Abstract At the Trans‐Atlantic Geotraverse hydrothermal field, metalliferous sediments cover extinct hydrothermal mounds and the surrounding seafloor. Here, we report the morphological, mineralogical and geochemical processes that deposit these sediments, remobilize their metals, and affect their preservation. We found that the initial sediment metal tenor is controlled by physical transport of hydrothermal material from its source, followed by diagenetic redistribution and potentially diffuse fluid flow after high‐temperature hydrothermal activity has ceased. We distinguished three different environments: (a) proximal metalliferous sediments on top of extinct mounds are mainly derived from oxidative weathering of primary sulfide structures and are predominantly composed of Fe oxyhydroxides with low contents of Cu, Co, and Zn; metal enrichments in specific layers are likely related to upward flow of low‐temperature hydrothermal fluids; (b) medial distant metalliferous sediments found at the base of the mounds, deposited by mass transport, contain cm‐thick layers of unsorted sulfide sands with high base metal contents (e.g., up to 28% Cu); these buried sulfides continue to undergo dissolution, resulting in metal release into porewaters; (c) distal metalliferous sediments, found in depositional basins a few hundreds of meters from the extinct mounds, include fining‐upwards sequences of thin sulfide sand layers with Fe oxyhydroxides and were deposited by recurrent turbiditic flows. Dissolved metals (e.g., Cu2+ and Mn2+) diffuse upwards under reducing conditions and precipitate within the sediment. Hence, when using hydrothermal sediments to construct reliable geochronological records of hydrothermal activity, distance from source, local seafloor morphology, mass‐transport and depositional, and diagenetic modification should all be considered.

The last eruption on Fogo Island (Cape Verde Archipelago) occurred in 2014-2015, with mostly hawaiian and strombolian but sometimes vulcanian activity, with variable emission rates of lava flows, pyroclasts and gases (SO2 and CO2). Some lava flows, mainly from the first stage of the eruption enclosed small granular ultramafic nodules (1-3cm), with angular to rounded shapes. The host rocks are porphyritic tephrites, with Ti-augite and Ti-magnetite phenocrysts and, sometimes, amphibole xenocrysts in a brown glassy matrix including laths of plagioclase, clinopyroxene and Ti-mgnetite. The nodules are composed of an early crystallization phase olivine, in subheuedral crystals devoid of kink-bands or in rounded crystals enclosed in clinopyroxene oikocrysts. Clinopyroxene occurs in subeuhedral to anhedral zoned crystals, sometimes partially patchy replaced by late igneous amphibole which also occurs as primary crystals as well as in some xenocrysts. In both cases they frequently show reaction rims with transformation in rhonite, most probably resulting from degassing. Oxide minerals are present as a minor component occurring in sub-euhedral to anhedral crystals as inclusions in olivine and clinopyroxene or interstitially between silicate minerals. The typical cumulus textures, and the mineral chemistry already obtained for the ultramafic nodules from the 2014 eruption at Fogo strongly suggest that they have a cumulate origin and are cognate with the host magmas. Indeed, the similar composition of Ti- augites from the nodules and phenocrysts (Wo49-51 En42-36 Fs12-10), as well as the olivine Fo contents and high CaO contents in olivine are explained by crystal segregation from the same magma of the host rock. Geothermobarometric calculations point to crystallization temperatures for the cumulates between 1150 and 1200 ºC and pressures around 7- 10 kbar, while phenocrysts in host rocks crystallized at around 1000 ºC, and pressures of 3-4. These data confirm the existence of a polybaric plumbing system feeding the 2014-15 Fogo eruption, with some of the reservoirs having developed at mantle depths (at least 22 km). This research received financial support from FCT (Fundação para a Ciência e Tecnologia) through project FIRE (PTDC/GEO-GEO/1123/2014).

The on-board identification of ore minerals during a cruise is often postponed until long after the cruise is over. During the M127 cruise, 21 cores with deep-seafloor sediments were recovered in the Trans-Atlantic Geotraverse (TAG) field along the Mid Atlantic Ridge (MAR). Sediments were analyzed on-board for physicochemical properties such as lightness (L*), pH and Eh. Selected samples were studied for mineral composition by X-ray powder diffraction (XRD). Based on XRD data, sediment samples were separated into high-, low- and non-carbonated. Removal of carbonates is a common technique in mineralogical studies in which HCl is used as the extraction agent. In the present study, sequential extraction was performed with sodium acetate buffer (pH 5.0) to remove carbonates. The ratio between the highest calcite XRD reflection in the original samples (Iorig) vs its XRD-reflection in samples after their treatment with the buffer (Itreat) was used as a quantitative parameter of calcite removal, as well as to identify minor minerals in carbonated samples (when Iorig/Itreat > 24). It was found that the lightness parameter (L*) showed a positive correlation with calcite XRD reflection in selected TAG samples, and this could be applied to the preliminary on-board determination of extraction steps with acetate buffer (pH 5.0) in carbonated sediment samples. The most abundant minerals detected in carbonated samples were quartz and Al- and Fe-rich clays. Other silicates were also detected (e.g., calcic plagioclase, montmorillonite, nontronite). In non-carbonated samples, Fe oxides and hydroxides (goethite and hematite, respectively) were detected. Pyrite was the dominant hydrothermal mineral and Cu sulfides (chalcopyrite, covellite) and hydrothermal Mn oxides (birnessite and todorokite) were mineral phases identified in few samples, whereas paratacamite was detected in the top 20 cm of the core. The present study demonstrates that portable XRD analysis makes it possible to characterize mineralogy at cored sites, in particular in both low- and high-carbonated samples, before the end of most cruises, thus enabling the quick modification of exploration strategies in light of new information as it becomes available in near-real time.

•Generic geological model of hydrothermally extinct seafloor massive sulphide.•Sub-surface characterisation by combined drilling and geophysics.•New sulphide resource estimate for slow-spreading mid-ocean ridges.•Holistic approach to assessing seafloor massive sulphide deposits.

New geochemical, isotopic (Sr-Nd-Hf-Pb) and K-Ar data, are presented here on samples from the Southern Azores Seamount Chain (SASC) located south of the Azores Plateau. The SASC also includes the Great Meteor, Small Meteor and Closs seamounts, morphologically connected by a saddle at −4100m deep. We conclude that the SASC are characterized by a narrow isotopic variability that falls within the Azores isotopic field. Although each seamount has its own isotopic signature, their mantle source must comprise four local mantle end-members, three of which are common to the Azores, e.g. Plato isotopic signature results from the mixing between HIMU and N-MORB while Great Meteor signature results from this mix with the Azores Common Component (AzCC). A fourth end-member with high 208Pb/204Pb and decoupled Th/U ratios (Δ8/4 up to 59.2) is identified on Great Meteor northern flank. New K-Ar ages on Plato (33.4±0.5Ma) and Small Hyeres (31.6±0.4Ma) show nearly coeval volcanism, which is contemporaneous with the E-MORBs erupted at the MAR, drilled on oceanic crust with 30-34Ma (DSDP82). This study endorses the genetic link between the Azores Archipelago and the SASC to the long-term activity of the Azores plume and the large-scale ridge-hotspot interaction, contributing to better constrain the temporal-spatial evolution of this region of the North Atlantic.

Recurrent eruptions at very active ocean island volcanoes provide the ideal means to gain insight on the scale of spatial variations at the mantle source and on temporal changes of magma genesis and evolution processes. In 2014, after 19years of quiescence, Fogo volcano (Cape Verde Archipelago) experienced a new eruption, with the vents located 200m from those of the 1995 eruption, and less than 2000m from those of the 1951 event. This offered a unique opportunity to investigate the existence of small-scale mantle heterogeneities and the short-term compositional evolution of magmas erupted by a very active oceanic volcano like Fogo. Here we present petrological and geochemical data from the early stages of the Fogo's most recent eruption – started on November 23, 2014 – and compare them with the signature of previous eruptions (particularly those of 1995 and 1951).The magmas erupted in 2014 are alkaline (up to 23.4% and 0.94% of normative ne and lc, respectively) with somewhat evolved compositions (Mg #<56), ranging from tephrites to phonotephrites. The eruption of phonotephritic lavas preceded the effusion of tephritic ones. Lavas carried to the surface clinopyroxene and kaersutite phenocrysts and cognate megacrysts, which indicate that the main stages of magma evolution occurred in magma chambers most probably located at mantle depths (25.6±5.5km below sea level). This was followed by a shallower (<1.5km below sea level) and shorter (≈50days) magma stagnation before the eruption. 2014 magmas have more unradiogenic Sr and more radiogenic Nd compositions than those of the previous 1951 and 1995 eruptions, which generally have less radiogenic Pb ratios. These isotopic differences – coming from quasi-coeval materials erupted almost in the same place – are remarkable and reflect the small-scale heterogeneity of the underlying mantle source. Moreover, they reflect the limited isotopic averaging of the source composition during partial melting events as well as the inefficient homogenization within the plumbing system when on route to the surface. The lid effect of an old and thick lithosphere is considered of utmost importance to the preservation of a significant part of source heterogeneity by erupted magmas. The decrease in the contribution of an enriched component to the Fogo magmas in the 2014 eruption marks a change on the volcano short-term evolution that was characterized by a progressive increase of the importance of such a component. Nb/U ratios of the 2014 lavas are similar, within 2σ, to the mean value of OIB, but significantly lower than those reported for the 1995 and 1951 eruptions. This is considered to reflect the lack of significant mixing of the 2014 magmas with lithospheric melts, as opposed to what is here hypothesised for the two previous eruptions.

No paleomagnetic data exist for Paleo-Mesoproterozoic times of the West African Craton (WAC). Therefore, paleogeographic reconstructions for such old geological times are difficult to constrain. Gaps on the sedimentary record and intense remagnetizations are the major problems that paleomagnetic studies come across. Recent geochronological results for dyke swarms that intrude several Proterozoic inliers of WAC in the Anti-Atlas Belt (southern Morocco) revealed ages between Paleoproterozoic and early Neoproterozoic, opening for the first time a window of opportunity to conduct paleomagnetic studies and tentatively infer about the paleoposition of WAC during Proterozoic. On this scope we conducted a paleomagnetic study on seven Proterozoic dykes of the Iguerda inlier. The meaning of the obtained paleomagnetic directions was evaluated by rock magnetic and mineral analyses, complemented by petrographic observations. Our samples record the presence of a complex history of remagnetization, mostly assigned to several Phanerozoic thermal/chemical events, in particular to the late stages of Pan African orogeny (s.l.), to the Late Carboniferous Variscan orogeny, and even to more recent events. The recognized remagnetization processes are related to widespread metamorphic events under greenschist facies followed by low-temperature oxidation, both responsible for the formation of new magnetic phases, like magnetite and hematite. These events obliterated the primary (magmatic) thermo-remanent magnetization and promoted multiple remagnetizations of the dykes, thermally and chemically. For only one dyke the presence of primary magnetization is possible to infer, though not to confirm, and would place WAC at an equatorial position around 1750 Ma.

The West Iberian Margin (WIM) preserves onshore testimonies of three Mesozoic magmatic cycles. In this paper we present and discuss 40Ar/39Ar ages and geochemical data for the second cycle, which occurred at least from 148Ma to 140Ma, during the late stages of an important extensional event associated with the Iberia–Newfoundland rifting. The related lithospheric stretching induced magma genesis by adiabatic decompression. Primitive rocks are mildly alkaline but evolved to SiO2-saturated and oversaturated rocks at “high” pressure. Magmas sampled a source of fairly homogenous composition characterized by Sr and Nd isotopic compositions (εNdi from +1.6 to +4.2), more enriched than the typical N-MORB source. Magmas were generated at the top of the garnet zone. Considering the thickness of the lithosphere and the geochemical constraints, an origin by melting of a metasomatized domain of the lithosphere is favored. The composition of these onshore magmas is somewhat distinct from the quasi coeval magmas emplaced offshore, which is interpreted as a result of the less important onshore lithospheric stretching, leading to lower degrees of partial melting. This favored the contribution of lithospheric metasomatized domains to onshore magmas. Rocks intruded two sectors of the Lusitanian Basin separated by the Nazaré Fault and characterized by distinct subsidence rates during the Jurassic. The fact that the rocks to the north of the Nazaré Fault are significantly more evolved indicates the more important development of magma chambers in the north, suggesting distinct thermal profiles for those two sectors. Such magma chambers enabled the “high-pressure” fractionation necessary to drive magma compositions from Ne-normative to SiO2- saturated and -over-saturated. The rocks cropping out south of the Nazaré fault are clearly less evolved, and its variability is mostly due to different partial melting events. Some rocks present evidence of post-magmatic processes involving neighboring Jurassic evaporite materials, leading to an increase in the Na2O content and 87Sr/86Sr ratio.

The islands of the Azores archipelago emerge from an oceanic plateau built on lithosphere increasing in age with distance from the Mid-Atlantic Ridge from 10 to 45 Ma. Here, we present the first comprehensive major and trace element and Sr–Nd–Pb isotope data from Santa Maria, the easternmost island of the archipelago, along with published data from the other Azores islands situated much closer to the Mid-Atlantic Ridge axis. We can show that the distinctively more variable and more enriched trace element ratios at Santa Maria combined with a relatively small range in Sr–Nd–Pb isotope ratios are the result of low degrees of partial melting of a common Azores mantle plume source underneath thicker lithosphere. This implies that melt extraction processes and melting dynamics may be able to better preserve the trace element mantle source variability underneath thicker lithosphere. These conclusions may apply widely for oceanic melts erupted on relatively thick lithosphere. In addition, lower Ti/Sm and K/La ratios and SiO 2 contents of Santa Maria lavas imply melting of a carbonated peridotite source. Mixing of variable portions of deep small-degree carbonated peridotite melts and shallow volatile-free garnet peridotite could explain the geochemical variability underneath Santa Maria in agreement with the volatile-rich nature of the Azores mantle source. However, Santa Maria is the Azores island where the CO 2 -rich nature of the mantle source is more evident, reflecting a combination of a smaller extent of partial melting and the positioning at the edge of the tilted Azores mantle plume.

Three volcano-stratigraphic units were identified at Brava Island in the Cape Verde Archipelago on the basis of field relationships, geologic mapping and Ar-40/Ar-39 and U-Th ages. The Lower Unit comprises a 2-to-3 Maold submarine volcanic sequence that represents the seamount stage. It is composed of nephelinitic/ankaramitic hyaloclastites and pillow lavas, which are cut by abundant co-genetic dikes. Plutonic rocks of an alkaline-carbonatite complex, which intruded the submarine sequence 1.8 to 1.3 Ma ago, constitute the Middle Unit. A major erosional surface developed between 1.3 and similar to 0.25 Ma. The post-erosional volcanism recorded in the Upper Unit started 0.25 Ma ago and is dominated by phonolitic magmatism. This phase is characterised by explosive phreato-magmatic and magmatic activity that produced block and ash flow, surge, and pyroclastic fall deposits and numerous phreato-magmatic craters. Effusive events are represented by lava domes and coulees. One peculiarity of Brava is the occurrence of carbonatites in both the plutonic complex and the post-erosional phase as extrusive volcanics. The intrusive carbonatites are younger than those occurring on Fogo, Santiago and Maio islands. Young (Upper Pleistocene to Holocene) extrusive carbonatites occurring in the late stages of volcanism are unknown in other Cape Verde islands.The occurrence of pillow lavas and hyaloclastites above the present sea level (up to 400 m) and raised Upper Pleistocene beaches indicates continuous uplift of Brava since the seamount stage. By dating raised marine markers, uplift rates were estimated at between 0.2 and 0.4 mm/a. The evolution of Brava was controlled by faults with directions similar to those described for Fogo, suggesting a common stress field. A detailed geological map (1/25,000) of Brava is presented. (C) 2010 Elsevier B.V. All rights reserved.

Lavas from Santiago Island attest to a complex magmatic history, in which heterogeneous mantle source(s) and the interactions of advecting magmas with thick metasomatised oceanic lithosphere played an important role in the observed isotopic and trace element signatures. Young (<3.3 Ma) primitive lavas from Santiago Island are characterised by pronounced negative K anomalies and trace element systematics indicating that during partial melting DK>DCe. These features suggest equilibration with an oceanic lithospheric mantle containing K-rich hydrous mineral assemblages, consistent with the occurrence of amphibole + phlogopite in associated metasomatised lherzolite xenoliths, where orthopyroxene is partially replaced by newly formed olivine + (CO2 + spinel + carbonate inclusion-rich) clinopyroxene. Metasomatism induced a decrease in \( a ^{{{\text{melt}}}}_{{{\text{SiO}}_{{\text{2}}} }} \) and Ti/Eu ratios, as well as an increase in fO 2 , Ca/Sc and Sr/Sm in the Santiago magmas, suggesting a carbonatitic composition for the metasomatic agent. Santiago primitive lavas are highly enriched in incompatible elements and show a moderate range in isotopic compositions (87Sr/86Sr = 0.70318–0.70391, 143Nd/144Nd = 0.51261–0.51287, 176Hf/177Hf = 0.28284–0.28297). Elemental and isotopic signatures suggest the involvement of HIMU and EM1-type mantle end-members, in agreement with the overall isotopic characteristics of the southern Cape Verde Islands. The overall geochemical characteristics of lavas from Santiago Island allow us to consider the EM1-like end-member as resulting from the involvement of subcontinental lithospheric mantle in the genesis of magmas on Santiago.

Santiago ultramafic xenoliths preserve mineralogical and geochemical evidence for the occurrence of metasomatic processes in the mantle beneath the island. Indeed, systematic orthopyroxene substitution by clinopyroxene in former harzburgites, occurrence of hydrated minerals and calcite, as well as LREE enriched patterns, strongly suggest involvement of a low-aSiO2 carbonatitic-like melt on such metassomatic process. This feature is supported by significant occurrence of carbonatite magmatic activity in Santiago Island.