Hydrothermal vent fields harbour a heterogeneous seascape owing to complex topography and vent fluid emissions, including in peripheral areas beyond active venting. At the Lucky Strike hydrothermal vent field, a remotely operated vehicle was used to acquire two hectares of seabed imagery to evaluate the role of environmental conditions on the structure and distribution of benthic communities. Our analyses revealed that large and mature edifices are potential keystone structures supporting hotspots of vent specialists (e.g., Bathymodiolus azoricus) and associated fauna (e.g., Zoanthidae). Higher densities of mobile bathyal shrimps were found in slabs adjacent to active venting areas. Communities occupying areas with lower hydrothermal exposure displayed significantly higher diversity than areas located closer (≤ 40 m) to vent emissions. Between 40 and 120 m from active sites, community diversity was greatest on hard substrata dominated by sessile organisms—such as octocorals, Cladorhizid and Hexactinellid sponges—in greater densities when these substrates were composed of basalt with complex topography. While the gradient in hydrothermal exposure creates a spatial zonation of benthic communities, abundance and diversity patterns remain locally conditioned by seabed topography and substratum hardness. This interplay fosters a patchwork of faunal communities at the decametre scale. These results underscore the complexity of designing surveys encompassing the diversity of habitat conditions within the deep hydrothermal seascape. Although historically overlooked, the diversity of distinct benthic communities beyond venting zones emphasises the ecological significance and vulnerability of areas that could be targeted by mining.

Observations at active magma-poor mid-oceanic ridges during ultraslow spreading (< 20 km/Myr full rate) are crucial for understanding the oceanization processes taking place during tectonic plate breakup. Particularly along magma-poor rifted margins, where subcontinental mantle is exhumed prior to the onset of oceanic spreading. It is hypothesized that this exhumation, occurring along detachment faults, is accompanied by a progressive increase in the magmatic budget, ultimately leading to the formation of a spreading ridge. These exhumation processes are believed to be similar to those observed in magma-poor areas along ultra-slow-spreading ridges, such as the easternmost part of the Southwest Indian Ridge (SWIR).There, dredging revealed an oceanic basement composed of serpentinized exhumed mantle intruded by gabbros and locally overlain by variable amounts of basalts (Sauter et al., 2013). The morphology of the serpentinite ridges allowed to propose a "flip-flop" evolution of the detachment faults, characterized by alternating fault vergences. In this study, we analyse large-scale seismic reflection profiles of the Sismosmooth cruise (2014), over a series of peridotite ridges formed by flip-flop detachment processes. The absence of sedimentary cover allows for direct observation and ground-truthing of the nature of the exhumed basement at the seafloor (dredges, sub-marine images, bathymetry, TOBI side-scan sonar data). However, seismic reflection data are challenging to interpret due to the high impedance contrast between the water column and the basement, which limits wave penetration in the basement (Canales et al., 2004).Our objective is to identify new criteria for identifying flip-flop detachment faults in contexts where the basement surface is covered by sediments, i.e. at continental margins. We also aim at identifying differences between flip-flop faulting at mid-ocean ridges and magma-poor rifted-margins. Detachment fault blocks in the easternmost SWIR form large amplitude, regularly spaced (11-18 km), mostly rounded and asymmetric ridges that expose serpentinized peridotites, locally with a thin basaltic cover. Seismic reflection data shows that the reflective top basement is locally affected by normal faults dipping mostly toward the ridge axis. Deep reflectors parallel to the top basement (~0.8 s TWT below top basement) occur locally, mostly beneath the inward-facing slopes of ridges, where the basement top is concave. We propose that they result from magma entrapment in the axial rift, when a new, antithetic, detachment fault cuts the previous one. Higher heat flow and hydrothermalism in the fault damage zone could prevent melt ascension to the seafloor.We next look for these features (smooth reflective top basement ridges and reflectors ~0.8 s TWT below top basement) in seismic reflection profiles acquired across magma-poor rifted margins where flip-flop processes are suspected. We propose an interpretation of smooth basement ridges in the most distal magma-poor rifted margins as proto-oceanic or oceanic domains. We apply this approach to the Iberia and Antarctica fossil margins and show how this new criteria, allowing us to propose that flip-flop detachment processes took place during or directly after the final breakup of the lithospheric mantle, may help map and interpret key domains of the most distal part of magma-poor rifted margins.

The detailed mineralogical study of serpentinized peridotites collected at 13 localities at mid-ocean ridges, in ophiolites and ultramafic bodies shows the formation of two alteration zones around olivine grains. At the olivine contact, a fine-grained mixture of serpentine, Fe-brucite ($\frac{Mg}{Mg + Fe}$ molar ratio comprised between 0.66 and 0.82) and awaruite occurs (Reaction Zone 1). X-ray mapping indicates limited mass transfer during Reaction Zone 1 formation, suggesting isochemical serpentinization except for the addition of water. The measured distribution of Fe and Mg between serpentine and brucite in Reaction Zone 1 is well reproduced with thermodynamic modelling incorporating the latest data for the Fe$\left ( OH ight )_{2}$ endmember. Thermodynamic modelling also reveals that, at low water to rock ratio, awaruite formation limits H$_{2}$ production to values more than one order of magnitude lower than previous estimates. The predicted H$_{2,aq}$ concentrations are comprised between 10$<^>{-3}$ and 10$<^>{-2}$ mol/kg, that is in the same range than the maximum values measured in fluids expelled at ultramafic-hosted hydrothermal sites. At a water to rock ratio of 1, the updated thermodynamic model only predicts magnetite formation after olivine at temperatures above 300 $<^>{\circ }$C, that is above the temperature estimates for serpentinization based on published oxygen isotope data. Nevertheless, a second assemblage composed of Ni-bearing magnetite, serpentine and Mg-brucite ($\frac{Mg}{Mg + Fe}$ molar ratio comprised between 0.83 and 0.98) can be found at the mesh rim (Reaction Zone 2). Reaction Zone 2 can display a symplectite microtexture. Transmission electron microscopy reveals the presence of porosity and Fe-brucite relics in the symplectite, suggesting Reaction Zone 2 formation after Reaction Zone 1 by dissolution-precipitation. Significant mass transfer is observed in Reaction Zone 2 at the mesh rim, probably associated with the solid volume decrease of 7$\%$ during Reaction Zone 1 reaction to form Reaction Zone 2. The mineralogy and the composition of Reaction Zone 2 are reproduced with thermodynamic modelling of olivine serpentinization not by increasing the water to rock ratio but rather by removing H$_{2}$ from the system. This indicates that H$_{2}$ diffusion is the main driver for magnetite formation during serpentinization. The H$_{2,aq}$ concentrations at the equilibrium with Reaction Zone 2 fall in the 10$<^>{-7}$-10$<^>{-3}$ mol/kg range. The relative proportion of Reaction Zone 1 and Reaction Zone 2 in serpentinized peridotites has first-order impacts on H$_{2}$ production.

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.

Fault scarps at Mid-Ocean Ridges (MOR) are well recognized on the seafloor and often measured to estimate the tectonic component of plate spreading. However, tectonic strain estimates based on the dimensions of fault scarps that can be traced on seafloor topographic maps (which we refer to as apparent tectonic strain) differ from the actual whole tectonic strain. This is clearly the case at relatively melt-poor slow-ultraslow ridge segment ends, where strain is accommodated by detachment faults that do not produce linear fault scarps at the seafloor. This contribution explores the relation between actual and apparent tectonic strain in magma robust MOR regions (at fast, intermediate spreading ridges, and in the magmatically robust segment centers of slow-ultraslow ridges). We use high-resolution (1-2 m) bathymetry data at 8 MOR sites, which span a broad range of spreading rates (14-110 km/Ma) and melt fluxes. To the first degree, apparent tectonic strain is highest at slow spreading ridges, which have the lowest melt fluxes, and decreases as melt flux increases (fast spreading ridges). We examine how faults nucleate and evolve on the young axial seafloor, while establishing the relationships to volcanism. Apparent tectonic strain derived from the dimensions of fault scarps on the young seafloor is reduced due to lava flows that cover pre-existing faults. Apparent tectonic strain also includes a component of strain that is not related to far-field tectonic stresses but to stalled dike intrusions that induce extensional faults in the shallow crust. This mechanism is probably responsible for the high apparent tectonic strain estimated at domal volcanos found at the center of intermediate-slow-ultraslow ridge segments. Apparent tectonic strain at and near MOR thus poorly reflects the real tectonic component of plate divergence, instead relating to the interplay between tectonic and magmatic processes over different time scales.

We used broad-band ocean bottom seismometer data from the RHUM-RUM (Reunion Hotspot and Upper Mantle - Reunions Unterer Mantel) experiment to derive the compliance function and estimate the shear velocity (V-s) structure of the subsurface at several sites beneath the Indian Ocean. The primary objective is to map the geological features of poorly explored marine regions, utilizing the compliance function, a measure of seafloor deformation in response to infragravity pressure signals at low frequencies (0.003 to 0.04 Hz). Compliance is the transfer function between vertical displacement and pressure, which is most sensitive to subsurface shear velocities. Our analytical process involves several data processing steps, including the removal of glitches, filtering out seismic events, minimizing tilt effects, calibrating pressure gauges, searching over the frequency and coherence domains to determine the optimal data window and performing depth-velocity inversion using Monte Carlo method, specifically the Metropolis-Hastings algorithm. We present the 'ComPy' software, which automates these processing steps for seafloor compliance analysis. The data, recorded over 13 months in 2012-2013 over a large region stretching from La Reunion Island to the Central Indian Ridge and the South-West Indian Ridge (SWIR) (water depths of 3 to 5 km), confirm the stability of the compliance function over time. Depth-velocity inversions of the derived compliance measurements, using the Metropolis-Hastings algorithm, illuminate the V-s structure of the oceanic crust down to 8 km. Low V-s anomalies in the crust at the SWIR are consistent with significant serpentinization of a crustal component of tectonically exhumed mantle-derived peridotites.

The study of the geological processes associated with the formation of the ocean floor is fundamental to understand hydrothermal systems in terms of genesis, evolution, duration, cyclicity and spatial distribution as well as the colonization of these systems by living beings. In this project, we will use music to communicate about these processes to the public. The ocean floor is constructed by the interaction in time and space of three major processes: volcanism, tectonics and hydrothermalism. This last process is fundamental in the cooling and transformation, through the alteration of rocks, of the oceanic floor. Finally, sedimentation gradually covers the floor constructed by these three processes. Instabilities and landslides will affect the sedimentary cover and volcanoes and thus modify the underwater landscape. Time is fundamental when studying these processes. The ocean floor is constantly changing. Eruptions occur suddenly, last a few hours or extend over several days. A fault can rupture, producing earthquakes which may cause major landslides. All of these processes have a direct impact on the distribution and dynamics of hydrothermal circulations. An earthquake can open new fractures allowing seawater to penetrate into the crust, creating new vents. Another earthquake or volcanic eruption can seal these circulation paths, leading to the cessation of the outflow of fluids and the death of the associated ecosystem. The lifespan of a hydrothermal site is therefore strongly dependent on this dynamic. In this dynamic landscape, organisms evolve, move, colonize chimneys, multiply and eventually disappear when the fluid output stops. It is this constant mutation, on variable time scales, ranging from a few years to a few tens of thousands of years, that we wish to transcribe into music. Over the years, our team built an immense collection of images of the ocean floor and hydrothermal vents. These documents are rarely released to the general public. When exposed, they often speak little because even if the images are beautiful and impressive, the processes and time scales behind them are difficult to grasp. The idea of this project is to create a musical piece telling the stories of the formation of the ocean floor and of hydrothermal fields, on different time scales. We will tell a story of a changing landscape, of the creation of oases of life, from their beginning until the death of the colonies. We will interact with the composer through videos and images, accompanied by explanations of the processes. The exchanges allow the composer to explain his musical choices which will be his way of perceiving these complex developments. The production of this piece will be entrusted to the orchestra of the University of Brest. Here too, the interaction between researchers and the musicians is at the heart of the project. Discussions and scientific explanations of the images will accompany the musical work. The work will therefore be the result of group construction. It will be presented during the university Art & Science festival and during scientific events or maritime festivals.

Abstract Using bathymetry and ROV dives, we investigate two successive flip‐flop detachment faults (D1 active, D2 older) in the near‐amagmatic 64°35′E region of the SWIR. Kilometer‐sized benches on the upper slopes of D1 footwall form the D1 degraded breakaway. Scarps at the top expose the D2 fault zone with deformed serpentinized peridotite, sigmoidal phacoids, planar fractures, and serpentinite microbreccia/gouge horizons. Two ROV sections of the D1 footwall show contrasting deformation styles, corresponding to distinct morphological domains, which relate to contrasting fault and footwall strength. One section documents corrugations, outcrops dominated by sigmoidal phacoids, and planar fractures with thin, discontinuous serpentinite microbreccia/gouge horizons. ROV dives in this corrugated domain show that NNE‐trending km‐spaced ridges and WNW‐trending narrow benches in the shipboard bathymetry correspond, respectively, to broad undulations (mega‐corrugations) of the D1 fault and to several antithetic minor normal faults (cumulated horizontal offset of ∼285 m). The other section, lacking corrugations, broad ridges, and antithetic fault, has thicker and more continuous serpentinite microbreccia/gouge horizons, indicating a weaker fault. The abundance of such weak gouges probably reflects hydrous fluid availability during deformation. We link mega‐corrugations in the western domain and km‐scale lobes of D1 emergence to a broad detachment damage zone with up to ∼600 m‐thick mega‐phacoids of less deformed serpentinized peridotite. Small antithetic normal faults in the corrugated domain are interpreted as due to bending forces in the D1 footwall. Our findings highlight the three‐dimensional, non‐planar structural and morphological variability of the exhumed D1 detachment fault zone along the ridge‐axis.

Secondary hydrothermal circulation is created by local entrainment of cold seawater into the shallow subseafloor by the intense discharge of high temperature fluids through focused vents at black smoker hydrothermal fields. To improve our understanding of secondary hydrothermal circulation, we tested a new instrumentation consisting of a small-scale (~150 m aperture) four-hydrophone network connected to single datalogger, which could potentially record very small and shallow microearthquakes associated with near-seafloor upflow of hydrothermal fluids and mineral precipitation. We report on a one-year catalog of near-seafloor microearthquakes, recorded with this hydrophone network deployed around the Tour Eiffel black smoker vent site of the Lucky Strike hydrothermal field. We detect 740 shallow microearthquakes (depth between 0 and 300 m, local magnitude between -4.0 and -0.5) that may be generated by reaction-driven cracking due to anhydrite precipitation in the shallow subseafloor. The seismicity rate is low, and the events are not focused around the black smokers, suggesting that heating of entrained seawater and anhydrite precipitation is not prevalent near these vents. This hypothesis is supported by time-series analysis of diffuse fluid samples from the Tour Eiffel site, which show little to no chemical evidence for anhydrite precipitation.

Large-offset detachment faults are commonly observed at slow-spreading mid-ocean ridges (MORs), typically in areas with a moderate to low magma supply (e.g., 13º20'N on the Mid-Atlantic Ridge). Detachments are also found at nearly amagmatic sections of ultraslow MORs (e.g., 64ºE on the Southwest Indian Ridge), where the seismogenic lithosphere is unusually thick (> 15 km). There, detachments of opposing polarity form in sequence and cross-cut each other in a "flip-flop" regime. Prior studies have shown that marked strength contrasts, resulting from reduced cohesion and/or friction in fault zones, promote stable detachments. Here we present 2-D thermo-mechanical models based on geological observations to examine how strength contrasts between fault zones and the adjacent lithosphere impact the modes of faulting at an ultraslow and nearly amagmatic ridge axis.We model the brittle lithosphere as a Mohr-Coulomb elasto-plastic material, where cohesion and friction diminish with increasing plastic strain. We explore a broad range of cohesion and friction contrasts between deformed and intact material. We also consider the influence of a strong, viscous lower lithosphere on the brittle deformation of the upper lithosphere by comparing simulations that use a dry olivine flow law with models where the brittle lithosphere sharply transitions into a low-viscosity asthenosphere. Fluid circulation in the shallow axial lithosphere is also considered, parameterizing both the cooling and the mechanical effect of hydrothermal circulation.Our simulations produce three distinct regimes: (1) sequential development of horsts bound by two active antithetic faults, (2) formation of intersecting “flip-flopping” detachments, (3) runaway detachments. The latter case describes models in which a single detachment remains active. In nature, this endmember case is not observed, probably because it results in an excessive migration of the detachment toward its hanging wall. We show that these 3 regimes transition over a narrow range of cohesion and friction contrasts between deformed and intact material (the contrast in friction coefficient over which our simulations transition from regimes 1 to 3 is only 0.1- 0.2). Distributed footwall damage produces antithetic proto-faults, but their ability to mature as major seafloor-breaching faults depends on the degree of rheological weakening. A stronger lower lithosphere promotes such distributed faulting and modifies the onset of the persistent detachment regime to greater strength contrasts. The impact of hydrostatic fluid pressure on tectonic styles is relatively minor compared to fault weakening.The results of these simulations are consistent with an analytical force balance model that compares the (localizing) loss of fault strength in the detachments to the (delocalizing) flexural force that develops in the surrounding lithosphere. Detachments persist when the magnitude of fault strength loss exceeds the maximum bending force. We find that runaway detachments require a total loss of integrated strength in excess of 1.5e12 N.m, equivalent in our models to a drop in friction coefficient by ~0.25–0.3 in fault zones. Thus, even a moderate frictional weakening, such as that allowed by the presence of lizardite in the fault zone (frictional strength of 0.45) enables large-offset (>15 km) faulting.

Oceanic detachment faults (ODFs) are critical drivers of plate separation in slow-to-ultraslow spreading mid-ocean ridges (MORs). Numerous previous studies have shown that the anatomy of ODFs varies significantly between more magmatic and nearly amagmatic ridges sections. More magmatic ODFs are typically dome-shaped, corrugated, and face volcanic seafloor on the hanging wall side, while the footwall exhumes upper crustal and mantle-derived rocks, including gabbro, serpentinized peridotite, basaltic breccia, and diabase. In contrast, nearly amagmatic ODFs (e.g., at 64°E Southwest Indian Ridge SWIR) are characterized by long (up to ~95 km) broad and smooth ridges with no visible corrugations in the shipboard bathymetry. These ODFs form in alternate polarity and exhume serpentinized peridotite to the seafloor on both plates. Here, we present the anatomy of the exposed fault zone in the footwall of D1, a young active ODF in the nearly amagmatic 64°35'E region of the SWIR, utilizing shipboard bathymetry, micro-bathymetry, and ROV dive observations to document their footwall geology, deformation patterns, and along-strike variations.The axial valley wall in the study area corresponds to the footwall of D1, and high-resolution bathymetry shows that it exposes two distinct domains. The western domain displays corrugations similar to those documented in more magmatic, domal ODFs, while the eastern domain is smooth. The western, corrugated domain also displays small offset ESE-trending antithetic normal faults and several hecto-to-kilometers-wide NNE-trending ridges, interpreted as mega-corrugations that formed due to hecto-to-kilometer-scale phacoids between linked fault splays in the detachment damage zone. These ridges and the antithetic minor faults are absent in the smooth eastern domain. Outcrop scale ROV dive observations show one significant difference in the geology of the exposed fault zone between the two domains: in the east, the fault zone comprises up to 10 m-thick intervals of serpentine microbreccia and chrysotile gouge, while in the west, these highly deformed horizons are a few decimeter-thick at the most, surrounding phacoids of less deformed serpentinites and therefore less pervasive at the outcrop scale. These observations suggest a stronger fault and footwall in the corrugated region. Microstructural observations also suggest that hydrous fluids facilitated the formation of the gouges and that non-brittle mechanisms (serpentine dissolution and precipitation) were involved.Compared with more magmatic, domal corrugated ODFs, the smooth eastern part of our study area exposes thicker and more pervasive intervals of cataclastic microbreccia and gouge. In contrast, previous work on domal ODFs shows that strongly deformed intervals there consist mostly of talc±tremolite±chlorite±serpentine schist. Experimental studies suggest that the frictional strength of talc and chrysotile gouge at the sample scale are comparable. However, the gouge outcrops in the eastern part of the D1 footwall are thick and promote the formation of a planar (smooth) exposed fault surface. By contrast, highly deformed talc-bearing schists at domal ODFs are found around meter to decameter-scale phacoids of less deformed rocks, which are probably the cause of the observed corrugations.

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)‐(H 2 O)‐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 H 2 O, 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 vast majority of the Earth’s volcanism takes place in the deep ocean along mid-ocean ridges (MORs), yet because it is difficult to detect and observe, it is also relatively poorly understood. MOR volcanism occurs where tectonic plates spread apart and mainly produces effusive basaltic fissure eruptions where dikes reach the surface. The character and frequency of volcanism varies greatly as a function of spreading rate and magma supply, as does the morphology of the ridge crest, the balance between volcanic construction and tectonic faulting, and the scales of ridge segmentation. The depth and continuity of magma storage in the crust beneath MORs also depends greatly on the local magma supply in space and time. The generation of MOR magmas is ultimately due to partial melting processes in the mantle where it rises beneath ridges due to plate spreading.

Hydrothermal convection in young oceanic lithosphere accounts for ~25% of the total global heat flow, and thus plays a critical role in Earth's thermal evolution. The permeability structure of the lithosphere is a key factor governing how efficiently heat tapped from magma bodies or hot upwelling mantle can be transferred to the overlying ocean. Drill hole measurements and laboratory experiments unambiguously show that permeability decreases with depth (i.e., pressure), either exponentially or through some power law relations. However, the impact of depth-decreasing permeability on the depth extent and heat output of seafloor hydrothermal systems has not been explored systematically.Here we present 2-D numerical simulations of hydrothermal convection treated as Darcy porous flow, with fluid properties corresponding to a 3.2 wt% NaCl-H2O mixture, and depth (i.e., pressure)-dependent permeability fields. We consider an empirical exponential dependence as well as a more recently proposed power-law-type dependence rooted in micromechanical modeling of experimental data. In reference simulations with uniform permeability, we find that, for a given basal temperature (TH) imposed at the model bottom, the hydrothermal heat output at the seafloor increases with permeability, but is largely independent of the depth extent of the model domain. On the other hand, in simulations with depth-decreasing permeability, the depth extent of hydrothermal convection (ZH) may be significantly lower than the height of the model domain. In such systems, heat extraction is intuitively more efficient when the heat source lies at a shallower depth. We find that the heat output in these simulations is primarily controlled by the harmonic mean of permeability in the hydrothermal system.To further quantify this finding, we investigate the relationship between our simulations' Rayleigh number (Ra, estimated from model inputs using the harmonically-averaged permeability) and Nusselt number (Nu, measured from simulation results). We find that the linear relationship Nu=Ra/Rac that is typical of porous convection holds for Ra > 103, with a critical Rayleigh number (Rac) on the order of 102. This relationship allows us to build an analytical model that predicts ZH, given the heat output, basal temperature (TH), and exponentially-decreasing permeability with depth Z: k= k0 e(-cZ). Fitting parameters against observed magma-fueled hydrothermal systems at mid-ocean ridges suggests that permeability at the seafloor (k0) is on the order of 10-12 - 10-11 m2, in agreement with independent estimates based on drill hole measurements and the poro-elastic tidal modulation of venting temperatures, and that the constant c is on the order of 1-4×10-3 m-1. Our findings further suggest that for convection to reach depths > 13 km, as has been proposed near oceanic detachment faults, permeability at the seafloor would need to be extremely large (k0> 10-10 m2). It remains unclear whether such conditions can be attained in the damage zone of a detachment fault.

Abstract 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.

Abstract 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.

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.

At ultra-slow spreading ridges, with full spreading rates less than similar to 20 mm/yr, spreading is accommodated both by highly spatially and temporally segmented magmatism, and tectonic extension along large-scale detachment faults that exhume ultramafic material to the seafloor. In the most magma-poor regions, detachment faulting alternates in polarity over time, producing a "flip-flopping" effect of subsequent detachment dips. The resulting seafloor in these regions displays a morphology termed "smooth seafloor" comprising elongate, broad ridges with peridotite/serpentinite lithologies. We conducted tomographic travel-time inversion of a 3-D wide-angle seismic data set acquired over a region of smooth seafloor around 64 degrees 30 ' E along the Southwest Indian Ridge (SISMOSMOOTH; Cruise MD199), to produce a seismic velocity volume through the crustal section and into the uppermost mantle. We observe patterns of velocity anomalies that correspond with variations in the bathymetry arising from the mode of spreading and are interpreted as changes in the degree of alteration with depth resulting from spatial and temporal variations in fluid-rock interaction, controlled by faulting and tectonic damage processes. The detachment faults do not show simple planar structures at depth but instead mirror the shapes of the bathymetric ridges that they exhume. Magmatic input is overall highly limited, but there is one region on the lower part of an exhumed detachment footwall where a thickness of volcanic material is observed that suggests a component of syn-tectonic volcanism, which could contribute to detachment abandonment. Ultra-slow spreading ridges are the slowest spreading type of mid-ocean ridge. At these mid-ocean ridges, instead of spreading through volcanism, the plate separation dominantly takes place along large shallow-dipping "detachment" faults. This process results in the formation of topographic ridges of "smooth seafloor" where mantle rocks are exposed. We use signals from sound sources at the sea surface recorded on receivers on the seabed to map variations in sound speed in the upper similar to 4 km below the seabed at an ultra-slow spreading location on the Southwest Indian Ridge. We observe patterns in the sound speed variation that we interpret in the context of spreading via detachment faults, which allow varying amounts of fluid to access the subsurface, leading to varying degrees of chemical alteration. At depth, the detachment faults show variations that mirror the shapes of the ridges they exhume, indicating that this mode of spreading is a strongly 3-D process. There is also evidence for some limited magma input in this area, some of which may have occurred during the end of the activity of a detachment fault. Ultra-slow spreading via detachment faulting producing "smooth seafloor" is a highly 3-D process Spatially and temporally variable fracturation and alteration of peridotites to serpentinites produces characteristic velocity patterns Localized lava flows and intrusions during the late stages of a detachment produce slow velocity anomalies in the upper similar to 1.5 km bsf

At ultra-slow ridges, tectonics, hydrothermalism, serpentinization and magmatism interact to build the oceanic crust. How this heterogenous crust forms and relates to faulting remains poorly understood, but is key for elucidating hydrothermal flow patterns and their implications for ocean-lithosphere element exchange. Along the melt-poor Southwest Indian Ridge (SWIR) at 64°30' East, crustal thickness varies across the ridge strike, with crustal thickening attributed to serpentinization extending downward along detachment faults, DFs. This observation calls into question the commonly assumed relationship between local crustal thickening and magma-supply increase. Here we use 2D numerical models to analyze how coupled tectonics, mantle melting, magma emplacement and serpentinization interact. Our model includes hydrothermal cooling, ocean loading, and the oceanic crust density. We reproduce the observed bathymetry at SWIR, 64°30'E, which is shaped by alternating DFs formed in flip-flop mode. Our results show that the offset and duration of DFs are controlled by ocean loading and crustal density. Importantly, shallow faulting and deeper mantle flow are coupled: long-lived DFs result in relatively slower mantle upwelling, lower melt supply, but crustal thickening due to deeper serpentinization, ~5 km, consistent with the observed thick ultramafic crust in nature. In between alternating DFs, mantle upwelling is faster, melt supply higher, and serpentinization shallower, < 2km. Since magmatic crustal thickness is overall very small, 1.5-2 km, changes in faulting-induced serpentinisation depth, are the main cause for observed variations in crustal thickness, 2-7 km. We conclude that, at melt-poor ridges, tectonics controls both crustal thickness variations and melt supply oscillations.

The Indomed-Gallieni supersegment (46-52 & DEG;E) of the Southwest Indian Ridge (SWIR) has been of interest as a melt-rich endmember of ultraslow spreading mid-ocean ridges, attributed to enhanced magmatism since 11 Ma. This provides an excellent opportunity to study the evolution and the impact of enhanced magmatism at mid-ocean ridges. Here we combine shipboard bathymetry and gravity data over the SWIR 46-53.5 & DEG;E. We illus-trate that the enhanced magmatism initiated in the western part of the Indomed-Gallieni supersegment with decreasing crustal productions and no propagation. In the eastern part, the enhanced magmatism propagated to the east at decreasing propagating velocities, producing an increasingly thick crust. The present propagation tip is at 51.9 & DEG;E,-30 km to the west of the Gallieni transform fault (TF). The most magmatically-robust area of the propagation has been located near 50.5 & DEG;E since-6 Ma. We hypothesize that a mantle melting anomaly with heterogenous upwelling rates and/or geometry was responsible for the change of the enhanced magmatism. The enhanced magmatism plays significant roles in ridge segmentation and seafloor spreading modes. The degree of along-axis melt focusing was lowered, causing the segmentation to be typical of slow spreading ridges. As magmatism dramatically decreases over 160 km along the ridge axis from the most robust (producing a 9.5-km-thick crust at 50.5 & DEG;E) to nearly-amagmatic near the Gallieni TF, this ridge section exhibits the full spectrum of ultraslow spreading modes, changing from magmatically-controlled (intense volcanism and almost no faulting) to tectonically-dominated (smooth ridges and extensive peridotites outcrops) modes.