Mass-independent Fractionation Of Sulfur Isotopes Research Articles

Multiple sulfur isotope and mineralogical constraints on the genesis of Ni-Cu-PGE magmatic sulfide mineralization of the Monchegorsk Igneous Complex, Kola Peninsula, Russia

We present the results of a pilot investigation of multiple sulfur isotopes for the Ni-Cu-PGE sulfide mineralization of the ∼2.5 Ga Monchegorsk Igneous Complex (MIC). Base Metal Sulfide (BMS) compositions, Platinum Group Element (PGE) distributions, and Platinum Group Mineral (PGM) assemblages were also studied for different types of Ni-Cu-PGE mineralization. The uniformly low S content of the country rocks for the MIC as well as variable Sm-Nd isotope systematics and low-sulfide, PGE-rich mineralization of the MIC suggest that S saturation was reached via assimilation of silicates rather than assimilation of sulfur-rich lithologies. R-factor modeling suggests that the mixing ratio for silicate-to-sulfide melt was very high, well above 15,000 for the majority of our mineralized samples, as might be expected for the low-sulfide, PGE-rich mineralization of the MIC. Small, negative Δ33S values (from −0.23 to −0.04 ‰) for sulfides in strongly metamorphosed MIC-host rocks indicate that their sulfur underwent mass-independent sulfur isotope fractionation (MIF) in the oxygen-poor Archean atmosphere before it was incorporated into the protoliths of the host paragneisses and homogenized during metamorphism. Ore minerals from the MIC have similar Δ33S values (from −0.21 to −0.06 ‰) consistent with country rock assimilation contributing to sulfide saturation, but, also importantly, our dataset suggests that Δ33S values decrease from the center to the margin of the MIC as well as from early to late magmatic phases, potentially indicating that both local assimilation of host rocks and S homogenization in the central part of the large intrusion took place.

Paleoarchean sulfur cycling: Multiple sulfur isotope constraints from the Barberton Greenstone Belt, South Africa

Mass-dependent and mass-independent sulfur isotope fractionation archived in volcanic and sedimentary rocks from the Barberton Greenstone Belt (3550–3215Ma), South Africa, provide constraints for sulfur cycling on the early Earth. Four different sample suites were studied: komatiites and tholeiites, barite, massive and disseminated sulfide ores, and non-mineralized black shales.Variable but generally slightly positive δ34S values between −0.7 and +5.2‰, negative Δ33S values between −0.50 and −0.09‰, and a negative correlation between δ34S and Δ33S as well as between Δ33S and Δ36S for komatiites and tholeiites from the Komati Formation and from the Weltevreden Formation are outside the expected range of unfractionated juvenile sulfur. Instead, results suggest alteration of oceanic crustal rock sulfur through interactions with fluids that most likely derived their sulfur from seawater.Barite from the Mapepe Formation displays positive δ34S values between +3.1 and +8.1‰ and negative Δ33S values between −0.77 and −0.34‰. The mass-independent sulfur isotope fractionation indicates an atmospheric sulfur source, notably photolytic sulfate, whereas the positive δ34S values suggest bacterial sulfate reduction of the marine sulfate reservoir.Non-mineralized black shale samples from the presumed stratigraphic equivalent of the Mapepe Formation show positive δ34S values between 0.0 and +1.3‰ and positive Δ33S values between +0.59 and +2.45‰. These results are interpreted to result from the reduction of photolytic elemental sulfur, carrying a positive Δ33S signature.Positive δ34S values ranging from +0.7 to +3.5‰ and slightly negative Δ33S values between −0.17 and −0.12‰ characterize massive and disseminated sulfides from the Bien Venue Prospect. Results suggest unfractionated juvenile magmatic sulfur source as the primary sulfur source, but a contribution from recycled seawater sulfate, which would be indicative of submarine hydrothermal activity, cannot be ruled out.Massive and disseminated sulfides from the M’hlati prospect are distinctly different from massive and disseminated sulfide from the Bien Venue Prospect. They show negative δ34S values between −1.2 and −0.1‰ and positive Δ33S values between +2.66 and +3.17‰, thus, displaying a sizeable mass-independent sulfur isotopic fractionation. Again, these samples clearly exhibit the incorporation of an atmospheric MIF-S signal. The source of sulfur for these samples has positive Δ33S values, suggesting a connection with photolytic elemental sulfur.In conclusion, the sulfur isotope signatures in Paleoarchean rocks from the Barberton Greenstone Belt are diverse and indicate the incorporation of different sources of sulfur. For komatiites and tholeiites, barite and massive and possibly also disseminated sulfides from Bien Venue, multiple sulfur isotopes are related to ambient seawater sulfate and its photolytic origin, while massive and disseminated sulfides from M’hlati and non-mineralized black shales are related to a second (photolytic elemental sulfur) end member.

SO<sub>2</sub> photolysis as a source for sulfur mass-independent isotope signatures in stratospehric aerosols

Abstract. Signatures of sulfur isotope mass-independent fractionation (S-MIF) have been observed in stratospheric sulfate aerosols deposited in polar ice. The S-MIF signatures are thought to be associated with stratospheric photochemistry following stratospheric volcanic eruptions, but the exact mechanism responsible for the production and preservation of these signatures is debated. In order to identify the origin and the mechanism of preservation for these signatures, a series of laboratory photochemical experiments were carried out to investigate the effect of temperature and added O2 on the S-MIF produced by two absorption band systems of SO2: photolysis in the 190 to 220 nm region and photoexcitation in the 250 to 350 nm region. The SO2 photolysis (SO2 + hν → SO + O) experiments showed S-MIF signals with large 34S/34S fractionations, which increases with decreasing temperature. The overall S-MIF pattern observed for photolysis experiments, including high 34S/34S fractionations, positive mass-independent anomalies in 33S, and negative anomalies in 36S, is consistent with a major contribution from optical isotopologue screening effects and data for stratospheric sulfate aerosols. In contrast, SO2 photoexcitation produced products with positive S-MIF anomalies in both 33S and 36S, which is different from stratospheric sulfate aerosols. SO2 photolysis in the presence of O2 produced SO3 with S-MIF signals, suggesting the transfer of the S-MIF anomalies from SO to SO3 by the SO + O2 + M → SO3 + M reaction. This is supported with energy calculations of stationary points on the SO3 potential energy surfaces, which indicate that this reaction occurs slowly on a single adiabatic surface, but that it can occur more rapidly through intersystem crossing. Based on our experimental results, we estimate a termolecular rate constant on the order of 10−37 cm6 molecule−2 s−1. This rate can explain the preservation of mass independent isotope signatures in stratospheric sulfate aerosols and provides a minor, but important, oxidation pathway for stratospheric SO2. The production and preservation of S-MIF signals requires a high SO2 column density to allow for optical isotopologue screening effects to occur and to generate a large enough signature that it can be preserved. In addition, the SO2 plume must reach an altitude of around 20 to 25 km, where SO2 photolysis becomes a dominant process. These experiments are the first step towards understanding the origin of the sulfur isotope anomalies in stratospheric sulfate aerosols.

Rovibrational bound states of SO2 isotopologues. I: Total angular momentum J = 0–10

Isotopic variation of the rovibrational bound states of SO2 for the four stable sulfur isotopes 32–34,36S is investigated in comprehensive detail. In a two-part series, we compute the low-lying energy levels for all values of total angular momentum in the range J=0–20. All rovibrational levels are computed, to an extremely high level of numerical convergence. The calculations have been carried out using the ScalIT suite of parallel codes. The present study (Paper I) examines the J=0–10 rovibrational levels, providing unambiguous symmetry and rovibrational label assignments for each computed state. The calculated vibrational energy levels exhibit very good agreement with previously reported experimental and theoretical data. Rovibrational energy levels, calculated without any Coriolis approximations, are reported here for the first time. Among other potential ramifications, this data will facilitate understanding of the origin of mass-independent fractionation of sulfur isotopes in the Archean rock record—of great relevance for understanding the “oxygen revolution”.

Stability and abundance of the trisulfur radical ion [formula omitted] in hydrothermal fluids

The interpretation of sulfur behavior in geological fluids and melts is based on a long-standing paradigm that sulfate, sulfide, and sulfur dioxide are the major sulfur compounds. This paradigm was recently challenged by the discovery of the trisulfur ion S3− in aqueous S-bearing fluids from laboratory experiments at elevated temperatures. However, the stability and abundance of this potentially important sulfur species remain insufficiently quantified at hydrothermal conditions. Here we used in situ Raman spectroscopy on model thiosulfate, sulfide, and sulfate aqueous solutions across a wide range of sulfur concentration (0.5–10.0 wt%), acidity (pH 3–8), temperature (200–500 °C), and pressure (15–1500 bar) to identify the different sulfur species and determine their concentrations. Results show that in the low-density (<0.2 g/cm3) vapor phase, H2S is the only detectable sulfur form. By contrast, in the denser liquid and supercritical fluid phase, together with sulfide and sulfate, the trisulfur radical ion S3− is a ubiquitous and thermodynamically stable species from 200 °C to at least 500 °C. In addition, the disulfur radical ion S2− is detected at 450–500 °C in most solutions, and polymeric molecular sulfur with a maximum abundance around 300 °C in S-rich solutions. These results, combined with revised literature data, allow the thermodynamic properties of S3− to be constrained, enabling quantitative predictions of its abundance over a wide temperature and pressure range of crustal fluids. These predictions suggest that S3− may account for up to 10% of total dissolved sulfur (Stot) at 300–500 °C in fluids from arc-related magmatic–hydrothermal systems, and more than 50% Stot at 600–700 °C in S-rich fluids produced via prograde metamorphism of pyrite-bearing rocks. The trisulfur ion may favor the mobility of sulfur itself and associated metals (Au, Cu, Pt, Mo) in geological fluids over a large range of depth and provide the source of these elements for orogenic Au and porphyry-epithermal Cu–Au–Mo deposits. Furthermore, the ubiquity of S3− in aqueous sulfate–sulfide systems offers new interpretations of the kinetics and mechanisms of sulfur redox reactions at elevated temperatures and associated mass-dependent and mass-independent fractionation of sulfur isotopes.

Multiple sulfur and oxygen isotope compositions in Beijing aerosol

Multiple sulfur isotopes (32S, 33S, 34S, 36S) and oxygen isotopes (16O, 18O) in Beijing aerosols were measured with MAT-253 isotope mass spectrometer. The δ 34S values of Beijing aerosol samples range from 1.68‰ to 12.57‰ with an average value of 5.86‰, indicating that the major sulfur source is from direct emission during coal combustion. The δ 18O values vary from -5.29‰ to 9.02‰ with an average value of 5.17‰, revealing that the sulfate in Beijing aerosols is mainly composed of the secondary sulfate. The main heterogeneous oxidation of SO2 in atmosphere is related to H2O2 in July and August, whereas H2O2 oxidation and Fe3+ catalytic oxidation with SO2 exist simultaneously in September and October. Remarkable sulfur isotope mass-independent fractionation effect is found in Beijing aerosols, which is commonly attributed to the photochemical oxidation of SO2 in the stratosphere. In addition, thermochemical reactions of sulfur-bearing compounds might be also a source of sulfur isotope anomalies based on the correlation between Δ33S and CAPE.

Sulfur isotope mass-independent fractionation in impact deposits of the 3.2 billion-year-old Mapepe Formation, Barberton Greenstone Belt, South Africa

Theoretical and experimental studies have shown that atmospheric SO2 isotopologue self-shielding effects in the 190–220nm region of the solar spectrum are the likely cause for mass independent fractionation of sulfur isotopes (S-MIF). The main products of this photochemical reaction – SO3 and S0 – typically define a compositional array of ca. Δ33S/δ34S=0.06–0.14. This is at odds with the generally observed trend in Archean sulfides, which broadly defines an array of ca. Δ33S/δ34S=0.9. Various explanations have been proposed, including a diminution of δ34S caused by chemical and biogenic mass-dependent fractionation of sulfur isotopes (S-MDF), mixing with photolytic products produced during felsic volcanic events, or partial blocking of the low-wavelength part of the spectrum due to the presence of reduced atmospheric gases or an organic haze. Early in Earth history large meteorite impacts would have ejected dust and gas clouds into the atmosphere that shielded solar radiation and affected global climate. It is thus likely that at certain time intervals of high meteorite flux the atmosphere was significantly perturbed, having an effect on atmospheric photochemistry and possibly leaving anomalous sulfur isotopic signatures in the rock record. Here we describe the sulfur isotopic signatures in sulfides of spherule beds S2, S3 and S4 of the Barberton Greenstone Belt, South Africa. In particular, in spherule bed S3 – and to a lesser extent S4 – a trend of ca. Δ33S/δ34S=0.23 is observed that closely follows the expected trend for SO2-photolysis in the 190–220nm spectral range. This suggests that an impact dust cloud (deposited as spherule beds), which sampled the higher region of the atmosphere, specifically incorporated products of SO2 photolysis in the 190–220nm range, and blocked photochemical reactions at higher wavelengths (250–330nm band). By implication, the generally observed Archean trend appears to be the result of mixing of different MIF-S sources arising from a variety of photochemical reactions that took place in the lower part of the atmosphere.

Sulfur sources of sedimentary “buckshot” pyrite in the Auriferous Conglomerates of the Mesoarchean Witwatersrand and Ventersdorp Supergroups, Kaapvaal Craton, South Africa

Large rounded pyrite grains (>1 mm), commonly referred to as “buckshot” pyrite grains, are a characteristic feature of the auriferous conglomerates (reefs) in the Witwatersrand and Ventersdorp supergroups, Kaapvaal Craton, South Africa. Detailed petrographic analyses of the reefs indicated that the vast majority of the buckshot pyrite grains are of reworked sedimentary origin, i.e., that the pyrite grains originally formed in the sedimentary environment during sedimentation and diagenesis. Forty-one of these reworked sedimentary pyrite grains from the Main, Vaal, Basal, Kalkoenkrans, Beatrix, and Ventersdorp Contact reefs were analyzed for their multiple sulfur isotope compositions (δ34S, Δ33S, and Δ36S) to determine the source of the pyrite sulfur. In addition, five epigenetic pyrite samples (pyrite formed after sedimentation and lithification) from the Middelvlei and the Ventersdorp Contact reefs were measured for comparison. The δ34S, Δ33S, and Δ36S values of all 41 reworked sedimentary pyrite grains indicate clear signatures of mass-dependent and mass-independent fractionation and range from −6.8 to +13.8 ‰, −1.7 to +1.7 ‰, and −3.9 to +0.9 ‰, respectively. In contrast, the five epigenetic pyrite samples display a very limited range of δ34S, Δ33S, and Δ36S values (+0.7 to +4.0 ‰, −0.3 to +0.0 ‰. and −0.3 to +0.1 ‰, respectively). Despite the clear signatures of mass-independent sulfur isotope fractionation, very few data points plot along the primary Archean photochemical array suggesting a weak photolytic control over the data set. Instead, other factors command a greater degree of influence such as pyrite paragenesis, the prevailing depositional environment, and non-photolytic sulfur sources. In relation to pyrite paragenesis, reworked syngenetic sedimentary pyrite grains (pyrite originally precipitated along the sediment-water interface) are characterized by negative δ34S and Δ33S values, suggesting open system conditions with respect to sulfate supply and the presence of microbial sulfate reducers. On the contrary, most reworked diagenetic sedimentary pyrite grains (pyrite originally precipitated below the sediment-water interface) show positive δ34S and negative Δ33S values, suggesting closed system conditions. Negligible Δ33S anomalies from epigenetic pyrite suggest that the sulfur was sourced from a mass-dependent or isotopically homogenous metamorphic/hydrothermal fluid. Contrasting sulfur isotope compositions were also observed from different depositional environments, namely fluvial conglomerates and marine-modified fluvial conglomerates. The bulk of the pyrite grains from fluvial conglomerates are characterized by a wide range of δ34S values (−6.2 to +4.8 ‰) and small Δ33S values (±0.3 ‰). This signature likely represents a crustal sulfate reservoir derived from either volcanic degassing or from weathering of sulfide minerals in the hinterland. Reworked sedimentary pyrite grains from marine-modified fluvial conglomerates share similar isotope compositions, but also produce a positive Δ33S/δ34S array that overlaps with the composition of Archean barite, suggesting the introduction of marine sulfur. These results demonstrate the presence of multiple sources of sulfur, which include atmospheric, crustal, and marine reservoirs. The prevalence of the mass-dependent crustal sulfur isotope signature in fluvial conglomerates suggests that sulfate concentrations were probably much higher in terrestrial settings in comparison to marine environments, which were sulfate-deficient. However, the optimum conditions for forming terrestrial sedimentary pyrite were probably not during fluvial progradation but rather during the early phases of flooding of low angle unconformities, i.e., during retrogradational fluvial deposition, coupled in some cases with marine transgressions, immediately following inflection points of maximum rate of relative sea level fall.

Geochemistry and Si–O–Fe isotope constraints on the origin of banded iron formations of the Yuanjiacun Formation, Lvliang Group, Shanxi, China

Banded iron formations (BIFs) within the Lvliang region of Shanxi Province, China, are hosted by sediments of the Yuanjiacun Formation, part of the Paleoproterozoic Lvliang Group. These BIFs are located in a zone where sedimentation changed from clastic to chemical deposition, indicating that these are Superior-type BIFs. Here, we present new major, trace, and rare earth element (REE) data, along with Fe, Si, and O isotope data for the BIFs in the Yuanjiacun within the Fe deposits at Yuanjiacun, Jianshan, and Hugushan. When compared with Post Archean Australian Shale (PAAS), these BIFs are dominated by iron oxides and quartz, contain low concentrations of Al2O3, TiO2, trace elements, and the REE, and are light rare earth element (LREE) depleted and heavy rare earth element (HREE) enriched. The BIFs also display positive La, Y, and Eu anomalies, high Y/Ho ratios, and contain 30Si depleted quartz, with high δ18O values that are similar to quartz within siliceous units formed during hydrothermal activity. These data indicate that the BIFs within the Yuanjiacun Formation were precipitated from submarine hydrothermal fluids, with only negligible detrital contribution. None of the BIF samples analyzed during this study have negative Ce anomalies, although a few have a positive Ce anomaly that may indicate that the BIFs within the Yuanjiacun Formation formed during the Great Oxidation Event (GOE) within a redox stratified ocean. The positive Ce anomalies associated with some of these BIFs are a consequence of oxidization and the formation of surficial manganese oxide that have preferentially adsorbed Ho, LREE, and Ce4+; these deposits formed during reductive dissolution at the oxidation–reduction transition zone or in deeper-level reducing seawater. The loss of Ce, LREE, and Ho to seawater and the deposition of these elements with iron hydroxides caused the positive Ce anomalies observed in some of the BIF samples, although the limited oxidizing ability of surface seawater at this time meant that Y/Ho and LREE/HREE ratios were not substantially modified, unlike similar situations within stratified ocean water during the Late Paleoproterozoic. Magnetite and hematite within the BIFs in the study area contain heavy Fe isotopes (56Fe values of 0.24–1.27‰) resulting from the partial oxidation and precipitation of Fe2+ to Fe3+ in seawater. In addition, mass-independent fractionation of sulfur isotopes within pyrite indicates that these BIFs were deposited within an oxygen-deficient ocean associated with a similarly oxygen-deficient atmosphere, even though the BIFs within the Yuanjiacun Formation formed after initiation of the GOE.

Mass-independent fractionation of sulfur isotopes during broadband SO2 photolysis: Comparison between 16O- and 18O-rich SO2

This paper describes a comparison of ultraviolet photolysis experiments undertaken with SO2 (oxygen with isotopes at natural abundance levels) and S18O2 (18O-substituted oxygen). Experiments were conducted in a closed photocell using a deuterium lamp (principally 190–235nm) under pressure regimes (5–25Torr) that produced optically thick conditions for 32SO2 and variable optical depths for other isotopologues. The experiments, which were designed to examine the effects of intramolecular isotopic substitution of oxygen atoms on the S-MIF produced during UV photolysis of SO2, reveal generally reduced sulfur fractionation for 18O-rich SO2 as compared to 16O-rich SO2. Model shielding calculations were undertaken using spectra that were shifted due to changes in rotational and vibrational energy levels. The model calculations suggest that processes in addition to rotational and vibrational shifts in absorption spectra play a role in the experimentally produced isotope effects. Such additional processes may include differences in primary photoexcitation arising from smaller peak-to-valley amplitudes for fine structure of 18O-rich SO2 absorption spectra or an isotopically selective process associated with transitions between excited states.

Multiple sulfur and carbon isotope composition of sediments from the Belingwe Greenstone Belt (Zimbabwe): A biogenic methane regulation on mass independent fractionation of sulfur during the Neoarchean?

To explore the linkage between mass-independent sulfur isotope fractionation (MIF-S) and δ13Corg excursions during the Neoarchean, as well as the contemporary redox state and biogeochemical cycling of carbon and sulfur, we report the results of a detailed carbon and multiple sulfur (δ34S, δ33S, δ36S) isotopic study of the ∼2.7Ga Manjeri and ∼2.65Ga Cheshire formations of the Ngezi Group (Belingwe Greenstone Belt, Zimbabwe).Multiple sulfur isotope data show non-zero Δ33S and Δ36S values for sediments older than 2.4Ga (i.e. prior to the Great Oxidation Event, GOE), indicating MIF-S thought to be associated with low atmospheric oxygen concentration. However, in several 2.7–2.5Ga Neoarchean localities, small-scale variations in MIF-S signal (magnitude) seem to correlate with negative excursion in δ13Corg, possibly reflecting a global connection between the relative reaction rate of different MIF-S source reaction and sulfur exit channels and the biogenic flux of methane into the atmosphere during periods of localized, microbiologically mediated, shallow surface-water oxygenation.The Manjeri Formation black shales studied here display a wide range of δ13Corg between −35.4‰ and −16.2‰ (average of −30.3±6.0‰, 1σ), while the Cheshire Formation shales have δ13Corg between −47.7‰ and −35.1‰ (average −41.3±3‰, 1σ). The δ34S values of sedimentary sulfides from Manjeri Formation vary between −15.15‰ and +2.37‰ (average −1.71±4.76‰, 1σ), showing very small and mostly negative Δ33S values varying from −0.58‰ to 0.87‰ (average 0.02±0.43‰, 1σ). Cheshire Formation black shale sulfide samples measured in this study have δ34S values ranging from −2.11‰ to 2.39‰ (average 0.25±1.08‰, 1σ) and near zero and solely positive Δ33S anomalies between 0.14‰ and 1.17‰ (average 0.56±0.29‰, 1σ). Moreover, Δ36S/Δ33S in the two formations are comparable with a slope of −1.38 (Manjeri Formation) and −1.67 (Cheshire Formation), respectively. This differs from the Archean reference line (i.e. −0.9).The sulfur and carbon isotopic signatures recorded here likely reflect different environmental conditions and/or ecosystems within the sampled Cheshire and Manjeri formations paleofacies. The differences in carbon isotopes in different sedimentological facies are interpreted as recording different metabolic pathways, including photosynthesis, methanogenesis and methanotrophy. Low δ13Corg values in the Cheshire Formation black shales suggest a strong influence of methanotrophy (likely preceded by bacterial methanogenesis), while the large range of Manjeri Formation δ34S may record bacterial sulfate reduction. The C and multiple S isotopic variations recorded here may imply different patterns of C and S fractionation, perhaps dependent on variations in the biogenic egress of methane to the atmosphere.

No mass-independent sulfur isotope fractionation in auriferous fluids supports a magmatic origin for Archean gold deposits

The source of the auriferous fluids that formed Archean greenstone gold deposits remains controversial; metamorphic fluids, fluids derived from felsic magmas, mantle fluids, and meteoric water have all been suggested. We report the results of ion microprobe (sensitive high-resolution ion microprobe stable isotopes) multiple sulfur isotope analyses ( 32 S, 33 S, 34 S) conducted to constrain the source of sulfur in four Archean gold deposits. The analyses are of pyrite samples that are genetically related to the gold mineralization from the Bellerophon, Victory, and Wallaby deposits in the Eastern Goldfield superterrane of the Yilgarn craton, Western Australia, together with sulfide from black shale from the Golden Mile deposit and St. Ives gold camp. Ore-associated pyrite from the Beattie gold deposit of the Abitibi greenstone belt of the Superior craton in Canada was measured to test whether the results obtained from the Yilgarn craton apply to other Archean gold deposits. Sulfur analyses from sulfide from shale show mass-independent fractionation (MIF) with Δ 33 S as high as +1.8‰, whereas those from the 4 Archean gold deposits are dominated by mass-dependent fractionation, with only a minor contribution of MIF sulfur. In this respect, Archean gold deposits contrast with Archean NiS deposits, which are dominated by MIF sulfur. The absence of MIF in the pyrite from the four Archean gold deposits analyzed in this study is consistent with the auriferous fluid originating from a felsic magma or mantle source, but inconsistent with the metamorphic and meteoritic fluid hypotheses.

The Archean komatiite-hosted, PGE-bearing Ni–Cu sulfide deposit at Vaara, eastern Finland: evidence for assimilation of external sulfur and post-depositional desulfurization

Archean komatiites host important resources of Ni, Cu, Co, and PGE, particularly in Western Australia and Canada. In Finland, several small, low-grade sulfide deposits have been found in komatiites, including the ca. 2.8 Ga Vaara deposit in the Archean Suomussalmi greenstone belt. It occurs in the central part of the serpentinized olivine cumulate zone of a komatiitic extrusive body and is composed of disseminated interstitial sulfides consisting of pyrite, pentlandite, millerite, violarite, and chalcopyrite accompanied by abundant magnetite. Although currently subeconomic, the mineralization is interesting due to the very high chalcophile element contents of the sulfide fraction (38 wt% Ni, 3.4 wt% Cu, 0.7 wt% Co, 22.4 ppm Pd, and 9.5 ppm Pt). The sulfides occur in relatively Cr-poor olivine cumulates suggesting involvement of a chromite-undersaturated magma. The parental magma was an Al-undepleted komatiite with an estimated MgO content of at least 24 wt%. In contrast to the common komatiite types in the eastern Finland greenstone belts, the Vaara rocks are moderately enriched in LREE relative to MREE, suggesting that crustal contamination played an important role in the genesis of the Vaara deposit. Multiple sulfur isotope data reveal considerable mass-independent sulfur isotope fractionation both in country rock sedimentary sulfides (Δ33S ranges from −0.50 to +2.37 ‰) and in the Vaara mineralization (Δ33S ranges from +0.53 to +0.66 ‰), which provides strong evidence for incorporation of crustal sulfur. Extensive replacement of interstitial sulfides by magnetite and the presence of millerite- and violarite-bearing, pyrrhotite-free sulfide assemblages indicate significant post-magmatic, low-temperature hydrothermal oxidation of the primary magmatic pyrrhotite-pentlandite-chalcopyrite assemblages and associated sulfur loss that led to a significant upgrading of the original metal tenors of the Vaara deposit.

Production, preservation, and biological processing of mass-independent sulfur isotope fractionation in the Archean surface environment

Mass-independent fractionation of sulfur isotopes (S MIF) in Archean and Paleoproterozoic rocks provides strong evidence for an anoxic atmosphere before ~2,400 Ma. However, the origin of this isotopic anomaly remains unclear, as does the identity of the molecules that carried it from the atmosphere to Earth's surface. Irrespective of the origin of S MIF, processes in the biogeochemical sulfur cycle modify the primary signal and strongly influence the S MIF preserved and observed in the geological record. Here, a detailed model of the marine sulfur cycle is used to propagate and distribute atmospherically derived S MIF from its delivery to the ocean to its preservation in the sediment. Bulk pyrite in most sediments carries weak S MIF because of microbial reduction of most sulfur compounds to form isotopically homogeneous sulfide. Locally, differential incorporation of sulfur compounds into pyrite leads to preservation of S MIF, which is predicted to be most highly variable in nonmarine and shallow-water settings. The Archean ocean is efficient in diluting primary atmospheric S MIF in the marine pools of sulfate and elemental sulfur with inputs from SO2 and H2S, respectively. Preservation of S MIF with the observed range of magnitudes requires the S MIF production mechanism to be moderately fractionating ( 20-40‰). Constraints from the marine sulfur cycle allow that either elemental sulfur or organosulfur compounds (or both) carried S MIF to the surface, with opposite sign to S MIF in SO2 and H2SO4. Optimal progress requires observations from nonmarine and shallow-water environments and experimental constraints on the reaction of photoexcited SO2 with atmospheric hydrocarbons.

Atmospheric sulfur rearrangement 2.7 billion years ago: Evidence for oxygenic photosynthesis

Mass-independently fractionated sulfur isotopes (MIF-S) provide strong evidence for an anoxic atmosphere during the Archean. Moreover, the temporal evolution of MIF-S shows increasing magnitudes between 2.7 and 2.5Ga until the start of the Great Oxidation Event (G.O.E.) at around 2.4Ga. The conclusion of a completely anoxic atmosphere up to the G.O.E. is in contrast to recent studies on redox-sensitive elements, which suggest slightly oxidizing conditions during continental weathering already several hundred million years prior to the G.O.E. In order to investigate this apparent inconsistency, we present multiple sulfur isotopes for 2.71Ga pyritic black shales derived from the Kidd Creek area, Ontario, Canada. These samples display high positive Δ33S values up to 3.8‰ and the typical late Archean slope in Δ36S/Δ33S of −0.9. In contrast, the time period before (3.2–2.73Ga) is characterized by greatly attenuated MIF-S magnitudes and a slope in Δ36S/Δ33S of −1.5. We attribute the increase in Δ33S magnitude as well as the contemporaneous change in the slope of Δ36S/Δ33S to changes in the relative reaction rate of different MIF-S source reactions and changes in atmospheric sulfur exit channels. Both of these are dependent on atmospheric CH4:CO2 and O2 mixing ratios. We propose a distinct change in atmospheric composition at 2.7Ga resulting from increased fluxes of oxygen and methane as the best explanation for the observed Neoarchean MIF-S record. Our data and modeling results suggest that oxygenic photosynthesis was a major contributor to primary productivity 2.7 billion years ago.

Primary origin of marginal Ni-Cu-(PGE) mineralization in layered intrusions: 33S evidence from The Platreef, Bushveld, South Africa

The margins of layered igneous intrusions can host ore deposits of nickel, copper, and platinum group elements (PGEs). These marginal deposits are characterized by a complex geologic history, which obscures direct evidence for the mineralization process. Mass-independent fractionation of sulfur isotopes is a distinguishing feature of Archean sedimentary rocks, and reflects processes operating solely in the Archean surface environment. As a result, mass-independent fractionation of sulfur isotopes is a chemically conservative tracer that indicates the involvement of crustal sulfur in marginal Ni-Cu-(PGE) deposits. In this study we use mass-independent fractionation of sulfur isotopes to evaluate the sulfur budget of the Platreef—the marginal PGE-bearing horizon of the northern limb of the Bushveld Complex, South Africa. Our multiple sulfur isotope measurements demonstrate that crustal sulfur in the Platreef originated from a restricted stratigraphic horizon (the Duitschland Formation) in the Platreef footwall. As this signature is spread out over the ≈100-km strike length of the Platreef, it does not reflect sulfur inputs from the local footwall. Nonlocal crustal sulfur isotope anomalies are unexpected given the high metal tenors in the Platreef. These features can be reconciled if a preformed Ni-Cu-(PGE)–rich sulfide melt assimilated sulfur in a staging chamber bounded completely by the Duitschland Formation, and then was emplaced into the Platreef. Such a model is supported by long-standing observations of lithologic, geochemical, and stratigraphic similarities between the Merensky Reef and the Platreef, as well as by recent suggestions that the northern limb represents an “escape structure” for the rest of the Bushveld Complex. Crustal sulfur assimilation is often thought to initiate the ore-forming process by producing an immiscible sulfide melt that can collect base metals and PGEs. In the Platreef, however, crustal sulfur assimilation apparently diluted a preexisting PGE-rich sulfide melt instead.

Sulfur- and oxygen-isotope constraints on the sedimentary history of apparent conglomerates from the Nuvvuagittuq Greenstone Belt (Nunavik, Québec)

Mafic igneous rocks of the Nuvvuagittuq Greenstone Belt (NGB) crystallized before 3.8Ga and possibly as early as 4.3Ga, potentially making the belt the oldest known supracrustal sequence on Earth. However, detrital zircons from a rare quartz-biotite schist in the NGB yield significantly younger ages of ≈3.77Ga or less. These appear to be inconsistent with the ages of the mafic igneous rocks, as the quartz-biotite schist has been interpreted as a metaconglomerate, formed by the dismantling of preexisting lithologies. In order to assess this genetic interpretation, we performed a sulfur and oxygen isotope study of the quartz-biotite schist. Sulfide grains found in quartz clasts and the matrix show significant mass-independent fractionation of sulfur isotopes (+0.2‰≤Δ33S‰≤+1.0‰; mean Δ33S=+0.5±0.1‰). Secondary sulfides from crosscutting veins do not show mass-independent fractionation of sulfur isotopes (−0.1‰≤Δ33S≤+0.3‰; mean Δ33S=+0.1±0.1‰). Oxygen isotope compositions of quartz from clasts, matrix and a fine-grained lens are highly enriched in 18O (16.9‰≤δ18O≤26.7‰). Non-zero Δ33S values indicate a surficial origin for sulfur, probably the Eoarchean atmosphere, while high δ18O values suggest a low-temperature (65±18°C) origin for the quartz, likely as chemical precipitation of a chert precursor from Si-saturated seawater. Therefore, the coupled S- and O-isotope measurements show that primary isotopic signatures characteristic of surficial environments survived the protracted metamorphic history of the NGB, and suggest that the quartz-biotite schist contains material that originated as chemical metasediments. The near mono-mineralic compositions of the clasts (quartz) and their shared 18O-enrichment suggest that they had a common protolith, which was deposited prior to the formation of the schist, and subsequently reworked. Whether the quartz-biotite schist represents a metaconglomerate or a structural melange, it preserves remnants of some of the oldest chemical sediments on Earth.

Excitation band dependence of sulfur isotope mass-independent fractionation during photochemistry of sulfur dioxide using broadband light sources

Ultraviolet photolysis of sulfur dioxide (SO2) is hypothesized to be the source of the sulfur isotope mass-independent fractionation (S-MIF) observed in Archean sulfate and sulfide minerals and modern stratospheric sulfate aerosols. A series of photochemical experiments were performed to examine the excitation band dependence of S-MIF during the photochemistry of SO2 under broadband light sources (a xenon arc lamp and a deuterium arc lamp). Optical filters (200±35nm bandpass and 250nm longpass filters) were used to separately access two different excitation bands of SO2 in the 190–220nm and the 250–330nm absorption regions, respectively.UV irradiation of SO2 in the 190–220nm and 250–330nm regions both produced elemental sulfur (S0) and sulfur trioxide (SO3) as end products but yielded very different sulfur isotope signatures. The elemental sulfur products from direct photolysis in the 190–220nm region were characterized by high δ34S values (154.7–212.0‰), modest Δ33S anomalies of 21±3‰, and relatively constant 33λ (=ln(δ33S+1)/ln(δ34S+1)) values of 0.64±0.3, all with respect to the initial SO2. Photoexcitation in the 250–330nm region produced elemental sulfur with δ34S values of 7.7–29.1‰ and Δ33S values of 15.0±1.6‰. In both excitation regions, the SO3 products were mass dependently fractionated relative to the SO2 reservoir. The two different absorption regions produced contrasting Δ36S/Δ33S signatures in the elemental sulfur products, with Δ36S/Δ33S=−1.9±0.3 and 0.64±0.3 for the 190–220nm and 250–330nm bands, respectively.Our results provide several critical constraints on the origin of the S-MIF signatures observed in modern stratospheric aerosols and in the Archean geological record. A lack of S-MIF in the sulfate product and positive Δ36S/Δ33S ratios for the elemental sulfur from SO2 photo-oxidation demonstrate that photoexcitation in the 250–330nm region is not a likely source for the S-MIF observed in modern stratospheric aerosols. Large δ34S fractionation, 33λ values, and Δ36S/Δ33S ratios observed for the 190–220nm band are qualitatively consistent with predictions from synthetic isotopologue-specific cross sections. These isotope patterns, however, are not compatible with the Archean rock record. We explore the possibility that S-MIF from both the 190 to 220nm and the 250 to 330nm absorption bands could have contributed to the Archean S-MIF signatures.

Mass-dependent and mass-independent sulfur isotope fractionation (δ34S and δ33S) from Brazilian Archean and Proterozoic sulfide deposits by laser ablation multi-collector ICP-MS

Mass-independent sulfur isotope fractionation (MIF) has been observed in rocks of the geological record older than about 2.45Ga, a characteristic which is thought to be related to processes in the Neoarchean atmosphere. Samples recording a MIF effect therefore have to contain sulfur of the exogenic sulfur cycle, while endogenic sulfur should not show this effect. The sulfides analysed from six Brazilian deposits represent either exogenic or endogenic sulfur sources, with supposed ages ranging from about 1.9 to 2.7Ga. Sulfur isotopes were analysed by in‐situ laser ablation MC-ICP-MS. A range of experiments were conducted using international and in-house isotope standards, which were run under various conditions and set-ups. These include the reference materials IAEA-S1, IAEA-S3, NBS123 (sphalerite), NBS127 (barite), and in-house standards BSB-py (pyrite) and BSB-cpy (chalcopyrite). During six days of analysis, an internal precision of sulfide analyses of 0.10–0.15‰ (1s) for δ34S and 0.40–0.60‰ (1s) for δ33S, and an accuracy of ~0.30‰ for δ34S and δ33S was achieved. The standard measurements define a δ33S/δ34S relationship of δ33S=0.513*δ34S+0.149, with R²=0.9997, which is close to the theoretical relationship for mass-dependent fractionation. The by far best analytical errors were obtained for natural pyrite, reaching a within-run precision of about 0.05–0.15‰ for δ34S (1s), and 0.10–0.15‰ (1s) for δ33S determinations. Including all precision and accuracy data, we arrive at the following 1s error limits to which sulfur isotope analyses for MIF studies obtained with this method are reliable: 0.32, 0.34 and 0.46‰ for pyrite (δ34S, δ33S and Δ33S determinations, respectively), 0.34, 0.42 and 0.54‰ for pyrrhotite, and 0.34, 0.50 and 0.58‰ for chalcopyrite.Pyrites and pyrrhotites from 2200Ma old sediments and volcanics associated with the Pilar greenstone belt, Goiás, have δ34S from −15.0 to +2.0‰, and δ33S from −8.0 to +1.0‰, with Δ33S being slightly but not significantly positive. Pyrites from supposed ca. 2060Ma old clastic sediments of the Águas Claras Formation (Carajás, Pará), have δ34S=9.5–12.5‰ and δ33S=5.0–6.5‰ with Δ33S close to zero. Chalcopyrites from the Salobo and Sequerinha sulfide deposits, also Carajás, are genetically related to 2570Ma old A-type alkaline granites with endogenic sources. They have δ34S=1.6–2.9‰ and δ33S=1.0–1.5‰, and δ34S=0.2–0.9‰ and δ33S=0.65‰, respectively, with Δ33S ranging from −0.15 to 0.56. The only Δ33S values of this study which clearly suggest sulfur isotope MIF come from the mineralized banded iron-formations of the Quadrilátero Ferrífero, Minas Gerais. Pyrites from the Mina de Cuiabá deposit (2700Ma) have δ34S=3.8–5.1‰ and δ33S=2.9–4.8‰ with Δ33S=0.9–2.2‰, while pyrites and chalcopyrites from São Bento (2720Ma) have δ34S=1.6–3.2‰ and δ33S=1.5–2.8‰ with Δ33S=0.2–1.5‰. The sulfides represent an epigenetic mineralization, derived from supracrustal rocks making part of an exogenic sulfur cycle. The study shows, that in-situ laser ablation ICP-MS is sufficiently sensitive for detecting MIF effects in Neoarchean rocks. Whether or not such samples show MIF-related Δ33S values depends on the age and provenance of the sulfur.

Sulfur mass-independent fractionation in liquid phase chemistry: UV photolysis of phenacylphenylsulfone as a case study

The sulfur isotope mass-independent fractionation (S-MIF) represents a distinct geochemical signature commonly observed in Archean rocks. Although S-MIF is generally thought to be exclusive to gas phase chemistry, several studies have suggested that liquid phase or heterogeneous chemistry may also produce S-MIF signatures.This study investigates the potential contribution of the poorly explored mass-independent effects from liquid phase sulfur chemistry. Our investigation focused on laboratory experiments of the UV photolysis of phenacylphenylsulfone as a model system. This system was chosen due to previous measurements of Δ33S (not Δ36S) by a low precision SO2 method indicating the occurrence of anomalous fractionation in 33S.The photolysis of PPS in micellar solution produced MIF of 33S in residual PPS ranging from depletion by −2.1‰ to enrichment by 6.4‰, with small mass-dependent fractionation of δ34S up to 2.1‰. While this magnitude of 33S anomaly is comparable to the range of values observed in Archean rocks, no anomaly in 36S was detected for the 32S–34S–36S system beyond analytical precision (0.19‰). These results confirm the anomalous fractionation to be caused most likely by magnetic isotope effects (MIE), affecting only the nuclear spin possessing 33S among the four stable isotopes of sulfur. The observed initial depletion of 33S in the reactant PPS was unexpected and suggests changing contributions of at least two spin-selective processes. The results of this study demonstrate that liquid phase processes, such as the photolysis of certain organic sulfur compounds, can produce anomalous 33S abundance.