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A predicted equilibrium intramolecular isotope distribution (Intra-ID) serves as a reference for measured position-specific (PS) isotope composition variation in an organic molecule. Equilibrium Intra-ID can be estimated from calculated reduced partition function ratios (RPFR or β factor), which are largely absent to date. For relatively small molecules, the PS β factor can be calculated directly. However, estimating the PS β factor considering an entire, large organic molecule is computationally prohibitive. The isotope effect is local in that the vibrational frequencies of an atom are only affected by its proximal bonding environment. Therefore, the cutoff calculation, which simplifies the calculation of an entire molecule to a local area, was previously proposed for large organic molecules. However, the cutoff size was not validated, which has hindered the application of the cutoff calculation. Here, we calculated a series of small organic molecules with 2–18 carbon atoms to test the influence of cutoff size on the 13β value estimation of a target carbon position in a carbon chain or a carbon ring. We calculated nineteen small molecules that have a methyl carbon and a functional group that is at least three bonds away from the target methyl position. The result showed that the equilibrium 13C enrichment of the methyl group relative to CO 2 at 25 °C (ln13 α (eq)) for the nineteen molecules varied in a small range, with a standard deviation of 0.2‰. Fourteen aromatic hydrocarbons with a benzene and one adjacent functional group were calculated to test the influence of different adjacent functional groups on similar carbon positions in benzene. The results showed that different adjacent functional groups had significant influence only on the predicted ln13 α (eq) value of the carbon position directly connected to them (standard deviation = 1.0‰, n = 14), with a negligible influence on the predicted ln13 α (eq) value of the remaining carbons in benzene (standard deviation = 0.2‰, n = 14). The PS 13β value of a specific carbon position in CoA calculated by the cutoff calculation differed from that of the entire-molecule calculation by 0.0–0.3‰. We concluded that the cutoff calculation simplified the calculation of a target position from an entire molecule to a cluster of three proximal bonds in a chain and/ or an adjacent ring, providing PS 13β values of sufficient accuracy for large organic molecules. [ABSTRACT FROM AUTHOR]

Understanding the origin of ocean island basalts (OIB) has important bearings on Earth's deep mantle. Although it is widely accepted that subducted oceanic crust, as a consequence of plate tectonics, contributes material to OIB's formation, its exact fraction in OIB's mantle source remains ambiguous largely due to uncertainties associated with existing geochemical proxies. Here we show, through theoretical calculation, that unlike many known proxies, triple oxygen isotope compositions (i.e. Δ17O) in olivine samples are not affected by crystallization and partial melting. This unique feature, therefore, allows olivine Δ17O values to identify subducted oceanic crusts in OIB's mantle source. Furthermore, the fractions of subducted ocean sediments and hydrothermally altered oceanic crust in OIB's mantle source can be quantified using their characteristic Δ17O values. Based on published Δ17O data, we estimated the fraction of subducted oceanic crust to be as high as 22.3% in certain OIB, but the affected region in the respective mantle plume is likely to be limited. [ABSTRACT FROM AUTHOR]

Abstract Methane has both biotic and abiotic origins, and the identification of its origins has important implications in understanding terrestrial processes as well as in searching for extraterrestrial life. Bulk stable isotope ratios (13C/12C and D/H) and multiply substituted isotopologues (13CH 3 D and 12CH 2 D 2) have been used to distinguish methane's origins, and kinetic models are essential to understanding and quantifying these isotopologue signatures. Although different kinetic models have been developed, inconsistencies exist within these models. Here we constructed an isotopologue-specific kinetic model to quantify isotopologue signatures of methane generated by biological- and metal-catalyzed CO 2 methanation. The Monte Carlo method and data from two experiments were used to constrain the kinetic reversibility and associated isotope effects. The results show that the reversibility can be the same for the biotic and abiotic CO 2 methanation pathways, and their distinct isotopic signatures depend on their respective hydrogen sources. Biotic methane sources hydrogen from carbon-hydrogen and sulfur-hydrogen bonds while abiotic methane sources from metal-hydrogen adsorption bonds. This hydrogen source difference results in abiotic methane being more depleted in 12CH 2 D 2 relative to biotic methane. Although kinetic and dynamic complexities may prevent differentiation between biotic and abiotic methane in multiple and clumped isotope spaces, our proposed molecular-level difference in hydrogen source provides a universal basis for analyzing the isotopologue signatures of methane of diverse origins. [ABSTRACT FROM AUTHOR]

Isotope fractionation during the evaporation of silicate melt and condensation of vapor has been widely used to explain various isotope signals observed in lunar soils, cosmic spherules, calcium-aluminum-rich inclusions, and bulk compositions of planetary materials. During evaporation and condensation, the equilibrium isotope fractionation factor (α) between high-temperature silicate melt and vapor is a fundamental parameter that can constrain the melt’s isotopic compositions. However, equilibrium α is difficult to calibrate experimentally. Here we used Mg as an example and calculated equilibrium Mg isotope fractionation in MgSiO3 and Mg2SiO4 melt-vapor systems based on first-principles molecular dynamics and the high-temperature approximation of the Bigeleisen-Mayer equation. We found that, at 2500 K, δ25Mg values in the MgSiO3 and Mg2SiO4 melts were 0.141 ± 0.004 and 0.143 ± 0.003‰ more positive than in their respective vapors. The corresponding δ26Mg values were 0.270 ± 0.008 and 0.274 ± 0.006‰ more positive than in vapors, respectively. The general α-T equations describing the equilibrium Mg α in MgSiO3 and Mg2SiO4 melt-vapor systems were: αMgl-Mgg=1+5.264×105T21m-1m′ and αMgl-Mgg=1+5.340×105T21m-1m′, respectively, where m is the mass of light isotope 24Mg and m′ is the mass of the heavier isotope, 25Mg or 26Mg. These results offer a necessary parameter for mechanistic understanding of Mg isotope fractionation during evaporation and condensation that commonly occurs during the early stages of planetary formation and evolution. [ABSTRACT FROM AUTHOR]

The equilibrium isotope fractionation factor α eq is a fundamental parameter in the study of stable isotope effects. Experimentally, it has been difficult to establish that a system has attained equilibrium. The three-isotope method, using the initial trajectory of changing isotope ratios (e.g. 16 O, 17 O, and 18 O) to deduce the final equilibrium point of isotope exchange, has long been hailed as the most rigorous experimental approach. However, over the years some researchers have cautioned on the limitations of this method, but the foundation of three-isotope method has not been properly examined and the method is still widely used in calibrating α eq for both traditional and increasingly non-traditional isotope systems today. Here, using water-water and dissolved CO 2 -water oxygen exchange as model systems, we conduct an isotopologues-specific kinetic analysis of the exchange processes and explore the underlying assumptions and validity of the three-isotope method. We demonstrate that without knowing the detailed exchange kinetics a priori the three-isotope method cannot lead to a reliable α eq . For a two-reservoir exchanging system, α determined by this method may be α eq , kinetic isotope effect, or apparent kinetic isotope effect, which can all bear different values. When multiple reservoirs exist during exchange, the evolving trajectory can be complex and hard to predict. Instead of being a tool for α eq determination, three-isotope method should be used as a tool for studying kinetic isotope effect, apparent kinetic isotope effect , and detailed exchange kinetics in diverse systems. [ABSTRACT FROM AUTHOR]

The element oxygen has three stable isotopes: 16O, 17O, and 18O. For a defined process, a change in 18O/16O scales with the corresponding change in 17O/16O, or the fractionation factors 18α and 17α have a relationship of θ = ln17α/ln18α, in which the triple oxygen isotope exponent θ is relatively fixed but does vary with reaction path, temperature, and species involved. When the small variation is of interest, the distinction of three concepts-θ, S (a slope through data points in δ17O-δ18O space), and C (an arbitrary referencing number for the degree of 17O deviation)-becomes important. Triple oxygen isotope variations can be measured by modern instruments and thus offer an additional line of information on the underlying reaction processes and conditions. Analytical methods and Earth science applications have recently been developed for air oxygen, carbon dioxide, water, silicates, oxides, sulfates, carbonates, and phosphates. [ABSTRACT FROM AUTHOR]

Small differences in triple isotope relationships, or Δ 17 O in the case of oxygen, have been increasingly used to study a range of problems including hydrological cycles, stratosphere-troposphere exchange, biogeochemical pathways and fluxes, and the Moon’s origin in the geochemical and cosmochemical communities. A Δ 17 O value depends on the triple isotope exponent θ of involved reaction steps. However, the probabilistic distribution of the intrinsic and apparent θ values has not been examined for elemental processes and for processes that are out of equilibrium or bearing reservoir-transport complexities. A lack of knowledge on the confines of θ may hamper our understanding of the subtle differences among mass-dependent processes and may result in mischaracterization of a set of mass-dependent processes as being in violation of mass-dependent rules. Here we advocate a reductionist approach and explore θ confines starting from kinetic isotope effects ( KIE s) within the framework of transition state theory (TST). The advantage of our KIE approach is that any elemental or composite, equilibrium or non-equilibrium process can be reduced to a set of KIE s with corresponding θ KIE . We establish that the KIE between a reactant and a transition state (TS) is intrinsic. Given a range of KIE s known for Earth processes involving oxygen, we use a Monte Carlo calculation method and a range of oxygen-bonded molecular masses to obtain a distribution of θ KIE values and subsequently that of θ eq . Next, complexities are examined by looking into expected effects due to reaction progress, unbalanced fluxes, and reference frame. Finally, compounded reservoir-transport effects are examined using two simple processes – Rayleigh Distillation (RD) and Fractional Distillation (FD). Our results show that the apparent θ values between two species or two states of the same evolving species have much broader confines than the commonly used “canonical” confines of 0.51–0.53, particularly when the overall fractionation factors are close to 1.000. Equilibrium processes exhibit the narrowest α-θ value distribution. More complex processes or non-equilibrium further broaden the confines of the apparent θ values for reaction systems. The compounded reservoir-transport effects of RD and FD demonstrate that non-canonical apparent θ and large non-zero Δ 17 O values are achievable even when all involved elemental steps are strictly mass-dependent. This study calls for a research effort to determine KIE and θ KIE for important natural processes, and for cautions in interpreting a slope value drawn in a δ′ 17 O–δ′ 18 O space, as the slope is only a superficial manifestation of a set of complex reaction pathways and dynamics. [ABSTRACT FROM AUTHOR]

Ceria (CeO 2 ) is a heavily studied material in catalytic chemistry for use as an oxygen storage medium, oxygen partial pressure regulator, fuel additive, and for the production of syngas, among other applications. Ceria powders are readily reduced and lose structural oxygen when subjected to low pO 2 and/or high temperature conditions. Such dis-stoichiometric ceria can then re-oxidize under higher pO 2 and/or lower temperature by incorporating new oxygen into the previously formed oxygen site vacancies. Despite extensive studies on ceria, the mechanisms for oxygen adsorption–desorption, dissociation–association, and diffusion of oxygen species on ceria surface and within the crystal structure are not well known. We predict that a large kinetic oxygen isotope effect should accompany the release and incorporation of ceria oxygen. As the first attempt to determine the existence and the degree of the isotope effect, this study focuses on a set of simple room-temperature re-oxidation experiments that are also relevant to a laboratory procedure using ceria to measure the triple oxygen isotope composition of CO 2 . Triple-oxygen-isotope labeled ceria powders are heated at 700 °C and cooled under vacuum prior to exposure to air. By combining results from independent experimental sets with different initial oxygen isotope labels and using a combined mass-balance and triangulation approach, we have determined the isotope fractionation factors for both high temperature reduction in vacuum (⩽10 −4 mbar) and room temperature re-oxidation in air. Results indicate that there is a 1.5‰ ± 0.8‰ increase in the δ 18 O value of ceria after being heated in vacuum at 700 °C for 1 h. When the vacuum is broken at room temperature, the previously heated ceria incorporates 3–19% of its final structural oxygen from air, with a δ 18 O value of 2.1 - 4.1 + 7.7 ‰ for the incorporated oxygen. The substantial incorporation of oxygen from air supports that oxygen mobility is high in vacancy-rich ceria during re-oxidation at room temperature. The quantified oxygen isotope fractionation factors are consistent with the direct involvement of O 2 in the rate limiting step for ceria reoxidation in air at room temperature. While additional parameters may reduce some of the uncertainties in our approach, this study demonstrates that isotope effects can be an encouraging tool for studying oxygen transport kinetics in ceria and other oxides. In addition, our finding warns of the special cares and limits in using ceria as an exchange medium for laboratory triple oxygen isotope analysis of CO 2 or other oxygen-bearing gases. [ABSTRACT FROM AUTHOR]

The stable isotope composition of O 2 , O 3 , and H 2 O in the geological past conveys rich information on history of the Earth system. However, few compounds are known to record O 2 or O 3 isotope signals directly and reliably. Sulfate (SO 4 2 − ), a non-labile oxyanion capable of forming weakly soluble minerals, plays important roles in global sulfur, carbon, and oxygen cycles. Here I review publications on the triple oxygen isotope composition of sulfate which has been recently expanded, demonstrating that in addition to H 2 O isotope signals, sulfate can carry isotope signatures from O 2 and O 3 . I argue that sulfate, to this point, is the only compound from which direct atmospheric O 2 and O 3 signals from the distant past can be retrieved. If the current understanding of the Earth's surface oxygenation history holds, we expect to observe little to no measurable deviation from a “normal” triple oxygen isotope composition for sulfate throughout the Archean, but both positive and negative deviations since the early Proterozoic. The full potential of this unique proxy can be achieved by further study on isotope kinetics of sulfur redox cycling and by filling gaps in geological records. [ABSTRACT FROM AUTHOR]

The isotope composition of seawater sulfate is an important tracer of sulfur, carbon, and oxygen cycles in Earth's deep past. Carbonate- associated sulfate (CAS) extracted by acid digestion is widely used as a proxy for sulfate in paleo-seawater from which the carbonate minerals precipitated. Early and late diagenesis, weathering, and laboratory processing can in some cases compromise original seawater sulfate signals. Here, we report that extracted CAS can also be severely contaminated by recent atmospheric sulfate, especially when the sampled carbonates are from outcrops in arid to semi-arid climates or in heavily polluted regions. Our evidence comes from triple oxygen isotope compositions of sequentially extracted water-leachable sulfate and acid-leachable sulfate from carbonates of diverse ages from northwestern and north-central China and southwestern North America. Independent of the age of the rocks, almost all the waterleachable sulfates and half of the acid-leachable sulfates bear positive 17O anomalies, clearly distinguishable from those of typical seawater sulfate. Because secondary atmospheric sulfate (SAS) is the only source of sulfate known to bear positive 17O anomalies, we conclude that sulfate extracted from carbonate outcrops in these regions has a significant component of SAS. Because SAS generally has a much lower δ34S value than paleo-seawater sulfate, it could shift the δ34S of the extracted CAS to lower values and in some cases even lower than that of the co-occurring pyrite, i.e., the "super-heavy pyrite" enigma reported in geological records. Our findings call for a re-evaluation of many published, outcrop-based CAS data and conclusions. [ABSTRACT FROM AUTHOR]

Abstract: Sulfate aerosol is an important atmosphere constituent that can be formed secondarily through the oxidation of sulfur gases. Atmospheric sulfur oxidation can take different pathways depending on meteorological conditions, which affects sulfate aerosol size and composition and therefore local or global climate. The magnitude of 17O enrichment (Δ17O) in secondary atmospheric sulfate (SAS) is a tracer for the apportionment of different sulfur oxidation pathways. Atmospheric chemistry-transport models predict a low 17O enrichment (Δ17O < 1‰) for SAS in mid-latitude continental sites. However, there are few long-term site observations to test the prediction, and data from interior metropolitan sites are entirely absent. We report here multiple oxygen and sulfur isotope compositions (Δ17O, δ18O, and δ34S) of SAS collected over a 950-day period in the city of Wuhan, central China, and to compare to data from a similar sampling campaign in the city of Baton Rouge, LA, U.S.A. The isotope compositions of bulk atmospheric sulfate closely reflect those of SAS in Wuhan, with the Δ17O ranging from 0.14‰ to 1.02‰, the δ18O from 8.0‰ to 16.1‰, and the δ34S from 2.1‰ to 7.3‰. The average Δ17O value at 0.53‰–0.59‰ is consistent with model prediction for continental interior, mid-latitude sites. The Asian monsoon-influenced meteorological condition in Wuhan appears to produce a weak but discernible seasonal pattern for Δ17O and δ18O of the SAS. The average rainwater pH value is higher in Wuhan than in Baton Rouge (5.47 versus 4.78) while the two cities have a statistically identical average SAS Δ17O value. We suggest that the higher pH does result in a higher fraction of SAS generated by aqueous O3 oxidation, but the resulted higher Δ17O value for SAS is diluted by the 17O-normal SAS generated from an enhanced transition-metal-catalyzed O2 oxidation pathway. The enhancement is corroborated with the much higher content of atmospheric particulate matter especially mineral dusts in Wuhan, a point to be considered by future effort to quantify the climate impact of SO2. [ABSTRACT FROM AUTHOR]

Abstract: We have recently reported non-mass-dependent 17O depletion in sulfate deposited after the Marinoan glacial meltdown at ca. 635 million years ago. Further investigation linking the Δ17O of barite to its sedimentological–geological context in Marinoan South China reveals that the 17O depletion in sulfate is most pronounced at sites near paleo-continents, supporting the hypothesis that atmospheric O2 was the source of the depletion which was transferred to sulfate via oxidative weathering of sulfides. Host minerals or rocks for the Marinoan 17O depletion have been limited to barite (southern China and western Africa) and carbonate-associated sulfate (CAS) in limestone lenses within a diamictite (Svalbard). If the Marinoan 17O depletion event is indeed related to an extraordinary atmospheric condition, the signal should be global in its distribution. The Kimberley region of Western Australia was close to a continent in the late Neoproterozoic and will serve as a test of our hypothesis that the anomalous 17O depletion may be widely recorded in the rock records of this time period. We report here that the CAS in the Moonlight Valley (MV) cap dolostones, Texas/Mabel Downs (TMD), the eastern Kimberley region, Western Australia is anomalously depleted in 17O (Δ17O value as low as −0.68‰). The magnitude of the anomaly decreases gradually toward the overlying Ranford mud-silt-sandstones. This finding not only expands the geographic distribution of the depleted 17O signal, but also the type of host rocks or minerals that the anomalous sulfate resides. The CAS in the MV cap dolostones in Palm Spring (PS) section, ∼150km south of TMD, however, does not bear a 17O depletion. Neither does the CAS in the Egan cap dolostones. The presence and absence of pronounced 17O anomalies in the two time-equivalent yet spatially different MV cap dolostones are consistent with the paleogeography that indicates TMD was close to the continent while PS was at an open-ocean environment. While sharing some of the same sedimentological features with that of the MV cap dolostones at TMD, the Egan cap has distinct δ13C and δ18O values for dolostones and distinct Δ17O, δ34S and δ18O values for CAS, supporting an earlier assignment of the Egan glaciation to be younger than Marinoan in the late Neoproterozoic. [Copyright &y& Elsevier]

Abstract: Distinct, non-mass-dependent 17O depletion was reported in barite from Marinoan cap dolostones (~635Ma) in South China, and has been interpreted as an indication of an extremely high-pCO2 atmosphere (Bao et al., 2008). There is a significant variation in the magnitude of the 17O anomaly even among samples from the same outcrops. Understanding the origins of this barite and particularly the source of sulfate is critical to interpreting the anomalous 17O signature and its implication on sulfur and oxygen cycles at the aftermath of Marinoan glacial meltdown. In this study, we analyzed 160 micro-sampled barite samples from 20 hand specimens collected from two cap dolostone sections in South China. Together with field, petrographic, and Sr isotope data, the expanded dataset 1) confirms the large variability in ∆17O (from −0.09‰ to −0.87‰, VSMOW), δ18O (+13.3‰ to +23.5‰, VSMOW), and δ34S (+20.0‰ to +45.5‰, VCDT) of barite; 2) demonstrates a hyperbolic relationship between the ∆17O and δ34S of various barite samples with more negative Δ17O values generally corresponding to lower δ34S values; 3) reveals that individual barite crystal fans and fans of the same layer possess a well-clustered set of δ18O, Δ17O, and δ34S values; and 4) shows, correspondingly, that barite crystal fans of different layers bear different sets of Δ17O, δ18O, and δ34S values. The study suggests that 17O-depleted barite crystals were formed under supersaturation when Ba2+ from sulfate-free deepwater came to mix with sulfate-bearing shallow water. There were multiple upwelling episodes which resulted in multiple barite layers. The large variability in sulfur and triple-oxygen isotope composition and the high 87Sr/86Sr ratios indicate that the two sites from South China were sufficiently close to the continent so that the isotopic composition of sulfate was easily influenced by changes in riverine flux when seawater sulfate concentration at that time was exceedingly low. Our depositional model for the barite is consistent with the proposed sequence of events after the meltdown of Marinoan glaciation in South China by Zhou et al. (2010), and is also consistent with the proposed atmospheric origin of sulfate 17O depletion by Bao et al. (2008). [Copyright &y& Elsevier]

Abstract: Aqueous oxidation of sulfide minerals to sulfate is an integral part of the global sulfur and oxygen cycles. The current model for pyrite oxidation emphasizes the role of Fe2+–Fe3+ electron shuttling and repeated nucleophilic attack by water molecules on sulfur. Previous δ18O-labeled experiments show that a variable fraction (0–60%) of the oxygen in product sulfate is derived from dissolved O2, the other potential oxidant. This indicates that nucleophilic attack cannot continue all the way to sulfate and that a sulfoxyanion of intermediate oxidation state is released into solution. The observed variability in O2% may be due to the presence of competing oxidation pathways, variable experimental conditions (e.g. abiotic, biotic, or changing pH value), or uncertainties related to the multiple experiments needed to effectively use the δ18O label to differentiate sulfate–oxygen sources. To examine the role of O2 and Fe3+ in determining the final incorporation of O2 oxygen in sulfate produced during pyrite oxidation, we designed a set of aerated, abiotic, pH-buffered (pH=2, 7, 9, 10, and 11), and triple-oxygen-isotope labeled solutions with and without Fe3+ addition. While abiotic and pH-buffered conditions help to eliminate variables, triple oxygen isotope labeling and Fe3+ addition help to determine the oxygen sources in sulfate and examine the role of Fe2+–Fe3+ electron shuttling during sulfide oxidation, respectively. Our results show that sulfate concentration increased linearly with time and the maximum concentration was achieved at pH 11. At pH 2, 7, and 9, sulfate production was slow but increased by 4× with the addition of Fe3+. Significant amounts of sulfite and thiosulfate were detected in pH⩾9 reactors, while concentrations were low or undetectable at pH 2 and 7. The triple oxygen isotope data show that at pH⩾9, product sulfate contained 21–24% air O2 signal, similar to pH 2 with Fe3+ addition. Sulfate from the pH 2 reactor without Fe3+ addition and the pH 7 reactors all showed 28–29% O2 signal. While the O2% in final sulfate apparently clusters around 25%, the measurable deviations (>experimental error) from the 25% in many reaction conditions suggest that (1) O2 does get incorporated into intermediate sulfoxyanions (thiosulfate and sulfite) and a fraction survives sulfite–water exchange (e.g. the pH 2 with no Fe3+ addition and both pH 7 reactors); and (2) direct O2 oxidation dominates while Fe3+ shuttling is still competitive in the sulfite–sulfate step (e.g. the pH 9, 10, and 11 and the pH 2 reactor with Fe3+ addition). Overall, the final sulfate–oxygen source ratio is determined by (1) rate competitions between direct O2 incorporation and Fe3+ shuttling during both the formation of sulfite from pyrite and from sulfite to final sulfate, and (2) rate competitions between sulfite and water oxygen exchange and the oxidation of sulfite to sulfate. Our results indicate that thiosulfate or sulfite is the intermediate species released into solution at all investigated pH and point to a set of dynamic and competing fractionation factors and rates, which control the oxygen isotope composition of sulfate derived from pyrite oxidation. [Copyright &y& Elsevier]

Volcanic eruptions release a large amount of sulphur dioxide (SO2) into the atmosphere. SO2 is oxidized to sulphate and can subsequently form sulphate aerosol, which can affect the Earth's radiation balance, biologic productivity and high-altitude ozone concentrations, as is evident from recent volcanic eruptions. SO2 oxidation can occur via several different pathways that depend on its flux and the atmospheric conditions. An investigation into how SO2 is oxidized to sulphate—the oxidation product preserved in the rock record—can therefore shed light on past volcanic eruptions and atmospheric conditions. Here we use sulphur and triple oxygen isotope measurements of atmospheric sulphate extracted from tuffaceous deposits to investigate the specific oxidation pathways from which the sulphate was formed. We find that seven eruption-related sulphate aerosol deposition events have occurred during the mid-Cenozoic era (34 to 7 million years ago) in the northern High Plains, North America. Two extensively sampled ash beds display a similar sulphate mixing pattern that has two distinct atmospheric secondary sulphates. A three-dimensional atmospheric sulphur chemistry and transport model study reveals that the observed, isotopically discrete sulphates in sediments can be produced only in initially alkaline cloudwater that favours an ozone-dominated SO2 oxidation pathway in the troposphere. Our finding suggests that, in contrast to the weakly acidic conditions today, cloudwater in the northern High Plains may frequently have been alkaline during the mid-Cenozoic era. We propose that atmospheric secondary sulphate preserved in continental deposits represents an unexploited geological archive for atmospheric SO2 oxidation chemistry linked to volcanism and atmospheric conditions in the past. [ABSTRACT FROM AUTHOR]

Abstract: Acritarchs from the Chuanlinggou Formation (ca. 1700Ma) were first reported by Xing and Liu (1973) [Xing, Y.S., Liu, G.Z., 1973. On Sinian micro-flora in Yenliao Region of China and its geological significance. Acta Geological Sinica 1, 1–64] and some were later described as eukaryotes by Yan (1982) [Yan, Y.Z., 1982. Schizofusa from the Chuanlinggou Fromation of Changzhougou System in Jixian county. Bulletin of the Tianjin Institute of Geology and Mineral Resources 6, 1–7]. The eukaryotic affinity has been questionable, however, for lack of indisputable morphological evidence. Chuanlinggou acritarchs are some of the oldest acritarchs found on Earth and are well preserved for ultrastructure study. In this study, Chuanlinggou acritarchs were examined by jointly using optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). In most cases, an ovoidal group of acritarchs represents a half vesicle following a complete longitudinal rupture, which is a morphological model different from a whole envelope with medial splits as proposed by earlier studies. This ovoidal group displays a bipolar morphology, longitudinal rupture, and occasionally striated wall structures that are consistent with a eukaryotic affinity. Thus, the Chuanlinggou ovoidal acritarchs probably extends the eukaryotes body fossil record into the Paleoproterozoic, ∼200 million years earlier than the morphologically more complex acritarchs from Mesoproterozoic Roper Group (ca. 1500Ma). [Copyright &y& Elsevier]

The oxygen isotope composition of terrestrial sulfate is affected measurably by many Earth-sudace processes. During the Neoproterozoic, severe "snowball" gladations would have had an extreme impact on the biosphere and the atmosphere. Here, we report that sulfate extracted from carbonate lenses within a Neoproterozoic gladal diamictite suite from Svalhard, with an age of ~635 million years ago, falls well outside the currently known natural range of triple oxygen isotope compositions and indicates that the atmosphere had either an exceptionally high atmospheric carbon dioxide concentration or an utterly unfamiliar oxygen cycle during deposition of the diamictites. [ABSTRACT FROM AUTHOR]

Abstract: The discovery of 33S anomalies in Archean sedimentary rocks has established that the early Earth before ∼2.2Ga (billion years ago) had a very different sulfur cycle than today. The origin of the anomalies and the nature of early sulfur cycle are, however, poorly known and debated. In this study, we analyzed the total sulfur and oxygen isotope compositions, the δ18O, Δ17O, δ34S, Δ33S, and Δ36S, for the >3.2Ga Fig Tree barite deposits from the Barberton Greenstone Belt, South Africa. The goal is to address two questions: (1) was Archean barite sulfate a mixture of 33S-anomalous sulfate of photolysis origin and 33S-normal sulfate of other origins? (2) did the underlying photochemical reactions that generated the observed 33S anomalies for sulfide and sulfate also generate 17O anomalies for sulfate? We developed a new method in which pure barite sulfate is extracted for oxygen and sulfur isotope measurements from a mixture of barite sands, cherts, and other oxygen-bearing silicates. The isotope data reveal that (1) there is no distinct 17O anomaly for Fig Tree barite, with an average Δ17O value the same as that of the bulk Earth (−0.02±0.07‰, N =49); and (2) the average δ18O value is +10.6±1.1‰, close to that of the modern seawater sulfate value (+9.3‰). Evidence from petrography and from the δ18O of barites and co-existing cherts suggest minimum overprinting of later metamorphism on the sulfate’s oxygen isotope composition. Assuming no other processes (e.g., biological) independently induced oxygen isotope exchange between sulfate and water, the lack of reasonable correlation between the δ18O and Δ33S or between the δ34S and Δ33S suggests two mutually exclusive scenarios: (1) An overwhelming majority of the sulfate in the Archean ocean was of photolysis origin, or (2) The early Archean sulfate was a mixture of 33S-normal sulfates and a small portion (<5%?) of 33S-anomalous sulfate of photolysis origin from the atmosphere. Scenario 1 requires that sulfate of photolysis origin must have had only small 33S or 36S anomalies and no 17O anomaly. Scenario 2 requires that the photolysis sulfate have had highly negative δ34S and Δ33S values, recommending future theoretical and experimental work to look into photochemical processes that generate sulfate in Quadrant I and sulfide in Quadrant III in a δ34S (X)–Δ33S (Y) Cartesian plane. A total sulfur and oxygen isotope analysis has provided constraints on the underlying chemical reactions that produced the observed sulfate isotope signature as well as the accompanying atmospheric, oceanic, and biological conditions. [Copyright &y& Elsevier]

Abstract: Secondary atmospheric sulfates (SAS) is the ultimate oxidation product and sink for sulfur gases of biological, volcanic, and anthropogenic origins on Earth. Their presence in the atmosphere as aqueous or solid phases contributes to acid rain and climate change, thus, understanding SAS formation pathways is pertinent. There has been extensive measurement of δ 34S values for SAS, which mainly aimed at source identification. Relatively fewer oxygen isotope compositions (δ 18O, Δ17O), which are most useful for resolving competing oxidation pathways, were available, however. This study represents the first effort to characterize the Δ17O, δ 18O, and δ 34S simultaneously for SAS in a tropospheric air shed. We measured a total of 20 samples collected in Baton Rouge (LA, USA) during a 600-day period. The isotope compositions for atmospheric sulfate range from +0.25‰ to +1.43‰ for Δ17O, +11.8‰ to +19.3‰ for δ 18O, and −1.4‰ to +3.8‰ for δ 34S. No apparent correlation is found among Δ17O, δ 18O, or δ 34S values. The Δ17O has no seasonal variation and its values are consistent with an oxidation pathway dominated by aqueous H2O2. The δ 18O and δ 34S are within the range of those observed in other sites around the world and are not characteristic for Baton Rouge. Despite the huge variability in atmospheric condition among mid-latitude sites, the long-term average Δ17O value for SAS appears to fall within a fairly narrow range from +0.6‰ to +0.8‰, which is ∼1‰ to 2‰ lower than those in polar sites. [Copyright &y& Elsevier]

Abstract: A large amount of calcite pseudomorphs after gypsum were discovered in a volcanic ash bed and its underlying sand/siltstone in the Gering Formation (late Oligocene), northwestern NE, USA. The highly positive 17O anomalies in the residual sulfate indicate that the sulfate could only have come from the oxidation of sulfur gases by ozone in the atmosphere. We previously proposed that it was most likely the result of an extreme tropospheric sulfate haze (dry fog) event that was associated with volcanic eruptions in the west. To test the dry fog hypothesis, we examine here two alternative scenarios that also involve atmospheric sulfate deposition but do not imply a dramatic volcanic and dry fog source: (1) is it possible that some playas in arid or semiarid regions accumulate the type of atmospheric sulfate deposits as we see in the mid-Gering bed by the inflow of surface water or groundwater? (2) Could a period of hyperaridity and stability during the late Oligocene have caused a significant accumulation of atmospheric background sulfate onto the mid-Gering ash bed surface? Sulfates from a variety of playa settings were collected and measured for their 17O anomalies. The sulfate isotope (both O and S) and concentration profiles of two volcanic ash beds that are currently exposed in the central Atacama Desert (Chile) and in Taylor Valley of the Antarctica Dry Valleys were studied along with their sedimentary features. These two ash beds were chosen for comparison because of their significant levels of atmospheric sulfate accumulated during prolonged and hyperarid climatic conditions. Results showed no or minimal 17O anomalies in any of the playa sulfate samples from around the world, and none of the parameters in the mid-Gering ash profile matches those in either of the two volcanic ash profiles in modern hyperarid settings. Conclusion: The observed 17O-anomalous gypsum deposit in the mid-Gering ash bed is most likely the result of a series of extreme dry fogs that resulted in an intense pulse of atmospheric sulfate deposition to a semiarid playa setting. [Copyright &y& Elsevier]

Sulfate is one of the major salts accumulated in the hyperarid Atacama Desert, Chile. Its origin has been attributed to local weathering, volcanic source, sea salt, and atmosphere deposition. Previous studies concluded that atmospheric sulfate is present in the desert, but this term was poorly defined in terms of chemical origin, and its contribution to the total sulfate budget in soil is debated. The contribution of each potential sulfate source is expected to vary geographically due to different source locations. Efforts to quantify the contribution from each source have been hampered by the lack of well-defined isotopic compositions for its end-members. In this study, we discard the vague term of “atmospheric sulfate” by recognizing two major sulfate sources on the basis of their formation pathways: primary and secondary sulfate. Primary sulfate, including those from rock weathering, sea salt, and volcanic ash leachates, do not have an 17O anomaly. Secondary sulfate, derived from the oxidation of reduced sulfur gases (biological or volcanic emissions) by atmospheric ozone or hydrogen peroxide, however, has an 17O anomaly. We developed a method to extract water-soluble anions sequentially or entirely from bulk soil samples, and analyzed the sulfate concentrations and 17O anomalies for four geographically representative soil profiles in the central Atacama Desert (23°S to 25.5°S).We found that sequential sulfate extractions on the same soil sample produce sulfate with decreasing Δ17O values. Except for a positive bulge at 20 to 50 cm in depth, there is a general increase in the Δ17O value for total sulfate with soil depth for all the four soil profiles that we examined. These findings are consistent with the hypothesis that secondary sulfates (17O-anomalous sulfates) are more soluble and thus preferentially leached in liquid water than primary sulfate. The finding also warns that incomplete sulfate extraction from sulfate-rich soils yields unpredictable results. We infer a Δ17O value of ∼4.6‰ for secondary sulfate based on data from partial sulfate extraction experiments. Total contribution of secondary sulfate in the central Atacama Desert is estimated to range from 9% to 24%, with the highest percentage in the Central Depression. [Copyright &y& Elsevier]

The Neoproterozoic Quruqtagh Group in eastern Chinese Tianshan consists of, in ascending order, the Bayisi, Zhaobishan, Altungol, Tereeken, Zhamoketi, Yukkengol, Shuiquan, and Hankalchough Formations. Diamictite units occur in the Bayisi–Altungol, Tereeken, and Hankalchough Formations. Our field observations confirm the glacial origin of the Tereeken and Hankalchough diamictites, but the glacial origin of diamictites in the Bayisi and Altungol Formations remain ambiguous. While siliciclastic strata dominate the succession, carbonate units are recognized between the diamictites and immediately atop the Tereeken and Hankalchough diamictites. Carbonates in the Altungol Formation are characterized by highly positive δ13C values (up to +10.4‰ versus PDB), comparable to those of pre-Marinoan but post-Sturtian carbonates. High-resolution sampling of the pinkish cap dolostone (ca. 10 m-thick) overlying the Tereeken diamictite shows a consistent δ13C chemostratigraphic pattern (ca. −5‰ at the base and slightly decreasing upsection) similar to that of Marinoan cap carbonates. This cap dolostone is also characterized by macropeloids similar to those in the Marinoan-age Raventhroat cap carbonate, Mackenzie Mountains. Chemostratigraphic and sedimentary features are suggestive of a Marinoan age for the Tereeken glaciation. Thus, the younger Hankalchough diamictite may represent a post-Marinoan glaciation. This interpretation is consistent with the occurrence of vendotaenid fossils in the Shuiquan Formation. The alternative interpretation—the Tereeken and Hankalchough diamictites are, respectively, Sturtian and Marinoan in age—is not consistent with the above evidence or the chemostratigraphic features of the Shuiquan Formation and the Hankalchough cap carbonate. A post-Marinoan age for the Hankalchough and other glaciogenic deposits indicates that the disappearance of Doushantuo-type acritarchs and the subsequent radiation of macroscopic animals are closely linked, in geochronological terms, with a Neoproterozoic glaciation. [Copyright &y& Elsevier]

Reports measurements of delta [sup18]O and delta [sup17]O values of sulphates from a range of dry-valley soils. Suggestion that Antarctic sulphate comes not just from sea salt but also from the atmospheric oxidation of reduced gaseous sulphur compounds; Possibility that the distributions of sulphate are largely explained by differences in particle size and transport mode which exist between sea-salt aerosols and aerosols formed from biogenic sulphur emission.

Rationale: The Δ36S standard deviation measured in a conventional isotope ratio mass spectrometer such as MAT 253 is ca 0.1‰ to 0.3‰. At this precision, it is difficult to resolve the origin of non‐mass‐dependent sulfur isotope fractionation in tropospheric sulfate aerosol and in Martian meteorites or small deviations from the canonical mass‐dependent fractionation laws. Interfering ions with m/z at 131 of 36SF5+ are suggested by the community as the cause of the poor precision, but the exact ion species has not been identified or confirmed. Methods: Here we examined the potential interfering ions by using a Thermo Scientific ultrahigh‐resolution isotope ratio mass spectrometer to measure SF6 working gas and SF6 gases converted from IAEA‐S1/2/3 Ag2S reference materials. Results: We found that there are two resolvable peaks to the right of the 36SF5+ peak when a new filament was installed, which are 186WF42+ followed by 12C3F5+. However, only the 12C3F5+ interference peak was observed after more than three days of filament use. 12C3F5+ is generated inside the instrument during the ionization process. Avoiding the interfering signals, we were able to achieve a Δ36S standard deviation of 0.046‰ (n = 8) for SF6 zero‐enrichment and 0.069‰ (n = 8) for overall measurement start from silver sulfide IAEA‐S1. Conclusions: Aging the filament with SF6 gas can avoid the interference of 186WF42+. Minimizing the presence of carbon‐bearing compounds and avoiding the interfering signals of 12C3F5+ from 36SF5+, we can improve Δ36S measurement accuracy and precision, which helps to open new territories for research using quadruple sulfur isotope composition. [ABSTRACT FROM AUTHOR]

Rationale: When isotope composition is measured in dual‐inlet mode with an isotope ratio mass spectrometer (IRMS), reference gas may be gradually enriched in heavy isotopes due to preferential loss of light ones from the bellows over time. Quantifying the degree of isotopic enrichment of the reference gas is imperative for high‐precision isotopic analysis (i.e. at per meg level). Methods: O2 and CO2 leaking experiments were performed with the dual‐inlet system of an IRMS (Thermo Fisher® MAT 253 Plus). During each experiment, the drop of gas pressure in the bellows with time was recorded and isotope ratios of residual gas at various time intervals were analyzed. Results: Isotopic enrichment of residual O2 gas could be as large as 1‰ for δ18O when a large fraction (>75%) of initial gas was lost. The evolution of isotope compositions of the remaining gas can be well described by a pressure‐dependent Rayleigh fractionation equation. When the pressure in the bellows is within 10–50 mBar, the isotope fractionation factor (α18O) for O2 leaking ranges from 0.99911 to 0.99982 and the characteristic relationship of α17O and α18O is from 0.5123 to 0.5124. Conclusions: Isotope fractionation associated with capillary leaking from bellows is pressure‐dependent. We recommend that the reference gas should be reloaded frequently, especially after a measurement with a low analyzing pressure for the analysis of small amounts of sample. [ABSTRACT FROM AUTHOR]

Provides information on a study which explored the atmospheric processes in the use of sulfur isotopes to study onset and consequences of microbial fractionation processes in Earth's early history. Examination of sulfur isotope variability; Importance of photochemical reactions in the early Proterozoic and Archean atmosphere; Description of the basic models for Earth's early sulfur cycle.

Presents a study of the oxygen-isotope values of two massive sulphate mineral deposits which show large isotopic anomalies despite being formed in surface environments. Origination of deposits in the central Namib desert and the sulphate-bearing Miocene volcanic ash-beds in North America; Questions regarding the source of the oxygen isotope; Possibility of the chemical origin of variable isotope anomalies on planets other than Earth.

Frequently occurred severe haze events in heavily populated China in recent years have beenattributed to extensive industrial and residential emissions, together with enhancedsecondary aerosol formation under stagnant meteorological condition, especially inwinter. Sulfate, as the major inorganic component of fine particles (PM2.5), has beenfound to form rapidly under high relative humidity conditions. Currently, a largegap exists in model-estimated sulfate concentration and field observation, withthe latter often being magnitudes higher. While model simulation is improving,multiple oxygen and sulfur isotopic compositions of atmospheric sulfate offer anindependent, often specific constraint on atmospheric secondary sulfate formationpathways. Here we report inorganic components, together with sulfur (δ34S) and triple-oxygenisotopes (δ18O, Δ17O) of sulfate in PM2.5 samples collected every 12 hours in the city ofBeijing, northern China in winter 2015. During the sampling period, PM2.5 concentrationwent up to as high as 178.9 ± 154.9 μg m−3. Sulfate had an average concentration of 20.9 ±20.9 μg m−3, with a mean δ34S values of 5.5±2.4‰ ranging from 1.3‰ to 9.4‰Ṫhe δ34Sis higher during heavily polluted days (average at 6.8‰) than that during relatively cleandays (average at 2.0‰). Meanwhile, the average sulfate Δ17O (Δ17O = δ18O -0.5305×δ17O) during heavy haze episodes was 0.62 ± 0.33‰ suggesting that aqueousoxidation pathway of S(IV)–H2O2 played a significant role on the enhanced sulfateformation during the evolution of haze episodes associated with fog processing. [ABSTRACT FROM AUTHOR]

Triple oxygen isotope (Δ′17O) analysis of CO 2 or carbonate is increasingly applied in diverse fields in earth and environmental sciences. One of the widely-used analytical methods for Δ′17O of CO 2 is through Pt-catalyzed oxygen isotope exchange between CO 2 and O 2. However, large inter-laboratory inconsistencies in oxygen isotope fractionation between the two gases have been reported, potentially hindering data comparison across different laboratories. By examining the oxygen isotope fractionations in thermal diffusion experiments for pure O 2 and CO 2 gases as well as CO 2 -O 2 mixtures, we demonstrate that thermal-gradient-induced (TG-induced) isotope fractionation is the main reason for the inconsistencies. The degree of TG-induced oxygen isotope fractionation is affected by temperature contrasts and volume ratios between the hot and cold parts of the extraction line as well as gas composition. Larger thermal contrasts generated larger oxygen isotope fractionations. CO 2 fractionated less than O 2 in the same thermal gradient conditions. For CO 2 -O 2 mixtures, TG-induced oxygen isotope fractionation of O 2 was reduced while that of CO 2 was enhanced, compared with the pure gases. Based on the results of thermal diffusion experiments, we found that the anomalous mass fractionation exponents (0.2 ∼ 0.9) observed in the Pt-catalyzed CO 2 -O 2 isotope exchange experiments can be rationalized quantitatively by a combination of isotopic exchange equilibrium and TG-induced oxygen isotope fractionation. Our model suggests that by weakening TG-induced oxygen isotope fractionation and by increasing the volume fraction of hot zone of a reactor, we can achieve a high precision (<10 per meg) for Δ′17O CO2 through CO 2 -O 2 exchange. Considering the widely existing thermal-gradient conditions in nature, the findings in this study may shed light on the origin of non-mass-dependent isotope fractionation. [ABSTRACT FROM AUTHOR]

Red mud, a waste residue of aluminium industry, was used as modified asphalt material to prepare red mud modified asphalt and red mud modified asphalt under freeze-thaw cycles. The matrix asphalt (MA), red mud modified asphalt (RMMA), and red mud modified asphalt under freeze-thaw cycles (RMMAFC) were studied by scanning electron microscopy (SEM), Fourier transform infrared spectrometry (FTIR), atomic force microscopy (AFM), and differential scanning calorimetry (DSC). Microscopic experiments were conducted to investigate the modification performance and mechanism. The modification mechanism of red mud modified asphalt was investigated using molecular dynamics simulation in this study. The results show that red mud can form a uniform and stable blending system with base asphalt after adding base asphalt. The structure of asphalt after adding red mud and adding red mud and freezing-thawing cycles does not change. The bee-structure decreases obviously with the addition of red mud by atomic force microscopy (AFM). Density decreases gradually, but bee-structure height increases obviously; bee-structure of red mud modified asphalt is destroyed after freeze-thaw cycles. Through differential scanning calorimetry (DSC), after adding red mud, heat absorption decreases. Freeze-thaw cycles greatly reduce heat absorption of red mud modified asphalt. Constructing molecular model of major components of red mud (Fe2O3, Al2O3) and asphaltene, simulation results show that the interfacial energy between asphaltene and red mud's main components Fe2O3 and Al2O3 at −10°C, 25°C, and 170°C is stronger than that of Fe2O3. The results of calculating the interfacial energy of asphaltene on the chemical composition surface of red mud are negative. It can be seen that there are adsorption effects on the surface of asphaltene and red mud. Therefore, increasing the content of Al2O3 or decreasing the content of Fe2O3 in red mud is beneficial to the adsorption of asphaltene. [ABSTRACT FROM AUTHOR]

The estimated contribution of "primary" combustion sulfate to the total atmospheric sulfate flux is inconsistent, hindering our interpretation of the atmospheric "missing sulfate". We expect combustion sulfate to inherit the characteristic triple oxygen isotope composition of air O 2 to variable degrees. To test the hypothesis, we extracted sulfate from soot samples collected from 48 residential chimneys throughout China. The mean sulfate's Δ′17O 0.5305 and δ18O values are −0.27 ± 0.06‰ and 12.8 ± 3.6‰, respectively, and the Δ′17O 0.5305 and δ18O values correlate negatively and extend to air O 2 's triple oxygen isotope composition as an endmember, suggesting that combustion sulfate is generated dominantly by S(IV) multiphase oxidation via O 2 in the plume, with possible additional pathways through •OH in aqueous films or droplets and NO 2 at soot surface. Oxygen sources (i.e., air O 2 , fuel, ambient moisture) and oxidation kinetics determine the triple oxygen isotope composition of combustion sulfate. The characterization of combustion sulfate's triple isotope composition helps to improve global and regional atmospheric sulfur chemistry models. • Triple oxygen isotope composition of combustion sulfate has an average value of −0.27‰ for the Δ′17O and 12.8‰ for the δ18O. • Oxygen sources (O 2 , fuel, water) and oxidation kinetics determine combustion sulfate's triple oxygen isotope composition. • The "primary" combustion sulfate is in fact "secondary" sulfate formed in chimney conduits and hot plumes. [ABSTRACT FROM AUTHOR]

Abstract Sulfate-driven anaerobic oxidation of methane (SD-AOM) plays a critical role in regulating the global methane budget. Determination of the diagnostic triple isotope exponent θ 33 (≡ ln 33 ⁡ α / ln 34 ⁡ α) for SD-AOM can help to identify and quantify microbial sulfate reduction via SD-AOM in the environment. The history of Earth's surface redox conditions can also be examined through the measurement of triple sulfur isotope compositions in sedimentary rocks. Due to difficulties in both culturing anaerobic methanotrophs and sampling pore-water sulfate in SD-AOM-dominated environments, however, the θ 33 values for the processes of SD-AOM have not been constrained. We propose that a set of modern cold-seep associated barite samples with low Δ δ 18 O / Δ δ 34 S values bear a record of residual pore-water sulfate during SD-AOM, and therefore the triple sulfur isotope composition of these barites can be used to deduce θ 33 values. We applied a 1-D diagenetic reaction–transport model to fit Δ 33 S and δ ′ 34 S results from modern cold seep barites collected from five sites in the Gulf of Mexico. Based on revealed negative correlations (R 2 = 0.77) between Δ 33 S and δ ′ 34 S values we calculated an upper-limit θ 33 value of 0.5100 to 0.5112 (±0.0005) given a 1000 ln 34 ⁡ α value of − 30 ‰ to − 10 ‰. This θ 33 value is distinctively lower than that of organoclastic sulfate reduction (OSR) in marine environments where the diagnostic isotope fractionation (1000 ln 34 ⁡ α) is typically more negative than that of SD-AOM. In addition, cold seep barite data display a negative Δ 33 S – δ ′ 34 S correlation whereas pore-water sulfates of all OSR-dominated settings show a positive one. Therefore, the diagnostic triple-sulfur isotope exponent and associated negative Δ 33 S – δ ′ 34 S correlation may allow for the identification of SD-AOM in sedimentary records. Highlights • A negative Δ 33 S – δ 34 S correlation associated with SD-AOM is archived in seep barite. • A diagnostic θ 33 value for SD-AOM ranges from 0.5100 to 0.5112 (±0.0005). • Triple sulfur isotope can be used to identify geological SD-AOM activities. [ABSTRACT FROM AUTHOR]

Abstract We report a petrography, mineral chemistry, oxygen and chromium isotopic study of Grove Mountains (GRV) 020043 together with a subset of other acapulcoites and lodranites. GRV 020043 is a petrologic type 4 chondrite, with chondrules of diverse types and sizes, and is composed of low-Ca pyroxene (40 vol.%), olivine (24 vol.%), diopside (8 vol.%), plagioclase (10 vol.%), Fe-Ni metal (kamacite and taenite), troilite and some accessory minerals (chromite and apatite). The olivine in GRV 020043 has an average fayalite content (Fa) of 10.7 mol.% with the low-Ca pyroxene having an average ferrosilite content (Fs) of 10.8 mol.%. The whole rock oxygen isotopic composition of GRV 020043 is +3.226 ± 0.267‰, +0.797 ± 0.131‰, and −0.927 ± 0.017‰ for δ18O, δ17O, and Δ17O, respectively, with a bulk chromium isotopic compositions of ε54Cr = −0.48 ± 0.10. These characteristics of GRV 020043 are different from all established or ungrouped chondrites but agree with those of the acapulcoite-lodranite clan. We therefore suggest that GRV 020043 represents the chondritic precursor of acapulcoite-lodranite parent body. The similarity of bulk oxygen and chromium isotopic compositions among GRV 020043, Acapulco, Northwest Africa (NWA) 468 (metal-rich lodranite), NWA 8118 (lodranite), NWA 8287 (acapulcoite), and NWA 8422 (lodranite) indicates that they originated from a common oxygen and chromium reservoir in the protoplanetary disk or may have derived from a parent body with a differentiated multilayer structure. [ABSTRACT FROM AUTHOR]

Rationale: The oxygen isotope composition of phosphate (δ18OPO4) is widely employed for reconstructing paleotemperature and tracing biogeochemical phosphorus cycling. However, existing phosphate purification protocols do not work well for igneous rocks and igneous weathering profiles (IWPs). A reliable purification method is needed for measuring δ18OPO4 in these materials. Methods: Our phosphate purification method includes two steps of cation exchange resin treatment separated by a step of calcium phosphate precipitation (R‐Ca‐R method). In addition, the silver phosphate precipitation in our procedure is featured by slow evaporation to crystallization until the appearance of ammonium nitrate or silver nitrate crystals. We evaluated our methods on weathered and pristine igneous rocks, phosphorite rocks, KH2PO4, and (NH4)2HPO4 solutions. Results: Our purification method converted over 99.9% phosphate in solution to calcium phosphate, which can be easily decalcified by cation resin. The improved silver phosphate precipitation method produced high phosphate yields (97.1%–99.5%) and retained original δ18OPO4 within analytical uncertainty (2σ = 0.6‰). We applied the purification and precipitation method on five igneous rocks and IWPs, and obtained δ18OPO4 values ranging from 6.4‰ to 8.0‰. Duplicate phosphate extractions yielded δ18OPO4 values differing by less than 0.3‰. Conclusions: We developed a new phosphate purification method that is applicable to phosphate extraction in igneous rocks and IWPs. We also proposed a reliable indicator for the termination of silver phosphate precipitation. Our method can achieve high phosphate yield and reproducible δ18OPO4 value. [ABSTRACT FROM AUTHOR]

The field of isotope geochemistry began with the study of oxygen isotope geothermometry, most notably for carbonates. For traditional oxygen isotope geothermometry only the relationship between one rare isotope, oxygen-18, and the common isotope, oxygen-16, is used because for most terrestrial processes the 17 O- 16 O relationship scales with the 18 O- 16 O relationship and is thought to not grant any new information. However, theoretical analysis predicts a small temperature-dependence of the equilibrium triple oxygen isotope relationship and instrumentation and techniques now allow for high-precision determination of the oxygen isotope composition for all three oxygen isotopes for a variety of sample types. To set the groundwork for triple oxygen isotope geothermometry, here we present new calibrations based on statistical thermodynamics and density functional theory for both the traditional two isotope and the recently introduced triple isotope thermometer for pairs of quartz, calcite, dolomite, fluorapatite, hematite, magnetite and liquid water. The results compare well with previous studies on 18 O/ 16 O fractionation where theoretical and experimental data are available. Of the models given here, pairs of quartz, calcite, dolomite and fluorapatite with water, hematite or magnetite show promising temperature sensitivities as triple isotope thermometers with acceptable uncertainties for surface and low-T hydrothermal environments. [ABSTRACT FROM AUTHOR]

The Marinoan panglaciation ( ca 650 to 635 Ma) is represented in north-east Svalbard by the 130 to 175 m thick Wilsonbreen Formation which contains syn-glacial carbonates in its upper 100 m. These sediments are now known to have been deposited under a CO2-rich atmosphere, late in the glaciation, and global climate models facilitate testing of proposed analogues. Precipitated carbonates occur in four of the seven facies associations identified: Fluvial Channel (including stromatolitic and intraclastic limestones in ephemeral stream deposits); Dolomitic Floodplain (dolomite-cemented sand and siltstones, and microbial dolomites); Calcareous Lake Margin (intraclastic dolomite and wave-rippled or aeolian siliciclastic facies); and Calcareous Lake (slump-folded and locally re-sedimented rhythmic/stromatolitic limestones and dolomites associated with ice-rafted sediment). There is no strong cyclicity, and modern analogues suggest that sudden changes in lake level may exert a strong control on facies geometry. Both calcite and dolomite in stromatolites and rhythmites display either primary or early diagenetic replacive growth. Oxygen isotope values (−12 to +15‰VPDB) broadly covary with δ13C. High δ13C values of +3·5 to +4·5‰ correspond to equilibration with an atmosphere dominated by volcanically degassed CO2 with δ13C of −6 to −7‰. Limestones have consistently negative δ18O values, while rhythmic and playa dolomites preserve intermediate compositions, and dolocretes possess slightly negative to strongly positive δ18O signatures, reflecting significant evaporation under hyperarid conditions. Inferred meltwater compositions (−8 to −15·5‰) could reflect smaller Rayleigh fractionation related to more limited cooling than in modern polar regions. A common pseudomorph morphology is interpreted as a replacement of ikaite (CaCO3·H2O), which may also have been the precursor for widespread replacive calcite mosaics. Local dolomitization of lacustrine facies is interpreted to reflect microenvironments with fluctuating redox conditions. Although differing in (palaeo)latitude and carbonate abundance, the Wilsonbreen carbonates provide strong parallels with the McMurdo Dry Valleys of Antarctica. [ABSTRACT FROM AUTHOR]

Two global glaciations occurred during the Neoproterozoic. Snowball Earth theory posits that these were terminated after millions of years of frigidity when initial warming from rising atmospheric CO2 concentrations was amplified by the reduction of ice cover and hence a reduction in planetary albedo. This scenario implies that most of the geological record of ice cover was deposited in a brief period of melt-back. However, deposits in low palaeo-latitudes show evidence of glacial-interglacial cycles. Here we analyse the sedimentology and oxygen and sulphur isotopic signatures of Marinoan Snowball glaciation deposits from Svalbard, in the Norwegian High Arctic. The deposits preserve a record of oscillations in glacier extent and hydrologic conditions under uniformly high atmospheric CO2 concentrations. We use simulations from a coupled three-dimensional ice sheet and atmospheric general circulation model to show that such oscillations can be explained by orbital forcing in the late stages of a Snowball glaciation. The simulations suggest that while atmospheric CO2 concentrations were rising, but not yet at the threshold required for complete melt-back, the ice sheets would have been sensitive to orbital forcing. We conclude that a similar dynamic can potentially explain the complex successions observed at other localities. [ABSTRACT FROM AUTHOR]

• Massively bedded barite (MBB) deposits occur mostly in Paleozoic sedimentary basins. • Sulfate-limited euxinic water is prone to be sulfate-free which accumulates hydrothermal Ba2+. • MBB deposits form when Ba2+-rich water encountering sulfate-bearing water. • Petrology, hyalophane, and δ 34 S and Sr isotope data support the new model. Existing models for massively bedded barite (MBB) deposits (e.g., sedimentary exhalative and diagenetic/ cold-seep) satisfy some geological and geochemical observations, but none explain the Paleozoic clustering of MBB deposits in Earth history. Here we bring seawater redox history into the picture and propose a sulfate-limited euxinic seawater (SLES) model where hydrothermally sourced Ba2+ accumulates as dissolved ion while other metal ions precipitate as insoluble metal sulfides. A subsequent encounter of this Ba2+-rich water mass with a sulfate-bearing one results in the deposition of MBB, thus physically separated from the accompanied metal sulfide deposits. We tested the SLES model using early Cambrian MBB deposits in South China through petrographic and isotope analyses. Syngenetic sphalerite and barite layers show a clear separation. A wide range of 87Sr/86Sr values (0.7082 to 0.7120) of the MBB supports a mixing of seawater and hydrothermal sources. A large range of δ 34 S values (32.2 to 61.1‰) of the MBB and the occurrence of mineral hyalophane supports an overall sulfate-limited but sulfate-concentration temporally and spatially heterogeneous ocean. Our model established the genetic link between MBB deposits and the accompanied metal sulfide deposits in the Paleozoic. The dearth of MBB deposits before and after the Paleozoic is due to widespread ferruginous oceans with little sulfate and complete oxygenated oceans with too much sulfate, respectively. The Paleozoic clustering of the MBB deposits is a consequence of a critical redox transition in Earth history. [ABSTRACT FROM AUTHOR]

• Thin beds of barite crystal fans occur in the basal Ediacaran, East Finnmark, Norway; • Barite formed in a shallow marine to subaerial, freshwater-dominated setting; • Barite's Δ17O and δ18O values are among the lowest in all palaeocontinents; • Barite in cracks and fissures is post-depositional. Basal Ediacaran barite, which has mass-anomalous depleted 17O, supporting Snowball Earth models, likely grew in shallow-marine settings, but sedimentological constraints lack details. Environmental conditions in the Varanger Palaeovalley during the basal Ediacaran are well preserved in the Nyborg Formation, where barite forms < 15 mm high syn syn-sedimentary crystal fans on Archean basement or thin intervening sediments. Sedimentary evidence suggests crystal fans formed in low-energy, very shallow-marine to subaerial facies. Rare earth element and yttrium signatures in associated carbonates suggest non-saline growth environments. Post-depositional fluid-flow replaced crystal fan barite with silica (now quartz) and redeposited barite as: <1 × 0.2 mm "dispersed" grains in sediments below fans; <5 mm grains replacing calcite filling desiccation cracks; <1 mm grains in basement fissures; <3 µm grains replacing matrix calcite; irregular grains in detrital sheet-silicate + calcite grains. Caledonian dissolution-reprecipitation reworked barite into brittle fractures and stylolites. Barite Δ17O values are amongst the most negative (-0.9 – -1.25 ‰) and δ18O values the lowest recorded (9.9 – 16.9 ‰) anywhere, consistent with a high latitude Baltica at ∼ 635 Ma. δ34S values (17.7 – 24.5 ‰) lie within the known range for basal Ediacaran barite. [ABSTRACT FROM AUTHOR]

Abstract: In both laboratory experiments and natural environments where microbial dissimilatory sulfate reduction (MDSR) occurs in a closed system, the ((34S/32S)sample/(34S/32S)standard −1) for dissolved SO4 2− has been found to follow a typical Rayleigh-Distillation path. In contrast, the corresponding ((18O/16O)sample/(18O/16O)standard)−1) is seen to plateau with an apparent enrichment of between 23‰ and 29‰ relative to that of ambient water under surface conditions. This apparent steady-state in the observed difference between and can be attributed to any of these three steps: (1) the formation of adenosine-5′-phosphosulfate (APS) from ATP and SO4 2−, (2) oxygen exchange between sulfite (or other downstream sulfoxy-anions) and water later in the MDSR reaction chain and its back reaction to APS and sulfate, and (3) the re-oxidation of produced H2S or precursor sulfoxy-anions to sulfate in environments containing Fe(III) or O2. This study examines the first step as a potential pathway for water oxygen incorporation into sulfate. We examined the structures and process of APS formation using B3LYP/6-31G(d,p) hybrid density functional theory, implemented in the Gaussian-03 program suite, to predict the potential for oxygen exchange. We conducted a set of in vitro, enzyme-catalyzed, APS formation experiments (with no further reduction to sulfite) to determine the degree of oxygen isotope exchange between the APS–sulfate and water. Triple-oxygen-isotope labeled water was used in the reactor solutions to monitor oxygen isotope exchange between water and APS sulfate. The formation and hydrolysis of APS were identified as potential steps for oxygen exchange with water to occur. Quantum chemical modeling indicates that the combination of sulfate with ATP has effects on bond strength and symmetry of the sulfate. However, these small effects impart little influence on the integrity of the SO4 2− tetrahedron due to the high activation energy required for hydrolysis of SO4 2− (48.94kcal/mol). Modeling also indicates that APS dissociation via hydrolysis is achieved through cleavage of the P–O bond instead of S–O bond, further supporting the lack of APS–H2O–oxygen exchange. The formation of APS in our in vitro experiments was verified by HPLC fluorescence spectroscopy, and triple-oxygen isotope data of the APS–sulfate indicate no oxygen isotope exchange occurred between APS–sulfate and water at 30°C for an experimental duration ranging from 2 to 120h. The study excludes APS formation as one of the causes for sulfate–oxygen isotope exchange with water during MDSR. [Copyright &y& Elsevier]

- The Grove Mountains (GRV) 021663 meteorite was collected from the Grove Mountains region of Antarctica. The meteorite is composed primarily of olivine (Fa5.4), orthopyroxene (Fs4.7Wo3.0), chromian diopside (En53.6Fs2.4Wo44), troilite, kamacite, and plagioclase (Ab74.5Or4An21.5). Minor phases include schreibersite and K-feldspar. The meteorite is highly weathered (W3) and weakly shocked (S2). We determine a whole rock oxygen isotopic composition of δ18O = 7.50‰, δ17O = 3.52‰. Comparisons of these data with other primitive achondrites have resulted in the reclassification of this meteorite as a member of the winonaite group. The occurrences of troilite, metal, and schreibersite in GRV 021663 indicate that these minerals were once completely molten. Euhedral inclusions of pyroxene within plagioclase further suggest that these may have crystallized from a silicate melt, while the depletion of plagioclase, metal, and troilite indicates that GRV 021663 could represent a residuum following partial melting on its parent asteroid. Trace element distributions in silicate minerals do not, however, confirm this scenario. As with other winonaite meteorites, the formation of GRV 021663 probably relates to brecciation and mixing of heterogeneous lithologies, followed by varying degrees of thermal metamorphism on the parent body asteroid. Peak metamorphic conditions may have resulted in localized partial melting of metal and silicate mineralogies, but our data are not conclusive. [ABSTRACT FROM AUTHOR]

Abstract: In order to understand spatial variations of stable isotope geochemistry in the Quruqtagh basin (northwestern China) in the aftermath of an Ediacaran glaciation, we analyzed carbonate carbon isotopes (δ 13 C carb), carbonate oxygen isotopes (δ 18 O carb), carbonate associated sulfate sulfur (δ 34 S CAS) and oxygen isotopes (δ 18 O CAS), and pyrite sulfur isotopes (δ 34 S py) of a cap dolostone atop the Ediacaran Hankalchough glacial diamictite at four sections. The four studied sections (YKG, MK, H and ZBS) represent an onshore–offshore transect in the Quruqtagh basin. Our data show a strong paleobathymetry-dependent isotopic gradient. From the onshore to offshore sections, δ 13 C carb values decrease from −2‰ to −16‰ (VPDB), whereas δ 18 O carb values increase from −4‰ to −1‰ (VPDB). Both δ 34 S CAS and δ 34 S py show stratigraphic variations in the two onshore sections (MK and YKG), but are more stable in the two offshore sections (H and ZBS). δ 18 O CAS values of onshore samples are consistent with terrestrial oxidative weathering of pyrite. We propose that following the Hankalchough glaciation seawater in the Quruqtagh basin was characterized by a strong isotopic gradient. The isotopic data may be interpreted using a three-component mixing model that involves three reservoirs: deep-basin water, surface water, and terrestrial weathering input. In this model, the negative δ 13 C carb values in the offshore sections are related to the upwelling of deep-basin water (where anaerobic oxidation of dissolved organic carbon resulted in 13 C-depleted DIC), whereas sulfur isotope variations are strongly controlled by surface water sulfate and terrestrial weathering input derived from oxidative weathering of pyrite. The new data provide evidence for the oceanic oxidation following the Hankalchough glaciation. [Copyright &y& Elsevier]

• Calcites precipitated from the solutions with identical chemical and isotopic compositions have different δ 13 C values. • The DBL thickness affects the pH and the δ 13 C value of HCO − 3 at the mineral-solution interface. • Our finding offers a mechanistic interpretation on the variability of the ε calcite / DIC in riverine or cave environments. It is often assumed that the carbon isotope composition (δ 13 C) of carbonate minerals records that of the dissolved inorganic carbon (DIC) species with insignificant disequilibrium effect. However, results from field observations and laboratory experiments have shown that the δ 13 C difference between calcite and solution can vary up to 3‰ even under a similar set of solution composition and temperature, raising uncertainties on the δ 13 C's paleoclimate and paleoecology implications. One likely cause is the variable calcite precipitation rates and pH values induced by different thicknesses of the stagnant liquid layer between solid and well mixed bulk solution (i.e., diffusion boundary layer, DBL). To test this hypothesis, we selected a well-studied natural travertine deposit (Baishuitai, SW China) which consists of meter-scale travertine terraces. Calcite crystals at both the pool bottom and the rim of the terraces are inorganically precipitated from solutions identical in chemical, isotopic composition and temperature, but different only in hydrodynamics. This difference results in thicker DBL for water at the pool-bottom than for water flowing across the rim. We found that the δ 13 C and Mg/Ca ratios are higher while Sr/Ca ratios are lower for the pool-bottom calcites than for the rim calcites. By applying a mass transfer model, we quantitatively link the differences in carbon isotope and elemental compositions of the abiogenic calcites to the different hydrodynamic conditions. The inverse variation in Mg/Ca and Sr/Ca ratios in calcites arises from the different precipitation rates between at the pool-bottom and at the rim, while the consistently lower δ 13 C for calcites at the rim is due to their higher pH at the solid-solution interface than for calcites precipitated at the pool-bottom. In contrast, calcite precipitation rate has little influence on carbon isotope fractionation between calcite and HCO − 3 . Our results demonstrate the role of DBL thickness in governing the δ 13 C of HCO − 3 at mineral surface, which can assist to interpret the variable δ 13 C values of calcites in riverine or cave environments. [ABSTRACT FROM AUTHOR]

Silicate melts have served as transport agents in the chemical and thermal evolution of Earth. Molecular dynamics simulations based on a deep neural network potential trained by ab initio data show that the viscosity of MgSiO3 melt decreases with increasing pressure at low pressures (up to ∼6 GPa) before it starts to increase with further compression. The melt electrical conductivity also behaves anomalously; first increasing and then decreasing with pressure. The melt accumulation implied by the viscosity turnover at ∼23 GPa along mantle liquidus offers an explanation for the low‐velocity zone at the 660‐km discontinuity. The increase in electrical conductivity up to ∼50 GPa may contribute to the steep rise of Earth's electrical conductivity profiles derived from magnetotelluric observations. Our results also suggest that small fraction of melts could give rise to detectable bulk conductivity in deeper parts of the mantle. Plain Language Summary: Dynamical behavior of silicate melts at high temperature and pressure controls melt distribution in Earth's interior. However, experimental data on the viscosity and electrical conductivity of silicate melts are limited to relatively low pressure. Moreover, the pressure dependence of viscosity is in debate, especially for depolymerized silicate melts. Using a new deep learning‐based simulation approach, we find that both the viscosity and electrical conductivity of MgSiO3 melt behave anomalously with pressure, which could have important implications for zones with seismic velocity anomalies and high electrical conductivity in Earth's interior. Key Points: The viscosity of MgSiO3 melt behaves anomalously at relatively low temperatures; first decreasing and then increasing with pressureThe electrical (ionic) conductivity shows anomalous pressure behavior at all temperaturesDeep potential molecular dynamics simulations offer us a reliable and complete data set to study transport properties of silicate melts [ABSTRACT FROM AUTHOR]

Silicate melts have served as transport agents in the chemical and thermal evolution of Earth. Molecular dynamics simulations based on a deep neural network potential trained by ab initio data show that the viscosity of MgSiO3 melt decreases with increasing pressure at low pressures (up to ∼6 GPa) before it starts to increase with further compression. The melt electrical conductivity also behaves anomalously; first increasing and then decreasing with pressure. The melt accumulation implied by the viscosity turnover at ∼23 GPa along mantle liquidus offers an explanation for the low‐velocity zone at the 660‐km discontinuity. The increase in electrical conductivity up to ∼50 GPa may contribute to the steep rise of Earth's electrical conductivity profiles derived from magnetotelluric observations. Our results also suggest that small fraction of melts could give rise to detectable bulk conductivity in deeper parts of the mantle. Plain Language Summary: Dynamical behavior of silicate melts at high temperature and pressure controls melt distribution in Earth's interior. However, experimental data on the viscosity and electrical conductivity of silicate melts are limited to relatively low pressure. Moreover, the pressure dependence of viscosity is in debate, especially for depolymerized silicate melts. Using a new deep learning‐based simulation approach, we find that both the viscosity and electrical conductivity of MgSiO3 melt behave anomalously with pressure, which could have important implications for zones with seismic velocity anomalies and high electrical conductivity in Earth's interior. Key Points: The viscosity of MgSiO3 melt behaves anomalously at relatively low temperatures; first decreasing and then increasing with pressureThe electrical (ionic) conductivity shows anomalous pressure behavior at all temperaturesDeep potential molecular dynamics simulations offer us a reliable and complete data set to study transport properties of silicate melts [ABSTRACT FROM AUTHOR]

Several hundred meteorites with a total mass of over 100 kg were collected as the Alatage Mountain (AM) strewn field located in the Kumtag desert, Xinjiang Province, China. Twelve AM meteorites were studied in this work. Petrography, mineralogy, bulk chemistry, bulk oxygen isotopic compositions, and light noble gas concentrations and isotopic compositions were determined for all or for a selection of the meteorites. The studied meteorites are L‐chondrites that suffered a very strong impact; impact melt veins and melt pockets are widely distributed. More than 50% of the troilite exists in the form of blebs and veins in olivine and pyroxene. Some of these meteorites are impact melt recrystallized rocks (e.g., AM 037). The strong impact caused the decomposition of troilite, which led in AM 034 to the sulfidization reaction of olivine. The metal in most meteorites is almost completely altered, and the troilite has been significantly oxidized. Weathering resulted in the depletion of Mg, Fe, Co, and Ni, and the enrichment of Sr, Ba, Pb, and U in these meteorites. The cosmic ray exposure (CRE) ages measured for these specimens range between 6.2 ± 1.9 Ma and 9.0 ± 2.7 Ma, depending on the cosmogenic nuclide used. The average CRE age is 7.6 ± 1.3 Ma. Both 4He and 40Ar gas retention ages indicate that the strong impact which caused the shock effects occurred about 320 Ma ago. [ABSTRACT FROM AUTHOR]

Diffusional isotope fractionation has been widely used to explain lithium (Li) isotope variations in minerals and rocks. Isotopic mass dependence of Li diffusion can be empirically expressed as D 7 Li D 6 Li = 6 7 β , where D is the diffusivity of a Li isotope. The knowledge about temperature and compositional dependence of the β factor which is essential for understanding diffusion profiles and mechanisms remains unclear. Based on the potential energy and interatomic forces generated by deep neural networks trained with ab initio data, we performed deep potential molecular dynamics (DPMD) simulations of several Li pseudo-isotopes (with mass = 2, 7, 21, 42 g/mol) in albite, hydrous albite, and model basalt melts to evaluate the β factor. Our calculated diffusivities for 7Li in albite and model basalt melts at 1800 K compare well with experimental results. We found that β in albite melt decreases from 0.267 ± 0.006 at 4000 K to 0.225 ± 0.004 at 1800 K. The presence of water appears to slightly weaken the temperature dependence of β , with β decreasing from 0.250 ± 0.012 to 0.228 ± 0.031 in hydrous albite melt. The calculated β in model basalt melt takes much smaller values, decreasing from 0.215 ± 0.006 at 4000 K to 0.132 ± 0.015 at 1800 K. Our prediction of β in albite and hydrous albite melts is in good agreement with experimental data. More importantly, our results suggest that Li isotope diffusion in silicate melts is strongly dependent on melt composition. The temperature and compositional effects on β can be qualitatively explained in terms of ionic porosity and the coupled relationship between Li diffusion and the mobility of the silicate melt network. Two types of diffusion experiments are suggested to test our predicted temperature and compositional dependence of β. This study shows that DPMD is a promising tool to simulate the diffusion of elements and isotopes in silicate melts. [ABSTRACT FROM AUTHOR]

Diffusional isotope fractionation occurs in geochemical processes (such as magma mixing, bubble growth, and crystal growth), even at magmatic temperatures. Isotopic mass dependence of diffusion is commonly expressed as D i D j = m j m i β , where D i and D j are diffusion coefficients of two isotopes whose masses are m i and m j. How the dimensionless empirical parameter β depends on temperature, pressure, and composition remains poorly constrained. Here, we conducted a series of first-principles molecular dynamics simulations to evaluate the β factor of Mg isotopes in MgSiO 3 and Mg 2 SiO 4 melts using pseudo-isotope method. In particular, we considered interactions between Mg isotopes by simultaneously putting pseudo-mass and normal-mass Mg atoms in a simulation supercell. The calculated β for Mg isotopes decreases linearly with decreasing temperature at zero pressure, from 0.158 ± 0.004 at 4000 K to 0.121 ± 0.017 at 2200 K for MgSiO 3 melt and from 0.150 ± 0.004 at 4000 K to 0.101 ± 0.012 at 2200 K for Mg 2 SiO 4 melt. Moreover, our simulations of compressed Mg 2 SiO 4 melt along the 3000 K isotherm show that the β value decreases linearly from 0.130 ± 0.006 at 0 GPa to 0.060 ± 0.011 at 17 GPa. Based on our diffusivity results, the empirically established positive correlation between β and solvent-normalized diffusivity (D i / D Si) seems to be applicable only at constant temperatures or in narrow temperature ranges. Analysis of atomistic mechanisms suggests that the calculated β values are inversely correlated with force constants of Mg at a given temperature or pressure. Good agreement between our first principles results with available experimental data suggests that interactions between isotopes of major elements must be considered in calculating β for major elements in silicate melts. Also, we discuss diffusion-controlled crystal growth by considering our calculated β values. [ABSTRACT FROM AUTHOR]

The aim of this paper is to study the feasibility of using an electrolytic manganese residue (EMR) as modified asphalt. In this paper, after grinding the electrolytic manganese residue (EMR) into asphalt, the electrolytic manganese residue- (EMR-) modified asphalt was prepared with different mix ratios. The three major indicators of the modified asphalt were studied, and its modification mechanism was studied by differential scanning calorimetry, infrared spectroscopy, and atomic force microscopy. The adhesion force, surface energy, and dissipation energy of the asphalt before and after modification were analyzed by a force curve. The results show that the surface energy of the electrolytic manganese residue (EMR) is increased after grinding, the high temperature performance of the asphalt is improved, and the temperature sensitivity of the asphalt is decreased; however, the low-temperature performance is not improved obviously. When the powder oil ratio is 9%, the comprehensive performance of the asphalt is the best. The results of the infrared spectrum analysis show that the mixture of the electrolytic manganese residue (EMR) and asphalt does not produce new functional groups, and thus, the preparation method is a physical modification method. The differential scanning calorimetry (DSC) results show that the electrolytic manganese residue (EMR) can enhance the high-temperature stability of asphalt. It is found that the stability and antideformation ability of the modified asphalt improved. [ABSTRACT FROM AUTHOR]