1 Introduction

Accessory minerals in igneous and metamorphic rocks are extremely important in understanding the crystallization evolution of the magmas and their petrologic links with major silicate assemblages besides their utility in geochronology (Hetherington et al. 2008; Suzuki and Kato 2008; Pandit 2018). Chemical dating of accessory minerals such as monazite, zircon, xenotime, and other Th–U–Pb bearing minerals is considered as one of the key geochronology tools for understanding the evolutionary history of magmatic and metamorphic systems. This is due to their potential to track multiple growth/deformation events in various magmatic and metamorphic conditions and also due to their ubiquitous presence in numerous rocks (Suzuki and Adachi 1991a, b; Montel et al. 1996; Verts et al. 1996; Williams et al. 1999; Asami et al. 2002, 2005; Pyle et al. 2005; Pant et al. 2009; Chew et al. 2011). Xenotime is one of the accessory minerals well established as a tool for geochronology, geochemistry and petrological investigations. It was first described by Swedish chemist, Jons Jacob Berzelius in 1824 from a pegmatite locality. Xenotime [Y(HREE)PO4] belongs to tetragonal system with a zircon type structure and hosts remarkable concentration of heavy rare earth elements (HREEs) and occurs over a vast P–T regime (Hetherington et al. 2008; Suzuki and Kato 2008). It is an ideal mineral for U–Th–Pb geochronology because of adequate amounts of uranium–thorium concentration and is extremely resistant to diffusional Pb loss (Compston and Mathai 1994; Dahl 1997; Griffin et al. 2000; Fletcher et al. 2004; Cherniak 2006; Hetherington et al. 2008). Like in the case of monazite (Burger et al. 1965; Grauert et al. 1974; Köppel 1974), the concordance in U–Pb and Th–Pb ages is also observed for xenotime (Hawkins and Bowring 1997) which makes it a robust mineral for geochronology. However, xenotime is less commonly observed in many rocks due to its smaller grain size. EPMA technique has proved to be a significant tool for U–Th–Pb chemical dating (Montel 2000) and as a result, measuring the REE orthophosphate compositions of monazite and xenotime by in-situ methods has become increasingly common in recent years (Suzuki and Aadachi 1991a, b; Hetherington et al. 2008; Pandey et al. 2019). Recently, Hazarika et al. (2017) proposed an analytical protocol for U–Th–Pb dating of xenotime based on EPMA technique. They have achieved an analytical uncertainty of less than 10% in U, Th and Pb concentration.

An advanced model of EPMA, viz., SXFive of M/s CAMECA, France, was installed in April, 2016 at the Department of Geology, Institute of Science, Banaras Hindu University (BHU). The analytical conditions for silicate mineral phases as well as protocol for U–Th–Pb chemical dating of monazite using the BHU-EPMA have already been reported (Pandey et al. 2017, 2019). Hetherington et al. (2008) have suggested that EPMA methodologies applied for monazite dating are also relevant for xenotime dating because of the similar properties in the absence of reference standard. Earlier studies of xenotime chemical ages were always reported with large errors (Chatterjee et al. 2007; Das et al. 2015). Therefore, the objective of this paper is to present the analytical conditions for xenotime chemical dating based on U–Th–Pb concentration with higher accuracy and precision. For this purpose, xenotime from the TTG (tonalite-trondhjemite-granodiorite) gneiss samples (HC-25 and HC-36) from the Baghaura area (N25°10′7.2″; E78°29′5.9″) of the Bundelkhand Craton, central India, were studied and subjected to microprobe analysis (figure 1).

Figure 1
figure 1

(A) Map showing the major cratons of India. (B) Geological map of the Bundelkhand Craton (after Basu 1986). Sample locations of the present study are marked. Ages reported by various authors are shown on the map (Sarkar et al. 1996; Mondal et al. 2002; Kaur et al. 2016).

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2 Analytical techniques and conditions for calibration and analysis

Two samples of tonalite-trondhjemite-granodiorite (TTG) gneisses, which are regarded as the basement of the Bundelkhand Craton, central India (Basu 1986; Chauhan et al. 2018; Pati 2020) were selected for the present research work. Well-polished thin sections were first coated by using LEICA-EM ACE 200 carbon coater to acquire thin carbon layer of 20 nm. Major and trace element analysis of xenotime were carried out by using a CAMECA SXFive EPMA equipped with five wavelength dispersive spectrometers (WDS) at the Department of Geology (Centre of Advanced Study), Institute of Science, Banaras Hindu University, Varanasi. The instrument is functional by LaB6 electron gun source at a voltage of 15 kV, current of 40 nA and beam size of 1 µm. Five different crystals were used: thallium acid phthalate (TAP), lithium fluoride (LIF), large pentaerythritol (LPET), large thallium acid phthalate (LTAP), and pentaerythritol (PET). Internal standard-andradite has been used to verify the positions of crystals by using wavelength dispersive spectrometers (WDS). Quantification of REEs in xenotime was performed in two ways: (i) at first, the analysis of REEs was acquired at a voltage of 15 kV and 40 nA current and (ii) at second run, the analysis of other trace elements for chemical dating was acquired at a voltage of 15 kV and 200 nA current. Beam damage effects are quite obvious when high beam current density and longer beam exposure time are involved during the trace element analysis (Hetherington et al. 2008). For quantification and routine calibration, synthetic glass standards of all REEs have been used which were supplied by CAMECA-AMETEK. The list of the standards used in the analysis is shown in table 1.

Table 1 Detailed calibration setting for xenotime chemical dating by using BHU-EPMA (CAMECA SXFive). This includes X-ray spectral lines, crystal, background position, natural and synthetic standards used during the analysis.
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The CAMECA SXFive Package, with SxSAB version 6.1, Cameca’s PC automation (PeakSightTM) and SX-Results software, was used to carry out routine calibration, overlap correction, data acquisition, quantification, age calculations and data processing. PAP (Pouchou and Pichoir 1985) was used for matrix correction. REE analysis (La to Lu) in xenotime was carried out on LIF crystal and yttrium (Y) on LTAP, whereas Pb, Th and U were analysed on spectrometers LPET crystal and PET crystal. Selection of background values are of prime importance for trace element analysis in xenotime and background intensity for Pb, Th and U. In this protocol, background values are defined in the same way as it was mentioned in monazite dating protocol reported earlier by us (Pandey et al. 2019). The background measurements of peak positions for Pb, Th and U were calculated from a non-linear regression of high precision wavelength dispersive scans (Williams et al. 2006; Jercinovic et al. 2008) and for other elements background values are based on linear interpolation of intensities between paired off-peak wavelength positions. For the evaluation of these analytical conditions, xenotime dating has been carried out from 20th September 2019 to 26th September 2019 (7 days), covering a time span of 153 hrs and 18 min without any interruption. X-ray spectral lines for all elements which are used in the analysis are also provided in table 1.

In this protocol, calibration for Th, U and Pb was carried out simultaneously in two different spectrometers LPET and PET. For Th analysis, Mα X-ray line was chosen and Th glass was used as a standard for the calibration of Th Mα. Background offset ranges from −1000 to +1000 at 200 nA. Counting time for Th calibration analysis was set to 600 s each for the background and the peak counting time fixed at 1200 s. In the U analysis, Mβ line was selected and calibration for UMβ was carried out using U-glass standard at 200 nA current in spectrometers LPET and PET. Background was set between −1000 and +1000 for 600 s and peak counting time was also fixed at 1200 s. For Pb analysis, Mβ line was preferred to avoid interference of PbMα with YLα lines. The calibration for PbMβ was carried out using a crocoite standard at 200 nA current through linear mode method of background estimation for 600 s and peak counting time fixed at 1200 s. The detection limits measured during this study are 70, 47, and 69 ppm for Pb, Th and U, respectively. Hetherington et al. (2008) mentioned that any slight change in the background measurement wavelength can affect the accuracy of the calculated age (table 2) and WDS step scans recorded in our CAMECA-SXFive instrument were used to obtain the background offsets for X-rays of UMβ, ThMα and PbMβ lines. Hence, we have selected the background from −1000 to +1400 as discussed by Hetherington et al. (2008), whereas Hazarika et al. (2017) have selected the background of +1350 as it was observed to be free from any interference. Similarly, all the rare earth elements (REEs) were calibrated on LIF crystal. The light rare earth elements (LREEs), based on the interference characteristics, were analysed on their specific spectral line (table 1). The calibration for the REEs was carried out with a background and peak count time of 30 s at 40 nA. For the calibration of La–Lα, an La-glass standard was used with a background value of −500 to +500. For the Ce–Lα calibration, the Ce-glass was used as a standard with the background range of −600 to +400 by avoiding any interference. In xenotime, the concentration of yttrium (Y) is much higher as compared to monazite, and an extensive care is, therefore, required while selecting the appropriate background without any interference. For Y, a YAG standard with Lα by LTAP with the background range of −500 to +500 at a peak count time of 30 s were measured. The background values for Pr–Lβ, Nd–Lα, Sm–Lβ, Eu–Lα, Gd–Lβ, Tb–Lα, Dy–Lα, Ho–Lα, Er–Lα, Tm–Lα, Yb–Lα and Lu–Lα− all range from −500 to +500.

Table 2 Major and trace elements analysis and U–Pb–Th ages of xenotime grains.
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3 Results

TTGs from the Baghaura area consist of quartz, plagioclase, biotite, K-feldspar and hornblende. Accessory phases include zircon, chlorite, xenotime, sphene and monazite. BSE images clearly show the occurrence of variable subhedral to anhedral grains of xenotime of different sizes (figure 2A–D). Mostly, the xenotime is found to be associated within the quartz groundmass and at the contact with biotite and feldspars (figure 2). No compositional zoning is observed in any of the xenotime grains. A total of 20 chemical ages from 20 xenotime grains of two samples (HC-25 and HC-36) and one line profile of 46 out of 50 points in one single large grain of xenotime in the sample number HC-36 were recorded. The contents of U–Th–Pb in various xenotime grains of both the samples are provided in table 2 and the age profile data is shown in table 3. The probability density plot and weighted average age distribution was acquired by using the ISOPLOT program (Ludwig 2011, version 4.2) and shown in figure 3. Both the samples yielded an age population at 2929±23 Ma with 95% confidence level. A continuous line scan (figure 4B) of 46 points also provided an age of 2900 Ma. Interestingly, the consistent line age data of 46 points in one single xenotime grain have recorded very low uncertainties, i.e., <3%. The chemical ages in xenotime with such very low uncertainties and high precision earlier were achieved only by Hetherington et al. (2008) with the help of very large pentaerythritol (VLPET) crystal installed in the CAMECASX-Ultrachron at University of Massachusetts, USA. However, BHU-EPMA has proved to be very efficient and achieved the chemical ages in xenotime with such very low uncertainties and high precision as well. These ages compare well with the published available ages (ranging from 2.35 to 3.50 Ga) for the TTGs from the same domain (Baghaura area) of the Bundelkhand craton (table 4). As these published ages were obtained by deploying other techniques and materials, viz., 3503±99 Ma (Rb–Sr whole rock isochron; Sarkar et al. 1996), 2697 ± 3 Ma (207Pb/206Pb zircon ages by ion microprobe; Mondal et al. 2002) and 2358 ± 46 Ma (U–Pb zircon by LA-ICP-MS; Kaur et al. 2016), the efficacy of our methodology is supported. It should be pointed out here that the age of the TTG suite from the Bundelkhand Craton display a wide range from 3.59 to 2.6 Ga (see Verma et al. 2016 and references therein). However, most reported ages are in the range of 3.5–3.0 Ga (Sarkar et al. 1984; Mondal et al. 2002; Kaur et al. 2014, 2016; Saha et al. 2016; Joshi et al. 2017). Therefore, our obtained xenotime age of 2.9 Ga from this domain is within the range of earlier reported age from the geochronologically well constrained TTGs from the Bundelkhand craton.

Figure 2
figure 2

Backscattered electron images (BSE) of xenotime grains of variable shape and size. (A) Euhedral big grain of xenotime observed in the groundmass of plagioclase in sample HC-25. (B) Xenotime occurring as inclusion within biotite and is surrounded by the plagioclase in sample HC-25. (C–D) Xenotime present as inclusion in plagioclase and biotite in sample HC-36. Abbreviations are Xtm: Xenotime, Qtz: Quartz, Pl: Plagioclase, Bt: Biotite, Ap: Apatite, and vug: cavity in a mineral. Analysis spot numbers are shown on the BSE images of xenotime grains (A–D) as displayed in table 2.

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Table 3 Line profile data of U–Th–Pb ages in xenotime from TTG sample (HC-36) of this study.
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Figure 3
figure 3

ISOPLOT diagram (Ludwig 2011) plotted for (A) weighted-average ages and (B) probability density ages for xenotime from TTG samples (HC-25 and HC-36) of this study with 2σ uncertainty and 20 number of point analysis.

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Figure 4
figure 4

(A) BSE image of the selected single xenotime grain (sample HC-36) for the line profile analysis and (B) the continuous age profile graph of 46 points for single xenotime grain of this study. Abbreviations are same as mentioned in figure 2. Analysis spot numbers for X–Y line profile are shown as displayed in table 3.

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Table 4 Published Archaean TTG ages from the Baghaura area of the Bundelkhand Craton and the results from the present study.
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4 Conclusions

This study presents highly improvised analytical protocol that has achieved very low uncertainties (<3%), than reported before, in the U–Th–Pb dating of xenotime using EPMA instrument at BHU and demonstrates its reliability and efficiency of the BHU-EPMA for the U–Th–Pb chemical dating of xenotime.