Hidra aeschynite-(Y): a potential natural reference material for microbeam Pb–Pb and Lu–Hf geochronology

Abstract

Niobium (Nb) is a critical strategic metal essential for advanced industrial and aerospace technologies. As aeschynite represents one of the primary natural sources of Nb, its geochronological characterization is of significant interest. In situ microbeam geochronology of aeschynite can provide a direct approach to constraining the timing of niobium mineralization and regional geological evolution. Accurate and precise in situ microbeam dating using SIMS or LA-ICP-MS requires well-characterized matrix-matched reference materials to correct for matrix-induced elemental fractionation. However, a critical gap exists due to the current absence of natural aeschynite reference materials. To address this deficiency, we propose Hidra aeschynite-(Y) from the Urstad Feldspar Mine, Norway, as a candidate of natural reference material for Pb–Pb and Lu–Hf dating. Multiple SIMS spot analyses demonstrate that it has a homogeneous age, with an arithmetic mean ²⁰⁷Pb/²⁰⁶Pb age of 902.1 ± 21.3 Ma (2SD; n = 46), corroborated by three LA-ICP-MS/MS analytical sessions (arithmetic mean ²⁰⁷Pb/²⁰⁶Pb age = 902.5 ± 22.5 Ma, 2SD; n = 55). Six ID-TIMS analyses yield an arithmetic mean ²⁰⁷Pb/²⁰⁶Pb age of 912.8 ± 6.9 Ma (2SD), recommended as the best estimate of the crystallization age. In addition, in situ LA-ICP-MS/MS Lu–Hf analyses of Hidra aeschynite-(Y) calibrated with NIST SRM 610 yield an isochron age of 912.5 ± 6.5 Ma (2σ; MSWD = 1.4; n = 68). The convergence of microbeam ²⁰⁷Pb/²⁰⁶Pb ages with the high-precision ID-TIMS benchmark, combined with reproducible Lu–Hf results, establishes Hidra aeschynite-(Y) as the first potential matrix-matched reference material for microbeam in situ Pb–Pb and Lu–Hf geochronology of aeschynite.

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1. Introduction

Niobium (Nb) exhibits exceptional properties, including high-temperature stability, corrosion resistance, and superconductivity,^1–3 enabling diverse applications in high-strength low-alloy steels, aerospace components, superconducting devices, advanced electronics, medical implants, and nuclear technologies.^2,3 Rising global demand, combined with constrained supply, has elevated Nb's economic criticality and supply-risk indices, generating urgent exploration imperatives.^2,3 However, in many deposits, the timing of mineralization and the mechanisms responsible for Nb enrichment remain poorly understood due to insufficient geochronological constraints, which in turn hampers resource development and utilization of Nb resources.^4,5 As a principal Nb-bearing mineral, aeschynite has attracted significant economic and scientific interest^6,7 and represents a potential target for Nb recovery.¹ Consequently, achieving direct high-precision geochronology of aeschynite is particularly valuable, as ore minerals always provide robust constraints on the timing of mineralization and subsequent geological evolution, thereby offering key insights into ore-forming processes and guiding the exploration of new Nb resources.^8–10

Aeschynite is an orthorhombic mineral group with the general formula AB₂O₆.¹ The large-cation A site is predominantly occupied by rare earth elements (REEs), calcium (Ca), thorium (Th), and uranium (U), while titanium (Ti), niobium (Nb), and tantalum (Ta) occupy the B site.¹ Given U incorporation at the A-site, both U–Pb and Pb–Pb isotopic systems enable aeschynite geochronology via isotope dilution-thermal ionization mass spectrometry (ID-TIMS), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS),¹¹ and secondary ionization mass spectrometry (SIMS).¹² Although ID-TIMS delivers benchmark precision and accuracy,¹² it has not been widely adopted due to its time-consuming dissolution and column chemistry procedures, which requires sample preparation in a specialized clean laboratory. Furthermore, the poor spatial resolution of ID-TIMS leads to the averaging of isotopic zonation over a single aliquot, limiting its application in certain geochronological studies.^13–15

The SIMS and LA-ICP-MS methods offer significant advantages, including high spatial resolution and rapid analytical speed.^15–21 However, both techniques require matrix-matched reference materials to calibrate matrix effects and to monitor the quality of U–Pb and Pb–Pb ages.^22–26 These reference materials are also crucial for method refinement, quality assurance, and cross-laboratory assessments.^27–32 For aeschynite geochronology, the natural aeschynite reference materials for U–Pb and Pb–Pb geochronology are currently unavailable, highlighting their urgent need. Here, we characterize Hidra aeschynite-(Y) as a new potential reference material for in situ Pb–Pb and Lu–Hf dating. The homogeneity of its ²⁰⁷Pb/²⁰⁶Pb ages was assessed using both SIMS and LA-ICP-MS. Based on the results, the TIMS-derived ²⁰⁷Pb/²⁰⁶Pb age of Hidra aeschynite-(Y) is recommended as the refernce age for this material. Furthermore, the high lutetium (Lu) content of Hidra aeschynite-(Y) renders it a promising candidate for Lu–Hf geochronology, and we assess the feasibility of its Lu–Hf dating using the innovative LA-ICP-MS/MS technique.

2. Materials and methods

The Hidra aeschynite-(Y) specimen (Fig. 1), sourced from the Urstad Feldspar Mine in Hitterø, Hidra, Flekkefjord, Vest-Agder, Norway, was purchased from a Gem dealer. It has a total weight of ∼20 g (Fig. 1). This size is enough for the future distribution in microbeam laboratories (e.g., LA-ICP-MS and SIMS) as well as for the inter-laboratory comparison project. The timing of formation for the geological setting of Hidra remains a subject of debate.³³ Various studies have suggested ages ranging from 1040 ± 17 Ma for an orthopyroxene megacryst to 910.7 ± 3.1 Ma for pegmatite formation.³⁴ Zircon dating has provided emplacement dates for anorthosite (929 ± 2 to 932 ± 3 Ma) and norite at 920 ± 3 Ma.³⁵ Additionally, zircon U–Pb dating of a charnockitic dyke cutting the Hidra pluton yields an age of 931 ± 10 Ma,³⁶ while monazite from the Hidra leuconorite pluton provides a U–Pb age of 932 ± 9 Ma.³⁷ A separate U–Pb date of 910.7 ± 3.1 Ma has been obtained for columbite in a pegmatite from Hidra Island.³⁴ These reported diverse ages reflect the complex geological evolution of the Hidra area.

Fig. 1 Hand specimen of Hidra aeschynite-(Y). The sample has a weight of ∼20 g.

2.1 Raman analysis and BSE and TIMA imaging

Raman spectra were acquired using a WITec confocal Raman microscope alpha 300R at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS). Spectra were collected in reflection mode using a laser excitation wavelength of 532 nm and a 300 gr per mm diffraction grating. Spectra in the 100–4000 cm⁻¹ region were obtained with a collection time of 2 s and 20 accumulations at a resolution of 4.8 cm⁻¹. The laser power on the sample was set to 5.8 mW. Imaging was conducted using a Zeiss EC Epiplan optical microscope with a 50× magnification.

The BSE and TIMA analyses were conducted on carbon-coated thin sections using a TESCAN TIMA GMS at the Mining Research Institute of Baotou Steel (Group) Corp. The analyses were performed with an acceleration voltage of 25 kV and a probe current of 12 nA, with a working distance of 15 mm. Pixel spacing was set to 1 μm. The current and the backscattered electron (BSE) signal intensity were calibrated using a platinum Faraday cup through the automated procedure. The performance of energy-dispersive spectroscopy (EDS) was verified using a manganese standard. The samples were scanned using the TIMA liberation analysis module.

2.2 EPMA major element analysis

EPMA analysis was performed at the Mining Research Institute of Baotou Steel (Group) Corp. using the JXA-iSP100 instrument. The operating conditions included an accelerating voltage of 15 kV and a beam current of 10 nA, with a beam spot diameter ranging from 1 to 10 μm. The following specific elemental standards were used: Ca-apatite; Ti-rutile; Fe-magnetite; Nb-LiNbO₃; U-uraninite; Th-ThO₂; Ta-LiTaO₃; Ce-H₂CeO₅P; Nd-H₂NdO₅P; Sm-H₂SmO₅P; Gd-H₂GdO₅P; Dy-H₂DyO₅P; Er-H₂ErO₅P; Tm-H₂TmO₅P; Yb-H₂YbO₅P; Lu-LSO; Y-H₂YO₅P.

2.3 LA-ICP-MS trace element analysis

Trace element contents of Hidra aeschynite-(Y) were determined using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) with an Agilent 7500a Q-ICP-MS instrument (Agilent Technologies, USA) coupled to a 193 nm ArF excimer laser system (Geolas HD, Lambda Physik, Göttingen, Germany) at the IGGCAS. The method follows the approach outlined by Wu et al.³⁸ with isotopes measured in peak-hopping mode. The laser beam diameter was approximately 44 μm, and the repetition rate was set to 5 Hz. The laser energy density was ∼4.0 J cm⁻² and helium was employed as the ablation gas to improve the transporting efficiency of ablated aerosols. ARM-1 reference glass^39,40 was used for calibration, and NIST SRM 610 and BCR-2G were analyzed for data quality control. Niobium (⁹³Nb) was used as an internal standard. The data were processed using the GLITTER program.⁴¹ For most trace elements (>0.10 μg g⁻¹), the accuracy was better than ±10% with an analytical precision (1 RSD) of ±10%.

2.4 TIMS Pb–Pb age determination

The Hidra aeschynite-(Y) grains underwent multiple washings with alcohol and acetone before being dissolved in 150 μL of 29 mol per L HF at 220 °C for 48 hours. Afterward, the solution was dried to form salts and redissolved in 100 μL of 1.1 mol per L HBr at 150 °C overnight, then dried again. Once the sample residue was redissolved in 100 μL of 1.1 mol per L HBr, Pb was separated through HBr-based anion-exchange chromatography using 50 μL shrink Teflon FEP micro-columns.⁴²

Pb isotope ratios were measured at the IGGCAS using a Thermo-Electron TRITON Plus thermal ionization mass spectrometer with a secondary electron multiplier (SEM). The Pb isotope data obtained from the TRITON Plus mass spectrometer were processed with Tripoli software to remove outliers.⁴³ The fractionation effects of Pb isotopes were corrected based on the measurement results of the reference standard NBS981 Pb, with a mass fractionation coefficient of 0.16 ± 0.08% a.m.u. The total procedural Pb blank in this study was approximately 3–5 pg.

2.5 LA-ICP-MS U–Pb and Pb–Pb dating

In situ U–Pb dating of Hidra aeschynite-(Y) was performed using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) with an Element XR HR-ICP-MS instrument (Thermo Fisher Scientific, USA) coupled to a 193 nm ArF excimer laser system (Geolas HD, Lambda Physik, Göttingen, Germany) at the IGGCAS. A laser beam diameter of 32 μm, a repetition rate of 2 Hz, and an energy density of ∼3.0 J cm⁻² were employed. Helium was used as the ablation gas to improve the transport efficiency of ablated aerosols. Due to the lack of matrix-matched reference materials for aeschynite, the coltan reference materials were used for external calibration. Coltan 139 (ref. 44) served as the primary calibration material, and SN3 and ZKW were analyzed as quality control materials.⁴⁴ The U–Pb age was calculated and plotted using IsoplotR.⁴⁵ The individual data are summarized in SI Table S1. Fourteen analyses of Coltan 139 yielded a concordia age of 506 ± 14 Ma (MSWD = 0.01). Seven analyses of SN3 yielded a concordia age of 384.8 ± 8.7 Ma (MSWD = 1.5). Five analyses of ZKW yielded a concordia age of 206 ± 11 Ma (MSWD = 2.6). These data are generally matched with the recommended ages.

In situ Pb–Pb dating of Hidra aeschynite-(Y) was performed using an iCap TQ Triple Quadrupole ICP-MS coupled with an Analyte G2 193 nm ArF excimer laser ablation system (Teledyne CETAC, Omaha, USA) to determine ²⁰⁴Pb/²⁰⁶Pb and ²⁰⁷Pb/²⁰⁶Pb ratios. The analytical approach employed a laser spot size of 44 μm, an ablation frequency of 5 Hz, and an energy density of ∼5.0 J cm⁻². Helium was used as the ablation gas to enhance the transport efficiency of the ablated aerosols. The ARM-1 glass reference material was used as a calibrator for Pb isotopic ratios, and NIST SRM 610 (ref. 40 and 46) was used for data quality control. High-purity (99.999%) NH₃ was used as the reaction gas to completely remove the ²⁰⁴Hg interference on ²⁰⁴Pb. The analytical precision of ²⁰⁴Pb/²⁰⁶Pb and ²⁰⁷Pb/²⁰⁶Pb was better than 1.5%. Systematic uncertainties were estimated to be ∼2.0%, incorporating uncertainties from the reference material, long-term variation of the validation material, and other potential sources. This uncertainty was propagated to the final reported uncertainties. The ²⁰⁷Pb/²⁰⁶Pb ages were calculated using the ISOPLOT 3.0 software package.⁴⁷ Three independent sessions were carried out. The individual data are summarized in SI Table S2. For session 1, a total of 23 spot measurements were made on a large grain (∼1 cm). For session 2, a total of 18 spot measurements were made on 6 individual grains, with each grain containing 3 randomly selected spots. For session 3, a total of 14 spot measurements were made on 3 individual grains, with each grain containing 4–5 randomly selected spots. These measurements could evaluate the intra-grain, inter-grain, or inter-mount homogeneity of Hidra aeschynite-(Y).

2.6 SIMS Pb–Pb dating

The Pb–Pb isotope determination of Hidra aeschynite-(Y) was conducted using a CAMECA IMS 1280-HR SIMS at the IGGCAS. A primary ion beam of O₂⁻ produced by the Duo-plasma ion source was accelerated at a potential of −13 kV. Kohler illumination mode was used with a 200 μm aperture to create an ellipsoidal primary beam spot of 20 × 30 μm in size. The primary beam intensity was about 2 nA during the analyses. Before each analysis, a sputtering procedure was performed on the sample surface for 120 s using a 25 μm raster to clean the coating layer and remove surface contaminants. The positive secondary ions were accelerated at a potential of +10 kV. The combination of the field aperture, entrance slit, and energy slit was set to 1800 μm, 7 μm, and 30 eV bandwidth, respectively, to achieve a high mass resolution of 20 000 (50% peak height).

The axial EM of the mono-collector was employed as the detector for all the ion species. The mass peaks of ⁹³Nb₂¹⁶O⁺, ¹⁸⁶W¹⁶O⁺, ²⁰⁴Pb⁺, ²⁰⁶Pb⁺, ²⁰⁷Pb⁺, ²⁰⁸Pb⁺, ²³²Th⁺, ²³⁸U⁺, ²³²Th¹⁶O₂⁺ and ²³⁸U¹⁶O₂⁺ were determined successively by peak jumping. The counting time for the aforementioned peaks in one cycle was 2 s, 2 s, 6 s, 6 s, 8 s, 6 s, 4 s, 4 s, 2 s, and 2 s, respectively. Each analysis took approximately 11 minutes, including seven acquisition cycles. The ⁹³Nb₂¹⁶O⁺ and ¹⁸⁶W¹⁶O⁺ peaks were used to monitor the interference. All analyzed aeschynite grains, along with the glass reference materials NIST SRM 610 (ref. 48) and ARM-1 glass,³⁹ were mounted in an epoxy resin mount and carefully polished using diamond paste with successive grades down to 0.1 μm to achieve optically flat surfaces in the analysis areas, thereby minimizing relief-related effects.^49–52

Peripheral positioning can introduce analytical biases, including stage tilt, differential charging, inhomogeneous magnetic fields, and beam transmission.^50,52 To minimize these effects, all analytical spots were confined to within ∼5 mm of the central x–y coordinates of the epoxy mount.^50,53 The information for the spot position is provided in SI Table S2, with coordinate ranges of −1752 to 2991 μm in the X direction and −3632 to −2104 μm in the Y direction. A total of 46 spot analyses were performed on a large grain (∼1 cm) to assess inter-grain homogeneity. This grain was mounted within ∼5 mm of the central x–y coordinates of the sample mount. Individual spot analysis has a spatial distance of 100 μm to several hundred μm.

The Pb isotopic compositions of Hidra aeschynite-(Y) were calibrated by measuring the ARM-1 glass standard, which yielded a ²⁰⁷Pb/²⁰⁶Pb ratio of 0.8602 ± 0.0041 (n = 8), with a mass fractionation of approximately 0.6% a.m.u. All ages were calculated using the decay constants recommended by Steiger and Jäger,⁵⁴ and the calculation routines of IsoplotR.⁴⁵ The uncertainties of isotopic ratios are reported at the 1σ level. To monitor external uncertainties during the calibration process, the NIST SRM 610 glass standard was measured alongside Hidra aeschynite-(Y). All eight NIST SRM 610 analyses resulted in a ²⁰⁷Pb/²⁰⁶Pb ratio of 0.911 ± 0.013 (n = 8), consistent within error with the previously reported value of 0.9096 ± 0.0003.⁵⁵ The individual data are summarized in SI Table S2.

2.7 LA-ICP-MS/MS Lu–Hf geochronology

In situ Lu–Hf dating was carried out using a Photo Machine Analyst G2 laser ablation system (Teledyne CETAC, Omaha, USA) coupled to an iCap TQ ICP-MS/MS (Thermo Fisher Scientific, Bremen, Germany) at the IGGCAS. The method largely followed the procedures outlined by Wu et al.,⁵⁶ Wang et al.⁵⁷ and Yan et al.⁵⁸ High-purity (99.999%) NH₃ was used as the reaction gas due to its higher efficiency compared to the pre-mixed NH₃–He gas.⁵⁶ Prior to analysis, the reaction cell was flushed for at least 12 hours with NH₃ to minimize instrument drift. A small amount of N₂ (4.0 mL min⁻¹) was added to the carrier gas after the sample chamber to enhance sensitivity.^59,60 The reaction product of ^(176+82)Hf (expressed for ¹⁷⁶Hf[¹⁴N¹H][¹⁴N¹H₂]₂[¹⁴NH₃]₃, at a mass shift of +82) was measured to distinguish ¹⁷⁶Hf from ¹⁷⁶Lu and ¹⁷⁶Yb. A short dwell time of 1 ms for ¹⁷⁵Lu was used to reduce the potential interference of ^(175+83)Lu on ^(176+82)Hf by decreasing the amount of ¹⁷⁵Lu ions entering the reaction cell.

The laser spot diameter was set to 50 or 100 μm. ¹⁷⁵Lu was monitored as a proxy for ¹⁷⁶Lu, and the present-day ¹⁷⁶Lu/¹⁷⁵Lu ratio of 0.02655 was used to calculate the contents of ¹⁷⁶Lu. The instrumental drift of elemental and isotopic ratios was corrected using NIST SRM 610. The elemental fractionation of ¹⁷⁶Lu/¹⁷⁷Hf and mass bias of ¹⁷⁶Hf/¹⁷⁷Hf were first externally corrected by analyzing NIST SRM 610, using the recommended values of 0.1379 ± 0.0050 and 0.282111 ± 0.000009, respectively, as determined by ID-MC-ICP-MS.⁶¹ The Lu–Hf isotope data were processed using Iolite v.4.0 software and the Visual Lu–Hf age plugin.²⁷ The Lu–Hf ages were calculated using IsoplotR with conventional isochron regression. The individual data are summarized in SI Table S3.

3. Results

3.1 Raman crystal structure and BSE and TIMA images

Fig. 2 presents the Raman spectral data for Hidra aeschynite-(Y). The Raman spectrum at room temperature indicates a lack of crystallinity, with only a few broad vibration bands observed at approximately 130, 379, 589, 743, 2100 and 3516 cm⁻¹. These bands differ from those typically seen in aeschynite-(Y). The observed peaks at 743 cm⁻¹ and 589 cm⁻¹ are attributed to the stretching vibrations of the (Nb,Ti)O₆ octahedral group, while the peak at 379 cm⁻¹ corresponds to the bending vibrations of the same group.⁶² The peak at 130 cm⁻¹ is assigned to lattice vibrations, and the broad band at 2100 cm⁻¹ is likely related to fluorescence induced by rare earth elements. The broad band observed at 3516 cm⁻¹ suggests the potential presence of OH⁻ in the sample.⁶³

Fig. 2 Raman spectra of Hidra aeschynite-(Y), indicating structural disorder or amorphous characteristics in the lattice structure.

Compared to RRUFF reference database spectra, Hidra aeschynite-(Y) exhibits significant Raman peak broadening. This broadening is indicative of partial metamictization, reflecting structural disorder or amorphous characteristics in the lattice structure.⁶⁴

Fig. 3 presents BSE images and TIMA results for Hidra aeschynite-(Y) to evaluate its homogeneity and purity. The BSE images reveal minor grayscale contrast variations within the grain, showing no discernible growth zoning (Fig. 3a). The TIMA analysis confirms the aeschynite grains are inclusion-free and primarily coexist with minor orthoclase and quartz (Fig. 3b).

Fig. 3 BSE images and TIMA results for Hidra aeschynite-(Y): (a) BSE image; (b) TIMA result. The aeschynite grains are inclusion-free and primarily coexist with minor orthoclase and quartz.

3.2 Chemical compositions

Table 1 summarizes the EPMA results for Hidra aeschynite-(Y), listing the following oxides: TiO₂, Nb₂O₅, Y₂O₃, ThO₂, Yb₂O₃, UO₂, Ta₂O₅, Gd₂O₃, Er₂O₃, Dy₂O₃, Nd₂O₃, Tm₂O₃, Sm₂O₃, CaO, FeO, Lu₂O₃, and Ce₂O₃ (Table 1). The Y-dominant A-site and Ti-dominant B-site occupancy align with the crystallographic formula of aeschynite-(Y) (Fig. 4). The ThO₂ (8.44 to 9.46 wt%) and UO₂ (3.73 to 4.43 wt%) contents further validate the metamictization potential observed in Raman data.

Table 1 Elemental compositions of nine investigated Hidra aeschynite-(Y) samples by EPMA. The data are given in wt%. “—” represents that the data are not given due to below detection limits

Sample	Hidra-1	Hidra-2	Hidra-3	Hidra-4	Hidra-5	Hidra-6	Hidra-7	Hidra-8	Hidra-9	Hidra-10	Hidra-11	Hidra-12	Hidra-13	Hidra-14	Hidra-15	Hidra-16
CaO	0.16	0.68	0.42	0.47	0.42	—	0.11	0.07	1.14	1.31	1.38	1.31	1.85	2.44	1.73	1.97
Y2O3	16.94	16.25	16.80	16.63	16.70	17.21	16.98	16.60	16.28	16.76	16.27	15.75	15.50	15.02	15.95	15.51
Nb2O5	18.08	17.48	17.73	16.94	17.46	17.91	18.02	17.33	17.10	16.28	16.32	16.99	16.88	16.44	17.55	16.81
ThO2	9.34	9.02	9.42	9.01	8.53	9.46	9.19	9.12	8.51	8.44	8.53	8.70	8.85	8.98	8.92	8.96
UO2	4.22	4.21	4.25	3.76	4.02	3.84	4.43	4.31	3.80	3.73	3.90	3.83	4.22	4.25	4.35	4.29
TiO2	32.38	33.05	32.87	33.29	32.54	32.90	33.17	32.85	30.88	30.50	31.32	32.04	30.84	31.32	31.25	30.48
Ce2O3	0.43	0.33	0.16	0.22	0.39	0.35	0.43	0.37	0.25	0.32	0.26	0.51	0.32	0.33	0.36	0.20
Nd2O3	1.08	1.20	1.08	1.22	1.73	1.10	0.98	1.13	0.94	0.99	1.18	1.29	1.02	0.90	0.95	1.15
Sm2O3	1.01	1.18	1.08	1.74	1.26	1.11	0.89	1.02	1.14	1.19	0.97	1.22	0.90	0.95	0.48	1.14
FeO	0.98	0.92	0.99	0.83	1.12	0.45	0.65	0.34	0.61	0.77	0.70	0.59	0.59	0.65	0.41	0.42
Gd2O3	1.75	1.95	1.54	1.79	1.66	1.49	1.65	1.68	1.54	1.21	1.90	1.87	1.55	1.41	1.34	1.49
Dy2O3	1.08	2.33	0.83	1.02	2.47	2.16	0.78	2.71	0.63	1.15	1.38	1.22	0.32	0.83	0.30	0.58
Er2O3	1.35	—	2.36	1.42	1.80	1.13	2.42	0.32	1.08	1.50	1.95	0.92	0.12	1.03	1.76	0.68
Tm2O3	0.96	1.48	1.31	0.87	0.39	1.42	0.98	1.13	1.76	0.75	1.04	1.12	0.86	0.97	1.08	1.34
Yb2O3	4.99	5.98	5.43	6.32	6.11	6.08	5.02	5.70	6.25	5.51	6.15	5.48	5.22	2.84	4.96	4.62
Lu2O3	1.07	0.41	1.08	0.91	0.59	0.22	0.70	0.74	0.70	0.68	0.45	0.05	0.57	0.89	0.37	0.57
Ta2O5	2.98	2.98	2.96	1.95	3.02	2.98	3.04	2.82	1.94	1.98	2.73	2.77	2.80	2.60	2.70	1.93
Total	98.79	99.44	100.31	98.41	100.22	99.82	99.42	98.20	94.55	93.07	96.43	95.67	92.40	91.83	94.43	92.13

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Fig. 4 The elemental compositions of Hidra aeschynite-(Y) in the A site and B site. The data indicate that the Hidra sample is aeschynite-(Y).

Table 2 details the LA-ICP-MS/MS instrumentation used in this study. The LA-ICP-MS trace element compositions confirm that the main elements in Hidra aeschynite-(Y) are Ti, Y, Nb, and Th (Table 3), consistent with EPMA results presented in Table 1. The chondrite normalized REE patterns plotted based on the REE concentrations of Hidra aeschynite-(Y) (Fig. 5) exhibit marked Eu depletion and pronounced M-HREE enrichment (M-HREE: Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y). These compositional features demonstrate trace element homogeneity in Hidra aeschynite-(Y).

Table 2 Summary information of LA-ICP-MS/MS instrumentation in this study

Laser ablation system
Make, model & type	Photon Machines Analyte G2
Ablation cell & volume	HelEx ablation cell
Laser wavelength	193 nm
Pulse width	5 ns
Energy density/fluence	4 J cm^−2
Repetition rate	10 Hz
Spot size	50–110 μm
Sampling mode/pattern	Single hole drilling, two cleaning pulses
Ablation gas flow (He)	900 mL min^−1
Ablation duration	25 s
ICP-MS/MS
Make, model & type	Thermal iCap TQ
RF power	1350 W
Sample cone	High sensitivity
Skimmer cone	High sensitivity
Coolant gas flow (Ar)	15.00 L min^−1
Auxiliary gas flow (Ar)	0.80 L min^−1
Carrier gas flow (Ar)	0.65 L min^−1
Enhancement gas flow (N2)	4.0 mL min^−1
Scan mode	Peak jump
Isotopes measured (m/z) + sample time	^27Al (2 ms), ^43Ca (2 ms), ^89Y (1 ms), ^90Zr (2 ms), ^172Yb (1 ms), ^(172+82)Yb (100 ms), ^175Lu (1 ms), ^(175+16)Lu (50 ms), ^(175+82)Lu (50 ms), ^(176+82)Hf (300 ms), ^(176+82)Hf (100 ms) and ^(178+82)Hf (100 ms)
Detection system	Single SEM in double mode, counting and analog
Resolution	∼300
Total integration time per reading	0.658 s
Lens parameters
Major
Extraction lens 2 (V)	−140.000
Angular deflection (V)	−250.000
Q1 entry lens (V)	−126.830
Q1 focus lens (V)	−0.690
Focus lens (V)	1.750
Q1 pole bias (V)	0.000
CR bias (V)	−6.080
Minor
Extraction lens 1 positive (V)	0.000
Deflection entry lens (V)	−30.000
CR entry lens (V)	−161.830
CR amplitude (V)	189.300
CR exit lens (V)	−40.230
D1 lens (V)	−350.000
D2 lens (V)	−148.750
Q3 entry lens (V)	−35.000

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Table 3 Elemental compositions of nine investigated aeschynite samples determined by LA-ICP-MS. The data are given in μg g⁻¹. Pb* (μg g⁻¹) = 23.6% × ²⁰⁶Pb (μg g⁻¹) + 22.6% × ²⁰⁷Pb (μg g⁻¹) + 52.3% × ²⁰⁸Pb (μg g⁻¹). Trace element mass fractions were calibrated against ARM-1. “—” represents that the data are not given due to below detection limits

Element	Mass	Hidra aeschynite-(Y) (μg g^−1)
		1	2	3	4	5	6	7	8	9
B	11	914	886	808	215	—	898	790	794	784
Na	23	728	686	853	1069	343	741	807	865	872
Mg	25	46	87	41	56	39	37	55	51	48
Al	27	202	282	278	394	210	178	272	253	235
Si	29	1006	1289		1720	778	—	534	826	741
K	39		32	23	41	—	33	31	23	27
Ca	43	13 845	14 836	13 257	4044	—	13 713	14 212	12 095	12 354
Sc	45	16	17	17	21	15	14	19	19	18
Ti	49	157 888	153 522	156 187	159 045	167 226	161 483	162 917	167 087	168 721
V	51	—	1	—	—	2	—	1	1	1
Mn	55	587	2193	385	652	40	192	284	352	250
Fe	57	5210	5497	4713	5939	3823	3819	4779	5028	4512
Cu	63	2	—	—	—	—	3	2	2	2
Zn	66	22	21	23	18	22	35	20	18	17
Ga	71	307	287	291	301	292	254	300	300	296
Ge	73	753	748	761	783	732	684	789	759	759
As	75	227	228	231	244	220	208	241	237	230
Rb	85	62	63	63	66	60	64	63	61	62
Sr	88	247	290	256	111	28	259	278	258	245
Y	89	137 822	139 016	137 463	141 079	131 197	134 440	132 827	130 004	135 203
Zr	90	127	168	164	192	154	138	185	175	183
Nb	93	100 825	107 424	104 929	105 472	114 219	107 341	106 781	102 714	102 258
Mo	98	56	92	95	104	69	65	104	107	122
Sn	118	2248	3303	3492	3797	3309	2936	3271	3601	3568
Sb	121	—	—	—	—	—	—	—	—	—
Cs	133	2	2	2	2	2	2	2	2	2
Ba	137	70	299	47	56	2	118	56	46	47
La	139	199	177	158	225	235	149	214	203	196
Ce	140	2745	2375	2360	2683	2772	1992	2599	2550	2471
Pr	141	970	924	941	1015	984	842	970	987	973
Nd	146	8906	8685	9002	8953	8680	8094	8784	8758	8673
Sm	147	12 788	12 754	13 082	12 927	12 580	12 211	12 687	12 429	12 353
Eu	153	67	60	55	64	65	67	75	79	53
Gd	158	23 248	24 496	24 415	24 379	22 615	22 674	23 410	23 414	23 045
Tb	159	5851	5931	6001	5751	5488	5701	5805	5842	5831
Dy	163	38 685	39 530	39 628	39 120	36 824	38 858	38 456	37 878	37 685
Ho	165	7381	7614	7641	7581	7037	7479	7232	7182	7354
Er	166	20 917	21 192	21 202	21 032	19 739	20 708	20 041	20 265	20 499
Tm	169	3431	3458	3522	3437	3212	3400	3312	3355	3403
Yb	173	23 029	22 624	22 107	23 261	22 161	22 931	22 437	22 612	23 233
Lu	175	2619	2580	2531	2621	2331	2517	2386	2418	2494
Hf	177	11	17	20	18	14	11	17	17	19
Ta	181	26 421	28 184	28 144	27 123	27 298	26 774	26 027	25 514	25 548
W	182	19 575	18 925	19 160	21 440	17 342	19 917	19 014	19 619	22 937
Pb*	—	10 379	8025	10 280	9911	14 057	10 350	9308	10 606	9740
Th	232	95 171	89 672	91 763	89 388	92 966	88 976	88 675	90 255	84 410
U	238	43 399	40 535	41 103	42 509	46 858	43 139	43 098	45 689	40 276
REEt		150 836	152 400	152 645	153 049	144 723	147 623	148 408	147 972	148 263
Th/U		2.2	2.2	2.2	2.1	2.0	2.1	2.1	2.0	2.1
Lu/Hf		238.1	151.8	126.6	145.6	166.5	228.8	140.4	142.2	131.3
Y/Lu		52.6	53.9	54.3	53.8	56.3	53.4	55.7	53.8	54.2

---

Fig. 5 The chondrite normalized REE patterns of Hidra aeschynite-(Y). REEs used in chondrite normalization are from Taylor and McLennan.⁶⁵ Hidra aeschynite-(Y) shows marked Eu depletion and pronounced M-HREE enrichment.

3.3 SIMS Pb–Pb dating

In situ ²⁰⁷Pb/²⁰⁶Pb isotope ratios and corresponding ²⁰⁷Pb/²⁰⁶Pb ages measured on Hidra aeschynite-(Y) are summarized in SI Table S2. The ²⁰⁷Pb/²⁰⁶Pb ratios range from 0.068549 to 0.069949, yielding an arithmetic mean of 0.069115 with 1.04% repeatability (2SD). In total, forty-six individual analyses of Hidra aeschynite-(Y) crystals were conducted, yielding an arithmetic mean ²⁰⁷Pb/²⁰⁶Pb age of 902.1 ± 21.3 Ma (2SD). This tight clustering reinforces data robustness and confirms isotopic coherence across all sampled domains.

3.4 LA-ICP-MS U–Pb dating

A total of thirty spots on Hidra aeschynite-(Y) were analyzed for U–Pb dating via LA-ICP-MS. As shown in Table 3, the crystals contain high concentrations of U and Th, ranging from 40 276 to 46 858 μg g⁻¹ and 84 410 to 95 171 μg g⁻¹, respectively. The complete U–Th–Pb dataset is provided in SI Table S1. Due to the lack of matrix-matched reference material for aeschynite, we used a coltan reference material for external calibration (Coltan 139). The data are plotted in SI Fig. S1. The resulting ²⁰⁶Pb/²³⁸U ratios exhibit a clear variation and bias (SI Fig. S1), reflecting both matrix effects and Pb loss after crystallization. This bias is illustrated in Fig. 7, where the data show significant deviations from the concordia line on the concordia diagram (SI Fig. S1), highlighting the combined influence of matrix mismatch and post-crystallization Pb loss (SI Fig. S1).

For the LA-ICP-MS Pb–Pb dating results, the ²⁰⁷Pb/²⁰⁶Pb ratios in three independent sessions range from 0.068345 to 0.069973, yielding an arithmetic mean of 0.069126 with 1.10% repeatability (2SD, n = 55). For session 1, twenty-three analytical spots yielded an arithmetic mean ²⁰⁷Pb/²⁰⁶Pb age of 900.9 ± 23.7 Ma (2SD). For session 2, eighteen analytical spots yielded an arithmetic mean ²⁰⁷Pb/²⁰⁶Pb age of 906.0 ± 10.0 Ma (2SD). For session 3, fourteen analytical spots yielded an arithmetic mean ²⁰⁷Pb/²⁰⁶Pb age of 900.6 ± 29.0 Ma (2SD). These results are presented in Fig. 6 and SI Table S2.

Fig. 6 The arithmetic mean ²⁰⁷Pb/²⁰⁶Pb ages of Hidra aeschynite-(Y) obtained from TIMS, SIMS and LA-ICP-MS/MS. These ages match with each other within analytical uncertainty.

Fig. 7 The ²⁰⁶Pb/²³⁸U ratios of Hidra aeschynite-(Y). The high variability of ²⁰⁶Pb/²³⁸U ratios indicates a disturbed U–Pb system, likely due to Pb loss after crystallization.

3.5 TIMS Pb–Pb isotope ratios

A total of six sub-samples of Hidra aeschynite-(Y) were analyzed by TIMS for Pb–Pb dating (Table 4). The measured ²⁰⁷Pb/²⁰⁶Pb ratios range from 0.069327 to 0.069686, yielding an arithmetic mean of 0.069474 with 0.36% repeatability (2SD). These ratios correspond to an arithmetic mean ²⁰⁷Pb/²⁰⁶Pb age of 912.8 ± 6.9 Ma (2SD, SI Table S1 and Fig. 6), providing an independent constraint on the crystallization age of Hidra aeschynite-(Y).

Table 4 Pb–Pb dating results for Hidra aeschynite-(Y) obtained by TIMS

No.	^206Pb/^204Pb	1 se	^207Pb/^204Pb	1 se	^207Pb/^206Pb	1 se
1	9745	50	678.2	3.9	0.069504	0.000058
2	12 932	47	899.2	3.8	0.069521	0.000053
3	13 998	246	976	18	0.069425	0.000078
4	18 342	77	1272.0	6.1	0.069327	0.000055
5	15 091	64	1053.3	5.1	0.069686	0.000053
6	20 473	204	1423	16	0.069384	0.000059

---

3.6 LA-ICP-MS/MS Lu–Hf dating

Three Lu–Hf dating sessions were conducted on Hidra aeschynite-(Y), with results summarized in SI Table S3. Session 1 comprised thirty-one analyses using a 110 μm spot size. The anchored isochron yielded an age of 906.6 ± 9.8 Ma (n = 31; MSWD = 2.2), consistent within errors with the unanchored isochron produced an age of 915 ± 18 Ma (n = 31; MSWD = 2.3), with an initial ¹⁷⁶Hf/¹⁷⁷Hf ratio of 0.2788 ± 0.0054 (Fig. 8). Session 2 included eighteen analyses, also with a 110 μm spot size. The anchored isochron yielded an age of 913.6 ± 7.1 Ma (n = 18; MSWD = 0.4), and the unanchored isochron gave 919 ± 36 Ma (n = 18; MSWD = 0.5), with an initial ¹⁷⁶Hf/¹⁷⁷Hf ratio of 0.280 ± 0.012 (Fig. 8). Session 3 involved nineteen analyses using a smaller 50 μm spot size. The anchored isochron yielded an age of 912.8 ± 7.1 Ma (n = 19; MSWD = 0.3), and the unanchored isochron yielded 922 ± 61 Ma (n = 19; MSWD = 0.4), with an initial ¹⁷⁶Hf/¹⁷⁷Hf ratio of 0.278 ± 0.027 (Fig. 8). It is interesting to note, despite calibration using NIST SRM 610, the Lu–Hf ages are in excellent agreement with the SIMS, LA-ICP-MS, and TIMS ²⁰⁷Pb/²⁰⁶Pb ages, indicating that the potential matrix effects between NIST SRM 610 and aeschynite-(Y) are little, if any, for LA-ICP-MS/MS Lu–Hf dating.

Fig. 8 The in situ LA-ICP-MS/MS Lu–Hf ages of Hidra aeschynite-(Y). Three independent LA-ICP-MS/MS Lu–Hf analytical sessions define well-constrained isochrons. These converge to an isochron age of 912.5 ± 6.5 Ma (MSWD = 1.4).

4. Discussion

4.1 Homogeneity of ²⁰⁷Pb/²⁰⁶Pb ages

Homogeneity is a fundamental prerequisite for any reference material;⁶⁶ however, it is not an inherent characteristic of the material itself but rather depends on the specific element being analyzed and the scale of the analysis.⁶⁷ This study evaluated the homogeneity of the ²⁰⁷Pb/²⁰⁶Pb age at the microscale (20–80 μm), a critical scale for assessing the isotopic consistency across the sample. This evaluation employed two high-precision analytical techniques (SIMS and LA-ICP-MS/MS), both capable of producing detailed isotopic measurements at different spatial resolutions.

Forty-six randomly selected grains of Hidra aeschynite-(Y) were analyzed by SIMS, yielding an overall ²⁰⁷Pb/²⁰⁶Pb ratio repeatability of 1.04% (2SD), demonstrating an excellent precision in the isotopic measurements, even at the microscale. This high repeatability confirms that the ²⁰⁷Pb/²⁰⁶Pb ratios (and their ²⁰⁷Pb/²⁰⁶Pb ages) are highly consistent across grains, with minimal variability attributable to analytical or sample heterogeneity.

Complementary three LA-ICP-MS/MS session analyses of fifty-five randomly selected grains yielded ²⁰⁷Pb/²⁰⁶Pb ratios with 1.10% (2SD) repeatability (SI Table S2), further supporting the reliability of the isotopic dataset. The slightly lower repeatability observed with LA-ICP-MS/MS compared to SIMS likely reflects differences in instrumental precision and ablation volumes between the techniques.

Collectively, both SIMS and LA-ICP-MS/MS analyses confirm the homogeneity of the ²⁰⁷Pb/²⁰⁶Pb ratios in Hidra aeschynite-(Y) at microscale resolution. This agreement validates the robustness of the isotopic dataset (SIMS: 1.04%; LA-ICP-MS/MS: 1.10%; 2SD) and establishes its utility for constructing a high-precision geochronological framework. It should be mentioned that all the homogeneity evaluation experiments were carried out at a single institute. For the wider use of this material as a reference material, inter-laboratory comparisons are necessary, and this will be the focus of future studies.

4.2 U–Pb and Pb–Pb ages

Concordant U–Pb ages are generally considered reliable only when certain conditions for accurate age determination are met, including: (a) the mineral remaining closed to the U–Pb isotopic system, and all intermediate daughter isotopes throughout its history, thereby preventing isotopic exchange; (b) the use of accurate initial Pb isotope ratios; and (c) assurance that all analytical results are precise and free from systematic errors.^67,68 However, the elevated concentrations of U and Th in most aeschynites make them especially susceptible to disturbance of metamictization, which can readily disrupt the U–Pb isotopic system.^63,69,70 Such disturbance may lead to partial Pb loss, thereby destroying the isotopic composition required for accurate dating.⁷¹

The Raman spectroscopy data indicate that the crystal structure of Hidra aeschynite-(Y) has undergone significant alteration (Fig. 2), implying that metamictization processes may have disrupted the isotopic integrity of the mineral. The U–Pb results obtained from LA-ICP-MS confirm that the U–Pb system has been compromised by varying degrees of Pb loss, which precludes a reliable estimate of U–Pb age concordance (SI Fig. S1 and 7).

In contrast, the Pb–Pb method is less sensitive to recent loss of U and Pb, as such disturbances have minimal impact on radiogenic Pb isotopic ratios, enabling more reliable age determinations even in isotopically disturbed samples.⁷² All the measured ²⁰⁷Pb/²⁰⁶Pb ages by SIMS, LA-ICP-MS, and TIMS analyses are mutually consistent within analytical uncertainty.

Given the overall consistency and lower associated uncertainty, we consider the arithmetic mean ²⁰⁷Pb/²⁰⁶Pb age of 912.8 ± 6.9 Ma (2SD) obtained by TIMS to be the most accurate and reliable estimate for the crystallization age of Hidra aeschynite-(Y).

4.3 Potential aeschynite-(Y) Lu–Hf dating reference material

Hidra aeschynite-(Y) contains high Lu concentrations (∼2500 μg g⁻¹) and exhibits a relatively high Lu/Hf ratio (∼100), making it a promising candidate for Lu–Hf geochronology. Several previous studies indicate that the Lu–Hf system may be more resistant to disturbance than other isotopic systems such as U–Pb.⁷³ While the LA-ICP-MS results reveal disturbance in the U–Pb system (SI Fig. S1 and 7), three independent LA-ICP-MS/MS Lu–Hf analytical sessions define well-constrained isochrons (Fig. 8), with individual ages of 906.6 ± 9.8 Ma, 913.6 ± 7.1 Ma, and 912.8 ± 7.1 Ma. These converge to an isochron age of 912.5 ± 6.5 Ma (MSWD = 1.4), confirming an undisturbed Lu–Hf system. It should be emphasized that under our instrument parameters, there is a negligible matrix effect of Lu/Hf between NIST SRM 610 and Hidra aeschynite-(Y). Future research on additional aeschynite samples will help to confirm the broader applicability of this mineral as a Lu–Hf dating reference material.

Moreover, a critical step in establishing reference material is interlaboratory calibration,^55,74 which evaluates consistency and reproducibility by analyzing the material across multiple laboratories. We therefore identify it as a prerequisite for the material's widespread adoption as a certified reference material.

The material will be provided free of charge for academic research, and we encourage potential collaborators to become involved in the inter-laboratory comparison project. The corresponding author can be contacted at E-mail: shitou.wu@mail.iggcas.ac.cn for the material distribution and the inter-laboratory comparison project.

5. Conclusions

This study establishes Hidra aeschynite-(Y) as a potential natural reference material for microbeam Pb–Pb and Lu–Hf geochronology. Pb–Pb dating confirms its isotopic homogeneity, with SIMS analyses yielding an arithmetic mean ²⁰⁷Pb/²⁰⁶Pb age of 902.1 ± 21.3 Ma (2SD; n = 46) and three LA-ICP-MS/MS session analyses giving an arithmetic mean ²⁰⁷Pb/²⁰⁶Pb age of 902.5 ± 22.5 Ma (2SD; n = 55). The recommended age value is obtained by TIMS, where six measurements yield an arithmetic mean ²⁰⁷Pb/²⁰⁶Pb age of 912.8 ± 6.9 Ma (2SD). In situ Lu–Hf analyses from three independent sessions yielded an isochron age of 912.5 ± 6.5 Ma (2σ; MSWD = 1.4; n = 68), calibrated with NIST SRM 610. These results demonstrate excellent agreement with ²⁰⁷Pb/²⁰⁶Pb ages, further validating the reliability of this material. Hidra aeschynite-(Y) therefore represents a valuable addition to the suite of reference materials available for microbeam in situ Pb–Pb and Lu–Hf geochronology (e.g., SIMS and LA-ICP-MS).

Author contributions

Bo Yang and Shitou Wu conceived the study, conducted the experimental analyses, interpreted the data and wrote the initial manuscript; Xianhua Li and Shitou Wu led the project, conceived the study, interpreted the data and substantially edited the manuscript; Xiao-Xiao Ling, Yu Liu, Zhao-Xue Wang, and Zhu-Yin Chu provided technical support for SIMS and TIMS analyses; Hao Wang and Yue-Heng Yang revised the manuscript, interpreted the data and provided supervision; and all authors finalized the manuscript for publication.

Conflicts of interest

There are no conflicts of interest to declare.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article and its supplementary information (SI). Supplementary information: Fig. S1: the LA-ICP-MS U–Pb concordia diagram; Table S1: the U–Pb age data of Hidra and reference materials. Table S2: the Pb–Pb data of TIMS, SIMS and LA-ICP-MS/MS; Table S3: the in situ Lu–Hf data of Hidra. See DOI: https://doi.org/10.1039/d5ja00326a.

Acknowledgements

We thank Ms Hongxia Ma for her assistance in sample preparation and Dr Xiaoguang Li for the Raman analysis. This work was supported by the National Natural Science Foundation of China (Grant 92262303, 42273034, 42522302, and 42430105), Baotou Iron and Steel (Group) Co., Ltd, the Joint Funds of State Key Laboratory of Baiyunobo Rare Earth Resource Researches and Comprehensive Utilization (GZ-2023-1-LH-001/002), the Natural Science Foundation of Inner Mongolia Autonomous Region of China (2025QN04037), the Baiyunobo Mineral Resources Comprehensive Utilization Academician Workstation of Baotou Iron and Steel (Group) Corp. (2024YSZ0007), the Class A Project of Baotou Steel (Group) Corp. (BGKYKJ-ZY-2025-Z-01), the Youth Innovation Promotion Association of the Chinese Academy of Sciences (Grant 2022066), and the Key Research Program of the Institute of Geology and Geophysics, Chinese Academy of Sciences (Grant IGG-CAS-2-21-1 and IGG-CAS-202204).

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