External Reproducibility Research Articles

In situ petrographic thin section U–Pb dating of zircon, monazite, and titanite using laser ablation–MC–ICP-MS

A laser ablation–multiple collector–inductively coupled plasma mass spectrometry (LA–MC–ICP-MS) analytical protocol is used to date accessory minerals (zircon, monazite, and titanite) at high spatial resolution (5–40 μm) using standard petrographic thin sections. The MC–ICP-MS instrument is equipped with a modified collector array containing a combination of Faraday buckets and multiple ion counters, which produces accurate and precise geochronological data using small sample volumes (pit depth ≤2 μm at 5 μm and ≤15 μm at 40 μm spot sizes). Standardization and normalization factors for the 206Pb/ 238U and 207Pb/ 235U values are calculated based on well-characterized external mineral standards previously dated by high precision ID-TIMS analysis. During an analytical session, the 2 σ relative standard deviation (i.e., external reproducibility) for the 206Pb/ 238U and 207Pb/ 235U values is ≤3%. The measured 207Pb/ 206Pb value is simultaneously corrected for instrumental mass bias by the aspiration of a Tl solution resulting in a 2 σ relative standard deviation of between ∼0.3 and 1%. The accuracy of the analytical protocol was verified on petrographic thin sections of several samples previously dated by ID-TIMS. The capacity of this new, ‘small volume’ in situ dating technique to provide contextual, relatively rapid and accurate age information is a substantial improvement in reconnaissance-style studies of geological areas with scarce geochronological age information.

A calculation method to eliminate gain effect on isotopic measurement using the virtual amplifier multi-collector mass spectrometer

The virtual amplifier design employed in the ThermoElectron multi-collector mass spectrometers is a novel innovation for high precision isotopic measurement. With the virtual amplifiers, uncertainties of gain factors of amplifiers can be effectively averaged out, so that accuracy and external reproducibility on isotopic measurement can be improved. This paper presents a new calculation method on isotopic measurement using the virtual amplifier mass spectrometers such as ThermoElectron Triton and Neptune. With the calculation method, gain factors of the amplifiers can be completely eliminated out for both raw measured isotopic ratios and normalized isotopic ratios when exponential or power law is used to correct isotope fractionation effect, and therefore, gain calibration is unnecessary even for the high precision measurement purpose.

Quantitative extraction and high precision isotope measurements of nickel by MC-ICPMS

This paper presents a new technique for the precise and accurate determination of Ni isotopic composition in geological samples, involving a two-stage ion-exchange procedure for nickel extraction and the use of a large-geometry high-resolution MC-ICPMS instrument. The key step of the chemical separation is an ion-exchange procedure based on the complexation of Ni with dimethylglyoxime which replaces the previous liquid–liquid extraction technique. The new method allows for quantitative recovery of Ni even for silicate samples as opposed to the previous technique. Measurements of an Ni standard doped with Fe or Zn show that isobaric interferences of these elements can be accurately corrected as long as Fe/Ni < 0.1 and Zn/Ni < 0.0006. Replicate measurements of a standard solution yield long-term external reproducibilities (2σ) of ±50 ppm for 60Ni/58Ni and ±120 ppm for 61Ni/58Ni, when ratios are normalized to 62Ni/58Ni.

Iron isotopes in natural carbonate minerals determined by MC-ICP-MS with a 58Fe– 54Fe double spike

We have developed a method for iron isotope analysis by multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) using a 58Fe– 54Fe double spike. A 20 min analysis produces mass-bias-corrected iron isotope data with an external reproducibility of ±0.05 (2 SD) on δ 56Fe, which represents a decrease in analysis time compared to sample-standard bracketing techniques. The estimation of external reproducibility is based on replicate analysis of the ETH hematite in-house standard. The double spike method has two advantages. First, matrix effects during MC-ICP-MS analysis are decreased with tests showing that accurate iron isotope data can, in some cases, be obtained even when matrix levels exceed iron concentration (Na/Fe, Mg/Fe, and Ca/Fe up to 5, 2, and 0.1, respectively). Because chemical separation reduces matrix/Fe to levels more than three orders of magnitude lower than this, measured Fe isotope compositions are unlikely to be compromised by matrix effects. Second, it is possible to spike samples before chemical purification, which enables any isotopic fractionation effect because of incomplete recovery of iron from a sample to be accounted for. This may be important where obtaining quantitative iron yields from samples is difficult, such as the extraction of dissolved iron from water samples. Fe isotope data on a set of standard reference materials (igneous rocks, ferromanganese nodules, sedimentary rocks, and ores) are presented, which are in agreement with previously published data considering analytical uncertainties. Mantle-derived standard rock samples that are the source of iron for surficial, (bio)geochemical cycling yield a mean δ 56Fe of 0.041 ± 0.11‰ ( n = 8; 2 SD) with reference to IRMM-14. Hydrothermal and metamorphic calcium carbonate rocks with a relatively low iron content (100–4000 ppm) have δ 56Fe = −1.25 to −0.07‰. Structural Fe(II) in hydrothermal calcites has δ 56Fe = −1.25 to −0.27‰. The light iron in this range of carbonate minerals may reflect the iron isotope composition of the hydrothermal fluids from which the carbonate precipitated, or the presence of Fe(III) and/or organic material in the hydrothermal fluids during calcite precipitation.

SIMS analyses of oxygen isotopes: Matrix effects in Fe–Mg–Ca garnets

Large instrumental mass fractionation (IMF) may occur during measurements of oxygen isotope ratios by SIMS. Part of this fractionation depends on crystal structure and mineral composition. In order to improve the accuracy of SIMS measurements, we gathered 6 commonly used garnet standards and prepared 6 others to adequately cover the composition range Alm 0–73, Prp 0–99, Grs 0–20. Electron microprobe analyses were performed at UBP-Clermont to check the chemical homogeneity of these standards. Oxygen isotope compositions were determined by laser fluorination and mass spectrometry at UW-Madison. Ten SIMS sessions and 336 δ 18O measurements at CRPG-Nancy, on a Cameca IMS1270 instrument, demonstrate that the standards are homogeneous with external reproducibility of 0.3‰ (1 σ). In terms of δ 18O, SIMS measurements indicate that, during a single session, IMF can vary up to 6.3‰ from one garnet standard to another. In most of the sessions, IMF can be correlated with the grossular content. However, for a satisfactory correction scheme, we suggest the combination of the 3 main components (Ca, Fe, Mg). This is done using a simple least square calculation routine. The correction coefficients determined for each session can be used to calculate the IMF and correct the measured isotopic ratio of a garnet of known chemical composition. This way, we were able to reproduce the δ 18O values of most of the Fe–Mg–Ca garnet standards within ± 0.6‰. Interestingly, the use of only 3 end-member standards (AlmCMG, PrpMM, GrsSE) plus a standard of intermediate composition (e.g. UWG-2) is sufficient to reproduce δ 18O within the same precision. Thus, linear interpolation among end-member standards is satisfactory in the case of the garnet solid-solutions. Two studies carried out on zoned garnets from the Alps and the Pyrenees indicate that matrix effects become significant when variations in grossular contents are important (> 10%). In order to obtain reliable isotope ratio measurements on Fe–Mg–Ca garnets using a SIMS, we suggest a correction scheme using at least 3 reliable end-member standards plus a standard of intermediate composition (a garnet standard closest to the average composition of the analysed garnet). This allows cross-checking and incorporates a correction based on the variations in composition of zoned crystals.

High precision Faraday collector MC-ICPMS thorium isotope ratio determination

Uranium-series dating of carbonate materials requires precise determination of the spike sample thorium isotope ratio, 230Th/ 229Th. This ratio is commonly measured using ion counting techniques, however the precision of analyses using ion counting devices suffers from beam intensity limitations, drift in multiplier gain and non-linearities in electron multiplier response. Here, we describe the application of multiple-collector inductively coupled plasma mass spectrometry (MC-ICPMS) to determine thorium isotope ratios at hitherto unattained precision. For the first time, thorium isotope analyses were performed using only Faraday collectors coupled to 10 11 Ω feedback resistors in the amplifier system. Spiked thorium solutions were concentrated to produce 230Th and 229Th signal intensities of around 50 mV and 100 mV, respectively (across a 10 11 Ω resistor) and are run at high intensity for a short period of time (∼1 min). These analyses yield a 230Th/ 229Th external reproducibility of better than 0.3‰ for ∼25–30 pg of consumed 230Th. This is a factor of two better than the best published thermal ionisation mass spectrometry (TIMS) and MC-ICPMS techniques for similar sample sizes, and represents up to an order of magnitude improvement over many other established protocols. Combined with new techniques for high precision Faraday measurement of uranium isotopic composition [1], this permits improvements in the uncertainty of U-series ages to better than 0.1 thousand years (ka) at 100 ka and 1 ka at 300 ka. It should also be possible to resolve events to ∼14 ka at 600 ka. Using these techniques, the U-series dating limit can be extended from 500–600 ka to 800 ka enabling a more detailed study of the frequency of late Pleistocene climate events.

Separation of U, Pb, Lu, and Hf from single zircons for combined U–Pb dating and Hf isotope measurements by TIMS and MC-ICPMS

The U–Pb system has long been used to precisely date zircons because the high U-to-common-Pb ratio in zircon produces very radiogenic Pb isotope compositions over time. In contrast to U/Pb, zircon has very low Lu/Hf and therefore unradiogenic Hf, making this mineral ideally suited for determining the initial Hf composition of its original host rock. A new chemical separation technique presented here enables the determination of both U–Pb age and initial Hf isotope composition of individual zircon grains. The acquisition of such complementary information for single detrital zircons is especially useful for provenance analyses and crustal growth studies. Zircons are spiked with mixed 176Lu– 180Hf and 233U– 205Pb tracers and then digested in HF–HNO 3. Lead, Lu, U, and Hf are sequentially separated from the zircon matrix on a single ion exchange column filled with Eichrom® Ln Spec resin. Using only ∼100 μl of resin for the separation keeps Pb blanks low (∼5 pg) while achieving better than 90% yields for each of the four elements. Hafnium isotope compositions and Lu concentrations are measured with multiple collector-inductively coupled plasma-mass spectrometry (MC-ICPMS), whereas U and Pb are analyzed by thermal ionization mass spectrometry (TIMS). The minimum grain size that can be processed is dictated by the amounts of Pb and Hf needed for an analysis. The smallest grains we currently analyze, as small as 50 μm (∼12pg of Pb and ∼3ng of Hf), can be analyzed with an external 176Hf/ 177Hf precision of ∼100 ppm (2 s.d.). The utility of this method is demonstrated with a population of detrital zircons from a Cambrian sediment of the Takaka Terrane, New Zealand. In addition, the technique has been used for 14 analyses of the standard zircon 91500, which yield a mean present-day 176Hf/ 177Hf of 0.282305 ± 12 (2 s.d., i.e., an external reproducibility of 43 ppm). The Hf isotopic compositions and U–Pb ages presented here are in good agreement with those of previous studies (e.g., Wiedenbeck, M., Alle, P., Corfu, F., Griffin, W.L., Meier, M., Oberli, F., von Quadt, A., Roddick, J.C., Spiegel, W., 1995. Three natural zircon standards for U-Th-Pb, Lu-Hf, Trace element and REE analyses. Geostandards Newsletter 19, 1-23.), but our data suggest that the 91500 zircon is heterogeneous with respect to Lu and Hf contents and Lu/Hf.

ZINC ISOTOPE VARIATION IN HYDROTHERMAL SYSTEMS: PRELIMINARY EVIDENCE FROM THE IRISH MIDLANDS ORE FIELD

Little is known about the range or controls on the zinc isotope composition of terrestrial materials and no systematic studies have been carried out on ore-forming systems. We have obtained zinc isotope data from 19 sphalerite samples, formed over a range of well-constrained precipitation conditions, from the Irish Zn-Pb ore field. The results reveal variation in δ 66Zn (where δ 66Zn = [(66Zn/64Zn)sample/(66Zn/64Zn)standard– 1] × 1000), from –0.17 to 1.33 per mil relative to the Lyon JMC 3-0749L zinc standard. This variation is significant compared to the external reproducibility (±0.12‰, 2 σ ), and the data show very good mass-dependency with δ 67Zn and δ 68Zn values. Thus, natural variations in the zinc isotope composition of these ore minerals can be resolved. Our results span the entire range of δ 66Zn values measured on terrestrial geologic samples to date. The data suggest that variations in the primary source rock composition or precipitation temperature are unlikely to be important controls on the zinc isotope composition of sphalerite in the ore field. We suggest that the variation is most likely due to a kinetic fractionation involving the preferential incorporation of light zinc isotopes in sphalerite precipitated rapidly under disequilibrium conditions. However, we cannot rule out the possibility of mixing of zinc derived from two isotopically distinct sources. The significant variation in zinc isotope compositions we have observed in the Irish ore field confirms that such fractionations can provide new insights into mineralizing processes in the Earth’s crust.

Accurate measurement of silver isotopic compositions in geological materials including low Pd/Ag meteorites

Very precise silver (Ag) isotopic compositions have been determined for a number of terrestrial rocks, and high and low Pd/Ag meteorites by utilizing multicollector inductively coupled plasma mass spectrometry (MC-ICPMS). The meteorites include primitive chondrites, the Group IAB iron meteorites Canyon Diablo and Toluca, and the Group IIIAB iron meteorite Grant. Silver isotopic measurements are primarily of interest because 107Ag was produced by decay of the short-lived radionuclide 107Pd during the formation of the solar system and hence the Pd-Ag chronometer has set constraints on the timing of early planetesimal formation. A 2σ precision of ±0.05‰ can be obtained for analyses of standard solutions when Ag isotopic ratios are normalized to Pd, to correct for instrumental mass discrimination, and to bracketing standards. Caution must be exercised when making Ag isotopic measurements because isotopic artifacts can be generated in the laboratory and during mass spectrometry. The external reproducibility for geological samples based on replicate analyses of rocks is ±0.2‰ (2σ). All chondrites analyzed have similar Ag isotopic compositions that do not differ significantly (>0.3‰) from the ‘terrestrial’ value of the NIST SRM 978a Ag isotope standard. Hence, they show no evidence of excess 107Ag derived from 107Pd decay or, of stable Ag isotope fractionation associated with volatile element depletion within the accretion disk or from parent body metamorphism. The Group IAB iron meteorite samples analyzed show evidence of complex behavior and disturbance of Ag isotope systematics. Therefore, care must be taken when using this group of iron meteorites to obtain chronological information based on the Pd-Ag decay scheme.

External factors and reproducibility of the behaviour test in German shepherd dogs in Switzerland

The Swiss German Shepherd Club (SC) has applied a standardised behaviour test for over 50 years. A successful test is a prerequisite for breeding approval. The aim of the study was to investigate the influence of external factors like socialisation, husbandry, training and others on the results of the behaviour test, and to verify if these results were still consistent after a year. The tested traits were self-confidence, nerve stability, hardness, sharpness, defence drive, reaction to gunfire, and temperament. Information about husbandry, training, socialisation, and the dog's behaviour in certain situations, etc. was collected by a questionnaire. From a total of 185 owners, 149 handlers with their dogs were willing to take part in this study. After 1 year, 38 dogs were tested a second time and their owners filled out another questionnaire very similar to the first one. Logistic regression analyses were used to measure the association between the results of the behaviour traits and the different external factors. Training of the young dog and contact with school aged children were significantly associated with one or more of the behaviour traits. Significant odds ratios were found for the associations between the puppy training and nerve stability and self-confidence, as well as between young dog training and the same character traits (nerve stability and self-confidence). A further positive association was found between defence drive and the contact of the dogs with school age children. Reproducibilities of the results of the behaviour test varied between traits, so the average scores for sharpness and defence drive significantly increased from the first to the second test, for temperament however, the scores decreased. Lower scores meant a more desired behaviour as rated by the club. The results of the other traits were similar in the two tests.

An assessment of the accuracy of stable Fe isotope ratio measurements on samples with organic and inorganic matrices by high-resolution multicollector ICP-MS

Multicollector ICP-MS-based stable isotope procedures provide the capability to determine small variations in metal isotope composition of materials, but they are prone to substantial bias introduced by inadequate sample preparation. Such a “cryptic” bias is not necessarily identifiable from the measured isotope ratios. The analytical protocol for Fe isotope analyses of organic and inorganic materials described here identifies and avoids such pitfalls. In medium-mass resolution mode of the ThermoFinnigan Neptune MC-ICP-MS, a 1-ppm Fe solution with an uptake rate of 50–70 μL min −1 yielded 3 × 10 −11 A on 56Fe for the ThermoFinnigan stable introduction system and 1.2–1.8 × 10 −10 A for the ESI Apex-Q uptake system. Sensitivity was increased again 3–5-fold when using Finnigan X-cones instead of the standard H-cones. The combination of the ESI Apex-Q apparatus and X-cones allowed the determination of the isotope composition on as little as 50 ng of Fe. Fe isotope compositions were corrected for mass bias with both the standard-sample bracketing (SSB) method, and by using the 65Cu/ 63Cu ratio of added synthetic copper (Cu-doping) as internal monitor of mass discrimination. Both methods provide identical results on high-purity Fe solutions of either synthetic or natural samples. We prefer the SSB method because of its shorter analysis time and more straightforward correction of instrumental mass bias compared to Cu-doping. Strong error correlations of the data are observed in three isotope diagrams. Thus, we suggest that the quality assessment in such diagrams should be performed with error ellipses rather than error bars. Reproducibility of δ 56Fe, δ 57Fe and δ 58Fe values of natural samples alone is not a sufficient criterion for accuracy. A set of tests is lined out that identify cryptic matrix effects and ensure a reproducible level of quality control. Using these criteria and the SSB correction method, we determined the external reproducibilities for δ 56Fe, δ 57Fe and δ 58Fe at the 95% confidence interval from 318 measurements of 95 natural samples to be 0.049, 0.071 and 0.28‰, respectively.

Mercury isotope fractionation in fossil hydrothermal systems

The Hg isotopic compositions of samples throughout the vertical extent of two fossil hydrothermal systems were analyzed by multicollector inductively coupled plasma-mass spectrometry. Results show >5‰ (δ 2 0 2 Hg/ 1 9 8 Hg; relative to NIST 3133) fractionation, more than 50 times greater than the 0.1‰ (2σ) external reproducibility of the analyses. The Hg isotope compositions from both hydrothermal systems can be grouped by dominant mineralogy and position; δ 2 0 2 Hg/ 1 9 8 Hg values at the tops of the systems are -3.5‰ to -0.4‰ in cinnabar-dominant sinter and -0.2‰ to +2.1‰ in metacinnabar-dominant sinter, and the underlying veins have δ 2 0 2 Hg/ 1 9 8 Hg values of -1.4‰ to +1.3‰. These differences probably resulted from the combination of boiling of the hydrothermal fluid, oxidation near the surface, and kinetic effects associated with mineral precipitation. The natural variation in Hg isotopic compositions observed in this study is higher than that expected from the trend of decreasing mass-dependent fractionation with increasing mass extrapolated from stable isotope systems up to Z = 26 (Fe), confirming that even the heaviest elements undergo significant stable isotope fractionation in hydrothermal systems.

Measurement of plutonium isotope ratios in nuclear fuel samples by HPLC-MC-ICP-MS

Radioactive isotopes are traditionally quantified by means of radioactivity counting techniques (α, β, γ). However, these methods often require extensive matrix separation and sample purification before the identification of specific isotopes and their relative abundance is possible as it is necessary in the frame of post-irradiation examinations on nuclear fuel samples. The technique of multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) is attracting much attention because it permits the precise measurement of the isotope compositions for a wide range of elements combined with excellent limits of detection due to high ionization efficiencies. The present paper describes one of the first applications of an online high-performance liquid chromatographic separation system coupled to a MC-ICP-MS in order to overcome isobaric interferences for the determination of the plutonium isotope composition and concentrations in irradiated nuclear fuels. The described chromatographic separation is sufficient to prevent any isobaric interference between 238Pu present at trace concentrations and 238U present as the main component of the fuel samples. The external reproducibility of the uncorrected plutonium isotope ratios was determined to be between 0.04 and 0.2% (2 s) resulting in a precision in the ‰ range for the isotopic vectors of the irradiated fuel samples.

Neodymium isotope fractionation in the mass spectrometer and the issue of 142Nd anomalies in early Archean rocks

Efforts to search for 142Nd excesses in ancient rocks with high apparent ε Nd 143 over 10 years ago yielded conflicting results and indicated that better mass spectrometers/techniques needed to be devised to resolve the anomalies. Recent searches again give apparently conflicting data: Caro et al. [Caro, G., Bourdon, B., Birck, J.L., Moorbath, S., 2003. 146Sm– 142Nd evidence from Isua metamorphosed sediments for early differentiation of the Earth's mantle. Nature 423, 428–432.] use a new generation Thermal ionization mass spectrometer (Triton) and find that all rocks from 3.7 to 3.8 Ga Isua Greenstone Belt, West Greenland give +15 ppm excesses in 142Nd/ 144Nd ratio over Ames Nd standard. In contrast, Papanastassiou et al. [Papanastassiou, D.A., Sharma, M., Ngo, H.H. Wasserburg, G.J., Dymek, R.F., 2003. No 142Nd excess in early Archean Isua gneiss IE 715-28. Lunar Planet. Sci. Conf., p. 1851.] using another Triton find that an Isua felsic gneiss (IE 715-28; initial ε Nd 143 = + 4.0 ) contains no 142Nd excess over CIT nNd-β standard. Whereas it is possible to imagine early Archean rocks with variable 142Nd anomalies there appears to be a critical issue of the manner in which the two sets of data were reduced. Papanastassiou et al. (2003) used the exponential law to correct for mass dependent isotope fractionation. Caro et al. (2003) found that they needed to use a second-order correction to remove the observed positive correlation between exponential law normalized 142Nd/ 144Nd and 150Nd/ 144Nd ratios. Such a correlation between average ratios of a suite of samples could arise from non-ideal mass fractionation correction if the samples are variably fractionated. It could also be a result of ion optical aberrations. In this paper we investigate if, during thermal ionization, there is a systematic fine structure to the observed mass dependent isotope fractionation of Nd isotopes, which violates the exponential law. We performed nineteen high precision Nd isotopes analyses of CIT nNd-β. The data were obtained for a limited range of mass fractionation (±0.1% per amu), which permited to isolate the fine structure from ion optical effects. The raw data were corrected for mass fractionation using the Rayleigh and exponential laws. Both the Rayleigh and exponential law normalized data sets give an external reproducibility of 6 ppm (2 σ). We also find that while there is no positive relationship between exponential law normalized 142Nd/ 144Nd and 150Nd/ 144Nd ratios, a positive correlation exists between 148Nd/ 144Nd and 150Nd/ 144Nd; the latter is found to be associated with ion beam growth and possibly related to effects arising from fractionation and mixing of different reservoirs on the filament. We conclude that if the data are obtained for a limited range of fractionation and the mixing between enriched and depleted reservoirs on the filament is minimized (by controlling the beam glowth), the exponential law (or Rayleigh law) would provide adequate correction for mass fractionation so that a second-order correction would not be necessary. No ion optical aberrations were observed at 6 ppm level. By analyzing enriched standards we are certain that we can resolve a 142Nd anomaly of ≥+10 ppm. We confirm that the level of 142Nd excess in IE 715-28 cannot be resolved from the baseline nNd-β standard. As IE 715-28 is a metasedimentary rock likely reflecting derivation from older continental crustal material with a high average initial ε Nd 143 , the absence of 142Nd anomaly in this rock indicates that highly depleted ( f Sm/Nd > 0.3) reservoirs were present in the late Hadean (<∼4.25 Ga). Such reservoirs were short-lived.

Development of high precision Rb–Sr phlogopite and biotite geochronology; an alternative to 40Ar/ 39Ar tri-octahedral mica dating

Potassium–Ar and Rb–Sr dating of minerals was fundamental in early efforts to date magmatic and metamorphic processes and paved the way for geochronology to become an important discipline within the earth sciences. Although K–Ar and, in particular, 40Ar/ 39Ar dating of micas is still widely applied, Rb–Sr dating of micas has declined in use, even though numerous studies demonstrated that tri-octahedral mica yields geologically realistic, and more reliable and reproducible Rb–Sr ages than the K–Ar or 40Ar/ 39Ar system. Moreover, a reduction of uncertainties typically reported for Rb–Sr ages (ca. 1%) can now be achieved by application of multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS) rubidium isotope dilution measurements (<0.3%). Replicate Rb–Sr biotite ages from the Oslo rift, Norway, yield an external reproducibility of ±0.3% ( n=4) and an analytical error of ±0.8 Ma for individual ages that vary between 276.9 and 275.5 Ma. Conventional thermal ionisation mass spectrometry (TIMS) Rb analysis on the same mineral separates yields ages between 276.1 and 271.7 Ma, three times the spread compared to Rb MC-ICPMS data. Biotite and phlogopite from the central Nagssugtoqidian orogen, West Greenland, yield 40Ar/ 39Ar plateau ages (ca. 1700 Ma) with a spread of ±150 Ma, while Rb–Sr ages on either biotite or phlogopite separates have a much narrower range of ±10 Ma. This comparison of Rb–Sr and 40Ar/ 39Ar ages demonstrates the robustness of the Rb–Sr system in tri-octahedral micas and cautions against the sole use of 40Ar/ 39Ar tri-octahedral mica ages to date geological events. Analytical errors of 16 Ma for these Rb–Sr mica ages determined by TIMS are reduced to <±5 Ma when the Rb concentration is determined by MC-ICPMS. All the TIMS and MC-ICPMS data from the Nagssugtoqidian orogen agree within assigned analytical uncertainties. However, high precision Rb–Sr dating by MC-ICPMS can resolve geological information obscured by TIMS age determinations. TIMS data for seven phlogopite samples form an isochron age of 1645±6 Ma, and thus, no differentiation in age between the different samples can be made. In contrast, MC-ICPMS Rb measurements on the same samples reveal two distinct populations with ages of 1633±3 or 1652±5 Ma. Combining the mica Rb–Sr geochronological data with the well-constrained thermal history of this ancient orogen, we estimate the closure temperature of the Rb–Sr system in 1–2 mm slowly cooled phlogopite crystals, occurring in a matrix of calcite and plagioclase to be ∼435 °C, and at least 50 °C above that of biotite.

Pb isotopic analysis of standards and samples using a 207Pb– 204Pb double spike and thallium to correct for mass bias with a double-focusing MC-ICP-MS

A Pb isotope analytical method using a 207Pb– 204Pb double spike (DS) and a double-focusing MC-ICP-MS routinely yields standard and sample data with an external reproducibility <100 ppm for all Pb isotope ratios. With modest increases in analysis times (<10 min), external reproducibility of ≤50 ppm is attainable, e.g., SRM 981— 206Pb/ 204Pb=16.9418±6; 207Pb/ 204Pb=15.5000±6; 208Pb/ 204Pb=36.7265±19 ( n=26; 2 S.D.). This is achievable even when unspiked and spiked runs are analysed sequentially, demonstrating that memory due to use of the DS can be easily overcome by short and careful washout routines coupled with on-peak zeroes measurements. Replicate analyses of three mixtures of SRM 981 and SRM 982, in variable proportions, lie within 42 ppm of a mixing line between SRM 981 and SRM 982 Pb isotope ratios. Coupled use of thallium to correct for mass bias during the same analyses of standards and samples does not produce Pb isotopic data of comparable quality to the DS-corrected analyses. While the approach of Woodhead (A simple method for obtaining highly accurate Pb isotope data by MC-ICP-MS, J. Anal. At. Spectrom., 17 (2002) 1381–1385) allows us to produce accurate thallium-corrected Pb isotopic data, the DS data are at least five times more precise and accurate than those obtained when using thallium to correct for mass bias. Relatively rapid analytical throughput (20–25 samples/day), elimination of differential loading blanks between unspiked and spiked runs, easy optimal spiking and the lack of anomalous 207Pb behaviour as observed sometimes during TIMS analysis make DS analysis by MC-ICP-MS an attractive procedure. New Pb DS data on lavas from the Torfajökull volcanic centre (Iceland) define near-perfect binary mixing within our errors, even over a very small range in 207Pb/ 204Pb (<300 ppm), comparable to or smaller than the reproducibility of thallium-corrected Pb isotopic data. We also show that recently published Pb isotopic data for Theistareykir picrites (Stracke et al., Theistareykir revisited, Geochem. Geophys. Geosyst., (2003) Paper no. 2001GC000201), corrected for mass bias using thallium, are up to 3300 ppm inaccurate compared to our DS data and also that of Thirlwall (Interlaboratory and other errors in Pb isotope analyses investigated using a 207Pb– 204Pb double spike, Chem. Geol., 163 (2000) 299–322) and Thirlwall et al. (Mantle components in Iceland and adjacent ridges investigated using double-spike Pb isotope ratios, Geochim. Cosmochim. Acta., 68, (2004) 361–386). These Icelandic data highlight both the potential and the dangers of Pb isotopic analysis by MC-ICP-MS and demonstrate the need for all MC-ICP-MS Pb isotopic studies to include analyses of well-characterized and isotopically homogeneous rock standards in addition to SRM 981. To this end, we present ca. 100 analyses of international standard reference materials, including the NIST 610–612–614 glasses that are used for in situ Pb isotope analysis by laser ablation ICP-MS and SIMS. While the Pb isotope compositions of some standards show poor reproducibility due to variable contamination during initial preparation, four of the reference materials (JB-2, NOD-A-1, AGV-1, BCR-1) and the NIST 610 and 612 glasses reproduce to <200 ppm. We also note that Pb isotopes are fractionated during normal anion exchange chemical separation of Pb, as collection of Pb in HCl significantly fractionates Pb isotope ratios, with the lighter isotopes preferentially eluted first. Provided a sufficiently large volume of ≥6 M HCl is collected (>10× the resin volume), there is no measurable (<50 ppm) fractionation of the Pb isotopes as the light-isotope-depleted tail is fully stripped from the resin.

Toward epsilon levels of measurement precision on 234U/ 238U by using MC-ICPMS

Variations in 234U/ 238U have wide-ranging applications as tracers for ground- and river-water fluxes and is an essential component in U-series dating. Analytical developments for measuring 234U/ 238U have progressed from direct alpha-counting, with precisions at the percent level, to thermal ionization and multiple-collector inductively coupled plasma mass spectrometry (TIMS and MC-ICPMS, respectively) isotopic measurement techniques. However, 234U/ 238U is difficult to measure with better than permil precision because of the small atomic ratios for most geological samples (∼10 −4 range). Using a Nu Instruments Nu Plasma MC-ICPMS, we have developed two analytical techniques for the precise measurement of 234U/ 238U: (1) a conventional standard-bracketing protocol using multiple Faraday cups and electron multipliers with ion counting capabilities (FM) and (2) a standard-bracketing Faraday cup protocol (FF). Both are capable of measuring 234U/ 238U with precisions at the epsilon level (1 epsilon = 1 part in 10 4): (1) The conventional standard-bracketing FM measurements are conducted as static measurements with the minor 234U isotope measured in a conventional discrete dynode electron multiplier (SEM) equipped with ion counter and a retardation filter. The Faraday-multiplier gain is measured using bracketing measurements of the U metal standard CRM-145. The external reproducibility of 234U/ 238U (reformulated into δ-notation as δ 234U), interspersed with frequent measurements of the gain, is at the ±0.6‰ level (2 σ) for both uraninite and carbonate standards, takes ∼75 min and consumes ∼120 ng of U per measurement. (2) The static standard-bracketing FF protocol measures all three natural U isotopes in Faraday collectors. This is not usually possible using a standard multiple-Faraday array due to the large differences in the abundances of naturally occurring U isotopes. In our study, this is achieved by replacing the standard 10 11 Ω resistor for the 238U Faraday cup with a 10 9 Ω resistor. The 10 9 Ω resistor enables the measurement of ion beams that are ∼100 times larger than can be accommodated by the normal 10 11 Ω resistor, so 238U and 234U are measured simultaneously in Faraday cups with intensities of ∼9 × 10 −9 and ∼5 × 10 −13 A, respectively. All measurements are normalized to bracketing CRM-145 standard measurements (measured with similar 238U signal intensities) thereby correcting for significant tailing from the large 238U ion beam below the smaller 234U beam. Measurements are conducted over 2 min on-peak and 400–650 ng of 238U is required per analysis. External reproducibility for samples with low matrix/U ratios (e.g., uraninites) is better than ±0.3‰ (2 σ). Coral samples show a slightly poorer external reproducibility of ±0.4‰ (2 σ) due to a higher matrix/U ratios of these samples. Repeat measurements of CRM-145 give respective δ 234U values of −36.44 ± 0.10‰ (2 σ m, n = 9) and −36.50 ± 0.14‰ (2 σ m, n = 54) using the FF and FM analytical technique, assuming Harwell uraninite (HU-1) is in secular equilibrium with respect to 234U/ 238U. The improved analytical precision achieved in this study for 234U/ 238U measurement is superior to any other reported measurements and is of great importance for U-series dating errors, particularly for samples older than 300,000 years before present.

Isotopic variations of Zn in biological materials.

Variations in the isotopic composition of Zn present in various biological materials were determined using high-resolution multicollector inductively coupled plasma mass spectrometry (MC-ICPMS), following digestion and purification by anion exchange chromatography. To correct for differences in instrumental mass discrimination effects between samples and standards, Cu was employed as an elemental spike. Complementary analyses of Zn separates by sector field ICPMS instruments revealed that the concentrations of the majority of potentially interfering elements were reduced to negligible levels. Residual spectral interferences resulting from (35)Cl(16)O(2)(+), (40)Ar(14)N(2)(+), and (40)Ar(14)N(16)O(+) could be instrumentally resolved from the (67)Zn, (68)Zn, and (70)Zn ion beams, respectively, during measurement by MC-ICPMS. The only other observed interference in the Cu and Zn mass range that could not be effectively eliminated by high-resolution multicollection resulted from (35)Cl(2)(+), necessitating modification of the sample preparation procedure to allow accurate (70)Zn detection. Complete duplication of the entire analytical procedure for human whole blood and hair, as well as bovine liver and muscle, provided an external reproducibility of 0.05-0.12 per thousand (2sigma) for measured delta(66/64)Zn, delta(67/64)Zn, and delta(68/64)Zn values, demonstrating the utility of the method for the precise isotopic analysis of Zn in biological materials. Relative to the selected Zn isotopic standard, delta(66/64)Zn values for biological samples varied from -0.60 per thousand in human hair to +0.56 per thousand in human whole blood, identifying the former material as the isotopically lightest Zn source found in nature to date.

Accurate and High‐Precision Measurement of Lithium Isotopes in Two Reference Materials by MC‐ICP‐MS

We report here a newly developed method for measurement of Li isotopes by use of multi‐collector ICP‐MS (Neptune) allowing rapid and high precision determination of Li isotope ratios at low levels of lithium (15–20 ng). The lithium reference sample solution IRMM‐016 was analysed over a period of ten months with an external reproducibility of 0.24% (2s, n = 52). Chemical separation of Li from matrix was performed on the seawater sample IRMM BCR‐403, for which a mean δ7Li value of + 31.0 ± 0.1 % (2s/√n, n = 31) was obtained. This mean value is in good agreement with those previously published for other seawater samples. BCR‐403 seawater being readily available, we propose that this seawater sample be used as a reference sample for Li isotope measurements.

The precise and accurate determination of thallium isotope compositions and concentrations for water samples by MC-ICPMS

We present optimised analytical protocols for the precise and accurate determination of Tl concentrations and isotope compositions (IC) for water samples by multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS). The Tl abundance measurements utilise the isotope dilution (ID) technique. About 50 ml of seawater or river water, with ∼150 to 500 pg of Tl, are spiked with a 203Tl tracer and thallium is then separated from the sample matrix using a single anion-exchange column. The isotope compositions of spiked samples are measured by MC-ICPMS using either admixed Pb or Pt for mass bias correction. Replicate analyses of three natural water samples with Tl contents of 3 to 10 pg/g displayed external reproducibilities (1 S.D.) of 0.2% to 1.5%. The determination of Tl isotope compositions requires a minimum of 5 ng of Tl, equivalent to about 0.5 to 1.5 l of seawater or river water. Separation of Tl is achieved with a two-step anion-exchange procedure that generates Tl separates that are essentially Pb-free and that display very low overall matrix contents. The Tl isotope compositions are determined by MC-ICPMS using admixed NIST SRM 981 Pb for the mass discrimination correction. With these techniques, replicate analyses of a seawater and a river water sample displayed an external precision (2 S.D.) of about ±0.1‰. A detailed study was carried out to identify possible sources of systematic error. These investigations indicate that the Tl stable isotope data are not biased by blanks, chemical fractionation during laboratory handling, mass spectrometric errors, spectral interferences, matrix effects, or use of an unsuitable method of mass bias correction. Therefore, the Tl isotope data can be assumed to be accurate, to within the stated uncertainties. Analyses of the international seawater standard NASS-5 were carried out, to provide a reference for future Tl isotope studies. This sample was found to have a Tl concentration of 9.37±0.02 pg/g and a Tl isotope composition of ε 205Tl=−5.0±1.0 (ε 205Tl is the deviation of the 205Tl/ 203Tl isotope ratio of a sample from the NIST SRM 997 Tl isotope standard in parts per 10 4).