Rayleigh Distillation Research Articles

Precipitation <i>δ</i><sup>18</sup>O on the Himalaya–Tibet orogeny and its relationship to surface elevation

Abstract. The elevation history of the Himalaya–Tibet orogen is central to understanding the evolution and dynamics of both the India–Asia collision and the Asian monsoons. The surface elevation history of the region is largely deduced from stable isotope (δ18O, δD) paleoaltimetry. This method is based on the observed relationship between the isotopic composition of meteoric waters (δ18Op, δDp) and surface elevation, and the assumption that precipitation undergoes Rayleigh distillation under forced ascent. Here we evaluate how elevation-induced climate change influences the δ18Op–elevation relationship and whether Rayleigh distillation is the dominant process affecting δ18Op. We use an isotope-enabled climate model, ECHAM-wiso, to show that the Rayleigh distillation process is only dominant in the monsoonal regions of the Himalayas when the mountains are high. When the orogen is lowered, local surface recycling and convective processes become important, as forced ascent is weakened due to weaker Asian monsoons. As a result, the δ18Op lapse rate in the Himalayas increases from around −3 to above −0.1 ‰ km−1, and has little relationship with elevation. On the Tibetan Plateau, the meridional gradient of δ18O decreases from ∼1 to ∼0.3 ‰ ∘−1 with reduced elevation, primarily due to enhanced sub-cloud reevaporation under lower relative humidity. Overall, we report that using δ18Op or δDp to deduce surface elevation change in the Himalayan–Tibetan region has severe limitations and demonstrate that the processes that control annual-mean precipitation-weighted δ18Op vary by region and with surface elevation. In summary, we determine that the application of δ18O paleoaltimetry is only appropriate for 7 of the 50 sites from which δ18O records have been used to infer past elevations.

Seasonal Deuterium Excess Variations of Precipitation at Summit, Greenland, and their Climatological Significance

AbstractThe hydrogen and oxygen isotopic composition of ice cores from Summit, Greenland, has provided invaluable information about variations in past climate. However, interpretations of these isotopic data have been made despite a paucity of direct isotopic studies of Summit precipitation. We provide insight to such interpretations by examining the annual cycle of deuterium excess (d‐excess) in precipitation samples from Summit and by considering the climatic controls on the annual cycle. Precipitation was collected daily from July 2011 to September 2014 at heights of 1, 2, and 4 m. The isotopic composition of precipitation sampled at 4 m above the snow surface is free of contamination from blowing snow. Precipitation d‐excess is high in the summer and low in the winter, a pattern opposite to that found at most high‐latitude locations, where summer d‐excess is low relative to winter. Low winter d‐excess values at Summit can be explained by varying degrees of Rayleigh distillation of moisture sourced from isotopically similar marine sources. However, the observed summer d‐excess maximum at Summit is anomalously high compared with other Arctic locations, and we propose that this is due to high d‐excess moisture contributed by sublimation of surface snow on the Greenland Ice Sheet. We demonstrate the plausibility of this hypothesis through simple isotopic mass balance calculations, analyses of cloud heights, and back trajectories to identify moisture sources. We show that Rayleigh distillation, sublimation, and the phase of the d‐excess annual cycle are all important factors that should be considered in ice core d‐excess interpretations.

Improving the organic solvents purity for microextraction preconcentration of impurities

The purity of different industrially produced extractants, such as hexane, n-octane, carbon tetrachloride, and ethanol, was studied using gas chromatography-mass spectrometry and gas chromatography. It was shown that the most characteristic impurities in these extractants were volatile organochlorine compounds ( LHS ) and hard volatile esters of o-phthalic acid ( o -phthalates ). The impurity content of these substances in extractants could reach tens of mg/l. Based on the calculation of the distribution coefficients of impurities in the “liquid-vapor” system, the methods of Rayleigh distillation and rectification were proposed for removing the impurities from the extractants. Gas chromatographic mass spectrometric and gas chromatographic analysis of purified extractants showed that the effective removal of impurities of o-phthalic acid esters was possible using the Rayleigh distillation during the distillation of extractants at an evaporation rate of no more than 6·10 –4 ml·cm –2 ·s –1 . This allowed obtaining a volume of extractant enough to carry out 50-55 microextractions in 5 minutes. The effective removal of volatile LHS was achieved by a multi-stage distillation method - rectification. The concentration of impurities of the investigated substances in the purified extractants did not exceed 10 –3 –10 –5 mg/l. The liquid-phase microextraction preconcentration with the use of purified extractants decreased the detection limits of LHS and o-phthalates in the water to 2∙10 -7 -1∙10 -5 and 4·10 -6 ‒1·10 -5 mg/l, respectively. Keywords: microextraction preconcentration, rectification, Rayleigh distillation, detection limits, extractant, gas chromatography, gas chromatography mass spectrometry (Russian) DOI: http://dx.doi.org/10.15826/analitika.2019.23.1.008 V.A. Krylov 1 , P.V. Mosygin 1 , L.V. Smirnova 1 , S.A. Bulanova 1 , I.A. Zhituhina 1 , G.V. Pushkarev 2 1 N.I. Lobachevsky Nizhny Novgorod State University, Russian Federation, 603950, Nizhny Novgorod, Gagarin pr., 23 2 Joint –Stock Company «Scientific - Production Enterprise «Salyut», Russian Federation, 603950, Nizhny Novgorod, Larina Str., 7

Water vapor exposure chamber for constant humidity and hydrogen and oxygen stable isotope composition.

Water vapor exposure experiments have applications for studying water physisorption and chemisorption hydration and hydroxylation reactions on a wide variety of material surfaces. The stable isotopes of hydrogen and oxygen in the water molecule are useful tracers of water exchange mechanisms and/or rates in such vapor exposure experiments. We designed and built a humidity chamber system that uses membrane-mediated liquid-vapor exchange of water followed by mixing with dry air to control the relative humidity of air and its δ2 H and δ18 O isotopic composition. We tested the stability and precision of the humidity and its isotopic composition on hourly to 90-day timescales. The humidity chamber design reported here is capable of providing relative humidity control to within ±1%, and consistent δ2 H and δ18 O values of the water vapor that are similar to our cavity ringdown spectroscopy (CRDS) measurement precision (δ2 Hvap ± 0.7‰ and δ18 Ovap ± 0.24‰). We quantify the isotopic enrichment effects of Rayleigh distillation in the system and provide information on water reservoir sizes large enough to buffer isotopic enrichment effects to within measurement precision. The humidity chamber design reported here provides a means to create constant δ2 H and δ18 O values over the course of an exposure experiment. The design has applications to a wide range of studies of water sorption on material surfaces from foods and pharmaceuticals to geological materials.

Predicting High-Dimensional Isotope Relationships from Diagnostic Fractionation Factors in Systems with Diffusional Mass Transfer

A dual or multiple stable isotope relationship, for example, a trajectory in a δ−δ (or δ′−δ′) space, can be used to deduce the relationship of underlying diagnostic isotope fractionation factors (α) and therefore reveal the mechanism of a reaction process. While temporal data sampled from a closed-system can be treated by a Rayleigh distillation model, spatial data should be treated by a reaction-transport model. Owing to an apparent similarity between the temporal and spatial trajectories, the research community has often ignored this distinction and applied a Rayleigh distillation model to cases where a reaction-transport model should be applied. To examine the potential error of this practice, here we compare the results of a Rayleigh distillation model to a diffusional reaction-transport model by simulating the trajectories in nitrate’s δ′18O−δ′15N space during a simple denitrification process. We found that an incorrect application of a Rayleigh distillation model can underestimate the degree of a di...

Formation and evolution of the Changbaishan volcanic geothermal system in a convergent plate boundary back-arc region constrained by boron isotope and gas data

The goal of this study is to analyze fluid geochemistry, boron isotope and gas data to reassess the formation and evolution processes of the Changbaishan volcanic geothermal system (Jilin, China). Due to the presence of hot fumaroles (with temperatures of higher than 100 °C) and sinters, we believe that phase separation is occurring underground, which is also indicated by the characteristics of the B and gas data. Based on their hydrochemical data and B/Cl ratios, the thermal water samples in this area can be divided into three groups: 1) group 1 is steam-heated/condensate water (group 1-1: JJ, JLQ; group 1-2: SBDG, XRQ); 2) group 2 is karst water (CR3 and LSD); and 3) group 3 is residual water (CR1 and CR2). The preliminary discrimination of samples using B and gas data implies that group 1-1 around the crater is controlled by a second phase separation process, which is induced by the input of hot magmatic material; group 1-2 in the basin area is controlled by the transformation of clay minerals, and the XRQ water sample shows contamination by sewage; group 2 represents karst water that has leached metamorphic rock and been directly heated by the input of hot magmatic volatiles; and group 3 is the result of one seawater phase separation process. The mixing and fluid-rock interaction models based on the mass balance of boron concentration and isotope imply that the phase separation and input of magmatic volatiles are more important than fluid-rock interactions in the formation of the Changbaishan geothermal system, and the first phase separation process in the deep carbonate reservoir occurred in a late stage, with approximately 75%–87.5% of water becoming vapor. Additionally, the Rayleigh distillation of B isotope implies that new magma is being generated below this area, with the low B concentrations and high boron isotopic values of its gases implying that the mixed component may be derived from the subducted Pacific plate (e.g., marine sediment). The addition of “excess heat” due to the continued subduction of the Pacific plate has further encouraged the phase separation process, and the subduction process has also accelerated the transition of the hydrothermal system into a non-hydrothermal system.

A Review on the Methods for Observing the Substance and Energy Exchange between Atmosphere Boundary Layer and Free Troposphere

Atmosphere boundary layer (ABL or BL) acts as a pivotal part in the climate by regulating the vertical exchange of moisture, aerosol, trace gases and energy between the earth surface and free troposphere (FT). However, compared with research on the exchange between earth surface and ABL, there are fewer researches on the exchange between ABL and FT, especially when it comes to the quantitative measurement of vertical exchange flux between them. In this paper, a number of various methodologies for investigating the exchange of the substance and energy between ABL and FT are reviewed as follows: (1) methods to obtain entrainment rate, which include method by investigating the height of inversion layer, method of flux-jump, estimating with dataset from the ASTEX Lagrangian Experiments and method of using satellite observations and Microwave Imager; (2) mass budget method, which can yield quantitative measurements of exchange flux between ABL and FT; (3) qualitative measurements: method based on Rayleigh distillation and mixing processes, methods of ground-based remote sensing and airborne tracer-tracer relationship/ratio method.

Use of carbon isotopic ratios in nontargeted analysis to screen for anthropogenic compounds in complex environmental matrices

Analytical data for ultra-high-performance liquid chromatography (UHPLC), nontargeted, high-resolution, mass-spectrometry (HR/MS) molecular features from a wide array of samples are used to calculate 13C112C(n-1)/12Cn isotopologue ratios. These ratios increase with molecular carbon number roughly following a trend defined by atmospheric carbon. When the effective source reservoir 13C/12C ratio is calculated from the isotopologue ratio (assuming a fractionation factor of unity), features in biotic samples uniformly are tightly grouped, proximate to atmospheric 13C/12C ratio. In contrast, features in soil natural organic matter (NOM), dust NOM and anthropogenic compounds range from proximate to relatively divergent from atmospheric 13C/12C. For the NOM, 13C/12C ratios are consistent with an expected preferential volatilization of 12C, rendering features in soil NOM 13C-enriched and some features in dust NOM 13C-depleted. Anthropogenic compounds tend to diverge most dramatically from atmospheric 13C/12C, generally toward 13C-depletion, but pesticides we tested tended toward 13C-enriched. This pattern is robust and evident in: i) anthropogenic vs natural features in dust; ii) perfluorinated compounds in standards and as soil contaminants; and iii) sunscreen compounds in commercial products and wastewater. Considering the observed wide 13C/12C range for anthropogenic compounds, we suggest Rayleigh distillation during synthetic processes commonly favors one isotope over the other, rendering a source reservoir that is progressively depleted as synthesis proceeds and, consequently, generates a wide variation in 13C/12C for man-made products. However, kinetic-isotopic effects and/or synthesis from petroleum/natural gas might contribute to the anthropogenic isotopic signature as well. Regardless of cause, 13C/12C can be used to cull HR/MS molecular features that are more likely to be of anthropogenic or non-biotic origin.

Magnesium isotope fractionation during granite weathering

The Mg isotope compositions of weathering profiles developed on granite and granodiorite bedrock have been analyzed to investigate Mg isotope behavior during the weathering of felsic rocks in a continental environment. δ26Mg generally exhibits a small increase from the bedrock values, which range from −0.17 ± 0.06‰ to +0.07 ± 0.06‰, until the upper two meters in which a decrease is observed, producing near surface values between −0.14 ± 0.06‰ and +0.09 ± 0.06‰. The small observed increase in isotopic composition below 2 m is likely due to Mg loss to the hydrosphere during biotite weathering and in situ illite production, while the decrease at depths <2 m is likely due to illite weathering near the surface, with a possible minor contribution from downslope deposition of less weathered material. One hydrothermally altered profile shows more extreme fractionation, reaching δ26Mg values as heavy as +0.55 ± 0.06‰ in the near surface component of the section. The fractionation associated with Mg loss during weathering can be modeled by Rayleigh distillation with fractionation factors between the residual rock and fluid ranging from α = 1.00001 to 1.00040. The Mg isotope composition is primarily controlled by the behavior of illite in the weathering profile, as Mg isotope composition is positively correlated with the illite fraction of major Mg-bearing minerals. A thorough understanding of Mg isotope systematics in these complex clay minerals may permit the use of Mg isotopes in paleosols as paleoclimate indicators.

Quantifying carbon isotope disequilibrium during in-cave evolution of drip water along discreet flow paths

Paleoclimate reconstructions that use speleothem proxy data have increased our understanding of terrestrial climate change, but gaps remain in our understanding of in-cave processes that influence speleothem chemistry. The δ13C values of speleothem calcite are typically influenced by kinetic isotope effects that operate during CO2 degassing and calcite precipitation. Therefore the identification and quantification of these isotopic effects is important in interpreting speleothem stable isotope records. Here we studied the change in water chemistry and δ13C values of dissolved inorganic carbon (DIC) along discreet flow paths at multiple drip sites in Inner Space Cavern, central Texas. We quantified the extent to which the water remains in C isotopic equilibrium during flow along speleothem surfaces as CO2 degasses and calcite precipitates.Two locations in the study cave that have long in-cave flow paths were examined to determine the geochemical evolution and its driving processes along these paths. At each location cave water was sampled at two points 1–2 meters apart along each flow path. Among the key spatial changes observed is a <∼1‰ to ∼4‰ increase in δ13C values of DIC along the flow paths. The magnitude of the increase in δ13C values is controlled by the extent of DIC loss to CO2 degassing. The extent of DIC loss and CO2 degassing is controlled by the pCO2 gradient between drip water and cave air. If the DIC loss is less than 15%, then the evolution of the δ13C value of the DIC reservoir can be accounted for by a Rayleigh distillation model with equilibrium C-isotope fractionation factors for (CO2(g)-HCO3−(aq)) and (CaCO3-HCO3−(aq)). As the depletion of the DIC reservoir exceeds 15% the DIC δ13C values become progressively higher such that the (HCO3−(aq)-CO2(g)) fractionation values needed to explain the observations change from equilibrium values of ∼8‰ to non-equilibrium values of up to ∼25‰. This variance in magnitude of C-isotope fractionation during CO2 degassing cannot be attributed to changes in temperature, and thus we infer significant kinetic isotope effects at higher rates of DIC loss. Such kinetic effects have significant implications for speleothem C-isotope proxy interpretations, as these kinetic isotope effects are of a similar magnitude as those used to infer past changes in drought and vegetation.

Dynamics of pyrite formation and organic matter sulfurization in organic-rich carbonate sediments

Organic-rich carbonate sediments are deposited in a range of environments today and in the geologic past. A significant part of organic matter (OM) degradation in such sediments often occurs by microbial reduction of seawater sulfate, and the sulfide product may be preserved in pyrite and in organic sulfur (S) compounds. The isotopic composition (δ34S) of these phases can provide valuable information about S cycling in the ocean and in sediment porewaters, but only insofar as the processes governing these δ34S values are understood. To this end, we investigated the pathways, timing and interactions between pyrite and organic S formation during the deposition of organic-rich chalks. As a test case, we studied cores representing the thickest (∼350 m) and most complete Late Cretaceous organic-rich sequence along the southern Paleotethyan margin (central Israel). The organic S and OM contents show an inverse relation with the pyritic S content, which together with the uniform FePy/FeHR ratio (∼40%), suggest competition between organic S and pyrite formation. Both kerogen and pyritic S are 34S-depleted relative to Late Cretaceous marine sulfate (δ34S∼17–20‰), but the kerogen S is consistently and unusually 34S-enriched relative to coexisting pyrite by up to ∼38‰. Large S isotope fractionation (∼60‰) during microbial sulfate reduction necessary to reproduce the lowest pyrite δ34S values in the core, and relatively invariant δ34S values in organic S suggests that this large fractionation was approximately constant during deposition of the chalks in the core. Higher pyrite δ34S values observed in the most organic-rich parts of the core may be explained by Fe-limited pyrite formation, perhaps due to the reaction of Fe (e.g., complexation, sorption) with organic compounds. Lesser Fe availability, relative to the OM available for sulfate reduction, limits the ultimate abundance of pyrite, but importantly, it delays the formation of pyrite to deeper below the sediment-water interface, from 34S-enriched sulfide produced by Rayleigh distillation of a dwindling sulfate reservoir. Thus, it appears that competing Fe-OM, S-OM and Fe-S reactions can significantly affect the δ34S values recorded in pyrite in organic-rich carbonate sediments despite large and relatively constant microbial S isotope fractionation.

Collisional and alteration history of the CM parent body

Boriskino is a little studied CM2 chondrite composed of millimeter-sized clasts of different lithologies and degrees of alteration. Boriskino thus offers a good opportunity to better understand the preaccretionary alteration history and collisional evolution that took place on the CM parent body. The least altered lithology displays 16O-poor Type 1a calcite and aragonite grains (δ18O ≈ 30–37‰, δ17O ≈ 15–18‰ and Δ17O ≈ −2 to 0‰, SMOW) that precipitated early, before the establishment of the petrofabric, from a fluid whose isotopic composition was established by isotopic exchange between a 16O-poor water and 16O-rich anhydrous silicates. In contrast, the more altered lithologies exhibit 16O-rich Type 2a and veins of calcite (δ18O ≈ 17–23‰, δ17O ≈ 6–9‰ and Δ17O ≈ −4 to −1‰, SMOW) that precipitated after establishment of the deformation, from transported 16O-rich fluid in preexisting fractures. From our petrographic and X-ray tomographic results, we propose that the more altered lithologies of Boriskino were subjected to high intensity impact(s) (10–30 GPa) that produced a petrofabric, fractures and chondrule flattening. Taking all our results together, we propose a scenario for the deformation and alteration history of Boriskino, in which the petrographic and isotopic differences between the lithologies are explained by their separate locations into a single CM parent body. Based on the δ13C-δ18O values of the Boriskino Type 2a calcite (δ13C ≈ 30–71‰, PDB), we propose an alternative δ13C-δ18O model where the precipitation of Type 2a calcite can occurred in an open system environment with the escape of 13C-depleted CH4 produced from the reduction of C-bearing species by H2 released during serpentinization or kamacite corrosion. Assuming a mean precipitation temperature of 110 °C, the observed δ13C variability in T2a calcite can be reproduced by the escape of ≈15–50% of dissolved carbon into CH4 by Rayleigh distillation.

Tracking Physicochemical Conditions of Evaporite Deposition by Stable Magnesium Isotopes: A Case Study of Late Permian Langbeinites

AbstractMagnesium isotopic compositions of evaporite deposits may record information concerning brine evolution during deposition. We report Mg isotopic values (δ26MgDSM3) measured from an evaporite deposit of langbeinite (K2Mg2(SO4)3) found in the Permian Salado Formation. We used these data to model Mg isotope fractionation between langbeinite and its parent brine. In addition, both measured and theoretical results are used to estimate precipitation temperature and interpret depositional environment. The Salado langbeinite δ26Mg values are relatively low and fall within a relatively narrow range (–4.12 ± 0.03‰ to −3.81 ± 0.07‰). Equilibrium fractionation factors between langbeinite and aqueous Mg2+ solutions were calculated using quantum chemical density functional theory. All computations were performed at the B3LYP/6‐31 + G(d,p) level. Solvation effects were addressed using a solvent model (“water‐droplet” approach) and mineral structures were investigated using volume variable cluster models (VVCM). The equilibrium Mg isotopic fractionation factors α between langbeinite and model brine solution we obtained are 1.0005, 1.0004, and 1.0003 (Δ26Mglangb‐water≈103lnα = 0.473‰, 0.390‰, and 0.322‰) at 10°C, 25°C, and 40°C, respectively. These relatively large equilibrium fractionation factors indicate significant Mg isotope fractionation between langbeinite and its parent brine during precipitation, as langbeinite preferentially incorporates the heavier 26Mg and 25Mg isotopes. Rayleigh distillation modeling of the Salado langbeinite's relatively light Mg isotopic composition requires δ26MgDSM3 values of −4‰ for the parent brine. Models favor a precipitation temperature as high as 40°C under equilibrium conditions. Potential disequilibrium precipitation conditions suggested by Mg isotopic data also imply rapid deposition in a hot, arid sedimentary environment prevailing in the southwestern U.S. during the Late Permian.

Liquid–Vapor Equilibria in the SiCl4–A (A = SiCl4–nFn (n = 1–4) Impurity) Systems

The vapor–liquid separation factors of SiCl4–A (A = SiCl4–nFn (n = 1–4) silicon fluorochloride impurities in silicon tetrachloride have been determined experimentally at 298 K by statically balancing phases. The values obtained for the SiCl4–SiCl3F, SiCl4–SiCl2F2, SiCl4–SiClF3, and SiCl4–SiF4 systems are 4.9 ± 0.8, 8.2 ± 1.2, 29.8 ± 6.1, and 42.0 ± 7.5, respectively. In the SiCl4–SiCl3F system, the vapor–liquid separation factor has also been determined by the Rayleigh distillation method: α = 5.0. The experimental data agree with the vapor–liquid separation factors estimated in conformal solution theory using parameters of the Lennard- Jones potential.

Magnesium isotope record of fluid metasomatism along the slab-mantle interface in subduction zones

Fluids released from subducting slabs play an important role in mantle metasomatism and arc magmatism. Magnesium could be mobile in subduction-zone fluids and Mg isotopic signatures of mélange rocks may provide information on crust-mantle interactions at subduction zones. To investigate the behaviour of Mg isotopes during fluid metasomatism along the slab-mantle interface, we report Mg isotopic compositions of a set of well-characterized samples from the ultramafic blocks in the Franciscan Complex of California. The partially serpentinized and completely serpentinized peridotites have δ26Mg values (from −0.26 to −0.14) clustered around the mantle value, indicating small Mg isotope fractionation during peridotite serpentinization. Samples with serpentine completely replaced by talc have lower MgO contents and heavier Mg isotopic compositions (δ26Mg of −0.13 to −0.01) than serpentinized peridotites, implying that Mg was non-conservative during this metasomatic replacement. The shift toward higher δ26Mg values of talc-dominated metaperidotites reflects the preferential loss of light Mg isotopes into fluids via Rayleigh distillation during the dehydration reactions that produced talc from serpentine. On the other hand, samples with talc completely replaced by tremolite have lighter Mg isotopic compositions (δ26Mg of −0.50 to −0.41) and higher CaO contents than the talc-dominated samples. These features suggest a metasomatic process involving isotopically light carbonate-rich fluids derived from the subducting slab. Collectively, large Mg isotope fractionation occurs during multistage fluid-rock interactions within the Franciscan subduction channel, which provides direct evidence for significant Mg isotopic variation during fluid metasomatism along the slab-mantle interface. This may account for the observed heterogeneous Mg isotopic compositions of arc lavas. Magnesium isotopes thus could be a potentially useful tracer of crust-mantle interactions at subduction zones.

Insight into hydrothermal and subduction processes from copper and nitrogen isotopes in oceanic metagabbros

The transfer of trace elements from external terrestrial reservoirs (i.e. atmosphere, hydrosphere, crust) to the mantle is mainly controlled by their geochemical behaviors during hydrothermal alteration of the oceanic crust and subsequent subduction metamorphism. Deciphering fluid–rock partitioning and element pathways during hydrothermal and subduction metamorphism is complicated due to a variety of pressure-temperature conditions and variable compositions of fluids interacting with the lithosphere. Fluid–rock interactions can, however, be traced by coupling multiple isotopic systems in a single and uniform type of lithology. Here we present Cu and N isotope relationships in Alpine metagabbros, which were hydrothermally-altered on the seafloor and subsequently buried to different depths in a subduction zone. Our sample set covers non-subducted, as well as blueschist- and eclogite-facies low-strain metagabbros, and veins and mylonites equilibrated in eclogite facies conditions. The low-strain metagabbros have Cu contents ranging from 22.5 to 73.2 ppm, and δ65Cu values from −1.66 to −0.02‰. In contrast, eclogite-facies veins and mylonites have lower Cu contents, below 11.1 ppm (except one sample at 27.7 ppm), but a very large range of δ65Cu values from −3.12 to 1.76‰. Nitrogen contents and δ15N values on the same samples were reported in a previous study and vary between 2.6 and 55 ppm, and from 0.8 to 8.1‰, respectively. In Cu vsδ65Cu and N vsδ15N diagrams, the low-strain metagabbros show clear trends, which are not related to subduction-zone metamorphic conditions, and more likely reflect hydrothermal processes. We propose that N and Cu were released concomitantly from metagabbros to hydrothermal fluids. This produced a decrease of N and Cu concentrations in the residual rocks, together with an increase of δ15N and a decrease of δ65Cu. The Cu isotope data, coupled to mineralogical analyses of the metagabbros by SEM, are compatible with partial dissolution of chalcopyrite under oxidative conditions. Nitrogen is not detected in chalcopyrite or in any major or minor mineral phases, probably reflecting the presence of ammonium (NH4+) in trace amounts substituting for Na+ and/or Ca2+ in major phases. The release of Cu and N during hydrothermal alteration is best modeled by Rayleigh distillation in an open system, although a batch equilibrium model cannot be completely discarded. During subduction of the oceanic crust, N was further released to fluids, while Cu was mostly retained in the rocks, a conclusion supported from isotopic and mineralogical analyses. This step resulted in a decrease of N concentration with moderate increase of δ15N (<0.5‰), and constant Cu contents and δ65Cu values. Accordingly, Cu isotope variability inherited from hydrothermal alteration was preserved in subducted metagabbros, although 90% of the fluids were lost during eclogitization. The present results imply that Cu depletion associated with light isotope enrichments in metagabbros can record the oxidative nature of hydrothermal alteration on the seafloor and be preserved despite late metamorphic overprint.

Nitrate Isotope Distribution in the Subarctic and Subtropical North Pacific

AbstractNitrogen and oxygen isotopic compositions of nitrate (δ15NNitrate and δ18ONitrate) were determined along 47°N in the subarctic North Pacific and 149°E in the western North Pacific. In the western subarctic gyre, known as a high‐nutrient, low‐chlorophyll region, the δ15NNitrate and the differences between δ15NNitrate and δ18ONitrate (herein Δ(15–18)) in the intermediate water were significantly lower than in the surrounding area. The 15N‐depleted nitrate was generated by nitrified nitrate from the remineralization of organic matter produced by the partial consumption of the surface nitrate pool. Between the western subarctic gyre and the Alaskan gyre, there was a 0.2‰ increase in δ15NNitrate and a 0.3‰ increase in Δ(15–18) in the intermediate water associated with a 7.2‐μM decrease in the nitrate concentration at the surface. Assuming Rayleigh distillation kinetics, the increase in utilization of the surface nitrate pool between the western subarctic gyre and the Alaskan gyre (from 29% to 85%) corresponds to a change in δ15N of organic matter from 2.6‰ to 4.9‰. The 15N enrichment of intermediate nitrate toward the east resulted from the increase in utilization. Moreover, the 15N‐depleted nitrate in the mixed‐layer water of the western subtropical gyre was affected by N2 fixation in the Kuroshio water, whereas the 15N‐enriched nitrate in the intermediate water at the western margin of North America was affected by water column denitrification. The δ15N sediment record in the western subarctic North Pacific likely reflects past changes in the high‐nutrient, low‐chlorophyll region but may also be controlled by water column denitrification and N2 fixation.

Mg isotope response to dolomitization in hinterland-attached carbonate platforms: Outlook of δ26Mg as a tracer of basin restriction and seawater Mg/Ca ratio

Magnesium isotopes in early diagenetic dolomite have been proposed as a potential tracer for seawater chemistry and global Mg cycles. However, the applicability of Mg isotopes of early diagenetic dolomite in studies of ancient seawater requires a detailed understanding of the behavior of Mg isotopes during dolomitization in a variety of geological settings. Hinterland attached carbonate platform is an important sink of seawater Mg through dolomitization, and basin restriction is a common feature in hinterland attached carbonate platforms, yet its effects on Mg isotope systematics in carbonates have not been well documented.The upper Albian Hevyon Formation in southern Israel was deposited in a typical hinterland attached carbonate platform setting and provides an ideal case for investigating the Mg isotope behavior during dolomitization in environments of frequent basin restrictions. The abundance of dolomite increases up-section in the Hevyon Formation and correlates with the appearance of microbial deposits and disappearance of metazoans, reflecting water level fluctuations in multiple exposure and recharging events. In conjunction with sedimentary facies analysis, multiple geochemical proxies (Ni/Co, V/Cr, Ce/Ce∗, and δ13C) indicate the development of anoxic conditions. These two lines of evidence, together with textural indicators and 87Sr/86Sr ratios, suggest dolomitization was microbial and penecontemporaneous in a shallow low circulation water body. δ26Mg of dolomite increases from −1.99 ± 0.12‰ in the lower part of the sequence to a value of −1.52 ± 0.02‰ in the middle interval, then decreases back to −2.06 ± 0.18‰ in the upper unit.Overall, Mg isotope compositions co-vary with δ13C, Mg/Ca ratios, as well as crystal size of dolomite, which implies that variations in δ26Mg value reflect syn-depositional changes in water chemistry as a result of dolomitization in episodically restricted basins. δ26Mg tracks the fluctuating marine connection and provides a powerful tracer for the level of restriction. Furthermore, by pairing Mg isotope variability of the dolomite with a Rayleigh distillation model and elemental mass balance in aqueous solution, it is possible to constrain the Mg/Ca ratio of seawater. Based on the records from the Hevyon Formation, Mg/Ca ratio of seawater during the Albian is constrained between 1.5 and 1, which is consistent with results from other records.

Reconstructing mantle carbon and noble gas contents from degassed mid-ocean ridge basalts

The fluxes of volatile elements from the mantle have long been used to understand mantle structure and evolution, and are critical controls on Earth's climate stability. Because of the ubiquity of magmatic degassing, inferring pre-degassing volatile concentrations from measured basalts requires the application of a degassing model. Such models, including the commonly-applied equilibrium Rayleigh distillation, typically assume equilibrium or solubility-based partitioning between melt and vapor. Here, we demonstrate that ratios of radiogenic isotopes of He, Ne, Ar and, especially, Xe measured in global mid-ocean ridge basalts (MORBs) are inconsistent with equilibrium degassing models, even when not considering He. We conclude that kinetic disequilibrium is a crucial process affecting volatile abundances during degassing. We present a simple disequilibrium Rayleigh distillation model to reconstruct pre-degassing MORB noble gas and carbon concentrations, which predicts that He and Ne achieve nearly equilibrium partitioning between melt and vesicles, but the slower-diffusing heavier noble gases are strongly affected by disequilibrium, resulting in non-equilibrium fractionation of noble gas elemental ratios, and other volatile element ratios like CO2/He.We apply our model to a large set of MORB data, and find average pre-degassing 3He, 22Ne, and 36Ar concentrations of 4.4 ± 0.9 × 10−10, 6.6 ± 1.4 × 10−11, and 6.8 ± 4.5 × 10−10 ccSTP/g (2σ), with variations of approximately 2 orders of magnitude, similar to other highly incompatible elements. Pre-degassing noble gas concentrations imply a mid-ocean ridge 3He flux of 800 ± 170 mol/yr and upper mantle 3He/22Ne and 3He/36Ar ratios of 6.6 ± 2.0 and 0.64 ± 0.44, but substantial variability in these ratios between samples. Applying our model to CO2, we calculate an average mantle CO2/3He molar ratio of 1.67 ± 0.21 × 109, which, when combined with our estimate of 3He flux, implies an upper mantle CO2 flux of 5.9 ± 1.0 × 1013 g/yr and a CO2 concentration of approximately 110 ppm.Our estimate of the mantle 3He flux is the first determined independently of oceanographic 3He measurements, and consequently represents a time-integrated flux substantially longer than ∼1000 years. And although at the high end of the range of previous estimates, our estimate does not resolve the long-standing heat-helium paradox. Additionally, our requirement for heterogeneous pre-degassing 3He/22Ne and 3He/36Ar ratios between samples is contrary to conclusions of previous applications of disequilibrium degassing models, which advocated uniform ratios. Furthermore, we find that CO2/Ba ratios are highly variable in MORB samples, but still consistent with an average mantle mass ratio of ∼100. However, estimating pre-degassing CO2 concentrations and the mantle CO2 flux depend strongly on the poorly-constrained carbon diffusivity. Consequently, our demonstration of the prevalence of disequilibrium during mid-ocean ridge degassing, and the potential for disequilibrium in other volcanic settings, highlights the need for better characterization of the physical parameters associated with volcanic degassing.

Archival processes of the water stable isotope signal in East Antarctic ice cores

Abstract. The oldest ice core records are obtained from the East Antarctic Plateau. Water isotopes are key proxies to reconstructing past climatic conditions over the ice sheet and at the evaporation source. The accuracy of climate reconstructions depends on knowledge of all processes affecting water vapour, precipitation and snow isotopic compositions. Fractionation processes are well understood and can be integrated in trajectory-based Rayleigh distillation and isotope-enabled climate models. However, a quantitative understanding of processes potentially altering snow isotopic composition after deposition is still missing. In low-accumulation sites, such as those found in East Antarctica, these poorly constrained processes are likely to play a significant role and limit the interpretability of an ice core's isotopic composition. By combining observations of isotopic composition in vapour, precipitation, surface snow and buried snow from Dome C, a deep ice core site on the East Antarctic Plateau, we found indications of a seasonal impact of metamorphism on the surface snow isotopic signal when compared to the initial precipitation. Particularly in summer, exchanges of water molecules between vapour and snow are driven by the diurnal sublimation–condensation cycles. Overall, we observe in between precipitation events modification of the surface snow isotopic composition. Using high-resolution water isotopic composition profiles from snow pits at five Antarctic sites with different accumulation rates, we identified common patterns which cannot be attributed to the seasonal variability of precipitation. These differences in the precipitation, surface snow and buried snow isotopic composition provide evidence of post-deposition processes affecting ice core records in low-accumulation areas.