Instrumental Mass Fractionation Research Articles

High–precision determination of carbon stable isotope in silicate glasses by secondary ion mass spectrometry: Evaluation of international reference materials

Secondary ion mass spectrometry (SIMS) has been used for isotope analysis of volatile components dissolved in silicate melts for decades. However, carbon in situ stable isotope analysis in natural silicate glasses has remained particularly challenging, with the few published attempts yielding high uncertainties. In this context, we characterized 31 reference silicate glasses of basaltic and basanitic compositions, which we then used as reference materials to calibrate δ13C − value analyses in silicate glasses by SIMS. This set of reference materials covers a wide range of CO2 concentrations (380 ppm – 12,000 ppm) and δ13C values (−28.1 ± 0.2 to −1.1 ± 0.2 ‰, ±1σ). The sets of reference materials were analyzed using large−geometry SIMS at two ion microprobe facilities to test reproducibility across different instrumental setups. The instrumental mass fractionation (IMF) varied widely with two different large−geometry SIMS instruments as well as with different analytical parameters such as field aperture size and primary beam intensity. We found that a precision better than ±1.1 ‰ (both average internal and external precision, ±1σ) for CO2 content higher than 1800 ppm could be achieved using a primary beam intensity of less than 5 nA, resulting in a final spot size of 10–20 μm, allowing precise analysis of δ13C in mineral−hosted melt inclusions. This level of precision was achieved at CO2 concentrations as low as 1800 ppm. This advance opens a wide range of new possibilities for the study of δ13C − value in mafic melts and their mantle sources. The reference materials are now available at the CNRS–CRPG ion microprobe facility in Nancy, France and will be deposited at the Smithsonian National Museum of Natural History, Washington, USA where they will be freely available on loan to any researcher.

Synthesis and Characterization of Metallic (Fe‐Ni, Fe‐Ni‐Si) Reference Materials for SIMS 34S/32S Measurements

Secondary ion mass spectrometry (SIMS) is often used to determine the sulfur contents and isotope ratios of metallic alloys in meteorites or high‐pressure experimental samples. However, SIMS analyses involve calibration and the determination of instrumental mass fractionation in reference materials with a matrix composition similar to that of the unknown samples. To provide metallic reference materials adapted to S measurements via SIMS, we synthesised a series of twenty‐eight alloys comprising four FeNi(±Si) compositions (Fe95Ni5, Fe90Ni10, Fe80Ni20, and Fe80Ni15Si5) with S contents varying from 100 μg g−1 to 4 g/100g using the “melt spinning” method, which guarantees that the metal alloys are rapidly quenched at ~ 106 K s−1. Sulfur contents were determined at the Service d'Analyse des Roches et Minéraux at the CRPG and absolute δ34S values were determined by multi‐collector ICP‐MS (MC‐ICP‐MS, ThermoScientific Neptune) and isotope ratio mass spectrometry (Thermoscientific Delta V). A δ34S value of 16.01 ± 0.31‰ was consistently obtained using the MC‐ICP‐MS, which was indistinguishable of the δ34S value of the FeS starting material (15.95 ± 0.08‰). It suggests that S did not undergo isotopic fractionation during the melting process. Of fifteen samples containing ≤ 5000 μg g−1 S, SIMS measurements with 15‐μm‐diameter spots were repeatable to within 10% relative (1 standard deviation, 1s) for S contents and 2‰ for δ34S values. However, samples containing > 5000 μg g−1 S showed FeNi–FeS immiscibility, leading to minor dispersion of the S mass fractions and δ34S values. No matrix effect was observed for Fe‐Ni, Si, or S contents in terms of the calibration curves and instrumental mass fractionation. We ultimately recommend eight samples as reliable reference materials for S isotopic measurements by SIMS, which we can share worldwide with other laboratories.

Unravelling instrumental mass fractionation of MC-ICP-MS using neodymium isotopes

Since the initial discovery of the non-exponential mass fractionation (non-EMF) of Nd isotopes analysis in 2002, similar deviations from an EMF pattern have been reported for measurements of a number of isotope systems (e.g., Si, Ge, Sr, Sn, Ba, Yb, W, Os, Hg and Pb) with MC-ICP-MS. However, the previous controversial reports on the magnitude of the deviations from EMF suggest that instrumental mass bias behaviour of MC-ICP-MS is neither fully understood nor well-characterised. Consequently, the standard approach of using a mass dependent fractionation (MDF) correction model (e.g., exponential law) may lead to both inaccurate and imprecise results. In this study, we systematically characterise the instrumental mass fractionation of MC-ICP-MS using Nd isotope measurements carried out under different plasma conditions, quantified using the normalised argon index (NAI) as an estimate of plasma temperature. Our results indicate that the mass bias of MC-ICP-MS is not always a simple exponential function of mass but shows systematic deviations from an EMF behaviour, which are closely associated with decreased NAIs. As a result, the conventional exponential correction yields a 143Nd/144Nd value of 0.512257 for the reference material BHVO-2 when the NAI is low, which is 722 ppm lower than the reported value of 0.512979. By tuning the plasma to higher NAIs (higher plasma temperatures), the deviations from the EMF array are systematically attenuated and the exponential correction is able to correct for the instrumental mass bias under high NAIs. In contrast, a regression correction model for Nd isotopes is developed to account for the observed mass fractionation behaviour that does not follow EMF under low NAIs, given that the regression correction relies on the observed loglinear fractionation of different isotope pairs and does not require both isotope ratios to undergo EMF. We expect that the analytical protocol and fundamental insights gained in this study are applicable to a wide range of other isotope measurements with MC-ICP-MS.

Chromite Reference Materials and Matrix Effects in SIMS Oxygen Isotope Measurements

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 XMag = 0.01 to 0.05, and XSpl = 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.

SIMS Iron Isotope Measurements of the Balmat Pyrite Reference Material: A Non‐Unique δ56Fe Signature

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.

Precise Measurement of Magnesium Isotopes in Fe‐Mg Minerals Using a Multi‐collector SHRIMP Ion Microprobe

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.

Sources of Inaccuracy in Boron Isotope Measurement Using LA‐MC‐ICP‐MS

Laser ablation multi‐collector‐inductively coupled plasma‐mass spectrometry (LA‐MC‐ICP‐MS) has become a valuable tool for the in situ measurement of the boron isotope composition of geological samples at high (tens to hundreds of μm) spatial resolution. That said, this application suffers from significant analytical challenges. We focus in this study on the underlying processes of two of the main causes for inaccuracies using this technique. We provide empirical evidence that not only Ca ions (Sadekov et al. 2019, Standish et al. 2019, Evans et al. 2021) but also Ar ions, that are reflected within the flight tube of the mass spectrometer, are the source for previously reported issues with spectral baselines. We also address the impact of plasma conditions on the instrumental mass fractionation as a source for matrix‐ and mass‐load‐related analytical biases. Comparing experimental data with the results of a dedicated release and diffusion model (RDM) we estimate that a close to complete (~ 97%) release of boron from the sample aerosol is needed to allow for consistently accurate LA boron isotope measurement results without the need for corrections.

A silica-related matrix effect on NanoSIMS Li isotopic analysis of glasses and its online calibration

NanoSIMS Li isotope analysis of silicate glasses is affected by instrumental mass fractionation (IMF) due to the matrix effect. Here, we found that the IMF is correlated with the silica content and can be well corrected.

A LA‐ICP‐MS Comparison of Reference Materials Used in Cassiterite U‐Pb Geochronology

Laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) is used to compare the suitability of four cassiterite (SnO2) materials (SPG, Yankee, AY‐4 and Jian‐1), and three matrix‐mismatched reference materials (NIST SRM 612, NIST SRM 614 and 91500 zircon) for normalisation of U‐Pb and Pb‐Pb isotope ratios in cassiterite. The excess variance of ages determined by LA‐ICP‐MS is estimated to be ±0.33% for 207Pb/206Pb vs. 208Pb/206Pb isochron ages and ± 1.8% and for U‐Pb ages. Incorporation of this excess variance in cassiterite ages is necessary for realistic uncertainties. 207Pb‐206Pb ages are advantageous for dating Precambrian cassiterite such as SPG compared with U‐Pb ages as matrix effect on instrumental mass fractionation of Pb isotopes are generally considered to be minor. We note minor bias in 207Pb/206Pb vs. 208Pb/206Pb isochron ages (~ 0.6%) when using either the NIST SRM 614 or 91500 zircon reference materials and emphasise the requirement for uncertainty propagation of all sources of error and reference materials with comparable U and Pb mass fraction to the cassiterite. The 238U/206Pb isotopic ratios from normalisation to matrix‐mismatched reference materials show varied results, which emphasises the need to use matrix‐matched reference materials for calculating U‐Pb ages. When cross‐calibrated against each other, LA‐ICP‐MS U‐Pb ages of the ca. 1535 Ma SPG, ca. 245 Ma Yankee and ca. 155 Ma Jian‐1 cassiterites are all consistent with their ID‐TIMS values.

NanoSIMS as an analytical tool for measuring oxygen and hydrogen isotopes in clay minerals from palaeosols: Analytical procedure and preliminary results

Oxygen and hydrogen isotope composition of neoformed clay minerals is commonly used as a palaeoclimatic proxy. Usually, sediments and rocks (including palaeosols) contain not only neoformed clay minerals, but also detrital and/or diagenetic ones. Together with the usually small size (micro- or nanometric) of the clay minerals in these materials, this can generate difficulties during isotopic analyses by conventional spectrometry. To avoid this problem, we used nanoscale secondary ion mass spectrometry (NanoSIMS) to analyse neoformed clay minerals included in palaeosol levels in early Barremian continental profiles located in NE Spain. The bottom levels of the profiles are rich in kaolinite, whereas the top levels contain smectite (beidellite-type). The isotopic compositions of pure powder standards of kaolinite and beidellite were measured in bulk by conventional mass spectrometry to obtain reference values for calibrating the instrumental mass fractionation. Two common preparation techniques for geological samples were tested (thin sections and thick polished sections), revealing that thin sections are more suitable for NanoSIMS analysis due to their lower resin content. The high spatial resolution of the instrument allowed the elemental mapping of the samples, permitting the localization of the minerals of interest and the measurement of isotopic ratios at selected points (1 x 1 μm2) within the samples. The preliminary isotopic results allowed to distinguish a decrease in the average 18O/16O and D/H ratios from the kaolinite in the bottom levels to the smectite in the top levels, reflecting a change in the climatic conditions. The average δ18O and δD values obtained for kaolinite (δ18OSMOW=18±15‰; δDSMOW=-82±36‰ and δ18OSMOW=14±4‰; δDSMOW=-97±37‰) and smectite (δ18OSMOW=13±14‰; δDSMOW=-167±87‰ and δ18OSMOW=11±6‰; δDSMOW=-180±40‰;), respectively, are consistent with the crystallization of the clays in weathering conditions. Despite the uncertainties of these preliminary isotopic measures, the results obtained allowed to estimate an average temperature for kaolinite formation of 21-22°C, and of 16-17°C for smectite. Given suitable calibration using pure isotopic standards and adequate sample preparation, NanoSIMS can thus have a great applicability in palaeoclimatic studies involving the oxygen and hydrogen isotopic composition of nanometre-sized clay minerals from palaeosols samples.

COM-1 and Hongcheon: New monazite reference materials for the microspot analysis of oxygen isotopic composition

BackgroundMonazite, a moderately common light rare earth element (LREE) and thorium phosphate mineral, has chemical, age, and isotopic characteristics that are useful in the investigation of the origin and evolution of crustal melts and fluid-rock interactions. Multiple stages of growth and partial recrystallization commonly observed in monazite inevitably require microspot chemical and isotopic analyses, for which well-characterized reference materials are essential to correct instrumental biases. In this study, we introduce new monazite reference materials COM-1 and Hongcheon for the use in the microspot analysis of oxygen isotopic composition.FindingsCOM-1 and Hongcheon were derived from a late Mesoproterozoic (~ 1080 Ma) pegmatite dyke in Colorado, USA, and a Late Triassic (~ 230 Ma) carbonatite-hosted REE ore in central Korea, respectively. The COM-1 monazite has much higher levels of Th (8.77 ± 0.56 wt.%), Si (0.82 ± 0.07 wt.%) and lower REE contents (total REE = 49.5 ± 1.2 wt.%) than does the Hongcheon monazite (Th, 0.23 ± 0.11 wt.%; Si, < 0.1 wt.%; total REE, 59.9 ± 0.7 wt.%). Their oxygen isotopic compositions (δ18OVSMOW) were determined by gas-source mass spectrometry with laser fluorination (COM-1, 6.67 ± 0.08‰; Hongcheon-1, 6.60 ± 0.02‰; Hongcheon-2, 6.08 ± 0.07‰). Oxygen isotope measurements performed by a Cameca IMS1300-HR3 ion probe showed a strong linear dependence (R2 = 0.99) of the instrumental mass fractionation on the total REE contents.ConclusionsWe characterized chemical and oxygen isotopic compositions of COM-1 and Hongcheon monazites. Their internal homogeneity in oxygen isotopic composition and chemical difference provide an efficient tool for calibrating instrumental mass fractionation occurring during secondary ion mass spectrometry analyses.

Sn Isotopic Values in Ten Geological Reference Materials by Double‐Spike MC‐ICP‐MS

The tin mass fraction and the δ120/118Sn relative to NIST SRM 3161a were measured for ten geological reference materials among which the Sn isotopic values of W‐2a, JA‐2, JG‐2, G‐2, GBW07316, NOD‐A‐1 and SGR‐1 are reported for the first time. The sample preparation procedure implemented in this study was based on acid digestion and matrix separation by two‐stage chromatographic using AG‐MP‐1 and TRU‐Spec resin. Isotopic measurements were carried out by multi‐collector inductively coupled plasma‐mass spectrometry (MC‐ICP‐MS), using a 117Sn–119Sn double‐spike technique to correct for the instrumental mass fractionation effects. The overall precision of this method is estimated to be ± 0.03‰ (2s, n = 81) and ± 0.07‰ (2s, n = 6) for the NIST SRM 3161a and a geological reference material BHVO‐2, respectively.

Accurate and Efficient SIMS Oxygen Isotope Analysis of Composition-Variable Minerals: Online Matrix Effect Calibration for Dolomite.

High-quality oxygen isotope analysis of composition-variable minerals (e.g., ubiquitous carbonates) using secondary ion mass spectrometry (SIMS) is extremely challenging. The classical off-line procedure, which requires additional electron probe microanalyzer (EPMA) chemical compositions for calibrating instrumental mass fractionation (IMF), is inherently inaccurate and analytically inefficient. In this study, the first accurate and paired SIMS analysis of δ18O and Fe# [molar Fe/(Mg + Fe)] in dolomite is reported. Based on five newly developed dolomite O-isotopic standards with an Fe# range of 0.01-0.35 obtained by SIMS, a novel accurate and rapid online matrix effect calibration method for dolomite O-isotope analysis was developed using concurrent SIMS 18O-16O-56Fe16O-24Mg16O measurements without additional chemical electron probe microanalysis. A logistic equation was proposed as the best-fit curve to represent the δ18O matrix effect based on the 56Fe16O/24Mg16O ratios. For CTD-4 carbonatitic dolomite with variable Fe# but homogeneous oxygen isotopes, the off-line method exhibited highly variable apparent δ18O values in the range of 5.74-10.11‰. The online method yielded a homogeneous δ18O value of 7.94 ± 0.34‰ (2SD, n = 40), which is comparable with that of bulk analysis (7.94 ± 0.20‰; 2SD). Comprehensive analyses validated the online method as the best strategy for performing accurate δ18O analysis of samples with highly heterogeneous compositions. Based on its accuracy, simplicity, and economic feasibility, this method has potential applications in the analysis of composition-complex dolomites, detrital dolomites, and other precious terrestrial and extraterrestrial materials.

Mass Bias Corrections for Hydrogen and Oxygen Isotope Analysis of Tourmaline by Secondary Ion Mass Spectrometry

ABSTRACT Hydrogen (δ2H) and oxygen (δ18O) stable isotopes are used to trace fluid sources, metals, and contaminants in the environment and Earth's subsurface. Tourmaline-supergroup minerals provide an opportunity to quantify both δ2H and δ18O from the same grain using in situ analytical techniques (e.g., Secondary Ion Mass Spectrometry – SIMS). These minerals occur in a wide variety of geological environments and have a wide range of chemical compositions. However, large differences in chemical composition are problematic during SIMS analysis, as instrumental mass fractionation (IMF) often varies with the chemical composition of the mineral. Therefore, calibration models derived by analyzing tourmalines of different chemical composition must be developed for accurate analysis by SIMS. Hydrogen and oxygen isotope analysis was done on six reference tourmaline samples using a CAMECA 7f SIMS instrument operating at extreme energy filtering. Spot-to-spot repeatability for tourmalines was in the range 4–5‰ and 0.6–1.0‰ for δ2H and δ18O, respectively. There is a strong correlation between IMF and several elements (B, Si, Ca, Fe, and Fe#). Iron content is the most robust predictor of IMF, and we report two calibration curves for the correction of δ2H and δ18O measured by SIMS using reference tourmaline crystals with different Fe contents, ranging from 0.00 to 14.00 wt.% Fe. This is the first calibration curve used to correct for the fractionation of hydrogen isotope ratios in tourmaline as measured by SIMS. Tourmaline-supergroup minerals require a suite of at least three, with a range of Fe content, to ensure accurate and precise H and O analysis by SIMS. Crystallographic orientation effects were not observed for these tourmalines.

Episodic fluid flow in an eclogite-facies shear zone: Insights from Li isotope zoning in garnet

Abstract Episodic fluid overpressure and escape is invoked as a cause or consequence of many subduction-zone seismic phenomena but can be challenging to constrain in exhumed high-pressure metamorphic rocks. In situ measurements of lithium isotopes in garnet reveal evidence of episodic fluid transport in a subduction shear zone now exposed in the Monviso ophiolite (Western Alps). Garnet from an eclogite block and associated metasomatic reaction rind was analyzed by secondary ion mass spectrometry (SIMS). All analyzed garnet preserves core-rim zoning in δ7Li and large negative δ7Li excursions (NEs) in mantles. These excursions cannot be explained by instrumental mass fractionation during analysis, equilibrium fractionation, or intracrystalline diffusion of Li within garnet. Instead, NEs were produced by kinetic fractionation of Li isotopes during bulk diffusion through a pore fluid, and the fractionated isotopic compositions were incorporated into garnet by syn-diffusion growth. Disequilibrium garnet growth textures associated with negative δ7Li support this interpretation and suggest metasomatism drove rapid garnet growth. Four distinct NEs were identified requiring that at least four pulses of fluid were transported within the adjacent shear zone. This evidence of episodic fluid transport along a subduction shear zone at eclogite facies supports models of intermediate-depth seismicity that rely on cyclic fluid overpressure and escape.

Improved Uranium Particle Analysis by SIMS using O3 - Primary Ions.

We have investigated the use of negative molecular oxygen primary ion beams (i.e., O2 - and O3 -) to determine the benefits of using such beams for uranium particle SIMS analyses. Typically, O- is the most practical negative primary ion species from the conventional duoplasmatron ion source for both age dating and uranium isotopic analysis of particles. Newer RF plasma ion sources make it possible to use O2 - and O3 - due to higher brightness and primary ion fluence, and the increased abundance of molecular species in the plasma relative to the duoplasmatron. We have determined that by using an O3 - beam, the ionization yield can be increased by a factor of approximately two over an O- beam, up to 4.7%, a substantial improvement that positively impacts measurement precision and detection limits. We also investigated the effect of the molecular oxygen beams on uranium isotope mass fractionation and the Th/U relative sensitivity factor for SIMS analyses in comparison to O- beams. We found that O3 - reduced instrumental mass fractionation and matrix/substrate effects relative to the other negative ion beams. Particle measurements using O3 - were improved compared to conventional O- beam analyses due to higher yields, smaller corrections, and reduced substrate effects.

A Method for Secondary Ion Mass Spectrometry Measurement of Lithium Isotopes in Garnet: The Utility of Glass Reference Materials

We present a novel method for the measurement of lithium isotopes in garnet utilising glass reference materials and secondary ion mass spectrometry (SIMS). Measured lithium isotopic compositions of natural garnets are heterogeneous, making them unreliable reference materials for in situ determination. However, SIMS lithium isotope measurements of glasses derived from these natural garnets are isotopically identical to their parent garnets and more homogeneous, demonstrating that they can be used as reliable reference materials. To characterise the composition dependence of instrumental mass fractionation (IMF), oxide and silicate powders were used to synthesise custom‐made glass reference materials (CGRMs) with garnet‐equivalent compositions. Results for six CGRMs measured by SIMS show a significant linear relationship between IMF and FeO and MnO contents. Corrections for this compositional IMF result in changes of up to 12‰ within the compositional range explored. Uncertainty in IMF‐corrected SIMS analyses with a 20 μm spot is in the range of 2.5 to 4.5‰, depending on the garnet composition and reference materials used. The method for in situ lithium isotope measurement in garnet by SIMS presented here is highly adaptable, valid across a range of Al‐rich garnet compositions, and yields spatial resolution and precision necessary to address a range of geological applications.

SIMS oxygen isotope matrix effects in silicate glasses: Quantifying the role of chemical composition

Instrumental mass fractionation (IMF) associated with Secondary Ion Mass Spectrometry (SIMS) measurements of oxygen isotope compositions in silicate glasses was studied using a set of 27 synthesized glasses spanning a compositionally broad range of six major oxides: SiO2, TiO2, Al2O3, total FeO (FeOt), MgO and CaO. The impact of chemical composition on the IMF values was investigated using a Cameca 1280-HR during a single SIMS analytical session operated under constant instrumental conditions. The data measured were compared with the δ18O values obtained by laser fluorination gas source mass-spectrometry (LF). The offset between the δ18O(LF) and δ18O(SIMS) was found to reach up to 5‰. Our data document that SIMS oxygen isotope matrix effects in silicate glasses strongly depend on the chemical composition of silicate glasses, here the cation‑oxygen bond strength was found to have a strong influence on the IMF value. We tested a variety of models based on single oxide contents and various composition-dependent parameters, but none were fully satisfactory for predicting IMF. Neither mean atomic mass nor NBO/T (the ratio of non-bridging oxygens per tetrahedrally coordinated cation) show a strong correlation with the IMF values (R2 of 0.45 and 0.46, respectively). Among the single oxides, only the model based on the SiO2 content is useful for prediction of the IMF in silicate glasses, but this model has a large standard error (1σ = ± 0.90‰) and was also found to break down for glasses with high Na and K contents. We propose an empirical model based on the correlation of six major element oxides that shows a strong correlation with IMF (R2 = 0.98, 1σ = ± 0.40‰). This model describes the experimental data with uncertainties that are roughly a factor of two better than the correction methods proposed in earlier studies. We also investigated using the correlation between IMF and isotope I-18O index, which describes the correlation between atomic bond strength and relative oxygen isotope enrichment in silicate substances (R2 = 0.87). Although our efforts provide refinements to the SIMS determination of δ18O in natural silicate glasses, truly accurate IMF corrections will need further refinements related to the impact of alkali elements.

Instrumental mass fractionation during sulfur isotope analysis by secondary ion mass spectrometry in natural and synthetic glasses

Sulfur isotope ratios are among the most commonly studied isotope systems in geochemistry. While sulfur isotope ratio analyses of materials such as bulk rock samples, gases, and sulfide grains are routinely carried out, in-situ analyses of silicate glasses such as those formed in magmatic systems are relatively scarce in the literature. Despite a number of attempts in recent years to analyse sulfur isotope ratios in volcanic and experimental glasses by secondary ion mass spectrometry (SIMS), the effects of instrumental mass fractionation (IMF) during analysis remain poorly understood. In this study we use more than 600 sulfur isotope analyses of nine different glasses to characterise the matrix effects that arise during sulfur isotope analysis of glasses by SIMS. Samples were characterised for major element composition, sulfur content, and sulfur isotope ratios by independent methods. Our glasses contain between 500 and 3400 ppm sulfur and cover a wide compositional range, including low-silica basanite, rhyolite, and phonolite, allowing us to investigate composition-dependent IMF. We use SIMS in multi-collection mode with a Faraday cup/electron multiplier detector configuration to achieve uncertainty of 0.3‰ to 2‰ (2σ) on measured δ34S. At high sulfur content, the analytical error of our SIMS analyses is similar to that of bulk analytical methods, such as gas-source isotope ratio mass spectrometry. We find IMF causes an offset of −12‰ to +1‰ between bulk sulfur isotope ratios and those measured by SIMS. Instrumental mass fractionation correlates non-linearly with glass sulfur contents and with a multivariate regression model combining glass Al, Na, and K contents. Both ln(S) and Al-Na-K models are capable of predicting IMF with good accuracy: 84% (ln(S)) and 87% (Al-Na-K) of our analyses can be reproduced within 2σ combined analytical uncertainty after a correction for composition-dependent IMF is applied. The process driving IMF is challenging to identify. The non-linear correlation between glass S content and IMF in our dataset resembles previously documented correlation between glass H2O abundance and IMF during D/H ratio analyses by SIMS, and could be attributed to changes in 32S− and 34S− ion yields with changing S content and glass composition. However, a clear correlation between S ion yields and S content cannot be identified in our dataset. We speculate that accumulation of alkalis at the SIMS crater floor may be the principal driving force of composition-dependent IMF. Nonetheless, other currently unknown factors could also influence IMF observed during S isotope ratio analyses of glasses by SIMS. Our results demonstrate that the use of multiple, well-characterised standards with a wide compositional range is required to calibrate SIMS instruments prior to sulfur isotope analyses of unknown silicate glasses. Matrix effects related to glass Al-Na-K contents are of particular importance for felsic systems, where alkali and aluminium contents can vary considerably more than in mafic magmas.

Mass fractionation of Rb and Sr isotopes during laser ablation-multicollector-ICPMS: in situ observation and correction

BackgroundOne of the most critical issues concerning in situ mass spectrometry lies in accounting for elements and molecules that overlap target isotopes of analytical interest in a sample. This study traced the instrumental mass fractionation of Rb and Sr isotopes during laser ablation-multicollector-inductively coupled plasma mass spectrometry (LA-MC-ICPMS) to obtain reliable 87Sr/86Sr ratios for high-Rb/Sr samples.FindingsIn the LA-MC-ICPMS analysis, Kr interferences were corrected using Ar and He gas blanks measured without ablating material. Contributions from doubly charged Er and Yb ions were corrected using the intensities of half masses and isotopic compositions reported in the literature. After Kr correction, the calculated 166Er2+ intensity of NIST SRM 610 approached the measured intensity at mass 83, and the 173Yb2+/171Yb2+ ratio agreed with the recommended value within error ranges. Kr- and REE2+-stripped peak intensities were further corrected for Rb interference. Use of the Sr mass bias factor for the calculation of measured 87Rb/85Rb yielded 87Sr/86Sr ratios consistent with the recommended and expected values for low-Rb/Sr materials, such as NIST SRM 616, modern shark teeth, and plagioclase collected from Jeju Island, but failed to account for the 87Rb interference from high-Rb/Sr materials including NIST SRM 610 and SRM 612. We calculated in situ mass bias factor of Rb from the known 87Sr/86Sr ratios of the standards and observed a correlation between Rb and Sr mass fractionation, which allowed inference of the Rb bias from the standard run. Reliable 87Sr/86Sr and 85Rb/86Sr ratios were obtained for SRM 610 and SRM 612 using the inferred mass bias factor of Rb calculated by the standard bracketing method.ConclusionsThis study revealed that Rb and Sr isotopes behave differently during LA-MC-ICPMS and suggests the potential usefulness of the standard bracketing method for measuring the Rb–Sr isotopic compositions of high-Rb/Sr materials.