Abnormal Hot Blank of Oxygen-free Copper Sample Holder and Implications for Laser 40Ar/39 Ar Dating
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摘要:
激光40Ar/39Ar定年方法中扣除的本底是样品测试过程中的系统冷本底。在满足激光加热样品时样品盘升温幅度有限和样品盘已完全脱气两个条件的情况下,这一处理方式的有效性才能得到保证。本文利用具有不同大气暴露史的无氧铜样品盘结合透长石标准样品YBCs,使用相同的脱气及测试流程,对比分析了不同样品盘的冷、热本底以及放置于不同样品盘时YBCs的大气氩含量。分析结果表明,放置于暴露大气14个月的样品盘内时,YBCs透长石大气氩含量高达约34.4%,使用预先激光去气的样品盘此值可降低至约2%;暴露大气约10个月的样品盘,激光加热其两个样品孔时,40Ar脱气量可达约1.6×10−14~3.1×10−14mol;暴露时长约为26个月的样品盘,40Ar含量升高至约0.8×10−13~2.0×10−13mol;它们均远高于系统冷本底3.8×10−16~6.2×10−16mol。两个样品盘热本底40Ar/36Ar值约为310,高于大气氩比值。因此,对于暴露大气时间较长的样品盘,约150℃去气四天的流程不足以使其完全脱气。激光加热样品时会导致样品盘局部升温,脱气不完全的样品盘会释放出大量热本底。模拟以及标准样品测试均显示了这种情况会影响辐照参数J值以及年龄的计算。激光微量年轻样品40Ar/39Ar定年过程中,建议装样后对无氧铜样品盘进行300~400℃至少5h的预脱气,以保证测试数据质量。实验室不具备预脱气条件时,持续使用同一样品盘也可以有效地降低异常热本底对测试结果的影响。
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关键词:
- 激光40Ar/39Ar定年 /
- 无氧铜样品盘 /
- 热本底 /
- 脱气 /
- 质谱分析
Abstract:BACKGROUNDOxygen-free copper (OFC) tray is usually used as a container for samples of laser 40Ar/39Ar dating. The blank of the mass spectrometry system needs to be subtracted from the sample signal before age calculation. The tray’s hot blank of argon, including its amount and isotope ratio will affect the 40Ar/39Ar age calculation. However, the material and structure of the laser window make the laser chamber susceptible to temperatures higher than 150°C during the degassing procedure of laser 40Ar/39Ar dating[4,10]. This temperature may be too low to degas the OFC tray completely during the experiment. Besides, the wells loading the mineral samples limit the direct acquisition of the hot blank of the system. These two points make it very difficult to accurately deduct the blank signal of laser 40Ar/39Ar dating.
OBJECTIVESTo testify to the effectiveness of the traditional degassing procedure on the sample holder and evaluate the effect of hot blank on age calculation of laser 40Ar/39Ar dating.
METHODSTo confirm the effectiveness of the degassing procedure, the hot blank of OFC trays, which had been exposed to atmosphere for different time intervals were measured by mass spectrometry, after degassing the laser chamber to 150℃ for four days. Loading the standard mineral YBCs sanidine in the well in OFC trays with different duration in air, the argon isotopes were measured after the same degassing procedure. The change of the proportion of atmospheric argon of YBCs will verify if laser energy heats the OFC tray and mineral sample simultaneously.
RESULTSThe temperature of the sample holder increases during laser heating of samples. For standard mineral YBCs sanidine placing in OFC tray C, exposed to the atmosphere for about 14 months, the proportion of atmospheric argon was 34.4% (Table 1). This value decreased to 2.0% when YBCs was placed in tray D and degassed by laser at higher energy. This implies that the tray temperature will increase during sample heating by laser, and a considerable amount of gas will be released if the tray is not completely degassed. It was proved that heated to about 150℃ was not enough to completely degas the OFC tray during experiment. The hot blank of another two planchettes following the same degassing procedure were measured. For tray A with exposure to the atmosphere for 10 months, the amount of 40Ar released from two wells reached 1.6×10−14−3.1×10−14mol; for tray B with exposure to the atmosphere for 26 months, the 40Ar content increased to 0.8×10−13−2.0×10−13mol. These levels were much higher than the cold background of 3.8×10−16−6.2×10−16mol. Incomplete degassing of the tray may lead to argon isotopic fractionation, resulting in the value of 40Ar/36Ar of the hot blank rise to 310, which is higher than the value of atmospheric argon. Under this condition, using 40Ar/36Ar=295.5[16] or 298.56[17] to correct the atmospheric argon will add extra 40Ar to the sample’s signal and lead to an older age. The OFC tray was degassed efficiently while heated with a higher laser power. Under the same degassing procedure, the hot blank of the empty wells in tray D was similar to the cold blank of the system. The OFC trays exposed to air also gave the same conclusion. When the wells in the tray were heated by a lower power energy following a higher power energy, the hot blank dropped to the cold blank level (Fig.2). Assuming all of the atmospheric argon contributed from the hot blank with 40Ar/36Ar=310, its effect on 40Ar/39Ar age calculation with different combination of 40Ar*/39ArK, J-value and atmospheric argon content was calculated. Results show that the percent change of age(t%) was mainly controlled by the atmospheric argon content. When the atmospheric argon content increased to 50% (Fig.3), it elevated the age by 5%. Under the same condition, the J-value was about 5.2% lower(Fig.4). In practice, however, the different capacity of adsorption and thermal desorption of gas of different minerals[18-19] makes the effects difficult to quantify.
CONCLUSIONSCareful degassing of the OFC tray is needed, especially in very young samples dating through the laser heating method. The temperature of the sample holder increases when the sample is heated by laser, releasing considerable amounts of gas if the holder is incompletely degassed. The limited temperature of the degassing procedure cannot degas the sample holder completely, if the holder adsorbs a lot of gases. There are two possible solutions to solve this problem. The first is pre-degassing the sample holder at 300-400℃ for at least 5 hours at high vacuum furnace after sample exchange. The second is reusing the same sample holder continuously in laser 40Ar/39Ar dating. A combination of both solutions is likely to be most effective.
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磷灰石是各种地质环境中广泛存在的副矿物[1],通常含有一定数量的铀(几μg/g到数百μg/g以上)[2-3],因此磷灰石U-Pb定年常用于限定成岩成矿和化石形成等重要地质作用时代[4-8]。激光剥蚀电感耦合等离子体质谱(LA-ICP-MS)因其样品制备简单、分析效率高、可提供微区原位信息等优点[9-11],是开展副矿物U-Pb年龄微区分析的重要手段之一[4,12-14]。对于LA-ICP-MS磷灰石U-Pb定年测定,Chew等[12]于2011年采用激光线扫描剥蚀法联合溶液校正建立LA-ICP-MS磷灰石U-Pb定年方法,此后该技术在磷灰石U-Pb年龄微区分析中应用广泛[15-19]。
由于磷灰石中铀含量通常较低[3],采用激光微束分析时进样量少,其Pb/U分析精度是首要考虑的问题。Thomson等[20]采用激光剥蚀联用MC-ICP-MS分析,分别用法拉第杯接收238U、232Th、208Pb、207Pb和206Pb,在激光剥蚀束斑65μm时获得的磷灰石标准样品MAD的206Pb/238U单点分析精度为4%~40%(2SD)。Chew等[21]采用四极杆电感耦合等离子体质谱开展磷灰石U-Pb年龄分析,在剥蚀束斑50μm时获得的Durango磷灰石206Pb/238U单点分析精度为8%~10% (2SD),而在束斑130μm时才达到满足激光分析要求的精度(约2%)。因此,开展激光微区磷灰石U-Pb定年分析时,评估仪器灵敏度对分析精度的影响是获得准确精密分析结果的基础。另一方面,磷灰石在结晶过程中矿物晶格中可能混入Pb元素,此类Pb不由U、Th等放射性衰变产生,即初始铅或普通铅[4]。不同成因的磷灰石普遍含有普通铅,已报道的用于磷灰石U-Pb定年微区分析的标准样品也均含有不同程度的普通铅[20-21]。U-Pb定年分析标样需具有均匀的206Pb/238U和207Pb/206Pb比值,当磷灰石标样中有不同含量的普通铅时,标样实测的206Pb/238U和207Pb/206Pb比值是其放射性成因比值和不同含量普通铅的混合信息,若直接用于Pb/U分馏和仪器漂移校正,则会对磷灰石样品的定年结果和初始铅组成造成影响。前人开展磷灰石U-Pb定年分析时均采用基体匹配的磷灰石MAD[20]作外标校正分析未知样品年龄,研究者多采用207Pb法先对MAD进行普通铅校正[16]。由于MAD标样普通铅含量低(约1%的年龄不谐和)[20],采用207Pb法可获得准确的分析结果,但该方法不适用于普通铅含量较高的标样分析。随着磷灰石原位U-Pb定年技术的发展和微区分析实验室数量的增加,低普通铅磷灰石MAD逐渐缺乏。为获得准确的激光微区U-Pb定年结果,因此需要对磷灰石标样中普通铅校正方法进行系统研究和开发非基体匹配分析方法。对当前特定副矿物U-Pb年龄微区分析高质量标样极度匮乏的问题,部分学者尝试开发了非基体匹配分析方法[4]。如以储量丰富、不含普通铅的锆石标样校正分析榍石[22]和金红石[23]等。Luo等采用水蒸气辅助激光剥蚀方法实现以锆石或NIST玻璃作为外标分析榍石[13]、独居石[13]、磷钇矿[13]、氟碳铈矿[24]以及黑钨矿[25]等副矿物的U-Th-Pb年龄。采用非基体匹配分析方法,不仅可以有效地解决基体匹配标样缺乏的问题,也可一定程度地避免标样中普通铅组成对分析结果造成的偏差。
因此,针对当前激光微区磷灰石U-Pb定年分析时标样中普通铅影响分析结果准确度和精密度等问题,本文将定量评估分析灵敏度对磷灰石U-Pb定年分析精度的影响,对比研究磷灰石标样中普通铅组成对分析结果造成的偏差;尝试建立磷灰石非基体匹配分析方法以消除普通铅对分析结果的影响,建立了准确的高精度激光微区磷灰石U-Pb定年新方法。
1. 实验部分
1.1 实验仪器
本实验在中国地质大学(武汉)地质过程与矿产资源国家重点实验室进行,采用Agilent 7900四极杆等离子体质谱仪(Agilent Technology,Tokyo,Japan)联合相干公司的193nm准分子纳秒激光(GeoLas HD,MicroLas Göttingen,Germany)。详细的实验参数列于表1。
表 1 LA-ICP-MS磷灰石U-Pb定年实验仪器参数Table 1. Instrumental parameters for LA-ICP-MS U-Pb dating of apatite.LA参数 GeoLas HD 193nm准分子激光 波长 193nm 激光能量密度 10J/cm2 剥蚀频率 5Hz 剥蚀时间 50s 背景时间 20s He流速 650mL/min 束斑直径 60~90μm ICP-MS参数 Agilent 7900型 等离子体功率 1400W 样品气 0.86L/min 检测元素 29Si, 42Ca,49Ti,51V,89Y, 93Nb, 139La,140Ce,
141Pr,146Nd,147Sm,151Eu,157Gd,159Tb,163Dy,
165Ho,166Er,169Tm,173Yb,175Lu,179Hf,181Ta,
201Hg,204Pb,206Pb,207Pb,208Pb,232Th,238U1.2 实验样品和分析方法
MAD磷灰石来自于马达加斯加的“1st Mine Discovery”。ID-TIMS年龄测试出两个年龄486±0.85Ma和474.25±0.41Ma,Th/U值约为15~30。MAD磷灰石是目前激光微区磷灰石U-Pb年龄分析常用的外标[2,26],本研究中基体匹配分析实验也采用MAD作为年龄校正标样。Durango磷灰石产于墨西哥杜兰戈市的露天铁矿中,磷灰石的形成与长英质侵入体有关,产出于火山口的两个熔结凝灰岩之间[27],同时结晶的四个单晶透长-歪长石的40Ar-39Ar年龄为31.44±0.18Ma (2SD) [27]。Durango磷灰石由于年轻且U含量较低,Pb同位素难以准确测定,在测试中需要采用大束斑测量,并固定初始铅组成,或分析较大范围区域从而获得更为广泛的Pb/U组成分布[21]。Otter Lake磷灰石产于加拿大魁北克省,该地区的岩石经历了多期构造活动,产自该地区的磷灰石样品通常呈深绿色-棕色长六角棱柱体,对同一磷灰石晶体使用溶液法测试其铅同位素,在207Pb/204Pb-206Pb/204Pb图解中获得的等时线年龄为913±7Ma (2SD,MSWD=0.24),该年龄作为Otter Lake磷灰石推荐年龄被广泛采用[21,28]。为解决基体匹配标样缺乏的问题,研究者们常采用美国国家标准与技术研究所(NIST)的合成玻璃NIST610、NIST612或NIST614[29]作为U-Pb年龄分析时的外标校正仪器分馏和信号漂移[13,30-32],本研究选用U含量与磷灰石较为相当的NIST612玻璃作为非基体匹配分析时的外部校正标样。数据处理采用Iolite软件[33]和Excel程序,U-Pb谐和图由Isoplot R软件绘制[34]。
2. 结果与讨论
2.1 信号强度对分析精度的影响
U-Pb定年的测试精度取决于U和Pb的信号强度,而信号强度与样品中U、Pb的含量及进样量有关。由于含普通铅矿物的U-Pb年龄需要对其进行普通铅校正,因此对于磷灰石U-Pb定年来说,除了关注Pb/U的分析不确定度外,还需注意Pb/Pb的分析不确定度。而U-Pb体系中的比值精度主要取决于低含量元素的信号强度。图1展示了在不同激光条件下同时测试磷灰石参考物质MAD和Durango获得U-Pb年龄的内部不确定度与Pb信号强度的关系。对于206Pb/238U,当206Pb计数达到6000cps时,其不确定度可以缩小到2%;当206Pb计数超过6000cps时,其不确定度下降变缓,超过20000cps时可以达到~1%的不确定度。对于207Pb/206Pb,其不确定度主要取决于含量更低的207Pb的信号强度,因此其不确定度较206Pb/238U相比偏高。当207Pb计数为~600cps时,其不确定度可以达到~5%,随后缓慢降低,直到~2400cps时达到~2%的水平。从图1还可以看到,对于年轻的样品Durango,由于其Pb含量过低,U-Pb年龄的内部精度始终较低。
2.2 磷灰石U-Pb年龄基体匹配分析
副矿物U-Pb定年测试一般采用标样、样品间插分析以校正元素分馏和仪器漂移。而当外部校正标样中含一定程度普通铅时,需要先对标样进行普通铅扣除,使用经过校正后的Pb/U比值对未知样品进行校正。本研究对比了磷灰石MAD作外标时直接校正和选用不同方法进行普通铅校正后对待测样品分析结果的影响。
2.2.1 磷灰石MAD直接校正结果
如图2展示在不对标样MAD进行普通铅校正情况下,直接用其校正监控样品的结果。对于Otter Lake,在不固定普通铅组成得到的Tera-Wasserburg谐和图交点年龄与推荐值的偏差在1%内,但是初始铅组成与推荐值产生了较大偏差,偏低36%;固定Tera-Wasserburg谐和图普通铅组成(207Pb/206Pb=0.9,由地球铅同位素演化模型[35]确定),交点年龄偏差为2.5%。对于Durango磷灰石,在不固定初始铅组成(207Pb/206Pb=0.84)得到的Tera-Wasserburg谐和图交点年龄偏差达到了30%,初始铅组成与推荐值产生了较大偏差,偏低60%;固定Tera-Wasserburg谐和图普通铅组成,交点年龄偏差为6%。研究结果表明使用含普通铅的磷灰石标样作为外标时,若不对其进行普通铅校正,最终获得的被测样品年龄和初始铅组成均会产生较大的系统偏差。
图 2 使用磷灰石MAD作为外标直接校正获得的磷灰石样品U-Pb年龄(a) 磷灰石Otter Lake,不固定初始铅同位素;(b) 磷灰石Otter Lake,固定初始铅同位素;(c) 磷灰石Durango,不固定初始铅同位素;(d) 磷灰石Durango,固定初始铅同位素。Figure 2. U-Pb ages results obtained by calibrating with apatite MAD as the external standard: (a) Apatite Otter Lake, without anchored initial Pb isotopic composition; (b) Apatite Otter Lake, with anchored initial Pb isotopic composition; (c) Apatite Durango, without anchored initial Pb isotopic composition; (d) Apatite Durango, with anchored initial Pb isotopic composition.2.2.2 207Pb法普通铅校正结果
对含普通铅的标准样品,常需对标样进行普通铅校正,再采用经过校正后的各标样对未知样品进行元素分馏和仪器漂移校正[4]。本研究选用207Pb法先对磷灰石标样MAD进行校正[4],其校正原理和操作步骤简述如下。用于分馏校正的磷灰石标样中含普通铅时,需先对标样中普通铅进行校正。
若采用207Pb法校正:(a)先对每个标样测试点的U、Pb信号扣除背景;(b)对每个标样测试点进行普通铅校正,计算公式如下所示。
$$ ^{206}\mathrm{P}\mathrm{b}_{\mathrm{r}}=^{206}\mathrm{P}\mathrm{b}_{\mathrm{m}}\times(1-\mathrm{\mathit{f}}_{206}) $$ (1) $$ ^{207}\mathrm{P}\mathrm{b}_{\mathrm{r}}=^{207}\mathrm{P}\mathrm{b}_{\mathrm{m}}-^{206}\mathrm{P}\mathrm{b}_{\mathrm{m}}\times\left(^{207}\mathrm{P}\mathrm{b}/^{206}\mathrm{P}\mathrm{b}\right)_{\mathrm{c}}\times f_{206} $$ (2) $$ \mathit{\mathrm{\mathit{f}}}_{206}=\frac{\left(^{207}\mathrm{Pb}/^{206}\mathrm{Pb}\right)_{\mathrm{m}}-\left(^{207}\mathrm{Pb}^*/^{206}\mathrm{Pb}^*\right)}{\left(^{207}\mathrm{Pb}/^{206}\mathrm{Pb}\right)_{\mathrm{c}}-\left(^{207}\mathrm{Pb}^*/^{206}\mathrm{Pb}^*\right)} $$ (3) 式中:206Pbr为普通铅校正后的瞬时206Pb信号;206Pbm为测试的瞬时206Pb信号;207Pbr为普通铅校正后的瞬时207Pb信号;207Pbm为测试的瞬时207Pb信号;(207Pb/206Pb)m为测试的207Pb/206Pb比值( 207Pb/206Pb)c为初始的207Pb/206Pb比值。
(c)经过普通铅校正后的各标样点再用于Pb/U分馏校正。具体校正步骤可在Iolite软件[33]中的VizualAge UComPbine功能实现。
图3展示了MAD经过普通铅校正后分析磷灰石Otter Lake和Durango的结果。对于Otter Lake,在不固定普通铅组成得到的Tera-Wasserburg谐和图交点年龄偏差为−1.5%,普通铅组成与推荐值的偏差为−10%;固定Tera-Wasserburg谐和图普通铅组成,交点年龄偏小于1%。对于Durango磷灰石,在不固定普通铅组成得到的Tera-Wasserburg谐和图交点年龄偏差为−31.4%,普通铅组成与推荐值偏差为−45.4%;固定Tera-Wasserburg谐和图普通铅组成,交点年龄偏差为−1.7%。磷灰石标样MAD先经过普通铅校正,后校正获得的磷灰石Otter Lake和Durango年龄与其推荐值相比偏差均在±2%以内,达到了目前国际上激光剥蚀微区分析磷灰石U-Pb定年的普遍水平[16,21,36-37]。
图 3 使用207Pb法校正MAD普通铅后获得的磷灰石样品的U-Pb年龄(a) 磷灰石Otter Lake,不固定初始铅同位素;(b) 磷灰石Otter Lake,固定初始铅同位素;(c) 磷灰石Durango,不固定初始铅同位素;(d) 磷灰石Durango,固定初始铅同位素。Figure 3. U-Pb age results obtained after prior correction for common Pb in MAD with 207Pb method: (a) Apatite Otter Lake, without anchored initial Pb isotopic composition; (b) Apatite Otter Lake, with anchored initial Pb isotopic composition; (c) Apatite Durango, without anchored initial Pb isotopic composition; (d) Apatite Durango, with anchored initial Pb isotopic composition.2.2.3 Tera-Wasserburg图解法普通铅校正结果
207Pb法假定样品的206Pb/238U和207Pb/235U年龄谐和,其数学校正原理可在Tera-Wasserburg图解上清晰呈现[4]。因此有学者采用标样测试点构筑的不一致线在Tera-Wasserburg图解下交点与其推荐值之比进行未知样品的分馏校正[4,32]。其操作步骤为:①分别计算出由所有测试点构筑的不一致线与X轴的交点;②计算经过标样推荐年龄的不一致线与X轴的交点,两交点比值即为Pb/U分馏校正系数;③将该系数乘以待测样品的Pb/U比值便得到待测样品经过校正后的Pb/U比值,而未知样品的207Pb/206Pb比值则由Pb同位素组成均匀的标样(如NIST玻璃)直接校正获得。
图4展示了使用Tera-Wasserburg图下交点法校正得到的结果。将未经普通铅校正的磷灰石MAD测试点固定上交点于0.87(由地球铅同位素演化模型[35]确定),其不一致线与X轴的交点为18.33,将MAD推荐年龄(475Ma)[20]也投入图中,得到经过推荐年龄的不一致线与X轴的交点为14.14 (图4a),将两者的比值(0.79)校正到磷灰石Otter Lake和Durango各测试点(图4中b,c),最终得到的Tera-Wasserburg谐和图交点年龄分别为929.0±7.1Ma (2σ,MSWD=1.2)和29.3±0.5Ma(2σ,MSWD=1.0),与其各自推荐值偏差分别为1.7%和6.6%。磷灰石Durango的结果呈现较大偏差,可能是由于其普通铅含量较高,分析获得的Pb/U比值不够分散,因此其下交点年龄出现较大偏差。这也说明为获得准确的分析结果,采用Tera-Wasserburg图解校正法时,分析样品需有较大范围的Pb/U比值。
2.3 磷灰石U-Pb年龄非基体匹配分析
水蒸气辅助激光剥蚀法现已广泛应用于榍石、独居石、磷钇矿和氟碳铈矿等副矿物U-Th-Pb年龄的非基体匹配分析[4,13]。为避免标样中普通铅对分析结果造成的影响,本研究也采用水蒸气辅助激光剥蚀方法,以NIST612玻璃作为外标,校正磷灰石MAD、Otter Lake和Durango的U-Pb年龄。水蒸气引入方法在本团队以往研究中已有详细介绍[4,13,24],本研究通过在剥蚀池前引入4.0mg/min水蒸气,选用NIST612玻璃作为外标直接校正磷灰石标样,分析结果如图5所示。磷灰石MAD、Otter Lake和Durango在Tera-Wasserburg图解下交点年龄分别为474.7±2.7Ma (2σ,MSWD=1.6)、934.8±2.1Ma(2σ,MSWD=2.1)和31.2±1.2Ma (2σ,MSWD=1.6),与其推荐值均在误差范围内一致。
2.4 不同分析方法结果对比
本研究为获得准确的磷灰石U-Pb年龄,分别采用磷灰石MAD和NIST612玻璃作为外标校正分析,并对比了207Pb法和Tera-Wasserburg图解法对MAD标样中普通铅的校正结果(表2)。由于测试所得磷灰石Otter Lake和Durango的Pb/U比值较为集中,且含较高程度普通铅,此处仅列出固定普通铅时计算的年龄结果。磷灰石MAD作为外标直接校正时,固定初始铅组成,获得的Otter Lake下交点年龄与推荐值的偏差为2.5%;Durango磷灰石的下交点年龄与推荐值的偏差为6%。使用207Pb法先对磷灰石标样MAD进行普通铅校正,获得的Otter Lake和Durango的交点年龄与其推荐值的偏差分别为0.03%和1.7%。采用Tera-Wasserburg图解法校正MAD中普通铅后获得的Otter Lake和Durango的交点年龄与其推荐值的偏差分别为1.7%和6.6%,其中Durango的结果呈现较大偏差,可能因为分析点的Pb/U比值不够分散,获得的Tera-Wasserburg图解下交点有较大偏离导致。以NIST612玻璃作为外标,测试的磷灰石MAD、Otter Lake和Durango在Tera-Wasserburg图解下交点年龄分别为474.7±2.7Ma(2σ,MSWD=1.6)、934.8±2.1Ma(2σ,MSWD=2.1)和31.2±1.2Ma(2σ,MSWD=1.6),与其推荐值的偏差分别为0.1%、2.3%和0.8%。虽然本研究中采用207Pb法和Tera-Wasserburg图解法都可以获得磷灰石Otter Lake和Durango准确的U-Pb年龄,但在标样普通铅含量较高时,采用207Pb法可能会存在过校正或校正不足的情况,造成样品年龄偏差,因此207Pb法适用于普通铅含量较低的标样校正;而Tera-Wasserburg图解法需要测试标样的Pb/U比值分散程度较大,构筑不一致线以获得准确的下交点,因此Tera-Wasserburg图解法适用于标样Pb/U比值分散程度较大的情况,应用范围广,但操作较为复杂。采用水蒸气辅助激光剥蚀非基体匹配分析,可避免标样中普通铅对分析结果的影响,同时有效地克服磷灰石标样匮乏的瓶颈。
表 2 不同测量条件下获得的磷灰石Tera-Wasserburg谐和图下交点年龄与推荐值的偏差Table 2. Deviations of Tera-Wasserburg values and recommended values for apatite U-Pb age obtained under different measurement conditions.激光条件 外标 普通铅校正方法 分析样品 Tera-Wasserburg谐和图下交点年龄与
推荐值的偏差(%)60μm,5Hz MAD 无普通铅校正 Otter Lake 2.5 Durango 6.0 60μm,5Hz MAD 207Pb Otter Lake 0.03 Durango 1.7 60μm,5Hz MAD Tera-Wasserburg图解法 Otter Lake 1.7 Durango 6.6 60μm,5Hz NIST612 无普通铅校正 MAD 0.1 Otter Lake 2.3 Durango 0.8 3. 结论
系统评价了磷灰石标样中不可避免的普通铅组成对U-Pb定年分析结果的影响。采用磷灰石MAD作外标直接开展U-Pb年龄分析,获得的被测样品年龄和初始铅组成均会产生显著的系统偏差(6%)。分别采用207Pb法或Tera-Wasserburg图解法校正标样中普通铅,再使用校正后结果进行元素分馏和仪器漂移校正,最终获得准确的磷灰石U-Pb年龄,测试值与推荐值的相对偏差在2%误差范围内。此外,为完全消除标样中普通铅的影响,本文通过在剥蚀池前引入4.0mg/min水蒸气,实现了以NIST612玻璃作为外标校正磷灰石MAD、Otter Lake和Durango的U-Pb年龄。该方法简单有效,可以缓解激光微区磷灰石U-Pb定年分析高质量标样匮乏的难题。
本文通过建立基体匹配和非基体匹配分析方法,有效地解决了磷灰石U-Pb定年分析时标样中普通铅对分析结果的影响,成功实现磷灰石U-Pb定年准确分析。未来研究仍需加强低普通铅磷灰石U-Pb定年标准样品研制,促进非基体匹配磷灰石U-Pb定年方法的推广,为磷灰石U-Pb年代学在地球科学研究中的应用提供支撑。
致谢:感谢审稿人对本文提出的宝贵修改建议。
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图 1 YBCs透长石A5和A5-1使用大气值(40Ar/36Ar=295.5)和热本底比值(40Ar/36Ar=310)校正大气氩时,对J值计算的影响程度对比。a为在放置14个月的样品盘C内的测试结果,变化幅度约为2.5%; b为置入激光去气后的盘D的测试结果,变化幅度约为0.9‰
Figure 1. Comparison of J-value calculation under different 40Ar/36Ar values with standard YBCs sanidine. a: Results from the oxygen-free copper (OFC) tray exposed to air for fourteen months; J-value changed about 2.5% using 40Ar/36Ar=310 to air correction. b: Results from the reusing OFC tray degassed by laser; J-value changed about 0.9‰.
图 2 系统冷本底、无氧铜盘实际热本底及其同位素比值随激光能量变化的情况。其中,a、b为A盘的结果;c、d为B盘的结果;e、f为D盘的结果。预先经过激光加热的D盘获得了与冷本底一致的40Ar信号量以及40Ar/36Ar比值
Figure 2. The amount of hot blank and isotope ratio characteristic of OFC tray after subtract cold blank. a and b come from tray A; c and d come from tray B; e and f come from tray D. The amount of 40Ar and 40Ar/36Ar value are similar with the cold blank of the system. Green solid circle means cold blank, blue circle means hot blank from first well in copper tray, and red circle means hot blank from second well.
图 3 样品大气氩来源于异常热本底时,使用大气氩初始值(40Ar/36Ar=295.5)扣除样品大气氩,样品年龄偏离真实值的百分比(t/(%))。假定热本底40Ar/36Ar=310;x、y、z轴分别指示模拟计算中样品大气氩含量、40Ar*/39ArK以及 J 值的变化范围
Figure 3. Percent change of 40Ar/39Ar age (t/(%)) resulting from calculating ages with the atmospheric 40Ar/36Ar of 295.5, while sample’s atmospheric argon comes from tray’s abnormal hot blank. Assuming the hot blank has an isotopic composition of 40Ar/36Ar=310; x, y and z axes indicate the range of atmospheric argon content of sample, 40Ar*/39ArK ratio and J-value used in the calculation.
图 4 标准样品大气氩来源于异常热本底时,使用大气氩初始值(40Ar/36Ar=295.5)扣除样品大气氩, J 值偏离真实值的百分比(t/(%))。假定热本底40Ar/36Ar=310, x轴为计算中标准样品大气氩含量的变化范围
Figure 4. Percent change of J-value (J(%)) resulting from calculating J-value with the atmospheric 40Ar/36Ar of 295.5, while standard mineral’s atmospheric argon comes from tray’s abnormal hot blank. Assuming the hot blank has an isotopic composition of 40Ar/36Ar=310, x axe indicates the range of the atmospheric argon content of standard material used in the calculation.
表 1 透长石YBCs样品A5及A5-1测试结果对比:A5显示样品各阶步均含有大量大气氩,A5-1大气氩含量正常(初始氩校正采用40Ar/36Ar=295.5)
Table 1 Result comparison of sanidine YBCs. A5 placed on an OFC tray exposures to air for fourteen months; A5-1 on a reusing tray; data calculation under 40Ar/36Ar=295.5. The proportion of atmospheric argon of steps of A5 is high, and return to normal of A5-1.
透长石YBCs样品A5 激光能量
(W)是否可以形成
坪的阶步40Ar/39Ar 36Ar/39Ar 40Ar*/39Ark 40Ar*(%) 辐照剂量监测 J值 ±2σ 0.24 - 10.319367 0.016838 5.34346 51.78 0.0030628 ±0.0000796 0.39 - 7.088052 0.006638 5.12619 72.32 0.0031926 ±0.0000388 0.54 - 8.306639 0.010221 5.28589 63.63 0.0030961 ±0.0000489 0.70 - 7.192548 0.007138 5.08307 70.67 0.0032197 ±0.0000417 1.32 - 7.205940 0.007393 5.02091 69.68 0.0032595 ±0.0000454 全熔 - - - - 65.59 0.0031648 ±0.0000226 透长石YBCs样品A5-1 激光能量
(W)是否可以形成
坪的阶步40Ar/39Ar 36Ar/39Ar 40Ar*/39Ark 40Ar*(%) 辐照剂量监测 J值 ±2σ 0.16 - 5.363579 0.000023 5.35657 99.87 0.0030553 ±0.0000345 0.39 √ 5.127897 0.000060 5.10986 99.65 0.0032028 ±0.0000205 0.54 √ 5.194142 0.000406 5.07372 97.68 0.0032256 ±0.0000205 0.70 √ 5.312808 0.000752 5.09012 95.81 0.0032152 ±0.0000204 1.32 √ 5.186170 0.000242 5.11425 98.61 0.0032000 ±0.0000194 全熔 - - - - 97.95 0.0032045 ±0.0000106 注:“√”表示可以形成坪的阶步;“-”表示不可以形成坪的阶步。 -
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