Tephrostratigraphy in marine cores is an important tool for dating sedimentary sequences along the North Atlantic and in Western Europe. Most previously analyzed marine tephra were produced by Icelandic volcanoes, albeit rarely from the Krafla volcano, and require land-based records for validation. Two series of rhyolitic tephra emitted to the east of the Krafla volcano were recorded in the Eemian Rangá Formation in Northern Iceland, the two series being separated by the Grímsvötn 1 tephra known as “5e low bas IV” in marine core (∼127 ka). These rhyolitic deposits were seemingly ejected from the Hágangnahali paleocrater row (North of the Hágöng) and overlay the older Halarauður Ignimbrite. This rhyolitic activity persisted for ∼20 ka to the end of the Eemian last thermal optimum. probably initiated by deglaciation (MIS 6a/5e). It postdated the Halarauður ignimbrite. The ignimbrite and caldera formation were correlated with an ODP907 tephra, yielding a possible 207 ka age, but possibly older. These results constrain the main rhyolitic activity of Krafla to after the mid-Pleistocene transition, as observed for other “two-magma” volcanoes. Rhyolitic activity is non-systematically triggered by deglaciation, at least as analysed during the period 80–400 ka, but this genetic link is possibly valid for the late Pliocene of Iceland.

There is ongoing debate as to when oxygenic photosynthesis evolved on Earth1,2. Geochemical data from ancient sediments indicate localized or ephemeral photosynthetic O2 production before the Great Oxidation Event (GOE) approximately 2.5-2.3 billion years ago (Ga), and currently suggest Archaean origins, approximately 3 Ga or earlier3-9. However, sedimentary records of the early Earth often suffer from preservation issues, and poor control on the timing of oxidation leaves geochemical proxy data for the ancient presence of O2 open to critique10-13. Here, we report rare Earth element data from three different Archaean carbonate platforms preserved in greenstone belts of the northwest Superior Craton (Canada), which were deposited by the activity of marine photosynthetic bacteria 2.87 Ga, 2.85 Ga and 2.78 Ga. All three indicate O2 production before the GOE in the form of significant depletions in cerium (Ce), reflecting oxidative Ce removal from ancient seawater, as occurs today14. Using 138La-138Ce geochronology, we show that La/Ce fractionation, and thus Ce oxidation, occurred at the time of deposition, making these the oldest directly dated Ce anomalies. These results place the origin of oxygenic photosynthesis in the Mesoarchaean or earlier and bring an important new perspective on a long-standing debate regarding Earth's biological and geochemical evolution.

The Plouaret-Commana-Huelgoat (PCH) vaugnerite-granite complex, emplaced at around 315 Ma on both sides of the North Armorican Shear Zone (NASZ) in the Armorican Massif (France), can be regarded as the "axial pivot" around which a large part of the history of the Armorican magmatism was revolving. Indeed, four main magmatic cycles can be distinguished in the Ordovician/Early Permian time-span over much of the considered areas, i.e. North and Central Armorican Domains (N/CAD): (1) several anorogenic mafic complexes (plutons, dykes and/or lavas) were successively emplaced from the Ordovician to Early Carboniferous; (2) near the Bashkirian/Moscovian time-boundary, when the N/CAD started to record the effects of the Variscan collision, magmatism became predominantly lamprophyric/vaugneritic (K-rich West-Armorican kersantites and PCH vaugnerites), prior to (3) the generalized granitic peak during the Moscovian-Kasimovian ages, in turn followed by (4) a K-rich intermediate magmatic activity, including again vaugnerites and the last granitic intrusions, during the late-/post-collision stages in the upper Pennsylvanian-Early Permian time-period. The sharp geochemical break evidenced in the present work between an early anorogenic magmatic group and a syn- to post-collisional system in the N/CAD emphasizes the original magmatic history recorded by part of Armorica, including probably the Central Iberian Zone (CIZ), during the nearly entire Paleozoic era compared to those commonly applied to the rest of the European Variscan orogen. This evolution is assumed to result from: (1) large-scale asthenospheric upheavals due to the downward straightening and retreat of variously plunging slabs during the Paleozoic times, (2) partial melting of metasomatized lithospheric/asthenospheric mantle due to fluid release from fragments of buried continental crust, and (3) crustal anatexis.

Mid-ocean ridges serve as key sites for understanding the composition of the mantle, but extensive melting usually masks its lithological diversity. This study explores how cold mid-ocean ridge segments, such as the eastern Romanche ridge-transform intersection (ERRTI), provide unique insights into mantle heterogeneity. Here, a thick cold lithosphere faces the warm ridge segment efficiently cooling the ridge tip, thus reducing melting and mixing, and allowing distinct short-scale lithologies to be sampled. Our findings reveal a mosaic of mantle components with diverse geochemical and isotopic signatures, reflecting dynamic mantle processes over time. By examining these cold regimes, this research sheds light on the mantle's compositional complexity and its evolution, offering a fresh perspective on lithospheric dynamics and melt generation in settings independent of hotspot influences.

The Bafoussam area in western Cameroon is part of the central Cameroon Volcanic Ligne (CVL). This study presents the mineralogy, major and trace element compositions, Sr-Pb-Hf isotopes, and new K–Ar geochronological data about mafic and felsic volcanic rocks. These rocks belong to two different series: A transitional series made of basalts, basaltic andesite, and trachytes and an alkaline mafic series with basalts, hawaiites, and basanites. New age data show that the transitional series belongs to the oldest part of the CVL and was emplaced between 47 and 35 Ma. The alkaline volcanism is younger, with ages ranging from 10 to 4.5 Ma. Magmatic evolution in both series is accomplished through a fractional crystallization process, with the removal of olivine and clinopyroxene, while plagioclase does not seem to be a major crystallizing phase. All the samples are enriched in incompatible trace elements, but the rocks from the alkaline series have more fractionated REE patterns and high Nb content compared to the transitional mafic lavas. Alkaline lavas have lower initial 87Sr/86Sr and higher 176Hf/177Hf and Pb isotopic ratios than the transitional lavas. Low La/Nb and high 87Sr/86Sri ratio are among chemical characteristics that show that some samples from the transitional series have interacted with a crustal component during their evolution in the crust. They cannot be used for discussing the mantle source of the volcanic rocks from this series. Trace elements show that primary magmas for both series formed in a garnet-bearing mantle source, with higher partial melting degrees (3–5

The first terrestrial habitats would have developed coincident with the generation of continental crust and the emergence of continents. Associated with this critical Earth-Life transition would have been the initiation of substantial subaerial weathering. Recently, fluvial sediments and nitrogen isotope-based evidence for a terrestrial biosphere have been reported from the 3.2 billion year old Moodies Group. Here we report hafnium isotope values for marine banded iron formations from the same succession. Values observed are amongst the most extreme values reported. These data exhibit a systematic departure from the terrestrial array, and if primary, can be explained by transport and weathering of felsic minerals under subaerial conditions, with values comparable to modern Amazon river waters and acid leachates of granite. These data from marine sediments likely constitute the earliest geochemical evidence of subaerial weathering processes affecting oceanic chemistry. This would imply that continental crust was an ecological niche and source of nutrients by the time they formed, at 3.22 billion years ago at the end of the Paleoarchean, consistent with interpretations of terrestrial sediments from the same succession that indicate microbial life and biogeochemical cycling were present at this time.

We present the first K-Ar geochronological and geochemical data (major and trace elements, as well as Sr, Nd, Pb, and Hf isotopes) on volcanic rocks from the Fotouni lavas in the central part of the Cameroon volcanic line (CVL). Two distinct compositional groups can be discriminated: alkaline and transitional lavas. The alkaline series ranges in composition from basanite and basalt to hawaiite and formed 14 to 3 Ma ago while the transitional series ranges from basalt and trachybasalt to basaltic trachyandesite and formed 47 to 38 Ma ago. All samples display trace element patterns dominated by light rare-earth element enrichments compared to heavy rare-earth elements. The alkaline samples have (87Sr/ 86 Sr)i ranging from 0.70292 to 0.70340, ε Nd = 3.45 to 5.98, ɛ Hf = 1.95 to 5.63, and relatively radiogenic Pb isotopic compositions: 206 Pb/ 204 Pb = 19.891–20.592; 207 Pb/ 204 Pb = 15.639–15.693; 208 Pb/ 204 Pb = 39.566–40.044. Transitional basalts are characterized by relatively radiogenic Sr and unradiogenic Nd, Hf, and Pb: (ɛ Nd )i = − 10.82 to + 3.43; ɛ Hf = − 14.00 to + 3.13; 206 Pb/ 204 Pb = 18.0716 to 18.9274; 207 Pb/ 204 Pb = 15.532 to15.704; 208 Pb/ 204 Pb = 37.8141 to 39.4735. Several geochemical characteristics indicate that the transitional lavas have interacted with the continental crust while alkaline lavas did not suffer significant amounts crustal contamination. Both transitional and alkaline lava series can be explained by mixing between two mantle components: an enriched component and a HIMU-like component. These two mantle sources are probably located within the lithospheric mantle and are different from both the Saint Helena and the Mont Cameroon mantle.

The Mozambique Channel plays a key role in the exchange of water masses between the Indian and Atlantic Oceans, which include the North Atlantic Deep Water (NADW) inflow from the south and the North Indian Deep Water (NIDW), an aged form of the NADW spreading poleward from the northern and equatorial Indian Ocean basin. Several authors assume that the Davie Ridge acts as a topographic barrier to the northward advection of NADW, which would therefore be absent in the Comoros Basin. Other studies suggest that the NADW flows from the south of the Mozambique Channel to the Comoros Basin, indicating that the Davie Ridge may not currently constitute a blocking topographic barrier to deep water mass circulation. To address this question, we studied ferromanganese (Fe, Mn) crusts collected over 2000 km in the Mozambique Channel, from the Agulhas Plateau to the Glorieuses Islands. Neodymium (Nd) isotope compositions (epsilon(Nd)) of surface scrapings range between epsilon(Nd) = -10.1 above the Agulhas Plateau, which might reflect the NADW inflow, and more radiogenic values between epsilon(Nd) = -8.0 and -8.2 in the Glorieuses area, highlighting the NIDW influence. However, value of epsilon(Nd) = -9.4 measured north of the Davie Ridge cannot be explained by the sole influence of the NIDW and therefore highlights the advection of the NADW northeast of the Comoros Basin. We estimate that the contribution of the NADW through the channel is up to 68% in the Agulhas Plateau and 60% north of the Davie Ridge. These findings are consistent with previous hydrographic studies and suggest that the Davie Ridge does not currently act as topographic barrier to deep currents.

The Early Holocene (12–8.2 cal ka) deglaciation and pulsed warming was associated in Iceland with two major generations of jökulhlaups around the Vatna ice-cap (Vatnajökull), at ca 11.4–11.2 cal ka and ca 10.4–9.9 cal ka, and major tephra emissions from the Grímsvötn and Bárðarbunga subglacial volcanoes. The earliest flood events were recorded inland during the Middle Younger Dryas and their deposits were overlain by the Early Preboreal Vedde Ash (11.8 cal ka). The first Holocene flood events (ca 11.4–11.2 cal ka) are issued from a glacial advance. The second, and major, set of floods was partly driven by the Erdalen cold events and advances (10.1–9.7 10 Be ka) initially issued from the Bárðarbunga (10.4, 10.1–9.9 ka) and Grímsvötn volcanoes (Saksunarvatn tephra complex, ca. 10.2–9.9 cal ka). These floods were also fed by the residual glacio-isostatic depressions below the Vatnajökull that enabled the storage of meltwaters in large subglacial lakes or aquifers until ca. 9.3 cal ka. This storage was enhanced by ice-damming and permafrost, especially during the twinned Erdalen events. Due to the glacio-isostatic rebound, the general slope was nearly flat, and the valley was partly filled with sediments until ca 10.8 cal ka. Temporary lacustrine deposits in this valley resulted from the very broad splay of waters as for the ca 11.2 cal ka and ca 10.1–9.9 cal ka flood, due to regional permafrost. These floods had a potential duration of several months as they were mostly fed by climate-driven meltwater. The maximal volume evacuated by these events did not greatly exceed 1 × 10 6 m 3 s −1 from the flood-affected transverse profile of the valleys that remain partly filled with sediments.

Precambrian banded iron formations (BIF) are chemical sedimentary deposits whose trace element signatures have been widely used to interrogate the chemical composition and redox state of ancient seawater. Here we investigated trace element signatures in BIF of the 3.22 Ga Moodies Group, Barberton Greenstone Belt (South Africa), which are interbedded with near-shore siliciclastic sedimentary rocks and represent one of the oldest known shallow-water occurrences of BIF. Unusual rare earth element (REE) signatures, notably with pronounced negative Ce anomalies in shale-normalized spectra, have been previously reported for chemical sediments of the Moodies Group, which we confirm here through an expanded dataset for Moodies BIF spanning three different localities. We find negative Ce anomalies as low as 0.2 Ce/Ce* that are associated with unusual enrichment of LREE relative to HREE in the sample set. While total REE abundances and certain REE features appear strongly related to the concentration of detrital indicators (e.g., Zr), and are likely primary, other features, notably LREE enrichment, cannot be explained as a primary feature of the sediment. This is better explained by later addition of REE from a LREE-enriched but Ce-depleted fluid that generated the significant negative Ce anomalies observed in surface samples of Moodies Group BIF. This REE addition event influenced both Sm-Nd and La-Ce isotope systematics, the latter yielding an isochron of 60 +/- 32 Ma, thus constraining the timing of emplacement of the negative Ce anomalies to the past 100 Ma, possibly upon surface exposure of the Barberton Greenstone Belt to wetter conditions during the Cenozoic. Our findings constitute a cautionary tale in that even the most immobile elemental redox proxies may be more sensitive to post-depositional modification than previously thought, and demonstrate the clear advantage offered by paleoredox proxies coupled to radiometric geochronometers to enable the direct dating of ancient signals of Earth surface oxygenation. (C) 2020 The Authors. Published by Elsevier B.V.

Marine mud belts represent potential continuous high-resolution climatic, environmental and anthropogenic archives. In this study, a geochemical record of the Gulf of Lions mud belt, which receives sediments from the Rhône watershed and to a lesser extent from the Languedoc region, is reported from Core KSGC-31. The effects of natural climatic changes and possible anthropogenic disturbances on Holocene sedimentation were ascertained by analysing sedimentation rates, chemical weathering (Al2O3/K2O) and sediment-source shifts (neodymium isotopic ratios; εNd). Measurements of elemental and isotopic lead were used to trace the source and determine the potential vectors of anthropogenic contaminations over the Holocene. High εNd values, recorded from 9000 to 3000 calibrated annum before present (cal. a BP) and around 1500 and 600 cal. a BP, are interpreted as an increase in sediment transport from the Alpine crystalline massifs to the sea induced by enhanced hydro-sedimentary conditions upstream. During the early and middle Holocene, low and stable weathering conditions were persistent, while the late Holocene was characterized by higher and more fluctuating weathering conditions. Sudden changes in the 206Pb/207Pb ratio observed during the Roman and Medieval periods suggest clear shifts in lead source from a natural Holocene background to late Holocene anthropogenic contaminations. Even though those shifts are coeval with atmospheric lead contaminations from Spain and Germany recorded in several sediment and ice archives, the local origin (the Cévennes) and the fluvial contamination is more likely in these cases. Those findings are contemporaneous with historical mining records in the Cévennes and point to an intensification of the merchant shipping.

While hydrothermal vents are now thought to be a major source of dissolved iron to the oceans, they have always been considered to be a sink for the dissolved rare-earth elements (DREEs). However, true dissolved REE observations in hydrothermal plumes are still lacking. Here we report for the first time the DREE concentrations and neodymium isotopic compositions (DεNd) of buoyant hydrothermal fluids at Lucky Strike (Mid-Atlantic Ridge). We find that 27 to 62% of total hydrothermal DREEs are rapidly scavenged by anhydrite precipitation at the onset of buoyant plume formation. After this initial loss, all DREEs behave quasi-conservatively within the buoyant plume. Dissolved phase εNd (DεNd) in the evolving plume are identical to black smoker DεNd of +9.0 and contrast radically with DεNd of the local deep water mass at −12.0. Plume DεNd as low as +6.6 may be reconciled by dissolution of newly formed barite in the local environment and carrying seawater DεNd signature. We find, based on the first plume DREE observations, that hydrothermal plumes are in fact a source of DREE to the North Atlantic Deep Water. Precipitation/dissolution processes of hydrothermally-derived minerals, i.e. sulfates in the buoyant plume and Fe oxy-hydroxide in the non-buoyant plume, will likely affect the fate of other trace metals and their isotopic composition.

Distributions of dissolved rare earth element (REE) concentrations and neodymium isotopic compositions (expressed as eNd) of seawater over and off the Kerguelen Plateau in the Southern Ocean are presented. The sampling took place during the austral spring bloom in October-November 2011 (KEOPS2 project, GEOTRACES process study) and aimed to further the investigations of the KEOPS1 austral summer study in terms of sources and transport of lithogenic material, and to investigate the impact of local biogeochemical cycles on the REE distributions. The REE signature of the coastal eastern Kerguelen Islands waters was characterized by negative europium anomalies (Eu/Eu*) and negative eNd in filtered samples. By contrast, the unfiltered sample showed a positive Eu/Eu* and more radiogenic eNd. These distinct signatures could reflect either differential dissolution of the local flood basalt minerals or differential leaching of local trachyte veins. The dissolved Kerguelen coastal REE patterns differ from those observed close to Heard Island, these latter featuring a positive Eu/Eu* and a less radiogenic eNd (Zhang et al., 2008). These differences enabled us to trace the transport of waters (tagged by the Kerguelen REE signature) 200 km downstream from the coastal area, north of the Polar Front. Northward transport of the central Plateau shallow waters, enriched by both local vertical supplies and lateral advection of inputs from Heard Island, was also evident. However, the transport of Kerguelen inputs southeastward across the Polar Front could not be discerned (possibly as a result of rapid dilution or scavenging of REE signatures), although evidence for such transport was found previously using Ra isotopes (Sanial et al., 2015). Comparison of the REE patterns at stations sampled prior, during and at the demise of the bloom revealed diverse fractionations, including production of significant lanthanum and europium anomalies, which are tentatively ascribed to chemical reactions with various inorganic and biogenic phases, including surface coatings, barite crystals, and biogenic silica.

We investigated mafic and felsic volcanic rocks from the Bamoun plateau, a magmatic province located north of Mount Cameroon, in the continental part of the Cameroon Volcanic Line (CVL). Basalts and dacites were probably emplaced more than 40 Ma ago, while basanites represent very young volcanic eruptions. Among the basalts, some of them have suffered crustal contamination during their uprise through the continental crust, and their primary trace element and isotopic compositions have been slightly modified. The formation of the dacites was also accompanied by some crustal contamination. Non-contaminated rocks show that the oldest magmas are transitional basalts formed by relatively high degrees of partial melting of a moderately enriched mantle source, probably containing pyroxenites. Recent basanites were produced by very low partial melting degrees of an enriched mantle source with HIMU composition, but different from the source of the nearby Mount Cameroon lavas. The mantle beneath the CVL is thus very heterogeneous, and the tendency towards more alkaline mafic-ultramafic compositions in the youngest volcanic manifestations along the CVL seems to be a general feature of all CVL.

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. OIB/seamount recycling as a possible process for E-MORB genesis Marc Ulrich, Christophe Hémond, Philippe Nonnotte, Klaus Peter Jochum

Epithermal mineralization was recently described in synrift volcanic rocks from the SE Afar Rift in Djibouti. To infer fluid sources in this system, coupled sulfur and strontium isotopic analyses were performed on mineralized samples from hydrothermal veins and stockwork zones. Mineralization includes gypsum and chalcedony, and/or quartz±carbonate containing gold and sulfides. Sulfur isotopic compositions of sulfides and sulfates were determined using Multicollector-Inductively Coupled Plasma Mass Spectrometer (MC ICPMS); Laser Ablation MC ICPMS; and Continuous Flow Elemental Analyzer-Isotope Ratio Mass Spectrometry while strontium isotopes were determined by Thermal Ionisation Mass Spectrometry (TIMS) methods. Sulfur isotopic composition of sulfides (mainly pyrite) varies from −9.2 to +6.6‰. These values are classically reported for hydrothermal sulfides resulting from the mixing of variable S sources such as seawater sulfate and magmatic source. In addition, individual pyrite grains within the same sample show both positive and negative δ34S values suggesting multi-phase mineralization processes. Coupled sulfur and strontium isotopic compositions of seven sulfate samples hosted in (i) gypsum mound, (ii) evaporites, (iii) stockwork veins in sediments or in volcanic rocks were also investigated to characterize the sources of Sr and S. The δ34S values of the sulfate ranged from −0.8 to +14.3‰ while the 87Sr/86Sr ranged from 0.70389 to 0.70639 consistent with variable mixing ratios of Sr and S derived from volcanic sources and seawater-derived sulfate. This study shows that, although precious-metal deposition is related to low-sulfidation epithermal environment, both acidic fluid of magmatic and saline origins contributed to the hydrothermal system in the SE Afar Rift.

The age and origin of the volcanism along the Cameroon Volcanic Line (CVL) are still a matter of debate. We present major and trace element compositions as well as Sr–Nd–Pb–Hf isotopic results for mafic rocks from the Bamoun area, in the central part of the Cameroon Volcanic Line, as well as for two samples from recent eruptions of Mt. Cameroon. Lava flows are mostly basalts and hawaiites with transitional affinity. Some samples are among the oldest rocks of the CVL, with ages older than 51 My while some rocks are very young, around 0.05 My. All the samples are enriched in incompatible elements, indicating that melts were formed in a garnet-bearing mantle source. Different mantle sources participated to the formation of the Bamoun lavas. One source is isotopically similar to the main volcanic rocks of the CVL and probably represents an important part of the subcontinental lithospheric mantle. The second source is enriched in incompatible elements and shows a marked positive Eu anomaly, probably related to the participation of pyroxenites in the partial melting processes. The third mantle source is similar to the source of the Mt. Cameroon. This mantle source was known previously only in the Mt. Cameroon lavas, and we report its occurrence for the first time in old lavas and in other location along the CVL.