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Abstract The exceptionally organic-rich rocks of the 1.98 Ga Zaonega Formation deposited in the Onega Basin, NW Russia, have refined our understanding of Earth System evolution during the Paleoproterozoic rise in atmospheric oxygen. These rocks were formed in vent- or seep-influenced settings contemporaneous with voluminous mafic volcanism and contain strongly 13C-depleted organic matter. Here we report new isotopic (δ34S, Δ33S, Δ36S, δ13C org) and mineralogical, major element, total sulphur and organic carbon data for the upper part of the Zaonega Formation, which was deposited shortly after the termination of the Lomagundi-Jatuli positive carbon isotope excursion. The data were collected on a recently obtained 102 m drill-core section and show a δ13C org shift from −38‰ to −25‰. Sedimentary sulphides have δ34S values typically between +15‰ and +25‰ reflecting closed-system sulphur isotope behaviour driven by high rates of microbial sulphate reduction, high sulphate demand, hydrothermal activity and hydrocarbon seepage. Four intervals record δ34S values that exceed +30‰. We interpret these unusually 34S-enriched sulphides to be a result of limited sulphate diffusion into pore waters due to changes in sedimentation and/or periods of basinal restriction. Additionally, there are four negative δ34S excursions that are interpreted to reflect changes in the open/closed-system behaviour of sulphate reduction or availability of reactive iron. Our findings highlight the influence of basinal processes in regulating sulphur isotope records and the need for care before interpreting such signals as reflecting global conditions. Highlights • Zaonega Formation Δ33S-δ34S data track basin-specific rather than global processes. • Opposing Δ33S-δ34S relationship relates to rapid microbial sulphate reduction. • Positive δ34S excursions reflect partial basinal restriction. [ABSTRACT FROM AUTHOR]

Xenotime (YPO4) is a trace component in many metasedimentary and some igneous rocks, altered rocks and many hydrothermal ore assemblages, where it forms in response to a range of different processes from igneous crystallisation to low-temperature early diagenesis. Due to its wide range of formation temperatures, its suitability and isotopic robustness for reliable U-Pb geochronology, dissolution/precipitation characteristics during overprint events, and widespread occurrence, it is one of the most valuable minerals for U-Pb geochronology leading to four-dimensional (4D) studies of many terranes. The formation environment of xenotime can be deduced from careful petrography and reliable U-Pb in-situ geochronology within a 4D framework. Further, xenotime is a physically robust mineral during sediment transport and may carry distinctive geochemical fingerprints, including age, to secondary environments. From published works, we review the U-Th-contents of well-characterised examples of xenotime formation to provide chemical fingerprints, which assist in identifying the xenotime formation environment. Although the U-Th characteristics of xenotime from all formation environments show considerable overlap, we make the following observations which may be distinctive of some ore formation environments: (i) hydrothermal xenotime formed from low salinity ore fluids (e.g. iron ore and orogenic gold deposits) trend to have the lowest U-contents (< 100 ppm U) and U/Th (< 4), in contrast to xenotime formed from higher salinity ore fluids (Sn-W and base metal deposits); (ii) xenotime associated with unconformity-related U-ores have the highest U-contents at U/Th > 10; (iii) diagenetic xenotime has the most variable and highest U/Th, whereas (iv) xenotime from Precambrian orogenic gold deposits has the least variable and lowest U/Th. Petrogenetic inferences from these observations for ore deposit research and exploration include: (i) support for the homogenising effect of the crustal scale of the fluid system in Precambrian orogenic gold deposits, and (ii) the potential for distinctive U-Th geochemical fingerprints for U ores and some Au ores. These characteristics may be reflected in detrital xenotime grains recovered from routine exploration sampling programs. [ABSTRACT FROM AUTHOR]

Subduction of sediments affects the composition of the Earth's mantle and the formation of melts in subduction zones. The sediment thickness of ~8 km along the Hellenic Trench is the largest observed globally and significant volumes of these sediments are subducted. Magmas erupting in the Aegean Arc reflect sediment subduction but they also assimilated sediments during ascent, making distinction of the two processes difficult. In this study, we present geochemical data from olivines in lavas and enclaves from the peninsula of Methana in the western Aegean Arc. The olivines have high forsterite (Fo) contents of 90–92% and contain Cr-spinel with Cr# >60 originating from a primitive melt in equilibrium with a highly depleted mantle wedge. The high δ 18 O VSMOW of ~6.5‰ of the olivines imply input of up to 6.5% sediment component into the mantle wedge. Incompatible element and radiogenic isotope ratios suggest that the sediment component is added as a carbonate-rich melt. Low Ni contents and small rounded sulphide inclusions in the Fo-rich olivines indicate the formation of an immiscible sulphide liquid during olivine crystallization. The input of carbonate-rich sediment reduced the mantle and the co-existing primitive melts, and possibly added S leading to sulphide saturation. This study shows that olivine crystals and their inclusions provide important insights into the mantle wedge, sediment subduction and its effects on the primary magma composition and can indeed be used to distinguish between a mantle and crustal origin of the sediment signature. [ABSTRACT FROM AUTHOR]

The composition of seawater changed dramatically during the initial rise of atmospheric oxygen in the earliest Paleoproterozoic, but the emerging view is that atmosphere–ocean system did not experience an irreversible transition to a well-oxygenated state. Instead, it has been suggested that the oxygen content of the atmosphere–ocean system decreased considerably after ca. 2.06 billion years ago (Ga), which resulted in a crash in marine sulfate concentrations. The end of the deposition of major granular iron formations at ca. 1.85 Ga has been linked either to the development of extensive euxinic conditions along continental shelves or a decrease in hydrothermal flux. The record of oceanic redox state is not well constrained for the period between ca. 2.06 Ga, the end of the Lomagundi positive carbon isotope excursion, and ca. 1.88 Ga when major granular iron formations appeared. We address this gap by presenting new iron-speciation, major and trace element data, as well as sulfur, organic carbon, and molybdenum isotopic data for greenschist facies organic matter-rich mudrocks (ORMs) of the ca. 1.93 Ga Bravo Lake Formation, Piling Group, Baffin Island. The iron speciation data suggest deposition of the Bravo Lake Formation under a euxinic (anoxic and sulfidic) water column. Trace metal enrichments and Mo isotope data suggest extensive marine euxinia ca. 90 million years before the disappearance of large-scale, economic granular iron formations. The addition of new Mo data in this time interval is important, as it contributes to filling in the sparse Proterozoic record. Lastly, this work provides further support for the idea that there was widespread anoxia shortly after the end of the Lomagundi Event. [ABSTRACT FROM AUTHOR]

The reaction of ferric (hydr)oxides with dissolved sulfide does not lead to the instantaneous production of thermodynamically stable products but can induce a variety of mineral transformations including the formation of metastable intermediates. The importance of the various transformation pathways depends, among other factors, on the characteristics of the ferric (hydr)oxides but a mechanistic model which relates the mineralogy of the ferric (hydr)oxides to the type of reaction products and their evolution over time is still missing. Here, we investigate the kinetics of the reaction between dissolved sulfide (6.7–7.5 mmol L − 1 ) with ferrihydrite (Fh, 12 mmol L − 1 ), lepidocrocite (Lp, 26.6 mmol L − 1 ), and goethite (Gt, 22 mmol L − 1 ) in batch experiments at pH 7 and room temperature. The time evolution of solution and solid phase composition was monitored over 2 weeks while TEM, and Mössbauer spectroscopy were used to characterize the transformations of the solid phases. Dissolved sulfide was consumed within 2 (Fh, Lp) to 8 h (Gt) with methanol extractable sulfur and HCl extractable Fe(II) (Fe(II) HCl ) being the main products after this time. The mass balances of Fe and S indicated that a large fraction of the Fe(II) HCl in the reactions with Fh (46% of Fe(II) HCl ) and Lp (36% of Fe(II) HCl ) was solid-phase bound but not associated with sulfur. This excess Fe(II) exceeded the adsorption capacity of the solids and remained associated with the oxides. Over the time scale of days, the concentrations of MES and Fe(II) HCl decreased and this process was accompanied by the formation of secondary iron oxides and pyrite in all experiments. The pyrite yield after two weeks showed the same trend as the amounts of intermediately produced excess Fe(II): Fh (84% of initial S(− II)) > Lp (50%) > Gt (13%). Besides the formation of pyrite, Fh transformed completely into thermodynamically more stable iron oxides such as hematite or magnetite. In contrast, formation of other iron oxides was only minor when Lp or Gt reacted with sulfide. We propose that the extent of pyrite and secondary iron mineral precipitation is controlled by the ratio between the competing formation rates of excess Fe(II) and surface bound FeS (FeS s ) in the early stage of the reaction. Formation of excess Fe(II) is a prerequisite for rapid pyrite formation and induces secondary formation of iron oxides. The competition between excess Fe(II) and FeS s formation, in turn, is ruled by two factors: 1) the ratio between added sulfide and available surface area, and 2) the capability of the iron(hydr)oxide to conduct electrons from surface bound Fe(II) to bulk Fe(III) and to accommodate structural Fe(II). This capability is largest for Fh and explains the most pronounced excess Fe(II) production and, by this, the greatest pyrite yield in experiments with Fh. During the reaction with Gt, in contrast, formation of FeS s outcompetes the accumulation of excess Fe(II) and consequently the precipitation of pyrite is only minor. This conceptual model constrains conditions at which relatively fast pyrite formation within the time scale of days or weeks might be relevant in natural environments. Suitable conditions are expected in environments with low sulfide levels in which formation of reactive iron (hydr)oxides is stimulated by redox oscillations (e.g., wetlands, riparian soils, tidal flats). [ABSTRACT FROM AUTHOR]