Further Characterization of the RW-1 Monazite: A New Working Reference Material for Oxygen and Neodymium Isotopic Microanalysis

Abstract

The oxygen (O) and neodymium (Nd) isotopic composition of monazite provides an ideal tracer of metamorphism and hydrothermal activity. Calibration of the matrix effect and monitoring of the external precision of monazite O–Nd isotopes with microbeam techniques, such as secondary ion mass spectrometry (SIMS) and laser ablation-multicollector-inductively coupled plasma-mass spectrometry (LA-MC-ICPMS), require well-characterized natural monazite standards for precise microbeam measurements. However, the limited number of standards available is impeding the application of monazite O–Nd isotopes. Here, we report on the RW-1 monazite as a potential new working reference material for microbeam analysis of O–Nd isotopes. Microbeam measurements by electron probe microanalysis (EPMA), SIMS, and LA-MC-ICPMS at 10–24 µm scales have confirmed that it is homogeneous in both elemental and O–Nd isotopic compositions. SIMS measurements yield δ¹⁸O values consistent, within errors, with those obtained by laser fluorination techniques. Precise analyses of Nd isotope by thermal ionization mass spectrometry (TIMS) are consistent with mean results of LA-MC-ICPMS analyses. We recommend δ¹⁸O = 6.30‰ ± 0.16‰ (2SD) and ¹⁴³Nd/¹⁴⁴Nd = 0.512282 ± 0.000011 (2SD) as being the reference values for the RW-1 monazite.

1. Introduction

Monazite is a light rare earth element (LREE)-bearing phosphate mineral and has an extremely variable composition, e.g., monazite-(Ce), monazite-(La), monazite-(Nd), and monazite-(Sm) [1]. It is a ubiquitous accessory mineral in peraluminous granite, pegmatite, metapelite, metapsammite, carbonatite, and hydrothermal veins [1]. It also occurs as a detrital mineral in many clastic sedimentary rocks. Monazite is sensitive to relatively low-grade metamorphism (down to greenschist facies) [2] and hydrothermal fluids [3]. Due to its high U and Th contents, low initial nonradiogenic Pb content, and apparent resistance to metamictization, monazite has been widely acknowledged as a good geochronometer (e.g., [1,4,5]). More importantly, monazite is the major carrier of rare earth elements (REEs) and, therefore, is considered the best proxy for use in the estimation of Nd isotopic compositions of large continental areas [6]. Furthermore, the oxygen isotopic composition of monazite is considered an essential monitor of fluid infiltration during metamorphism [7]. The elemental composition of monazite is also used to calculate the temperatures of metamorphism where it coexists with xenotime and garnet [8,9]. Therefore, the combination of monazite U–Th–Pb ages and O–Nd isotopic and elemental compositions may provide a comprehensive view of many geological processes and continental evolution [10].

Current secondary ion mass spectrometry (SIMS) and laser ablation-multicollector-inductively coupled plasma-mass spectrometry (LA-MC-ICPMS) techniques provide rapid data output in monazite U–Th–Pb age determinations and O–Nd isotopic microanalyses, with higher spatial resolution and less sample destruction than traditional bulk dissolution analytical techniques (e.g., [7,11,12]). However, such microbeam techniques require matrix-matched reference materials to correct for matrix effects and to evaluate the external precision. So far, many monazite U–Th–Pb age standards have been developed (

Table 1. Compilation of previously investigated monazite reference materials.

Name	Th Content (wt.%)	2SD a	δ18OV-SMOW (‰) b	2SD	Reference Age (Ma)	2SD	143Nd/144Nd	2SD	Host Lithology (Location)	Quantity	References
RW-1	8.3	0.4	6.30	0.16	904.2	0.3	0.512282	0.000011	pegmatite (Norway)	limited	this study, [17]
eBay	0.02–0.34 (heterogeneous)	-	9.34 (heterogeneous)	0.62	-	-	-	-	unknown (Hiddenite, North Carolina)	limited	[7,13,18]
			9.52	-
Diamantina	0.2	0.1	-	-	495.3	0.5	0.511427	0.000023	hydrothermal quartz vein (Brazil)	unlimited	[19]
UNIL1-Mnz1	0.6	0.3	8.45	0.38	-	-	-	-	carbonatite (Namibia)	limited	[15]
USGS-44069	2.1–5.5 (heterogeneous)	-	7.67	0.52	424.9	0.4	0.512175	0.000040	metapsamittic layer (USA)	unlimited	[12,14,15,20]
554	3.3	-	7.54 (heterogeneous)	0.24	45.3	1.4	0.512075	0.000041	granite (Santa Catalina, Mountains, Arizona)	unlimited	[7,12,21]
Itambe	6.3 (heterogeneous)	1.2	0.46	0.42	~509–514	-	-	-	pegmatite (Brazil)	limited	[14]
Moacyr	6.5	0.4	1.45	0.10	506.4	0.7	0.512421	0.000011	pegmatite (Brazil)	unlimited	[15,22]
UNIL-Mnz2	9.4 (heterogeneous)	1	9.51	0.54	-	-	-	-	pegmatite (Madagascar)	limited	[15]
Manangoutry	12.1	0.2	10.19	0.16	555	2	0.511044	0.000022	charnockite (Madagascar)	limited	[12,15,23]
Namaqualand	8.3	0.2	-	-	~1033	-	0.511894	0.000025	monazite vein (South Africa)	unlimited	[12,24]
Brazil	6.1	-	1.43	0.16	-	-	-	-	unknown	limited	[7,25]
Brazil c	13.3	0.5	7.89	0.28	-	-	-	-	unknown	limited	[13]
M1	7.9–14.3 (heterogeneous)	-	-	-	~535	-	0.511716	0.000052	unknown	limited	[12]
M4	6.4–8.5 (heterogeneous)	-	-	-	~525	-	0.511761	0.000020	unknown	limited	[12]

^a SD = standard deviation. ^b V-SMOW = Vienna Standard Mean Ocean Water; δ¹⁸O value is determined by laser or conventional fluorination. ^c A different crystal from Ayers et al. [7].

), while monazite standards for O–Nd isotopic microanalysis are limited. Breecker and Sharp [13] found that instrumental mass fractionation (IMF) in the SIMS analysis of the monazite oxygen isotope is significantly correlated with the Th content. More recently, Rubatto et al. [14] and Didier et al. [15] refined in situ monazite analytical procedures and attempted to correct IMF with a set of reference materials of different Th and YREEPO₄ contents (XREEY). Monazite composition varies greatly due to the substitution of huttonite (ThSiO₄) and cheralite [CaTh(PO₄)₂] [1]. The Th content of these monazite-group minerals varies from a few hundred ppm up to 45 wt.% [16]. Therefore, the availability of compositionally diverse standards covering the range of natural monazite compositions is fundamental for precise oxygen isotopic analysis by SIMS.

An ideal monazite standard would be homogeneous for U–Th–Pb ages and O–Nd isotopic and elemental compositions, as well as being easily available to the scientific community. To date, ten monazite reference materials have been reported for the O isotope, and nine for the Nd isotope (e.g., [12,13,14,15,17,18,19,20,21,22,23,24,25], Table 1). Among them, six are heterogeneous in elemental compositions (eBay, USGS-44069, Itambe, UNIL-Mnz2, M1, and M4), two are heterogeneous in O isotopic compositions (eBay and 554), and Manangoutry monazite is of limited quantity. Only one reference material, Moacyr, is ideal for U–Th–Pb and O–Nd isotopic microanalysis, therefore, new monazite standards are urgently required. This study reports a comprehensive analysis of the elemental and O–Nd isotopic compositions of the RW-1 monazite, which has been reported as a reference material for U–Th–Pb microbeam dating [17]. We investigate its crystal structure using Raman Spectroscopy for the first time and assess its compositional homogeneity using electron probe microanalysis (EPMA), including X-ray mapping, cathodoluminescent (CL) imaging, and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICPMS). We assess its homogeneity of O–Nd isotopic compositions using SIMS and LA-ICPMS at a scale of ~20 µm, and precisely determine isotopic compositions using laser fluorination isotope ratio mass spectrometry (LF-IRMS) and thermal ionization mass spectrometry (TIMS). Results indicate that the RW-1 monazite may be a new high-Th (i.e., enriched in Th content) monazite reference material for microbeam O–Nd isotopic analyses.

2. Sample Descriptions

The RW-1 monazite was collected from a pegmatite dike located in the Landsverk 1 quarry in the Evje-Iveland district, Southern Norway [17]. It is a yellowish-brown crystal with a weight of 44 g, provided by the Evje og Hornnes Geomuseum Fennefoss, Norway. It is transparent under a binocular microscope and homogeneous in terms of its gray-scale intensity in back-scattered electron (BSE) images [17]. It has been recently developed as a reference material for U–Th–Pb microbeam dating with a recommended reference ²⁰⁷Pb/²³⁵U age of 904.15 ± 0.26 Ma [17].

The RW-1 crystal was crushed into fragments of around 50–200 µm diameter. Monazite fragments were randomly selected by handpicking under a binocular microscope and cast in a standard epoxy mount with a diameter of 2.5 cm along with Diamantina, UNIL-Mnz1, USGS-44069, Itambe, and Manangoutry monazite standards. The mount was polished to expose fragment interiors. Monazite grains were documented with transmitted and reflected light photomicrographs and analyzed by Raman spectrometry. Cathodoluminescence (CL) images revealed internal structures and mineral inclusions. In addition, the fragments were investigated by X-ray mapping to understand the elemental (Si, Th, Ce, and Y) distributions.

3. Analytical Methods

All spectroscopic and image studies and compositional and isotopic analyses were performed at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGG-CAS), Beijing, China (except LF-IRMS; see Section 3.4).

3.1. Spectroscopic and Image Investigations

Raman spectra were acquired with a Horiba–Jobin Yvon LabRam HR 800 Raman spectroscopy system using a 633 nm He–Ne laser for excitation, with a spectral resolution of up to 0.65 cm⁻¹. CL images were obtained using a Nova NanoSEM 450 field emission scanning electron microscope (FE-SEM) equipped with a Gatan Mono CL4 detector, with an accelerating voltage of 10 kV and working distance of 13.5 mm. X-ray mapping used a CAMECA SXFiveFE EPMA and Bruker energy-dispersive X-ray spectrometer (EDS) operating in BSE mode with an accelerating voltage of 10 kV, primary beam current of 100 nA, 0.4 × 0.4 µm step size, and dwell time of 30 ms per step (with 3 h acquisition per map).

3.2. Major and Trace Element Compositions

The major element composition of RW-1 fragments, including P, La, Ce, Pr, Nd, Sm, Gd, Dy, Y, Si, Ca, Th, U, and Pb, was determined using a CAMECA SXFiveFE EPMA equipped with thallium acid phthalate (TAP), and large LiF and polyethylene terephthalate (PET) diffraction crystals, with an accelerating voltage of 10 kV, beam current of 200 nA, defocused beam diameter of 10 µm, 20–30 s counting time per element (120 s for Pb), and a total acquisition time of ~6 min. Standards were a natural apatite [Ca₅PO₄(F, Cl, OH)] for P and Ca; a natural wollastonite (CaSiO₃) for Si; a natural crocoite (PbCrO₄) for Pb; two Si–Al–Ca glasses containing 5 wt.% Th and U; and nine Si–Al–Ca glasses containing ~10 wt.% of each REE.

Concentrations of middle (MREEs) and heavy REEs (HREEs), including Eu, Tb, Ho, Er, Tm, Yb, and Lu, were determined for 23 monazite fragments using an Agilent 7500a quadrupole-inductively coupled plasma-mass spectrometry (Q-ICPMS) system (Agilent Technologies, Santa Clara, CA, USA) coupled to a 193 nm ArF excimer laser system (Geolas HD, Lambda Physik, Göttingen, Germany). Procedures were similar to those described by Wu et al. [26] with isotopes measured in peak hopping mode with a laser beam diameter of 24 µm, pulse repetition rate of 3 Hz, beam flux of 5 J/cm², acquisition times of ~20 s for background and 40 s for sample data, and a He–Ar carrier gas. Trace element concentrations were calibrated against National Institute of Standards and Technology (NIST) Standard Reference Material (SRM) 610 as an external reference material. The average Ce content (determined by EPMA) was used as an internal standard to correct for differences in ablation yields between monazite and glass reference materials. US Geological Survey (USGS) BCR-2G glass was analyzed for further quality control purposes using the internal standard Ce. Concentrations were calculated using the GLITTER 4.0 program [27]. For most trace elements with concentrations of >0.5 ppm, the accuracy was better than 10% relative standard deviation (2RSD), with a precision (2RSD) of 20%.

3.3. SIMS Oxygen Isotopic Analysis

The RW-1 oxygen isotopic composition (δ¹⁸O) was determined using a Cameca IMS-1280 SIMS system following analytical procedures similar to those described by Li et al. [28] and Tang et al. [29]. The Gaussian-focused Cs⁺ ion beam was accelerated at 10 kV, with an intensity of 1.3–2.0 nA and raster size of 10 µm. A normal incidence electron-flood gun was used to compensate for sample charging and to maintain the voltage stability. Negative secondary ions were extracted with a −10 kV potential. ¹⁶O and ¹⁸O ions were detected simultaneously by two Faraday cups (multicollection mode). The nuclear magnetic resonance (NMR) controller was used to stabilize the magnetic field. The entrance slit was set at ~125 µm, the field aperture was 6000 µm, the energy slit was 40 eV, and the exit slit was ~300 µm. The signal intensity of ¹⁶O was ~1.6 × 10⁹ cps, and the primary beam size was typically 10 × 15 µm. Each spot analysis involved presputtering, beam centering, and signal collection, with a total analytical time per spot of ~3 min, including 2 min for presputtering and centering of the secondary beam, and 1 min for collection of 16 cycles of ¹⁶O and ¹⁸O signals. Measured ¹⁸O/¹⁶O ratios were normalized to Vienna Standard Mean Ocean Water compositions (V-SMOW; ¹⁸O/¹⁶O = 0.0020052), and results are given in the δ notation. The internal precision of a single analysis was generally 0.2–0.3‰ (2SE).

3.4. LF-IRMS Oxygen Isotopic Analysis

RW-1 oxygen isotopic compositions were also determined by LF-IRMS at the Stable Isotope Laboratory of the University of Lausanne, Switzerland using procedures similar to those described by Didier et al. [15]. Transparent and inclusion-free monazite fragments were selected and crushed into powders and 1–2 mg of powder was loaded into pits in a Pt sample holder. After a two-step prefluorination of 10–12 h and 10 min, O₂ was extracted by a CO₂ laser in an F₂-atmosphere (100 mbar). The gas mixture was purified and analyzed in a dual inlet gas-source mass spectrometer (Finnigan MAT 253). The NBS-28 quartz standard was alternately analyzed with RW-1 samples, as an external reference material. Seven repeated measurements on NBS-28 quartz yielded a δ¹⁸O value of 9.55‰ ± 0.11‰ (2SD) that is identical, within errors, to the recommended value of 9.64‰ ± 0.12‰ (2SD) [30].

3.5. Sm–Nd Isotopic Composition

RW-1 Sm–Nd isotopic compositions were determined using a Neptune plus MC-ICPMS coupled to a 193 nm ArF excimer laser ablation system following procedures similar to those described by Liu et al. [12] and Yang et al. [31]. Individual fragments were analyzed in static mode with ablation pits 16 μm in diameter, a pulse repetition rate of 2 Hz, and a laser beam flux of 5 J/cm² with a He carrier gas. N₂ was mixed with the Ar to enhance the signal intensity and to reduce the production of NdO⁺. Each spot analysis was carried out in one block over 200 cycles, with an integration time of 0.262 s per cycle and a total time per spot of ~60 s. Because the Sm content of monazite is relatively high (~3 wt.%), a ¹⁴⁷Sm/¹⁴⁹Sm ratio of 1.08680 [32] and ¹⁴⁴Sm/¹⁴⁹Sm ratio of 0.22332 [33] was applied in correcting the isobaric interference of ¹⁴⁴Sm on ¹⁴⁴Nd. Measured ¹⁴³Nd/¹⁴⁴Nd ratios were normalized to ¹⁴⁶Nd/¹⁴⁴Nd = 0.7219 using the exponential law. A total of 214 analyses of Sm–Nd isotopes were carried out in two sessions. The Namaqualand monazite standard yielded a ¹⁴³Nd/¹⁴⁴Nd ratio of 0.511874 ± 0.000040 (2SD; n = 34), consistent, within errors, with the recommended value of 0.511894 ± 0.000025 (2SD) [12]. The Iveland monazite standard yielded a ¹⁴³Nd/¹⁴⁴Nd ratio of 0.512927 ± 0.000030 (2SD; n = 34), also consistent, within errors, with the recommended value of 0.512963 ± 0.000093 (2SD) [12].

3.6. TIMS Nd Isotopic Analysis

Monazite fragments (1–2 mg) were handpicked under a binocular microscope for Sm–Nd TIMS isotopic analysis. The fragments were cleaned ultrasonically three times for 10 min with Milli-Q > 18.2 MΩ water, dried, and split into several aliquots, which were digested in a PFA Teflon screw-top vial in 6 M HCl at 160 °C for 24 h. Chemical separation involved conventional two-stage ion exchange purification techniques [34] with REEs being separated from matrix elements by cation exchange (Bio-Rad AG50W-X12; 200–400 mesh resin) and Nd being purified with Ln Spec (Eichrom Technologies, Lisle, IL, USA) resin. Nd isotopic ratios were determined using a Triton Plus TIMS instrument (Thermo Fisher, Waltham, MA, USA) in static mode with Faraday cups [34]. The Nd procedural blank was <50 pg, requiring no correction of isotopic ratios. Measured ¹⁴³Nd/¹⁴⁴Nd ratios were normalized to ¹⁴⁶Nd/¹⁴⁴Nd = 0.7219 using the exponential law. The standard solution JNdi-1 [35] was used to evaluate instrumental bias during data collection. The JNdi-1 standard yielded a ¹⁴³Nd/¹⁴⁴Nd ratio of 0.512107 ± 0.000006 (n = 3; 2SD). Seven measurements of RW-1 Nd isotopic compositions were carried out by TIMS. Raw data were corrected to the JNdi-1 standard according to the reference ¹⁴³Nd/¹⁴⁴Nd value of 0.512115 ± 0.000007 (2SD) [35].

4. Results and Discussion

4.1. RW-1 Mineralogical Features and Elemental Compositions

RW-1 fragments are transparent, but occasionally opaque with some fractures and inclusions under transmitted and reflected light (Appendix A, Figure A1). It has four main Raman bands at approximately 470, 620, 970, and 1050 cm⁻¹ (Figure 1), typical of internal modes of the noninteracting PO₄ units within the monazite system [36,37]. The RW-1 Raman shift was consistent with those of UNIL-Mnz1, USGS-44069, and Manangoutry monazite standards (Figure 1). The Itambe monazite has an ambiguous peak at ~1050 cm⁻¹, while low-Th monazites such as UNIL-Mnz1 and Diamantina tend to have an additional peak at ~990 cm⁻¹ and a more conspicuous Raman band at ~470 cm⁻¹ relative to the high-Th monazites, such as Itambe and Manangoutry. RW-1 displayed no visible zoning and no conspicuous inclusions in CL images (Appendix A, Figure A1). The high-resolution X-ray map indicates that RW-1 is homogeneous without discernible Si, Th, Ce, or Y compositional zonation (Appendix A, Figure A1).

The inter-shard variation of major oxide compositions was investigated by EPMA in three sessions (1, 2, and 3, Table S1), involving individual analyses of 23, 21, and 36 fragments, respectively (Sessions 1 and 3 did not include UO₂ and PbO, respectively). Overall, all EPMA analyses of RW-1 fragments indicated high ThO₂ (9.48 ± 0.50 wt.%; 2SD) and low UO₂ (0.23 ± 0.05 wt.%; 2SD) contents, and high Th/U ratios (41.4 ± 9.2; 2SD). The most abundant REE is Ce₂O₃ (25.05 ± 0.60 wt.%; 2SD), followed by Nd₂O₃ (14.26 ± 0.28 wt.%; 2SD) and La₂O₃ (7.85 ± 0.28 wt.%; 2SD) (

Table 2. Elemental compositions of the RW-1 monazite.

Methods	EPMA, wt.% *
	SiO2	CaO	P2O5	Y2O3	ThO2	UO2	PbO	La2O3	Ce2O3	Pr2O3	Nd2O3
Average	1.81	0.66	28.03	2.39	9.48	0.23	0.39	7.85	25.05	3.68	14.26
2SD	0.20	0.06	0.76	0.17	0.50	0.05	0.03	0.28	0.60	0.51	0.28
2RSD (%)	10.77	8.51	2.73	7.21	5.32	22.39	8.62	3.53	2.41	13.92	1.98
Methods	EPMA, wt.%				LA, ppm, N = 23
	Sm2O3	Gd2O3	Dy2O3	Total	Eu	Tb	Ho	Er	Tm	Yb	Lu
Average	3.60	1.65	0.28	99.12	33	972	254	407	49	306	27
2SD	0.35	0.30	0.05	1.38	2	58	18	34	5	42	5
2RSD (%)	9.85	18.09	19.14	1.39	5	6	7	8	10	14	18

* Every oxide underwent 80 analyses except UO₂ (57 analyses) and PbO (44 analyses).

). Y₂O₃ contents are in the range of 2.16–2.55 wt.%. Overall, inter-shard variation of major elements was limited to 2–5% 2RSD for high-concentration (>4 wt.%) elements, such as Ce, P, Nd, Th, and La, and 7–22% 2RSD for low-concentration (<4 wt.%) elements, including Pr, Sm, Y, Gd, Si, Ca, Pb, Dy, and U. In the ThSiO₄–YREEPO₄–CaTh(PO₄)₂ ternary diagram (Figure 2), the RW-1 monazite displays a reasonably homogeneous composition, plotted near high-Th UNIL-Mnz2 monazite. The structural formulae based on four oxygens [8] have an average end-member composition of 6.3% huttonite (ThSiO₄), 5.7% cheralite (CaTh(PO₄)₂), and 88.0% monazite (YREEPO₄) (Table S1).

Variations in MREE and HREE contents of RW-1, including Eu, Tb, Ho, Er, Tm, Yb, and Lu, were also investigated by LA-Q-ICPMS. Twenty-three analyses revealed average Eu and Lu contents of 33 ± 2 ppm (2SD) and 27 ± 5 ppm (2SD), respectively (Table 2). The relative standard deviations of trace element contents were generally <20% (2RSD). RW-1 chondrite-normalized REE patterns exhibited light REE (LREE) enrichment with [La/Dy]_N ~28 (Figure 3; subscript N denotes chondrite normalized value [38]). The small fractionation in MREEs and HREEs ([Gd/Lu]_N ~66), large negative [Eu/Eu*]_N anomaly (~0.005, [Eu/Eu*]_N = [Eu]_N/([Sm]_N × [Gd]_N)^0.5), and low [Th/U]_N ratio (~10) are considered typical characteristics of monazite in garnet-absent metamorphic or igneous rocks [39]. Compared with monazite of different origins (see integrated data in Table S2), RW-1 appears compositionally similar to the Moacyr and UNIL-Mnz2 monazite in Th-Th/Ce and Si/Ce-Ca/Ce-Y/LREE diagrams (Figure 4). The monazite from hydrothermal rocks or carbonatite is characteristically low in Th content (<1 wt.%) and Th/Ce ratio (<0.04) (Figure 4a), such as the UNIL-Mnz1 and Diamantina monazite (Table 1). The igneous monazite has large ranges, focused in the center of the Si/Ce-Ca/Ce-Y/LREE diagram (Figure 4b), like the RW-1, Itambe, Moacyr, and UNIL-Mnz2 monazite, while the metamorphic monazite is scattered to the low “Si/Ce” and low “Y/LREE” fields, like the USGS-44069 and Manangoutry monazite (Table 1).

4.2. Oxygen Isotopic Compositions

SIMS oxygen isotopic analyses were investigated for 303 RW-1 fragments in 3 sessions to assess the homogeneity of RW-1 oxygen isotopic compositions at a scale of ~20 µm, with Sessions 1, 2, and 3 involving 125, 74, and 104 fragments, respectively (Appendix A, Figure A2). δ¹⁸O values (with the average measured δ¹⁸O value normalized to the LF-IRMS value, below) from each session yielded Gaussian distributions (Figure 5) with 2SDs of 0.39‰, 0.40‰, and 0.37‰ for Sessions 1, 2, and 3, respectively (Appendix A, Figure A2). Overall, the 303 measured δ¹⁸O values formed a Gaussian distribution (Figure 5) with a 2SD of 0.38‰, indicating homogeneity in oxygen isotopic composition.

A further 70 and 50 RW-1 fragments were analyzed in Sessions 4 and 5, respectively, together with the USGS-44069, Itambe, and Manangoutry monazite to monitor the external precision and to correct for IMF. The 2SD values of RW-1, USGS-44069, Itambe, and Manangoutry were 0.39–0.46‰, 0.40–0.70‰, 0.40–0.59‰, and 0.40–0.52‰, respectively. The goodness of fit for IMF-Th was nearly the same as that of IMF-XREEY using the York linear regression equation [40] (Appendix A, Figure A3). After IMF-correction using IMF-Th curve fitting, RW-1 δ¹⁸O values exhibited a Gaussian distribution with average (± 2SD) values of 5.97‰ ± 0.39‰ and 6.05‰ ± 0.46‰ for Sessions 4 and 5, respectively (Figure 6). Overall, the 120 measured δ¹⁸O values formed a Gaussian distribution with a mean of 6.00‰ ± 0.43‰ (2SD; Figure 6). After IMF-correction using IMF-XREEY curve fitting, the 120 measured δ¹⁸O values yielded a mean of 5.86‰ ± 0.43‰ (2SD; Table S3) that is also identical, within errors, to the aforementioned result of IMF-Th curve fitting.

RW-1 oxygen isotopic compositions were also determined in four analyses by LF-IRMS, with an oxygen yield of 90–97% (

Table 3. Results of LF-IRMS determinations of RW-1 δ¹⁸O values.

Monazite	Date	Weight (mg)	δ18OVSMOW (‰)	Yield (%)
RW-1	2017/11/8	2.04	6.34	90
	2017/11/8	2.03	6.20	92
	2018/1/25	1.75	6.37	97
	2018/1/25	1.87	6.28	93
Average			6.30
2SD			0.16

). The measured δ¹⁸O values ranged from 6.20‰ to 6.37‰, with a mean value of 6.30‰ ± 0.16‰ (2SD), which is consistent, within errors, with the above SIMS results. Therefore, we recommend a δ¹⁸O value of 6.30‰ ± 0.16‰ (2SD) as the best estimate for the RW-1 monazite.

4.3. Nd Isotopic Compositions

A total of 214 Sm–Nd isotopic analyses of monazite RW-1 were conducted by LA-MC-ICPMS in two sessions (119 in Session 1 and 95 in Session 2) to determine the homogeneity of Nd isotopic compositions (Table S4). The measured ¹⁴³Nd/¹⁴⁴Nd ratio in Session 1 ranged from 0.512249 ± 0.000024 (2SE) to 0.512375 ± 0.000032 (2SE), forming a Gaussian distribution with a mean of 0.512309 ± 0.000048 (2SD; Figure 7a). The measured ¹⁴³Nd/¹⁴⁴Nd ratio in Session 2 ranged from 0.512223 ± 0.000022 (2SE) to 0.512367 ± 0.000034 (2SE), forming a Gaussian distribution with a mean of 0.512288 ± 0.000064 (2SD; Figure 7b). The 214 values form a Gaussian distribution, suggesting that the RW-1 monazite is homogeneous in terms of the Nd isotopic composition, with an overall mean of 0.512300 ± 0.000060 (2SD; Figure 7c). Furthermore, the 214 measurements yielded a mean ¹⁴⁷Sm/¹⁴⁴Nd ratio of 0.1627 ± 0.0098 (2SD) and an initial εNd value of −2.7 ± 0.6 (2SD; Table S4), calculated using its TIMS age of 904.15 ± 0.26 Ma (2SD) [17].

Seven TIMS Nd isotopic analyses yielded ¹⁴³Nd/¹⁴⁴Nd ratios of 0.512274 ± 0.000008 to 0.512290 ± 0.000008 (Table S4), with a mean of 0.512282 ± 0.000011 (2SD; Figure 7d), identical, within errors, to that yielded by LA-MC-ICPMS analysis. Therefore, we recommend a ¹⁴³Nd/¹⁴⁴Nd ratio of 0.512282 ± 0.000011 (2SD) as the best estimate for RW-1.

5. Conclusions

Numerous EPMA, SIMS, and LA-MC-ICPMS analyses confirm that the RW-1 monazite is homogeneous in both elemental and O–Nd isotopic compositions at a scale of ~20 µm. Mean O and Nd isotopic compositions determined, respectively, by SIMS and LA-MC-ICPMS are identical within errors to those determined, respectively, by LF-IRMS and TIMS: δ¹⁸O = 6.30‰ ± 0.16‰ (2SD) and ¹⁴³Nd/¹⁴⁴Nd = 0.512282 ± 0.000011 (2SD). Therefore, we consider that RW-1 can be used as a new working reference material for in situ compositional and O–Nd isotopic determinations with samples of relatively high Th content, or as an external standard in assessing analytical accuracy. Along with the Moacyr and Manangoutry monazite, RW-1 is one of three monazite reference materials that have been well characterized, with homogeneous elemental and O–Nd isotopic compositions and known U–Th–Pb ages. The limitation of RW-1 is that it contains some visible fractures and inclusions, therefore, microbeam analysis of RW-1 should be guided by CL or BSE images to avoid these minor imperfections. It is available on request from the corresponding author of this paper.