LKZ-1: A New Zircon Working Standard for the In Situ Determination of U–Pb Age, O–Hf Isotopes, and Trace Element Composition

Abstract

This study introduces a new zircon reference material, LKZ-1, for the in situ U–Pb dating and O–Hf isotopic and trace element analyses. The secondary ion mass spectrometric analyses for this gem-quality single-crystal zircon yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 572.6 ± 2.0 Ma (2σ, n = 22, MSWD = 0.90), with moderately high U concentrations (619 ± 21 ppm, 1 SD), restricted Th/U ratios (0.146 ± 0.002, 1 SD), and negligible common Pb content (²⁰⁶Pb_c < 0.2%). A comparable ²⁰⁶Pb/²³⁸U age (570.0 ± 2.5 Ma, 2σ) was produced by the isotope dilution-thermal ionization mass spectrometry. The secondary ion mass spectrometric and laser ablation-assisted multiple collector inductively coupled plasma mass spectrometer analyses respectively showed that LKZ-1 had little variation in O (δ¹⁸O_V-SMOW = 10.65 ± 0.14‰; laser fluorination value = 10.72 ± 0.02‰; 1 SD) and Hf (¹⁷⁶Hf/¹⁷⁷Hf = 0.281794 ± 0.000016, 1 SD) isotopic compositions. LKZ-1 was also fairly homogeneous in its chemical composition (RSD of laser ablation ICPMS data ≤ 10%), displaying a relatively uniform chondrite-normalized rare earth element pattern ((Lu/Gd)_N = 31 ± 3, Eu/Eu^* = 0.43 ± 0.17, Ce/Ce^* = 44 ± 32; 1 SD). These consistencies suggest that the LKZ-1 zircon is a suitable working standard for geochronological and geochemical analyses.

1. Introduction

Geochemical and geochronological data for rocks and minerals have long been obtained by wet chemical and isotopic analyses [1,2]. Despite the constraints of time-consuming and labor-intensive sample preparation, wet analysis is still widely used, particularly when a high level of precision is demanded. However, if the sample is composed of chemically or isotopically heterogeneous domains that have their own petrogenetic significance, microsampling strategies are inevitably required.

Technical advances during the past several decades have facilitated the routine analysis of tiny single crystals at the subgrain scale. The high spatial resolution of microbeam techniques, typically of the micrometer- or submicrometer-scale [3,4], provides an excellent opportunity to integrate the chemical and isotopic data from individual subgrain domains with textural observations (e.g., [5]). Zircon (ZrSiO₄) has been a prime target of in situ analysis using secondary ion mass spectrometry (SIMS) [6,7] or laser ablation (LA)-assisted inductively coupled plasma mass spectrometry (ICPMS) [8,9], mainly for its ability to retain the original isotopic signature due to its strong resistance to physicochemical breakdown and slow intracrystalline diffusion of constituent ions [10]. Zircon can provide U–Pb age constraints, as well as trace element and O–Hf isotope data for given microdomains. The integration of such multifaceted zircon data has proven to be a powerful tool for addressing the diverse issues related to crustal evolution [11,12,13,14].

Microbeam analysis can be performed rapidly and requires minimal sample preparation; however, the measured data should be carefully corrected and calibrated for spectral and isobaric interference and instrumental fractionation. This process is generally achieved using matrix-matched standards, which are used as a reference material to check the data accuracy and, more importantly, as a primary standard to calculate the inter-elemental isotope ratios or instrumental mass fractionation (IMF) factors. In the latter case, the relative isotopic composition of the standard should be uniform and accurately known, because it directly affects the results calculated for unknown samples. Many zircon standards have been suggested for U–Th–Pb dating and chemical/isotopic analyses. Among these, 91500 [15], FC1 [16], Temora [17,18], and Plešovice [19] are widely used; however, their supply is quite limited. For example, the International Association of Geoanalysts provides only a limited amount of zircon 91500 per laboratory, with the recommendation that it be used initially to establish an in-house reference material. It is also noted that some zircon standards are chemically inhomogeneous [19,20]. Thus, there is still demand for high-quality zircon standards.

This study introduces a new zircon reference material LKZ-1. We measured its U–Pb age and Hf isotopic composition using a high-resolution (HR)-SIMS, isotope dilution thermal ionization mass spectrometry (ID-TIMS), and LA-multiple collector (MC)-ICPMS. The oxygen isotopic composition of LKZ-1 was measured using a laser fluorination system and HR-SIMS. The chemical composition of LKZ-1 was measured using a quadrupole ICPMS connected to a femtosecond LA system.

2. Materials and Methods

2.1. LKZ-1

LKZ-1 is a single transparent pale yellow megacryst of gem-quality zircon with no visible inclusions or cracks (Figure 1). The total mass was 0.83 g. The LKZ-1 was imported from Sri Lanka by an online gem dealer and purchased by the Korea Basic Science Institute (KBSI) in 2014. No information about the nature of its host rock is available.

Our previous LA-MC-ICPMS analyses confirmed that this zircon is in a ²³⁸U–²³⁰Th radioactive equilibrium [21]. It is noted that the preliminary ion microprobe U–Pb data for LKZ-1, presented by Kim et al. (2015) [22], are revised in this study.

2.2. HR-SIMS U–Pb Analysis

Fragments of the LKZ-1 zircon were mounted in epoxy with reference zircons for in situ isotopic and chemical analyses. The U–Th–Pb isotopic analyses were conducted using a sensitive high-resolution ion microprobe (SHRIMP IIe/MC) at the KBSI Ochang Center. Before the SHRIMP analyses, cathodoluminescence (CL) and backscattered electron (BSE) images of the fragments were examined using a scanning electron microscope (SEM) (JEOL JSM-6610LV). 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 about 5000 at 1% peak height. We used the 91500 (²⁰⁶Pb/²³⁸U age = 1062.4 Ma, [15]) and SL13 (U = 238 ppm) standard zircons for Pb/U calibration and to determine the U abundance, respectively. The Pb/U ratios were calibrated against the 91500 zircon according to the power law relationship between Pb⁺/U⁺ and UO⁺/U⁺. The Th/U ratios were estimated using a fractionation factor derived from the measured ²³²Th¹⁶O⁺/²³⁸U¹⁶O⁺ versus ²⁰⁸Pb/²⁰⁶Pb of the SL13 standard. The common Pb was corrected using the ²⁰⁷Pb method [23]. Data processing was conducted using the SQUID 2.50 and Isoplot 3.75 software [24,25]. The weighted mean ages were calculated after excluding the outliers using the Student’s t-test and reported at the 95% confidence level.

2.3. ID-TIMS U–Pb Analysis

ID-TIMS U–Pb isotopic analyses were performed at the Jack Satterly Geochronology Laboratory in the Department of Earth Sciences, University of Toronto (Toronto, ON, Canada). Four fragments were selected for analyses. The fragments were chemically abraded prior to dissolution [26]. This process involved thermal annealing in a muffle furnace at 900 °C for 48 h followed by a chemical etch for 9 h in ~0.10 mL concentrated hydrofluoric acid and ~10 µL 8N nitric acid in Teflon dissolution vessels at 200 °C. The fragments were rinsed with distilled water, and washed in 8N HNO₃. A mixed ²⁰⁵Pb–²³⁵U spike was added to the Teflon dissolution capsules during sample loading. The zircon-spike mixture was dissolved using ~0.10 mL concentrated hydrofluoric acid and ~0.02 mL 8N nitric acid at 200 °C [27] for 5 days, dried to a precipitate, and re-dissolved in ~0.15 mL 3N hydrochloric acid. The U and Pb were isolated from the zircon solutions using anion exchange chromatography, dried in dilute phosphoric acid, and deposited onto outgassed rhenium filaments with silica gel [28]. The U and Pb were analyzed using a VG M354 mass spectrometer with multiple Faraday collectors in static mode for Pb and a single Daly pulse-counting system in dynamic mode for U measurements. The dead time of the Daly measuring system for U was 14.5 ns. The mass discrimination correction for the Daly detector was constant at 0.03%/atomic mass unit. The amplifier gains and Daly characteristics were monitored using the SRM 982 Pb standard. A thermal mass fractionation correction of 0.1(±0.05)% per atomic mass unit for Pb and U was applied. The total common Pb in each zircon analysis was assumed to have the isotopic composition of laboratory blank (²⁰⁶Pb/²⁰⁴Pb = 18.49 ± 0.4%; ²⁰⁷Pb/²⁰⁴Pb = 15.59 ± 0.4%; ²⁰⁸Pb/²⁰⁴Pb = 39.36 ± 0.4%). The total amount of common Pb in the present analyses ranged from 0.5 to 4.7 picograms. The Pb/U and Pb/Pb isotopic ratios were corrected for IMF, common Pb in the spike, and blank. The Th/U ratios were calculated from the radiogenic ²⁰⁸Pb/²⁰⁶Pb ratio and ²⁰⁷Pb/²⁰⁶Pb age assuming concordance. Corrections for ²³⁰Th disequilibrium in ²⁰⁶Pb/²³⁸U and ²⁰⁷Pb/²⁰⁶Pb were made assuming Th/U of 4.2 in the magma. We used the decay constants of Jaffey et al. (1971) [29] (²³⁸U and ²³⁵U are 1.55125 × 10⁻¹⁰ and 9.8485 × 10⁻¹⁰ per year, respectively). The ²³⁸U/²³⁵U ratio of 137.88 was used for the ²⁰⁷Pb/²⁰⁶Pb model age calculations. All the age errors quoted in the text and table, and error ellipses in the concordia diagram are given at 2σ. Plotting and age calculations were obtained using the Isoplot/Ex 3.00 software [30].

2.4. Oxygen Isotope Analysis

Zircon oxygen isotopes were measured using a laser fluorination system at the Korea Polar Research Institute (KOPRI), and the Cameca IMS 1280 ion probe at the SIMS laboratory of the Institute of Geology and Geophysics, Chinese Academy of Sciences (CAS) in Beijing, China.

The laser fluorination system at the KOPRI for oxygen isotopic analysis is well described elsewhere [31]. About 3 mg of the LKZ-1 zircon was loaded in the nickel sample holder, and then the sample holder was placed in the reaction chamber. The chamber that was assembled with a purification line was evacuated to 10⁻³ mbar or better, and heated for over 10 h by an external heater to eliminate the absorbed moisture inside the chamber. Before the sample was analysed, the chamber was pre-fluorinated by a small amount of BrF₅ for 1 h at room temperature to completely remove the moisture absorbed on the sample surfaces and inside the chamber. Any remaining gases in the reaction chamber were thoroughly evacuated by a diffusion pump (<10⁻⁴ mbar), and then sufficient BrF₅ was introduced into the chamber for laser fluorination of the sample. The sample was gradually heated using a defocused CO₂ infrared laser beam by automatically increasing the lasing power up to 60%. All the gaseous species derived from the samples were introduced into the purification line consisting of two cryogenic traps (liquid nitrogen, −196 °C) and a heated KBr getter. A molecular sieve (MS13X) pellet on the final cryogenic trap collected the pure O₂ for 10 min at liquid nitrogen temperature. The oxygen yield of the sample was calculated by the pressure value of the recovered O₂ gas from MS13X. The oxygen isotopic composition of the sample gas was analyzed using a dual-inlet isotope ratio mass spectrometer (MAT 253 Plus, Thermo Fisher Scientific, Waltham, MA, USA) connected on-line to the laser fluorination system. The measured ¹⁸O/¹⁶O ratios were normalized to the Vienna standard mean oceanic water (V-SMOW) (¹⁸O/¹⁶O = 0.0020052, [32]) and presented as δ¹⁸O notation. The long-term reproducibility of the laser fluorination system based on the repeated analyses (n = 28) of in-house obsidian standard is ±0.08‰ (1σ) for δ¹⁸O [31].

For the SIMS analyses at the CAS, the Gaussian focused Cs⁺ primary ion beam was accelerated at 10 kV, with an intensity of ~1.6 nA. The spot size was approximately 20 μm in diameter (10 μm beam + 10 μm raster). A normal incidence electron flood gun was used to compensate for sample charging. The magnetic field was stabilized using a nuclear magnetic resonance controller. Negative secondary ions were extracted with a potential of −10 kV. The field aperture was 6000 × 6000 μm². A ~120 μm entrance slit, 40 eV energy slit, ~133 transfer magnification, and 500 μm exit slit provided a mass resolution of ~2500 at 1% peak height. Under these conditions, the count rate of ¹⁶O^– was typically ~1 × 10⁹ cps/nA. The ¹⁶O and ¹⁸O ions were detected simultaneously using two Faraday cups with 10¹⁰ and 10¹¹ Ω resistors, respectively. The in-run precision was typically better than 0.2 ‰ (2 standard errors). The IMF was corrected based on the 91500 zircon value suggested by Valley (2003) [7] (δ¹⁸O_V-SMOW = 10.07 ± 0.03‰).

2.5. LA-MC-ICPMS U–Pb and Lu–Yb–Hf Isotope Analysis

Zircon U–Pb and Lu–Yb–Hf isotopes were measured using a Plasma II MC-ICPMS (Nu Instruments) equipped with an NWR193-nm ArF Excimer laser ablation system at the KBSI Ochang Center. The instrumental parameters for the U–Pb and Lu–Yb–Hf isotopic analyses are summarized in

Table 1. LA-MC-ICPMS instrument and operational parameters for U–Pb dating and Lu–Yb–Hf isotope analysis.

Parameter	U–Pb	Lu–Yb–Hf
MC-ICPMS
Instrument	Nu Plasma II
RF power	1300 W	1300 W
Reflected power	<1 W	<1 W
Mixed gas & flow rate	Ar, ~0.9 L/min	Ar, ~0.7 L/min
Auxiliary gas & flow rate	Ar, 0.9 L/min	Ar, 0.9 L/min
Cool gas & flow rate	Ar, 13 L/min	Ar, 13 L/min
Sampler cone	Ni (1 mm orifice)	Ni (1 mm orifice)
Skimmer cone	Ni (0.7 mm orifice)	Ni (0.7 mm orifice)
Data acquisition mode	Time resolved analysis	Time resolved analysis
Integration time	0.2 s	0.2 s
Collectors	2 Faraday cups and 5 ion counters	10 Faraday cups
Measured isotopes	238U, 232Th, 208Pb, 207Pb, 206Pb, 204Pb, 202Hg	172Yb, 173Yb, 174(Hf + Yb), 175Lu, 176(Hf + Yb + Lu), 177Hf, 178Hf, 179Hf, 180Hf, 183W
Laser ablation system
Laser	ESI machines 193nm ArF excimer laser
Ablation mode	Single hole drilling
Cell	Two volume 2 cell (10 × 10 cm)
Sample transport tubing	0.5 m length	0.5m length
Pulse repetition rate & width	5Hz, <4 ns	10Hz, <4 ns
Pulse energy density	~3 J/cm2	~ 8 J/cm2
Ablation duration	30 s	60 s
Carrier gas & flow rate	He, 0.65 L/min	He, 0.65 L/min; N2, 0.002 L/min
Crater size	~15 µm	~50 µm
Drill depth	~4 µm	~20 µm

. The raw data were processed using Iolite 2.5 within the Igor Pro 6.3.5.5 software [33], and corrected for the background. The instrumental mass discrimination and laser-induced elemental fractionation during the U–Pb analysis were corrected by calibration against the 91500 zircon. No common Pb correction was performed. The ages were calculated using the Isoplot 3.75 software [25]. All the ratios were calculated with 2σ errors.

The ¹⁷⁶Lu and ¹⁷⁶Yb interferences on the ¹⁷⁶Hf signal were corrected using the isotopic values suggested by Chu et al. (2002) [34] and Vervoort et al. (2004) [35], respectively, on the basis of previous MC-ICPMS results for mixed standard solutions [36]. The mass bias of the measured Hf isotopic ratios was corrected to ¹⁷⁹Hf/¹⁷⁷Hf = 0.7325 using an exponential correction law. The ¹⁷⁶Lu/¹⁷⁷Hf and ¹⁷⁶Yb/¹⁷⁷Hf ratios were calculated following the method of Iizuka and Hirata (2005) [37]. The ε_Hf values were calculated using a ¹⁷⁶Lu decay constant of 1.865 × 10⁻¹¹ per year [38] and the chondritic values suggested by Blichert-Toft and Albarède (1997) [39].

2.6. LA-Quadrupole ICPMS Analysis

The minor and trace elements were analyzed using a 343 nm femtosecond laser ablation microprobe (J200 LA model, Applied Spectra Inc., Fremont, CA, USA) coupled to an iCapQ model (Thermo Fisher Scientific, Bremen, Germany) quadrupole ICPMS at the Core Research Facilities, Pusan National University, Korea. The zircon surfaces were ablated at a pulse repetition rate of 10 Hz and a pulse energy of 250 μJ, producing a ~30-μm-diameter crater pit. Helium (700 mL/min) was flushed into the sample cell to increase the sample transport efficiency and reduce aerosol deposition around the ablation pit [40]. The instrumental parameters of ICPMS were optimized to provide the highest sensitivity, whilst maintaining the ratio of ThO⁺/Th⁺ below 0.005. The zircon data were externally calibrated using NIST 610 standard glass, and internally normalized using ²⁹Si and ⁹¹Zr. The reference values for the NIST 610 were taken from Jochum et al. (2011) [41], and the stoichiometric values of zircon (Si = 15.3%, Zr = 47.6%) were used for normalization.

3. Results and Discussion

3.1. U–Pb Age

As shown in Figure 1, the LKZ-1 fragments were relatively dark under CL. The CL and BSE images showed no clear zonation patterns. The HR-SIMS (SHRIMP) U–Th–Pb isotope data are presented in

Table 2. SHRIMP U–Th–Pb results for the LKZ-1 zircon.

Spot No.	206Pbc1 (%)	206Pb/238U2	±%	207Pb/206Pb2	±%	Th (ppm)	U (ppm)	Disc.%	Date3 (Ma)
LKZ_1.1	0.01	0.2322	2.62	0.0590	0.84	97.4	652	−1.1	570.3	±	4.5
LKZ_1.2	0.04	0.2310	3.17	0.0595	0.85	93.4	636	0.6	569.1	±	4.5
LKZ_2.1	0.13	0.2220	0.45	0.0592	0.88	87.0	603	−6.3	565.7	±	9.4
LKZ_2.2	0.07	0.2157	2.50	0.0590	0.89	87.7	607	−3.8	566.9	±	4.5
LKZ_3.1	0.05	0.2227	2.80	0.0600	0.87	90.3	619	2.7	572.5	±	4.6
LKZ_3.2	0.07	0.2299	1.86	0.0591	2.31	86.3	597	−3.7	572.2	±	4.6
LKZ_4.1	0.06	0.2272	1.53	0.0602	0.86	92.4	626	3.2	572.4	±	4.6
LKZ_4.2	0.01	0.2210	2.11	0.0603	0.88	92.6	631	6.4	570.5	±	4.5
LKZ_5.1		0.2248	1.42	0.0587	0.89	90.3	621	−3.8	580.8	±	4.7
LKZ_5.2		0.2189	1.76	0.0584	0.89	90.3	627	−5.9	577.8	±	4.6
LKZ_6.1		0.2224	2.14	0.0600	0.88	89.1	619	8.3	568.0	±	4.5
LKZ_6.2		0.2185	0.45	0.0591	0.89	96.3	656	1.1	580.1	±	4.6
LKZ_7.1	0.07	0.2261	1.90	0.0593	1.55	82.0	576	−2.8	573.4	±	4.6
LKZ_7.2	0.08	0.2294	1.41	0.0599	0.89	86.4	599	0.0	574.3	±	4.6
LKZ_8.1	0.16	0.2272	1.78	0.0587	1.69	89.7	620	−15.1	580.0	±	6.8
LKZ_8.2	0.03	0.2228	1.98	0.0583	0.91	89.8	619	−9.2	580.9	±	7.8
LKZ_9.1	0.01	0.2261	1.76	0.0586	0.91	85.4	598	−4.6	572.6	±	4.6
LKZ_9.2		0.2245	2.20	0.0587	1.57	86.8	607	−3.6	578.7	±	4.7
LKZ_10.1		0.2275	2.00	0.0598	0.85	96.8	649	6.2	571.1	±	4.6
LKZ_10.2	0.02	0.2303	2.05	0.0596	0.85	97.7	652	2.0	570.2	±	4.6
LKZ_11.1		0.2313	1.84	0.0593	0.88	87.7	602	3.8	564.7	±	4.6
LKZ_11.2	0.07	0.2297	1.72	0.0572	0.99	89.3	608	−19.7	571.3	±	4.6

¹²⁰⁴Pb-based common ²⁰⁶Pb proportion; ² Uncorrected data; ^{3 207}Pb-corrected ²⁰⁶Pb/²³⁸U date.

. The common Pb proportion in ²⁰⁶Pb, calculated using the ²⁰⁴Pb counts and an assumed Pb isotopic composition after the model of Stacey and Kramers (1975) [42], was mostly less than 0.1%. The SHRIMP analyses produced consistently concordant data (Figure 2). The ²⁰⁷Pb-corrected data generated a weighted mean ²⁰⁶Pb/²³⁸U age of 572.6 ± 2.0 Ma (n = 22, MSWD = 0.90), which was consistent with the ²⁰⁴Pb-corrected ²⁰⁷Pb/²⁰⁶Pb age (weighted mean = 573 ± 12 Ma, n = 21, MSWD = 1.4). The relatively uniform concentrations of U (619 ± 21 ppm, 1 standard deviation, same hereafter unless otherwise stated) and Th (90.2 ± 4.2 ppm) yielded highly consistent Th/U ratios of 0.146 ± 0.002. The Plešovice zircon (ID-TIMS ²⁰⁶Pb/²³⁸U age = 337.13 ± 0.37 Ma; [19]), analyzed together with LKZ-1, yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 338.7 ± 1.4 Ma (n = 15, MSWD = 0.81) (Table S1).

The LA-MC-ICPMS U–Pb results for the LKZ-1 zircon are listed in

Table 3. LA-MC-ICPMS U–Pb results for the LKZ-1 zircon.

Spot No.	206Pb/238U	2SE	Date (Ma)	2SE	207Pb/206Pb	2SE	Date (Ma)	2SE
LKZ-1-1	0.0929	0.0011	574.1	6.6	0.0594	0.0006	580	11
LKZ-1-2	0.0934	0.0011	575.6	6.2	0.0589	0.0007	562	17
LKZ-1-3	0.0931	0.0008	573.9	4.7	0.0593	0.0005	571	10
LKZ-1-4	0.0928	0.0013	572.2	7.7	0.0592	0.0007	564	13
LKZ-1-5	0.0924	0.0008	569.8	4.6	0.0589	0.0005	566	12
LKZ-1-6	0.0942	0.0013	580.4	7.8	0.0589	0.0005	566	10
LKZ-1-7	0.0935	0.0013	576.9	7.5	0.0592	0.0005	572	10
LKZ-1-8	0.0943	0.0014	581.0	8.0	0.0589	0.0005	568	10
LKZ-1-9	0.0944	0.0015	581.4	9.0	0.0588	0.0005	559	11
LKZ-1-10	0.0937	0.0015	577.9	8.7	0.0587	0.0004	555	9
LKZ-1-11	0.0935	0.0015	576.1	8.6	0.0587	0.0005	559	11
LKZ-1-12	0.0923	0.0013	569.1	7.9	0.0587	0.0005	553	11
LKZ-1-13	0.0932	0.0015	574.3	8.9	0.0589	0.0005	561	12
LKZ-1-14	0.0923	0.0013	568.8	7.5	0.0585	0.0006	547	12
LKZ-1-15	0.0936	0.0016	576.6	9.5	0.0584	0.0006	547	13
LKZ-1-16	0.0927	0.0008	571.2	4.8	0.0588	0.0006	561	14
LKZ-1-17	0.0923	0.0009	569.0	5.5	0.0592	0.0005	577	10
LKZ-1-18	0.0932	0.0009	574.4	5.2	0.0591	0.0005	565	10
LKZ-1-19	0.0928	0.0008	572.0	4.6	0.0585	0.0005	559	9
LKZ-1-20	0.0934	0.0009	575.4	5.1	0.0588	0.0006	554	12
LKZ-1-21	0.0943	0.0015	580.8	8.8	0.0590	0.0011	574	19
LKZ-1-22	0.0927	0.0009	571.6	5.3	0.0591	0.0007	567	14
LKZ-1-23	0.0933	0.0010	575.1	5.9	0.0589	0.0008	573	14
LKZ-1-24	0.0936	0.0012	576.9	7.2	0.0584	0.0007	532	16
LKZ-1-25	0.0933	0.0008	574.7	4.7	0.0587	0.0006	553	13
LKZ-1-26	0.0941	0.0013	579.9	7.5	0.0587	0.0007	563	14
LKZ-1-27	0.0941	0.0013	579.3	7.7	0.0586	0.0005	552	12
LKZ-1-28	0.0939	0.0015	578.5	9.0	0.0583	0.0006	544	14
LKZ-1-29	0.0938	0.0015	577.6	8.9	0.0583	0.0006	541	12
LKZ-1-30	0.0941	0.0014	579.9	8.1	0.0588	0.0007	547	14

, and shown graphically in Figure 3. The LA-MC-ICPMS analyses produced concordant age data, with a weighted mean ²⁰⁶Pb/²³⁸U age of 574.8 ± 1.3 Ma (n = 28, MSWD = 1.13).

Four U-Pb ID-TIMS zircon analyses produced two overlapping, concordant results and two that were slightly discordant (1.2%), but within error of the concordant results (

Table 4. U–Pb isotopic data for the LKZ-1 zircon obtained by ID-TIMS.

No.	Weight (mg)	U (ppm)	Th/U	Pbc (pg)	206Pb/204Pb	207Pb/235U	2σ	206Pb/238U	2σ	Error Corr.
sk40p10	0.151	464	0.149	1.4	292650	0.7524	0.0026	0.09216	0.00029	0.976
sk40p11	0.165	494	0.147	2.0	238760	0.7571	0.0019	0.09273	0.00020	0.959
sk40p12	0.101	818	0.147	4.7	104445	0.7583	0.0049	0.09286	0.00057	0.935
sk40p42	0.060	183	0.150	0.5	118809	0.7540	0.0018	0.09232	0.00017	0.925
No.	207Pb/206Pb	2σ	206Pb/238U Age (Ma)	2σ	207Pb/235U Age (Ma)	2σ	207Pb/206Pb Age (Ma)	2σ	Disc. %
sk40p10	0.059212	0.000046	568.31	1.74	569.63	1.51	574.90	1.67	1.2
sk40p11	0.059218	0.000045	571.64	1.17	572.34	1.10	575.13	1.66	0.6
sk40p12	0.059224	0.000136	572.43	3.35	573.02	2.84	575.34	5.00	0.5
sk40p42	0.059237	0.000056	569.23	1.00	570.55	1.01	575.81	2.04	1.2

Pbc is total common Pb, and assumes all has laboratory blank isotopic composition; Error Corr. is correlation coefficients of X-Y errors on the concordia plot; Disc. is percent discordance for the given ²⁰⁷Pb/²⁰⁶Pb age.

, Figure 4). Four U-Pb data together yielded a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 575.22 ± 0.98 Ma (MSWD = 0.16, probability = 0.92). The weighted mean ²⁰⁶Pb/²³⁸U age of the four data was 570.0 ± 2.5 Ma (MSWD = 5.4, probability = 0.001), but if only the two concordant results are considered, a more precise ²⁰⁶Pb-²³⁸U age of 571.7 ± 1.1 Ma is obtained. The Th/U ratios (0.148 ± 0.001) were consistent with those from the SHRIMP analyses (0.146 ± 0.002). The ID-TIMS results may indicate that discordant parts remained in the analyzed fragments in spite of thermal annealing and chemical etching. The discordance of U–Pb age is not attributable to metamictization because the U contents and ²⁰⁶Pb/²³⁸U dates of the SHRIMP spots did not show any particular trend (Figure S1). If the discordance of our ID-TIMS ages reflects the thermal disturbance experienced by LKZ-1, then the concordance of SHRIMP U–Pb dates may have been caused by the selective analysis of inner, unaffected parts of the crystal. The SEM observation was not helpful to identify the discordant parts because the fragments appeared homogeneous in CL and BSE (Figure 1).

In fact, zircon megacrysts commonly show U–Pb age discordance [6]. The discordancy is typically small. For example, the ID-TIMS results for the 91500 zircon reported by Wiedenbeck et al. (1995) [15] were ~0.3% discordant (²⁰⁷Pb/²⁰⁶Pb age = 1065.4 ± 0.3 Ma, ²⁰⁶Pb/²³⁸U age = 1062.4 ± 0.4 Ma). The ID-TIMS results for SL13, listed by Claoué-Long et al. (1995) [43], were also discordant. The weighted mean ²⁰⁷Pb/²⁰⁶Pb and ²⁰⁶Pb/²³⁸U ages were 576.3 ± 0.8 Ma and 572.1 ± 0.4 Ma, respectively. The discordance of SL13 data was attributed to the extraneous addition of ²³¹Pa into the crystal or uncertainties of the U decay constants [44,45]. The identical U–Pb ages of the SL13 and LKZ-1 zircons may represent an important geologic event in Sri Lanka.

We conclude that the best ²⁰⁶Pb/²³⁸U ratio of LKZ-1 is equivalent to the SHRIMP age (572.6 ± 2.0 Ma), considering that discordant parts in the zircon fragments may have been included in the ID-TIMS analyses. As stated earlier, this age is consistent with the ID-TIMS ²⁰⁶Pb/²³⁸U age constrained by concordant results (571.7 ± 1.1 Ma).

3.2. O and Lu–Y–Hf Isotopic Compositions

The O and Lu–Yb–Hf isotopic compositions of LKZ-1 are presented in

Table 5. HR-SIMS O isotopic results for the LKZ-1 zircon.

Spot No.	δ18OV-SMOW (‰)	2SE	Spot No.	δ18OV-SMOW (‰)	2SE
LKZ-1@1	10.58	0.18	LKZ-1@11	10.77	0.19
LKZ-1@2	10.79	0.28	LKZ-1@12	10.88	0.22
LKZ-1@3	10.52	0.25	LKZ-1@13	10.48	0.21
LKZ-1@4	10.37	0.34	LKZ-1@14	10.68	0.12
LKZ-1@5	10.59	0.43	LKZ-1@15	10.63	0.29
LKZ-1@6	10.60	0.33	LKZ-1@16	10.62	0.28
LKZ-1@7	10.96	0.28	LKZ-1@17	10.54	0.23
LKZ-1@8	10.60	0.45	LKZ-1@18	10.81	0.31
LKZ-1@9	10.56	0.32	LKZ-1@19	10.74	0.17
LKZ-1@10	10.69	0.23	LKZ-1@20	10.56	0.31
			Average	10.65
			SD	0.14

Laser fluorination value = 10.72 ± 0.02‰ (1SD).

and

Table 6. LA-MC-ICPMS Lu–Yb–Hf results for the LKZ-1 zircon.

Spot No.	176Hf/177Hf	2SE (ppm)	178Hf/177Hf	2SE (ppm)	176Lu/177Hf	2SE (%)	176Yb/177Hf	2SE (%)	εHf(0)	εHf(t)
LKZ-1	0.281788	92	1.467234	45	0.000104	0.13	0.00394	0.64	−34.8	−22.3
LKZ-2	0.281816	89	1.467252	53	0.000104	0.12	0.00392	0.61	−33.8	−21.3
LKZ-3	0.281776	96	1.467270	75	0.000102	0.14	0.00388	0.62	−35.2	−22.7
LKZ-4	0.281805	82	1.467186	57	0.000104	0.13	0.00394	0.56	−34.2	−21.7
LKZ-5	0.281808	85	1.467226	48	0.000104	0.12	0.00395	0.63	−34.1	−21.6
LKZ-6	0.281800	85	1.467237	42	0.000104	0.12	0.00393	0.61	−34.4	−21.8
LKZ-7	0.281803	92	1.467215	51	0.000103	0.13	0.00393	0.64	−34.3	−21.7
LKZ-8	0.281785	89	1.467261	50	0.000104	0.13	0.00392	0.61	−34.9	−22.4
LKZ-9	0.281777	92	1.467226	58	0.000104	0.13	0.00393	0.61	−35.2	−22.6
LKZ-10	0.281779	82	1.467236	55	0.000104	0.14	0.00393	0.54	−35.1	−22.6
LKZ-11	0.281789	92	1.467268	55	0.000103	0.16	0.00376	0.48	−34.8	−22.2
LKZ-12	0.281796	96	1.467236	57	0.000106	0.17	0.00393	0.46	−34.5	−22.0
LKZ-13	0.281797	92	1.467261	61	0.000103	0.16	0.00379	0.45	−34.5	−21.9
LKZ-14	0.281805	96	1.467233	51	0.000106	0.18	0.00397	0.53	−34.2	−21.7
LKZ-15	0.281792	96	1.467264	65	0.000103	0.15	0.00383	0.47	−34.7	−22.1
LKZ-16	0.281815	85	1.467229	44	0.000103	0.14	0.00391	0.59	−33.8	−21.3
LKZ-17	0.281807	82	1.467233	51	0.000104	0.12	0.00392	0.54	−34.1	−21.6
LKZ-18	0.281790	89	1.467161	59	0.000104	0.14	0.00397	0.55	−34.7	−22.2
LKZ-19	0.281793	89	1.467256	57	0.000103	0.13	0.00392	0.61	−34.6	−22.1
LKZ-20	0.281811	85	1.467302	62	0.000104	0.14	0.00394	0.58	−34.0	−21.4
LKZ-21	0.281805	99	1.467251	50	0.000102	0.17	0.00377	0.50	−34.2	−21.7
LKZ-22	0.281773	89	1.467230	52	0.000103	0.15	0.00381	0.47	−35.3	−22.8
LKZ-23	0.281792	89	1.467250	52	0.000102	0.16	0.00381	0.55	−34.7	−22.1
LKZ-24	0.281772	99	1.467269	55	0.000102	0.16	0.00377	0.53	−35.4	−22.8
LKZ-25	0.281779	85	1.467225	52	0.000102	0.15	0.00385	0.65	−35.1	−22.6
LKZ-26	0.281775	106	1.467352	44	0.000105	0.21	0.00310	0.52	−35.3	−22.7
LKZ-27	0.281756	99	1.467289	42	0.000107	0.20	0.00320	0.50	−35.9	−23.4
LKZ-28	0.281805	92	1.467365	39	0.000104	0.19	0.00318	0.47	−34.2	−21.7
LKZ-29	0.281795	103	1.467350	44	0.000104	0.19	0.00320	0.50	−34.6	−22.0
LKZ-30	0.281825	103	1.467347	44	0.000104	0.19	0.00323	0.50	−33.5	−21.0
LKZ-31	0.281798	117	1.467337	48	0.000104	0.21	0.00318	0.53	−34.4	−21.9
LKZ-32	0.281787	103	1.467351	44	0.000103	0.20	0.00318	0.53	−34.8	−22.3
LKZ-33	0.281787	103	1.467360	42	0.000105	0.19	0.00324	0.49	−34.8	−22.3
LKZ-34	0.281787	110	1.467324	46	0.000101	0.23	0.00307	0.59	−34.8	-22.3
LKZ-35	0.281791	106	1.467343	40	0.000105	0.21	0.00321	0.56	−34.7	−22.2
LKZ-36	0.281793	103	1.467316	44	0.000103	0.23	0.00318	0.57	−34.6	−22.1
LKZ-37	0.281815	106	1.467329	41	0.000104	0.23	0.00321	0.56	−33.8	−21.3
LKZ-38	0.281805	103	1.467327	42	0.000108	0.24	0.00334	0.54	−34.2	−21.7
LKZ-39	0.281821	96	1.467361	42	0.000106	0.21	0.00328	0.55	−33.6	−21.1
LKZ-40	0.281799	131	1.467363	40	0.000106	0.21	0.00328	0.61	−34.4	−21.9
LKZ-41	0.281838	106	1.467339	36	0.000105	0.20	0.00327	0.55	−33.0	−20.5
LKZ-42	0.281789	117	1.467322	42	0.000103	0.23	0.00315	0.60	−34.8	−22.2
LKZ-43	0.281771	114	1.467375	40	0.000101	0.26	0.00311	0.64	−35.4	−22.9
LKZ-44	0.281777	124	1.467349	36	0.000104	0.23	0.00320	0.63	−35.2	−22.7
LKZ-45	0.281785	131	1.467365	41	0.000101	0.21	0.00315	0.64	−34.9	−22.4
Average	0.281794		1.467286		0.000104		0.00358		−34.6	−22.0
SD	0.000016		0.000057		0.000001		0.00035		0.6	0.6

, respectively. The δ¹⁸O_V-SMOW value of LKZ-1 measured by the laser fluorination system was 10.72 ± 0.02 ‰. The oxygen yield of LKZ-1 was 97.8%, meaning that almost all the oxygen was released from the sample. Thus, any kinetic isotopic fractionation effect caused by incomplete fluorination of sample can be ignored.

During the analytical session at the CAS, the 91500 and Penglai [46] zircons yielded an average measured δ¹⁸O_V-SMOW value of 15.16 ± 0.17 ‰ and 10.40 ± 0.21‰, respectively. Detailed data, including ¹⁶O intensities, ¹⁸O/¹⁶O ratios, and spot locations on the mount, are given in Table S2. The interlaboratory calibration work of Wiedenbeck et al. (2004) for the 91500 zircon [47] reported laser fluorination δ¹⁸O_V-SMOW values between 10.07‰ and 9.74‰. Although the simple mean was 9.86 ± 0.11‰ (n = 13), the high-end value (10.07‰) was preferred in this study to correct the IMF because the simple mean yielded a slightly underestimated δ¹⁸O_V-SMOW average (5.08‰) for the Penglai zircon (laser fluorination δ¹⁸O_V-SMOW = 5.31 ± 0.12‰; [46]). An IMF value of –5.09‰, with reference to the 91500 zircon value of 10.07‰, generated an average δ¹⁸O_V-SMOW value of 10.65 ± 0.14‰ for the LKZ-1 zircon (Figure 5). This value was indistinguishable from that obtained by the laser fluorination method (10.72 ± 0.02‰). The Penglai zircon measured during several analytical sessions yielded an IMF-corrected δ¹⁸O_V-SMOW of 5.29 ± 0.34 ‰ (n = 23) (Table S2).

The LA-MC-ICPMS analyses produced an average ¹⁷⁶Hf/¹⁷⁷Hf of 0.282294 ± 0.000021 (n = 41) for the 91500 zircon (Table S3), which was consistent with the MC-ICPMS result obtained by solution chemistry (0.282308 ± 0.000006; [48]). The ¹⁷⁶Lu/¹⁷⁷Hf results (0.00028 ± 0.00010) for the 91500 zircon (Table S3) were also consistent with the solution chemistry-based data (0.00031 ± 0.00007; [48]). The LKZ-1 zircon showed little intra- and inter-grain variation in ¹⁷⁶Hf/¹⁷⁷Hf (0.281794 ± 0.000016; Figure 6), ¹⁷⁶Lu/¹⁷⁷Hf (0.000104 ± 0.000001), and ¹⁷⁶Yb/¹⁷⁷Hf (0.00358 ± 0.00035) (n = 45). The ¹⁷⁸Hf/¹⁷⁷Hf ratios were indistinguishable between 91500 (1.467289 ± 0.000059) and LKZ-1 (1.467286 ± 0.000057), with their ¹⁷⁶Lu/¹⁷⁷Hf and ¹⁷⁶Yb/¹⁷⁷Hf ratios showing a good positive correlation (R² = 0.97). The initial ε_Hf values calculated from the ¹⁷⁶Hf/¹⁷⁷Hf and ¹⁷⁶Lu/¹⁷⁷Hf ratios and SHRIMP ²⁰⁶Pb/²³⁸U age (572.6 Ma) varied narrowly and gave an average value of –22.0 ± 0.6. The ¹⁷⁶Hf/¹⁷⁷Hf and ¹⁷⁶Lu/¹⁷⁷Hf ratios did not show a clear trend (Figure S2).

3.3. Chemical Composition

The LA-quadrupole ICPMS results for the 91500 and LKZ-1 zircons are summarized in

Table 7. Summary of chemical data for the 91500 and LKZ-1 zircons (unit: ppm).

								91500								LKZ-1
	[15,47]			[49]			[50]			[51]				This Study	SD (n = 37)	This Study	SD (n = 28)
P	35	±	17											14	7	17	8
Ca														102	66	68	50
Y	153	±	14	147	±	22								148	36	86	6
La	0.013	±	0.01	<0.08	±	0.05	0.014	±	0.011	0.031	±	0.038		0.013	0.025	0.003	0.002
Ce	2.56	±	0.20	2.5	±	0.5	2.59	±	0.43	2.77	±	0.41		2.19	0.34	0.84	0.08
Pr	0.020	±	0.01	<0.05	±	0.05	0.035	±	0.015	0.022	±	0.021		0.019	0.027	0.015	0.012
Nd	0.25	±	0.01	<0.35	±	0.07	0.295	±	0.064	0.324	±	0.085		0.25	0.14	0.15	0.05
Sm	0.38	±	0.03	0.41	±	0.2	0.278	±	0.084	0.624	±	0.246		0.52	0.24	0.29	0.08
Eu	0.19	±	0.02	0.37	±	0.17	0.191	±	0.065	0.310	±	0.112		0.26	0.11	0.09	0.03
Gd	1.76	±	0.39	2.1	±	0.35	1.60	±	0.39	2.719	±	0.658		2.50	0.90	1.53	0.17
Tb	0.78	±	0.12				0.75	±	0.20	1.11	±	0.25		0.94	0.31	0.61	0.06
Dy	10.3	±	0.9	12	±	2.1	12.3	±	2.5	14.2	±	2.5		11.8	3.4	7.3	0.6
Ho	4.60	±	0.46	4.9	±	0.7	5.1	±	1.4	5.83	±	0.90		4.92	1.37	2.93	0.26
Er	23.7	±	2.6	26	±	3.6	28.6	±	6.2	28.2	±	4.6		27.1	7.0	15.3	1.2
Tm	5.95	±	0.71				7.5	±	1.8	8.44	±	1.27		6.81	1.63	3.44	0.32
Yb	60.1	±	8.4	66	±	7.3	77	±	18	101	±	16		69	17	32	3
Lu	14.1	±	1.7	14	±	1.9	17.1	±	3.8	13.9	±	2.0		13.8	3.1	5.9	0.5
Hf	5895			6400	±	900								5780	340	7740	310
Th	28.61	±	0.07	31	±	4.9								30	6	94	6
U	81.2			88	±	15								83	14	654	31

For LKZ-1, U and Th concentrations were normalized by ²⁹Si. All the other elements were normalized by ⁹¹Zr. [15]; ID-TIMS results (Hf, Th, U), [47]; P, EPMA result. Y and REE concentrations are working values from inter-laboratory SIMS analyses. [49,51]; LA-ICPMS results, [50]; HR-SIMS results.

with literature data [15,47,49,50,51]. The individual spot data analysed in this study are listed in Table S4.

For the 91500 zircon, the internal normalization by ⁹¹Zr generated concentration results close to the literature data, except for U and Th. We employed ²⁹Si-normalization for the calculation of U and Th concentrations because the ⁹¹Zr-normalization for these elements yielded results consistently ~15% lower than the recommended values. The ²⁹Si-normalized results for U (83 ± 14 ppm) and Th (30 ± 6 ppm), and the ⁹¹Zr-normalized result for Hf (5780 ± 340 ppm) agreed well with the ID-TIMS data ([U] = 81.2 ppm, [Th] = 28.61 ppm, [Hf] = 5895 ppm; [15]). The ⁹¹Zr-normalized results for P (14 ± 7 ppm) and Y (148 ± 36 ppm) were marginally or closely similar to the literature data [[P] = 35 ± 17 ppm, [Y] = 153 ± 14 ppm; [47]). As shown in Figure 7a, the chondrite-normalized pattern of our ⁹¹Zr-normalized rare earth element (REE) data generally matched the recommended values. The slightly negative Eu anomalies in our data (Eu/Eu^* = 0.72 ± 0.17) are consistent with the data previously obtained by inter-laboratory SIMS analyses [47].

For most elements >1 ppm, the relative standard deviation values for the concentration data for the LKZ-1 zircon were <10%. The chondrite-normalized REE pattern of LKZ-1 (Figure 7b) was highly consistent, with prominently positive Ce (Ce/Ce^* = 44 ± 32) and negative Eu (Eu/Eu^* = 0.43 ± 0.17) anomalies. The ratios between the heavy and middle REEs were also consistent ((Lu/Gd)_N = 31 ± 3). The U (654 ± 31 ppm) and Th (94 ± 6 ppm) concentrations, and their ratios (Th/U = 0.144 ± 0.004), were comparable to the SHRIMP results. The positive correlations observed between U and Th concentrations and Yb and Y concentrations (Figure 7c–f) indicate that our data reflect inter- and intra-grain chemical variation, rather than random analytical errors.

4. Conclusions

The results of our ID-TIMS, HR-SIMS, and LA-MC-ICPMS analyses demonstrate that the LKZ-1 zircon is nearly concordant in U–Pb age, with negligible common Pb content and a consistent Th/U ratio. LKZ-1 is fairly homogeneous in O–Hf isotopic and chemical composition. These consistencies suggest that LKZ-1 can be used as a suitable working standard for geochronological and isotopic/geochemical analyses. Fragments of the LKZ-1 zircon are available upon request from the lead author of this paper.