Low-temperature Isotopic Exchange Research Articles

Carbon isotopic fractionation in molecular clouds

Context. Carbon fractionation has been studied from a theoretical point of view with different models of time-dependent chemistry, including both isotope-selective photodissociation and low-temperature isotopic exchange reactions. Aims. Recent chemical models predict that isotopic exchange reactions may lead to a depletion of 13C in nitrile-bearing species, with 12C/13C ratios two times higher than the elemental abundance ratio of 68 in the local interstellar medium. Since the carbon isotopic ratio is commonly used to evaluate the 14N/15N ratios with the double-isotope method, it is important to study carbon fractionation in detail to avoid incorrect assumptions. Methods. In this work, we implemented a gas-grain chemical model with new isotopic exchange reactions and investigated their introduction in the context of dense and cold molecular gas. In particular, we investigated the 12C/13C ratios of HNC, HCN, and CN using a grid of models, with temperatures and densities ranging from 10 to 50 K and 2 × 103 to 2 × 107 cm−3, respectively. Results. We suggest a possible 13C exchange through the 13C + C3 → 12C +13CC2 reaction, which does not result in dilution, but rather in 13C enhancement, for molecules that are formed starting from atomic carbon. This effect is efficient in a range of time between the formation of CO and its freeze-out on grains. Furthermore, the parameter-space exploration shows, on average, that the 12C/13C ratios of nitriles are predicted to be a factor 0.8–1.9 different from the local 12C/13C of 68 for high-mass star-forming regions. This result also affects the 14N/15N ratio: a value of 330 obtained with the double-isotope method is predicted to vary in the range 260–630, up to 1150, depending on the physical conditions. Finally, we studied the 12C/13C ratios of nitriles by varying the cosmic-ray ionisation rate, ζ: the 12C/13C ratios increase with ζ because of secondary photons and cosmic-ray reactions.

Mineralogical and geochemical studies of brecciated ores in the Dalucao REE deposit, Sichuan Province, southwestern China

The Dalucao deposit, located in western Sichuan Province, southwestern China, in the western part of the Yangtze Craton, is one of the largest and most extensive rare earth element (REE) deposits in the Himalayan Mianning–Dechang REE belt. Moreover, the Dalucao deposit is the only deposit identified in the southern part of the belt. The Dalucao deposit contains the No. 1, 2, and 3 orebodies; the No. 1 and 3 orebodies are both hosted in two breccia pipes, located in syenite–carbonatite host rocks. Both pipes have elliptical cross-sections at the surface, with long-axis diameters of 200–400m and short-axis diameters of 180–200m; the pipes extend downwards for >450m. No. 1 and No. 3 have total thickness varying between 55 and 175m and 14 to 58m respectively. The REE mineralization is associated with four brecciation events, which are recorded in each of the pipes. The ore grades in the No. 1 and 3 orebodies are similar, and consist of 1.0%–4.5% rare earth oxides (REOs). The No. 1 orebody is characterized by a Type I mineral assemblage (fluorite+barite+celestite+bastnäsite), whereas the No. 3 orebody is characterized by a Type II assemblage (fluorite+celestite+pyrite+muscovite+bastnäsite+strontianite). Argon (40Ar/39Ar) dating of hydrothermal muscovite intergrown with REE minerals in typical ores from the No. 1 and 3 orebodies yielded similar ages of 12.69±0.13 and 12.23±0.21Ma, respectively, which suggest that both mineral assemblages formed coevally, rather than in paragenetic stages. Both ages are also similar to the timing of intrusion of the syenite–carbonatite complex (12.13±0.19Ma). The ore-mineral assemblages occur in breccias, veinlets, and in narrow veins. The ore veinlets, which usually show a transition to mineralized breccia or brecciated ores, are commonly enveloped by narrow veins and stringer zones with comparable mineral assemblages. The brecciated ores form 95% of the volume of the deposit, whereas brecciated ores are only a minor constituent of other deposits in the Mianning–Dechang REE belt. The carbonatite in the syenite–carbonatite complexes contains high concentrations of S (0.07–2.32wt.%), Sr (16,500–20,700ppm), Ba (3600–8400ppm), and light REEs (LREE) (2848–10,768ppm), but is depleted in high-field-strength elements (HFSE) (Nb, Ta, P, Zr, Hf, and Ti). The syenite is moderately enriched in large-ion lithophile elements (LILE), Sr (155–277ppm), and Ba (440–755ppm). The mineralized, altered, and fresh syenites and carbonatites exhibit similar trace element compositions and REE patterns. Brecciation events, and the Dalucao Fault and its secondary faults around the deposit, contributed to the REE mineralization by facilitating the circulation of ore-forming fluids and providing space for REE precipitation. Some hydrothermal veins composed of coarse-grained fluorite and quartz are distributed in the syenite–carbonatite complex. The oxygen isotope compositions of ore-forming fluids in equilibrium with quartz at 215°C are −4.95‰ to −7.45‰, and the hydrogen isotope compositions of fluid inclusions in coarse-grained quartz are −88.4‰ to −105.1‰. The syenite–carbonatite complex and carbonatite are main contributors to the mineralization in the geological occurrence. Thus, the main components of the ore-forming fluids were magmatic water, meteoric water, and CO2 derived from the decarbonation of carbonatite. According to the petrographic studies, bastnäsite mineralization developed during later stages of hydrothermal evolution and overprinted the formation of the brecciated fluorite–quartz hydrothermal veins. As low-temperature isotope exchange between carbonates of the carbonatite and water-rich magmatic fluids will lead to positive shifts in δ18O values of the carbonates, C–O isotopic compositions from the bulk primary carbonatite to hydrothermal calcite and bastnäsite changed (δ18OV-SMOW from 8.0‰ to 11.6‰, and δ13C V-PDB from −6.1 to −8.7‰). According to the chemical composition of syenite and carbonatite, REE chloride species are the primary complexes for the transport of the REEs in the hydrothermal fluids, and the presence of bastnäsite and parisite means the REE were precipitated as fluorocarbonates. High contents of Sr, Ba and S in the syenite–carbonatite complex led to the deposition of large amount of barite and celestite.

Development of an analytical diffusion model for modeling hydrogen isotope exchange

We create a model for H retention depth profiles in W and subsequently model how this profile changes after isotope exchange. This is accomplished by calculating how trapping defects in W accumulate D (or H) inventory as W is being exposed to plasma. Each trapping site is characterized by a trapping rate and a release rate, where the only free parameters are the distribution of these trapping sites in the material. The filled trap concentrations for each trap type are modeled as a diffusion process because post-mortem deuterium depth profiles indicate that traps are filled well beyond the ion implantation zone (2–5nm). Using this retention model, an isotope exchange rate is formulated. The retention model and isotope exchange rate are compared to low temperature isotope exchange experiments in tungsten with good agreement. The limitations of the current model highlight important physics and motivate future work.

Catalytic Role of O•– Radicals in the Low-Temperature Isotopic Exchange in Dioxygen

In spite of a long investigation history, the low-temperature isotopic exchange in dioxygen taking place without involvement of the catalyst oxygen (R0 exchange) is still an exotic and poorly understood phenomenon in heterogeneous catalysis. Although very strong bonds are to be cleft in O2 molecules (118 kcal/mol), over some metal oxides R0 can be observed even at the temperature of liquid nitrogen. In this work, we studied the R0 exchange over a FeZSM-5 zeolite at 233 K and discovered for the first time a linear dependence of the R0 rate on the concentration of O•– radicals (α-oxygen), which identifies a catalytic role of these species. Upon transition to cryogenic temperatures, O•– species lose the ability to exchange themselves with dioxygen, but start functioning as a principal part of unique active sites capable of catalyzing a very smooth redistribution of the oxygen–oxygen bonds in adsorbed O2 molecules. Running by a highly concerted mechanism, this remarkable process leads to the R0 exchange with ...

Nitrogen isotopes in the recent solar wind from the analysis of Genesis targets: Evidence for large scale isotope heterogeneity in the early solar system

We have analyzed nitrogen, neon and argon abundances and isotopic ratios in target material exposed in space for 27 months to solar wind (SW) irradiation during the Genesis mission. SW ions were extracted by sequential UV (193 nm) laser ablation of gold-plated material, purified separately in a dedicated line, and analyzed by gas source static mass spectrometry. We analyzed gold-covered stainless steel pieces from the Concentrator, a device that concentrated SW ions by a factor of up to 50. Despite extensive terrestrial N contamination, we could identify a non-terrestrial, 15N-depleted nitrogen end-member that points to a 40% depletion of 15N in solar-wind N relative to inner planets and meteorites, and define a composition for the present-day Sun ( 15N/ 14N = [2.26 ± 0.67] × 10 −3, 2σ), which is indistinguishable from that of Jupiter’s atmosphere. These results indicate that the isotopic composition of nitrogen in the outer convective zone of the Sun has not changed through time, and is representative of the protosolar nebula. Large 15N enrichments due to e.g., irradiation, low temperature isotopic exchange, or contributions from 15N-rich presolar components, are therefore required to account for inner planet values.

Low-temperature isotopic exchange in obsidian: Implications for diffusive mechanisms

While a great deal is known about the interaction between water and rhyolitic glasses and melts at temperatures above the glass transition, the nature of this interaction at lower temperatures is much more poorly understood. This paper presents the results of a series of isotopic exchange experiments aimed at further elucidating this process and determining the extent to which a point-by-point analysis of the D/H or 18O/ 18O isotopic composition across the hydrated rim on a geological or archaeological obsidian sample can be used as a paleoclimatic monitor. Experiments were performed by first hydrating the glass for 5 days in water of one isotopic composition, followed by 5 days in water of a second composition. Because waters of near end-member compositions were used (nearly pure 1H 2 16O, 1H 2 18O, and D 2 16O), the relative migration of each species could be ascertained easily by depth-profiling using secondary ion mass spectrometry (SIMS). Results suggest that, during hydration, both the isotopic composition of the waters of hydration, as well as that of intrinsic water remaining from the initial formation of the glass vary dramatically, and a point-by-point analysis leading to paleoclimatic reconstruction is not feasible.

Fluid geochemistry in the Ivigtut cryolite deposit, South Greenland

The 1.27 Ga old Ivigtut (Ivittuut) intrusion in South Greenland is world-famous for its hydrothermal cryolite deposit [Na 3AlF 6] situated within a strongly metasomatised A-type granite stock. This detailed fluid inclusion study characterises the fluid present during the formation of the cryolite deposit and thermodynamic modelling allows to constrain its formation conditions. Microthermometry revealed three different types of inclusions: (1) pure CO 2, (2) aqueous-carbonic and (3) saline-aqueous inclusions. Melting temperatures range between − 23 and − 15 °C for type 2 and from − 15 to − 10 °C for type 3 inclusions. Most inclusions homogenise between 110 and 150 °C into the liquid. Stable isotope compositions of CO 2 and H 2O were measured from crushed inclusions in quartz, cryolite, fluorite and siderite. The δ 13C values of about − 5‰ PDB are typical of mantle-derived magmas. The differences between δ 18O of CO 2 (+ 21 to + 42‰ VSMOW) and δ 18O of H 2O (− 1 to − 21.7‰ VSMOW) suggest low-temperature isotope exchange. δD (H 2O) ranges from − 19 to − 144‰ VSMOW. The isotopic composition of inclusion water closely follows the meteoric water line and is comparable to Canadian Shield brines. Ion chromatography revealed the fluid's predominance in Na, Cl and F. Cl/Br ratios range between 56 and 110 and may imply intensive fluid–rock interaction with the host granite. Isochores deduced from microthermometry in conjunction with estimates for the solidification of the Ivigtut granite suggest a formation pressure of approximately 1–1.5 kbar for the fluid inclusions. Formation temperatures of different types of fluid inclusions vary between 100 and 400 °C. Thermodynamic modelling of phase assemblages and the extraordinary high concentration in F (and Na) may indicate that the cryolite body and its associated fluid inclusions could have formed during the continuous transition from a volatile-rich melt to a solute-rich fluid.

Copper-mediated isotopic exchange between [125I]iodide and bis(triethylammonium) undecahydro-12-iodo-closo-dodecaborate in aqueous media

Derivatives of dodecahydro-closo-dodecaborate(2-) anion were proposed as boron-rich compounds for boron-neutron capture therapy (BNCT) of malignant tumours. Labeling of such tumour-targeting compounds with radioisotopes would facilitate the investigation of their pharmacokinetics and help to optimize patient treatment protocols. Earlier, we reported the feasibility of labeling of closo-dodecaborate(2-) by isotopic exchange in molten acetamide. In this study, the feasibility of low-temperature isotopic exchange in the system [125I]iodide - bis(triethylammonium) undecahydro-12-iodo-closo-dodecaborate was investigated. Our attempts to perform the exchange in solvents such as methylene chloride, acetone and acetonitrile in the presence of phase-transfer catalysts were unsuccessful. However, copper mediated exchange in aqueous media was possible. Isotopic exchange of triethylammonium undecahydro-12-iodo-closo-dodecaborate provided a 90-95% labeling yield after heating for three and half hours at 100 °C.

Burbankite group minerals and their alteration in rare earth carbonatites—source of elements and fluids (evidence from C–O and Sr–Nd isotopic data)

The 370–380 Ma Khibina and Vuoriyarvi complexes on the Kola Peninsula, Russia, which form part of the Palaeozoic Kola Alkaline Province, contain REE-rich carbonatites with burbankite (Na,Ca) 3(Sr,Ca,REE,Ba) 3(CO 3) 5 or calcioburbankite (Na,Ca) 3(Ca,Sr,REE,Ba) 3(CO 3) 5 as the principal primary REE mineral. Within each complex the C–O and Sr–Nd isotopic data are similar for burbankite group minerals and co-existing calcite or dolomite (Khibina: δ 13C(V-PDB)=−6.4 to−5.8‰, δ 18O(V-SMOW)=7.3–7.7‰, ( 87Sr/ 86Sr) 370=0.70390–0.70404 and ( 143Nd/ 144Nd) 370=0.51230–0.51235; Vuoriyarvi: δ 13C=−4.2 to −3.0‰, δ 18O=8.1–9.4‰, ( 87Sr/ 86Sr) 370=0.70313–0.70315 and ( 143Nd/ 144Nd) 370=0.51243–0.51245). This indicates that the REE mineralization and its host carbonatites in each complex are derived from the same source and are co-genetic. There is, however, a great difference between the Sr, Nd and C isotopic signatures from Khibina and Vuoriyarvi, whereas the δ 18O ranges are similar. This suggests that the REE carbonatites of the two complexes originate from sources with different isotopic signatures. At least three mantle components are needed to explain the variations in Sr and Nd compositions in the carbonatites from Kola. The δ 13C ranges of primary carbonatites with low δ 18O values are quite different for Khibina and Vuoriyarvi and show correlation with the radiogenic isotope compositions. The data may be best explained by subduction-related source contamination that caused δ 13C variations in different mantle components. During late-stage processes burbankite and calcioburbankite have been replaced by various assemblages of REE–Sr–Ba minerals. The alteration of burbankite group minerals is an open-system hydrothermal process leading to multiple element transfer. It has produced mineral assemblages which are characterized by high δ 18O values (Khibina: δ 18O(V-SMOW)=11.4–13.9‰ and Vuoriyarvi: δ 18O=17.1–18.0‰) compared to primary burbankite and calcioburbankite. Co-existing calcite and dolomite have retained their original C and O isotope compositions, and one calcite sample from Khibina shows strong positive δ 13C– δ 18O shifts similar to those of the pseudomorph. The high δ 18O and sometimes high δ 13C values can be attributed to low-temperature isotope exchange between minerals and fluid with variable CO 2/H 2O ratio taking place during and/or after crystallization as usually observed in carbonatites. The Sr and Nd isotope compositions of pseudomorphs and associated calcite/dolomite in general are identical to those of burbankite/calcioburbankite and associated carbonates suggesting that the fluids which caused burbankite alteration are from the same source, i.e. carbonatitic. Small variations in the Sr and Nd isotope signatures point to interaction of the pseudomorph-forming fluid with alkali silicate wall rocks.

Primary and secondary features of analcimes formed in carbonate-zeolite ocelli of alkaline basalts (Mecsek Mts., Hungary): textures, chemical and oxygen isotope compositions.

Analcime is common in magmatic rocks of the alkaline basalt-phonotephrite-phonolite suite of the Mecsek Mts., Hungary. Besides the occurrence of xenomorphic groundmass analcime, wedge-shaped crystals between feldspars, and products of feldspar and nepheline alteration, analcime also occurs in calcite ocelli formed in basaltic dikes. Microscopic textures of these ocelli are characteristic of rapidly crystallized carbonate melt, suggesting that the ocelli are droplets of carbonate-rich melt separated from the silicate magma by liquid immiscibility. This carbonate would have crystallized below magmatic temperatures, and above hydrothermal ones. Thus, the analcime might be regarded as transitional between primary magmatic (P type) and hydrothermal (H type) analcimes. SEM studies reveal that the analcimes of the ocelli form euhedral crystals whose surfaces are smooth without signs of the porous texture characteristic for products of leucite alteration. Chemical compositions determined by electron microprobe are close to the theoretical NaAlSi2O6·H2O formula, with minor Ca substitution. XSi values (0.642 to 0.686) fall between the ranges of analcimes considered to be of primary magmatic and hydrothermal origins, whereas the low Fe contents indicate relationships with H type ones. Oxygen isotope compositions of silicate minerals in lavas and dikes and calcite ocelli in dikes have been determined in order to investigate the preservation of magmatic compositions, and the effect of low-temperature isotope exchange. The most positive δ18O values among the studied mineral separates were found in the analcimes (17.5 to 19.0‰). Based on comparisons with oxygen isotope compositions of calcites of the ocelli (13.0 to 13.3‰) and amphiboles (7.4‰) of their host rock, this 18O-enrichment could be a result of retrograde oxygen isotope exchange with magmatic fluids at decreasing temperatures. Effects of low-temperature isotope exchange appear also in the amphiboles, biotites and feldspars of the Mecsek series, resulting in increasingly more positive δ18O ranges (5.1 to 7.4‰, 7.2 to 7.4‰, and 7.6 to 15.0‰, respectively) as a function of sensitivity to retrograde isotope exchange. Primary magmatic compositions have been preserved in pyroxenes (6.0 to 6.5‰), indicating generation of basaltic melt by low degree partial melting of mantle peridotite.

Stable isotope and major element compositions of fluid inclusions in Devonian and Cambrian dolomite cements, western Canada

Homogenization and melting temperatures, major element and hydrogen and oxygen isotope data for fluid inclusions, and carbon and oxygen isotope data for dolomite from the Manetoe Facies (Devonian) and Cathedral Formation (Cambrian) of western Canada indicate that the dolomite cements formed from alteration of the host limestone by heated mixtures of evaporated seawater or residual evaporite brines and meteoric water. The measured δ 18O values for fluid inclusion waters are much more negative (about 8‰) than those calculated for waters in isotope equilibrium with dolomite at the moderately high temperatures of formation. Fluid inclusion waters appear to have exchanged oxygen isotopes (reequilibrated) with the host dolomite as they cooled. The data may provide low temperature isotope exchange equilibrium fractionation factors between dolomite and water. Since no hydrogen-bearing compounds other than water were found in these samples using Quadrupole Mass Spectrometry, the inclusion fluids likely retained their initial hydrogen isotope compositions. Therefore, their 8D values (−81 to −42%o) are more useful for determining the origin of the dolomitizing fluids.

Low temperature isotopic exchange of molecular oxygen via the reaction of NO, NH3 and O2 over supported vanadia and molybdena catalysts

This study describes the behaviour of NO, NH3 and18O2 over supported molybdena and vanadia catalysts. The study was carried out using differential scanning calorimetry. The catalysts show different behaviour with respect to the reaction of NO, NH3 and O2 and similar behaviour with respect to the formation of water.