In situ S‐isotope compositions of sulfate and sulfide from the 3.2 Ga Moodies Group, South Africa: A record of oxidative sulfur cycling

Sulfate minerals are rare in the Archean rock record and largely restricted to the occurrence of barite (BaSO4). The origin of this barite remains controversially debated. The mass‐independent fractionation of sulfur isotopes in these and other Archean sedimentary rocks suggests that photolysis of volcanic aerosols in an oxygen‐poor atmosphere played an important role in their formation. Here, we report on the multiple sulfur isotopic composition of sedimentary anhydrite in the ca. 3.22 Ga Moodies Group of the Barberton Greenstone Belt, southern Africa. Anhydrite occurs, together with barite and pyrite, in regionally traceable beds that formed in fluvial settings. Variable abundances of barite versus anhydrite reflect changes in sulfate enrichment by evaporitic concentration across orders of magnitude in an arid, nearshore terrestrial environment, periodically replenished by influxes of seawater. The multiple S‐isotope compositions of anhydrite and pyrite are consistent with microbial sulfate reduction. S‐isotope signatures in barite suggest an additional oxidative sulfate source probably derived from continental weathering of sulfide possibly enhanced by microbial sulfur oxidation. Although depositional environments of Moodies sulfate minerals differ strongly from marine barite deposits, their sulfur isotopic composition is similar and most likely reflects a primary isotopic signature. The data indicate that a constant input of small portions of oxidized sulfur from the continents into the ocean may have contributed to the observed long‐term increase in Δ33Ssulfate values through the Paleoarchean.Centre National de la Recherche ScientifiqueDeutsche ForschungsgemeinschaftH2020 European Research Counci

Asthenosphere-induced melting of diverse source regions for East Carpathian post-collisional volcanism

The occurrence of post-subduction magmatism in continental collision zones is a ubiquitous feature of plate tectonics, but its relation with geodynamic processes remains enigmatic. The nature of mantle sources in these settings, and their interaction with subduction-related components, are difficult to constrain using bulk rocks when magmas are subject to mixing and assimilation within the crust. Here we examine post-collisional magma sources in space and time through the chemistry of olivine-hosted melt inclusions and early-formed minerals (spinel, olivine and clinopyroxene) in primitive volcanic rocks from the Neogene–Quaternary East Carpathian volcanic range in Călimani (calc-alkaline; 10.1–6.7 Ma), Southern Harghita (calc-alkaline to shoshonitic; 5.3–0.03 Ma) and the Perșani Mountains (alkali basaltic; 1.2–0.6 Ma). Călimani calc-alkaline parental magma compositions indicate a lithospheric mantle source metasomatised by ~ 2% sediment-derived melts, and are best reproduced by ~ 2–12% melting. Mafic K-alkaline melts in Southern Harghita originate from a melt- and fluid-metasomatised lithospheric mantle source containing amphibole (± phlogopite), by ~ 5% melting. Intraplate Na-alkaline basalts from Racoș (Perșani) reflect small-degree (1–2%) asthenosphere-derived parental melts which experienced minor interaction with metasomatic components in the lithosphere. An important feature of the East Carpathian post-collisional volcanism is that the lithospheric source regions are located in the lower plate (distal Europe-Moesia), rather than the overriding plate (Tisza-Dacia). The volcanism appears to have been caused by (1) asthenospheric uprise following slab sinking and possibly south-eastward propagating delamination and breakoff, which induced melting of the subduction-modified lithospheric mantle (Călimani to Southern Harghita); and (2) decompression melting as a consequence of minor asthenospheric upwelling (Perșani)

Mass-dependent Selenium Isotopic Fractionation during Microbial Reduction of Seleno-oxyanions by Phylogenetically Diverse Bacteria

Selenium (Se) isotope fractionation has been widely used for constraining redox conditions and microbial processes in both modern and ancient environments, but our knowledge of the controls on fractionation during microbial reduction of Se-oxyanions is based on a limited number of studies. Here we complement and expand the currently available pure culture data for Se isotope fractionation by investigating for the first time six phylogenetically and physiologically non-respiring bacterial strains that reduce Se-oxyanions to elemental Se [Se(0)]. Experiments were performed with either selenate [Se(VI)] or selenite [Se(IV)] at lower, more environmentally-relevant concentrations (9 to 47 μM) than previously investigated. Enterobacter cloacae SLD1a-1, Desulfitobacterium chlororespirans Co23 and Desulfitobacterium sp. Viet-1 were incubated with Se(VI) and Se(IV). Geobacter sulfurreducens PCA, Anaeromyxobacter dehalogenans FRC-W and Shewanella sp. (NR) were examined for their ability reducing Se(IV) to Se(0). Our data confirm that microbial reduction of both Se-oxyanions is accompanied by large kinetic isotopic fractionation (reported as 82/76ε =1000*(82/76α-1) ‰). Under our experimental conditions, microbial reduction of Se(VI) shows consistently greater isotope fractionation (ε= -9.2‰ to -11.8‰) than reduction of Se(IV) (ε= -6.2 to -7.8‰) confirming the difference in metabolic pathways for the reduction of the two Se-oxyanions. For Se(VI), the inverse relationship between normalized cell specific reduction rate (cSRR) and Se isotope fractionation suggests that the kinetic isotope effect for Se(VI) reduction is governed by an enzymatically-specific pathway related to the bacterial strain-specific physiology. In contrast, the lack of correlation between normalized cSRR and isotope fractionation for Se(IV) reduction indicates a non-enzyme specific pathway which is dominantly extracellular. Our study highlights the importance to understand microbially-mediated Se isotope fractionation depending on Se species, and cell-specific reduction rates before Se isotope ratios can become a fully applicable tool to interpret Se isotopic changes in modern and ancient environments

Multiple subduction imprints in the mantle below Italy detected in a single lava flow

Post-collisional magmatism reflects the regional subduction history prior to collision but the link between the two is complex and often poorly understood. The collision of continents along a convergent plate boundary commonly marks the onset of a variety of transitional geodynamic processes. Typical responses include delamination of subducting lithosphere, crustal thickening in the overriding plate, slab detachment and asthenospheric upwelling, or the complete termination of convergence. A prominent example is the Western-Central Mediterranean, where the ongoing slow convergence of Africa and Europe (Eurasia) has been accommodated by a variety of spreading and subduction systems that dispersed remnants of subducted lithosphere into the mantle, creating a compositionally wide spectrum of magmatism. Using lead isotope compositions of a set of melt inclusions in magmatic olivine crystals we detect exceptional heterogeneity in the mantle domain below Central Italy, which we attribute to the presence of continental material, introduced initially by Alpine and subsequently by Apennine subduction. We show that superimposed subduction imprints of a mantle source can be tapped during a melting episode millions of years later, and are recorded in a single lava flow

Arc‐Type Magmatism Due to Continental‐Edge Plowing Through Ancient Subduction‐Enriched Mantle

The puzzling... (See article for full abstract)

Tectonic significance of changes in post-subduction Pliocene-Quaternary magmatism in the south east part of the Carpathian-Pannonian Region

The south-eastern part of the Carpathian–Pannonian region records the cessation of convergence between the European platform/Moesia and the Tisza–Dacia microplate. Plio-Quaternary magmatic activity in this area, in close proximity to the ‘Vrancea zone’, shows a shift from normal calc-alkaline to much more diverse compositions (adakite-like calc-alkaline, K-alkalic, mafic Na-alkalic and ultrapotassic), suggesting a significant change in geodynamic processes at approximately 3 Ma. We review the tectonic setting, timing, petrology and geochemistry of the post-collisional volcanism to constrain the role of orogenic building processes such as subduction or collision on melt production and migration. The calc-alkaline volcanism (5.3–3.9 Ma) marks the end of normal subduction-related magmatism along the post-collisional Călimani–Gurghiu–Harghita volcanic chain in front of the European convergent plate margin. At ca. 3 Ma in South Harghita magma compositions changed to adakite-like calc-alkaline and continued until recent times (< 0.03 Ma) interrupted at 1.6–1.2 Ma by generation of Na and K-alkalic magmas, signifying changes in the source and melting mechanism. We attribute the changes in magma composition in front of the Moesian platform to two main geodynamic events: (1) slab-pull and steepening with opening of a tear window (adakite-like calc-alkaline magmas) and (2) renewed contraction associated with deep mantle processes such as slab steepening during post-collisional times (Na and K-alkalic magmas). Contemporaneous post-collisional volcanism at the eastern edge of the Pannonian Basin at 2.6–1.3 Ma was dominated by Na-alkalic and ultrapotassic magmas, suggesting a close relationship with thermal asthenospheric doming and strain partitioning related to the Adriatic indentation. Similar timing, magma chamber processes and volume for K-alkalic (shoshonitic) magmas in the South Apuseni Mountains (1.6 Ma) and South Harghita area at a distance of ca. 200 km imply a regional connection with the inversion tectonics

Geochemical constraints on the geodynamic setting of Alborz-Azerbaijan Cenozoic magmatism

The Alborz Mountains in northern Iran form part of the Tethyan orogenic belt and surround the South Caspian Basin. The geology of the western Alborz Mountains is dominated by Eocene mafic to intermediate high-K calc-alkaline-alkaline shoshonitic and minor Oligo-Miocene magmatic rocks, displaying arc geochemical characteristics (e.g., negative Nb, Ta, Ti anomalies). Cenozoic magmatism across this region in western Asia has been explained by a diversity of contrasting geodynamic models involving (multiple slab) subduction and slab-breakoff. The aim of this study is to better constrain the geodynamic setting of magmatism during regional convergence through the investigation of the relatively unstudied Alborz-Azerbaijan magmatic belt. Incompatible trace element geochemistry of Eocene lavas from this belt is distinctive and indicates that they were generated by relatively low-degrees of partial melting of the subcontinental lithospheric mantle with a contribution of asthenosphere melts. Miocene lavas from the Alborz and northern Urmia–Dokhtar magmatic arc (UDMA) share a common arc geochemical signature. Zircon εHf(t) values of the Miocene magmatic rocks from the Alborz and northern UDMA range from −0.4 to 11.7, suggesting incorporation of older continental crust mixed with a more juvenile component. New thermochronological data (fission track and (U-Th)/He on apatite) from the late Eocene plutonic bodies in the Tarom area track exhumational cooling at moderate rates following rapid post-emplacement magmatic cooling at ca. 40 Ma. The geochemical data in conjunction with geological and published geophysical results imply a bending or disruption in the subducting slab under the Tarom area, associated with slab roll-back during the Eocene. This process led to the arc-front displacement and a greater contribution of deep enriched mantle in the Alborz magmas compared to those from the high-flux magmatic event along the Alborz and Urmia–Dokhtar magmatic arc (UDMA), triggered by asthenospheric upwelling and mixing with melts derived from earlier metasomatized subcontinental lithospheric mantle

Plumbing System Architecture and Differentiation Processes of the Nyiragongo Volcano, DR Congo

The Nyiragongo volcano is one of the most alkali-rich volcanic centres on the planet (Na2O + K2O generally >10 wt.%, agpaitic index up to 1.34), characterized by a semi-permanently active lava lake which hosts silica-undersaturated (SiO2 < 40 wt.%), low viscosity lavas. To improve our understanding of this unique magmatic system, we present a set of 291 samples, acquired during new field excursions between 2017 and 2021. The major and trace element composition of all samples was measured, revealing a lithological range extending from primitive picrites (Mg# 82) erupted from parasitic cones to a variety of highly evolved nephelinites, leucitites, and melilitites erupted from the main edifice as recently as 2002, 2016, and 2021. We measured major and trace element compositions from the full spectrum of minerals present in all sampled lithologies. From these we calculated that the main magma reservoirs feeding Nyiragongo are at approximately 9-15 and 21-33 km depth, in agreement with recent seismic observations. Fractional crystallization modelling using observed mineral compositions and proportions was performed to quantitatively link the lithologies to specific residual liquid fractions assuming evolution from an olivine-melilite parental melt. Our modelling indicates that fractionation and cumulate formation in deep chambers reduces the remaining melt fraction to ∼60%, after which melts are injected into upper, liquid dominated magma chambers where fractionation and accumulation of clinopyroxene, melilite, and feldspathoids dominates. Characterisation of mineral textures and geochemistry reveals high crystal mobility in a repeatedly recharging plumbing system split between liquid-dominated, evolved magma chambers and more solid-dominated, primitive mushes, decreasing in liquid fraction with depth

Causes of Cretaceous subduction termination below South China and Borneo: Was the Proto-South China Sea underlain by an oceanic plateau?

The South China, Indochina, and Borneo margins surrounding the South China Sea contain long-lived arcs that became inactive at approximately 85 Ma, even though an embayment of oceanic crust (the ‘Proto-South China Sea’) remained in the intervening region. This oceanic crust eventually subducted in the Cenozoic below Borneo and the Cagayan arc, while the modern South China Sea opened in its wake. To investigate the enigmatic cessation of Mesozoic subduction below South China and Borneo, we studied a fragment of oceanic crust and overlying trench-fill sediments that accreted to NW Borneo during the final stages of Paleo-Pacific subduction. Based on radiolarian biostratigraphy of cherts overlying the pillow basalts and detrital zircon geochronology of the trench-fill, we constrained the minimum age of the oceanic crust during accretion to 40 Ma. This shows that subduction cessation was not related to ridge subduction. Geochemical analysis of pillow basalts revealed an enriched mid-ocean ridge basalt signature comparable to oceanic plateaus. Using paleomagnetism, we show that this fragment of oceanic crust was not part of the Izanagi Plate but was part of a plate (the ‘Pontus’ Plate) separated from the Izanagi Plate by a subduction zone. Based on the minimum 40 Ma age of the oceanic crust and its geochemistry, we suggest that Mesozoic subduction below South China and Borneo stopped when an oceanic plateau entered the trench, while the eastern plate margin with the Izanagi Plate remained active. We show how our findings offer opportunities to restore plate configurations of the Panthalassa-Tethys junction region

Precessional pacing of early Proterozoic redox cycles

Regularly alternating reduction-oxidation (redox) patterns attributed to variations in the Earth's orbit and axis (Milankovitch cycles) are widely recorded in marine sediment successions of the Phanerozoic and attest to a dynamic history of biospheric oxygen in response to astronomically forced climate change. To date, however, such astronomical redox control remains elusive for much older, Precambrian intervals of the geological record that were characterized by a globally anoxic and iron-rich ocean, i.e., prior to Earth's atmospheric oxygenation (ca. 2.4–2.2 billion years ago). Here we report a detailed cyclostratigraphic and geochemical investigation of marine-sedimentary redox cycles identified in the ca. 2.46 billion-year-old Joffre Member of the Brockman Iron Formation, NW Australia, suggesting the imprint of Earth's climatic precession cycle. Around the base and top of regularly intercalated mudrock layers, we identify sharp enrichments in redox sensitive elements (Fe, S, Ca, P) that appear to represent chemical reaction fronts formed during nonsteady state diagenesis. Using a reactive transport model, we find that the formation of characteristic double S peaks required periods of increased organic matter deposition, coupled to strongly declining Fe2+ concentrations in the overlying water column. This combination, in turn, implies a periodic deepening of the iron chemocline due to enhanced oxygenic photosynthesis in marine surface waters, and is interpreted as the result of precession-induced changes in monsoonal intensity that drove variations in runoff and associated nutrient delivery. Our study results point to a dynamic redox evolution of Precambrian oceanic margin environments in response to Milankovitch forcing, and offer a temporal framework to investigate linkages between biological activity and the early build-up of oxygen in Earth's ocean-atmosphere system