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Volume 30 Issue 5
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Electron Probe Microanalysis of Monazite and Its Applications to U-Th-Pb Dating of Geological Samples

  • Corresponding author: Junpeng Wang, wangjp@cug.edu.cn
  • Received Date: 2019-02-02
  • Electron probe microanalysis (EPMA) dating of monazite has been developed over decades. However, limited by the detectability and analytical sensitivity of dating-related elements (Th, Pb, U and Y), the EPMA dating has been restricted to geological research. In this study, various probe currents, beam diameters and counting times have been utilized on a JEOL JXA-8230 electron microprobe to determine the optimal experimental conditions for measuring Th, Pb, U and Y in monazite. The optimal conditions are:(1) accelerating voltage is 15 kV; (2) probe current is 100 nA; (3) beam diameter is 1 μm; (4) the peak and background counting time of U and Pb are 200 and 100 s; and (5) the peak and background counting time of Th and Y are 100 and 50 s. We apply this method to monazite from garnet-bearing biotite gneiss in the Zanhuang area of the Central Orogenic Belt of the North China Craton. The PbO-ThO2* isochron age calculated by EPMA data is 1 812±17 Ma (MSWD=2.06), which is similar to the weighted mean 207Pb/206Pb age (1 805±12 Ma, MSWD=1.07) obtained by LA-ICP-MS. This study suggests that EPMA dating of monazite as a powerful dating technique can be widely used in geochronological study.
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Electron Probe Microanalysis of Monazite and Its Applications to U-Th-Pb Dating of Geological Samples

    Corresponding author: Junpeng Wang, wangjp@cug.edu.cn
  • State Key Laboratory of Geological Processes and Mineral Resources, Hubei Key Laboratory of Critical Zone Evolution, Center for Global Tectonics, School of Earth Sciences, China University of eosciences, Wuhan 430074, China

Abstract: Electron probe microanalysis (EPMA) dating of monazite has been developed over decades. However, limited by the detectability and analytical sensitivity of dating-related elements (Th, Pb, U and Y), the EPMA dating has been restricted to geological research. In this study, various probe currents, beam diameters and counting times have been utilized on a JEOL JXA-8230 electron microprobe to determine the optimal experimental conditions for measuring Th, Pb, U and Y in monazite. The optimal conditions are:(1) accelerating voltage is 15 kV; (2) probe current is 100 nA; (3) beam diameter is 1 μm; (4) the peak and background counting time of U and Pb are 200 and 100 s; and (5) the peak and background counting time of Th and Y are 100 and 50 s. We apply this method to monazite from garnet-bearing biotite gneiss in the Zanhuang area of the Central Orogenic Belt of the North China Craton. The PbO-ThO2* isochron age calculated by EPMA data is 1 812±17 Ma (MSWD=2.06), which is similar to the weighted mean 207Pb/206Pb age (1 805±12 Ma, MSWD=1.07) obtained by LA-ICP-MS. This study suggests that EPMA dating of monazite as a powerful dating technique can be widely used in geochronological study.

0.   INTRODUCTION
  • Uranium and Pb bearing accessory minerals (such as monazite, zircon, apatite, titanite, etc.) exist widely in various types of rocks and ores. Their U-Th-Pb ages can be used to constrain the formation and evolution of geological bodies. Monazite is a monoclinic phosphate mineral with a general chemical formula of (Ce, La, Nd, Th)PO4. It occurs mainly in Al-rich intermediate-acidic magmatic rocks and metamorphic rocks, and also exists in sedimentary rocks (Qiu and Yang, 2011). Like zircon, monazite is an ideal mineral for U-Th-Pb isotopic system dating. However, the monazite U-Th-Pb isotopic system has the closure temperature of about 750 ℃ (Engi et al., 2017; Smith and Giletti, 1997; Copeland et al., 1988), which is significantly lower than the closure temperature of zircon (~900 ℃, Chemiak et al., 2004; Lee et al., 1997). Therefore, compared with zircon, monazite is more suitable for age determination of medium- and low-grade metamorphism, deformation and fluid activity events (Ba et al., 2018; Liu et al., 2007). In particular, monazite with complex compositional zonation may record multi-stage metamorphic or fluid activity events.

    Suzuki (1991) firstly proposed the method of using electron probe microanalysis (EPMA) to analyze U, Th and Pb contents of monazite to calculate their "chemical age" (CHIME, Chemical Th-U-total Pb Isochron Method), which has been widely used since then (Montel et al., 2018; Qiao and Wu, 2018; Zhao et al., 2018; Tango et al., 2017; Faure et al., 2014; Suzuki and Kato, 2008; Liu et al., 2007, 2006, 2004; Faure, 2005; Cocherie and Albarede, 2001; Williams et al., 1999; Montel et al., 1996; Suzuki et al., 1991). EPMA dating of monazite has a high spatial resolution (1 μm, Williams et al., 1999) and fast analysis speed, which is an in-situ non-destructive dating technique (Zhang et al., 2003). However, since the contents of Pb and U in monazite are close to the detection limit of EPMA (100 ppm-1 000 ppm, Montel et al., 2001), the accuracy of EPMA dating is usually lower than that of mass-spectrometry (MS). Therefore, it is crucial for monazite geochronology to improve the accuracy of electron probe measurement by selecting the optimal analytical conditions.

    The Zanhuang massif is located at the boundary between the Eastern Block (EB) and the Central Orogenic Belt (COB) of the North China Craton (NCC) (Wang et al., 2017, 2013; Kusky et al., 2016), and its Precambrian tectonic evolution can provide us an insight into the assemblage pattern of NCC. Being focused on zircon, most previous studies on the geochro-nology of Zanhuang area lacked the geochronological study of other accessory minerals with lower closure temperature, such as monazite and titanite (Xiao et al., 2014, 2011; Trap et al., 2011, 2009).

    In this paper, we try to develop an experimental scheme that can accurately measure the contents of Th, U, Pb in monazite. These include optimized measurement conditions (accelerating voltage, probe current, beam diameter and counting time), background measurement and deduction, and spectral interference correction. The improved experimental method can be applied to monazite from a garnet-bearing biotite gneiss in the Zanhuang area, then we compare the EPMA ages with the ones obtained from LA-ICP-MS analysis to find out whether our experimental scheme is reliable.

1.   THEORETICAL BASIS
  • Monazite generally contains a large amount of Th (3 wt.%-15 wt.%, a few up to 25 wt.%). The radiogenic Pb can be measured above the detection limit of the EPMA at 100 million years (Braun et al., 1998; Montel et al., 1996). There are two hypothetical preconditions for the EPMA dating of monazite. Firstly, there is no common Pb in the monazite. Compared with radiogenic Pb, the content of common Pb is considered to be less than 1 ppm (Williams et al., 2017; Rhede et al., 1996; Parrish, 1990; Corfu, 1988; Williams et al., 1983), so it can be neglected. Secondly, the isotopic system must remain closed, such that there is no loss of Pb. Recently, many studies show that the diffusion of Th and Pb in monazite is negligible up to temperatures of about 750 ℃ (Bosse and Villa, 2019; Cherniak and Pyle, 2008; Chemiak et al., 2004; Rhede et al., 1996). In addition, EPMA analysis has a high resolution (up to 1 μm), allowing monazite crystals on the micro scale be regarded as a closed system (Suzuki and Kato, 2008; Cocherie and Albarede, 2001; Schärer and Allègre, 1982). The Th-Pb system used in the monazite EPMA dating is also more stable than the U-Pb system (Barth et al., 1994). However, monazite that has undergone complicated deformation processes in the presence of igneous or metamorphic fluids, should be carefully considered during age interpretation.

    The radioactive Pb isotope composition can be expressed as

    The total Pb content of the mineral in closed system is equal to the radiogenic Pb formed by the decay of Th and U, namely

    It is assumed that the isotopic system is concordant, that is, the age obtained by the above three isotopic system are the same, and the current 235U/238U=1/137.88, so the formula for EPMA dating is as follows

    Lead, Th, and U are measured in ppm, and λ232, λ235 and λ238 are the radioactive decay constants of 232Th, 235U and 238Pb, respectively (Steiger and Jäger, 1977). According to Suzuki (1991), the UO2 concentration can be converted into an equivalent ThO2 content and this value is added to the measured values of ThO2, which results in an apparent ThO2* amount, which allows for the linear relationship between ThO2* and PbO to be satisfied

    In the above formula, m is the slope and q is the y-axis intercept. Therefore, an isochron can be obtained by converting the measured values at each analysis point into PbO-ThO2* diagrams, and the slope m can be obtained. The age is calculated using the following formula.

    WTh and WPb are the molecular masses of ThO2 and PbO, respectively. For Th-rich monazite, the Th-Pb system dominates, so WTh is equal to 264(=232+32) and WPb is equal to 224(=208+16).

2.   SAMPLE PREPARATION
  • Monazite is a monoclinic phosphate mineral. The content of rare earth elements (REEs) in monazite is 50 wt.%-68 wt.%, and the content of ThO2 is 3 wt.%-15 wt.% (some as high as 25 wt.%). Monazite under the microscope is yellow or colorless, often in the form of euhedral-anhedral granular, short monoclinic crystals. The refractivity increases with increasing Th content, and monazite often shows a polychromatic halo due to the radioactivitive decay of U and Th. Under backscattered electron (BSE) imaging, monazite is much brighter than zircon and various Fe oxides and sulfides.

    Monazite grain separates were mounted on an epoxy resin target and polished down to an approximate cross-section, so as to maximize the internal structure of the monazite. The advantage of this method is that many monazite grains could be examined and dated at the same time. The separation and mounting of monazite grains were completed in Yuneng Mining and Rock Technology Service Company in Langfang City, Hebei Province. See Suzuki and Kato (2008) for detailed sample processing. The grains mount was coated with a 20 μm conductive carbon film before measurement. Carbon coating is carried out by ion sputtering instrument (Leica EM ACE200) of the EPMA Laboratory of the School of Earth Sciences, China University of Geosciences (Wuhan). The thickness of carbon coating can be accurately controlled by a quartz thickness gauge equipped in the instrument, which keeps the thickness of carbon as same as the standards (the thickness of carbon film of standards is 20 μm), so as to reduce matrix effects.

3.   ANALYTICAL METHODS
  • All the experiments were carried out in the EPMA laboratory of the School of Earth Sciences, China University of Geosciences (Wuhan) using JEOL JXA-8230 EPMA, which is equipped with four Wave Dispersive Spectrometers (WDS), including a WDS with small Roland Circle (radius of 100 mm). Each spectrometer contains two crystals. The matrix corrections of the intensity measurements were made using a ZAF (atomic number, absorption and fluorescence) correction procedure (Wang et al., 2019).

  • Standards used for EPMA measurement should be homogeneous on the micro-scale, well-characterized, and similar in composition to the samples in order to minimize matrix effects (Carpenter, 2008). Monazite contains 15-20 elements, such as Si, P, Ca, Y, Th, U, Pb, REE, etc. Among these elements in monazite, only Th, U, Pb and Y were measured. The standards selected for each element are listed as follows.

    SPI (Structure Probe Inc.) monazite standard (from Buenopolis, Minas Gerais, Brazil) for Th, metal U for U, crocoite (PbCrO4) for Pb, and Y-Al garnet (YAG) for Y. Note that pure galena (PbS) cannot be used for the standard of Pb, because the Pb Mα spectral line in galena will produce characteristic X-ray spectral interference with the S Kα spectral line, which will result in the inaccurate calibration of standard.

    The selected crystals for measurements are: crystal PETJ for Th, Pb and crystal PETH for U, Y. The measured spectral lines are: Th Mα, Pb Mα, U Mβ (see Section 4.1 for details), and Y Lα. The measuring positions of the upper and lower background are: U (-4 to +4 mm), Pb (-3 to +5 mm), and others are -5 to +5 mm (see Section 4.1 for details).

  • In order to improve the detection limit of trace elements by EPMA, Zhou (1988) suggested that the accelerating voltage, beam intensity and counting time should be increased simultaneously. However, as the accelerating voltage increases, the excitation volume will be larger, resulting in an increase in the absorption correction, which in turn increases the quantitative analysis error. Therefore, in order to obtain the highest peak-to-background ratio (P/B) and the best spatial resolution, the optimal accelerating voltage used in this experiment was 15 kV.

    The peak-to-background ratio (P/B) and counting intensity of trace elements will be higher as the probe current increases. However, if the probe current is too large, it will easily damage the surface of the sample and reduce spatial resolution (Zhang et al., 2019). In order to find the optimal probe current in monazite analysis, the SPI monazite was selected for preliminary measurement. The accelerating voltage, the beam diameter and counting time remain constant, and the net peak counts for each element are measured by changing probe current. The experimental conditions are as follows.

    The accelerating voltage is 15 kV, and the selection of crystals and spectral lines for each element is the same as in Section 3.1. The beam diameter is 1 μm, and the counting times of U, Th, Pb, Y is 40 s for peak position and 20 s for background position. The probe current is 20, 50, 100, 150, 200 and 300 in units of nA, respectively.

    Table 1 lists the contents of ThO2, Y2O3, PbO, UO2 of monazite and its standard deviations (S.D.%) and the X-ray intensity values of the corresponding elements under different beam intensities. Each element was measured 20 times under the same probe current and the values in this table are the average values of 20 measurements. The results show that the X-ray intensity of U, Th, Pb and Y increases linearly with increasing probe current. As the probe current increases, the four elements become unstable (shown as standard deviations (S.D.%) in Fig. 1 and Table 1). Under 100 nA or less probe current, the contents of ThO2, Y2O3, PbO and UO2 become more stable with increasing probe current, which is mainly due to an improved detection limit and peak-to-background ratio (P/B). However, when the probe current is higher than 100 nA, the S.D.% values of ThO2 and PbO become larger with increasing current. It may be that high probe current damages the sample's surface, reducing spatial resolution, and lowering the statistical accuracy of the peak counts for ThO2 and PbO. The S.D.% values of Y2O3 and UO2 increase slightly with increasing beam current, then decrease greatly, and their S.D.% values are the largest at 150 nA. But in general, the contents of Y2O3 and UO2 become more and more accurate with increasing probe current. However, due to the fact that PbO has the greatest influence on the age calculation in monazite dating, the probe current was chosen to be 100 nA.

    Probe current (nA) Content of oxides (wt.%) Standard deviation of oxides (S.D.%) Net peak counts (cps)
    ThO2 Y2O3 PbO UO2 ThO2 Y2O3 PbO UO2 Th Y Pb U
    Standard value (wt.%) 11.80 2.00 0.36 0.245
    20 11.645 7 1.996 5 0.351 2 0.208 1 19.300 5.720 4.930 5.600 300.39 103.27 4.03 22.33
    50 11.765 5 2.010 3 0.373 5 0.235 7 12.020 3.117 4.527 2.871 770.60 263.24 10.88 64.21
    100 11.771 6 2.016 9 0.368 0 0.240 0 6.624 3.593 2.442 1.975 1 563.53 538.31 21.47 130.94
    150 11.827 3 2.030 1 0.369 8 0.255 7 8.732 3.949 2.539 3.075 2 355.52 812.45 32.33 204.05
    200 11.829 4 1.998 8 0.378 2 0.249 3 8.692 1.825 4.137 1.082 3 161.92 1 092.56 44.40 273.95
    300 11.888 0 1.999 0 0.385 9 0.253 7 13.773 1.479 4.085 1.357 4 737.62 1 627.62 67.47 415.22

    Table 1.  Values of ThO2, Y2O3, PbO, UO2 along with standard deviations and the X-ray intensity of the corresponding elements in monazite for different probe currents (n=20)

    Figure 1.  The contents of ThO2, Y2O3, PbO, UO2 and S.D.% values in monazite for different probe currents.

  • Because of the difference in crystal resolution, the influence of beam diameter on characteristic X-ray of different elements and crystals is different. For example, crystal TAP is of high resolution. When the beam diameter increases from 1 to 50 μm, the X-ray intensity of Si (measured by TAP) drops by 4% (Zhou, 1988). The results show that when the beam size becomes smaller, better spatial resolution can be obtained without loss of X-ray intensity (Jercinovic et al., 2008). However, in some cases, it is necessary to increase the beam size. For example, when testing hydrous silicate minerals and silicate minerals containing alkali-metal elements, large beam diameter is generally selected to prevent migration and escape of certain components in the mineral during analysis (Reed, 2002). The experimental conditions are as follows.

    The accelerating voltage is 15 kV, and the selection of crystals and spectral lines for each element is the same as in Section 3.1. The probe current is 200 nA, and the counting time of U, Th, Pb, Y is 40 s for peak position and 20 s for background position. The beam diameter is 1, 3, 5, 10, and 20 in units of μm, respectively.

    Table 2 shows that as the beam diameter increases, the peak counts of each element do not change significantly, which is different from the case where the probe current is increased. This means that the beam diameter does not significantly affect the X-ray intensity. Table 2 and Fig. 2 show that as the beam size increases, the S.D.% values of ThO2, Y2O3, PbO and UO2 become larger at first, then decrease, and reach the maximum at 3 μm. Generally, at a beam diameter of 1 μm, the spatial resolution was highest and the S.D.% values of all elements were the lowest.

    Beam diameter (μm) Content of oxides (wt.%) Standard deviation of oxides (S.D.%) Net peak counts (cps)
    ThO2 Y2O3 PbO UO2 ThO2 Y2O3 PbO UO2 Th Y Pb U
    Standard value (wt.%) 11.80 2.00 0.36 0.245
    1 11.829 4 1.998 8 0.378 2 0.249 3 8.692 1.825 4.137 1.082 3 161.92 1 092.56 44.40 273.95
    3 11.930 9 2.038 2 0.418 0 0.272 8 18.352 4.639 7.259 3.158 3 149.95 1 045.55 48.02 298.64
    5 11.833 8 2.001 6 0.379 3 0.251 6 12.474 1.856 4.479 1.278 3 147.56 1 088.17 39.56 275.07
    10 11.820 9 2.025 1 0.382 5 0.248 8 12.210 3.203 4.221 1.251 3 158.18 1 086.10 44.86 273.22
    20 11.873 5 2.009 5 0.387 4 0.249 9 11.555 2.520 4.422 1.501 3 191.65 1 085.30 45.72 275.94

    Table 2.  Values for ThO2, Y2O3, PbO and UO2 along with standard deviations and the X-ray intensity of the corresponding elements in monazite for different beam diameters (n=20)

    Figure 2.  Contents of ThO2, Y2O3, PbO, UO2 and S.D.% values in monazite for different beam diameters.

    The above experiments were carried out under the probe current of 200 nA. In order to further verify the experimental results, we tested the changes of contents of Th, Y, Pb and U at 100 nA and the beam sizes were 1 and 3 μm, respectively. The experimental results are shown in Table 3. The results show that under the same beam diameter, the X-ray intensity becomes larger as the probe current increases, which is consistent with the results of Section 3.2. Under the same probe current, as the beam diameter increases, the X-ray intensity is approximately constant within the error range. In terms of elemental contents, when the probe current increases, the S.D.% values of Th, Y, Pb and U become larger. Under the same probe current, the instability of these four elements increases with increasing beam diameter. Tables 2 and 3 show that the S.D.% values of ThO2, Y2O3, PbO and UO2 are the smallest under the conditions of 100 nA and 1 μm. Under this condition, the spatial resolution of the analysis is the highest and the result is the most accurate. Therefore, 100 nA and 1 μm are suitable conditions for monazite analysis.

    Beam diameter (μm) Content of oxides (wt.%) Standard deviation of oxides (S.D.%) Net peak counts (cps)
    ThO2 Y2O3 PbO UO2 ThO2 Y2O3 PbO UO2 Th Y Pb U
    Standard value (wt.%) 11.80 2.00 0.36 0.245
    100 (nA) 1 11.771 6 2.016 9 0.368 0 0.240 0 6.624 3.593 2.442 1.975 1 563.53 538.31 21.47 130.94
    3 11.938 8 2.016 4 0.408 8 0.273 5 15.494 2.887 5.502 3.085 1 728.22 565.58 23.48 150.22
    200 (nA) 1 11.829 4 1.998 8 0.378 2 0.249 3 8.692 1.825 4.137 1.082 3 161.92 1 092.56 44.40 273.95
    3 11.930 9 2.038 2 0.418 0 0.272 8 18.352 4.639 7.259 3.158 3 149.95 1 045.55 48.02 298.64

    Table 3.  The values for ThO2, Y2O3, PbO and UO2 along with standard deviations and the X-ray intensity of the corresponding elements in monazite for different beam diameters, 100 and 200 nA (n=20)

  • Peak and background counting time are considered during the analysis. One of the difficulties in determination of trace elements is that there is little difference between the peak and background X-ray intensity, such that the peak-to-background ratio (P/B) is close to 1. This makes it impossible for the EPMA to recognize the peak position of trace elements. Therefore, for accurate analysis of trace elements in monazite, we must increase the peak-to-background ratio (P/B). Long counting times can significantly lower the detection limit and increase the peak-to-background ratio (P/B) of trace elements (Reed, 1995; Goldstein et al., 1986). However, if counting time is too long, it may cause the sample to be contaminated. Long counting times are also subject to the stability of EPMA. To explore the appropriate counting time, the following experiments are designed.

    The accelerating voltage is 15 kV, and the selection of crystals and spectral lines for each element is the same as in Section 3.1. The probe current is 100 nA, and the beam diameter is 1 μm. The counting time of each element from short to long is divided into five experimental conditions. Detailed counting time conditions and results of measurement of Th, Y, Pb and U are shown in Table 4. The interference of Th and Y on U and Pb should be noticed. So these two elements need a long counting time, which can be shorter than that of Pb and U.

    Counting time (s) Content of oxides (wt.%) Standard deviation of oxides (S.D.%) Net peak counts (cps)
    ThO2 Y2O3 PbO UO2 ThO2 Y2O3 PbO UO2 Th Y Pb U
    Peak (s) Background (s) 11.80 2.00 0.36 0.245
    Th Y Pb U Th Y Pb U
    10 10 10 10 5 5 5 5 11.801 0 2.006 0 0.375 0 0.252 0 15.927 6.444 5.656 2.306 1 559.84 523.88 21.79 137.15
    40 40 40 40 20 20 20 20 11.771 6 1.981 4 0.368 0 0.240 0 7.224 3.680 2.975 2.712 1 563.53 538.31 21.47 130.94
    100 100 200 200 50 50 100 100 11.824 4 2.033 3 0.384 2 0.240 5 5.547 4.420 2.538 2.009 1 581.65 537.07 22.57 132.11
    200 200 300 300 100 100 150 150 11.779 2 2.006 7 0.377 2 0.243 6 5.445 2.370 2.333 2.892 1 566.02 535.68 22.02 133.08
    300 300 500 500 150 150 200 200 11.844 3 2.012 6 0.391 9 0.247 2 13.680 3.384 3.949 1.494 1 614.77 550.61 23.45 138.48

    Table 4.  The values for ThO2, Y2O3, PbO and UO2 along with standard deviations and the X-ray intensity of the corresponding elements in monazite for different counting times (n=20)

    Table 4 shows that the net peak counts of each element remain the same as the counting time increases. It is worth noting that the peak counts for Th, Y, Pb and U increase (less than 8%) under the longest test times, but they are basically unchanged within the error range. Therefore, these results indicate that as the counting time increases, the X-ray intensity of each element remains substantially unchanged. Table 4 and Fig. 3 show that as the counting time increases, the S.D.% values of ThO2, Y2O3, PbO and UO2 decrease gradually, indicating that the repeatability of measurement is getting better. The S.D.% values are basically the smallest under conditions 2, 3 and 4. However, when the peak and background position counting time of Th are increased to 300 and 150 s, the S.D.% value of Th becomes larger, which is consistent with that described in Yao (2008). With increasing counting times, the measurement of the trace elements is closer to the standard value, while the analytical precision for the major elements becomes worse. The reason may be that as the X-ray intensity of major elements increases, the accuracy of the counting statistics becomes worse, and the machine also becomes unstable during long time analysis. Since Th and Pb are the most important elements for monazite dating, the analyzed contents of ThO2 and PbO under conditions 3 and 4 are the most accurate, but conditions 3 and 4 take 16 and 22 minutes to analyze a single point, respectively. We choose the counting times of condition 3 for monazite analysis, that is, the peak and background counting time of U and Pb are 200 and 100 s, and the peak and background counting time of Th and Y are 100 and 50 s.

    Figure 3.  The contents of ThO2, Y2O3, PbO, UO2 and S.D.% values in monazite for different counting times. For condition 1, the peak and background counting times of Th, U, Pb and Y are 10 and 5 s, respectively. For condition 2, the peak and background counting times of Th, U, Pb and Y are 40 and 20 s, respectively. For condition 3, the peak and background counting times of Th and Y are 100 and 50 s, and the counting times of Pb and U are 200 and 100 s, respectively. For condition 4, the peak and background counting times of Th and Y are 200 and 100 s, and the counting times of Pb and U are 300 and 150 s, respectively. For condition 5, the peak and background counting times of Th and Y are 300 and 150 s, and the counting times of Pb and U are 500 and 200 s, respectively.

    In order to test the credibility of the optimal counting time, we designed comparison experiments at 200 nA with different counting times. The detailed counting time and results are shown in Table 5. The results show that the measurement of Th, Y, Pb and U is more accurate with increasing counting times under 100 and 200 nA, where the result under 100 nA is better than that under 200 nA. It further shows that 100 nA is the most suitable probe current for analyzing monazite. Therefore, the conditions used in the monazite analysis are as follows: (1) probe current is 100 nA; (2) beam diameter is 1 μm; (3) the peak and background counting time of U and Pb are 200 and 100 s; and (4) the peak and background counting time of Th and Y are 100 and 50 s.

    Counting time (s) Content of oxides (wt.%) Standard deviation of oxides (S.D.%) Net peak counts (cps)
    ThO2 Y2O3 PbO UO2 ThO2 Y2O3 PbO UO2 Th Y Pb U
    Probe current (nA) Peak (s) Background (s) 11.80 2.00 0.36 0.245
    Th Y Pb U Th Y Pb U
    100 10 10 10 10 5 5 5 5 11.801 0 2.006 0 0.375 0 0.252 0 15.927 6.444 5.657 2.306 1 559.84 523.88 21.79 137.15
    40 40 40 40 20 20 20 20 11.771 6 1.981 4 0.368 0 0.240 0 7.224 3.680 2.975 2.712 1 563.53 538.31 21.47 130.94
    100 100 200 200 50 50 100 100 11.824 4 2.033 3 0.384 2 0.240 5 5.547 4.420 2.538 2.009 1 581.65 537.07 22.57 132.11
    200 10 10 10 10 5 5 5 5 11.772 5 2.006 7 0.380 1 0.249 0 11.801 2.832 5.691 2.063 3 120.94 1 068.00 44.26 271.14
    40 40 40 40 20 20 20 20 11.829 4 1.998 8 0.378 2 0.249 3 8.692 1.825 4.137 1.082 3 161.92 1 092.56 44.40 273.95
    100 100 200 200 50 50 100 100 11.850 6 2.000 4 0.388 1 0.248 8 7.694 1.654 3.463 0.658 3 116.52 1 074.39 44.80 268.875

    Table 5.  The values for ThO2, Y2O3, PbO and UO2 along with standard deviations and the X-ray intensity of the corresponding elements in monazite for different counting times, 100 and 200 nA (n=20)

4.   SPECTRAL INTERFERENCE CORRECTION
  • The characteristic X-rays between these elements in monazite are prone to overlap, resulting in the peak intensity of the test elements being higher than the actual X-ray intensity as represented by the actual amount of the element, such as the spectral overlap of Th Mγ on U Mβ (Dutch, 2009; Suzuki and Kato, 2008). In addition, the peak X-ray intensity of trace elements is not much greater than that of the background intensity. Even small interference and measurement position deviations will have a greater impact on the result (Yao et al., 2008). Therefore, in order to measure monazite trace elements accurately, the selection of spectral lines, the deduction of background intensity and the spectral interference correction are key points.

  • In general, for elements with atomic numbers (Z) less than 32, the K spectral line is selected. The L spectral line is chosen when the atomic number of analytic elements is 72≥Z≥32. The M spectral line is selected when Z > 72 (Jiao and Li, 2011). For the analysis of Th, U, and Pb in monazite, the excitation voltage of L spectral line is much greater than 15 kV. Hence the M spectral line is generally used for measurement (Reed, 2002).

    Figure 4a shows that the L value (peak position) of the U Mα spectral line is 125.258 mm, but it is overlapped by the nearby Th Mβ spectral line, which makes the measurement of its background intensity particularly difficult. The L value of the U Mβ spectral line is 119.198 mm. Although the upper and lower background are interfered by Th Mγ and K Kα, the overlapped intensity is not too high, and can be stripped. There is no K in monazite, thus only the interference of Th Mγ on U Mβ is considered.

    Figure 4.  X-ray spectrum near U (a), Pb (b) in monazite (modified after Yao, 2008).

    For the U Mβ spectral line, if the default upper and lower background measurement position (peak position ±5 mm) is used, the lower background position (114.198 mm) has interference with the Er Lα spectral line (Fig. 4a), and the upper background position (124.198 mm) has U Mα interference (Fig. 4a). In this study, we use the optimal upper and lower background measurement position of U Mβ obtained by the "two-point linear difference" and "multi-point background acquisition" method by Yao (2008a), that is, peak position ±4 mm.

    Figure 4b shows that there are strong interferences in the vicinity of Pb Mα and Pb Mβ. Considering the extremely low PbO content, the higher characteristic X-ray intensity of Pb Mα spectral line is selected. The Pb Mα spectral line shows strong interference with Y Lγ, Th Mζ1 and Th Mζ2, but due to the extremely low intensity of Th Mζ1 and Th Mζ2, the interference on the Pb Mα spectral line is small (several ppm, Yao, 2008). It should be noted that if the samples contain S, the interference of S on the Pb Mα spectral line should also be considered. The monazite tested in this study does not contain S, so we only consider the spectral interference of Y Lγ on Pb Mα.

    The L value of the Pb Mα spectral line is 169.324 mm. By default, the lower background measurement position is 164.324 mm. This position is overlapped by the high-intensity Ce Lα2 spectral line (Fig. 4b), while the upper background measurement position (174.324 mm) has no spectral line overlap. Yao (2008) suggested that the lower background position for Pb Mα spectral line should be -3 mm from the peak position, which allows it to avoid interference. Therefore, the Pb Mα spectral line is measured with upper and lower background measurement positions at -3 and +5 mm.

    There is no spectral line interference around the Th with high X-ray intensity, so the Th Mα spectral line is chosen (Fig. 4a). Also, the Y Lα spectral line is selected.

    In summary, the spectral line interferences to be considered in this experiment are the spectral interference of Th Mγ on U Mβ and YLγ on Pb Mα.

  • The method commonly used to avoid spectral interference is the "correction factor method". Firstly, the correction factor of Th on U is measured using the Th-bearing standard without U and Pb, and Y on Pb using the Y-bearing standard without Pb. Secondly, these correction factors are applied to the spectral interference correction for Pb and U (Allaz et al., 2019; Konečný et al., 2018; Jercinovic, 2005; Zhang et al., 2003; Amli and Griffin, 1975).

    The characteristic X-ray intensities of U and Pb are calculated using the following formula.

    Here, IUMβ (net) is the true X-ray intensity of UMβ. IUMβ(obs) is the observed X-ray intensity of UMβ. IThMα(obs) is the observed Th Mα X-ray intensity. IPbMα(net) is the true Pb Mα X-ray intensity. IPbMα(obs) is the observed PbMα X-ray intensity, and IYLα(obs) is the observed Y Lα X-ray intensity in the sample. The correction factors can be calculated using the following relationships

    In this study, fThMγ and fYLγ were measured by using ThSiO2 and Y-Al garnet (YAG) standards, which give the correction factors for fThMγ and fYlγ of 0.024 38 and 0.012 29, respectively. It should be noted that different EPMA instruments may have different correction factors due to the differences in the mechanical structures, the crystal reflectance properties and the take-off angles of the spectrometer (Jercinovic, 2005; Geisler and Schleicher, 2000).

5.   APPLICATION OF THE MONAZITE EPMA DATING PROGRAM
  • In this study, a geological sample containing monazite was chosen to carry out EPMA dating. The results were compared with those of LA-ICP-MS analysis of the same monazite in the sample to verify the reliability of the above analytical protocols. The sample (17XT-1-1) is from the Zanhuang area (GPS: 36°59′7.5"N, 113°58′0.5"E). Recently, Wang et al.(2017, 2013) and Deng et al. (2013) identified an Archean tectonic mélange in the central Zanhuang massif, named "Zanhuang mélange", which represents the suture zone between the Eastern Block and the Central Orogenic Belt of NCC. The samples were collected from a passive continental margin sedimentary sequence adjacent to the Zanhuang mélange unit in the Zanhuang massif, and occurred as interbeds of marble (Fig. 5a). The sample consists of garnet-bearing biotite gneiss, which is mainly composed of coarse-grained K-feldspar (40 vol.%-45 vol.%), plagioclase (15 vol.%-20 vol.%), quartz (20 vol.%-25 vol.%), biotite (10 vol.%-15 vol.%), garnet (5 vol.%) and fibrous sillimanite (5 vol.%). Accessory minerals include muscovite, magnetite, ilmenite, zircon, and monazite (Fig. 5b).

    Figure 5.  (a) Field and (b) microscopic photographs of the garnet-bearing biotite gneiss. Mineral abbreviation: Kfs. K-feldspar; Pl. plagioclase; Qtz. quartz; Grt. garnet; Sil. sillimanite.

    Only smooth, unaltered, crack-free areas of the monazite grains were investigated by EPMA following a detailed examination of BSE images and transmitted-light photomicrographs of monazite grain cross-sections. The EPMA analysis positions were recorded (Fig. 6a), and the LA-ICP-MS analyses were carried out near the EPMA analysis locations.

    Figure 6.  (a) Backscattered electron (BSE) images of the analyzed monazites labeled with EPMA (red numbers) and LA-ICP-MS analytical positions (yellow numbers); (b) concordia plot and weighted average of 207Pb/206Pb age diagram for monazites analyzed by LA-ICP-MS; (c) isochron age and weighted average age for monazites analyzed by EPMA.

    The EPMA measurement conditions were made under the analytical conditions concluded as optimal obtained in Section 3, i.e., accelerating voltage is 15 kV. Th, U, Pb, Y standards are monazite, metal U, crocoisite (PbCrO4), Y-Al garnet (YAG) respectively. Crystal PETJ is utilized for Th, Pb and crystal PETH for U, Y. The measured spectral lines are Th Mα, Pb Mα, U Mβ and Y Lα. The measuring positions of the upper and lower background are U (-4 to +4 mm), Pb (-3 to +5 mm), and others are -5 to +5 mm. The probe current is 100 nA, and the beam diameter is 1 μm. The peak and background counting time of U and Pb are 200 and 100 s, and the peak and background counting time of Th and Y are 100 and 50 s.

    The results are shown in Table 6. The amounts of PbO and UO2 are corrected values using correction factors to strip spectral line interferences. The method is to multiply the observed counting intensity of Th by the correction factor fThMγ obtained in Section 4.2, and calculate the intensity of the Th Mγ spectral line overlapped on the U Mβ spectral line, and then subtract the counting intensity of the overlapped U from the observed counting intensity of U to obtain the true intensity of U Mβ in order to calculate the true amount of UO2. The amount of interference on the U Mβ spectral line calculated for the sample is about 8%-15% of the original content. The interference amount of YLγ on the Pb Mα spectral line calculated using the correction factor fYLγ obtained above is about 2%-4%.

    Label PbO Err (Pb) UO2 Err (U) ThO2 Err (Th) Y2O3 Err (Y) ThO2* Age (Ma) Err (T, Ma) LA-ICP-MS age (Ma)
    1.1 0.393 0.005 5 0.240 0.003 4 3.923 0.010 0 0.328 0.003 8 4.823 9 1 857 25 1 833±28
    1.2 0.422 0.005 6 0.267 0.003 5 4.192 0.010 0 0.338 0.003 9 5.190 4 1 851 23
    1.3 0.412 0.005 5 0.281 0.003 5 4.135 0.009 9 0.401 0.004 0 5.183 0 1 815 23
    1.4 0.402 0.005 5 0.282 0.003 5 3.987 0.009 9 0.385 0.004 1 5.039 3 1 819 24
    1.5 0.375 0.005 6 0.200 0.003 4 3.967 0.009 9 0.450 0.003 5 4.713 1 1 813 26
    1.6 0.398 0.005 4 0.246 0.003 4 3.976 0.009 6 0.358 0.003 8 4.897 3 1 852 25
    1.7 0.417 0.005 6 0.270 0.003 3 4.153 0.009 8 0.314 0.003 4 5.162 5 1 840 23
    1.8 0.406 0.005 6 0.267 0.003 3 3.998 0.009 7 0.295 0.003 2 4.997 3 1 849 24
    2.1 0.570 0.005 7 0.256 0.003 5 6.201 0.010 0 0.485 0.004 0 7.157 1 1 816 17 1 803±29
    2.2 0.640 0.005 6 0.270 0.003 5 7.210 0.010 0 0.513 0.003 7 8.212 0 1 778 15
    2.3 0.699 0.005 7 0.218 0.003 5 8.182 0.010 2 0.411 0.004 0 8.992 5 1 773 14
    2.4 0.665 0.005 7 0.276 0.003 5 7.209 0.010 1 0.404 0.003 9 8.240 2 1 839 15
    2.5 0.603 0.005 7 0.184 0.003 5 7.041 0.010 1 0.354 0.003 9 7.724 0 1 781 16
    2.6 0.656 0.005 8 0.283 0.003 5 7.244 0.010 3 0.425 0.003 8 8.300 1 1 803 15
    2.7 0.663 0.005 5 0.284 0.003 4 7.294 0.009 9 0.418 0.003 8 8.353 0 1 808 15
    2.8 0.655 0.005 2 0.137 0.003 4 7.955 0.009 6 0.379 0.003 9 8.463 4 1 766 15
    3.1 0.738 0.006 0 0.246 0.003 5 8.260 0.010 0 0.472 0.003 5 9.178 4 1 830 14 1 807±31
    3.2 0.711 0.005 7 0.438 0.003 5 7.447 0.010 0 0.515 0.004 0 9.076 3 1 787 14
    3.3 0.898 0.005 8 0.492 0.003 5 9.324 0.010 0 0.343 0.003 9 11.161 7 1 833 11
    3.4 0.762 0.005 7 0.421 0.003 5 7.750 0.010 0 0.486 0.004 0 9.328 8 1 860 13
    3.5 0.657 0.005 7 0.348 0.003 5 7.109 0.010 1 0.456 0.003 8 8.403 8 1 784 15
    3.6 0.650 0.005 6 0.285 0.003 3 7.311 0.009 8 0.332 0.003 3 8.307 2 1 783 15
    3.7 0.658 0.005 6 0.278 0.003 3 7.285 0.009 8 0.284 0.003 9 8.321 4 1 803 16
    3.8 0.584 0.005 6 0.256 0.003 4 6.549 0.009 8 0.410 0.003 7 7.500 3 1 777 17
    4.1 0.724 0.005 6 0.295 0.003 5 7.885 0.010 3 0.394 0.003 9 8.988 5 1 834 14 1 854±34
    4.2 0.611 0.005 7 0.271 0.003 5 6.530 0.010 0 0.391 0.003 6 7.544 9 1 844 16
    4.3 0.718 0.005 5 0.301 0.003 5 7.875 0.010 2 0.299 0.003 5 8.999 1 1 819 13
    4.4 0.804 0.005 7 0.260 0.003 5 9.046 0.010 0 0.398 0.003 8 10.019 2 1 828 12
    4.5 0.815 0.005 7 0.264 0.003 5 8.845 0.010 0 0.360 0.003 8 9.836 8 1 885 13
    4.6 0.612 0.005 5 0.281 0.003 4 6.318 0.009 7 0.425 0.003 8 7.373 2 1 887 16
    4.7 0.543 0.005 5 0.309 0.003 4 5.517 0.009 8 0.437 0.003 5 6.674 2 1 854 18
    4.8 0.593 0.005 6 0.356 0.003 4 5.807 0.009 8 0.458 0.004 0 7.146 1 1 889 17
    5.1 0.653 0.005 6 0.256 0.003 5 7.423 0.010 3 0.407 0.004 1 8.375 3 1 779 15 1 802±30
    5.2 0.525 0.005 8 0.343 0.003 5 5.528 0.010 0 0.401 0.004 0 6.799 7 1 763 18
    5.3 0.490 0.006 1 0.342 0.003 5 5.014 0.010 0 0.495 0.004 1 6.285 4 1 779 21
    5.4 0.502 0.005 7 0.366 0.003 5 5.057 0.100 0 0.505 0.003 9 6.417 5 1 786 19
    5.5 0.506 0.005 7 0.312 0.003 5 5.069 0.100 0 0.482 0.004 0 6.236 5 1 849 20
    5.6 0.647 0.005 5 0.272 0.003 4 7.205 0.009 7 0.387 0.003 8 8.216 3 1 795 16
    5.7 0.568 0.005 5 0.364 0.003 4 5.704 0.009 7 0.508 0.003 9 7.063 5 1 833 18

    Table 6.  EPMA Th-U-Pb (wt.%) and LA-ICP-MS 207Pb/206Pb ages of monazites from the garnet-bearing biotite gneiss

    The apparent age of a single analysis point was calculated according to the age equation of Suzuki et al. (1991), and the single-point age error was calculated using the average of the multiple measurements of the detection limits (Cocherie and Albarede, 2001), all calculations were done using the "ChemAge" software of Geisler and Schleicher (2000). The amount of common Pb is assumed to be negligible per the above discussion. LA-ICP-MS analyses were conducted at Sample Solution Analytical Technology Co., Ltd., Wuhan, China. Spot analyses (16 μm) were carried out on the monazite grains. The monazite standards GBW44069 with 424.9±0.4 Ma was used to correct the age of the monazite. Detailed operating conditions for the laser ablation system and the ICP-MS instrument and data reduction are the same as described by Liu et al. (2008).

    The concordia plot and the mean 207Pb/206Pb age calculations were made at 2σ (95% confidence limit) using the Ludwig (2000) ISOPLOT/EX program. Twenty-eight LA-ICP-MS point analyses were performed on 20 monazite grains. All ages were concordant (> 98%) (see Supplementary Table S1). The upper intercept of all 28 points in concordia plots at 1 811±9 Ma (MSWD=1.2), and the weighted mean 207Pb/206Pb age is 1 805±12 Ma (MSWD=1.07) (Fig. 6b).

    Table 6 shows that 39 EPMA chemical ages obtained on 5 monazite grains plotted on PbO-ThO2* diagram fall on an isochron of 1 812±17 Ma (MSWD=2.06) (Fig. 6c), which is close to the LA-ICP-MS age from the same monazites, with an error of less than 2%. Some analysis points on the same grain may differ in content due to inhomogeneous or local diffusion of Pb caused by microcracks, but all analyses are consistent with the ages from LA-ICP-MS taken near the specific EMP analytical points. The weighted mean age of the 39 EPMA analyses is 1 813±11 Ma (MSWD=4.1) (Fig. 6c), which is consistent with the weighted mean 207Pb/206Pb age measured by LA-ICP-MS. Therefore, the experimental scheme for measuring the chemical age of monazite by EPMA is reliable.

    The advantages of dating monazite by EPMA lie in its high spatial resolution, such that extremely narrow growth margins of monazite can be measured, which is beyond the capability of the LA-ICP-MS method. However, the damage to the monazite grain surface by the electronic beam should be considered before analysis. For example, although the beam diameter used in the above experiments is 1 μm, the actual excitation volume in the monazite has a diameter of about 8 μm. Therefore, when choosing analysis spots for monazite, the area selected should be far from the cracks or edges of minerals. The ages of the growth margins of monazite analyzed in this study are slightly younger than that of the core (e.g., point 3.8, < 40 Ma), suggesting that they are the products of the same prolonged tectonic event. It is speculated that these monazites are metamorphic monazites, and the metamorphic event in Zanhuang area occurred at ca. 1.8 Ga, which is consistent with the metamorphic ages reported by Xiao et al. (2011) and Trap et al. (2009).

6.   CONCLUSIONS
  • (1) Using JEOL JXA-8230 electron probe microanalysis, we selected monazite, crocoite (PbCrO4), metal U, Y-Al garnet (YAG) as the standards of Th, Pb, U and Y, respectively. The optimal experimental conditions were determined as follows: (1) accelerating voltage is 15 kV; (2) probe current is 100 nA; (3) beam diameter is 1 μm; (4) The peak and background counting time of U and Pb are 200 and 100 s; and (5) the peak and background counting time of Th and Y are 100 and 50 s.

    (2) ThSiO2 and Y-Al garnet (YAG) are used to measure the spectral interference of Th Mγ on U Mβ and YLγ on Pb Mα. The obtained correction factors fThMγ and fYLγ are 0.024 38 and 0.012 29, respectively.

    (3) Monazites from garnet-bearing biotite gneiss in Zanhuang area of North China Craton, were analyzed using the above experimental conditions, and LA-ICP-MS test was carried out near the EPMA analytical positions. The results show that the PbO-ThO2* isochron age obtained by EPMA is 1 812±17 Ma (MSWD=2.06), and the weighted mean 207Pb/206Pb age measured by LA-ICP-MS is 1 805±12 Ma (MSWD=1.07), which are consistent within the error range. The result indicates that our EPMA dating scheme is reliable and can be widely used in the geochronological study of monazite-bearing rocks.

ACKNOWLEDGMENTS
  • This work was supported by the National Natural Science Foundation of China (No. 41602234), the Fundamental Research Funds for the Central Universities, China University of Geosciences, Wuhan, China (Nos. CUGL180406, CUGCJ 1707), and Open Fund (No. GRMR20 1901) from the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan. We are grateful to Daniel Harlov, Timothy Kusky and Wang Lu for providing valuable suggestions and revising this paper. We also appreciate the editor and two anonymous reviewers for constructive comments, which helped to improve this manuscript significantly. The final publication is available at Springer via https://doi.org/10.1007/s12583-019-1020-8.

    Electronic Supplementary Material: Supplementary material (Table S1) is available in the online version of this article at https://doi.org/10.1007/s12583-019-1020-8.

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