Stable Chromium Isotopes Research Articles

Light stable Cr isotope compositions of mid-ocean ridge basalts: Implications for mantle source composition

Stable isotopes of chromium (Cr) and iron (Fe), both multivalent elements in silicate melts, are valuable proxies of redox conditions in magmatic systems. Partial melting and fractional crystallization have been identified as the dominant fractionation processes for Cr and Fe isotopes in high-temperature reservoirs that can lead to small but detectable isotope variations in igneous rocks. More recently, however, the source mineralogy has been suggested to play an important role, which can ultimately affect the magnitude of Cr and Fe isotopic fractionation during partial melting and thus the Cr and Fe isotopic composition of the originating melt.With the aim to investigate the role of magmatic processes and the source Cr and Fe isotope signatures on the isotope composition of these transition metals in normal mid-ocean ridge basalts (N-MORB), we analyzed 21 well-characterized MORB glasses from the southern East-Pacific Rise, Pacific-Antarctic Ridge and Mid-Atlantic Ridge. The Cr isotope compositions of N-MORB show a narrow range from −0.278 to −0.186 ‰ in δ53/52Cr (i.e., difference of a sample’s 53Cr/52Cr ratio relative to the international reference material NIST SRM979), with an average value of −0.237 ± 0.050 ‰ (2SD; n = 19). As such, the average N-MORB δ53/52Cr value is significantly lower than that of Bulk Silicate Earth of −0.12 ± 0.06 ‰ and the Cr isotope compositions reported for most ocean island basalts ranging from ∼ −0.23 to 0.00 ‰. Iron isotopic compositions of these MORB range from +0.032 to +0.137 ‰ in δ56/54Fe (i.e., difference of a sample’s 56Fe/54Fe ratio relative to the international reference material IRMM014) and are within the range of previously published MORB data. The lack of correlations between N-MORB Cr isotope signatures and indices of magmatic differentiation suggests a negligible control of this process on Cr stable isotopes. Partial melting modeling using pMELTS provides evidence that the significant offset towards lower δ53/52Cr values of N-MORB cannot be produced by decompression partial melting of a lherzolithic source such as the depleted upper mantle alone. Instead, we suggest that the lower δ53/52Cr values of MORB could be linked to intrinsic small-scale 53Cr-depleted pyroxene-rich domains in the upper mantle that influence the melt’s Cr isotopic budget during partial melting.

UHAS-MIDA Software Package: Mass Isotopomer Distribution Analysis-Applying the Inverse Matrix for the Determination of Stable Isotopes of Chromium Distribution during Red Blood Cell Labelling

Clinical assessment of fluid volume status in children during malaria can be taxing and often inaccurate. During malaria, changes in fluid volume are rather multifarious and estimating this parameter, especially in sick children is very challenging for clinicians who frequently rely on indices such as long capillary refill times, tachycardia, central venous pressure and decreased urine volume as guides. Here, we present the UHAS-MIDA, an open-source software tool that calculates the red blood cell (RBC) concentration and blood volume during malaria in children determined using a stable isotope of chromium (53Cr as the label) by gas chromatography-mass spectrometry in selective ion monitoring (GC/MS-SIM) analysis. A key component involves the determination of the compositions of the most abundant naturally occurring isotopes of Cr (50Cr, 52Cr, 53Cr), and converting the proportions into a 3 × 3 matrix. To estimate unknown proportions of chromium isotopic mixtures from the measured abundances of three ions, an inverse matrix was calculated. The inverse together with several inputs is then used to calculate the corrected MS ion abundances. Thus, we constructed the software tool UHAS- MIDA using HTML, CSS/Bootstrap, JavaScript, and PHP scripting languages. The tool enables the user to efficiently determine RBC concentration and fluid volume. The source code, binary packages and associated materials for UHAS-MIDA are freely available at https://github.com/bentil078/Abaye-et-al_UHASmida

Chromium Isotope Systematics in Modern and Ancient Microbialites

Changes in stable chromium isotopes (denoted as δ53Cr) in ancient carbonate sediments are increasingly used to reconstruct the oxygenation history in Earth’s atmosphere and oceans through time. As a significant proportion of marine carbonate older than the Cambrian is microbially-mediated, the utility of δ53Cr values in ancient carbonates hinges on whether these sediments accurately capture the isotope composition of their environment. We report Cr concentrations (Cr) and δ53Cr values of modern marginal marine and non-marine microbial carbonates. These data are supported by stable C and O isotope compositions, as well as rare earth elements and yttrium (REY) concentrations. In addition, we present data on ancient analogs from Precambrian strata. Microbial carbonates from Marion Lake (Australia, δ53Cr ≈ 0.99‰) and Mono Lake (USA, ≈0.78‰) display significantly higher δ53Cr values compared with ancient microbialites from the Andrée Land Group in Greenland (720 Ma, ≈0.36‰) and the Bitter Springs Formation in Australia (800 Ma, ≈−0.12‰). The δ53Cr values are homogenous within microbialite specimens and within individual study sites. This indicates that biological parameters, such as vital effects, causing highly variable δ53Cr values in skeletal carbonates, do not induce variability in δ53Cr values in microbialites. Together with stable C and O isotope compositions and REY patterns, δ53Cr values in microbialites seem to be driven by environmental parameters such as background lithology and salinity. In support, our Cr and δ53Cr results of ancient microbial carbonates agree well with data of abiotically precipitated carbonates of the Proterozoic. If detrital contamination is carefully assessed, microbialites have the potential to record the δ53Cr values of the waters from which they precipitated. However, it remains unclear if these δ53Cr values record (paleo-) redox conditions or rather result from other physico-chemical parameters.

Oxidized conditions in iron meteorite parent bodies

Some iron meteorite parent bodies may have formed beyond Mars under oxidizing conditions, according to analyses of chromium isotopes. Magmatic iron meteorites are thought to originate from the cores of differentiated planetary embryos, but it is unclear where they formed in the circumstellar disk. Oxygen fugacity (fO2) is thought to have varied radially across the disk, affecting planetary accretion and core formation. Metal–silicate equilibration experiments show that the chromium concentration in metallic melt increases with decreasing fO2. Chromium can therefore be used to reconstruct fO2 during metal segregation. Here, we report mass-dependent fractionation of stable chromium isotopes measured for several groups of magmatic iron meteorites. We find that chromium concentrations are variable and most samples are isotopically heavy compared with the bulk silicate Earth and chondrites. Although it is evident that the chromium isotopes were systematically affected by fractional crystallization, we use the mass-dependent fractionation behaviour to derive initial chromium concentrations for the planetary embryo cores. We find that the cores were diverse in fO2 and demonstrate that core formation in some planetary embryos occurred under relatively oxidizing conditions (fO2 relative to iron–wustite (IW) buffer (ΔIW) = −0.95 ± 0.15). It is difficult to reconcile this with the view that iron meteorite parent bodies formed closer to Earth and subsequently migrated outwards. Instead, we suggest that these parent bodies formed at greater distances, beyond Mars.

Calculated differential and double differential cross section of DT neutron induced reactions on natural chromium (Cr)

Chromium is an important alloying element of stainless steel (SS) and SS is the main constituent of structural material proposed for fusion reactors. Energy and double differential cross section data will be required to estimate nuclear responses in the materials used in fusion reactors. There are no experimental data of energy and double differential cross section, available for neutron induced reactions on natural chromium at 14 MeV neutron energy. In this study, energy and double differential cross section data of (n,p) and (n,α) reactions for all the stable isotopes of chromium have been estimated, using appropriate nuclear models in TALYS code. The cross section data of stable isotopes are later converted into the energy and double differential cross section data of natural Cr using the isotopic abundance. The contribution from compound, pre-equilibrium and direct nuclear reaction to total reaction have also been calculated for 52,50Cr(n,p) and 52Cr(n,α). The calculation of energy differential cross section shows that most of emitted protons and alpha particles are of 3 and 8 MeV respectively. The calculated data is compared with the data from EXFOR data library and is found to be in good agreement.

Mass dependent fractionation of stable chromium isotopes in mare basalts: Implications for the formation and the differentiation of the Moon

We present the first stable chromium isotopic data from mare basalts in order to investigate the similarity between the Moon and the Earth’s mantle. A double spike technique coupled with MC-ICP-MS measurements was used to analyse 19 mare basalts, comprising high-Ti, low-Ti and KREEP-rich varieties. Chromium isotope ratios (δ53Cr) for mare basalts are positively correlated with indices of magmatic differentiation such as Mg# and Cr concentration which suggests that Cr isotopes were fractionated during magmatic differentiation. Modelling of the results provides evidence that spinel and pyroxene are the main phases controlling the Cr isotopic composition during fractional crystallisation. The most evolved samples have the lightest isotopic compositions, complemented by cumulates that are isotopically heavy. Two hypotheses are proposed to explain this fractionation: (i) equilibrium fractionation where heavy isotopes are preferentially incorporated into the spinel lattice and (ii) a difference in isotopic composition between Cr2+ and Cr3+ in the melt. However, both processes require magmatic temperatures below 1200°C for appreciable Cr3+ to be present at the low oxygen fugacities found in the Moon (IW −1 to −2 log units). There is no isotopic difference between the most primitive high-Ti, low-Ti and KREEP basalts, which suggest that the sources of these basalts were homogeneous in terms of stable Cr isotopes. The least differentiated sample in our sample set is the low-Ti basalt 12016, characterised by a Cr isotopic composition of −0.222±0.025‰, which is within error of the current BSE value (−0.124±0.101‰). The similarity between the mantles of the Moon and Earth is consistent with a terrestrial origin for a major fraction of the lunar Cr. This similarity also suggests that Cr isotopes were not fractionated by core formation on the Moon.

Application of chromium stable isotopes to the evaluation of Cr(VI) contamination in groundwater and rock leachates from central Euboea and the Assopos basin (Greece)

Major and trace elements (a) in groundwater, ultramafic rocks from natural outcrops and soil samples from cultivated sites of Central Euboea and Assopos basin, and (b) in experimentally produced laboratory water leachates of rocks and soils were investigated by SEM/EDS, XRD and ICP/MS. In addition, stable chromium isotopes (expressed as δ53Cr values) were measured in groundwater and leachates in order to identify potential sources for Cr-contamination.The higher Cr(VI) concentrations in soil leachates compared to those in the rock pulp leachates potentially can be explained by the presence of larger amounts of Fe (Fe(II)) and Mn (Mn-oxides acting as oxidizing catalysts). Assuming that redox processes produce significant Cr isotope fractionation (groundwater δ53Cr values range between 0.8 and 1.98‰), the compilation of the obtained analytical data suggests that the dominant cause of Cr isotope fractionation is post-mobilization reduction of Cr(VI). However, the lack of a very good negative relationship between Cr(VI) concentrations and δ53Cr values may reflect that sorption, precipitation and biological processes (fractionation during uptake by plants) complicate the interpretation of the Cr isotope signatures.The variation in δ53Cr values (0.84 to 1.98‰ in groundwater from Euboea, and from 0.98 to 1.03‰ in samples from the Assopos basin) imply initial oxidative mobilization of Cr(VI) from the ultramafic host rocks, followed by reductive processes that lead to immobilization of portions of Cr(III). Using a Rayleigh distillation model and different fractionation factors of Cr(VI) reduction valid for aqueous Fe(II) and Fe(II)-bearing minerals, we calculate that more than ~53%, but maximum ~94%, of the originally mobile Cr(VI) pool was reduced to immobile Cr(III) in the waters investigated. This indicates that efficient processes in the aquifers may facilitate natural attenuation of the toxic Cr(VI) to less harmful Cr(III).

Chromium isotopes in carbonates — A tracer for climate change and for reconstructing the redox state of ancient seawater

Strontium and carbon isotopes of marine carbonates are routinely applied for chemostratigraphic cross correlations of time-equivalent sedimentary sequences and for calibration of the compositional evolution of seawater throughout Earth's history, mainly for the purpose of reconstructing ancient climatic changes. We here present results of a new isotopic tracer system – stable chromium isotopes – applied to a late Ediacaran marine carbonate sequence exposed in the Calera de Recalde syncline, Arroyo del Soldado Group, Uruguay. The aim was to compare Cr isotope signatures directly to δ 13C, 87Sr/ 86Sr and 143Nd/ 144Nd fluctuations in a well defined stratigraphic profile comprising sediments that were deposited during cold–warm periods accompanied by sea-level changes in response to glaciation–deglaciation at higher latitudes. The studied section is characterized by a pronounced negative (down to − 3.3‰) δ 13C excursion in carbonates paralleled by a decrease of 87Sr/ 86Sr values. Chromium isotope signatures over this section also show a correlated decrease in δ 53Cr (δ 53Cr = [(( 53Cr/ 52Cr) sample / ( 53Cr/ 52Cr) SRM979) − 1] × 1000) values from ~+0.29 to − 0.17‰ which mirrors a decrease in positively fractioned seawater signatures to slightly negative values characteristic of high-temperature magmatic sources. Linear correlations between δ 53Cr and εNd(570 Ma), 87Sr/ 86Sr and Cr concentrations can be explained by mixing between two major input sources of Cr, Nd and Sr into the shallow seawater: 1) a source characterized by negative δ 53Cr values of ~−0.2‰ , low 87Sr/ 86Sr values of ~ 0.707, and elevated 147Sm/ 144Nd values of ~ 0.13, recognized as a subaqueous hydrothermal dominated input source, and 2) a source characterized by positively fractionated δ 53Cr values of ~+0.2‰, higher 87Sr/ 86Sr values of ~ 0.708, and lower 147Sm/ 144Nd values of ~ 0.11, a source which is strongly affected by continentally derived input. Chromium isotopes provide a powerful tool for reconstructing the redox state of ancient seawater since positive values indicate that, at least locally, Neoproterozoic shallow ocean waters were sufficiently oxidized to fractionate chromium and/or that oxygen levels of the atmosphere were sufficient to transform Cr(III) into the more mobile hexavalent Cr(VI) formed during weathering processes on land. The fact that 87Sr/ 86Sr values, despite δ 13C fluctuations, remain low (indicative of a strong hydrothermal input into the basin at his time) implies that CO 2 limitation was the cause of negative δ 13C and δ 53Cr excursions in otherwise nutrient rich late Neoproterozoic basins, and that glaciation is only one more consequence of a tectonically driven, biologically mediated system. In such a scenario, glaciation acts as an amplifier of δ 53Cr signals. These signals in marine carbonates are a sensitive tracer for redox processes in the ocean and/or on land and have the potential to contribute significantly, in combination with the other commonly used isotopic tracers, to the reconstruction of climatic changes, particularly those that are associated with major glaciation periods in Earth's history.

NEUTRON-RICH CHROMIUM ISOTOPE ANOMALIES IN SUPERNOVA NANOPARTICLES

Neutron-rich isotopes with masses near that of iron are produced in type Ia and II supernovae. Traces of such nucleosynthesis are found in primitive meteorites in the form of variations in the isotopic abundance of 54Cr, the most neutron-rich stable isotope of chromium. The hosts of these isotopic anomalies must be presolar grains that condensed in the outflows of supernovae, offering the opportunity to study the nucleosynthesis of iron-peak nuclei in ways that complement spectroscopic observations and can inform models of stellar evolution. However, despite almost two decades of extensive search, the carrier of 54Cr anomalies is still unknown, presumably because it is fine-grained and is chemically labile. Here we identify in the primitive meteorite Orgueil the carrier of 54Cr-anomalies as nanoparticles, most likely spinels that show large enrichments in 54Cr relative to solar composition (54Cr/52Cr ratio >3.6xsolar). Such large enrichments in 54Cr can only be produced in supernovae. The mineralogy of the grains supports condensation in the O/Ne-O/C zones of a type II supernova, although a type Ia origin cannot be excluded. We suggest that planetary materials incorporated different amounts of these nanoparticles, possibly due to late injection by a nearby supernova that also delivered 26Al and 60Fe to the solar system. This idea explains why the relative abundance of 54Cr and other neutron-rich isotopes vary between planets and meteorites. We anticipate that future isotopic studies of the grains identified here will shed new light on the birth of the solar system and the conditions insupernovae.

Parameter sensitivities in nuclear reaction cross-section calculations

The present work explores the relationships between measured neutron reaction data and the parameters of nuclear models used to calculate comparable results as well as to interpolate and extrapolate into domains where measurements are not feasible. In particular, dimensionless sensitivity parameters are exploited as a potential method for quantifying these relationships. A dimensionless sensitivity parameter is defined as the ratio of the fractional change in a calculated observable physical quantity to a specific fractional change of a selected nuclear model parameter. The basic approach is described and it is illustrated by considering several neutron activation reactions associated with stable isotopes of chromium.

Yrast spectroscopy of54Cr

High-spin levels in the heaviest stable isotope of chromium were observed following the fusion-evaporation reaction C-12(Ca-48, alpha 2n)Cr-54 at a beam energy of 157 MeV, using the GAMMASPHERE and MICROBALL detector arrays. Numerous yrast and near yrast levels are reported, and previously assigned 7(+) and 9(+) levels are suggested to be 5(+) and 7(+), respectively. The inferred level scheme is compared with shell model calculations.

Comparison of total blood volume in normal, preeclamptic, and nonproteinuric gestational hypertensive pregnancy by simultaneous measurement of red blood cell and plasma volumes

OBJECTIVE: Our purpose was to demonstrate reduced blood volume in preeclampsia compared with nonproteinuric gestational hypertension and normal pregnancy by use of independent measures of red blood cell and plasma volumes. STUDY DESIGN: Red blood cells labeled with a nonradioactive stable isotope of chromium and Evans’ blue were infused in subjects with preeclampsia or gestational hypertension and normotensive pregnant controls. Blood was sampled eight times over 60 minutes for dye concentration and at 30 minutes for chromium analysis. RESULTS: Total blood and plasma volumes are decreased in preeclampsia (2660 ± 382 mL/m 2 and 1790 ± 332 mL/m 2, respectively) compared with normotensive subjects (3217 ± 391 mL/m 2, P < 0.001 and 2279 ± 325 mL/m 2, P < .001) and gestational hypertension (3139 ± 272 mL/m 2, P < .001 and 2132 ± 265 mL/m 2, P = .003). Total body/peripheral hematocrit ratio is increased in preeclampsia. CONCLUSIONS: Blood volume, by measurement of red blood cell and plasma volumes, is reduced and has altered distribution in preeclampsia but is normal in gestational hypertension. (Am J Obstet Gynecol 1998;179:87-93.)

Distribution of a stable isotope of chromium (53Cr) in serum, urine, and breast milk in lactating women

To determine the fate and distribution of chromium during lactation, six lactating women (25-38 y old) were given three doses of the tracer 53Cr (7.55 micromol/d, or 400 microg/d) on days 1, 2, and 3 of the study. Diet records, blood samples taken while subjects were fasting, and 24-h composite milk and urine samples were collected from day 0 to day 6. Fasting blood samples, morning milk samples, and 24-h urine samples were also collected on days 8, 10, 15, 30, 60, and 90. 53Cr and natural and total chromium concentrations in biological fluids were measured with gas chromatography-mass spectrometry and total urinary chromium was measured with atomic-absorption spectrometry. 53Cr was detectable in serum 2 h after dosing and continued to be detected from day 30 to day 60. Changes in total serum chromium concentration in response to the oral dose suggested that chromium concentrations in blood were not tightly regulated. 53Cr was not detected in breast milk and no significant changes in natural chromium concentration in milk were observed in response to the oral doses, suggesting that breast-milk chromium concentrations are independent of intake. The estimated chromium intake of exclusively breast-fed infants was 2.5 nmol/d (0.13 microg/d), below the lower end of the range of estimated safe and adequate daily dietary intakes (10-40 microg/d) for infants 0-6 mo of age. The baseline chromium concentration in urine and the minimum 53Cr absorption in lactating women were comparable with values for nonpregnant, nonlactating subjects. Chromium losses in breast milk do not appear to be compensated for via increased absorption or decreased excretion.

Red Cell Volume Determination Using a Stable Isotope of Chromium

To validate and improve a method of red cell volume determination by use of a stable isotope of chromium. Twelve subjects were sequentially injected with red blood cells labeled with a stable isotope of chromium (53Cr) and red blood cells labeled with radioisotopic chromium (51Cr). Measurement of 53Cr dilution was by gas chromatography/mass spectrometry. Measurement of 51Cr dilution was by gamma counter. Comparison of the two methods led to results that differed on average by 34.5 +/- 45.0 mL (1.8 +/- 2.2%), 0.3 to 3.2%, 95% confidence interval. Measurement of red cell volume by use of a stable isotope of chromium is accurate, with potential applications including measurement in pregnant women and children or other groups in whom exposure to radioisotopes is undesirable.

Measurement of red blood cell mass by labelling with enriched stable isotopes of chromium

Measurement of red blood cell mass by labelling with enriched stable isotopes of chromium

Numerical investigation of the separation of sulfur isotopes in a single gas centrifuge

The method developed was used to analyze the separation of stable chromium isotopes (in the form CrO2F2) as well as some other elements in the type of centrifuge studied. It was determined that the efficiency as a function of the parameter studied in the present work is similar. Moreover, the results made it possible to determine the dependence of the separation capability on the molecular mass and physical properties of the working gases.

Measurement of blood volume with an enriched stable isotope of chromium (53Cr) and isotope dilution by combined gas chromatography-mass spectrometry

We used an established isotope dilution gas chromatography-mass spectrometry method to measure blood volumes with an enriched stable isotope of chromium. The results were compared with those of the conventional method, which involves radioactive 51Cr. The two methods were compared simultaneously in two male subjects of different sizes, and essentially identical volumes were obtained. The isotope dilution method is insensitive to contamination with natural (unenriched) chromium and can be used to measure all of the stable isotopes of chromium. The method has potential applications in simultaneous erythrocyte survival studies. The absence of radioactivity makes possible an accurate means of studying blood volume changes in children and during pregnancy.

ENDF/B-VI Evaluations for Isotopes of CR, FE, NI, CU, and PB

Evaluations have been done for each of the stable isotopes of chromium, iron, nickel, copper, and lead. They are based on analysis of experimental data and results of nuclear model calculations whi...

Synthesis of the Heaviest Chemical Elements—Results and Perspectives

AbstractAll 17 artificially produced elements heavier than uranium were discovered by nuclear‐chemical synthesis. The three heaviest ones–elements 107, 108 and 109–were synthesized at the heavy‐ion accelerator UNILAC in Darmstadt by nuclear fusion from the heaviest stable nuclei, lead‐208 and bismuth‐209, and the most neutron‐rich stable isotopes of chromium and iron: element 107 from bismuth‐209 (atomic number Z = 83) and chromium‐54 (Z = 24), element 108 from lead‐208 (Z = 82) and iron‐58 (Z = 26), and element 109 from bismuth‐209 and iron‐58. The first isotopes detected were those with mass numbers 262 (Z = 107), 265 (Z = 108) and 266 (Z = 109); these nuclei are short‐lived α‐emitters with half‐lives of 8.2 ms, 1.8 ms and 3.4 ms, respectively. The yields of these reactions are extremely small; only three atoms of element 109 have ever been observed. Experiments to synthesize element 110 have given ambiguous results. All attempts to detect the “superheavy” elements with proton numbers near Z = 114 and neutron numbers near N = 184 have thus far failed. These elements have been predicted theoretically, and many attempts to synthesize them have been made, e.g. at UNILAC by fusion of calcium‐48 (Z = 20) with curium‐248 (Z = 96), or by transference of protons in the collision of two very heavy nuclei such as uranium‐238 (Z = 92). Surprisingly, the heaviest known nuclei are far more stable toward spontaneous fission into two fragments than was expected, but their synthesis is strongly hindered–much more than initially anticipated. It is this hindrance rather than the decreasing nuclear stability which seems to presently limit the extent of the periodic table: even heavier elements should be able to exist, but no way has yet been found to produce them.

Magnetic moments of excited states in the stable chromium isotopes.

The g factors of the first ${2}_{1}^{+}$ states in $^{50}\mathrm{Cr}$ and $^{54}\mathrm{Cr}$ have been measured by the perturbed angular correlation/transient field technique. The results, g(${2}_{1}^{+}$, $^{50}\mathrm{Cr}$)=+0.45(15) and g(${2}_{1}^{+}$, $^{54}\mathrm{Cr}$)=+0.53(12), are compared to shell model calculations.