U–Th isotopic microanalysis of zircon reference materials and KBSI working standards

Abstract

Background

The ²³⁸U–²³⁰Th disequilibrium dating of the mineral zircon (ZrSiO₄) provides an efficient tool for investigating the time scales of Quaternary magmatic processes. In situ mass spectrometric U–Th microanalysis of zircon requires careful calibration and correction of the measured isotope data, particularly for the instrumental fractionation of U and Th isotopes.

Findings

For the selection of suitable calibration materials for U–Th isotopic analysis using a laser ablation multiple collector inductively coupled plasma mass spectrometer (LA-MCICPMS), we estimated the homogeneity of four reference zircons (91500, TEMORA 2, FC1, and Plešovice) and two zircon working standards (LKZ-1 and BRZ-1) in terms of their ²³²Th/²³⁸U ratios, based on the Pb isotopic compositions measured by a sensitive high-resolution ion microprobe (SHRIMP). The measured LA-MCICPMS ²³²Th/²³⁸U ratios of the zircons were calibrated externally using the SHRIMP ²⁰⁸Pb/²⁰⁶Pb-based average value of the 91500 zircon, ²³²Th/²³⁸U = 0.351 ± 0.035 (error corresponds to 1 standard deviation). The molecular interference-corrected ²³⁰Th/²³²Th ratios of the zircons were calibrated based on the assumption that the Plešovice zircon is in ²³⁸U–²³⁰Th secular equilibrium. After the calibration and correction, the activity ratios of ²³⁰Th/²³²Th and ²³⁸U/²³²Th for all reference zircons and working standards were plotted on the equiline.

Conclusions

This study confirms that the 91500 zircon is relatively homogeneous in terms of U/Th ratios (relative standard deviation = ~ 10%) and does not support a recent claim that the Plešovice zircon is not in ²³⁸U–²³⁰Th radioactive equilibrium. The working standards LKZ-1 and BRZ-1 can be used to check the reliability of U–Th isotopic analyses for Quaternary zircons.

Introduction

Geochronological methods using parent–daughter isotope pairs with long half-lives, such as ⁸⁷Rb–⁸⁷Sr (half-life = 48.8 Gy), ¹⁴⁷Sm–¹⁴³Nd (106 Gy), ²³⁸U–²⁰⁶Pb (4.47 Gy), and ⁴⁰K–³⁹Ar (1.25 Gy) systems, pose many challenges for the dating of Quaternary rocks and minerals, mainly because the amounts of daughter isotopes accumulated are extremely small. Also, in the case of ⁴⁰K (or ⁴⁰Ar)–³⁹Ar dating, problems associated with excess argon cannot always be avoided (e.g., Esser et al. 1997). On the other hand, the widely used radiocarbon dating method is only applicable to the last ca. 50,000 years because of the short half-life of ¹⁴C (= 5730 years). The geochronological gaps between the ranges of these dating methods are filled by uranium-series geochronometers using short-lived radionuclides in the decay chains, such as ²³⁰Th (half-life = 75,600 years) and ²³¹Pa (32,500 years).

The mineral zircon (ZrSiO₄) presents many advantages as a ²³⁸U–²³⁰Th geochronometer. It has a relatively high uranium content (ca. 100–1000 ppm) and is strongly resistant to physicochemical breakdown. Furthermore, the intracrystalline diffusion of its constituent cations is relatively slow (Cherniak and Watson 2003). Therefore, Quaternary zircons have been a prime target for ²³⁸U–²³⁰Th disequilibrium dating since the advent of in situ microanalytical techniques that employ secondary ion mass spectrometry or laser ablation-assisted inductively coupled plasma mass spectrometry (LA-ICPMS) (Reid et al. 1997; Zou et al. 2010; Ito et al. 2013). These techniques have made it possible to investigate the sub-grain time records in single crystals but require careful calibration and correction of the measured isotope data. The zircon reference material plays a critical role in the calibration of LA-ICPMS data (Guillong et al. 2016).

For the selection of suitable reference materials for calibration of laser ablation data, we estimated the homogeneity of four reference zircons (91500, TEMORA 2, FC1, and Plešovice) and two zircon working standards (LKZ-1 and BRZ-1) in terms of their ²³²Th/²³⁸U ratios, based on Pb isotopic compositions measured with a sensitive high-resolution ion microprobe (SHRIMP). Then, we assessed the reliability of the calibration for LA-multiple collector (MC) ICPMS zircon U–Th isotope data using the ²⁰⁸Pb/²⁰⁶Pb-based average ²³²Th/²³⁸U ratio in the most homogeneous zircon reference material (the 91500 zircon), in addition to the ²³⁸U–²³⁰Th radioactive equilibrium assumed for the Plešovice zircon.

Materials

According to Wiedenbeck et al. (1995), the 91500 zircon specimen stored at the Harvard Mineralogical Museum originally consisted of a single crystal presumably collected from a porphyroblastic syenite gneiss in Ontario, Canada. The authors reported its chemical composition and isotope dilution-thermal ionization mass spectrometric (ID-TIMS) ²⁰⁷Pb/²⁰⁶Pb age of 1065.4 ± 0.3 Ma. Its average U concentration and Th/U ratio were estimated to be 81.2 ppm and 0.3444 ± 0.0009, respectively. The sub-grain distributions of trace elements in this reference zircon were investigated by Wiedenbeck et al. (2004). The 91500 zircon is a widely used reference material for O (δ¹⁸O_V-SMOW = 10.07 ± 0.03‰; Valley 2003) and Hf isotopes (¹⁷⁶Hf/¹⁷⁷Hf = 0.282308 ± 0.000006; Blichert-Toft 2008).

The TEMORA 2 zircon, introduced by Black et al. (2004), was collected from a gabbroic diorite stock in the Lachlan Orogen of Eastern Australia. The ID-TIMS analyses for this zircon yielded a ²⁰⁶Pb/²³⁸U age of 416.78 ± 0.33 Ma, moderate U concentrations of 320–82 ppm, and Th/U ratios of 0.51–0.33 (Black et al. 2004).

The FC1 zircon was collected from an olivine gabbroic anorthosite in northeastern Minnesota. Paces and Miller Jr (1993) reported its ID-TIMS ²⁰⁷Pb/²⁰⁶Pb age as 1099.0 ± 0.6 Ma, with U concentrations ranging from 1510 to 218 ppm and Th/U ratios of 0.604–0.519.

The Plešovice zircon came from a potassic granulite in the southern Bohemian Massif, Czech Republic. The ID-TIMS analyses of this zircon by Sláma et al. (2008) yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 337.13 ± 0.37 Ma and relatively low Th/U ratios of 0.16–0.04. The U concentrations of the pristine domains in the Plešovice zircon, determined by LA-ICPMS, range from 1106 to 465 ppm (Sláma et al. 2008).

The Korea Basic Science Institute (KBSI) is developing zircon working standards, namely LKZ-1 and BRZ-1. The former, from Sri Lanka, is a single transparent pale yellow crystal (length to width = ~ 1.7:1) with dark cathodoluminescence (CL) emissions, and the latter, from Brazil, consists of translucent dark brown crystals and exhibits a symmetric CL banding (Fig. 1). The SHRIMP analyses by Kim et al. (2015) yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 591.1 ± 2.7 Ma (n = 29, mean square of weighted deviates [MSWD] = 2.1) for LKZ-1 and 572.2 ± 4.9 Ma (n = 30, MSWD = 4.1) for BRZ-1. The LKZ-1 is fairly high in U content, at 646 ± 45 ppm (hereinafter, errors correspond to 1 standard deviation (SD) unless otherwise stated), whereas the BRZ-1 has a significantly lower U content (90 ± 32 ppm) (Kim et al. 2015).

Fig. 1

Photographs of KBSI working standards (a) LKZ-1 and (b) BRZ-1 and their representative cathodoluminescence images

Methods

For Pb isotopic measurements using the KBSI SHRIMP, the primary O₂⁻ beam was focused into a ~ 25-μm-diameter spot at an accelerating voltage of 10 kV. The collector slit width was fixed at 100 μm, achieving a mass resolution of ~ 5000 at 1% peak height. The common Pb was removed by ²⁰⁴Pb correction method (Williams 1998) using the model of Stacey and Kramers (1975). Data processing was conducted using the SQUID 2.50 program (Ludwig 2009).

Zircon U–Th isotopic analyses were carried out at the KBSI using a Plasma II MCICPMS (Nu Instruments) equipped with an NWR193-nm ArF excimer laser ablation system (ESI). Typical laser ablation and ICPMS parameters are summarized in Table 1. Signal intensities were measured with Faraday collectors (for ²³⁸U and ²³²Th) and ion counters (for ²³⁰Th and 228 mass) simultaneously. The raw data were processed using Iolite 2.5 running within Igor Pro 6.3.5.5 software (Paton et al. 2011). The activity ratios were calculated using the decay constants proposed by Steiger and Jäger (1977) (for ²³⁸U and ²³²Th) and Cheng et al. (2013) (for ²³⁰Th).

Table 1 LA-MCICPMS instrument and operational parameters

Results and discussion

SHRIMP analysis

Generally, the mass fractionation of Pb in the SHRIMP analysis occurs within the limits of analytical precision (Williams 1998 and references therein), and thus, no correction was applied to the measured ²⁰⁸Pb/²⁰⁶Pb ratios. The common Pb-corrected ²⁰⁸Pb/²⁰⁶Pb ratios of the zircon reference materials and KBSI working standards are listed in Additional file 1: Table S1. The ²³²Th/²³⁸U ratios were calculated by assuming that the U–Pb and Th–Pb isotopic systems in the zircons were concordant (Eq. 1):

$$ {}^{208}\mathrm{Pb}{/}^{206}{\mathrm{Pb}}_{\mathrm{SHRIMP}}{=}^{232}\mathrm{Th}{/}^{238}{\mathrm{U}}_{\mathrm{CONCORDANCE}}\left[\left({e}^{\lambda_{232}\cdot t}-1\right)/\left({e}^{\lambda_{238}\cdot t}-1\right)\right] $$

(1)

where λ₂₃₂, λ₂₃₈, and t are the ²³²Th decay constant (4.9475 × 10⁻¹¹ y⁻¹), the ²³⁸U decay constant (1.55125 × 10⁻¹⁰ y⁻¹) (Steiger and Jäger 1977), and zircon crystallization age, respectively. The ²³²Th/²³⁸U ratios were also calculated by the SQUID program, using the experimental relationship proposed by Williams et al. (1996) (Eq. 2):

$$ {}^{232}\mathrm{Th}{/}^{238}{\mathrm{U}}_{\mathrm{SQUID}}=\left[0.03446\left({}^{238}{\mathrm{U}\mathrm{O}}^{+}{/}^{238}{\mathrm{U}}^{+}\right)+0.868\right]\left({}^{232}{\mathrm{ThO}}^{+}{/}^{238}{\mathrm{U}\mathrm{O}}^{+}\right) $$

(2)

The average ²³²Th/²³⁸U_CONCORDANCE of the 91500 zircon (0.352 ± 0.035, relative standard deviation [RSD] = 10%, n = 48) is in agreement with the previous ID-TIMS (0.356 ± 0.001; Wiedenbeck et al. 1995) and LA-ICPMS ([Th] = 29.9 ppm (RSD = 7%), [U] = 80.0 ppm (RSD = 10%), ²³²Th/²³⁸U = 0.386 ± 0.047; Wiedenbeck et al. 2004) values within the measurement uncertainties. The other reference zircon materials and working standards were less homogeneous in terms of Th/U ratios (Fig. 2). However, the average ²³²Th/²³⁸U_CONCORDANCE ratios of TEMORA 2 (0.451 ± 0.080, RSD = 18%, n = 47), FC1 (0.577 ± 0.125, RSD = 22%, n = 149), and Plešovice zircons (0.181 ± 0.157, RSD = 87%, n = 31) are consistent with the previous ID-TIMS and LA-ICPMS results (Paces and Miller Jr 1993; Black et al. 2004; Sláma et al. 2008). The Plešovice zircon showed the widest variation in ²³²Th/²³⁸U (0.52–0.07). The LKZ-1 and BRZ-1zircons yielded an average ²³²Th/²³⁸U_CONCORDANCE of 0.139 ± 0.015 (RSD = 11%, n = 6) and 0.381 ± 0.092 (RSD = 24%, n = 23), respectively. As shown in Fig. 3a, the ²³²Th/²³⁸U ratios calculated by the two equations (i.e., ²³²Th/²³⁸U_CONCORDANCE and ²³²Th/²³⁸U_SQUID) agree well with each other. The ²³²Th/²³⁸U_SQUID ratios with > 1% standard error (SE) are not displayed in this figure. It should also be noted that the ²⁰⁸Pb/²⁰⁶Pb ratios with poor internal precision (defined here as SE > 5%) were not considered in the calculation of ²³²Th/²³⁸U_CONCORDANCE. The ²³²Th/²³⁸U_CONCORDANCE ratios calculated for the 91500 zircon follow a normal distribution (Fig. 3b).

Fig. 2

The SHRIMP ²⁰⁸Pb/²⁰⁶Pb and calculated ²³²Th/²³⁸U_CONCORDANCE for the zircons. Each analytical session is marked in shaded and unshaded areas

Fig. 3

a Comparison between ²³²Th/²³⁸U ratios calculated using SHRIMP ²⁰⁸Pb/²⁰⁶Pb data and the assumption that the U–Pb and Th–Pb isotopic systems in the zircons are concordant (²³²Th/²³⁸U_CONCORDANCE) and those calculated by the SQUID program (²³²Th/²³⁸U_SQUID). b The frequency histogram of ²³²Th/²³⁸U_CONCORDANCE ratios for the 91500 zircon

LA-MCICPMS analysis

Essentially, correction of inter-elemental isotope ratios measured by the LA-ICPMS is unavoidable because the sensitivity varies between elements due to mass bias and differences in the vaporization and ionization efficiency of the aerosol introduced into the plasma. Matrix-matched and homogeneous standard materials are required for the external calibration of inter-elemental LA-ICPMS data. Our SHRIMP analyses confirm that the 91500 zircon is the most homogeneous in terms of the Th/U ratio among the analyzed reference materials. We employed its ²³⁸U/²³²Th_CONCORDANCE (= 2.85 ± 0.29) for the calibration of LA-MCICPMS ²³⁸U/²³²Th data. The relative sensitivity factor (RSF = 2.85 over ²³⁸U/²³²Th_measured) was fairly constant during single analytical tests (RSD < 3%) but varied significantly from run to run (Fig. 4a); this means that the RSF value needs to be checked before and after the sample analysis.

Fig. 4

Variation of a the relative sensitivity factor (RSF) measured for the 91500 zircon (= 2.85 over ²³⁸U/²³²Th_measured), b the count rate at mass 228, and c the correction factor (CF) measured for the Plešovice zircon [= (²³⁸U/²³²Th)_corrected/(²³⁰Th/²³²Th)_measured] in the LA-MCICPMS analysis. Symbols are the same as in Fig. 3

Of all the reference zircons analyzed in this study, the Plešovice zircon showed the widest variation in ²³²Th/²³⁸U. Ito (2014) argued that the Plešovice zircon is not in secular equilibrium, but this hypothesis may have been skewed by biased corrections for abundance sensitivity and molecular interferences (Guillong et al. 2015). During our LA-MC-ICP-MS analyses, the count rate at mass 228 was relatively constant (Fig. 4b), supporting the interpretation of Guillong et al. (2015) that mass 230 contains contributions from molecular zirconium sesquioxide ions. We removed these contributions using the isotopic abundance ratio proposed for ⁹⁰Zr₂ and (⁹⁰Zr⁹²Zr + ⁹¹Zr₂) (= 0.7142; Guillong et al. 2015). Following this, the ²³⁸U–²³⁰Th radioactive equilibrium assumed for the Plešovice zircon was used for the correction and calibration of the peak tail of ²³²Th and the Faraday–ion counter efficiency. The correction factor measured for the Plešovice zircon [= (²³⁸U/²³²Th)_corrected/(²³⁰Th/²³²Th)_measured] also varied significantly from run to run (Fig. 4c). After calibration and correction, we confirmed that the activity ratios of ²³⁰Th/²³²Th and ²³⁸U/²³²Th for all zircons were plotted on the equiline as displayed in Fig. 5 and listed in Additional file 2: Table S2.

Fig. 5

²³⁰Th/²³²Th vs. ²³⁸U/²³²Th activity ratios for the reference zircon materials and KBSI working standards. Note that all zircons demonstrate secular equilibrium and plot along the equiline. Symbols are the same as in Fig. 3

Conclusions

This study confirms that the 91500 zircon is relatively homogeneous in terms of U/Th ratios (RSD = ~ 10%). After calibration and correction using the SHRIMP ²⁰⁸Pb/²⁰⁶Pb-based average ²³²Th/²³⁸U ratio for the 91500 zircon (= 0.351 ± 0.035), and the ²³⁸U–²³⁰Th radioactive equilibrium assumed for the Plešovice zircon, the activity ratios of ²³⁰Th/²³²Th and ²³⁸U/²³²Th were plotted on the equiline for all zircon reference materials and KBSI working standards. The LKZ-1 and BRZ-1 employed by the KBSI can be used to check the reliability of U–Th isotopic analyses for Quaternary zircons.