Geostandards and Geoanalytical Research最新文献

Stable Mg delta values (the relative deviation from a certain reference material, e.g., DSM-3 here and expressed as δ²⁶Mg_DSM-3) were routinely measured by the sample-standard bracketing (SSB) method on a multi-collector ICP-MS, as only three isotopes (i.e., ²⁴Mg, ²⁵Mg and ²⁶Mg) naturally exist. Due to potential inaccuracy in correcting mass bias during measurements, considerable measurement bias has been reported among laboratories. Recently, a critical mixture double spike (CMDS) technique has been developed and demonstrated to accurately correct mass bias of measurement results with precision of ± 0.03‰ for δ²⁶Mg. Here, we measured thirty-one geological reference materials including igneous, metamorphic and sedimentary rocks, sediments and minerals using the CMDS technique, with the purpose of better characterising their δ²⁶Mg_DSM-3 values. Aligning with the data previously reported, uncorrected bias, on average measured by ∆²⁶Mg_SSB-CMDS (i.e., δ²⁶Mg_SSB - δ²⁶Mg_CMDS) as -0.071 ± 0.092‰ (2s, n = 42), has been reaffirmed for the traditional SSB method. Such uncorrected bias positively correlates with sample Mg/(Si+Al+Ca), and thus may result from the accumulative effect of residual matrix elements. The new data set herein can aid future inter-laboratory comparison and data quality control.

Tourmaline serves as a vital recorder of geological processes. However, suitable tourmaline reference materials (RMs) for elemental and stable isotopic composition in situ measurements are still limited. In this study, three tourmaline megacrysts (MD-B66, IM-B232 and BR-DG68) were characterised as potential RMs for B isotope measurements by LA-MC-ICP-MS and mass fraction determinations by LA-ICP-MS. Over 251 measurements with LA-MC-ICP-MS on ten randomly selected fragments from MD-B66 consistently yielded B isotope ratios of -7.74 ± 0.25‰ (2s), establishing MD-B66 as a suitably homogeneous primary reference material for high-precision, in situ microbeam B isotope measurements. Notably, the (long-term) intermediate precision of 0.25‰ (2s) for in situ B isotope measurements obtained using this reference material is comparable to that reported from solution MC-ICP-MS methods in the literature. Two other tourmaline megacrysts, with intermediate precision ranging from ± 0.48‰ to ± 0.61‰ (2s) for δ¹¹B measurement, can be employed as secondary RMs for quality control. The mean δ¹¹B values determined by solution MC-ICP-MS for MD-B66 (-7.71 ± 0.32‰, n = 12), IM-B232 (-13.17 ± 0.62‰, n = 8) and BR-DG68 (-13.85 ± 0.32‰, n = 12), with expanded uncertainties at the 95% confidence level, are consistent with those determined by LA-MC-ICP-MS. Among these three new RMs, BR-DG68 displays relatively homogeneous major and trace element mass fractions. Characterisation using both in situ and wet chemical techniques demonstrated the suitability of BR-DG68 as the first tourmaline reference material for elemental measurement by LA-ICP-MS, which would permit matrix-matched and therefore more accurate elemental measurement in tourmalines. Unlike electron probe microanalysis with B and Li contents calculated based on stoichiometric assumptions, direct and accurate measurements of the two low atomic number elements, along with other major and trace elements can be achieved by LA-ICP-MS with the aid of the newly developed tourmaline reference material BR-DG68. Overall, current and future studies in geochemistry may benefit from these newly proposed tourmaline RMs, which should lead to significantly improved precision and accuracy for in situ B isotope and elemental measurement.

A common problem of trace element measurements by LA-ICP-MS is the existence of interferences that cannot be resolved instrumentally. The software G.O.Joe is developed to calculate trace element mass fractions in solid samples analysed by LA-ICP-MS, offering the opportunity to correct for isobaric interference (including mass bias) and abundance sensitivity. Designed as a platform-independent web application, G.O.Joe is written in the Dart programming language and runs on all web browsers supported by Flutter (i.e., Chrome, Safari, Edge, Firefox) without the need for installation. G.O.Joe is freely accessible from any computer with an internet connection to facilitate immediate data evaluation and the efficient processing of large datasets (> 400 analyses). G.O.Joe features an intuitive user interface that simplifies the selection of peak and background signals, import of instrument settings and reference material compositions to convert the measured raw signals into element mass fractions. Key functions of G.O.Joe are presented by processing the analysis of tungstates and silicates (i.e., scheelite and garnet) including specific correction methods to demonstrate their large effect on interfered masses/isotopes achievable with only little effort. This study introduces G.O.Joe as a simple, time-efficient and flexible software to significantly improve data quality in various trace element studies utilising LA-ICP-MS.

The rubidium (Rb) isotope system has the potential to trace planetary evolution, magmatic-fluid interaction and chemical weathering. These applications are based on Rb isotope measurement results with precisions fit for purpose, but measurements of low-Rb geological materials are challenging due to large sample consumption and overload of ion-exchange resin. Here we developed a measurement procedure for Rb isotope data (δ⁸⁷Rb_SRM984) of low-Rb geological materials using MC-ICP-MS. Using an Aridus III desolvator and Ni standard sampler + Ni X skimmer cone combination, the Rb loading amount was reduced significantly to 20 ng. A comparison between two methods for instrumental mass-bias correction, the sample-standard bracketing and combined sample-standard bracketing and internal (Zr) normalisation (C-SSBIN), shows that C-SSBIN could produce Rb isotope data with better intermediate measurement precisions but strictly restricted to optimal Zr/Rb ratio. The robustness of this method was demonstrated by monitoring δ⁸⁷Rb_SRM984 data of two in-house Rb isotope standards, replicates, some reference materials with δ⁸⁷Rb_SRM984 values previously reported, and element-doped and matrix-spiked synthetic solutions. Based on repeated measurements of Rb isotope standards and reference materials, the long-term (over one year) intermediate precision was better than 0.05‰ (2s, standard deviations). We additionally recommend thirteen reliable reference materials for future Rb isotope ratio measurements.

Baddeleyite is an important U-Pb geochronometer and Hf isotope tracer that commonly occurs as an accessory phase in silica-undersaturated igneous rocks of terrestrial and extra-terrestrial origin. Currently, very few well-characterised, large sized reference materials are available for baddeleyite U-Pb geochronology and Hf isotope measurement. In this study, we document a baddeleyite reference material (SK10-3) of Cenozoic age. SK10-3 is inclusion-free and does not contain secondary alteration minerals. The baddeleyite has uniform U-Pb ages and Hf isotope ratios, within analytical uncertainty, as demonstrated by multiple LA-ICP-MS spot analyses (weighted mean ²⁰⁶Pb/²³⁸U age: 31.59 ± 0.11 Ma, MSWD = 0.7, n = 197) and LA-MC-ICP-MS analyses (arithmetic mean ¹⁷⁶Hf/¹⁷⁷Hf ratio: 0.282741 ± 59, 2s, n = 188). Seven ID-TIMS analyses yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 31.592 ± 0.020/0.022/0.040 Ma (n = 7, 2s, MSWD = 2.2). Nine aliquots of MC-ICP-MS analyses yielded an arithmetic mean ¹⁷⁶Hf/¹⁷⁷Hf ratio of 0.282742 ± 8 (2s). We further demonstrate that the method of shallow-pit (~ 2 μm depth) ablation substantially improves the precision and accuracy of baddeleyite U-Pb ages. SK10-3 has a relatively high ¹⁷⁶Yb/¹⁷⁷Hf ratio (~ 0.007) compared with most other baddeleyites, allowing the precise measurement of β_Yb and may be useful in generating the β_Yb-β_Hf relationship during LA-MC-ICP-MS Hf isotope measurement. SK10-3 may be a useful addition to previously distributed baddeleyite reference materials for microbeam-based U-Pb geochronology and Hf isotope measurements.

Tin isotope ratios of eight different geological reference materials were determined using Neptune and Neoma (Thermo Fisher, USA) multi-collector-inductively coupled plasma-mass spectrometry (MC-ICP-MS) systems using the ¹¹⁷Sn-¹²²Sn double-spike technique. Tin was purified by two-stage ion-exchange chromatography with a recovery of 85% ± 5%, with isobarically interfering matrix elements being reduced to negligible levels. Doping experiments indicated that the Neoma instrument was less affected by isobaric interferences than the Neptune instrument. The determined δ^124/116Sn_SRM3161a values are consistent with published values, and the long-term (> 6 months) intermediate precision was better than ±0.01‰ amu⁻¹ for both instruments. The method allows the Sn isotope ratios of samples containing ~ 2 ng of Sn to be precisely determined with an Sn solution concentration of 1 ng g⁻¹.

Iron isotope ratios in magnetite have been widely used to reveal critical geological and biological processes. Laser ablation multi-collector inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS) is ideally suited for measurement of Fe isotope ratios, however the lack of suitable reference materials poses a significant challenge for in situ Fe isotopic measurements in magnetites. In this study, five high-quality natural magnetite crystals were characterised for Fe isotope ratios using solution nebulisation (SN)-MC-ICP-MS and LA-MC-ICP-MS. The effects of LA-MC-ICP-MS analytical conditions were investigated to obtain precise and accurate Fe isotope ratios. The yielded intermediate measurement precisions for the δ⁵⁶Fe values in the five investigated magnetites were ± 0.05–0.06‰ (2s) using SN-MC-ICP-MS and ± 0.08–0.14‰ (2s) using LA-MC-ICP-MS. Magnetites with homogeneous Fe isotopic compositions in hand-specimen measurements and microanalysis can serve as potential reference materials for in situ Fe isotopic measurement. Furthermore, the Fe isotope ratios in the magnetites from the Jinchuan Ni-Cu-PGE sulfide deposit were measured using LA-MC-ICP-MS with natural magnetite as the bracketing calibrator. The increase in the Fe isotopic composition with magmatic sulfide evolution was primarily dominated by oxygen fugacity (fO₂) and hydrothermal fluids. This finding implies that the Fe isotopic composition of magnetite can serve as a potential geochemical indicator of magmatic Ni-Cu sulfide mineralisation.

Stratigraphic ages from conventional glauconite geochronology are commonly younger than those obtained via high temperature chronometers. The widely used glauconite reference material GL-O, for example, has a K-Ar age (95.03 ± 1.11 Ma) ~ 5 Ma younger than its expected stratigraphic age. To identify the influences on glauconite ages and assess the suitability of GL-O as a reference material for in situ Rb-Sr geochronology, we separated GL-O grains based on colour and morphology. Each fraction was characterised petrographically and compositionally before in situ Rb-Sr dating. Separate aliquots were dated via conventional isotope dilution (ID) Rb-Sr geochronology. We find a ~ 10 Ma spread in the in situ Rb-Sr ages of GL-O fractions, where more rapid maturation and isotopic closure of darker grains yields ages closer to the depositional age, whereas smaller, more porous light green grains show evidence for delayed maturation leading to continued Rb uptake during burial as well as Sr isotope exchange with connate fluids. Discrepancies between ID and in situ Rb-Sr ages are explained by (i) core-rim age zonation, (ii) the presence of alteration resistant, Sr-rich apatite inclusions, (iii) differences in laser-induced fractionation. We recommend additional purification steps before use of GL-O as reference material for in situ Rb-Sr geochronology.