Double Spike Technique Research Articles

ダブルスパイクを用いた隕石試料の高精度84Sr/86Srおよび88Sr/86Sr比分析方法の開発

ダブルスパイクを用いた隕石試料の高精度84Sr/86Srおよび88Sr/86Sr比分析方法の開発

Strontium isotope fractionation of planktic foraminifera and inorganic calcite

We have investigated the strontium isotope fractionation (Δ88/86Srcarb–aq) between inorganic calcite and aqueous Sr2+ ions by precipitation experiments at a constant temperature of 25°C and precipitation rates (R) ranging from 102.3 to 104.2μmol/m2/h. Strontium isotope ratios were measured using the 87Sr–84Sr double spike technique. It was found that strontium isotope fractionation in these calcites is strongly dependent on the precipitation rate:Δ88/86Srcarb–aq=-0.08∗log(R[μmol/m2/h])+0.08The measured δ88/86Sr values are significantly correlated with previously measured δ44/40Ca and Sr/Ca values of the same calcite samples: Δ88/86Srcarb–aq=+0.18∗Δ44/40Cacarb–aq-0.01Δ88/86Srcarb–aq=-1.5∗KdSr-0.03 The slope of 88Sr/86Sr versus 44Ca/40Ca fractionation is 0.18±0.04 and compatible with a kinetic fractionation during dehydration of the strontium and calcium ions, but not with isotope fractionation in a diffusive boundary layer. Using published equilibrium Δ44/40Cacarb–aq and KdSr values we estimate the equilibrium isotope fractionation of strontium to be very close to zero (Δ88/86Sreq(carb–aq)=−0.01±0.06‰). This estimate is confirmed by strontium isotope values of natural inorganic calcites that precipitated very slowly in basalts of the ocean crust.The results from the inorganic calcites are used to explain strontium isotope fractionation of planktic foraminifera. Specimens of two warm water species (Globigerinoides ruber and Globigerinoides sacculifer) were picked from the Holocene section of a Caribbean sediment core. We found no significant difference in δ88/86Sr between the two species. In addition, G. ruber specimens from Marine Isotope Stage 2 in the same core show δ88/86Sr values identical to the Holocene specimens.The strontium isotopes of both foraminifera species are strongly fractionated (Δ88/86Srcarb–aq=−0.248±0.005‰) when compared to published data of other major marine calcifiers. Applying the results from the inorganic precipitation experiments we find that the strong foraminiferal strontium isotope fractionation can be explained by calcification in a largely open system at high precipitation rates, comparable in magnitude to rates known from scleractinian reef corals. This interpretation is in good agreement with the kinetic calcification model for planktic foraminifera by Kisakürek et al. (2011), which was based on calcium isotopes and elemental Sr/Ca ratios.

Optimizing sample and spike concentrations for isotopic analysis by double-spike ICPMS

Double spike techniques are widely used for measuring the isotopic composition of natural samples. In order to achieve the most accurate results by double spike analysis, it is important to choose an appropriate double-spike composition, analyte concentration, and spike to natural ratio (Cspk/Cnat) where Cnat is the concentration of a sample or standard with a natural abundance of the isotopes and Cspk is the concentration of an added spike with an unnatural isotope composition. Here, the effect of varying these parameters is explored using a Monte Carlo technique which simulates error from counting statistics, Johnson noise, and isobaric interferences. Typically, optimal spike composition and Cspk/Cnat are calculated under the constraint that total concentration of spike plus sample (Cspk + Cnat) must remain constant, so that as the amount of double spike is increased, the amount of sample is decreased. In practice, there is no reason for Cspk + Cnat to be held constant, because an analyst with a fixed quantity of sample may add any amount of spike to this sample as long as detector limits are not exceeded. Therefore, here, double spikes are here optimized while allowing Cspk and Cnat to vary independently. For thirty three different elements, this new approach of allowing Cspk and Cnat to vary independently led to a decrease in theoretical error of up to ∼30% in the absence of isobaric interferences. In the presence of isobaric interferences, this approach can deliver even larger improvements in accuracy and precision. Theoretical error is then compared to observed error both for δ56Fe standards and for δ56Fe, δ66Zn, and δ114Cd measured in seawater. Theoretical error and measured error for real seawater samples are highly correlated, with 78%, 85%, and 96% of observed error in δ56Fe, δ66Zn, and δ114Cd, respectively, accounted for using an error model which includes only Johnson noise and counting statistics. This confirms that such models, which minimize theoretical error, can be used to optimize spike composition, Cspk, and Cnat in order to increase accuracy and precision for analysis of natural samples.

Determination of mass-dependent variations in nickel isotope compositions using double spiking and MC-ICPMS

We present a new technique for the accurate and precise determination of mass-dependent variations in nickel isotope compositions in geological materials. Our method involves an ion-exchange procedure comprising three columns and utilising the ability of Ni to form strong complexes with both ammonia and dimethylglyoxime. The separation procedure is independent of sample pH and works even for samples with large matrix to analyte ratios. Processed Ni solutions are free of matrix elements and direct isobars of Ni, and the yield is normally 85–95%. The purified Ni solutions were analysed using a Nu Plasma, multi-collector inductively coupled plasma mass spectrometer (MC-ICPMS), where instrumental mass fractionation—together with potential isotopic fractionation during chemical separation due to incomplete yield—was corrected for by a double-spike technique, where samples were spiked prior to column chemistry. Tests performed on both mixtures of synthetic and natural terrestrial standards demonstrates that the method is accurate. Replicate measurements of USGS reference materials (peridotite PCC-1, basalt BHVO-2, and shale SCo-1) yield a long-term external reproducibility (2 s.d.) of typically ± 0.07‰, ± 0.1‰, and ± 0.14‰ for 60Ni/58Ni, 61Ni/58Ni, and 62Ni/58Ni respectively.

Precise determination of triple Sr isotopes (δ87Sr and δ88Sr) using MC-ICP-MS

The non-traditional stable strontium (Sr) isotopes have received increasing attention recently as new geochemical tracers for studying Sr isotopic fractionation and source identification. This has been attributed to the advancement in multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS), allows to determine precisely and simultaneously of the triple Sr isotopes. In this study, we applied a modified empirical external normalization (EEN) MC-ICPMS procedure for mass bias correction in Sr isotopic measurement using 92Zr/90Zr. High-purity Zr Standard was spiked into sample solutions and the degree of fractionation was calculated off-line using an exponential law. The long-term external reproducibility for NIST SRM 987 δ87Sr and δ88Sr was better than 0.040‰ and 0.018‰ (2SD), respectively. The IAPSO standard seawater was used as a secondary standard to validate the analytical protocol and the absolute ratios measured were 0.709161±0.000018 for 87Sr/86Sr, 0.177±0.021‰ for δ87Sr, and 0.370±0.026‰ for δ88Sr (2SD, n=7). These values are in good agreement with the literature data analyzed by thermal ionization mass spectrometry (TIMS) double spike technique. Rock standards, BHVO-2, BCR-2 and AGV-2 were also analyzed to validate the robustness of the methodology and showed identical results with literature data. Compared to previous 91Zr/90Zr correction, we obtained improved results based on 92Zr/90Zr, probably due to similar mass difference between 92Zr/90Zr and measured Sr isotopes. The new analytical protocol presented in this study not only improves the analytical precision but also increases sample efficiency by omitting the use of the standard-sample bracketing (SSB) procedure.

Zinc isotope fractionation analyses by thermal ionization mass spectrometry and a double spiking technique

The aim of this work was to develop thermal ionization mass spectrometry (TIMS) isotopic procedures to measure Zn isotope fractionation (δZn) in natural materials. This work represents the most recent development of Zn isotope measurements and the first δZn identification in terrestrial materials using TIMS and a double spike technique. The developed procedures evaluate and solve several critical analytical issues involved in TIMS Zn isotope analysis. For example, no more than 1 μg Zn was used for isotopic analyses which, considering the high ionization potential and low thermal ionization of Zn, represents a useful breakthrough in Zn isotope TIMS analysis. The effect of the ion exchange process on δZn was assessed and found equal to +0.07 ± 0.02‰ amu −1 per column. The ionization efficiency of Zn was enhanced to 0.22 ± 0.07%, which is four times more than what was achieved previously. The magnitude of δZn accompanied by the 95% confidence associated uncertainties were calculated relative to the IRMM 3702, using a Monte Carlo approach for each individual analysis, while the calculated average of δZn for number of analysis was accompanied by a 95% confidence calculated using GUM Workbench software. δZn values where always calculated using two different sets of isotopes which always agreed within uncertainty. These developments enabled sub-per mil δZn to be revealed relative to δZn zero for natural materials. Most of the samples measured are Standard Reference Materials SRMs, where, except for BCR-1 and BIR-1, this is the first time Zn isotopic fractionation has been measured in these samples. No previously published results for Zn isotopic fractionation have been published on these samples using double spiking. Consistent δZn of ∼+ 0.3‰ amu −1 was found in 5 sediments from a range of localities. δZn in two metamorphic samples is similar to that found in igneous rocks but different to that found in sedimentary rocks, which is consistent with our understanding that high temperature and pressure processes do not fractionate the composition of chalcophile elements. The isotope fractionation of Zn in a clay sample is within uncertainties the same as the sediments. The isotope fractionation of Zn of −0.088 ± 0.070‰ amu −1 was also measured in a standard rice sample. δZn in Antarctic Krill of +0.21 ± 0.11‰ amu −1 was found to be similar to the average δZn of +0.281 ± 0.083‰ amu −1 for marine sediments. River water was fractionated by −1.09 ± 0.70 ‰ amu −1, while restrained tap water yielded the maximum isotope fractionation of −6.39 ± 0.62‰ amu −1. δZn in high pure Zn standard materials ranged from −5.11 ± 0.36‰ amu −1 for AE 10760 to +0.12 ± 0.16‰ amu −1 for Zn IRMM 10440 with some evidence for a relationship between Zn isotope fractionation and its purity. All of the measured isotope fractionation yields an atomic weight within the IUPAC atomic weight of Zn.

Isotope fractionation and concentration measurements of Zn in meteorites determined by the double spike, IDMS‐TIMS techniques

Abstract– The isotope fractionation of Zn in meteorites has been measured for the first time using thermal ionization mass spectrometry and a double spiking technique. The magnitude of δZn ranged from −0.29 to +0.38‰ amu−1 for five stone meteorites whereas the iron meteorite Canyon Diablo displays δZn of 1.11 ± 0.11‰ amu−1. The results for chondrites in this work can be divided into positive and negative δZn, supporting a previous proposal that chondrites are a mixture of materials from two different temperature sources. The Zn isotope fractionation present in meteorites may represent a primordial heterogeneity formed in the early solar system. An anomalous isotopic composition of Zn obtained for the Redfields iron meteorite suggests large‐scale inherited isotope heterogeneity of the protosolar nebula, or the presence of a parent body that has formed within its own isotopically anomalous reservoir. These anomalies are in the same direction but smaller than nuclear field shift effects observed in chemical exchange reactions. The isotope dilution mass spectrometry (IDMS) technique was used to measure Zn concentration, yielding a range from 20.1 μg g−1 to 302 μg g−1 in five stone meteorites and from 0.019 to 26 μg g−1 in seven iron meteorites. The IDMS‐measured abundance of Zn in Orgueil is 302 ± 14 μg g−1 and should be considered for future compilations of the abundance of Zn in the solar system.

Measurement of zinc stable isotope ratios in biogeochemical matrices by double-spike MC-ICPMS and determination of the isotope ratio pool available for plants from soil.

Analysis of naturally occurring isotopic variations is a promising tool for investigating Zn transport and cycling in geological and biological settings. Here, we present the recently installed double-spike (DS) technique at the MAGIC laboratories at Imperial College London. The procedure improves on previous published DS methods in terms of ease of measurement and precisions obtained. The analytical method involves addition of a 64Zn–67Zn double-spike to the samples prior to digestion, separation of Zn from the sample matrix by ion exchange chromatography, and isotopic analysis by multiple-collector inductively coupled plasma mass spectrometry. The accuracy and reproducibility of the method were validated by analyses of several in-house and international elemental reference materials. Multiple analyses of pure Zn standard solutions consistently yielded a reproducibility of about ±0.05‰ (2 SD) for δ66Zn, and comparable precisions were obtained for analyses of geological and biological materials. Highly fractionated Zn standards analyzed by DS and standard sample bracketing yield slightly varying results, which probably originate from repetitive fractionation events during manufacture of the standards. However, the δ66Zn values (all reported relative to JMC Lyon Zn) for two less fractionated in-house Zn standard solutions, Imperial Zn (0.10 ± 0.08‰: 2 SD) and London Zn (0.08 ± 0.04‰), are within uncertainties to data reported with different mass spectrometric techniques and instruments. Two standard reference materials, blend ore BCR 027 and ryegrass BCR 281, were also measured, and the δ66Zn were found to be 0.25 ± 0.06‰ (2 SD) and 0.40 ± 0.09‰, respectively. Taken together, these standard measurements ascertain that the double-spike methodology is suitable for accurate and precise Zn isotope analyses of a wide range of natural samples. The newly installed technique was consequently applied to soil samples and soil leachates to investigate the isotopic signature of plant available Zn. We find that the isotopic composition is heavier than the residual, indicating the presence of loosely bound Zn deposited by atmospheric pollution, which is readily available to plants. FigureZinc isotope ratio pools of bulk soil and the associated acid leach (estimated plant available pool) as measured by double-spike MC-ICPMS. δxZnLyon-JMC=(Rsample/RJMC-Lyon -1)x103, where Rsample and RJMC-Lyon denote the xZn/64Zn isotope ratio of the sample and standard (JMC-Lyon), respectively, and where x denotes either 66 or 68.

Efficiency of the double spiking technique in Pb mass spectrometric analysis for geochronology

Efficiency of the double spiking technique in Pb mass spectrometric analysis for geochronology

Molybdenum isotope mass fractionation in iron meteorites

An examination of the isotope fractionation of molybdenum in a range of iron meteorites has been carried out by thermal ionization mass spectrometry incorporating the double spike technique. Ten iron meteorites were dissolved in hydrochloric acid and double spiked with a 92Mo– 98Mo mixture of enriched isotopes. An ion exchange-based chemical separation process was then used to remove any vestiges of zirconium and ruthenium from the samples, so as to eliminate any possibility of isobaric interferences, and to provide a final molybdenum extract suitable for mass spectrometric analysis. The molybdenum from each extracted meteorite was analysed under similar mass spectrometric conditions to a Laboratory Standard. The double spike technique enabled the magnitude of the isotope fractionation in the meteorites to be determined with high accuracy relative to the Laboratory Standard, whose absolute isotopic composition is known. The ten iron meteorites gave isotope fractionations in the range −0.5‰ to +1.2‰/mass unit with respect to the terrestrial standard, the isotopic composition of which is identical, within experimental uncertainty, to other terrestrial samples.

Calcium Isotopes (δ44/40Ca) in MPI‐DING Reference Glasses, USGS Rock Powders and Various Rocks: Evidence for Ca Isotope Fractionation in Terrestrial Silicates

We report δ44/40Ca(SRM 915a) values for eight fused MPI‐DING glasses and the respective original powders, six USGS igneous rock reference materials, the U‐Th disequilibria reference material TML, IAEA‐CO1 (Carrara marble) and several igneous rocks (komatiites and carbonatites). Sample selection was guided by three considerations: (1) to address the need for information values on reference materials that are widely available in support of interlaboratory comparison studies; (2) support the development of in situ laser ablation and ion microprobe techniques, which require isotopically homogenous reference samples for ablation; and (3) provide Ca isotope values on a wider range of igneous and metamorphic rock types than is currently available in the scientific literature. Calcium isotope ratios were measured by thermal ionisation mass spectrometry in two laboratories (IFM‐GEOMAR and Saskatchewan Isotope Laboratory) using 43Ca/48Ca‐ and 42Ca/43Ca‐double spike techniques and reported relative to the calcium carbonate reference material NIST SRM 915a. The measurement uncertainty in both laboratories was better than 0.2‰ at the 95% confidence level. The impact of different preparation methods on the δ44/40Ca(SRM 915a) values was found to be negligible. Except for ML3‐B, the original powders and the respective MPI‐DING glasses showed identical δ44/40Ca(SRM 915a) values; therefore, possible variations in the Ca isotope compositions resulting from the fusion process are excluded. Individual analyses of different glass fragments indicated that the glasses are well homogenised on the mm scale with respect to Ca. The range of δ44/40Ca(SRM 915a) values in the igneous rocks studied was larger than previously observed, mostly owing to the inclusion of ultramafic rocks from ophiolite sections. In particular, the dunite DTS‐1 (1.49 ± 0.06‰) and the peridotite PCC‐1 (1.14 ± 0.07‰) are enriched in 44Ca relative to volcanic rocks (0.8 ± 0.1‰). The Carrara marble (1.32 ± 0.06‰) was also found to be enriched in 44Ca relative to the values of assumed precursor carbonates (< 0.8‰). These findings suggest that the isotopes of Ca are susceptible to fractionation at high temperatures by, as yet, unidentified igneous and metamorphic processes.

The double spike toolbox

The double spike technique is a well established method for correcting for instrumental mass fractionation in mass spectrometry. The precision of the technique is controlled by the choices of double spike composition and the proportions in which the double spike and sample are mixed. To make these choices easier, we provide software (“the double spike toolbox”) for calculating optimal double spikes, which are chosen purely on the basis of minimising error propagation. In addition, we provide “cocktail lists” of optimal double spikes for all 33 elements that have 4 or more naturally occurring isotopes, using some sensible default parameters. As examples, we discuss the application of the software to Fe, Pb, and Ca isotopes.

High performance automated ion chromatography separation for Ca isotope measurements in geological and biological samples

Natural mass-dependent fractionation of calcium isotopes is a promising tool for investigating Ca pathways and cycling in geological and biological materials. But since natural isotope fractionation of Ca appears to be extremely limited (∼1.25‰/amu), excellent external precision and sensitivity are needed to make full use of its potential. Here, we describe a new Ca purification procedure that consists of a high selectivity automated ionic chromatography separation protocol, which is suitable for Ca isotope measurements by mass spectrometry and applicable to multiple natural matrixes (waters, mineral and organic samples). The analytical progress in this automated technique are multiple: (1) saving time with a minimum of handling, (2) unique operating protocol whatever the nature of the sample, (3) complete separation of Ca from K, Mg and Sr, avoiding isobaric interferences which are critical during TIMS analysis, and (4) Ca separation by peak recognition optimising the full recovery of Ca even if its retention time is shifted from one sample to another. The two latter advantages ensure a Ca recovery yield close to 100%, leading to the absence of any fractionation of Ca isotopes during the chemical clean-up. Thus, this chemical separation will be of special interest for applications not compatible with the use of the double spike technique such as MC-ICP-MS and 40Ca excesses measurements. Additionally this procedure leads to a twofold improvement of the long-term repeatability of the Ca isotopes determination by TIMS (±0.11 δ44/40Ca, 2SD) as compared with a classical resin chemistry protocol and is similar to the best repeatability published so far (±0.10 δ44/40Ca, 2SD).

Mass-dependent cadmium isotopic variations in nature with emphasis on the marine environment

We report a survey of natural mass-dependent cadmium isotope fractionation measured by thermal ionization mass spectrometry using a double-spike technique (DS-TIMS). Over sixty samples of natural terrestrial Cd from diverse environments, including MORB, OIB, continental loess, hydrogenic and hydrothermal ferromanganese deposits, and sphalerites (both oceanic and from major continental ore deposits) were analysed. Our results are expressed in terms of ε 112/110Cd, which are deviations in 112Cd/ 110Cd from our in-house JMC Cd standard in parts per 10 4. The total ε 112/110Cd variation is relatively small, with a range of only 5 ε-units, and is one-to-two orders of magnitude smaller than that previously found in meteorites. The MORB, OIB and loess ε 112/110Cd values are similar and provide a good estimate for the bulk silicate Earth (BSE) value which is − 0.95 ± 0.12 relative to our Cd standard (ε 112/110Cd = + 0.16 relative to Münster JMC Cd). Taken together, these data suggest little Cd isotope fractionation takes place during crust–mantle segregation. Cd isotopic compositions of continental sphalerite (ZnS) deposits worldwide and high-temperature oceanic hydrothermal sulphides show remarkably similar ε 112/110Cd values, consistent with our estimate for the BSE. In contrast, mid-temperature oceanic sulphides from a single extinct hydrothermal chimney display over 4 ε-units variation — along with the most negative values. These variations are most probably caused by precipitation/redissolution of sulphide phases en route within the hydrothermal system. The ε 112/110Cd variability found in worldwide marine Fe–Mn deposits reflects the seawater Cd isotope signal upon precipitation from ambient seawater. A decrease in ε 112/110Cd is observed in passing from shallow-water Fe–Mn deposits to those from deeper waters (> 2000 m depth). This shift is explained by biological fractionation related to the uptake of dissolved seawater Cd by phytoplankton in the upper water column. The relatively uniform ε 112/110Cd values close to zero at great depths are consistent with regeneration and remineralization of Cd at depth. Our data suggest that Cd isotopes – much like the Cd/Ca ratio in foraminifera – could potentially serve as a proxy for past changes in biological productivity. The temporal Cd isotope record in a Fe–Mn crust archive at 2000 m depth from the NE Atlantic suggests no gross long-term changes in Cd cycling took place over the past 8 Ma.

Seasonal δ 34S variations in two high elevation snow pits measured by 33S– 36S double spike thermal ionization mass spectrometry

δ 34S and sulfate concentrations were determined in snow pit samples using a thermal ionization mass spectrometric technique capable of 0.2‰ accuracy and requires ≈5 μg (0.16 μmol) natural S. The technique utilizes a 33S– 36S double spike for instrumental mass fractionation correction, and has been applied to snow pit samples collected from the Inilchek Glacier, Kyrgyzstan and from Summit, Greenland. These δ 34S determinations provide the first high-resolution seasonal data for these sites, and are used to estimate seasonal sulfate sources. Deuterium (δD) and oxygen (δ 18O) isotope data show that the Inilchek and Summit snow pit samples represent precipitation over ≈20 months. The δ 34S values for the Inilchek ranged from +2.6 ± 0.4‰ to +7.6 ± 0.4‰ on sample sizes ranging from 0.3 to 1.8 μmol S. δ 34S values for Greenland ranged from +3.6 ± 0.7‰ to +13.3 ± 5‰ for sample sizes ranging from 0.05 to 0.29 μmol S. The SO 4 2 - concentration ranged from 92.6 ± 0.4 to 1049 ± 4 ng/g for the Inilchek and 18 ± 9 to 93 ± 6 ng/g for the Greenland snow pit. Anthropogenic sulfate dominates throughout the sampled time interval for both sites based on mass balance considerations. Additionally, both sites exhibit a seasonal signature in both δ 34S and SO 4 2 - concentration. The thermal ionization mass spectrometric technique has three advantages compared to gas source isotopic methods: (1) sample size requirements of this technique are 10-fold less permitting access to the higher resolution S isotope record of low concentration snow and ice, (2) the double spike technique permits δ 34S and S concentration to be determined simultaneously, and (3) the double spike is an internal standard.

Evaluation of the readsorption of plutonium and americium in dynamic fractionations of environmental solid samples

A dynamic extraction system exploiting sequential injection (SI) for sequential extractions incorporating a specially designed extraction column is developed to fractionate radionuclides in environmental solid samples such as soils and sediments. The extraction column can contain up to 5 g of a soil sample, and under optimal operational conditions it does not give rise to creation of back pressure. Attention has been placed on studies of the readsorption problems during sequential extraction using a modified Standards, Measurements and Testing (SM&T) scheme with four-step sequential extractions. The degree of readsorption in dynamic and conventional batch extraction systems is compared and evaluated by using a double-spiking technique. A high degree of readsorption of plutonium and americium (>75%) was observed in both systems, and they also exhibited similar distribution patterns of the two radionuclides. However, the dynamic system is fully automated, eliminates manual separations, significantly reduces the operational time required, and offers detailed kinetic information.

The stable Cr isotope inventory of solid Earth reservoirs determined by double spike MC-ICP-MS

We present the first comprehensive set of stable Cr isotope data for the major igneous silicate Earth reservoirs, Cr(III)-rich ores and minerals, and hydrothermal chromates. These were determined by MC-ICP-MS using a double spike technique. Mantle xenoliths, ultramafic rocks, cumulates, as well as oceanic and continental basalts share a common Cr isotope composition with an average δ 53/52Cr value of − 0.124 ± 0.101‰ (2 SD) relative to the isotopically certified chromium standard NIST SRM 979. An isotopic difference between mantle xenoliths and basalts, as was reported for iron, was not observed for chromium. Thus, the change in oxidation state that is observed when solid mantle rocks, containing only trivalent chromium, partially melt to form basaltic melts, which predominantly contain bivalent chromium, does not cause any measurable Cr isotope fractionation. Chromite separates from major chromitite seams of the Bushveld and Great Dyke layered igneous complexes are invariable in their Cr isotope compositions and reproduce within uncertainties the average δ 53/52Cr SRM 979 value of igneous silicate Earth reservoirs. This is important for environmental stable Cr isotope studies, because it labels the approximate isotope composition of industrial Cr(VI) pollutants. The Cr isotope compositions of hydrothermal lead chromates (crocoites PbCr VIO 4) from various localities yield δ 53/52Cr SRM 979 values between 0.640 and 1.037‰; these Cr isotope compositions are substantially heavier than those of igneous silicate rocks from which the chromium was leached. Precipitation experiments revealed an isotope fractionation of Δ 53/52Cr (crocoite–Cr(VI)aq) of only ca. + 0.10 ±0.05‰ at a temperature of 20 °C. Thus, the heavy Cr isotope signature of crocoites is most likely the result of repeated redox cycling of chromium in hydrothermal processes.

Precise determination of cadmium isotope fractionation in seawater by double spike MC-ICPMS

A new technique has been developed for the accurate and precise determination of the stable Cd isotope composition of seawater. The method utilizes a 110Cd– 111Cd double spike, and it involves separation of Cd from seawater by column chromatography and isotopic analyses by multiple collector inductively coupled plasma mass spectrometry. As a by-product, it also generates precise Cd concentration data. Repeated analyses of three pure Cd reference materials and three seawater samples yielded reproducibilities of about ±1.0 to ±1.6 ε 114/110Cd (2 SD), based on measurements that each consumed about ∼8 ng of natural Cd ( ε 114/110Cd is the deviation of the 114Cd/ 110Cd isotope ratio of a sample from the standard in parts per 10,000). This demonstrates that the new double spike technique is superior to published methods of Cd isotope analyses, with regard to the acquisition of precise data for samples of limited size. Additional experiments showed that as little as 1–5 ng of seawater Cd could be analyzed with a precision of about ±2 to ±6 ε 114/110Cd (2 SD). The accuracy of the seawater isotope data was ascertained by experiments in which a Cd-free seawater matrix was doped with small quantities of isotopically well-characterized Cd. Repeated mass scans that were carried out on purified Cd fractions of several samples furthermore demonstrated the absence of significant spectral interferences. The isotope data that were acquired for the three seawater samples reveal, for the first time, small but resolvable Cd isotope fractionations in the marine environment. Cadmium-rich intermediate water from the North Pacific was found to have an isotope composition of ε 114/110Cd = 3.2 ± 1.0. In contrast, Cd-depleted seawater from the upper water column of the Atlantic and Arctic Oceans displayed isotope compositions of ε 114/110Cd = 6.4 ± 1.1 and 6.6 ± 1.6, respectively. These observations are in accord with the interpretation that the isotope effects are due to the biological fractionation that occurs during the uptake of dissolved seawater Cd by phytoplankton.

Accurate Isotopic and Concentration Analyses of Small Amounts of Pb Using Isotope Dilution Coupled with the Double Spike Technique

A new method has been developed for the simultaneous determination of Pb abundance and Pb isotopic composition with high precision and accuracy for small test portion masses by thermal ionisation mass spectrometry. In this method, a 205pb‐204pb double spike is added to samples prior to the chemical separation of Pb, and the isotopic composition of the spike‐sample mixture is determined rigorously by the double spike technique using a 207Pb‐204Pb spike. The isotopic composition and concentration of Pb in the sample are then obtained by utilising the principle of isotope dilution. Using this technique, replicate determinations of Pb from NIST SRM 981 and GSJ JP‐1 (peridotite; 0.07 μg g−1 Pb) were performed. The measured concentration and isotopic data were identical, within uncertainty, to published data or to data that were determined independently in this study. The application of this method to U‐Pb dating and the determination of the “initial” Pb isotopic composition was also tested. Lead isotopic compositions and the concentrations of Pb, Th and U were determined for a single batch of samples, through the addition of 205pb‐204pb, 230Th and 235U spikes to samples prior to chemical separation. Also in these experiments, we confirmed that this routine gives accurate data for Pb, Th and U concentrations and Pb isotopic compositions.

Cadmium isotopic composition in the ocean

The oceanic cycle of cadmium is still poorly understood, despite its importance for phytoplankton growth and paleoceanographic applications. As for other elements that are biologically recycled, variations in isotopic composition may bring unique insights. This article presents (i) a protocol for the measurement of cadmium isotopic composition (Cd IC) in seawater and in phytoplankton cells; (ii) the first Cd IC data in seawater, from two full depth stations, in the northwest Pacific and the northwest Mediterranean Sea; (iii) the first Cd IC data in phytoplankton cells, cultured in vitro. The Cd IC variation range in seawater found at these stations is not greater than 1.5 ε Cd/amu units, only slightly larger than the mean uncertainty of measurement (0.8 ε Cd/amu). Nevertheless, systematic variations of the Cd IC and concentration in the upper 300 m of the northwest Pacific suggest the occurrence of Cd isotopic fractionation by phytoplankton uptake, with a fractionation factor of 1.6 ± 1.4 ε Cd/amu units. This result is supported by the culture experiment data suggesting that freshwater phytoplankton ( Chlamydomonas reinhardtii and Chlorella sp.) preferentially take up light Cd isotopes, with a fractionation factor of 3.4 ± 1.4 ε Cd/amu units. Systematic variations of the Cd IC and hydrographic data between 300 and 700 m in the northwest Pacific have been tentatively attributed to the mixing of the mesothermal (temperature maximum) water ( ε Cd/amu = −0.9 ± 0.8) with the North Pacific Intermediate Water ( ε Cd/amu = 0.5 ± 0.8). In contrast, no significant Cd IC variation is found in the northwest Mediterranean Sea. This observation was attributed to the small surface Cd depletion by phytoplankton uptake and the similar Cd IC of the different water masses found at this site. Overall, these data suggest that (i) phytoplankton uptake fractionates Cd isotopic composition to a measurable degree (fractionation factors of 1.6 and 3.4 ε Cd/amu units, for the in situ and culture experiment data, respectively), (ii) an open ocean profile of Cd IC shows upper water column variations consistent with preferential uptake and regeneration of light Cd isotopes, and (iii) different water masses may have different Cd IC. This isotopic system could therefore provide information on phytoplankton Cd uptake and on water mass trajectories and mixing in some areas of the ocean. However, the very small Cd IC variations found in this study indicate that applications of Cd isotopic composition to reveal aspects of the present or past Cd oceanic cycle will be very challenging and may require further analytical improvements. Better precision could possibly be obtained with larger seawater samples, a better chemical separation of tin and a more accurate mass bias correction through the use of the double spiking technique.