Mass Fractionation Correction Research Articles

Origin of carbonatites and associated silicate rocks revealed by Mg triple-isotope approach

Carbonatites are rare carbonate-rich igneous rocks derived from carbon and carbonate-rich regions of Earth's mantle. Although a number of igneous processes are recognized to have controlled their compositions, the origin of the carbonate-rich nature of these magmas remains debated and has been linked to various mantle-related processes, including subduction and plume–lithosphere interaction. High-precision isotope measurements can provide insights into carbonatite petrogenesis, including the identification of subducted crustal material in their source region. In particular, combining mass-dependent and kinetically corrected Mg isotope data, also known as the triple-isotope approach, provides insights into mass fractionation processes driving mass dependent fractionation, which in turn allows to distinguish between equilibrium and kinetic processes. In this work, we report high-precision Mg stable isotope data for 59 carbonatites and associated silicate rocks from different localities and ages ranging from 3000 Ma to present-day, as well as C and O isotopic analysis for 38 carbonatites and Sr isotopic analysis for 41 carbonatites and silicate rocks. In addition, we also report Mg isotope data for 17 Phanerozoic carbonate rocks, with the aim of identifying the isotopic signature of carbonate-rich material potentially recycled to the carbonatite mantle source regions. Collectively, the data reveal a range of stable Mg isotope compositions from δ25/24MgDTS-2 = −1.20 ± 0.01‰ to +0.08 ± 0.01‰. We observe positive residual deviations after kinetic mass fractionation correction of the Mg isotope data for our carbonatites; an isotope signal that is also present in Phanerozoic carbonates. This observation establishes that a component of the Mg present in these samples experienced mass-dependent equilibrium isotopic fractionation processes, which are significantly larger at low temperatures. Given that the Mg results do not covary with C and O isotopic signals, the magnitude of the fractionation following the equilibrium law observed in carbonatites provides strong evidence for recycled material from the Earth's surface in the mantle source of Ca- and Mg-rich carbonatites. Furthermore, associated silicate rocks present mantle-like Mg isotopic compositions in contrast with the genetically linked carbonatites - based on the Sr isotopes - for some complexes, which is best explained if the carbonatites and associated silicate rocks represent distinct generation of partial melts of a mantle source containing recycled carbonate.

Development of three chalcopyrites and one copper metal as potential reference materials for copper isotopic analysis by LA-MC-ICP-MS.

Laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) has become a powerful technique for in situ Cu isotopic analysis in natural geological samples. Cu isotopic compositions in natural chalcopyrites have been used to reveal aspects of the mineralization processes directly. However, internationally or commercially available matrix-matched chalcopyrite reference materials for mass fractionation correction or quality control purposes are still lacking for in situ Cu isotopic analysis using LA-MC-ICP-MS. Three natural chalcopyrites 14ZJ12-1, JGZ-29 and JGZ-78, and one copper metal GBW02141 with different Cu isotopic compositions were investigated as potential microanalytical reference materials by LA-MC-ICP-MS. Ga element was used as an internal standard to correct the mass fractionation of Cu isotopes during LA-MC-ICP-MS analysis. A large number of Cu isotope ratio measurements using femtosecond LA-MC-ICP-MS were conducted and produced good intermediate precision of δ65 CuNIST976 (0.07-0.08‰, 2 standard deviations), demonstrating the homogeneous Cu isotopic distribution in the recommended samples. The mean δ65 CuNIST976 values of -0.21 ± 0.04‰, 0.46 ± 0.04‰, -0.06 ± 0.04‰ and 0.11 ± 0.05‰ (2 standard deviations) in 14ZJ12-1, JGZ-29, JGZ-78 and GBW02141, respectively, were obtained using solution-MC-ICP-MS in the four recommended samples. Here, we describe three natural chalcopyrites 14ZJ12-1, JGZ-29, JGZ-78, and one copper metal GBW02141 as the potential Cu isotopic reference materials for LA-MC-ICP-MS analysis. Our analyses demonstrate that these recommended materials have a high degree of elemental and isotopic homogeneity, indicating that they are suitable for microanalysis techniques for data quality assurance or interlaboratory calibration.

Rapid and accurate SIMS microanalysis of monazite oxygen isotopes

28Si–18O–16O simultaneous measurement by SIMS enables matrix-effect correction of monazite oxygen isotopes.

Accurate and precise measurement of Ce isotope ratios by thermal ionization mass spectrometry (TIMS)

The La-Ce isotope system has been rarely applied, mostly due to analytical difficulties. Recent technical refinements of mass spectrometers have overcome some of these technical limitations, but Ce isotope analyses still face considerable analytical challenges. These are mainly related to the low abundance of the minor isotopes 136Ce and 138Ce relative to the main isotopes 140Ce and 142Ce (136Ce=0.19%, 138Ce=0.25%, 140Ce=88.45%, 142Ce=11.11%). Hence simultaneous measurement of ion beams over a large dynamic range is required, resulting in large differences in count statistical uncertainty on the individual ion beams. In addition, the large abundance of 140Ce introduces a tailing effect of the large 140CeO ion beam onto the 136CeO and 138CeO ion beams, which requires adequate correction. Here, we present a chemical purification scheme and high-precision thermal ionization mass spectrometric (TIMS) method for analyzing CeO isotope ratios in silicate samples. The advantages and disadvantages of different mass spectrometric strategies for data acquisition by TIMS were evaluated, including the use of 1010, 1011, and 1012Ω amplifiers and different strategies for the 140CeO tail correction. An optimization scheme was developed for different on-peak and off-peak collection schemes, in combination with different tail and baseline correction methods. It is shown that, as long as the integration times for the on-peak, off-peak (half-mass), and baseline signals are adequately optimized for the employed collection scheme and tail correction method, different strategies yield Ce isotope ratios with similar precision of 20–40ppm (2 S.D.). In contrast to previous studies, we have acquired 140CeO by using a 1010Ω amplifier, and have determined a long-term average 140Ce/142Ce of=7.94319±2. Using a common 136Ce/142Ce=0.01688 for mass fractionation correction, the 136Ce/138Ce of the international rock reference materials BCR-1, BCR-2, and BHVO-2 of this study agree well with those recently reported, when all Ce isotope ratios are reported relative to a common 138Ce/136Ce=1.337366 for the average Ames Ce metal. In addition, Ce isotope ratios for several other widely available international rock reference materials (AGV-2, BE-N, BIR-1, DNC-1, W2A) are presented, and facilitate easy inter-laboratory comparison.

Role of fractionation correction in accurate determination of 142Nd/144Nd by TIMS: A case study of 1.48 Ga alkaline rocks from Khariar, India

The short-lived isotopic systematics of 146Sm-142Nd is a tracer of early silicate Earth differentiation events. Evidence for these events comes from anomalous 142Nd/144Nd, defined in terms of μ142Nd (μ142Nd=[{(142Nd/144Nd)sample/(142Nd/144Nd)standard}−1]×106) with respect to a terrestrial standard representing the modern accessible mantle. This requires measurement of accurate and highly precise 142Nd/144Nd, which is carried out by Thermal Ionisation Mass Spectrometry (TIMS). Since multiple factors affect the accuracy of the final results, we carried out a detailed investigation on the effect of various data acquisition, fractionation correction and normalization methods on the accuracy of 142Nd/144Nd determinations. Based on the analyses of Ames Nd standard using various combinations of the most commonly employed methods we observed that for a multi-dynamic mode of data acquisition, the power-normalised exponential law is the most appropriate method for mass fractionation correction. The time delays between successive sequences in a multi-dynamic mode had little effect on the final value of 142Nd/144Nd. The different standards have different 142Nd/144Nd ratios and therefore, their uses yield different μ142Nd values for the same sample. We extended this information to understand the two contradicting results from 1.48Ga alkaline rocks from Khariar, India, carried out on the same sample aliquots (Upadhyay et al. 2009; Roth et al. 2014b). A confirmation of 142Nd anomalies in such younger rocks is important because it could establish the longevity of early silicate differentiation signatures beyond Archean. Our experiment on freshly collected samples from the same outcrops, using identical analytical procedures, could not reproduce the results of Upadhyay et al. (2009). We did, however, observe slightly negative μ142Nd values with respect to Ames Nd, which became normal with respect to JNdi-1.

Determination of the Tin Stable Isotopic Composition in Tin‐bearing Metals and Minerals by MC‐ICP‐MS

This study uses MC‐ICP‐MS for the precise analysis of the stable tin isotopic composition in ore minerals of tin (cassiterite, stannite), tin metal and tin bronze. The ultimate goal is to determine the provenance of tin in ancient metal objects. We document the isotope compositions of reference materials and compare the precision of different isotope ratios and the accuracy of different procedures of mass fractionation correction. These data represent a base with which isotopic data of future studies can be directly compared. The isotopic composition of cassiterite and stannite can be determined after reduction to tin metal and bronze, respectively. Both metals readily dissolve in HCl, but while the solutions of tin metal can be directly measured, the bronze solutions must be purified with an anion exchanger. The correction of the mass bias is best performed with an internal Sb standard and an empirical regression method. A series of Sn isotope determinations on commercially available mono‐element Sn solutions as well as reference bronze materials and tin minerals show fractionations ranging from about −0.09‰ to 0.05‰/amu. The combined analytical uncertainty (2s) was determined by replicate dissolutions of reference materials of bronze (BAM 211, IARM‐91D) and averages at about 0.005‰/amu.

Correction of mass fractionation for isotope dilution analysis by MC-ICP-MS

Correction of mass fractionation for isotope dilution analysis by MC-ICP-MS

High precision Os isotopic measurement using N-TIMS: Quantification of various sources of error in 186Os/188Os measurements

The 190Pt–186Os system could be useful to study a number of different geologic processes. Over the last decade, this system has received considerable attention owing to its potential to detect core–mantle signatures in plume-derived lavas. Additionally, this system may also be useful as a proxy for melt depletion, examining the role of pyroxenites/sulfides in mantle melting and for dating of Platinum Group Element (PGE) based ores. The total natural variation of 186Os/188Os is limited (~150ppm in the mantle), which thus requires extremely high levels of accuracy and precision to discern natural variations. 186Os/188Os ratios were measured in laboratory standards and natural samples to examine and quantify the sources of error affecting high precision 186Os/188Os measurements and a set of refined analytical procedures to minimize and reduce these various sources of error were recommended.Inter and intra-run variations in oxygen isotopic composition used in oxide correction of OsO3− peaks can produce systematic shifts in 186Os/188Os. Useful analysis time is wasted in measuring O isotopic composition line-by-line and it is sufficient to utilize pre- and post-run determination of O isotopic composition for performing oxide correction. Although the use of 189Os/188Os as a normalizing ratio for mass fractionation results in lower uncertainty in Os isotopes from oxide correction, this benefit is overwhelmed by the increased error propagation from mass fractionation. Instead, usage of the 192Os/188Os normalization generates the least propagated error due to mass fractionation and is recommended. Earlier studies have variably used different O isotopic compositions for oxide correction and different normalizations for mass fractionation correction, which can generate systematic shifts in the reduced data. Use of mutually consistent normalizing values for mass fractionation largely reduces the offsets between reduced data using different normalizing schemes, thus improving data comparison across laboratories.For the quantities of Os and beam intensities of 186Os utilized in previous 186Os/188Os studies (several 10's of ng Os, 80–250mV of 186Os with a 1011 Ω resistor), the largest source of analytical error is derived from Johnson noise on baseline integrations. Increasing the duration of baseline measurement significantly reduces the Johnson noise error, allowing to achieve maximum external precision for a given sample size. Several 186Os/188Os analyses reported in the literature seem to have been compromised by an interference, which may be WO3−, organics or some other unidentified species that affects both the 186Os/188Os and the 184Os/188Os ratios. This makes it extremely critical to monitor and correct PtO2− interferences, which could mask other potential interferences that directly affect 186Os/188Os measurements.

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

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

High precision 87Sr/86Sr measurements by MC-ICP-MS, simultaneously solving for Kr interferences and mass-based fractionation

Sr isotope measurements by multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) are complicated by Kr interferences at atomic masses 84 and 86. Particularly the interference on 86Sr is problematic for 87Sr/86Sr measurements, since it is the normalizing stable isotope, and as such also used in the mass-based fractionation correction using 86Sr/88Sr. Although on-peak baseline measurements can subtract an average Kr contribution, signal variations and errors within-run are propagated into the final error, and signal-to-noise ratios are lower than combined Sr+Kr signals. Alternatively, intensities of interference-free 82Kr or 83Kr isotopes can be monitored to correct the interfering Kr masses, although the errors for these signals are amplified and propagated into the 86Kr and 84Kr correction. Instead, our approach depends on the use of the most abundant Kr isotope (84Kr) for correction, which can then be used to estimate interference by 86Kr on 86Sr. This is accomplished by solving how much Kr is needed to explain the difference between I84/I88 and I86/I88 versus 84Sr/88Sr and 86Sr/88Sr (I8×: signal intensity at mass 8×). A solution with the traditional isotope dilution equation would require off-line iteration to include an exponential mass fractionation correction. Instead, we formulate an online exponential correction of the Sr data without iterating, relying on a separate Kr correction that estimates Kr mass fractionation with either the linear law or a series expansion of the exponential law. Both methods advance MC-ICP-MS long-term 87Sr/86Sr precision for NIST SRM 987 to ±16∗10−6 or 22.5ppm 2σ, outperforming an 83Kr-based correction and an on-peak baseline correction applied to the same raw data. Both our methods find identical results for USGS rock standards that agree with published (TIMS) values.

A new approach to Pb evaporation and condensation from zircon enabling sub-Ma age determination on Precambrian rocks

Sub-million year age precision on low to moderately metamict Precambrian zircon is often reproducible using the isotope dilution thermal ionization technique (ID-TIMS) exploiting U–Pb double spiking for precise mass fractionation correction. However, this requires total dissolution of sample and laborious chemical processing. On the other hand whole grain evaporation techniques of Pb extraction are simple and less time consuming but provide relatively inefficient ionization and are not amenable to double Pb isotope spiking for precise mass fractionation correction, which limits the precision of ages that they can provide (often restricted to ±few million years). A modified evaporation–condensation method is described that overcomes these limitations with the ability to use 202Pb–205Pb double spiking for precise mass fractionation correction in order to achieve sub-million year age precision on low to highly metamict Precambrian samples. Examples are shown of age determinations on previously dated zircon populations from the Marmion tonalite and Nevison Lake rhyolite, western Superior province (U<200ppm), which give 3001.8±0.5Ma and 2998.2±0.3Ma, respectively, and from the black norite and the felsic norite in the Sudbury igneous complex (U>1000ppm), which give 1848.9±0.2Ma and 1851.3±0.5Ma, respectively (all errors are 95% confidence).

K–Ar analyses of the post-caldera lavas of Bratan volcano in Bali Island, Indonesia — Ar isotope mass fractionation to light isotope enrichment

The post-caldera lavas of Bratan volcano in Bali Island, Indonesia were collected for whole rock chemical analyses and K–Ar analyses. Major and trace element chemistry shows that the lavas are basalts to andesites and typical of subduction-related tectonic setting. The 38Ar/36Ar ratios are 0.1851±3–0.1875±2 and the 40Ar/36Ar, 294.3±0.3–301.6±0.1, which strongly suggest that the mass fractionation to light isotope enrichment took place. The effect of the groundwater on magma is common on the basis of systematic mass fractionation of the atmospheric Ar enriched in lighter isotopes. This case was under the mass fractionation law analyzed numerically, giving the mass fractionation correction ages (14±15, 31±6, 55±22, 66±23, 94±32 and 125±51ka) consistent with the volcano stratigraphy though the magma composition that changed frequently in time.

Improvements in 230Th dating, 230Th and 234U half-life values, and U–Th isotopic measurements by multi-collector inductively coupled plasma mass spectrometry

We have developed techniques for measuring 234U and 230Th on Faraday cups with precisions of 1–3 epsilon units (1 ε-unit=1 part in 104) using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). Using a Thermo-Scientific Neptune with desolvation nebulization, we obtained ionization/transmission efficiencies of 1–2% for both U and Th. We set up protocols to correct for tailing, prepared U and Th gravimetric standards, tested a Th mass fractionation correction procedure based on U isotopes, and identified natural calcite samples likely to be in U–Th isotopic secular equilibrium. The measured atomic ratios, 234U/238U=54.970 (±0.019)×10−6 and 230Th/238U=16.916 (±0.018)×10−6, for these calcite samples were identical within errors (quoted 2σ uncertainties calculated combining all sources of error). Half-life values calculated from these ratios are consistent with previous values, but have much smaller errors: 245,620±260a for 234U and 75,584±110a for 230Th (quoted 2σ uncertainties calculated using all sources of error). In calculating a 230Th age, some of the systematic errors included in estimating the full error in the half-lives effectively cancel. Removing these uncertainties (uncertainty in the 238U half-life value, uncertainty in our gravimetric uranium and thorium standards, and uncertainty in the absolute isotopic composition of the uranium standard), yields effective uncertainties for the purposes of 230Th dating of ±70a for the 234U half-life value and ±30a for the 230Th half-life value. Under ideal circumstances, with our methods, the 2σ uncertainty in age, including uncertainty in half-life values is ±10a at 10ka, ±100a at 130ka, ±300a at 200ka, ±1ka at 300ka, ±2ka at 400ka, ±6ka at 500ka, and ±12ka at 600ka. The isotopic composition of a sample with an age <800ka can clearly be resolved from the isotopic composition of a sample in secular equilibrium, assuming closed system behavior. Using these techniques, we analyzed a Sanbao Cave (Hubei, China) stalagmite that formed between 510 and 640ka ago. As the half-life values were determined independent of the Sanbao Cave ages, the observed co-variation between stalagmite δ18O and Northern Hemisphere summer insolation is consistent with accurate ages and half-life values.

A new analytical bias correction for in situ Sr isotope analysis of plagioclase crystals using laser-ablation multiple-collector inductively coupled plasma mass spectrometry

A new analytical protocol was developed for correcting baseline-induced biases during the analysis of Sr isotope ratios in plagioclase using excimer laser-ablation Aridus-dual-intake-system multiple-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS). Residual analytical biases of 84Sr/86Sr and 87Sr/86Sr were observed after applying on-peak background subtractions and mass-fractionation corrections using the conventional 86Sr/88Sr exponential internal normalization. The residual biases occurred only for samples analysed with LA and not for solution analyses using Aridus with the same instrumental setup. Based on observations from the ablation of NaCl and olivine crystals, we concluded that this was due to suppression and enhancement of the Kr baseline by loading of the LA sample aerosols and by the introduction of Kr from the samples, respectively. We also found that both the 84Sr/86Sr and 87Sr/86Sr isotope ratios were affected proportionally by the baseline biases of the LA analyses of an isotopically/compositionally homogeneous anorthite plagioclase (MkAn) from Miyakejima, Japan, and similar results were seen in theoretical calculations. Therefore, the bias correction for the target 87Sr/86Sr ratios was available using the shift in the simultaneously measured 84Sr/86Sr from the natural ratio. We then determined the correlation factors between 84Sr/86Sr and 87Sr/86Sr using numerical simulations and confirmed them by analysing MkAn, which closely reproduced the theoretical factors. By applying the new theoretical correction protocol, the accurate measurement of unknown plagioclase samples was possible without using any standard. This was confirmed by comparison with the results of micro-milling thermal ionization mass spectrometry (MM-TIMS) of the same plagioclase crystals with various compositions. The new correction technique improved reproducibility by a factor of four, providing a basis for an accurate Sr isotope analysis using LA-MC-ICP-MS.

The measurement of Pb isotope ratios in sub-ng quantities by fast scanning single collector sector field-ICP-MS

This work describes an analytical protocol for the measurement of isotope ratios in samples containing small amounts of lead (i.e. ≤0.2ng total Pb) using the Nu Attom single collector-ICP-MS. The method was evaluated using the NIST SRM 981 isotopic standard and standard-sample bracketing for correction of the instrumental mass fractionation. The long-term precision (n=32, ±2SD) was 0.11% for the 208Pb/206Pb and 207Pb/206Pb ratios and 0.21–0.23% for the 20xPb/204Pb ratios (where x=208, 207, 206). The precision of the measured Pb isotope ratios reported here (for equivalent total Pb quantities) is comparable to multi-collector (MC) methodologies, using either an array of ion counters (Cocherie and Robert, 2007, Chem. Geol. 243, 90–104) or conventional Faraday collectors coupled with a high efficiency sample cone (Makishima and Nakamura, 2010, J. Anal. At. Spectrom. 25, 1712–1716). The accuracy of the measured Pb isotope ratios and the effect of the sample matrix were evaluated for a series of USGS rock standard digests and doped solutions of the NIST SRM 981 standard. For total cation concentrations <10μgmL−1 and concomitant low concentrations (<1.5μgmL−1) of easily ionisable elements (i.e. Na and K), matrix effects were not resolvable at the level of precision reported. However, significant matrix effects were associated with sample compositions typical of natural waters, and removal of (at least) the major cations is required for accurate isotope ratio measurements.

Anomalous isotopic shifts associated with organic resin residues during cadmium isotopic analysis by double spike MC-ICPMS

Previous studies have reported that matrix effects during analysis by multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS), leading to highly anomalous isotopic results, can be derived from ion exchange separation procedures, as resin-derived organics are stripped from the column together with the element of interest. In low concentration samples, where matrix to analyte ratios are high, these artifacts cannot be corrected for using sample-standard bracketing or external normalization techniques to monitor instrumental mass fractionation. It has been suggested that such artifacts can be overcome by implementing a double spike for the correction of instrumental mass fractionation. We present new Cd isotopic results showing that although a double spike reliably corrects for instrumental mass fractionation during sample analysis, the effects of these resin-derived organics introduce very anomalous shifts in bracketing standards that have not been chemically processed using ion exchange resins. These inaccurate results appear to be the consequence of one or more polyatomic interference/s or mass independent isotopic fractionation as opposed to the previously proposed matrix effects. These analytical artifacts can be completely alleviated by the oxidation of samples with solutions of either H2O2/HNO3 or HClO4/HNO3 prior to analysis. Although the present results have direct relevance for Cd analysis, it is probable that other isotopic systems are affected by similar resin-derived organics. The increased sensitivity of the next generation of MC-ICPMS instruments means that progressively smaller sample sizes are accessible for analysis, leading to a larger proportion of resin-derived matrix in many samples, such that oxidation of samples will prove to be essential to circumvent the issue of inaccurate isotopic measurement.

Isotope Ratio Analysis of Lead in Blood and Environmental Samples by Multi-collector Inductively Coupled Plasma Mass Spectrometry

It is widely recognized that lead (Pb) affects children's cognitive function, even at relatively low blood lead levels (<10 µg dL(-1)). The determination of the source of Pb in children is essential for effective risk management. The use of multi-collector ICPMS (MC-ICPMS) for isotope ratio measurements of Pb in environmental and biological samples was examined for this purpose. MC-ICPMS with an instrumental mass fractionation correction by Tl allowed accurate isotope ratio measurements of the Pb isotopic reference material NIST SRM 981. However, the presence of matrix elements (Al, Ca, Fe and Na) at more than 10 mg kg(-1) in the sample solution significantly deteriorated the accuracy. The separation of Pb from the matrix is necessary for accurate measurements of the isotope ratio of Pb in environmental and biological samples. Bromide-complexation, followed by anion exchange was found to be satisfactory in terms of the recovery of Pb (90 to 104%) and the efficiency of matrix separation. The procedure was applied to a preliminary source analysis of Pb in the blood of Japanese children, and a significant contribution of indoor dust was demonstrated.

Calculation methods for direct internal mass fractionation correction of spiked isotopic ratios from multi-collector mass spectrometric measurements

It is difficult to do internal mass fractionation corrections for isotope dilution analysis by thermal ionization mass spectrometry (TIMS) or multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS), especially for MC-ICP-MS. In this study, calculation methods for direct internal fractionation correction of spiked isotope analysis by TIMS or MC-ICP-MS cycle by cycle for elements having at least two internal reference isotopic ratios are presented. For TIMS, direct internal mass fractionation correction calculation methods, based on both power and exponential laws, are derived; whereas for MC-ICP-MS, due to larger mass fractionation effects, only exponential law is considered. These calculation strategies can be applied for both static and multi-dynamic measurements. For multi-dynamic measurements, the isotope fractionation effect, gain and cup efficiency effects of different collectors, as well as ion beam fluctuation effects are all simultaneously eliminated. The calculation methods were verified by Sr isotopic analyses of spiked NBS987 standard solutions by TIMS and Hf isotopic analyses of spiked geological reference materials by MC-ICP-MS. In addition, precise and accurate calibrations of isotopic ratios of the spikes, based on the calculation methods, are discussed.

Simplified technique for the measurements of Re-Os isotope by multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS)

washing the inlet system with 5% HCl‐EtOH. The external reproducibility of the 187 Os/ 188 Os ratio of the Johnson-Matthey Chemical ICP (JMC) Os standard solution, as measured using the multi-ion counter system, is 0.07% (2σ, n = 12) for analyte amounts ranging from 0.1 to 1 ng total Os. Using MC-ICP-MS, the mean values of 187 Os/ 188 Os obtained for standards were found to be within analytical uncertainty of values obtained by negative thermal ionization mass spectrometry (N-TIMS). An improved mass-fractionation correction technique using Ir was applied to the isotopic analysis of Re. The present methods demonstrate the feasibility of MC-ICP-MS for precision measurements of the 187 Re‐ 187 Os isotope system.

Response to Comment on “Neodymium-142 Evidence for Hadean Mafic Crust”

Andreasen and Sharma raise concerns about the neodymium-142 data and age that we reported for rocks from the Nuvvuagittuq greenstone belt in Quebec, Canada. We agree that the issue of accurate mass fractionation correction is important, but stand by our discussion of this issue in our original report and our conclusion that the variation in 142 Nd/ 144 Nd ratios reflects the decay of 146 Sm caused by Sm-Nd fractionation within 300 million years of Earth’s formation.