Ross W. Williams

Magnesium isotope heterogeneity of the isotopic standard SRM980 and new reference materials for magnesium-isotope-ratio measurements

Multicollector ICP-MS has been used for the precise measurement of variations in the isotopic composition of the isotopic standard of magnesium (SRM980) provided by the National Institute of Standards and Technology (Gaithersburg, MD, USA). The SRM980 consists of metal chips weighing between 1 and 50 mg and each unit delivered by the National Institute of Standards and Technology corresponds to a bottle containing about 0.3 g. Height units were analysed. Variations in sample 25Mg/24Mg, and 26Mg/24Mg ratios are expressed as δ25Mg and δ26Mg units, respectively, which are deviations in parts per 103 from the same ratio in a standard solution. The differences in δ25Mg and δ26Mg of the SRM980 are up to 4.20 and 8.19‰, respectively, while the long-term repeatability of δ25Mg and δ26Mg are 0.09 and 0.16‰, respectively, at 95% confidence. However, when plotted in a three-isotope diagram, all the data fall on a single mass fractionation line. Overall limits of error of the SRM980 reported here fall within the previously reported overall limits of error. The isotopic heterogeneity not only corresponds to differences among units but has been found at the chip-size level. This result, due to the precision of the MC-ICP-MS technique, makes the SRM980 inappropriate for the international isotopic standard of magnesium. The SRM980 can still be used to report the excess of 26Mg, which is defined by the deviation from the mass-dependent relationship between 25Mg/24Mg, and 26Mg/24Mg ratios. Two large batches (around 10 g of Mg in each) of pure Mg solutions (in 0.3 M HNO3) have been prepared and characterised. These 2 solutions (DSM3 and Cambridge 1) are suitable reference material because they are immune to heterogeneity. DSM3 and Cambridge 1 are isotopically different (by 1.3‰ per u) and are available upon request from the first author. In addition, DSM3 has an isotopic composition very similar to the Mg-isotopic composition of carbonaceous chondrites (Orgueil and Allende). Because of the lack of heterogeneity and the cosmochemical and geochemical significance of DSM3, we urge the use of DSM3 as the primary isotopic reference material to report Mg-isotopic variations.

Across‐arc geochemical trends in the Izu‐Bonin arc: Contributions from the subducting slab, revisited

New Sr, Nd, Hf, and Pb isotope and trace element data are presented for basalts erupted in the Izu back arc. We propose that across-arc differences in the geochemistry of Izu-Bonin arc basalts are controlled by the addition of aqueous slab fluids to the volcanic front and hydrous partial melt of the slab to the back arc. The volcanic front has the lowest concentrations of incompatible elements, the strongest relative enrichments of fluid-mobile elements, and the most radiogenic Sr, Nd, Hf, and Pb, suggesting the volcanic front is the result of high degrees of partial melting of a previously depleted mantle source caused by an aqueous fluid flux from the slab. Relative to the volcanic front, the back arc has higher concentrations of incompatible elements and elevated La/Yb and Nb/Zr, suggesting lower degrees of partial melting of a less depleted or even enriched mantle source. Positive linear correlations between fluid-immobile element concentrations and the estimated degree of mantle melting suggest the slab contribution added to the mantle wedge in the Izu back arc is a supercritical melt. Pb, Nd, and Hf isotopes and Th/La systematics of back-arc basalts are consistent with a slab melt composed of >90% altered oceanic crust and <10% sediment; that is, altered oceanic crust, not subducted sediment, dominates the slab contribution. High field strength element systematics require supercritical melts to be in equilibrium with residual rutile and zircon.

235U–231Pa age dating of uranium materials for nuclear forensic investigations

Age dating of nuclear material can provide insight into source and suspected use in nuclear forensic investigations. We report here a method for the determination of the date of most recent chemical purification for uranium materials using the 235U-231Pa chronometer. Protactinium is separated from uranium and neptunium matrices using anion exchange resin, followed by sorption of Pa to an SiO2 medium. The concentration of 231Pa is measured by isotope dilution mass spectrometry using 233Pa spikes prepared from an aliquot of 237Np and calibrated in-house using the rock standard Table Mountain Latite and the uranium isotopic standard U100. Combined uncertainties of age dates using this method are 1.5 to 3.5 %, an improvement over alpha spectrometry measurement methods. Model ages of five uranium standard reference materials are presented; all standards have concordant 235U-231Pa and 234U-230Th model ages.

Validation of reference materials for uranium radiochronometry in the frame of nuclear forensic investigations

The results of a joint effort by expert nuclear forensic laboratories in the area of age dating of uranium, i.e. the elapsed time since the last chemical purification of the material are presented and discussed. Completely separated uranium materials of known production date were distributed among the laboratories, and the samples were dated according to routine laboratory procedures by the measurement of the (230)Th/(234)U ratio. The measurement results were in good agreement with the known production date showing that the concept for preparing uranium age dating reference material based on complete separation is valid. Detailed knowledge of the laboratory procedures used for uranium age dating allows the identification of possible improvements in the current protocols and the development of improved practice in the future. The availability of age dating reference materials as well as the evolvement of the age dating best-practice protocol will increase the relevance and applicability of age dating as part of the tool-kit available for nuclear forensic investigations.

Round-robin 230Th–234U age dating of bulk uranium for nuclear forensics

In an inter-laboratory measurement comparison study, four laboratories determined 230Th–234U model ages of uranium certified reference material NBL U050 using isotope dilution mass spectrometry. The model dates determined by the participating laboratories range from 9 March 1956 to 19 October 1957, and are indistinguishable given the associated measurement uncertainties. These model ages are concordant with to slightly older than the known production age of NBL U050.

Developing 226Ra and 227Ac age-dating techniques for nuclear forensics to gain insight from concordant and non-concordant radiochronometers

The model age or ‘date of purification’ of a nuclear material is an important nuclear forensic signature. In this study, chemical separation and MC-ICP-MS measurement techniques were developed for 226Ra and 227Ac: grand-daughter nuclides in the 238U and 235U decay chains, respectively. The 230Th–234U, 226Ra–238U, 231Pa–235U, and 227Ac–235U radiochronometers were used to calculate model ages for CRM-U100 standard reference material and two highly-enriched pieces of uranium metal from the International Technical Working Group Round Robin 3 Exercise. Results demonstrate the accuracy of the 226Ra–238U and 227Ac–235U chronometers and provide information about nuclide migration during uranium processing.

Measuring 0.01‰ to 0.1‰ isotopic variations by MC-ICPMS—testing limits for the first time with Pb δ-iCRMs

A blind comparison on Pb-isotope δ-scale measurements by MC-ICPMS of 0.01‰ to 0.1‰ level was organised, involving five laboratories. Test samples were obtained from the series of candidate ERM-3810 δ-isotopic Certified Reference Materials (δ-iCRMs), and comprise four pairs of a material with ∼natural Pb-isotopic composition (‘delta zero’ or ‘δ-0’) and the same natural Pb progressively enriched in 207Pb (with δ207Pb values certified to ∼0.1% relative uncertainty, k = 2). Participants were free to apply the measurement strategy of their choice. A result was considered ‘acceptable’ only when, simultaneously, there was agreement within stated uncertainties with the corresponding reference value and the relative uncertainty stated by the participant was < 100%. This study illustrates the high degree of difficulty inherent to these δ-scale measurements by ‘routine’ MC-ICPMS methodologies (in this case, three participants reported 55% of their results which were deemed accurate, and the other two reported none). The closer to unity the isotope ratio value the better the results became (‘acceptable’ results mostly for δ7/6‰ and δ7/8‰ measurements). This first experiment of its kind demonstrates that Pb δ-scale isotopic measurements by MC-ICPMS can be reliably carried out down to 0.05‰ levels (two participants delivered accurate results above this threshold systematically for δ7/6‰, δ7/8‰ and δ7/4‰). Below this limit, at ∼0.01‰ and ∼ 0.03‰ levels, results are no longer consistent or reproducible and appear to be susceptible to a number of effects introducing error (such as short term changes in mass discrimination) which are either not well understood, or not controlled and/or not corrected for at a sufficiently low level of uncertainty. These results also suggest that ‘routine’ methods for absolute (calibrated) Pb-isotope ratio determination by MC-ICPMS produce relative combined uncertainties on results which are unlikely to be better than 0.05‰ (k = 2).

Application of the 226Ra–230Th–234U and 227Ac–231Pa–235U radiochronometers to uranium certified reference materials

Uranium certified reference materials (CRM) issued by New Brunswick Laboratory were subjected to dating using four independent uranium-series radiochronometers. In all cases, there was acceptable agreement between the model ages calculated using the 231Pa–235U, 230Th–234U, 227Ac–235U or 226Ra–234U radiochronometers and either the certified 230Th–234U model date (CRM 125-A and CRM U630), or the known purification date (CRM U050 and CRM U100). The agreement between the four independent radiochronometers establishes these uranium certified reference materials as ideal informal standards for validating dating techniques utilized in nuclear forensic investigations in the absence of standards with certified model ages for multiple radiochronometers.

Nuclear forensic analysis of uranium oxide powders interdicted in Victoria, Australia

Abstract Nuclear forensic analysis was conducted on two uranium samples confiscated during a police investigation in Victoria, Australia. The first sample, designated NSR-F-270409-1, was a depleted uranium powder of moderate purity (∼ 1000 μg/g total elemental impurities). The chemical form of the uranium was a compound similar to K2(UO2)3O4 · 4H2O. While aliquoting NSR-F-270409-1 for analysis, the body and head of a Tineid moth was discovered in the sample. The second sample, designated NSR-F-270409-2, was also a depleted uranium powder. It was of reasonably high purity (∼ 380 μg/g total elemental impurities). The chemical form of the uranium was primarily UO3 · 2H2O, with minor phases of U3O8 and UO2. While aliquoting NSR-F-270409-2 for analysis, a metal staple of unknown origin was discovered in the sample. The presence of 236U and 232U in both samples indicates that the uranium feed stocks for these samples experienced a neutron flux at some point in their history. The reactor burn-up calculated from the isotopic composition of the uranium is consistent with that of spent fuel from natural uranium (NU) fueled Pu production. These nuclear forensic conclusions allow us to categorically exclude Australia as the origin of the material and greatly reduce the number of candidate sources.

Plutonium(IV) and (V) Sorption to Goethite at Sub-Femtomolar to Micromolar Concentrations: Redox Transformations and Surface Precipitation

Pu(IV) and Pu(V) sorption to goethite was investigated over a concentration range of 10(-15)-10(-5) M at pH 8. Experiments with initial Pu concentrations of 10(-15) - 10(-8) M produced linear Pu sorption isotherms, demonstrating that Pu sorption to goethite is not concentration-dependent across this concentration range. Equivalent Pu(IV) and Pu(V) sorption Kd values obtained at 1 and 2-week sampling time points indicated that Pu(V) is rapidly reduced to Pu(IV) on the goethite surface. Further, it suggested that Pu surface redox transformations are sufficiently rapid to achieve an equilibrium state within 1 week, regardless of the initial Pu oxidation state. At initial concentrations >10(-8) M, both Pu oxidation states exhibited deviations from linear sorption behavior and less Pu was adsorbed than at lower concentrations. NanoSIMS and HRTEM analysis of samples with initial Pu concentrations of 10(-8) - 10(-6) M indicated that Pu surface and/or bulk precipitation was likely responsible for this deviation. In 10(-6) M Pu(IV) and Pu(V) samples, HRTEM analysis showed the formation of a body centered cubic (bcc) Pu4O7 structure on the goethite surface, confirming that reduction of Pu(V) had occurred on the mineral surface and that epitaxial distortion previously observed for Pu(IV) sorption occurs with Pu(V) as well.