is discussed and advice on the selection of instruments and reduction software is provided.
CONCLUSIONSIn the future, the development of large-n detrital zircon U-Pb geochronology has great prospects, and will play a greater role in the study of provenance tracing and stratigraphic dating.
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水生氮是水生生态系统中的重要营养元素，但过量的氮会导致水体富营养化[1]。沉积物作为水环境中各污染元素的蓄积库或释放源，不仅能间接反映水体的污染情况，在物理化学因素制约下，还会向水体中释放营养元素，影响富营养化过程[2]。由氮污染引发的湖泊、河口和近海水体富营养化现象仍是当今世界面临的一个全球性重大环境问题。
沱江发源于四川盆地北部的九顶山，是长江左岸流域全部在四川境内的一级支流，是长江五大支流之一。沱江途经四川盆地的重要农业区，于泸州注入长江，受沿岸人为活动影响较为明显。2004年2月，发生在四川沱江的特大水污染事故，给沱江流域的生态系统及沿岸的水体环境造成了直接和潜在的危害[3]。为评价沱江流域的环境污染状况及生态恢复情况，学者们对其沉积物-水界面的影响因素进行了研究，大多集中在沉积物中氮的赋存状态分布特征、与上覆水氮的交换通量模拟计算以及探讨沉积物中氮的污染来源等方面[4-6]。例如，张蓉[7]对冬季沱江流域沉积物-水界面氮的赋存状态及其环境地球化学进行了研究；吴怡[8]对夏季沱江流域沉积物-水界面氮的赋存状态及其环境地球化学进行了研究，对沱江沉积物中氮的污染状况及沉积物-水界面氮赋存状态及其随季节变化的迁移转化规律研究取得了相应的进展，但缺少对沉积物中氮的各种赋存状态随时间变化的迁移转化规律的系统研究。
近年来，受工业、农业和沿岸居民生活的影响，沱江的生态环境依然遭受着一定程度的污染，但对沱江流域氮的环境地球化学研究甚少。本文选取沱江流域上游金堂段的沉积物为研究对象，对沉积物中氮的赋存状态进行了研究，旨在分析河流沉积物中氮赋存状态的垂向分布行为，以揭示其存在的地球化学特征，并与十年前该地区沉积物中氮的相关研究结果进行对比分析，探讨了十年前后氮赋存状态变化趋势，研究成果对于掌握该地区生态环境状况以及预测未来可能存在的风险具有指导意义。
1. 实验部分
1.1 采样点概况
沱江是四川农业区域和工业城市最集中的河流，流域内有成都、德阳、内江、自贡、泸州5座大中城市，大中型工厂多达千余座。采样点(图 1)位于沱江上游金堂地区(E104°31′24.35″，N30°43′51.97″)，采样时间为2017年1月2日。金堂县属四川省东部地台区，跨“成都断陷”、“龙泉山褶皱带”和“川中台拱”三大构造单元。沱江流域上游的三河(毗河、中河、北河)汇聚于此地形成沱江干流。采样点上游矿产资源较少，亦不存在矿产开发，该地区生态环境良好，沿江的网箱养殖渔业较多。
1.2 样品采集
采样器预先用1%硝酸(V/V)浸泡3天后，先后用一次蒸馏水和二次蒸馏水洗净，再用保鲜膜、保鲜袋保护，采样时先放入充氮气的取样袋中1 h。为使分析数据与2007年的数据具有对比性，根据GPS坐标定位，采样位置与张蓉[7]选取的采样位置一致。于2017年1月2日在选定的采样点，将采样器缓缓垂直插入沉积物中，采样柱体高度约20 cm。为防止样品被氧化，在充氮气的取样袋中，将沉积物柱样随即从下到上每隔1 cm进行分取，去除大块的沙砾和植物残根，采用四分法取样装入聚氯乙烯瓶中，拧紧瓶盖。所有样品用液氮罐冷藏保存，快速运回实验室后，立即将样品置于超低温(-20℃)冰箱中冷冻保存，并尽可能在短时间内完成样品的分析工作。
1.3 分析方法
可交换态氨氮(AN)：参照Mackin等[9]应用的沉积物中吸附态氨氮的提取方法，在1 g左右的沉积物中加入2 mol/L氯化钾溶液10 mL，振荡器中振荡1 h(250 r/min)，经0.45 μm微孔滤膜过滤后，按照HJ535—2009《国家环境保护标准》，采用纳氏试剂分光光度法测定提取液中氨氮含量。氨氮含量按沉积物干样计算。
总氮(TN)：参照Smart等[10]建立的沉积物中总氮的提取方法，利用碱性过硫酸钾在高温(120℃)下将沉积物中氮形态蒸馏消解为硝酸盐，按照HJ636—2012《国家环境保护标准》，用紫外分光光度法测定消解液中硝酸盐含量。总氮含量按沉积物干样计算。
有机氮(ON)：由总氮和可交换态氨氮差减得到。
含水率(%)：称取湿沉积物样品5 g于坩埚中，记录总量，放入烘箱于105℃烘干24 h后取出称重，记录二者差值计算含水率。
可挥发性物质含量(TVS)：称取烘干后的沉积物0.5 g于马弗炉中在750℃焙烧4 h后，放入干燥器中冷却至室温后称量，计算沉积物中可挥发性物质含量。
2. 结果与讨论
2.1 沉积物中氮赋存状态的垂向分布特征
沱江流域金堂段沉积物中氮赋存状态及TVS、含水率的垂向分布如图 2所示。由图 2可见，TN含量在518.913~4386.899 mg/kg之间波动，平均含量为1256.270 mg/kg。随着深度的增加，TN的变化趋势和有机氮的变化趋势类似，总体呈现出逐渐减小的趋势，这可能是由于该采样点离岸较近，陆源补充比较丰富，有机质含量会随着TN的变化而变化。TN的最大值(4386.899 mg/kg)出现在沉积物表层，最小值(518.913 mg/kg)出现在-13 cm处。TN含量的垂向分布特征，可能造成的原因有：①外源氮污染的输入，大量于沉积物表层沉积以及内源氮污染向沉积物表层释放；②沉积物中的氮在微生物作用下不断矿化分解进入间隙水中，从而使TN含量随沉积物深度的增加呈下降趋势[11]。
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图 2 金堂沉积物中氮赋存状态、TVS及含水率垂向分布(2017)Figure 2. Profiles of nitrogen species, TVS and moisture in 2017 in sediments of Jintang, Tuo RiverON含量在101.531~3793.683 mg/kg之间波动，平均含量为747.883 mg/kg，占TN平均含量的59.53%。在沉积物表层，ON含量最高，可能是由于有机质的矿化作用大都在表层含氧区内发生[12]。随着深度的增加，ON总体呈现出逐渐减小的趋势，-13 cm处达到最小值为101.531 mg/kg，在沉积物最下层ON含量略有增加。ON的这种垂向分布特征主要取决于水体中生物有机体的沉积作用以及微生物的分解作用。微生物在有氧条件下，将沉积物中的ON通过氨化作用分解为铵态氮，铵态氮又可以通过硝化作用继续转化为硝态氮。但溶解氧在沉积物中的渗透深度只有表层的几厘米，在这种缺氧的环境下微生物分解有机物的作用将大大减弱，并且，分解有机质的微生物大多数存在于沉积物表层中，且随着沉积物深度的增加其数量逐渐减少，所以ON含量会随着沉积物深度的增加而逐渐减少[6]。
AN含量在364.410~633.884 mg/kg之间波动，平均含量为508.387 mg/kg，占TN平均含量的40.47%。表层沉积物中AN含量较高，可能造成的原因有：①在沉积物表层氨化作用突出，随着沉积物深度的增加，氧含量的迅速减少，沉积物中各种微生物数量与活性减少，氨化作用降低，同时还原环境促进了反硝化作用的进行，大量氮元素通过反硝化作用转化为N2，从而使得AN含量开始下降[13]；②NH4+-N本身带正电荷，易于被表层带负电的沉积物颗粒胶体吸附而不易发生淋失[14-15]。在沉积物最下层，AN含量略有增加。AN含量在-2 cm处最高，为633.884 mg/kg，在-11 cm处最低，为364.410 mg/kg。
2.2 沉积物含水率和TVS的垂向分布特征
沉积物含水率在30.524%~56.643%之间波动，平均值为38.895%；TVS在8.855%~13.647%之间波动，平均值为10.996%。含水率和TVS的总体变化随深度增加而减小，峰值均出现在沉积物-水界面处，沉积物表层的含水率较高，可以反映表层沉积物孔隙度相对较大，可在一定的水动力条件下再悬浮，从而造成二次污染[16]。
2.3 沉积物中各氮赋存状态、TVS及含水率的相关性分析
用SPSS软件对金堂沉积物中氮赋存状态及TVS、含水率做相关性分析，结果见表 1。从表 1中可以看出，AN与ON的相关系数r=0.537(0.01＜P≤0.05)，AN与TN的相关系数r=0.618(0.01＜P≤0.05)，ON与TN的相关系数r=0.998(0.001＜P≤0.01)，AN、ON与TN两两之间均呈正相关关系，ON与TN的正相关关系尤为明显，说明三者在沉积物中存在着动态平衡的关系。
表 1 金堂沉积物中氮赋存状态、TVS及含水率的相关性分析(2017)Table 1. The correlation coefficient (r) among nitrogen species, TVS and moisture in sediments of Jintang, Tuo River分析项目 AN ON TN TVS 含水率 AN 1 - - - - ON 0.537* 1 - - - TN 0.618* 0.998** 1 - - TVS 0.199 0.743** 0.727** 1 - 含水率 0.649** 0.838** 0.848** 0.754** 1 注：标注“*”的数据代表在0.05水平上(双侧)显著相关；
标注“**”的数据代表在0.01水平上(双侧)显著相关。ON与TVS的相关系数r=0.743(0.001＜P≤0.01)，TN与TVS的相关系数r=0.727(0.001＜P≤0.01)，沉积物中TVS含量分布在很大程度上影响着ON的垂向分布特征，从金堂段的ON含量占TN的比例及其相关关系来看，TN在沉积物中的分布特征受ON含量的分布影响较大，可以推测沉积物中有机质的矿化分解过程会促进ON的释放。
含水率与AN、ON、TN及TVS的相关系数分别为0.649(0.001＜P≤0.01)、0.838(0.001＜P≤0.01)、0.848(0.001＜P≤0.01)、0.754(0.001＜P≤0.01)。沉积物中含水率对AN、ON、TN及TVS含量分布影响较大，而含水率与沉积物的粒度组分、黏度性质等相关。由此推测，金堂地区沉积物的粒度组成、黏度等对沉积物-间隙水中有机污染物的分布发挥了主导作用，进而影响氮赋存状态在沉积物和水体中的分布特征[11]。
2.4 沉积物中各氮赋存状态、TVS、含水率十年前后垂向分布的时空对比
结合本项目组在沱江流域金堂沉积物2007年氮赋存状态的研究结果[7]，十年前后沉积物中氮赋存状态及TVS、含水率的垂向分布对比如图 3所示。
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图 3 金堂沉积物中氮赋存状态、TVS及含水率在2007年和2017年的垂向分布对比Figure 3. A comparison of the vertical distributions of nitrogen species, TVS and moisture in the years of 2007 and 2017 in the sediments of Jintang, Tuo RiverAN2017含量均大于AN2007含量，两者在沉积物中的分布特征大致相同，总体呈现减小的趋势。在-3 cm以上，ON2017平均含量远远大于ON2007平均含量；在-3 cm以下，ON2017含量均小于ON2007含量。两者均是随着沉积物深度的增加，总体上呈现逐渐减小的趋势，且在沉积物-水界面出现最大值。在-3 cm以上，TN2017含量均大于TN2007含量；在-3 cm以下，TN2017含量与TN2007含量相比有大有小，且二者相差不大，随着沉积物深度的增加，总体均呈现逐渐减小的趋势，且在沉积物-水界面出现最大值。
在沉积物-水界面氮的迁移转化是一个涉及物理、化学及生物等多种因素的复杂的地球化学循环过程，包括了氮的输入、氮的固定，有机质的矿化、硝化、反硝化，硝酸盐的氨化等反应过程[17]。由十年前后各形态氮的分布规律可见，AN含量的总体增加是外源氮输入以及内源氮释放综合作用的结果；而ON与TN基本在-3 cm以上增加，在-3 cm以下减小，推测外源氮的输入是以一部分氨氮和一部分有机氮的形式进入沉积物-水界面。外源输入的ON不易通过分子扩散的形式扩散至间隙水或沉积物中，只能通过微生物作用发生矿化反应而形成无机氮形态，这个过程是相对缓慢和复杂的。随着沉积物深度的增加，含氧量逐渐减少，使得沉积物下层容易呈现出相对还原环境，有机质的矿化作用及硝化作用逐渐减弱，由有机质矿化作用而产生的无机氮形态减弱，沉积物中无机氮的形态是以AN为主，因此在沉积物下层来自有机质矿化作用产生的AN应该在AN分布中占的比例较小。由此推测，AN总体含量的增加受外源氮输入的影响较大。这与2016年新华网报道的氨氮再次污染事件以及近年来学者们针对沱江流域生态环境的研究结果一致，氨氮是重要的超标因子之一[18-19]。
AN在十年前后呈现基本相同的垂向分布规律，与沉积物的沉积环境、沉积物的组分及含水率等密切相关，表层沉积物的含氧量高于深层沉积物的含氧量，无论是由于浓度梯度扩散至间隙水的AN，或者有机质矿化产生的AN，还是硝酸盐氨化产生的AN，由于逐渐形成的厌氧环境常常发生反硝化过程，最终生成NO或N2，进入大气圈或者再进入氮的地球化学循环过程，导致AN均大致呈现随深度减小的趋势。在-3 cm以下，ON含量相比十年前减小，说明ON作为内源氮污染的来源被微生物分解形成无机氮形态进入下一地球化学循环过程。
对比沱江流域金堂段2007年和2017年沉积物中各氮赋存状态的含量可见，随着时间的推移，AN含量是明显增加的，深度在-3 cm以下ON含量是减小的，-3 cm以上ON含量是增加的，可以推测金堂地区沉积物中的氮已经作为内源氮释放至间隙水甚至上覆水中，使得沉积物表层ON以及TN含量增加明显。沱江流域水环境污染需要解决的主要超标因子从过去的有机污染物转变为难以治理的氨氮污染物[20]。
TVS2017含量在深度-8 cm以上基本是减小的，而在-8 cm以下TVS2017含量大于TVS2007。在-4 cm以上及-7 cm以下，含水率2017大于含水率2007；在-5 cm至-7 cm，含水率2017小于含水率2007。
3. 结论
本文系统地研究了沱江流域金堂地区沉积物中氮的不同赋存状态的垂向分布特征，并对比了十年前后氮赋存状态的变化。结果表明：-3 cm以上有机氮是总氮的主要赋存状态，随着深度的增加，总氮与有机氮的变化趋势类似，在-3 cm以上含量迅速减小，最大值均出现在沉积物表层，且二者与TVS的垂向分布特征密切相关。对比2007年和2017年沉积物中不同赋存状态氮的含量发现，可交换态氨氮含量是明显增加的，-3 cm以下有机氮与总氮含量是减小的，-3 cm以上有机氮与总氮含量是增加的，可推测沱江流域沉积物中的氮已经作为内源氮释放至间隙水甚至上覆水中，同时存在外源污染，使得沉积物表层有机氮以及总氮含量升高明显。本研究成果对评价水环境质量、治理环境污染、维护生态平衡具有重要意义。
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图 1 气溶胶快速引入系统(Aerosol Rapid Introduction System，ARIS)与激光和ICP-MS连接示意图：激光端与PEEK管相接，并通过ARIS转接器将气溶胶传输至ICP-MS(据文献[25])
Figure 1. Diagram of aerosol rapid introduction system connected with both laser and ICP-MS: The port of laser is connected with the PEEK tube, and the aerosol is transmitted to ICP-MS through the ARIS adapter (Modified from Reference[25])
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图 2 单颗粒分析时间分别为30s(a)、12s(b)、6s(c)和3s(d)条件下的信号特征。信号强度以对数坐标呈现，不同颜色的线条分别表示不同同位素的信号强度(据文献[26])
Figure 2. Signal characteristics under the condition of single grain analysis time of 30s (a), 12s (b), 6s (c) and 3s (d). The signal intensity is presented in logarithmic coordinates, and the lines of different colors represent the intensity of different isotopes (Modified from Reference [26])
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图 5 (a) Wetherill谐和图示意图，误差椭圆三个方向分别表示三组比值的误差；(b)T-W谐和图示意图
Figure 5. (a) Wetherill concordia diagram, and the three directions of the error ellipse represent errors of three ratios, respectively; (b) T-W concordia diagram
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图 6 两个不谐和度对数比值距离的定义：da表示数据点到谐和线的垂直Aitchison距离；dc表示数据点与计算的谐和年龄(tc)之间的Aitchison距离(据文献[60])
Figure 6. Illustration of the two log ratio distance definitions of discordance: da is the perpendicular Aitchison distance from the measured data to the concordia line; dc is the Aitchison distance from the measured data to concordia age (tc) (Modified from Reference [60])
表 1 不谐和度计算方法及特点
Table 1 Different definitions of discordance and their characteristics
参数 计算方法 表示公式 特点 dr[61] 相对年龄差异 dr=1- t68/ t76 滤除年轻的年龄组分 dt[57] 绝对年龄差异 dt =t76- t68 滤除年老的年龄组分 dp[55] U-Pb比值 dp=Prob(s＞S~χ22) 影响准确度和精度 dsk[62] 地幔演化模型 $d_{\mathrm{sk}}=1-\left[\frac{{ }^{238} \mathrm{U}}{{ }^{206} \mathrm{~Pb}}\right] /\left[\frac{{ }^{238} \mathrm{U}}{{ }^{206} \mathrm{~Pb}}\right]^*$ 滤除年老组分 da[60] Aitchison距离 $d_{\mathrm{a}}=\mathrm{d} x\left(t_{68}\right) \sin \left[\arctan \frac{\mathrm{d} y\left(t_{76}\right)}{\mathrm{d} x\left(t_{68}\right)}\right]$ 滤除1000~2000Ma的锆石颗粒 dc[60] Aitchison谐和距离 $d_{\mathrm{c}}=\operatorname{sgn}\left(t_{76}-t_{68}\right) \sqrt{\mathrm{d} x\left(t_{\mathrm{c}}\right)^2+\mathrm{d} y\left(t_{\mathrm{e}}\right)^2}$ 最合理 DMS[63] 年龄差异绝对值的较小值 $D_{\mathrm{MS}}=\left\{\left|t_{68}-t_{75}\right|, \left|t_{76}-t_{75}\right|\right\}$ 滤除较老的年龄组分 
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