Gas Source Mass Spectrometer Research Articles

A robust methodology for triple (∆47, ∆48, ∆49) clumped isotope analysis of carbonates

Analysis of ∆48 and ∆47 values of CO2 released from phosphoric acid digestion of carbonates enables the rate-limiting kinetic processes involved in carbonate mineralization to be identified and formation temperatures to be determined by accounting for these kinetic biases. This method has the benefit of only requiring analysis of a single carbonate phase. To resolve temperature from the kinetic information, mass spectrometric measurements of highest accuracy and precision are required.Here, we outline a robust methodology for high-precision triple (∆47, ∆48, ∆49) clumped isotope analysis using turbo-pumped gas and carbonate preparation lines, a common acid bath and the dual inlet of a Thermo Scientific 253plus gas source mass spectrometer, equipped with additional m/z47.5 half mass cup. By analysis of >400 replicates each, we report long-term (50 months) ∆47, CDES 90 and ∆48, CDES 90 values for three internationally accepted (ETH-1, -2 and -3) and two internal carbonate standards (GU1, Carrara) unbiased by scale compression. In order to obtain highest accuracy and precisions closest to shot noise limit it is essential to account for a sample size effect, correct raw intensities for a negative background utilizing the half mass cup, avoid pressure imbalances between sample and reference gas and distribute samples evenly across a given analytical session.We confirm ∆47, CDES 90 values that were recently assigned to ETH-1 and ETH-2. Our long-term average ∆47, CDES 90 value of 0.6032 ‰ for ETH-3, on the contrary, occurs 10 ppm lower than its recently assigned value. As a consequence, ∆47, CDES 90 values of samples having relatively low temperatures of formation are not consistent with ∆47, I-CDES values. We show that GU1, a synthetic carbonate deriving from the hydroxylation of CO2 and exhibiting a strong anti-clumped ∆48, CDES 90 value, appears to be homogeneous in its isotopic composition. If used in combination with ETH-1, −2 and − 3, it may allow projection of background corrected raw data without any bias into ∆47-∆48 CDES 90 space, provided our long-term dual clumped isotope values for these carbonate anchors are considered.Stability of mass spectrometric measurements is related to filament age and/or room temperature fluctuations and can be monitored using reference gas m/z49 intensity. Under the most stable analytical conditions, the ∆49, CDES 90 values of equilibrated gases and carbonates reveal a trend with formation temperature. Using data from the most stable sessions, we present a temperature calibration for ∆49, CDES 90 in CO2 evolved from phosphoric acid digestion of carbonates.

Combined high-precision ∆48 and ∆47 analysis of carbonates

High-precision analysis of the excess abundance (relative to the stochastic distribution) of mass 48 isotopologues in CO2 evolved from acid digestion of carbonates (∆48) has not been possible until recently due to the relatively low natural abundance of 18O. Here we show that the 253 Plus™ gas source mass spectrometer equipped with Faraday cups and 1013 Ω resistors can perform combined ∆47 and ∆48 analyses on carbonates with external reproducibilities (1SD) of 0.010 ‰ and 0.030 ‰, respectively.~10 mg aliquots of five carbonate reference materials (ETH 1, ETH 2, ETH 3, ETH 4, and Carrara) are digested with phosphoric acid at 90 °C using a common acid bath. The evolved CO2 is purified using an automated gas preparation system (including cryotraps and a GC) and analyzed for its ∆47 and ∆48 compositions using the dual inlet system of a 253 Plus™ gas source mass spectrometer. Raw ∆47 and ∆48 values are finally normalized to the Carbon Dioxide Equilibrium Scale (CDES).In ∆47, CDES 90°Cvs. ∆48, CDES 90°C space, calcite reference materials Carrara, ETH 3 and ETH 4 agree with the equilibrium curve for calcite after adding semi-empirically determined 90 °C acid fractionation factors of 0.196 ‰ (for ∆47) and 0.136 ‰ (for ∆48) to theoretical ∆63 and ∆64 data. Agreement between measured and theoretically expected ∆48, CDES 90°C highlights the accuracy of our high-precision clumped isotope analytical setup. Combined analysis of the abundances of mass 47 and mass 48 isotopologues in CO2 evolved from acid digestion of natural carbonates has excellent potential for the determination of accurate and highly precise paleotemperatures as well as for the identification of rate-limiting kinetic processes involved in biomineralization. A formation temperature of 15(±2) °C is obtained on the 95 % confidence level for the Upper Cretaceous chalk sample ETH 3.

A reconnaissance study of 13C–13C clumping in ethane from natural gas

Ethane is the second most abundant alkane in most natural gas reservoirs. Its bulk isotopic compositions (δ13C and δD) are used to understand conditions and progress of cracking reactions that lead to the accumulation of hydrocarbons. Bulk isotopic compositions are dominated by the concentrations of singly-substituted isotopologues (13CH3–12CH3 for δ13C and 12CDH2–12CH3 for δD). However, multiply-substituted isotopologues can bring additional independent constraints on the origins of natural ethane. The 13C2H6 isotopologue is particularly interesting as it can potentially inform the distribution of 13C atoms in the parent biomolecules whose thermal cracking lead to the production of natural gas. This work presents methods to purify ethane from natural gas samples and quantify the abundance of the rare isotopologue 13C2H6 in ethane at natural abundances to a precision of ±0.12‰ using a high-resolution gas source mass spectrometer. To investigate the natural variability in carbon-carbon clumping, we measured twenty-five samples of thermogenic ethane from a range of geological settings, supported by two hydrous pyrolysis of shales experiments and a dry pyrolysis of ethane experiment. The natural gas samples exhibit a range of ‘clumped isotope’ signatures (Δ13C2H6) at least 30 times larger than our analytical precision, and significantly larger than expected for thermodynamic equilibration of the carbon-carbon bonds during or after formation of ethane, inheritance from the distribution of isotopes in organic molecules or different extents of cracking of the source. However we show a relationship between the Δ13C2H6 and the proportion of alkanes in natural gas samples, which we believe can be associated to the extent of secondary ethane cracking. This scenario is consistent with the results of laboratory experiments, where breaking down ethane leaves the residue with a low Δ13C2H6 compared to the initial gas. Carbon-carbon clumping is therefore a new potential tracer suitable for the study of kinetic processes associated with natural gas.

Competition influence in the segregation of the trophic niche of otariids: a case study using isotopic Bayesian mixing models in Galapagos pinnipeds.

The feeding success of predators is associated with the competition level for resources, and, thus, sympatric species are exposed to a potential trophic overlap. Isotopic Bayesian mixing models should provide a better understanding of the contribution of preys to the diet of predators and the feeding behavior of a species over time. The carbon and nitrogen isotopic signatures from pup hair samples of 93 Galapagos sea lions and 48 Galapagos fur seals collected between 2003 and 2009 in different regions (east and west) of the archipelago were analyzed. A PDZ Europa ANCA-GSL elemental analyzer interfaced with a PDZ Europa 20-20 continuous flow gas source mass spectrometer was employed. Bayesian models, SIAR and SIBER, were used to estimate the contribution of prey to the diet of predators, the niche breadth, and the trophic overlap level between the populations. Statistical differences in the isotopic values of both predators were observed over the time. The mixing model determined that Galapagos fur seals had a primarily teutophagous diet, whereas the Galapagos sea lions fed exclusively on fish in both regions of the archipelago. The SIBER analysis showed differences in the trophic niche between the two sea lion populations, with the western rookery of the Galapagos sea lion being the population with the largest trophic niche area. A trophic niche partitioning between Galapagos fur seals and Galapagos sea lions in the west of the archipelago is suggested by our results. At intraspecific level, the western population of the Galapagos sea lion (ZwW) showed higher trophic breadth than the eastern population, a strategy adopted by the ZwW to decrease the interspecific competition levels in the western region.

Timing of isotopic integration in marine mammal skull: comparative study between calcified tissues

Tissues with different turnover rates have different isotope compositions and reflect the different periods in an animal's life when the isotopes are incorporated during the growth of tissues. Bone is one of the most used tissues for reconstruction of an animal's diet; however, the time of isotope integration remains unknown for many species. The δ(15)N and δ(13)C values in tooth dentine and bone tissue from the maxilla and mandible of 21 stranded northern elephant seals, Mirounga angustirostris, collected on the San Benito and Magdalena Islands, Mexico, between 2000 and 2008 were compared. Bone and dentine samples from each growth layer within the tooth were analyzed using a PDZ Europa ANCA-GSL elemental analyzer interfaced with a PDZ Europa 20-20 continuous flow gas source mass spectrometer. The δ(15)N and δ(13)C values were not different between bone structures, indicating similar turnover rates, metabolic activity and amino acid compositions. The differences in the observed δ(13)C values between tissues are probably indicative of differences in their amino acid compositions, although the similarities in δ(15)N values indicated less variation from different amino acids. Correlation of the analyses between isotopic values of tissues suggests that the maxilla and/or mandible of M. angustirostris might reflect the δ(15) N signal incorporated during the last 5 years of life of the individuals. This study demonstrated the usefulness of the applied approach for providing a best approximation of the timing of isotopic integration into the skull of a marine mammal, thereby reducing uncertainty in exploring historic changes in the species' feeding behavior.

Uranium hexafluoride (UF6) gas source mass spectrometry for certification of reference materials and nuclear safeguard measurements at IRMM

Uranium hexafluoride gas source mass spectrometry at IRMM is based on two foundations, firstly the operation of a UF6 gas source mass spectrometer (GSMS) and secondly the preparation of primary UF6 reference materials, which were converted from gravimetrically prepared mixtures of highly enriched oxides of 235U and 238U. Recently a new GSMS for uranium isotopic measurements using UF6 gas, the “URANUS” from Thermo Fisher, was installed at IRMM, which also allows measurements of the so-called “minor” isotope ratios n(234U)/n(238U) and n(236U)/n(238U). In this paper the design and the implementation of measurement techniques for the new URANUS GSMS are described. This includes the “single standard” and the “double standard” (DS) method as well as the newly developed “memory corrected double standard” method (MCDS). This required a detailed investigation of memory effects within the GSMS instrument, in particular regarding the dependence of memory effects on the isotope ratios of samples and standards. The results of this study led to new recommendations for the selection of the standards for a given sample and for suitable measurement procedures. The measurement performance for the “major” isotope ratio n(235U)/n(238U) as well as the “minor” isotope ratios n(234U)/n(238U) and n(236U)/n(238U) is presented and compared with other mass spectrometric techniques. With the installation and validation of the new URANUS GSMS instrument IRMM has established two new complementary techniques for measuring the full isotopic composition of uranium samples. UF6 GSMS in combination with the MCDS method is considered the preferred technique for samples in the UF6 form and for smaller uncertainties for measurements of the major ratio n(235U)/n(238U), while thermal ionization mass spectrometry (TIMS), in combination with the “modified total evaporation” (MTE) method as well as ion counting and high abundance sensitivity for the detection of 236U, provides a superior measurement performance for the minor isotope ratios n(234U)/n(238U) and n(236U)/n(238U).

A high-resolution gas-source isotope ratio mass spectrometer

We describe a new high-resolution, multi-collector gas source mass spectrometer designed for isotopic analysis of volatile and semi-volatile molecules: the Thermo Scientific MAT253-Ultra, a prototype double-focusing isotope ratio mass spectrometer installed in the Caltech laboratories for stable isotope geochemistry. This instrument achieves mass resolving power of up to ∼27,000 (M/ΔM) and can analyze diverse gases and semi-volatile compounds using a conventional dual inlet and/or a carrier gas. It has a multi-collector array comprised of 7 detector positions with adjustable spacing, all of which can register ions through an SEM or Faraday cup and spanning up to a 1013 range in signal strength. Abundance sensitivity in the He mass range is as good as 10−12, and precision commonly approaches the counting statistics limit down to 0.1‰ (SEM) or 0.01‰ (Faraday) for a range of analytes. This instrument permits resolution of isobaric interferences arising from both contaminants and multiple isotopologues of an analyte that share a cardinal mass, enabling direct isotopic analysis of molecules with complex mass spectra such as hydrocarbons. This ability should enable the measurement of position-specific isotopic compositions, including multiple substitutions, by comparing isotope ratios of molecular ions with those of daughter fragment ions (assuming products of recombination and other source reactions are recognized and corrected for). The combination of high mass resolution with stable multicollection will provide a wide range of potential new tools for isotope geochemistry, including (but not limited to): singly and multiply substituted methane and larger hydrocarbons; position-specific 13C analysis of propane and larger hydrocarbons; precise analysis of 17O/16O and 18O/16O on fragment ions from CO2 and other molecules; analysis of a variety of N2O isotopologues (including 18O, 17O, position-specific 15N, and various ‘clumped’ species); and high precision and abundance sensitivity noble gas analyses. These capabilities greatly extend the scope of stable isotope variations that can be utilized for problems in forensics, environmental geochemistry, biochemistry, and Earth and planetary sciences.

Process methodology for the small scale production of m6N5 purity zinc using a resistance heated vacuum distillation system

Ultra high purity Zn (m6N5) was obtained through triple vacuum distillation using an m4N5 Zn as input material. High-volatile impurities were removed from the zinc matrix by vacuum evaporation, while traces having lower volatility than the matrix remained in the residual material after finishing of the entire purification process. The time required for the removal of the main high-volatile impurity (Cd) was monitored using a connected gas source mass spectrometer. During distillation the Zn vapor of the distillate was condensed on the cap of a glassy carbon crucible which was water cooled by a movable copper block. The temperatures were optimized for the removal of the volatiles and for the distillation process using vapor pressure data. High resolution glow discharge mass spectrometry was applied for the analysis and purity evaluation of the distilled and input zinc. The analysis confirmed the reduction of the metallic impurities from 43 mg kg −1 to 0.5 mg kg −1 (m6N5) after three consecutive vacuum distillations. An increase in the grain size and a decrease in the micro-hardness were observed for the purified Zn material.

Silicon isotope abundance ratios and atomic weights of NBS-28 and other reference materials

The silicon isotope abundance ratios and atomic weights of NBS-28 and several other silicon isotope reference materials were determined in this study. For the calibration of the measurement procedure, two new synthetic isotope mixtures were prepared gravimetrically from highly enriched silicon isotope materials in the form of SiO 2. All materials were converted into SiF 4 gas and subsequently their silicon isotope ratios were measured on the SiF 3 + species by using a gas source mass spectrometer MAT-253. The calibrated isotope abundance ratios of NBS-28 are 0.0507446 (26) for 29Si/ 28Si and 0.0341465 (15) for 30Si/ 28Si. The corresponding isotopic abundances are 92.17515(28)% for f( 28Si), 4.67739(24)% for f( 29Si) and 3.14746(14)% for f( 30Si). The silicon molar mass of NBS-28 is determined to be 28.08653(11), which is 0.001 larger than previously reported values. In the meanwhile, the 29Si/ 28Si and 30Si/ 28Si isotope abundance ratios and atomic weights of SRM-990, IRMM-017, IRMM-018, GBW-04421 and GBW-04422 are also calibrated. These new calibrated data can improve the reliability and comparability of silicon isotope results using above calibrators.

Analytical Methods for Non-Traditional Isotopes

This chapter is devoted to the analytical methods employed for making high precision isotope ratio measurements that preserve naturally occurring mass-dependent isotopic variations. The biggest challenge in making these types of measurements is deconvolving mass-dependent isotopic fractionation produced in the laboratory and mass spectrometer, from naturally occurring mass-dependent isotopic fractionation, because the patterns of isotope variation produced by these processes are identical. Therefore, the main theme of this chapter is the description and mathematical treatment of mass-dependent isotopic variations and the possible pitfalls in deconvolving instrumental mass bias from naturally occurring mass-dependent isotopic variations. This chapter will not attempt to catalog methods for isotopic analysis of different elements. These details are better discussed in later chapters where ‘element specific’ analytical issues are covered. Rather, the effort in the chapter will be to focus on those specific items that make mass analysis of non-traditional isotopes challenging and unique, and the methods that can be employed to make precise and accurate isotope ratios. Isotopic analysis of non-traditional isotopes is made using three main types of mass spectrometers. Elements that can easily be introduced as gases, such as Cl or Br, are typically analyzed using a gas source mass spectrometer. In contrast, metal elements are analyzed using either a thermalionization mass spectrometer (TIMS) or a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS). The main difference between these three types of instruments is how the sample is introduced into the instrument, and how the sample is ionized. In contrast, the analyzer part of each instrument is similar. All three types of instruments use a stack of lenses with variable potential to focus the ion beam, a magnet to resolve the ion beam into different masses, and a series of collectors to measure ion currents of different isotopes simultaneously. The core of the mass analyzer …

Calibrated sulfur isotope abundance ratios of three IAEA sulfur isotope reference materials and V-CDT with a reassessment of the atomic weight of sulfur

Calibrated values have been obtained for sulfur isotope abundance ratios of sulfur isotope reference materials distributed by the IAEA (Vienna). For the calibration of the measurements, a set of synthetic isotope mixtures were prepared gravimetrically from high purity Ag2S materials enriched in32S, 33S, and 34S. All materials were converted into SF6 gas and subsequently, their sulfur isotope ratios were measured on the SF5+ species using a special gas source mass spectrometer equipped with a molecular flow inlet system (IRMM’s Avogadro II amount comparator). Values for the 32S/34S abundance ratios are 22.650 4(20), 22.142 4(20), and 23.393 3(17) for IAEA-S-1, IAEA-S-2, and IAEA-S-3, respectively. The calculated 32S/34S abundance ratio for V-CDT is 22.643 6(20), which is very close to the calibrated ratio obtained by Ding et al. (1999). In this way, the zero point of the VCDT scale is anchored firmly to the international system of units SI. The 32S/33S abundance ratios are 126.942(47), 125.473(55), 129.072(32), and 126.948(47) for IAEA-S-1, IAEA-S-2, IAEA-S-3, and V-CDT, respectively. In this way, the linearity of the V-CDT scale is improved over this range. The values of the sulfur molar mass for IAEA-S-1 and V-CDT were calculated to be 32.063 877(56) and 32.063 911(56), respectively, the values with the smallest combined uncertainty ever reported for the sulfur molar masses (atomic weights).

In situ Measurement of Oxygen Isotope Ratios by Laser Ablation Mass Spectrometry — an Example from the Torre Alfina Volcano, Central Italy

The Torre Alfina volcano is located in the southern part of the Tuscan Magmatic Province, Central Italy. The rocks of the volcano have a rather constant mineralogical and geochemical composition and are slightly porphyric with olivine microphenocrysts set in a microcrystalline groundmass consisting of biotite and feldspar. They belong to the ultrapotassic rock family (Peccerillo, 1994). Corresponding to the classification of Foley et al. (1987) the Torre Alfina rocks are transitional rocks between lamproites and Roman-type rocks. Frequently mantle xenoliths and crustal xenoliths are embedded. Lamproitic magma modified by crustal assimilation is assumed as source material for the Torre Alfina transitional rocks (Peccerillo, 1994). The genesis of lamproitic magma is considered by partial melting of a refractory harzburgitic mantle, which was metasomatized by a melt with a composition similiar to pelitic sediments (Conticelli and Peccerillo, 1992). The metasomatism of the mantle probably occurs in the formation of mica rich veins (Foley, 1992). Since the late 80th's laser microprobe techniques have been developed to measure oxygen isotope ratios in single minerals or in small rock sections. Refractory minerals like olivine and garnet can be analysed with the UV laser ablation technique without any fractionation due to attainable high temperatures (Wiechert and Hoefs, 1995). The applied laser microprobe device is built up by an optical system for beam adjustment and sample observation, a vacuum system to purify the sample gas and to transform the oxygen gas into CO2 with heated diamonds, and a gas source mass spectrometer. The reproducibility for in situ measurements of 8180 is about +0.2%0. In order to prohibit recombination of the vapourized oxygen and metal atoms during laser ablation the sample chamber is filled with fluorine gas. Rocks consisting of a fine-grained or vitrious and feldspar rich matrix and of porous structures as well as carbonates and/or water containing minerals start to react with the fluorine before laser treatment, which leads to high blanks. To reduce the blank a special acid treatment has been developed prior to the introduction of the sample into the sample chamber. Despite this treatment some samples still generate a high blank. In these cases the blank can be reduced by the fluorination of the reactive part of the sample. One peridotitic xenolith and one crustal xenolith have been selected for this study and analysed in detail. Oxygen isotope ratios have been measured using an ArF-excimer laser ablation technique. Figure 1 shows a traverse of oxygen isotope analyses in a peridotitic xenolith, which mainly consists of olivines and minor amounts of orthopyroxenes and clinopyroxenes. Olivines are both finegrained (about 200 gm) as well as coarse-grained (about 1 mm). Oxygen isotope ratios of olivines

The role of off-line mass spectrometry in nuclear fission.

The role of mass spectrometry in nuclear fission has been invaluable since 1940, when A. O. C. Nier separated microgram quantities of (235) U from (238) U, using a gas source mass spectrometer. This experiment enabled the fissionable nature of (235) U to be established. During the Manhattan Project, the mass spectrometer was used to measure the isotope abundances of uranium after processing in various separation systems, in monitoring the composition of the gaseous products in the Oak Ridge Diffusion Plant, and as a helium leak detector. Following the construction of the first reactor at the University of Chicago, it was necessary to unravel the nuclear systematics of the various fission products produced in the fission process. Off-line mass spectrometry was able to identify stable and long-lived isotopes produced in fission, but more importantly, was used in numerous studies of the distribution of mass of the cumulative fission yields. Improvements in sensitivity enabled off-line mass spectrometric studies to identify fine structure in the mass-yield curve and, hence, demonstrate the importance of shell structure in nuclear fission. Solid-source mass spectrometry was also able to measure the cumulative fission yields in the valley of symmetry in the mass-yield curve, and enabled spontaneous fission yields to be quantified. Apart from the accurate measurement of abundances, the stable isotope mass spectrometric technique has been invaluable in establishing absolute cumulative fission yields for many isotopes making up the mass-yield distribution curve for a variety of fissile nuclides. Extensive mass spectrometric studies of noble gases in primitive meteorites revealed the presence of fission products from the now extinct nuclide (244) Pu, and have eliminated the possibility of fission products from a super-heavy nuclide contributing to isotopic anomalies in meteoritic material. Numerous mass spectrometric studies of the isotopic and elemental abundances of samples from the Oklo Natural Reactor have enabled the nuclear parameters of the various reactor zones to be calculated, and the mobility/retentivity of a number of elements to be established in the reactor zones and the surrounding rocks. These isotopic studies have given valuable information on the geochemical behavior of natural geological repositories for radioactive waste containment. © 1997 John Wiley & Sons, Inc.

On the measurement of the valve-mixing correction of the changeover valve of a dual-inlet gas source mass spectrometer

One of the most important instrumental correction factors in the measurement of stable isotope ratios from dynamic gas source mass spectrometers is the so-called mixing correction for the changeover valve of the inlet system. Methods for the determination of this parameter are discussed and it is demonstrated that with a particular valve a commonly used technique gives values of the correction factor which are not necessarily applicable to normal operation. A more rigorous procedure is suggested, and details are given of the mixing investigated with argon, carbon dioxide and nitrogen.

Isotopic analysis of microgram quantities of lead iodide using an electron-bombardment source mass spectrometer

A method for routine isotopic analysis of small amounts of lead using PbI 2 + ions in a modified commercial gas-source mass spectrometer has been developed. Samples as small as 2 μg of lead have been analyzed successfully. Mass fractionation averages + 0.06% per mass unit with maximum observed fractionation of + 0.13% per mass unit. Chemical separation of lead by a standard anion exchange column produces samples of sufficient purity for mass spectrometry by PbI 2.

Precision measurement of lead isotopes ratios: preliminary analyses from the U.S. mine, Bingham Canyon, Utah

A gas-source mass spectrometer has been constructed for the precise measurement of lead isotope ratios. Sixteen analyses on 4 different preparations of the same galena made over a period of 2 months gave 95% confidence limits (per analysis) of 206Pb/ 204Pb= 0.080%, 207Pb/ 206Pb= 0.042%and 208Pb/ 206Pb= 0.046% . Eight samples from the U.S. mine in the Bingham district have a linear relationship over the 1% range of their 206Pb/ 204Pb ratios. The simplest model fitting these data suggests that the lead was separated from a primary system (μ=8.98) 1630±150m.y. ago and subsequently mixed with a radiogenic lead of similar age; biotite K-Ar dates for altered intrusive rocks associated with the ores provide an apparent age of mineralization and suggest that isotopic evolution of the lead was terminated about 36 m.y. ago.

The anomalous nature of Ivigtut lead

Kanasewich and Slawson (1964) have recently suggested that the lead of Ivigtut, GreenLond, which is very unusual isotopically, is anomalous in the sense that it has developed in a multi-stage system. This interpretation is based on eight galena analyses from Ivigtut, which agree with their mean ratio within 1/8 of one per cent, and one analysis from Laxebunden, 12 km N.E. of Ivigtut. This paper presents five additional analyses from this area which are considered to establish definitively the anomalous nature of the lead. The samples, which consist of pyrite and galena, were compared with a standard in a gas-source mass spectrometer using a free radical technique for the preparation of lead tetramethyl. The lead isotope ratios from Ivigtut are linearly related on a plot of Pb 207/Pb 204 vs. Pb 206/Pb 204 with a slope of 0.113 ± 0.004. The deviation of points from their best straight line is 0.03 per cent of the average Pb 207/Pb 204 ratio. The isotopic analyses are interpreted to indicate that the Ivigtut leads were deposited at a time not earlier than 1100 ±40 m.y . ago and that the source material for the anomalous lead component came into existence between 1880 ± 60 m.y . and 1100 ± 40 m.y . ago. Ordinary lead mineralization occurred at least 2980 ± 10 m.y . ago. A possible application of lead isotope studies to problems of continental drift is discussed.

Gas source mass spectrometry of trace leads from Sudbury, Ontario

The measurement of lead isotope abundances with a gas source mass spectrometer has been limited to lead ores and minerals. A free radical technique for the preparation of lead tetramethyl is described which has reduced by a substantial amount the sample size necessary for a precise gas source analysis. In this way, the range of minerals which may be studied by means of a gas source mass spectrometer has been greatly extended. The free radical method has been applied to the study of lead isotope abundance variations in the mining district of Sudbury, Ontario. The analysis of lead from various sulphides has yielded lead isotope ratios which are linearly related on a plot of Pb 207/Pb 204 vs. Pb 206/Pb 204 with a slope of 0.131 ± 0.003; the standard deviation of points from their best straight line is 0.37 per cent of the average Pb 207/Pb 204 value. (The intercomparison technique previously used by us to determine the ratios of ordinary leads was not used for these analyses.) It is concluded that the linear relationship is the result of just two geological events, which fact simplifies the possible interpretations. The maximum times of these two events are 21.30 ± 50 m. y. and 1280 ± 50 m. y. respectively. Two interpretations of the results are suggested. The first gives ages of 1950 and 350 m.y. for the primary and secondary events. The two events in the second interpretation are considered to have occurred 1600 and 950 m.y. ag. The second interpretation is more easily defended on geological grounds.