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The intersection of the Foundation Plume with the Pacific-Antarctic Ridge is a key location in global geodynamics where a mantle plume is approached by and interacting with a fast-spreading mid-ocean ridge. Here, we discuss a comprehensive major and trace element and Sr-Nd-Pb isotope dataset of new and existing samples from the young Foundation Seamount Chain (<5 Ma) and adjacent section of the Pacific-Antarctic Ridge. We use the geochemistry of axial, off-axis and intraplate lavas to map the spatial extent of plume dispersal underneath the ridge as well as the internal zonation of the upwelling plume. We show that the unusual length, increased crustal thickness and occurrence of silicic rocks on the axis of the Foundation Segment are the direct result of plume being tapped by the axial melting zone. We demonstrate that the plume is not homogeneous but shows a HIMU-like (high time-integrated [sup 238]U/[sup 204]Pb) OIB (Ocean Island Basalt) component characterized by [sup 206]Pb/[sup 204]Pb of up to 20.5 in its center and a more EM1-like (Enriched Mantle one) OIB component characterized by low U/Pb and [sup 206]Pb/[sup 204]Pb but high Rb/Nb and [sup 87]Sr/[sup 86]Sr towards its edges. Plume entrainment leads to a high magma supply rate that fosters the formation of silicic rocks and triggers the lengthening of the segment over time. However, plume dispersal is not symmetric as the geochemical tracers for the OIB component are extending <100 km northwards but >300 km southwards. We relate this to the current plate tectonic framework in which the obliquity between the migrating ridge and the absolute plate motions induces a sub-axial asthenospheric flow that preferentially channels plume material southwards. [ABSTRACT FROM AUTHOR]

In a deforming partially molten rock, melt concentrates into a grain‐scale melt pocket aligned at a preferred orientation (melt‐preferred orientation, or MPO). However, observing this texture alone provides limited information on the 3D orientation and geometry of these melt pockets, which are critical parameters for estimating permeability. Here, we modeled the MPO of experimentally deformed peridotites by simulating melt streaks arising from melt pockets of various shapes and 3D orientations. The model aims to identify 3D distribution and characteristics of melt pockets that could account for the observed length, thickness, and the probability of melt streaks. Results show that melt pockets at preferred orientation exhibit greater length, thickness, and number density compared to those perpendicular. These results can be incorporated into the simulation of melt flow through individual melt pockets, which allows us to estimate the permeability corresponding to the observed MPO. We found that the permeability of vertically compressed peridotites increases with increasing compressive strain and a more elongated and thickened shape for melt pocket aligned at preferred orientation. The vertical permeability in the sample with 30% compressive strain is at least 40 times larger than that of an undeformed sample. For peridotites deformed under simple shear, the permeability exhibits an anisotropy of at least three. Such anisotropic permeability, coupled with the formation of melt‐rich bands and other melt channels, is believed to cause lateral melt focusing beneath mid‐ocean ridges. Plain Language Summary: The distribution of melt at the grain scale controls the permeability in partially molten rock. While observe melt streaks on thin sections show variations in length and thickness with respect to the orientation, the geometry and distribution of melt pockets in 3D are poorly constrained. Here, we use an improved statistical model to identify the dependence of melt pocket dimensions as functions of orientation. We further calculate melt flux through individual melt streaks and estimate permeability corresponding to the observed melt distribution texture. We found that deformation in the mantle can significantly accelerate melt extraction and potentially bend the melt flow using anisotropic permeability. Key Points: We parameterized the 3D shapes and orientations of melt pockets under the constraints of observed melt‐preferred orientation (MPO)Permeability in peridotites deformed under vertical compression increases with compressive strain and demonstrates anisotropy up to twoPermeability in peridotites deformed under simple shear demonstrates anisotropy of at least three [ABSTRACT FROM AUTHOR]

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 intersection of the Foundation Plume with the Pacific-Antarctic Ridge is a key location in global geodynamics where a mantle plume is approached by and interacting with a fast-spreading mid-ocean ridge. Here, we discuss a comprehensive major and trace element and Sr-Nd-Pb isotope dataset of new and existing samples from the young Foundation Seamount Chain (300 km southwards. We relate this to the current plate tectonic framework in which the obliquity between the migrating ridge and the absolute plate motions induces a sub-axial asthenospheric flow that preferentially channels plume material southwards.

Decades of research have resulted in characterization of the ocean floor manifestations of mid-ocean ridge (MOR) hydrothermal systems, yet numerical models accounting for the connections between heat transfer, hydrology and geochemistry have been slow to develop. The Thermo-hydro-chemical code ToughReact can be used to describe the coupled effects of fluid flow, heat transfer, and fluid-rock chemical interactions that occur in MOR systems. We describe the results of 2-dimensional simulations of steady state flow in fractured diabase with mineral-fluid chemical reactions. Basal heating and specified permeability yield maximum temperature of 400°C. Total fluid flux and high fracture flow velocities are in accord with observations. Fluid chemistry, mineralogical changes and 87Sr/86Sr ratios can be compared to observations to assess and calibrate models. Simulated high temperature fracture fluids have Mg and SO4 near zero, elevated Ca and 87Sr/86Sr of about 0.7040. Total alteration is 10%–50% for simple models of spreading. Anhydrite forms mainly near the base of the upwelling zone and results in substantial local fracture porosity reduction. A calibrated model is used to predict how Sr isotopes and other features of altered oceanic crust would be different in the Cretaceous (95 Ma) early Proterozoic (1,800 Ma) and Archean (3,800 Ma), when seawater may have had high Ca and Sr concentrations, lower pH, higher temperature, and lower Na, Mg, and SO4. The simulations are offered as a start on what ultimately may require a longer-term community effort to better understand the role of MOR thermo-hydro-chemical systems in Earth evolution.

The redox state of the convective asthenospheric mantle governs the speciation of volatile elements such as carbon and, therefore, influences the depth at which (redox) melting can occur, with implications for seismic signals. Geophysical observations suggest the potential presence of carbonatite melts at a depth of 200–250 km. However, thermodynamic models indicate that the onset of (redox) melting would occur at 100–150 km for a mantle with 3%–4% of Fe3+/∑Fe. Here, we present a new oxybarometer that is based on the V/Sc exchange coefficient between olivine and the melt, which is insensitive to surficial alteration, volatile degassing, electron exchange reactions and fractional crystallization. By applying this method to primary mid‐ocean ridge basalts (MORBs) from Southwest Indian Ridge and East Pacific Rise, we demonstrate that the average oxygen fugacity (fo2) of MORBs corrected for the depth of formation is 0.78 ± 0.26 (1σ) log units above the fayalite‐magnetite‐quartz (FMQ) buffer, which is slightly more oxidized than previously estimated (near FMQ buffer). Our findings indicate that the convective asthenospheric mantle exhibits higher oxygen fugacity than the continental lithospheric mantle. Along an adiabat, carbonatitic melts can form from a CO2‐bearing source at a depth of 200–250 km, explaining the asthenospheric mantle's electrical conductivity and seismic velocity anomaly. Plain Language Summary: The oxidized form of carbon, carbonate, plays a pivotal role in the melting of the deep mantle by substantially lowering the mantle solidus and instigating the formation of carbonatite melt. The presence of a high‐conductivity and seismic low‐velocity layer at depths of 220–300 km in the asthenospheric mantle suggests the presence of carbonatite melts. Nevertheless, the traditional view posits that the asthenospheric mantle at these depths is reduced, with carbon primarily in the form of diamonds, making it unlikely to induce mantle melting and carbonatite melt formation. In this study, we present a novel oxybarometer based on the V/Sc exchange coefficient between olivine and melt and calculate the oxygen fugacity of the Southwest Indian Ridge and East Pacific Rise MORBs. Our findings reveal that global MORBs and their mantle sources are more oxidized than previously thought. In such an oxidized environment, the conversion of diamond to carbonate and the formation of carbonatite melts occur at depths of 220–250 km, providing a reasonable explanation for the high conductivity and low‐velocity layer in the deep asthenosphere. Key Points: The exchange of V/Sc between the olivine and melts can serve as a robust oxybarometerMid‐ocean ridge basalts are oxidized with oxygen fugacity of FMQ + 0.46 ± 0.26Carbonatitic melts can form at a depth of 200–250 km beneath mid‐ocean ridges [ABSTRACT FROM AUTHOR]

Mid-ocean ridge and oceanic transforms are among the most prominent features on the seafloor surface and are crucial for understanding seafloor spreading and plate tectonic dynamics, but the deep structure of the oceanic lithosphere remains poorly understood. The large number of microearthquakes occurring along ridges and transforms provide valuable information for gaining an in-depth view of the underlying detailed seismic structures, contributing to understanding geodynamic processes within the oceanic lithosphere. Previous studies have indicated that the maximum depth of microseismicity is controlled by the 600-°C isotherm. However, this perspective is being challenged due to increasing observations of deep earthquakes that far exceed this suggested isotherm along mid-ocean ridges and oceanic transform faults. Several mechanisms have been proposed to explain these deep events, and we suggest that local geodynamic processes (e.g., magma supply, mylonite shear zone, long-lived faults, hydrothermal vents, etc.) likely play a more important role than previously thought. [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 ultraslow‐spreading Mohns Ridge is a key supersegment of the Arctic Mid‐Ocean system, where it represents a boundary between the Jan Mayen hotspot in the south and the highly anomalous Knipovich Ridge to the north. Understanding the timing and mode of Plio‐Pleistocene seafloor spreading along this ridge segment is critical for establishing the recent geodynamic evolution of the Norwegian‐Greenland Sea. To investigate magmatic accretion at this ultraslow spreading ridge, we collected samples from the Schulz Massif, which is located off‐axis at 73.4°N and exposes gabbroic intrusives and mantle peridotite. The petrology and petrography of these samples indicate that the exposed crustal section underwent multiple episodes of magmatism, which are characterized by distinct crystal sizes and geochemistry. To calibrate the age of the seafloor, we combined high‐resolution and high‐precision single zircon U‐Pb geochronology. Our data suggest that seafloor spreading has been nearly symmetrical for the last ∼4.6 Myr with a time‐averaged half‐spreading rate of ∼7.4 mm yr−1. Crystal size analysis of olivine in porphyric intrusions suggests that the crustal section was fed crystal‐laden melts with recurrence rates predicted to stabilize fault‐dominated seafloor spreading. Our combined geochronological and crystal size approach gives a critical perspective on the mode of seafloor spreading in the Mohns Ridge and allows insights into accretionary mechanisms and crustal structures during symmetric seafloor spreading. Key Points: We investigated a unique suite of lower crustal rocks recovered from a tectonic window off‐axis of the ultraslow‐spreading Mohns RidgeZircon U‐Pb geochronology suggests 4.6 Myr of nearly symmetrical seafloor spreading at a time‐averaged half‐spreading rate of ∼7.4 mm yr−1Small‐scale magmatic injections were important for crustal construction [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]

Abstract In a deforming partially molten rock, melt concentrates into a grain‐scale melt pocket aligned at a preferred orientation (melt‐preferred orientation, or MPO). However, observing this texture alone provides limited information on the 3D orientation and geometry of these melt pockets, which are critical parameters for estimating permeability. Here, we modeled the MPO of experimentally deformed peridotites by simulating melt streaks arising from melt pockets of various shapes and 3D orientations. The model aims to identify 3D distribution and characteristics of melt pockets that could account for the observed length, thickness, and the probability of melt streaks. Results show that melt pockets at preferred orientation exhibit greater length, thickness, and number density compared to those perpendicular. These results can be incorporated into the simulation of melt flow through individual melt pockets, which allows us to estimate the permeability corresponding to the observed MPO. We found that the permeability of vertically compressed peridotites increases with increasing compressive strain and a more elongated and thickened shape for melt pocket aligned at preferred orientation. The vertical permeability in the sample with 30% compressive strain is at least 40 times larger than that of an undeformed sample. For peridotites deformed under simple shear, the permeability exhibits an anisotropy of at least three. Such anisotropic permeability, coupled with the formation of melt‐rich bands and other melt channels, is believed to cause lateral melt focusing beneath mid‐ocean ridges.

Abstract 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.

Owing to their abundance and relative availability on Earth's seafloor, mid‐ocean ridge basalts (MORBs) have a well‐defined chemical element budget, reflected by the low standard deviation associated with typical normal MORB (N‐MORB) composition. However, the exact mechanisms leading to magma differentiation and MORB generation remain debated, which hinders our ability to evaluate MORB parental magma composition. In this study, we leverage the predictive power of the BDD21 numerical framework to obtain a representative trace element budget of parental MORB magma and assess its ability to fractionate into the N‐MORB composition. Utilizing revised parameterizations for mineralogy, melting, and partitioning, we couple BDD21 with numerical simulations of a MOR system driven by seafloor spreading in which we track the evolution of partial melting, mineral modal abundances, and concentrations of incompatible elements. Parental magma compositions are determined once simulations reach a steady state, and magma chamber replenishment models are employed to predict the trace element budget of the erupted liquid. We explore a range of geophysical and geochemical parameters to evaluate their effect on computed trace element concentrations. Previous magma chamber replenishment models are extended to account for multiple crystallization events and melt‐crystal interaction. Modeling outcomes suggest that petrologically constrained fractionation of parental magma compositions obtained through BDD21 yields glass compositions compatible with the N‐MORB budget. Nevertheless, our results show a systematic underestimation of Sr concentration, indicating the presence of recycled oceanic crust in the MORB source region. Plain Language Summary: Beneath Earth's oceans, tectonic plates spread apart at mid‐ocean ridges, generating volcanic regions whose magmatic output forms Earth's oceanic crust. The rocks composing the oceanic crust represent the solidification of magma produced by partially melting deeper mantle rocks. Accordingly, they carry a message about the nature of Earth's interior. To decipher this message, scientists recover rock samples from the seabed and analyze their composition to infer the nature of that otherwise inaccessible deep layer. Unfortunately, the chemical inventories obtained for these rocks reveal that significant processing of the magma occurs as it ascends toward the surface. Therefore, the relationship between crustal rocks and their deep parental counterparts is indirect. Here, we address this complexity using a two‐step process. First, we simulate the generation and composition of partial melts derived from mantle peridotites (archetypal upper‐mantle rocks) using a mid‐ocean ridge numerical model. Second, we employ models of magma chamber processes to account for the expected magmatic evolution of the generated liquid. By optimizing and extending existing magma chamber models, we obtain an accurate representation of rock compositions sampled from Earth's seabed. Our findings provide a new opportunity to decipher the nature of Earth's interior. Key Points: Refined the BDD21 peridotite melting and melt chemistry framework to reproduce geochemical observations of the mid‐ocean ridge systemImproved calibration of peridotite melting parameterization revises solidus and yields appropriate crustal thickness for mid‐ocean ridgesDepleted mantle compositions underestimate Sr concentration in basalts, requiring a prevalent contribution from recycled oceanic crust [ABSTRACT FROM AUTHOR]

Partial melting beneath a mid‐ocean ridge creates a chemically depleted layer in the uppermost mantle. This chemical depletion lowers the density of the lithosphere compared with the unmelted mantle. Furthermore, dehydration associated with depletion leads to an increase in mantle viscosity that may affect the structure and dynamics of the oceanic plate. Previous studies have mainly considered the formation of this depleted upper‐mantle layer in a two‐dimensional mid‐ocean ridge setting, leaving the dynamics of mid‐ocean ridge–transform fault systems largely unexplored. In this study, spatial variations in the degree of depletion in the uppermost mantle are predicted for a mid‐ocean ridge–transform fault system using a three‐dimensional thermomechanical model. The degree of depletion generally increases with increasing half‐spreading rate. Less depletion is predicted beneath the transform fault and fracture zone compared with the surrounding mantle. Lateral differences in the degree of depletion in the ridge‐parallel direction are reduced when plastic yielding is considered. The degree of variation in the predicted depletion is related to the transform fault length especially at a low spreading rate, thereby suggesting that the large scatter in observed abyssal peridotite compositions with slow spreading rates could be partly attributed to the length of the fault. Plain Language Summary: Oceanic plates are a key component in the theory of plate tectonics. Partial melting beneath a mid‐ocean ridge, where oceanic plates are formed, results in residual melt‐depleted mantle that is less dense and more viscous than the unmelted mantle. It is therefore important to understand the degree of depletion in this tectonic setting to better constrain the structure and dynamics of oceanic plates. Previous studies have mainly considered the formation of depleted mantle in a two‐dimensional mid‐ocean ridge setting. However, this layer of depleted mantle likely undergoes a more complex formation process if transform faults, which connect neighboring mid‐ocean ridges, are located nearby. In this study, spatial variations in the degree of depletion in the uppermost oceanic mantle are predicted for a mid‐ocean ridge–transform fault system using three‐dimensional computational simulations. The degree of depletion generally increases with the rate of plate motion. The degree of depletion is lower beneath transform faults and their extension compared with the surrounding mantle. Key Points: Thermomechanical modeling is used to predict the degree of mantle depletion in a mid‐ocean ridge–transform fault systemThe degree of depletion can be significantly lower beneath the transform fault and fracture zone compared with the surrounding mantleModeling results indicate that chemical heterogeneity due to depletion changes the structure and dynamics of the oceanic plate [ABSTRACT FROM AUTHOR]

Two‐phase flow, a system where Stokes flow and Darcy flow are coupled, is of great importance in the Earth's interior, such as in subduction zones, mid‐ocean ridges, and hotspots. However, it remains challenging to solve the two‐phase equations accurately in the zero‐porosity limit, for example, when melt is fully frozen below solidus temperature. Here we propose a new three‐field formulation of the two‐phase system, with solid velocity (vs), total pressure (Pt), and fluid pressure (Pf) as unknowns, and present a robust finite‐element implementation, which can be used to solve problems in which domains of both zero porosity and non‐zero porosity are present. The reformulated equations include regularization to avoid singularities and exactly recover to the standard single‐phase incompressible Stokes problem at zero porosity. We verify the correctness of our implementation using the method of manufactured solutions and analytic solutions and demonstrate that we can obtain the expected convergence rates in both space and time. Example experiments, such as self‐compaction, falling block, and mid‐ocean ridge spreading show that this formulation can robustly resolve zero‐ and non‐zero‐porosity domains simultaneously, and can be used for a large range of applications in various geodynamic settings. Plain Language Summary: The Earth's interior is hot and consists of highly viscous rocks that are deformable over geological time scales. In some regions, such as mid‐ocean ridges or hotspots, mantle temperature is high enough such that mantle rocks become partially molten, forming low‐viscosity magma. Migration of magma toward Earth's surface is a process of a low‐viscosity fluid flowing in a deformable high‐viscosity matrix, which is a two‐phase system. Investigating magma migration requires to solve the fully coupled two‐phase system. However, at lower temperatures magma disappears because of solidification, such that the system reduces to a single (solid) phase, which is mathematically problematic because the linear system for the coupled two‐phase problem is singular and under‐determined in this case. In this work, we propose a new formulation for the coupled two‐phase system, which works for both single‐phase and two‐phase cases. The new formulation is implemented and benchmarked against a set of analytical solutions. Using selected example experiments, which have well‐known solutions and contain zero‐porosity domains, we demonstrate that the model works consistently and robustly for a large range of geodynamic settings. Key Points: We propose a new three‐field two‐phase formulation that works consistently in the zero‐porosity single‐phase limitBenchmarks demonstrate that our implementation on a Q1‐P0 element has the expected spatial and temporal convergence rateThe approach is robust and suitable to resolve geological problems in various settings including vanishing porosity [ABSTRACT FROM AUTHOR]

The mantle plumes modify geophysical and geochemical features along and across mid‐ocean ridges. Despite abundant studies of plume‐ridge interaction, few geodynamic studies focus on the Arctic Ocean. The Jan Mayen Hotspot is located at the southern end of the Mohns Ridge and offset by the Jan Mayen Transform Fault, which creates an ideal area to study plume‐ridge‐transform fault interaction at the ultra‐slow spreading ridge. Through analyzing geophysical observations, we revealed that the M factor and crustal thickness decrease and the axial relief increases northeastward along the Mohns Ridge within a distance of ∼370 km to the Jan Mayen Hotspot. Combined with modeling results, the properties of the Jan Mayen plume were estimated, which has a diameter of 75 km, a temperature anomaly of 100°C, and a buoyancy flux of 0.22 Mg/s. Additionally, our model results indicate that the along‐ridge dispersion of plume is slightly enhanced by the transform fault. Plain Language Summary: Mid‐ocean ridges are places in the ocean where the Earth's plates separate and new crust is formed. Hotspots and oceanic transform faults modify the seafloor topography and chemical composition of the igneous crust. In the Arctic Ocean, the Mohns Ridge is located between 70°N and 74°N, with the nearby Jan Mayen Hotspot and Jan Mayen Transform Fault to the south, making it a natural laboratory for the investigation of plume‐ridge‐transform interaction. This study combines different types of geophysical data and numerical models to quantify how plumes and transform faults influence the seafloor topography and crustal structure at the ultra‐slow spreading Mohns Ridge. The findings reveal that the Jan Mayen mantle plume affects the seafloor togography and crustal thickness over several hundreds of kilometers along the Mohns Ridge. The presence of an oceanic transform fault above the mantle plume reduces the thermal anomaly for adjacent ridge segments but enhances the along‐ridge plume dispersion. By comparing the observed data with model results, the properties (i.e., diameter, temperature amomaly, and buoyancy flux) of the Jan Mayen mantle plume were estimated. This study provides valuable insights into how plumes and transform faults modify ridge spreading processes. Key Points: The crustal thickness, axial relief, and the fraction of magma emplacement to plate separation (M) along the Mohns Ridge are calculatedNumerical models estimate a diameter of 75 km, a thermal anomaly of 100°C, and a buoyancy flux of 0.22 Mg/s for the Jan Mayen mantle plumePlume‐ridge interactions affect along‐ridge topography and crustal thickness, and transform faults could enhance the interaction distance [ABSTRACT FROM AUTHOR]

Abstract The ultraslow‐spreading Mohns Ridge is a key supersegment of the Arctic Mid‐Ocean system, where it represents a boundary between the Jan Mayen hotspot in the south and the highly anomalous Knipovich Ridge to the north. Understanding the timing and mode of Plio‐Pleistocene seafloor spreading along this ridge segment is critical for establishing the recent geodynamic evolution of the Norwegian‐Greenland Sea. To investigate magmatic accretion at this ultraslow spreading ridge, we collected samples from the Schulz Massif, which is located off‐axis at 73.4°N and exposes gabbroic intrusives and mantle peridotite. The petrology and petrography of these samples indicate that the exposed crustal section underwent multiple episodes of magmatism, which are characterized by distinct crystal sizes and geochemistry. To calibrate the age of the seafloor, we combined high‐resolution and high‐precision single zircon U‐Pb geochronology. Our data suggest that seafloor spreading has been nearly symmetrical for the last ∼4.6 Myr with a time‐averaged half‐spreading rate of ∼7.4 mm yr−1. Crystal size analysis of olivine in porphyric intrusions suggests that the crustal section was fed crystal‐laden melts with recurrence rates predicted to stabilize fault‐dominated seafloor spreading. Our combined geochronological and crystal size approach gives a critical perspective on the mode of seafloor spreading in the Mohns Ridge and allows insights into accretionary mechanisms and crustal structures during symmetric seafloor spreading.

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 (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.

The unique ecosystems and biodiversity associated with mid-ocean ridge (MOR) hydrothermal vent systems contrast sharply with surrounding deep-sea habitats, however both may be increasingly threatened by anthropogenic activity (e.g., mining activities at massive sulphide deposits). Climate change can alter the deep-sea through increased bottom temperatures, loss of oxygen, and modifications to deep water circulation. Despite the potential of these profound impacts, the mechanisms enabling these systems and their ecosystems to persist, function and respond to oceanic, crustal, and anthropogenic forces remain poorly understood. This is due primarily to technological challenges and difficulties in accessing, observing and monitoring the deep-sea. In this context, the development of deep-sea observatories in the 2000s focused on understanding the coupling between sub-surface flow and oceanic and crustal conditions, and how they influence biological processes. Deep-sea observatories provide long-term, multidisciplinary time-series data comprising repeated observations and sampling at temporal resolutions from seconds to decades, through a combination of cabled, wireless, remotely controlled, and autonomous measurement systems. The three existing vent observatories are located on the Juan de Fuca and Mid-Atlantic Ridges (Ocean Observing Initiative, Ocean Networks Canada and the European Multidisciplinary Seafloor and water column Observatory). These observatories promote stewardship by defining effective environmental monitoring including characterizing biological and environmental baseline states, discriminating changes from natural variations versus those from anthropogenic activities, and assessing degradation, resilience and recovery after disturbance. This highlights the potential of observatories as valuable tools for environmental impact assessment (EIA) in the context of climate change and other anthropogenic activities, primarily ocean mining. This paper provides a synthesis on scientific advancements enabled by the three observatories this last decade, and recommendations to support future studies through international collaboration and coordination. The proposed recommendations include: i) establishing common global scientific questions and identification of Essential Ocean Variables (EOVs) specific to MORs, ii) guidance towards the effective use of observatories to support and inform policies that can impact society, iii) strategies for observatory infrastructure development that will help standardize sensors, data formats and capabilities, and iv) future technology needs and common sampling approaches to answer today’s most urgent and timely questions.

The 4.5 km deep IDDP-2 drill hole was drilled at Reykjanes, Iceland, in an active seawater-recharged hydrothermal system on the landward extension the Mid-Atlantic Ridge. Drilling targeted a well-defined hydrothermal up-flow zone feeding the Reykjanes geothermal reservoir, which produces fluids compositionally equivalent to basalt-hosted deep sea hydrothermal vents. Spot cores recovered from depths between 3,648 to 4,659 m depths consist of pervasively altered sheeted diabase dikes. The shallowest core has an alteration mineral assemblage similar to the producing geothermal reservoir but is being overprinted by the underlying Hornblende Zone assemblage dominated by labradorite and hornblende. The deepest cored section (4,634–4,659 m) retains a diabasic texture, but all primary minerals are replaced or have changed composition. Plagioclase ranges from An30 to An99 and igneous augite is replaced by intergrown hornblende, clinopyroxene, and orthopyroxene. Hydrothermal biotite and potassium feldspar formed locally due to reaction with a phase-separated brine, indicated by co-existing vapor-rich and salt + vapor-rich fluid inclusions. Seawater-derived supercritical hydrothermal fluid entering the bottom of the bore hole actively phase separates due pressure drop controlled by the fluid levels in the drill hole. Felsic veins, present in trace amounts, record a continuous transition from magmatic to hydrothermal conditions, including incipient hydrous melting. The vertical changes observed in mineralogy and mineral chemistry indicate that fluids from the deep high temperature reaction zone can undergo significant cooling and reaction with host rocks along their path to the seafloor.

Abstract Partial melting beneath a mid‐ocean ridge creates a chemically depleted layer in the uppermost mantle. This chemical depletion lowers the density of the lithosphere compared with the unmelted mantle. Furthermore, dehydration associated with depletion leads to an increase in mantle viscosity that may affect the structure and dynamics of the oceanic plate. Previous studies have mainly considered the formation of this depleted upper‐mantle layer in a two‐dimensional mid‐ocean ridge setting, leaving the dynamics of mid‐ocean ridge–transform fault systems largely unexplored. In this study, spatial variations in the degree of depletion in the uppermost mantle are predicted for a mid‐ocean ridge–transform fault system using a three‐dimensional thermomechanical model. The degree of depletion generally increases with increasing half‐spreading rate. Less depletion is predicted beneath the transform fault and fracture zone compared with the surrounding mantle. Lateral differences in the degree of depletion in the ridge‐parallel direction are reduced when plastic yielding is considered. The degree of variation in the predicted depletion is related to the transform fault length especially at a low spreading rate, thereby suggesting that the large scatter in observed abyssal peridotite compositions with slow spreading rates could be partly attributed to the length of the fault.

Abstract Owing to their abundance and relative availability on Earth's seafloor, mid‐ocean ridge basalts (MORBs) have a well‐defined chemical element budget, reflected by the low standard deviation associated with typical normal MORB (N‐MORB) composition. However, the exact mechanisms leading to magma differentiation and MORB generation remain debated, which hinders our ability to evaluate MORB parental magma composition. In this study, we leverage the predictive power of the BDD21 numerical framework to obtain a representative trace element budget of parental MORB magma and assess its ability to fractionate into the N‐MORB composition. Utilizing revised parameterizations for mineralogy, melting, and partitioning, we couple BDD21 with numerical simulations of a MOR system driven by seafloor spreading in which we track the evolution of partial melting, mineral modal abundances, and concentrations of incompatible elements. Parental magma compositions are determined once simulations reach a steady state, and magma chamber replenishment models are employed to predict the trace element budget of the erupted liquid. We explore a range of geophysical and geochemical parameters to evaluate their effect on computed trace element concentrations. Previous magma chamber replenishment models are extended to account for multiple crystallization events and melt‐crystal interaction. Modeling outcomes suggest that petrologically constrained fractionation of parental magma compositions obtained through BDD21 yields glass compositions compatible with the N‐MORB budget. Nevertheless, our results show a systematic underestimation of Sr concentration, indicating the presence of recycled oceanic crust in the MORB source region.

Abstract The mantle plumes modify geophysical and geochemical features along and across mid‐ocean ridges. Despite abundant studies of plume‐ridge interaction, few geodynamic studies focus on the Arctic Ocean. The Jan Mayen Hotspot is located at the southern end of the Mohns Ridge and offset by the Jan Mayen Transform Fault, which creates an ideal area to study plume‐ridge‐transform fault interaction at the ultra‐slow spreading ridge. Through analyzing geophysical observations, we revealed that the M factor and crustal thickness decrease and the axial relief increases northeastward along the Mohns Ridge within a distance of ∼370 km to the Jan Mayen Hotspot. Combined with modeling results, the properties of the Jan Mayen plume were estimated, which has a diameter of 75 km, a temperature anomaly of 100°C, and a buoyancy flux of 0.22 Mg/s. Additionally, our model results indicate that the along‐ridge dispersion of plume is slightly enhanced by the transform fault.

The mid-ocean rift in the Red Sea is one of the youngest rifting systems on Earth. Only recently, state-of-the-art methods and modern deep-sea instruments have been used to explore this young and unique volcanic system. During the first autonomous underwater vehicle surveys of the Red Sea Rift in Spring 2022, we collected multibeam bathymetry, backscatter, sub-bottom profiler data, and water column data over a 9 km long ridge segment in the Hadarba Deep between 22.49°N and 22.56°N to investigate the volcano-tectonic processes of this ultra-slow spreading segment (12 mm/year spreading rate). The high-resolution hydroacoustic data was used to (1) delineate and quantify the geometry of tectonic structures and individual lava flows, (2) define lava flow morphology and eruption style, (3) estimate relative ages of flows and features, and (4) retrace the evolution of the volcanic activity. In addition, the geochemistry of several young lava flows provides information on the relation between the different magma that supply these eruptions. About 90 eruptive units with variable sedimentary cover have been identified within the 43 km2 mapped region. The oldest lava flows are buried under 3 to 4.2 m of sediment, indicating ages of up to ~30 ka based on average sedimentation rate estimates (~14 cm/ka), while the youngest eruptions are covered by

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.

[Display omitted] • Competition between continental margins and mid-ocean ridges affects subduction initiation. • Weakened continental margins primarily promote subduction development along margins. • Model results shed new light on the distribution of Cenozoic subduction zones. Where subduction zones develop is a fundamental issue in subduction dynamics. Intra-oceanic settings and continental margins are the main regions for subduction development, and various tectonic settings are further proposed based on these two categories, including extinct mid-ocean ridges and passive continental margins. Mid-ocean ridges often locate adjacent to passive continental margins since they formed in successive extension processes. However, it remains enigmatic how these two tectonic settings compete in terms of subduction initiation and development. Here, we use a 2-D numerical model to investigate how subduction develops in the tectonic setting where mid-ocean ridges and passive continental margins are juxtaposed. The results exhibit three oceanic plate subduction types: continental margin subduction, ridge-inversed subduction, and a transitional type featured by double subduction along ridges and margins. The physical parameters that influence the formation of the three types of subduction, including the geometry of weakened passive margins, the cooling age of extinct mid-ocean ridges, the direction and magnitude of convergence rates, and the ridge-margin distance, are systematically investigated. Small cooling ages, pushing from oceanic plates, short ridge-margin distances, and strong continental margins favor ridge-inversed subduction, while large cooling ages, pushing from overriding plates, long ridge-margin distances, and weakened continental margins facilitate continental margin subduction. We emphasize the importance of preexisting weakness along the passive margins, which primarily promote subduction development along margins. Our model results shed new light on the natural examples of subduction development along extinct mid-ocean ridges and weakened passive margins. [ABSTRACT FROM AUTHOR]

Mapping and sampling four sections of the slow‐spreading Reykjanes Ridge provide insight into how tectonic and volcanic activity varies with distance from the Iceland plume. The studied areas are characterized by significant variations in water depth, lava chemistry, crustal thickness, thermal structure, and ridge morphology. For each study area, fault pattern and dimension, tectonic strain, seamount morphology, and density are inferred from 15 m‐resolution bathymetry. These observations are combined with geochemical analysis from glass samples and sediment thickness estimations along Remotely Operated Vehicle‐dive videos. They reveal that (a) tectonic and volcanic activity along the Reykjanes Ridge, do not systematically vary with distance from the plume center. (b) The tectonic geometry appears directly related to the deepening of the brittle/ductile transition and the maximum change in tectonic strain related to the rapid change in crustal thickness and the transition between axial‐high and axial valley (∼59.5°N). (c) Across‐axis variations in the fault density and sediment thickness provide similar widths for the neo‐volcanic zone except in regions of increased seamount emplacement. (d) The variations in seamount density (especially strong for flat‐topped seamounts) are not related to the distance from the plume but appear to be correlated with the interaction between the V‐shape ridges (VSR) flanking the ridge and the ridge axis. These observations are more compatible with the buoyant upwelling melting instability hypothesis for VSR formation and suggest that buoyant melting instabilities create many small magma batches which by‐pass the normal subaxial magmatic plumbing system, erupting over a wider‐than‐normal area. Plain Language Summary: Volcanic eruptions and faults growth are two important geologic processes taking place along seafloor spreading centers. Their variations in space and time are displayed in the morphology of the spreading centers. Investigating these morphological variations is key to understanding the deeper processes of the oceanic crust formation. South of Iceland, the Reykjanes Ridge is the location of increased volcanism due to the interaction between the mid‐ocean ridge and the Iceland hotspot. Using high‐resolution seafloor topographic data, chemical analyses of volcanic rock, and videos of the seafloor from a remotely operated vehicle, we investigated how volcanism and faulting change along the ridge. The increase in fault dimensions (height, length) with distance from the plume center is probably the result of the crust and mantle becoming cooler and stiffer and thus able to support larger faults. Fault density and thickness of the sediment covering the lava flows near the ridge axis are used to delimit the region of young volcanism. Seamounts often emplaced beyond that region. A peak in seamount abundance near 60°N suggests that the thick crust here is generated from numerous small batches of magma possibly resulting from a migrating instability in the melt production process beneath the axis. Key Points: The distance from the plume center is not the only factor controlling tectonic and volcanic activity along the Reykjanes RidgeFault dimensions are primarily controlled by the variation of crustal thermal structure with distance from the hotspotFlat‐topped seamount abundances peak where a V‐shaped ridge intersects the axis, consistent with a buoyant upwelling melting instability [ABSTRACT FROM AUTHOR]

Fissures and faults provide insight into how plate separation is accommodated by magmatism and brittle deformation during crustal accretion. Although fissure and fault geometry can be used to quantify the spreading process at mid‐ocean ridges, accurate measurements are rare due to insufficiently detailed mapping data. Here, fissures and faults at the fast‐spreading 9°50′N segment of the East Pacific Rise were mapped using bathymetric data collected at 1‐m horizontal resolution by autonomous underwater vehicle Sentry. Fault dip estimates from the bathymetric data were calibrated using co‐registered near‐bottom imagery and depth transects acquired by remotely operated vehicle Jason. Fissures are classified as either eruptive or non‐eruptive (i.e., cracks). Tectonic strain estimated from corrected fault heaves suggests that faulting plays a negligible role in the plate separation on crust younger than 72 kyr (<4 km from the ridge axis). Pre‐ and post‐eruption surveys show that most fissures were reactivated during the eruptions in 2005–2006. Variable eruptive fissure geometry could be explained by the frequency with which each fissure is reactivated and partially infilled. Fissure swarms and lava plateaus in low‐relief areas >2 km from the ridge are spatially associated with off‐axis lower‐crustal magma lenses identified in multichannel seismic data. Deep, closely spaced fissures overlie a relatively shallow portion of the axial magma lens. The width of on‐axis fissures and inferred subsurface dike geometry imply a ∼9‐year long diking recurrence interval to fully accommodate plate spreading, which is broadly consistent with cycle intervals obtained from estimates of melt extraction rates, eruption volumes, and spreading rate. Plain Language Summary: New oceanic crust is created by seafloor spreading at mid‐ocean ridges, where lava erupts as the tectonic plates spread apart. Plate separation is accommodated by a combination of slip along dipping faults and the opening of magma‐filled cracks, called dikes. We present bathymetric and profile mapping data collected from near the seafloor on a volcanically active portion of the East Pacific Rise near 9°50′N. The data show faults and fissures (i.e., open cracks) in remarkable detail, with meter‐scale resolution. A total of 707 fissures and 42 faults were identified and measured, suggesting that the amount of plate separation accommodated by faulting is minimal compared to that by dike intrusion causing open cracks. About one‐third of the fissures mapped are located within the region covered by the most recent eruptions in 2005–2006, and most of these fissures seem to have been reactivated from previous eruptions. Based on measurements of fissure width, we estimate that the interval between diking events is ∼9 years, which agrees with previous independent estimates. The analysis in this study reveals the relative importance of faults and fissures in seafloor spreading, and in the magmatic cycles that continuously re‐pave the ocean floor. Key Points: >700 fissures mapped at 1‐m bathymetric resolution at 9°50′N East Pacific Rise, most of which were active during the 2005–2006 eruptionsFault slip and crack opening accommodate <0.3% of plate separation, indicating the dominant role of diking at fast‐spreading ridgesFissures and lava flow located >2 km from the ridge axis are spatially associated with off‐axis lower crustal off‐axis magma lens [ABSTRACT FROM AUTHOR]

Abyssal hills are potential proxy records of mid‐ocean ridge faulting and volcanism, as well as the spreading rates and mantle properties that influence these processes. Satellite gravity‐based global prediction of abyssal hill root‐mean‐square (RMS) heights could provide such a proxy, but are broadly influenced off‐axis by pelagic sediment cover, which can lower their values by preferentially filling lows. Here I formulate a sediment‐corrected RMS height prediction by estimating an empirical relationship between mean RMS and sediment thickness as a function of spreading rate. I utilize these values to investigate regional variations in the spreading‐rate dependence of mean RMS across eight regions world‐wide, focusing on half spreading rates <40 mm/yr where spreading rate dependence is strongest. I find that regional variations in this relationship are significant, likely indicating heterogeneity in mantle properties between the different regions. Plain Language Summary: Abyssal hills are formed by faults and lavas at the mid‐ocean ridges, where tectonic plates spread apart, and cover much of the ocean floor. We can use their heights to infer the history and spatial pattern of the spreading process. Here I do an analysis with a global map of abyssal hill heights, applying corrections to account for the effect of sediment cover. I look at the relationship between spreading rate and height in eight different regions in the Atlantic, Pacific and Indian Oceans. Abyssal hills are typically larger where spreading rates are slower. However, changes in this trend can provide clues about differences in Earth properties beneath spreading ridges. My results did show important trend differences across these regions. I conclude that the Earth's properties below the mid‐ocean ridges change significantly from one region to the next. Key Points: Global predicted abyssal hill root‐mean square (RMS) heights can be corrected for sediment thickness using empirical relationships dependent on spreading rateSignificant regional variations are observed in the relationship between mean abyssal hill RMS height and spreading rateGeographic variations in abyssal hill roughness, independent of spreading rate, are likely indicative of heterogeneity in mantle properties [ABSTRACT FROM AUTHOR]

Hydrothermal vent fields located in the gap between known sites in Guaymas Basin and 21°N on the East Pacific Rise were discovered on the Alarcón Rise and in southern Pescadero Basin. The Alarcón Rise spreading segment was mapped at 1-m resolution by an autonomous underwater vehicle. Individual chimneys were identified using the bathymetric data. Vent fields were interpreted as active from temperature anomalies in water column data and observed and sampled during remotely operated vehicle dives. The Ja Sít, Pericú, and Meyibó active fields are near the eruptive fissure of an extensive young lava flow. Vent fluids up to 360 °C from Meyibó have compositions similar to northern East Pacific Rise vents. The Tzab-ek field is 850 m west of the volcanic axis, and active chimneys rise up to 33 m above a broad sulfide mound. The inactive field is 10 km north-northeast along the rift axis, and most sulfide chimneys are enriched in Zn and associated elements that are transported at lower temperature compared to the more Cu-rich active fields. In southern Pescadero Basin, the Auka field is on the margin of a sediment-filled graben at 3,670-m depth. Discharging fluids are clear, contain hydrocarbons, and have neutral pH, elevated salinity, and temperatures up to 291 °C. They have deposited massive mounds of calcite with minor sulfide. The fluids are compositionally similar to those in Guaymas Basin, produced by high-temperature basalt-seawater interaction followed by reaction with sediment. The paucity of sulfide minerals suggests subsurface deposition of metals.

Abstract Fissures and faults provide insight into how plate separation is accommodated by magmatism and brittle deformation during crustal accretion. Although fissure and fault geometry can be used to quantify the spreading process at mid‐ocean ridges, accurate measurements are rare due to insufficiently detailed mapping data. Here, fissures and faults at the fast‐spreading 9°50′N segment of the East Pacific Rise were mapped using bathymetric data collected at 1‐m horizontal resolution by autonomous underwater vehicle Sentry. Fault dip estimates from the bathymetric data were calibrated using co‐registered near‐bottom imagery and depth transects acquired by remotely operated vehicle Jason. Fissures are classified as either eruptive or non‐eruptive (i.e., cracks). Tectonic strain estimated from corrected fault heaves suggests that faulting plays a negligible role in the plate separation on crust younger than 72 kyr (2 km from the ridge are spatially associated with off‐axis lower‐crustal magma lenses identified in multichannel seismic data. Deep, closely spaced fissures overlie a relatively shallow portion of the axial magma lens. The width of on‐axis fissures and inferred subsurface dike geometry imply a ∼9‐year long diking recurrence interval to fully accommodate plate spreading, which is broadly consistent with cycle intervals obtained from estimates of melt extraction rates, eruption volumes, and spreading rate.

Abstract Mapping and sampling four sections of the slow‐spreading Reykjanes Ridge provide insight into how tectonic and volcanic activity varies with distance from the Iceland plume. The studied areas are characterized by significant variations in water depth, lava chemistry, crustal thickness, thermal structure, and ridge morphology. For each study area, fault pattern and dimension, tectonic strain, seamount morphology, and density are inferred from 15 m‐resolution bathymetry. These observations are combined with geochemical analysis from glass samples and sediment thickness estimations along Remotely Operated Vehicle‐dive videos. They reveal that (a) tectonic and volcanic activity along the Reykjanes Ridge, do not systematically vary with distance from the plume center. (b) The tectonic geometry appears directly related to the deepening of the brittle/ductile transition and the maximum change in tectonic strain related to the rapid change in crustal thickness and the transition between axial‐high and axial valley (∼59.5°N). (c) Across‐axis variations in the fault density and sediment thickness provide similar widths for the neo‐volcanic zone except in regions of increased seamount emplacement. (d) The variations in seamount density (especially strong for flat‐topped seamounts) are not related to the distance from the plume but appear to be correlated with the interaction between the V‐shape ridges (VSR) flanking the ridge and the ridge axis. These observations are more compatible with the buoyant upwelling melting instability hypothesis for VSR formation and suggest that buoyant melting instabilities create many small magma batches which by‐pass the normal subaxial magmatic plumbing system, erupting over a wider‐than‐normal area.

Abstract Abyssal hills are potential proxy records of mid‐ocean ridge faulting and volcanism, as well as the spreading rates and mantle properties that influence these processes. Satellite gravity‐based global prediction of abyssal hill root‐mean‐square (RMS) heights could provide such a proxy, but are broadly influenced off‐axis by pelagic sediment cover, which can lower their values by preferentially filling lows. Here I formulate a sediment‐corrected RMS height prediction by estimating an empirical relationship between mean RMS and sediment thickness as a function of spreading rate. I utilize these values to investigate regional variations in the spreading‐rate dependence of mean RMS across eight regions world‐wide, focusing on half spreading rates

Although positive buoyancy of young lithosphere near spreading centers does not favor spontaneous subduction, subduction initiation occurs easily near ridges due to their intrinsic rheological weakness when plate motion reverses from extension to compression. It has also been repeatedly proposed that inherited detachment faults may directly control the nucleation of new subduction zones near ridges subjected to forced compression. However, recent 3D numerical experiments suggested that direct inversion of a single detachment fault does not occur. Here we further investigate this controversy numerically by focusing on the influence of brittle‐ductile damage on the dynamics of near‐ridge subduction initiation. We self‐consistently model the inversion of tectonic patterns formed during oceanic spreading using 3D high‐resolution thermomechanical numerical models with strain weakening of faults and grain size evolution. Numerical results show that forced compression predominantly reactivates and rotates inherited extensional faults, shortening and thickening the weakest near‐ridge region of the oceanic lithosphere, thereby producing ridge swellings. As a result, a new megathrust zone is developed, which accommodates further shortening and subduction initiation. Furthermore, brittle/plastic strain weakening has a key impact on the collapse of the thickened ridge and the onset of near‐ridge subduction initiation. In contrast, grain size evolution of the mantle only slightly enhances the localization of shear zones at the brittle‐ductile transition and thus plays a subordinate role. Compared to the geological record, our numerical results provide new helpful insights into possible physical controls and dynamics of natural near‐ridge subduction initiation processes recorded by the Mirdita ophiolite of Albania. Plain Language Summary: Intra‐oceanic subduction comprises 40% of the subduction margins, but the mechanisms behind their initiation remain controversial. Although positive buoyancy of young oceanic plates compared to underlying mantle impedes the spontaneous subduction initiation near mid‐ocean ridges, the subduction can easily initiate there during the forced compression. In addition, the weak faults formed at the spreading stage facilitate the formation of subduction initiation. To clarify how subduction initiates at mid‐ocean ridges, we run 3D numerical models to self‐consistently explore the process of oceanic spreading followed by a gradual transition to forced plate convergence. Numerical results demonstrate that under such conditions, two previously diverging oceanic plates can subsequently collide and thicken at the spreading center, thereby producing a new lithospheric‐scale shear zone and causing the subsequent near‐ridge subduction initiation. Strong weakening of the shear zone rocks promotes the subduction initiation, which fails without such weakening. In turn, thermal and compositional heterogeneities inherited from the seafloor spreading stage control geometry and polarity of the resulting young subduction zone. Key Points: Under forced compression, subduction initiation starts along the newly formed shear zoneStrain weakening plays a key role in the new shear zone and subduction initiationGrain size reduction slightly enhances the localization of new shear zones [ABSTRACT FROM AUTHOR]

Aim: Maintaining genetic connectivity is crucial for species that inhabit the disjunct and unstable deep‐sea hydrothermal vents. We aimed to re‐assess the connectivity of the annelid genus Hesiolyra distributed at hydrothermal vents along the East Pacific Rise (EPR). A previous study detected a major clade among five Hesiolyra populations spanning from 13°N to 21°S and a minor sympatric southern clade with ~1% divergence from the major clade. However, this study was based on a short locus of 366‐bp COI gene, which might not contain sufficient informative sites. Location: East Pacific Rise. Methods: We sequenced 188 specimens of Hesiolyra from five hydrothermal vent fields along the East Pacific Rise for six mitochondrial and two nuclear loci to infer their genetic divergence, population diversity and gene flow. Results: We found a minor southern clade (Hesiolyra aff. bergi) which was genetically distinct from Hesiolyra bergi sensu stricto for all gene markers explored except 16S. We also found shared genotypes between H. bergi and H. aff. bergi likely resulted from incomplete lineage sorting. For H. bergi s.s., we found a low but fixed divergence between the north and south EPR populations. We estimated the northern and southern metapopulations of H. bergi s.s. split ~0.45 Mya (HPD: 0.27–0.74 Mya). The northern metapopulation had a higher haplotype diversity than the southern metapopulation, indicating historical gene flow and loss of genetic diversity in the southern clade. Main conclusions: We confirmed that the equatorial dispersal filter observed previously on several other vent species also applies to H. bergi. Our results highlight the power of the multi‐locus approach in revealing the divergence and population genetic history of marine species with strong dispersal capabilities, indicating that the NEPR and the SEPR should be considered as separate biogeographic regions in environmental management and biological conservation. [ABSTRACT FROM AUTHOR]

The rise of the mantle asthenosphere and tectonic activity are linked to travertine and mid-ocean ridges, although their relationship has not been clarified. To investigate the connection between travertine and mid-ocean ridges, we gathered information on the geographic distribution of travertine from around the world, plotted it, and linearized it. Making a map of the locations of linearized travertine, mid-ocean ridges, and 2016-2020 (M>5) earthquakes (which may or may not constitute seismic belts) and assessing its global distribution. The sedimentary models of travertine and mid-ocean ridges, on the other hand, were drawn based on previous studies, comparing and analyzing the similarities and differences between the sedimentary models of travertine and mid-ocean ridges. The results indicate that: 1) travertine and mid-ocean ridges are both primarily distributed in the seismic belts, 2) their deposition is closely related to the mantle asthenosphere, and 3) travertine occurs on land but mid-ocean ridges are in the ocean, which is a significant difference. This study examines the relationship between travertine and mid-ocean ridges are related analogs regarding geographical location and sedimentary model and suggests that travertine, like mid-ocean ridges, may be a driving force for plate drift. Researchers may gain a better understanding of the otherwise difficult-to-study mid-ocean ridge system by analyzing similarities between travertine and mid-ocean ridges. [ABSTRACT FROM AUTHOR]

• We show the patterns of deformation, mixing, and decoupling of chemical heterogeneities marked by Nd-Hf isotope ratios in the residue at mid-ocean ridges. • The observed melt fraction (∼1%) beneath fast-spreading ridge would produce strong vertical dispersion of chemical heterogeneity. • If the melt fraction is less than 0.2%, probably in the case of ultraslow-spreading ridge, heterogeneities larger than 2 km in diameter would experience negligible dispersion. • A previous melting event could produce the residual peridotite with extremely depleted Hf isotope ratio that eventually gets recycled into the asthenosphere. The common assumption that residual peridotites retain the Nd-Hf isotope ratios in the mantle source is debated because melt and solid of different isotopic compositions could undergo chemical exchange during melt migration, altering the isotopic signature of the source. By modeling the transport of chemical heterogeneities in the melting region beneath a mid-ocean ridge, we show that the shape of a chemical heterogeneity marked by Nd or Hf isotope ratio changes systematically through subvertical dispersion, stretching, compression, and shearing. The isotope ratios inside the chemical heterogeneity decay toward the values of background mantle. The amount of decay depends on the strength of dispersion, which itself is strongly dependent on the melt fraction in the melting region. When the maximum melt fraction is greater than 1%, buoyancy-driven melt flow relative to the solid causes subvertical dispersion of isotopic signals in the solid. Differential flows of the melt and solid also produce chromatography fractionation of Nd with respect to Hf, causing their isotope ratios to decouple. Compositions of the residue in Nd-Hf isotope ratio diagram do not record the endmembers in the source, instead they represent an area that covers part of the binary mixing line between the background mantle and the original heterogeneity. In the case of small melt fraction (<0.2%), the low permeability results in sluggish melt flow, weak dispersion, and negligible chromatography fractionation. Consequently, Nd and Hf isotope ratios in the residue remain coupled, representing the endmember isotope ratios in the source. The ridge model with larger melt fraction may correspond to the fast-spreading ridge, while the model with smaller melt fraction may correspond to the ultraslow-spreading ridge. The present study underscores the importance of melt migration processes beneath mid-ocean ridges on the deformation, mixing and decoupling of Nd-Hf isotope ratios in residual peridotites. [ABSTRACT FROM AUTHOR]

In this study, we examine the influence of the mantle and large-scale tectonics on the global mid-ocean ridge (MOR) system. Using solely seismically-inferred upper mantle temperatures below the melting zone (260-600 km) and an interpretable machine learning model (Random Forest and Principal Component Analysis), we can predict a priori the location (ocean basin and ridge system) of global MOR with up to 90% accuracy. Two features provide > 50% of the discriminative power: the temperature difference between the mid-layer (340-500 km) and other depths, and the depth-averaged temperature of the upper mantle. We suggest long-term (100s Myr) tectonic and large-scale convective processes primarily driven by deep subduction left ample, distinct but hidden fingerprints in the mantle that allow us to separate regions at ∼1000 km scale. Our result implies that the large-scale geophysical and geochemical differences observed along the MOR system are reflective, not primarily of shallow processes associated with melting, but of the integrated long-term tectonic, subduction, and convective flow history that determines the present-day upper mantle temperature structure. • Mantle temperatures beneath global mid-ocean ridges exhibit inter- basin differences. • We use machine learning to predict the geographic location of ridge segments based on the sub-ridge upper mantle temperature. • The integrated history of convection and tectonics is recorded in the large-scale patterns observed at mid-ocean ridges. [ABSTRACT FROM AUTHOR]

Abstract Decades of research have resulted in characterization of the ocean floor manifestations of mid‐ocean ridge (MOR) hydrothermal systems, yet numerical models accounting for the connections between heat transfer, hydrology and geochemistry have been slow to develop. The Thermo‐hydro‐chemical code ToughReact can be used to describe the coupled effects of fluid flow, heat transfer, and fluid‐rock chemical interactions that occur in MOR systems. We describe the results of 2‐dimensional simulations of steady state flow in fractured diabase with mineral‐fluid chemical reactions. Basal heating and specified permeability yield maximum temperature of 400°C. Total fluid flux and high fracture flow velocities are in accord with observations. Fluid chemistry, mineralogical changes and 87Sr/86Sr ratios can be compared to observations to assess and calibrate models. Simulated high temperature fracture fluids have Mg and SO4 near zero, elevated Ca and 87Sr/86Sr of about 0.7040. Total alteration is 10%–50% for simple models of spreading. Anhydrite forms mainly near the base of the upwelling zone and results in substantial local fracture porosity reduction. A calibrated model is used to predict how Sr isotopes and other features of altered oceanic crust would be different in the Cretaceous (95 Ma) early Proterozoic (1,800 Ma) and Archean (3,800 Ma), when seawater may have had high Ca and Sr concentrations, lower pH, higher temperature, and lower Na, Mg, and SO4. The simulations are offered as a start on what ultimately may require a longer‐term community effort to better understand the role of MOR thermo‐hydro‐chemical systems in Earth evolution.

Continental formation models invoke subduction or plume‐related processes to create the buoyant, refractory character of continental lithospheric mantle (CLM). From similarities in melt depletion, major element composition, modal clinopyroxene, and Os isotope systematics it has been proposed that oceanic mantle lithosphere is the likely protolith to non‐cratonic CLM, however, a direct link between the two has been difficult to ascertain. Using dredged mantle peridotite xenoliths from the Ferrel Seamount, off the west coast of Baja California, Mexico, we show that tectonic isolation of an oceanic plate may lead to formation of non‐cratonic CLM. Ferrel xenoliths are coarse‐grained spinel lherzolite, or rare harzburgite. Bulk‐rock and clinopyroxene trace element compositions reveal two‐stages of melt refertilization following melt depletion, with infiltration by mid‐ocean ridge basalt‐type melts, followed by melt addition from host alkali basalt. Melt depletion correlations with 187Os/188Os and highly siderophile element abundances indicate preserved melt depletion and refertilization processes are ancient. From these observations, the Ferrel xenoliths represent lithosphere from the abandoned Pacific‐Farallon ridge. The history of melt depletion, followed by MORB‐melt refertilization is consistent with the peridotites representing oceanic mantle lithosphere that was subsequently incorporated into the Baja‐Guadalupe microplate during "ridge jump." These peridotites demonstrate that isolation of oceanic lithosphere that is rafted onto a continental margin provides a viable means for producing non‐cratonic CLM. We suggest that continuation of late‐stage, low degree melt refertilization may provide a link between oceanic lithosphere and non‐cratonic CLM and propose a tectonic model to preserve and facilitate this continued evolution. Key Points: Ancient melt depletion can be preserved despite metasomatism and suggest Ferrel Seamount peridotite was formed by mid‐ocean ridge processesFerrel Seamount peridotites likely represent preserved Pacific‐Farralon lithosphereA tectonic process, ridge jump, can isolate oceanic lithosphere and provides a starting point for the formation of continental lithosphere [ABSTRACT FROM AUTHOR]

Seismic anisotropy produced by aligned olivine in oceanic lithosphere offers a window into mid‐ocean ridge (MOR) dynamics. Yet, interpreting anisotropy in the context of grain‐scale deformation processes and strain observed in laboratory experiments and natural olivine samples has proven challenging due to incomplete seismological constraints and length scale differences spanning orders of magnitude. To bridge this observational gap, we estimate an in situ elastic tensor for oceanic lithosphere using co‐located compressional‐ and shear‐wavespeed anisotropy observations at the NoMelt experiment located on ∼70 Ma seafloor. The elastic model for the upper 7 km of the mantle, NoMelt_SPani7, is characterized by a fast azimuth parallel to the fossil‐spreading direction, consistent with corner‐flow deformation fabric. We compare this model with a database of 123 petrofabrics from the literature to infer olivine crystallographic orientations and shear strain accumulated within the lithosphere. Direct comparison to olivine deformation experiments indicates strain accumulation of 250%–400% in the shallow mantle. We find evidence for D‐type olivine lattice‐preferred orientation (LPO) with fast [100] parallel to the shear direction and girdled [010] and [001] crystallographic axes perpendicular to shear. D‐type LPO implies similar amounts of slip on the (010)[100] and (001)[100] easy slip systems during MOR spreading; we hypothesize that grain‐boundary sliding during dislocation creep relaxes strain compatibility, allowing D‐type LPO to develop in the shallow lithosphere. Deformation dominated by dislocation‐accommodated grain‐boundary sliding (disGBS) has implications for in situ stress and grain size during MOR spreading and implies grain‐size dependent deformation, in contrast to pure dislocation creep. Plain Language Summary: Earth's upper mantle is composed primarily of the mineral olivine, which responds to deformation by organizing its seismically fast axis in the flow direction. During seafloor spreading, olivine crystals align with the spreading direction and become frozen into the lithosphere preserving the near‐ridge deformation history. The resulting rock fabric can be observed in place via the directional dependence of seismic wavespeeds (seismic anisotropy) as well as in hand‐sample rocks collected from the field. However, interpreting seismic anisotropy observations in the context of laboratory and field data remains a challenge, due to large differences in length scale and incomplete seismic constraints. Here, we bridge this observational gap by incorporating multiple data types to solve for the complete anisotropic structure of oceanic lithosphere that formed ∼70 Myr ago. By comparing our model to laboratory data, we infer the magnitude of shear strain and style of olivine deformation during seafloor spreading for the first time. Our results indicate large shear strains and an olivine fabric type different to that typically assumed, consistent with deformation that is sensitive to the presence of grain boundaries even though most of the strain is produced by dislocations. This has new implications for the formation and evolution of oceanic plates. Key Points: We develop an in situ elastic tensor for oceanic lithosphere that incorporates co‐located compressional and shear wavespeed anisotropy constraintsSeismic anisotropy is consistent with corner flow during spreading and shear strains of 250%–400%Girdled D‐type olivine fabric implies activation of multiple olivine easy slip systems during mid‐ocean ridge spreading [ABSTRACT FROM AUTHOR]

Along ultraslow spreading ridges melt is distributed unequally, but melt focusing guides melt away from amagmatic segments toward volcanic centers. An interplay of tectonism and magmatism is thought to control melt ascent, but the detailed process of melt extraction is not yet understood. We present a detailed image of the seismic velocity structure of the Logachev volcanic center and adjacent region along the Knipovich Ridge. With travel times of P‐ and S‐waves of 3,959 earthquakes we performed a local earthquake tomography. We simultaneously inverted for source locations, velocity structure and the Vp/Vs‐ratio. An extensive low velocity anomaly coincident with high Vp/Vs‐ratios >1.9 lies underneath the volcanic center at depths of 10 km below sea level in an aseismic area. More shallow, tightly clustered earthquake swarms connect the anomaly to a shallow anomaly with high Vp/Vs‐ratio beneath the basaltic seafloor. We consider the deep low‐velocity anomaly to represent an area of partial melt from which melts ascent vertically to the surface and northwards into the adjacent segment. By comparing tomographic studies of the Logachev and Southwest Indian Ridge Segment‐8 volcano we conclude that volcanic centers of ultraslow spreading ridges host spatially confined, circular partial melt areas below 10 km depth, in contrast to the shallow extended melt lenses along fast spreading ridges. Lateral feeding over distances of 35 km is possible at orthogonal spreading segments, but limited at the obliquely spreading Knipovich Ridge. Plain Language Summary: Mid‐ocean ridges mark the tectonic plate boundaries, where the plates drift apart. Fresh magma rises into the gap and builds new seafloor. The slower the plates drift apart, the less magma is present underneath the ridge. At very slow spreading ridges there is not enough magma to build new seafloor along the entire length of the ridge. Rather, melt is guided toward individual volcanic centers spaced at about 100 km, where melt accumulates and ascents. In our study we try to find melt storage areas and ascent paths of such a volcanic center. With velocities of different seismic wave types from earthquakes we map the velocity structure of the area underneath the major Logachev volcanic center. Lower velocities indicate an area partly including melt at depths of more than 10 km, far deeper than at mid‐ocean ridges with sufficient melt supply. From the deep magma reservoir, many earthquake swarms map the long ascent path of melt to the surface. The interplay of magmatic and tectonic activity is important here. In a comparison with results from another volcanic center, we find that lateral magma feeding is possible in orthogonal spreading, but limited in oblique spreading, as at the Knipovich Ridge. Key Points: Active volcanic centers at ultraslow spreading ridges host deeper and more confined partial melt areas than faster spreading ridgesEarthquake swarms delineate melt ascent paths from the partial melt area to the surfaceLateral feeding at shallow depths into subordinate segments is prevented by ridge obliquity [ABSTRACT FROM AUTHOR]

Abstract Continental formation models invoke subduction or plume‐related processes to create the buoyant, refractory character of continental lithospheric mantle (CLM). From similarities in melt depletion, major element composition, modal clinopyroxene, and Os isotope systematics it has been proposed that oceanic mantle lithosphere is the likely protolith to non‐cratonic CLM, however, a direct link between the two has been difficult to ascertain. Using dredged mantle peridotite xenoliths from the Ferrel Seamount, off the west coast of Baja California, Mexico, we show that tectonic isolation of an oceanic plate may lead to formation of non‐cratonic CLM. Ferrel xenoliths are coarse‐grained spinel lherzolite, or rare harzburgite. Bulk‐rock and clinopyroxene trace element compositions reveal two‐stages of melt refertilization following melt depletion, with infiltration by mid‐ocean ridge basalt‐type melts, followed by melt addition from host alkali basalt. Melt depletion correlations with 187Os/188Os and highly siderophile element abundances indicate preserved melt depletion and refertilization processes are ancient. From these observations, the Ferrel xenoliths represent lithosphere from the abandoned Pacific‐Farallon ridge. The history of melt depletion, followed by MORB‐melt refertilization is consistent with the peridotites representing oceanic mantle lithosphere that was subsequently incorporated into the Baja‐Guadalupe microplate during “ridge jump.” These peridotites demonstrate that isolation of oceanic lithosphere that is rafted onto a continental margin provides a viable means for producing non‐cratonic CLM. We suggest that continuation of late‐stage, low degree melt refertilization may provide a link between oceanic lithosphere and non‐cratonic CLM and propose a tectonic model to preserve and facilitate this continued evolution.

Abstract Seismic anisotropy produced by aligned olivine in oceanic lithosphere offers a window into mid‐ocean ridge (MOR) dynamics. Yet, interpreting anisotropy in the context of grain‐scale deformation processes and strain observed in laboratory experiments and natural olivine samples has proven challenging due to incomplete seismological constraints and length scale differences spanning orders of magnitude. To bridge this observational gap, we estimate an in situ elastic tensor for oceanic lithosphere using co‐located compressional‐ and shear‐wavespeed anisotropy observations at the NoMelt experiment located on ∼70 Ma seafloor. The elastic model for the upper 7 km of the mantle, NoMelt_SPani7, is characterized by a fast azimuth parallel to the fossil‐spreading direction, consistent with corner‐flow deformation fabric. We compare this model with a database of 123 petrofabrics from the literature to infer olivine crystallographic orientations and shear strain accumulated within the lithosphere. Direct comparison to olivine deformation experiments indicates strain accumulation of 250%–400% in the shallow mantle. We find evidence for D‐type olivine lattice‐preferred orientation (LPO) with fast [100] parallel to the shear direction and girdled [010] and [001] crystallographic axes perpendicular to shear. D‐type LPO implies similar amounts of slip on the (010)[100] and (001)[100] easy slip systems during MOR spreading; we hypothesize that grain‐boundary sliding during dislocation creep relaxes strain compatibility, allowing D‐type LPO to develop in the shallow lithosphere. Deformation dominated by dislocation‐accommodated grain‐boundary sliding (disGBS) has implications for in situ stress and grain size during MOR spreading and implies grain‐size dependent deformation, in contrast to pure dislocation creep.

Abstract Along ultraslow spreading ridges melt is distributed unequally, but melt focusing guides melt away from amagmatic segments toward volcanic centers. An interplay of tectonism and magmatism is thought to control melt ascent, but the detailed process of melt extraction is not yet understood. We present a detailed image of the seismic velocity structure of the Logachev volcanic center and adjacent region along the Knipovich Ridge. With travel times of P‐ and S‐waves of 3,959 earthquakes we performed a local earthquake tomography. We simultaneously inverted for source locations, velocity structure and the Vp/Vs‐ratio. An extensive low velocity anomaly coincident with high Vp/Vs‐ratios >1.9 lies underneath the volcanic center at depths of 10 km below sea level in an aseismic area. More shallow, tightly clustered earthquake swarms connect the anomaly to a shallow anomaly with high Vp/Vs‐ratio beneath the basaltic seafloor. We consider the deep low‐velocity anomaly to represent an area of partial melt from which melts ascent vertically to the surface and northwards into the adjacent segment. By comparing tomographic studies of the Logachev and Southwest Indian Ridge Segment‐8 volcano we conclude that volcanic centers of ultraslow spreading ridges host spatially confined, circular partial melt areas below 10 km depth, in contrast to the shallow extended melt lenses along fast spreading ridges. Lateral feeding over distances of 35 km is possible at orthogonal spreading segments, but limited at the obliquely spreading Knipovich Ridge.