Kinetic Isotope Fractionation Research Articles

Nitrogen isotopes of marine oils in the Tarim Basin, China: Implications for the origin of organic matters and the paleoenvironment

Nitrogen isotope compositions (δ15N) of sedimentary rocks are usually used to reconstruct the paleoenvironment and nitrogen (N) biogeochemical cycle. The δ15N values of crude oils inherit the characteristics of relevant source rocks and can well reflect the information of hydrocarbon-forming organisms and environment in ancient water column. However, studies on the δ15N of crude oils are limited due to the low N content. In this study, a new efficient method is applied to the marine oils from the Bashituo (BST) and Halahatang (HLHT) areas of the Tarim Basin to obtain the nitrogenous components (i.e., nonhydrocarbons and asphaltenes) for the achievement of N concentration. The carbon and nitrogen isotopes of these components and the biomarkers of oils were measured. The δ15N values in asphaltenes (δ15NAsp) are significantly heavier than those in nonhydrocarbons (δ15NNSOs) in these oils, which are attributed to the potential directional N transfer and kinetic isotope fractionation during the thermal evolution of organic matters (OM). The δ15NAsp values have significant correlations with OM origin associated parameters and weak correlations with environmental parameters, suggesting that the difference in δ15NAsp values is mainly resulted from biological source rather than redox conditions. The δ15NNSOs values have a closer relationship with the redox condition than biological characteristics, indicating that they have a good response to paleoenvironmental variation in the water column, which is not completely overprinted by the difference of OM origin. Different redox conditions give rise to distinct nitrogen cycles, resulting in various δ15N values. Anammox occurs in the water column of the Early Cambrian dominated by physically stratified conditions with significant isotope fractionation, resulting in relatively heavier δ15N of OM in the BST area. In the Middle–Late Ordovician period, the limited suboxic zone leads to an insignificant positive bias of δ15N caused by partial denitrification in the HLHT oils. The evaluation of δ15N in nitrogenous fractions enables a more comprehensive reconstruction of N cycle for ancient oceans.

Insights on the origin of oldhamite in enstatite meteorites from Ca stable isotopes

In order to understand the origin of oldhamite (CaS) in enstatite meteorites, we report Ca isotopic compositions (δ44/40Ca) of oldhamite (obtained from water leachate of bulk chondrites and aubrites and mineral separates from the Norton County aubrite) and silicate minerals from different types of enstatite chondrites and aubrite. The δ44/40Ca of the bulk enstatite chondrites range from 1.05 ‰ to 1.24 ‰, with an average of 1.13 ± 0.12 ‰, higher than that of the estimate of the bulk silicate earth (∼0.94 ‰). Major and trace element analyses show that the water leachates of enstatite chondrites are mainly composed of oldhamite, and they take over 20.6–68.5 % Ca of the bulk meteorite Ca budget. The Ca isotope fractionation between oldhamite and residual silicate (Δ44/40Caoldhamite-silicate) for the studied enstatite chondrites is minimum (−0.44 ‰) for Abee (impact-melt breccia) and maximum (+0.16) for St.Marks (EH5). The Ca isotope fractionation between oldhamite (individual mineral grains and leachate) and silicates in Norton County varies from −0.47 ‰ to −0.31 ‰ with an average of −0.41 ‰. These Δ44/40Caoldhamite-silicate correlate well with previous theoretical calculation and suggests that the oldhamites in Norton County are in isotopic equilibrium with co-existing silicates, and therefore were formed during magmatic processes. However, in enstatite chondrites, the large variation on Δ44/40Caoldhamite-silicate and its negative correlation with metamorphic temperature reflects the redistribution and equilibration of Ca isotopes during metamorphism. The variable Δ44/40Caoldhamite-silicate found in unequilibrated chondrites reflect kinetic Ca isotope fractionation between oldhamite and nebular gas and therefore is evidence for the formation of oldhamite by condensation in the solar nebula.

Isotope fractionation of alkaline and alkaline-earth elements (Li, K, Rb, Mg, Ca, Sr, Ba) during diffusion in aqueous solutions

In this study, we used an improved “diffusion cell” method to precisely determine the diffusion-driven kinetic isotope fractionation factors of the Li, K, Rb, Mg, Ca, Sr, and Ba cations in aqueous solutions under room temperature. The obtained isotope fractionation factors (±2σ errors) are, α7/6Li = 0.996139 ± 0.000140, α41/39K = 0.998572 ± 0.000072, α87/85Rb = 0.999333 ± 0.000020, α26/24Mg = 0.999877 ± 0.000010, α44/42Ca = 0.999704 ± 0.000010, α88/86Sr = 0.999781 ± 0.000014, α138/135Ba = 0.999716 ± 0.000018. The results show that the charge of the cation and the ion-water bond length for the aquo ions are the two predominant factors affecting the mass dependence of isotope fractionation (β factor) during cation diffusion in aqueous solutions. Cations with higher charge numbers and shorter ion-water bond lengths exhibit less kinetic isotope fractionation during diffusion. Therefore, the isotope separation effect during diffusion (or β factor) in fluids is fundamentally controlled by the intensity of ion-water interaction. Weaker ion-water interaction (e.g., lower charge number, longer ion-water bond length) leads to less prominent hydrodynamic behavior for diffusing ions at the molecular level, thus more significant isotope fractionation in bulk solutions, and vice versa. Ions of larger radius would show stronger mass dependence of isotope fractionation (β factor), which can cancel the effect of decreasing relative isotope mass difference for heavier elements, thus kinetic isotope fractionation during diffusion in aqueous solutions remains prominent even for heavy elements such as Rb, Sr, and Ba. The diffusion-driven kinetic isotope fractionation factors measured in this study could provide a useful basis for interpreting specific natural isotopic variability of alkaline and alkaline-earth elements in supergene environments where chemical diffusion takes place.

Titanium-rich basaltic melts on the Moon modulated by reactive flow processes

The origin of titanium-rich basaltic magmatism on the Moon remains enigmatic. Ilmenite-bearing cumulates in the lunar mantle are often credited as the source, but their partial melts are not a compositional match and are too dense to enable eruption. Here we use petrological reaction experiments to show that partial melts of ilmenite-bearing cumulates react with olivine and orthopyroxene in the lunar mantle, shifting the melt composition to that of the high-Ti suite. New high-precision Mg isotope data confirm that high-Ti basalts have variable and isotopically light Mg isotope compositions that are inconsistent with equilibrium partial melting. We employ a diffusion model to demonstrate that kinetic isotope fractionation during reactive flow of partial melts derived from ilmenite-bearing cumulates can explain these anomalously light Mg isotope compositions, as well as the isotope composition of other elements such as Fe, Ca and Ti. Although this model does not fully replicate lunar melt–solid interaction, we suggest that titanium-rich magmas erupted on the surface of the Moon can be derived through partial melting of ilmenite-bearing cumulates, but melts undergo extensive modification of their elemental and isotopic composition through reactive flow in the lunar mantle. Reactive flow may therefore be the critical process that decreases melt density and allows high-Ti melts to erupt on the lunar surface.

Gallium isotope fractionation during precipitation of α-GaOOH from aqueous solution

Gallium (Ga) has two naturally-occurring stable isotopes: 69Ga (60.1%) and 71Ga (39.9%). Their isotopic ratios were employed as a potential proxy to trace the geochemical behavior of Ga and its analogue Al. Ga is also an important component of many semiconductors. When exposed to aqueous solutions, it may be dissolved and then precipitate as Ga-bearing crystals (e.g., GaOOH). Thus, the exploration of Ga isotope fractionation during α-GaOOH precipitation could be helpful for understanding geochemical behaviors of Ga and other metals (including Al and Fe) during oxyhydroxide precipitation. In this study, three series of precipitation experiments with variable initial aqueous Ga concentrations (about 9.2, 2.0, and 1.8 ppm) were carried out to determine Ga isotope fractionation factor during α-GaOOH precipitation at ∼20 °C and neutral pH as a function of time. The experimental results could be modeled using a Rayleigh precipitation process, in which fractionation factors between crystals and solution (Δ71Gasolid-solution) are between −0.28 and − 0.37‰, depending on the initial Ga concentration and type of matrix anion (Cl− vs. NO3−). This isotope fractionation is significantly smaller in magnitude than the values reported for Ga adsorption on goethite (−0.89‰) and calcite (−1.27‰). The isotope fractionation is mainly caused by the increase in GaO bond length from ∼1.84 Å in aqueous GaOH4— to ∼1.991 Å in α-GaOOH solid, with a simultaneous change of Ga coordination number from 4 to 6, although kinetic isotope fractionation may play an important role. Our study provides the first experimental evidence of significant Ga isotope fractionation during kinetic oxyhydroxide precipitation, and suggests that the measurable Ga isotope fractionation occurred at water-solid interface and during precipitations can be potentially used for tracing global geochemical cycling of Ga and its analogue Al and Fe.

Climate monitoring in the Caumont cave and quarry system (northern France) reveal near oxygen isotopic equilibrium conditions for carbonate deposition

The study of modern cave deposits forming under near isotopic equilibrium conditions can potentially help disentangle the processes influencing the oxygen isotope system and suitability of stalagmites as archives of past hydrological or thermal changes. We used cave monitoring to evaluate the impact of kinetic isotope fractionation and assess the conditions under which modern cave carbonates form in the Caumont cave and quarry system, located in Normandy, northwest France. Over 20 months, we collected climatological data, dripwater, and modern carbonate samples at 2–4-week intervals at three different stations inside the Caumont cave and quarry system. We find highly stable (10.4 ± 0.3°C – 11.3 ± 0.1°C) temperature in the deeper sections of the Caumont cave and quarry system. The temporal dynamics of δ18Odrip indicates that the drip water composition in Caumont reflects the original (though subdued) signal of precipitation, rather than the impact the seasonal to interannual cave air temperature has on isotopic fractionation. The monitoring reveals that δ13C of modern carbonate is influenced by prior carbonate precipitation that occurs during the summer season when evapotranspiration can minimize effective infiltration. Comparison of δ18O from dripwater and modern calcite, precipitated on glass plates and collected every two to four weeks, reveals that modern calcite forms near oxygen isotope equilibrium. A Hendy test on modern carbonate deposited on a stalagmite-shaped glass flask over 20 months confirms this finding because neither does δ13C increase with distance from the apex, nor are δ13C and δ18O positively correlated. We conclude that the δ13C signal in speleothems reflect summer (and longer-term) prior carbonate precipitation in response to effective infiltration dynamics, and that the δ18O signal likely reflects annual to multi-annual changes in the composition of precipitation above the cave.

Complexation of Zr and Hf in fluoride-rich hydrothermal aqueous fluids and its significance for high field strength element fractionation

Zirconium and hafnium behave nearly identically in most geological processes due to their identical nominal ionic charge and similar radius. Some of the most pronounced exceptions from this rule are observed in fluoride-rich aqueous systems, suggesting that aqueous fluoride complexation may be involved in Zr/Hf fractionation. To understand the mechanisms causing this phenomenon, we investigated complexation of Zr4+ and Hf4+ in fluoride-rich (1.0 mol/kg HF) aqueous solutions at 40 MPa and 100–400 °C, using synchrotron X-ray absorption spectroscopy (X-ray absorption near edge structure and extended X-ray absorption fine structure) combined with classical and ab initio molecular dynamics simulations. The dominant experimentally observed complexes are [Zr(F,OH)4·2H2O]0 and [Hf(F,OH)4·2H2O]0, respectively. The first coordination shell comprises a distorted octahedron, with fluoride and hydroxide ligands at a similar mean radial distance (1.9–2.0 Å) from the central cation, and H2O ligands at a slightly greater distance (>2.1 Å). With increasing temperature, the H2O ligands move further out, causing first an increasing distortion of the octahedron and subsequently a partial transition to less hydrated complexes as a certain fraction of the H2O molecules move to the second shell at > 3 Å. As a consequence, the radial distance of the F- and OH– anions from the central cation, as well as the overall average radial distance of the first shell decreases due to decreased steric repulsion from the H2O ligands. Both experiments and simulations agree in that Hf forms slightly shorter bonds to its nearest neighbors than Zr. The results suggest two hypotheses for the mechanism of Zr/Hf fractionation during precipitation of minerals from fluoride-rich hydrothermal solutions: 1) The heavy twin (Hf) prefers the lower coordination (shorter bonds) and is thus less likely to enter into the higher coordination found in the solids. This mechanism would be analogous to equilibrium isotope fractionation. 2) The change of Hf into a higher coordination environment (e.g., from solution to solid) is slower because it forms stronger ligand-bonds than Zr. This would be analogous to reactive kinetic isotope fractionation. In either case mass dependent fractionation qualitatively matches the observations but mass independent effects on bond strength may also be significant. Quantitative investigations of these effects are needed and may also shed light on the currently still somewhat enigmatic fractionation behavior of Zr isotopes.

Petrogenesis of the Wushi carbonatites in the northwestern Tarim Basin: Implications to deep carbon recycling

Carbonatites are rarely exposed on the Earth's surface, and most of them are supposed to have been mantle-derived. However, in the most recent years, carbonatites with pronounced crustal signatures had also been recognized. Although carbonatites have attracted the attention of many researchers, their genesis remains controversial. In this study, we present a systematic C–O–Sr–Nd–Mg isotopic investigation on the carbonatites exposed in Wushi County located at the northwestern margin of the Tarim Basin to determine their petrogenesis. The Wushi carbonatites, containing the xenoliths of wall rocks, intruded into Precambrian sandstones as dykes. Carbonatites predominantly comprise calcite and accessory minerals, such as dolomite, apatite, mica and quartz, and thus are classified as calciocarbonatites. Trace element geochemistry indicates that the total rare earth element (REE) concentrations in Wushi carbonatites range from 13.6 to 57.3 ppm, with an average of 30.9 ppm. The chondrite-normalized REE patterns of the test samples indicate that light rare earth elements (LREE) in the samples have been enriched relative to their heavy rare earth elements (HREE) and that the LREE/HREE ratios vary from 6.0 to 9.1. The patterns also show negative Eu anomalies (δEu = 0.65–0.75) and weak negative Ce anomalies (δCe = 0.75–0.99). The primitive mantle-normalized trace element spider diagrams display the enrichment of large ion lithophile elements (LILE), such as Pb, Sr and Ba, and the strong depletion of high field strength elements (HFSE), such as Nb, Ta, Zr, Hf and Ti. The δ18OV-SMOW (15.9‰–18.2‰) and δ13CV-PDB (−2.97‰–3.36‰) values are more consistent with those of crust-derived carbonatites than with mantle-derived carbonatites. The values of δ18OV-SMOW and δ13CV-PDB are consistent with the high values of 87Sr/86Sr (0.7096–0.7110) and low values of 143Nd/144Nd (0.5118–0.5120). Typically, δ26Mg of the carbonatites (−1.76‰ to −1.27‰) is much lighter than the δ26Mg of the mantle (−0.25‰ ± 0.04‰) but heavier than the δ26Mg of sedimentary carbonates (−2.72‰ to −2.15‰) obtained from the neighbouring Western Tianshan orogenic belt. The light Mg isotope anomalies cannot be explained using secondary alteration, continental crustal contamination, kinetic isotope fractionation and fractional crystallization. Thus, these anomalies could be due to the isotope exchange reactions occurring between the oceanic crust and sedimentary carbonates during subduction. In this study, we propose a two-stage model to explain Wushi carbonatite formation based on its isotope characteristics. During the early stage of carbonatite formation, sedimentary carbonate rocks located along the northern margin of the Tarim Craton and oceanic crust were subducted into the deep mantle. Isotope exchange and carbonation occurred during subduction to form carbonated eclogite. In the second stage of carbonatite formation, the heat produced by the Permian mantle plume in the Tarim Basin melted the carbonated eclogite to produce carbonatite melt. Thus, the petrogenesis of the Wushi carbonatites revealed by the presence of multiple isotopes indicates that the carbonatite produced by carbonated eclogite in the deep mantle would exhibit typical crustal isotopic characteristics and provide new insights into deep carbon recycling.

Copper isotope fractionation by diffusion in a basaltic melt

Copper shows limited isotopic variation in equilibrated mantle-derived silicate rocks, but large isotopic fractionation during kinetic processes. For example, lunar and terrestrial samples that have experienced evaporation were found to have an isotopic fractionation of up to 12.5‰ in their 65Cu/63Cu ratios, while komatiites, lherzolites, mid-ocean ridge and ocean island basalts show negligible Cu isotope fractionation as a result of equilibrium partial melting and crystal fractionation. The contrast between the observed magnitudes of equilibrium and kinetic isotope fractionation for Cu calls for a better understanding of kinetic Cu isotope fractionation. One of the mechanisms for creating large kinetic isotopic fractionation even at magmatic temperatures is diffusion. In this study, we performed Cu isotopic measurements on Cu diffusion couple experiments to constrain the beta factor for Cu isotopic fractionation by diffusion. We demonstrate a Monte Carlo approach for the regression and error estimation of the measured isotope profiles, which yielded beta values of 0.16 ± 0.03 and 0.18 ± 0.03 for the two experimental charges measured. Our results are subsequently applied to a quantitative model for the evaporation of a molten sphere to discuss the role of diffusion in affecting the bulk Cu isotopic fractionation between liquid and vapor during evaporation. We apply the model to Cu evaporation experiments and tektite data to show that convection primarily governs mass transport for evaporation during tektite formation. In addition, we show that Cu isotopes can be used as a tool to test the role of kinetics during various magmatic processes such as magmatic sulfide ore deposit formation, porphyry-type ore deposit formation, and fluid-rock interactions.

Linking Significant Zr Isotopic Fractionation in Magmatic Zircons With Petrographic Textures

AbstractThe origin of crystal‐scale Zr isotopic variation remains enigmatic. Here, we report high‐precision Zr isotopic compositions of thin‐sectioned and hand‐picked zircon crystals from monzodiorites and K‐feldspar granites, which represent the mafic and felsic products of a differentiating magma. Our results reveal that zircons from both rock associations are dominated by a rim‐ward increase in the 94Zr/90Zr ratio and accompanied by ∼1‰ intracrystalline and intercrystalline variations, confirming the preferential incorporation of light Zr isotopes and thus kinetic isotopic fractionation during zircon growth. Notably, zircons enclosed by K‐feldspar in monzodiorites generally yield lower 94Zr/90Zr and higher Zr/Hf ratios than that of separated crystals, while in K‐feldspar granites, tiny zircons intergrown with apatite obtain higher 94Zr/90Zr but lower Zr/Hf ratios relative to that of sizable separates. The diversity in petrographic textures and sizes of zircon crystals, corresponding to their crystallization sequence, potentially accounts for some of the heterogeneity of Zr isotopes at the mineral scale.

Experimental and theoretical investigations of stable Sr isotope fractionation during its incorporation in aragonite

Strontium partitioning and isotope fractionation between aragonite and fluid have been determined experimentally at low values of the fluid saturation state (Ω) with respect to this mineral (1.1 ≤ Ωaragonite ≤ 2.2), and the measured isotope fractionation has been compared with the results of first-principles simulations. For the latter, Density Functional Theory (DFT) was used for estimation of the equilibrium Sr isotope fractionation between aragonite and Sr2+(aq). The obtained results suggest that, for values of Ωaragonite close to unity, the apparent distribution coefficient of Sr in aragonite (DSr,aragonite=XSrCO3XCaCO3(CaSr)fluid) exhibits values higher than one that rapidly decrease at increasing aragonite growth rate. Under equilibrium conditions (i.e. Ωaragonite=1) a DSr,aragoinite value of 2.7 can be extrapolated. Additionally, for aragonite growth rates rp ≤ 10−8.0±0.2 (mol/m2/s) the Sr isotope fractionation between aragonite and the fluid (i.e. Δ88/86Sraragonite-fluid) shows a constant value of −0.1±0.05‰, whereas it decreases to −0.40‰ when the growth rate increases to 10−7.7(mol/m2/s). The surface reaction kinetic model (SRKM) developed by DePaolo (2011) has been used to describe the dependence of DSr,aragonite and Δ88/86Sraragonite-fluid on mineral growth rate. In this model the best fit for DSr,aragonite and Δ88/86Sraragonite-fluid were 4 and −0.01‰, respectively, whereas the kinetic isotope fractionation factor for Δ88/86Sraragonite-fluid was −0.6‰. The results of first-principles calculations yield an equilibrium Sr isotope fractionation factor of −0.04‰ which is in excellent agreement with the experimental value of the present study. These results are the first experimental measurements of Sr isotope fractionation during inorganic aragonite precipitation as a function of growth rate and the first DFT calculations of Sr equilibrium fractionation in the aragonite-fluid system. The results of this study provide new insight into the mechanisms controlling stable Sr isotope composition in aragonite, which has implications for using Sr isotopes for paleo-reconstructions of natural archives, particularly those of abiogenic origin.

Impact of wet-dry cycles on enhanced rock weathering of brucite, wollastonite, serpentinite and kimberlite: Implications for carbon verification

Enhanced rock weathering is a proposed CO2 removal strategy for mitigating climate change; its implementation can be facilitated by improving carbon verification methods. In this study, weathering experiments exposed brucite, wollastonite skarn, serpentinite, and kimberlite residues (Venetia Diamond Mine, South Africa), to wetting and drying cycles (4/day) for 1 yr to elucidate reaction pathways and rates while evaluating carbon verification tools, including mineral quantification, total inorganic carbon (TIC), and stable and radiogenic carbon isotopes. Two primary reaction pathways were identified: A) silicate and hydroxide dissolution leading to carbonate precipitation (desirable), and B) dissolution and reprecipitation of carbonates (undesirable). The kimberlite residues, containing serpentine (19–33 wt%) and calcite (4–5 wt%), experienced variable and minor changes in total inorganic carbon. The δ13C and δ18O values of the carbonate minerals approached those expected for atmospheric-derived CO2 and are best explained by the exchange of carbonate CO2 with atmospheric CO2 (pathway A) as opposed to net CO2 sequestration. In contrast, the reaction of brucite (1.22% to 5.98%) and wollastonite skarn (0.22% to 1.01%) with atmospheric CO2 was substantial, as indicated by the increases in TIC (pathway B) yet decreases in δ13C values were inconsistent with the incorporation of atmospheric CO2 as a result of kinetic isotope fractionation due to carbonation being CO2 supply limited. The pulverized serpentinite experience insufficient carbonation to be detected by TIC. Unweathered (blue ground) and naturally weathered (yellow ground) kimberlite from the Voorspoed Diamond Mine, South Africa were compared to kimberlite residues used in experiments. Yellow ground contained twice the TIC content compared to blue ground whereas quantitative scanning electron microscopy revealed calcite veinlets that are characteristic of secondary mineralization. While the blue and yellow ground samples were essentially radiocarbon dead (0.03 and 0.05 F14C, respectively), substantial modern carbon was incorporated into the kimberlite (up to +0.27 F14C; CO2 exchange), brucite (+0.55 F14C), and wollastonite skarn (+0.53 F14C; CO2 sequestration) powders during weathering experiments. Although stable and radiogenic carbon isotopes have been used to verify the mineralization of atmospheric CO2, these methods are affected by CO2 exchange and kinetic fractionation effects during weathering that can be misleading and, thus, should not be relied on as sole methods for carbon verification.

Geological control on carbon isotope equilibrium and kinetic fractionation of CH4-CO2-HCO3− in microbial coalbed and shale gas systems

Microbial coalbed gas (CBG) and shale gas (SG), predominately composed of methane (CH4) and carbon dioxide (CO2), are important economic resources and potent greenhouse gases. Although isotopic equilibrium of CH4 and CO2 has been observed in microbial CBG and SG basins, it is difficult to judge under what geological conditions equilibrium is achieved. Moreover, the effects of CO2 dissolution on the isotopic fractionation process need to be considered. We use data from eight microbial CBG and SG basins to discuss the geological conditions in which equilibrium and kinetic isotopic fractionation in CH4-CO2-HCO3− system is achieved. Based on isotopic equilibrium temperatures calculated using computer codes developed in MatLab software, we show that, in deep and closed reservoirs, the CH4-CO2 and CH4-HCO3− are close to carbon isotope equilibrium. In contrast, in shallow and open reservoirs, they are in disequilibrium. The CO2-HCO3− is in disequilibrium in most reservoirs. We propose that both low free energy gradients and long coexisting time of CH4 and CO2/HCO3− are necessary to attain isotopic equilibrium. However, it is difficult to accurately estimate the timescale for attaining isotope equilibrium among them. In general, a closed and deep CBG/SG reservoir is likely to be geologically and geochemically stable over long timescales, favoring isotopic equilibrium of CH4-CO2 and CH4-HCO3−. However, a shallow and open reservoir is unfavourable for their isotopic equilibrium due to shorter timescales for the coexistence of CH4-CO2-HCO3−. Using data from systems close to equilibrium, we estimated the percentage of CO2 in total CH4 and CO2 in CBG reservoirs in various basins to be from 27% to 50%, where methanogenesis is mainly by CO2 reduction. This is significantly higher than the CO2 content (1% to 15%) in gaseous CH4 and CO2 in these basins but is consistent with those (36% to 48%) from culture experiments for coal conversion by methanogenesis. Further study shows that 53–99% of the CO2 formed during CBG generation has dissolved into groundwaters to form dissolved inorganic carbon (DIC) in CBG reservoirs. We propose that CO2 dissolution likely has significantly affect the abundance and isotopic compositions of gaseous CO2 in subsurface.

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.

A pilot study of stable isotope fractionation in Bombyx mori rearing

Hydrogen, oxygen, carbon, and nitrogen isotopes derived from three different strains of silkworms at different life stages involved in silkworm rearing, were measured to understand the fractionation characteristics of stable isotopes at different stages of silkworm development, and to trace the movement of these isotopes from food to larva to excrement and finally to silk. We found that silkworm strain had little effect on δ2H, δ18O and δ13C values. However, a large difference was found in the δ15N levels of newly-hatched silkworms between Jingsong Haoyue and Hua Kang No. 3 orthogonal strains, suggesting that the mating and egg laying differences may result in an inconsistent kinetic nitrogen isotope fractionation. The δ13C values of silkworm pupae and silkworm cocoon also displayed significant differences, suggesting that heavy carbon isotopes are greatly fractionated from the larva to the silk during cocoon formation. Overall, these results may be used to clarify the relationship between isotope fractionation and the ecological process of the Bombyx mori and expand our ability to resolve stable isotope anomalies at a small regional-scale level.

Triple silicon isotope insights into the formation of Precambrian cherts

Cherts are pervasive in the Precambrian and their O and Si isotopic compositions have been used extensively for reconstructing seawater paleo-temperatures. However, these reconstructions are obscured by the fact that cherts exhibit, at a given age, a large isotopic (O and Si) heterogeneity at bulk and micrometer scales. This is understood as reflecting the fact that cherts are an assemblage of silica precursors having different origins, and that diagenesis and metamorphism have modified the isotopic compositions acquired during the last equilibration with seawater. To test the contributions of the different processes that control the Si isotopic composition of cherts, we developed high-precision triple Si isotope measurements by Multicollector-Inductively Coupled-Plasma Mass-Spectrometer (MC-ICP-MS). This approach allows to identify deviations from the equilibrium Si isotopic fractionation at the level of a few ppm. Triple Si isotope data combined with trace elements and O isotope data in Precambrian cherts demonstrate that three different sources of silica were involved in the formation of cherts, and that both equilibrium and kinetic isotopic fractionation took place for Si. Archean seawater is predicted to have a triple Si isotopic composition kinetically fractionated relative to bulk silicate Earth. This requires a large kinetic isotopic fractionation during hydrothermalism of the oceanic crust and appears consistent with warm oceans in the Precambrian. However more work is required on the triple isotope systematics of Si inputs and output to seawater by rivers and authigenic clays formation. Triple Si isotope variations can thus be used as a new tool to unravel the origin of cherts and of their O and Si isotopic compositions. The present evidence of kinetic isotopic fractionations for Si isotopes also implies that Si isotopes should be associated to triple O isotope for paleotemperature reconstructions.

Kinetic zinc isotope fractionation in olivine phenocrysts records magma evolution history of intra-plate basalts

In the past few years, zinc isotope systematics of basaltic magmas have been widely used as novel proxies for terrestrial mantle heterogeneity induced by recycled crustal materials and for planet formation and evolution. The influence of crystal-melt isotope disequilibrium during magma differentiation on the zinc isotopic composition of basaltic melts, nonetheless, has received little attention. In addition, no quantitative constraint has yet been given for the diffusion-driven Zn isotope fractionation between olivine crystals and melts. Here we present high-precision zinc isotope data (δ66ZnJMC-Lyon) for a series of olivine phenocrysts separated from intra-plate alkali basalts from the Jiaodong Peninsula in Eastern China, together with in-situ chemical analysis. Olivine phenocrysts have Zn isotopic compositions which are too light in comparison with the host basaltic melts (Δ66Znol-melt = −0.41‰ to −0.10‰; n = 16) to be explained by equilibrium isotope fractionation at magmatic temperatures. Instead, the decrease of δ66Zn values with decreasing Mg# (i.e., Mg/[Mg + Fe2+] × 100) and increasing Zn + Fe2+ contents in olivine phenocrysts suggests diffusion-driven kinetic fractionation during olivine crystallization. The data is well-fitted with a diffusion model in which Zn together with Fe2+ diffuse from surrounding melt into olivine crystals and approximately equal flux of Mg diffuses into melt due to the large chemical gradient, yielding a kinetic Zn isotope fractionation factor βZn of 0.07. By utilizing the fractional crystallization model under equilibrium or disequilibrium conditions, it is suggested that diffusion-induced isotope disequilibrium during olivine crystallization may drive Zn isotopic composition of the residual melt toward heavier values. When using zinc isotope systematics of any basaltic magma to probe the source heterogeneity, this impact must be taken into account if the Zn isotope disequilibrium widely exists in olivine phenocrysts. Based on the time-related quantitative diffusion model, our results show that the magma residence time for strongly alkali basaltic lavas (∼20 days) is 10–60 times shorter than that for weakly alkali basaltic lavas (6 months to 4 years). This supports the generation of weakly alkali basalts via interaction between silica-undersaturated melts and the surrounding lithospheric mantle, implying a probable common mechanism in intra-plate basalts that could be comparable globally. Thus, Zn isotope disequilibrium between olivine crystals and melts could provide valuable information on the evolution history of intra-plate basaltic magmas.

Non-Linear Clumped Isotopes from DIC Endmember Mixing and Kinetic Isotope Fractionation in High pH Anthropogenic Tufa

Clumped isotope values (Δ47) of carbonates forming in high pH conditions do not correspond to mineral precipitation temperatures due to certain effects including kinetic isotope fractionation and dissolved inorganic carbon (DIC) endmember mixing. Field-based archives of these carbonate environments are needed to evaluate and quantify these effects accurately. In this study, we measure the clumped isotope values of anthropogenic carbonates for the first time. Tufa layers were analyzed from samples precipitating in a high pH (>10) stream that drains a major slag heap in north east England. Δ47 values are 0.044‰–0.183‰ higher than expected equilibrium values. Non-linear distribution of clumped isotope data is diagnostic of DIC endmember mixing, rather than partial equilibration of DIC. Episodic dilution of hydroxide-rich stream waters by equilibrated rainfall surface runoff provides the mechanism by which mixing occurs. Δ47 values are ~0.010‰–0.145‰ higher than linear clumped isotope mixing profiles, suggesting that the majority of Δ47 increase results from a combination of endmember non-linear mixing effects and an atmosphere-hydroxide sourcing of DIC. The diagnostic trends and variation in clumped isotope values present in these results demonstrates the potential of anthropogenic carbonate systems as a useful archive for studying and quantifying kinetic effects in clumped isotopes.

Tellurium isotope fractionation during evaporation from silicate melts

As a moderately volatile, redox-sensitive chalcophile and siderophile element, Te and its isotopic composition can inform on a multitude of geochemical and cosmochemical processes. However, the interpretation of Te data from natural settings is often hindered by an insufficient understanding of the behavior of Te in high-temperature conditions. Here, we present the results of Te evaporation and isotopic fractionation in silicate melting experiments. The starting material was boron-bearing anorthite-diopside glass with 1 wt% TeO2. The experiments were conducted over the temperature range of 868–1459 °C for 15 min each, and at oxygen fugacities (logfO2) relative to the fayalite-magnetite-quartz buffer (FMQ) of FMQ−6 to FMQ+1.5, and in air. Evaporation of Te decreases with decreasing fO2. For high-temperature experiments performed at >1200 °C Te loss is accompanied by Te isotope fractionation towards heavier compositions in the residual glasses. By contrast, Te loss in experiments performed at temperatures <1200 °C typically resulted in lighter Te isotopic compositions in the residues relative to the starting material. In air, Te evaporates as TeO2, whereas at lower oxygen fugacities we predict the evaporation of Te2, using Gibbs free energy minimization calculations. In air, the experimentally determined kinetic isotopic fractionation factor for δ128/126Te at T > 1200 °C is αK = 0.99993. At reducing conditions, Te likely substitutes as Te2− for O2– in the melt structure and becomes increasingly soluble at highly reducing conditions. Consequently, Te evaporation is not predicted for volcanic processes on reduced planetary bodies such as the Moon or Mercury.

Zinc isotope fractionation in mantle rocks and minerals, and a revised δ66Zn value for the Bulk Silicate Earth

The isotope composition of zinc (Zn) can be an important tracer of planetary formation and differentiation, but estimates of the Bulk Silicate Earth (BSE) and the extent and origin of Zn isotope variations in the mantle continue to be debated. We report 153 high-precision Zn isotope analyses of whole-rock samples and minerals of well-characterized, mantle-derived spinel and garnet peridotites from five sites in Siberia, Mongolia and China: 38 xenoliths in basaltic rocks (fertile to strongly melt-depleted as well as variably metasomatized or veined peridotites) and 3 orogenic peridotites. Data quality was rigorously tested by twenty-two duplicated analyses using several dissolution techniques. Spinel (Sp) contains 760–1460 μg/g Zn and hosts up to 58 % of whole-rock Zn budget in the peridotites followed by olivine (Ol, 30–66 % of whole-rock Zn budget), orthopyroxene (Opx), clinopyroxene (Cpx) and garnet, with abundance ratios of [Zn]Sp/[Zn]Ol = 15–47, [Zn]Ol/[Zn]Opx = ∼1.5, [Zn]Ol/[Zn]Cpx = ∼6. Spinel has higher δ66Zn values than coexisting silicates (∼0.12 ‰ higher than in Ol). Minerals in orogenic peridotites show disequilibrium Zn element and isotope partitioning due to slow inter-grain Zn diffusion at retrograde metamorphism.Fertile peridotites (≥3 wt% Al2O3) contain 52–60 μg/g Zn, usually higher than melt-depleted (<3 wt% Al2O3) rocks (40–53 μg/g). The δ66Zn values in the majority of spinel and garnet peridotites range from 0.11 to 0.26 ‰ and show no systematic co-variations with melt extraction indices (e.g., Al2O3, Mg#), suggesting, unlike some previous studies, that partial melting does not appreciably affect Zn isotope composition of residual mantle. Garnet lherzolites yield an average δ66Zn of 0.22 ± 0.09 ‰ (2SD), similar to that (0.18 ± 0.07 ‰, 2SD) for shallower spinel peridotites, indicating uniform Zn isotope composition with depth. Four fertile lherzolites from earlier work, duplicated in this study to assess inter-laboratory bias, fall in the same δ66Zn range, suggesting that previously reported higher δ66Zn for these samples could be due to analytical problems. We thus estimate the δ66Zn of the BSE at 0.19 ± 0.08 ‰ (2SD) based on suitable fertile peridotites from several localities in this study and the literature.The δ66Zn values (−0.10 to 0.04 ‰) are much lower in Fe-rich (9.3–13.1 wt% FeOT) and veined spinel peridotites as well as in spinel pyroxenite veins from Mongolia (−0.20 to 0.11 ‰). The Fe-rich peridotites and pyroxenites are rich in Zn (up to 113 μg/g) and were previously found to have anomalously low δ56Fe (−0.33 to − 0.27 ‰) and δ53Cr (−1.36 to − 0.05 ‰). We attribute these features to kinetic isotope fractionation during reaction with mafic melts migrating in conduits in the lithospheric mantle or intrusion of Fe-Zn-rich melts with light Fe-Zn isotope compositions. Kinetic Zn isotope fractionation, rather than equilibrium partial melting, may be a major cause of Zn isotope heterogeneity in the mantle.