Geostandards and Geoanalytical Research最新文献

Secondary ion mass spectrometry (SIMS) measurements of δ¹⁸O in nine chromite (Mg, Fe)(Al, Cr, Fe)₂O₄ samples (Cr# = 0.58–0.79, Mg# = 0.39–0.72) from chromitites and harzburgite have confirmed their intra- and inter-grain isotopic homogeneity, making them suitable reference materials for in situ analysis. The SIMS measurements of the oxygen isotopic composition of these chromites exhibit a range of instrumental mass fractionation (IMF) as a function of the end-member compositions of the chromite, particularly the magnetite (FeOFe₂O₃) and spinel (MgAl₂O₄) components. Functional relationships have been developed to correct for IMF over the compositional range X_Mag = 0.01 to 0.05, and X_Spl = 0.14 to 0.22. We conclude that, to be accurate, SIMS measurements of δ¹⁸O must be combined with electron probe microanalyses of the individual analysed spots.

In situ iron isotope ratios (δ⁵⁶Fe) in sulfide measured by secondary ion mass spectrometry (SIMS) can provide valuable information on several of Earth's surface processes. SIMS relies on the use of a matrix-matched reference material to correct for instrumental mass fractionation. To date Balmat pyrite has been widely used as a reference material, on the assumption of its homogeneous δ⁵⁶Fe composition. However, several studies have reported divergent bulk δ⁵⁶Fe values, which may jeopardise its use. Here, we combined bulk solution MC-ICP-MS and in situ SIMS δ⁵⁶Fe measurements on two Balmat batches: the Balmat-Original published in Whitehouse and Fedo (2007) and Balmat-UNIL. Despite similar compositions, this study demonstrates the existence of two isotopically distinct Balmat populations. With respect to Balmat-Original (δ⁵⁶Fe = -0.39 ± 0.05‰, 2s), Balmat-UNIL is isotopically 'lighter' with a bulk solution MC-ICP-MS composition of -1.46 ± 0.024‰. Additionally, Balmat-UNIL has two subpopulations: the first is characterised by δ⁵⁶Fe values of -1.46 ± 0.25‰, whereas the second agrees with the original Balmat batch. In each Balmat-UNIL subpopulation, the intra-grain and inter-grain variabilities are sufficient to use Balmat as a reference material for δ⁵⁶Fe isotope measurements by SIMS. This study revealed at least two end-member compositions of Balmat pyrite and calls for a careful batch-specific determination of bulk δ⁵⁶Fe.

The distribution of Mg isotopes in minerals is becoming increasingly relevant in Earth science. Usually, they are determined by dissolving mineral concentrates and, after purifying Mg with ion exchange resins, analysing the resulting solutions by TIMS or, most often, MC-ICP-MS. When applied to individual minerals, these methods are slow and prone to contamination from impurities in the concentrates, inconveniences that may be avoided using spot analysis techniques such as LA-MC-ICP-MS or SIMS, albeit at the price of a large instrumental mass fractionation (IMF) and isobaric interferences, most prominent in the former. Here, we studied the potential of the multi-collector SHRIMP II ion microprobe for measuring Mg isotopes in Fe-Mg silicates and oxides. We found that, when corrected for the divergence of the Mg ion paths within the sample chamber caused by the Earth's magnetic field, the SHRIMP's IMF overwhelmingly depends on the mineral species, and the effects of variable chemical composition are negligible. We propose that the IMF is caused by the force constant difference, ∆F, between "hard" and "soft" bonds linking the ions of the studied element to the mineral lattice. Given that ∆F is a constant for each mineral species, we calculated IMF-correction factors for the most common Mg-bearing minerals. The thus-calculated correction factors permit the analysis in the same session, and with reasonable accuracy (within ~ 0.3‰ of the δ²⁶Mg determined by SN-MC-ICP-MS analyses of concentrates), of samples from different mineral species, facilitating the application of Mg isotopes to terrestrial studies.

Polyphase groundmasses (micro-scale minerals with or without glass) are generated from silicate liquids during the cooling of natural lavas often alongside larger minerals formed long before eruption. Many researchers have posited that compositions gleaned from the analysis of groundmasses closely approximate the compositions of the melts they were derived from, and these have been used frequently to model pre-eruptive magma conditions. However, it is difficult to confidently identify and sample these groundmasses once they are formed. Using a sample of lava that exhibits a wide degree of textural variation (ranging from holocrystalline to hypohyaline) we show that compositions of groundmasses sampled using defocused electron beams are significantly different from glass compositions in terms of mean composition and covariance. Despite this, several groundmass compositions qualify as ‘in equilibrium’ with matrix/rim olivine. When processed using available thermometers, however, modelled equilibrium temperatures are significantly higher than those produced using glass data, on average. Because of this, we prescribe caution in using polyphase groundmass data generated using defocused beam analysis even as a rudimentary approach.

A new matrix-matched reference material (NWU-Brt) with sulfur isotope ratios resembling those of natural barites has been developed for in situ S isotope measurements by laser ablation multi-collector inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS). A 100 g quantity of natural barite crystal was milled to ultra-fine particles and sintered to a solid block using a fast hot-pressing sintering technique (FHPS). We report δ³⁴S ratios determined by isotope ratio mass spectrometry (IRMS), solution nebuliser multi-collector inductively coupled plasma-mass spectrometry (SN-MC-ICP-MS) and LA-MC-ICP-MS, involving up to six participating laboratories. The homogeneity of δ³⁴S ratios of the synthesised barite was tested by LA-MC-ICP-MS with an analytical spot size of 53 μm. The LA-MC-ICP-MS results show that NWU-Brt demonstrates a satisfactory homogeneous composition and is an appropriate material for calibrating δ³⁴S ratios in barite. IRMS and SN-MC-ICP-MS produced mean δ³⁴S values of +14.17 ± 0.42‰ (2s) and +14.27 ± 0.23‰ (2s), respectively. The results of LA-MC-ICP-MS analyses are consistent with the IRMS and SN-MC-ICP-MS ratios within uncertainty. Overall, our results confirm the suitability of NWU-Brt for the calibration of δ³⁴S ratios in barite using LA-MC-ICP-MS.

Apatite fission track (AFT) dating is now routinely performed using LA-ICP-MS, and U-Pb age data are often acquired at the same time, allowing double dating of individual grains. Furthermore, additional geochemical data, such as REE mass fractions are readily acquired during the analysis. It is therefore of practical interest to identify a single material as a quality control on ages and composition. Here we present AFT and U-Pb data for the McClure Mountain Syenite (MMS) apatite, already widely used as an age reference material by the U-Pb community. We performed double dating on 238 grains, in thirteen measurement sessions over two years. The AFT data show remarkable reproducibility with an overall central age of 254.1 ± 5.1 Ma. The U-Pb results agree with the reference age for individual sessions (and for all data together). The results show that that the McClure Mountain apatite is a valid quality control material for double dating, allowing the use of the same material for both methods. We also propose to extend the quality control using MMS to trace element measurements performed in conjunction AFT and U-Pb dating.

Plagioclase, one of the most common rock-forming minerals, can provide useful information on the crystallisation environment, magmatic evolution and thermal history of host rocks. Owing to matrix effects, trace element and Sr isotope in situ measurements in plagioclase have been hindered by a shortage of reference materials for quality control and method validation. A natural plagioclase sample from Madagascar was evaluated for its feasibility as a reference material for Sr isotope and element determination. Measurement results from EPMA, LA-ICP-MS and LA-MC-ICP-MS indicate that MGP-1 plagioclase is homogeneous at the μm–mm scale with respect to major elements (RSD < 4%), most trace elements (RSD < 15%), and ⁸⁷Sr/⁸⁶Sr ratio (2s = 0.00013). Major and trace elements were also determined using XRF and solution ICP-MS, showing good agreement within 10% from in situ measurement results. The ⁸⁷Sr/⁸⁶Sr ratio obtained by TIMS ranged from 0.703441 to 0.703477, with a mean value of 0.703459 (2s = 0.000028), consistent with the LA-MC-ICP-MS mean value. Reference values and uncertainties for major and trace elements and ⁸⁷Sr/⁸⁶Sr ratio are presented in this study. Consequently, the MGP-1 plagioclase can be used as a reference material for in situ measurement of trace elements and ⁸⁷Sr/⁸⁶Sr ratios, contributing to research on the formation and evolution of micro-zoned plagioclase and host rocks.