XBD-1 and PT-1 scheelites: potential reference materials for SIMS oxygen isotope analysis

Abstract

Scheelite is a common accessory mineral and an essential component of tungsten ores. Its U–Pb dating, chemical compositions, and oxygen isotopes provide critical information on the timing and genesis of scheelite-bearing ores and the nature and evolution of the ore-forming fluids. In situ analysis by secondary ion mass spectrometry (SIMS) is the only option for oxygen isotope investigation of natural scheelite crystals, as they commonly exhibit complex growth zoning and contain inclusions. However, there is a current lack of well-characterized scheelite reference materials for SIMS oxygen isotope analysis. This study thus characterizes two natural scheelite samples (XBD-1 and PT-1) as working reference materials for in situ oxygen isotopic analysis of this mineral by employing secondary ion mass spectrometry (SIMS). Our SIMS analyses reveal that both XBD-1 and PT-1 scheelite samples exhibit homogeneous oxygen isotopic compositions with their 1SD being 0.2‰ (n = 117) and 0.3‰ (n = 101), respectively, supporting their use as reference materials for high-precision SIMS δ¹⁸O analysis of scheelite. Laser fluorination isotopic ratio mass spectrometry yields mean δ¹⁸O values of 8.57 ± 0.20‰ (1SD, n = 2) for XBD-1 and −6.21 ± 0.20‰ (1SD, n = 3) for PT-1, which are recommended as reference oxygen isotopic values of these materials.

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1 Introduction

Scheelite (CaWO₄) is a widespread accessory mineral that occurs in diverse geological settings worldwide and serves as an important component of tungsten ores. It commonly occurs in different types of ore deposits, including porphyry type, skarn type, quartz vein-hosted gold deposits, and pegmatite-related deposits.^1–7 Scheelite often coexists spatially and temporally with minerals such as native gold, pyrite, beryl, muscovite, tourmaline, feldspar, quartz, and cassiterite.^2,8 Due to its notable resistance to both chemical alteration and physical weathering, scheelite serves as a robust proxy for tracing and characterizing various ore-forming systems.^9,10 U–Pb dating of U-rich scheelite is essential in setting precise constraints on the timing of formation of tungsten ores.^11,12 In addition, major, minor, and trace elemental characteristics of scheelite offer critical information on the genesis of ore deposits.^10,13,14In situ U–Pb dating by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS)^11,15,16 and in situ measurements of major, minor, and trace elements by electron probe microanalysis (EPMA) and LA-ICP-MS^3,12,16,17 have become routine in geological studies.

Oxygen isotope compositions (δ¹⁸O) of scheelite provide further essential details on the nature and evolution of ore-forming fluids.^3,16,18 However, scheelite crystals generally have internal zonings visible in cathodoluminescence (CL) indicating the complex history of their growth.⁹ Furthermore, these crystals may contain abundant mineral and fluid inclusions.^3,19–21 Therefore, oxygen isotope analysis of scheelite conducted by using bulk methods²² can compromise the results of bulk δ¹⁸O measurements.

Secondary ion mass spectrometry (SIMS) has become a vital technique for in situ oxygen isotope analysis in mineralogical and geochemical research. SIMS offers high spatial resolution (10–15 μm lateral, ∼1 μm depth) in combination with a good analytical precision (2SE = 0.1–0.3‰).^23,24 Therefore, application of the in situ technique to oxygen isotope analysis of scheelite is required to overcome the limitations of bulk techniques.

However, results of secondary ion mass spectrometry measurements are generally sensitive to crystal structures and chemical compositions of analyzing materials. This issue is commonly referred to as the “matrix effect”.^25,26 Hence, it is essential to use matrix-matched (i.e., of the same crystal structure and chemistry) standard reference materials (SRMs) during secondary ion mass spectrometry analysis in order to achieve accurate and reliable results. Although there have been recent achievements in the development of oxygen isotope analysis of scheelite by secondary ion mass spectrometry (SHRIMP),²⁶ no well-characterized scheelite SRMs are currently available to the scientific community. As such, we here characterize two scheelite samples, XBD-1 and PT-1, whose homogeneous chemical compositions and δ¹⁸O compositions suggest that they are promising candidates as SRMs for in situ oxygen isotope analysis of scheelite.

2 Sample information and preparation

The scheelite sample XBD-1 (Fig. 1) weighing at 30.3 g is a yellow–orange single crystal of gem-quality with well-developed crystal faces. This crystal is transparent and coarse-grained reaching up to 2 cm in the biggest dimension. This sample originates from the Xuebaoding W–Sn–Be deposit located approximately 14.5 km to the northwest from Huya Town, Mianyang City, Sichuan Province, China.^27–29

Fig. 1 Photograph of XBD-1 (left) and PT-1 (right) scheelite samples.

The scheelite sample PT-1 represents one scheelite crystal, reaching up to 7 cm in length (Fig. 1). This sample is predominantly gray in color and macroscopically translucent, with a total weight of 123.1 g. It was sourced from the Piaotang tungsten deposit in the eastern Nanling Range, Jiangxi Province, China. Ore mineralization at the Piaotang deposit is primarily characterized by wolframite-dominated quartz veins, with subordinate occurrences of scheelite-bearing skarn and porphyry-type mineralization.^30,31

In this study, scheelite crystals (XBD-1 and PT-1) were mechanically crushed into fragments of approximately 100 μm in size. The resulting grains were randomly embedded into epoxy resin together with silicate glass chips of the NIST SRM 610 glass.³² Three mounts were prepared: the first containing 50 grains of XBD-1, the second containing 50 grains of PT-1, and the third combining 35 grains from each of the two samples. All these mounts were carefully polished to achieve flat grain surfaces, minimizing topographic effects during subsequent EPMA and SIMS measurements.²⁴ Selected fragments of both XBD-1 (Fig. 2a–d) and PT-1 (Fig. 2e–h) scheelite are homogeneous, exhibiting no zoning visible in back-scattered electron (BSE) images.

Fig. 2 BSE photos for the XBD-1 (a–d) and PT-1 (e–h) scheelites.

3 Analytical methods

3.1 EPMA analysis

Major and minor element compositions were determined using a JXA-iHP200F electron probe microanalyzer equipped with five wavelength-dispersive spectrometers at the MNR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences. The analytical conditions were similar to those described by Yang et al.:³³ an accelerating voltage of 15 kV, a beam current of 20 nA, and a beam spot size of 10 μm. Synthetic zinc tungstate (ZnWO₄) was employed as the calibration standard, and all the measurements were corrected using the conventional ZAF routine.

3.2 Isotope ratio mass spectrometry (IRMS) analysis

Oxygen isotope bulk analysis of scheelite was conducted by employing the laser fluorination technique at the International Center for Isotope Effects Research, School of Earth Sciences and Engineering, Nanjing University. To minimize the influence of inclusions and secondary alteration, XBD-1 and PT-1 scheelite grains were carefully hand-picked under a binocular microscope prior to the laser fluorination analysis. Each aliquot weighing at around 3 mg was reacted with purified BrF₅ in an evacuated chamber, using a CO₂ infrared laser operating at 20 W. The liberated O₂ was then analyzed with a Thermo Fisher MAT 253 dual-inlet mass spectrometer. Analytical protocols described by Wostbrock et al.^34,35 and Miller et al.³⁶ were followed. All δ¹⁸O values are reported relative to the VSMOW scale. San Carlos olivine (δ¹⁸O = +5.25‰; Eiler et al.³⁷) was used as a reference material to calculate final values of scheelite samples. San Carlos olivine was measured concurrently, yielding a reproducibility of ±0.20‰ (1SD, n = 20).

3.3 SIMS analysis

Oxygen isotope analyses of scheelite were performed using a CAMECA IMS-1280 secondary ion mass spectrometry at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS), Beijing. We followed the analytical procedures described by Li et al.²³ and Tang et al.²⁴ A Cs⁺ primary ion beam was accelerated at 10 kV, with a spot size of 10–15 μm and a raster size exceeding 10 μm. Measurements were conducted in multi-collection mode. The measured ¹⁸O/¹⁶O ratios were normalized to the Vienna Standard Mean Ocean Water (VSMOW; ¹⁸O/¹⁶O = 0.0020052). Analyses of reference glass NIST610 (ref. 32) were interspersed with the scheelite samples to monitor the instrumental stability throughout the analytical sessions. Internal uncertainties on single analyses are between 0.1 and 0.3‰ (2SE).

4 Results and discussion

4.1 Major and minor element compositions

In this study, a total of 20 electron microprobe analyses were performed on both the XBD-1 and PT-1 scheelite samples (SI Table S1). The XBD-1 scheelite exhibits WO₃ contents ranging from 79.39 to 80.94 wt%, CaO ranging from 19.26 to 20.12 wt%, and ZnO ranging from 0.09 to 0.45 wt%. The PT-1 scheelite yields similar chemical compositions: WO₃ concentrations of 79.46 to 80.88 wt%, CaO between 19.26 and 20.47 wt%, and ZnO varying from <0.07 to 0.47 wt%.

4.2 IRMS oxygen isotope results

Based on the bulk IRMS analysis, the resulting mean δ¹⁸O value for XBD-1 is 8.57 ± 0.20‰ (1SD, n = 2), while that for PT-1 is −6.21 ± 0.20‰ (1SD, n = 3) (SI Table S2). These values are considered to be the most reliable reference δ¹⁸O values for the XBD-1 and PT-1 scheelite materials and were used for final calibration of subsequent SIMS analyses obtained in this study (Section 4.3).

4.3 SIMS oxygen isotope results

SIMS oxygen isotope analyses were conducted in three analytical sessions. The first two sessions consisted of individual measurements of 50 XBD-1 scheelite grains (Session 1) and 50 PT-1 scheelite grains (Session 2), respectively. Each group was embedded in a separate mount to assess inter-grain isotopic variability. The third session involved a combined mount containing 34 XBD-1 spots and 30 PT-1 spots (SI Table S3).

Across these three sessions, 117 SIMS analyses were conducted for XBD-1 and 101 SIMS analyses were conducted on PT-1. The measured δ¹⁸O values were calibrated to the reference XBD-1 and PT-1 values determined by IRMS (see Section 4.2). Analytical uncertainty of SIMS analyses was assessed using repeated measurements of the NIST SRM 610 glass, yielding a 1SD reproducibility of ±0.17‰ (n = 21), ±0.21‰ (n = 18), and ±0.14‰ (n = 20) for Sessions 1, 2, and 3, respectively. This analytical reproducibility indicates stable instrumental conditions throughout the analyses.

The δ¹⁸O values for XBD-1 scheelite display limited variation, with a 1SD of 0.21‰ (n = 83, Session 1) and 0.17‰ (n = 34, Session 3). For PT-1 scheelite, the corresponding values are 0.30‰ (n = 71, Session 2) and 0.31‰ (n = 30, Session 3). These results demonstrate high analytical reproducibility for both samples. Considering the random crystallographic orientations of the analyzed grains, it is suggested that crystal orientation may not significantly affect SIMS-based oxygen isotope determinations in scheelite.

Overall, the δ¹⁸O values from all 117 SIMS measurements on XBD-1 conform to a Gaussian distribution (Fig. 3a), with a 1SD of 0.20‰ (Fig. 3b). Similarly, the 101 measurements from PT-1 also follow a Gaussian distribution (Fig. 3c), with a 1SD of 0.30‰ (Fig. 3d).

Fig. 3 SIMS results of the δ¹⁸O homogeneity test conducted on the XBD-1 and PT-1 scheelites. Plot (a) and plot (b) are δ¹⁸O values for XBD-1 from session 1 and session 3. Plot (c) and plot (d) show all obtained δ¹⁸O values for PT-1 from session 2 and session 3. All the SIMS data were normalized to a δ¹⁸O value of 8.57‰ for XBD-1 and −6.21‰ for PT-1 defined by IRMS.

5 Conclusions

Our results demonstrate that both XBD-1 and PT-1 scheelite samples are isotopically homogeneous with respect to oxygen isotope compositions. Therefore, these materials are suitable candidates for use as working recommended materials in SIMS-based oxygen isotope analysis of scheelite. We propose δ¹⁸O values of 8.57 ± 0.20‰ (1SD) for XBD-1 and −6.21 ± 0.20‰ (1SD) for PT-1 as reference δ¹⁸O values.

It is also important to note that scheelite from W–Mo deposits can show significant substitution of Mo for W, with MoO₃ concentrations reaching up to 30 wt% in some cases.^4,38 In contrast, both XBD-1 and PT-1 contain no detectable MoO₃, making them more appropriate for calibration of in situ oxygen isotope measurements of Mo-free scheelite using secondary ion mass spectrometry.

Author contributions

J.Li prepared the original manuscript. A. Melnik and Q.-L. Li contributed to manuscript revision. G.-Q. Tang, Y. Liu and X.-X. Ling conducted the SIMS experiments. F.-T. Tong, M.-C. Li and Y.-B. Peng carried out the IRMS analysis. H.-X. Ma contributed to sample preparation. X.-H. Li supervised this study.

Conflicts of interest

There are no conflicts to declare.

Data availability

Data for this article, including EPMA, SIMS, and IRMS data, have been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5ja00311c.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (grant number 42225301). We thank Dr Yan Liu for donating the XBD-1 scheelite sample. We are thankful to the two anonymous reviewers for their fruitful comments.

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