Instrumental Mass Fractionation Research Articles

Tracing metal–silicate segregation and late veneer in the Earth and the ureilite parent body with palladium stable isotopes

Stable isotope studies of highly siderophile elements (HSE) have the potential to yield valuable insights into a range of geological processes. In particular, the strong partitioning of these elements into metal over silicates may lead to stable isotope fractionation during metal–silicate segregation, making them sensitive tracers of planetary differentiation processes. We present the first techniques for the precise determination of palladium stable isotopes by MC-ICPMS using a 106Pd–110Pd double-spike to correct for instrumental mass fractionation. Results are expressed as the per mil (‰) difference in the 106Pd/105Pd ratio (δ106Pd) relative to an in-house solution standard (Pd_IPGP) in the absence of a certified Pd isotopic standard. Repeated analyses of the Pd isotopic composition of the chondrite Allende demonstrate the external reproducibility of the technique of ±0.032‰ on δ106Pd. Using these techniques, we have analysed Pd stable isotopes from a range of terrestrial and extraterrestrial samples. We find that chondrites define a mean δ106Pdchondrite=−0.19±0.05‰. Ureilites reveal a weak trend towards heavier δ106Pd with decreasing Pd content, similar to recent findings based on Pt stable isotopes (Creech et al., 2017), although fractionation of Pd isotopes is significantly less than for Pt, possibly related to its weaker metal–silicate partitioning behaviour and the limited field shift effect. Terrestrial mantle samples have a mean δ106Pdmantle=−0.182±0.130‰, which is consistent with a late-veneer of chondritic material after core formation.

Quantification of oxygen isotope SIMS matrix effects in olivine samples: Correlation with sputter rate

We investigated the magnitude and reproducibility of instrumental mass-dependent fractionation of oxygen isotopes in secondary ion mass spectrometry (SIMS) analyses of olivine crystals of different major element chemistry (from Mg-rich to Fe-rich) in order to improve the accuracy of in-situ O-isotope measurements in geochemical/cosmochemical olivine samples. We found that oxygen isotope SIMS matrix effects are reproducible, and developed a model curve that can be used for correcting instrumental mass fractionation of olivine samples of intermediate chemical composition. The changes in instrumental mass fractionations were likely caused by differing Cs concentrations in the near surface regions of the samples due to different sample sputtering rates.

Investigation of the quasi-simultaneous arrival (QSA) effect on a CAMECA IMS 7f-GEO.

IMS 7f-GEO isotope ratio applications increasingly involve analyses (e.g., S- or O- isotopes, coupled with primary ion currents <30 pA) for which quasi-simultaneous arrival (QSA) could compromise precision and accuracy of data. QSA and associated correction have been widely investigated for the CAMECA NanoSIMS instruments, but not for the IMS series. Sulfur and oxygen isotopic ratio experiments were performed using an electron multiplier (EM) detector, employing Cs+ primary ion currents of 1, 2, 5 and 11.5 pA (nominal) and a variety of secondary ion transmissions to vary QSA probability. An experiment to distinguish between QSA undercounting and purported aperture-related mass fractionation was performed using an EM for 16 O- and 18 O- plus an additional 16 O- measurement using a Faraday cup (FC) detector. An experiment to investigate the accuracy of the QSA correction was performed by comparing S isotopic ratios obtained using an EM with those obtained on the same sample using dual FCs. The QSA effect was observed on the IMS-7f-GEO, and QSA coefficients (β) of ~0.66 were determined, in agreement with reported NanoSIMS measurements, but different from the value (0.5) predicted using Poisson statistics. Aperture-related fractionation was not sufficient to explain the difference but uncertainties in primary ion flux measurement could play a role. When QSA corrected, the isotope ratio data obtained using the EM agreed with the dual FC data, within statistical error. QSA undercounting could compromise isotope ratio analyses requiring ~1×105 counts per second for the major isotope and primary currents <20 pA. The error could be >8‰ for a 1 pA primary current. However, correction can be accurately applied. For instrumental mass fractionation (IMF)-corrected data, the magnitude of the error resulting from not correcting for QSA is dependent on the difference in secondary ion count rate between the unknown and standard analyses. Copyright © 2017 John Wiley & Sons, Ltd.

Accurate and Precise Silicon Isotope Analysis of Sulfur‐ and Iron‐Rich Samples by MC‐ICP‐MS

Silicon isotope determination of sulfur‐rich samples by MC‐ICP‐MS can be challenging because cation‐exchange chromatography used for Si purification does not efficiently remove anionic sulfur species. Results for pure Si standard solutions with addition of sulfate showed shifts of up to +1.04 ± 0.10‰ (2s) in δ30Si. Doping of both standard solutions and samples with S to a fixed S/Si ratio can eliminate the relative change in instrumental mass fractionation due to variable S/Si in samples and also boosts the relative sensitivity of Si by up to 66%. Moreover, Fe hydroxide precipitation during sample processing adsorbs Si resulting in isotopic fractionations. Tests using Fe‐rich samples showed that this could be a major factor for observed shifts in δ30Si. Acidification of the sample and standard solutions to a pH &lt; 1 aggressively dissolved any Fe hydroxide precipitates, even in relatively Fe‐rich samples such as chondrite meteorites. The pH values of the sample solutions were subsequently adjusted to a range of 2–3 by adding ultra‐pure NaOH solutions. The combination of sulfur doping and the pH adjustment protocol ensured a full recovery of Si and proved to be an efficient and reliable method for Si isotope determination of S‐ and Fe‐rich materials.

Influence of glass composition on secondary ion mass spectrometry instrumental mass fractionation for Si and Ca isotopic analyses.

In situ secondary ion mass spectrometry (SIMS) analysis requires the use of standards to unravel the instrumental mass fractionation (IMF) induced by the analytical procedures. Part of this IMF might be caused by the nature of the sample and the differences in composition and structure between the sample and the standards. This "matrix effect" has been tentatively corrected for by using standards with chemical compositions equivalent to the samples, or by the empirical use of chemical parameters. However, these corrections can only be applied to a narrow compositional range and fail to take proper account of the matrix effect when a wider chemical field is tested. We synthesized a series of glasses whose compositions span a very large part of the NCMAS (Na2 O-CaO-MgO-Al2 O3 -SiO2 ) system. Si and Ca isotopic analyses were performed on two ion microprobes (Cameca IMS-1270 and IMS-1280). The matrix effect observed may reach 20‰ between extreme compositions and cannot be accounted for by the previously used "chemical" parameters (e.g. SiO2 , SiO2 /(SiO2 +Al2 O3 )) nor by the NBO/T parameter. It therefore appears necessary to consider not only the structure of the glasses, but also the nature of the different atoms. Consequently, we assessed the use of another concept, the optical basicity, based on the electronegativities of the constitutive elements of glass. We show that this parameter significantly improves the efficiency of the matrix-effect correction and that it can be applied across the entire NCMAS compositional range studied here. Furthermore, the use of optical basicity reduces the number of glass standards required for a reliable isotopic study, and it can also be used to probe the structure of the glass. Copyright © 2016 John Wiley & Sons, Ltd.

Predicting instrumental mass fractionation (IMF) of stable isotope SIMS analyses by response surface methodology (RSM)

Instrumental mass fractionation (IMF) of isotopic SIMS analyses (Cameca 1280HR, CRPG Nancy) was predicted by response surface methodology (RSM) for the 18O/16O determination of plagioclase, K-feldspar, and quartz.

Evaluation of potential monazite reference materials for oxygen isotope analyses by SIMS and laser assisted fluorination

Monazite can record several episodes of fluid/rock interaction within a single grain. Coupled in situ determination of age and oxygen isotope composition by SIMS (secondary ion mass spectrometer) may characterize such events. Yet, monazite reference materials for oxygen isotope analyses are scarce. Ion probe (CAMECA IMS 1280) and laser fluorination measurements of δ18O values from four monazites are presented. Monazites (YREEPO4) with variable cheralite [CaTh(PO4)2] and huttonite (ThSiO4) substitutions, which span a wide compositional range covering most magmatic and metamorphic, as well as hydrothermal monazites were selected. Three monazites, Moacyr, Manangoutry and UNIL-Mnz1, have a homogeneous chemical (determined by electron microprobe analysis and back-scattered imaging) and oxygen isotopic composition with uncertainties (2 SD) in the range of 0.3 to 0.4‰ for ion probe measurements. These three samples are thus potentially suitable as reference materials, and sample UNIL-Mnz1 is available for inter-laboratory comparison. Sample UNIL-Mnz2 is less homogeneous, but it is still useful for internal monitoring. Uncertainties (2 SD) on the ion probe data are in the range of 0.5 to 0.6‰.Using the investigated monazites as well as literature data, a new calibration curve has been established to account for instrumental mass fractionation (IMF) due to the solid solution in natural monazites. The data illustrate that the IMF is primarily a function of the YREEPO4 content, while the influence of the Th-content seems to be less important.The δ18O values for the four studied monazites have been determined by laser fluorination (LF). They are as follows (1 SD): 1.5±0.1‰ for Moacyr, 10.2±0.1‰ for Manangoutry, 8.5±0.2‰ for UNIL-Mnz1, and 9.5±0.3‰ for the slightly less homogenous UNIL-Mnz2. The analytical protocol adopted here results in consistently high yields (>90%) and reproducible values, and hence it is argued that LF is a viable method for the calibration of phosphate reference materials.

Silicon isotope ratio measurements by inductively coupled plasma tandem mass spectrometry for alteration studies of nuclear waste glasses

High-level, long-lived nuclear waste arising from spent fuel reprocessing is vitrified in silicate glasses for final disposal in deep geologic formations. In order to better understand the mechanisms driving glass dissolution, glass alteration studies, based on silicon isotope ratio monitoring of 29Si-doped aqueous solutions, were carried out in laboratories.This work explores the capabilities of the new type of quadrupole-based ICP-MS, the Agilent 8800 tandem quadrupole ICP-MS/MS, for accurate silicon isotope ratio determination for alteration studies of nuclear waste glasses. In order to avoid silicon polyatomic interferences, a new analytical method was developed using O2 as the reaction gas in the Octopole Reaction System (ORS), and silicon isotopes were measured in mass-shift mode. A careful analysis of the potential polyatomic interferences on SiO+ and SiO2+ ion species was performed, and we found that SiO+ ion species suffer from important polyatomic interferences coming from the matrix of sample and standard solutions (0.5M HNO3). For SiO2+, no interferences were detected, and thus, these ion species were chosen for silicon isotope ratio determination. A number of key settings for accurate isotope ratio analysis like, detector dead time, integration time, number of sweeps, wait time offset, memory blank and instrumental mass fractionation, were considered and optimized. Particular attention was paid to the optimization of abundance sensitivity of the quadrupole mass filter before the ORS. We showed that poor abundance sensitivity leads to a significant shift of the data away from the Exponential Mass Fractionation Law (EMFL) due to the spectral overlaps of silicon isotopes combined with different oxygen isotopes (i.e. 28Si16O18O+, 30Si16O16O+). The developed method was validated by measuring a series of reference solutions with different 29Si enrichment. Isotope ratio trueness, uncertainty and repeatability were found to be <0.2%, <0.5% and <0.6%, respectively. These performances meet the requirements of the studies of nuclear glasses alteration and open up possibilities to use this method for precise determination of silicon content in natural samples by Isotope Dilution.

Improvements in Cd stable isotope analysis achieved through use of liquid-liquid extraction to remove organic residues from Cd separates obtained by extraction chromatography.

Organic compounds released from resins that are commonly employed for trace element separations are known to have a detrimental impact on the quality of isotopic analyses by MC-ICP-MS. A recent study highlighted that such effects can be particularly problematic for Cd stable isotope measurements (M. Gault-Ringold and C. H. Stirling, J. Anal. At. Spectrom., 2012, 27, 449-459). In this case, the final stage of sample purification commonly applies extraction chromatography with Eichrom TRU resin, which employs particles coated with octylphenyl-N,N-di-isobutyl carbamoylphosphine oxide (CMPO) dissolved in tri-n-butyl phosphate (TBP). During chromatography, it appears that some of these compounds are eluted alongside Cd and cannot be removed by evaporation due to their high boiling points. When aliquots of the zero-ε reference material were processed through the purification procedure, refluxed in concentrated HNO3 and analyzed at minimum dilution (in 1 ml 0.1 M HNO3), they yielded Cd isotopic compositions (ε114/110Cd = 4.6 ± 3.4, 2SD, n = 4) that differed significantly from the expected value, despite the use of a double spike technique to correct for instrumental mass fractionation. This result was accompanied by a 35% reduction in instrumental sensitivity for Cd. With increasing dilution of the organic resin residue, both of these effects are reduced and they are insignificant when the eluted Cd is dissolved in ≥3 ml 0.1 M HNO3. Our results, furthermore, indicate that the isotopic artefacts are most likely related to anomalous mass bias behavior. Previous studies have shown that perchloric acid can be effective at avoiding such effects (Gault-Ringold and Stirling, 2012; K. C. Crocket, M. Lambelet, T. van de Flierdt, M. Rehkämper and L. F. Robinson, Chem. Geol., 2014, 374-375, 128-140), presumably by oxidizing the resin-derived organics, but there are numerous disadvantages to its use. Here we show that liquid-liquid extraction with n-heptane removes the organic compounds, dramatically improving quality of the Cd isotope data for samples that are analyzed at or close to minimum dilution factors. This technique is quick, simple and may be of use prior to analysis of other isotope systems where similar resins are employed.

Hydrogen isotopic fractionation in secondary ion mass spectrometry using polyatomic ions

Isotopic measurements at the micron scale using secondary ion mass spectrometry (SIMS) require an accurate control on the instrumental mass fractionation (IMF). The IMF correction is challenging since substantial mass fractionation can occur during both emission and detection processes. In this work, we report hydrogen isotopic measurements on a series of organic standards using polyatomic ions (CH−, CD−, C2H−, C2D−) with the NanoSIMS-50 instrument. We show that the D/H ratios resulting from the measurement of CD−/CH− and C2D−/C2H− ratios are close to the true D/H ratios of the samples over more than 3 decades and allow accurate hydrogen isotopic measurements to be performed in solid samples. Large secondary emission yields are usually associated with high electron affinities and correlate presently with low IMF effects. Moreover, we show that even when large variations in secondary ionic current are observed, an accurate isotopic ratio can still be inferred.

TIMS measurements of full range of natural Ca isotopes with internally consistent fractionation correction

This study presents static measurements of the Ca isotopic composition of standard reference materials SRM 915 a/b on a Triton Plus™ thermal ionization mass spectrometer with a specially developed Faraday cup array allowing simultaneous measurement of 40Ca and 48Ca. The total amount of Ca in all analyses was kept <1μg. With this setup the measurement uncertainties were 0.06‰ for 40Ca/44Ca and 0.12‰ for 48Ca/40Ca. Measuring all isotopes simultaneously better allows to test the internal consistency of different Ca isotope abundances reported in the literature. The exponential law was observed to correct incompletely instrumental mass fractionation. An improved fractionation correction based on the exponential law is proposed. It changes the 40Ca/44Ca ratio of SRM 915a (corrected relative to 42Ca/44Ca=0.31221; 48Ca/44Ca=0.08871) from 47.1635±0.0028 to 47.1649±0.0047. The measurements of SRM 915b were performed with different analytical conditions (runs were prolonged till complete filament load depletion). Even if the 40Ca/44Ca ratio of SRM 915b, when corrected with the simple exponential law, appears different (47.1532±0.0038) from that of SRM 915a, it becomes coincident (47.1613±0.0028) when corrected with a second-order refinement. This supports the use of the improved exponential law to obtain internally consistent Ca isotope ratio for natural samples.

High-precision measurements of seawater Pb isotope compositions by double spike thermal ionization mass spectrometry

A new method for the determination of seawater Pb isotope compositions and concentrations was developed, which combines and optimizes previously published protocols for the separation and isotopic analysis of this element. For isotopic analysis, the procedure involves initial separation of Pb from 1 to 2L of seawater by co-precipitation with Mg hydroxide and further purification by a two stage anion exchange procedure. The Pb isotope measurements are subsequently carried out by thermal ionization mass spectrometry using a 207Pb–204Pb double spike for correction of instrumental mass fractionation. These methods are associated with a total procedural Pb blank of 28±21pg (1sd) and typical Pb recoveries of 40–60%. The Pb concentrations are determined by isotope dilution (ID) on 50mL of seawater, using a simplified version of above methods. Analyses of multiple aliquots of six seawater samples yield a reproducibility of about ±1 to ±10% (1sd) for Pb concentrations of between 7 and 50pmol/kg, where precision was primarily limited by the uncertainty of the blank correction (12±4pg; 1sd). For the Pb isotope analyses, typical reproducibilities (±2sd) of 700–1500ppm and 1000–2000ppm were achieved for 207Pb/206Pb, 208Pb/206Pb and 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb, respectively. These results are superior to literature data that were obtained using plasma source mass spectrometry and they are at least a factor of five more precise for ratios involving the minor 204Pb isotope. Both Pb concentration and isotope data, furthermore, show good agreement with published results for two seawater intercomparison samples of the GEOTRACES program. Finally, the new methods were applied to a seawater depth profile from the eastern South Atlantic. Both Pb contents and isotope compositions display a smooth evolution with depth, and no obvious outliers. Compared to previous Pb isotope data for seawater, the 206Pb/204Pb ratios are well correlated with 207Pb/206Pb, underlining the significant improvement achieved in the measurement of the minor 204Pb isotope.

Hydrogen and copper isotope analysis of turquoise by SIMS: calibration and matrix effects

The hydrogen isotope system is used extensively to provide information on the genesis of minerals (e.g., source of fluids and mechanisms of precipitation). The copper isotopic system is less well understood, but has the potential to provide valuable insight on mineral precipitation, particularly supergene Cu-rich minerals. Here we present a rapid and precise method for measuring hydrogen and copper isotopes in semi-precious gem-quality turquoise (Cu(Al,Fe3+)6(PO4)4(OH)8·4H2O) by secondary ion mass spectrometry (SIMS). The suitability of standards for instrumental mass fractionation (IMF) calibration was assessed by external precision of SIMS measurements for each standard (1–4‰ for δDIMF and 0.1–0.4‰ for δ65CuIMF). IMF in turquoise was correlated with H and Fe contents for D/H measurements and Fe content for 65Cu/63Cu measurements. Based on these correlations, IMF can be corrected to enable δD and δ65Cu analyses by SIMS with accuracies of ±5‰ and ±0.5‰, respectively. The precision and accuracy of SIMS thus rivals those of other mass spectrometric methods for H and Cu isotopes and demonstrates the potential of SIMS applications in identifying gemstones provenance and understanding the genesis of turquoise deposits.

Strontium isotope analysis of curved tooth enamel surfaces by laser-ablation multi-collector ICP-MS

Laser ablation multi-collector ICP-MS (LA-MC-ICP-MS) is increasingly applied to measure the strontium isotope composition (87Sr/86Sr) of fossil or modern tooth samples in order to address questions about mobility. Recently, concerns have been raised that an analytical bias due to instrumental isotopic mass fractionation might be introduced when this method is used to sample across naturally curved tooth enamel surfaces.We address this concern by reanalyzing data that were originally produced using 750μm linear LA-MC-ICP-MS scans on the external, slightly curved surfaces of fossil hominin teeth (Australopithecus and Paranthropus) from South Africa. By re-integrating the first and last 1/3 of the data along the original 750μm linear scans, we compared 87Sr/86Sr results produced by sections of each scan with varying degrees of laser focus. The results show no evidence of any analytical scatter between strontium isotope values for the different sections of the curved tooth surfaces and results for the different sections of each linear scan agree well within the precision of the method (external 2σ ± 0.0004). Furthermore, each analytical session was bracketed by analysis of a curved rodent (Otomys sp.) tooth in-house standard with up to ±100μm vertical change along the 750μm linear scan. Long-term, average LA-MC-ICP-MS 87Sr/86Sr results for one such Otomys tooth standard (0.72989±0.00029, n=71) agree well with the solution MC-ICP-MS analysis of the same tooth (0.72976±0.00002). New LA-MC-ICP-MS 87Sr/86Sr ratios, using both a 213nm and solid-state 193nm laser ablation system, for sets of consecutive 750μm linear scans along the curved, outer tooth enamel surface on another single Otomys tooth in-house standard agree within ±0.0004 regardless of progressively more extreme vertical change during analysis (±10μm to ±900μm).These results are similar to LA-MC-ICP-MS Sr isotope analyses, using the same instrumentation and methodology, of an in-house, polished, flat clinopyroxene mineral standard (LA: 0.70482±0.00029, n=155; solution: 0.70495±0.00003).We conclude that, provided the stable 86Sr/88Sr ratio is used in conjunction with the exponential law to constantly correct for instrumental isotopic mass fractionation during acquisition, curved tooth enamel surfaces represent no impediment to accurate LA-MC-ICP-MS strontium isotope analyses.

Improved precision and spatial resolution of sulfur isotope analysis using NanoSIMS

High precision analyses of all four sulfur isotopes in four pyrite and three sphalerite standards and in working reference samples were carried out using a CAMECA NanoSIMS 50L instrument. The measurements were made using three different settings of the Faraday cup (FC) and/or electron multiplier (EM) detectors, which meet different requirements for spatial resolution. The effects of EM aging and quasi-simultaneous arrival were corrected before the calibration of instrumental mass fractionation by a standard–sample–standard bracket method using the standards measured together with the samples. High analytical precision was achieved by counting 32S, 33S and 34S with the FCs and 36S with the EM (i.e. the FC–FC–FC–EM mode) using a 0.7 μm diameter ∼350 pA Cs+ primary beam and scanning over areas of 5 × 5 μm2. The standard deviations of spot-to-spot and grain-to-grain (external reproducibility 1 SD) measurements were less than 0.3, 0.3 and 0.7‰ for δ33S, δ34S and δ36S, respectively. To achieve a higher lateral resolution of ≤2 × 2 μm2, the Cs+ beam was reduced to 7–10 pA with a diameter of ∼200 nm; 32S was measured with the FC and the other signals were measured with the EMs. The external reproducibility (1 SD) was better than 0.5‰ for both δ33S and δ34S and was 3‰ for δ36S. To achieve the highest lateral resolution for the analysis of submicron-sized sulfides, a ∼0.7 pA Cs+ beam of ∼100 nm diameter was used, scanning over areas of 0.5 × 0.5 μm2, and all 32S, 33S and 34S were counted with the EMs. The external reproducibility (1 SD) was better than 1.5‰ for both δ33S and δ34S. These three modes have important applications in the isotope analysis of micron-sized sulfur samples, such as pyrite framboids and areas of complex zoning in sulfide minerals.

Tracing lead pollution sources in abandoned mine areas using stable Pb isotope ratios

This study focused on Pb isotope ratios of sediments in areas around an abandoned mine to determine if the ratios can be used as a source tracer. For pretreatment, sediment samples were dissolved with mixed acids, and a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS, Nu plasma II) was used to investigate the Pb isotopic composition of the samples. The measured isotope ratios were then corrected for instrumental mass fractionation by measuring the (203)Tl/(205)Tl ratio. Repeated measurements with the NIST SRM 981 reference material showed that the precision of all ratios was below 104 ppm (±2σ) for 50 ng/g. The isotope ratios ((207)Pb/(206)Pb) found were 0.85073 ± 0.0004~0.85373 ± 0.0003 for the main stream, while they were 0.83736 ± 0.0010 for the tributary and 0.84393 ± 0.0002 for the confluence. A binary mixing equation for isotope ratios showed that the contributions of mine lead to neighboring areas were up to 60%. Therefore, Pb isotope ratios can be a good source tracer for areas around abandoned mines.

Oxygen three-isotope ratios of silicate particles returned from asteroid Itokawa by the Hayabusa spacecraft: A strong link with equilibrated LL chondrites

Oxygen three-isotope ratios of four mineral phases (olivine, low-Ca and high-Ca pyroxene, and plagioclase) in seven silicate particles from asteroid Itokawa were measured to investigate oxygen isotope systematics of asteroidal materials. In order to obtain highly precise and accurate oxygen isotope ratios (∼±0.3‰ in δ18O), we used a new indium mounting method that minimizes potential instrumental mass fractionation due to surface topography. The oxygen isotope data of the seven Itokawa particles are distributed above the terrestrial fractionation line with indistinguishable ΔO17(=δO17−0.52×δO18) values of +1.34±0.36‰ (2SD; n=22), which is similar to those of Itokawa particles analyzed by Yurimoto et al. (2011) and Nakamura et al. (2012). However, δ18O values of the seven Itokawa particles are tightly clustered (+3.9‰ to +5.9‰ VSMOW) compared to previous analyses of the Itokawa particles (+1.4‰ to +8.8‰).The average Δ17O values and 2SD of the two equilibrated ordinary chondrites (EOCs), which were analyzed for comparison, are +1.31±0.49‰ for St. Séverin (LL6) and +0.83±0.64‰ for Guareña (H6). The Δ17O values of the seven Itokawa particles are similar to those of St. Séverin. The average bulk δ18O value of the seven Itokawa particles is estimated as +4.8‰, which is within the δ18O range of bulk LL5 and LL6 chondrites. It is suggested that Itokawa particles are similar to those of equilibrated LL chondrites, which strengthens a link between asteroid Itokawa and equilibrated LL chondrites that fell to Earth.The seven Itokawa particles and two EOCs have δ18O values of high-Ca pyroxene that are lower than those of olivine, indicating that oxygen isotopes between the two mineral phases are not in equilibrium. Apparent oxygen isotope equilibrium temperatures estimated from a pair of low-Ca pyroxene and olivine are excessively higher than those from a pair of plagioclase and high-Ca pyroxene (∼800 °C) and peak metamorphic temperatures. Oxygen isotopes in olivine and low-Ca pyroxene do not represent equilibrium. On the other hand, oxygen isotope ratios in high-Ca pyroxene and plagioclase were mass-dependently fractionated accompanied by recrystallization (and possibly by diffusion) during thermal metamorphism. Values of δ18O in high-Ca pyroxene were lowered by exchange, which may have resulted in the reverse δ18O fractionation between olivine and high-Ca pyroxene.

Accurate Mg/Ca, Sr/Ca, and Ba/Ca ratio measurements in carbonates by SIMS and NanoSIMS and an assessment of heterogeneity in common calcium carbonate standards

As archives of past climate variability, the micron and sub-micron scales of element:calcium (Me/Ca) variability in both biogenic and inorganic carbonates contain important geochemical information. Ideally working at smaller and smaller scales leads to higher temporal resolution of past changes, but more often it has revealed the strong overprint of other processes on the climate signal. Therefore, the role of SIMS and NanoSIMS techniques in studying paleoenvironmental proxies continues to increase. We evaluate the accuracy and precision of the CAMECA ims 7F-GEO and NanoSIMS-50L ion probes for measurements of Sr/Ca, Mg/Ca, and Ba/Ca ratios in carbonate minerals. Nine carbonate reference materials were examined for their 88Sr/42Ca, 24Mg/42Ca, and 138Ba/42Ca ratios using a primary O− beam with spot sizes of 20–40μm (SIMS) and 0.8–2μm (NanoSIMS). To assess accuracy, seven of these standards were analyzed for Sr/Ca and Mg/Ca with ID-ICP-MS. Variability in the elemental ratios arising from drift and changes in the tuning of the ims 7F-GEO over a nine month period is smaller than the chemical heterogeneity of the most frequently analyzed standards (OKA and Blue-CC). Across a whole crystal, Blue-CC is more homogeneous (1σ of 2.39% and 1.60% for Sr/Ca and Mg/Ca respectively) than OKA, but in the bulk matrix of OKA there is even less variability (1σ of 0.85% and 0.83% respectively). We find that carbonate samples can be accurately normalized to carbonate standards with significantly different absolute Me/Ca ratios. NanoSIMS intensity ratios follow counting statistics better than ±1% (2σ) at any one spot, but conversion to Me/Ca ratios increases the uncertainty. Two factors give rise to this limitation. First, the spatial heterogeneity of nominally homogeneous standards can lead to accuracy offsets that are as large as the ranges quoted above. Second, the NanoSIMS generates higher instrumental mass fractionation relative to SIMS. The combination of three different analytical techniques demonstrates that Blue-CC and homogeneous calcite zones in OKA are promising reference materials for precise analyses. Accuracy is crucially dependent on making independent measurements on exactly the same crystal of standard used in the SIMS and NanoSIMS machines.

Matrix Corrections and Error Analysis in High‐Precision SIMS18O/16O Measurements of Ca–Mg–Fe Garnet

We report technical and data treatment methods for making accurate, high‐precision measurements of 18O/16O in Ca–Mg–Fe garnet utilising the Cameca IMS 1280 multi‐collector ion microprobe. Matrix effects were similar to those shown by previous work, whereby Ca abundance is correlated with instrumental mass fractionation (IMF). After correction for this effect, there appeared to be no significant secondary effect associated with Mg/Fe2+ for routine operational conditions. In contrast, investigation of the IMF associated with Mn‐ or Cr‐rich garnet showed that these substitutions are significant and require a more complex calibration scheme. The Ca‐related calibration applied to low‐Cr, low‐Mn garnet was reproducible across different sample mounts and under a range of instrument settings and therefore should be applicable to similar instruments of this type. The repeatability of the measurements was often better than ± 0.2‰ (2s), a precision that is similar to the repeatability of bulk techniques. At this precision, the uncertainties due to spot‐to‐spot repeatability were at the same magnitude as those associated with matrix corrections (± 0.1–0.3‰) and the uncertainties in reference materials (± 0.1–0.2‰). Therefore, it is necessary to accurately estimate and propagate uncertainties associated with these parameters – in some cases, uncertainties in reference materials or matrix corrections dominate the uncertainty budget.

Evaluation of the effects of composition on instrumental mass fractionation during SIMS oxygen isotope analyses of glasses

Significant instrumental mass fractionation (IMF) occurs during measurements of oxygen isotope ratios in magmatic glasses by SIMS. In order to characterise and correct for this fractionation, we measured oxygen isotope ratios in a range of international and internal glass standards ranging in composition from basalt (47wt.% SiO2) to rhyolite (72wt.% SiO2) and with known major element compositions. Oxygen isotope ratios were determined by laser fluorination at SUERC, East Kilbride, or taken from previously published values. A total of 1105 δ18O measurements were made over nine sessions on a Cameca IMS-1270 ion microprobe at the University of Edinburgh. SIMS measurements on glass standards had external precision better than ±0.36‰ (1σ), and the reference material analysed alongside the unknown samples, USGS synthetic glass GSA-1G, had an average external precision of ±0.14‰. The selected standards are thus sufficiently homogeneous in δ18O to be suitable calibration standards. In terms of δ18O, the SIMS measurements show that, within a single session, IMF may vary by up to 4.7‰ from one glass standard to another. IMF is strongly correlated with SiO2 and CaO. A least squares regression calculation was used to explore potential univariate and multivariate correction schemes. For each correction scheme, the correction coefficients determined for each session were then used to calculate the IMF and correct the measured isotopic ratio of each glass standard. A univariate correction scheme using only SiO2 to correct for IMF reproduced 75% of the glass standards to within ±0.2‰ of their true δ18O, and 95% to within ±0.4‰. Bivariate correction schemes using SiO2–CaO and FeO–CaO produced similar results, but did not significantly improve on the SiO2 correction. The correction schemes were applied to δ18O measurements made on melt inclusions and glasses from the Askja volcanic system, North Iceland. The uni- and bivariate correction schemes tested produced δ18O values within the published range for Icelandic basalts. We recommend a simple correction scheme based on the SiO2 content of appropriate standards, which should span a suitable compositional range from basalt to rhyolite.