Electron Probe Microanalysis of Hf and Ti in Zircon: Significance and Advantage
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摘要:
锆石是一种重要的定年矿物,其微量元素地球化学行为是解释锆石年龄地质意义的重要依据。锆石微量元素分析一般采用的是在大束斑条件下(10~50μm)的质谱仪测试方法,其结果反映的是在该束斑范围内,元素的平均含量信息。相比之下,电子探针显微分析可以在极小微区范围内(< 5μm)进行元素的定量分析,更能有效地揭示元素的地球化学行为,可作为研究锆石微量元素的重要技术补充。本文以锆石定年中常用标准锆石(TEMORA、Qinghu和Plešovice)作为研究对象,在20kV加速电压、50~300nA轰击电流以及2~5μm束斑条件下,对其中的Hf和Ti进行了定量分析,包括常规的点和线分析(Point/Line Analysis)以及网格分析(Grid Analysis),并以此为基础进行了线性拟合并建模,对元素的面分布情况进行了探讨。在较长的计数时间条件下(300s),本文得到了20μg/g(1σ)的Ti含量检测限。实验结果表明:锆石中的Zr与Hf之间具有负相关性,反映了两者的类质同象替代;其次,在概率统计方面,电子探针Hf和Ti的定量分析结果明显高于LA-ICP-MS方法,反映了其在更小微区下的含量信息以及地球化学行为,须在诸如(锆石)Ti温度计的应用中特别注意。再者,测试所用标准锆石样品的Hf含量(和Zr/Hf比值)在微区下具有环带分布特征,表现为从核部到边部具有升高(降低)的趋势,这与锆石结晶生长过程中的熔体分异程度有关。另一方面,标准锆石样品中的Ti并未表现出类似Hf的分布特征,且Hf(和Zr/Hf比值)与Ti含量之间也没有发现明显的相关性。因此,对于前人提出的Hf具有类似Ti一样的温度计指示功能的观点仍有待进一步探讨。
Abstract:BACKGROUND Zircon is an important refractory mineral that is ubiquitous in most magmatic, metamorphic and sedimentary rocks. It can provide robust U-Pb age(s) for most geologic processes. The trace elements in zircon are of great importance in the interpretation of zircon U-Pb age and relevant geological significance. Commonly, the trace elements abundance in zircon is analyzed by mass spectrometry methods, such as laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS), sensitive high-resolution ion microprobe (SHRIMP) and others, which usually require a large spot diameter (10-50μm). Electron probe micro-analyzer (EPMA) is capable of conducting elemental quantitative analyses at micro scales (< 5μm) with relatively high spatial resolution and analytical accuracy/precision. Under appropriate analytical condition settings, the detection limit (D.L.) and analytical uncertainty (standard deviation) of EPMA can be comparable to other high-sensitive methods (e.g. LA-ICP-MS) with only a small difference of one order of magnitude. Besides, EPMA is an accessible instrument in most facilities and is much cheaper and efficient than other techniques. Hafnium is an important trace element in zircon that often substitutes the zirconium ion, since they possess almost the same effective ionic radius in the eightfold coordination, and the zircon has a complete solid solution with the hafnon (HfSiO4). The Hf abundance in zircon is of importance in the zircon U-Pb age interpretation, and so is the Zr/Hf ratio—it commonly relates with magmatic differentiation: the higher Hf or lower Zr/Hf ratio domain is indicative of zircon growth from a more fractionated melt. Besides, the Hf-related geochemistry is also informative of certain tectonic settings, in which zircon formed. Titanium is an important temperature sensor in zircon that can be utilized in the crystallization temperature estimate. The Ti cation prefers to substitute Si in the four-fold coordination instead of replacing the eightfold Zr in the zircon lattice. Currently, the Ti-in-zircon thermometer has been widely used in studies of magmatic and metamorphic rocks, although certain discrepancy may occur compared to other approaches in temperature estimate. Moreover, some researchers reported a negative correlation between Ti and Hf contents in the igneous zircons and thus argued for a potential Hf-related temperature sensor similar to the Ti thermometry.
OBJECTIVES This work investigates the standard zircons conventionally utilized in U-Pb dating: TEMORA (TEM) and Qinghu (QH) zircons of plutonic origin and Plešovice (PLE) zircon of ultrahigh-temperature metamorphic origin. They were provided by the Beijing SHRIMP center (TEM) and Ion Probe Facility Institute of Geology and Geophysics CAS (QH and PLE) as polished epoxy resins targets. Suitable analytical conditions are designed on those standard zircons based on conditional analyses in order to acquire accurate and reliable Hf and Ti contents in zircon.
METHODS EPMA model of JEOL JAX-8100 equipped with four wavelength dispersive spectrometers (WDS) was utilized at Peking University. The standard materials for Zr, Si, Ti and Hf quantification were zircon (Zr, Si), rutile (Ti) and pure metal Hf from the national standard samples collection. Based on the results in the conditional analyses, two separate experiments (Exp-Ⅰ and Exp-Ⅱ) with nuanced analytical conditions were designed at 20kV accelerating voltage. In Exp-Ⅰ measurement, the current beam was set at 50nA and the counting time was adjusted to 100s for Ti and 10s for the rest; Hf (Lα) was measured by a LIF crystal in the high-resolution H type spectrometer (LIFH), Zr (Lα) and Si (Kα) were measured by TAP crystals in two separate spectrometers, and Ti (Kα) was measured by a PET crystal. In Exp-Ⅱ measurement, the current beam was set at 300nA, the counting time for Ti was adjusted to 300s and a PET crystal in the high-resolution H type spectrometer (PETH) was utilized; to avoid possible dead time effects in measuring Zr and Hf, EPMA CAL function was performed with fixed Zr and Hf contents using their average values obtained in Exp-Ⅰ. The background values were selected empirically in the conditional analyses. In both experiments, the spectrometer's pulse height analyzer (PHA) was set in differentiation mode (Dif) with empirical HV values in order to filter the high order X-ray signals of Hf and Zr that might potentially interfere with the measured characteristic X-ray spectra of Ti (Kα). As monitors, standard materials of Zr-free rutile and Ti-free zircon were also analyzed at the end of each measurement. In all, the detection limit for Ti was 60μg/g with relatively higher uncertainties and 20μg/g with moderate-low standard deviation errors in Exp-Ⅰ and Exp-Ⅱ, respectively.
RESULTS In Exp-Ⅰ measurement, a negative correlation is observed between Zr and Hf that corresponds with the isomorphic substitution of Zr and Hf in zircon. The Hf abundance in TEM standard zircon is relatively lower than that in PLE and QH standard zircons. However, there is no obvious correlation, neither negative nor positive, between Hf and Ti and is observed in all analyzed standard zircons. Generally, the Hf content in PLE, QH and TEM standard zircons decreases successively, as does the Zr/Hf ratio; while the Ti content is mostly either close to or below the detection limit (60μg/g) that requires Exp-Ⅱ to re-examine further.
Note, compared to other high precision techniques such as LA-ICP-MS, the content of Hf obtained by EPMA is about 10% higher in general and on average, despite a relatively wider variation range. The content of Ti acquired in Exp-Ⅱ by EPMA is quite comparable in the average with the LA-ICP-MS analyses, but in general, the EPMA analyses for Ti fluctuate greatly, similarly to the case of Hf analyses (by EPMA): this perhaps relates to the different order of magnitude of presence for each element (Hf vs. Ti) in zircon. Nevertheless, in general, both Hf and Ti analyses obtained by EPMA show relatively higher abundances in smaller spot analytical conditions, compared to the LA-ICP-MS measurement with larger spot: the latter method de facto measures an approximate average elemental concentration in the spot domain (e.g.32μm), while EPMA measures elements in a much smaller spot (2μm); it also explains the broad variation of EPMA analyses for both elements, since they perhaps distribute unevenly in zircon. To be noted, the Ti content obtained by EPMA indicates a much higher and expectable (Ti-) temperature that seems to best match the actual peak temperature attainment of parent rocks, derived by alternative thermometric approaches, while the conventional LA-ICP-MS analyses often yielded somewhat underestimated Ti-in-zircon temperatures. This is probably a potential key factor that explains the mismatch of Ti-temperature, derived from LA-ICP-MS analyses, with other methodological outcomes, particularly in the studies of ultrahigh-temperature metamorphic rocks: the peak metamorphic temperature inferred from pseudosection calculation or else is often higher than the zircon Ti temperature in practice.
In the line analysis (Exp-Ⅰ), a pronounced zonation of Hf content (increase) and Zr/Hf ratio (decline) is noticed from core to rim in the standard zircons, along their oscillatory annulus under cathodoluminescence, although it remains ambiguous for Ti because of its lower detection limit and poor reliability. The Hf and Zr/Hf zonation in zircon corresponds well with the zircon crystallization from magmatic melt and may provide an implication for melt presence in a relatively high temperature condition. Thus, such a pattern of Hf and Zr/Hf in zircon is possibly also applicable in the high temperature metamorphism that could help to identify the zircon (over)growth environment.
In the grid analysis (Exp-Ⅰ), several small domains of 40μm×40μm and 60μm×60μm sizes were selected, and the move step at 10-15μm and probe diameter of 5μm were adjusted appropriately. The measurement outcomes were then processed by the software OriginPro (ver. 2020b) in linear calculation mode and were plotted in colored contours to simulate conventional map scan results. The built elemental maps of those small domains show that there is no apparent correlation between Ti and Hf or Zr/Hf: it is in accordance with the above conclusions derived from statistic evaluation and line analyses. Nevertheless, PLE standard zircon seems to possess higher Ti content than the others in general, as being revealed above. Also, there is no obvious correlation between the Hf-Ti distribution in zircon and its oscillatory texture under electron or cathodoluminescence imaging, which commonly relate to other trace elements such as U, Th and REE.
CONCLUSIONS In this work a reliable EPMA method for the quantitative analyses of Hf and Ti in zircon is established. Exp-Ⅰ provides a reliable Hf measurement in zircon, while Exp-Ⅱ provides a lower detection limit (20μg/g) and standard deviation for Ti in EPMA analyses that can be utilized further in practice. EPMA analyses with a small probe diameter show a generally higher outcome compared to other large-spot techniques, such as LA-ICP-MS, despite the relatively wide variation range. This could explain why the Ti-in-zircon temperature, acquired by LA-ICP-MS analyses, was often lower than the actual peak temperature attainment inferred by other methods. Thus, EPMA analyses are recommended as an optimal method for elemental quantification in the first place. The Hf content in zircon is indicative of melt presence, from which zircon crystalized; thus, profile (line) analyses for Hf are informative for zircon origin deciphering. In both igneous and high-temperature metamorphic (standard) zircons, there is no obvious correlation between Ti and Hf or Zr/Hf, thus, it remains unclear whether Hf is temperature related as proposed by other researchers.
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Key words:
- zircon /
- trace element Hf and Ti /
- Zr/Hf ratio /
- electron microprobe analyzer /
- quantitative analyses
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锆石(ZrSiO4)是一种重要的副矿物,在岩浆岩、变质岩和沉积岩中均有广泛的分布,并且在月岩、陨石等地外物质中也有发现[1]。作为一种耐蚀的难熔矿物,锆石在绝大多数的地质过程中(如风化、蚀变、变质作用等),都可以保持很好的物理化学稳定性,并能够有效地保存其生长(结晶)信息,记录地质演化进程中的重要事件/时间[2-3]。锆石是一种重要的定年矿物[4],并且具有极高的封闭温度(>900℃[5]),可以对岩浆岩结晶年龄和高级变质岩变质峰期年龄进行准确测定。锆石常富含各种微量元素,如REE、Y以及Hf和Ti等,是诠释其U-Pb年龄地质意义以及探讨母岩变质演化过程的重要研究对象[6-7],具有关键研究价值。
例如,锆石中的Ti含量与结晶温度具有正相关性,可用于温度的定量计算[8-10],并在实际工作中得到了很好应用[11]。然而,在某些岩浆岩[12]和变质岩[13]研究中,锆石Ti温度计结果往往与体系真实的温度峰期有明显差异(多数情况下是偏低的[12-13])。一般认为,这与锆石结晶时的环境压力、体系中SiO2+TiO2组分的化学活度、其他微量元素(如Hf、REE、Y、P等)对锆石晶格的破坏和替代,以及流体活动等客观因素有关[8-9, 14-15];而且其并不适用于低温变质岩(<650℃[16])。除此之外,测试Ti含量的实验分析方法选择应该也是一个重要因素。例如,在金红石Zr分析中,电子探针测试结果明显要优于LA-ICP-MS方法[17-18]。锆石中Hf含量则更加丰富,其含量可达0.5%~10%(质量分数, wt%)[6],这是因为锆石与铪石(HfSiO4)是固溶体组分,HfⅣ和ZrⅣ可相互替代,并且Zr/Hf比值还可以反映锆石与(岩浆)熔体之间的分异程度[19]。因此,锆石Hf的绝对含量或Zr/Hf比值,与锆石Lu-Hf同位素体系一样[20],都可以对锆石的成因及其演化过程提供重要的指示意义。此外,有研究发现,(岩浆)锆石中Hf和Ti具有(负)相关性,有望为锆石Hf温度计的开发提供重要信息[21-23]。
目前,锆石微量元素分析主要依靠高灵敏度高分辨离子探针(SHRIMP)[24-26]、二次离子探针(SIMS)[27]以及激光剥蚀电感耦合等离子体质谱仪(LA-ICP-MS)[28-29]等高精度、高灵敏度的技术方法。但是,这些分析方法均需要使用较大的测试束斑(10~50μm),无法对更小微区(< 5μm)条件下的微量元素含量提供更加精细的信息。相比之下,具有快速、无损、原位、高精准度技术特征的电子探针分析(EPMA)[30-31]则是一种理想的技术手段。但受电子探针工作原理限制,即检测的是外层电子跃迁激发的X射线信号,其微量元素(< 0.01wt%)分析能力在很大程度上受实验条件设置的制约,包括测试灵敏度或检测限(Detection Limit),以及分析精度或标准偏差(Standard Deviation)等重要指标,因此,一般需要建立特殊的条件实验来进行测试。
在提高电子探针微量元素分析可靠性方面,除了可以通过设置更高的激发能量(加速电压、轰击电流)和更长的元素峰位测量时间来提高分析准确度,还需要考虑如何选择适宜的分光晶体以及设定合适的背景值,并对测试结果进行相关的统计学平均含量分析[32]。而对于潜在的元素干扰峰(重叠)的甄别与剥离,可启用波谱仪的脉冲幅度分析器(PHA/SCA)的分离模式(Differential Mode, 简写Dif),来对不同能量特征的X射线进行“过滤”[32-33]。此外,还需要注意的是,在高能量电子轰击下被测样品是否会发生“损伤”而导致测试不准确。例如,对石英进行Ti和Al含量分析时,有研究人员提出了采用多道波谱仪同时测试的实验条件来获得一个相对稳定可靠的结果[34-35]。最后,如果客观条件允许,可使用配备有大晶体的波谱仪进行测试,来提高分析的灵敏度[36]。
在锆石Hf、Ti等微量元素分析中,一般多采用的是上述大束斑如LA-ICP-MS测试方法,包括目前广泛应用的锆石Ti温度计[8-10],而很少采用极小束斑条件的电子探针分析进行探讨,而后者往往可以提供更精细的信息[17-18]。本文以锆石U-Pb定年中常用的标准锆石为测试对象,在高加速电压(20kV)和强轰击电流(50~300nA)的条件下,对锆石中Hf和Ti进行了点(Point)、线(Line)和面分布网格式(Grid)的定量分析测试,并与LA-ICP-MS分析测试结果进行对比,进而探讨了电子探针相关测试结果的地球化学意义。该实验方法有望推广到岩石学和矿物学的其他研究领域。
1. 实验部分
1.1 实验样品
在锆石U-Pb定年中,合格的锆石标准样品是保证分析结果准确的关键。目前,在同位素(U-Pb-Lu-Hf-O等)测试中常用的标准锆石包括91500[37]、TEMORA(简称TEM[38-39])、Plešovice(简称PLE[40])以及Qinghu(简称QH[41])。本文测试样品为TEM(来自北京离子探针中心)、PLE和QH(来自中国科学院地质与地球物理研究所离子探针实验室)。91500锆石因其极低的Ti含量(< 1 μg/g [37]),接近电子探针测试的极限灵敏度,因而未纳入测试。
TEM和QH锆石分别来自中性岩浆岩闪长岩和酸性深成岩浆岩(石英)二长岩,根据文献资料显示,其LA-ICP-MS测试Ti含量结果分别为 < 10μg/g和6.1~60μg/g[38-39, 41],而Hf含量在TEM锆石中约为8310~7801μg/g[38-39],在QH锆石中则未知。PLE锆石来自高温变质岩富钾麻粒岩(900~1050℃,1.5~2GPa),据文献资料,其Ti和Hf含量的LA-ICP-MS测试结果分别为40~100μg/g和8477~14431μg/g[40]。
TEM和QH锆石为典型的岩浆锆石类型,具有自形度较高的均匀形态(柱状晶体),几乎不含矿物包裹体。在电子图像下,如背散射电子(BSE)和阴极发光(CL)图像,两者锆石均发育有典型的岩浆锆石特征震荡环带或韵律带状结构[42]。PLE锆石则具有典型的高级变质作用锆石特征,为自形-半自形的等径状或柱状晶体,有类似岩浆锆石一样的韵律带状结构,或冷杉状、扇形分带结构(CL图像)。本文研究的锆石均已在相关单位完成了环氧树脂样品靶的制备,主要作为其他未知锆石样品U-Pb定年的标准锆石使用。
1.2 测试方法
实验仪器型号为JXA-8100日本电子JEOL电子探针,所有测试在北京大学造山带与地壳演化教育部重点实验室完成。分析中使用的标准物质为国家标准样品金红石(样品编号Y21、GBW04-1416-2001、ZBA Oxide Set 36)和中国微束分析标准化技术委员会认证的参考标样锆石(样品编号K5、ZKW Mineral Set 36)以及金属单质Hf(样品编号J42、ZJ Metal Set 36),分别用于Ti以及Zr、Si和Hf的定量分析。
本文建立了两个系列的不同电流和计数时间的实验条件(表 1):①以测试Hf为主的实验-Ⅰ,采用了加速电压20kV和激发电流50nA;②以测试Ti为主的实验-Ⅱ,采用了加速电压20kV和激发电流300nA。并且使用电子探针分析软件的CAL功能[32]:以实验-Ⅰ中的测试结果为依据,对Zr(ZrO2, 65wt%)和Si(SiO2, 34wt%)的含量进行了预设(Fixed)而未加测试。
表 1. 锆石的电子探针分析条件Table 1. EPMA analytical conditions for zircon实验-Ⅰ:加速电压20kV,激发电流50nA 谱仪通道 元素 特征谱线 分光晶体 计数模式 计数时间
(s)背景测量
位置(mm)背景值计数
时间(s)标样 检测限
(1σ, μg/g)标准偏差
(%)1 Ti Kα PET Dif 10 +1/-5 5 金红石 60 4~800 2 Zr Lα TAP Dif 10 +5/-4 5 锆石 220 0.25 3 Hf Lα LIFH Dif 10 ±5 5 铪金属 120 0.25 4 Si Kα TAP Dif 10 ±5 5 锆石 40 3~5 实验-Ⅱ:加速电压20kV,激发电流300nA 谱仪通道 元素 特征谱线 分光晶体 计数模式 计数时间
(s)背景测量
位置(mm)背景值计数
时间(s)标样 检测限
(1σ, μg/g)标准偏差
(%)3 Ti Kα PETH Dif 300 +2.5/-1.5 5 金红石 20 5~100 在实验-Ⅰ中,Hf元素的测试可使用TAP或LIF分光晶体。Hf的特征X射线Lα谱线能量计数(Count)在TAP晶体谱仪中比LIF晶体谱仪测量结果更高,理论上更适合进行Hf的分析。然而在本文实验中,TAP晶体谱仪通道已承担Si和Zr两个主量元素的分析任务,因此,为避免潜在的元素峰位重叠并提高分析效率,这里使用了配备有LIF晶体的高计数率H型号小罗兰圆谱仪对Hf进行分析。该谱仪通道(LIFH)具有与TAP晶体谱仪等效的Hf元素Lα谱线能量计数性能,满足微量元素测试要求[32]。此外,还需注意对不同元素特征X射线谱线重叠峰的识别与区分。Hf元素在PET晶体中的二阶Lβ1谱线(Lβ12nd的L值:88.023mm)与Ti的特征峰位有重叠,可能会影响Ti含量的计数;主量元素Zr在LIF晶体中会出现二阶Kα1谱线(Kα12nd的L值:109.134mm),与Hf的特征峰位有重叠,可能会影响Hf含量的测试。鉴于此,本文实验对所有测试谱仪的脉冲幅度分析器(PHA)启用了X射线信号的分离模式(Dif),来对高阶的干扰谱线进行“滤波”处理:两者的能量(Hf Lβ12nd=9.0229keV, Zr Kα12nd=15.7755keV)正好是Ti(4.509keV)和Hf(7.8992keV)特征谱线能量的2倍,因此,可通过设置合适的PHA甄别电压来对高阶的干扰X射线能量进行过滤(Ti PET和Hf LIFH波谱仪的HV值分别为1720V和1746V)。
在实验-Ⅱ中,为了对Ti进行更准确的分析,尤其是降低分析结果的检测限和相应的标准偏差,采用了更强的轰击电子束流和更长的元素计数时间(表 1)。同时,为了避免其他较高含量的元素的计数率出现死时间(Dead Time)效应,这里采用了定量分析软件中的CAL功能,对主量元素Zr和Si含量进行了预设(实验-Ⅰ中测得的结果平均值),并忽略Hf元素的测试——其已在实验-Ⅰ中完成。Ti元素的测试可使用PET或LIF分光晶体。其中,在使用PET晶体进行测试时,Ti的特征X射线谱线Kα的峰位(L值:88.072mm)比较接近谱仪下极限(86.000mm),对背景(Background, BG)的测量位置(±5mm)造成一定影响。因此,本组实验预先对背景值扣除区间进行了条件实验(以标准样品金红石为测试对象),确定了一个更加收敛、符合测试要求的扣除区间(+2.5,-1.5)。在使用LIF晶体测试时,Ti元素的峰位计数率比PET晶体低了近一半,这将对作为微量元素Ti的测试造成显著的影响,导致更低的检测灵敏度(高检测限)和峰位辨析能力。权衡之下,本组实验采用了计数率更高的配有PET晶体的H型波谱仪(PETH)对Ti进行定量分析,并更改了测试峰位的背景值扣除区间。
2. 结果与讨论
2.1 锆石中Hf和Ti含量分析结果
实验-Ⅰ结果显示,Ti的检测限为60μg/g,标准偏差波动范围极大,部分数据甚至超过了100%,可靠性较低。这一方面应该是由于Ti本身在样品中大多具有较低的含量所致,其次,也有可能与未使用H型高计数率波谱仪通道有关(见实验-Ⅱ),因此这部分数据未纳入本文讨论中。在实验-Ⅱ中,Ti的检测限为20μg/g,标准偏差在合理范围内波动(< 100%),数据质量相对更稳定可靠。其中,检测限反映的是在一定实验条件下可探测到的元素最小含量,即所测元素特征峰位的强度计数(NP)接近此处背景的强度计数(NB),此时的X射线特征峰位强度计数的标准偏差可描述为
σ=√2NB ,而通过与工作曲线斜率dN/dC(N为计数,C为含量浓度,即方法之灵敏度[31])相乘即可计算出该条件下的检测限值。对于低于检测限的定量分析结果,其既可能表示(待测元素)含量过低而无法测量,也可能表示并不存在(含量为零)。鉴于此,本文对一些低于检测限且标准偏差小于100%的数据,也进行了谨慎地讨论。测试结果显示,锆石中Zr和Hf含量之间表征出一定的负相关性(图 1a),而Zr含量与Zr/Hf比值之间具有正相关性,这确认了锆石中Hf与Zr的类质同象替代[6]。与此同时,TEM锆石中的Hf含量测定值相比PLE、QH锆石的Hf测定值偏低(图 1a,图 2),但是Ti含量波动范围却较大(图 1b):从检测限60μg/g到300μg/g之间都有不同概率的分布,并且与Hf含量(Zr/Hf比值)之间并未表征出明显的相关性,这似乎并不支持Hf在锆石中具有Ti一样的地球化学行为,即对温度的指示意义(温度计)[21-23];此外,在不同类型的锆石之间——岩浆岩类型TEM、QH锆石与高温变质岩PLE锆石,Ti含量并未表现出与Hf类似的差异性。
图 1. 电子探针测试标准锆石(PLE、QH和TEM)中Hf和Ti含量实验结果a—Zr与Hf含量之间的关系;b—Ti与Hf含量之间的关系。低于电子探针分析检测限(D.L.)结果区间已用黄色阴影标出,下同。Figure 1. Standard zircons (PLE, QH and TEM) EPMA analyses for Hf and Ti.(a) Correlation between Zr and Hf; (b) Correlation between Ti and Hf. Lower detection limit (D.L.) range is illustrated by yellow shade (same below).从统计概率计算分析的角度观察(图 2),PLE、QH和TEM锆石中Hf含量的统计平均值依次降低,分别为12296μg/g、10814μg/g和9414μg/g。相应的,其Zr/Hf比值分别为39、45和52。与此同时,测得Ti含量从趋势上看,在PLE锆石中相对较高(90μg/g),而在QH(44μg/g)和TEM锆石(45μg/g) 中则比较近似。不过,由于其大都低于检测限,可靠性有待验证。QH锆石Ti含量的文献参考值为6~60μg/g(LA-ICP-MS测试结果)[41],这与本文实验-Ⅰ的测试结果类似(图 2a),并在实验-Ⅱ中也得到确认(图 3)。本文使用的概率统计学平均值计算方法已在相关文献中有详细介绍[32],该方法可有效地评估微量元素平均含量问题,并且可为测试所需的最小数据量提供统计学意义上的限定。
图 3. 标准锆石的Hf与Ti含量箱型图a—PLE和TEM锆石中Hf含量的电子探针与LA-ICP-MS测试结果对比;b—PLE锆石中Ti含量的电子探针与LA-ICP-MS测试结果对比;c—PLE和QH锆石中Ti含量的电子探针测试结果对比;d—PLE和QH中锆石的Ti温度计结果对比(电子探针数据)。Figure 3. Box plots of Hf and Ti in the standard zircons: (a) Hf content in PLE and TEM zircons by EPMA and LA-ICP-MS; (b) Ti content in PLE zircon by EPMA and LA-ICP-MS; (c) Ti content in PLE and QH zircons by EPMA; (d) Ti temperature of PLE and QH zircons (EPMA data).2.2 电子探针与LA-ICP-MS测试结果对比
与高精度的LA-ICP-MS测试方法相比,本文采用电子探针分析锆石的Hf含量(TEM和PLE锆石,图 3a)大约偏高了近10%(约1000μg/g),且具有较大的波动范围(本文中的LA-ICP-MS数据均在北京大学造山带与地壳演化教育部重点实验室完成)。在实验-Ⅱ中,Ti的测试结果(PLE锆石, 图 3b)的平均值在两种测试方法中则吻合得较好,不过,电子探针分析结果的波动范围更大。然而对于QH锆石,大部分测试结果仍低于测试检测限(20μg/g, 图 3c),总体上比PLE锆石中的Ti含量要低,这与实验-Ⅰ中的初步结论一致(图 2中a, b)。另一方面,PLE和QH锆石的Ti温度计计算结果显示[9],两者的Ti温度平均值分别为983℃和902℃(图 3d):前者与母体岩石经历的高温变质作用的峰期温度一致性较好(900~1050℃[40]),而后者也符合母体岩石的理论结晶温度范围(800~900℃[43])。不过,鉴于两种分析方法使用的束斑直径差异较大:电子探针分析使用的束斑直径(2μm)远远低于LA-ICP-MS的束斑直径(32μm),对于Hf的测试,在更小束斑条件下其测试结果应该会偏高。这符合微区分析技术特点:较大束斑条件下的测试结果反映了在该束斑范围内元素的平均含量(分布)。因此,可进一步推断,Hf在锆石中的替代概率并不一致,即非均匀发生(否则两种束斑条件下的测试结果应最大限度地接近)。而电子探针和LA-ICP-MS两种测试方法测定的Ti含量的平均值的一致性比较好,这一方面确定了本文实验方法的可靠性,另一方面也反映出Ti在锆石微区范围内的分布具有规律性,符合锆石Ti温度计的适用性原理[8, 10]。然而需要注意的是,从整体来看,电子探针分析Ti含量的个别结果仍是要高于LA-ICP-MS分析方法的结果,与上述Hf含量的测试结果情况类似。
2.3 锆石微区Hf和Ti线性分布特征
前人研究发现,锆石的Ti与Hf或与Zr/Hf比值之间具有相关性,并且Hf含量可反映锆石结晶时的(岩浆)熔体分异程度[21-23]。本文研究的标准锆石在电子图像下(BSE和CL)都表现出明显的环带结构,这一般与一些微量元素如REE、Y、U、Th、Ti的含量有关,反映了锆石的生长过程[42]。具有典型岩浆成因震荡CL环带的QH锆石(图 4a)的剖面分析结果显示,Hf含量从核部(8000μg/g)到边部(13000μg/g)具有显著的升高趋势,即Zr/Hf比值向边部呈降低趋势(60→40);与此同时,由于Ti含量的测试结果均低于或接近检测限(60μg/g),其有效性还有待确认。类似的,在TEM锆石(图 4b)和PLE锆石(图 4c)中,Hf和Zr/Hf比值也表现出类似的环带分布特征,同样,其中部分Ti含量测试结果由于低于检测限仍有待进一步确认。不过,在前人研究中发现,(花岗岩)岩浆锆石中的Ti和Hf含量的剖面环带是具有负相关性的(LA-ICP-MS测试结果[22-23])。
在此,本文实验表明,岩浆锆石和(超)高温变质锆石具有相似的Hf含量分布特征:在锆石从熔体中结晶生长的过程中,体系中的Hf元素倾向于(分异)进入锆石晶体,使得锆石具有从核部到边部Hf含量增加的环带结构,相应的,Zr/Hf比值则从核部到边部是降低的(图 4),这与其他研究人员的发现是一致的[22-23]。不过,这似乎有悖于我们对矿物结晶过程的一般性认识:如果Hf优先进入锆石,熔体中的Hf含量应该逐渐降低,因此结晶锆石的Hf含量环带应该是降低的(核到边部),遵循瑞利分馏(Rayleigh Fractionation)原理。然而,瑞利分馏过程仅在封闭体系下有效[44],即系统中的Zr/Hf比值是固定的。
在自然界开放体系下,当Zr与Si达到饱和形成锆石时,熔体可能尚未完成结晶分异过程(尤其是花岗岩类岩),体系中仍有相对(Zr而言)较多Hf能够继续进入锆石晶体发生类质同象替代,并导致锆石Zr/Hf比值相对减小。可见,锆石局部的高Hf或低Zr/Hf比值反映了其生长环境中有高度分异的熔体存在[22],而且此时整个体系是处于固相线温度以上,因此,也可以认为对环境温度具有指示意义[21-23]。另外,在此过程中,锆石从熔体中结晶生长,可能会有局部的Zr-Hf浓度变化,或者其他的环境变化(外来流体/熔体混染、温压变化等),导致该Hf的成分环带出现局部性的波动(图 4中b, c)。
2.4 锆石微区Hf和Ti面分布特征
在具有明显震荡环带(BSE和CL图像下)的QH(图 5中a, b)和PLE锆石(图 5中c, d)中,本文对其中某个细小微区范围(40μm×40μm和60μm×60μm)内进行了网格分析(grid analysis)及面分布建模。电子束斑大小设定为5μm,步长设定为10~15μm,每个网格进行5×5点位的定量分析(实验-Ⅰ条件);测试结果数据在OriginPro软件中进行了线性计算(linear) 的面分布建模(contour plot)。结果显示,微区范围下锆石的Hf和Zr之间的耦合性明显,表现为Hf含量和Zr/Hf比值的完全互补性;而对于Ti含量的分布,通过对高于检测限的结果进行面分布建模发现,其与Hf、Zr/Hf比值之间并没有出现明显的相关性,这与上述的线性剖面分析的结论是一致;并且,PLE锆石中的Ti似乎比QH锆石要更富集且含量更高(例如图 5c),这也与前面利用统计概率计算平均值的结果一致(图 2);此外,锆石的震荡环带与Ti、Hf和Zr/Hf比值之间也没有出现相关性,这说明震荡环带结构与Ti或Hf没有直接关系,而可能与U-Th和/或REE等微量元素含量有关[42]。最后,两种不同类型的锆石-岩浆锆石QH和高温变质锆石PLE中的Ti和Hf似乎具有类似的地球化学行为,这符合通常对高温变质岩与(高温)岩浆岩在成岩机制上具有可类比性的认识。
图 5. 标准锆石阴极发光图像以及选择的微区范围(白色点线线框)的电子探针网格分析Ti、Hf和Zr/Hf结果的面分布图建模a、b—QH锆石;c、d—PLE锆石。半透明圆圈为电子束流束斑。Figure 5. Standard zircon CL images and the selected micro-zone (by white dotted squares) EPMA grid analyses of Ti, Hf and Zr/Hf: (a-b) QH zircon and (c-d) PLE zircon. The translucent circles are electron probe beams.3. 结论
本文建立了电子探针定量分析锆石Hf和Ti的实验条件,并以定年工作中常用的标准锆石为研究对象进行了测试,在20kV加速电压、300nA轰击电流、300s峰位计数时间条件下,测得20μg/g(1σ)的Ti含量检测限。实验发现,电子探针测试锆石Hf和Ti含量结果要高于LA-ICP-MS方法(近10%),这反映了电子探针优异的微束分析技术特征,预示着在更小的微区条件下(< 2μm),锆石中Hf和Ti(以及其他微量元素)可能具有更高的含量。这对常用的LA-ICP-MS测试微量元素方法提出了挑战,需要在诸如Ti温度计的应用中特别注意。
电子探针测试标准锆石Hf和Ti含量结果显示,锆石Hf与Zr之间具有负相关性,符合两者的类质同象替代关系。而且Hf在锆石中具有环带分布特征,反映了体系中熔体的分异(结晶)程度,对锆石的成因环境具有重要的指示意义。另外,Hf与Ti在锆石中的含量与其电子图像下的(震荡)环带结构之间并无直接关联,可能与其他微量元素有关。与此同时,Hf与Ti之间在微区条件下并没有明显的相关性,尚不支持前人提出的Hf具有类似Ti一样的温度计功能的观点,仍有待进一步解释和更多更详细的测试分析实验说明。
相比其他诸如LA-ICP-MS大束斑条件测试方法,电子探针能够在极小微区条件下对微量元素进行分析,具有显著的技术优势,这在研究元素富集、扩散、迁移等行为特征中具有重要意义。
致谢
感谢北京大学造山带与地壳演化教育部重点实验室LA-ICP-MS实验室马芳高级工程师在标准锆石微量分析上的协助。特别感谢两位匿名审稿人为完善本文给予的专业评审意见。
要点
(1) 锆石Hf和Ti含量电子探针小束斑测试结果高于LA-ICP-MS方法。
(2) 锆石Hf和Zr/Hf比值与体系中熔体状态有关。
(3) 锆石Hf与Ti无明显耦合性,不具备温度指示性。
HIGHLIGHTS
(1) EPMA analyses of Hf and Ti in zircon are higher than those of LA-ICP-MS.
(2) Zircon Hf and Zr/Hf are indicative of melt status in the system.
(3) Hf in zircon seems to be irrelevant with Ti, and possesses no clear temperature indication.
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表 1 锆石的电子探针分析条件
Table 1. EPMA analytical conditions for zircon
实验-Ⅰ:加速电压20kV,激发电流50nA 谱仪通道 元素 特征谱线 分光晶体 计数模式 计数时间
(s)背景测量
位置(mm)背景值计数
时间(s)标样 检测限
(1σ, μg/g)标准偏差
(%)1 Ti Kα PET Dif 10 +1/-5 5 金红石 60 4~800 2 Zr Lα TAP Dif 10 +5/-4 5 锆石 220 0.25 3 Hf Lα LIFH Dif 10 ±5 5 铪金属 120 0.25 4 Si Kα TAP Dif 10 ±5 5 锆石 40 3~5 实验-Ⅱ:加速电压20kV,激发电流300nA 谱仪通道 元素 特征谱线 分光晶体 计数模式 计数时间
(s)背景测量
位置(mm)背景值计数
时间(s)标样 检测限
(1σ, μg/g)标准偏差
(%)3 Ti Kα PETH Dif 300 +2.5/-1.5 5 金红石 20 5~100 -
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