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Seafloor abyssal peridotites are products after partial melting and melt-rock interaction, providing constraints on upper mantle processes and heterogeneity. Despite extensive studies of ophiolitic peridotites and a burgeoning dataset for dredged abyssal peridotites, limited studies have been undertaken on abyssal peridotite drill core samples. This omission means a lack of comparison between modern ocean mantle lithosphere and that of ophiolites, where field context is possible in the latter, but not the former. Ocean Drilling Program (ODP) Leg 153 near the Kane fracture zone, and ODP Leg 209, near the Fifteen-Twenty fracture zone, both on the mid-Atlantic Ridge but ∼1000 km distant from one another, recovered serpentinized mantle peridotites. We report whole-rock major-, trace- and highly siderophile-element (HSE: Re, Pd, Pt, Ru, Ir, Os) abundances and 187Os/188Os ratios of peridotites from three cores (maximum depth of 200.8 m) from the two Legs to explore mantle heterogeneity within the Atlantic Ocean basin. Peridotites from ODP Leg 153 are relatively fertile, with Al 2 O 3 contents ranging from 1.1 to 2.7 wt%. They are characterized by relatively radiogenic 187Os/188Os (0.1259 ± 0.0017; 2 S.D.) and uniform HSE abundances. The ODP Leg 209 peridotites are highly refractory, with Al 2 O 3 contents ranging from 0.1 to 0.8 wt%, and imply that they record an extensively depleted mantle. They have 187Os/188Os of 0.1207 ± 0.0023 (2 S.D.) and fractionated HSE abundances, indicative of depletion occurring > 1 billion years ago. Marked heterogeneity both in the extent and timing of depletion are therefore evident along a large segment of the mid-Atlantic ridge that produces zero-age basalts. Measurement of Os isotopes in spatially associated mid-ocean ridge basalts (MORB) is challenging due to their sub ng/g Os contents. Instead, Nd isotopes of MORB are used to show that they are unlikely to have formed directly from abyssal peridotite-like sources. Abyssal peridotites therefore represent long-lived refractory residues during mantle upwelling with the sources of MORB being more enriched. Comparison between drilled abyssal peridotites and ophiolitic mantle reveals larger compositional variations in the latter. The overall higher melting extent in ophiolite mantle is likely reflective of a more complex life cycle than the modern oceanic lithosphere. [ABSTRACT FROM AUTHOR]

Aim: Predicting spatial and temporal changes in species distributions is difficult for highly mobile species, making effective management challenging. We aim to understand the seasonal variability in habitat suitability for Atlantic bluefin tuna (Thunnus thynnus; ABFT) by using tracking data to create species distribution models. Location: North‐east Atlantic Ocean. Methods: Forty‐nine ABFT were tagged with pop‐up archival satellite tags along the west coast of Ireland during successive autumns from 2016 to 2021. Four thousand two hundred and sixteen daily locations were used to build a habitat model to describe ABFT environmental preferences and habitat suitability. For each location, 100 'pseudoabsence' locations were simulated using correlated random walks. Results: Tags had a mean retention rate of 237 ± 107 days, resulting in 11,602 cumulative days at liberty, and a mean daily distance travelled of 51 ± 143 km. ABFT showed two primary migration routes from Ireland, travelling to the Bay of Biscay or across the Mid‐Atlantic ridge in the winter. However, in 2019 six individuals travelled northwards towards Iceland, likely in response to a marine heatwave. Eight ABFT entered the Mediterranean in May/June for putative spawning, with six returning to Irish waters a year later. Five environmental covariates best described ABFT habitat, with habitat suitability highest at sea surface temperatures (SST) of 15–18°C, a mixed layer depth between ~200 and 300 m, high relief bathymetric features (i.e. shelf breaks and seamounts) and frontal regions as revealed by non‐linear preferences of eddy kinetic energy and SST standard deviation. Main Conclusions: In the last decade, ABFT have been re‐establishing historic foraging grounds in higher latitudes, for example, Irish and Nordic waters. Our model shows the extent of suitable habitat in these regions and how recent ocean warming may have contributed to northern expansions. Understanding the drivers behind these reappearances is fundamental to successful future management. Achoimre: Aidhm: Tá sé deacair athraithe i ndáileadh ama agus spásdháileadh speiceas atá an‐soghluaiste a thuar, rud a chiallaíonn go bhfuil sé dúshlánach bainistiú éifeachtach a dhéanamh. Tá muid ag iarraidh inathraitheacht shéasúrach in oiriúnacht ghnáthóg an Tuinnín Gorm Atlantach (Thunnus thynnus; TGA) a thuiscint tríd sonraí rianaithe a úsáid chun samhail dháileadh speiceas a chruthú. Suíomh: Oirthuaisceart an Atlantaigh. Modh: Rinneadh clibeadh ar 49 TGA le clibeanna cartlainne a thagann go barr uisce ar chósta iarthar na hÉireann le linn ceithre fhómhair i ndiaidh a chéile ó 2016–2021. Úsáideadh 4216 suíomhanna laethúil chun samhail ghnáthóg a chruthú a rinne cur síos ar roghanna timpeallachta agus oiriúnacht ghnáthóg an TGA. I gcomhair gach suíomh, ionsamhlaíodh 100 suíomhanna bréag‐neamhláithreacht ag úsáid siúlóidí randamacha comhghaolaithe. Torthaí: Bhí meán ráta coinneála de 237 ± 107 lá ag na clibeanna, rud a thug 11,602 lá taifeadta i ndiaidh a chéile agus meán fhad laethúil taistealta de 51 ± 143 km dá bharr. Léirigh TGA dhá phríomhbhealaí imirce ó hÉirinn, ag taisteal go Bá na Bioscáine nó thar Droim an Atlantaigh Láir i rith an gheimhridh. Ach i 2019 thaisteal sé iasc ó thuaidh i dtreo an Íoslainn, is dócha mar gheall ar thonn teasa mara. Chuaigh ocht TGA isteach sa Mheánmhuir i rith mí Bealtaine/mí an Mheitheamh chun uibheacha a sceitheadh de réir dealraimh. Tháinig sé dóibh ar ais go huiscí na hÉireann an bhliain dar gcionn. Déanann cúig athróg timpeallachta an cur síos is fearr ar ghnáthóg an TGA, tá oiriúnacht ghnáthóg is airde ag teocht barr uisce (TBA) de 15–18°C, nuair atá doimhneacht an chisil mheasctha idir ~200 agus 300 m, ag gnéithe bataiméadrach ard‐rilíf (i.e. scoilteanna scairbhe agus tuláin grinnill), agus ag réigiúin frontacha léirithe ag roghanna neamh‐líneacha fuinneamh cinéatach guairneánach agus diall caighdeánach TBA. Príomhchonclúidí: Le leathscór bliain anuas tá TGA tar éis réigiúin sealgaireachta stairiúil a athbhunú in áiteanna le hard‐domhanleithead, ar nós uiscí na hÉireann agus uiscí Nordacha. Taispeánann ár samhail achar an ghnáthóg oiriúnach sna réigiúin seo agus an bhaint féideartha a bhí ag téámh aigéanach le gairid ar leathadh ó thuaidh. Tá sé buntábhachtach na cúiseanna don athfhilleadh seo a thuiscint i gcomhair bainistiú rathúil amach anseo. [ABSTRACT FROM AUTHOR]

The structure of the lithosphere and the associated magmatic systems found in different locations along slow‐spreading ridges can vary dramatically, from melt‐starved to magmatically robust segments. A growing number of studies suggest that the evolution of the magmatic crust being governed solely by fractional crystallization is too simplistic. Reactions between migrating melts and their surroundings play a key role during accretion, yet the full extent of their impact is still to be resolved. We present here the results of a petrological, microstructural, and in situ geochemical study of two drilled sequences from the Kane Megamullion and Atlantis Massif oceanic core complexes. We show that melt‐mush reactions generate locally strong textural and/or geochemical heterogeneity at the cm‐scale, but their impact can also be identified at the 100 m‐scale. We found evidence for assimilation at various degrees of primitive lithologies of potential mantle origin within the gabbroic sequence at both locations, in addition to typical melt‐mush reactions previously described in other slow‐spread magmatic systems. Observations and numerical modeling confirm the similarity of the reactions impacting both sequences. However, the regime of the reactions (ranges of assimilation to crystallization ratios) seems to vary between Kane Megamullion and Atlantis Massif, variations which likely result from differences in melt fractions present during melt‐mush reactions. We infer relying on our observations and previous studies that the regime of the reactions is most likely controlled by the melt flux during the formation of the two sections. Key Points: Kane Megamullion and Atlantis Massif oceanic core complexes record widespread melt‐mush reactions at different scalesIn addition to typical melt‐mush reactions, assimilation of primitive (mantle?) lithologies is observed within gabbro sequencesDiscrepancies in the melt‐mush reaction signatures of the two sequences reveal variable melt dynamics during accretion [ABSTRACT FROM AUTHOR]

Complex deformation is observed along the plate boundary between the Africa and Eurasia plates, this complexity is highlighted by the faulting mechanism changing from normal faulting at the Mid-Atlantic Ridge to thrust and strike-slip faulting in the Ibero-Maghreb region (Iberia, Morocco, Algeria and Tunisia). The geodynamics of the study area shows the occurrence of NW-SE convergence between the two plates, with anticlockwise rotation. An updated scheme of the pattern of the tectonic stress direction from the Azores Archipelago to the Tunisian Atlas is presented, along with the analysis of the principal stress axis orientations (Shmax = a1, Shmin = a3) from the inversion of fault plane solutions. We used a catalogue of 557 fault plane solutions with only main shocks without considering the related aftershock solutions for the period from 1931 to 2020. This study complements previous work limited to Algeria and eastern Morocco by inverting earthquake mechanisms of aftershock sequences of strong events that occurred in Al Hoceima (Morocco), El Asnam, Chenoua-Tipasa, Zemmouri and Constantine (Algeria). The present work includes the area from Tunisia to the Mid-Atlantic Ridge. The inversion considers only the earthquake mechanisms of events 4.0

The South Atlantic Ocean hosts several well‐studied volcanic ridges and seamount chains, but the origin of their associated mantle plumes is debated. Reduced seismicity on the southern Mid‐Atlantic Ridge (MAR) suggests anomalously ductile thermomechanical conditions at 52°S and 47.5°S. These low seismicity patches extend 120–560 km along‐axis, and correspond with axial high spreading ridge morphology, geochemical anomalies, and mantle wave speed patterns likely associated with the Shona and Discovery plumes. Bathymetric data show that the northern extent of the Shona swell is associated with increased volcanism, elevated axial bathymetry, and a series of northward‐propagating rifts, with the overall swell geometry suggesting a buoyancy flux of 0.4–0.5 Mg s−1. The nearby Bouvet Island may be a product of a branch of the larger Shona plume swell, which has influenced crustal accretion on the southern MAR for the past 24 million years. Plain Language Summary: Mantle plumes are upwellings of hot material that are thought to originate deep inside Earth's mantle, reaching the base of the tectonic plates and creating oceanic islands like Hawaiian‐Emperor seamount chain. Although these seamount chains help us understand processes within the mantle, the precise location of their associated mantle plumes is surprisingly poorly known. Several prominent seamount chains are located in the south Atlantic ocean, however severe sea conditions mean that they have only been sparsely sampled and mapped, and the location of the Shona, Discovery, and Bouvet mantle plumes have been debated since the 1970s. Prominent gaps in records of earthquake seismicity suggest that hot material from the Shona and Discovery plumes is reaching the Mid‐Atlantic ridge today, inhibiting slip on faults. Associated slow mantle wave speeds, elevated seafloor depths, and the composition of basalts dredged from the seafloor are all consistent with the idea that thermal anomalies associated with the Discovery and Shona plumes are beneath the ridge axis today. The Shona plume is likely to exert significant control on how new crust forms in the southern Atlantic Ocean, and could explain the formation of Bouvet Island, which could be associated with a branch of the main Shona hotspot. Key Points: Lack of seismicity between 47–48°S and 50.5–53.8°S on Mid‐Atlantic Ridge (MAR), coincident with Discovery and Shona mantle plumesAseismic segments align with geochemical anomalies, regional swells, and slow mantle wave speeds, consistent with ridge‐plume interactionBouvet Island could be a branch of the main Shona plume, which has buoyancy flux of 0.4–0.5 Mg s−1 [ABSTRACT FROM AUTHOR]

The structure of King's Trough and its surroundings (King's mesostructural cluster), located on the eastern flank of the Mid-Atlantic Ridge in the North Atlantic, is described in this paper. This work is based on geological and geophysical data obtained during the 55th expedition of the R/V Akademik Nikolaj Strakhov. Six provinces were identified within the southeastern part of King's Trough, based on the results of bathymetric survey. Each province has its own morphostructural feature resulting from multistage tectonic and volcanic processes, which alternate and conjugate with each other in time. According to seismoacoustic profiling data, three main types of seismic facies have been identified: (a) pelagic complexes; (b) deposits of turbidite flows; and (c) chaotic facies of gravity origin. It is shown that the anomalous magnetic field of the study area is the superposition of linear and isometric anomalies. The first were formed during the formation of the oceanic crust in the axial zone of spreading. The second are associated with volcanic massifs formed under intraplate conditions. The obtained data confirm the assumption that the formation of King's Trough was preceded by the formation of an elongated arched rise, which became a scene of intense intraplate volcanism that increased from southeast to northwest. This stage was followed by subsidence of the axial part of the rise with the formation of King's Trough and the Peake and Freen Troughs. [ABSTRACT FROM AUTHOR]

The Doldrums transform system, located in the Equatorial Atlantic at 7–8°N, is a 110 km-wide multi-fault shear zone, with five active transform faults separated by four short intra-transform ridge segments (ITRs). The two central ITRs, ITR-2 and ITR-3, are significantly deeper than the peripheral ridge segments, suggesting differences in the thermal conditions of the sub-ridge mantle. New chemical and radiogenic isotope data from on-axis lavas erupted across the transform domain reveal that the basalts from ITR-3 are enriched in alkalis (Na 2 O + K 2 O = 4.3 wt%; Na 8 up to 3.7) and light rare earth elements (La/Sm) N = 0.86–0.97). However, these basalts have lower Sr and Pb isotope ratios than MORB from the Equatorial Atlantic (i.e., 87Sr/86Sr ∼ 0.70237 and 206Pb/204Pb ∼ 18), and relatively high Nd and Hf isotope ratios (143Nd/144Nd = 0.51315–0.51325; 176Hf/177Hf = 0.2832–0.28325). Hence, the mantle underlying ITR-3 is, on average, depleted in highly incompatible elements. Thermal models of the sub-ridge mantle show that it is also the coldest mantle region under the Doldrums system with the lowest crust production. Considering its location in the central transform domain, it is likely that the local mantle underwent melting beneath adjacent segments of the Mid-Atlantic Ridge (MAR) before remelting below ITR-3. We propose that the initial melting event beneath the MAR selectively removed the most fusible, geochemically enriched mantle components, leaving behind a predominantly peridotitic source, characterized by comparatively low Sr and Pb isotope ratios. Therefore, MORB from intra-transform ridge segments, such as those within the Doldrums transform system, provide a rare opportunity to constrain the isotopic composition of highly incompatible element depleted peridotitic mantle, which is a ubiquitous, but otherwise often camouflaged component of Earth's mantle. [ABSTRACT FROM AUTHOR]

The Azores archipelago, situated east of the Mid-Atlantic Ridge, comprises volcanic islands arranged along sub-parallel spreading systems and rests on a thick oceanic crust. Magma is supplied directly from the roots of the volcanic systems. Located at or nearby the boundary between the crust and the mantle, they consist of mafic cumulates and mafic mush layers. This work focuses on tephra samples and a submarine lava younger than 40.000 years, collected from both central volcanoes and fissure zones. Our report details a new dataset of major, trace, and volatile elements analysed in glassy melt inclusions trapped in olivine (Fo75.8–85.6) which are extracted from cumulative bodies at the vicinity of the crust-mantle boundary. Their compositions cover a range from subalkaline to mildly alkaline basalt, and trachybasalt, which match those of Azores lavas. They registered a chemical evolution through fractional crystallisation of olivine alone, as well as olivine and clinopyroxene, as both the FeOt/MgO (1.4–3.1) and CaO/Al2O3 (0.4–1.0) ratios of the melt decrease. Incompatible element ratios of Zr (40–352 ppm), Ba (135–612 ppm), and Rb (5–77 ppm), as compared to Nb (5–82 ppm), exhibit variability within a limited but significant range of values. The ranges in the Nb/Zr, Ba/Nb and Rb/Nb ratios recorded by melt inclusions possibly reveal distinct geochemical sources (at least two), and mixing between partial melts as they move upward. The halogen signature is characteristic of the shallow mantle. The majority of melt inclusions show Cl/K ratio (0.06) similar to E-MORB, although some of them are comparable to N-MORB (Cl/K = 0.03). Their F/Nd ratio may achieve a rather high value (27.8). [ABSTRACT FROM AUTHOR]

Trace metals (TMs) manganese (Mn), cobalt (Co), and aluminium (Al) have important geochemical and biological roles in the ocean. Here, we present full depth profiles of dissolved (d) and particulate Al, Mn, and Co along the latitude of 40 °S in the South Atlantic Ocean from the GEOTRACES GA10 cruises that operated in austral spring 2010 and summer 2011. The region is characterized by enhanced primary productivity and forms a key transition zone between the Southern Ocean and South Atlantic Subtropical Gyre. The mean concentrations of dAl, dCo, and dMn (±standard deviation) were 3.36 ± 2.65 nmol kg−1, 35.3 ± 17.6 pmol kg−1, and 0.624 ± 1.08 nmol kg−1, respectively. Their distributions in surface waters were determined by external sources and complex internal biogeochemical processes. Specifically, surface ocean dCo was controlled by the interplay between phytoplankton uptake, remineralization and external inputs; dMn was likely determined by the formation and photoreduction of Mn-oxides; and dAl was supplied by atmospheric deposition and removed by scavenging onto particles. Fluvial and sedimentary inputs near the Rio de La Plata estuary and benthic sources from the Agulhas Bank resulted in elevated dTM concentrations in near-shore surface waters. These externally sourced dTMs were effectively delivered to the open ocean by offshore diffusion and/or advection, and potentially facilitated enhanced primary productivity along the transect. The distributions of dTMs at depth were predominantly controlled by the mixing of North Atlantic Deep Water (NADW) and waters of Antarctic origin (e.g., Upper Circumpolar Water (UCDW) and Antarctic Bottom Water (AABW)). The calculated endmember concentrations of dAl and dCo in NADW showed minor decreases in the SASTG following north–south transport, suggesting removal rates of 0.064 nM/year and 0.035–0.075 pM/year, respectively. The endmember concentration of dCo in AABW was maintained at ∼30 pmol kg−1 without evidence for scavenging removal in the Southern Ocean and SASTG (time frame >400 years). The concentrations of dMn in NADW and AABW were between 0.1 and 0.16 nmol kg−1, and any elevated dMn concentrations were ascribed to local external inputs (e.g., from sediments in the Argentine Basin and hydrothermal activity near the Mid-Atlantic Ridge). Hence, four controlling factors (sources, internal cycling, water mass mixing and time) need to be considered when assessing TM distributions in the global ocean, even for TMs that are vulnerable to scavenging removal processes. Because the deep waters formed in high latitude oceans are crucial components of the global thermohaline overturning system, any processes (e.g., glacier melting, upwelling and sinking, and biological activity) that impact the preformed dTM concentrations in high latitude oceans will determine the downstream dTM distributions. Therefore, the sources and sinks of TMs and associated biological activity in high latitude oceans could engender basin to global scale impacts on seawater distributions of Al, Co, and Mn and their stoichiometric relationships with macronutrients, and the global biogeochemical cycles of these scavenged-type TMs. [ABSTRACT FROM AUTHOR]

Scleractinian corals are the main modern builders of coral reefs, which are major hot spots of marine biodiversity. Southern Atlantic reef corals are understudied compared to their Caribbean and Indo‐Pacific counterparts and many hypotheses about their population dynamics demand further testing. We employed thousands of single nucleotide polymorphisms (SNPs) recovered via ezRAD to characterize genetic population structuring and species boundaries in the amphi‐Atlantic hard coral genus Favia. Coalescent‐based species delimitation (BFD* – Bayes Factor Delimitation) recovered F. fragum and F. gravida as separate species. Although our results agree with depth‐related genetic structuring in F. fragum, they did not support incipient speciation of the 'tall' and 'short' morphotypes. The preferred scenario also revealed a split between two main lineages of F. gravida, one from Ascension Island and the other from Brazil. The Brazilian lineage is further divided into a species that occurs throughout the Northeastern coast and another that ranges from the Abrolhos Archipelago to the state of Espírito Santo. BFD* scenarios were corroborated by analyses of SNP matrices with varying levels of missing data and by a speciation‐based delimitation approach (DELINEATE). Our results challenge current notions about Atlantic reef corals because they uncovered surprising genetic diversity in Favia and rejected the long‐standing hypothesis that Abrolhos Archipelago may have served as a Pleistocenic refuge during the last glaciations. [ABSTRACT FROM AUTHOR]

Lightening of bottom water is required to close the abyssal overturning circulation, believed to play an important role in the climate system. A tracer release experiment and turbulence measurement programs have revealed how bottom water is lightened, and illuminated the associated circulation in the deep Brazil Basin, a representative region of the global ocean. Tracer was released on an isopycnal surface about 4000 m deep, over one of the fracture zones emanating from the Mid-Atlantic Ridge (MAR). Tracer that mixed toward the bottom moved toward the MAR across isopycnal surfaces that bend down to intersect the bottom at a rate implying a near-bottom buoyancy flux of 1.5 × 10−9 m2 s−3, somewhat larger than inferred from dissipation measurements. The diffusivity at the level of the tracer release is estimated at 4.4 ± 1 × 10−4 m2 s−1, again larger than inferred from dissipation rates. The main patch moved southwest at about 2 cm s−1 while sinking due to the divergence of buoyancy flux above the bottom layer. The isopycnal eddy diffusivity was about 100 m2 s−1. Westward flow away from the MAR is the return flow balancing the eastward near-bottom upslope flow. The southward component of the flow is roughly consistent with conservation of potential vorticity. The circulation as well as the pattern of diapycnal flux are qualitatively the same as in St. Laurent et al. (2001) but are more robust. The results indicate that diapycnal diffusivity is about twice that invoked by Morris et al. (2001) in calculating the basinwide buoyancy budget. Significance Statement: Buoyancy flux into the abyssal waters is required to close the overturning circulation of those waters, an important part of the climate system. This contribution presents a robust view of the strength of that buoyancy flux and the associated circulation. [ABSTRACT FROM AUTHOR]

Magmatic and tectonic processes in the formation of oceanic lithosphere at slow–ultraslow-spreading mid-ocean ridges (MORs) are more complicated relative to faster-spreading ridges, as their melt flux is overall low, with highly spatial and temporal variations. Here, we use the teleseismic catalog of magnitudes over 4 between 1995 and 2020 from the International Seismological Center to investigate the characteristics of magmatic and tectonic activities at the ultraslow-spreading Southwest Indian Ridge and Arctic Gakkel Ridge and the slow-spreading North Mid-Atlantic Ridge and Carlsberg Ridge (total length of 14,300 km). Using the single-link cluster analysis technique, we identify 78 seismic swarms (≥8 events), 877 sequences (2–7 events), and 3543 single events. Seismic swarms often occur near the volcanic center of second-order segments, presumably relating to relatively robust magmatism. By comparing the patterns of seismicity between ultraslow- and slow-spreading ridges, and between melt-rich and melt-poor regions of the Southwest Indian Ridge with distinct seafloor morphologies, we demonstrate that a lower spreading rate and a lower melt supply correspond to a higher seismicity rate and a higher potential of large volcano-induced seismic swarms, probably due to a thicker and colder lithosphere with a higher degree of along-axis melt focusing there. [ABSTRACT FROM AUTHOR]

The Lucky Strike volcano is the central edifice of the Lucky Strike segment on the slow spreading Mid‐Atlantic Ridge. The volcano summit hosts one of the largest known deep‐sea hydrothermal fields, and overlies an axial magma chamber (AMC) whose summit reflector lies 3–3.8 km beneath the seafloor. We present a 12‐year microearthquake catalog beneath the volcano, which we constructed using data collected from 2007 to 2019 as part of the EMSO‐Azores observatory. The catalog reveals continuous low magnitude seismicity (85% with ML < 0), focused mainly north‐northwest of the hydrothermal field and 0–2 km above the AMC reflector. Focal mechanisms estimated only from the 2010–2011 deployment show a mixture of thrust, normal and strike/slip faulting mechanisms. We propose that the observed microearthquake activity is due to thermal cracking, tectonic cracking, and possible small dike injections occurring at the base of hydrothermal downflow zones. We document a ∼800 m eastward shift of the seismicity sometime during a catalog gap from June 2013 to April 2015, accompanied by a change in the seismicity pattern from a patch just above the AMC to a more vertically aligned structure. We also identify several higher seismicity periods, often including relatively large magnitude events (ML > 0.8). We interpret the spatio‐temporal variations in seismicity patterns and the correlation between larger magnitude events and higher seismicity periods to indicate that the hydrothermal domain is a dynamic interface, where the variations are caused by changes in permeability and/or temperature induced by tectonic and small episodic melt injections. Plain Language Summary: Hydrothermal circulation at mid‐ocean ridges allows chemical and heat exchange between the solid Earth and the oceans. The Lucky Strike hydrothermal field sits at the summit of a volcano on the Mid‐Atlantic Ridge. We present a 12‐year catalog of microearthquakes recorded beneath the volcano summit from 2007 to 2019 using a seafloor seismometer network. This record shows that seismicity is continuous and low‐magnitude. It is focused north of the hydrothermal field and evolves over time from a patch just above the magma chamber summit to a vertical pipe‐like structure 800 m to the east of the original patch. This distribution and our estimated geometry of the earthquake sources suggest that there is a mixture of sources for the seismicity, including cracking due to hydrothermal cooling, cracking due to tectonic stresses and possibly small magma intrusions from the magma chamber. Our results suggest that the hydrothermal circulation domain is a dynamic interface whose bounds change over time due to changes in permeability and possibly to episodic melt injections. Key Points: Seismicity record at a slow spreading ridge center shows persistent microearthquakes due to processes at the hydrothermal downflow baseSpatio‐temporal variations in seismicity suggest a dynamic hydrothermal flow domain with permeability changes and possible dike injectionsA spatial shift in seismicity cluster with a change in its pattern indicates a change in the magmato‐tectonic activity of the volcano [ABSTRACT FROM AUTHOR]

Abstract The structure of the lithosphere and the associated magmatic systems found in different locations along slow‐spreading ridges can vary dramatically, from melt‐starved to magmatically robust segments. A growing number of studies suggest that the evolution of the magmatic crust being governed solely by fractional crystallization is too simplistic. Reactions between migrating melts and their surroundings play a key role during accretion, yet the full extent of their impact is still to be resolved. We present here the results of a petrological, microstructural, and in situ geochemical study of two drilled sequences from the Kane Megamullion and Atlantis Massif oceanic core complexes. We show that melt‐mush reactions generate locally strong textural and/or geochemical heterogeneity at the cm‐scale, but their impact can also be identified at the 100 m‐scale. We found evidence for assimilation at various degrees of primitive lithologies of potential mantle origin within the gabbroic sequence at both locations, in addition to typical melt‐mush reactions previously described in other slow‐spread magmatic systems. Observations and numerical modeling confirm the similarity of the reactions impacting both sequences. However, the regime of the reactions (ranges of assimilation to crystallization ratios) seems to vary between Kane Megamullion and Atlantis Massif, variations which likely result from differences in melt fractions present during melt‐mush reactions. We infer relying on our observations and previous studies that the regime of the reactions is most likely controlled by the melt flux during the formation of the two sections.

Unlike their eruptive counterparts, plutonic rocks often experience post-cumulus evolution prior to their final solidification. These processes have the potential to exert significant control on the distributions of elements and their isotopic compositions of plutonic rocks. However, our understanding in these effects remains limited, particularly for metal stable isotopes. Here, we carried out high-precision δ65Cu measurements for a suite of primitive to evolved cumulates from the Kane area, 23oN of Mid-Atlantic Ridge. Most mid-ocean ridge basalts (MORBs) exhibit uniformly bulk silicate Earth (BSE)-like δ65Cu of + 0.09 ± 0.08 ‰, indicating limited Cu isotopic fractionation in crustal magma reservoirs prior to melt extraction. However, our new Cu isotopic data on these cumulates show that large Cu isotopic variations (δ65Cu = -0.6 ‰ to + 0.9 ‰) occur in variably evolved cumulates (MgO/total FeO < 3.0), while the most primitive cumulates (MgO/total FeO > 3.0) exhibit BSE-like δ65Cu (+0.06 ± 0.06 ‰). Petrological and geochemical observations indicate limited hydrothermal effects on Kane cumulate Cu isotopes. Modeling results indicate that the variable δ65Cu in Kane cumulates are unlikely to result from magmatic processes during the cumulus stage. Instead, the large Cu isotopic variation in Kane cumulates requires post-cumulus melt evolution involving fractionation between sulfide and intercumulus melts at relatively low temperatures. Textures and mineral chemistry indicate pervasive post-cumulus modifications of Kane cumulates by porous melt flow. Thermodynamic calculations indicate that intercumulus melts persist at temperatures even to 200 °C lower than the initial liquidus temperatures. Our data indicate that lower temperatures in the post-cumulus melt evolution and lower sulfide Ni contents result in lower sulfide-silicate melt Cu isotope fractionation factor (α sulfide-melt), then causing greater Cu isotopic variations in cumulates. We suggest that post-cumulus melt evolution reconcile both the Cu depletion and large Cu isotope fractionation in oceanic cumulates that are unexpected by a simple process of crystal accumulation from MORBs. This study highlights the essential role of post-cumulus evolution in controlling metal budget and their stable isotope fractionation during intra-crustal differentiation, which might also be applied to island arc and continental settings. [ABSTRACT FROM AUTHOR]

14°N on the Mid-Atlantic ridge (MAR) is one of only a few locations worldwide where volatile-saturated, geochemically enriched mid-ocean ridge basalts (E-MORBs) have been recovered. These basaltic glasses are so gas-rich that CO 2 -filled bubbles may "pop" when brought to the surface, due to the pressure and temperature change. Although these "popping rocks" have long been regarded as representative samples of un-degassed magmas sourced from the upper mantle, uncertainties regarding both their generation mechanism(s) and the potential effects of gas loss/accumulation processes have hampered unambiguous quantification of the upper mantle volatile element (water, carbon, nitrogen, noble gas) inventory. Fortunately, the extent and consequences of gas loss/accumulation processes can be tested by studying characteristic changes in volatile elements compositions, including 4He/40Ar* (where 40Ar* is 40Ar corrected for atmospheric contamination). To document the mechanism of popping rock generation and potential effects of degassing and gas accumulation processes on MORB volatile systematics, we present a comprehensive volatile characterization (carbon, nitrogen and noble gas systematics) of popping rocks and associated MORBs (n = 19) recently sampled at 14°N on the MAR, including 2 normal MORBs (N-MORB) from an oceanic core complex (OCC) and 17 E-MORBs. In line with previous studies, we find that PR exhibit the lowest 4He/40Ar* (1.08 ± 0.04) among all MORB samples, lower than the conventional mantle production ratio of 3 ± 1. Such low 4He/40Ar* could either (i) derive from accumulation of first-generated bubbles originating from open-system degassing of underlying magmas, or (ii) represent the actual upper mantle production ratio. We summarize the arguments in favor of each of these two scenarios (including the required accumulation times for radiogenic noble gases accumulation, the K/U and 232Th/238U of the upper mantle, popping rock vesicle size distributions, and physical considerations for vesicle growth and upwelling through a basaltic magma), and discuss their implications for the volatile composition of un-degassed magmas from the upper mantle. We find homogenous N isotope compositions (average δ15N of −4.49 ± 1.40 ‰ at 14°N) but variable δ13C (from −11 ‰ to −3.4 ‰), potentially compatible with the expectations for residual dissolved gas after Rayleigh fractionation. However, explaining the light δ13C signatures of PR via this process appears incompatible with any of the two scenarios proposed for explaining their low 4He/40Ar*, which would predict (i) 13C-enrichements and (ii) no fractionation relative to an initial composition at δ13C ∼ -5 ‰, respectively. After correction for solubility-controlled degassing fractionation and potential gas accumulation processes using 4He/40Ar* systematics, we find relatively homogeneous C/3He ((2.65 ± 0.51) × 109) but variable C/N (from 125 up to 4578), whose potential origins are discussed as part of a companion paper (Part B: Source effects). [ABSTRACT FROM AUTHOR]

Most animal species have a singular developmental pathway and adult ecology, but developmental plasticity is well-known in some such as honeybees where castes display profoundly different morphology and ecology. An intriguing case is the Atlantic deep-sea hydrothermal vent shrimp pair Rimicaris hybisae and R. chacei that share dominant COI haplotypes and could represent very recently diverging lineages or even morphs of the same species. Rimicaris hybisae is symbiont-reliant with a hypertrophied head chamber (in the Mid-Cayman Spreading Centre), while R. chacei is mixotrophic with a narrow head chamber (on the Mid-Atlantic Ridge). Here, we use X-ray micro-computed tomography and fluorescence in situ hybridization to show that key anatomical shifts in both occur during the juvenile–subadult transition, when R. hybisae has fully established symbiosis but not R. chacei. On the Mid-Atlantic Ridge, the diet of R. chacei has been hypothetically linked to competition with the obligatorily symbiotic congener R. exoculata , and we find anatomical evidence that R. exoculata is indeed better adapted for symbiosis. We speculate the possibility that the distinct development trajectories in R. hybisae and R. chacei may be determined by symbiont colonization at a "critical period" before subadulthood, though further genetic studies are warranted to test this hypothesis along with the true relationship between R. hybisae and R. chacei. [ABSTRACT FROM AUTHOR]

The link between weather regimes (WRs) and polar low (PL) activity is examined over the North Atlantic and North Pacific basins. Compared to earlier studies based on limited, regional PL datasets, our study conducts a more complete evaluation of regional WR–PL relationships using an expanded PL climatology. Our findings show that PL activity is increased over the Norwegian and Barents Seas during the Atlantic ridge regime and decreased over the former region during the Scandinavian blocking regime, with negative impacts also stretching to the Irminger Sea. Over the Labrador and Irminger Seas, PL activity is modulated strongest by the North Atlantic Oscillation (NAO), with positive impacts during the positive phase and vice versa. Over the North Pacific, the Arctic low contributes to increased PL activity over most regions, while the opposite is true for the Pacific wave train regime. The variability of PL activity associated with WRs is strongly related to changes in key environmental conditions. In general, regions of enhanced (reduced) PL activity are coincident with anomalous low-level northerly (southerly) flow and reduced (increased) static stability. Further analysis shows that certain persistent WRs can strongly modulate PL activity over some regions, either due to the amplification or propagation of favorable or unfavorable conditions, which cautions the limitation of regarding WRs as stationary patterns. A previously developed PL genesis potential index is shown to represent the observed impacts well, which serves to confirm the robustness of our findings and suggests the potential applicability of WRs to the subseasonal prediction of PLs. Significance Statement: Polar lows are intense small-scale (∼300 km) cyclones that form over high-latitude oceanic regions. The hazardous impacts they pose to coastal communities and maritime and air operations in the Arctic motivate their skillful prediction, which remains a major challenge at lead times beyond a few days. In this study, we relate PLs and the key environmental conditions that favor their development to weather regimes, which are recurrent large-scale circulation patterns that can persist for weeks at a time. We find that weather regimes have strong impacts on polar low activity through the modulation of key environmental conditions. These relationships can potentially be utilized in the extended-range prediction of polar lows. [ABSTRACT FROM AUTHOR]

We report on a 3 years monitoring experiment of low to medium temperature diffuse venting at two vent sites (Tour Eiffel and White Castle) of the Lucky Strike, black smoker‐type hydrothermal field, Mid‐Atlantic Ridge. Diffuse vents account for a large part of the energy flux of mid‐ocean ridges hydrothermal fields and provide key habitats for the hydrothermal fauna. We document the time and space variability of diffuse venting temperature and chemistry, describe the effect of tidal loading and currents and discuss the extent of mixing, cooling of black smoker fluids, heating of entrained seawater and anhydrite precipitation/dissolution in the substratum. We emphasize the role of a thin (<2 m) volcaniclastic formation capping the brecciated basalt substratum. This formation is porous, but becomes impermeable when indurated by hydrothermal precipitates. It forms an intermediate layer between the vents at the seabed and the fluids as they discharge out of the brecciated basalts. Diffuse fluids inferred to discharge out of meter‐spaced cracks in the brecciated basalts beneath this volcaniclastic layer are hot (>80°C) and contain >10% of the hot endmember fluid component, over distances of up to 25 m from the black smokers. These results provide a geologically integrated framework in which to study site‐scale, near seafloor hydrothermal circulation and associated vent habitats at Lucky Strike and other black smoker‐type hydrothermal fields. They suggest diffuse heat fluxes in the upper range of previously published estimates at the two studied Lucky Strike hydrothermal vent sites. Plain Language Summary: Mid‐ocean ridges (MOR) are a key feature of plate tectonics, extending some 60,000 km in all the major oceans. MOR hydrothermal circulations transfer heat and chemical compounds from the solid earth to the ocean and provide habitats for the hydrothermal fauna. The vents include black smokers that expel the hottest fluids and diffuse vents that expel lower temperature fluids at lower rates but over larger surfaces. The contribution of diffuse vents to the energy and chemical fluxes of MOR hydrothermal systems is still largely an open question. In this paper, we address it by using data from a 3 years monitoring experiment of diffuse vents at two sites (Tour Eiffel and White Castle) of the Lucky Strike, a black smoker‐type hydrothermal field in the Mid‐Atlantic Ridge. We document the time and space variability of venting temperature and derive chemical constraints on the extent of mixing of black smoker fluids with entrained seawater and of mineral precipitation/dissolution in the substratum of the vents. Our results suggest diffuse heat fluxes in the upper range of previously published Lucky Strike hydrothermal field estimates and provide a geologically integrated framework in which to study diffuse vent habitats at Lucky Strike and other black smoker‐type hydrothermal fields. Key Points: Time variability of both fluid temperature and fluid chemistry at diffuse vents of the Lucky Strike mid‐ocean ridge hydrothermal fieldHot (>80°C) and hydrothermal endmember‐rich diffuse fluids (>10%) come out of the basalts up to 25 m from the black smokersFluids that come out of basalt substratum are modified in volcaniclastic layer before coming out at vents that host the hydrothermal fauna [ABSTRACT FROM AUTHOR]

The correct and precise crystal forms and the crystallographic orientation relationships (CORs) of ten main magnetite needle groups in the plagioclase host were unequivocally determined by transmission electron microscopy from >100 selected magnetite needles with well defined shape orientations in petrographic thin sections of a gabbro core sample drilled from the Mid‐Atlantic Ridge. The as‐observed CORs of magnetite in plagioclase are in accordance with the optimal matching in the oxygen sublattices of the two structures via the close alignment between two sets of close‐packed (c.p.) oxygen arrays parallel to {111}m in magnetite (m) and two sets of quasi‐c.p. oxygen arrays parallel to (150)p, (150)p, (112)p, (312)p, (021)p or (121)p in plagioclase (p), as demonstrated for the first time in the accumulated oxygen number versus normal distance [from the (hkl) plane] plots. Such a simple oxygen sublattice matching principle does not rely on the length matching for the specific pair of lattice directions or on the d‐spacing matching for the specific pair of lattice planes of the two structures, and therefore must be generally applicable for any mineral/oxide inclusions with a c.p. or quasi‐c.p. oxygen sublattice formed via precipitation in open or closed systems. The topotaxial nuclei then undergo extended growth along the kinetically fast growth direction of the two structures with the most similar oxygen distribution patterns, i.e. along the normal direction of the aligned pair of oxygen arrays, thereby yielding the pronounced shape orientations such as magnetite needles in plagioclase. [ABSTRACT FROM AUTHOR]

Oceanic core complexes provide an accessible window into deep processes occurring at slow and ultra-slow-spreading mid-ocean ridges. We analyse samples from IODP ocean drilling of core complexes at the Atlantis Bank, Atlantis Massif, and near the Kane Transform at the South West Indian and Mid-Atlantic ridges. We correlate secondary minerals, including oxides, with sites of melt migration. We interpret changes to mineral assemblage and microchemistry, reaction textures and melt-pseudomorph microstructures as fingerprints of open-system melt-mediated processes. This micro-scale information is combined with a macro-scale review of legacy mineral chemistry data to show that melt-fluxed rocks share remarkably similar characteristics across the three core complexes investigated. These are rich in oxides and have olivine, orthopyroxene and clinopyroxene grains that are chemically distinct from oxide-poor gabbros. We propose that oceanic crust fluxed with external melt can be recognised by the following key features: (1) high modes of secondary minerals, such as oxides and olivine, (2) microstructural evidence for the former presence of melt, and (3) mineral chemistry differences between primary and secondary olivine, orthopyroxene, clinopyroxene and plagioclase. Importantly, olivine has previously only been reported as primary, that is, having crystallised from magma. However, our results show that gabbros with secondary olivine are reliable indictors of melt–rock interaction during deformation-assisted diffuse melt migration through the gabbroic oceanic crust. Finally, we propose a new model for the formation of oceanic core complexes where deformation-assisted melt migration plays a critical role in strain localisation, exhumation and evolution of the core complex. Secondary mineral microstructures indicate the former presence of melt. Olivine, plagioclase and pyroxene chemistry fingerprint melt-fluxed oceanic crust. Evidence of multiple fluxes of external melt associated with strain localisation. High oxide mode and secondary minerals are fingerprints of melt-fluxed zones in oceanic core complexes. [ABSTRACT FROM AUTHOR]

We set out to characterize the fine‐scale processes acting on interannual dynamics of deep‐sea vent fauna by using a novel approach involving a 5‐yr time series of 3D photogrammetry models acquired at the Eiffel Tower sulfide edifice (Lucky Strike vent field, Mid‐Atlantic Ridge). Consistently, with the overall stability of the vent edifice, total mussel cover did not undergo drastic changes, suggesting that they have been at a climax stage for at least 25 yr based on previous data. Successional patterns showed consistency over time, illustrating the dynamic equilibrium of the ecological system. In contrast, microbial mats significantly declined, possibly due to magmatic events. The remaining environmental variability consisted of decimeter‐scale displacement of vent outflows, resulting from their opening or closure or from the progressive accretion of sulfide material. As a result, vent mussels showed submeter variability in the immediate vicinity of vent exits, possibly by repositioning in response to that fine‐scale regime of change. As former studies were not able to quantify processes at submeter scales in complex settings, this pioneering work demonstrates the potential of 3D photogrammetry models for conducting long‐term monitoring in the deep sea. We observed that the ability of mussels to displace may enable them to cope with changing local conditions in a stable system. However, the long‐term stability of mussel assemblages questions their capacity to withstand large‐scale disturbances and may imply a low resilience of these "climax" communities. This suggests that they may be particularly vulnerable to the negative effects of mining activities in hydrothermal ecosystems. [ABSTRACT FROM AUTHOR]

Carbonate‐brucite chimneys are a characteristic of low‐ to moderate‐temperature, ultramafic‐hosted alkaline hydrothermal systems, such as the Lost City hydrothermal field located on the Atlantis Massif at 30°N near the Mid‐Atlantic Ridge. These chimneys form as a result of mixing between warm, serpentinization‐derived vent fluids and cold seawater. Previous work has documented the evolution in mineralogy and geochemistry associated with the aging of the chimneys as hydrothermal activity wanes. However, little is known about spatial heterogeneities within and among actively venting chimneys. New mineralogical and geochemical data (87Sr/86Sr and stable C, O, and clumped isotopes) indicate that the brucite and calcite precipitate at elevated temperatures in vent fluid‐dominated domains in the interior of chimneys. Exterior zones dominated by seawater are brucite‐poor and aragonite is the main carbonate mineral. Carbonates record mostly out of equilibrium oxygen and clumped isotope signatures due to rapid precipitation upon vent fluid‐seawater mixing. On the other hand, the carbonates precipitate closer to carbon isotope equilibrium, with dissolved inorganic carbon in seawater as the dominant carbon source and have δ13C values within the range of marine carbonates. Our data suggest that calcite is a primary mineral in the active hydrothermal chimneys and does not exclusively form as a replacement of aragonite during later alteration with seawater. Elevated formation temperatures and lower 87Sr/86Sr relative to aragonite in the same sample suggest that calcite may be the first carbonate mineral to precipitate. Plain Language Summary: At the Lost City hydrothermal field, warm alkaline fluids are discharging out of uplifted mantle rocks. When vent fluids mix with seawater at the seafloor, carbonate and brucite minerals form spectacular towers up to 60 m high. Systems like Lost City are important because the reaction between water and rocks provides carbon and energy sources for microbial life. However, we still do not fully understand what controls the mineralogy and geochemistry of the Lost City hydrothermal chimneys. In this paper, we suggest that the extent of mixing between the hydrothermal fluids and seawater influences the mineralogy and geochemistry of the chimneys. Calcite, which was previously thought to form only during alteration of aragonite by seawater, can also form during seawater‐hydrothermal fluid mixing. Both calcite and brucite form in the interior of the chimneys where vent fluid is more dominant. Aragonite, on the other hand, forms in the exterior of the structures from seawater‐rich fluids. Lastly, because minerals precipitate rapidly during fluid mixing, the stable isotope geochemistry of the carbonates mostly records the composition and temperature of seawater and not the mixed fluid. Thus, care should be exercised in interpreting mineral geochemical data from similar systems. Key Points: The mineralogy and geochemistry of Lost City chimneys are controlled by the extent of mixing between hydrothermal fluids and seawaterBrucite and calcite precipitate in vent fluid dominated zones while aragonite forms in the exterior of the structures in seawater‐rich zonesCarbonates precipitate in isotopic disequilibrium and record the O and C stable isotope composition of seawater dissolved inorganic carbon [ABSTRACT FROM AUTHOR]

In this work, we present both 1-D and 3-D shear wave velocity (Vs) models of the oceanic crust and uppermost mantle below the Deep OCean Test ARray area, located ∼ 70 km north of the central section of the Gloria Fault, in the eastern North-Atlantic Ocean. The velocity models are inferred from the dispersion of surface waves recorded on ocean bottom seismometers. Dispersion measurements are obtained from the analysis of ambient seismic noise at short periods (< 14 s) and teleseismic surface waves at long periods (> 14 s) using the two-station method. The 1-D Vs model is inferred from the joint inversion of Rayleigh wave phase velocities and Love wave group and phase velocities. The 3-D tomographic model is obtained by inversion of 2-D Love wave group velocity maps as a function of depth, further constrained by the average of Love wave phase velocities obtained from ambient noise (4–9 s) and the average Rayleigh and Love wave phase velocities calculated from teleseismic data (14–44 s). The 1-D Vs model shows a sediment layer with a low velocity of 1.05 km s−1, similar to previous studies in the region. Below the sediments, we find an oceanic crust with velocities ranging from 3.3 to 4.5 km s−1. The model reaches an unusually high velocity of 4.9 km s−1 in a 20 km thick layer at depths between 16 and 36 km. We interpret this fast velocity layer as indicative of the presence of harzburgite, a residue of enhanced melting that might have been formed by the proximity between the Mid-Atlantic Ridge and the Azores mantle plume. At greater depths the velocity decreases, forming a low-velocity zone that reaches a minimum at ∼ 70 km depth, which we interpret as the maximum depth for the lithosphere–asthenosphere boundary. The 3-D model shows a structure that is mostly horizontally layered, with Vs isocontours at 3.5–4.5 km s−1 highlighting oscillations of the crustal structure with wavelengths of ∼25–30 km. These oscillations may be due to changes in the rate of mantle upwelling and magma supply rate. [ABSTRACT FROM AUTHOR]

The compositions of metalliferous sediments associated with hydrothermal vents can provide key geochemical data for locating seafloor sulfides. In this study, we present the geochemistry of seabed sediments from the Xunmei hydrothermal field (HF) in the South Mid-Atlantic Ridge (SMAR). The results indicate that the sediments are mainly composed of pelagic material (biogenic calcium components), basaltic debris, iron-manganese oxides, and hydrothermal components. The sediments are significantly enriched in Cu, Zn, Fe, and Co deriving from hydrothermal fluids, as well as Mn, V, Mo, U, and P, which are primarily scavenged from seawater. The northeastern Xunmei has the highest concentrations of Cu and Zn, while the northeastern, northern, and southern regions are characterized by great inputs of Fe. Manganese and Mo are mainly enriched in the western and southern parts and show a strong positive correlation, indicating that Mo is mainly scavenged by Mn oxides. Uranium, P, and Fe exhibit strong positive correlations, suggesting that they coprecipitate with Fe from hydrothermal plumes. Vanadium and Co are introduced into sediments in different ways: V is scavenged and coprecipitated by hydrothermal plumes, and Co is derived from sulfide debris. Based on the contents of Cu and Zn and Cu/Fe (0.159), Zn/Fe (0.158), and Fe/Mn (1440) ratios, it can be inferred that a high-temperature hydrothermal vent existed in northeastern Xunmei. In combination with the distribution patterns of the above elements, the hydrothermal vents in the southern part ceased erupting after a short period of activity. In addition, the high Mn anomaly and the high U/Fe ratios at the boundaries of the investigated area indicate the presence of a relatively oxidized environment in southwestern Xunmei. [ABSTRACT FROM AUTHOR]

Hydrothermal fluids in ultramafic‐hosted hydrothermal systems associated with oceanic detachment faults can be more oxidizing compared to mafic‐hosted vent sites. These fluids form a mineral assemblage of pyrite, magnetite and hematite. At 13°30′N on the Mid‐Atlantic Ridge, chlorite‐quartz breccias recovered from an exposed fault scarp contain pyrite, with abundant magnetite and hematite, indicating that the redox of the fluids was variable. In primary micron‐scale zonations in pyrite, Ni, Co, and Se have a decoupled relationship, recording fluctuations in the chemical composition and temperature of hydrothermal fluid as the grains grew. Secondary zonations that erase and overprint primary zonations are limited to the grain margin and permeable regions within the grain core. Secondary zonations formed via two processes: (a) grain dissolution followed by overgrowth, and (b) remobilization of metals during oxidizing fluid flow events. In both instances, Ni and Co have been mobilized and concentrated, and are not lost to the hydrothermal fluid. Superimposed on these features is evidence of grain scale deformation related to periods of fault movement along the detachment surface. Sulfur isotope ratios (δ34S) in pyrite systematically decrease from the grain margin to the grain core, indicating that increased amounts of sulfur were derived from thermochemical sulfate reduction of seawater. Thus, pyrite records the evolution of fluid flow and deformation events during exhumation along the detachment surface from ∼1 to 2 km below the seafloor at the base of the lava pile, with temporal fluctuations in fluid redox identified as an important process in controlling Ni and Co enrichment in pyrite. Plain Language Summary: Detachment faults are long lived faults that can expose ultramafic rocks at the seafloor. We aim to investigate the links between hydrothermal activity and detachment fault formation. To do this we use pyrite as a tape recorder for past fluid flow events. Across individual mineral grains, distinct zonations in metal content and sulfur isotope ratios show that the incursion of seawater occurred periodically during pyrite growth, increasing during fault movement events that lead to changes in the temperature and pH of the fluids in the fault zone. These changes concentrated metals toward the center of individual mineral grains. Zonations were then overprinted by later deformation‐related events, providing evidence that the samples formed at deeper crustal levels below the seafloor and were progressively exhumed at the seafloor over time. Key Points: Microtextural, geochemical, and isotopic variations in subseafloor pyrite record the history of sample exhumation along a detachment faultNickel and Co are remobilized and concentrated in pyrite across individual mineral grains in response to fluctuating fluid redox conditionsEvidence of pyrite deformation and alteration mineralogy of samples indicates sample exhumation from a depth of 1–2 km [ABSTRACT FROM AUTHOR]

The results of 230Th/U dating of massive sulfides and metalliferous sediments (288 and 245 analyses, respectively) of hydrothermal sites in the northern equatorial area of the Mid-Atlantic Ridge (MAR) from 12° to 20° N are presented. The maximum and average ages of sulfides are 223 ± 38.2 and 80.5 ka, respectively. The comparison of ages of massive sulfides from various MAR hydrothermal sites showed asynchronous stages of hydrothermal activity along the ridge axis indicating the features of tectonic activity and magmatism, which control the hydrothermal activity within each region. The glaciation periods, which record the activation and low- and high-temperature hydrothermal activity, are an exception. A direct positive correlation between the age of a hydrothermal site and its distance from the axial ridge zone is typical of mafic-related hydrothermal sites in contrast to ultramafic-related hydrothermal sites, which is related to the different scenarios of the formation of these two types of sulfide mineralization within slow-spreading ridges. The ages of sulfides and sediments have been jointly analyzed based on the example of the Pobeda and Semenov hydrothermal sites. The interpretation of data on sediments indicates the continuous character of hydrothermal activity, which is partly supported by the age of sulfides. [ABSTRACT FROM AUTHOR]

This paper presents the new geochemical isotope (Sr–Nd system, H2O, Cl) data obtained for basalt glasses of the Mid-Atlantic Ridge (MAR) sampled from six areas of the MAR axial zone between 31° and 12° N. The data are consistent with the existing ideas about large-scale geochemical segmentation of the MAR. It is shown that samples from predominantly serpentinite segments have a narrower range of variations of strontium isotopic composition (87Sr/86Sr = 0.7027–0.7032) in comparison with samples collected from the areas where the crustal section is dominated by basalts (87Sr/86Sr = 0.7024–0.7041). The variation ranges of the neodymium isotopic composition in these two groups of samples are almost identical (εNd = +4.9 to +10.9 and +5.9 to +11.6 in serpentinite and basalt segments, respectively), although, in general, serpentinite segments have a slightly more enriched composition. The wide variations of the neodymium isotopic composition and increased contents of Cl, H2O, and U, as well as increased K2O/TiO2 and La/Sm ratios, in samples from serpentinites can most probably be related to the participation of different geochemically heterogeneous sources in the magmatism of the MAR axial zone. The influence of enriched plume-type matter cannot be excluded in some segments. The isotopic composition of noble gases may shed light on the subject. [ABSTRACT FROM AUTHOR]

The composition of basalts dredged from the slopes of a single seamount (guyot), which is located south of the eastern end of the Charlie Gibbs Fracture Zone in the North Atlantic, is studied. Groups of high- and low-Ti rocks, which belong to oceanic tholeiites of the E-MORB and T-MORB types, respectively, are recognized. The first group of basalts have higher contents of TiO2, Na2O, K2O, P2O5, Fe2O3, and incompatible trace elements. The basalts of both groups are similar in the normalized lithophile element patterns indicating their melting from a similar mantle substrate. The difference in their composition is probably caused by melting of high-Ti volcanic rocks at a deeper intermediate level between spinel and garnet facies. Our studies give grounds to believe that the seamount also formed as a large neovolcanic rise 64–67 Ma ago under the influence of a microplume, which was a branch of the Milne deep mantle plume. [ABSTRACT FROM AUTHOR]

Continuous two-year studies of particle fluxes and associated environmental parameters in the axial zone of the Arctic segment of the Mid-Atlantic Ridge at the junction of the Mohns and Knipovich ridges (Norwegian Sea) have been carried out for the first time. Sediment traps were deployed from the R/V Akademik Mstislav Keldysh in June 2019 in the northeastern part of the Mohns Ridge with revision in August 2020 and lifting in July 2021. It is shown that the sedimentation of particles in the study area was affected by the global transport of water masses in the northeasterly direction, with Atlantic waters in the subsurface layer and water masses of Arctic origin recirculating deeper in the subsurface. A weak positive temperature trend at a depth of more than 2500 m (0.02°C over two years) is detected. The bimodal vertical distribution of fluxes and changes in the composition of sinking particles corresponded to the pattern of sediment recycling in the ocean. The annual course of fluxes in the subsurface layer is determined by the activity of Si-concentrating and calcifying plankton, while the maximum bloom period is also manifested in the deep layer. At the same time, the main significant source of the flux deeper than 2000 m is the supply of lithogenic matter from the near-bottom nepheloid layer. [ABSTRACT FROM AUTHOR]

Hydrothermal vents have emerged as an important source of iron to seawater, yet only a subset of iron is soluble and persists long enough to be available for surface biological uptake. The longevity and solubility of iron in seawater is governed by strong organic ligands, like siderophores, that are produced by marine microorganisms and are a part of the ocean's dissolved iron-binding ligand pool. These ligands have been hypothesized to aid in the persistence of dissolved iron in hydrothermal environments. To explore this hypothesis, we measured iron, iron-binding ligands, and siderophores from 11 geochemically distinct sites along a 1,700 km section of the Mid-Atlantic Ridge. Siderophores were found in hydrothermal plumes at all sites, with proximity to the vent playing an important role in dictating siderophore types and diversity. The notable presence of amphiphilic siderophores may point to microbial utilization of siderophores to access particulate hydrothermal iron, and the exchange of dissolved and particulate iron. The tight coupling between strong ligands and dissolved iron within neutrally buoyant plumes across six distinct hydrothermal environments, and the presence of dissolved siderophores with siderophore-producing microbial genera, suggests that biological production of siderophores exerts a key control on hydrothermal dissolved iron concentrations. [ABSTRACT FROM AUTHOR]

The structure of the ocean floor and the composition of basalts and dolerites of the MAR segment between the Maxwell and Charlie Gibbs FZs (North Atlantic) were studied based on the data obtained during the 53rd cruise of the R/V Akademik Nikolaj Strakhov organized and performed by the Geological Institute of the Russian Academy of Sciences over the period of July 7–August 15, 2022. In this segment, areas of greater and lesser magmatic productivity, which correspond to higher and lower bottom relief, alternate along the spreading axis. In high-relief areas, spreading cells form in the axial zone, and rises of tectonic and volcanic genesis dominate in the crest zone. In low-relief areas, the rift valley consists of deep rift basins; low ridges are developed on the flanks, which are separated by wide depressions. Oceanic N- and T-type tholeiites and E-MORB-type basalts and dolerites are distinguished among the studied volcanics. The N-tholeiites are widespread and were derived from mainly depleted mantle (source of DM). E-MORB-type basalts and dolerites are found in high-relief areas. Their mantle substrate is formed by a mixture of DM and EM-2 material with the subordinate role of HIMU. T-MORB volcanics are mainly localized on large volcanic rises in the southern part of the studied MAR segment and were melted from a substrate formed by a mixture of DM and HIMU material with the subordinate role of EM-2. Two types of mantle inhomogeneities involved in melting are reconstructed: passive and active. The former are represented by blocks of the transformed continental lithosphere that are similar in composition to the EM-2 mantle source. Active inhomogeneities associate with the uplift near Maxwell FZ of the microplume of the enriched mantle with a composition close to HIMU and with its subaxial flow in the north direction up to the Charlie Gibbs FZ. [ABSTRACT FROM AUTHOR]

The paper deals with the description of new geomagnetic data collected in the North Atlantic during cruises of research vessels of the Russian Academy of Sciences performed in 2020–2022. Modern technologies of geomagnetic measurements in the ocean and methods of their processing are considered. The obtained geomagnetic data indicate the heterogeneous nature of the sources of magnetic anomalies in the Charlie–Gibbs fracture zone and adjacent areas of the Mid-Atlantic Ridge and allow solving a wide range of geological and tectonic problems. Priority directions for further research aimed at adjusting spreading parameters and constructing models of the magnetic layer typical for transform faults and amagmatic segments of the Mid-Atlantic Ridge are formulated. [ABSTRACT FROM AUTHOR]

The geochemical peculiarities of sample collection of MORB chilled glasses obtained in six areas of the axial zone of the Mid-Atlantic Ridge (MAR), 12°–31° N have been studied. The results of this study provided information on the composition of the parental melts for these glasses and made it possible to assess the probable geochemical effects reflecting the interaction of magmatic melts with hydrothermal systems of the MAR axial zone or with altered oceanic crust (AOC). It is shown that basalts of the E-MORB family which includes most samples are localized mainly in the "cold" segments of the MAR, in the crustal section dominated by serpentinites. On the other hand, samples with depleted signatures (N-MORB) belong to MAR segments where serpentinite outcrops either are absent or play a subordinate role. The E-MORB chilled glasses from "cold" segments of the MAR show signs of contamination of basaltic melts with components assimilated either from the host serpentinites or from aqueous–saline fluids circulating in hydrothermal systems located in serpentinites ("serpentinite hosted"). Judging by the data obtained on the nature of Cl, U, and Sr variations in the studied chilled glasses ascribed to the N-MORB family, there are no signs of intracrustal contamination. It is assumed that relics of the ancient continental lithosphere preserved under axial zone of the MAR and involved in the partial melting of the shallow mantle took part in the formation of E-MORB parental melts in some MAR segments. [ABSTRACT FROM AUTHOR]

In June 1982 F.R.V. Walther Herwig II (WH-II) cruise 52, leg 2, investigated the bathy- and benthopelagic fish fauna along the Azores Biscay Rise and the Mid-Atlantic Ridge (MAR) north of the Azores. In total 678 decapod cephalopods were collected as bycatch and identified belonging to about 33 species in 17 families. The five most abundant families were Histioteuthidae, Cranchiidae, Mastigoteuthidae, Octopoteuthidae, and Chiroteuthidae. A comparison with the cephalopod species captured on a cruise of R.V. G.O. SARS (G.O. SARS) to the North Atlantic between 60°-41°N in June-July 2004 and their distribution patterns and biodiversity have been done. The use of different types of trawls applied on G.O. SARS and WH-II 52(2) regarding selection, species composition and biogeography are discussed. Locations with high species abundance and diversity were observed on WH-II 52(2) along the mid-Atlantic Ridge at about 50°-43°N. [ABSTRACT FROM AUTHOR]

Studies of oceanic crust, which covers a large proportion of the Earth's surface, have provided significant insight into the dynamics of crustal accretion processes at mid‐ocean ridges. It is now recognized that the nature of oceanic crust varies fundamentally as a function of spreading rate. Ocean Drilling Program (ODP) Hole 1256D (eastern Pacific Ocean) was drilled into the crust formed at a superfast spreading rate, and hence represents a crustal end member. Drilling recovered a section through lava and sheeted dykes and into the plutonic sequence, the study of which has yielded abundant insight into magmatic and hydrothermal processes operating at high spreading rates. Here, we present zircon U‐Pb dates for Hole 1256D, which constrain the age of the section, as well as the duration of crustal accretion. We find that the main pulse of zircon crystallization within plutonic rocks occurred at 15.19 Ma, consistent with magnetic anomalies, and lasted tens of thousands of years. During this episode, the main plutonic body intruded, and partial melts of the base of the sheeted dykes crystallized. One sample appears to postdate this episode by up to 0.25 Myr, and may be an off‐axis intrusion. Overall, the duration of crustal accretion was tens to several hundreds of thousands of years, similar to that found at the fast‐spreading East Pacific Rise and the slow‐spreading Mid‐Atlantic Ridge. This indicates that crustal accretion along slow‐ to superfast‐spreading ridges occurs over similar time scales, with substantially longer periods of accretion occurring at ultraslow‐spreading ridges characterized by thick lithosphere. Plain Language Summary: The oceanic crust paves approximately 2/3 of the Earth's surface. It is formed at mid‐ocean ridges, where tectonic plates separate and new crust is formed by the solidification of magma. This magma is formed by partial melting of the upper mantle beneath the ridge axis. Plates spread at different rates at different mid‐ocean ridges, and the fastest‐known spreading occurred some 11–18 million years ago in the eastern Equatorial Pacific. A section of the crust formed during this episode of superfast‐spreading was recovered by scientific drilling in the framework of the Integrated Ocean Drilling Program (IODP). This study presents age data that determine when this section of superfast‐spreading crust formed, and how long it took to build the crust. We find that the age of the section is 15.19 Ma, and that crustal formation lasted between tens and several hundreds of thousands of years. This duration is similar to that found at mid‐ocean ridges with slow‐ to fast‐spreading rates, such as the Mid‐Atlantic Ridge and East Pacific Rise. However, it is much shorter than the formation of crust at ultraslow‐spreading ridges, where the cool and thick nature of the lithosphere leads to prolonged episodes of crustal formation. Key Points: The main pulse of crystallization of superfast‐spreading crust at Hole 1256D occurred at 15.19 MaCrustal accretion lasted between tens and several hundreds of thousands of yearsCrustal accretion along slow‐ to superfast‐spreading ridges occurs over similar time scales [ABSTRACT FROM AUTHOR]

At mid-ocean ridges (MORs), seawater carrying dissolved sulfate (SO4) infiltrates the oceanic crust. Hydrothermal fluid emissions from such systems have much lower δ34S and sulfur is mostly present as reduced sulfide, albeit in lower total sulfur concentrations than in seawater. This has been explained by anhydrite formation and sulfate reduction based on petrographic evidence and mass balance considerations. Here, we utilize the chemical and stable isotope (δ34S, δ18O) systematics in natural anhydrite and pyrite from various locations along the submarine and on-land section of the Mid-Atlantic ridge near Iceland to quantify the key variables that control anhydrite formation and sulfate recycling in the oceanic crust. Hydrothermal anhydrite exhibited δ34S values of +20.6 ± 1.0‰ and δ18O values between +2.4 to +25.3‰. Volcanogenic anhydrite in encrustations showed δ34S values of −1.7 to +21.4‰ and δ18O values between +1.4 and +38.0‰. Hydrothermal pyrite exhibited δ34S values ranging from +3.4 and +19.7‰. Comparison of the natural dataset with results from geochemical isotope modelling revealed that δ34S and δ18O values of anhydrite and pyrite were dependent on the isotope composition of the source fluid, extent of water–rock interaction, temperature, and redox conditions. Departures of δ34S and δ18O values in anhydrite from the source fluid were caused by progressive fluid-basalt interaction where lower δ34S and δ18O values reflected a change in sources of S and O from solely fluid to basaltic origin. The δ18O values of anhydrite were additionally affected by temperature. Quantitative formation of anhydrite mainly occurred at temperatures < 150 °C, whereas at elevated temperatures (>200 °C) reduction of seawater-sulfate to H2S and subsequent pyrite precipitation were found to limit anhydrite formation. Extending our calculations to the oceanic crust revealed that the majority of seawater-sulfate is sequestered into anhydrite (3–38 Tg S yr−1) in vicinity of MORs at < 200 °C at shallow depth (

At Mid‐Ocean Ridges, hot, reduced, acidic, and metal‐rich fluids are responsible for the formation of ultramafic‐hosted seafloor massive sulfide deposits (UM‐SMSs), where mantle exhumation efficiently operates. As UM‐SMSs display great structural, mineralogical, and geochemical variabilities from site to site, a simple genetic model cannot be applied. Notably, fluid circulation and Fe‐Ca metasomatism are reported in ultramafic‐hosted hydrothermal deposits exposed in ophiolites, suggesting it might have genetic implications on the formation of mineralized systems. Similar Fe‐Ca metasomatism was reported in drilled mantle rocks at the Mid Atlantic Ridge Kane (MARK) area, offering access to the vertical dimension beneath an exhumed oceanic core complex to provide an integrative study of the nature and geometry of deep magmatic and hydrothermal processes. At MARK, mantle rocks underwent complex processes of melt‐rock and fluid‐rock interactions. Magma channeling and interactions with surrounding rocks enriched mantle silicates in Fe, Co, and Zn. There, subsequent hydrothermal alteration allowed to stabilize Fe‐rich silicates. Mineralogy and geochemistry of hydrothermal phases at MARK suggest mineral crystallization under temperatures from ∼830° down to 350°C during early mantle exhumation at a depth <6.5 km below seafloor, followed by serpentinization of the massif during progressive mantle denudation. Considering the lithological heterogeneity at (ultra)slow‐spreading ridges, metal enrichment in deep mantle rocks during melt‐rock interactions may be a widespread process. In ultramafic‐dominated environments where extensional tectonics allow fluid flows to these deep zones, fluids may leach and transport metals to the surface, accounting for metal entrapment in UM‐SMSs. Plain Language Summary: Mantle exhumation along detachment faults is a common tectonic process that efficiently operates at (ultra)slow spreading ridges where the supply of magma is locally insufficient to fully accommodate the lithospheric divergence. This leads to the exposure at seawater of important volume of deep‐seated magmatic and mantle rocks. Detachment faults also represent formidable permeable pathways for fluids, enhancing seawater circulation at depth and fluid‐rock interactions with the exhuming rocks. This notably modifies the rheological, thermal, and chemical properties of the oceanic lithosphere and results in a diversity of types of hydrothermal alteration in rocks. At depth, the fluid gets heated (>350°C), decreasing its density and allowing its rise to the seafloor where the mixing with ambient, cold seawater may produce metal‐rich sulfide mineralizations. The genetic links between deep‐seated hydrothermal alterations and the final venting of the hydrothermal fluid at the seafloor are cryptic because of the difficulty to access these environments. Key Points: Mantle rocks at Mid Atlantic Ridge Kane recorded complex melt‐rock and fluid‐rock interactions during incipient mantle exhumation, prior to serpentinizationMagma channelizing and melt‐rock interactions are responsible for metal enrichment (Fe, Co, Zn) in the surrounding mantle silicatesMetal leaching during hydrothermal alteration of enriched mantle rocks may account for important metal budget at seafloor massive sulfides [ABSTRACT FROM AUTHOR]

The complex structure of the region has been formed as a result of ​kinematic reorganization of plate boundaries, accompanied by the extinction of old spreading centers, the formation of new ones, and plume magmatic activity in the South Atlantic southeastern part. These processes resulted in the formation of a system of ridges, rises, and plateaus with different morphologies and geochemical characteristics. The results of density modeling of the structure of the crust and tectonosphere along the profiles, which extend from the Falkland Plateau to the Mozambique Ridge and cross a series of rises and ridges divided by deep-water basins, showed that the rises have different crustal structures indicative of their different origins. The formation conditions of various submarine rises have been studied on the basis of physical modeling. The new experimental model of the formation of the lithosphere and submarine plateaus of the region is provided, in which the important role belongs to the breakup of the Agulhas large igneous province on the Agulhas Plateau and Northeast Georgia Rise, the accretion of the oceanic crust on the Agulhas spreading ridge, and a further jump of the Agulhas spreading ridge axis leading to the termination of spreading at this ridge and the formation of a southern segment of the Mid-Atlantic Ridge and conjugated Meteor and Islas Orcadas Rises. Spreading axis jumps, which are accompanied by periodic activity of hotspots, played an important role in the formation of the submarine rises of various genetic types that, in turn, determined the different structures of their crust. [ABSTRACT FROM AUTHOR]

This article presents results of the structural and morphological analysis of the fracture zones that are part of Doldrums Megatransform System (DMS), located in the northern part of the Equatorial Atlantic (6.5°–9° N) that include Vernadskiy and Bogdanov transform faults and the Doldrums and Pushcharovskiy megatransforms. Bathymetric map, based on the multibeam echo sounding data, collected during the 45th cruise of the R/V Akademik Nikolaj Strakhov was used for this analysis. It was established that large-scale variations in the width of fracture zone valleys are determined by the distribution of stresses perpendicular to the fracture zone. In the areas with compressive stresses, the fracture zone valleys are narrower and the extension areas are wider. The difference in geodynamic settings within the DMS is due to the difference in spreading directions, which change from 89° to 93° when moving from south to north. The depth of fracture zone valleys consistently increases from the periphery of the DMS (Bogdanov and Doldrums faults) to the center (Pushcharovskiy fracture zone) in accordance with a decrease in the upper mantle temperature. In each fracture zone, the valley depth decreases from the rift- fracture zone intersections towards the center of the active part to a certain background depth. It is assumed that this phenomenon is the result of the uplift of the valley bottom, which occurred due to the decompaction of the lithosphere, caused by the serpentinization of ultramafic rocks. The violation of the revealed variations in the width and depth of fracture zone valley patterns occurs as a result of various ridges and uplifts formation in the fracture zone. In the axial zones of the active parts of the fracture zone valleys median ridges are widespread, extending parallel to the fracture zone and representing serpentinite diapirs squeezed out above the bottom surface. Transverse ridges that were formed 10‒11 million years ago as a result of the lithospheric plate edge flexural bending under extensional conditions are now located in the western passive parts on the southern sides of the of Doldrums and Pushcharovskiy fracture zone valleys. The transverse ridge on the northern side of the Vernadskiy fracture zone, which includes Mount Peyve, was formed between 3.65‒2.4 Ma. Due to the frequent jumps of the spreading axis in this region, it was divided into three segments. There are interfracture zone ridges in megatransforms, which in the active part consist of two fracture zone valleys. The times of their formation were in the Pushcharovskiy megatransform, 30‒32 million years ago and in the Doldrums megatransform, about 4 million years ago. Due to the curvilinearity of the outlines and under the pressure of moving lithospheric plates, the interfracture zone ridges experience longitudinal (along the fault) compressive and tensile stresses, which are compensated by vertical uplifts of their separate blocks and the formation of depressions, pull apart depressions, and spreading centers (the latter are only in Pushcharovskiy megatransform). The structure-forming processes that determine the patterns and morphology of the fracture zones as a part of the DMS are related in their origin to the spreading and transform geodynamic systems. [ABSTRACT FROM AUTHOR]

Detailed models of crustal structure at volcanic passive margins offer insight into the role of magmatism and the distribution of igneous addition during continental rifting. The Eastern North American Margin (ENAM) is a volcanic passive margin that formed during the breakup of Pangea ∼200 Myr ago. The offshore, margin‐parallel East Coast Magnetic Anomaly (ECMA) is thought to mark the locus of syn‐rift magmatism. Previous widely spaced margin‐perpendicular studies seismically imaged igneous addition as seaward dipping reflectors (SDRs) and high velocity lower crust (HVLC; >7.2 km/s) beneath the ECMA. Along‐strike imaging is necessary to more accurately determine the distribution and volume of igneous addition during continental breakup. We use wide‐angle, marine active‐source seismic data from the 2014–2015 ENAM Community Seismic Experiment to determine crustal structure beneath a ∼370‐km‐long section of the ECMA. P‐wave velocity models based on data from short‐period ocean bottom seismometers reveal a ∼21‐km‐thick crust with laterally variable lower crust velocities ranging from 6.9 to 7.5 km/s. Sections with HVLC (>7.2 km/s) alternate with two ∼30‐km‐wide areas where the average velocities are less than 7.0 km/s. This variable structure indicates that HVLC is discontinuous along the margin, reflecting variable amounts of intrusion along‐strike. Our results suggest that magmatism during rifting was segmented. The HVLC discontinuities roughly align with locations of Mid‐Atlantic Ridge fracture zones, which may suggest that rift segmentation influenced later segmentation of the Mid‐Atlantic Ridge. Plain Language Summary: The East Coast of the United States is a passive margin that formed during continental breakup of Pangea, the most recent supercontinent. Although passive margins are generally not locales of active faulting and magmatism, by investigating their current structure, we gain insight into processes during past rifting. We know that extensive volcanism and magmatic addition to the Earth's crust occurred during the breakup of Pangea, but we do not fully understand how the distribution changes from north to south along the margin. To study these rocks, which are buried 10–20 km beneath the seafloor, we use ocean bottom seismometers to determine the sound speed of rocks beneath the surface, which can tell us about composition and rock type. We found that the thickness and extent of magma‐derived rocks are variable along the margin, with gaps up to 30 km wide. This variability is likely related to processes happening in the continental rift which may have influenced the structure of the Mid‐Atlantic Ridge that formed after rifting ended. Key Points: High velocity lower crust beneath the East Coast Magnetic Anomaly is variable along‐strike and discontinuous in some placesThe lower crust variability reflects variable intrusion into lower continental crust and represents magmatic segmentation during riftingThe high velocity lower crust discontinuities are coincident to Mid‐Atlantic Ridge fracture zones, suggesting a rift‐to‐ridge connection [ABSTRACT FROM AUTHOR]

Abstract—Over the whole area of the Eurasian lithospheric plate considered in the paper—from the Atlantic to the Pacific Ocean—the velocity vectors of the horizontal eastward displacements of GPS sites form a gentle, smooth, huge arc, convex towards the north. Three relatively narrow arc-shaped bands can be distinguished within the arc. The southernmost band, most provided with measurement data (referred to as a arc-shaped band A in this paper) is discussed in detail in (Shevchenko et al., 2021). The direct, real results of the published geodetic measurements have shown that the length of this band increases by 5 to 10 mm annually. The arc is lengthening. In the cited paper, we considered and rejected five interpretations of this lengthening, which involve the idea of external action on the rocks of the Earth's crust/lithosphere of this arc-shaped band. It is most likely that the elongation is caused by the internal process of the increase in the volume of rocks within the band due to the supply of additional mineral material by deep fluids and its subsequent crystallization. In this paper, we present the results of similar geodetic measurements in other two arc-shaped bands, B and C. It turns out that the arc-shaped band C located to the north of the other bands is lengthening (and at a similar rate), as is band A located to the south of the other bands. At the same time, the arc-shaped band B, which is located between bands A and C, is moving eastward without lengthening. Geodetic measurements in the axial part of the Mid-Atlantic Ridge on the Iceland island suggest that the general expansion of the Atlantic Ocean and the corresponding eastward motion of the Eurasian lithospheric plate are associated with the injection of plastic magmatic wedges (of basic composition) in the axial, rift zone of the ridge. Against the background of this general eastward motion of the lithospheric plate, its constituent parts (whose markers are the arc-shaped bands A and C) are moving in the same eastward direction with a velocity increasing from west to east. We attribute this increase in the velocity and the corresponding lengthening of the two bands to the above inflow of the additional mineral material and its subsequent crystallization. [ABSTRACT FROM AUTHOR]

SUMMARY: Seismicity along transform faults provides important constraints for our understanding of the factors that control earthquake ruptures. Oceanic transform faults are particularly informative due to their relatively simple structure in comparison to their continental counterparts. The seismicity of several fast-moving transform faults has been investigated by local networks, but as of today there been few studies of transform faults in slow spreading ridges. Here, we present the first local seismicity catalogue based on event data recorded by a temporary broad-band network of 39 ocean–bottom seismometers located around the slow-moving Chain Transform Fault (CTF) along the Mid-Atlantic Ridge (MAR) from 2016 to 2017 March. We locate 972 events in the area by simultaneously inverting for a 1-D velocity model informed by the event P- and S-arrival times. We refine the depths and focal mechanisms of the larger events using deviatoric moment tensor inversion. Most of the earthquakes are located along the CTF (700) and Romanche transform fault (94) and the MAR (155); a smaller number (23) can be observed on the continuing fracture zones or in intraplate locations. The ridge events are characterized by normal faulting and most of the transform events are characterized by strike-slip faulting, but with several reverse mechanisms that are likely related to transpressional stresses in the region. CTF events range in magnitude from 1.1 to 5.6 with a magnitude of completeness around 2.3. Along the CTF we calculate a b-value of 0.81 ± 0.09. The event depths are mostly shallower than 15 km below sea level (523), but a small number of high-quality earthquakes (16) are located deeper, with some (8) located deeper than the brittle-ductile transition as predicted by the 600 °C-isotherm from a simple thermal model. The deeper events could be explained by the control of sea water infiltration on the brittle failure limit. [ABSTRACT FROM AUTHOR]

The processes driving the evolution of crystal mushes are often documented in complex systems where crystallization, assimilation, magma replenishment and mixing occur concurrently and are generally overprinted by compaction and deformation. Documenting the characteristics of an undisturbed crystal mush is thus of upmost importance; it highlights the initial conditions with which complex crystal mush processes proceed. We here present the structure and composition of an oceanic crystal mush through detailed petro‐structural and chemical study of metre‐scale intrusions from the Mid‐Atlantic Ridge. Textures, bulk‐rock and mineral compositions indicate closed‐system crystallization of primitive melts, undisturbed by dissolution–precipitation reactions and subsequent deformation. These frozen crystal mushes record the simplest possible evolution of small‐scale intrusions and can be used as a baseline to pinpoint the impact of crystal mush processes on the evolution of complex systems. Any divergence from this reference results from processes occurring concomitantly to the progressive closure of the magmatic system. [ABSTRACT FROM AUTHOR]

The work is devoted to the study of ore minerals from the surface horizon of ore-bearing sediments in the Pobeda hydrothermal cluster using the following methods: optical microscopy, scanning electron microscopy, and X-ray spectral microanalysis. It was found that ore minerals are represented by fragments of Cu–Fe sulfides (isocubanite, chalcopyrite, and pyrite), newly formed iron hydroxides, and atacamite. In addition, barite and edaphogenic material as talcified silicate clasts, sometimes with sulfide inclusions, are present. Structural-morphological types of iron hydroxides are distinguished. Based on the hydrophysical data, the location of assumed active hydrothermal vent in the Pobeda-3 ore occurrence area was updated. Distribution of the studied minerals depending on the location relative to active hydrothermal vents is described. Decrease in the size and amount of hydrothermal mineral clasts and edaphogenic material, as well as increase in the degree of sulfide replacement by iron hydroxides, were observed when moving away from the sources. Moreover, decrease in the Cu/Fe ratio in the chemical composition of Cu–Fe sulfides is also noted. An unidentified phase of Cu3.57‒4.22Fe1.71‒2.19S4.99‒5.31 with the chalcopyrite lamellas was established in the surface horizon of the column at station 37L245g. [ABSTRACT FROM AUTHOR]

A section of 46 CTD/Lowered Acoustic Doppler Current Profiler stations along the Vema Fracture Zone (VFZ) was made for the first time with a repeat of the transverse section on the main sill (at 41° W) for the sixth time. Potential temperature of Antarctic Bottom Water when it flows into the fracture increases from 1.3°С to 1.6°C. This temperature increase is much smaller than in the Romanche Fracture Zone because in the VFZ there are not many transverse sills that force the flow to become turbulent. A zone with minimal velocities of the bottom flows exists in the western part of the VFZ up to the main sill. A current with velocities of 0.25 m/s flows above the low velocity zone at a depth of about 4,000 m. Underwater spillways with hydraulically controlled flow of bottom water down the slope from 4,500 m to 5,000 m at speeds up to 0.40 m/s were found east of the sill (in the central and southern channels). The bottom current accelerates as it flows down, but then its kinetic energy decreases. The bottom current slows down and mixes with surrounding waters. A thin (∼30 m) bottom flow descends further to the deep depression. In the southern channel of the VFZ, bottom water flows into a deep (5,400 m) basin. The coldest and densest water reaches this depression during rare inflows. Plain Language Summary: Antarctic Bottom Water formed on Antarctic slopes slowly flows northward. The flow passes many deep basins and abyssal channels between them until it reaches the Vema Fracture Zone (VFZ). The VFZ is the main conduit in the Mid‐Atlantic Ridge for the bottom water flow to the Northeast Atlantic; this flow fills the abyssal basins of the Northeast Atlantic. Unlike the Romanche and Chain fracture zones near the equator, Antarctic Bottom Water only slightly warms here while flowing through the fracture because there are not many transverse sills that can prevent the bottom flow and induce mixing with North Atlantic Deep Water occupying the overlying water. The fastest current flows over a low velocity zone with low friction. After overflowing the main sill, the water descends from 4,500 m to 5,000 m as a spillway, in which the flow partly mixes with the surrounding waters. Key Points: Antarctic Bottom Water flows in the Vema Fracture Zone (VFZ) without strong warming because it flows over a low velocity zone with less frictionTwo underwater spillways were found in the VFZ in which bottom water flows down from 4,500 m to 5,000 mThe downflow of water in the spillways is governed by hydraulic control [ABSTRACT FROM AUTHOR]

Because new observations have revealed that the Labrador Sea is not the primary source for waters in the lower limb of the Atlantic Meridional Overturning Circulation (AMOC) during the Overturning in the Subpolar North Atlantic Programme (OSNAP) period, it seems timely to re‐examine the traditional interpretation of pathways and property variability for the AMOC lower limb from the subpolar gyre to 26.5°N. In order to better understand these connections, Lagrangian experiments were conducted within an eddy‐rich ocean model to track upper North Atlantic Deep Water (uNADW), defined by density, between the OSNAP line and 26.5°N as well as within the Labrador Sea. The experiments reveal that 77% of uNADW at 26.5°N is directly advected from the OSNAP West section along the boundary current and interior pathways west of the Mid‐Atlantic Ridge. More precisely, the Labrador Sea is a main gateway for uNADW sourced from the Irminger Sea, while particles connecting OSNAP East to 26.5°N are exclusively advected from the Iceland Basin and Rockall Trough along the eastern flank of the Mid‐Atlantic Ridge. Although the pathways between OSNAP West and 26.5°N are only associated with a net formation of 1.1 Sv into the uNADW layer, they show large density changes within the layer. Similarly, as the particles transit through the Labrador Sea, they undergo substantial freshening and cooling that contributes to further densification within the uNADW layer. Plain Language Summary: The North Atlantic Deep Water is a cold and fresh water mass formed at high latitudes and advected southward across the North Atlantic as part of the Atlantic Meridional Overturning Circulation. The upper part of this water mass (uNADW) has long been considered to be mainly formed in the Labrador Sea. However, new observations have revealed that the Labrador Sea is not the primary source for uNADW, suggesting that the dense water observed at 26.5°N is not necessarily related to Labrador Sea convection. Here, we perform Lagrangian experiments between the subpolar gyre and 26.5°N within an eddy‐rich ocean model to show that the Labrador Sea is a main gateway for uNADW sourced from the Irminger Sea and is associated with large property changes within the uNADW layer. Additionally, we reveal direct interior pathways connecting uNADW sourced from the Iceland Basin and Rockall Trough to 26.5°N. Key Points: The large majority of upper North Atlantic Deep Water (uNADW) sourced from the Irminger Sea transits through the Labrador Sea before reaching 26.5°NInterior pathways along the eastern flank of the Mid‐Atlantic Ridge connect the Iceland Basin and Rockall Trough to 26.5°NThough uNADW is mainly sourced in the eastern subpolar gyre, its transit in the Labrador Sea is associated with further property changes [ABSTRACT FROM AUTHOR]

The Azores region straddles the Mid-Atlantic Ridge and comprises volcanic islands and seamounts interspersed with non-magmatic oceanic basins arranged along subparallel slow-spreading systems. The cross-check of CO2-rich fluid inclusions barometry from lavas and tephras erupted during the last 20 ka with the sources of seismicity of the last 22 years traced the deepest magma accumulation zone at a regional scale. These zones are considered to represent the boundary between the dense rocks of magma systems and the lithospheric mantle. This boundary is at a depth range of ~ 17 to 20 km beneath the islands close to the Mid-Atlantic Ridge, which are younger than 500 ka, and up to ~ 29.4 km beneath the eastern island of S. Miguel, younger than 800 ka and the nearby Dom João de Castro Seamount and the Hirondelle Basin. The same method has been applied to the ~ 2 Ma-old volcanic products of Flores, the ~ 1.9 Ma old S. Jorge and the 5 Ma-old lavas of Santa Maria. Results revealed a depth of ~ 30.5 km beneath the Flores and ~ 26 km beneath the ~ 1.9 Ma-old S. Jorge. These two islands are west and east of the Mid-Atlantic Ridge, within 200 km. Finally, the crust-mantle transition has been found below Santa Maria, the easternmost island, ~ 500 km away from the Mid-Atlantic Ridge at a depth of ~ 33 km. Overall, repeated mafic magma injections at the base of each magma system generated vertically stacked layered ultramafic mush bodies, which thickened the crust overtime. [ABSTRACT FROM AUTHOR]

The upper mantle is heterogeneous in Fe isotope compositions, but the origin of the heterogeneity needs understanding. Recent studies on oceanic basalts demonstrate that the upper mantle Fe isotope heterogeneity results from low‐degree melt metasomatism. However, whether this hypothesis is of global significance needs testing in continental settings. Here we present an Fe isotopic study on intraplate continental alkali basalts from continental China, which are shown to have derived from upper mantle containing more metasomatized lithologies. The results show first‐order positive correlations of δ56Fe with indices of low‐degree melt metasomatism (e.g., [La/Sm]N), which, together with the so‐far published mid‐ocean ridge basalts data, define global trends, substantiating the global significance of low‐degree melt metasomatism in causing upper mantle Fe isotope variation. We, thus, suggest that low‐degree melt metasomatism is a globally effective process to preferentially concentrate heavy Fe isotopes in metasomatic lithologies at upper mantle conditions both today and in Earth's history. Plain Language Summary: Previous studies have demonstrated that the upper mantle of the Earth has a heterogeneous iron isotope composition, but the cause to such a heterogeneity remains not well understood. By studying oceanic basalts from Mid‐Atlantic Ridge and East Pacific Rise, we suggest that oceanic upper mantle heterogeneous in iron isotope is the result of the mantle metasomatism, which is an enrichment process by low‐degree partial melt at upper mantle condition that progressively enrich more incompatible trace element as well as heavy iron isotopes (57,56Fe vs. 54Fe). However, whether this deep mantle process is globally widespread needs further testing using continental basalts. We here present an iron isotope study on intraplate alkali basalts from eastern continental China. The results are in line with the hypothesis based on the oceanic basalts, substantiating the global significance of melt metasomatism‐induced upper mantle iron isotope fractionation. We further suggest that this deep mantle process fractionate iron isotopes through preferentially concentrating heavy Fe isotopes in metasomatic lithologies at upper mantle conditions, both in Earth's history and at present. Key Points: Iron isotope study on continental alkali basalts verifies the global significance of low‐degree melt metasomatism in causing upper mantle Fe isotope fractionationLow‐degree melt metasomatism is a globally effective process to concentrate heavy Fe isotopes at upper mantle conditionsThe low‐degree melt metasomatism associated Fe isotope fractionation could be both recent and ancient [ABSTRACT FROM AUTHOR]

A seismic swarm affected the 53.3°–54.3° Latitude North section of the Mid‐Atlantic Ridge from 26 September to 10 December 2022. We rely on regional, teleseismic and array data to relocate 61 hypocenters and derive 77 moment tensors. The 2022 swarm released a cumulative moment equivalent to Mw 6.3. Seismicity was shallow (7 ± 3 km depth). Most earthquakes are located along the ridge axis with typical, NS oriented normal faulting mechanisms, but a few among the largest and latest earthquakes have unusual thrust mechanisms and locations as far as ∼25 km from the ridge. We attribute the swarm to a shallow magmatic intrusion, with a vertical dike first propagating ∼60 km along axis, accompanied by shallow normal faulting, and then thickening and triggering thrust earthquakes off the ridge, in response to compressive stress buildup. The unrest provides a rare example of an energetic, magmatic driven swarm episode at the mid‐ocean ridge. Plain Language Summary: The largest plate boundary systems on Earth are Mid‐ocean ridges (MOR), where the plates continuously drift apart and new lithosphere is constantly being formed. Although the process is well understood, we rarely detect spreading events at MOR, mainly because these regions are remote and local monitoring is rarely possible. In September–November 2022 a large, unusual seismic swarm occurred along a spreading center ridge segment of the North Mid‐Atlantic Ridge. Despite the remoteness of the region, we managed to model regional and teleseismic data to perform earthquake relocation, depth estimation and moment tensor inversion. In this way, we could reconstruct the geometry and the evolution of the seismicity. We found that in the early days of the swarm, seismicity migrated unilaterally over ∼60 km along the ridge axis, from North to South, triggering normal faulting earthquakes, which are typical at MOR. Later, large thrust mechanisms, anomalous in an extensional environment, appeared and quickly became predominant. We explain seismological observations by a magmatic intrusion, which first propagated southward, producing shallow normal faulting earthquakes above the vertical magma dike, and later thickened, increasing compressional stresses on its sides, and triggering large thrust earthquakes. Key Points: Analysis of a short, intense seismic swarm at the Mid‐Atlantic RidgeIdentification of unusual, thrust focal mechanisms in an extensional environmentSwarm triggered by dike intrusion at the mid‐ocean ridge [ABSTRACT FROM AUTHOR]