Multiple Collector Inductively Coupled Plasma Mass Spectrometer Research Articles

Simultaneous In Situ Determination of Pb Isotope Ratios and Trace Element Concentrations in Melt Inclusions by LASS‐ICP‐MS

AbstractMelt inclusions provide unique insights into magmatic sources and processes. This study presents a method for simultaneously measuring Pb isotopes and trace elements in melt inclusions using a 193 nm excimer laser ablation system coupled to a multiple‐collector inductively coupled plasma‐mass spectrometer (MC‐ICP‐MS) and a sector‐field ICP‐MS (SF‐ICP‐MS). The ablated aerosol was split with a Y‐shaped connector into two gas streams, which were simultaneously transported into two mass spectrometers. Lead isotopes were measured on the MC‐ICP‐MS, while trace element concentrations were determined on the SF‐ICP‐MS. Little analytical bias was observed when the aerosol is divided into variable proportions (5:5, 6:4, 7:3, 8:2–9:1) to the two instruments. The precision and accuracy of this method were evaluated by analyzing five geological reference glasses with two laser spot diameters (33, and 45 μm). All the 33 measured trace element concentrations are consistent with the reference values (±10%) and have precisions less than 15% (2RSD). The precisions of the 208Pb/206Pb, and 207Pb/206Pb ratios correlate with Pb concentrations. From BHVO‐2G (Pb = 1.6 ppm) to TB‐1G (Pb = 16.1 ppm), the precision on these ratios improve from about 0.4%–0.2% with a 33 μm laser spot diameter. The mean Pb isotope ratios for the five reference glasses are within 0.15% of the reference values. Using our new method, Pb isotope ratios and trace element contents of plagioclase‐hosted melt inclusions in a mid‐ocean ridge basalt from the Indian Ocean were measured.

Effects of different cone combinations on accurate and precise determination of Li isotopic composition by MC-ICP-MS

The multiple collector inductively coupled plasma mass spectrometer (MC-ICP-MS) is booming to be a high-precision, fast, and accurate instrument in measuring lithium (Li) isotopes. Modified highly sensitive Jet sample and X skimmer cones have largely upgraded analytical sensitivity and reduced sample consumption with distinct instrumental mass bias behaviors. Herein, four available combinations of the sample and skimmer cones [Jet sample cone + X skimmer cone (Jet + X), Standard sample cone + X skimmer cone (Standard + X), Standard sample cone + H skimmer cone (Standard + H), and Jet sample cone + H skimmer cone (Jet + H)] were tested for their effects on peak shape, sensitivity, mass bias behavior, and accuracy and precision of Li isotopic measurements in solution mode. The results showed that all four combinations were able to attain an ideal peak shape by adjusting the associated parameters, with a positively linear relation between 7Li (and 6Li) signals and Li concentrations. For a given Li concentration, the sensitivities were enhanced 3–7 times using the Standard + X, the Jet + H, and the Jet +X combinations compared to that of using the Standard + H. The enhanced sensitivity is attributable to more plasma ions introduced into the mass spectrometer through the Jet and X cones. Mass bias is distinct for different cone combinations, with a slight difference with various Li concentrations, indicating the neccesity of concentration match during measurement. Among the four cone combinations, the best reproducibility can be obtained by using the Standard + X cones to determine precise and accurate Li isotopes for samples. Furthermore, a pretreatment by 5% HNO3 and 0.1% HF, and then 2% HNO3 prior to analyses can minimize the memory effect of Li.

Decoupled carbonate chemistry controls on the incorporation of boron into <i>Orbulina universa</i>

Abstract. In order to fully constrain paleo-carbonate systems, proxies for two out of seven parameters, plus temperature and salinity, are required. The boron isotopic composition (δ11B) of planktonic foraminifera shells is a powerful tool for reconstructing changes in past surface ocean pH. As B(OH)4− is substituted into the biogenic calcite lattice in place of CO32−, and both borate and carbonate ions are more abundant at higher pH, it was suggested early on that B ∕ Ca ratios in biogenic calcite may serve as a proxy for [CO32−]. Although several recent studies have shown that a direct connection of B ∕ Ca to carbonate system parameters may be masked by other environmental factors in the field, there is ample evidence for a mechanistic relationship between B ∕ Ca and carbonate system parameters. Here, we focus on investigating the primary relationship to develop a mechanistic understanding of boron uptake. Differentiating between the effects of pH and [CO32−] is problematic, as they co-vary closely in natural systems, so the major control on boron incorporation remains unclear. To deconvolve the effects of pH and [CO32−] and to investigate their impact on the B ∕ Ca ratio and δ11B, we conducted culture experiments with the planktonic foraminifer Orbulina universa in manipulated culture media: constant pH (8.05), but changing [CO32−] (238, 286 and 534 µmol kg−1 CO32−) and at constant [CO32−] (276 ± 19.5 µmol kg−1) and varying pH (7.7, 7.9 and 8.05). Measurements of the isotopic composition of boron and the B ∕ Ca ratio were performed simultaneously using a femtosecond laser ablation system coupled to a MC-ICP-MS (multiple-collector inductively coupled plasma mass spectrometer). Our results show that, as expected, δ11B is controlled by pH but it is also modulated by [CO32−]. On the other hand, the B ∕ Ca ratio is driven by [HCO3−], independently of pH. This suggests that B ∕ Ca ratios in foraminiferal calcite can possibly be used as a second, independent, proxy for complete paleo-carbonate system reconstructions. This is discussed in light of recent literature demonstrating that the primary relationship between B ∕ Ca and [HCO3−] can be obscured by other environmental parameters.

Evaluation of lead isotope compositions of NIST NBS 981 measured by thermal ionization mass spectrometer and multiple-collector inductively coupled plasma mass spectrometer

Because Pb isotopes can be used for tracing, they are widely used in many disciplines. The detection and analysis of Pb isotopes of bulk samples are usually conducted using thermal ionization mass spectrometer (TIMS) and multiple-collector inductively coupled plasma mass spectrometer (MC-ICP-MS), both of which need external reference materials with known isotopic compositions to correct for the mass discrimination effect produced during analysis. NIST NBS 981 is the most widely used reference material for Pb isotope analysis; however, the isotopic compositions reported by various analytical laboratories, especially those using TIMS, vary from each other. In this study, we statistically evaluated 229 reported TIMS analysis values collected by GeoReM in the last 30 years, 176 reported MC-ICP-MS analysis values, and 938 MC-ICP-MS analysis results from our laboratory in the last five years. After careful investigation, only 40 TIMS results were found to have double or triple spikes. The ratios of the overall weighted averages, 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb, obtained from 40 spiked TIMS reports and 1114 MC-ICP-MS results of NIST NBS 981 isotopes were 16.9406 ± 0.0003 (2s), 15.4957 ± 0.0002 (2s), and 36.7184 ± 0.0007 (2s), respectively.

An estimate of the Bulk Silicate Earth potassium isotopic composition based on MC-ICPMS measurements of basalts

We report a method for high-precision potassium isotope measurements that improves by an order of magnitude the precision compared to previous methods. The purification of K is achieved through ion exchange chromatography on AG50W-X8 cation exchange resins. The 41K/39K ratios are analyzed with a GV Instruments IsoProbe P Multiple-Collector Inductively Coupled Plasma Mass Spectrometer (MC-ICP-MS) equipped with a hexapole collision gas cell. With this technique, the argon-hydride interferences as well as the large 40Ar+ peak from the Ar-plasma are eliminated. The 41K/39K ratios can thus be obtained with a precision of 0.05permil. The precision and accuracy of this method is validated with gravimetrically determined 41K or 39K enriched standards. New precise K isotopic compositions of three terrestrial basalts (BCR-2, BHVO-1 and a MORB) are also reported. These basalts have indistinguishable K isotopic compositions and are used to define a reference value of −0.479±0.027permil for the 41K/39K ratio of the Bulk Silicate Earth (BSE) relative to a Merck Suprapur potassium nitrate standard. Seawater and sylvites from evaporite deposits have similar 41K/39K ratios, but higher by up to 0.227permil compared to the Merck Suprapur standard. Other commercially available K-salts/solutions also have 41K/39K ratios similar to the Merck standard, while a SPEX K-chloride was higher by 0.384permil. This shows that K isotope variations will be useful as a tracer in low-temperature geochemistry processes.

The anomalous Ca cycle in the Ediacaran ocean: Evidence from Ca isotopes preserved in carbonates in the Three Gorges area, South China

The Ediacaran–Cambrian was one of the most important periods for the evolution of life. Recent studies have provided detailed chemostratigraphies that decipher the linkages between ambient surface environmental changes and biological evolution. The occurrence of skeletal fossils in Ediacaran rocks suggests that the possible onset of Ca-biomineralization started in the latest Neoproterozoic. Molecular clocks also predict the emergence of animals containing Ca-carbonate spicules in the Neoproterozoic. Therefore, it is important to estimate the transition of the Ca cycle in seawater. Ancient Ca cycles in the oceans are estimated from the calcium isotopic compositions of carbonate rocks. However, the lack of continuous Ca isotopic data in the Ediacaran leaves the Ca cycle unresolved.The almost continuously exposed, Ediacaran and Cambrian strata in South China mainly comprise carbonate rocks with subordinate black shales and sandstones, which contain many fossils, suitable for the study of environmental and biological changes. We conducted drilling from the Liantuo, via the Nantuo, Doushantuo, Dengying and Yanjiahe to the Shuijingtuo formation at four sites in the Three Gorges area in order to obtain continuous, fresh samples without surface alteration and oxidation. We analyzed 44Ca/42Ca ratios in carbonate rocks with a multiple collector-inductively coupled plasma-mass spectrometer (MC-ICP-MS) screened for diagenetic alteration.The resultant δ44/42Ca ratios exhibit a smooth curve. The values of δ44/42Ca are anomalously high relative to those in the Phanerozoic, and they gradually decrease towards the end of the Ediacaran. Our new Ca isotope chemostratigraphy suggests that a different Ca cycle had existed during the Ediacaran. The long-term, high δ44/42Ca is not simply explicable by changes in temperature, differences in carbonate mineralogy, or changes in the input/output fluxes of Ca to the ocean. We propose three possible explanations for the observed high δ44/42Ca ratios in the Ediacaran: high δ44/42Ca ratios of Ca inputs, potential undiscovered sinks of Ca with low δ44/42Ca values, and negligible isotope fractionation between carbonate and seawater; we consider that one or all of these contributed to the high δ44/42Ca ratios. Of these, we prefer the small isotope fractionation, which implies that the Ca concentration in seawater was initially low in the early Ediacaran and increased throughout that period.

High-Precision Measurement of Variations in Calcium Isotope Ratios in Urine by Multiple Collector Inductively Coupled Plasma Mass Spectrometry

We describe a new chemical separation method to isolate Ca from other matrix elements in biological samples, developed with the long-term goal of making high-precision measurement of natural stable Ca isotope variations a clinically applicable tool to assess bone mineral balance. A new two-column procedure utilizing HBr achieves the purity required to accurately and precisely measure two Ca isotope ratios ((44)Ca/(42)Ca and (44)Ca/(43)Ca) on a Neptune multiple collector inductively coupled plasma mass spectrometer (MC-ICPMS) in urine. Purification requirements for Sr, Ti, and K (Ca/Sr > 10 000; Ca/Ti > 10 000 000; and Ca/K > 10) were determined by addition of these elements to Ca standards of known isotopic composition. Accuracy was determined by (1) comparing Ca isotope results for samples and standards to published data obtained using thermal ionization mass spectrometry (TIMS), (2) adding a Ca standard of known isotopic composition to a urine sample purified of Ca, and (3) analyzing mixtures of urine samples and standards in varying proportions. The accuracy and precision of δ(44/42)Ca measurements of purified samples containing 25 μg of Ca can be determined with typical errors less than ±0.2‰ (2σ).

Improved method for Hf separation from silicate rocks for isotopic analysis using Ln-spec resin column

An improved chemical separation method for Hf isotope ratio measurement using both the thermal ionization mass spectrometer (TIMS) and the multiple collector-inductively coupled plasma-mass spectrometer (MC-ICP-MS) is presented in this paper. In the first column (2.5-ml Eichrom Ln-spec resin), Hf cut was collected with 2M HCl-0.2M HF after washing the major elements, HREE, Ti, Nb, and Zr. For further Hf purification, the second column (1-ml Ln-spec resin) chemistry was conducted in a manner similar to that of the first column. The first column is designed for treatment with a 0.5-g silicate rock sample for TIMS analysis. Thus, because the Hf amount required for MC-ICP-MS analysis is much lesser than that required for TIMS analysis, the column chemistry for MC-ICP-MS analysis can be scaled down, depending on the amount of digested sample. Although there is a need to improve the TIMS technique, the TIMS Hf data obtained for geological reference rocks and Ethiopian flood basalts after the application of the proposed separation methods are consistent, within analytical error, with the previously reported data obtained using the MC-ICP-MS. The advantages of the proposed method include a reduction in the amount of reagents used (hence, a consequent reduction in the blank contribution), reduction in the time required, and a simplified preparation requiring a fewer number of acids.

Stable silicon isotope analysis on nanomole quantities using MC-ICP-MS with a hexapole gas-collision cell

We demonstrate in this study that a single focusing multiple collector inductively coupled plasma mass spectrometer (MC-ICP-MS) equipped with a hexapole gas-collision cell (GV-instrument®Isoprobe) can precisely determine the δ29Si (2S.D., 0.2‰) using a total Si consumption of less than 14 nmole (390 ng Si). Testing and evaluation of background, rinse time, and major matrix effects have been performed in a systematic way to establish a procedure to measure δ29Si in small quantities. Chemical purification prior to analysis is required to remove potential interferences. For data collected during a four-year period, the average δ29Si value of IRMM-018 relative to NBS-28 was found to be −0.95‰ (n = 23, 2S.D. 0.16‰) with a 95% confidence interval (−0.95 ± 0.028‰). The mean δ29Si value of the Big-Batch standard was found to be −5.50‰ (n = 6, 2S.D. 0.26‰). Although determination of the δ30Si measurements is not possible, with our current instrument we demonstrate that this system provides a fast and long-term reliable method for the analysis of δ29Si in purified samples with low Si concentration (18 µM Si).

The Ediacaran radiogenic Sr isotope excursion in the Doushantuo Formation in the Three Gorges area, South China

The Ediacaran period was one of the most important times for the evolution of life. However, the scarcity of well-preserved outcrops of Ediacaran rocks still leaves ambiguity in decoding ambient surface environmental changes and biological evolution. The Ediacaran strata in South China are almost continuously exposed, comprise mainly carbonate rocks with subordinate black shales and sandstones, and they contain many fossils, suitable for study of environmental and biological changes in the Ediacaran. We conducted drilling through the Doushantuo Fm at four sites in the Three Gorges area to obtain continuous, fresh samples without surface alteration and oxidation. We analyzed 87Sr/ 86Sr and 88Sr/ 86Sr ratios of the fresh carbonate rocks, selected on the basis of microscopic observations and the geochemical signatures of Sr contents, Mn/Sr and Rb/Sr ratios, and δ 18O values, with a multiple collector-inductively coupled plasma-mass spectrometer (MC-ICP-MS). The chemostratigraphy of the 87Sr/ 86Sr ratios of the drilled samples displays a smooth curve and two large positive shifts during Ediacaran time. The combination of the detailed chemostratigraphies of δ 13C, δ 18O and 87Sr/ 86Sr values and Mn and Fe contents enables us to decode the surface environmental changes and their causes in the Ediacaran. The first large positive excursion of 87Sr/ 86Sr occurred together with negative δ 13C and positive δ 18O excursions. The higher 87Sr/ 86Sr values indicate an enhancement of continental weathering, whereas the positive δ 18O excursion suggests global cooling. Global regression due to global cooling enhanced the oxidative decay of exposed marine organic sediments and continental weathering. Accelerated influx of nutrients promoted primary productivity, resulting in oxidation of dissolved organic carbon (DOC), whereas active sulfate reduction due to a higher sulfate influx from the continents caused remineralization of the large DOC, both of which caused a negative δ 13C anomaly. The 580 Ma Gaskiers glaciation accounts for the close correlation among the positive 87Sr/ 86Sr, negative δ 13C and positive δ 18O excursions. The second large positive shift of 87Sr/ 86Sr firstly accompanied a positive δ 13C excursion, and continued through the Shuram δ 13C negative excursion. The positive correlation of δ 13C and 87Sr/ 86Sr values is consistent with an enhanced continental weathering rate due to continental collisions that built Trans-Gondwana mountain chains, and with a higher primary activity due to the enhancement of continental weathering and consequent higher nutrient contents in seawater. The accompanied increase in Mn and Fe contents implies a gradual decline of the seawater oxygen content due to more active aerobic respiration and oxidation of reductive materials flowing in the oceans. In the Shuram excursion, higher 87Sr/ 86Sr values and a transition from increase to decrease in Mn and Fe contents were accompanied by the large negative δ 13C excursion. The higher 87Sr/ 86Sr values are the first compelling evidence for enhanced continental weathering, which was responsible for the large δ 13C anomaly through the remineralization of the DOC by more active sulfate reduction due to a higher sulfate influx. Higher Mn and Fe contents in the early and middle stages of the excursion suggest a decline in the oxygen content of seawater due to oxidative decay of the DOC, whereas in the late stages the decrease in Mn and Fe contents is consistent with oceanic oxygenation. The emergence of Ediacara biota after the Gaskiers glaciation and the prosperity of the latest Ediacaran is concomitant with the formation of more radiogenic seawater with high 87Sr/ 86Sr values, suggesting that enhanced continental weathering, and the consequent higher influx of nutrients, played an important role in biological evolution.

Separation and Measurement of Pa, Th, and U Isotopes in Marine Sediments by Microwave-Assisted Digestion and Multiple Collector Inductively Coupled Plasma Mass Spectrometry

This manuscript describes a new protocol for determination of Pa/Th/U in marine sediments. It is based on microwave-assisted digestion and represents an important reduction of working time over conventional hot-plate digestion methods, and the use of HClO(4) is avoided. Although Th and U are completely dissolved with a first microwave step, around 40% of (231)Pa remains undissolved, and a short hot-plate step with reverse aqua regia is required to achieve total digestion and spike equilibration. Next, the method involves a separation of these elements and a further purification of the Pa fraction using Dowex AG1-X8 resin. Separation with Bio-Rad and Sigma-Aldrich resins was compared; although both perform similarly for Th and U, Pa yields are higher with Bio-Rad. Finally, samples are measured using a Nu instruments multiple collector inductively coupled plasma mass spectrometer (MC-ICPMS). Overall chemical yields range around 50% for Pa, 60% for Th, and 70% for U.

In situ determination of precise stable Si isotope ratios by UV-femtosecond laser ablation high-resolution multi-collector ICP-MS

Abstract A new method for the in situ measurement of stable Si isotope ratio using UV-femtosecond laser ablation connected to a multiple-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) has been established. The use of medium mass resolution mode (with a resolving power m /Δ m = 8000) permits to resolve spectral interferences on 28 Si, 29 Si and 30 Si allowing for determination of the 29 Si/ 28 Si and 30 Si/ 28 Si ratios with high accuracy and precision on wide, interference-free plateaus. A three-isotope plot demonstrated that interferences, if existing, are negligible for Si isotope ratios. The laser spot size is 35 μm and measurements are made using square rasters of 150 μm side length. Different types of elemental Si, Si-oxide and silicate matrices have been analysed by laser ablation using the international standard NBS28 as the bracketing standard. Thus δ 29 Si and δ 30 Si have been determined for the silicon isotope standard IRMM-017 ( δ 30 Si ± 2 S.D. = − 1.26 ± 0.24‰; n = 89) and BigBatch ( δ 30 Si = − 10.55 ± 0.42‰; n = 15), as well as San Carlos Olivine ( δ 30 Si = − 0.81 ± 0.19‰; n = 14), Caltech Rose Quartz ( δ 30 Si = 0.10 ± 0.13‰; n = 14) sponge needles ( Stylocordyla borealis ; δ 30 Si = − 2.19 ± 0.32‰; n = 14) and JER-diopside glass ( δ 30 Si = 0 ± 0.09‰; n = 14) samples. The overall repeatability achievable is 0.15‰ (2 S.D.) on δ 29 Si and 0.24‰ (2 S.D.) on δ 30 Si. The silicon isotope standard IRMM-018 has also been measured and was confirmed to be heterogeneous. The two isotope ratios follow an equilibrium mass-dependent fractionation law which can be represented as δ 30 Si = 1.93 × δ 29 Si. Published extractions methods have been used to wet-chemically purify Si from the JER-diopside glass. The Si extracted was presented as Si-gel and ablated like a solid. The results show excellent agreement with in situ measurements of this glass that confirms that this technique can be employed to a wide variety of matrices, including Si purified from solution. The new technique presents a viable alternative to solution MC-ICP-MS for bulk measurements and the most precise technique so far for in situ measurement of Si isotope ratios.

Automation and miniaturization of an on-line flow injection Sr/matrix separation method for accurate, high throughput determination of Sr isotope ratios by MC-ICP-MS

In this work we present a fully automated Sr/matrix separation method, based on flow injection (FI) hyphenated to a multiple collector-inductively coupled plasma-mass spectrometer (MC-ICP-MS). Our method compares favourably to the common, manual, batch Sr/matrix separation procedure, with respect to labour and reagent investment. We validated the system analyzing NIST SRM 987 Sr isotope reference material and digests of Asparagus officinalis at Sr concentrations of approximately 30–40 ng g−1 total Sr using separation columns containing approximately 40 μL of Sr specific resin. 33 consecutive runs of the NIST SRM 987 Sr isotope reference material could be repeated with 0.03% (2 RSD), yielding an average 87Sr/86Sr isotope ratio of 0.71030 which is well within the certified range. 5 Hungarian and 5 Austrian samples were determined to have 87Sr/86Sr isotope ratios ranging from 0.70504–0.70746 and 0.70884–0.70923 respectively. 87Sr/86Sr isotope ratios of single elutions could be measured with typical precisions of 0.02–0.1% (2 RSD) depending on signal intensities. 6 successive measurements could be repeated with 0.006–0.04% (2 RSD). Results presented in this work are in agreement with results previously obtained via off-line Sr/matrix separation and subsequent measurement of 87Sr/86Sr isotope ratios by MC-ICP-MS. We were able to use a single 40 μL column for Sr/matrix separation of 66 samples.

Application of the isotope dilution technique for 93Zr determination in an irradiated cladding material by multiple collector-inductively coupled plasma mass spectrometry

The determination of 93Zr concentration, a long-lived radionuclide present in spent nuclear fuel and in the structural components of nuclear reactors, is a major issue for nuclear waste disposal purpose and to validate neutronic calculation codes. To measure 93Zr concentration in irradiated cladding material with a high precision, an analytical method based on the use of multiple collector-inductively coupled plasma mass spectrometer (MC-ICPMS) combined to isotope dilution technique was developed. First a radiochemical separation of zirconium from a zircaloy sample (a zirconium alloy used as a cladding material for nuclear fuel elements), has allowed to obtain a very pure zirconium fraction with no potential isobaric interferences for mass spectrometric measurements. Then as the determination of all zirconium isotope ratios in the sample is necessary for the isotope dilution method, a MC-ICPMS procedure was developed to perform these precise measurements. Finally, the determination of 93Zr concentration in the same sample was performed, after preparation and calibration of a 96Zr spike solution. The uncertainties obtained on isotope ratios of zirconium by MC-ICPMS were in the order of 0.1%. The final uncertainty obtained on the 93Zr concentration in the nuclear material used and after chemical purification was lower than 0.6%.

Ca isotopic compositions of dolomite, phosphorite and the oldest animal embryo fossils from the Neoproterozoic in Weng'an, South China

Deciphering the drastic changes of surface environment and the emergence of animals after the Neoproterozoic Snowball Earth event are important for understanding the changes of surface environment and its influence on the evolution of life through geologic time. Especially, the emergence of two types of Metazoan animals such as animal embryo fossils, cnidarians or sponges, and Ediacaran fauna in the late Neoproterozoic was one of the critical turning points in the biological evolution. Calcium is one of the essential elements for the growth of most animals. In this study, in order to evaluate the Ca cycles in the Neoproterozoic, we have measured Ca isotopic ratios ( 44Ca/ 42Ca and 43Ca/ 42Ca) for phosphorite, dolostone and phosphatic animal embryo fossils with a multiple collector, inductively coupled plasma-mass spectrometer (MC-ICP-MS). The resulting 44Ca/ 42Ca ratio defined by the relative deviation from the ratio of NIST SRM915a ( δ 44/42Ca NIST915) for phosphorite and dolostone ranges from 0.83 to 0.95‰, demonstrating that the fractionation between phosphorite/dolostone and seawater was very small. This evidence indicates that at the emergence of the Weng'an biota seawater was deficient in Ca probably due to mass deposition of phosphorite/dolostone and to the beginning of Ca-biomineralization. Three phosphatic animal embryo fossils have lower δ 44/42Ca values than the phosphorite and dolomite, implying that the precursor of the phosphatic embryo fossils was able to fractionate Ca isotopes through Ca-biomineralization, consistent with marine gastropods.

Isotope fractionation studies of molybdenum

Mass spectrometric studies of the isotopic composition of molybdenum have become an active area of research in stable isotope geochemistry, biogeochemistry and cosmochemistry. The redox chemistry of Mo, together with its proclivity for covalent bonding, indicates its importance in isotope fractionation studies such as palaeoceanography. The measurement of the magnitude of isotope fractionation of Mo in natural systems is a challenging task, in that natural fractionation has to be carefully distinguished from chemical and instrumental isotope fractionation. An ion exchange chemical separation procedure has been developed with high efficiency and low blank, to ensure that the isobaric elements Zr and Ru are removed from the samples before mass spectrometric analysis. The isotope fractionation resulting from this procedure is 0.14‰ per u. The isotopic composition of Mo of a Laboratory Standard has been measured by positive and negative thermal ionization mass spectrometry (P-TIMS and N-TIMS, respectively), to give an isotope fractionation of 6.4‰ and 0.5‰ per u, respectively, with respect to the absolute isotope abundances of Mo. In both cases the lighter isotopes are enhanced with respect to the heavier isotopes. An ascorbic acid activator has enabled the sensitivity of P-TIMS to be improved as compared to traditional methods. The same experiment was repeated using a multiple collector-inductively coupled plasma-mass spectrometer (MC-ICP-MS) to give an isotope fractionation of approximately 17.0‰ per u. In this case the heavier isotopes are enhanced with respect to the lighter isotopes. The strengths and weaknesses of these three mass spectrometric techniques are evaluated. We conclude that MC-ICP-MS is the optimum mass spectrometric method for accurately measuring the isotope fractionation of Mo in natural materials, provided chemical and instrumental isotope fractionation can be resolved from naturally induced isotope fractionation. There is an urgent need for an internationally accepted calibrated reference material to be developed. Our Laboratory Standard has recently been calibrated at Curtin University by measurements of gravimetric mixtures of two enriched isotopes of Mo, to obtain the absolute isotope abundances of Mo. This Laboratory Standard can be made available to those laboratories in which isotopic studies of Mo are executed, to enable meaningful inter-laboratory comparisons to be conducted.

In situ iron isotope ratio determination using UV-femtosecond laser ablation with application to hydrothermal ore formation processes

The feasibility of in situ stable Fe isotope ratio measurements using UV-femtosecond laser ablation connected to a multiple-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) has been investigated. Different types of matrices, independently determined by solution MC-ICP-MS after chromatographic separation of Fe, have been analysed by laser ablation using the isotopically certified iron reference material IRMM-014 as the bracketing standard. The samples have been pure iron metal (JM Puratronic), Fe-meteorites (North Chile, Glenormiston and Toluca), the meteorite phases kamacite and taenite in Toluca and Fe-sulphides. Furthermore, Fe isotope ratios from hydrothermal hematite, siderite and goethite from an old mining area in the Schwarzwald, Germany, and of magnetite from the metamorphic Biwabik iron formation have been determined. The results show that a precision of better than 0.1‰ (2 sigma) can be achieved with laser ablation and that all the results obtained agree with those determined by solution ICP to better than 0.1‰. This precision and accuracy is achievable in both raster and spot ablation mode. A matrix-matched bracketing standard is not required , and all these materials can be measured accurately against a metal standard. The hydrothermal minerals show significant Fe isotope zonations. In some samples the range of δ 56Fe in a single aggregate encompasses the entire spectrum of ratios found by bulk solution analyses in multiple samples distributed over the whole mining district. For example, isotopic zonations found in secondary fibrous hematites show a continuous change in the δ 56Fe values from −0.5‰ in the core to −1.8‰ in the rim. Primary hydrothermal siderite shows the reverse pattern with lighter values in the core than in the rim. While the siderite is thought to record primary fluid histories, the hematite pattern is interpreted as a reworked isotopic signature generated by oxic dissolution of primary zoned siderite and immediate close range re-precipitation of the oxidized Fe. Abrupt changes are documented for secondary goethite showing a distinct overgrowth that is 0.4‰ lighter than the core of the grain. If indeed Fe isotopes in secondary minerals from hydrothermal ore deposits record the initial isotopic signatures of their precursor minerals, and these in turn record hydrothermal fluid histories, then the tools are in place for a detailed reconstruction of the deposit‘s genesis. We expect similar observations from other Fe-rich deposits formed at intermediate and low-temperatures (e.g. banded iron formations). Laser ablation now provides us with the spatial resolution that adds a further dimension to our interpretation of stable Fe-isotope fractionation.

Sr isotope ratio measurements by double-focusing MC-ICPMS: techniques, observations and pitfalls

Routine techniques for Sr isotope analysis on a double-focusing multiple collector inductively coupled plasma mass spectrometer (MC-ICPMS) are described that for unspiked samples yield accurate and precise results (e.g., 87 Sr/ 86 Sr=0.710255±24 ; 0.003%; 2 S.D.) similar to those obtainable by thermal ionisation mass spectrometry. Prior to a 5-min analysis, backgrounds are measured on-mass to correct for interferences on masses 84 and 86 derived from small amounts of Kr in the Ar supply, and stable, residual memory Sr and Rb signals from sample material on the torch and cones. Each analysis consumes ca. 300 ng of Sr, which compares unfavourably to TIMS where loads of ca. 1 ng can be measured. Simultaneous ionisation of all elements during plasma-based ionisation proves to be a disadvantage during Sr isotope analysis as extra chemical clean-up steps, beyond those necessary for TIMS analyses, are required to remove potential isobaric interference problems such as Rb, doubly charged rare earth element (REE) and Ca dimers and argides. Reproducibility for spiked Sr samples is significantly worse than for unspiked samples, largely due to an inability to determine the exact isotopic composition of interfering Rb. This problem can be avoided by splitting the digested sample into isotopic composition and dilution aliquots prior to spiking. Laser ablation (LA)-MC-ICPMS Sr isotope ratio measurements of various geological and biological samples have shown significant deviations in the naturally constant 84 Sr/ 86 Sr ratio and the radiogenic 87 Sr/ 86 Sr ratio from values obtained by analysis of Sr chemically separated from the sample matrix. The precise reasons for this remain unclear but likely reflect a combination of isobaric interferences from Ca dimers and Ca argides and doubly charged REE, as well as disruption of mass bias effects due to differential loading of the plasma. It is also unclear whether this is an instrument- or technique-specific problem and we suggest great care be taken when attempting Sr isotope analyses by in situ LA-MC-ICPMS techniques.

Rapid and highly reproducible analysis of rare earth elements by multiple collector inductively coupled plasma mass spectrometry

A method has been developed for the rapid chemical separation and highly reproducible analysis of the rare earth elements (REE) by isotope dilution analysis by means of a multiple collector inductively coupled plasma mass spectrometer (MC-ICP-MS). This technique is superior in terms of the analytical reproducibility or rapidity of analysis compared with quadrupole ICP-MS or with thermal ionization mass spectrometric isotope dilution techniques. Samples are digested by standard hydrofluoric–nitric acid–based techniques and spiked with two mixed spikes. The bulk REE are separated from the sample on a cation exchange column, collecting the middle-heavy and light REE as two groups, which provides a middle-heavy REE cut with sufficient separation of the light from the heavier REE to render oxide interferences trivial, and a Ba-free light REE cut. The heavy (Er-Lu), middle (Eu-Gd), and light REE (La-Eu) concentrations are determined by three short (1 to 2 min) analyses with a CETAC Aridus desolvating nebulizer introduction system. Replicate digestions of international rock standards demonstrate that concentrations can be reproduced to <1%, which reflects weighing errors during digestion and aliquotting as inter-REE ratios reproduce to ≤0.2% (2 SD). Eu and Ce anomalies reproduce to <0.15%. In addition to determining the concentrations of polyisotopic REE by isotope dilution analysis, the concentration of monoisotopic Pr can be measured during the light REE isotope dilution run, by reference to Pr/Ce and Pr/Nd ratios measured in a REE standard solution. Pr concentrations determined in this way reproduce to <1%, and Pr/REE ratios reproduce to <0.4%. Ce anomalies calculated with La and Pr also reproduce to <0.15% (2 SD). The precise Ce (and Eu) anomaly measurements should allow greater use of these features in studying the recycling of materials with these anomalies into the mantle, or redox-induced effects on the REE during recycling and dehydration of oceanic lithosphere, partial melting, metamorphism, alteration, or sedimentation processes. Moreover, this technique consumes very small amounts (subnanograms) of the REE and will allow precise REE determinations to be made on much smaller samples than hitherto possible.

Isotope-ratio measurements of lead in NIST standard reference materials by multiple-collector inductively coupled plasma mass spectrometry.

The capability of a second-generation Nu Instruments multiple collector inductively coupled plasma mass spectrometer (MC-ICP-MS) has been evaluated for precise and accurate isotope-ratio determinations of lead. Essentially the mass spectrometer is a double-focusing instrument of Nier-Johnson analyzer geometry equipped with a newly designed variable-dispersion ion optical device, enabling the measured ion beams to be focused into a fixed array of Faraday collectors and an ion-counting assembly. NIST SRM Pb 981, 982, and 983 isotopic standards were used. Addition of thallium to the lead standards and subsequent simultaneous measurement of the thallium and lead isotopes enabled correction for mass discrimination, by use of the exponential correction law and 205Tl/203Tl = 2.3875. Six measurements of SRM Pb-982 furnished the results 206Pb/204Pb = 36.7326(68), 207Pb/204Pb = 17.1543(30), 208Pb/204Pb = 36.7249(69), 207Pb/206Pb = 0.46700(1), and 208Pb/206Pb = 0.99979(2); the NIST-certified values were 36.738(37), 17.159(25), 36.744(50), 0.46707(20), and 1.00016(36), respectively. Direct isotope lead analysis in silicates can be performed without any chemical separation. NIST SRM 610 glass was dissolved and introduced into the MC-ICP-MS by means of a micro concentric nebulizer. The ratios observed were in excellent agreement with previously reported data obtained by TIMS and laser ablation MC-ICP-MS, despite the high Ca/Pb concentration ratio (200/1) and the presence of many other elements at levels comparable with that of lead. Approximately 0.2 microg lead are sufficient for isotope analysis with ratio uncertainties between 240 and 530 ppm.