Mass-dependent Isotope Fractionation Research Articles

Role of natural isotopic fractionation in isotope geo- and cosmo-chronology: A theoretical investigation

We introduce a new isotope chronological model in which the natural mass-dependent isotopic fractionation effects of the radioactive (“parent”) and radiogenic (“daughter”) elements are systematically and rigorously considered. Using this model, we show that internally-normalized radiogenic isotopic ratios, commonly determined for daughter elements such as Sr, Nd, Cr, Ni, Hf, W, and Os, are dependent on the extent of natural isotopic fractionation of the daughter and parent elements at the time of system closure. This dependence indicates that (1) in two samples derived from the same isotopically homogeneous source at the same time and with identical radiogenic ingrowth over time, the present-day internally-normalized radiogenic isotope ratios would be different if they were initially fractionated to different degrees, and (2) if different internally-normalized radiogenic isotopic ratios are observed for two co-genetic objects, the difference between them would include contributions from both radiogenic ingrowth and natural isotopic fractionation. Consequently, the isochron dating equations employed in traditional chronological studies will yield inaccurate results when significant natural isotopic fractionation are present among the studied samples. Modified isochron equations that can be used to retrieve correct chronological information from isotopically-fractionated samples are presented. These theoretical considerations are applied to the 87Rb–87Sr, 147Sm–143Nd, and 146Sm–142Nd isotope systems of calcium–aluminium-rich inclusions (CAIs), a set of samples that have undergone significant natural Sr, Nd, and Sm isotope fractionation during their formation. The large natural Sr isotope fractionation (up to ca. 5.3 ‰ for 88Sr/86Sr) in fine-grained CAIs can generate analytically well-resolvable biases (>120 ppm) in the internally-normalized 87Sr/86Sr ratios and lead to significant scatters of their 87Rb–87Sr isochron (in conjunction with scatters induced by open-system disturbances). The 87Rb–87Sr systems of coarse-grained CAIs, on the contrary, are essentially not affected by natural Sr isotopic fractionation due to their much subdued fractionation degrees, resulting in a more robust isochron. Similarly, the large natural Nd (up to ca. 4.0 ‰ for 146Nd/144Nd) and Sm (up to ca. 7.1 ‰ for 152Sm/148Sm) isotopic fractionation in fine-grained CAIs can induce significant scatters of the 147Sm–143Nd isochron if the natural fractionation followed the kinetic or power law, and 146Sm–142Nd isochron if the natural fractionation followed the equilibrium, Rayleigh, or power law. This implies that when studying radioactive isotope systems in objects whose daughter and parent elements can undergo significant isotope fractionation in nature, accompanying stable isotope analyses are necessary for accurate chronological interpretations.

Organic matters, but inorganic matters too: column examination of elevated mercury sorption on low organic matter aquifer material using concentrations and stable isotope ratios

Abstract. Sorption of mercury (Hg) in soils is suggested to be predominantly associated with organic matter (OM). However, there is a growing collection of research that suggests that clay minerals and Fe/Mn oxides are also important solid phases for the sorption of soluble Hg in soil–groundwater systems. We use a series of (60 mL syringe based) column experiments to examine sorption and subsequent desorption of HgCl2 solutions (experiment 1 (EXP1): 46.1 ± 1.1 mg L−1; experiment 2 (EXP2): 144 ± 6 mg L−1) in low-OM (0.16 ± 0.02 %) solid-phase aquifer materials. Analyses of total Hg concentrations, Hg speciation (i.e. pyrolytic thermal desorption (PTD)), and Hg stable isotopes are performed on both solid- and liquid-phase samples across sorption and desorption phases of the experiments. The sorption breakthrough curve best fitted a Freundlich model. Despite the very low-OM content, the Hg equilibrium sorptive capacity in these columns is very high: 1510 ± 100 and 2320 ± 60 mg kg−1 for EXP1 and EXP2, respectively, and it is similar to those determined for high-OM soils. Data from the experiments on mass-dependent Hg stable isotope fractionation data from these experiments (described by δ202Hg) support preferential sorption of lighter isotopes to the solid-phase materials with results indicating an isotopically heavier liquid phase and an isotopically lighter solid phase. Desorption fits exponential decay models, and 46 ± 6 % and 58 ± 10 % of the sorbed Hg is removed from the solid-phase materials at the termination of desorption in EXP1 and EXP2, respectively. The divergence of δ202Hg values between liquid and solid phases also continues into desorption. This desorption profile is linked to the initial release of easily exchangeable Hg(II) species physically sorbed to Fe/Mn oxides and clay mineral surfaces (liquid phase enriched in heavy isotopes) and then slower release of Hg(II) species that have undergone secondary reaction to more stable/less-soluble Hg(II) species and/or diffusion/transport into the mineral matrices (processes favouring lighter isotopes; solid phase enriched in lighter isotopes). The secondary production of Hg(0) within the columns is confirmed by PTD analyses that indicate distinct Hg(0) release peaks in solid-phase samples at <175 ∘C, which again agrees with field observations. Retardation (RD) and distribution (KD) coefficients are 77.9 ± 5.5 and 26.1 ± 3.0 mL g−1 in EXP1, respectively, and 38.4 ± 2.7 and 12.4 ± 0.6 mL g−1 in EXP2, respectively. These values are similar to values derived from column experiments on high-OM soil and provide the basis for future Hg fate and transport modelling in soil–groundwater systems.

Nature and origin of primary ore-forming fluids in the highly metamorphosed Archean Hongtoushan VMS deposit, North China: Insights from in situ S isotopes and trace elements of sulfides

Many ancient volcanogenic massive sulfides (VMS) deposits underwent various degrees of deformation and metamorphism, which modified the primary orebody geometries and sulfide textures, resulting in the great challenge of revealing the source and physicochemical properties of primary ore-forming fluids and original mineralization processes. Here, we observed the textural evolution of sulfides in the highly metamorphosed Archean VMS Hongtoushan Cu-Zn deposit, China, to constrain the histories and P-T conditions of metamorphic remobilization and to evaluate the influence of metamorphism on geochemical characteristics of sulfide. Further, the in situ sulfur isotopic and trace elemental analyses were utilized to reveal the source and physicochemical conditions of initial fluids. During metamorphism, pyrite developed elongated texture, bulging recrystallization, coarse porphyroblasts, and foam texture. These textures all occurred at a temperature of 550 – 700 °C, at which sulfide remobilization and deformation occurred in an anhydrous environment. The geochemical exchange among sulfides did not happen during metamorphism. The sulfur isotope and trace elements of sulfides thus can be used to track the source and physicochemical properties of primary ore-forming fluids. In situ sulfur isotopic analyses of sulfides yielded values of δ34S approaching zero and Δ33S situating mass-dependent isotope fractionation (MDF) area, indicating that the sulfur of the original submarine hydrothermal system was mainly derived from the magmatic fluids. The content of Se in pyrite revealed the primary fluids were acidic and reductive and were used to constrain the temperature of ore-forming fluids, which was higher than 250 °C. The ratio of Co/Ni of pyrite was higher than 10, indicating primary fluids were of magmatic-hydrothermal origin. The high content of Se and As in pyrite and Cd and Hg in sphalerite documented that these elements were partially derived from the magmatic volatiles or magma degassing process. Overall, our study demonstrates that coupling textures and in situ geochemical features of sulfides could be a helpful tool for tracking the primary ore-forming processes of the metamorphosed massive sulfide deposits.

Identification of alkaline amendment sources (slash and burn versus marling) for cereal crops grown in the North of France: A multiple isotope approach (87Sr/86Sr, δ44/40Ca, δ88/86Sr)

Early farmers used at least two types of agrarian amendments that could raise pH and base saturation levels to allow the cultivation of cereals: marling and plant ash. Ash can be input in many different ways: felling and burning in place, transferring plant material from wastelands and spreading the ash, charring sod or peat blocks, burning stubble after harvest. Marling includes all the practices of amending limestone, from marl to chalky limestone composed of 99% CaCO3. In order to understand the evolution of these agricultural practices, it is important to identify which of the two techniques was used to amend cereal crops in the past. In order to test the potential of δ44/40Ca-δ88/86Sr-87Sr/86Sr multiple isotope approach for archaeological samples, we first applied the technique to currently grown crops, amended either with marl or with ash from freshly-cut and burned trees. We found that this approach makes it possible to discriminate cereal grains amended either by marling (less radiogenic Sr) or with tree ash (more radiogenic Sr). We also identified a positive correlation between stable Ca and Sr isotope values, suggesting that the Ca and Sr came from similar sources and had undergone similar mass-dependent isotopic fractionation mechanisms. Consequently, we later on mainly focused on stable and radiogenic Sr isotopes. Stable Sr isotope fractionation was also observed between different locations, different organs of a given cereal species and between different cereal types, but also within the same cereal species or the number of grains studied for a given locality, pointing to biological fractionation combined with source variation.

Origin of carbonatites and associated silicate rocks revealed by Mg triple-isotope approach

Carbonatites are rare carbonate-rich igneous rocks derived from carbon and carbonate-rich regions of Earth's mantle. Although a number of igneous processes are recognized to have controlled their compositions, the origin of the carbonate-rich nature of these magmas remains debated and has been linked to various mantle-related processes, including subduction and plume–lithosphere interaction. High-precision isotope measurements can provide insights into carbonatite petrogenesis, including the identification of subducted crustal material in their source region. In particular, combining mass-dependent and kinetically corrected Mg isotope data, also known as the triple-isotope approach, provides insights into mass fractionation processes driving mass dependent fractionation, which in turn allows to distinguish between equilibrium and kinetic processes. In this work, we report high-precision Mg stable isotope data for 59 carbonatites and associated silicate rocks from different localities and ages ranging from 3000 Ma to present-day, as well as C and O isotopic analysis for 38 carbonatites and Sr isotopic analysis for 41 carbonatites and silicate rocks. In addition, we also report Mg isotope data for 17 Phanerozoic carbonate rocks, with the aim of identifying the isotopic signature of carbonate-rich material potentially recycled to the carbonatite mantle source regions. Collectively, the data reveal a range of stable Mg isotope compositions from δ25/24MgDTS-2 = −1.20 ± 0.01‰ to +0.08 ± 0.01‰. We observe positive residual deviations after kinetic mass fractionation correction of the Mg isotope data for our carbonatites; an isotope signal that is also present in Phanerozoic carbonates. This observation establishes that a component of the Mg present in these samples experienced mass-dependent equilibrium isotopic fractionation processes, which are significantly larger at low temperatures. Given that the Mg results do not covary with C and O isotopic signals, the magnitude of the fractionation following the equilibrium law observed in carbonatites provides strong evidence for recycled material from the Earth's surface in the mantle source of Ca- and Mg-rich carbonatites. Furthermore, associated silicate rocks present mantle-like Mg isotopic compositions in contrast with the genetically linked carbonatites - based on the Sr isotopes - for some complexes, which is best explained if the carbonatites and associated silicate rocks represent distinct generation of partial melts of a mantle source containing recycled carbonate.

Spurious molybdenum isotope anomalies resulting from non-exponential mass fractionation

Mass-independent (nucleosynthetic) Mo isotope anomalies are uniquely useful for constraining genetic relationships among meteoritic and planetary materials and, by extension, the origin and nature of Earth's late-stage building blocks. The meaningful interpretation of such data, however, critically depends on the accurate correction of any natural and analytical mass-dependent isotope fractionation, which is commonly assumed to follow the ‘exponential law’. Here, using new high-precision Mo isotope data for a diverse set of terrestrial samples, we show that mass-dependent Mo isotope fractionation in nature typically does not adhere to this law, but is instead dominated by equilibrium and Rayleigh processes. We demonstrate that even moderate degrees of such non-exponential fractionation (i.e., mass-dependent isotope fractionation deviating from the exponential law) can result in significant spurious mass-independent Mo isotope anomalies that, when misinterpreted as nucleosynthetic anomalies, can lead to erroneous conclusions, particularly with respect to Earth's accretion history. Consequently, assessing the magnitude and origin of mass-dependent fractionation will be essential for future efforts to precisely determine the mass-independent Mo isotope composition of bulk silicate Earth and to identify potential nucleosynthetic isotope anomalies in terrestrial rocks.

Application of the δ44/40Ca-δ88/86Sr multi-proxy to Namibian Marinoan cap carbonates

Several studies have exploited the stable calcium (Ca) and strontium (Sr) isotope compositions of marine carbonate rocks to investigate ancient carbon cycle dynamics, seawater geochemistry, and controls on carbonate production and preservation. However, the two proxies have rarely been applied together. Based on knowledge achieved to date, the δ44/40Ca-δ88/86Sr multi-proxy can distinguish signatures imparted by mass-dependent fractionation and reservoir mixing, including early diagenesis and shifts in the isotopic composition of seawater. In detail, the δ44/40Ca-δ88/86Sr multi-proxy represents a five-isotope system because the determination of δ88/86Sr values requires analysis of fractionation-corrected 87Sr/86Sr ratios, which provide additional constraints on mixing. Here, we apply the novel δ44/40Ca-δ88/86Sr multi-proxy to two Marinoan (ca. 635 Ma) “cap carbonate” sequences from Namibia. Cap carbonates were widely deposited after the Neoproterozoic Marinoan Snowball Earth Event, but controversy surrounds their origin. We find that the rocks archive primary environmental signals deriving from a combination of seawater-glacial meltwater mixing and kinetic isotope effects. In an outer platform section, dolostone δ44/40Ca and δ88/86Sr values define a line predicted for kinetic mass-dependent isotope fractionation. This dolostone mostly precipitated from meltwater. Moreover, stratigraphically higher samples exhibiting the fastest precipitation rates display elevated 87Sr/86Sr ratios, consistent with long-held expectations that a rapid deglacial weathering pulse forced cap carbonate formation. An inner-platform dolostone shows greater effects from water-mass mixing but still reveals that precipitation rates increased up-section. Overlying limestones show greater Ca and Sr contributions from seawater. Amplification of local coastal processes during global ice sheet collapse offers a simple but sufficient proposition to explain the Ca isotope heterogeneity of cap carbonates. Detection of kinetic isotope effects provides evidence for predominantly rock-buffered diagenesis and further offers a basis for developing the δ44/40Ca-δ88/86Sr multi-proxy as an indicator of saturation state and pCO2.

Reliable uncertainties: Error correlation, rotated error bars, and linear regressions in three-isotope plots and beyond

Correlated errors of experimental data are a common but often neglected problem in physical sciences. Various tools are provided here for thorough propagation of uncertainties in cases of correlated errors. Discussed are techniques especially applicable to three-isotope plots common in geo- and cosmochemistry, where common denominators in isotope ratios, as well as instrumental effects, such as mass-dependent isotope fractionation, could lead to significant correlation of errors. Furthermore, various techniques for calculating linear regressions are compared to each other, showing that the method suggested by Mahon (1996) [1] gives the best results after correcting for some typographical errors therein and eliminating a minor mistake. We also provide a method for calculating linear regressions through a fixed point, avoiding previously made mistakes.

Barium isotope fractionation in barite–fluid systems at chemical equilibrium

The barium isotope composition of sedimentary barite (BaSO4, barium sulfate) is emerging as a powerful tracer of the sources and cycling of Ba in modern and ancient marine environments. To reliably use Ba isotopes to interrogate the marine Ba cycle, it is important to identify and constrain processes that fractionate the isotope composition of Ba in BaSO4. Of particular interest is ion exchange: micro-scale dissolution and precipitation that occurs in mineral–fluid systems at chemical equilibrium. This process is often important in systems where minerals, such as BaSO4, and a fluid remain in contact for prolonged periods of time; however, the impact of ion exchange on Ba isotope compositions in BaSO4 is unknown. To constrain the rate and isotopic effect associated with ion exchange in BaSO4–fluid systems, we conducted a series of experiments under marine-relevant conditions and interpreted the results using a multi-phase time-dependent numerical reactor model. From a series of isotope-tracer experiments, we find that BaSO4–fluid ion exchange progresses at a rate between 5 and 53 pmol m−2 s−1. In a parallel set of experiments used to assess mass-dependent isotope fractionation of Ba, the combined effect of BaSO4 dissolution and precipitation while at chemical equilibrium was found to result in the continued evolution of Ba isotopes and produced a modeled offset of Δ138Babarite–dBa = −0.10 ± 0.05 ‰ at isotopic equilibrium. We then constrained the magnitude of isotopic fractionation during BaSO4 dissolution by fitting our data in the numerical reactor model and using previous estimates of Ba isotope fractionation during BaSO4 precipitation (⍺precipitation = 0.99968 ± 0.00002). At chemical equilibrium, we find our data are best explained by an ⍺dissolution = 0.99978 ± 0.00006, implying that BaSO4 dissolution releases isotopically ‘light’ Ba to solution. Since the magnitude of the isotope effects associated with BaSO4 precipitation and dissolution are imbalanced, ion exchange will tend to alter the isotope composition of co-located BaSO4 and fluids until the two phases are offset by ≈0.10 ‰. The importance of this effect on sedimentary BaSO4 likely depends on several factors and we suggest multiple site-screening criteria to maximize the utility of this emerging proxy.

Mass-independent Sn isotope fractionation and radiogenic 115Sn in chondrites and terrestrial rocks

Tin has ten stable isotopes, providing the opportunity to investigate and discriminate nucleosynthetic isotope anomalies from mass-dependent and mass-independent isotope fractionation. Novel protocols for chemical separation (based on TBP-resin) and MC-ICP-MS analyses are reported here for high precision Sn isotope measurements on terrestrial rocks and chondrites. Relative to the Sn reference standard (NIST SRM 3161a), terrestrial basalts and chondrites show isotope patterns that are consistent with mass-dependent and mass-independent isotope fractionation processes as well as with 115Sn radiogenic ingrowth from 115In.Two different mass-independent isotope effects are identified, namely the nuclear volume (or nuclear field shift) and the magnetic isotope effect. The magnetic isotope effect dominates in the two measured ordinary chondrites, while repeated analyses of the carbonaceous chondrite Murchison (CM2) display a pattern consistent with a nuclear volume effect. Terrestrial basalts show patterns that are compatible with a mixture of nuclear volume and magnetic isotope effects. The ultimate origin of the isotope fractionation is unclear but a fractionation induced during sample preparation seems unlikely because different groups of chondrites show distinctly different patterns, hence pointing towards natural geo/cosmochemical processes. Only the carbonaceous chondrite Murchison (CM2) shows a Sn isotope pattern similar to what expected for nucleosynthetic variations. However, this pattern is better reproduced by nuclear volume effects. Thus, after considering mass-independent and mass-dependent effects, we find no evidence of residual nucleosynthetic anomalies, in agreement with observations for most other elements with similar half-mass condensation temperatures.Most chondrites show a deficit in 115Sn/120Sn (typically −150 to −200 ppm) relative to terrestrial samples, with the exception of one ordinary chondrite that displays an excess of about +250 ppm. The 115Sn/120Sn data correlate with In/Sn, being consistent with the β− decay of 115In over the age of the solar system. This represents the first evidence of the 115In-115Sn decay system in natural samples. The radiogenic 115Sn signature of the BSE derives from a suprachondritic In/SnBSE, which reflects preferential partitioning of Sn into the Earth’s core.

Rubidium and potassium isotopic variations in chondrites and Mars: Accretion signatures and planetary overprints

As moderately volatile elements, isotopes of Rb and K can trace volatilization processes in planetary bodies. Rubidium isotopic data are however very scarce, especially for non-carbonaceous meteorites. Here, we report combined Rb and K isotopic data (δ87/85Rb and δ41/39Κ) for 7 ordinary, 6 enstatite, and 4 Martian meteorite falls to understand the causes for the variations in volatile abundances and isotopic compositions. Bulk Rb and K isotopic compositions of planetary bodies are estimated to be (Table 1): Mars +0.10 ± 0.03 ‰ for Rb and −0.26 ± 0.05 ‰ for K, bulk OCs -0.12-0.24+0.15 ‰ for Rb and -0.72-0.41+0.28 ‰ for K, bulk ECs +0.02-0.26+0.29 ‰ for Rb and -0.33-0.23+0.37 ‰ for K. The bulk K isotopic compositions of subgroup OCs are estimated to be -0.72-0.55+0.26 ‰ for H chondrites, -0.71-0.39+0.23 ‰ for L chondrites, and -0.77-0.30+0.63 ‰ for LL chondrites. A broad correlation between the Rb and K isotopic compositions of planetary bodies is observed. The correlation follows a slope that is consistent with kinetic evaporation and condensation processes, suggesting volatility-controlled mass-dependent isotope fractionation (as opposed to nucleosynthetic anomalies).Individual ordinary and enstatite chondrites show large Rb and K isotopic variations (−1.02 to +0.29 ‰ for Rb and −0.91 to −0.15 ‰ for K). Samples of lower metamorphic grades display correlated elemental and isotopic fractionations between Rb and K, while samples of higher metamorphic grades show great scatter, suggesting that chondrite parent-body processes have decoupled the two elements and their isotopes at the sample scale. Several processes could have contributed to the observed isotopic variations of Rb and K, including (i) chondrule “nugget effect”, (ii) volatilization during parent-body thermal metamorphism (heat-induced vaporization and gas transport within parent bodies), (iii) thermal diffusion during parent-body metamorphism, and (iv) impact/shock heating. Quantitative modeling of the first two processes suggests that neither of them could produce isotopic variations large enough to explain the observed isotopic variations. Volatilization during parent-body thermal metamorphism [the scenario (ii)], which has been commonly invoked to explain the isotopic variations of volatile elements, is gas transport-limited and its effect on isotopic fractionations of moderately volatile elements should be negligible. Modeling of diffusion processes suggests that (iii) could produce K isotopic variation comparable to the observed variation. The large isotopic variations in non-carbonaceous meteorites are thus most likely due to diffusive redistribution of K and Rb during metamorphism and/or shock-induced heating and vaporization.

COSMO: Double spike optimization for sample-limited analyses of isotopically anomalous materials

The double spike (DS) technique is the gold standard by which high-precision and high-accuracy mass-dependent isotope fractionations are quantified, and has played a critical role in the recent development of numerous non-traditional stable isotope systems. The democratization of the technique is in great part due to the availability of the so-called ‘DS toolbox’, a software suite that allows for the straightforward identification of optimal DS compositions. As new applications for DS measurements arise, some additional considerations must be taken into account in deciding on an optimal double spike. In particular, sample-limited investigations of combined mass-dependent and independent isotope effects (e.g., in Early Solar System materials) present an additional challenge in determining optimal spikes.Here, we describe the cosmo software package, which specifically addresses this upcoming need in cosmochemistry/isotope geochemistry to optimize DS measurements for small samples with mass-independent anomalies (e.g., nucleosynthetic anomalies, radiogenic ingrowth). These measurements are subject to additional errors from a complementary unspiked measurement, which is necessary to properly quantify mass-dependent isotope effects during the DS inversion. The software package addresses this additional complication by offering users the ability to (i) specify additional parameters relevant to practical sample-limited analyses (e.g., instrumental transmission efficiency, number of cycles of analyses), (ii) optimize how a sample is split between unspiked and spiked measurements, and (iii) identify the internal normalization scheme that leads to the lowest uncertainty on the mass-dependent fractionation factor, α, and/or the isotope anomalies, ε. These additional functionalities were designed to operate within the DS toolbox framework and expands its applicability to a wider array of samples (i.e., extraterrestrial samples) and measurement scenarios to push the limits of new and improved instrumentation.

Equilibrium mass-dependent isotope fractionation of antimony between stibnite and Sb secondary minerals: A first-principles study

Antimony (Sb) isotopes are gaining increasing interest for their potential as geochemical tracers in geological, environmental and archaeological studies. However, little is known about the parameters controlling Sb isotope fractionation, which is essential to interpret variations of isotopic signature in natural systems. In this study, equilibrium mass-dependent isotope fractionation factors (β-factor) were calculated between different Sb-bearing minerals commonly found in mining environments including primary Sb sulphide (stibnite Sb2S3) and its oxidation products (valentinite Sb2O3, senarmontite Sb2O3, cervantite Sb2O4) and synthetic antimony pentoxide Sb2O5. First-principles calculations within the Density Functional Theory (DFT) were performed with different functionals to test the robustness of the method. Among the studied minerals, stibnite has the lowest β-factor (ln(β) = 0.71 ‰ at 22°C), then β-factors progressively increase from valentinite (ln(β) = 1.64 ‰ at 22°C), to senarmontite (ln(β) = 1.80 ‰ at 22°C), cervantite (ln(β) = 2.20 ‰ at 22°C) and antimony pentoxide (ln(β) = 3.03 ‰ at 22°C). The factors that most fractionate Sb isotopes are found to be i) the change of Sb oxidation state (Sb isotope ratio in Sb(V)-bearing minerals is higher than in Sb(III)-bearing minerals), ii) the change of first neighbour of Sb (Sb isotope ratio in SbO bonds is higher than in SbS bonds) and iii) distortion of the atomic SbO polyhedrons. The negligible differences in the β-factors obtained with different functionals showed the robustness of the approach for the calculation of β-factors, despite differences in the calculated mineral lattice and Raman frequencies. The results of this study provide a theoretical basis to interpret natural Sb isotope variations. The results suggest that a significant enrichment in the heavy isotope could occur during oxidative dissolution of stibnite and subsequent precipitation of Sb(III) and Sb(V) oxides in sulphide environments. More generally, this work strongly supports that Sb isotopes may be a useful tracer of Sb transformation processes in nature.

The effect of alkalinity on Ni–O bond length in silicate glasses: Implications for Ni isotope geochemistry

Equilibrium mass-dependent (“stable”) isotopic fractionation of an element during magmatic processes is driven by a contrast in bonding environment between minerals and silicate melt, which is expressed as an isotopic fractionation factor. A quantitative understanding of such isotopic fractionation factors is vital to interpret observed isotopic variations in magmatic rocks. It is well known that the local environment and the bond strength of an element dictate the sign and magnitude of isotopic fractionation between minerals, but it is uncertain how the structure and chemical composition of a silicate melt can affect mineral-melt isotopic fractionation factors. To explore this, we studied the coordination environment of nickel (Ni) in different silicate glasses using extended X-ray absorption fine structure (EXAFS) measurements at the German synchrotron X-ray source (DESY).We determined Ni–O bond lengths in a suite of synthetic but near-natural silicate glasses using EXAFS and found that the former vary systematically with melt alkalinity, which is best described by the parameter ln[1 + (Na + K)/Ca]. With increasing melt alkalinity, Ni occupies more IV-fold coordinated sites, which are associated with a shorter Ni–O bond length. Next, we use the ionic model, which allows to predict isotopic fractionation factors based on the difference in bond length between two phases. We find that more alkaline melts have a stronger preference for the heavier isotopes of Ni than less alkaline melts. This implies that the magnitude of mineral-melt Ni isotope fractionation factors, for instance between olivine and melt, will depend on the alkalinity of the melt. At magmatic temperatures, however, the variation in fractionation factors caused by melt alkalinity will rarely exceed 0.05 ‰ and is thus mostly negligible, in particular in the realm of basaltic melt compositions. Nevertheless, the relationship between melt alkalinity and fractionation factor reported here can be used to extrapolate empirical data for mineral-melt Ni isotope fractionation factors, once such data become available, to the full range of magma compositions on Earth and other Solar System bodies.

Kinetic isotope effects in H2O2 self-decomposition: Implications for triple oxygen isotope systematics of secondary minerals in the solar system

Hydrogen peroxide (H2O2) is a ubiquitous molecule in nature that shapes the redox state of planetary surfaces. Given that H2O2 is a major oxidant, isotope effects associated with H2O2 chemistry play a key role in determining triple oxygen isotopic compositions (δ17O and δ18O) of secondary aerosols and minerals, which are powerful proxies for understanding terrestrial/Martian atmospheric chemistry, and chemical evolution in the solar nebular. However, isotope effects in H2O2 self-decomposition processes, which actively occur in nature due to the thermal instability of H2O2, remains poorly understood. Here, we report a hitherto overlooked large and mass-dependent isotope fractionation in aqueous H2O2 self-decomposition processes quantified in a series of kinetic experiments. δ18O in remaining H2O2 is several tens of per mil (‰) with respective to initial H2O2. By synthesizing triple oxygen isotope measurements of natural H2O2, ozone, various oxyanions formed in the atmosphere (sulfate, nitrate, perchlorate, and carbonate), and oxygen-bearing secondary minerals in meteorites, we find that a decoupled Δ17O–δ18O pattern in natural H2O2 is attributed in part to the high degree of mass-dependent δ18O variation in H2O2 decomposition, and further argue that this unique signature may play a crucial role in triple oxygen isotope systematics in a board spectrum of secondary minerals in our solar system including aerosols, sediments, and meteorites. Our results shed fresh insights into recent debates on the role of H2O2 in the formation of these secondary minerals in the modern Earth, geological past, and other planets. The isotope effect experimentally quantified in this study are needed for future improvements of planetary atmosphere and solar nebular evolution models. We highlight the importance of further experimental and theoretical efforts to quantify isotope effects in H2O2 chemistry that are representative of natural systems.

Mass-independent fractionation of oxygen isotopes during thermal decomposition of divalent metal carbonates: Crystallographic influence, potential mechanism and cosmochemical significance

Few physical or chemical processes defy well-established laws of mass-dependent isotopic fractionation. A surprising example, discovered two decades ago, is that thermal decomposition of calcium and magnesium carbonate minerals (conducted in vacuo, to minimise back-reaction and isotopic exchange) causes the oxygen triple-isotope compositions of the resulting solid oxide and CO2 to fit on parallel mass-dependent fractionation lines in ln(1 + δ17O) versus ln(1 + δ18O) space, with anomalous depletion of 17O in the solid and equivalent enrichment of 17O in the CO2. By investigating the thermal decomposition of other natural divalent metal carbonates and one synthetic example, under similar conditions, we find that the unusual isotope effect occurs in all cases and that the magnitude of the anomaly (Δ′17O) seems to depend on the room temperature crystallographic structure of the carbonate. A lower cation coordination number (as associated with smaller cation radius) correlates with a Δ′17O value closer to zero. Local symmetry considerations may therefore be influential. Relative to a reference fractionation line of slope 0.524 and passing through VSMOW, solid oxides produced by thermal decomposition of orthorhombic carbonates were characterised by Δ′17O = −0.367 ± 0.004‰ (standard error). The comparable figure from rhombohedral examples was −0.317 ± 0.010‰, whereas from the sole monoclinic (synthesised) specimen it was −0.219 ± 0.011‰. The numerical values are, to some extent, dependent on details of the experimental procedure. We discuss potential origins of the isotopic anomaly, including the possibility of hyperfine coupling between 17O nuclei and unpaired electrons of transient radicals (the ‘magnetic isotope effect’). A new mechanism based on the latter process is proposed. The associated transition state is compatible with that suggested by recent quantum chemical and kinetic studies of the thermal decompositions of calcite and magnesite. An earlier suggestion based on the magnetic isotope effect is shown to be incompatible with the generation of a 17O anomaly, regardless of the identity of the carbonate. We cannot exclude the possibility that a Fermi resonance between states leading to dissociation may additionally affect the magnitude of Δ′17O in some cases. Our findings have cosmochemical implications, with thermal processing of carbonates providing a potential mechanism for the mass-independent fractionation of oxygen isotopes in protoplanetary systems.

Experimental investigation of Ru isotope fractionation between metal, silicate and sulfide melts

To improve the understanding of large-scale planetary processes, i.e. differentiation and core formation, of Earth and other planetary bodies, we performed experiments at 1 GPa in a range of temperatures to investigate mass-dependent isotope fractionation of ruthenium (Ru) between metal, silicate, and sulfide melts. Metal silicate fractionation is 102Ru/99Rusilicate - 102Ru/99Rumetal = 0.02 ± 0.02‰ (95% confidence interval) at 1600 °C and therefore negligible for Earth's core formation. However, there is resolvable Ru isotope fractionation between liquid metal and liquid sulfide: The 102Ru/99Ru ratio of liquid sulfide is 0.11 ± 0.03‰ lighter than that of liquid metal at 1400 °C in sulfur (S)-bearing experiments. The unexpected lighter Ru isotope composition of the sulfide can be best explained with different RuS bonding environments. Our results show furthermore, that addition of tin (Sn) instead of S to experimental charges affects Ru isotope fractionation significantly. The 102Ru/99Ru ratios in the Sn-bearing phase are 0.18‰ ± 0.01‰ heavier than metal; hence, the presence of Sn not only changes the magnitude of the Ru isotope fractionation but also its direction. The observed Ru isotope fractionations are too small to preserve a resolvable isotope fractionation signature during core formation or the Hadean matte scenario at very high temperatures.

Industrially Purified Nd Materials Identified by Distinct Mass-Dependent Isotopic Composition

Rare earth elements (REEs) are considered emerging anthropogenic pollutants. Anthropogenic lanthanum, cerium, samarium, and gadolinium alone, or excess of all the REEs have already been reported in the environment. In addition, it is only a matter of time for neodymium (Nd) of anthropogenic origin to be reported disseminated in the environment, given its growing demand for new technologies and its use in permanent magnets of wind turbine. So far, only in a few cases was the addition of anthropogenic Nd detected in soils and sediments by the measurements of REE concentrations. For this reason, we propose to use the Nd isotopic composition to help the distinction of pollution. The isotopic tracing of Nd using variations in the abundance of 143Nd from the radioactive decay of 147Sm (Nd-radiogenic composition) is one option. Here, we expand the Nd isotopic fingerprinting by the investigation of the stable Nd isotopic composition expressed as δxNd, the relative permil (%0) deviation from the isotopic composition of the pure Nd JNdi-1 reference standard. The measurement of δxNd used a MC-ICPMS (multi-collector inductively coupled plasma mass spectrometry) with sample-standard bracketing technique, allowing the determination of precise and accurate Nd isotopic variations. Our results show that Nd-magnets (Neo) and man-made purified Nd materials are not significantly different on average (respectively, δ148Nd of −0.105 ± 0.023 and −0.120 ± 0.141%0). More importantly, they are different from terrestrial rocks (δ148Nd of −0.051 ± 0.031%0). Moreover, the Nd-radiogenic composition of Neo can be highly variable, even when they come from a single supplier. In addition, the study of all Nd stable isotopic compositions demonstrates that irrespective of their natural origin (witnessed by their Nd-radiogenic composition), all Nd from rocks and man-made materials are related by mass-dependent isotopic fractionation laws. We also have defined a parameter, the Δ148−150Nd′, allowing the distinction of thermodynamic isotopic fractionation (the Δ148−150Nd′ is invariant) from kinetic isotopic fractionation (the Δ148−150Nd′ is negatively correlated with the δ148Nd). Such covariation is observed for anthropogenic materials that could be seen as small deficit in 150Nd (around 5 ppm/%0/amu), but too small to be consistent with nuclear field effect. On the other hand, the anthropogenic material defines a covariation in the Δ148−150Nd'–δ148Nd space in full agreement with the theoretical expectation from mass-dependent kinetic isotopic fractionation. The mass-dependent fractionation of Nd by chromatographic separation is also consistent with a kinetic isotopic fractionation. The purification of Nd from other light REEs by industrial processes involves chromatographic separation and, therefore, is likely to produce anthropogenic Nd with low values for δ148Nd associated with high values for Δ148−150Nd′. Both are resolvable with current MC-ICPMS technology and could be useful to trace incoming anthropogenic pollution in the environment. In soils, the combination of low values for δ148Nd with high values for Δ148−150Nd′ is likely to be an unambiguous pollution signal from the degradation in the environment of Neo or other industrial products, especially if this is associated with an Nd-radiogenic composition inconsistent with the surrounding rocks and soils. In contrast, the industrial residue of Nd purification could be characterized by high δ148Nd with low values for Δ148−150Nd′, and the leak or the discharge of such residue could also be unambiguously distinguished.

Contamination levels and habitat use influence Hg accumulation and stable isotope ratios in the European seabass Dicentrarchus labrax

Hg accumulation in marine organisms depends strongly on in situ water or sediment biogeochemistry and levels of Hg pollution. To predict the rates of Hg exposure in human communities, it is important to understand Hg assimilation and processing within commercially harvested marine fish, like the European seabass Dicentrarchus labrax. Previously, values of Δ199Hg and δ202Hg in muscle tissue successfully discriminated between seven populations of European seabass. In the present study, a multi-tissue approach was developed to assess the underlying processes behind such discrimination.We determined total Hg content (THg), the proportion of monomethyl-Hg (%MeHg), and Hg isotopic composition (e.g. Δ199Hg and δ202Hg) in seabass liver. We compared this to the previously published data on muscle tissue and local anthropogenic Hg inputs.The first important finding of this study showed an increase of both %MeHg and δ202Hg values in muscle compared to liver in all populations, suggesting the occurrence of internal MeHg demethylation in seabass. This is the first evidence of such a process occurring in this species. Values for mass-dependent (MDF, δ202Hg) and mass-independent (MIF, Δ199Hg) isotopic fractionation in liver and muscle accorded with data observed in estuarine fish (MDF, 0–1‰ and MIF, 0–0.7‰). Black Sea seabass stood out from other regions, presenting higher MIF values (≈1.5‰) in muscle and very low MDF (≈-1‰) in liver. This second finding suggests that under low Hg bioaccumulation, Hg isotopic composition may allow the detection of a shift in the habitat use of juvenile fish, such as for first-year Black Sea seabass.Our study supports the multi-tissue approach as a valid tool for refining the analysis of Hg sourcing and metabolism in a marine fish. The study’s major outcome indicates that Hg levels of pollution and fish foraging location are the main factors influencing Hg species accumulation and isotopic fractionation in the organisms.

Simultaneous determination of mass-dependent Mg isotopic variations and radiogenic 26Mg by laser ablation-MC-ICP-MS and implications for the formation of chondrules

Improvements in our understanding of the formation of chondrules requires a better knowledge of the thermal histories and the nature of their solid precursors. We present an in situ nanosecond laser ablation multi-collector inductively-coupled-plasma mass-spectrometry (LA-MC-ICP-MS) technique to measure simultaneously mass-dependent Mg isotopic fractionations and radiogenic 26Mg in chondritic components, thus allowing us to investigate within a chronological framework the thermal processes redistributing Mg in chondrules and their precursors. The internal 26Al-26Mg isochrons provide initial 26Al/27Al ratios from 5.46 (±0.38) × 10−5 to 6.14 (±0.92) × 10−5 for amoeboid olivine aggregates (AOAs) and Ca-, Al-rich inclusions (CAIs), and from 0.16 (±0.08) × 10−5 to 1.87 (±0.92) × 10−5 for chondrules from Allende and Leoville chondrites, which are consistent with the previously reported values. The combination of these values with up to 2.5‰ variation of the 25Mg/24Mg ratio within the studied chondrules shows that: (i) AOAs and the precursors of chondrules were likely formed via condensation of rapid-cooling gas reservoirs, and (ii) Mg stable isotopes are probably at disequilibrium between olivines and mesostases in some chondrules, likely due to Mg loss by vaporization during chondrule formation. We use these new observations to propose that Mg isotopes can likely serve as a tracer for the thermal histories of chondrules. We present here a scenario taking into account Mg loss by vaporization from chondrule melt and Mg gain into the melt by olivine dissolution. The existing Mg isotopic observations in chondrule melts and olivines can be explained in a scenario with a homogeneous distribution of Mg isotopes and initial 26Al in the accretion disk, provided that chondrule precursors have been heated up to sufficiently high peak temperatures (up to 2123 K) and stayed above 1800 K for several tens of minutes to allow for significant Mg evaporation. These conditions are most consistent with a shock wave model for the origin of chondrules.