Exploring the CAM18 Crystal as a Potential Reference Material for U–Pb Analysis of Zircon

Abstract

In the process of in situ zircon U–Pb dating, it is an effective means to overcome the matrix effect by using a matrix-matched external reference material. However, the limited number of available zircon reference materials still makes it difficult to meet the research needs. In this paper, we performed a preliminary analysis of the gemological characteristics, trace elements and U–Pb ages of natural zircon CAM18 to assess its suitability as a reference material for laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb dating. This tawny, gem-quality zircon has no visible inclusions and weighs approximately 0.55 g. Its density, full width at half maximum (FWHM) of the Raman peak and alpha flux (Dα) indicate that the sample has suffered mild-to-moderate radiation damage without any thermal treatment. The LA-ICP-MS U–Pb dating results reveal that the trace elements content and U–Pb ages of the sample are fairly homogeneous at the 50 μm scale, and there is no obvious loss of radiogenic Pb. The ²⁰⁶Pb/²³⁸U age (571.0 ± 3.0 Ma, 2s) and ²⁰⁷Pb/²³⁵U age (573.4 ± 6.0 Ma, 2s) are consistent within the analytical uncertainty, and the calculated concordia age is 571.4 ± 1.4 Ma (2s, n = 20). The variation in the ²⁰⁶Pb/²³⁸U ages is small, with a measurement repeatability of 0.46% (RSD), which is within the uncertainty of the age accuracy obtained by LA-ICP-MS. The oscillatory zoning, Th/U ratio (0.2) and chondrite-normalized rare-earth element (REE) pattern imply a magmatic origin of zircon CAM18. The Ti-in-zircon temperature ranges from 714 to 742 °C, and the oxygen fugacity ranges from ΔFMQ−2.87 to ΔFMQ−3.17, suggesting that it is crystallized in a reducing environment. All the results show that zircon CAM18 may has great potential in LA-ICP-MS U–Pb dating.

1. Introduction

Gem-quality zircon has a subadamantine luster, high dispersion and high refractive index, giving it a diamond-like appearance. It is favored by consumers because it exhibits a series of rich colors, such as colorless, green, red, orange and yellow. Additionally, zircon is an accessory mineral found in magmatic, sedimentary and metamorphic rocks, possessing the following advantages: high U and Th content, low common Pb content, high closure temperature (>750 °C) and stable physico-chemical properties [1,2,3]. These properties make zircon an excellent dating mineral that can effectively record the timing of various geological events [4,5].

The chemical formula for zircon is ZrSiO₄, and it may contain small amounts of Hf, U, Th, Ti, Y, P and rare-earth elements (REEs) due to the isomorphic substitutions of Zr⁴⁺ and Si⁴⁺ in the structure. The crystal structure of zircon is composed of isolated [SiO₄] tetrahedra and [ZrO₈] dodecahedra. In the a-axis and b-axis directions, each [ZrO₈] dodecahedron is connected to the surrounding four [ZrO₈] dodecahedra by a shared edge, forming a long chain in the a-axis direction. In the c-axis direction, the [SiO₄] tetrahedra and the [ZrO₈] dodecahedra form [001] strong chains by means of sharing edges and corners. This structure determines that zircon often develops a prismatic habit, with high birefringence and hardness [6,7].

In recent years, the spatial resolution and analysis efficiency of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) have been rapidly developed, making it the primary method for in situ U–Pb dating and trace element analysis [8,9]. However, during laser ablation, aerosol transport and ionization in ICP, instruments often produce elemental fractionation, impacting the precision and accuracy of dating results. The utilization of reference materials offers a useful method to ameliorate these problems. First, it can help us to become acquainted with the operating status of the instrument to determine the optimal instrument parameters; second, it can well reflect the dynamic change in detector sensitivity and monitor the analytical accuracy; third, it can be used to calibrate the U–Pb data of unknown samples.

Many zircon reference materials often possess some crucial dating conditions, such as a homogeneous chemical composition and structure, high ²⁰⁶Pb/²⁰⁴Pb ratios (˃10,000, e.g., SA01), moderate content of U (tens to hundreds of ppm, e.g., 91500), few mineral inclusions and an appropriate volume (grains several mm to cm in diameter) [2,10,11]. Research on zircon reference materials has brought fruitful results, including 91500, GJ-1, M257, Plešovice, QGNG and FC-1, of which the first three zircons are gem-quality [10,11,12,13,14,15]. Nevertheless, the use of these zircon reference materials is mostly limited by small amounts of storage or difficulty in mining and can hardly meet the long-term development needs of laboratories. Therefore, it is urgent to develop novel zircon reference materials.

Compared with multi-crystal zircons, the advantages of gem-quality zircons are that (1) they have stable structures as well as uniform chemical compositions; (2) they have no obvious radiation damage; (3) they contain relatively few cracks and mineral inclusions; (4) they are more likely to be concordant at the accepted age. Thus, in this paper, a gem-quality zircon from Cambodia was studied in terms of spectroscopic analyses, trace elements and isotope measurements. On the one hand, this was accomplished to explore whether the crystal has the potential to be a reference material for LA-ICP-MS U–Pb dating; on the other hand, it was accomplished in order to discuss its possible genesis and provide an important basis for subsequent provenance studies. In addition, this study can provide useful reference information for future applications of the crystal. It is ideal to use the crystal as a reference material for unknown zircons with a similar U–Pb age and isotopic composition, which helps to alleviate the matrix effects that may result from zircon decay accumulated damage.

2. Materials and Methods

The sample is a gem-quality zircon crystal (CAM18) purchased from a gemstone dealer who claimed that it had been collected from Cambodia, though the exact provenance and petrological characteristics are unknown (Figure 1).

Routine gemological signature observation and spectroscopic testing of the sample were performed at the Gemmological Research Laboratory of the China University of Geosciences (Beijing). The color, luster, transparency, inclusions, extinction phenomena, dichroism, fluorescence weight and density were observed and recorded by using a 10× magnifying glass, gem photographic microscope, polariscope, dichroscope, ultraviolet and electronic balance.

The UV–vis spectrometer was the UV-3600 UV–vis spectrophotometer produced by Shimadzu Corporation in Japan. The sample was analyzed by absorption method, and the experimental conditions were as follows: the wavelength range was 300~800 nm, the slit width was 20 nm, the time constant was 0.1 s, and the sampling interval was 0.5 s.

The infrared spectrometer was the Tensor 27 Fourier infrared spectrometer, manufactured by Bruker in Germany. The sample was analyzed by reflection method, and the scanning range was set to 400~1200 cm⁻¹.

The Raman spectrum was collected in the range of 100~1100 cm⁻¹ by the Horiba HR-Evolution Raman microspectrometer. The conditions were as follows: 532 nm laser source, 50 nW laser power, 4 cm⁻¹ spectral resolution, 600 gr/mm grating and 100 μm slit width.

The preparation of sample target and the acquisition of related pictures were carried out at Beijing Zirconia Pilot Technology Co., Ltd. First, the selected sample and zircon reference materials were pasted on a glass plate with double-sided tape, and then the prepared epoxy resin was injected into the sample target for curing. Second, the cured sample target was removed from the glass plate for cleaning, sanding and polishing. Finally, the cathodoluminescence, transmitted light and reflected light images were photographed.

LA-ICP-MS U–Pb dating and trace element analysis were carried out at the mineral laser microprobe analysis laboratory of China University of Geosciences (Beijing). The instrument was mainly composed of NewWave 193UC ArF excimer laser and Angilent 7900 ICP-MS. The main tuning parameters are shown in

Table 1. Typical operation conditions for LA-ICP-MS analysis in Milma Lab.

ICP-MS Conditions		Dwell Time
RF power	1350 W	204(Pb + Hg)	20 ms
Feedback power	<5 W	206Pb	20 ms
RF matching	1.37 V	207Pb	30 ms
Sampling depth	7.0 mm	208Pb	15 ms
Plasma gas	15 L/min	232Th	10 ms
Auxiliary gas	1 L/min	238U	15 ms
Make-up gas (Ar)	0.8–1 L/min	Other elements	6 ms
Laser Parameters
Wavelength		193 nm
Pulse duration		5 ns
Ablation style		Single spot
Energy density		4–8 J/cm2
Carrier gas (He)		800–900 mL/min
Ablation spot size		35 μm
Repetition rate		6–8 Hz

.

Prior to performing the analysis, the ICP-MS was tuned by ablating the NIST SRM 610 to obtain a maximum signal intensity of ²³⁸U (1 × 10⁶ cps), while maintaining a low ThO⁺/Th⁺ ratio (<0.15%) and a U⁺/Th⁺ ratio close to 1. During the analysis, the laser ablation energy density, spot size and repetition rate were set to 4 J/cm², 50 μm and 8 Hz, respectively. The following elements were collected by ICP-MS: ²⁹Si, ⁴⁵Sc, ⁴⁹Ti, ⁸⁹Y, ⁹¹Zr, ⁹³Nb, ¹³⁹La, ¹⁴⁰Ce, ¹⁴¹Pr, ¹⁴³Nb, ¹⁴⁷Sm, ¹⁵¹Eu, ¹¹⁵Gd, ¹⁵⁹Tb, ¹⁶³Dy, ¹⁶⁵Ho, ¹⁶⁶Er, ¹⁶⁹Tm, ¹⁷³Yb, ¹⁷⁵Lu, ¹⁷⁹Hf, ¹⁸¹Ta and ²⁰²Hg, which were measured with dwell time of 6 ms; ²³²Th, which was measured with dwell time of 10 ms; ²³⁸U and ²⁰⁸Pb, which were measured with dwell time of 15 ms; ²⁰⁴(Pb + Hg) and ²⁰⁶Pb, which were measured with dwell time of 20 ms; ²⁰⁷Pb, which was measured with dwell time of 30 ms. Each analytical run included 2 analyses of zircon 91500 (primary reference material), 2 analyses of zircon GJ-1 (secondary reference material) and 10 analyses of unknowns. NIST SRM 610 was used as an external reference material, and ⁹¹Zr was used as an internal standard to calibrate the trace elements. Each single-spot analysis consisted of 20 s of background signal acquisition, 50 s of ablation and 30 s of washout. Isoplot R was used to calculate the weighted average age and for the generation of Concordia and Tera–Wasserburg diagrams [16].

3. Results

3.1. Gemmological Properties

Zircon CAM18 is a cut gemstone in a near-round shape, approximately 8 mm in diameter, with a density of 4.49 g/cm³ and a weight of 0.55 g. The crystal is yellow–brown and transparent and has a vitreous luster. The pleochroism is manifested as light fuchsia and light yellow. The sample is an anisotropic gemstone that shows one extinction for each 90-degree rotation under a polariscope. It is inert under long-wave UV light (365 nm) but fluoresces faintly yellow under short-wave UV light (254 nm). Under the gem microscope, we can see two of each pavilion facet of the sample, which is caused by its high birefringence (0.040~0.060), but no other inclusions are observed (Figure 2A). After crushing, the sample develops a conchoidal fracture that exhibits a greasy luster (Figure 2B).

3.2. Spectral Characteristics

3.2.1. FTIR Spectra

All five infrared spectral analyses of zircon CAM18 show four peaks at 435 cm⁻¹, 621 cm⁻¹, 937 cm⁻¹ and 1078 cm⁻¹ (Figure 3). The 621 cm⁻¹ reflectance peak is caused by the anti-symmetric bending vibration of υ₄ (SiO₄), corresponding to the internal A_2u infrared-active mode. The 937 cm⁻¹ and 1078 cm⁻¹ reflectance peaks are associated with the anti-symmetric stretching vibration of υ₃ (SiO₄), corresponding to the internal E_u and A_2u infrared-active modes, respectively [17]. The attribution of the 435 cm⁻¹ reflectance peak remains controversial. Nasdala et al. [17] and Dawson et al. [18] attributed it to the anti-symmetric bending vibration of υ₄ (SiO₄), corresponding to the internal E_u infrared-active mode, whereas Woodhead et al. [19] attributed it to the external vibration mode caused by the rotary vibration of the [SiO₄]⁴⁻ tetrahedron.

The width and intensity of the 435 cm⁻¹ and 621 cm⁻¹ peaks can reflect the degree of zircon metamictization, especially the width of the 621 cm⁻¹ peak, which is highly sensitive to metamictization and easy to quantitatively study, so these two peaks are a useful tool for determining the degree of zircon metamictization [20,21]. From the FTIR spectral analysis results of zircon CAM18, the width and intensity of the 435 cm⁻¹ and 621 cm⁻¹ peaks show a broadening trend and a weakening trend, respectively. This suggests that the sample has undergone a certain degree of metamictization.

3.2.2. UV–Vis–NIR Spectra

Although colorless zircons seldom exhibit prominent absorption peaks in the UV–vis–NIR spectra, zircons of different colors tend to show various absorption peaks, mainly due to the selective absorption of light by the transition metal elements (such as Nb, Ti and Fe) and rare-earth elements (such as Nd, Pr and Er) in the structure [22]. The UV–vis–NIR spectra of zircon CAM18 consists of eight peaks (Figure 4). The peaks at 652 nm and 689 nm are the characteristic peaks of zircon, mainly attributed to the substitution of U⁴⁺ for Zr⁴⁺ in the crystal lattice [23]. The appearance of other peaks is caused by the absorption of light by some impurity ions. Among them, the light brown of zircon CAM18 is mainly affected by the absorption peaks at 351 nm, 480 nm and 511 nm, and the origin may be related to the absorption of the color centers [23,24].

3.2.3. Raman Spectra

In Figure 5, the Raman peaks at 1003 cm⁻¹, 972 cm⁻¹ and 438 cm⁻¹ represent the internal structure vibration peaks of the lattice, caused by the anti-symmetric stretching vibration (B_1g, υ₃), symmetric stretching vibration (A_1g, υ₁) and symmetric bending vibration (A_1g, υ₂) of the [SiO₄]⁴⁻ tetrahedron, respectively. The Raman peaks at 223 cm⁻¹, 210 cm⁻¹ and 203 cm⁻¹ belong to the external structure vibration peaks, caused by Zr⁴⁺ and [SiO₄]⁴⁻ tetrahedral vibrations [17]. The origin of the 352 cm⁻¹ Raman peak (E_g) is controversial. Syme et al. [25] and Koleso et al. [26] considered it to be an external vibration mode caused by [SiO₄]⁴⁻ tetrahedral vibration, while Dawson et al. [18] specified it as an internal vibration mode, caused by υ₄ bending vibration.

The relative intensity variation in the 352 cm⁻¹ and 1003 cm⁻¹ Raman peaks are related to the anisotropy of the crystal. The strongest intensity of the 352 cm⁻¹ peak is obtained in the direction perpendicular to the c-axis of zircon, and the strongest intensity of the 1003 cm⁻¹ peak is obtained in the direction parallel to the c-axis [27]. With the increase in metamorphism, the structure of zircon becomes more and more disordered, which is reflected in the Raman spectra by the increase in the width of the Raman peaks and the decrease in the Raman shift [28]. Although the Raman peaks of zircon CAM18 have a small shift in the direction of the low wavenumber, the peak shapes are relatively sharp and small in width, so it can be judged that the sample is less affected by metamictization and still has a high degree of crystallization.

3.3. U–Pb Ages

In the CL image, most zircon fragments are grayish white with fewer cracks, reflecting less difference in trace element content (Figure 6). In addition, the sample developed a narrow and dense oscillatory zoning, indicating that it originated from magma [29]. The results of the LA-ICP-MS U–Pb dating are shown in

Table A1. LA-ICP-MS U–Pb dating results.

	204(Pb + Hg)	Radiogenic Isotope Ratios									Isotopic Ages (Ma)								Dα (×1018 α/g)
		207Pb/206Pb	1σ	208Pb/232Th	1σ	207Pb/235U	1σ	206Pb/238U	1σ	rho	207Pb/235U	1σ	206Pb/238U	1σ	207Pb/206Pb	1σ	208Pb/232Th	1σ
CAM18
1	0.53	0.0584	0.0007	0.0277	0.0004	0.7391	0.0093	0.0916	0.0006	0.4928	562	5.5	565	3.4	546	27.8	552	8.7	1.39
2	0.81	0.0588	0.0007	0.0278	0.0004	0.7517	0.0096	0.0926	0.0006	0.4895	569	5.6	571	3.4	561	27.8	555	8.5	1.37
3	0.58	0.0584	0.0007	0.0278	0.0004	0.7496	0.0092	0.0929	0.0006	0.5157	568	5.3	572	3.5	546	25.9	554	8.8	1.35
4	0.53	0.0585	0.0007	0.0278	0.0004	0.7444	0.0088	0.0921	0.0005	0.4894	565	5.1	568	3.2	550	24.1	555	7.6	1.40
5	bd (1)	0.0591	0.0006	0.0279	0.0004	0.7536	0.0087	0.0922	0.0005	0.4719	570	5.1	569	3.0	572	24.1	556	8.3	1.39
6	0.23	0.0580	0.0007	0.0280	0.0004	0.7411	0.0090	0.0925	0.0005	0.4807	563	5.3	570	3.2	532	30.6	558	8.5	1.34
7	0.62	0.0597	0.0007	0.0283	0.0004	0.7640	0.0093	0.0926	0.0006	0.4960	576	5.4	571	3.3	594	25.9	564	8.8	1.38
8	0.25	0.0585	0.0007	0.0282	0.0005	0.7465	0.0091	0.0923	0.0006	0.4935	566	5.3	569	3.3	550	25.9	562	9.0	1.40
9	0.44	0.0598	0.0007	0.0286	0.0004	0.7684	0.0096	0.0931	0.0006	0.4889	579	5.5	574	3.4	594	25.9	571	8.6	1.43
10	bd (1)	0.0600	0.0008	0.0283	0.0005	0.7615	0.0101	0.0919	0.0006	0.4606	575	5.8	567	3.3	606	27.8	564	9.5	1.38
11	0.34	0.0607	0.0007	0.0289	0.0004	0.7744	0.0093	0.0924	0.0006	0.5126	582	5.4	570	3.4	628	21.3	576	8.8	1.42
12	0.13	0.0591	0.0007	0.0287	0.0005	0.7567	0.0087	0.0928	0.0006	0.5554	572	5.0	572	3.5	569	25.9	573	9.0	1.37
13	0.20	0.0596	0.0007	0.0291	0.0004	0.7626	0.0091	0.0927	0.0005	0.4873	576	5.2	571	3.2	587	24.1	579	8.4	1.36
14	0.40	0.0596	0.0006	0.0299	0.0004	0.7682	0.0086	0.0934	0.0005	0.5161	579	4.9	575	3.2	587	22.2	595	8.7	1.37
15	0.22	0.0600	0.0007	0.0293	0.0005	0.7733	0.0092	0.0934	0.0006	0.5131	582	5.3	575	3.4	606	20.2	584	9.2	1.37
16	bd (1)	0.0598	0.0007	0.0294	0.0004	0.7667	0.0093	0.0928	0.0005	0.4769	578	5.4	572	3.2	598	30.5	586	8.1	1.68
17	0.59	0.0598	0.0006	0.0295	0.0005	0.7691	0.0083	0.0932	0.0006	0.5563	579	4.8	574	3.3	594	24.1	588	9.2	1.35
18	0.68	0.0590	0.0007	0.0288	0.0004	0.7548	0.0089	0.0926	0.0006	0.5237	571	5.2	571	3.4	569	24.1	574	8.5	1.36
19	0.020	0.0600	0.0008	0.0293	0.0005	0.7726	0.0102	0.0932	0.0006	0.4872	581	5.8	574	3.5	606	27.8	585	8.9	1.37
20	0.74	0.0594	0.0008	0.0299	0.0005	0.7624	0.0101	0.0929	0.0006	0.5133	575	5.8	572	3.7	583	27.8	595	9.3	1.34
91500
1	0.82	0.0763	0.0012	0.0563	0.0010	1.8885	0.0306	0.1792	0.0014	0.4876	1077	10.8	1063	7.8	1106	36.1	1108	19.7
2	0.38	0.0734	0.0017	0.0517	0.0013	1.8119	0.0399	0.1791	0.0015	0.3868	1050	14.4	1062	8.4	1026	41.7	1020	24.9
3	0.23	0.0752	0.0012	0.0548	0.0010	1.8577	0.0302	0.1791	0.0014	0.4735	1066	10.7	1062	7.6	1073	31.5	1078	19.1
4	0.35	0.0746	0.0014	0.0533	0.0014	1.8427	0.0354	0.1793	0.0014	0.4150	1061	12.7	1063	7.8	1057	38.6	1049	27.1
5	0.29	0.0763	0.0015	0.0526	0.0013	1.8887	0.0381	0.1795	0.0016	0.4419	1077	13.4	1064	8.8	1103	40.7	1036	24.1
6	0.22	0.0735	0.0013	0.0555	0.0011	1.8117	0.0307	0.1788	0.0014	0.4530	1050	11.1	1061	7.5	1028	35.2	1092	20.7
GJ-1
1	0.37	0.0587	0.0008	0.0300	0.0010	0.7971	0.0112	0.0983	0.0006	0.4602	595	6.4	604	3.8	567	29.6	598	19.2
2	0.28	0.0608	0.0008	0.0307	0.0008	0.8248	0.0114	0.0983	0.0006	0.4760	611	6.3	604	3.8	635	28.5	611	16.2
3	0.51	0.0604	0.0009	0.0318	0.0010	0.8152	0.0116	0.0977	0.0006	0.4556	605	6.5	601	3.7	617	30.4	632	19.3

, and the uncertainties are reported at the 1σ level. The ²⁰⁶Pb/²³⁸U ages range from 565 ± 3.4 to 575 ± 3.4 Ma, and the ²⁰⁷Pb/²³⁵U ages range from 562 ± 5.5 to 582 ± 5.4 Ma. The data are concentrated on and near the concordia line with high concordance, yielding a concordia age of 571.4 ± 1.4 Ma (2s) (Figure 7A). The ²³⁸U/²⁰⁶Pb and ²⁰⁷Pb/²⁰⁶Pb ratios (not corrected for common Pb) yield a lower-intercept age of 570.5 ± 4.0 Ma (2s) (Figure 7B). The weighted average ²⁰⁶Pb/²³⁸U age is 571.1 ± 3.0 Ma (2s), indicating that the sample is a product of Neoproterozoic magmatic activity (Figure 7C).

3.4. Trace Elements

The trace element data are shown in

Table A2. Trace element abundance, Ce anomalies, and Eu anomalies in zircon and Ti-in-zircon temperature.

	CAM 18-1	CAM 18-2	CAM 18-3	CAM 18-4	CAM 18-5	CAM 18-6	CAM 18-7	CAM 18-8	CAM 18-9	CAM 18-10	CAM 18-11	CAM 18-12	CAM 18-13	CAM 18-14	CAM 18-15	CAM 18-16	CAM 18-17	CAM 18-18	CAM 18-19	CAM 18-20
Ti (ppm)	3.08	3.28	3.27	3.32	3.23	2.61	3.22	3.61	3.33	3.44	3.40	3.35	3.60	3.39	3.57	3.47	3.49	3.64	3.47	3.48
La (ppm)	0.0049	0.0028	0.0014	0.0022	0.0041	0.0021	0.0028	bd (1)	0.0035	0.0030	0.0015	0.0028	0.0063	0.0042	0.0056	0.0001	0.0002	0.0008	bd (1)	0.0029
Ce (ppm)	2.32	2.29	2.28	2.26	2.26	2.23	2.31	2.35	2.30	2.34	2.36	2.27	2.22	2.32	2.29	2.36	2.29	2.25	2.26	2.26
Pr (ppm)	0.027	0.033	0.029	0.044	0.031	0.030	0.031	0.034	0.030	0.033	0.029	0.024	0.032	0.037	0.036	0.050	0.029	0.020	0.029	0.032
Nd (ppm)	0.51	0.57	0.53	0.60	0.60	0.57	0.53	0.55	0.59	0.51	0.55	0.55	0.54	0.51	0.60	0.59	0.52	0.50	0.56	0.51
Sm (ppm)	0.99	0.92	0.81	0.90	0.95	1.05	1.03	1.04	0.92	0.95	1.04	0.87	0.97	0.91	0.91	0.99	0.84	0.96	0.90	0.95
Eu (ppm)	0.18	0.17	0.17	0.19	0.18	0.16	0.17	0.17	0.21	0.18	0.18	0.19	0.21	0.17	0.16	0.19	0.18	0.16	0.16	0.19
Gd (ppm)	3.33	3.39	3.40	3.42	3.39	3.46	3.46	3.33	3.37	3.35	3.35	3.20	3.23	3.17	3.45	3.48	3.50	3.38	3.46	3.20
Tb (ppm)	0.98	0.90	0.89	0.90	0.93	0.91	0.90	0.90	0.91	0.91	0.91	0.82	0.89	0.87	0.85	0.94	0.87	0.94	0.90	0.88
Dy (ppm)	9.14	8.58	8.47	8.86	9.17	8.87	9.12	9.04	9.10	8.75	8.95	8.45	8.99	8.63	8.92	8.97	8.58	9.09	8.67	8.83
Ho (ppm)	3.02	3.03	2.90	3.06	2.95	3.05	2.92	3.03	3.09	3.01	3.11	2.99	3.01	2.94	2.98	3.10	2.95	2.99	3.00	2.98
Er (ppm)	13.14	13.09	12.57	13.09	12.81	12.86	12.84	12.76	12.93	12.63	12.96	12.76	12.85	12.78	12.77	13.42	12.76	12.86	13.06	12.58
Tm (ppm)	2.70	2.62	2.63	2.68	2.67	2.68	2.70	2.61	2.70	2.66	2.67	2.62	2.64	2.63	2.70	2.72	2.67	2.67	2.66	2.57
Yb (ppm)	24.86	23.54	23.49	24.24	24.66	24.56	24.45	24.96	24.78	23.86	24.79	24.58	23.89	24.28	24.57	25.22	24.14	23.97	24.57	24.21
Lu (ppm)	4.73	4.57	4.58	4.63	4.76	4.81	4.66	4.79	4.82	4.65	4.71	4.57	4.62	4.66	4.74	4.74	4.59	4.65	4.67	4.61
Hf (ppm)	10,778	10,776	10,738	10,793	10,774	10,789	10,783	10,727	10,716	10,750	10,689	10,758	10,708	10,695	10,755	10,672	10,721	10,765	10,724	10,731
total Pb (ppm)	67.70	67.73	66.61	69.06	68.44	65.96	68.53	69.00	71.09	68.05	70.64	68.03	67.64	68.40	68.56	71.28	67.45	67.37	68.48	66.41
Th (ppm)	136	135	130	137	137	129	137	138	140	137	140	135	134	132	135	142	134	134	136	133
U (ppm)	677	670	658	686	678	653	675	682	696	674	695	668	665	668	670	700	660	665	671	653
Th/U	0.20	0.20	0.20	0.20	0.20	0.20	0.20	0.20	0.20	0.20	0.20	0.20	0.20	0.20	0.20	0.20	0.20	0.20	0.20	0.20
δCe (2)	49.45	58.41	87.73	56.32	49.15	68.88	60.79	50.04	55.03	57.66	87.73	67.89	38.33	45.63	39.54	258.77	233.13	137.91	43.87	57.52
δEu (3)	0.30	0.29	0.31	0.33	0.31	0.26	0.28	0.28	0.36	0.31	0.29	0.35	0.36	0.31	0.28	0.31	0.32	0.27	0.28	0.33
ΔFMQ (4)	−2.98	−3.04	−3.03	−3.10	−3.06	−2.87	−3.02	−3.10	−3.08	−3.05	−3.06	−3.08	−3.17	−3.05	−3.12	−3.08	−3.09	−3.16	−3.12	−3.10
logfO2 (5)	−19.39	−19.32	−19.32	−19.35	−19.37	−19.63	−19.34	−19.17	−19.33	−19.23	−19.26	−19.31	−19.25	−19.26	−19.22	−19.24	−19.23	−19.22	−19.28	−19.25
T (°C) (6)	728	734	733	735	732	714	732	742	735	738	737	735	742	736	741	738	739	743	738	739

(1) bd = below detection. (2) $\mathsf{\delta }\mathrm{Ce}=\frac{{\left(\frac{\mathrm{Ce}}{\sqrt{\mathrm{La}\times \mathrm{Pr}}}\right)}_{\mathrm{zircon}}}{{\left(\frac{\mathrm{Ce}}{\sqrt{\mathrm{La}\times \mathrm{Pr}}}\right)}_{\mathrm{chondrite}}}$. (3) $\mathsf{\delta }\mathrm{Eu}=\frac{{\left(\frac{\mathrm{Eu}}{\sqrt{\mathrm{Sm}\times \mathrm{Gd}}}\right)}_{\mathrm{zircon}}}{{\left(\frac{\mathrm{Eu}}{\sqrt{\mathrm{Sm}\times \mathrm{Gd}}}\right)}_{\mathrm{chondrite}}}$. (4) $\mathsf{\Delta }\mathrm{FMQ}=3.998\times \log [\frac{\mathrm{Ce}}{\sqrt{{{(\mathrm{U}}_{\mathrm{i}} \times \mathrm{Ti})}^{\mathrm{z}}}}]+2.284$, wherein U_i denotes age-corrected initial U content, superscript z denotes zircon. (5) ${\mathrm{logfO}}_2{\left(\mathrm{sample}\right)=\mathrm{logfO}}_2\left(\mathrm{FMQ}\right)+3.998 \times \log [\frac{\mathrm{Ce}}{\sqrt{{{(\mathrm{U}}_{\mathrm{i}} \times \mathrm{Ti})}^{\mathrm{z}}}}]+2.284$, wherein U_i denotes age-corrected initial U content, FMQ represents the reference buffer fayalite + magnetite + quartz, superscript z denotes zircon. (6) $\mathrm{T}(°\mathrm{C})=[-4800+0.4748 \times (\mathrm{P} -{ 1000\left)\right]/(\mathrm{logTi}}^{\mathrm{zir}}- 5.711 -{ \mathrm{loga}}_{{\mathrm{TiO}}_2}^{\mathrm{melt}}{+\mathrm{loga}}_{{\mathrm{SiO}}_2}^{\mathrm{melt}})$, where P = 300 MPa, ${\mathrm{a}}_{{\mathrm{TiO}}_2}^{\mathrm{melt}}=0.8$ and ${\mathrm{a}}_{{\mathrm{SiO}}_2}^{\mathrm{melt}}=1$.

. The Th and U concentrations are 136 ± 3 ppm and 673 ± 14 ppm, respectively, with a Th/U ratio of 0.20; the REE content is 62.8~66.7 ppm, and the average value is 64.8 ppm; the LREE content is 3.82~4.18 ppm, and the average value is 4.00 ppm; the HREE content is 59.0~62.6 ppm, and the average value is 60.8 ppm. As shown in Figure 8, these points exhibit fairly uniform trace element abundances, with small ranges in the mass fractions. The average values of LREE/HREE and (La/Yb)_N are 0.07 and 0.00009, respectively, indicating that the enrichment degree of HREE is significantly higher than that of LREE. The average values of (La/Sm)_N and (Gd/Yb)_N are 0.0021 and 0.11, respectively, indicating that the degree of fractionation inside LREE is greater than that of HREE. In addition, Ce and Eu show obvious positive and negative anomalies, with average values of 80.2 and 0.31 for δCe and δEu, respectively. Overall, zircon CAM18 has REE characteristics that are similar to most magmatic zircons.

4. Discussion

4.1. Structural State and Radiation Damage

The radioactive decay of elements like U and Th can cause radiation damage to the crystal structure of zircon, resulting in a decrease in the crystallinity and physico-chemical stability. In this case, radiogenic Pb is easily lost, forming a discordant U–Pb age dataset. In order to comprehensively assess the structural state of zircon CAM18, we studied it based on the density, Raman spectroscopy parameters and α flux (Dα).

The density of the sample is 4.49 g/cm³, consistent with typical intermediate-type zircon, showing that the sample has experienced a volume expansion under the effect of α self-irradiation [30]. With the increase in radiation intensity, the υ₃ peak (1008 cm⁻¹) of crystalline zircon shows a decrease in Raman shift and an increase in full width at half maximum (FWHM). Five analyses of zircon CAM18 suggest that the υ₃ peak is located at 1003 cm⁻¹, and the FWHMs after correction using the Lorentzian–Gaussian function vary from 8.9 to 11.3 cm⁻¹ (Figure 5). These values reconfirm the mild-to-moderate radiation damage state of zircon CAM18 [31]. The α flux (Dα), calculated using U and Th concentrations and the U–Pb age (about 571 Ma) of the sample, represents the number of α-decay events per gram, and this value can be used to further quantify the radiative intensity since the time of closure of the U–Pb system [32]. The α flux of all analysis points (except for one analysis with a α flux of 1.68 × 10¹⁸ α/g) is between 1.34 and 1.43 × 10¹⁸ α/g, which may be related to the heterogeneity of radiation damage and still needs to be experimentally verified by more fragments (Table A1).

The thermal annealing caused by heating events can eliminate partial radiation damage. The density and Raman band broadening of the sample are consistent with the calculated α flux result, suggesting that the lattice damage is at a mild-to-moderate level. Thus, the occurrence of heat treatment can be ruled out.

4.2. U–Pb Geochronology

The variation in ²⁰⁶Pb/²³⁸U ages between different CAM18 zircon grains is small, with a measurement repeatability of 0.46% (RSD), within the uncertainty of the age accuracy obtained by LA-ICP-MS (about 1~2%, 2s) [33]. The weighted average ²⁰⁶Pb/²³⁸U and ²⁰⁷Pb/²³⁵U ages are 571.0 ± 3.0 Ma (2s, n = 20) and 573.4 ± 6.0 Ma (2s, n = 20), respectively, and both are in agreement within analytical uncertainty. This consistency suggests that no significant loss of radiogenic Pb has occurred in the U–Pb system since the zircon formation. Since the ²³⁵U signal is calculated from ²³⁸U, potential biases may be introduced when calculating the ²⁰⁷Pb/²³⁵U ratio. The measured ²³⁸U/²⁰⁶Pb and ²⁰⁷Pb/²⁰⁶Pb ratios are plotted on a Tera–Wasserburg plot, yielding a lower-intercept age of 570.5 ± 4.0 Ma (2s). The lower-intercept age is consistent with the weighted average ²⁰⁶Pb/²³⁸U age within error, indicating that these U–Pb data have high reliability.

From the age results of the sample, the moderate radiation damage generated by the α-decay events of U does not cause any significant loss of radiogenic Pb. Zircon reference materials 91500, TEMORA-1 and SA01 have average U concentrations of 80, 195 and 161 ppm, respectively. In contrast, a particularly advantageous property of zircon CAM18 is the high U mass fraction of 672 ppm, which produces high count rates and good counting statistics for LA-ICP-MS analyses (

Table 2. Summary of U–Pb age data for zircon CAM18 and other zircon reference materials.

Reference Material	Average U Concentration (ppm)	Age (Ma, 2s 1)	Reference
91500	80	1065.4 ± 0.6	[11,37]
TEMORA-1	195	416.8 ± 0.2	[38]
SA01	161	535.1 ± 0.4	[39]
M127	896	524.4 ± 0.2	[32]
Qinghu	1042	159.5 ± 0.2	[40]
CAM18	672	571.4 ± 3.0	This study

¹ 2s means 2 sigma uncertainties.

). Noticeable in this dataset, however, is the large ablation spot size that could exacerbate elemental fractionation and matrix effects, reducing the accuracy of the dating results, and it is difficult to perform effective analyses on natural zircons with complicated internal structures or small particles [34,35]. As a result, considering the effect of different beam spot diameters on the accuracy of U–Pb ages by LA-ICP-MS, it is essential to carry out dating analyses with small laser beam diameters in the future. Additionally, the isotope dilution thermal ionization mass spectrometry (ID-TIMS) technique has a higher analytical accuracy (0.1%) compared to the LA-ICP-MS technique [36]. To confirm the absolute accuracy of the LA-ICP-MS age, the ID-TIMS technique is also required for this sample to further study the distributed fragments.

4.3. Magmatic Genesis

The Th/U ratio is usually used as a criterion for distinguishing magmatic zircon (>0.4) and metamorphic zircon (<0.1), but a small number of magma zircons have extremely low Th/U ratios [41,42]. For example, magmatic zircons from Concordia and Kweekfontein granites exhibit high U (2060~4650 ppm) and low Th (80~380 ppm) concentrations, with a very low Th/U ratio of 0.06 [43]. The reason for this phenomenon is that these zircons crystallized from a melt enriched with U and are not related to the decoupling migration of Th and U in a metamorphic fluid. Therefore, it is necessary to combine this information with other evidence to interpret the origin of zircon CAM18.

Zircon CAM18 has a homogeneous Th/U ratio of 0.2 (Figure 9A). In addition, based on other magmatic characteristics, such as oscillatory zoning, the positive Ce anomaly, the negative Eu anomaly and the left-dipping REE pattern, it can be determined that the sample is of magmatic origin. The Th/U ratio in zircon is controlled by multiple factors, mainly including the element content in the melt, the distribution coefficients of elements between the melt and zircon, and the crystallization of Th-rich minerals [44]. In Figure 9B, the Th/U ratio shows a negligible change as the U content increases, implying that the Th and U content in the melt is not the main factor controlling the low Th/U ratio. The crystallization of Th-rich minerals (such as monazite) can consume the Th content in the melt, resulting in later crystallized zircon with low Th content. Therefore, the Th/U ratio of zircon CAM18 may be influenced by the crystallization of Th-rich minerals as well as the distribution coefficient of elements.

4.4. Magma Oxidization State

The chondrite-normalized REE pattern of zircon depends on the valence state and ion radius of the REE [45,46]. Compared with LREEs, HREEs have similar ionic radii to Zr⁴⁺, and they are easier to replace Zr⁴⁺ in the zircon lattice, so the REE distribution shows a left-dipping pattern. The Ce and Eu in magma exist in two valence states: Ce³⁺ (r = 1.14 Å), Ce⁴⁺ (r = 0.97 Å), Eu²⁺ (r = 1.25 Å) and Eu³⁺ (r = 1.07 Å) [47,48]. Since Ce⁴⁺ has the same charge and similar ion radius as Zr⁴⁺, it is more compatible in zircon than Ce³⁺, and, likewise, Eu³⁺ is more compatible in zircon than Eu²⁺. As the oxygen fugacity increases, the Ce³⁺ in magma is easily oxidized to Ce⁴⁺, which leads to Ce enrichment in zircon because the partition coefficient of Ce⁴⁺ is greater than that of La³⁺ and Pr³⁺. Similarly, the oxidized Eu³⁺ has better compatibility in zircon, so a small Eu depletion is observed in zircon. The Ce and Eu anomalies in magma zircon can be used as an indicator to characterize the relative oxygen fugacity during magmatic differentiation [46,49].

Trail et al. [50] derived a formula for calculating Ce and Eu anomalies based on the chondrite-normalized values, which is usually restricted by elemental detection limits and mineral inclusions. When LREE-rich inclusions (such as allanite, apatite, sphene and monazite) are also detected during laser ablation, the measured La and Pr content significantly increases. Except for the CAM18-8 and CAM18-19 analysis points, the La and Pr concentrations of other analysis points are higher than the detection limit of LA-ICP-MS and less than 1~200 × 10⁻³ and 10~500 × 10⁻³ ppm, respectively, so the influence of LREE-enriched mineral inclusions can be excluded [51,52,53]. As can be seen in Figure 10A, there is no positive correlation between δCe and δEu, implying that oxygen fugacity is not the main factor affecting Ce and Eu anomalies. Two reasons may cause the negative Eu anomaly of the sample. First, the Eu depletion of zircon CAM18 is inherited from the host magma; second, there is extensive plagioclase fractional crystallization prior to or during zircon crystallization, which consumes Eu in the melt, resulting in a significant negative Eu anomaly in the zircon [51]. In order to determine the magmatic oxygen fugacity, the newly proposed zircon oxygen fugacity meter by Loucks et al. [54] is used in this paper to calculate logfO₂. This method is not limited by crystallization temperature, pressure or parent magma composition, and only requires trace element concentrations (such as Ti, U and Ce) and U–Pb ages to obtain fO₂ with a standard error of ±0.6 log units. The oxygen fugacity and temperature results are shown in Table A2. The temperature ranges from 714 to 742 °C, and the oxygen fugacity is below the fayalite–magnetite–quartz (FMQ) buffer, ranging from ΔFMQ−2.87 to ΔFMQ−3.17, with an average value of ΔFMQ−3.07 (Figure 10B). These results show that zircon CAM18 crystallizes in a reducing environment with low oxygen fugacity.

5. Conclusions

Integrated analyses from multiple methods reveal that zircon CAM18 is free of mineral inclusions and has a crystalline structure and uniform trace element concentrations. In addition, it has homogeneous U–Pb ages, within the uncertainty of the age accuracy obtained by LA-ICP-MS. The crystal may be a potential reference material for LA-ICP-MS U–Pb dating. However, we do not know how well the LA-ICP-MS age matches its real age. In the future, the ID-TIMS technique with high analytical accuracy will be used to examine the absolute accuracy of LA-ICP-MS age.