Two potential natural calcite reference materials for laser in situ Sr isotope analysis

Abstract

Calcite is one of the common rock-forming minerals, and its Sr isotopic composition provides useful information for revealing various geological processes. For laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) analytical techniques, accurate and reliable Sr isotope data rely on high-quality calcite reference materials (RMs). However, calcite RMs with homogeneous composition and structure are rare, especially natural solid calcite RMs, making them inadequate to meet the needs of various applications within the scientific community. Here we identified two new natural calcite reference materials, HZZ-2 and TARIM, for laser in situ Sr isotope analysis. These two RMs exhibit uniform Ca and Sr contents, as confirmed by μ-XRF component analysis. EPMA and LA-ICP-MS methods indicate that they have homogeneous major and trace element compositions, both showing exceptionally low Rb/Sr ratios (<0.00001) and relatively moderate Sr contents (HZZ-2: ∼1100 μg g⁻¹ and TARIM: ∼620 μg g⁻¹). Bulk isotope analyses by TIMS yielded a mean ⁸⁷Sr/⁸⁶Sr ratio of 0.70941 ± 0.00001 (2s, n = 8) for HZZ-2 and 0.71042 ± 0.00001 (2s, n = 7) for TARIM. Multiple LA-MC-ICP-MS spot Sr isotope analyses on random pieces of HZZ-2 and TARIM yielded mean ⁸⁷Sr/⁸⁶Sr ratios of 0.70940 ± 0.00003 (2s, n = 318) and 0.71042 ± 0.00002 (2s, n = 305), respectively, consistent with the TIMS results. These results suggest that the HZZ-2 and TARIM calcite samples can be used as potential reference materials for determining high or low Sr samples by laser in situ Sr isotope analysis.

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1. Introduction

As a common natural carbonate mineral in the lithosphere, calcite can form in diverse geological environments through varying geological processes.¹ It carries abundant geological information, such as geochemical rare-earth-element proxies, U–Pb geochronology, and C, O, Sr isotope tracers,^2,3 attracting much attention in the geological scientific community. For example, the Sr isotopic composition in calcite is widely used to unravel geological processes such as ore-forming fluid sources, mineralization processes of ore deposits, the evolution of the mantle, and paleoclimate reconstruction.^4–9 Sr predominantly occurs in calcium-bearing minerals. Calcite commonly contains significant amounts of Sr (ranging from tens to thousands of μg g⁻¹), but has extremely low (<1.0 μg g⁻¹) Rb content.¹⁰ This characteristic renders the decay of ⁸⁷Rb into ⁸⁷Sr negligible after calcite formation, and its ⁸⁷Sr/⁸⁶Sr ratio remains stable over geological timescales. It follows that the measured ⁸⁷Sr/⁸⁶Sr ratio can be used to represent the initial Sr isotope composition. As a result, calcite Sr isotope geochemistry has been accepted as an emerging geochemical method.

Accurately determining the Sr isotopic composition of calcite is a prerequisite for its geological applications. Bulk solution thermal ionization mass spectrometry (TIMS) and multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) are traditional Sr isotope analysis methods, both of which have the advantage of high precision. However, these techniques necessitate complex chemical purification procedures prior to analysis, require relatively large sample sizes, and lack high spatial resolution. These disadvantages mean that they are not suitable for the analysis of rare samples or spatially heterogeneous samples. The advent of LA-MC-ICP-MS has significantly promoted the application of isotope analysis techniques,¹¹ as it offers advantages of high sensitivity, high spatial resolution, rapid analysis, and less sample consumption compared to traditional solution TIMS and MC-ICP-MS methods.¹² Since then, LA-MC-ICP-MS has been widely used for determining Sr isotope ratios at a grain or subgrain-scale.^13–15

However, in LA-MC-ICP-MS Sr isotope ratio analysis, challenges such as insufficient instrumental sensitivity, instrumental mass discrimination, and laser-induced isotopic fractionation persist. Additionally, it is critical to address isobaric interferences (from ⁸⁷Rb and Kr gases, REE²⁺, Ca-based molecules, etc.).¹⁶ To address these problems, numerous studies have tried to further improve the precision and accuracy of LA-MC-ICP-MS,^16–20 which can be effectively controlled with the help of matrix-matched reference materials (RMs) in combination with optimized calibration schemes.

High-quality reference materials are crucial for the acquisition of high-precision ⁸⁷Sr/⁸⁶Sr ratios in LA-MC-ICP-MS Sr isotope analysis in terms of quality assessment and method validation processes. While many reference materials for in situ Sr isotope analysis have been developed in various mineral phases, such as apatite,¹⁹ plagioclase,²¹ clinopyroxene,¹⁵etc., few focus on calcite reference materials, especially natural calcite reference materials. Weber et al. (2018, 2020)^22,23 suggested that aragonite (JCp-1 and JCt-1) and synthetic carbonate powder (NanoSr, MACS-1, and MACS-3) can be used for in situ Sr isotope analysis. However, these materials exhibit significant limitations, including susceptibility to contamination during sample preparation, poor long-term stability, and consequently, inadequate reliability as reference materials for precise analytical applications. Liang et al. (2023)²⁴ proposed a natural calcite reference material (MNP) for in situ analysis of Sr, but its Sr content was too high (7064–12893 μg g⁻¹) as most calcite in nature does not have such high Sr contents. Wu et al. (2023)²⁵ reported two types of natural calcite RMs (TLM and LSJ07) with low Sr contents (TLM: ∼100 μg g⁻¹ and LSJ07: ∼200 μg g⁻¹), which can meet the application requirements for low-Sr samples. Yin et al. (2024)²⁶ proposed two natural calcite RMs with low to moderate Sr contents (BZS: 212 μg g⁻¹ and WS-1: 598 μg g⁻¹), further complementing the existing calcite reference material system. Currently, natural calcite RMs have either too high or too low Sr content, while reference materials with moderate Sr content are still scarce. Reference materials with different Sr contents have their own specific range of application, given that the Sr content of natural calcite samples may vary significantly. In this case, it is particularly important to develop new calcite reference materials with relatively moderate Sr content.

In this study, we comprehensively developed two new natural calcite RMs (HZZ-2 and TARIM) for LA-MC-ICP-MS Sr isotope analyses through an integrated analytical approach employing μ-XRF, EPMA, TIMS and LA-MC-ICP-MS. The results indicate that both samples exhibit a homogeneous Sr isotopic composition and a moderate Sr content, simultaneously meeting the application requirements for both high-Sr and low-Sr calcite samples.

2. Sampling and analytical methods

2.1 Sample description and preparation

The HZZ-2 sample was provided by the Chengdu Geological Survey Center of the China Geological Survey, with its probable collection site located in Yunnan Province, China. The TARIM sample was collected from a carbonate geode in the Wushi region of the Tarim Basin, China. It also serves as a reference material for U–Pb dating of calcite in the Mineral Laser Microanalysis Laboratory (Milma Lab) at China University of Geosciences, Beijing (CUGB). HZZ-2 calcite is light white in color (Fig. 1a), and TARIM calcite is generally light white with a slightly yellowish surface (Fig. 1b). Both have a clean surface with no impurities. The reference age of TARIM calcite was 208.5 ± 0.6 Ma.²⁷ The common Pb composition of HZZ-2 calcite is too high to allow age determination. The HZZ-2 sample weighs ∼212 g and has a volume of ca. 5.5 × 5 × 2.5 cm³ and the TARIM sample weighs ∼1245 g and has a volume of ca. 11 × 10 × 6 cm³.

Fig. 1 (a) Photograph of the HZZ-2 calcite sample. (b) Photograph of the TARIM calcite sample. Image reproduced from Zhang et al. (2023).²⁷ (c) Photograph of the small pieces of HZZ-2 calcite. (d) Photograph of the small pieces of TARIM calcite. (e) Typical epoxy mounts of the HZZ-2 calcite. (f) Typical epoxy mounts of the TARIM calcite. The typical epoxy mount diameter is 2.54 cm.

The two samples were pretreated using the same method. A part of each sample was cut into small pieces using a diamond-trimmed tungsten wire, with sizes ranging from about 8 mm to 20 mm (Fig. 1c and d). Then, different pieces from these samples were randomly selected and embedded into epoxy resin, which was subsequently polished (Fig. 1e and f). The samples were analyzed in different laboratories by EPMA, LA-MC-ICP-MS and TIMS methods. Other calcite Sr isotope reference materials were also used and are discussed in this study (more details can be seen in Section 3.5).

2.2 μ-XRF image

The μ-XRF analysis was performed at the Key Laboratory of Micro- and Element Forms Analysis of China Geological Survey, using a Bruker M4 TORNADO instrument. The instrument is equipped with an Rh-target microbeam X-ray tube, a W tube and two large-area XFlash silicon drift detectors (resolution < 140 eV). The instrument parameters were set to 50 kV voltage and 600 μA current. All analyses were performed under vacuum conditions (20 mPa), using a pixel step size of 4–20 μm; the residence time of each pixel was 15 ms, and the testing time for a single sample was approximately 8 hours. In the end, only μ-XRF images of elements with higher contents (Ca, Sr) were generated.

2.3 Electronic probe microanalysis

The major-element analysis of both the HZZ-2 and TARIM calcite samples was carried out at the Hebei Key Laboratory of Earthquake Dynamics, Institute of Disaster Prevention, China, using a JEOL JXA-8230 Electron Probe X-ray Micro-Analyzer. EPMA analysis was performed at an accelerating voltage of 15 kV, a beam current of 20 nA, and a beam diameter of 5 μm. Random small calcite pieces were analyzed using EPMA for elemental mapping and compositional characterization. In addition, electron probe analysis was performed on eight small pieces of HZZ-2 calcite with a total of 60 points and four small pieces of TARIM calcite with a total of 20 points. To verify chemical homogeneity, each spot was randomly assigned to a small piece of the sample. The data quality was monitored using the US SPI mineral standards with an accuracy of 0.5% for Ca element and 5% for other elements, and the processing of the data was completed using the ZAF correction procedure.

2.4 LA-ICP-MS analysis

The trace-element contents of HZZ-2 and TARIM calcite were determined by LA-quadrupole(Q)-ICP-MS using a RESOlution S155 LR 193 nm ArF excimer laser ablation system with an Agilent 7900 Q-ICP-MS at the Mineral Laser Microprobe Analysis Laboratory (Milma Lab) of CUGB. The analytical approach was similar to that outlined by Duan et al. (2023).²⁸ During twelve in situ trace-element analysis sessions, the laser parameters were set at a repetition rate of 8 Hz, an energy density of 2.7 J cm⁻², and a beam spot of 150 μm. NIST SRM 610 glass served as the calibration reference material, with ⁴³Ca employed as the internal reference value, while BCR-2G was analyzed for data quality monitoring. Each sample was analyzed for twenty-eight distinct elements, including ²⁴Mg, ⁵¹V, ⁵³Cr, ⁵⁵Mn, ⁵⁷Fe, ⁷¹Ga, ⁸⁵Rb, ⁸⁸Sr, ⁸⁹Y, ⁹⁰Zr, ¹³⁷Ba, ¹³⁹La, ¹⁴⁰Ce, ¹⁴¹Pr, ¹⁴³Nd, ¹⁴⁷Sm, ¹⁵¹Eu, ¹⁵⁷Gd, ¹⁵⁹Tb, ¹⁶³Dy, ¹⁶⁵Ho, ¹⁶⁶Er, ¹⁶⁹Tm, ¹⁷³Yb, ¹⁷⁵Lu, ²⁰⁸Pb, ²³²Th, and ²³⁸U with isotope dwell times of 6 ms. Iolite V3.73 software²⁹ with Trace Element IS DRS were used to perform time drift correction and quantitative calibration. For most trace elements, the accuracy and precision are better than 10% (1 RSD). Detailed instrumental parameters are summarized in Table S1.†

2.5 Sr isotope analysis by TIMS

Bulk Sr isotope analyses of HZZ-2 and TARIM calcite were conducted using a TRITON Plus TIMS at the Laboratory of Isotope Geochemistry, located within the State Key Laboratory of Geological Processes and Mineral Resources, CUGB. This laboratory is a Class 1000 over-pressured clean facility. Small pieces of calcite were randomly selected from each sample. Specifically, each small piece of calcite was crushed separately and weighed. HZZ-2 calcite was divided into eight aliquots, while TARIM calcite was divided into seven aliquots. Each aliquot was precisely weighed to ∼20 mg to ensure adequate Sr recovery for high-precision isotopic analysis.

The calcite powder aliquots were individually transferred into PFA capsules (Savillex®) for dissolution using a mixed acid solution containing HCl, HF, and HNO₃. After complete dissolution, the mixture was evaporated at 130 °C. The reconstituted sample solution was then added to pre-washed 2.8 mL of AG50W-X12 resin (400 mesh), and Sr was purified by elution using 11 mL of 4 mol L⁻¹ HCl. Due to the high calcium content of calcite, the Sr chemical separation process was repeated three to four times. The purified solution was evaporated to dryness at 130 °C and subsequently dissolved in 3% HNO₃. The solution and the activator (TaF₅) were loaded onto a Re wire strip and slowly evaporated for TIMS testing. The data were corrected online using the exponential law, with a fixed ⁸⁶Sr/⁸⁸Sr ratio of 0.1194.³⁰ Instrument stability and data quality were monitored using NIST SRM987 (standard solution) and BCR-2 (USGS reference material), with ⁸⁷Sr/⁸⁶Sr ratios of 0.71025 ± 0.00002 (2se) and 0.70503 ± 0.00002 (2se), respectively, which were consistent with previously reported values within the uncertainty range.³¹

2.6 Sr isotope analysis by LA-MC-ICP-MS

In situ Sr isotope analyses of HZZ-2 and TARIM calcite were performed using a Neptune Plus MC-ICP-MS (Thermo Fisher Scientific, Germany) with a RESOlution S155 LR 193 nm ArF excimer laser ablation system at the Laboratory for Mineral Laser Microanalysis (Milma Lab) of CUGB. Eight pieces of HZZ-2 calcite (named A–H) and four pieces of TARIM calcite (named J–M) were randomly selected for analysis. Each TARIM piece was analyzed in two separate sessions to yield a total of eight sessions, matching HZZ-2. The laser parameters were set as follows: a spot size of 75 μm for HZZ-2 and 130 μm for TARIM (larger spot for TARIM to compensate for its lower Sr content), with an energy density of 5 J cm⁻² and a repetition rate of 6 Hz. Each analysis comprised 20 s of background measurement followed by 40 s of ablation and 10 s of signal washout. Argon and helium served as carrier gases during the laser ablation process, and a trace amount of nitrogen was added to enhance the instrument's sensitivity.³²⁸⁵Rb, ⁸⁴Sr, ⁸⁶Sr, ⁸⁷Sr and ⁸⁸Sr were collected using Faraday Cups L2, L3, C, H2, and H3 respectively, acquiring 250 cycles of 0.262 s each (Table S1†). During the sessions, data quality was monitored by interspersing analyses of two calcite reference materials, MNP²⁴ and LSJ07.²⁵

Correction or suppression of interferences is critical for LA-MC-ICP-MS Sr isotope analysis. Due to the coexistence of multiple interferences, Kr, REE²⁺ and Rb are corrected in sequence, according to the interference correction procedures summarized by previous researchers.¹⁶ Among Kr isotopes, ⁸⁴Kr and ⁸⁶Kr interfere with ⁸⁴Sr and ⁸⁶Sr, respectively. The interference was corrected using the natural abundance ratios of the Kr isotopes (⁸³Kr/⁸⁴Kr = 0.20175 and ⁸³Kr/⁸⁶Kr = 0.66474)^12,33 along with a 20-second gas blank. REE²⁺ interference arises from Er²⁺ (¹⁶⁸Er²⁺ and ¹⁷⁰Er²⁺) and Yb²⁺ (¹⁶⁸Yb²⁺, ¹⁷⁰Yb²⁺, ¹⁷²Yb²⁺, ¹⁷⁴Yb²⁺, and ¹⁷⁶Yb²⁺) and is corrected by monitoring the interference-free ¹⁶⁷Er²⁺ and ¹⁷³Yb²⁺ signals.^17,34 Since Rb and Sr exhibit similar isotopic fractionation behavior in mass spectrometry, the interference of ⁸⁷Rb on ⁸⁷Sr can be corrected using the exponential law (β_Rb = β_Sr), taking into account the Rb isotope abundance ratio (naturally occurring ⁸⁵Rb/⁸⁷Rb = 2.5926) and the ⁸⁵Rb signal.^12,17,35 Based on the exponential law, the ⁸⁷Sr/⁸⁶Sr ratios are calculated using the ratio of ⁸⁶Sr/⁸⁸Sr = 0.1194 for mass fractionation correction.³⁰ Iolite V3.73 software²⁹ was used for offline data reduction and processing. Using the calcite reference materials MNP and LSJ-07 for data quality monitoring, the average ⁸⁷Sr/⁸⁶Sr ratios obtained were 0.70605 ± 0.00014 (2s, n = 57) and 0.71015 ± 0.00008 (2s, n = 34), respectively, which are consistent with the recommended values reported in the literature within the error range.^24,25

3. Results and discussion

3.1 μ-XRF images

μ-XRF analysis was conducted on the HZZ-2 and TARIM calcite samples. The μ-XRF images of Ca and Sr, obtained following the analysis, are presented in Fig. 2. From the image, the Ca content in the two calcite samples is uniform (Fig. 2a and b), indicating that the major elemental compositions of the samples are homogeneous. The Sr content image of the HZZ-2 calcite is uniform (Fig. 2c), and the Sr content image of the TARIM calcite is slightly uneven (Fig. 2d), which may be affected by the surface roughness of the sample. However, the results of TIMS and LA-MC-ICP-MS analyses showed that the Sr isotopic composition of TARIM calcite is homogeneous (more details can be seen in Sections 3.3 and 3.4). On the whole, the major element abundances of HZZ-2 and TARIM calcite are homogeneous, and the homogeneity of HZZ-2 calcite is better than that of TARIM calcite.

Fig. 2 μ-XRF images of Ca and Sr elemental distributions in calcite samples: HZZ-2 calcite (a and c) and TARIM calcite (b and d).

3.2 Chemical composition

To determine the chemical composition of the calcite samples, forty single-spot EPMA analyses and thirty-five single-spot LA-ICP-MS analyses were performed on small pieces of HZZ-2 (A, B, C, D, E, F, G, and H), while twenty single-spot EPMA analyses and forty single-spot LA-ICP-MS analyses were performed on small pieces of TARIM (J, K, L, and M). EPMA major-element and LA-ICP-MS trace-element data are presented in Tables S2 and S3,† respectively. The EPMA elemental distribution maps of Mg, Mn, and Sr exhibit uniformity (Fig. 3), indicating a homogeneous distribution of these elements within the mineral particles. Furthermore, EPMA data reveal that the major element abundances in HZZ-2 and TARIM calcite are also homogeneous. The chemical composition of the sample closely matches the theoretical calcite composition, indicating minimal cationic substitution (e.g., Mg²⁺ or Fe²⁺ for Ca²⁺) during crystallization. HZZ-2 and TARIM calcite yield CaO mean contents of 56.42 wt% (RSD% = 0.73%) and 56.47 wt% (RSD% = 0.53%), respectively. Both HZZ-2 and TARIM calcite yield low magnesium content, and the chemical composition indicates that both reference materials are low-Mg calcite, with mean values of 0.21 wt% and 0.15 wt% for MgO, respectively. The Mn content in HZZ-2 and TARIM calcite ranges from 24.9 μg g⁻¹ to 43.2 μg g⁻¹ (mean = 29.5 μg g⁻¹) and 57.5 μg g⁻¹ to 84.1 μg g⁻¹ (mean = 64.5 μg g⁻¹), respectively. The Fe contents in HZZ-2 and TARIM calcite are similar, with mean values of 547 μg g⁻¹ and 541 μg g⁻¹, respectively. Both HZZ-2 and TARIM calcite yield extremely low Rb content, with mean values of 0.005 μg g⁻¹ and 0.006 μg g⁻¹, respectively. Compared to other calcite RMs (Table 1), HZZ-2 and TARIM calcite materials have relatively modest Sr content (mean values of 1100 μg g⁻¹ and 620 μg g⁻¹, respectively) and extremely low Rb/Sr ratios (<0.00001).

Fig. 3 EPMA elemental distribution mappings of Mg, Mn, and Sr in calcite samples: HZZ-2 calcite (a, b and c) and TARIM calcite (d, e and f).

Table 1 Summary of carbonate reference materials currently used for Sr isotope analysis from reported references

Name	Mineral/rock type	Origin	Sr content (μg g^−1)	^87Sr/^86Sr		Methods	References
				SN-TIMS (2s)	LA-MC-ICP-MS (2s)
NanoSr	Synthetic carbonate	Synthetic powder	∼500	0.70756 ± 0.00003	0.70753 ± 0.00007	TIMS and MC-ICP-MS	23
JCp-1	Coral	Shigaki Island (Japan)	7260–7500	0.70916 ± 0.00002	0.70918 ± 0.00006	TIMS and MC-ICP-MS	22
JCt-1	Giant clam	Kume Island (Japan)	∼1400	0.70917 ± 0.00001	0.70917 ± 0.00005	TIMS and MC-ICP-MS
MACS-1	Carbonate pellet	Synthetic powder	196–249	0.70795 ± 0.00001		MC-ICP-MS
MACS-3	Carbonate pellet	Synthetic powder	6260–8012	0.70754 ± 0.00001	0.70754 ± 0.00001	TIMS and MC-ICP-MS
MNP	Calcite	Maoniuping deposit (China)	7064–12893	0.70617 ± 0.00005	0.70617 ± 0.00005	TIMS and MC-ICP-MS	24
TLM	Calcite	Tarim Basin (China)	∼100	0.70970 ± 0.00003	0.70969 ± 0.00023	TIMS and MC-ICP-MS	25
LSJ07	Calcite	Hunan, China	∼200	0.71004 ± 0.00003	0.71006 ± 0.00015	TIMS and MC-ICP-MS
BZS	Calcite	A mineral fair	212	0.71181 ± 0.00001	0.71181 ± 0.00013	MC-ICP-MS	26
WS-1	Calcite	Yunnan, China	598	0.70872 ± 0.00003	0.70872 ± 0.00009	MC-ICP-MS
HZZ-2	Calcite	Yunnan, China	∼1100	0.70941 ± 0.00001	0.70940 ± 0.00003	TIMS and LA-MC-ICP-MS	This study
TARIM	Calcite	Tarim Basin (China)	∼620	0.71042 ± 0.00001	0.71042 ± 0.00002	TIMS and LA-MC-ICP-MS	This study

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The chondrite-normalized REE element patterns of both HZZ-2 and TARIM calcite are depicted in Fig. 4. The REE distribution pattern of HZZ-2 calcite shows a right-sloping trend, which is characterized by LREE enrichment, a relative deficit of HREE, and a slight positive Eu anomaly (Fig. 4a). Compared to HZZ-2 calcite, the REE distribution pattern of TARIM calcite shows a left-leaning trend, with a lower LREE content relative to HREE and a slight negative anomaly of Eu (Fig. 4b). The differences between the chondrite-normalized REE element patterns of HZZ-2 and TARIM calcite may indicate that their crystallization environments are different. HZZ-2 calcite yields Er and Yb mean contents of 0.023 μg g⁻¹ and 0.009 μg g⁻¹ and TARIM calcite yields Er and Yb mean contents of 0.390 μg g⁻¹ and 0.268 μg g⁻¹. Both HZZ-2 and TARIM calcite yield very low Er and Yb contents, indicating that the doubly charged Er²⁺ and Yb²⁺ interfere negligibly with the determination of Sr isotope composition.

Fig. 4 Rare earth element chondrite-normalized patterns plot for HZZ-2 calcite (a) and TARIM calcite (b). Chondrite normalizing values are from Sun and McDonough (1989).⁴⁰

3.3 TIMS Sr isotope ratios

The Sr isotope data of eight randomly selected fragments of HZZ-2 calcite and seven randomly selected fragments of TARIM calcite obtained by TIMS at CUGB are shown in Fig. 5 and Table 2. The results showed that eight analyses of HZZ-2 calcite yielded a mean ⁸⁷Sr/⁸⁶Sr ratio of 0.70941 ± 0.00001 (2s, n = 8, Fig. 5a) and seven analyses of TARIM yielded a mean ⁸⁷Sr/⁸⁶Sr ratio of 0.71042 ± 0.00001 (2s, n = 7, Fig. 5b). The mean ⁸⁷Sr/⁸⁶Sr ratios of 0.70941 ± 0.00001 (2s, n = 8) and 0.71042 ± 0.00001 (2s, n = 7) determined by TIMS analysis are suggested as recommended ⁸⁷Sr/⁸⁶Sr values for the HZZ-2 and TARIM calcite reference materials, respectively.

Fig. 5 Sr isotopic results of HZZ-2 calcite (a) and TARIM calcite (b) obtained by TIMS. The blue circles represent the ⁸⁷Sr/⁸⁶Sr values of the individual samples, and the red circles represent the mean values (indicated in red font). The grey shaded area represents 2sd of the mean values, and the error bars for individual values represent 2se.

Table 2 Sr isotopic analytical data by TIMS and the overall results of the laser in situ analysis

Name	Sample	^87Sr/^86Sr	2s	Number	Methods
TIMS analysis
HZZ-2	HZZ-2-1	0.70940	0.00002	1	TIMS
	HZZ-2-2	0.70940	0.00002	1	TIMS
	HZZ-2-3	0.70941	0.00002	1	TIMS
	HZZ-2-4	0.70941	0.00002	1	TIMS
	HZZ-2-5	0.70942	0.00002	1	TIMS
	HZZ-2-6	0.70940	0.00002	1	TIMS
	HZZ-2-7	0.70941	0.00002	1	TIMS
	HZZ-2-8	0.70940	0.00002	1	TIMS
	Mean	0.70941	0.00001
TARIM	TARIM-1	0.71042	0.00002	1	TIMS
	TARIM-2	0.71043	0.00002	1	TIMS
	TARIM-3	0.71042	0.00002	1	TIMS
	TARIM-4	0.71042	0.00002	1	TIMS
	TARIM-5	0.71042	0.00002	1	TIMS
	TARIM-6	0.71042	0.00002	1	TIMS
	TARIM-7	0.71043	0.00002	1	TIMS
	Mean	0.71042	0.00001
NISTSRM987		0.71025	0.00002	1	TIMS
BCR-2		0.70503	0.00002	1	TIMS
In situ analysis
HZZ-2	HZZ-2-A	0.70939	0.00006	40	LA-MC-ICP-MS
	HZZ-2-B	0.70942	0.00006	38	LA-MC-ICP-MS
	HZZ-2-C	0.70941	0.00007	40	LA-MC-ICP-MS
	HZZ-2-D	0.70940	0.00007	40	LA-MC-ICP-MS
	HZZ-2-E	0.70938	0.00006	40	LA-MC-ICP-MS
	HZZ-2-F	0.70939	0.00006	40	LA-MC-ICP-MS
	HZZ-2-G	0.70942	0.00006	40	LA-MC-ICP-MS
	HZZ-2-H	0.70941	0.00007	40	LA-MC-ICP-MS
	Total	0.70940	0.00003	318	LA-MC-ICP-MS
TARIM	TARIM-J	0.71042	0.00005	35	LA-MC-ICP-MS
	TARIM-K	0.71041	0.00008	35	LA-MC-ICP-MS
	TARIM-L	0.71047	0.00005	40	LA-MC-ICP-MS
	TARIM-M	0.71039	0.00008	40	LA-MC-ICP-MS
	TARIM-J2	0.71042	0.00006	40	LA-MC-ICP-MS
	TARIM-K2	0.71041	0.00005	40	LA-MC-ICP-MS
	TARIM-L2	0.71046	0.00005	40	LA-MC-ICP-MS
	TARIM-M2	0.71038	0.00005	35	LA-MC-ICP-MS
	Total	0.71042	0.00002	305	LA-MC-ICP-MS
MNP		0.70605	0.00014	57	LA-MC-ICP-MS
LSJ07		0.71015	0.00008	34	LA-MC-ICP-MS

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3.4 LA-MC-ICP-MS Sr isotopes

To assess the homogeneity of Sr isotopes in the calcite samples, a total of 318 LA-MC-ICP-MS spot measurements were performed in eight sessions on eight small pieces of HZZ-2 calcite, while a total of 305 LA-MC-ICP-MS spot measurements were performed in eight sessions on four small pieces of TARIM calcite. The results of LA-MC-ICP-MS Sr isotope analysis for HZZ-2 and TARIM calcite are shown in Fig. 6 and Table S4.† Using the same laser parameters (75 μm, 5 J cm⁻², 6 Hz) for HZZ-2 calcite, the mean ⁸⁷Sr/⁸⁶Sr ratios for the eight sessions were as follows: 0.70939 ± 0.00006 (2s, n = 40, piece A), 0.70942 ± 0.00006 (2s, n = 38, piece B), 0.70941 ± 0.00007 (2s, n = 40, piece C), 0.70940 ± 0.00007 (2s, n = 40, piece D), 0.70938 ± 0.00006 (2s, n = 40, piece E), 0.70939 ± 0.00006 (2s, n = 40, piece F), 0.70942 ± 0.00006 (2s, n = 40, piece G), and 0.70941 ± 0.00007 (2s, n = 40, piece H), respectively (Fig. 6a). A total of 318 Sr isotopic analyses yielded ⁸⁷Sr/⁸⁶Sr values from 0.70932 to 0.70950, following a Gaussian distribution (Fig. 6b) with an overall mean ⁸⁷Sr/⁸⁶Sr value of 0.70940 ± 0.00003 (2s, n = 318).

Fig. 6 The ⁸⁷Sr/⁸⁶Sr measurements using LA-MC-ICP-MS and TIMS and histogram of the LA-MC-ICP-MS ⁸⁷Sr/⁸⁶Sr ratios of HZZ-2 calcite (a and b) and TARIM calcite (c and d). The ⁸⁷Sr/⁸⁶Sr values and averages are shown as colorless circles. The ⁸⁷Sr/⁸⁶Sr values and the mean values with uncertainty (2sd) are represented by circles and fonts of different colors, respectively. Blue lines indicate the mean values and the error bars are 2se for individual values.

The laser parameters used (130 μm, 5 J cm⁻², 6 Hz) for TARIM calcite analysis were slightly different from those used for HZZ-2 calcite. Across eight sessions, TARIM calcite yielded mean ⁸⁷Sr/⁸⁶Sr values of 0.71042 ± 0.00005 (2s, n = 35, piece J), 0.71041 ± 0.00008 (2s, n = 35, piece K), 0.71047 ± 0.00005 (2s, n = 40, piece L), 0.71039 ± 0.00008 (2s, n = 40, piece M), 0.71042 ± 0.00006 (2s, n = 40, piece J), 0.71041 ± 0.00005 (2s, n = 40, piece K), 0.71046 ± 0.00005 (2s, n = 40, piece L), and 0.71038 ± 0.00005 (2s, n = 35, piece M), respectively (Fig. 6c). The two sessions labeled J (K, L, and M) correspond to two independent analyses of the same small pieces of TARIM calcite at different time times. We used TARIM-J (K, L, M) and TARIM-J2 (K2, L2, M2) to distinguish between them, as given in Table S4.† During eight sessions, 305 spot measurements yielded a mean ⁸⁷Sr/⁸⁶Sr value of 0.71042 ± 0.00002 (2s, n = 305), and the data followed a Gaussian distribution (Fig. 6d).

In summary, HZZ-2 and TARIM calcite show uniform Sr isotope ratios and appear to lack inter-grain variations at the sampling scale of LA-MC-ICP-MS. The results obtained using the LA-MC-ICP-MS in situ Sr isotope analysis for HZZ-2 and TARIM calcite are consistent with the TIMS results within analytical uncertainty.

3.5 Comparisons with other reference materials

Other carbonate reference materials that can be used for Sr isotope analysis are either used or discussed in this study, and information pertaining to these RMs is summarized in Table 1. The reference materials can be broadly divided into synthetic substances (JCp-1, JCt-1, MACS-1, MACS-3, and NanoSr) and natural solid calcite materials (MNP, TLM, LSJ07, BZS, WS-1, TARIM, and HZZ-2) and their Sr contents and isotopic ratios are shown in Fig. 7. JCp-1 and JCt-1 are biological sources and are mainly composed of aragonite and have a smaller range of applications. MACS-1 has a low Sr content (196–249 μg g⁻¹) but relatively high contents of REEs (Er = 128 ± 17 μg g⁻¹ and Yb = 133 ± 10 μg g⁻¹)³⁶ and Rb (0.17 ± 0.05 μg g⁻¹).³⁷ The combined interference of Er and Yb may result in a significant error in the ⁸⁷Sr/⁸⁶Sr measurements, and the high content of Rb will produce strong isotopic interference with Sr, so MACS-1 cannot be used for LA-MC-ICP-MS Sr isotope analysis. Weber et al. (2020)²³ proposed a new reference material (NanoSr) made of synthetic carbonate nano-powders for laser in situ Sr isotope analysis. Compared to traditional synthetic powder pellets, nano-synthetic powder pellets have better mechanical stability and homogeneity,^38,39 but they may be contaminated during the preparation of nanoparticles, resulting in abnormal content of some trace elements.³⁷

Fig. 7 Carbonate reference materials currently available for Sr isotope analysis. The circles represent the natural solid calcite reference materials, the triangles represent the synthetic powder particle reference materials. The red (HZZ-2) and orange (TARIM) circles represent the two new natural solid calcite reference materials presented in this study, respectively.

Compared with natural solid calcite, the synthetic carbonate powder pellet RMs may not meet the needs of long-term analytical testing due to oxidation and loss of cohesion. However, there are few reference materials of natural solid carbonates, creating an urgent need for the development of high-quality RMs. Liang et al. (2023)²⁴ developed a natural calcite reference material with high Sr content (7064–12893 μg g⁻¹) for laser in situ Sr isotope analysis; however there are few calcite samples with such high Sr content in nature. Wu et al. (2023)²⁵ proposed two low-Sr natural calcite RMs, TLM and LSJ07, with Sr content sof ∼100 μg g⁻¹ and ∼200 μg g⁻¹, respectively. Due to their low Sr content, they are actually suitable for low Sr calcite samples. In addition, the low Sr content also results in a corresponding reduction in the Sr signal strength detected by the instrument, which places higher demands on the instrument and makes the analysis more challenging. Yin et al. (2024)²⁶ proposed two natural calcite RMs (BZS and WS-1), which to some extent address the shortage of reference materials with moderate Sr content.

Two new natural calcite RMs (HZZ-2 and TARIM) can meet the analytical testing requirements for the vast majority of calcite samples because of the relatively moderate Sr contents (HZZ-2 = ∼1100 μg g⁻¹ and TARIM = ∼620 μg g⁻¹). TIMS and LA-MC-ICP-MS analyses demonstrate that they have homogeneous Sr isotope ratios. The ⁸⁷Sr/⁸⁶Sr ratios in each LA-MC-ICP-MS spot analysis was evaluated using two standard deviations (2s), while the homogeneity of the samples was further verified through the mean square of weighted deviates (MSWD). The MSWD values of ⁸⁷Sr/⁸⁶Sr for HZZ and TARIM calcite based on all LA-MC-ICP-MS spot analyses are 0.9 and 2.6, respectively. In the eight sessions, the MSWD values for HZZ-2 and TARIM calcite remained low, with little variation, ranging from 0.4 to 1.7 and 0.98 to 1.6, respectively. These results indicate that the ⁸⁷Sr/⁸⁶Sr values of HZZ-2 and TARIM calcite are fairly homogeneous. Due to the high level of common lead, the age of the HZZ-2 calcite cannot be accurately determined. Fortunately, TARIM serves as a reference material for in situ calcite U–Pb dating, with multiple LA-(MC/Q)-ICP-MS U–Pb dating analyses yielding a lower intercept age of 208.0 ± 0.4/3.2 Ma (2s, MSWD = 3.0, n = 515).²⁷ Serving as a dual-purpose reference material for simultaneous U–Pb dating and Sr isotope analysis via LA-MC-ICP-MS, TARIM calcite enables multiple geological applications within a single analytical session, significantly enhancing analytical efficiency while providing crucial constraints for geochronological interpretations. In addition, HZZ-2 and TARIM calcite materials are abundant (HZZ-2: ∼212 g and TARIM: ∼1245 g) enough to meet the needs of the scientific community for large quantities. Both HZZ-2 and TARIM calcite are important complements to the previously reported MNP, TLM, LSJ07, BZS, and WS-1 natural calcite RMs and can be used for laser in situ Sr isotope analysis of samples with either high or low Sr content.

4. Conclusions

Two new natural calcite reference materials (HZZ-2 and TARIM) were developed for laser in situ Sr isotope analysis using LA-MC-ICP-MS in this study. The analysis results obtained using μ-XRF, EPMA and LA-ICP-MS indicate that the two RMs have relatively homogeneous major and trace element compositions, very low Rb/Sr ratios (<0.00001), and relatively moderate Sr contents (HZZ-2 = ∼1100 μg g⁻¹ and TARIM = ∼620 μg g⁻¹). The TIMS analysis yielded mean ⁸⁷Sr/⁸⁶Sr ratios of 0.70941 ± 0.00001 (2s, n = 8) for HZZ-2 calcite and 0.71042 ± 0.00001 (2s, n = 7) for TARIM calcite, respectively. Multiple LA-MC-ICP-MS spot analyses show that the HZZ-2 and TARIM calcite reference materials have highly homogeneous Sr isotope ratios, and the results are consistent with the TIMS values within analytical uncertainty. We recommend using the TIMS ⁸⁷Sr/⁸⁶Sr values as the reference values for HZZ-2 and TARIM calcite. Both HZZ-2 and TARIM calcite reference materials are suitable for calcite samples with high or low Sr content and are important additions to previously reported reference materials.

The HZZ-2 and TARIM calcite RMs developed in this study are in sufficient quantities and the scientific community can obtain them by contacting the corresponding authors of this paper (email: E-mail: changialight@163.com).

Data availability

The authors confirm that the data supporting this study are available within the article and its ESI tables.†

Author contributions

Hao-Jie Li: methodology, investigation, writing-original draft. Zhi-Zhong Hu: sample (HZZ-2) provision. Liang-Liang Zhang: conceptualization, resources, writing-review & editing, supervision, funding acquisition. Di-Cheng Zhu: writing-review & editing, validation, resources. Jin-Cheng Xie: LA-MC-ICP-MS Sr methodology, validation. Qing Wang: supervision. Wen-Tan Xu: EPMA major element methodology. Li-Juan Xu: TIMS Sr isotopic methodology. Wei Guo: μ-XRF elemental analysis methodology. Jian Wu: resources, supervision.

Conflicts of interest

The authors declare that they have no conflict of interest.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (42473039, 42121002, and 41973052). We extend our gratitude to Lu Liang, Hao-Yin Chi, Xin-Meng Liu and Yu-Xuan Zhao for their help in the LA-MC-ICP-MS Sr isotope analysis experiment.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ja00103j

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