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1.
An anomalous enrichment in marine sulfate δ34SSO4 is preserved in globally-distributed latest Ediacaran-early Cambrian strata. The proximity of this anomaly to the Ediacaran-Cambrian boundary and the associated evolutionary radiation has invited speculation that the two are causally related. Here we present a high-resolution record of paired sulfate (δ34SSO4) and pyrite (δ34Spyr) from sediments spanning ca. 547-540 million years ago (Ma) from the Ara Group of the Huqf Supergroup, Sultanate of Oman. We observe an increase in δ34SSO4 from ∼20‰ to ∼42‰, beginning at ca. 550 Ma and continuing at least through ca. 540 Ma. There is a concomitant increase in δ34Spyr over this interval from ∼ −15‰ to 10‰. This globally correlative enrichment, here termed the Ara anomaly, constitutes a major perturbation to the sulfur cycle. The absolute values of δ34Spyr reported here and in equivalent sections around the world, require the isotopic composition of material entering the ocean (δ34Sin) to be significantly more enriched than modern (∼3‰) values, likely in excess of 12‰ during the late Ediacaran-early Cambrian. Against this background of elevated δ34Sin, the Ara anomaly is explained not by increased fractionation between sulfate and pyrite (Δδ34S), but by an increase in pyrite burial (fpyr), most likely driven by enhanced primary production and sequestration of organic carbon, consistent with earlier reports of elevated organic carbon burial and widespread phosphorite deposition.  相似文献   

2.
Sulfur isotope compositions of pumice and adsorbed volatiles on ash from the first historical eruption of Anatahan volcano (Mariana arc) are presented in order to constrain the sources of sulfur erupted during the period 10-21 May, 2003. The isotopic composition of S extracted from erupted pumice has a narrow range, from δ34SV-CDT +2.6‰ to +3.2‰, while the composition of sulfur adsorbed onto ash has a larger range (+2.8‰ to +5.3‰). Fractionation modeling for closed and open system scenarios suggests that degassing of SO2 raised the δ34SV-CDT value of S dissolved in the melt from an initial composition of between +1.6‰ and +2.6‰ for closed-system degassing, or between −0.5‰ and +1.5‰ for open-system degassing, however closed-system degassing is the preferred model. The calculated values for the initial composition of the magma represent a MORB-like (δ34SV-CDT ∼ 0‰) mantle source with limited contamination by subducted seawater sulfate (δ34SV-CDT +21‰). Modeling also suggests that the δ34SV-CDT value of SO2 gas in closed-system equilibrium with the degassed magma was between +0.9‰ and +2.5‰. The δ34SV-CDT value of sulfate adsorbed onto ash in the eruption plume (+2.8‰ to +5.1‰) is consistent with sulfate formation by oxidation of magmatic SO2 in the eruption column. The sulfur isotope composition of sulfate adsorbed to ash changes from lower δ34S values for ash erupted early in the eruption to higher δ34S values for ash erupted later in the eruption. We interpret the temporal/stratigraphic change in sulfate isotopic composition to primarily reflect a change in the isotopic composition of magmatic SO2 released from the progressively degassing magma and is attributed to the expulsion of an accumulated gas phase at the beginning of the eruption. More efficient oxidation of magmatic SO2 gas to sulfate in the early water-rich eruption plume probably contributed to the change in S isotope compositions observed in the ash leachates.  相似文献   

3.
Evaluation of the extent of volatile element recycling in convergent margin volcanism requires delineating likely source(s) of magmatic volatiles through stable isotopic characterization of sulfur, hydrogen and oxygen in erupted tephra with appropriate assessment of modification by degassing. The climactic eruption of Mt. Mazama ejected approximately 50 km3 of rhyodacitic magma into the atmosphere and resulted in formation of a 10-km diameter caldera now occupied by Crater Lake, Oregon (lat. 43°N, long. 122°W). Isotopic compositions of whole-rocks, matrix glasses and minerals from Mt. Mazama climactic, pre-climactic and postcaldera tephra were determined to identify the likely source(s) of H2O and S. Integration of stable isotopic data with petrologic data from melt inclusions has allowed for estimation of pre-eruptive dissolved volatile concentrations and placed constraints on the extent, conditions and style of degassing.Sulfur isotope analyses of climactic rhyodacitic whole rocks yield δ34S values of 2.8-14.8‰ with corresponding matrix glass values of 2.4-13.2‰. δ34S tends to increase with stratigraphic height through climactic eruptive units, consistent with open-system degassing. Dissolved sulfur concentrations in melt inclusions (MIs) from pre-climactic and climactic rhyodacitic pumices varies from 80 to 330 ppm, with highest concentrations in inclusions with 4.8-5.2 wt% H2O (by FTIR). Up to 50% of the initial S may have been lost through pre-eruptive degassing at depths of 4-5 km. Ion microprobe analyses of pyrrhotite in climactic rhyodacitic tephra and andesitic scoria indicate a range in δ34S from −0.4‰ to 5.8‰ and from −0.1‰ to 3.5‰, respectively. Initial δ34S values of rhyodacitic and andesitic magmas were likely near the mantle value of 0‰. Hydrogen isotope (δD) and total H2O analyses of rhyodacitic obsidian (and vitrophyre) from the climactic fall deposit yielded values οf −103 to −53‰ and 0.23-1.74 wt%, respectively. Values of δD and wt% H2O of obsidian decrease towards the top of the fall deposit. Samples with depleted δD, and mantle δ18O values, have elevated δ34S values consistent with open-system degassing. These results imply that more mantle-derived sulfur is degassed to the Earth’s atmosphere/hydrosphere through convergent margin volcanism than previously attributed. Magmatic degassing can modify initial isotopic compositions of sulfur by >14‰ (to δ34S values of 14‰ or more here) and hydrogen isotopic compositions by 90‰ (to δD values of −127‰ in this case).  相似文献   

4.
Previous geochemical and microbiological studies in the Cariaco Basin indicate intense elemental cycling and a dynamic microbial loop near the oxic-anoxic interface. We obtained detailed distributions of sulfur isotopes of total dissolved sulfide and sulfate as part of the on-going CARIACO time series project to explore the critical pathways at the level of individual sulfur species. Isotopic patterns of sulfate (δ34SSO4) and sulfide (δ34SH2S) were similar to trends observed in the Black Sea water column: δ34SH2S and δ34SSO4 were constant in the deep anoxic water (varying within 0.6‰ for sulfide and 0.3‰ for sulfate), with sulfide roughly 54‰ depleted in 34S relative to sulfate. Near the oxic-anoxic interface, however, the δ34SH2S value was ∼3‰ heavier than that in the deep water, which may reflect sulfide oxidation and/or a change in fractionation during in situ sulfide production through sulfate reduction (SR). δ34SH2S and Δ33SH2S data near the oxic-anoxic interface did not provide unequivocal evidence to support the important role of sulfur-intermediate disproportionation suggested by previous studies. Repeated observation of minimum δ34SSO4 values near the interface suggests ‘readdition’ of 34S-depleted sulfate during sulfide oxidation. A slight increase in δ34SSO4 values with depth extended over the water column may indicate a reservoir effect associated with removal of 34S-depleted sulfur during sulfide production through SR. Our δ34SH2S and Δ33SH2S data also do not show a clear role for sulfur-intermediate disproportionation in the deep anoxic water column. We interpret the large difference in δ34S between sulfate and sulfide as reflecting fractionations during SR in the Cariaco deep waters that are larger than those generally observed in culturing studies.  相似文献   

5.
This paper presents the results of an isotopic study of spring waters in ?wi?tokrzyski (Holy Cross Mountain) National Park (?NP), south-central Poland. The δ34SV-CDT and δ18OV-SMOW of soluble sulfates (n = 40) varied from 0.5‰ to 18.1‰ and from 3.5‰ to 12.2‰, respectively. The average δ34S values are closely similar to those of rainwater, soils and rocks (comprising scattered pyrite). This suggests that soluble sulfates in the springs originated from mixing of recent and historic deposition, sulfates derived from pyrite oxidation, and CS-mineralization in soils and debris. An additional anthropogenic sulfur input (inorganic fertilizer) occurs in the water of spring S-61 located in the ?wi?tokrzyski National Park buffer zone. The δ18OV-SMOW of spring waters (n = 4) were in the range of −10.6‰ to −10.2‰ indicating that they are derived from vadose groundwater in ?NP. This was the first isotope study of spring waters in the national parks of Poland. It enabled the determination of sulfur pathways and discrimination between natural and anthropogenic sources of this element in a relatively pristine area.  相似文献   

6.
The sulfur isotopic composition of the Herrin (No. 6) Coal from several localities in the Illinois Basin was measured. The sediments immediately overlying these coal beds range from marine shales and limestones to non-marine shales. Organic sulfur, disseminated pyrite, and massive pyrite were extracted from hand samples taken in vertical sections.The δ 34S values from low-sulfur coals (< 0.8% organic sulfur) underlying nonmarine shale were +3.4 to +7.3%0 for organic sulfur, +1.8 to +16.8%0 for massive pyrite, and +3.9 to +23.8%0 for disseminated pyrite. In contrast, the δ 34S values from high-sulfur coals (> 0.8% organic sulfur) underlying marine sediments were more variable: organic sulfur, ?7.7 to +0.5%0, pyrites, ?17.8 to +28.5%0. In both types of coal, organic sulfur is typically enriched in 34S relative to pyritic sulfur.In general, δ 34S values increased from the top to the base of the bed. Vertical and lateral variations in δ 34S are small for organic sulfur but are large for pyritic sulfur. The sulfur content is relatively constant throughout the bed, with organic sulfur content greater than disseminated pyrite content. The results indicate that most of the organic sulfur in high-sulfur coals is derived from post-depositional reactions with a 34S-depleted source. This source is probably related to bacterial reduction of dissolved sulfate in Carboniferous seawater during a marine transgression after peat deposition. The data suggest that sulfate reduction occurred in an open system initially, and then continued in a closed system as sea water penetrated the bed.Organic sulfur in the low-sulfur coals appears to reflect the original plant sulfur, although diagenetic changes in content and isotopic composition of this fraction cannot be ruled out. The wide variability of the δ 34S in pyrite fractions suggests a complex origin involving varying extents of microbial H2S production from sulfate reservoirs of different isotopic compositions. The precipitation of pyrite may have begun soon after deposition and continued throughout the coalification process.  相似文献   

7.
The acidophilic iron-oxidizing bacterium, Acidithiobacillus ferrooxidans, plays a part in the pyrite oxidation process and has been widely studied in order to determine the kinetics of the reactions and the isotopic composition of dissolved product sulphates, but the details of the oxidation processes at the surface of pyrite are still poorly known. In this study, oxygen and sulphur isotopic compositions (δ18O and δ34S) were analyzed for dissolved sulphates and water from experimental aerobic acidic (pH < 2) pyrite oxidation by A. ferrooxidans. The oxidation products attached to the pyrite surfaces were studied for their morphology (SEM), their chemistry (Raman spectroscopy) and for their δ18O (ion microprobe). They were compared to abiotically (Fe3+, H2O2, O2) oxidized pyrite surface compounds in order to constrain the oxidation pathways and to look for the existence of potential biosignatures for this system.The pyrite dissolution evolved from non-stoichiometric (during the first days) to stoichiometric (with increasing time) resulting in dissolved sulphates having distinct δ18O (e.g. +11.0‰ and −2.0‰, respectively) and δ34S (+4.5‰ and +2.8‰, respectively) values. The “oxidation layer” at the surface of pyrite is complex and made of iron oxides, sulphate, polysulphide, elemental sulphur and polythionates. Bio- and Fe3+-oxidation favour the development of monophased micrometric bumps made of hematite or sulphate while other abiotic oxidation processes result in more variable oxidation products. The δ18O of these oxidation products at the surface of oxidized pyrites are strongly variable (from ≈−40‰ to ≈+30‰) for all experiments.Isotopic fractionation between sulphates and pyrite, Δ34SSO4-pyrite, is equal to −1.3‰ and +0.4‰ for sulphates formed by stoichiometric and non-stoichiometric processes, respectively. These two values likely reflect either a S-S or a Fe-S bond breaking process. The Δ18OSO4-H2O and Δ18OSO4-O2 are estimated to be ≈+16‰ and ≈−25‰, respectively. These values are higher than previously published data and may reflect biological effects. The large δ18O heterogeneity measured at the surfaces of oxidized pyrites, whatever the oxidant, may be related (i) to the existence of local surface environments isolated from the solution in which the oxidation processes are different and (ii) to the stabilization at the pyrite surface of reaction intermediates that are not in isotopic equilibrium with the solution. Though the oxygen isotopic composition of surface oxidation products cannot be taken as a direct biosignature, the combined morphological, chemical and isotopic characterization of the surfaces of oxidized pyrites may furnish clues about a biological activity on a mineral surface.  相似文献   

8.
The main terminal processes of organic matter mineralization in anoxic Black Sea sediments underlying the sulfidic water column are sulfate reduction in the upper 2-4 m and methanogenesis below the sulfate zone. The modern marine deposits comprise a ca. 1-m-deep layer of coccolith ooze and underlying sapropel, below which sea water ions penetrate deep down into the limnic Pleistocene deposits from >9000 years BP. Sulfate reduction rates have a subsurface maximum at the SO42−-CH4 transition where H2S reaches maximum concentration. Because of an excess of reactive iron in the deep limnic deposits, most of the methane-derived H2S is drawn downward to a sulfidization front where it reacts with Fe(III) and with Fe2+ diffusing up from below. The H2S-Fe2+ transition is marked by a black band of amorphous iron sulfide above which distinct horizons of greigite and pyrite formation occur. The pore water gradients respond dynamically to environmental changes in the Black Sea with relatively short time constants of ca. 500 yr for SO42− and 10 yr for H2S, whereas the FeS in the black band has taken ca. 3000 yr to accumulate. The dual diffusion interfaces of SO42−-CH4 and H2S-Fe2+ cause the trapping of isotopically heavy iron sulfide with δ34S = +15 to +33‰ at the sulfidization front. A diffusion model for sulfur isotopes shows that the SO42− diffusing downward into the SO42−-CH4 transition has an isotopic composition of +19‰, close to the +23‰ of H2S diffusing upward. These isotopic compositions are, however, very different from the porewater SO42− (+43‰) and H2S (−15‰) at the same depth. The model explains how methane-driven sulfate reduction combined with a deep H2S sink leads to isotopically heavy pyrite in a sediment open to diffusion. These results have general implications for the marine sulfur cycle and for the interpretation of sulfur isotopic data in modern sediments and in sedimentary rocks throughout earth’s history.  相似文献   

9.
A laser fluorination microprobe system has been constructed for high-accuracy, high-precision multisulfur isotope analysis with improved spatial resolution. The system uses two lasers: (a) a KrF excimer laser for in situ spot analysis by ultraviolet (UV) photoablation with λ = 248 nm and (b) a CO2 laser for whole-grain analysis of powdered samples by infrared heating at λ = 10.6 μm. A CO2 laser is necessary for the analysis of interlaboratory isotope reference materials because they are supplied as powders. The δ34S and δ33S compositions of reference materials measured with a CO2 laser fluorination system agree (±0.2‰, 1σ) with the recommended values by the Sulfur Isotope Working Group of the International Atomic Energy Agency [Ding et al 2001] and [Taylor]. The precision of replicate analyses of powdered sulfide minerals with the CO2 laser is typically ±0.2‰ (1σ) for δ34S.The in situ fluorination of sulfides with a KrF excimer laser (λ = 248 nm) was validated by comparison of measurements of side-by-side laser craters and powders excavated from drill holes. Powders from drill holes were analyzed with the CO2 laser. In situ laser craters and drill hole powders give the same δ34SV-CDT and δ33SV-CDT values within 0.2‰. The δ34SV-CDT and δ33SV-CDT values of both powders and in situ analyses are independent of F2 gas pressure over a range of 15 to 65 torr. No dependence of δ34SV-CDT and δ33SV-CDT values on UV laser energy fluence has been observed. Mineral-specific fractionation of sulfur isotopes in analyzing pyrite, sphalerite, galena, troilite, and chalcopyrite has not been observed with a KrF excimer laser (λ = 248 nm). Test analyses with an ArF excimer laser (λ = 193 nm), however, gave fractionated sulfur isotope ratios.A range of Δ33S anomalies of from - 1.5 to +3.0‰ in Archean samples from the North Pole district, Pilbara Craton, Australia, and from black shale of the Lokamonna Formation, South Africa, were verified by in situ analysis of individual pyrite grains with a KrF excimer laser. These results show that a combination of high-accuracy, high-precision analyses with improved spatial resolution permits locating and analyzing host minerals of non-mass-dependent sulfur isotope anomalies.  相似文献   

10.
Jurassic high-sulfur coals from the Maghara area in Egypt were analyzed for the abundance and isotopic composition of different forms of sulfur. Analyses indicated that the sulfur occurs in the form of organic, pyrite, and sulfate forms. Pyrite sulfur represents the major fraction, while sulfate sulfur is minor and could be formed during sample preparation for the analyses.The δ34S CDT values of the organic sulfur are positive ranging between 1.0‰ and 13.5‰ with an average of 9.1‰. Pyrite δ34S values are also positive ranging between 1.5‰ and 15.4‰ with an average of 6.6‰. The high δ34S values of the organic sulfur in the Maghara coals suggest a freshwater origin of the organic components of these coals. The lack of correlation between pyrite and organic sulfur isotopes implies different incorporation mechanisms of sulfur. The high-sulfur contents along with the positive and high δ34S values suggest a marine origin of pyrite sulfur and support the geological interpretation of marine invasion after the peat formation that was responsible for the incorporation of the pyrite sulfur.The occurrence of pyrite as euhedral crystals as well as the high and positive δ34S values of the pyrite sulfur indicates the formation of pyrite during diagenesis as a result of marine water invasion of the preexisting peat in a brackish coastal plain environment.  相似文献   

11.
Sulfur isotopic compositions were determined by ion microprobe for 36 spots on anhydrite crystals in trachyandesitic pumices erupted from El Chichón Volcano in 1982. Individual anhydrite crystals are homogeneous in δ34S, within the ±1‰ (2σ) uncertainty of the method, but crystal-to-crystal variations are large (+2.5 to +10.9‰). The mean δ34S for anhydrite (+6.4 ± 2.1‰, 1σ) is significantly lower than earlier results for bulk anhydrite separates (+9.0 to +9.2‰). The difference between the mean δ34S values in these two populations may reflect a grain-size effect, with heavier sulfur concentrated in smaller anhydrite crystals, few of which were analyzed by ion microprobe. Variations in δ34S show no correlation with complex textures in anhydrite revealed by cathodoluminescence color. Ion-microprobe analyses of δ34S were also obtained on six ovoid-shaped inclusions of pyrrhotite, chalcopyrite, and/or intermediate sulfide solid solution hosted by silicate or oxide crystals, interpreted to be magmatic (δ34S = −0.1 to +2.7‰; mean +0.7‰), and on four irregularly shaped multiphase sulfide fragments in the matrix, interpreted as xenocrystic, which range widely in δ34S (−3.7 to +5.5‰). We evaluate four different mixing scenarios involving (1) magmatic anhydrite and sedimentary sulfate, (2) magmatic anhydrite and hydrothermal anhydrite, and anhydrite and coexisting sulfide crystals precipitated in different domains of a common magma reservoir that were affected by (3) different degrees of degassing or (4) different degrees of crustal sulfur contamination. The model involving physical contamination of sedimentary sulfate is considered untenable. The other three models are considered to be viable, but none of them can explain all observations. The results of this study and other recent investigations prompt a re-evaluation of the sulfur budget for the 1982 El Chichón eruption. We estimate that 2.2 × 1013 g of S was emitted, and that 58 wt.% of the sulfur was present as anhydrite prior to eruption, with the remainder in a vapor phase, with H2S/SO2 ≈ 9. The bulk magmatic δ34S value for the 1982 El Chichón trachyandesite is estimated as +4.1 to +5.8‰, typical of the relatively heavy sulfur isotopic compositions that characterize subduction-related magmas.  相似文献   

12.
Sulfide sulfur in mid-oceanic ridge hydrothermal vents is derived from leaching of basaltic-sulfide and seawater-derived sulfate that is reduced during high temperature water rock interaction. Conventional sulfur isotope studies, however, are inconclusive about the mass-balance between the two sources because 34S/32S ratios of vent fluid H2S and chimney sulfide minerals may reflect not only the mixing ratio but also isotope exchange between sulfate and sulfide. Here, we show that high-precision analysis of S-33 can provide a unique constraint because isotope mixing and isotope exchange result in different Δ33S (≡δ33S-0.515 δ34S) values of up to 0.04‰ even if δ34S values are identical. Detection of such small Δ33S differences is technically feasible by using the SF6 dual-inlet mass-spectrometry protocol that has been improved to achieve a precision as good as 0.006‰ (2σ).Sulfide minerals (marcasite, pyrite, chalcopyrite, and sphalerite) and vent H2S collected from four active seafloor hydrothermal vent sites, East Pacific Rise (EPR) 9-10°N, 13°N, and 21°S and Mid-Atlantic Ridge (MAR) 37°N yield Δ33S values ranging from −0.002 to 0.033 and δ34S from −0.5‰ to 5.3‰. The combined δ34S and Δ33S systematics reveal that 73 to 89% of vent sulfides are derived from leaching from basaltic sulfide and only 11 to 27% from seawater-derived sulfate. Pyrite from EPR 13°N and marcasite from MAR 37°N are in isotope disequilibrium not only in δ34S but also in Δ33S with respect to associated sphalerite and chalcopyrite, suggesting non-equilibrium sulfur isotope exchange between seawater sulfate and sulfide during pyrite precipitation. Seafloor hydrothermal vent sulfides are characterized by low Δ33S values compared with biogenic sulfides, suggesting little or no contribution of sulfide from microbial sulfate reduction into hydrothermal sulfides at sediment-free mid-oceanic ridge systems. We conclude that 33S is an effective new tracer for interplay among seawater, oceanic crust and microbes in subseafloor hydrothermal sulfur cycles.  相似文献   

13.
We present the results of compound-specific sulfur isotope analyses performed on organic sulfur compounds (OSCs) isolated from sediments deposited in the euxinic Cariaco Basin, Venezuela. Individual OSCs (sulfurized highly branched isoprenoids and malabaricatriene) have sulfur isotope compositions of ca. −15‰, which is 34S enriched by 5-15‰ relative to coeval bulk organic and inorganic sulfur pools. These observed differences in the sulfur isotope composition of bulk organic sulfur in the kerogen and bitumen pools and individual OSCs demonstrate that there are multiple pathways of organic sulfur formation operating simultaneously in marine sediments. Comparison of our measured compound-specific sulfur isotope data with values predicted using simple isotopic mass balance assumptions suggests that the sulfurization process likely involves multiple sources of inorganic sulfur. Further, the isotopic composition of these various precursor inorganic sulfur species and the specific pathway of sulfur incorporation into organic matter (OM) impart distinct isotopic compositions to the resulting organic sulfur compounds. These data represent the first compound-specific sulfur isotope measurements made in marine sediments, and demonstrate the utility of compound-specific sulfur isotope analysis in identification of inorganic sulfur sources for OM sulfurization and tracking pathways of sulfur incorporation, which will lead to a more complete understanding of diagenetic sulfurization of OM.  相似文献   

14.
We present multiple sulfur isotope measurements of sulfur compounds associated with the oxidation of H2S and S0 by the anoxygenic phototrophic S-oxidizing bacterium Chlorobium tepidum. Discrimination between 34S and 32S was +1.8 ± 0.5‰ during the oxidation of H2S to S0, and −1.9 ± 0.8‰ during the oxidation of S0 to , consistent with previous studies. The accompanying Δ33S and Δ36S values of sulfide, elemental sulfur, and sulfate formed during these experiments were very small, less than 0.1‰ for Δ33S and 0.9‰ for Δ36S, supporting mass conservation principles. Examination of these isotope effects within a framework of the metabolic pathways for S oxidation suggests that the observed effects are due to the flow of sulfur through the metabolisms, rather than abiotic equilibrium isotope exchange alone, as previously suggested. The metabolic network comparison also indicates that these metabolisms work to express some isotope effects (between sulfide, polysulfides, and elemental sulfur in the periplasm) and suppress others (kinetic isotope effects related to pathways for oxidation of sulfide to sulfate via the same enzymes involved in sulfate reduction acting in reverse). Additionally, utilizing fractionation factors for phototrophic S oxidation calculated from our experiments and for other oxidation processes calculated from the literature (chemotrophic and inorganic S oxidation), we constructed a set of ecosystem-scale sulfur isotope box models to examine the isotopic consequences of including sulfide oxidation pathways in a model system. These models demonstrate how the small δ34S effects associated with S oxidation combined with large δ34S effects associated with sulfate reduction (by SRP) and sulfur disproportionation (by SDP) can produce large (and measurable) effects in the Δ33S of sulfur reservoirs. Specifically, redistribution of material along the pathways for sulfide oxidation diminishes the net isotope effect of SRP and SDP, and can mask the isotopic signal for sulfur disproportionation if significant recycling of S intermediates occurs. We show that the different sulfide oxidation processes produce different isotopic fields for identical proportions of oxidation, and discuss the ecological implications of these results to interpreting minor S isotope patterns in modern systems and in the geologic record.  相似文献   

15.
The Navia gold belt is located in the West Asturian-Leonese Zone of the Iberian Variscan Orogen. The host rocks of the mineralization are quartzites, sandstones and black shales of Cambro-Ordovician age. The gold belt extends along 35 km and has five major veins: Penedela, Encarnita, Fornaza, Carmina and S. Jose. The ores belong to at least four associations having contrasting mineralogies and textures. The δ34S values for individual mineral phases reflect the polyphase metallogenic history. The older association (Stage 1) is Fe-Mn-rich and is made up of spessartine, grunerite-dannemorite and quartz, with magnetite, pyrrhotite and chalcopyrite as metallic phases. The mineralization of Stage 1 is followed by the As-rich Stage 2 with quartz, arsenopyrite and pyrite. The δ34S values for pyrite range from 14.9 to 19.9 per mil (n = 16), and for arsenopyrite from 13.2 to 17.3 per mil (n = 7). The observed isotopic homogeneity likely implies isotopic equilibrium at the scale of the gold vein. Stage 3 contains a coarse-grained base metal sulphide-rich association. The δ4S values for sphalerite range from 16.4 to 20.6 per mil (n= 16), and for galena from 17.0 to 18.7 per mil (n = 11). δ34Ssp > δ34Sgl suggests that the sulphur isotopic fractionation of the ore-forming system had reached equilibrium. The youngest crosscutting mineral association (Stage 4) consists of Pb-Sb sulphosalts, bornite, electrum and quartz. The δ34S values for sulphosalts range from 9.7 to 15.8 per mil, showing the lightest results of the Navia sulphides.The relatively tight clustering of δ34S values of the Au-related sulphides, and the results of fluid inclusions and paragenetic studies, can be interpreted to indicate that the hydrothermal fluids of the last three stages were dominated by H2S. In the H2S predominant field, sulphide minerals precipitating from solutions would exhibit δ34S values similar to the δ34SΣS value of the ore fluid. The heavy δ34SΣS of the Navia fluids is consistent with leaching of sulphur from the host rocks. The main sulphur source could be diagenetic pyrite from the siliciclastic rocks of the Cabos and Luarca Formations, which exhibit δ34S values from 8.3 to 21.2 per mil. An additional sulphur-source in Stage 3 would be the leaching of disseminated sphalerite and galena present in Cambrian carbonates.  相似文献   

16.
The coexistence of magmatic anhydrite and sulfide minerals in non-arc-related mafic magmas has only rarely been documented. Likewise the S isotope fractionation between sulfate and sulfide in mafic rocks has infrequently been measured. In the Kharaelakh intrusion associated with the world-famous Noril’sk ore district in Siberia coexisting magmatic anhydrite and sulfide minerals have been identified. Sulfur isotope compositions of the anhydrite-sulfide assemblages have been measured via both ion microprobe and conventional analyses to help elucidate the origin of the anhydrite-sulfide pairs. Magmatic anhydrite and chalcopyrite are characterized by δ34S values between 18.8‰ and 22.8‰, and 9.3‰ and 13.2‰, respectfully. Coexisting anhydrite and chalcopyrite show Δ values that fall between 8.5‰ and 11.9‰. Anhydrite in the Kharaelakh intrusion is most readily explained by the assimilation of sulfate from country rocks; partial reduction to sulfide led to mixing between sulfate-derived sulfide and sulfide of mantle origin. The variable anhydrite and sulfide δ34S values are a function of differing degrees of sulfate reduction, variable mixing of sulfate-derived and mantle sulfide, incomplete isotopic homogenization of the magma, and a lack of uniform attainment of isotopic equilibrium during subsolidus cooling. The δ34S values of sulfide minerals have changed much less with cooling than have anhydrite values due in large part to the high sulfide/sulfate ratio. Variations in both sulfide and anhydrite δ34S values indicate that isotopically distinct domains existed on a centimeter scale. Late stage hydrothermal anhydrite and pyrite also occur associated with Ca-rich hydrous alteration assemblages (e.g., thomsonite, prehnite, pectolite, epidote, xonotlite). δ34S values of secondary hydrothermal anhydrite and pyrite determined by conventional analyses are in the same range as those of the magmatic minerals. Anhydrite-pyrite Δ values are in the 9.1-10.1‰ range, and are smaller than anticipated for the low temperatures indicated by the silicate alteration assemblages. The small Δ values are suggestive of either sulfate-sulfide isotopic disequilibrium or closure of the system to further exchange between ∼550 and 600 °C. Our results confirm the importance of the assimilation of externally derived sulfur in the generation of the elevated δ34S values in the Kharaelakh intrusion, but highlight the sulfur isotopic variability that may occur in magmatic systems. In addition, our results confirm the need for more precise experimental determination of sulfate-sulfide sulfur isotope fractionation factors in high-T systems.  相似文献   

17.
Previous efforts to constrain the timing of Paleoproterozoic atmospheric oxygenation have documented the disappearance of large, mass-independent sulfur isotope fractionation and an increase in mass-dependent sulfur isotope fractionation associated with multiple glaciations. At least one of these glacial events is preserved in diamictites of the ∼2.4 Ga Meteorite Bore Member of the Kungarra Formation, Turee Creek Group, Western Australia. Outcrop exposures of this unit show the transition from the Boolgeeda Iron Formation of the upper Hamersley Group into clastic, glaciomarine sedimentary rocks of the Turee Creek Group. Here we report in situ multiple sulfur isotope and elemental abundance measurements of sedimentary pyrite at high spatial resolution, as well as the occurrence of detrital pyrite in the Meteorite Bore Member. The 15.3‰ range of Δ33S in one sample containing detrital pyrite (−3.6‰ to 11.7‰) is larger than previously reported worldwide, and there is evidence for mass-independent sulfur isotope fractionation in authigenic pyrite throughout the section (Δ33S from −0.8‰ to 1.0‰). The 90‰ range in δ34S observed (−45.5‰ to 46.4‰) strongly suggests microbial sulfate reduction under non-sulfate limiting conditions, indicating significant oxidative weathering of sulfides on the continents. Multiple generations of pyrite are preserved, typically represented by primary cores with low δ34S (<−20‰) overgrown by euhedral rims with higher δ34S (4-7‰) and enrichments in As, Ni, and Co. The preservation of extremely sharp sulfur isotope gradients (30‰/<4 μm) implies limited sulfur diffusion and provides time and temperature constraints on the metamorphic history of the Meteorite Bore Member. Together, these results suggest that the Meteorite Bore Member was deposited during the final stages of the “Great Oxidation Event,” when pO2 first became sufficiently high to permit pervasive oxidative weathering of continental sulfides, yet remained low enough to permit the production and preservation of mass-independent sulfur isotope fractionation.  相似文献   

18.
A diffusion-diagenesis model of the sulfur cycle is developed to calculate theoretical distributions of stable sulfur isotopes in marine sediments. The model describes the depth variation in δ34S of dissolved sulfate and H2S. and of pyrite. The effects of sulfate reduction, sulfate and H2S diffusion. and of sedimentation are considered as well as the bacterial isotope fractionation and the degree of pyrite formation. Under open system conditions of sulfur diagenesis the isotopic difference, ΔSO2?4 — H2S, tends to increase with depth being smaller than the bacterial fractionation factor near the sediment surface and larger in deeper layers. The two isotopes in SO2?4 or in H2S do not diffuse in the same proportion as they occur in the porewater. This explains why sulfur, which is incorporated from seawater sulfate by diffusion and precipitation as pyrite, can be enriched in 32S relative to the seawater sulfate. The model calculations demonstrate the importance of taking the whole dynamic sulfur cycle into account before drawing conclusions about sulfur diagenesis from the stable isotope distribution.  相似文献   

19.
Oxidation of pyrite by hydrogen peroxide (H2O2) at millimolar levels has been studied from 4 to 150 °C in order to evaluate isotopic effects potentially associated with radiolytic oxidation of pyrite. Gaseous, aqueous, and solid phases were collected and measured following sealed-tube experiments that lasted from 1 to 14 days. The dominant gaseous product was molecular oxygen. No volatile sulfur species were recovered from any experiment. Sulfate was the only aqueous sulfur species detected in solution, with sulfite and thiosulfate below the detection limits. X-ray diffraction patterns and images from scanning electron microscopy reveal solid residues composed primarily of hydrated ferric iron sulfates and sporadic ferric-ferrous iron sulfates. Hematite was detected only in solid residue produced during high temperature experiments. Elemental sulfur and/or polysulfides are inferred to be form on reacting pyrite surface based on extraction with organic solvents. Pyrite oxidation by H2O2 increases in rate with increasing H2O2concentration, pyrite surface area, and temperature. Rates measured in sealed-tube experiments at 25°C, for H2O2 concentration of 2 × 10−3 M are 8.8 × 10−9 M/m2/sec, which are higher than previous estimates. A combination of reactive oxygen species from H2O2 decomposition products and reactive iron species from pyrite dissolution is inferred to aggressively oxidize the receding pyrite surface. Competing oxidants with temperature-dependent oxidation efficiencies results in multiple reaction mechanisms for different temperatures and surface conditions. Sulfur isotope values of remaining pyrite were unchanged during the experiments, but showed distinct enrichment of 34S in produced sulfate and depletion in elemental sulfur. The Δsulfate-pyrite and Δelemental sulfur-pyrite was +0.5 to +1.5‰ and was −0.2 to −1‰, respectively. Isotope data from high-temperature experiments indicate an additional 34S-depleted sulfur fraction, with up to 4‰ depletion of 34S, in the hematite. Sulfur isotope trends were not influenced by H2O2 concentration, temperature, or reaction time. Results of this study indicate that radiolytically produced oxidants, such as hydrogen peroxide and hydroxyl radicals, could efficiently oxidize pyrite in an otherwise oxygen-limited environment. Although H2O2 is generally regarded as being of minor geochemical significance on Earth, the H2O2 molecule plays a pivotal role in Martian atmospheric and soil chemistry. Additional experimental and field studies are needed to characterize sulfur and oxygen isotope systematics during radiolytical oxidation of metallic sulfides and elemental sulfur.  相似文献   

20.
罗河铁矿(又称庐枞铁矿)是我国长江中下游火山岩地区大型铁矿床之一,是一种重要的矿床类型。十多年来,各研究单位和生产部门对该矿床进行了多方面、较系统的研究,取得可喜成果(张荣华等,1974;张荣华,1981;陈锦石等,1982)。然而,要进一步阐明这类矿床的成矿机理,还需要对该矿床的蚀变和成矿条件进行深入研究。本文着重讨论罗河铁矿的硫同位素分馏机制,并在此基础上结合地质和矿物包裹体的资料,近似定量地推算该矿床形成各阶段的物理化学条件范围及变化规律。  相似文献   

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