Sulfur Isotope Fractionation Research Articles

Redox states of Archean surficial environments: The importance of H2,g instead of O2,g for weathering reactions

Redox states of the Archean Eon have been constrained by various lines of evidence, including atmospheric, photochemical, and ecological models, mass-independent fractionations of sulfur isotopes, Fe-depletion of paleosols, and preservation of diagnostic detrital minerals. Although these lines of evidence present seemingly consistent upper limits on pO2,g, they are conceptually contradictory about the redox state of Archean surficial environments. Atmospheric, photochemical, and ecological modeling studies suggest weakly reducing environments under redox states represented by moderate H2,g levels. However, current interpretations of Fe-depletion in paleosols and the preservation of detrital minerals are based on low O2,g levels at which the reducing detrital minerals are thermodynamically unstable and survive because of slow kinetics of oxidative weathering.In this study, we show that under the redox state indicated by the Archean pH2,g range, Fe2+ and reducing Fe(II)-minerals are actually thermodynamically stable and have no tendency to be oxidized. We emphasize that pH2,g values are orders of magnitude higher than pO2,g in the Archean atmosphere and that H2,g and O2,g are not in equilibrium. The redox states of Archean surface environments behave as though they were controlled by the more abundant H2 instead of the very low O2, as in modern anoxic basins. Weathering in this case should have involved non-redox acidic dissolution of Fe(II)-species or reductive reaction of Fe(III)-species. Fe(II)-depleted paleosols and the preservation of relatively reduced detrital minerals are natural consequences of their thermodynamic stabilities in the Archean Eon's reducing environments rather than slow kinetics of oxidizing reactions. After the appearance of oxygenic photosynthesis, probably in the middle/late Archean, locally oxygenated environments could have existed, while the atmosphere as a whole remained anoxic. The profile of redox states on the Archean surface seems to be a reverse analogue to the modern Earth. Although oxidizing dissolution of transition metals could happen in O2-oases, quick reduction of oxyanions by abundant reductants, such as aqueous Fe2+, Fe(II)-minerals, and H2, might have restricted riverine transport of oxyanions and potentially complicate the interpretation of signals of O2-whiffs in marine sediments.

Experience in applying a local analysis an sulfur isotopes in the ore sulfides of the largest deposits of Bodaibinsky Synclinoria (Eastern Siberia)

The experience of LA—ICP determining of the isotopic composition of sulfur pyrite and arsenopyrite of gold deposits of the Baikal-patomsky plateau is considered. The technique is characterized. It is shown that all the studied samples have values δ34S and δ33S strictly corresponding to the law of mass-dependent fractionation of sulfur isotopes. A regular alleviation of the isotopic composition of sulfur of pyrite as its crystals grow for Sukhoi Log and the isotopic homogeneity of pyrite and pyrrhotite of the Golets Vysokhashiy deposit are established. It is concluded that the possible role of metamorphism of the Mamsko-Oronsky belt in the formation of isotopic homogeneity of ore sulfides.

ToF-SIMS analysis of abiotic and biotic iron sulfide layers formed in aqueous conditions on iron surfaces

An approach for the identification of the origin of iron sulfides formation on iron surfaces using Time-of-Flight Secondary Ions Mass Spectrometry (ToF-SIMS) is reported here in. Two different sulfidation processes of iron surfaces are presented (both at room temperature): the first abiotic procedure was carried out using chronoamperometry (E = −0.8 V/SCE) of pure iron in 10 mM of Na2S,9H2O (pH = 11) while the second biotic procedure was accomplished by immersing a pure iron sample in a medium containing HS− ions produced by a sulfate-reducing bacterium (SRB) of the genus Desulfovibrio at an open circuit potential (OCP). A surface analytical method using Time-of-Flight Secondary Ions Mass Spectrometry (ToF-SIMS) coupled with ToF-SIMS spectra peak-fitting and data processing was developed to calculate an accurate sulfur isotopic fractionation δ34SV-CDT on iron sulfide layers formed on iron surfaces, thus allowing to discriminate between abiotically versus biotically generated sulfides. This approach contributes to a better understanding of iron surface interactions with sulfur containing environmental species of abiotic or biotic origin.

Proteomic and Isotopic Response of Desulfovibrio vulgaris to DsrC Perturbation.

Dissimilatory sulfate reduction is a microbial energy metabolism that can produce sulfur isotopic fractionations over a large range in magnitude. Calibrating sulfur isotopic fractionation in laboratory experiments allows for better interpretations of sulfur isotopes in modern sediments and ancient sedimentary rocks. The proteins involved in sulfate reduction are expressed in response to environmental conditions, and are collectively responsible for the net isotopic fractionation between sulfate and sulfide. We examined the role of DsrC, a key component of the sulfate reduction pathway, by comparing wildtype Desulfovibrio vulgaris DSM 644T to strain IPFG07, a mutant deficient in DsrC production. Both strains were cultivated in parallel chemostat reactors at identical turnover times and cell specific sulfate reduction rates. Under these conditions, sulfur isotopic fractionations between sulfate and sulfide of 17.3 ± 0.5‰ or 12.6 ± 0.5‰ were recorded for the wildtype or mutant, respectively. The enzymatic machinery that produced these different fractionations was revealed by quantitative proteomics. Results are consistent with a cellular-level response that throttled the supply of electrons and sulfur supply through the sulfate reduction pathway more in the mutant relative to the wildtype, independent of rate. We conclude that the smaller fractionation observed in the mutant strain is a consequence of sulfate reduction that proceeded at a rate that consumed a greater proportion of the strains overall capacity for sulfate reduction. These observations have consequences for models of sulfate reducer metabolism and how it yields different isotopic fractionations, notably, the role of DsrC in central energy metabolism.

Extreme sulfur isotope fractionation in the Late Devonian Dry Creek volcanogenic massive sulfide deposit, central Alaska

The Dry Creek Zn-Pb-Cu-Ag-Au volcanogenic massive sulfide (VMS) deposit, in east-central Alaska, occurs in a Late Devonian sequence of peralkaline rhyolite tuff, minor graphitic argillite, and local peralkaline quartz-porphyry rhyolite intrusions. Principal mineralized facies are semi-massive and massive sulfide in variably silicified and graphitic rhyolite tuff, massive sulfide in graphitic argillite, disseminated sulfides in graphitic and non-graphitic rhyolite tuff, and vein-hosted sulfides in a subvolcanic, peralkaline quartz-porphyry intrusion. In situ analysis of the sulfur isotope composition of sulfide minerals from all facies of the deposit shows a total range in δ34S values from −48.0 to 23.1‰. This remarkable 71.1‰ variation is more than twice the largest range known for sulfides in nearly all individual VMS deposits, both modern and ancient, which typically is <25‰. Based on results of geochemical reaction modeling presented here, extreme sulfur isotope systematics in the Dry Creek deposit is attributed to two main processes: low-temperature (<100 °C) kinetically controlled bacterial sulfate reduction (BSR) in anoxic to locally sulfidic bottom waters and pore fluids, and higher temperature (100–300 °C) hydrothermal sulfate reduction (HSR) of seawater sulfate during water-rock reactions. Deposition of organic-rich sediments prior to and during mineralization provided the key environment that promoted microbially mediated fractionations with Δ34Ssulfate-sulfide of up to 70‰. During BSR reaction progress modeling, residual sulfate evolved by Rayleigh distillation in pore water, producing δ34SSO4 values up to 58‰; normal Late Devonian seawater sulfate had δ34SSO4 = 22‰. BSR produced FeS precipitates with δ34S values from −50 to −15‰. Integrated textural and δ34S data also suggest that isotopically light H2S produced in pore fluids by BSR was incorporated into hydrothermal fluids and early disseminated sulfide minerals during the emplacement of peralkaline quartz-porphyry rhyolite intrusions in the shallow subsurface, broadly coeval with VMS mineralization. Modeling of moderate- to high-temperature (100–300 °C) hydrothermal sulfate reduction (HSR) of seawater-rock interaction predicts sulfide minerals with δ34S values from −8 to 19‰.High-precision CA-ID-TIMS U-Pb geochronology of zircons from a synvolcanic peralkaline quartz porphyry intrusion yields a weighted mean 206Pb/238U date of 363.02 ± 0.43 Ma (2σ total uncertainty). This date indicates that the formation of graphitic and sulfidic sediments at Dry Creek, and contemporaneous VMS mineralization, occurred at least 3.6 m.y. before, and hence are unrelated to, widespread black shale deposition during the Hangenberg Event at ca. 359 Ma.

Large-volume air sample system for measuring &amp;lt;sup&amp;gt;34&amp;lt;/sup&amp;gt;S∕&amp;lt;sup&amp;gt;32&amp;lt;/sup&amp;gt;S isotope ratio of carbonyl sulfide

Abstract. Knowledge related to sulfur isotope ratios of carbonyl sulfide (OCS or COS), the most abundant atmospheric sulfur species, remains scarce. An earlier method developed for sulfur isotopic analysis for OCS using S+ fragmentation by an isotope ratio mass spectrometer is inapplicable for ambient air samples because of the large samples required (approx. 500 L of 500 pmol mol−1 OCS). To overcome this difficulty, herein we present a new sampling system for collecting approximately 10 nmol of OCS from ambient air coupled with a purification system. Salient system features are (i) accommodation of samples up to 500 L (approx. 10 nmol) of air at 5 L min−1; (ii) portability of adsorption tubes (1∕4 in. (0.64 cm) outer diameter, 17.5 cm length, approximately 1.4 cm3 volume) for preserving the OCS amount and δ34S(OCS) values at −80 ∘C for up to 90 days and 14 days; and (iii) purification OCS from other compounds such as CO2. We tested the OCS collection efficiency of the systems and sulfur isotopic fractionation during sampling. Results show precision (1σ) of δ34S(OCS) values as 0.4 ‰ for overall procedures during measurements for atmospheric samples. Additionally, this report presents diurnal variation of δ34S(OCS) values collected from ambient air at the Suzukakedai campus of the Tokyo Institute of Technology located in Yokohama, Japan. The observed OCS concentrations and δ34S(OCS) values were, respectively, 447–520 pmol mol−1 and from 10.4 ‰ to 10.7 ‰ with a lack of diurnal variation. The observed δ34S(OCS) values in ambient air differed greatly from previously reported values of δ34S(OCS) = (4.9±0.3) ‰ for compressed air collected at Kawasaki, Japan, presumably because of degradation of OCS in cylinders and collection processes for that sample. The difference of atmospheric δ34S(OCS) values between 10.5 ‰ in Japan (this study) and ∼13 ‰ recently reported in Israel or the Canary Islands indicates that spatial and temporal variation of δ34S(OCS) values is expected due to a link between anthropogenic activities and OCS cycles. The system presented herein is useful for application of δ34S(OCS) for investigation of OCS sources and sinks in the troposphere to elucidate its cycle.

Tracing water-rock interaction in carbonate replacement deposits: A SIMS pyrite S-Pb isotope perspective from the Chinese Xinqiao system

Deciphering the sources of metals is a prerequisite to establishing genetic models of ore formation, and is of importance for developing exploration models. For most magmatic-hydrothermal deposits, non-magmatic sourced components (e.g., fluids, sulfur and metals) are suggested to have minor or negligible contributions. On the other hand, there is substantial evidence to indicate that external ore-forming components could be crucial for the mineralization of some magmatic-hydrothermal deposits. Here, we conduct a detailed pyrite sulfur and lead isotope study on the Xinqiao stratabound deposit, South China, with the aim of constraining the contributions of ore-forming components from sedimentary rocks and existing mineralization through water-rock interaction. Traditionally, the source of mineralization at Xinqiao is debated. For example, a Cretaceous magmatic-hydrothermal origin was proposed as the sole source of metals. In sharp contrast, a Carboniferous sedimentary sulfide layer as proto-ore, which was enriched later by the Cretaceous magmatic-hydrothermal fluids, is required in a remobilization model. Crystallized pyrite grains from the stratabound orebody (py1) have a δ34S value of 4.09 ± 1.42 ‰ (2 SD), which is similar to that of pyrite grains from the garnet bearing skarn ore (py3). Colloform pyrite ore, which is composed of fine-grained (∼500 nm) cubic pyrite grains (py2a), has a δ34S value of 0.20 ± 1.14 ‰. The sulfur isotope composition of pyrite grains from the sandstone hosted quartz-pyrite veins is bimodal with δ34S values of 3.28 ± 6.46 ‰ (py4a) and −11.87 ± 5.43 ‰ (py4b). The Pb isotope compositions (206/204Pb ≈ 18.5, 207/204Pb ≈ 15.5) of py1, py4a and py4b are broadly the same as that of K-feldspar from the Cretaceous Jitou diorite, while py2a are more radiogenic (206/204Pb ≈ 21.5, 207/204Pb ≈ 18). Our pyrite S and Pb isotope data show no support for the presence of a Carboniferous sedimentary sulfide layer as proto-ore, but instead suggest that leaching sulfur and metals from Permian shales and an unexposed mineralization through water-rock interaction is a vital mechanism for the mineralization at Xinqiao. In line with previous studies, contributions from the late Permian shales are complementary for the stratabound orebody, and recycling existing Mo-Au rich mineralization is key for the formation of sandstone hosted gold rich pyrite-quartz veins. The existing mineralization is not exposed yet, and should be considered in future mineral exploration. Limited sulfur isotope fractionation between py1, py2 and py3 (0–4 ‰) further indicates a relatively reducing condition during their formation, while significant lower sulfur isotope composition (δ34S = −12 ‰) of py4 could imply an oxidizing condition during its formation. Our study highlights that detailed in-situ sulfur and lead isotope analyses under robust geological and petrographic frameworks can tightly constrain the source of ore-forming components, yield insights into oxygen fugacity during ore formation, and offer clues for future mineral exploration.

A Case History of Applying In situ Analysis of the Sulfur Isotopic Compositions of Sulfides from Ores of the Largest Deposits in the Bodaibo Synclinorium (Eastern Siberia)

The experience of applying the laser ablation technique to study the sulfur isotopic compositions of pyrite and arsenopyrite samples from gold deposits of the Baikal–Patom highland is discussed. The experimental procedure is described. All studied samples were found to display δ34S and δ33S values in strict compliance with the law of mass-dependent sulfur isotope fractionation. Regular lightening of the sulfur isotope composition in the course of crystal growth for pyrites from the Sukhoi Log deposit and the isotopic homogeneity of pyrite and pyrrhotite from the Golets Vysochaishii deposit were established. The conclusion that the sulfur isotopic homogeneity of sulfides in the ores occurs probably owing to metamorphism within the Mama–Oron belt was made.

Precise determination of equilibrium sulfur isotope effects during volatilization and deprotonation of dissolved H2S

Sulfide (H_2S, HS^−, and S^(2−)) is ubiquitous in marine porewaters as a result of microbial sulfate reduction, constituting the reductive end of the biogeochemical sulfur cycle. Stable isotopes have been widely used to constrain the sulfur cycle, because the redox transformations of sulfur compounds, such as microbial sulfate reduction, often exhibit sizable kinetic isotope effects. In contrast to sulfate ion (SO_4^(2−)), the most abundant form of dissolved sulfur in seawater, H2S is volatile and also deprotonated at near neutral pH. Equilibrium isotope partitioning between sulfide species can therefore overlap with kinetic isotope effects during reactions involving sulfide as either reactant or intermediate. Previous experimental attempts to measure equilibrium fractionation between H_2S and HS− have reached differing results, likely due to solutions of widely varying ionic strength. In this study, we measured the sulfur isotope fractionation between total dissolved sulfide and gaseous H2S at 20.6 ± 0.5 °C over the pH range from 2 to 8, and calculated the equilibrium isotope effects associated with deprotonation of dissolved H_2S. By using dilute solutions of Na2S, made possible by the improved sensitivity of mass spectrometric techniques, uncertainty in the first dissociation constant of H2S due to ionic strength could be better controlled. This in turn allowed us to close sulfur isotope mass balance for our experiments and increase the accuracy of the estimated fractionation factor. At equilibrium, aqueous H2S was enriched in ^(34)S by 0.7‰ and 3.1‰ relative to gaseous H_2S and aqueous HS−, respectively. The estimated fractionation between aqueous H_2S and HS^− lies between two earlier experimental reports, but agrees within the uncertainty of the measurements with a recent theoretical calculation.

Role of APS reductase in biogeochemical sulfur isotope fractionation

Sulfur isotope fractionation resulting from microbial sulfate reduction (MSR) provides some of the earliest evidence of life, and secular variations in fractionation values reflect changes in biogeochemical cycles. Here we determine the sulfur isotope effect of the enzyme adenosine phosphosulfate reductase (Apr), which is present in all known organisms conducting MSR and catalyzes the first reductive step in the pathway and reinterpret the sedimentary sulfur isotope record over geological time. Small fractionations may be attributed to low sulfate concentrations and/or high respiration rates, whereas fractionations greater than that of Apr require a low chemical potential at that metabolic step. Since Archean sediments lack fractionation exceeding the Apr value of 20‰, they are indicative of sulfate reducers having had access to ample electron donors to drive their metabolisms. Large fractionations in post-Archean sediments are congruent with a decline of favorable electron donors as aerobic and other high potential metabolic competitors evolved.

Sources of Sulfur for Sulfide Mineralization in the Archean Rocks of the Sharyzhalgai Uplift of the Siberian Craton Basement (from Multi-Isotope Data)

Abstract —We present results of study of sulfide ore occurrence in highly metamorphosed (granulite facies) Archean rocks of the Siberian craton basement. The host rocks and ore minerals are briefly described, and new data on the multiple sulfur isotope (δ33S, δ34S, Δ33S) composition of sulfides are presented. Application of high-resolution analytical methods enabling the assessment of the sulfur isotope behavior in situ made it possible to reveal mass-independent fractionation of sulfur isotopes in the rock samples. The isotopic composition of sulfur in the sulfides indicates its inflow from several sources, including the ancient Archean atmosphere, where primary sulfur has passed through a cycle of fractionation. Despite the high-gradient metamorphism, the subsequent ultrametamorphic and post-ultrametamorphic transformations accompanied by a change in the primary mineral composition of rocks and by chemogenic fractionation of sulfur, the signature of the sedimentary source of sulfur in sulfide ores has been well preserved. Analysis of the chemical composition of rocks and ore minerals and of sulfur isotopes has led to the conclusion that the studied rocks are metamorphosed Late Archean analogs of black shales and the sulfide mineralization is of stratiform pyrite type.

First-principles investigation of the effect of crystal structure on sulfur isotope fractionation in sulfide polymorphs

In order to study the effect of crystal structure on S isotope fractionation properties of sulfide polymorphs, the reduced partition function ratios of S-34/S-32 (10(3)ln beta(34-32)) for FeS2, CoAsS, ZnS, CdS and MnS polymorphs are calculated in the present study. Our calculations show that 10(3)ln beta(34-32) decreases in the order of pyrite > marcasite > cobaltite > alloclasite > sphalerite approximate to wurtzite > gamma-MnS > hawleyite approximate to greenockite > alabandite. The S coordination number exerts a strong influence on the equilibrium partitioning of S isotopes in these sulfides, and there exist significant S isotope fractionations between the sulfides with different S coordination numbers (CNS), such as alabandite (CNS = 6) and other sulfides (e.g., FeS2, CoAsS, ZnS and CdS polymorphs) (CNS = 4). There is a broad negative correlation between the average bond lengths of all the bonds formed by S and 10(3)ln beta(34-32) values. The compactness of atom packing plays an important role in determining S isotope fractionation properties of sulfides, and S-34 generally tends to concentrate in the phases with higher atomic packing factor (APF). The linear correlation between APF and 10(3)ln beta(34-32) can predict the change of 10(3)ln beta(34-32) with increasing pressure, but pressure has a negligible effect on S isotope fractionation factors between sulfides.

Deconstructing the Dissimilatory Sulfate Reduction Pathway: Isotope Fractionation of a Mutant Unable of Growth on Sulfate.

The sulfur isotope record provides key insight into the history of Earth's redox conditions. A detailed understanding of the metabolisms driving this cycle, and specifically microbial sulfate reduction (MSR), is crucial for accurate paleoenvironmental reconstructions. This includes a precise knowledge of the step-specific sulfur isotope effects during MSR. In this study, we aim at resolving the cellular-level fractionation factor during dissimilatory sulfite reduction to sulfide within MSR, and use this measured isotope effect as a calibration to enhance our understanding of the biochemistry of sulfite reduction. For this, we merge measured isotope effects associated with dissimilatory sulfite reduction with a quantitative model that explicitly links net fractionation, reaction reversibility, and intracellular metabolite levels. The highly targeted experimental aspect of this study was possible by virtue of the availability of a deletion mutant strain of the model sulfate reducer Desulfovibrio vulgaris (strain Hildenborough), in which the sulfite reduction step is isolated from the rest of the metabolic pathway owing to the absence of its QmoABC complex (ΔQmo). This deletion disrupts electron flux and prevents the reduction of adenosine phosphosulfate (APS) to sulfite. When grown in open-system steady-state conditions at 10% maximum growth rate in the presence of sulfite and lactate as electron donor, sulfur isotope fractionation factors averaged −15.9‰ (1 σ = 0.4), which appeared to be statistically indistinguishable from a pure enzyme study with dissimilatory sulfite reductase. We coupled these measurements with an understanding of step-specific equilibrium and kinetic isotope effects, and furthered our mechanistic understanding of the biochemistry of sulfite uptake and ensuing reduction. Our metabolically informed isotope model identifies flavodoxin as the most likely electron carrier performing the transfer of electrons to dissimilatory sulfite reductase. This is in line with previous work on metabolic strategies adopted by sulfate reducers under different energy regimes, and has implications for our understanding of the plasticity of this metabolic pathway at the center of our interpretation of modern and palaeo-environmental records.

Degassing-induced fractionation of multiple sulphur isotopes unveils post-Archaean recycled oceanic crust signal in hotspot lava

Mantle source regions feeding hotspot volcanoes likely contain recycled subducted material. Anomalous sulphur (S) isotope signatures in hotspot lavas have tied ancient surface S to this deep geological cycle, but their potential modification by shallow magmatic processes has generally been overlooked. Here we present S isotope measurements in magmatic sulphides, silicate melt inclusions and matrix glasses from the recent eruption of a hotspot volcano at El Hierro, Canary Islands, which show that degassing induces strongly negative δ34S fractionation in both silicate and sulphide melts. Our results reflect the complex interplay among redox conditions, S speciation and degassing. The isotopic fractionation is mass dependent (Δ33S = 0‰), thus lacking evidence for the recycled Archaean crust signal recently identified at other hotspot volcanoes. However, the source has an enriched signature (δ34S ~ + 3‰), which supports the presence of younger 34S-rich recycled oceanic material in the Canary Island mantle plume.

Study on pollution behavior and sulfate formation during the typical haze event in Nanjing with water soluble inorganic ions and sulfur isotopes

The typical haze event in Nanjing was selected to study pollution behavior and sulfate formation by field measurement. Based on the concentrations of water soluble inorganic ions in PM2.5, pollution characteristics of the haze were investigated with phase clustering analysis. Besides, δ34S values of SO2 and sulfate in PM2.5 were determined in order to explore sulfur sources and sulfate formation. The result showed that PM2.5 pollution during the haze event was significantly serious, which was mainly from coal combustion, vehicle exhaust emission and biomass burning. Sulfate formation was attributed to aqueous phase sulfur oxidation reactions promoted by high relative humidity and NO2 concentration under the alkaline condition. The color of sky on 22 Dec. was ascribed to the combination of sunset glow and fine particles in high-moisture atmosphere. δ34S values of SO2 are found to be lower than those of sulfate in PM2.5 indicating there was presence of sulfur isotopic fractionation during SO2 oxidation. The average contribution of SO2 homogenous oxidation to sulfate was about 51.2% during the haze events. The ratio of SO2 heterogeneous and homogeneous oxidation to sulfate was mainly attributed to the concentrations of gaseous pollutants (NO2, SO2 and O3) and relative humidity of the atmosphere.

Stepwise oxygenation of the Paleozoic atmosphere

Oxygen is essential for animal life, and while geochemical proxies have been instrumental in determining the broad evolutionary history of oxygen on Earth, much of our insight into Phanerozoic oxygen comes from biogeochemical modelling. The GEOCARBSULF model utilizes carbon and sulphur isotope records to produce the most detailed history of Phanerozoic atmospheric O2 currently available. However, its predictions for the Paleozoic disagree with geochemical proxies, and with non-isotope modelling. Here we show that GEOCARBSULF oversimplifies the geochemistry of sulphur isotope fractionation, returning unrealistic values for the O2 sourced from pyrite burial when oxygen is low. We rebuild the model from first principles, utilizing an improved numerical scheme, the latest carbon isotope data, and we replace the sulphur cycle equations in line with forwards modelling approaches. Our new model, GEOCARBSULFOR, produces a revised, highly-detailed prediction for Phanerozoic O2 that is consistent with available proxy data, and independently supports a Paleozoic Oxygenation Event, which likely contributed to the observed radiation of complex, diverse fauna at this time.

Genesis and evolution of framboidal pyrite and its implications for the ore-forming process of Carlin-style gold deposits, southwestern China

Southwestern Guizhou, an indispensable component of the “golden triangle” area of southwestern China, is famous for its numerous Carlin-style gold deposits. Unlike other sulfides in the deposits that have been well studied, the genesis of framboidal pyrite is still controversial because of its tiny size. Here, we first determined the chemical and in situ sulfur isotopic compositions of framboidal pyrites from three typical Carlin-style gold deposits (namely Shuiyindong, Taipingdong and Nibao) using secondary electron image (SEI), back scattered electron (BSE), electron probe micro analysis (EPMA), and nanometer-scale secondary ion mass spectroscopy (Nano-SIMS) methods. Our EPMA data show that the framboidal pyrites contain a minor amount of trace elements (Ni < 0.344 wt%, Co = 0.017–0.295 wt%, Cr = 0.009–0.292 wt% and As < 1.479 wt%). Incorporation of the trace elements into the framboidal pyrites resulted in a slight decrease in S and Fe concentrations compared to the theoretical values. The in situ δ34S values of the framboidal pyrites, ranging from −53.2‰ to −30.9‰, are in good agreement with the sulfur isotopic fractionation triggered by the bacterial sulfate reduction (BSR) process in an open-SO42− ocean condition. Combined with SEI, BSE, and Nano-SIMS high-resolution multiple elemental mapping, four consecutive processes responsible for their genesis and evolution are summarized as follows: (1) initial microcrystal nucleation via the BSR process, (2) abundant microcrystal aggregation to form individual framboid and multiple-framboids during sedimentation or diagenesis, (3) partial replacement and recrystallization of preexisting framboids by As-rich ore fluids, and (4) eventual framboidal pyrite transformation to larger euhedral grains via continuous recrystallization. These processes not only present compelling mechanisms responsible for the variations in morphology and chemical composition of the framboidal pyrites, from the initial framboid with a low 75As32S signal (Py1) to the framboid with a recrystallization origin (Py3), but also indicate that their genesis and evolution may have an intimate correlation with the formation of Carlin-style gold deposits.

Atmospheric SO2 oxidation by NO2 plays no role in the mass independent sulfur isotope fractionation of urban aerosols

Modern anthropogenic aerosols usually exhibit low but significant Δ33S signatures (−0.6 to 0.5‰) whose origin still remains unclear. While isotope fractionation factors associated with the oxidation of SO2 by O2+TMI (Transition Metal Ion), H2O2 or OH cannot lead to such extreme Δ33S-values, an increasing number of studies points to the significant role of NO2 as a contributing oxidant, especially in the urban environment.To address the possible relation between atmospheric NO2 and observed Δ33S-values in aerosols, we carried out laboratory experiments oxidizing SO2 by NO2 at temperatures ranging between −7 and 52 °C. Our results show that at temperatures ≥10 °C SO2 oxidation by NO2 is characterized by 1) a 34α-value whose temperature dependence (0.2437/T+0.0457) is distinct from those related to oxidation by O2+TMI, H2O2 and OH oxidation pathways and 2) 33β (0.514 ± 0.0003) and 36β (1.90 ± 0.002) values that are closer to the mass dependent values (0.515 and 1.89 respectively) than those reported for the other oxidation pathways. This implies that the NO2 oxidation pathway cannot explain the extreme Δ33S-values measured in urban aerosols.Our data show that if atmospheric SO2 oxidation by NO2 is neglected, both the O2+TMI and OH oxidation pathways would be overestimated in urban areas. Finally, we conclude that another oxidation reaction is responsible for the high Δ33S-values measured in urban aerosol samples.

The fluid evolution of the Nimbus Ag-Zn-(Au) deposit: An interplay between mantle plume and microbial activity

The Nimbus Ag-Zn-(Au) volcanic-hosted massive sulfide (VHMS) deposit represents an exceptional orebody in the Yilgarn Craton of Western Australia. It is located in a zone of juvenile crust running N-S through the Eastern Goldfields Superterrane. However, unlike most other VHMS occurrences associated with the 2690–2680 Ma rift sequence (Teutonic Bore, Jaguar, Bentley, Erayinia), it is hosted by the ca. 2705 Ma plume-related stratigraphy, which is more typically associated with komatiite-hosted nickel-sulfide mineralization. The Nimbus deposit displays hybrid VHMS-epithermal characteristics resulting from low temperature and shallow water conditions, which developed a quartz-carbonate-sericite dominated alteration assemblage in the dacite host rocks. Sulfide mineralization comprises pyrite, sphalerite, galena, arsenopyrite, Ag-Sb-Pb-Bi sulfosalts, and rare chalcopyrite. In this study, we applied a combination of in-situ analytical techniques to monitor the evolving sulfur isotope signature of the mineralizing fluids, and propose that the Nimbus VHMS deposit developed as a result of a bimodal fluid history. In the first stage, the hydrothermal system – powered by plume-related magmatism – deposited a series of barren pyrite lenses with colloform textures. Their variable but consistently negative mass-independent sulfur isotope fractionation signature (Δ33S = −0.81%) reflects the interaction between mantle-derived magmatic fluids and Archean seawater. The ubiquitous presence of carbon-rich porous textures and the remnants of carbonaceous “nests” in colloform pyrite also indicate a significant contribution of microbial sulfate reducers during the incipient stages of sulfide precipitation. The gradual formation of colloform lenses eventually sealed the hydrothermal system establishing a physical barrier that limited the interaction with seawater. This process promoted the onset of higher temperature and pressure conditions required to form the high-grade Zn-Ag mineralization, sourcing sulfur and fluids largely from a magmatic reservoir (Δ33S = +0.09%). Whereas the identification of a predominantly magmatic sulfur source is supported by sulfur isotope signatures, the recognition of a magmatic origin of the fluid may only be inferred indirectly. It is consistent with the observed quartz-sericite alteration style and the proposed mechanism of sulfide deposition via decompression, which does not require any interaction with seawater. The trace element distribution of the ore-related sulfides also supports a closed hydrothermal system developed through multiple fluid pulses. Indeed, this ore-forming scenario is commonly favored by high confining pressures, which at Nimbus were established following the deposition of the colloform pyrite lenses that progressively sealed the hydrothermal system creating the favorable conditions for mineralization.

Cenozoic record of δ34S in foraminiferal calcite implies an early Eocene shift to deep-ocean sulfide burial

Understanding the changes in, and drivers of, isotopic variability of sulfur in seawater sulfate (δ34SSO4-sw) over geological time remains a long-standing goal, particularly because of the coupling between the biogeochemical sulfur and carbon cycles. The early Cenozoic has remained enigmatic in this regard, as the existing seawater sulfate isotopic records appear to be decoupled from the well-defined carbon isotope composition of the ocean. Here, we present a new Cenozoic record of sulfur isotopes, using carbonate-associated sulfate hosted in the calcite lattice of single-species foraminifera. The vastly improved stratigraphy afforded by this record demonstrates that carbon and sulfur cycles, as recorded by their isotopes, are not fully decoupled in the early Cenozoic. With a model driven by partial coupling of the carbon and sulfur cycles, we demonstrate that a change in sulfur isotopic fractionation of the pyrite burial flux best explains the large increase in δ34SSO4-sw ~53 million years ago (Ma) and the subsequent long steady state. We suggest that the locus of pyrite burial changed from shallow epicontinental seas and shelf environments to more open-ocean sediments around 53 Ma. Loss of extensive shelf environments corresponds to Cretaceous–Palaeogene sea-level changes and tectonic reorganization, occurring as the Himalayan arc first collided with Asia. A Cenozoic reconstruction of the δ34S of marine sulfate suggests a shift in the locus of pyrite burial from shallow seas to the open ocean during the early Eocene.