Thermodynamic determination of condensation behavior for the precursory elements of radioxenon following an underground nuclear explosion.

The measurement of radioactive xenon isotopes (radioxenon) in the atmosphere is a tool used to detect underground nuclear explosions, provided that some radioxenon escaped containment and that fractionation leading to the alteration of the relative proportions of these isotopes, is accounted for. After the explosion, volatilization followed by melting of the surrounding rocks produces a magma where the more refractory radioactive species get dissolved while the more volatile ones contribute to the gas phase that might escape. Indium, tin, antimony, tellurium and iodine are the main fission products involved in the decay chains leading to radioxenon. In this study, condensation as a function of temperature for these precursors of radioxenon were determined using thermodynamic calculations for systems with complex chemical composition corresponding to major environments of known underground nuclear explosions and for a range of pressure values representative of the cavity evolution. Our results illustrate a large difference between the relevant condensation temperatures for the radioxenon precursors and the tabulated boiling temperatures of the pure compounds often used as indicators of their volatility. For some precursory elements such as tin, the often-considered Heaviside function represents an oversimplification of the concept of condensation temperature, as condensation occurs over a temperature range as large as 2000 K. This results from the speciation of the elements in the gas phase mainly driven by the formation of oxides. Condensation also strongly depends on pressure while it moderately depends on the bulk chemical composition of the system. This study shows the importance and complexity of the condensation process following underground nuclear explosions. It also shows how thermodynamic computations allow the prediction of the quantity and the relative proportions of radioactive xenon isotopes in the gas phase in the presence of magma, before their potential emission to the atmosphere. Better detection, discrimination and understanding of underground nuclear explosions should arise by taking into account the fractionation resulting from the condensation of the radionuclides producing radioxenon in nuclear cavities.

More than Five Percent Ionization Efficiency by Cavity Source Thermal Ionization Mass Spectrometry for Uranium Subnanogram Amounts.

Numerous applications require the precise analysis of U isotope relative enrichment in sample amounts in the subnanogram to picogram range; among those are nuclear forensics, nuclear safeguards, environmental survey, and geosciences. However, conventional thermal ionization mass spectrometry (TIMS) yields U combined ionization and transmission efficiencies (i.e., ratio of ions detected to sample atoms loaded) of less than 0.1% or 2% depending on the loading protocol, motivating the development of sources capable of enhancing ionization. The new prototype cavity source TIMS at ETH Zürich offers improvements from 4 to 15 times in combined ionization and transmission efficiency compared to conventional TIMS, yielding up to 5.6% combined efficiency. Uranium isotope ratios have been determined on reference standards in the 100 pg range bound to ion-exchange or extraction resin beads. For natural U standards, n(235U)/ n(238U) ratios are measured to relative external precisions of 0.5-1.0% (2RSD, 2 < n < 11, conventional source) or 2.0% (2RSD, n = 6, cavity source) and accuracies of 0.2-0.7% (conventional source) or 0.4-0.9% (cavity source). Meanwhile, n(234U)/ n(238U) ratios are determined to relative external precisions of 1.7-3.6% (2RSD, 2 < n < 11, conventional source) or 5.6% (2RSD, n = 6, cavity source) and accuracies of 0.1-2.5% (conventional source) or 0.5-8.3% (cavity source), which would benefit further from in-run organic interference and peak tailing corrections.

Mercury isotope signatures in contaminated sediments as a tracer for local industrial pollution sources.

Mass-dependent fractionation (MDF) and mass-independent fractionation (MIF) may cause characteristic isotope signatures of different mercury (Hg) sources and help understand transformation processes at contaminated sites. Here, we present Hg isotope data of sediments collected near industrial pollution sources in Sweden contaminated with elemental liquid Hg (mainly chlor-alkali industry) or phenyl-Hg (paper industry). The sediments exhibited a wide range of total Hg concentrations from 0.86 to 99 µg g(-1), consisting dominantly of organically-bound Hg and smaller amounts of sulfide-bound Hg. The three phenyl-Hg sites showed very similar Hg isotope signatures (MDF δ(202)Hg: -0.2‰ to -0.5‰; MIF Δ(199)Hg: -0.05‰ to -0.10‰). In contrast, the four sites contaminated with elemental Hg displayed much greater variations (δ(202)Hg: -2.1‰ to 0.6‰; Δ(199)Hg: -0.19‰ to 0.03‰) but with distinct ranges for the different sites. Sequential extractions revealed that sulfide-bound Hg was in some samples up to 1‰ heavier in δ(202)Hg than organically-bound Hg. The selectivity of the sequential extraction was tested on standard materials prepared with enriched Hg isotopes, which also allowed assessing isotope exchange between different Hg pools. Our results demonstrate that different industrial pollution sources can be distinguished on the basis of Hg isotope signatures, which may additionally record fractionation processes between different Hg pools in the sediments.

Stable Hg isotope signatures in creek sediments impacted by a former Hg mine.

The goal of this study was to investigate the Hg stable isotope signatures of sediments in San Carlos Creek downstream of the former Hg mine New Idria, CA, USA and to relate the results to previously studied Hg isotope signatures of unroasted ore waste and calcine materials in the mining area. New Idria unroasted ore waste was reported to have a narrow δ(202)Hg range (−0.09 to 0.16‰), while roasted calcine materials exhibited a very large variability in δ(202)Hg (−5.96 to 14.5‰). In this study, creek sediment samples were collected in the stream bed from two depths (0­10 and 10­20 cm) at 10 locations between the mine adit and 28 km downstream. The sediment samples were size-fractionated into sand, silt, and (if possible) clay fractions as well as hand-picked calcine pebbles. The sediment samples contained highly elevated Hg concentrations (8.2 to 647 µg g(­1)) and displayed relatively narrow mass-dependent fractionation (MDF, δ(202)Hg; ± 0.08‰, 2SD) ranges (−0.58 to 0.24‰) and little to no mass-independent fractionation (MIF, Δ(199)Hg; ± 0.04‰, 2SD) (0.00 to 0.10‰), similar to what was observed previously for the unroasted ore waste. However, due to the highly variable and overlapping δ(202)Hg signatures of the calcines, they could not be ruled out as source of Hg to the creek sediments. Overall, our results suggest that analyzing creek sediments downstream of former Hg mines can provide a more reliable Hg isotope source signature for tracing studies at larger spatial scales, than analyzing the isotopically highly heterogeneous tailing piles typically found at former mining sites. Creek sediments carry an integrated isotope signature of Hg transported away from the mine with runoff into the creek, eventually affecting ecosystems downstream.

Kinetics of Hg(II) exchange between organic ligands, goethite, and natural organic matter studied with an enriched stable isotope approach.

The mobility and bioavailability of toxic Hg(II) in the environment strongly depends on its interactions with natural organic matter (NOM) and mineral surfaces. Using an enriched stable isotope approach, we investigated the exchange of Hg(II) between dissolved species (inorganically complexed or cysteine-, EDTA-, or NOM-bound) and solid-bound Hg(II) (carboxyl-/thiol-resin or goethite) over 30 days under constant conditions (pH, Hg and ligand concentrations). The Hg(II)-exchange was initially fast, followed by a slower phase, and depended on the properties of the dissolved ligands and sorbents. The results were described by a kinetic model allowing the simultaneous determination of adsorption and desorption rate coefficients. The time scales required to reach equilibrium with the carboxyl-resin varied greatly from 1.2 days for Hg(OH)2 to 16 days for Hg(II)-cysteine complexes and approximately 250 days for EDTA-bound Hg(II). Other experiments could not be described by an equilibrium model, suggesting that a significant fraction of total-bound Hg was present in a non-exchangeable form (thiol-resin and NOM: 53-58%; goethite: 22-29%). Based on the slow and incomplete exchange of Hg(II) described in this study, we suggest that kinetic effects must be considered to a greater extent in the assessment of the fate of Hg in the environment and the design of experimental studies, for example, for stability constant determination or metal isotope fractionation during sorption.

Mercury isotope signatures as tracers for Hg cycling at the New Idria Hg mine.

Mass-dependent fractionation (MDF) and mass-independent fractionation (MIF) of Hg isotopes provides a new tool for tracing Hg in contaminated environments such as mining sites, which represent major point sources of Hg pollution into surrounding ecosystems. Here, we present Hg isotope ratios of unroasted ore waste, calcine (roasted ore), and poplar leaves collected at a closed Hg mine (New Idria, CA, U.S.A.). Unroasted ore waste was isotopically uniform with δ(202)Hg values from -0.09 to 0.16‰ (± 0.10‰, 2 SD), close to the estimated initial composition of the HgS ore (-0.26‰). In contrast, calcine samples exhibited variable δ(202)Hg values ranging from -1.91‰ to +2.10‰. Small MIF signatures in the calcine were consistent with nuclear volume fractionation of Hg isotopes during or after the roasting process. The poplar leaves exhibited negative MDF (-3.18 to -1.22‰) and small positive MIF values (Δ(199)Hg of 0.02 to 0.21‰). Sequential extractions combined with Hg isotope analysis revealed higher δ(202)Hg values for the more soluble Hg pools in calcines compared with residual HgS phases. Our data provide novel insights into possible in situ transformations of Hg phases and suggest that isotopically heavy secondary Hg phases were formed in the calcine, which will influence the isotope composition of Hg leached from the site.

Solution speciation controls mercury isotope fractionation of Hg(II) sorption to goethite.

The application of Hg isotope signatures as tracers for environmental Hg cycling requires the determination of isotope fractionation factors and mechanisms for individual processes. Here, we investigated Hg isotope fractionation of Hg(II) sorption to goethite in batch systems under different experimental conditions. We observed a mass-dependent enrichment of light Hg isotopes on the goethite surface relative to dissolved Hg (ε(202)Hg of -0.30‰ to -0.44‰) which was independent of the pH, chloride and sulfate concentration, type of surface complex, and equilibration time. Based on previous theoretical equilibrium fractionation factors, we propose that Hg isotope fractionation of Hg(II) sorption to goethite is controlled by an equilibrium isotope effect between Hg(II) solution species, expressed on the mineral surface by the adsorption of the cationic solution species. In contrast, the formation of outer-sphere complexes and subsequent conformation changes to different inner-sphere complexes appeared to have insignificant effects on the observed isotope fractionation. Our findings emphasize the importance of solution speciation in metal isotope sorption studies and suggest that the dissolved Hg(II) pool in soils and sediments, which is the most mobile and bioavailable, should be isotopically heavy, as light Hg isotopes are preferentially sequestered during binding to both mineral phases and natural organic matter.

Silicon isotope evidence against an enstatite chondrite Earth.

The compositions of Earth materials are strikingly similar to those of enstatite chondrite meteorites in many isotope systems. Although this suggests that Earth largely accreted from enstatite chondrites, definitive proof of this model has been lacking. By comparing the silicon (Si) isotope signatures of several extraterrestrial materials with terrestrial samples, we show that they cannot be explained by core-formation scenarios involving a bulk Earth of enstatite chondrite composition. Si isotope similarities between the bulk silicate Earth and the Moon preclude the existence of a hidden reservoir in the lower mantle, a necessary condition of the enstatite chondrite model, and require an equilibrium process after the Moon-forming impact. A three-end-member chondritic mixing model for Earth reconciles the Si isotope similarities between enstatite chondrites and Earth.

Iron isotope fractionation during Fe uptake and translocation in alpine plants.

The potential of stable Fe isotopes as a tracer for the biogeochemical Fe cycle depends on the understanding and quantification of the fractionation processes involved. Iron uptake and cycling by plants may influence Fe speciation in soils. Here, we determined the Fe isotopic composition of different plant parts including the complete root system of three alpine plant species (Oxyria digyna, Rumex scutatus, Agrostis gigantea) in a granitic glacier forefield, which allowed us, for the first time, to distinguish between uptake and in-plant fractionation processes. The overall range of fractionation was 4.5 per thousand in delta(56)Fe. Mass balance calculations demonstrated that fractionation toward lighter Fe isotopic composition occurred in two steps during uptake: (1) before active uptake, probably during mineral dissolution and (2) during selective uptake of Fe at the plasma membrane with an enrichment factor of -1.0 to -1.7 per thousand for all three species. Iron isotopes were further fractionated during remobilization from old into new plant tissue, which changed the isotopic composition of leaves and flowers over the season. This study demonstrates the potential of Fe isotopes as a new tool in plant nutrition studies but also reveals challenges for the future application of Fe isotope signatures in soil-plant environments.

Equilibrium mercury isotope fractionation between dissolved Hg(II) species and thiol-bound Hg.

Stable Hg isotope ratios provide a new tool to trace environmental Hg cycling. Thiols (-SH) are the dominant Hg-binding groups in natural organic matter. Here, we report experimental and computational results on equilibrium Hg isotope fractionation between dissolved Hg(II) species and thiol-bound Hg. Hg(II) chloride and nitrate solutions were equilibrated in parallel batches with varying amounts of thiol resin resulting in different fractions of thiol-bound and free Hg. Mercury isotope ratios in both fractions were analyzed by multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS). Theoretical equilibrium Hg isotope effects by mass-dependent fractionation (MDF) and nuclear volume fractionation (NVF) were calculated for 14 relevant Hg(II) species. The experimental data revealed that thiol-bound Hg was enriched in light Hg isotopes by 0.53 per thousand and 0.62 per thousand (delta(202)Hg) relative to HgCl(2) and Hg(OH)(2), respectively. The computational results were in excellent agreement with the experimental data indicating that a combination of MDF and NVF was responsible for the observed Hg isotope fractionation. Small mass-independent fractionation (MIF) effects (<0.1 per thousand) were observed representing one of the first experimental evidences for MIF of Hg isotopes by NVF. Our results indicate that significant equilibrium Hg isotope fractionation can occur without redox transition, and that NVF must be considered in addition to MDF to explain Hg isotope variations.

Early differentiation of the Earth and the Moon.

We examine the implications of new 182W and 142Nd data for Mars and the Moon for the early evolution of the Earth. The similarity of 182W in the terrestrial and lunar mantles and their apparently differing Hf/W ratios indicate that the Moon-forming giant impact most probably took place more than 60Ma after the formation of calcium-aluminium-rich inclusions (4.568Gyr). This is not inconsistent with the apparent U-Pb age of the Earth. The new 142Nd data for Martian meteorites show that Mars probably has a super-chondritic Sm/Nd that could coincide with that of the Earth and the Moon. If this is interpreted by an early mantle differentiation event, this requires a buried enriched reservoir for the three objects. This is highly unlikely. For the Earth, we show, based on new mass-balance calculations for Nd isotopes, that the presence of a hidden reservoir is difficult to reconcile with the combined 142Nd-143Nd systematics of the Earth's mantle. We argue that a likely possibility is that the missing component was lost during or prior to accretion. Furthermore, the 142Nd data for the Moon that were used to argue for the solidification of the magma ocean at ca 200Myr are reinterpreted. Cumulate overturn, magma mixing and melting following lunar magma ocean crystallization at 50-100Myr could have yielded the 200Myr model age.

Super-chondritic Sm/Nd ratios in Mars, the Earth and the Moon.

Small isotopic differences in the atomic abundance of neodymium-142 (142Nd) in silicate rocks represent the time-averaged effect of decay of formerly live samarium-146 (146Sm) and provide constraints on the timescales and mechanisms by which planetary mantles first differentiated. This chronology, however, assumes that the composition of the total planet is identical to that of primitive undifferentiated meteorites called chondrites. The difference in the 142Nd/144Nd ratio between chondrites and terrestrial samples may therefore indicate very early isolation (<30 Myr from the formation of the Solar System) of the upper mantle or a slightly non-chondritic bulk Earth composition. Here we present high-precision 142Nd data for 16 martian meteorites and show that Mars also has a non-chondritic composition. Meteorites belonging to the shergottite subgroup define a planetary isochron yielding an age of differentiation of 40 +/- 18 Myr for the martian mantle. This isochron does not pass through the chondritic reference value (100 x epsilon(142)Nd = -21 +/- 3; 147Sm/144Nd = 0.1966). The Earth, Moon and Mars all seem to have accreted in a portion of the inner Solar System with approximately 5 per cent higher Sm/Nd ratios than material accreted in the asteroid belt. Such chemical heterogeneities may have arisen from sorting of nebular solids or from impact erosion of crustal reservoirs in planetary precursors. The 143Nd composition of the primitive mantle so defined by 142Nd is strikingly similar to the putative endmember component 'FOZO' characterized by high 3He/4He ratios.

Insights into the dynamics of mantle plumes from uranium-series geochemistry.

The long-standing paradigm that hotspot volcanoes such as Hawaii or Iceland represent the surface expression of mantle plumes--hot, buoyant upwelling regions beneath the Earth's lithosphere--has recently been the focus of controversy. Whether mantle plumes exist or not is pivotal for our understanding of the thermal, dynamic and compositional evolution of the Earth's mantle. Here we show that uranium-series disequilibria measured in hotspot lavas indicate that hotspots are indeed associated with hot and buoyant upwellings and that weaker (low buoyancy flux) hotspots such as Iceland and the Azores are characterized by lower excess temperatures than stronger hotspots such as Hawaii. This direct link between buoyancy flux and mantle temperature is evidence for the existence of mantle plumes.

Trace-element fractionation in Hadean mantle generated by melt segregation from a magma ocean.

Calculations of the energetics of terrestrial accretion indicate that the Earth was extensively molten in its early history. Examination of early Archaean rocks from West Greenland (3.6-3.8 Gyr old) using short-lived 146Sm-142Nd chronometry indicates that an episode of mantle differentiation took place close to the end of accretion (4.46 +/- 0.11 Gyr ago). This has produced a chemically depleted mantle with an Sm/Nd ratio higher than the chondritic value. In contrast, application of 176Lu-176Hf systematics to 3.6-3.8-Gyr-old zircons from West Greenland indicates derivation from a mantle source with a chondritic Lu/Hf ratio. Although an early Sm/Nd fractionation could be explained by basaltic crust formation, magma ocean crystallization or formation of continental crust, the absence of coeval Lu/Hf fractionation is in sharp contrast with the well-known covariant behaviour of Sm/Nd and Lu/Hf ratios in crustal formation processes. Here we show using mineral-melt partitioning data for high-pressure mantle minerals that the observed Nd and Hf signatures could have been produced by segregation of melt from a crystallizing magma ocean at upper-mantle pressures early in Earth's history. This residual melt would have risen buoyantly and ultimately formed the earliest terrestrial protocrust.

146Sm-142Nd evidence from Isua metamorphosed sediments for early differentiation of the Earth's mantle.

Application of the 147Sm-143Nd chronometer (half-life of 106 Gyr) suggests that large-scale differentiation of the Earth's mantle may have occurred during the first few hundred million years of its history. However, the signature of mantle depletion found in early Archaean rocks is often obscured by uncertainties resulting from open-system behaviour of the rocks during later high-grade metamorphic events. Hence, although strong hints exist regarding the presence of differentiated silicate reservoirs before 4.0 Gyr ago, both the nature and age of early mantle differentiation processes remain largely speculative. Here we apply short-lived 146Sm-142Nd chronometry (half-life of 103 Myr) to early Archaean rocks using ultraprecise measurement of Nd isotope ratios. The analysed samples are well-preserved metamorphosed sedimentary rocks from the 3.7-3.8-Gyr Isua greenstone belt of West Greenland. Our coupled isotopic calculations, combined with an initial epsilon 143Nd value from ref. 6, constrain the mean age of mantle differentiation to 4,460 +/- 115 Myr. This early Sm/Nd fractionation probably reflects differentiation of the Earth's mantle during the final stage of terrestrial accretion.