Explore the vast range of titles available.

Lost City (mid-Atlantic ridge) is a unique oceanic hydrothermal field where carbonate-brucite chimneys are colonized by a single phylotype of archaeal Methanosarcinales, as well as sulfur- and methane-metabolizing bacteria. So far, only one submarine analog of Lost City has been characterized, the Prony Bay hydrothermal field (New Caledonia), which nonetheless shows more microbiological similarities with ecosystems associated with continental ophiolites. This study presents the microbial ecology of the ‘Lost City’-type Old City hydrothermal field, recently discovered along the southwest Indian ridge. Five carbonate-brucite chimneys were sampled and subjected to mineralogical and geochemical analyses, microimaging, as well as 16S rRNA-encoding gene and metagenomic sequencing. Dominant taxa and metabolisms vary between chimneys, in conjunction with the predicted redox state, while potential formate- and CO-metabolizing microorganisms as well as sulfur-metabolizing bacteria are always abundant. We hypothesize that the variable environmental conditions resulting from the slow and diffuse hydrothermal fluid discharge that currently characterizes Old City could lead to different microbial populations between chimneys that utilize CO and formate differently as carbon or electron sources. Old City discovery and this first description of its microbial ecology opens up attractive perspectives for understanding environmental factors shaping communities and metabolisms in oceanic serpentinite-hosted ecosystems.

Oceanic detachment faults play a central role in accommodating the plate divergence at slow-ultraslow spreading mid-ocean ridges. Successive flip-flop detachment faults in a nearly-amagmatic region of the ultraslow spreading Southwest Indian Ridge (SWIR) at 64°30’E accommodate ~100% of plate divergence, with mostly ultramafic smooth seafloor. Here we present microseismicity data, recorded by ocean bottom seismometers, showing that the axial brittle lithosphere is on the order of 15 km thick under the nearly-amagmatic smooth seafloor, which is no thicker than under nearby volcanic seafloor or at more magmatic SWIR detachment systems. Our data reveal that microearthquakes with normal focal mechanisms are colocated with seismically-imaged damage zones of the active detachment fault and of antithetic hanging-wall faults. The level of the hanging-wall seismicity is significantly higher than that documented at more magmatic detachments of slow-ultraslow ridges, which may be a unique feature of nearly-amagmatic flip-flop detachment systems. Oceanic detachment faults play a central role in accommodating the plate divergence at mid-oceanic ridges. Here, the authors show micro-seismicity of a nearly-amagmatic flip-flop detachment fault system at the ultraslow spreading Southwest Indian Ridge.

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 field Hot (>80°C) and hydrothermal endmember‐rich diffuse fluids (>10%) come out of the basalts up to 25 m from the black smokers Fluids that come out of basalt substratum are modified in volcaniclastic layer before coming out at vents that host the hydrothermal fauna

The kinetics of the reaction (Mg,Fe)‐olivine + H2O → serpentine + magnetite + brucite + H2 were investigated at 500 bars in the 250–350°C range using natural olivine (San Carlos; Fo91) with grain sizes between 1 and 150 μm and for run durations up to 514 d. The amount of magnetite produced, which directly relates to reaction progress, was accurately monitored using up to 24 time‐resolved magnetic measurements per experiment. Eighty percent of serpentinization was achieved after 60 d for olivine grain sizes of 5–15μm and after 500 d for grain sizes of 50–63 μm. Serpentinization kinetics were found to be inversely proportional to the geometrical surface area of the starting olivine grains. They were one or two orders of magnitude slower than serpentinization kinetics commonly used for modeling serpentinization‐related processes. The nature of the serpentine mineral product depended on the initial olivine grain size (IGS); for IGS in the 5–150μm range lizardite formed, and olivine dissolution was the rate‐limiting process. At IGS below 5μm, chrysotile crystallized instead of lizardite, and the relationship between olivine surface area and reaction kinetics no longer held. We infer that for such small olivine grain sizes dissolution is no longer the rate‐limiting process. Serpentinization in our experiments was associated with the creation of new reactive surface area according to two cooperative processes: etch pits formation associated with dissolution and grain fracturing for IGS above 20μm. Interestingly, fractures and etch pits with similar geometry and sizes were also observed for residual olivine (with a typical grain size of 50 μm) in serpentinized peridotite samples from the Southwest Indian Ridge. This suggests that the processes governing olivine serpentinization kinetics in our experiments are similar to those prevailing in natural systems. We therefore suggest that the new kinetic data set that we present here, which encompasses a range of olivine grain sizes and reaction temperatures, is relevant to the serpentinization of olivine in the oceanic crust insofar as water is available. Key Points Experimental study of the kinetics of olivine serpentinization Influence of temperature and olivine initial grain size (IGS) on the kinetics Reactive surface area increases with etch pits and fractures

Fluid composition and fluxes in subduction zones are primarily governed by the nature and degree of hydrothermal alteration of the subducting oceanic lithosphere. However, spatial and temporal heterogeneities inherited from mid‐ocean ridge and oceanic transform fault (OTF) systems introduce significant uncertainties in constraining these fluid variations. Here, we focus on the effect of subducting Fe‐Ti‐rich gabbroic rocks (oxide gabbros), which are commonly found in (ultra)slow‐spread oceanic crust along OTF walls, in detachment faults forming at the inside corners of ridge‐transform intersections (RTIs) and within subducted oceanic metamorphic units. We carried out a petrological and geochemical characterization of oxide gabbros from the Vema OTF which segments the mid‐Atlantic Ridge to document and discuss their abundance, composition, formation and transformation processes at RTIs. Results illustrate spatially variable magmatic and hydrothermal processes at RTIs, resulting in variable Fe‐Ti‐(P)‐(H2O)‐V enrichment (ilmenite + titanomagnetite ± apatite ± amphibole ± olivine) of primary gabbroic rocks. Thermodynamic modeling reveals significant variability in the stability of hydrated phases across different gabbroic compositions, indicating that, in subduction zones, Fe‐Ti‐enriched lithologies release fluids at shallower depths. Oxide gabbros, like the ones studied, represent a significant but often overlooked source of H2O, halogens and large ion lithophile elements to the mantle wedge. In addition, subducted P‐rich oxide gabbros may serve as a deep (>700 km) source of fluorine in the asthenosphere. Our results demonstrate that subduction of a compositionally heterogeneous slab containing significant amounts of oxide gabbros generates a broad dehydration domain with implications for seismicity, water transport along the subduction interface and fluid‐mediated tectonic slicing.

The global mid-ocean ridge system, where tectonic plates diverge, is traditionally thought of as the largest single volcanic feature on the Earth. Yet, wide expanses of smooth sea floor in the easternmost part of the Southwest Indian Ridge in the Indian Ocean lacks the hummocky morphology that is typical for submarine volcanism. At other slow-spreading ridges, the sea floor can extend by faulting the existing lithosphere, along only one side of the ridge axis. However, the smooth sea floor in the easternmost Southwest Indian Ridge also lacks the corrugated texture created by such faulting. Instead, the sea floor is smooth on both sides of the ridge axis and is thought to be composed of altered mantle-derived rocks. Here we use side-scan sonar to image the sea floor and dredge samples to analyse the composition of two sections of the Southwest Indian Ridge, between 62° 05′ E and 64° 40′ E, where the sea floor formed over the past 11 million years. We show that the smooth floor is almost entirely composed of seawater-altered mantle-derived rocks that were brought to the surface by large detachment faults on both sides of the ridge axis. Faulting accommodates almost 100% of plate divergence and the detachment faults have repeatedly flipped polarity. We suggest that this tectonic process could also explain the exhumation of mantle-derived rocks at the magma-poor margins of rifted continents. The sea floor at the easternmost Southwest Indian mid-ocean ridge is smooth, unlike that at other mid-ocean ridges. Sonar imaging and analysis of rock samples show that the sea floor here is composed almost entirely of sea-water-altered mantle rocks that have been brought to the surface by large faults on both sides of the ridge axis over the past 11 million years.

The Lucky Strike vent field, located on the Mid‐Atlantic Ridge (MAR), is hosted on enriched mid‐ocean ridge basalt associated with the nearby Azores hotspot. In this study, we present bulk rock geochemistry coupled with in situ sulfur isotope analysis of hydrothermal samples from Lucky Strike. We assess the geological controls on the differences in the major and trace element content and sulfur isotopic composition of the hydrothermal deposits within the vent field. The hydrothermal deposits contain elevated concentrations of elements typically enriched in mid‐ocean basalt (E‐MORB), such as Mo, Ba, and Sr, compared to typical values for other hydrothermal deposits hosted on the MAR. The range in sulfur isotope compositions of hydrothermal marcasite and chalcopyrite (−2.5 to 8.7‰) is similar to the range recorded at other sediment‐free basalt‐hosted seafloor hydrothermal sites. However, at Lucky Strike, the Capelinhos vent, situated 1.4 km east of the main field, is enriched in 34S (by ∼3.5‰ for both marcasite and chalcopyrite), relative to the main field. This difference reflects contrasting subseafloor fluid/rock interactions at these two sites, including subseafloor sulfide precipitation at the main field that results in <20% of reduced sulfur within the upwelling hydrothermal fluid reaching the seafloor. We also compare the geochemistry of the hydrothermal deposits at Lucky Strike to other hydrothermal sites along the MAR and show that the average hydrothermal deposit Ba/Co is useful to discriminate between E‐MORB and other mafic/ultramafic hosted deposits. Plain Language Summary We investigate the variations in composition of metal‐ and sulfur‐rich hydrothermal deposits that form on the seafloor at a cluster of high‐temperature hot springs called the Lucky Strike hydrothermal vent field, on the Mid‐Atlantic Ridge. We find that the mineralogy and geochemistry of the deposits do not vary spatially within this vent field. However, variations in the relative abundances of different sulfur isotopes within these deposits differ between the central cluster of vents and a newly discovered site called Capelinhos that is located 1.4 km east of the main vent field. Isotopic variations are usually interpreted to indicate differences in sulfur sources, with seawater and sulfur from the mantle as the two primary sources. However, our results instead show that significant mineral precipitation below the seafloor at the main vent cluster is the likely source of these isotopic variations. In addition, we show that the relative abundances of various trace elements within the hydrothermal deposits can be used to fingerprint the composition of the volcanic rocks that host these deposits. In particular, the ratio of Ba to Co can be used to fingerprint specific tectonic settings for different hydrothermal vent sites on mid‐ocean ridges. Key Points Spatial variations in in situ sulfur isotope compositions at Lucky Strike indicate differences in fluid/rock interactions in the sub‐surface In situ sulfur isotope data suggests that >80% of the available H2S in the ascending hydrothermal fluid has precipitated in the subseafloor The Ba/Co ratio of hydrothermal deposits discriminates those associated with enriched mid‐ocean basalts from other mafic/ultramafic hosted deposits

Passive seismic interferometry has become very popular in recent years in exploration geophysics. However, it has not been widely applied in marine exploration. The purpose of this study is to investigate the internal structure of a quasi-amagmatic portion of the Southwest Indian Ridge by interferometry and to examine the performance and reliability of interferometry in marine explorations. To reach this goal, continuous vertical component recordings from 43 ocean bottom seismometers were analyzed. The recorded signals from 200 station pairs were cross-correlated in the frequency domain. The Bessel function method was applied to extract phase–velocity dispersion curves from the zero crossings of the cross-correlations. An average of all the dispersion curves was estimated in a period band 1–10 s and inverted through a conditional neighborhood algorithm which led to the final 1D S-wave velocity model of the crust and upper mantle. The obtained S-wave velocity model is in good agreement with previous geological and geophysical studies in the region and also in similar areas. We find an average crustal thickness of 7 km with a shallow layer of low shear velocities and high Vp/Vs ratio. We infer that the uppermost 2 km are highly porous and may be strongly serpentinized.

A Good Spread Data from the recent seismic reflection survey over the Lucky Strike volcano and hydrothermal vent field in the Atlantic Ocean made from the French ship RV l'Atalante provide the first clear seismic image of an axial magma chamber beneath a slow spreading ridge. Faults can be seen penetrating down to the chamber, a formation that has never been imaged before. These findings provide an insight into the interaction between the volcanic and tectonic processes that contribute to crust formation at slow spreading ridges. The Lucky Strike volcano is situated at the centre of the Lucky Strike segment of the Mid-Atlantic Ridge, currently spreading at a rate of about 22 mm per year. Crust at slow-spreading ridges is formed by a combination of magmatic and tectonic processes, with magmatic accretion possibly involving short-lived crustal magma chambers 1 . The reflections of seismic waves from crustal magma chambers have been observed beneath intermediate 2 , 3 and fast-spreading centres 4 , 5 , but it has been difficult to image such magma chambers beneath slow-spreading centres 6 , 7 , owing to rough seafloor topography and associated seafloor scattering 7 , 8 . In the absence of any images of magma chambers 6 or of subsurface near-axis faults, it has been difficult to characterize the interplay of magmatic and tectonic processes in crustal accretion and hydrothermal circulation at slow-spreading ridges. Here we report the presence of a crustal magma chamber beneath the slow-spreading Lucky Strike segment of the Mid-Atlantic Ridge. The reflection from the top of the magma chamber, centred beneath the Lucky Strike volcano and hydrothermal field, is approximately 3 km beneath the sea floor, 3–4 km wide and extends up to 7 km along-axis. We suggest that this magma chamber provides the heat for the active hydrothermal vent field above it. We also observe axial valley bounding faults that seem to penetrate down to the magma chamber depth as well as a set of inward-dipping faults cutting through the volcanic edifice, suggesting continuous interactions between tectonic and magmatic processes.

The interaction between magmatic systems and ice sheet dynamics in polar ocean basins is a critical, yet poorly known Earth system process with implications for cryosphere evolution and planetary analogs. The submarine segment of the Terror Rift in the Ross Sea, Antarctica, represents a unique seafloor environment where volcanic activity occurs beneath and adjacent to multiple advances of major ice sheets. Here we present observational data acquired during the most recent U.S. research vessel Nathaniel B. Palmer expedition NBP25-01 (February-April 2025), including high-resolution seafloor bathymetry, rock dredging and seafloor imagery in the discovery of widespread explosive basaltic volcanism erupted along rift structures that occurred both synchronously with and following seafloor groundings of ice sheets, including during the Holocene. Our results support advanced concepts of mantle dynamics, magma-ice interactions and the evolution of rifted polar regions and establish the Terror Rift as an analog for cryovolcanic environments on icy-planets and moons. Regional volcanism in the Terror Rift is influenced by deep-seated tectonic and magmatic processes as well as surficial factors, such as the advancement and retreat cyclicity of ice sheets, as evidenced by mapping and sampling of seafloor volcanism in the southwestern Ross Sea.