OLG: a new potential reference material for apatite (U–Th)/He dating

Abstract

The (U–Th)/He dating technique has been widely applied for constraining shallow crustal geological processes, with apatite being the most commonly used mineral due to its low closure temperature and broad occurrence. Currently available (U–Th)/He reference materials, including Durango, Fish Canyon Tuff (FCT), Limberg t3 Tuff (LT3T), and MK-1, are relatively young (<40 Ma). This highlights the need for a new, homogeneous, and older reference. In this study, we characterize a new apatite sample, OLG, obtained from a mineral dealer and sourced from the Otter Lake area in the Grenville Province, Canada. Single-grain (U–Th)/He dating yields highly reproducible ages of 210.2 ± 1.4 Ma (2σ), significantly older than those of existing reference materials. BSE imaging, EPMA, and LA-ICP-MS analyses confirm its major-element homogeneity and relatively uniform trace-element distribution, although minor inclusions could be present. OLG shows strong U, Th, and He signals, with an average U concentration of ∼112.2 ppm, Th concentration of ∼923.7 ppm, and a Th/U ratio of ∼8.2. Its older age, high U and Th concentrations, and excellent compositional homogeneity indicate that OLG could serve as a robust new reference material for high-precision (U–Th)/He thermochronology.

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1 Introduction

The (U–Th)/He dating technique, which uses the decay of nuclides, such as ²³⁸U, ²³⁵U, ²³²Th, and ¹⁴⁷Sm, and the accumulation of their daughter isotope, ⁴He, to date minerals is widely used in studies of shallow crustal geological processes.¹ Apatite is the most commonly used mineral due to its low closure temperature² and ubiquity in various types of crustal rocks. (U–Th)/He geochronology is an absolute dating method, meaning that the results do not depend upon a measurement standard. However, because reference materials for dating are used to inspect experimental procedures, it is still very important to have a homogeneous reference mineral. In recent years, different (U–Th)/He laboratories have developed new apatite and zircon reference materials to meet increasingly greater demand and strict criteria.

Currently, there are four popular (U–Th)/He dating reference materials: Durango, Fish Canyon Tuff apatite (FCT), Limberg t3 Tuff (LT3T), and MK-1. FCT apatite is an old low-temperature thermochronology standard, a product of rapid cooling and crystallization following the eruption of the La Garita Caldera, located within the San Juan volcanic field in southwestern Colorado, USA.³ Previous studies have conducted multiple geochronological analyses on it, including apatite and zircon (U–Th)/He dating, U–Pb dating, titanite (U–Th)/He dating, and biotite and feldspar ⁴⁰Ar/³⁹Ar dating.^4–6 However, different sites have various ages, not all sites are suitable for low-temperature thermochronometers.⁷ The LT3T apatite is an excellent (U–Th)/He reference material, yielding a precise age of 16.8 ± 1.0 Ma (2σ). Its euhedral grains (>200 µm) allow reliable alpha emission corrections (>0.9), while high U–Th content (13–72 ppm U and 129–204 ppm Th) ensures analytical robustness.⁸ Concordance between Ar/Ar and fission track (FT) ages confirms its suitability for calibrating young (<20 Ma) thermochronometers. Durango apatite is the most popular (U–Th)/He dating reference; it is a big crystal (>1 cm) and has homogeneous U content; however, the age difference between crystals is much large than the analytical uncertainty.⁹ MK-1 apatite was collected from the Mogok metamorphic belt in Myanmar and is a gem apatite with high U concentration (>200 ppm) and used as a standard for (U–Th)/He dating.^10–12

All these age references have a young age, <35 Ma. Therefore, finding a new age and old reference is important, and the (U–Th)/He community would benefit from a homogeneous and reproducible apatite reference material. In this study, we analyzed an apatite (OLG), which was collected from the Otter Lake area, Grenville Province, Canada. Single fragment (U–Th)/He dating analysis yielded highly reproducible dates (∼210.2 Ma) which is much older than the present references. The high U (∼112.2 ppm) and Th (∼923.7 ppm) concentrations provide strong analytical signals, particularly advantageous for in situ helium analysis. In addition, OLG has a fission track age of ∼135.9 Ma, and its thermal history shows a multi-stage, monotonic cooling path without evidence of complex thermal events. Furthermore, rim-to-core age measurements show no systematic trend. These characteristics make OLG a unique and promising candidate for an apatite (U–Th)/He age reference.

2 Sample description

The OLG apatite from the Otter Lake area, Québec (Fig. 1), north of the Bancroft domain within the Grenville Province, Canada, yields a ²⁰⁷Pb/²⁰⁴Pb–²⁰⁶Pb/²⁰⁴Pb isochron age of 913 ± 7 Ma through HBr leaching and bulk dissolution.¹³ Additionally, ²³⁸U–²⁰⁶Pb ages of 932 ± 12 Ma and 920 ± 4 Ma were obtained on Otter Lake apatite by LA-ICP-MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry)¹⁴ and LA-ICP-MS (NH₃ mode),¹⁵ respectively. The crystal used in this study was commercially sourced and may differ slightly in age from that reported in previous studies. It is a large dark green-brown crystal, 25 mm in length and 8 g in weight. The crystal had its surface polished with a sandpaper to remove the alpha emission zone (∼20 µm), and the remaining portions were cut in to 8 large fragments along the c-axis. Fragments smaller than 100 mesh were selected for (U–Th)/He analysis.

Fig. 1 Optical image of the OLG apatite crystal used in this study. XYZ represents the axis of cutting.

3 Analytical methods

3.1 Backscattered electron imaging and electron probe microanalysis

The electron probe analysis (EPMA) and backscattered electron (BSE) imaging using an electron probe were conducted in the Electron Probe Laboratory of the School of Earth Sciences and Technology at Southwest Petroleum University. The electron probe model was a JEOL-JXA-8230, equipped with four spectrometers. Before analysis, a uniform carbon film of approximately 20 nm thickness was coated on the sample surface. The operating conditions of the electron probe were: 15 kV accelerating voltage, 20 nA accelerating current, and 10 µm beam diameter. All the test data underwent ZAF correction.

3.2 Fission track analysis

The laser-based method was used in this study. The analysis was conducted at the State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology (CDUT). Track analysis was performed using the Autoscan System from Australia.^16,17 The TrackWorks software was used to capture stacks of high-resolution digital images with a ×100 dry objective under both transmitted and reflected light using a highly sensitive iDS camera. Manual track counting of fission tracks was carried out using the FastTracks software and the coincidence mapping technique.¹⁸ Uranium concentrations were determined using LA-ICP-MS; the method is described in Section 3.3.

3.3 Trace element analysis

LA-ICP-MS analysis was also performed at CDUT, using a 193 nm RESO LR laser system coupled to an Agilent 7900 ICP mass spectrometer. The energy density, spot size and frequency of the laser were set to 3.5 J cm⁻², 50 µm and 8 Hz, respectively.¹⁷ NIST SRM 610 glass was used as the calibration reference material with ⁴³Ca used for internal normalization. NIST SRM 612 glass served as the secondary standard for element checks. The iolite 4 Trace Elements data reduction scheme¹⁹ was used in this study.

3.4 Conventional solution (U–Th)/He method

The (U–Th)/He dating is also done in CDUT. An Alphachron noble gas quadrupole mass spectrometer for He extraction and an Agilent 7900 ICP mass spectrometer for U and Th isotopic measurements were used for (U–Th)/He dating in this study.^11,20 The selected crystals were loaded into Pt capsules and thermally degassed under vacuum at ∼900 °C for 5 min with a 980 nm diode laser, which was sufficient to extract >99.9% of the total amount of ⁴He in most of the grains, as shown by replicate heating. Gas purification was achieved with two SAES AP-10-N getters; the purified gas was introduced into a quadrupole mass spectrometer (QMG 220 from Pfeiffer) for isotopic analysis. The measurement of ⁴He content uses isotope dilution with a pure ³He spike, stored in a fixed device in the purification system. Due to frequent use, the pressure in the ⁴He standard gas tank (Q Tank) declines over time, reducing the absolute quantity of gas delivered to the pipette. Regular calibration is therefore necessary. After helium extraction, Pt-wrapped grains were transferred to polypropylene (PP) vials for dissolution following the procedure of ref. 11. Following that, 25 µl of spike solution (stored in 50% v/v HNO₃) containing ca. 15 ng ml⁻¹ U and 5 ng ml⁻¹ Th (²³⁵U/²³⁸U = ∼254; ²³⁰Th/²³²Th = ∼10) was added to each vial. To completely dissolve U and Th, the PP vials were subjected to ultrasonic agitation for 15 min. After being left for at least 4 h at room temperature for full dissolution, 1000 µl of 5% v/v HNO₃ was added to the vials, diluting them to a total volume of 1025 µl. Fragments of Durango apatite age standard were analyzed with the samples for monitoring the measurement accuracy. Age calculation was conducted using a R-based program called IsoplotR.²¹

4 Results

4.1 Backscattered electron imaging and electron probe microanalysis results

The EPMA results for the 8 subsamples indicated that the OLG apatite is chemically pure and homogeneous (Fig. 2a). The in situ analysis was performed on 5 randomly selected points on each of the 8 subsamples (Fig. 2b). All analyzed points display a high and consistent abundance of CaO ranging from 53.143 to 54.274% m m⁻¹, with an arithmetic mean and standard deviation of 53.753 ± 0.243 (1σ)% m m⁻¹. The P₂O₅ abundance was also consistent, ranging from 37.127 to 39.018% m m⁻¹, with an arithmetic mean and standard deviation of 38.143 ± 0.41 (1σ)% m m⁻¹. Chlorine abundance was low, and fluorine abundance was 2.953 ± 0.302 (1σ)% m m⁻¹, implying that the OLG apatite is a fluorine apatite. During EPMA, neither inclusions nor compositional variations associated with chemical zoning were observed in any of the apatite fragments; this finding is consistent with BSE imaging results.

Fig. 2 (a) BSE images of the eight subsamples used for electron probe microanalysis. BSE images of OLG apatite fragments showing homogeneous textures without zoning. (b) EPMA compositional results for CaO (green line), P₂O₅ (red line) and F (blue line) of the eight subsamples of OLG apatite (data shown in Table 1). The black lines across the points are the mean value of each subsample.

4.2 In situ trace element analysis

For each subsample, 20 points were randomly chosen for measurement of the trace elements and calcium. Besides, 8 points for fission track analysis also provide trace element values. There is no significant difference in average values among the 8 subsamples (Fig. 3). Removing the three outliers in X1Y2Z2, the U content ranges from 104.2 to 128.6 ppm with an average of ∼112.2 ± 3.7 (1σ) ppm. Th ranges from 785.6 to 1003.4 with an average of ∼923.7 ± 26.7 (1σ) ppm (the detailed trace element result can be found in Table S1). The Th/U ratio is limited in range from 7.4 to 9.0 with an average of 8.2 ± 0.3 (1σ). Hidden inclusions could be a potential cause of the outliers. Some small inclusions were observed on the fragments, particularly in the X2Y2Z2 subsample. These inclusions may reflect variations in Ca concentration, introducing slightly more noise into the measurements. In contrast, the laser point used for fission track analysis shows no outliers. Maybe the counting area is a good place for LA-ICP-MS measurements.

Fig. 3 U and Th concentrations in eight apatite subsamples measured by LA-ICP-MS. Diamonds represent rejected outliers (2σ criterion). The data can be found in Table S1.

4.3 Fission track age

For each of the 8 subsamples, 8 specific areas were selected for fission track analysis. All tracks within each area were counted to obtain one age estimate per area, resulting in a total of 64 individual age determinations (Fig. 4). The data yield a central age of 135.9 ± 1.9 Ma (1σ; n = 64). The low mean square weighted deviation (MSWD) value of 0.68, together with a high p(χ²) value of 0.97, indicates that the single-grain age distribution is consistent with a single population and that the observed dispersion could be fully accounted for by analytical uncertainties alone. The He age (∼210.2 Ma) is largely older than the FT age (∼135.9 Ma). The main factors are the larger diffusion domain associated with the large crystal size^17,22 and the radiation damage resulting from the relatively high U (∼112.2 ppm) and Th (∼927.3 ppm) contents.²³ Normally, FT age uncertainties are heteroscedastic. However, the linear arrays observed in the radial plot indicate a near-perfect correlation between age and age uncertainty, which is unlikely to be realistic. This arises because, in laser-based FT dating, the main sources of uncertainty are (1) the U concentration and (2) the number of spontaneous tracks (N_s). In this sample, the U concentration is highly homogeneous, so the uncertainty is primarily controlled by N_s. Moreover, the counting areas (two boxes in FastTrack) for each age determination were similar, resulting in only minor variations in N_s. This, in turn, led to the observed linear relationship between FT age and its associated uncertainty.

Fig. 4 Fission track age (1σ) of 64 counting area. The age was calculated using the ICP absolute method in IsoplotR.²¹ The dataset is provided in Table S2.

4.4 Conventional He dating results

A total of 56 fragments, 7 from each of the 8 OLG subsamples, were selected for (U–Th)/He dating. It should be noted that ¹⁴⁷Sm is not measured in this study, and the ages may be slightly overestimated by several (2–3) million years. And the age of OLG would need to be recalculated/recalibrated in other labs that measure ¹⁴⁷Sm. Two fragments were lost during transfer from the Alphachron extraction line to PP vials due to incomplete sealing of the Pt capsules. Of the remaining 54 fragments, four were identified as outliers. The remaining 50 fragments yielded (U–Th)/He ages ranging from 194.5 to 229.6 Ma (Fig. 5), with a weighted mean age of 210.4 ± 1.6 Ma (2σ; Table 2). As with trace element analyses, the presence of minor inclusions may account for the outliers. Overall, the age data exhibit high reproducibility without systematic trends across the crystal. The dataset displays an MSWD of 2.6 and a low p-value (6.8 × 10⁻⁹), indicating minor overdispersion. This could result from the high concentrations of U, Th, and He, which generate strong signals and minimal single-grain analytical uncertainty. The average Th/U ratio of the 27 accepted fragments is 8.0 ± 0.6, although one fragment (X1Y2Z2-3) has a notably low value of 0.54, which inflates the overall uncertainty but still lies within the range observed by LA-ICP-MS (8.2 ± 0.3). For comparison, eight fragments (one from each subsample) were independently analyzed at the National Institute of Natural Hazards (NINH), yielding (U–Th)/He ages from 197.0 to 225.2 Ma, with a mean of 209.7 ± 2.7 Ma (2σ). The corresponding Th/U ratios ranged from 8.3 to 8.6, with a mean of 8.5 ± 0.1 (1σ). Both the ages and Th/U ratios obtained at CDUT and NINH are indistinguishable within analytical uncertainty, confirming the inter-laboratory reproducibility of the OLG apatite (Table 3).

Fig. 5 Conventional (U–Th)/He ages plotted against Th/U ratios for all analyzed fragments of OLG and Durango apatite. All ages are shown with 2σ uncertainties. The left y-axis corresponds to OLG apatite, while the right y-axis applies to Durango apatite.

Table 1 Electron probe microanalysis results for the 8 subsamples of apatite OLG

Sample	Na2O	MgO	SrO	CaO	SO3	P2O5	F	SiO2	Al2O3	FeO	BaO	Cl	Total
X1Y1Z1	0.049	0	0	54.099	0.746	38.343	2.634	1.693	0	0.032	0.02	0.03	96.53
X1Y1Z1	0.07	0.04	0	53.928	0.601	37.701	2.698	1.718	0	0.046	0.01	0.036	95.704
X1Y1Z1	0.034	0.003	0	53.793	0.694	38.198	3.287	1.553	0.004	0.019	0	0.031	96.225
X1Y1Z1	0.056	0	0	53.754	0.697	38.419	2.994	1.767	0	0.047	0	0.034	96.499
X1Y1Z1	0.05	0	0	53.975	0.61	38.583	2.556	1.705	0	0.042	0	0.037	96.474
X1Y2Z2	0.055	0.007	0	53.635	0.68	38.816	3.308	1.782	0.009	0.042	0.034	0.028	96.997
X1Y2Z2	0.031	0.01	0	53.419	0.657	38.065	2.717	1.906	0.001	0	0	0.019	95.677
X1Y2Z2	0.028	0.009	0	53.462	0.626	38.487	3.531	1.708	0	0.011	0	0.02	96.39
X1Y2Z2	0.069	0.001	0	53.682	0.63	37.636	2.72	1.915	0	0.047	0.003	0.03	95.581
X1Y2Z2	0.065	0	0	53.846	0.686	37.584	2.969	1.934	0	0.029	0	0.022	95.88
X2Y1Z1	0.046	0.017	0	54.182	0.698	38.971	2.638	1.801	0	0.018	0.024	0.04	97.315
X2Y1Z1	0.069	0.017	0	54.274	0.719	38.618	3.26	1.753	0	0.022	0	0.039	97.381
X2Y1Z1	0.043	0.017	0	53.776	0.648	38.702	3.008	1.92	0	0.006	0.069	0.042	96.944
X2Y1Z1	0.05	0.017	0	53.64	0.724	38.123	2.73	1.782	0.006	0.04	0.007	0.024	95.988
X2Y1Z1	0.043	0.017	0	54.061	0.673	39.018	2.679	1.844	0	0.037	0.035	0.04	97.316
X2Y2Z2	0.055	0.017	0	53.743	0.741	38.467	3.009	1.933	0	0.012	0	0.038	96.732
X2Y2Z2	0.049	0.017	0	53.977	0.653	37.919	2.694	1.85	0	0.015	0.003	0.029	96.061
X2Y2Z2	0.063	0.017	0	54.011	0.691	38.131	3.446	1.765	0.01	0	0.079	0.04	96.791
X2Y2Z2	0.064	0.017	0	53.512	0.733	37.696	2.99	1.824	0	0.051	0	0.04	95.649
X2Y2Z2	0.053	0.017	0	53.426	0.697	38.081	2.84	1.868	0	0.067	0	0.046	95.878
X3Y1Z1	0.049	0.017	0	53.645	0.669	38.467	2.574	1.905	0.008	0.034	0.01	0.032	96.302
X3Y1Z1	0.058	0.017	0	53.579	0.637	37.472	3.123	1.963	0.002	0.083	0.01	0.043	95.652
X3Y1Z1	0.033	0.017	0	53.143	0.607	38.265	2.979	1.981	0	0.007	0	0.038	95.806
X3Y1Z1	0.049	0.017	0	53.746	0.632	37.801	2.648	1.927	0	0.019	0	0.048	95.744
X3Y1Z1	0.054	0.017	0	53.486	0.722	37.909	2.506	1.907	0	0.056	0.034	0.03	95.644
X3Y2Z2	0.054	0.017	0	53.595	0.674	38.197	2.739	1.948	0	0.042	0.024	0.028	96.146
X3Y2Z2	0.052	0.017	0	53.629	0.659	37.866	3.391	1.872	0	0.034	0.048	0.029	96.149
X3Y2Z2	0.039	0.017	0	53.344	0.634	38.239	3.015	1.916	0	0.042	0.014	0.026	95.995
X3Y2Z2	0.043	0.017	0	53.64	0.693	37.912	3.368	1.83	0.007	0.02	0.058	0.032	96.185
X3Y2Z2	0.045	0.017	0	53.462	0.603	38.454	3.501	1.797	0.004	0.068	0.051	0.045	96.56
X4Y1Z1	0.049	0.017	0	53.79	0.581	38.085	3.036	1.725	0.004	0.034	0.017	0.027	96.064
X4Y1Z1	0.079	0.017	0	53.973	0.673	37.745	2.546	1.806	0	0.002	0.065	0.028	95.849
X4Y1Z1	0.064	0.017	0	53.938	0.679	38.519	3.506	1.563	0.013	0	0.034	0.034	96.87
X4Y1Z1	0.057	0.017	0	53.726	0.699	38.072	2.998	1.729	0	0.037	0	0.041	96.088
X4Y1Z1	0.057	0.017	0	53.863	0.695	37.957	3.123	1.79	0	0.001	0.014	0.034	96.214
X4Y2Z2	0.064	0.017	0	53.884	0.723	37.127	2.742	1.746	0.009	0.011	0.089	0.049	95.279
X4Y2Z2	0.051	0.017	0	53.957	0.671	37.769	2.624	1.721	0	0.041	0	0.038	95.785
X4Y2Z2	0.054	0.017	0	53.802	0.587	38.047	2.798	1.762	0.012	0.014	0.054	0.045	95.987
X4Y2Z2	0.067	0.017	0	53.705	0.624	38.286	3.066	1.755	0	0.01	0.078	0.03	96.335
X4Y2Z2	0.056	0.017	0	54.032	0.595	37.984	3.119	1.734	0	0.047	0	0.053	96.299
Minimum	0.028	0	0	53.143	0.581	37.127	2.506	1.553	0	0	0	0.019	95.279
Maximum	0.079	0.04	0	54.274	0.746	39.018	3.531	1.981	0.013	0.083	0.089	0.053	97.381
Average	0.053	0.008	0	53.753	0.667	38.143	2.953	1.81	0.002	0.03	0.022	0.035	96.224
Sigma	0.011	0.009	0	0.243	0.045	0.41	0.302	0.102	0.004	0.021	0.027	0.008	0.515

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Table 2 (U–Th)/He dating results for the OLG apatite and Durango apatite fragments analyzed at CDUT. The current amount of ⁴He of CDUT is presented in Table S3

Sample	U238_atoms	U238_atoms_SE	Th232_atoms	Th232_atoms_SE	He_atoms	He_atoms_SE	Age (Ma)	±σ (Ma)	Th/U	Comments
OLG apatite
X1Y1Z1-1	3.09 × 10^12	1.06 × 10^11	2.50 × 10^13	8.00 × 10^11	2.80 × 10^12	2.83 × 10^10	243.1	6.3	8.1	Outlier
X1Y1Z1-2	3.07 × 10^12	9.14 × 10^10	2.50 × 10^13	7.31 × 10^11	2.28 × 10^12	2.30 × 10^10	198.8	4.7	8.2
X1Y1Z1-3	2.55 × 10^12	9.30 × 10^10	2.04 × 10^13	6.84 × 10^11	2.05 × 10^12	2.08 × 10^10	217.8	5.9	8.0
X1Y1Z1-4	2.15 × 10^12	5.68 × 10^10	1.75 × 10^13	5.19 × 10^11	1.72 × 10^12	1.74 × 10^10	214.3	5.0	8.1
X1Y1Z1-5	2.37 × 10^12	8.39 × 10^10	2.10 × 10^13	7.01 × 10^11	2.05 × 10^12	2.12 × 10^10	218.8	5.9	8.8
X1Y1Z1-6	3.77 × 10^12	1.08 × 10^11	3.19 × 10^13	8.70 × 10^11	3.04 × 10^12	3.14 × 10^10	210.4	4.8	8.5
X1Y1Z1-7	3.12 × 10^12	1.12 × 10^11	2.76 × 10^13	1.21 × 10^12	2.66 × 10^12	2.76 × 10^10	216.0	7.1	8.9
X1Y2Z2-1	1.93 × 10^12	5.96 × 10^10	9.78 × 10^12	4.33 × 10^11	7.63 × 10^11	7.48 × 10^9	140.8	4.1	5.1	Outlier
X1Y2Z2-2	3.17 × 10^12	9.98 × 10^10	2.56 × 10^13	7.71 × 10^11	2.54 × 10^12	2.57 × 10^10	215.5	5.3	8.1
X1Y2Z2-3	2.19 × 10^12	6.32 × 10^10	1.16 × 10^13	3.09 × 10^11	1.23 × 10^12	1.25 × 10^10	194.5	4.2	5.3
X1Y2Z2-4	3.28 × 10^12	9.37 × 10^10	2.61 × 10^13	7.20 × 10^11	1.98 × 10^12	1.96 × 10^10	164.3	3.7	8.0	Outlier
X1Y2Z2-5	1.06 × 10^12	3.40 × 10^10	9.29 × 10^12	2.68 × 10^11	8.89 × 10^11	9.09 × 10^9	213.8	5.1	8.8
X1Y2Z2-6	1.10 × 10^12	4.01 × 10^10	1.02 × 10^13	2.66 × 10^11	9.25 × 10^11	9.49 × 10^9	206.4	4.8	9.2
X1Y2Z2-7	2.46 × 10^12	7.95 × 10^10	2.26 × 10^13	6.64 × 10^11	2.30 × 10^12	2.38 × 10^10	230.7	5.6	9.2
X2Y1Z1-1	1.80 × 10^12	2.42 × 10^12	8.24 × 10^12	6.23 × 10^12	3.49 × 10^12	3.54 × 10^10	704.6	519.1	4.6	Grain lost
X2Y1Z1-2	3.35 × 10^12	1.09 × 10^11	2.48 × 10^13	7.64 × 10^11	2.57 × 10^12	2.61 × 10^10	218.1	5.4	7.4
X2Y1Z1-3	3.33 × 10^12	8.53 × 10^10	2.69 × 10^13	8.08 × 10^11	2.63 × 10^12	2.66 × 10^10	212.4	5.0	8.1
X2Y1Z1-4	3.06 × 10^12	8.97 × 10^10	2.44 × 10^13	6.85 × 10^11	2.29 × 10^12	2.32 × 10^10	203.3	4.7	8.0
X2Y1Z1-5	3.81 × 10^12	1.22 × 10^11	2.98 × 10^13	1.11 × 10^12	3.00 × 10^12	3.12 × 10^10	216.2	6.1	7.8
X2Y1Z1-6	4.15 × 10^12	1.21 × 10^11	3.26 × 10^13	1.58 × 10^12	3.10 × 10^12	3.22 × 10^10	204.6	7.0	7.9
X2Y1Z1-7	5.45 × 10^12	2.53 × 10^11	4.18 × 10^13	1.36 × 10^12	3.94 × 10^12	4.05 × 10^10	201.1	5.7	7.7
X2Y2Z2-1	3.74 × 10^12	1.11 × 10^11	2.98 × 10^13	1.37 × 10^12	2.89 × 10^12	2.93 × 10^10	209.7	6.9	8.0
X2Y2Z2-2	1.98 × 10^12	5.60 × 10^10	1.60 × 10^13	4.54 × 10^11	1.66 × 10^12	1.67 × 10^10	225.3	5.2	8.1
X2Y2Z2-3	9.52 × 10^11	3.12 × 10^10	7.76 × 10^12	2.84 × 10^11	8.18 × 10^11	8.28 × 10^9	229.6	6.4	8.1
X2Y2Z2-4	2.88 × 10^12	9.68 × 10^10	2.38 × 10^13	7.06 × 10^11	2.36 × 10^12	2.39 × 10^10	217.1	5.3	8.3
X2Y2Z2-5	1.84 × 10^13	6.13 × 10^11	1.42 × 10^14	8.18 × 10^12	1.32 × 10^13	1.38 × 10^11	198.8	7.9	7.7
X2Y2Z2-6	9.21 × 10^12	2.60 × 10^11	6.86 × 10^13	4.42 × 10^12	6.46 × 10^12	6.67 × 10^10	198.8	8.5	7.4
X2Y2Z2-7	1.42 × 10^13	4.82 × 10^11	1.08 × 10^14	4.96 × 10^12	9.70 × 10^12	1.01 × 10^11	191.2	6.3	7.6
X3Y1Z1-1	6.64 × 10^11	7.73 × 10^11	5.38 × 10^12	7.99 × 10^12	1.61 × 10^12	1.62 × 10^10	637.5	641.7	8.1	Grain lost
X3Y1Z1-2	2.67 × 10^12	7.23 × 10^10	2.19 × 10^13	7.08 × 10^11	2.12 × 10^12	2.15 × 10^10	211.5	5.3	8.2
X3Y1Z1-3	3.55 × 10^12	9.71 × 10^10	2.85 × 10^13	7.52 × 10^11	2.79 × 10^12	2.81 × 10^10	212.3	4.6	8.0
X3Y1Z1-4	3.84 × 10^12	1.07 × 10^11	3.10 × 10^13	9.28 × 10^11	2.93 × 10^12	2.95 × 10^10	205.3	4.9	8.1
X3Y1Z1-5	6.15 × 10^12	2.08 × 10^11	4.51 × 10^13	1.53 × 10^12	4.73 × 10^12	4.97 × 10^10	219.9	5.8	7.3
X3Y1Z1-6	5.95 × 10^12	3.08 × 10^11	4.29 × 10^13	1.70 × 10^12	4.10 × 10^12	4.24 × 10^10	199.3	6.5	7.2
X3Y1Z1-7	3.68 × 10^12	1.95 × 10^11	2.68 × 10^13	1.24 × 10^12	2.67 × 10^12	2.77 × 10^10	208.5	7.6	7.3
X3Y2Z2-1	2.98 × 10^12	8.43 × 10^10	2.42 × 10^13	8.68 × 10^11	2.45 × 10^12	2.49 × 10^10	220.3	5.9	8.1
X3Y2Z2-2	3.52 × 10^12	1.12 × 10^11	2.91 × 10^13	9.08 × 10^11	2.95 × 10^12	2.98 × 10^10	221.9	5.6	8.2
X3Y2Z2-3	1.38 × 10^12	4.82 × 10^10	1.13 × 10^13	4.40 × 10^11	1.14 × 10^12	1.15 × 10^10	220.2	6.5	8.2
X3Y2Z2-4	1.44 × 10^12	4.80 × 10^10	1.18 × 10^13	5.10 × 10^11	1.09 × 10^12	1.10 × 10^10	201.8	6.4	8.2
X3Y2Z2-5	2.54 × 10^12	8.86 × 10^10	2.02 × 10^13	1.01 × 10^12	1.85 × 10^12	1.95 × 10^10	198.0	7.1	8.0
X3Y2Z2-6	3.27 × 10^12	1.06 × 10^11	2.62 × 10^13	1.40 × 10^12	2.41 × 10^12	2.49 × 10^10	199.4	7.5	8.0
X3Y2Z2-7	6.26 × 10^12	2.43 × 10^11	4.82 × 10^13	2.24 × 10^12	4.72 × 10^12	4.92 × 10^10	209.2	7.1	7.7
X4Y1Z1-1	3.31 × 10^12	1.03 × 10^11	2.75 × 10^13	9.69 × 10^11	2.59 × 10^12	2.62 × 10^10	206.7	5.6	8.3
X4Y1Z1-2	1.94 × 10^12	5.89 × 10^10	1.61 × 10^13	4.86 × 10^11	1.57 × 10^12	1.59 × 10^10	213.9	5.2	8.3
X4Y1Z1-3	1.09 × 10^12	3.50 × 10^10	8.68 × 10^12	2.90 × 10^11	8.02 × 10^11	8.12 × 10^9	199.9	5.2	8.0
X4Y1Z1-4	2.52 × 10^12	8.59 × 10^10	2.01 × 10^13	7.81 × 10^11	1.98 × 10^12	2.00 × 10^10	213.1	6.2	8.0
X4Y1Z1-5	3.11 × 10^12	8.71 × 10^10	2.19 × 10^13	8.72 × 10^11	2.17 × 10^12	2.29 × 10^10	204.8	5.8	7.0
X4Y1Z1-6	3.04 × 10^12	1.62 × 10^11	2.22 × 10^13	1.14 × 10^12	2.12 × 10^12	2.21 × 10^10	200.1	7.8	7.3
X4Y1Z1-7	7.55 × 10^12	3.13 × 10^11	5.46 × 10^13	1.66 × 10^12	4.86 × 10^12	5.04 × 10^10	186.0	4.9	7.2	Outlier
X4Y2Z2-1	1.57 × 10^12	4.35 × 10^10	1.31 × 10^13	4.90 × 10^11	1.18 × 10^12	1.20 × 10^10	198.1	5.5	8.3
X4Y2Z2-2	2.06 × 10^12	6.45 × 10^10	1.69 × 10^13	6.16 × 10^11	1.76 × 10^12	1.79 × 10^10	227.3	6.3	8.2
X4Y2Z2-3	1.42 × 10^12	3.63 × 10^10	1.18 × 10^13	5.02 × 10^11	1.16 × 10^12	1.18 × 10^10	215.7	6.6	8.3
X4Y2Z2-4	2.34 × 10^12	7.39 × 10^10	1.91 × 10^13	5.99 × 10^11	1.88 × 10^12	1.89 × 10^10	214.6	5.4	8.2
X4Y2Z2-5	4.71 × 10^12	2.80 × 10^11	4.10 × 10^13	2.03 × 10^12	3.95 × 10^12	4.08 × 10^10	214.7	8.5	8.7
X4Y2Z2-6	7.56 × 10^12	3.09 × 10^11	6.56 × 10^13	3.65 × 10^12	5.93 × 10^12	6.18 × 10^10	201.4	8.1	8.7
X4Y2Z2-7	6.35 × 10^12	2.13 × 10^11	5.18 × 10^13	2.12 × 10^12	5.04 × 10^12	5.25 × 10^10	212.1	6.5	8.2
Durango apatite–age standard
DUR-84	3.47 × 10^11	1.39 × 10^10	8.28 × 10^12	2.81 × 10^11	8.95 × 10^10	9.10 × 10^8	30.8	1.0	23.8
DUR-85	3.70 × 10^11	1.20 × 10^10	8.54 × 10^12	2.74 × 10^11	9.49 × 10^10	9.59 × 10^8	31.5	0.9	23.0
DUR-86	8.64 × 10^11	2.75 × 10^10	1.98 × 10^13	7.03 × 10^11	2.19 × 10^11	2.22 × 10^9	31.3	1.0	23.0
DUR-87	4.85 × 10^11	1.43 × 10^10	1.11 × 10^13	4.91 × 10^11	1.19 × 10^11	1.20 × 10^9	30.4	1.2	23.0
DUR-88	3.38 × 10^11	1.23 × 10^10	8.05 × 10^12	2.45 × 10^11	8.80 × 10^10	8.91 × 10^8	31.1	0.9	23.8
DUR-89	2.54 × 10^11	8.34 × 10^9	6.18 × 10^12	1.94 × 10^11	6.56 × 10^10	6.67 × 10^8	30.3	0.9	24.4
DUR-90	1.01 × 10^11	3.81 × 10^9	2.07 × 10^12	7.09 × 10^10	2.18 × 10^10	2.23 × 10^8	29.3	0.9	20.6
DUR-91	4.28 × 10^11	1.34 × 10^10	1.04 × 10^13	3.32 × 10^11	1.09 × 10^11	1.11 × 10^9	30.0	0.9	24.3
DUR-92	4.63 × 10^11	1.56 × 10^10	1.14 × 10^13	4.00 × 10^11	1.24 × 10^11	1.25 × 10^9	31.1	1.0	24.6
DUR-93	3.01 × 10^11	1.08 × 10^10	7.06 × 10^12	2.35 × 10^11	7.42 × 10^10	7.55 × 10^8	29.9	0.9	23.5
DUR-94	4.26 × 10^11	1.20 × 10^10	8.24 × 10^12	2.45 × 10^11	9.86 × 10^10	9.98 × 10^8	32.9	0.9	19.3
DUR-95	5.86 × 10^11	1.53 × 10^10	1.11 × 10^13	4.81 × 10^11	1.25 × 10^11	1.27 × 10^9	30.9	1.1	18.9
DUR-96	4.08 × 10^11	1.30 × 10^10	8.20 × 10^12	2.51 × 10^11	9.25 × 10^10	9.35 × 10^8	31.2	0.9	20.1
DUR-97	4.60 × 10^11	1.26 × 10^10	9.59 × 10^12	3.33 × 10^11	1.05 × 10^11	1.07 × 10^9	30.5	0.9	20.8
DUR-98	6.52 × 10^11	2.00 × 10^10	1.20 × 10^13	4.71 × 10^11	1.31 × 10^11	1.33 × 10^9	29.7	1.0	18.4
DUR-233	5.68 × 10^11	3.77 × 10^10	9.64 × 10^12	6.81 × 10^11	1.07 × 10^11	1.13 × 10^9	29.9	1.8	17.0
DUR-234	6.46 × 10^11	1.70 × 10^10	1.33 × 10^13	3.67 × 10^11	1.42 × 10^11	1.50 × 10^9	29.7	0.8	20.6
DUR-235	6.04 × 10^11	1.98 × 10^10	1.37 × 10^13	3.53 × 10^11	1.49 × 10^11	1.57 × 10^9	30.6	0.8	22.8
DUR-236	3.32 × 10^11	1.32 × 10^10	7.71 × 10^12	2.33 × 10^11	8.20 × 10^10	8.63 × 10^8	30.2	0.9	23.2
DUR-237	4.52 × 10^11	1.62 × 10^10	9.59 × 10^12	2.99 × 10^11	1.12 × 10^11	1.20 × 10^9	32.7	0.9	21.2
DUR-238	7.34 × 10^11	4.22 × 10^10	1.66 × 10^13	4.86 × 10^11	1.87 × 10^11	1.95 × 10^9	31.7	0.9	22.7
DUR-239	9.57 × 10^11	3.88 × 10^10	2.45 × 10^13	8.65 × 10^11	2.62 × 10^11	2.76 × 10^9	30.8	1.0	25.6
DUR-240	8.24 × 10^11	2.94 × 10^10	1.81 × 10^13	7.20 × 10^11	1.96 × 10^11	2.08 × 10^9	30.5	1.1	21.9
DUR-241	1.16 × 10^12	3.88 × 10^10	2.53 × 10^13	1.54 × 10^12	2.66 × 10^11	2.81 × 10^9	29.5	1.5	21.8

---

Table 3 (U–Th)/He dating results for the OLG apatite and Durango apatite fragments analyzed at NINH

Sample	^238U (mol)	1s	^232Th (mol)	1s	He (mol)	1s	Age (Ma)	1s	Th/U	Comments
OLG apatite
X1Y1Z1	6.64 × 10^−12	1.34 × 10^−13	5.53 × 10^−11	1.14 × 10^−12	4.97 × 10^−12	5.40 × 10^−14	197.0	3.6	8.3
X1Y2Z2	5.01 × 10^−12	9.80 × 10^−14	4.28 × 10^−11	8.75 × 10^−13	3.91 × 10^−12	4.16 × 10^−14	202.0	3.7	8.6
X2Y1Z1	4.29 × 10^−12	8.67 × 10^−14	3.68 × 10^−11	8.08 × 10^−13	3.73 × 10^−12	4.06 × 10^−14	224.3	4.3	8.6
X2Y2Z2	5.50 × 10^−12	1.12 × 10^−13	4.67 × 10^−11	9.37 × 10^−13	4.61 × 10^−12	5.06 × 10^−14	217.5	4.0	8.5
X3Y1Z1	3.27 × 10^−12	6.88 × 10^−14	2.83 × 10^−11	5.89 × 10^−13	2.87 × 10^−12	3.03 × 10^−14	225.2	4.2	8.6
X3Y2Z2	7.27 × 10^−12	1.46 × 10^−13	6.18 × 10^−11	1.22 × 10^−12	5.93 × 10^−12	6.72 × 10^−14	211.7	3.9	8.5
X4Y1Z1	5.18 × 10^−12	1.16 × 10^−13	4.46 × 10^−11	8.95 × 10^−13	4.14 × 10^−12	4.51 × 10^−14	205.5	3.8	8.6
X4Y2Z2	6.52 × 10^−12	1.29 × 10^−13	5.52 × 10^−11	1.07 × 10^−12	5.09 × 10^−12	5.45 × 10^−14	203.5	3.6	8.5
Durango apatite–age standard
DUR1205	1.82 × 10^−13	3.85 × 10^−15	3.76 × 10^−12	7.06 × 10^−14	4.33 × 10^−14	5.10 × 10^−16	32.0	0.6	20.7
DUR1206	2.71 × 10^−13	5.77 × 10^−15	5.75 × 10^−12	1.11 × 10^−13	6.47 × 10^−14	7.68 × 10^−16	31.4	0.6	21.2

---

5 Discussion

5.1 Compositional and age homogeneity of OLG

The most critical characteristic of a potential geochronological reference material is its compositional and textural homogeneity. To rigorously evaluate this aspect in OLG apatite, we conducted a series of petrographic and geochemical analyses. BSE imaging revealed a uniform internal texture, with no discernible zoning, fractures, or mineralogical heterogeneities, suggesting a promising degree of crystal integrity. To further investigate the intra-crystalline homogeneity, a representative OLG apatite crystal was sectioned longitudinally along the c-axis into eight large fragments (subsamples), ensuring coverage across different growth zones.

Quantitative analyses of U and Th concentrations were conducted on randomly selected fragments from each subsample using LA-ICP-MS. The results show negligible spatial variation in parent isotope concentrations across the crystal. Mean U and Th contents were measured as 112.2 ± 3.7 ppm (1σ) and 923.7 ± 26.7 ppm (1σ), respectively, yielding a relatively high and consistent Th/U ratio of ∼8.2 (Table 2). These data are in excellent agreement with the observed textural homogeneity, confirming the lack of significant geochemical zoning or inclusions that might compromise the reproducibility of (U–Th)/He dating results. The resulting dates are highly consistent, yielding a weighted mean age of 210.2 ± 1.4 Ma (2σ). The mean age determined at CDUT was 210.4 ± 1.6 Ma (2σ), while NINH yielded a comparable age of 209.7 ± 2.7 Ma (2σ). These results demonstrate excellent inter-laboratory reproducibility, reinforcing the reliability and robustness of OLG as a candidate reference material for high-precision (U–Th)/He thermochronology (Fig. 6).

Fig. 6 Reproducibility of (U–Th)/He ages for OLG apatite fragments. All ages are presented with 2σ uncertainties. Mean ages were calculated as error weighted means using IsoplotR (https://isoplotr.bgs.ac.uk) (Vermeesch, 2018). The 4 white boxes are outliers, which is not used for age calculation. The solid line and shaded gray band indicate the weighted mean age and its 2σ uncertainty for the entire dataset. Dashed lines represent the confidence interval accounting for overdispersion.²⁴ Colors correspond to the eight subsamples analyzed.

To examine the age variation from rim to core, we selected the sample X4Y2Z2 and analyzed eight fragments along its axis (Fig. 7). The core yielded an age of ∼204.5 Ma, while the rim was dated at ∼218.7 Ma. The weighted mean age of ∼215.3 Ma is consistent within error with previous values and shows no systematic trend, suggesting that the observed differences may be attributable to random errors. The observed homogeneity in both chemical composition and radiogenic helium retention, combined with consistent analytical results across laboratories, positions the OLG apatite as a strong candidate for a new community-wide standard, particularly suited to applications requiring an older reference age.

Fig. 7 (a) The measured (U–Th)/He ages (1σ) of fragments from the edge to the core of subsample X4Y2Z2 and (b) the (U–Th)/He ages (2σ) of the fragments.

5.2 Thermal history of OLG

In this study, QTQt 5.9.0 was used in thermal history modelling²⁵ (Fig. 8). The fission track annealing model follows Ketcham et al. (2007),^26,27 and the RDAAM model was applied to account for the radiation damage.^23,28 U and Th concentrations were set at ∼112.2 and ∼927.3 ppm, respectively. The helium age was 210.2 ± 0.7 Ma (1σ), with a radius of 12 000 µm estimated from the crystal size. A constraint box of 900 ± 50 Ma and 450 ± 100 °C was imposed as the start of the thermal history, based on the previous geochronological data^14,15 and closure temperature²⁹ of the U–Pb system. The constrained present-day temperature was set as 15 ± 15 °C. Notably, the He ages are significantly older than the FT ages. This may be attributed to the accumulation of radiation damage in apatite, which reduces He diffusivity and raises the closure temperature, thereby enhancing He retention within the crystal.^23,28,30 In addition, the relatively large grain size increases the diffusion distance, further promoting He retention.^22,31,32 The thermal evolution of the Otter Lake region in the Grenville Province documents a polyphase history that aligns with its tectonometamorphic framework. Apatite crystallization and isotopic closure occurred at ∼900 Ma, contemporaneous with late Grenvillian metamorphism and fluid–rock interaction during the Ottawan phase of the orogen.^13,33 Significant cooling between 250 and 150 Ma is consistent with regional uplift and erosion linked to Mesozoic tectonism, including rifting associated with the opening of the North Atlantic and reactivation of Grenvillian structures.^34,35 A final phase of accelerated cooling after ca. 20 Ma reflects near-surface exhumation, plausibly driven by enhanced denudation during Pleistocene glaciations and/or neotectonic uplift affecting eastern North America.^36–38 Overall, a single apatite grain cannot fully capture the complex thermal history of the region, and a more detailed investigation is therefore required.

Fig. 8 Thermal history of OLG apatite. Only the temperature range above the closure temperature of the AFT (∼140 °C) are shown. The figure is made with QTQt displayer (A. Derycke) available at https://deryckehub.ovh.

5.3 The advantage of old age reference

The currently used AHe dating standards range from 16 to 31 Ma, but employing older-age standards offers several key advantages that enhance the accuracy, precision, and reliability of the method. In conventional solution-based (U–Th)/He dating, the measurements of parent and daughter isotopes are fully decoupled, and the larger single grains can yield higher signals to achieve reliable results. However, in most routine practices, standard crystals are crushed to sizes comparable to unknown samples (generally <100 mesh). This is done primarily to avoid two major issues: (1) oversized fragments can generate excessively high signals, leading to strong memory effects^39,40 in ICP-MS—particularly for Th—that may affect subsequent analyses for small grains and (2) reducing grain size ensures that both standards and unknowns are fully degassed during helium extraction. Considering these factors, older standards remain particularly valuable. Younger apatites typically contain lower helium concentrations, making them more susceptible to analytical noise in mass spectrometry,^41,42 whereas older standards, with higher helium contents, significantly improve measurement precision and reduce uncertainties.^43,44 For in situ He analysis, a standard is required to calculate the κ value,⁴⁵ and standards with higher He signals allow the use of smaller analytical beam sizes,⁴² thereby enhancing the applicability of in situ (U–Th)/He dating. By incorporating older apatite standards, AHe thermochronology benefits from reduced diffusion uncertainties, minimized analytical errors, expanded applicability to deep-time studies, improved validation of helium retention, and refined diffusion models, ultimately leading to greater accuracy and reliability.

6 Conclusion

(1) The OLG apatite exhibits excellent age reproducibility (210.2 ± 1.4 Ma, 2σ) and homogeneous Th/U ratios in (U–Th)/He analyses from two independent laboratories, supporting its suitability as a reference.

(2) BSE imaging, EPMA, and LA-ICP-MS analyses confirm its major-element homogeneity and generally uniform trace-element distribution, though minor inclusions may exist.

(3) Compared to existing standards, the OLG apatite provides an older reference age (∼210.2 Ma) with strong U, Th, and He signals, making it a robust benchmark for high-precision (U–Th)/He thermochronology.

Conflicts of interest

The authors have no competing interests to declare that are relevant to the content of this article.

Data availability

For requests of OLG samples, please contact the corresponding or first author. We will provide 150–200 mg of material per laboratory.

The raw data are available in supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5ja00246j.

Acknowledgements

This research was financially supported by Sichuan Provincial Department of Science and Technology (No. 2025ZNSFSC1167) and National Natural Science Foundation of China (No. 42302342 and 42230310). The authors appreciate Zhang Tianyao and Tang Rong for help in data collection and Dr Liu Guoqi for data reduction.

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