Sulfur Isotope Exchange Research Articles

Platform for Multiple Isotope Labeling via Carbon-Sulfur Bond Exchange.

In this work, we explore a nickel-catalyzed reversible carbon-sulfur (C-S) bond activation strategy to achieve selective sulfur isotope exchange. Isotopes are at the foundation of applications in life science, such as nuclear imaging, and are essential tools for the determination of pharmacokinetic and dynamic profiles of new pharmaceuticals. However, the insertion of an isotope into an organic molecule remains challenging, and current technologies are element-specific. Despite the ubiquitous presence of sulfur in many biologically active molecules, sulfur isotope labeling is an underexplored field, and sulfur isotope exchange has been overlooked. This approach enables us to move beyond standardized element-specific procedures and was applied to multiple isotopes, including deuterium, carbon-13, sulfur-34, and radioactive carbon-14. These results provide a unique platform for multiple isotope labeling and are compatible with a wide range of substrates, including pharmaceuticals. In addition, this technology proved its potential as an isotopic encryption device for organic molecules.

Subduction zone sulfur mobilization and redistribution by intraslab fluid–rock interaction

Subduction zones mediate the sulfur exchange between Earth’s interior and surface, and play a critical role in the long-term global sulfur cycle. Dehydration of the subducted slab releases aqueous fluids, which in turn interact with the surrounding wall-rocks during reactive flow through the slab. Fluids are regarded as the key agent for sulfur transfer within the slab and subsequently into the mantle wedge. However, the mechanisms underlying sulfur mobilization and redistribution during fluid–rock interaction are not fully understood. Here, we investigate three high-pressure blueschist/eclogite–selvage–vein systems from the Southwestern Tianshan metamorphic belt in northwestern China to explore sulfur behavior during fluid–rock interaction along channelized fluid pathways and within their alteration halos. Petrologic observations on mineral parageneses and δ34S compositions of sulfides reveal two different scenarios of sulfur mobilization and redistribution during fluid–rock interaction at HP conditions. Fe-rich and medium-fS2 (near the pyrite–pyrrhotite buffer) fluids favor pyrrhotite stability over pyrite and can trigger the pyrite–to–pyrrhotite transition in the reaction halo. In this case the bulk-rock sulfur budget may not vary but the sulfur isotope exchange is still effective, producing a δ34S mixing trend in the metasomatic zones. In contrast, Fe-poor and high-fS2 fluids cause conversion of pyrrhotite to pyrite, significant sulfur addition, and sulfur isotope exchange in alteration selvages. Medium-fS2 fluids can transport sulfur and thus are effective in transferring sulfur out of the slab, whereas high-fS2 fluids cause sulfur sequestration along fluid pathways and considerably reduce long-distance sulfur transfer. Multiple fluid infiltration events with different fluid sources result in several pyrite overgrowth sequences, recorded by contrasting Co–Ni element distributions and in-situ δ34S zoning of pyrite. The heaviest δ34S value (+25‰) thus far reported in HP rocks has been found in vein pyrite, suggesting that a seawater sulfate-derived δ34S signature can be transported to great depths in a subduction zone, even though the timing and mechanism of sulfate–to–sulfide reduction in the subduction zone remain unconstrained. Our study provides natural evidence for fluid-mediated sulfide transformation, sulfur transport/storage, and sulfur isotope exchange during fluid–rock interaction, and illuminates the role these processes play in sulfur release from the subducting slab and sulfur subduction into the deep mantle.

Sulfur isotope exchange between thiophenes and inorganic sulfur compounds under hydrous pyrolysis conditions

Sulfur isotope fractionation during thermal degradation of aromatic organic sulfur compounds was studied utilizing quartz tube-hydrous pyrolysis at 360°C. BT (benzothiophene), DBT (dibenzothiophene), 3-PT (3-phenylthiophene) and 2-OT (2-octylthiophene) were used to represent organic sulfur compounds with a range of thermal stabilities. These compounds were heated for 40h with and without the addition of reduced inorganic sulfur species: elemental S, sodium sulfide (Na2S∗9H2O) and sodium sulfite (Na2SO3). The extent of degradation of each compound showed good correlations with its aromatic stabilization and strongly depended on the added inorganic sulfur species, with DBT being the most stable and 2-OT the least stable. Sulfur isotope enrichments up to 2.2‰ resulted from the thermal degradation of thiophenes. A significant decrease in stability of thiophenes was observed in the presence of reduced sulfur species. In contrast, BT and DBT did not show any exchange of S isotopes with external reduced S pool, while 3-PT did experience S isotope exchange. The results of the present study suggest that the main controls on the δ34S values of BT and DBT during TSR are the relative rates of their degradation and their formation from the reaction between TSR-H2S and petroleum hydrocarbons. This finding improves our confidence in using the δ34S of individual thiophenes as reliable proxies for the occurrence and extent of thermal maturation and thermochemical sulfate reduction.

Identification of sulfur sources and isotopic equilibria in submarine hot-springs using multiple sulfur isotopes

Multiple sulfur isotopes were measured in metal sulfide deposits, elemental sulfur, and aqueous hydrogen sulfide to constrain sulfur sources and the isotopic systematics of precipitation in seafloor hydrothermal vents. Areas studied include the Eastern Manus Basin and Lau Basin back-arc spreading centers and the unsedimented basalt-hosted Southern East Pacific Rise (SEPR) and sediment-hosted Guaymas Basin mid-ocean ridge spreading centers. Chalcopyrite and dissolved hydrogen sulfide (H2S) δ34S values range from −5.5‰ to +5.6‰ in Manus Basin samples, +2.4‰ to +6.1‰ in Lau Basin samples, and +3.7‰ to +5.7‰ in SEPR samples. Values of δ34S for cubic cubanite and H2S range from −1.4‰ to +4.7‰ in Guaymas Basin samples.Multiple sulfur isotope systematics in fluid-mineral pairs from the SEPR and Lau Basin show that crustal host rock and thermochemical reduction of seawater-derived dissolved sulfate (SO4) are the primary sources of sulfur in mid-ocean ridge and some back-arc systems. At PACMANUS and SuSu Knolls hydrothermal systems in the Eastern Manus Basin, a significant contribution of sulfur is derived from disproportionation of magmatic sulfur dioxide (SO2), while the remaining sulfur is derived from crustal host rocks and SO4 reduction. At the sedimented Guaymas Basin hydrothermal system, sulfur sources include crustal host rock, reduced seawater SO4, and biogenic sulfide. Vent fluid flow through fresher, less-mature sediment supplies an increased quantity of reactant organic compounds that may reduce 34S-enriched SO4, while fluid interaction with more highly-altered sediments results in H2S characterized by a small, but isotopically-significant input of 34S-depleted biogenic sulfides.Near-zero Δ33S values in all samples implicate the abiotic processes of SO4 reduction and leaching of host rock as the major contributors to sulfur content at a high temperature unsedimented mid-ocean ridge and at a back-arc system. Δ33S values indicate that SO2 disproportionation is an additional process that contributes sulfur to a different back-arc system and to acid spring-type hydrothermal fluid circulation. At the sedimented Guaymus Basin, near-zero Δ33S values are also observed, despite negative δ34S values that indicate inputs of biogenic pyrite for some samples.In contrast with previous studies reporting isotope disequilibrium between H2S and chalcopyrite, the δ34S values of chalcopyrite sampled from the inner 1–2mm of a chimney wall are within ±1‰ of δ34S values for H2S in the paired vent fluid, suggesting equilibrium fluid-mineral sulfur isotope exchange at 300–400°C. Isotopic equilibrium between hydrothermal fluid H2S and precipitating chalcopyrite implies that sulfur isotopes in the chalcopyrite lining across a chimney wall may accurately record past hydrothermal activity.

Os, Nd, O and S isotope constraints on country rock contamination in the conduit-related Eagle Cu–Ni–(PGE) deposit, Midcontinent Rift System, Upper Michigan

The Eagle Cu–Ni–(PGE) deposit is hosted by mafic to ultramafic intrusive rocks associated with the Marquette–Baraga dike swarm in northern Michigan. Sulfide mineralization formed in a conduit system during early stages in the development of the ∼1.1Ga Midcontinent Rift System. The conduit environment represents a prime location for melt–rock interaction. In order to better assess the extent of country rock contamination in the Eagle system, a combined trace element, Nd, Os, O and S isotope study of country rocks, sulfide-bearing igneous rocks and massive sulfide was undertaken.Both the Eagle and the weakly mineralized East Eagle intrusion show trace element patterns that are similar to those of picritic basalts that formed during early stages of rift development. The trace element, Os, Nd, and O isotopic values of the igneous rocks are consistent with <5% of bulk contamination by Paleoproterozoic and Archean country rocks. Both the Re–Os and Sm–Nd system provide isochrons that are in agreement with the 1107Ma U–Pb baddeleyite age of the intrusive rocks. Calculated γOs(1100) and εNd(1100) values for the magmas are +34 and −2. δ18O values of pyroxene in feldspathic pyroxenite range from 6.5‰ to 6.6‰ and provide the only indication that bulk contamination may locally have exceeded 20%. Sulfur isotopic values of disseminated and massive sulfide in the Eagle intrusion range from 0.3‰ to 4.6‰. The δ34S values are much lower than those that characterize most of the country rocks, but could still be indicative of a contribution of S from country rocks of up to ∼50%. Δ33S values of the disseminated and massive sulfides range from −0.10‰ to 0.09‰ indicating a source in Paleoproterozoic country rocks. Semi-massive sulfide in the Eagle deposit has δ34S values between 2.2‰ and 5.3‰, and Δ33S values show a broad range between −0.86‰ and 0.86‰ indicating a major contribution from an Archean source. Isotopic data from the Eagle deposit strongly indicate that multiple sources of sulfur were involved in the generation of the Ni–Cu–(PGE) mineralization, and magmas which traversed variable paths through the mantle and crust were focused and utilized the same conduit at the level of the Eagle deposit. Our results emphasize the fact that the sulfur isotopic values of immediate country rocks may not be appropriate as end-member values for mixing calculations; sulfur derivation from deeper in the conduit system may have involved rocks with very different sulfur isotope ratios. In addition, sulfur isotopic exchange reactions between passing magma and accumulated crystals or sulfide liquid in the conduit may have led to decreased sulfur isotope variability and δ34S values near those of mantle sulfur.

Processes affecting δ 34S and δ 18O values of dissolved sulfate in alluvium along the Canadian River, central Oklahoma, USA

The δ 34S and δ 18O values for dissolved sulfate in groundwater are commonly used in aquifer studies to identify sulfate reservoirs and describe biogeochemical processes. The utility of these data, however, often is compromised by mixing of sulfate sources within reservoirs and isotope fractionation during sulfur redox cycling. Our study shows that, after all potential sulfate sources are identified and isotopically characterized, the δ 34S SO4 and δ 18O SO4 values differentiate processes such as sulfate-source mixing, sulfide oxidation, barite dissolution, and organosulfur decomposition. During bacterial reduction of sulfate, the values reflect kinetic sulfur isotope fractionation and exchange of oxygen isotopes between sulfate and water. Detailed analysis of the chemistry (Cl and SO 4 concentrations) and isotopic composition (δ 2H H2O and δ 18O H2O) of groundwater in an alluvial aquifer in Central Oklahoma, USA allowed the identification of five distinct end members that supply water to the aquifer (regional groundwater flowing into the study area, river water, leachate from a closed landfill that operated within the site, rain, and surface runoff). The δ 34S SO4 and δ 18O SO4 values in each end member differentiated three sources of sulfate: sulfate dissolved from Early to Late Permian rocks within the drainage basin (δ 34S SO4 = 8–12‰ and δ 18O SO4 = 10‰), iron sulfides oxidized by molecular oxygen during low water-table levels (δ 34S SO4 = − 16‰ and δ 18O SO4 = 10‰), and organosulfur compounds (predominately ester sulfates) from decomposition of vegetation on the surface and from landfill trash buried in the alluvium (δ 34S SO4 = 8‰ and δ 18O SO4 = 6‰). During bacterial reduction of these sulfate sources, similar isotope fractionation processes are recorded in the parallel trends of increasing δ 34S SO4 and δ 18O SO4 values. When extensive reduction occurs, the kinetic sulfur isotope fractionation (estimated by ε H2S–SO4 = − 23‰) results in the steady increase of δ 34S SO4 values to greater than 70‰. Equilibrium isotope fractionation during exchange of sulfate oxygen and water oxygen, a process not commonly observed in field-based studies, is documented in δ 18O SO4 values asymptotically approaching 21‰, the value predicted for conditions at the study site ( ε SO4–H2O = 27‰). These results show that recognition of all potential sulfate sources is a critical first step to resolving complexities in δ 34S SO4 and δ 18O SO4 data. The approach taken in this study can be used in other aquifer systems where the identification of multiple sulfate sources and sulfur redox cycling is important to understanding natural processes and anthropogenic influences.

The HDS mechanism: Which “auxiliary” process takes place – sulfur isotopic exchange or replacement – and why is it important to know it?

In the light of general evaluation of the radioisotopic data obtained during the years of our research two possible ways of sulfur transfer from the catalyst into H 2S in the course of thiophene HDS have been reconsidered. These are (i) sulfur exchange between H 2S formed and the catalyst sulfur and (ii) extrusion of sulfide sulfur into the gas phase by the interaction of the catalyst SH group with H 2 with the further formation of H 2S. The data of our radioisotopic studies supports the second way, which proceeds within the “forcing out” mechanism. This mechanism assumes either of the two possible schemes of H 2S formation—(a) H 2 interaction with the catalyst bridge sulfur and (b) H 2 interaction with the SH group. The radioisotopic studies support the second possibility.

S-33 constraints on the seawater sulfate contribution in modern seafloor hydrothermal vent sulfides

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 H 2S 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 Δ 33S (≡δ 33S–0.515 δ 34S) values of up to 0.04‰ even if δ 34S values are identical. Detection of such small Δ 33S differences is technically feasible by using the SF 6 dual-inlet mass-spectrometry protocol that has been improved to achieve a precision as good as 0.006‰ (2 σ). Sulfide minerals (marcasite, pyrite, chalcopyrite, and sphalerite) and vent H 2S 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 Δ 33S values ranging from −0.002 to 0.033 and δ 34S from −0.5‰ to 5.3‰. The combined δ 34S and Δ 33S 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 δ 34S but also in Δ 33S 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 Δ 33S 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.

The δ34S values of the early-cleaved sulfur upon low temperature pyrolysis of a synthetic polysulfide cross-linked polymer

Closed and open kinetic pyrolysis experiments were conducted at 200 °C on synthetic sulfur cross-linked polymer. Sulfur isotopic analyses of the released H 2S and the residual organic sulfur revealed large discrimination (6–9.5‰) between the early-formed, isotopically light H 2S and the residual organic sulfur. This indicates that there is a relatively large kinetic isotope effect during thermal alteration of organic sulfur. This large discrimination decreases as the reaction time increases. Isotope exchange between organic sulfur and inorganic sulfur species rapidly occur under closed system conditions. Therefore, we suggest that the early isotopic discrimination in closed system is minimized due to both gradual release of heavier sulfur isotope and isotopic exchange processes as the pyrolysis experiments advance. These results have significant geochemical implications for source rock–oil–H 2S correlations.

Stable-isotope geochemistry of the Pierina high-sulfidation Au–Ag deposit, Peru: influence of hydrodynamics on SO 42−−H 2S sulfur isotopic exchange in magmatic-steam and steam-heated environments

The Pierina high-sulfidation Au–Ag deposit formed 14.5 my ago in rhyolite ash flow tuffs that overlie porphyritic andesite and dacite lavas and are adjacent to a crosscutting and interfingering dacite flow dome complex. The distribution of alteration zones indicates that fluid flow in the lavas was largely confined to structures but was dispersed laterally in the tuffs because of a high primary and alteration-induced permeability. The lithologically controlled hydrodynamics created unusual fluid, temperature, and pH conditions that led to complete SO 4 2−–H 2S isotopic equilibration during the formation of some magmatic-steam and steam-heated alunite, a phenomenon not previously recognized in similar deposits. Isotopic data for early magmatic hydrothermal and main-stage alunite (δ 34S=8.5‰ to 31.7‰; δ 18O SO 4 =4.9‰ to 16.5‰; δ 18O OH=2.2‰ to 14.4‰; δD=−97‰ to −39‰), sulfides (δ 34S=−3.0‰ to 4.3‰), sulfur (δ 34S=−1.0‰ to 1.1‰), and clay minerals (δ 18O=4.3‰ to 12.5‰; δD=−126‰ to −81‰) are typical of high-sulfidation epithermal deposits. The data imply the following genetic elements for Pierina alteration–mineralization: (1) fluid and vapor exsolution from an I-type magma, (2) wallrock buffering and cooling of slowing rising vapors to generate a reduced (H 2S/SO 4≈6) highly acidic condensate that mixed with meteoric water but retained a magmatic δ 34S ΣS signature of ∼1‰, (3) SO 2 disproportionation to HSO 4 − and H 2S between 320 and 180 °C, and (4) progressive neutralization of laterally migrating acid fluids to form a vuggy quartz→alunite–quartz±clay→intermediate argillic→propylitic alteration zoning. Magmatic-steam alunite has higher δ 34S (8.5‰ to 23.2‰) and generally lower δ 18O SO 4 (1.0 to 11.5‰), δ 18O OH (−3.4 to 5.9‰), and δD (−93 to −77‰) values than predicted on the basis of data from similar occurrences. These data and supporting fluid-inclusion gas chemistry imply that the rate of vapor ascent for this environment was unusually slow, which provided sufficient time for the uptake of groundwater and partial to complete SO 4 2−–H 2S isotopic exchange. The slow steam velocities were likely related to the dispersal of the steam column as it entered the tuffs and possibly to intermediate exsolution rates from magmatic brine. The low δD values may also partly reflect continuous degassing of the mineralizing magma. Similarly, data for steam-heated alunite (δ 34S=12.3‰ to 27.2‰; δ 18O SO 4 =11.7‰ to 13.0‰; δ 18O OH=6.6‰ to 9.4‰; δD=−59‰ to −42‰) are unusual and indicate a strong magmatic influence, relatively high temperatures (140 to 180 °C, based on Δ 18 O SO 4–OH fractionations), and partial to complete sulfur isotopic exchange between steam-heated sulfate and H 2S. Restricted lithologically controlled fluid flow in the host tuffs allowed magmatic condensate to supplant meteoric groundwater at the water table and create the high-temperature low-pH conditions that permitted unusually rapid SO 4 2−–H 2S isotopic equilibration (50–300 days) and (or) long sulfate residence times for this environment. Late void-filling barite (δ 34S=7.4‰ to 29.7‰; δ 18O SO 4 =−0.4‰ to 15.1‰) and later void-filling goethite (δ 18O=−11.8‰ to 0.2‰) document a transition from magmatic condensate to dominantly meteoric water in steam-heated fluids during cooling and collapse of the hydrothermal system. These steam-heated fluids oxidized the top ∼300 m of the deposit by leaching sulfides, redistributing metals, and precipitating barite±acanthite±gold and goethite–hematite±gold. Steam-heated oxidation, rather than weathering, was critical to forming the orebody in that it not only released encapsulated gold but likely enriched the deposit to ore-grade Au concentrations.

A review of the stable-isotope geochemistry of sulfate minerals in selected igneous environments and related hydrothermal systems

The stable-isotope geochemistry of sulfate minerals that form principally in I-type igneous rocks and in the various related hydrothermal systems that develop from their magmas and evolved fluids is reviewed with respect to the degree of approach to isotope equilibrium between minerals and their parental aqueous species. Examples illustrate classical stable-isotope systematics and principles of interpretation in terms of fundamental processes that occur in these systems to produce (1) sulfate in igneous apatite, (2) igneous anhydrite, (3) anhydrite in porphyry-type deposits, (4) magmatic-hydrothermal alunite and closely related barites in high-sulfidation mineral deposits, (5) coarse-banded alunite in magmatic-steam systems, (6) alunite and jarosite in steam-heated systems, (7) barite in low-sulfidation systems, (8) all of the above minerals, as well as soluble Al and Fe hydroxysulfates, in the shallow levels and surface of active stratovolcanoes. Although exceptions are easily recognized, frequently, the sulfur in these systems is derived from magmas that evolve fluids with high H 2S/SO 2. In such cases, the δ 34S values of the igneous and hydrothermal sulfides vary much less than those of sulfate minerals that precipitate from magmas and from their evolved fluids as they interact with igneous host rocks, meteoric water, oxygen in the atmosphere, and bacteria in surface waters. Hydrogen isotopic equilibrium between alunite and water and jarosite and water is always initially attained, thus permitting reconstruction of fluid history and paleoclimates. However, complications may arise in interpretation of δD values of magmatic-hydrothermal alunite in high-sulfidation gold deposits because later fluids may effect a postdepositional retrograde hydrogen–isotope exchange in the OH site of the alunite. This retrograde exchange also affects the reliability of the SO 4–OH oxygen–isotope fractionations in alunite for use as a geothermometer in this environment. In contrast, retrograde exchange with later fluids is not significant in the lower temperature steam-heated environment, for which SO 4–OH oxygen–isotope fractionations in alunite and jarosite can be an excellent geothermometer. Sulfur isotopic disequilibrium between coexisting (but noncontemporaneous) igneous anhydrite and sulfide may occur because of loss of fluid, assimilation of country-rock sulfur during crystallization of these minerals from a magma, disequilibrium effects related to reactions between sulfur species during fluid exsolution from magma, or because of retrograde isotope exchange in the sulfides. Anhydrite and coexisting sulfide from porphyry deposits commonly closely approach sulfur–isotope equilibrium, as is indicated by the general agreement of sulfur–isotope and filling temperatures (315 to 730 °C) in quartz. The data from anhydrite and coexisting sulfides also record a significant range in H 2S/SO 4 2− and δ 34S ΣS among deposits and even during the course of mineralization at a single deposit. Sulfur isotopic disequilibrium among aqueous sulfur species may occur in any hydrothermal environment except the relatively high-temperature (200–400 °C) low-pH (<3) magmatic-hydrothermal environment, in which SO 4 2− forms along with H 2S from the disproportionation of SO 2 during the condensation of a magmatic vapor plume. Magmatic–steam alunite forms from expanding SO 2-rich magmatic vapors that rise so rapidly that sulfur isotopic exchange between SO 4 2− and H 2S does not occur during disproportionation of SO 2. Such alunite has δ 34S values similar to that of the bulk sulfur in the magma. Residence times of SO 4 2− in steam-heated systems, although seldom long enough to permit more than partial sulfur–isotope exchange with streaming H 2S, often are long enough to permit oxygen–isotope equilibrium with water. In active stratovolcanoes, aqueous sulfate derived from the disproportionation of SO 2 and the oxidation of H 2S can also mix with that derived from the oxidation of pyrite near the surface. In the near-neutral low-sulfidation system at Creede, CO, isotopic exchange among hydrothermal aqueous species was so slow that sulfur and even oxygen isotopic signatures derived from bacteriogenic and thermochemical reduction of sulfate in moat sediments are observed in hydrothermal barite.

Kinetics of sulfur isotope exchange between aqueous sulfide and thiosulfate involving intra- and intermolecular reactions at hydrothermal conditions

Sulfur isotope exchange between sulfide (H 2S) and thiosulfate (HSSO 3H) can be described by the general rate law for a two-compound system (X and AB) with three exchangeable atoms (X, A, and B) proposed by [X. Chu, H. Ohmoto, Kinetics of isotope exchange reactions involving intra- and intermolecular reactions: I. Rate law for a system with two chemical compounds and three exchangeable atoms. Geochim. Cosmochim. Acta 55 1991 1953–1961]. According to the rate law, the isotope exchange reaction is comprised of one overall intramolecular exchange between sulfane (–SH or SH) and sulfonate (–SO 3H or SO 3H) sulfurs of thiosulfate (i.e., SH⇔SO 3H in thiosulfate) and two overall intermolecular exchanges between sulfide and sulfane sulfur of thiosulfate (i.e., H 2S⇔SH of thiosulfate) and between sulfide and sulfonate sulfur of thiosulfate (i.e., H 2S⇔SO 3H of thiosulfate). The rate constants for the three overall exchange reactions and the equilibrium isotopic fractionation factors among sulfide, sulfane, and sulfonate of thiosulfate were estimated by fitting [F. Uyama, H. Chiba, M. Kusakabe, H. Sakai, Sulfur isotope exchange reaction in the aqueous system: thiosulfate–sulfide–sulfate at hydrothermal temperature. Geochem. J. 19 1985 301–315] experimental data on sulfur isotope exchange between aqueous H 2S and sodium thiosulfate by the least squares method. At temperatures of 100–170 °C, the equilibrium fractionation factors (in per mil) can be expressed as: 1000 ⁢ ln α H 2 S – SH = − 0.327 ± 0.055 ( 10 12 / T 4 ) + 2.676 ± 0.341 ( 10 6 / T 2 ) 1000 ⁢ ln α SO 3 H – SH = − 0.352 ± 0.009 ( 10 12 / T 4 ) + 7.523 ± 0.054 ( 10 6 / T 2 ) and 1000 ⁢ ln α SO 3 H – H 2 S = − 0.0293 ± 0.058 ( 10 12 / T 4 ) − 4.871 ± 0.357 ( 10 6 / T 2 ) ( T in K). At near-neutral pH, the overall rate (m −1 s −1) for the sulfur isotope exchange between H 2S and –SO 3H of thiosulfate is described by log k SO 3 H ⇔ H 2 S = − 5.14 ( 10 3 / T ) + 10.35 ( T in K) with an activation energy of 98.3 kJ/mol at 100–170 °C. A comparison of the rates of sulfur exchanges among H 2S, –SH, and –SO 3H of thiosulfate with the rates of polysulfide–thiosulfate formation and disproportion reactions determined by [W.F. Giggenbach, Kinetics of the polysulfide–thiosulfate disproportionation up to 240 °C. Inorg. Chem. 13 1974b 1730–1733] suggests that the sulfur isotope exchanges between aqueous sulfide and thiosulfate may proceed via the formation and disproportionation of polysulfides (e.g., S 3S 2−, S 4S 2−, etc.): 10H 2S+3S 2O 3 2−=4S 3S 2−+2H ++9H 2O and S n S 2−+SSO 3 2−=S n+1 S 2−+SO 3 2− . The disproportionation reaction of polysulfides appears to control the exchange rate between S 2− and S 6+ atoms in thiosulfate and is considered the rate-determining step in the sulfate–sulfide exchange reaction rather than the intramolecular exchange of thiosulfate proposed by [H. Ohmoto, A.C. Lasaga, Kinetics of reactions between aqueous sulfates and sulfides in hydrothermal systems. Geochim. Cosmochim. Acta 46 1982 1727–1745]. Therefore, polysulfides may play an important role in the chemical and isotopic reactions between aqueous sulfide and sulfate under hydrothermal conditions.

SULFUR ISOTOPE EXCHANGE AND METAL ENRICHMENT IN THE FORMATION OF MAGMATIC Cu-Ni-(PGE) DEPOSITS

Sulfur isotope measurements provide a useful means of evaluating the source of sulfur in magmatic Cu-Ni-PGE deposits. Although low sulfur, PGE-rich deposits (e.g., J-M reef of the Stillwater Complex, Merensky reef of the Bushveld Complex) show little or no isotopic evidence for the involvement of crustally derived sulfur, several high sulfur deposits (e.g., Noril’sk, Duluth Complex) show strong evidence for contamination of mafic magmas by sulfur of crustal origin. Processes such as magma degassing or redox changes accompanying crystallization are ineffective in significantly modifying sulfur isotope signatures of sulfides that crystallize from immiscible liquids that exsolve from the mafic magmas. For this reason δ 34S values are primarily dependent on sulfur source characteristics, and variations can often be treated using a two-component mixing approach. However, deposits such as those at Noril’sk and Voisey’s Bay that appear to have formed in magma conduit systems present another possibility for modification of sulfur isotope signatures. These deposits are thought to have been upgraded in metal tenor by reaction with uncontaminated mantle-derived melts that traversed the accumulated sulfide on their way to the surface. Our calculations illustrate the efficiency of reactions that involve the exchange of Fe for Cu, Ni, or PGE in producing increases in the metal tenor of the accumulated sulfide. Sulfur isotope exchange between the crustally contaminated sulfide ore accumulation and sulfur of mantle origin may accompany chalcophile element exchange, and is capable of masking or limiting the degree of isotopic evidence for the initial stage of ore genesis that involved crustal sulfur. Sulfur isotope exchange is distinct from the addition of sulfur ( in the form of immiscible sulfide droplets from a sulfide-saturated magma) to the accumulated sulfide, although both additive mixing and exchange can lead to similar isotopic effects. Stable isotopic exchange provides a mechanism in addition to additive mixing that may lead to decoupling of different isotopic systems.

Sulfur Isotopic Composition of H2S and SO4 2− from Mineral Springs in the Polish Carpathians

A number of springs in Carpathian Mts. contain dissolved H2S and SO4 2- in concentrations above 10 mg/dm3. In this study we have investigated the sulfur isotope composition (δ34S) of the dissolved sulfur species in the springs from the flysch area in the Carpathian Mts. along the tectonic dislocation. It is believed that some of these springs may carry a major fraction of dissolved sulfur species of extremely deep sulfur (of mantle origin), which is subjected to SO4 2-—H2S isotope exchange at high temperatures. The original isotopic compositions may be modified by reduction/oxidation at low temperatures and by admixture of sulfur from other sources. In order to distinguish the sulfur of mantle origin we investigated δ34S of dissolved sulfide and sulfate and on the basis of known concentrations we calculated δ34S of total dissolved sulfur. The isotope fractionation between sulfate and sulfide helped to distinguish the sulfur origin. Evaluating the sulfur isotope exchange, we selected 4 springs which likely have only weakly disturbed sulfur of mantle origin.

The metamorphism of pyrite and pyritic ores: an overview

AbstractPyrite, the most widespread and abundant of sulphide minerals in the Earth's surficial rocks, commonly constitutes the primary opaque phase in ore deposits. Consequently, an understanding of the behaviour of pyrite and its relationships with coexisting phases during the metamorphism of pyritebearing rocks is vital to the interpretation of their genesis and post-depositional history. Metamorphism is commonly responsible for the obliteration of primary textures but recent studies have shown that the refractory nature of pyrite allows it to preserve some pre-metamorphic textures. Pyrrhotite in pyritic ores has often been attributed to the breakdown of pyrite during metamorphism. It is now clear that pyrrhotite can be primary and that the presence of pyrrhotite with the pyrite provides a buffer that constrains sulphur activity during metamorphism. Pyrite-pyrrhotite ratios change during metamorphism as prograde heating results in sulphur release from pyrite to form pyrrhotite and as retrograde cooling permits re-growth of pyrite as the pyrrhotite releases sulphur. Retrograde growth of pyrite may encapsulate textures developed during earlier stages as well as preserve evidence of retrograde events. Sulphur isotope exchange of pyrite with pyrrhotite tends to homogenise phases during prograde periods but leaves signatures of increasingly heavy sulphur in the pyrite during retrograde periods.

Sulfur isotope exchange between 35S-labeled inorganic sulfur compounds in anoxic marine sediments

Isotope exchange reactions between 35S-labeled sulfur compounds were studied in anoxic estuarine sediment slurries at 21°C and pH 7.4–7.7. Two experiments labeled with radioactive elemental sulfur ( 35S°) and one labeled with radioactive sulfate ( 35SO 4 2−) were performed as time-course experiments of up to 10 days. More than 85% of the added 35S° was distributed into the reduced sulfur pools comprising ΣHS −(=H 2S-HS −+S 2−), iron sulfide (FeS) and pyrite (FeS 2) in less than 10 min after 35S° labeling. When 1.2 μmol ΣHS − cm −3 was present in the slurry, 64% of the total 35S was recovered in the ΣHS − pool in less than 1.5 h. With no detectable σHS − (less than 1 μM) in the slurry, 58% of the total 35S was observed in the pyrite pool within 1.5 h. The FeS pool received up to 31% of all 35S added. The rapid 35S incorporation from 35S° into ΣHS − and FeS pools was explained by isotope exchange reactions. In contrast, there was evidence that the radioactivity observed in the ‘pyrite pool’ was caused by adhesion of the added 35S° to the FeS 2 grains. In all 35S°-labeled experiments we also observed oxidation, in the absence of oxygen, of reduced sulfur to 35SO 4 2−, which reached more than 24% of the total 35S after 72 h. In the 35SO 4 2− experiment, ΣH 35S − was formed and we observed a significant isotope exchange between H 35S − and S° as well as precipitation of Fe 35S. As a result of isotope exchange, specific radioactivities of the reduced sulfur pools were poorly defined and could not be used to calculate their rates of formation. Such isotope exchange reactions between the reduced inorganic sulfur compounds will affect the stable isotope distribution and are expected to decrease the isotopic fractionations. All 35S, however, that was incorporated into AVS (i.e. ΣHS −+FeS), S° and FeS 2 during short-term 35SO 4 2− incubations originated from the formed H 2 35S, and realistic sulfate reduction rates therefore were measured from the sum of the radioactivities of reduced sulfur compounds.

ChemInform Abstract: Sulfur Isotopic Exchange Between Two Thiophosphoryl Compounds.

ChemInform Abstract: Sulfur Isotopic Exchange Between Two Thiophosphoryl Compounds.

Sulphur isotope exchange between sulphides and fluid sulphur: consequences for geothermometric application

Sulphur isotope exchange between sulphides and fluid sulphur: consequences for geothermometric application

Kinetics and Mechanism of Isotopic Exchange Reaction Between Elemental Sulfur and O,O-Diaryl Dithiophosphates

Abstract The salts derived from O,O-disubstituted dithiophosphates and tertiary amines are commonly known as useful starting materials in a variety of 35 important organic syntheses including labelling with the 35S (1, 2). The sulfur isotopic exchange reaction occurs in dry toluene at (RO)2PSSHNRi + S. S8s (RO)2PSSHNR'3 + 8 ⇄(RO), sb.2PS*SHNR'3 + s8 R = Ar; R′= Alk, Ar z J o temperature above 100 C. The reaction is exactly first order with respect to dithiophosphate, whereas the order in sulfur decreases from 1 to 0 with increase of sulfur concentration. Electron-withdrawing substituents in ester groups and electron-realesing substituents at nitrogen increase the reaction rate. The sulfur isotopic exchange does not occur practically in diaryl dithiophosphoric acids and their tetraethylammonium salts. The reaction mechanism, in which an isomerization of trialkylammoniwn dithiophosphate to an ion-pair precedes the exchange proper reaction with elemental sulfur, has been supported by kinetic isotope effect kH/kD of ammonium hydrogen. For the isotope exchange with either elemental sulfur or dibenzyl trisulfide the same values of both first-order rate constant and activation parameters of the dithiophosphate isomerization reaction have been determined.

A mass-independent sulfur isotope effect in the nonthermal formation of S2F1

A non-mass-dependent sulfur isotope effect is present in the rotationally symmetric S2F10 molecule, produced in an electrical discharge through sulfur tetrafluoride. A similar isotopic fractionation was observed in the product S2F10 from the electrodissociation of SF5Cl, and reaction between fluorine atoms produced by F2 photolysis and SF4, collectively ruling out the SF5 formation process as the source of the mass-independent fractionation. The mass-independent mechanism is shown to occur in the SF5+SF5→S2F10 formation step. The secondary dissociation of S2F10 as a source of the mass-independent fractionation is ruled out by control S2F10 dissociation experiments which are shown to produce small mass-dependent fractionations. Mass-dependent effects such as sulfur isotopic exchange and secondary dissociation reactions are significant processes for the system under study and have been quantitatively accounted for. The role of symmetry in non-mass-dependent isotope effects is strengthened by the present experiments, and the search and characterization of mass-independent effects is extended to sulfur-containing molecules.