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Two plutonium oxides were prepared as unique reference materials for measurement of actinide elements present as trace constituents. Each reference material unit is approximately 200 mg of PuO2 powder in a quartz glass bottle. Characterized attributes of the oxides included mass fractions of plutonium, americium, neptunium, and uranium. Isotope-amount ratios were also determined for plutonium and uranium, but neptunium and americium were observed to be monoisotopic 237Np and 241Am. Measurements for characterization and verification of the attributes show that plutonium and trace actinides are homogeneous with the exception of limited heterogeneity for uranium, primarily observed for the 238U isotope. Model purification ages calculated from measured americium and uranium attribute values are consistent with material histories and indicate that these impurities are predominantly due to the decay of plutonium isotopes.

In 2017, the Nuclear Forensics International Technical Working Group organized their fifth Collaborative Materials Exercise (CMX-5). The exercise samples were two uranium dioxide fuel pellets manufactured from the same starting materials by different processes to have similar bulk isotopic composition, but different spatial uranium isotopic distributions. Sets of identical materials were sent to all participating laboratories, who then utilized their existing nuclear forensic capabilities to independently analyse fuel pellets and identify similarities and differences of the materials’ characteristics. Here we present and compare the ability of different analytical techniques to spatially resolve uranium isotopic heterogeneity in the uranium oxide fuel pellets.

During 2017–2018 Los Alamos National Laboratory, Lawrence Livermore National Laboratory and the China Institute of Atomic Energy collaborated in an interlaboratory 231Pa/235U radiochronometry exercise. The laboratories used different analytical methods to obtain a consensus model purification date for CRM U010 of December 28, 1958 ± 198 days and for CRM U850 of May 20, 1958 ± 363 days. These results agree with previously reported model dates using the 230Th/234U radiochronometer as well as the production histories of these materials. The concordance of interlaboratory data confirms the ability of laboratories to make reproducible radiochronometry measurements using distinct analytical approaches.

A highly-enriched (244)Pu isotope dilution reference material has been prepared and characterized for metrologically traceable measurements of very small quantities of plutonium. The amount of plutonium in samples associated with nuclear safeguards and nuclear forensic measurements can be significantly less than 1 ng. Accordingly, the ability to quantify the amount and isotopic composition of plutonium from a single mass-spectrometric analysis is particularly desirable. The highly-enriched (244)Pu reference material, described here, will minimize the magnitude of spike corrections necessary to obtain accurate information on plutonium isotopic composition from isotope dilution measurements.

Application of the 231Pa/235U radiochronometer for nuclear forensic investigations is challenged by a lack of certified reference materials with 231Pa/235U model purification dates. The Japan Atomic Energy Agency, Los Alamos National Laboratory, and Lawrence Livermore National Laboratory completed an interlaboratory study measuring 231Pa/235U model ages of New Brunswick Laboratory CRM U100. Results from independent laboratories were combined to calculate a consensus 231Pa/235U model purification date for CRM U100 of March 26, 1959 ± 237 days. This 231Pa/235U consensus date for CRM U100 may be used by the nuclear forensic community for quality control of 231Pa/235U radiochronometry measurements of unknown materials.

Radiochronometric data, a key signature in evaluating the provenance and process history of nuclear material out of regulatory control, are conventionally acquired via multi-collector mass spectrometry. Here we explore the potential of age-dating by single-collector mass spectrometry. To evaluate model age accuracy/precision across different instrument designs, we performed 230Th–234U and 231Pa–235U radiochronometry of CRM 125-A using two single-collector and one multi-collector plasma source mass spectrometers. Single-collector instruments produce accurate model ages for this uranium standard and thus hold promise for nuclear forensics radiochronometry. Increased acquisition of age information via multiple instrument designs will bolster the global response to nuclear interdictions.

A highly-enriched (233)U reference material (>0.99987 n((233)U)/n(U)) has been prepared and characterized for use as an isotope dilution mass spectrometry spike. An ion exchange separation was performed on 1 g of high purity (233)U to further reduce trace amounts of contaminant Pu in the material. The purified (233)U was then prepared as a master solution which was analyzed for molality of uranium by modified Davies and Gray titration. A portion of the master solution was quantitatively diluted and dispensed for reference material units. Selected units were analyzed for verification of uranium amount and to characterize uranium isotope amount ratios by multi-collector inductively couple plasma mass spectrometry. Modelling of spike-corrected isotopic data show that the new spike will enable simultaneous measurements of uranium amount and isotope amount ratios with resulting uncertainties that are substantially less sensitive to over spiking than widely used (233)U certified reference materials.

A (231)Pa reference material has been characterized for amount of protactinium. This reference material is primarily intended for calibration of (233)Pa tracers produced for (235)U–(231)Pa model age measurements associated with nuclear forensics and nuclear safeguards. Primary measurements for characterization were made by isotope dilution mass spectrometry of a purified (231)Pa solution using a (233)Pa isotopic spike. The spike was calibrated by allowing multiple aliquots of the (233)Pa spike solution to decay to (233)U and then measuring the ingrown (233)U by isotope dilution mass spectrometry using a certified uranium assay and isotopic standard as a reverse-spike. The new (231)Pa reference material will simplify calibration of the (233)Pa isotope dilution spikes, provide metrological traceability, and potentially reduce the overall measurement uncertainty of model ages.

The application of multiple radiochronometers in a forensic investigation of bulk uranium provides increased confidence for interpreting age-dating results. We have developed a streamlined method for the purification of Pa and Th from bulk uranium and have applied this method in analyzing uranium certified reference materials (CRMs) IRMM-1000 and CRM U100, using the 231Pa–235U and 230Th–234U radiochronometers. Our improved 233Pa spike calibration technique has replaced the time-intensive, conventional calibration using geologic rock standards. Paired Pa–U and Th–U age-dating analyses of CRM U100 produce concordant results. In contrast, analyses of IRMM-1000 demonstrate reproducibly-discordant Th–U and Pa–U age-dates.

Radiochronometry analyses of two Pu metals were performed using the 237Np–241Am–241Pu, 234U–238Pu, 235U–239Pu, and 236U–240Pu decay series. For one sample, all radiochronometers yield concordant model dates in 1959–1960, indicating that the aqueous processing method used to purify Pu effectively removed U, Am and Np decay products from the bulk Pu. The second sample yields discordant model dates that are also older than the known production date in 1982. The excess U, Am and Np present in the sample indicate that the sample was purified during at least two different episodes, using a combination of aqueous methods and molten salt extraction.

The 238U decay constant (λU-238) is fundamental to radioisotope-based chronometry in the Earth and planetary sciences, yet only a single published λU-238 value (Jaffey et al. in Phys Rev C 4(5):1889–1906, 1971) is widely applied. We have determined λU-238 via the novel approach of measuring of 234Th ingrowth in high-purity 238U solutions, using isotope dilution mass spectrometry (ID-MS). The 234Th decay constant (λTh-234) was measured via decay counting with high-purity Ge (HPGe) γ detectors. Preliminary results for λU-238 agree with the value determined by α-counting [1] within the elevated uncertainty of 0.462% (k = 2). Ongoing efforts to reproduce λU-238 with reduced experimental uncertainties will inform future conclusions.

Uranium-series radiometric dating was performed on two well-pedigreed UF6 cylinder samples containing highly enriched uranium that were in use at the Portsmouth Gaseous Diffusion Plant between the mid-1970s and mid-1980s. Daughter–parent (i.e. 231Pa–235U, 230Th–234U) radiometric ages determined on solid heel materials (e.g. UO2F2, UF4) recovered from the emptied cylinders are unrealistically old due to the preferential sequestration of progeny isotopes relative to uranium within the solid heels. However, limited elemental fractionation amongst the progeny isotopes themselves in the heel material allowed for realistic age constraints on the fill dates of the UF6 cylinders to be extracted from the granddaughter–daughter systems (i.e. 226Ra–230Th, 227Ac–231Pa).

In a recent international exercise, 10 international nuclear forensics laboratories successfully performed radiochronometry on three low enriched uranium oxide samples, providing 12 analytical results using three different parent-daughter pairs serving as independent chronometers. The vast majority of the results were consistent with one another and consistent with the known processing history of the materials. In general, for these particular samples, mass spectrometry gave more accurate and more precise analytical results than decay counting measurements. In addition, the concordance of the 235U–231Pa and 234U–230Th chronometers confirmed the validity of the age dating assumptions, increasing confidence in the resulting conclusions.

In 2014 the United States Department of Energy and the China Institute of Atomic Energy collaborated in a study measuring the model ages of uranium certified reference materials. Lawrence Livermore National Laboratory (LLNL) and Los Alamos National Laboratory (LANL), and the Chinese Institute of Atomic Energy (CIAE) determined 230Th–234U model ages for uranium certified reference materials U010 and U850. The aim of this work was to collaborate with CIAE and compare methods for measuring the age of bulk uranium materials using the 230Th–234U radiochronometer. Accurate results for age-dating depend on the accurate calibration of the tracer materials (e.g., 229Th and 233U) used for the isotope dilution mass spectrometry (IDMS) analyses. To facilitate inter-comparison of results, samples of a 230Th standard reference material were distributed to each laboratory for 229Th tracer inter-calibration. The resulting 230Th–234U model ages from this collaboration for U010 ranged from March 1956 to January 1959, which agree with the known material production date of June 5th, 1958. The determined model ages for U850 were from December 1955 to October 1957, which agree with the material production date of December 31, 1957. All three laboratories used independent methods to determine model ages for uranium standards that agree with known production ages and with previously reported results.

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.

A new determination of the (229)Th half-life was made based on measurements of the (229)Th massic activity of a high-purity solution for which the (229)Th molality had previously been measured. The (229)Th massic activity was measured by direct comparison with SRM 4328C using 4παβ liquid scintillation counting, NaI counting, and standard addition liquid scintillation counting. The massic activity was confirmed by isotope dilution alpha spectrometry measurements. The calculated (229)Th half-life is (7825 ± 87) years (k = 2), which is shorter than the three most recent half-life determinations but is consistent with these values within uncertainties.

An intercomparison of the radio-chronometric ages of four distinct plutonium-certified reference materials varying in chemical form, isotopic composition, and period of production are presented. The cross-comparison of the different 234U/238Pu, 235U/239Pu, 236U/240Pu, and 241Am/241Pu model purification ages obtained at four independent analytical facilities covering a range of laboratory environments from bulk sample processing to clean facilities dedicated to nuclear forensic investigation of environmental samples enables a true assessment of the state-of-practice in "age dating capabilities" for nuclear materials. The analytical techniques evaluated used modern mass spectrometer instrumentation including thermal ionization mass spectrometers and inductively coupled plasma mass spectrometers for isotopic abundance measurements. Both multicollector and single collector instruments were utilized to generate the data presented here. Consensus values established in this study make it possible to use these isotopic standards as quality control standards for radio-chronometry applications. Results highlight the need for plutonium isotopic standards that are certified for 234U/238Pu, 235U/239Pu, 236U/240Pu, and 241Am/241Pu model purification ages as well as other multigenerational radio-chronometers such as 237Np/241Pu. Due to the capabilities of modern analytical instrumentation, analytical laboratories that focus on trace level analyses can obtain model ages with marginally larger uncertainties than laboratories that handle bulk samples. When isotope ratio measurement techniques like thermal ionization mass spectrometry and inductively coupled plasma mass spectrometry with comparable precision are utilized, model purification ages with similar uncertainties are obtained.

A new determination of the

A highly-enriched 233U reference material (>0.99987 n(233U)/n(U)) has been prepared and characterized for use as an isotope dilution mass spectrometry spike. An ion exchange separation was performed on 1 g of high purity 233U to further reduce trace amounts of contaminant Pu in the material. The purified 233U was then prepared as a master solution which was analyzed for molality of uranium by modified Davies and Gray titration. A portion of the master solution was quantitatively diluted and dispensed for reference material units. Selected units were analyzed for verification of uranium amount and to characterize uranium isotope amount ratios by multi-collector inductively couple plasma mass spectrometry. Modelling of spike-corrected isotopic data show that the new spike will enable simultaneous measurements of uranium amount and isotope amount ratios with resulting uncertainties that are substantially less sensitive to over spiking than widely used 233U certified reference materials.

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 concentrations10(-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.

The results of an international comparison of activity measurements of a solution of 231Pa are reported and analysed. Prior to this, no known standardisation of 231Pa by activity measurement had been carried out. The comparison was run in 2017–2018 involving eight laboratories, and returned results with no identifiable inconsistencies between methods or laboratories. The results, including one mass determination, gave a231Pa activity concentration of 41.461(48) kBq g−1 and a231Pa atom concentration of 61.48(23) × 1015 atoms g−1, from which a half-life value of 32 570(130) years was derived.

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.

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 230Th/234U 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.

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.

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.

A new reference material was characterized for (229)Th molality and thorium isotope amount ratios. This reference material is intended for use in nuclear forensic analyses as an isotope dilution mass spectrometry spike. The reference material value and expanded uncertainty (k = 2) for the (229)Th molality is (1.1498 ± 0.0016) × 10(−10) mol g(−1) solution. The value and expanded uncertainty (k = 2) for the n((230)Th)/n((229)Th) ratio is (5.18 ± 0.26) × 10(−5) and the n((232)Th)/n((229)Th) ratio is (3.815 ± 0.092) × 10(−4).

Early in 2009, a state policing agency raided a clandestine drug laboratory in a suburb of a major city in Australia. During the search of the laboratory, a small glass jar labelled "Gamma Source" and containing a green powder was discovered. The powder was radioactive. This paper documents the detailed nuclear forensic analysis undertaken to characterise and identify the material and determine its provenance. Isotopic and impurity content, phase composition, microstructure and other characteristics were measured on the seized sample, and the results were compared with similar material obtained from the suspected source (ore and ore concentrate material). While an extensive range of parameters were measured, the key 'nuclear forensic signatures' used to identify the material were the U isotopic composition, Pb and Sr isotope ratios, and the rare earth element pattern. These measurements, in combination with statistical analysis of the elemental and isotopic content of the material against a database of uranium ore concentrates sourced from mines located worldwide, led to the conclusion that the seized material (a uranium ore concentrate of natural isotopic abundance) most likely originated from Mary Kathleen, a former Australian uranium mine.

In a recent international exercise, 10 international nuclear forensics laboratories successfully performed radiochronometry on 3 low-enriched uranium oxide samples, providing 12 analytical results using 3 different parent-daughter pairs serving as independent chronometers. The vast majority of the results were consistent with one another and consistent with the known processing history of the materials. In general, for these particular samples, mass spectrometry gave more accurate and more precise analytical results than decay counting measurements. In addition, the concordance of the 235U-231Pa and 234U-230Th chronometers confirmed the validity of the age dating assumption, increasing our confidence in the resulting conclusions.

In the framework of a collaboration between laboratories involved in bulk U and Pu analysis of environmental samples (DIF centre of the French Commissariat a l’Energie Atomique, US National Laboratories of New Brunswick, Lawrence Livermore, Pacific Northwest, Oak Ridge, and Los Alamos), two round-robins were organised, each one consisting of the complete analysis (chemical preparation and isotope measurement) of three Quality Control samples. The samples were 10 × 10 cm cotton tissues (“swipe samples”) containing low amounts of U (from ~20 to ~150 ng) and Pu (from ~0.15 to ~10 pg). Despite using different spikes, different methods of sample preparation and different analytical instrumentation, the results for U and Pu contents and isotopic compositions reported by all laboratories are globally in good agreement. All laboratories are able to measure sub-pg amounts of U and Pu isotopes with acceptable accuracy and reproducibility, even if limited discrepancies are observed affecting one or other measurement and/or laboratory. General and laboratory specific recommendations were discussed and adopted to continue to improve the accuracy and precision of the measurements.

By making modifications to our previously established measurement setup, we increased our abundance sensitivity for 233U by three orders of magnitude and can now measure 233U/238U ratios as low as 10−13. Because 233U has separate production pathways than 236U, it can provide valuable information on the particular source of anthropogenic uranium in a sample. We demonstrated the utility of our improved capability by using 233U to distinguish separate sources of anthropogenic uranium in a set of samples collected from a contaminated site. In the future, we plan to apply our new capability to characterizing 233U in a wide range of uranium materials.

Uranium (U) and plutonium (Pu) isotopes in coral soils, contaminated by nuclear weapons testing in the northern Marshall Islands, were isolated by ion-exchange chromatography and analyzed by mass spectrometry. The soil samples were spiked with 233U and 242Pu tracers, dissolved in minerals acids, and U and Pu isotopes isolated and purified on commercially available ion-exchange columns. The ion-exchange technique employed a TEVA® column coupled to a UTEVA® column. U and Pu isotope fractions were then further isolated using separate elution schemes, and the purified fractions containing U and Pu isotopes analyzed sequentially using multi-collector inductively coupled plasma mass spectrometer (MCICP-MS). High precision measurements of 234U/235U, 238U/235U, 236U/235U, and 240Pu/239Pu in soil samples were attained using the described methodology and instrumentation, and provide a basis for conducting more detailed assessments of the behavior and transfer of uranium and plutonium in the environment.

Runit Island on Enewetak Atoll was very heavily impacted by the U.S. nuclear testing campaign in the northern Marshall Islands (1946–58). The primary source of contamination on Runit Island was the 1958 Quince safety test where a large quantity of device plutonium (Pu) was scattered over the area near the GZ. A second low-yield device was detonated on the same site 10 days later, further disturbing the soil and leaving behind a very heterogeneous pattern of contamination including milligram-size particles of plutonium. A limited cleanup of the Fig-Quince zone was carried out in 1979. During this period, the effectiveness of the cleanup operations was primarily evaluated on the basis of bulk soil concentration data with little consideration given to the heterogeneity and long-term material-, biological-, and environmental-specific impacts of residual high activity (hot) particle contamination. The aim of the present study was twofold; (i) to characterize the levels and distribution of residual contamination in the Fig-Quince zone, and (ii) to develop pertinent data on the frequency distribution, elemental and isotopic composition, and physico-chemical properties of hot particles isolated from surface soils from Fig-Quince with a view towards providing recommendations on the future management and possible cleanup of the site. Today, Runit Island remains under an administrative quarantine.

For more than 20 years, countries and their agencies which monitor discharge sites and storage facilities have relied on the National Institute of Standards and Technology (NIST) Standard Reference Material (SRM) 4355 Peruvian Soil reference material. Its low fallout contamination makes it an ideal soil blank for measurements associated with terrestrial pathway to man studies. Presently, SRM 4355 is out of stock, and a new batch of the Peruvian soil is currently under development as future NIST SRM 4355A. Both environmental radioanalytical laboratories and mass spectrometry communities will benefit from this SRM. The former must assess their laboratory contamination and measurement detection limits by measurement of blank sample material. The Peruvian Soil is so low in anthropogenic radionuclides that it is a suitable virtual blank. On the other hand, mass spectrometric laboratories have high sensitivity instruments that are capable of quantitative isotopic measurements at low plutonium levels of the SRM 4355 (first Peruvian Soil SRM) that provided the mass spectrometric community with the calibration, quality control, and testing material needed for methods development, and legal defensibility. The quantification of the ultra-low plutonium content in the SRM 4355A was a considerable challenge for the mass spectrometric laboratories. Careful blank control andmore » correction, isobaric interferences, instrument stability, peak assessment, and detection assessment were necessary. Furthermore, a systematic statistical evaluation of the measurement results and considerable discussions with the mass spectroscopy metrologists were needed to derive the certified values and uncertainties. SRM 4355A will provide the mass spectrometric community with the quality control and testing material needed for higher sensitivity methods development, and legal defensibility.« less

Bulk chemical and O-, Mg- and Si-isotopic compositions were measured for each of 17 Types A and B refractory inclusions from CV3 chondrites. After bulk chemical compositions were corrected for non-representative sampling in the laboratory, the Mg- and Si-isotopic compositions of each inclusion were used to calculate its original chemical composition assuming that the heavy-isotope enrichments of these elements are due to Rayleigh fractionation that accompanied their evaporation from CMAS liquids. The resulting pre-evaporation chemical compositions are consistent with those predicted by equilibrium thermodynamic calculations for high-temperature nebular condensates, but only if different inclusions condensed from nebular regions that ranged in total pressure from 10−6 to 10−1 bar, regardless of whether they formed in a system of solar composition or in one enriched in dust of ordinary chondrite composition relative to gas by a factor of 10 compared to solar composition. This is similar to the range of total pressures predicted by dynamic models of the solar nebula for regions whose temperatures are in the range of silicate condensation temperatures. Alternatively, if departure from equilibrium condensation and/or non-representative sampling of condensates in the nebula occurred, the inferred range of total pressure could be smaller. Simple kinetic modeling of evaporation successfully reproduces observed chemical compositions of most inclusions from their inferred pre-evaporation compositions, suggesting that closed-system isotopic exchange processes did not have a significant effect on their isotopic compositions. Comparison of pre-evaporation compositions with observed ones indicates that 80% of the enrichment in refractory CaO + Al2O3 relative to more volatile MgO + SiO2 is due to initial condensation and 20% due to subsequent evaporation for both Types A and B inclusions.

Major, minor and trace uranium isotopes were measured at Lawrence Livermore National Laboratory in environmentally acquired samples using different instruments to span large variations in concentrations. Multi-collector inductively-coupled plasma mass spectrometry (MC-ICPMS) can be used to measure major and minor isotopes: 238 U, 235 U, 234 U and 236 U. Accelerator mass spectrometry (AMS) can be used to measure minor and trace isotopes: 234 U, 236 U and 233 U. The main limit of quantification for minor or trace uranium isotopes is the abundance sensitivity of the measurement technique; i.e. the ability to measure a minor or trace isotope of mass M in the presence of a major isotope at M ± 1 mass units. The abundance sensitivity for 236 U/ 235 U isotope ratio measurements using MC-ICPMS is around ∼2 × 10 −6 . This compares with a 236 U/ 235 U abundance sensitivity of ∼1 × 10 −7 for the current AMS system, with the expectation of 2–3 orders of magnitude improvement in sensitivity with the addition of another high energy filter. Comparing 236 U/ 234 U from MC-ICPMS and AMS produced agreement within ∼10% for samples at 236 U levels high enough to be measurable by both techniques.

To evaluate the time scale of fluid transfer and andesite differentiation at Arenal volcano in Costa Rica, we have measured trace-element concentrations and U-series disequilibria in whole rocks and mineral separates (pyroxene, plagioclase, magnetite) from lavas of the current eruption (1968 to 2003) by HR-ICP-MS, TIMS and PIMMS techniques. Whole rock and mineral separate analyses show a small but measurable variation in (230Th)/(232Th) (1.10 to 1.18). In contrast, (230Th)/(238U) range from 0.91 to 1.04 reflecting the moderate spread in Th/U. Stage 1 (1968–1971) whole rocks and mineral separates have both higher (230Th)/(232Th) and (238U)/(232Th) than to younger stage 2 lavas (1971 to present), which have lower, nearly constant (230Th)/(232Th) and lower, slightly variable (238U)/(232Th). 226Ra excesses exist in both whole rocks and mineral separates with (226Ra)/(230Th) ranging between 0.94 and 4.8. Whole rock (226Ra)/(230Th) are largest early in the eruption and decrease in the later lavas, which are influenced by newer recharge material. 238U–230Th whole rock and mineral data produce an inclined array on an equiline diagram, which we interpret to represent progressive melting of a variably fluxed mantle wedge and a Nicaraguan sediment component, and subsequent mixing. 238U–230Th internal isochrons suggest that minerals grew instantaneously with respect to the half-life of 230Th. Whole rock and mineral separate (226Ra)/(230Th) data indicate that melts were produced, transported, differentiated and erupted in

We present a new analytical method for U-series isotopes using the SHRIMP RG (Sensitive High mass Resolution Ion MicroProbe) mass spectrometer that utilizes the preconcentration of the U-series isotopes from a sample onto a single ion-exchange bead. Ion-microprobe mass spectrometry is capable of producing Th ionization efficiencies in excess of 2%. Analytical precision is typically better than alpha spectroscopy, but not as good as thermal ionization mass spectroscopy (TIMS) and inductively coupled plasma multicollector mass spectrometry (ICP-MS). Like TIMS and ICP-MS the method allows analysis of small samples sizes, but also adds the advantage of rapidity of analysis. A major advantage of ion-microprobe analysis is that U and Th isotopes are analyzed in the same bead, simplifying the process of chemical separation. Analytical time on the instrument is ∼60 min per sample, and a single instrument-loading can accommodate 15–20 samples to be analyzed in a 24-h day. An additional advantage is that the method allows multiple reanalyses of the same bead and that samples can be archived for reanalysis at a later time. Because the ion beam excavates a pit only a few μm deep, the mount can later be repolished and reanalyzed numerous times. The method described of preconcentrating a low concentration sample onto a small conductive substrate to allow ion-microprobe mass spectrometry is potentially applicable to many other systems.

The recent discovery of the migration of plutonium in groundwater away from underground nuclear tests at the Nevada Test Site has spawned considerable interest in the mechanisms by which plutonium may be released to the environment by a nuclear explosion. A suite of solid debris samples was collected during drilling through an expended test cavity and the overlying collapse chimney. Uranium and plutonium were analyzed for isotope ratios and concentrations using high precision magnetic sector inductively coupled mass spectrometry. The data unequivocally shows that plutonium may be dispersed throughout the cavity and chimney environment at the time of the detonation. The 239Pu/240Pu ratios are also fractionated relative to initial plutonium isotope ratio for the test device. Fractionation is the result of the volatilization of uranium and production of 239Pu by the reaction 238U(n,γ). We conclude that for the test under consideration plutonium was deposited outside of the confines of the cavity by dynamic processes in early-time and it is this plutonium that is most likely transferred to the groundwater regime.

Identifying and reducing impacts from mercury sources in the environment remains a considerable challenge and requires process based models to quantify mercury stocks and flows. The stable isotope composition of mercury in environmental samples can help address this challenge by serving as a tracer of specific sources and processes. Mercury isotope variations are small and result only from isotope fractionation during transport, equilibrium, and transformation processes. Because these processes occur in both industrial and environmental settings, knowledge of their associated isotope effects is required to interpret mercury isotope data. To improve the mechanistic modeling of mercury isotope effects during gas phase diffusion, an experimental program tested the applicability of kinetic gas theory. Gas-phase elemental mercury diffusion through small bore needles from finite sources demonstrated mass dependent diffusivities leading to isotope fractionation described by a Rayleigh distillation model. The measured relative atomic diffusivities among mercury isotopes in air are large and in agreement with kinetic gas theory. Mercury diffusion in air offers a reasonable explanation of recent field results reported in the literature.

A piece of scrap uranium metal bar buried in the dirt floor of an old, abandoned metal rolling mill was analyzed using multi-collector inductively coupled plasma mass spectroscopy (MC-ICP-MS). The mill rolled uranium rods in the 1940 and 1950s. Samples of the contaminated dirt in which the bar was buried were also analyzed. The isotopic composition of uranium in the bar and dirt samples were both the same as natural uranium, though a few samples of dirt also contained recycled uranium; likely a result of contamination with other material rolled at the mill. The time elapsed since the uranium metal bar was last purified can be determined by the in-growth of the isotope 230Th from the decay of 234U, assuming that only uranium isotopes were present in the bar after purification. The age of the metal bar was determined to be 61 years at the time of this analysis and corresponds to a purification date of July 1950 ± 1.5 years.

We describe a procedure for measurement of [sup 231]Pa in geologic samples by isotope dilution thermal ionization mass spectrometry, using [sup 233]Pa as a spike isotope, which provides marked improvements in precision and sample size relative to established decay counting techniques. This method allows determination of as little as a few tens of femtograms of [sup 231]Pa (approximately 10[sup 3] atoms) with a conservative estimated uncertainty of [+-]1% (95% confidence level). Applications of [sup 231]Pa-[sup 235]U systematics to uranium-series geochemistry and geochronology should be greatly enhanced by this approach. 31 refs., 4 figs., 1 tab.

Mass spectrometric measurements of235U231Pa and238U230Th disequilibria are used to further constrain processes producing U-series disequilibria in young mid-ocean ridge basalts (MORB) and to determine eruption ages for samples from 9–10°N East Pacific Rise and Juan de Fuca-Gorda ridges.231Pa/235U activity ratios for axial basalts are large and relatively uniform within ridge segments. The systematically large231Pa/235U,230Th/238U, and226Ra/230Th activity ratios in MORB are inconsistent with simple equilibrium melting models and crustal contamination or seawater alteration processes. More complicated melting processes involving disequilibrium or heterogeneous melting of mantle sources and disequilibrium transport of melts to the crust could produce the large U-series disequilibria in MORB. 231Pa and230Th model ages were determined by estimating initial231Pa/235U and230Th/232Th ratios from axial samples and by assuming constancy of these ratios over time. All231Pa and230Th ages agree to ±0–7 ka for samples with ages of 0–130 ka. These concordant ages validate the model assumptions of the two dating methods and suggest that mantle sources and melt generation processes have been uniform for these areas during the past 130 ka. Consequently,231Pa and230Th age-dates can be used to quantify the temporal and spatial dependence of volcanism at oceanic spreading centers during the past 375 ka.

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

230Th/232Th atom ratios as low as 2.75 × 10−6 have been measured with an average precision of 0.5% (2σmean) by thermal ionization mass spectrometry for leucite-bearing lamproites from the Gaussberg volcano, Antarctica. Most lavas have high and variable initial 230Th enrichments (45–60%), but constant Th/U ratios (7.55 ± 0.05). Th and Sr isotope ratios correlate negatively at constant eNd; higher 87Sr/86Sr ratios are associated with less excess 230Th and lower Sr concentrations. These differences are attributed to coupled variations in the degree of source metasomatism and percent melting. Low porosity melts are inferred to have been extracted fast enough that chemical equilibrium was not achieved. Source metasomatism has been multi-stage and increased the Th/U ratio, creating a Th-enriched reservoir in the subcontinental lithosphere that complements the Th-depleted asthenosphere.

The activities of 238U-series nuclides and 232Th have been measured by alpha spectrometry for nine alkali basalts from Heard island, Mt. Cameroon, Nyamuragira, and Nyiragongo. Large 226Ra enrichments are inferred to be absent from all, suggesting the absence of subsolidus fluids before magma genesis. Th isotope ratios are consistent with deep metasomatism forming carbonates beneath Nyiragongo, versus shallower metasomatism forming amphibole beneath Nyamuragira. In contrast, the Th/U ratio in the Nyamuragira source has increased with time, which is used to confirm that the Th/U ratio of the mantle is about 4.2.