Isotope Fractionation Behavior Research Articles

Antimony isotopic fractionation induced by Sb(V) adsorption on β-MnO2

Antimony (Sb) isotopes hold immense promise for unraveling Sb biogeochemical cycling in environmental systems. Mn oxides help control the fate of Sb via adsorption reactions, yet the behavior and mechanisms of Sb isotopic fractionation on Mn oxides are poorly understood. In this study, we examine the Sb isotopic fractionation induced by adsorption on β-MnO2 in different experiments (kinetic, isothermal, effect of pH). We observe that adsorption on β-MnO2 surfaces preferentially enriches lighter Sb isotopes through equilibrium fractionation, with Δ123Sbaqueous-adsorbed of 0.55–0.79 ‰. Neither the pH or surface coverage affects the fractionation magnitude. The analysis of extended X-ray absorption fine structure (EXAFS) demonstrates that the enrichment of light isotope results from the adsorption of inner-sphere complexation on solids. Our finding of this study enhances our comprehension of the impact of β-MnO2 on Sb isotopic fractionation behavior and mechanism and facilitate the applicability of Sb isotopes as effective tracers to elucidate the origins and pathways of Sb contamination in environmental systems, as well as provide a new insight into forecasting the isotopic fractionation of other similar metals adsorbed by manganese oxides.

Quantification of classical and non-classical crystallization pathways in calcite precipitation

Crystal precipitation from aqueous solution occurs through multiple pathways. Besides the classical ion-by-ion addition, non-classical crystallization mechanisms, such as multi-ion polymer and nano-particle attachment, could be significant. These non-classical crystallization processes have been observed with advanced microscopy, yet detailed quantification of their contributions remains challenging. Building from paired Ca and Sr isotope observations, we develop a theoretical framework to quantify the contributions of classical and non-classical crystallization pathways on the precipitation of the calcium carbonate mineral calcite, a common precipitate in nature. We demonstrate that the classical crystallization pathway alone is insufficient to account for the observed isotope behaviors and, thus, the entire calcite precipitation process. We further present a surface kinetic model that incorporates non-classical crystallization pathways. This model enables the characterization of the roles of classical and non-classical crystallization mechanisms in calcite precipitation. The results suggest that the relative contribution of non-classical crystallization pathways increases with saturation state and can, under high supersaturation levels, be comparable to or greater than the classical pathway. The presented theoretical framework readily explains observed trace element partitioning and isotope fractionation behaviors during calcite precipitation and can be further expanded onto other mineral systems to gain insights into crystal growth mechanisms.

Pentavalent U Reactivity Impacts U Isotopic Fractionation during Reduction by Magnetite.

Meaningful interpretation of U isotope measurements relies on unraveling the impact of reduction mechanisms on the isotopic fractionation. Here, the isotope fractionation of hexavalent U [U(VI)] was investigated during its reductive mineralization by magnetite to intermediate pentavalent U [U(V)] and ultimately tetravalent U [U(IV)]. As the reaction proceeded, the remaining aqueous phase U [containing U(VI) and U(V)] systematically carried light isotopes, whereas in the bicarbonate-extracted solution [containing U(VI) and U(V)], the δ238U values varied, especially when C/C0 approached 0. This variation was interpreted as reflecting the variable relative contribution of unreduced U(VI) (δ238U < 0‰) and bicarbonate-extractable U(V) (δ238U > 0‰). The solid remaining after bicarbonate extraction included unextractable U(V) and U(IV), for which the δ238U values consistently followed the same trend that started at 0.3-0.5‰ and decreased to ∼0‰. The impact of PIPES buffer on isotopic fractionation was attributed to the variable abundance of U(V) in the aqueous phase. A few extremely heavy bicarbonate-extracted δ238U values were due to mass-dependent fractionation resulting from several hypothesized mechanisms. The results suggest the preferential accumulation of the heavy isotope in the reduced species and the significant influence of U(V) on the overall isotopic fractionation, providing insight into the U isotope fractionation behavior during its abiotic reduction process.

High-temperature inter-mineral potassium isotope fractionation in ultrapotassic and granitic rocks: Implications for the potassium isotopic compositions of arc magmas

To constrain the mechanism and magnitude of inter-mineral K isotope fractionation during high-temperature magmatic processes, we conducted high-precision K isotope measurements on ultrapotassic and granitic rocks and mineral separates from the Qinling Orogenic Belt, China, as well as first-principles investigations of the corresponding K-bearing minerals. Our results show that the ultrapotassic rocks (−0.33 to −0.14‰) have considerably higher δ41K than the primitive mantle (−0.42 ± 0.08‰), confirming the previous conclusion that slab-derived fluids have contributed heavy K isotopic signatures to their mantle sources. Specifically, the K isotopic fractionation between biotite and microcline (Δ41Kbiotite-microcline = −0.11 to 0.05‰, with biotite and microcline ranging from −0.49 to −0.14‰ and − 0.48 to −0.12‰, respectively) agrees well with the calculated equilibrium isotopic fractionation scale. This evidence, together with the homogeneous mineral chemical compositions, suggests equilibrium K isotopic fractionation between biotite and microcline. Notably, amphiboles (δ41K ranging from −0.41 to 0.48‰) have heterogeneous K isotopic compositions and are generally heavier than coexisting microcline and biotite. In addition, the determined K isotopic fractionation between amphibole/plagioclase and microcline is inconsistent with the calculated results and is likely caused by the kinetic fractionation processes. To reveal the potential controls on the K isotopic compositions of arc magma, we utilized theoretical and measured K isotopic fractionation factors to model K isotopic fractionation behavior during amphibole crystallization. The results show that when Δ41Kamphibole-melt = 0.05‰, the fractional crystallization of amphibole shows negligible effect on the K isotopic composition of the residual melts. Even in the case of Δ41Kamphibole-melt = 0.24‰, fractional crystallization of amphibole exerts an insignificant control on the K isotopic composition of arc magma. Therefore, although amphibole may have heavier K isotopic compositions than the melts, its fractional crystallization may not distinctly decrease the K isotopic composition of residual melts.

Isotopic fractionation of chlorine and potassium during chloride sublimation under lunar conditions

The Moon is depleted in volatile elements and compounds, and lunar samples exhibit a wide range of Cl isotopic compositions, which is believed to result from the volatilization of metal chlorides (e.g., NaCl, KCl, and FeCl2). However, the Cl isotopic fractionation behavior during volatilization is not well constrained, particularly for metal chlorides. Furthermore, the effect of metal chloride evaporation on metal isotopes is poorly known. In the present study, we performed NaCl and KCl sublimation experiments to study Cl and K isotopic fractionations at temperatures ranging from 923 K to 1061 K and at pressures of 7×10–5 bar to 1 bar in an N2 atmosphere. The isotope fractionation factors of 37/35Cl(αgas–solid) from NaCl sublimation experiments are 0.9985±0.0002, 0.9958±0.0004, and 0.99807±0.00004 at 1, 10–2, and 7×10–5 bar, respectively. Those of 41/39K(αgas–solid) and 37/35Cl(αgas–solid) from KCl sublimation experiments are 0.99884±0.00004 and 0.9988±0.0003 at 1 bar, 0.9977±0.0002 and 0.9972±0.0003 at 10–2 bar, and 0.9989±0.0002 and 0.9989±0.0001 at 7×10–5 bar, respectively. Chlorine and K isotopes fractionate more at 10–2 bar than at 7×10–5 bar and 1 bar. The saturation index in all the sublimation experiments was >95%, which resulted in near-equilibrium isotopic fractionation at the sublimation interface. Therefore, the isotopic fractionation was controlled by mass transfer processes in the gas and solid phases. The isotopic fractionation at 10–2 bar was controlled by the chemical diffusion of sublimated gas in an N2 atmosphere with almost no convection effect, (i.e., Pe number close to zero), whereas the isotopic fractionation at 1 bar was suppressed by atmospheric convection with a turbulence factor of 0.4±0.1 (i.e., Pe number >1). The extremely high sublimation rate and the very slow diffusion in the sublimating solid at 7×10–5 bar suppressed isotopic fractionations. Based on our experimental results, calculations using Cl/K and Na/K in lunar materials reveal that degassing of KCl contributed very little (<0.2‰) to the K isotopic fractionation (>0.58‰) during lunar magma ocean degassing. The Cl isotopic fractionation factor from lunar samples is similar to our results at 10–2 bar. This similarity of Cl isotope fractionation indicates that there may have been a transient atmosphere above the lunar magma ocean.

In situ/operando XAFS investigation of the sorption/precipitation of Zn(II) on palygorskite surface at the molecular scale: Implications for Zn stable isotope fractionation

In aqueous geological environments, the fate and transport of Zn are controlled by geochemical reactions (e.g., sorption) occurring at the mineral/water interface. However, many of the underlying Zn fixation mechanisms, especially the kinetics of surface-induced precipitation, are still unclear. Additionally, Zn stable isotope fractionation during sorption has recently been shown to be an important process, but the relationship between Zn isotope fractionation and sorption mechanisms (e.g., surface complexation, precipitation) is also not well understood. To address these issues, we employed X-ray absorption fine structure (XAFS) spectroscopy, high-resolution transmission electron microscopy (HRTEM), and in situ/operando quick-scanning XAS (QXAS) spectroscopy to elucidate the sorption/precipitation mechanisms of Zn at palygorskite/solution interfaces. We also measured Zn isotope ratios during some sorption experiments to illustrate how Zn isotope fractionation behavior is affected by the evolution of coordination environments at different pH values, initial concentrations, and reaction times. Our results demonstrate that Zn sorption mechanisms vary as a function of pH, ionic strength, initial concentration, and reaction time. At low pH (pH < 7.0) and low ionic strength (I ≤ 0.01 M), Zn(II) predominantly forms an outer-sphere surface complex in octahedral coordination. At low pH (pH < 7.0) and high ionic strength (I = 0.1 M), Zn(II) is predominantly sorbed as an inner-sphere octahedral surface complex with insignificant isotopic fractionation (Δ66Znsorbed-aqueous = −0.02 ± 0.05‰). At high pH and high initial Zn concentration (pH 7.5, C0 ≥ 0.2 mM), the formation of octahedral Zn phyllosilicate precipitates is observed, yielding a small fractionation with an average Δ66Znsorbed-aqueous of 0.10 ± 0.07‰. In addition, in situ/operando QXAS of Zn sorption at pH 7.5 in a flow cell revealed that the predominant sorbed Zn species shifted from inner-sphere tetrahedral complexes during the initial stage (e.g., within minutes) to inner-sphere octahedral complexes during later stages (e.g., within hours), followed by the formation of Zn-phyllosilicate precipitates (e.g., within days). This molecular evidence coincides with the evolution of Zn isotope fractionation from a large Δ66Znsorbed-aqueous value (0.53 ± 0.05‰) to a small value of 0.05 ± 0.04‰ during the sorption process. The combination of QXAS and isotope fractionation results reveals a change in the Zn local environment from tetrahedral coordination to octahedral coordination. The findings in this study not only provide new insight into surface adsorption/precipitation mechanisms but also demonstrate that stable isotope fractionation is linked with the local molecular/bonding environments at mineral–water interfaces.

Performance of second generation ICP-TOFMS for (multi-)isotope ratio analysis: a case study on B, Sr and Pb and their isotope fractionation behavior during the measurements

The second generation ICP-TOFMS is subject to IIF that does not follow the known mass dependent fractionation laws and is possibly caused by non-mass dependent fractionation and/or multiple fractionation processes with varying contributions.

Magnesium isotope fractionation between calcite and aqueous solutions under elevated temperatures of 98–170 °C

The incomplete understanding of Mg isotope fractionation behavior between calcite and aqueous solutions has limited the interpretation and application of Mg isotope data in natural carbonates. In this study, we performed a series of aragonite-to-calcite conversion experiments under hydrothermal conditions to investigate the effects of temperature and organic ligands on elemental and isotopic partitioning of Mg between calcite and aqueous solutions. The experiments produced magnesian calcite via a dissolution-and-precipitation process. The Mg distribution coefficients between solid and aqueous solution (DMg, 0.08 to 0.21) showed negative correlations with Sr distribution coefficients (DSr, 0.04 to 0.48), as well as the proportion of calcite in the bulk solid, which indicates mixing of calcite (Mg-rich and Sr-poor) and aragonite (Sr-rich and Mg-poor) in the solid phases. The apparent Mg isotope fractionation factors between solid and aqueous solutions (Δ26Mgsolid-soln) show a positive correlation with reaction temperatures, where Δ26Mgsolid-soln increases from ca. −1.84 ‰ at 98 °C to −1.35 ‰ at 170 °C. In addition, the presence of oxalate contributed to slightly lower Δ26Mgsolid-soln values, which is interpreted to be related to the isotopic effect of Mg-oxalate speciation. Based on the experimental results, we propose a temperature-dependent function for Mg isotope fractionation between calcite and free Mg-aquo ions (i.e., Mg(H2O)62+): Δ26Mgcal-Mg(aq) = (−0.17 ± 0.01) × 106/T2- (0.52 ± 0.08). This equation is applicable to a wide range of temperatures, especially at hydrothermal conditions, thus enabling interpretations of Mg isotope data from calcite of hydrothermal and burial origin.

Lithium and strontium isotopic systematics in the Nalenggele River catchment of Qaidam basin, China: Quantifying contributions to lithium brines and deciphering lithium behavior in hydrological processes

The salt lakes fed by the Nalenggele River in the Qaidam Basin, located on the northeastern Qinghai Tibetan Plateau (QTP), are major brine lithium reservoirs in China. The quantitative identifications of the Li sources and their dynamic behavior shed light on the formation mechanism of the brine Li deposits in closed-basin of arid regions. This study presents Li and Sr isotope ratios (δ7Li, 87Sr/86Sr) and other hydrogeochemical parameters for river waters, springs, spring river waters, shallow groundwaters, surface brines, and intercrystalline brines around the Nalenggele River Basin (NRB) and its adjacent small watersheds. The results indicate that the Nalenggele River water is a mixing product of hot springs and regional surface meltwater, and the source of dissolved Li in the NRB is mainly supplied by the hot springs. With multi-tracer models, it is confirmed that around 84.9 %−93.4 % of dissolved Li is provided by the Li-rich hot spring waters, which account for only 3–5 % of the total water input. Furthermore, the dissolved Li in the streams of different hydrological regions shows various isotopic fractionation behavior. In general, the lower average δ7Li values (average of +5.6 ‰) with more enriched Li with respect to Na in the waters of the piedmont Gobi region indicates a significant additional input of dissolved Li from bedrock and sediments. The second-stage alluvial fan of the NRB produces 7Li enrichments and Li-depleted waters (compared with Na), which can be explained by the adsorption of clay or Fe (Mn) oxide phases that fractionate Li isotopes. The highest average δ7Li values (+12.76 ‰) are observed when Li is not adsorbed by the scarce fine clastic sediments in the hypersaline environment of the salt lake region. It can be concluded that the continuous halite precipitation during the brine evolution not only causes the abnormal enrichment of Li in the brine (compared with Na) but also continuously sequesters the 6Li from the brines, resulting in a significant 7Li enrichment. Overall, these findings advance the understanding of the lithium sources of Li-rich brines and its enrichment and metallogenic process during water migration in hyper-arid regions and have crucial indicative significance for lithium cycling under a strong evaporation environment.

Stable isotope geochemistry of silicon in granitoid zircon

Zircon is often used to study granites (sensu lato) and continental crust because it is very resistant and can be analyzed for various isotope systems that provide time and source information about their parental melts. Granites with different petrological histories have distinct bulk-rock silicon isotope compositions but it is unclear if these differences are also detectable in zircon because of superimposed fractionation effects (e.g., related to temperature, silica content, magmatic processes). The present study explores the Si isotope signatures of zircon from various granite types to constrain their isotope fractionation behavior and uses them as igneous petrogenetic tools, and possibly as granite source discriminators when zircon is found in detrital sediments. Our results show that although Si isotope compositions in zircon can be modified by secondary (post-crystallization) processes such as alteration/weathering and metamorphism, they are primarily controlled by zircon-melt isotope fractionation, which depends on both zircon crystallization temperature and magma silica content. Once these fractionation effects are understood and filtered out, a pattern emerges between Si isotope signatures of zircons from different granite types that is consistent with theoretical and experimental results as well as with known Si isotope differences at the bulk-rock scale. Silicon isotope ratios in zircon can track magma evolution (e.g., temperature and SiO2 changes) and, hence, reveal complex processes that involved magma mingling, fractional crystallization, and/or multiple sources. This study, therefore, illustrates that Si isotopes in zircon can be used to investigate magma evolution and represents a useful complement to existing techniques in granite studies involving zircon (e.g., U-Th-Pb, Lu-Hf and O isotopes) provided that it is not used as a stand-alone technique.

Extreme iron isotope variation of pyrite in the Muping gold deposit, Jiaodong: Implication for tracing metal origin

Pyrite has complicated iron isotope fractionation behavior in the hydrothermal condition. Here we present a detailed in-situ iron isotope analyses of different generation pyrite from the Muping gold deposit, eastern China. δ56Fe values of pyrite investigated range from −4.25‰ to +1.78‰, unlike previous bulk analytic data (−0.78‰ to +0.79‰). Iron isotopic homogeneity of single pyrite grain, which can be verified by in-situ analysis, was recommended as the preferred and reliable criterion to judge the isotopic fractionation condition in the hydrothermal gold deposit. Inter-grain variations in Fe isotopic composition suggest occurrence of both kinetic and equilibrium fractionation for pyrite at Muping. Electron backscattered diffraction analysis shows that the replacement likely occurred via dissolution-reprecipitation pathway. The replacement process resulted in obvious kinetic isotope effect, in which pyrites are possessed in extreme negative Fe isotopic composition (~−4‰). Maximum δ56Fe of pyrite up to +1.78‰ implied that kinetic isotopic effect also play a significant role in increasing the Fe isotopic composition of ore-forming fluids in the Muping gold deposit. Further model calculations suggested the kinetic isotopic effect was dominated by pyrite fast precipitation rather than pyrrhotite precipitation or pyrite precipitation via “FeS” pathway. This kinetic isotope effect ultimately will be partly or completely erased due to fast isotope exchange rate at medium–high temperature condition. Pyrite with homogeneous Fe isotopic composition can be used to trace iron source of hydrothermal gold deposit but be cautious.

Cadmium Isotope Fractionation during Adsorption and Substitution with Iron (Oxyhydr)oxides.

Cadmium (Cd) isotopes have great potential for understanding Cd geochemical cycling in soil and aquatic systems. Iron (oxyhydr)oxides can sequester Cd via adsorption and isomorphous substitution, but how these interactions affect Cd isotope fractionation remains unknown. Here, we show that adsorption preferentially enriches lighter Cd isotopes on iron (oxyhydr)oxide surfaces through equilibrium fractionation, with a similar fractionation magnitude (Δ114/110Cdsolid-solution) for goethite (Goe) (-0.51 ± 0.04‰), hematite (Hem) (-0.54 ± 0.10‰), and ferrihydrite (Fh) (-0.55 ± 0.03‰). Neither the initial Cd2+ concentration or ionic strength nor the pH influence the fractionation magnitude. The enrichment of the light isotope is attributed to the adsorption of highly distorted [CdO6] on solids, as indicated by Cd K-edge extended X-ray absorption fine-structure analysis. In contrast, Cd incorporation into Goe by substitution for lattice Fe at a Cd/Fe molar ratio of 0.05 preferentially sequesters heavy Cd isotopes, with a Δ114/110Cdsolid-solution of 0.22 ± 0.01‰. The fractionation probably occurs during the transformation of Fh into Goe via dissolution and reprecipitation. These results improve the understanding of the Cd isotope fractionation behavior being affected by iron (oxyhydr)oxides in Earth's critical zone and demonstrate that interactions with minerals can obscure anthropogenic and natural Cd isotope characteristics, which should be carefully considered when applying Cd isotopes as environmental tracers.

Differentiation of Hydrogen and Oxygen Isotopes in the Water Source Treatment Wetlands of Stream Networks

To explore the isotopic distribution and differentiation of water along the hydraulic flow gradients and plant-bed/ditch systems in constructed root-channel wetlands, surface and subsurface water samples were collected from four ecological wetlands, namely Shijiuyang and Guanjinggang in Jiaxing, as well as Changshuitang and Taishangang in Haining. All samples were collected along water flow pathways during the wet and rainy summer season in August 2019, except for those from Taishangang, which were collected within the plant-bed/ditch system during the dry and cold winter season in January 2020. The abundance of deuterium (δD) and δ18 O was determined in each functional area of the wetlands to assess the influence of wetlands on water differentiation. Stable isotope technology and mathematical statistics were used to analyze the distribution of δD and δ18 O in constructed root-channel wetlands and to reveal the influence of plant-bed/ditch systems on stable isotopes of water. A variety of data mining methods were used to examine the differentiation of stable isotopes of water, at various dimensions and scales, including nonparametric Kendall's tau-b correlation, stepwise regression, gray relational analysis, and machine learning (random forest) combined with scatter diagrams and model hypothesis diagnosis analysis. The main results were as follows:① The spatiotemporal variations in water isotopes of stream networks were largely affected by different water supply and evaporation enrichment effects. The slope and intercept of the wetland water line in Jiaxing were both significantly lower than the regional precipitation line of the adjacent Changshu Station (CHNIP). This showed that the wetlands area had undergone hydrogen and oxygen isotope enrichment. The δD values in Shijiuyang wetland water ranged from -52.2‰ to -49.4‰, and δ18 O values ranged from -7.6‰ to -6.9‰. In Guanjinggang wetland water samples, δD ranged from -48.1‰ to -45.1‰, and δ18 O ranged from -6.8‰ to -5.8‰. The δD values in Changshuitang wetland water ranged from -49.8‰ to -48.4‰, and δ18 O ranged from -7.2‰ to -6.6‰. The δD values in Taishangang wetland water ranged from -55.3‰ to -51.6‰, and δ18 O ranged from -7.8‰ to -7.2‰. ② Hydrogen and oxygen isotope abundance and composition of water showed complex nonlinear changes in the vertical and horizontal dimensions at different scales. At the regional scale, water level elevation in the vertical dimension had a greater impact on water isotope distribution than the length of the hydraulic flow pathway in the horizontal dimension. Water isotopes tended to be enriched in low-lying areas with low water levels. At the local scale, the influence of hydraulic process often played a greater role in determining water isotope distributions. The spatial variations of water isotopes were comprehensively determined by the evaporation of regional water and meandering hydraulic processes inside the wetland. ③ Compared with other wetland functional areas, the central constructed root-channel area (middle treatment zone) was more enriched in water isotopes. ④ The underground macropore network formed by plants with developed rhizomes or roots (e.g., Phragmites communis Trin. and Typha orientalis Presl), mineral-rich substrate soil, and aquatic plants in the plant bed had a significant influence on the abundance of hydrogen and oxygen isotopes in the plant-bed/ditch system. Therefore, when water passed through the plant-bed/ditch system, the values of δD and δ18 O in the lower ditch (outlet) were lower than those in the higher ditch (inlet). ⑤ The abrupt change in isotopic contents of the plant-bed/ditch system might indicate an inflection point in water quality purification. ⑥ The deuterium excess (d-excess) in subsurface water of the plant-bed/ditch system was significantly higher than that in ditch water, and the coefficient of variation in subsurface water was considerably greater than that in ditch water. The d-excess in the wetland root-channel ecological purification zone showed significant temporal differences and was negative in the summer and positive in the winter, which reflected the seasonal variation in water vapor sources and the spatial variation in isotope fractionation behavior in wetlands. These results provide some understanding of the distribution of water isotopes in constructed wetlands, which will strengthen their operation and management. This study also provides some ideas regarding new technologies for water quality improvement and shows that water isotope technology may be a reliable method for analyzing wetland hydrology.

Microbially induced chromium isotope fractionation and trace elements behavior in lower Cambrian microbialites from the Jaíba Member, Bambuí Basin, Brazil.

In east-central Brazil, the Ediacaran-Cambrian Bambuí Basin has the potential to provide a record of unique geochemical responses of Earth's ocean and atmosphere evolution during this key time interval. From this perspective, we studied an interval of the upper Bambuí Basin using sedimentologic, stratigraphic, and chemostratigraphic tools. The lower Cambrian Jaíba Member of the uppermost Serra da Saudade Formation is an interval of up to 60m-thick of carbonate rocks disposed into two shallowing upward trends. Inner to outer ramp and high-energy shoal deposits are described, in which laminated microbialites are the prevailing sedimentary facies. REE+Y data suggest contamination by iron (oxy)hydroxides that are dissociated from the riverine detritic flux. Sedimentary iron enrichment may be related to the settling of iron nanoparticles in coastal environments, diagenetic iron mobilization, or both. MREE enrichment is caused by microbial degradation of organic matter in the iron reduction zone during the anoxic early-diagenetic stage. Chromium isotopes yielded negatively fractionated values (δ53 Cr=-0.69 to -0.27‰), probably resulting from biotic and abiotic reduction of dissolved Cr(VI) to light and less toxic Cr(III) within pores of microbial mats. The δ53 Cr data of the Jaíba microbialite are thus a product of metabolic reactions in microbial mats and do not reflect seawater signal. The isotopic offset from seawater is feasible from molecular diffusion of Cr into pore water and reduction reactions occurring deep inside the mat, although the exact mechanism and consequences are not yet fully understood due to the poor preservation of metabolic reactions in the geological record. Our study suggests that Cr isotopes can be used to reconstruct Cr and other metals cycling within ancient microbial mats, and that caution should be taken when using past microbialites to infer seawater Cr records and redox state of the atmosphere and ocean.

STABLE MERCURY ISOTOPE FRACTIONATION BEHAVIOURS OF PLANTS

The overarching aim of this study is to define mercury (Hg) isotopic features of plants which have different photosynthetic pathways (C3, C4 and CAM) and to understand if different parts of the plants have different Hg isotopic fractionation behavior. For this, carbon isotopic values of terrestrial plants were analyzed which were used to determine the photosynthetic pathways of plants. Plants were sub-sampled into leaves, stems and roots and their Hg isotopic values were analyzed. &#x0D; Results showed that C3 and C4 plants exhibit mass dependent (even Hg isotopes) and mass independent Hg isotope fractionation (odd Hg isotopes). Both C3 and C4 plants are enriched in light isotopes, but the degree of mass fractionation is approximately three times greater in C3 plants, than in C4 plants. Hg in both C3 and C4 plants exhibit negative MIF isotope effect which reported as depletion “and no clear MIF effect. These findings suggest a connection between the Hg isotopic composition and the photosynthetic pathway.&#x0D; In addition, the leaves are slightly more fractionated than the roots. Differences in the degree of MIF between roots and leaves suggest that they obtain Hg from different sources.

Stable chromium isotope fractionation during magmatic differentiation: Insights from Hawaiian basalts and implications for planetary redox conditions

The stable isotope compositions of chromium (Cr) are fractionated during magmatic differentiation of lunar mare basalts, which might be attributed to redox-related mineral crystallization. It has yet to be demonstrated whether magmatic differentiation fractionates Cr isotope composition of terrestrial samples. Here, we present high-precision stable Cr isotope measurements, reported as δ53Cr relative to NIST SRM 979, of well-characterized Hawaiian tholeiitic basalts from Koolau, Mauna Kea and Kilauea. The studied Makapuu-stage Koolau lavas have MgO ranging from 6.58 to 21.54 wt.%, and they have homogeneous δ53Cr ranging from −0.21‰ to −0.17‰. Similarly, studied Mauna Kea lavas have MgO ranging from 9.11 to 17.90 wt.%, and they also have homogeneous δ53Cr ranging from −0.17‰ to −0.13‰. Some Makapuu-stage Koolau and Mauna Kea lavas experienced subaerial or submarine alteration. The homogenous δ53Cr within each sample suites implies that the post-magmatic alterations have not significantly changed the Cr isotope compositions of these samples. Conversely, nine Kilauea Iki basalts have MgO ranging from 7.77 to 26.87 wt.% reflecting varying degrees of magmatic differentiation, and they show resolvable Cr isotope variations with δ53Cr ranging from −0.18‰ to 0.00‰. The δ53Cr values of the Kilauea Iki samples are positively correlated with indicators of magmatic differentiation such as Cr and MgO contents, and Mg# values. The most evolved samples have the lightest isotope compositions, whereas the olivine-spinel cumulates display complementary heavy isotope compositions. This fractionation is most likely generated by the crystallization and accumulation of spinel, which is dominated by Cr3+ and, hence, enriched in heavier Cr isotopes relative to the residual melt. At a given MgO content, Kilauea and Mauna Kea lavas, both Kea-trend volcanoes, have higher δ53Cr than Makapuu-stage Koolau lavas, a Loa-trend volcano. This difference might reflect recycling of altered oceanic crusts or redox differences of their magmatic sources, with the mantle source of Makapuu-stage lavas being more reducing.To understand the different Cr isotope fractionation behaviors of terrestrial and extraterrestrial basalts, we present a quantitative model that relates the Cr isotope compositions of basalts from the Earth, the Moon and Vesta, to the crystallization assemblage, the degree of fractional crystallization, and the Cr2+/ΣCr ratios of minerals and melts, which are related to the oxygen fugacity during differentiation. The primitive Hawaiian basaltic magma for Kilauea Iki and Mauna Kea lavas is estimated to have δ53Cr of −0.15‰, which is close to the average value of the BSE (−0.14‰ to −0.12‰). We further speculate that the initial lunar mantle is relatively homogeneous with BSE-like isotope composition (−0.16‰ to −0.09‰). The observed low δ53Cr in lunar mafic rocks is the result of redox-dominated fractional crystallization and accumulation processes of lunar mafic magmas. These magmas might be derived from variable degrees of partial melting of the primitive lunar mantle.Combined with previous results on the variations in Cr valences and contents in silicate melts and minerals related to oxygen fugacity, Cr concentration and isotope composition can serve as a useful oxybarometer for understanding the redox conditions of planetary differentiation and magmatic evolution.

Observation of varied characteristics of chlorine isotope effects of organochlorines in dechlorination reactions on different types of electron ionization mass spectrometers

Dechlorination reactions of organochlorines can commonly take place in fragmentation on electron ionization mass spectrometry (EI-MS) and collision-induced dissociation (CID) on electron ionization tandem mass spectrometry (EI-MS/MS), possibly accompanied with chlorine isotope effects. However, it remains unclear whether characteristics of chlorine isotope effects of individual organochlorines occurring on different types of mass spectrometers are similar or different. Revelation of characteristics of chlorine isotope effects on different mass spectrometers may help to elucidate mechanisms of dechlorination reactions and fragmental pathways of organochlorines. This study investigated the characteristics of chlorine isotope effects of two model organochlorines, i.e., tetrachloroethylene and trichloroethylene, during fragmentation/CID on EI-double focusing magnetic-sector high resolution MS (EI-HRMS), EI-quadrupole MS (EI-qMS) and EI-triple-quadrupole MS/MS (EI-MS/MS) that were all coupled with gas chromatography (GC). The patterns of chlorine isotope ratios measured by different instruments were different, showing different characteristics of chlorine isotope effects. On EI-HRMS, both normal and inverse chlorine isotope effects were observed; while on EI-qMS, only normal isotope effects were found, and on EI-MS/MS merely inverse isotope effects were discovered. On EI-HRMS and EI-qMS, inter-ion and intra-ion chlorine kinetic isotope effects (Cl-KIEs) might participate in isotope fractionation processes and contribute to inverse inter-ion and normal intra-ion isotope effects, respectively, and the dominant isotope effects determined the direction of observed (apparent) isotope effects. On EI-MS/MS, both inter-ion and intra-ion Cl-KIEs were inverse at high collision energies (including 30 and 45 eV) and contributed to normal inter-ion and inverse intra-ion isotope effects, respectively, and the inverse intra-ion isotope effects were the dominant, leading to the inverse apparent isotope effects in all dechlorination reactions. Particularly, the chlorine isotope ratios measured by EI-qMS exponentially decreased with the increasing Cl atoms of ions, implying that the intra-ion isotope effects rose as broken C–Cl bonds increased. Moreover, fundamentals of chlorine isotope effects in the stepwise multibond dechlorination reactions were revealed with the aid of density functional theory calculations. The different measurement timescales and geometries of the mass spectrometers and the difference in EI-induced fragmentation and CID might result in the different characteristics of chlorine isotope effects. This study may be conducive to elucidating isotope fractionation behaviors and fragmental pathways of organochlorines on EI-MS(/MS), and provides new insights into the mechanisms of dechlorination reactions losing multiple chlorine atoms.

Iron isotopic fractionation in mineral phases from Earth's lower mantle: Did terrestrial magma ocean crystallization fractionate iron isotopes?

Iron is the most abundant transition metal in the Earth's interior, yet considerable uncertainties remain as to why mantle-derived rocks have diverse iron isotopic compositions. In particular, the isotopic fractionation behavior of iron in the lower-mantle minerals bridgmanite and ferropericlase are largely unexplored. The reason is that it is challenging to study isotopic fractionation at the high pressures relevant to the deep mantle. Here we report in situ measurements of the mean force constants of iron bonds in these minerals pressurized in diamond anvil cells using the technique of nuclear resonant inelastic X-ray scattering (NRIXS). We find that the transition from high- to low-spin iron in ferropericlase ((Mg0.75Fe0.25)O) at approximately 60 GPa drastically stiffens its iron bonds in the low-spin state. The mean force constant of iron bonds in both Fe-bearing and (Fe,Al)-bearing bridgmanite exhibits softening by 21% at approximately 40–60 GPa, which seems to be associated with changes in the iron local environment during the transition from low to high quadrupole splitting states. These results indicate that in the lower mantle, low-spin ferropericlase is enriched in heavy iron isotopes relative to bridgmanite and metallic iron by +0.15‰ and +0.12‰, respectively. Based on these results, we investigate whether terrestrial magma ocean crystallization could have fractionated iron isotopes. We conclude that this process cannot be responsible for the heavy iron isotope enrichment measured in terrestrial basalts.

Two-stage chromium isotope fractionation during microbial Cr(VI) reduction

Chromium isotope fractionation analysis is a promising approach for the assessment of microbial Cr(VI) reduction in groundwater. Understanding the mechanisms and other parameters that control Cr isotope fractionation factors (between the product Cr(III) and reactant Cr (VI)) in microbial Cr(VI) reduction is critical to this application. To date, such studies are very limited. Here, the influence of critical factors on observed Cr isotope fractionation during Cr(VI) reduction by Shewanella oneidensis MR-1 under various conditions was investigated. The Cr(VI) concentration and Cr isotope ratio measurements were conducted on unreacted Cr(VI) remaining in solution to determine Cr isotope fractionation factors. The changes in ambient environmental conditions (e.g., pH, temperature) have limited influence on Cr isotope fractionation factors. However, as a result of Cr(VI) consumption as the experiments proceed, the change in bioavailability of Cr(VI) has a significant impact on Cr isotope fractionation factors. For example, in temperature-controlled experiments, Cr isotope fractionation showed two-stage behavior: during Stage I, the values of ε were −2.81 ± 0.19‰ and −2.60 ± 0.14‰ at 18 °C and 34 °C, respectively; during Stage II, as Cr(VI) reduction progressed, Cr isotope fractionation was significantly masked, and the ε values decreased to −0.98 ± 0.49‰ and −1.01 ± 0.11‰ at 18 °C and 34 °C, respectively. Similar two-stage isotope fractionation behaviors were observed in pH-controlled experiments (pH = 6.0 and 7.2) and in experiments with and without the addition of a competing electron acceptor (nitrate). Masking of isotope fractionation in Stage II indicated restrictions on the bioavailability of Cr(VI) and mass-transfer limitations. This study provides an explanation for the variation in Cr isotope fractionation factors during microbial Cr(VI) reduction in the environment, furthering the viability of Cr isotope ratio analysis as an approach in understanding Cr biogeochemical cycling.

High-temperature inter-mineral Cr isotope fractionation: A comparison of ionic model predictions and experimental investigations of mantle xenoliths from the North China Craton

Recent works have proposed that chromium (Cr) isotopes could be fractionated during peridotite partial melting and basaltic magma crystallization. However, until now, inter-mineral Cr isotope fractionation behavior for major mantle minerals has been poorly constrained. To investigate the mechanism and magnitude of equilibrium inter-mineral Cr isotope fractionation and to explore its implications for planetary mantle and crust lithology evolutions, we performed a systematic study of equilibrium Cr isotope fractionation between major mantle minerals by coupling theoretical ionic modeling with laboratory isotope analyses of natural samples and Cr valence determinations by X-ray absorption near edge spectroscopy (XANES). The ionic model considers both charges and coordination environments of Cr species in mantle minerals, which have proven to be critical factors affecting inter-mineral isotope fractionation. The ionic modeling results predict a general order of spinel (Spl) > pyroxene (Py) ≥ olivine (Ol) in 53Cr/52Cr. Systematic Cr isotope analyses of coexisting mantle minerals of seventeen peridotite xenoliths from Beiyan in the North China craton were performed. Chromium isotope results for different mineral pairs of lherzolites (e.g., Δ53CrSpl–Ol of 0.11‰ to 0.16‰, Δ53CrSpl–Py of 0.04‰ to 0.11‰ and Δ53CrPy–Ol of 0.05‰ to 0.10‰ at 870 °C to 970 °C), document measurable and systematic inter-mineral Cr isotope fractionation, in excellent agreement with the modeling results predicted with XANES-determined Cr2+/ΣCr values of the mineral separates. This fractionation order could essentially account for Cr isotope behaviors during partial melting and magmatic crystallization observed in terrestrial peridotites and lunar basalts.In contrast, we found that metasomatisms could influence Cr isotope compositions of minerals from Beiyan metasomatized clinopyroxene- (Cpx-) rich lherzolites and wehrlites by means of mineral–melt interaction and/or kinetic diffusion, leading to disequilibrium inter-mineral Cr isotope fractionation.