Geostandards and Geoanalytical Research最新文献

Total sulfur (TS), chromium-reducible sulfur (CRS) and HCl-insoluble sulfur (HCl-insoluble S) contents (mass fractions), together with their δ³⁴S_TS, δ³⁴S_CRS and δ³⁴S_{HCl-insoluble S} values were measured in marine, stream and lacustrine geological reference materials (RMs). These were JMs-1, JMs-2, SBC-1, SCo-2, JSd-3, JSd-4 and JLk-1, which include modern surface and ancient sediments. Using these data, the mechanisms for driving sulfur isotopic distinctions between the δ³⁴S_TS, δ³⁴S_CRS and δ³⁴S_{HCl-insoluble S} values were systematically investigated. Detectable amounts of acid volatile sulfur were not found in all analysed RMs. On the other hand, sulfate in the analysed RMs was found to have the heaviest δ³⁴S values compared with other sulfur species, such as elemental sulfur, pyrite and organic sulfur, which is consistent with expectation, based on sulfur oxidation numbers. The most notable finding is that differences between δ³⁴S_CRS and δ³⁴S_{HCl-insouble S} values can be reasonably explained by the fact that the sulfur isotope ratio of organic sulfur has a wide range compared with that of other sulfur species. Consequently, this data set could contribute to a better understanding of elemental and isotopic S mass fractions of widely used geological RMs by different acid pre-treatments.

We introduce three new synthetic basalt reference materials and a new high-precision set-up for stable carbon isotope measurement in basaltic glasses using a large-geometry secondary ion mass spectrometry (SIMS) instrument. The new reference materials, characterised for carbon mass fraction and isotope composition, show homogeneity for in situ analysis for the reported set-up. Their bulk hydrogen mass fraction and isotope ratios are reported. Our SIMS protocol uses multi-collection, cycling between concurrent measurements of ¹²C and ¹³C on electron multipliers, and either ³⁰Si or ¹⁸O, as a reference mass, on a 10¹¹ Ω resistor Faraday cup. This set-up achieves high measurement repeatability for δ¹³C down to ± 0.35‰ 1RSE at 1706 ⁺⁸⁹/_-88 μg g^-1 CO₂, with ± 1.00‰ 1RSE or better between 163 ^+5.1/_-5.2 and 267 ^+8.9/_-8.9 μg g^-1 CO₂, using a 10 nA primary beam current and a 40 μm analytical pit over a 100 cycle analysis. Carbon blanks were characterised by measuring carbon-free olivines, allowing for blank corrections on δ¹³C measurements. After blank and instrument mass fractionation corrections, we measure δ¹³C in glasses down to 26.16 ^+0.85/_-0.86 μg g^-1 CO₂ with a final measurement standard sample deviation of ± 2.97‰ 1s. We report in situ measurements on an ocean floor basaltic glass and a set of synthetic basaltic glasses to demonstrate our approach. Reference materials and the SIMS set-up improve the accuracy and precision of δ¹³C measurements in natural basaltic glasses across a wide range of geologically relevant carbon contents.

Silver is a powerful geochemical tracer in various geological processes (e.g., hydrothermal deposit formation and evolution of the terrestrial and Martian mantles). The measurement of the ultra-trace Ag (< 0.1 μg g^-1) in geological samples by ICP-MS is often hindered due to serious polyatomic interferences arising from Zr, Nb and Y elements presented in samples. In this study, a simple and rapid ammonia coprecipitation method was developed to remove interference elements (Zr, Nb and Y) in NH₄HF₂ digests for the determination of Ag mass fractions in geological materials by isotope dilution ICP-MS (ID-ICP-MS). The removal of interference elements (Zr, Nb and Y) in digestion solutions of geological materials was achieved using ammonia coprecipitation, effectively eliminating their polyatomic interferences. To compensate for the incomplete recovery of Ag during ammonia coprecipitation, isotope dilution calibration was chosen over internal standard calibration to achieve final accurate results. The limit of quantitation for Ag using the proposed method was 1.83 ng g^-1. The results obtained for Ag mass fractionation in nine international geological reference materials were in good agreement with the published values obtained by ID-ICP-MS and ICP-MS/MS. The proposed simple, rapid and practical analytical method for the determination of Ag mass fractions improves our understanding of the behaviour of silver in cosmochemistry and geochemistry studies.

Potassium isotope ratios (δ⁴¹K) were determined for eighteen Chinese geological reference materials (RMs, GSR series) encompassing igneous, metamorphic and sedimentary rocks. K₂O mass fractions (w) ranged between 0.15 and 7.5% in these RMs. Measurements were conducted using a collision cell MC-ICP-MS with sample-standard bracketing technique. The intermediate precision of K isotopic determination, based on measurement results collected over 2.5 years, was 0.04‰ (2s, n = 69) for in-house GSB K solution and 0.05‰ (2s, n = 63) for basaltic RM BCR-2. The accuracy of our data was assessed by comparison with previously published data and/or utilising two or three different measurement procedures. The high-precision δ⁴¹K values for these eighteen RMs span a wide range from -1.15‰ (GBW07122, GSR 15, amphibolite) to 0.28‰ (GBW07120, GSR 13, limestone), and data for six RM are presented for the first time, making them valuable references for quality assurance and inter-laboratory comparisons.

During the burial of mudstones, the associated organic matter undergoes gradual thermal maturation, a key process that can influence the reactivity of organic matter during catagenesis, the formation of hydrocarbon deposits and the chemical weathering of mudstones. Conventional methods for assessing the thermal maturity of organic matter often fail to reflect the geochemical heterogeneity between individual organic phases in mudstone samples. Here, we report an alternative, non-destructive, surficial and micro-scale (analytical spot size of ~ 300 nm with about 4 μm diffusion depth for micrometre-size organic grains) method to evaluate the thermal maturity of organic matter in mudstones using the carbon Kα X-ray spectrum measured by field emission-electron probe microanalyser (FE-EPMA). Using this method, we observed correlations between parameter values derived from FE-EPMA spectra, including the peak position, the peak area and the intra-sample heterogeneity of these measurements, and independently measured vitrinite/solid bitumen reflectance for a suite of mudstones, representing different age, geological context and burial depth. With the increased values in peak area and position, we identified an increase in the carbon mass fraction of organic matter and the mean nominal oxidation state of carbon approaching zero. These trends, which are consistent with aromatisation and graphitisation, provide the rationale for using FE-EPMA to estimate the thermal maturity of organic matter. To explore some of these trends in more detail, we employed time-of-flight secondary ionisation mass spectrometry, X-ray photoelectron spectroscopy and optical reflectance measurements on a subset of samples.

Instrumental mass fractionation (IMF) accompanying oxygen isotope measurement by SIMS was studied for seventeen experimental alkali-rich (0–10 mole % of both K₂O and Na₂O) glasses along with a set of reference material glasses and minerals (quartz and olivine). Analyses were undertaken in two measurement sessions using a CAMECA IMS 1270-E7 ion microprobe operated under identical instrumental conditions. All employed experimental glasses and reference materials were re-analysed by laser fluorination gas source-mass spectrometry (LF GS-MS) to estimate their IMF values. The obtained IMF values, which ranged from -2.31 to +5.14‰, were combined with the IMF data for alkaline free glasses to estimate the compositional matrix effect for all major rock-forming oxides. Using this joint data set (including forty-four experimental glasses) a number of linear multiple variable regression models describing the link between IMF and major element chemical composition were established. To generalise the established models for the matrix effect (ME) correction we also evaluated an approach of routine measurement of chemically pure quartz with a known ¹⁸O/¹⁶O ratio along with the other glass reference materials and use it as the “zero” point for multi-component regression fit. Our choice was justified by the fact that the ME in quartz has to be strictly constant. The last represents an advantage of using quartz instead of olivine or any other reference materials for normalisation. Finally, we tested our best multi-component regression model (standard error as low as 0.4‰) established for silicate glasses, being presumably crystallographic-amorphous substances, in application to different structural groups of minerals such as olivine, clinopyroxene, garnet, feldspars and quartz. The difference between the measured ME values and the model-predicted varied systematically, and the magnitude of the observed deviations increased proportionally with increasing magnitude of the measured ME, pointing towards the role of crystallographic structure in producing a compositionally related matrix effect.

We report for the first time δ⁵³Cr_{SRM 979} values in sixteen geological reference materials (RMs) with varied lithological characteristics. The measured values range from -0.24‰ ± 0.05‰ (2s) to -0.08‰ ± 0.02‰ (2s) for igneous rocks, from -0.18 ± 0.04‰ (2s) to 0.08 ± 0.03‰ (2s) for sedimentary rocks and between -0.21‰ ± 0.04‰ (2s) and -0.10‰ ± 0.04‰ (2s) for metamorphic rocks, indicating significant isotopic fractionation across these RMs. Measurements were performed using thermal ionisation mass spectrometry with a ⁵⁰Cr-⁵⁴Cr double-spike technique following a two-column matrix separation. To validate our measurement procedure, we applied it to eighteen additional well-characterised RMs. Our results are in agreement with previously reported values, with precision typically below 0.05‰ (2s). This comprehensive data set provides a valuable reference for quality assurance schemes and inter-laboratory comparisons in Cr isotopic studies.

The stoichiometric calibration method for dual-energy computed tomography (DECT) can be used in geosciences to characterise materials based on their effective atomic number (Z_eff) and their electron density (ρ_e) without previous knowledge of the incident X-ray beam. A stoichiometrically calibrated DECT method was applied here to measure these two properties on three different sedimentary rocks using three different X-ray CT instruments to determine which one is best to reveal the chemical composition or the mineralogical variations at the meso-scale. The three tested instruments: (1) a medical CT, (2) a custom-built micro-CT, and (3) a commercial micro-CT. Several acquisition settings were tested to identify the most suitable parameters for the characterisation the samples. Some parameters such as incident energies, resolution and calibration materials proved to have a significant impact on the accuracy of the characterisation. The determination of a general measurement protocol for geological samples was found to be difficult because of several complicating factors, including the nature of the sample, objectives of the study, and instrumental limitations that influence DECT characterisation. Nonetheless, comparison of the results obtained by the three scanners brings out the key parameters to be considered to perform a useful rock sample characterisation with DECT.

With growing interest in the Earth's deep nitrogen cycle, electron probe microanalysis is increasingly used to quantify nitrogen in minerals and glasses. However, measuring nitrogen by electron microprobe comes with challenges, including beam sensitivity and differences in peak and background shapes between sample and reference materials. This study provides a robust analytical protocol to addresses these issues. We gathered mineral and glass reference materials with known nitrogen mass fractions from previous work, and synthesised additional glasses with nitrogen quantified independently by gas-source mass spectrometry. Our method uses nitrides as calibration materials because of their beam stability and high nitrogen mass fraction. We assess peak shapes to implement an area-peak factor correction, and account for curved backgrounds using a four-point background correction. Importantly, we describe how to estimate uncertainties on both background and area-peak factor corrections, which is required to assess detection limits but has been largely overlooked in previous studies. Our method yields nitrogen mass fractions within uncertainty for five out of seven reference materials, with two exceptions highlighting the need for further refinement. We provide Python code for applying the corrections and calculating uncertainties, allowing our method to be implemented by any researcher with access to an electron microprobe laboratory.

Elemental mercury (Hg) is routinely determined in crystalline rocks with mass fractions lower than 10 ng g^-1 by thermal decomposition using Direct Mercury Analyzer (DMA-80) or Lumex RA-915+ (equipped with a PYRO-915+ attachment) instruments, both based on atomic absorption spectroscopy. However, 223 analyses over the course of one year with DMA-80 and cold vapour-atomic fluorescence spectroscopy (CV-AFS) on three reference materials (RMs) and six crystalline rocks (granite, diorite, gabbro, spinel peridotite, phlogopite-rich peridotite, and sulfide-rich orthogneiss) from the exposed transcrustal section of the Ivrea-Verbano Zone and upper crustal Serie dei Laghi unit (western Alps, Italy) reveal that rock analyses using the DMA-80 are variably affected by different internal and external biases when Hg mass fractions are below 10 ng g^-1. Conversely, CV-AFS analyses are more precise, providing homogenous and repeatable results, even at ultra-low Hg mass fractions (< 1 pg g^-1). Furthermore, CV-AFS analyses show that gabbro and spinel peridotite powders roasted for analysis by DMA-80 still contain ~ 0.6 to ~ 1.4 ng g^-1 of Hg, implying inefficient release of Hg from basic/ultrabasic lithologies. Therefore, we recommend the use of CV-AFS for Hg measurements in crystalline rocks. We also propose a new Hg reference value of 3.9 ± 1.5 ng g^-1 for the GSJ granodiorite reference material JG-1a.