Intercomparison of Boron Isotope and Concentration Measurements. Part I: Selection, Preparation and Homogeneity Tests of the Intercomparison Materials

In 1999 the Istituto di Geoscienze e Georisorse (IGG), with the support of the International Atomic Energy Agency (IAEA), undertook the collection, preparation and distribution of eight geological materials intended for a blind interlaboratory comparison of measurements of boron isotopic composition and concentration. The materials came from Italian sources and consist of three natural waters (Mediterranean seawater and two groundwaters) and five rocks and minerals (tourmaline, basalt, obsidian, limestone and clay). The solid materials were crushed, milled and mixed, in preparation for distribution. Extensive assays performed at the IGG on these materials demonstrated that their boron isotopic and chemical compositions are homogeneous.
Additional homogeneity tests were carried out on solid material fragments at the GeoForschungsZentrum Potsdam, with the specific objective of investigating the suitability of some of them for the calibration in situ of micro-analytical techniques. Two materials, B4 (tourmaline) and B6 (obsidian), proved to be isotopically homogeneous and may become excellent references for in situ microanalyses of boron isotopes.
The materials described here were used as the basis of a major laboratory intercomparison study and are now available for further distribution from either the IAEA (solid materials) or the IGG (waters).     

Boron Isotopic Analysis of Proposed Borosilicate Mineral Reference Samples

This brief contribution presents new boron isotopic data for a set of proposed borosilicate reference minerals, most of which are described in the companion paper by Dyar et al.²⁰⁰¹. The results for a variety of minerals (tourmalines, danburite, prismatine, serendibite, ferroaxinite and a Li mica) show that it is generally possible to reproduce the ¹¹ B/¹⁰ B ratio within ± 0.5 per mil with replicate chemistry and mass spectrometry over long time spans. Because the accuracy of boron isotopic analysis is commonly determined by reference to secondary standards, it is suggested that some of the samples used in this study be adopted for interlaboratory comparisons and for quality control on boron isotopic analyses produced by a variety of analytical methods.     

Calcium Isotopic Compositions of Sixteen USGS Reference Materials

Calcium isotopic compositions of sixteen Ca‐bearing USGS geological reference materials including igneous and sedimentary rocks are reported. Calcium isotopic compositions were determined in two laboratories (GPMR, State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan; and CIG, Centre for Isotope Geochemistry, University of California, Berkeley) using the ⁴²Ca‐⁴⁸Ca double‐spike technique by thermal ionisation mass spectrometry. As opposed to common cation exchange resin, a micro‐column filled with Ca‐selective resin (DGA resin) was used in order to achieve high recovery (> 96%) and efficient separation of Ca from the sample matrix. The intermediate measurement precision was evaluated at 0.14‰ (2s) for δ^44/40Ca_SRM915a at GPMR, based on replicate measurements of pure Ca reference material NIST SRM 915a, NIST SRM 915b and seawater. Overall, the measurement uncertainties in both laboratories were better than 0.15‰ at the 2s level. Result validation was carried out for all available data sets. The Ca isotopic compositions of USGS reference materials are not only in agreement between GPMR and CIG, but also in agreement with previously published data within quoted uncertainties. The comprehensive data set reported in this study serves as a reference for both quality assurance and interlaboratory comparison of high precision Ca isotopic study.     

Development and Re‐Evaluation of Tourmaline Reference Materials for In Situ Measurement of Boron δ Values by Secondary Ion Mass Spectrometry

Six tourmaline samples were investigated as potential reference materials (RMs) for boron isotope measurement by secondary ion mass spectrometry (SIMS). The tourmaline samples are chemically homogeneous and cover a compositional range of tourmaline supergroup minerals (primarily Fe, Mg and Li end‐members). Additionally, they have homogeneous boron delta values with intermediate precision values during SIMS analyses of less than 0.6‰ (2s). These samples were compared with four established tourmaline RMs, that is, schorl IAEA‐B‐4 and three Harvard tourmalines (schorl HS#112566, dravite HS#108796 and elbaite HS#98144). They were re‐evaluated for their major element and boron delta values using the same measurement procedure as the new tourmaline samples investigated. A discrepancy of about 1.5‰ in δ¹¹B was found between the previously published reference values for established RMs and the values determined in this study. Significant instrumental mass fractionation (IMF) of up to 8‰ in δ¹¹B was observed for schorl–dravite–elbaite solid solutions during SIMS analysis. Using the new reference values determined in this study, the IMF of the ten tourmaline samples can be modelled by a linear combination of the chemical parameters FeO + MnO, SiO₂ and F. The new tourmaline RMs, together with the four established RMs, extend the boron isotope analysis of tourmaline towards the Mg‐ and Al‐rich compositional range. Consequently, the in situ boron isotope ratio of many natural tourmalines can now be determined with an uncertainty of less than 0.8‰ (2s).     

The Role of Isotopic Reference Materials for the Analysis of "Non-Traditional" Stable Isotopes

The advent of multiple collector-inductively coupled plasma-mass spectrometry has provided an impetus to the study of isotope abundance variations in natural materials. In particular, the study of "non-traditional" stable isotopes has revealed isotope fractionation variations caused by a range of physiochemical and biological mechanisms. The magnitude of these variations may be < 1 per mil per mass unit, but are significant in terms of the experimental uncertainties involved, provided rigorous mass spectrometric protocols are followed. The double spike technique can be used effectively to evaluate isotope fractionation effects for both multiple collector-inductively coupled plasma-mass spectrometry and thermal ionisation mass spectrometry. The demanding nature of this research implies the need for internationally-accepted reference materials so that interlaboratory comparisons can be made with confidence. At present, isotopically certified reference materials are unavailable for many elements, including Cu, Zn, Mo and Cd, and it is important that this situation be rectified as soon as practicable. Until such time as isotopically certified reference materials become available for every element, stable isotope geochemists should adopt a common reference material as the standard for each element so that rigorous interlaboratory comparisons can be made.     

Chemical Separation and Isotopic Variations of Cu and Zn From Five Geological Reference Materials

This paper presents an adapted anion exchange column chemistry protocol which allowed separation of high-purity fractions of Cu and Zn from geological materials. Isobaric and non-spectral interferences were virtually eliminated for consequent multiple-collector ICP-MS analysis of the isotopic composition of these metals. The procedure achieved ∼ 100% recoveries, thus ensuring the absence of column-induced isotopic fractionation. By employing these techniques, we report isotopic analyses for Cu and Zn from five geological reference materials: BCR-027 blende ore (BCR), δ⁶⁵Cu = 0.52 ± 0.15‰ (n = 10) and δ⁶⁶Zn = 0.33 ± 0.07‰ (n = 8); BCR-030 calcined calamine ore (BCR), δ⁶⁶Zn = -0.06 ± 0.09‰ (n = 8); BCR-1 basalt (USGS), δ⁶⁶Zn = 0.29 ± 0.12‰ (n = 8); NOD-P-1 manganese nodule (USGS), δ⁶⁵Cu = 0.46 ± 0.08‰ (n = 10) and δ⁶⁶Zn = 0.78 ± 0.09‰ (n = 9); SU-1 Cu-Co ore (CCRMP), δ⁶⁵Cu = -0.018 ± 0.08‰ (n = 10) and δ⁶⁶Zn = 0.13 ± 0.17‰ (n = 6). All uncertainties are ± 2s; copper isotope ratios are reported relative to NIST SRM-976, and zinc isotope ratios relative to the Lyon-group Johnson Matthey metal (batch 3-0749 L) solution, JMC Zn. These values agree well with the limited data previously published, and with results reported for similar natural sample types. Samples were measured using a GVi IsoProbe MC-ICP-MS, based at the Natural History Museum, London. Long-term measurement reproducibility has been assessed by repeat analyses of both single element and complex matrix samples, and was commonly better than ± 0.07‰ for both δ⁶⁶Zn and δ⁶⁵Cu.     

Calcium Isotopic Fractionation and Compositions of Geochemical Reference Materials

High‐precision calcium isotopic compositions of a set of geological reference materials from the IAG (OU‐6), ANRT (UB‐N), MPI‐DING, USGS and GSJ, relative to NIST SRM 915a, are reported here. Measurements were performed by thermal ionisation mass spectrometry (Triton instrument) using a ⁴²Ca–⁴³Ca double spike. δ^44/40Ca values of selected reference materials, mainly felsic rocks, are reported for the first time. Felsic rock values of δ^44/40Ca ranged from 0.13‰ to 1.17‰, probably implying Ca isotopic fractionation could occur during magma evolution. δ^44/40Ca values of ultramafic rocks, ranging from 0.74‰ to 1.51‰, were positively correlated with MgO and negatively with CaO contents, possibly owing to Ca isotopic fractionation during partial melting. δ^44/40Ca of intermediate‐mafic rocks were around 0.78‰ and displayed limited variation, suggesting Ca isotopic fractionation is insignificant during magma evolution processes. As expected, δ^44/40Ca of sedimentary and metamorphic rocks varied widely due to complex geological processes.     

Magnesium Isotopic Compositions of International Geological Reference Materials

Magnesium isotopic compositions are reported for twenty‐four international geological reference materials including igneous, metamorphic and sedimentary rocks, as well as phlogopite and serpentine minerals. The long‐term reproducibility of Mg isotopic determination, based on 4‐year analyses of olivine and seawater samples, was ≤ 0.07‰ (2s) for δ²⁶Mg and ≤ 0.05‰ (2s) for δ²⁵Mg. Accuracy was tested by analysis of synthetic reference materials down to the quoted long‐term reproducibility. This comprehensive dataset, plus seawater data produced in the same laboratory, serves as a reference for quality assurance and inter‐laboratory comparison of high‐precision Mg isotopic data.     

Boron Determination in Twenty One Silicate Rock Reference Materials by Isotope Dilution ICP-MS

The concentration of boron was determined in twenty one geochemical reference materials (silicate rocks) by isotope dilution inductively coupled plasma-mass spectrometry. Boron was extracted from the rocks using HF digestion, suppressing boron volatilisation through boron-mannitol complexation. Sample solutions, in a diluted HCl matrix, were analysed by ICP-MS without any separation of boron from the matrix elements. The results obtained were in agreement with the literature data and indicate that using the described procedure, trace amounts of boron can be very easily determined in complex matrices with rapidity and precision. With the instrumentation and reagents used in this study, this procedure can be used for the determination of 0.5 μg g⁻¹ boron in a 15 0 mg silicate rock sample. Replicate analyses of the twenty one geochemical reference materials (GRM), ranging in boron concentration from 1.35 to 15 7 μg g⁻¹, yielded precisions (relative standard deviation) varying between 0.9 and 9.8%.     

Iron Isotopic Compositions of Geological Reference Materials and Chondrites

High‐precision iron isotopic compositions for Fe‐bearing geological reference materials and chondrites with a wide range of matrices (e.g., silicates, oxides, organic‐bearing materials) are reported. This comprehensive data set should serve as a reference for iron isotopic studies across a range of geological and biological disciplines for both quality assurance and inter‐laboratory calibration. Where comparison is available, the iron isotopic compositions of most geological reference materials measured in this study were in agreement with previously published data within quoted uncertainties. Recommendations for the reporting of future iron isotopic data and associated uncertainties are also presented. Long‐term repeat analyses of all samples indicate that highly reproducible iron isotopic measurements are now obtainable (± 0.03‰ and ± 0.05‰ for δ⁵⁶Fe and δ⁵⁷Fe, respectively).     

New Reference Materials,Analytical Procedures and Data Reduction Strategies for Sr Isotope Measurements in Geological Materials by LA-MC-ICP-MS

Laser ablation multi-collector mass spectrometry (LA-MC-ICP-MS) has emerged as the technique of choice for in situ measurements of Sr isotopes in geological minerals. However, the method poses analytical challenges and there is no widely adopted standardised approach to collecting these data or correcting the numerous potential isobaric inferences. Here, we outline practical analytical procedures and data reduction strategies to help establish a consistent framework for collecting and correcting Sr isotope measurements in geological materials by LA-MC-ICP-MS. We characterise a new set of plagioclase reference materials, which are available for distribution to the community, and present a new data reduction scheme for the Iolite software package to correct isobaric interferences for different materials and analytical conditions. Our tests show that a combination of Kr-baseline subtraction, Rb-peak-stripping using βRb derived from a bracketing glass reference material, and a CaCa or CaAr correction for plagioclase and CaCa or CaAr + REE²⁺ correction for rock glasses, yields the most accurate and precise ⁸⁷Sr/⁸⁶Sr measurements for these materials. Using the analytical and correction procedures outlined herein, spot analyses using a beam diameter of 100 μm or rastering with a 50–65 μm diameter beam can readily achieve < 100 ppm 2SE repeatability ("internal") precision for ⁸⁷Sr/⁸⁶Sr measurements for materials with < 1000 μg g^-1 Sr.     

Intercomparison of Boron Isotope and Concentration Measurements. Part II: Evaluation of Results

The Istituto di Geoscienze e Georisorse (IGG), on behalf and with the support of the International Atomic Energy Agency (IAEA), prepared eight geological materials (three natural waters and five rocks and minerals), intended for a blind interlaboratory comparison of measurements of boron isotopic composition and concentration. The materials were distributed to twenty seven laboratories - virtually all those performing geochemical boron isotope analyses in the world -which agreed to participate in the intercomparison exercise. Only fifteen laboratories, however, ultimately submitted the isotopic and/or concentration results they obtained on the intercomparison materials. The results demonstrate that interlaboratory reproducibility is not well reflected by the precision values reported by the individual laboratories and this observation holds true for both boron concentration and isotopic composition. The reasons for the discrepancies include fractionations due to the chemical matrix of materials, relative shift of the zero position on the δ¹¹B scale and a lack of well characterized materials for calibrating absolute boron content measurements. The intercomparison materials are now available at the IAEA (solid materials) and IGG (waters) for future distribution.     

Biotite Reference Materials for Secondary Ion Mass Spectrometry 18O/16O Measurements

Five new biotite reference materials were calibrated at the SwissSIMS laboratory (University of Lausanne) for oxygen isotope determination by secondary ion mass spectrometry (SIMS) and are available to the scientific community. The oxygen isotope composition of the biotites, UNIL_B1 to B5, was determined by laser‐heating fluorination to be 11.4 ± 0.11‰, 8.6 ± 0.15‰, 6.1 ± 0.04‰, 7.1 ± 0.05‰ and 7.6 ± 0.04‰, respectively. SIMS analyses on spots smaller than 20 μm gave a measurement repeatability of 0.3‰ (2 standard deviation, 2s). The matrix effect due to solid solution in natural biotite could be expressed as a linear function of X_Mg and X_F for biotite. No effect was found for different crystallographic orientations. SIMS analysis allows the oxygen isotope composition of biotite to be measured with a measurement uncertainty of 0.3–0.4‰ (2s) for biotites with similar major element compositions. A measurement uncertainty of 0.5‰ (2s) is realistic when F poor biotites (lower than 0.2% m/m oxides) within the compositional range of X_Mg of 0.3–0.9 were compared from different sessions. The linear correlation with F content offers a reasonable working curve for F‐rich biotites, but additional reference materials are needed to confirm the model.     

Boron and Oxygen Isotope Composition of Certified Reference Materials NIST SRM 610/612 and Reference Materials JB-2 and JR-2

We present data on the concentration, the isotope composition and the homogeneity of boron in NIST silicate glass reference materials SRM 610 and SRM 612, and in powders and glasses of geological reference materials JB-2 (basalt) and JR-2 (rhyolite). Our data are intended to serve as references for both microanalytical and wet-chemical techniques. The δ¹¹ B compositions determined by N-TIMS and P-TIMS agree within 0.5% and compare with SIMS data within 2.5%. SIMS profiles demonstrate boron isotope homogeneity to better than δ¹¹ B = 2% for both NIST glasses, however a slight boron depletion was detected towards the outermost 200 μm of the rim of each sample wafer. The boron isotope compositions of SRM 610 and SRM 612 were indistinguishable. Glasses produced in this study by fusing JB-2 and JR-2 powder also showed good boron isotope homogeneity, both within and between different glass fragments. Their major element abundance as well as boron isotope compositions and concentrations were identical to those of the starting composition. Hence, reference materials (glasses) for the in situ measurement of boron isotopes can be produced from already well-studied volcanic samples without significant isotope fractionation. Oxygen isotope ratios, both within and between wafers, of NIST reference glasses SRM 610 and SRM 612 are uniform. In contrast to boron, significant differences in oxygen isotope compositions were found between the two glasses, which may be due to the different amounts of trace element oxides added at ten-fold different concentration levels to the silicate matrix.     

Thallium Mass Fraction and Stable Isotope Ratios of Sixteen Geological Reference Materials

Thallium stable isotope ratio and mass fraction measurements were performed on sixteen geological reference materials spanning three orders of magnitude in thallium mass fraction, including both whole rock and partially separated mineral powders. For stable isotope ratio measurements, a minimum of three independent digestions of each reference material was obtained. High‐precision trace element measurements (including Tl) were also performed for the majority of these RMs. The range of Tl mass fractions represented is 10 ng g^?1 to 16 μg g^?1, and Tl stable isotope ratios (reported for historical reasons as ε²⁰⁵Tl relative to NIST SRM 997) span the range ?4 to +2. With the exception – attributed to between‐bottle heterogeneity – of G‐2, the majority of data are in good agreement with published or certified values, where available. The precision of mean of independent measurement results between independent dissolutions suggests that, for the majority of materials analysed, a minimum digested mass of 100 mg is recommended to mitigate the impact of small‐scale powder heterogeneity. Of the sixteen materials analysed, we therefore recommend for use as Tl reference materials the USGS materials BCR‐2, COQ‐1, GSP‐2 and STM‐1; CRPG materials AL‐I, AN‐G, FK‐N, ISH‐G, MDO‐G, Mica‐Fe, Mica‐Mg and UB‐N; NIST SRM 607 and OREAS14P.     

Molybdenum Mass Fractions and Isotopic Compositions of International Geological Reference Materials

The double‐spike method with multi‐collector inductively coupled plasma‐mass spectrometry was used to measure the Mo mass fractions and isotopic compositions of a set of geological reference materials including the mineral molybdenite, seawater, coral, as well as igneous and sedimentary rocks. The long‐term reproducibility of the Mo isotopic measurements, based on two‐year analyses of NIST SRM 3134 reference solutions and seawater samples, was ≤ 0.07‰ (two standard deviations, 2s, n = 167) for δ^98/95Mo. Accuracy was evaluated by analyses of Atlantic seawater, which yielded a mean δ^98/95Mo of 2.03 ± 0.06‰ (2s, n = 30, relative to NIST SRM 3134 = 0‰) and mass fraction of 0.0104 ± 0.0006 μg g^?1 (2s, n = 30), which is indistinguishable from seawater samples taken world‐wide and measured in other laboratories. The comprehensive data set presented in this study serves as a reference for quality assurance and interlaboratory comparison of high‐precision Mo mass fractions and isotopic compositions.     

SA01 – A Proposed Zircon Reference Material for Microbeam U‐Pb Age and Hf‐O Isotopic Determination

A new natural zircon reference material SA01 is introduced for U‐Pb geochronology as well as O and Hf isotope geochemistry by microbeam techniques. The zircon megacryst is homogeneous with respect to U‐Pb, O and Hf isotopes based on a large number of measurements by laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) and secondary ion mass spectrometry (SIMS). Chemical abrasion isotope dilution thermal ionisation mass spectrometry (CA‐ID‐TIMS) U‐Pb isotopic analyses produced a mean ²⁰⁶Pb/²³⁸U age of 535.08 ± 0.32 Ma (2s, n = 10). Results of SIMS and LA‐ICP‐MS analyses on individual shards are consistent with the TIMS ages within uncertainty. The δ¹⁸O value determined by laser fluorination is 6.16 ± 0.26‰ (2s, n = 14), and the mean ¹⁷⁶Hf/¹⁷⁷Hf ratio determined by solution MC‐ICP‐MS is 0.282293 ± 0.000007 (2s, n = 30), which are in good agreement with the statistical mean of microbeam analyses. The megacryst is characterised by significant localised variations in Th/U ratio (0.328–4.269) and Li isotopic ratio (?5.5 to +7.9‰); the latter makes it unsuitable as a lithium isotope reference material.     

Mass‐Independent and Mass‐Dependent Ca Isotopic Compositions of Thirteen Geological Reference Materials Measured by Thermal Ionisation Mass Spectrometry

We report mass‐independent and mass‐dependent Ca isotopic compositions for thirteen geological reference materials, including carbonates (NIST SRM 915a and 915b), Atlantic seawater as well as ten rock reference materials ranging from peridotite to sandstone, using traditional ε and δ values relative to NIST SRM 915a, respectively. Isotope ratio determinations were conducted by independent unspiked and ⁴³Ca‐⁴⁸Ca double‐spiked measurements using a customised Triton Plus TIMS. The mean of twelve measurement results gave ε^40/44Ca values within ± 1.1, except for GSP‐2 that had ε^40/44Ca = 4.04 ± 0.15 (2SE). Significant radiogenic ⁴⁰Ca enrichment was evident in some high K/Ca samples. At an uncertainty level of ± 0.6, all reference materials had the same ε^43/44Ca and ε^48/44Ca values. We suggest the use of δ^44/42Ca to report mass‐dependent Ca isotopic compositions. The precision under intermediate measurement conditions for δ^44/42Ca over eight months in our laboratory was ± 0.03‰ (with n ≥ 8 repeat measurements). Measured igneous reference materials gave δ^44/42Ca values ranging from 0.27‰ to 0.54‰. Significant Ca isotope fractionation may occur during magmatic and metasomatism processes. Studied reference materials with higher (Dy_n/Yb_n) tend to have lower δ^44/42Ca, implying a potential role of garnet in producing magmas with low δ^44/42Ca. Sandstone GBW07106 had a δ^44/42Ca value of 0.22‰, lower than all igneous rocks studied so far.     

硅酸盐氧同位素标样研制

本文介绍了硅酸盐氧同位素标准物质(SH和H)的研制。均匀性检验证实该标准物质是均匀的。该标准物质采用直接比较测量法标定,即直接与国际标准V-SMOW和SLAP进行比较。 国际标准水样采用CO_2-H_2O平衡法制备CO_2;石英标样采用BrF_5法制备CO_2。将六个定值实验室提供的数据统一处理后,获得SH和H的δ~(18)Ox/v-s_(MOW)值分别为11.11±0.06(‰)和-1.75+0.08(‰)。该标准物质于89年12月被国家技术监督局批准为国家一级标准物质,山阳石英标样(SH)统一编号为G     

Precise/ Small Sample Size Determinations of Lithium Isotopic Compositions of Geological Reference Materials and Modern Seawater by MC-ICP-MS

The Li isotope ratios of four international rock reference materials, USGS BHVO-2, GSJ JB-2, JG-2, JA-1 and modern seawater (Mediterranean, Pacific and North Atlantic) were determined using multi-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS). These reference materials of natural samples were chosen to span a considerable range in Li isotope ratios and cover several different matrices in order to provide a useful benchmark for future studies. Our new analytical technique achieves significantly higher precision and reproducibility (< ± O.3%o 2s) than previous methods, with the additional advantage of requiring very low sample masses of ca . 2 ng of Li.