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Utrecht University, The Netherlands (1); Université du Québec à Montréal, Canada (2); University of Geneva, Switzerland (3); Royal Netherlands Institute for Sea Research, and Utrecht University, The Netherlands (4; Geological Survey of Western Australia (5) Large-scale banded iron formations (BIFs) were deposited during Neoarchean to early Paleoproterozoic and have been mainly linked to hydrothermal plume activity and the rise of oxygen in the ocean and atmosphere. However, the potential influence of astronomical “Milankovitch” forcing on their deposition has been largely neglected so far and was hindered by large uncertainties in BIF depositional rates. In a recent study, we showed that outcrops of the ~2.5 Ga Kuruman Banded Iron Formation (BIF) in South Africa reveal a characteristic pattern in weathering profile that can be laterally traced over ~250 km (Lantink et al., 2019)*. Cyclostratigraphic analysis combined with high-precision U-Pb dating indicated that this pattern reflects a hierarchy of two superimposed cycles linked to the 405 kyr and a 1.2 - 1.6 Myr eccentricity cycle. This pattern is mirrored in magnetic susceptibility and high-resolution XRF core scan data of a new drill-core through the Kuruman BIF, providing insight into the chemical variations and may be used for cyclostratigraphic correlations to the broadly time-equivalent6 Australian Dales Gorge Member BIF. Furthermore, the dominance of eccentricity implies that precession-related cycles should be visible as well within the BIF stratigraphy and may be used to reconstruct the Precambrian evolution of the Earth-Moon distance. *https://doi.org/10.1038/s41561-019-0332-8 Lantink-Astronomical climate forcing of ~25 Ga banded iron formations_Info.pdf

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.

Bulk δ82/76Se values of representative marine shales from the Paleoarchean to the present day vary between approximately −3 and + 3 ‰ with only local deviations beyond this range. This muted Se isotope variability in the shale record contrasts with the relatively large fractionations associated with abiotic and microbial Se oxyanion reduction seen in experimental studies. Long-term temporal trends in the bulk shale data do not directly correlate with changes in redox conditions of the global ocean, although a minor but significant shift towards more negative formation-averaged δ82/76Se values appears to track oxygenation of the deep ocean at the end of the Proterozoic. We hypothesize that extensive δ82/76Se variability in the shale data was suppressed due to the early emergence of biological assimilatory uptake and the resulting persistence of low seawater Se concentrations, coupled with small authigenic Se outputs throughout most of geological time. In the modern ocean, Se is an essential micronutrient with a relatively short residence time of about 11,500 yrs. The marine Se cycle is dominated by assimilation into biomass and subsequent recycling in the water column and surface sediments, i.e. processes that result in only minimal isotopic fractionation. We suggest that similar processes dominated back through the geological record to Archean times. Our model shows that paleoceanographic information could in principle be extracted from proxy data on the Se isotopic composition of seawater, once isotopic differences can be readily discerned between individual sedimentary Se pools.

Multiple sulfur isotope variability in Archean sedimentary rocks provides constraints on the composition of the Earth’s earliest atmosphere. The magnitude and sign of mass-independent anomalies reflect not only atmospheric processes, but also transformations due to the Archean marine sulfur cycle prior to preservation into sedimentary pyrite. The processes affecting the Archean marine sulfur cycle and the role of microbial or abiotic redox reactions during pyrite formation remain unclear. Here we combine iron (Fe) and multiple sulfur (S) isotope data in individual pyrite grains with petrographic information and a one-dimensional reactive transport model, to investigate the sources of Fe and S in pyrite formed in a Paleoarchean sedimentary basin. Pyrites were selected from mudstones, sandstones and chert obtained from a drill core in the ca. 3.2 Ga Mapepe and Mendon Formations of the Fig Tree and Onverwacht Groups, respectively, in the Barberton Greenstone Belt, Kaapvaal Craton, South Africa. Pyrite textures and δ 56 Fe distinguish early-diagenetic pyrite formed with pore-water ferrous iron (disseminated grains with average δ 56 Fe pyrite = 0‰) from late-diagenetic pyrite formed through sulfidation of iron oxide minerals (layered and aggregate forms with average δ 56 Fe pyrite = + 1‰). Mass dependent S isotope variability in pyrite was small (δ 34 S pyrite ranged from − 1.1 to + 3.3‰) with a correspondingly minor spread in Δ 33 S pyrite (ranging from + 0.3 to + 2.1‰) and Δ 36 S pyrite (ranging from − 3.08 to + 0.27‰) that indicates a lack of post-depositional re-working with other distinct sulfur sources. Our combined Fe and S isotope data are most readily explained with pyrite sulfide derived from microbial-reworking of solid elemental S. Iron oxide minerals were necessary to buffer sulfide concentrations and provide favorable conditions for microbial sulfur disproportionation to proceed. The lack of a negative Δ 33 S signal indicates that pyrite from relatively deep marine diagenetic environments only partially records the products of atmospheric photolysis, consistent with low sulfate concentrations in the Paleoarchean ocean.

The Bakhuis Granulite Belt in western Suriname is an ultrahigh-temperature (UHT) metamorphic terrain in the centre of the Paleoproterozoic (Transamazonian) Guiana Shield. Next to the UHT granulites, the belt contains a 30 by 30km body of orthopyroxene-bearing granitoids: the Kabalebo charnockites. This setting offers an excellent opportunity to investigate the source and origin of charnockite magmatism and the common association of charnockites with (ultra)high-temperature metamorphic terrains. We present a detailed geochemical dataset and LA-ICPMS zircon U/Pb ages with the aim to investigate the geochemical and geochronological relationship between charnockite magmatism and UHT metamorphism in the Bakhuis Granulite Belt. The Kabalebo charnockites have a characteristic trace element signature with elevated K2O, P2O5, Zr, REE and Ba coupled with mobile element depletion, which is a consequence of high-temperature melting of anhydrous but fertile granulitic crust. Field and geochemical evidence suggests that the intermediate granulites in the Bakhuis Granulite Belt are the source of the Kabalebo charnockites. The new U/Pb zircon ages indicate that charnockite magmatism (1993-1984Ma) postdates UHT metamorphism (2.07-2.05Ga) by at least 60Myr. We argue that it is not possible to maintain a thermal anomaly >200°C in excess of a normal geothermal gradient for such a prolonged period and hence conclude that the Bakhuis Granulite Belt has experienced two distinct periods in which temperatures >950°C were reached in the lower crust.The presence of comagmatic metadolerite enclaves in the charnockites establishes that mafic magmatism occurred contemporaneously with, and was the likely heat source for, charnockite magmatism at 1.99-1.98. Ga. In contrast, the 2.07-2.05. Ga UHT metamorphic event is not associated with felsic or mafic magmatism in the Bakhuis Granulite Belt or nearby Guiana Shield and postdates the suturing of the juvenile North Guiana TTG-greenstone belt with the West African Shield by at least 10. Myr. We postulate that the UHT metamorphism at 2.07-2.05. Ga is the result of mantle upwelling in a slab tear in the subducted West African slab that formed as the result of crustal scale shearing and boudinage. Prior to the final stabilisation of the Amazonian-West African Shield at 1.90. Ga, northward subduction at 1.99-1.98. Ga caused the emplacement of voluminous hot, mafic magma, resulting in partial melting of the Bakhuis granulite suite to form the Kabalebo charnockites. Charnockite magmatism was roughly contemporaneous with the emplacement of a large belt of shallow granites and felsic volcanic rocks in the SW Guiana Shield. Despite their similar age, the inherited zircon populations suggest that the charnockites are derived from a distinct, juvenile source while the felsic volcanic rocks include an Archaean protolith.

Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) line-scanning is a promising technique for producing high-resolution (µm-scale) geochemical records on resin-embedded sediments. However, this approach has not yet been thoroughly tested on sediment samples of known elemental compositions. Here, we address this through the analysis of resin-embedded quartz, calcite, and clay (montmorillonite) sediments spiked with Al, V, Mo, and Ba across a range of concentrations. LA-ICP-MS spot analyses on these samples were compared to data independently obtained by conventional techniques: solution nebulization-ICP-optical emission spectroscopy (SN-ICP-OES), SN-ICP-MS, and X-Ray Fluorescence on fused glass beads. Data were reported as (log-)ratios of the analyte elements normalized to Ca, Si, and Al for calcite, quartz and clay respectively to correct for varying ablation yields. Our LA-ICP-MS data demonstrate close agreement to within 6% of the reference values determined by the conventional techniques, with high correlation coefficients (R2 ˜ 1.0) across the full range of concentrations, and high precision (< 3% relative standard deviations after three repeated analyses). Barium was hosted in both aluminosilicates and carbonates in the clay matrix, giving different yields (elemental fractionation), and leading to variable accuracy (mean deviation of 15%). A selection of the spiked sediments was used to produce artificial laminated sediments to further test the effects of line scanning on LA-ICP-MS accuracy and precision in resin-embedded samples. It appears that LA-ICP-MS line-scan analyses have good accuracy (deviations from the reference values generally < 5%) and high precision (relative standard deviations of repeated analyses on the same sample generally < 5%) for our target elements. Moreover, the LA-ICP-MS line-scanning closely records the alternating geochemical profiles of our artificial laminations. However, calcium showed a clear tailing signal at the transition from a calcite to a quartz layer, indicating that geochemical signals can be smeared at transitions to sediment layers devoid of analyte element(s). We also analyzed resin-embedded natural sediments for comparison of LA-ICP-MS line-scan data to parallel sub-samples measured by SN-ICP-OES and SN-ICP-MS. In general, the results from LA-ICP-MS line-scans of natural sediments correspond well with reference values and with high reproducibility, corroborating the results of the artificial laminations. The results for Ca, V, Mn, Sr, Mo, Ba, and U (in log-ratios to Al) suggest that these elements can be analyzed semi-quantitatively, showing correlation coefficients (R2) ranging from 0.59 to 0.85 compared to conventional analytical techniques. However, we show that using non-matrix matched calibration standards such as NIST SRM 610 may induce deviations from the LA-ICP-MS values to the reference values. Consequently, we recommend a simple protocol for further data correction in which binned-mean LA-ICP-MS values are calibrated to a parallel series of discrete samples analyzed by these conventional techniques.

Mass-dependent and mass-independent sulfur isotope fractionation archived in volcanic and sedimentary rocks from the Barberton Greenstone Belt (3550–3215 Ma), South Africa, provide constraints for sulfur cycling on the early Earth. Four different samples suites were studied: komatiites and tholeiites, barite, massive and disseminated sulfide ores, and non-mineralized black shales. Previous multiple sulfur isotope centered either on specific areas and/or lithologies from the Barberton Greenstone Belt, while this study provides results for a much wider sample selection across the stratigraphic succession. Variable but generally slightly positive δ34S values between -0.7 and +5.0‰, negative Δ33S values between -0.51 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 indicate variable degrees of alteration through ambient seawater during serpentinisation of these rocks. 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‰. Moreover, samples reveal a linear negative correlation between Δ33S and Δ36S with a slope of -0.7. The mass-independent sulfur isotope fractionation indicates an atmospheric sulfur source, whereas the positive δ34S values suggest bacterial sulfate reduction of the marine sulfate reservoir. The latter process is further discernible through a weak positive correlation between δ34S and δ18O of the barite. Non-mineralized black shales from the presumed stratigraphic equivalent of the Mapepe Formation show positive δ34S values between 0.0 and 1.2‰ and positive Δ33S values between 0.59 and 2.48‰. These results are interpreted to result from the mixing between the two principal atmospheric sources, i.e. elemental sulfur, carrying a positive Δ33S signature and sulfate, carrying a negative Δ33S value, with its δ34S signature subsequently modified through bacterial sulfate reduction. Positive δ34S values ranging from +0.5 to +3.4‰ and slightly negative Δ33S values between -0.17 and -0.13‰ characterize massive and disseminated sulfides from the Bien Venue Prospect. Results suggest a mixture between the unfractionated juvenile magmatic sulfur source and a contribution from recycled seawater sulfate indicative of submarine hydrothermal activity. Considering the isotope values, these three sets of samples show a common source of sulfur, characterized by negative Δ33S and positive Δ36S, represented by seawater sulfate. Massive and disseminated sulfides from the M’hlati prospect are characterized by different values compared to massive and disseminated sulfide from the Bien Venue Prospect. They show negative δ34S values between -1.4 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, thus this is related to elemental sulfur. In conclusion, the sulfur isotope inventories in the Paleoarchean rocks and hydrothermal precipitates from the Barberton Greenstone Belt are quite diverse and indicate the incorporation of at least two sources of sulfur. Komatiites and tholeiites, barite and massive and disseminated sulfides from Bien Venue show a common sulfur source, related to seawater sulfate, while massive and disseminated sulfides from M’hlati are related to elemental sulfur and not mineralized black shales with a mixing between these two sulfur sources.