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As a result of a study of igneous rocks of the basalt - andesite series, dredged on the Shaka Ridge in the South Atlantic, it was found that they differ from the basalts of mid-ocean ridges and ocean islands, and have an age of 183.8 ± 2.2 Ma, comparable to the time of manifestation of the Karoo-Maud mantle plume in central Gondwana. Geochemical and Sr–Nd–Pb isotopic features of the studied igneous rocks show their similarity with the Jurassic mafic complexes of the Ferrar province in Antarctica and the Falkland Islands, formed during the intrusion of the Karoo-Maud plume and under the influence of paleo-Pacific subduction. However the supply of ice rafted debris into the study area due to ice transportation is considered unlikely. Based on the all data obtained, it was concluded that the Shaka Ridge is a continental block that was moved during the opening of the South Atlantic in the Early Cretaceous-Early Miocene from the continental margin of Africa along an extended transform fault into the present Bouvet triple junction area. [ABSTRACT FROM AUTHOR]

At slow to ultraslow spreading ridges, the limited melt supply results in tectonic accretion and the exhumation of mantle rocks. Melt supply is focused toward volcanic centers where magmatic accretion dominates. In areas where the ridges reorientate, both types of accretion can occur across the ridge axis with detachment faults developing on the inside corners and hydrothermal vent fields located in close proximity. Microseismicity studies improve the understanding of the tectonic processes at detachment faults and their interplay with hydrothermal vent systems, but are mostly limited to mature detachment faults or short deployment times. This study presents results from a ∼11 months ocean bottom seismometer deployment around the Loki's Castle hydrothermal vent field at the intersection of the slow to ultraslow spreading Mohns and Knipovich Ridge. We observe seismicity to be highly asymmetric with the majority of the plate divergence being accommodated by an emerging detachment fault at the inside corner of the intersection west of Loki's Castle. Seismic activity related to the detachment fault displays a distinct contrast, with continuous low‐magnitude events occurring at depth and episodic large‐magnitude events concentrated in clusters within the footwall. The detachment fault shows no significant roll‐over at shallow depths and the locus of spreading is located east of the detachment. These results suggest that the detachment fault west of Loki's Castle is at an early development stage. Plain Language Summary: At mid‐ocean ridges, Earth's tectonic plates spread apart and the created space is filled by rising magma. In areas where spreading is slowest, there is not enough magma available to create new oceanic crust and existing oceanic crust is being pulled upwards during spreading. This results in earthquakes along cracks known as detachment faults. Often close by to these faults, hot mineral‐rich water comes out through chimneys at the seafloor, known as hydrothermal vents. Seismometers placed along the seafloor can record small earthquakes related to spreading processes. Scientists have used these earthquakes to study such processes, but they mostly focus on long‐lived detachment faults or study them for short periods. This study looks at nearly a year of data from seismometers placed around the Loki's Castle hydrothermal vent field, located where two slow‐spreading ridges meet in the Norwegian‐Greenland Sea. The findings show that most of the tectonic activity is related to a detachment fault in its early stages west of Loki's Castle. The fault is continuously active with small earthquakes at depth, but occasional large earthquakes occur focused in areas below the fault's surface. Key Points: Most of the plate divergence at the Mohns‐Knipovich intersection is accommodated by an emerging detachment fault west of Loki's CastleThe largest magnitude events occur episodically within the footwall of the detachment, including indications for antithetic normal faultsLoki's Castle is located apart from the detachment fault where the brittle‐ductile‐transition shallows along the axial volcanic ridge [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 (<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]

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]

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]

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]

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]

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]

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]

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]

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]

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. Plain Language Summary: This paper describes the results of a numerical simulation of the thermal, hydrological, and chemical processes that occur in a simplified mid‐ocean ridge (MOR) hydrothermal system. The simulations exhibit the relationships between fluid flow, temperature, heat transfer, and mineral‐fluid reactions. A simplified rock composition is used corresponding to diabase. The model accounts for flow mainly in fractures, and can reproduce many of the key aspects of seafloor vent fluid chemistry and the altered rocks found in ophiolites. Sr isotopes are also tracked. There are several key issues that must be addressed in constructing such models, and many of these are described in detail. The model parameters are calibrated against modern seafloor hydrothermal systems and then used to predict how the geochemical processes, including Sr isotope exchange, would be affected by changing seawater chemical composition, as is likely to be the case for Cretaceous, Paleozoic and Precambrian oceans. The simulations presented are viewed as a first step in building a more complete simulation capability for MOR hydrothermal systems to better understand their role in Earth evolution. Key Points: Mid‐ocean ridge hydrothermal systems, a key component of global geochemical cycles, involve coupled heat transfer, fluid flow, and chemical reactionsThe thermo‐hydro‐chemical code ToughReact has the capabilities to simulate the coupled processes and improve understanding of these critical global systemsWe use the model to assess effects of different seawater chemistry in the Cretaceous and Precambrian, which include large differences in Sr isotope and Na exchange [ABSTRACT FROM AUTHOR]

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]

We analyze a large database of abyssal peridotite clinopyroxene compositions using log‐ratio transformation, principal component analysis (PCA) and k‐means clustering, to better understand clinopyroxene compositional systematics in abyssal peridotites. We combine this analysis with open‐system melting models to investigate the potential sources of compositional variation. PCA shows that 84% of the variation in clinopyroxene compositions can be represented using only 2‐dimensional information. We use k‐means clustering to classify clinopyroxene compositions into four clusters. Clusters 1–3, representing 85% of the data, show progressive depletions in LREE/HREE, and are associated with decreases in Na2O in clinopyroxene, and general increases in Cr# of spinel. We interpret peridotites with clinopyroxene compositions from clusters 1–3 to represent residues of partial melt extraction. The degree of melt extraction increases from cluster 1 to 3, and exerts a primary control on compositional variations. The presence or absence of garnet‐field melting prior to spinel‐field melting and the retained melt fraction during partial melting exert secondary controls on clinopyroxene compositions. Cluster 4 clinopyroxenes, representing show less fractionated LREE/HREE with low‐HREE abundances, elevated Sr, and depleted signatures in their host peridotites. Clinopyroxene compositions in cluster 4 cannot be modeled by melt depletion alone. Instead, they are only reconstructed in our models where melt‐rock interaction, suggesting that peridotites with cluster 4 clinopyroxenes have experienced both of these processes. Clusters 1–4 are observed in most ridges, reflecting compositional heterogeneity on each ridge. This variability reflects variations in the degree of partial melting, amount of garnet‐field melting, retained melt fractions, and melt‐rock interaction. Plain Language Summary: "Abyssal peridotites" are rocks that come from the Earth's mantle, beneath the oceanic crust. They are exposed on the seafloor near mid‐ocean ridges, the sites at which new oceanic crust forms by melting the mantle below. We analyzed a large database of chemical analyses of the mineral "clinopyroxene" from these abyssal peridotites using statistical methods, to try and understand what controls their chemical compositions. Our results simplify the complex chemical variations in these clinopyroxenes to just two main variables, and allow us to divide the compositions into 4 groups. The first three groups (85% of the clinopyroxene database) can be explained by forming in mantle rocks that melted by varying amounts, during the melting process that leads to the formation of oceanic crust. The last group (15% of the clinopyroxene database) cannot be explained by simple melting processes. Instead, these mantle rocks were first partially melted, and then reacted with melts that originated deeper in the mantle and passed through them. All clinopyroxene groups are present in most mid‐ocean ridges, showing that the composition of abyssal peridotites is extremely variable even on each ridge. Key Points: We classify database of abyssal peridotite clinopyroxene compositions using statistical methods and compare the data with melting modelsEighty five percent of abyssal peridotites can be residues following melt extraction but 15% experienced both partial melting and melt‐rock interactionAll clinopyroxene compositional clusters are observed in most ridges, reflecting compositional heterogeneities on each ridge [ABSTRACT FROM AUTHOR]

Seismicity along mid‐ocean ridges and oceanic transform faults provides insights into the processes of crustal accretion and strike‐slip deformation. In the equatorial Atlantic ocean, the slow‐spreading Mid‐Atlantic Ridge is offset by some of the longest‐offset transform faults on Earth, which remain relatively poorly understood due to its remote location far from land‐based teleseismic receivers. A catalog of T‐phase events detected by an array of 10 autonomous hydrophones deployed between 2011 and 2015, extending from 20°N to 10°S is presented. The final catalog of 6,843 events has a magnitude of completeness of 3.3, compared to 4.4 for the International Seismic Center teleseismic catalog covering the same region, and allows investigation of the dual processes of crustal accretion and transform fault slip. The seismicity rate observed at asymmetric spreading segments (those hosting detachment faults) is significantly higher than that of symmetric spreading centers, and 74% of known hydrothermal vents along the equatorial Mid‐Atlantic Ridge occur on asymmetric spreading segments. Aseismic patches are present on nearly all equatorial Atlantic transform faults, including on the Romanche transform where regional rotation and transpression could explain both bathymetric uplift and reduction in seismic activity. The observed patterns in seismicity provide insight into the thermal and mechanical structure of the ridge axis and associated transform faults, and potentially provide a method for investigating the distribution of hydrothermal vent systems. Plain Language Summary: New oceanic seafloor is created at mid‐ocean ridges where tectonic plates move apart, causing seafloor volcanism and deep‐sea hydrothermal venting. Meanwhile, changes in the direction of plate motion occur along strike‐slip faults which can be hundreds of kilometers long, called oceanic transforms, and offset the mid‐ocean ridge. Neither of these processes are fully understood, but constantly generate numerous small earthquakes which in places like the remote equatorial Atlantic Ocean, often go undetected by far‐away instruments on land. Using a network of hydrophones spaced thousands of kilometers apart and moored ∼800 m below the sea surface, over 6,000 earthquakes in the equatorial Atlantic ocean were located from 2011 to 2015, roughly 10 times more than would have been detected by land‐based stations alone. Similar to previous studies, we find that transform faults have patches where very few earthquakes occur and hence may be locked or creeping, likely due to spatial variability in the thermal properties of the lithosphere and underlying mantle. More earthquakes occurred in places where the plates spread apart asymmetrically, and hydrothermal vents are also more likely to occur in places where spreading is asymmetric. Key Points: Catalog of 6843 hydroacoustic events recorded by autonomous hydrophones in equatorial Atlantic Ocean between August 2011 and January 2015Asymmetric spreading segments have higher hydroacoustic event rate, and frequently host hydrothermal vent systemsAll equatorial Atlantic transforms host aseismic patches, and precursors are common ∼2–24 hr before mainshocks of normal faulting events on the ridge axis [ABSTRACT FROM AUTHOR]

Melt extraction from the partially molten mantle is among the fundamental processes shaping the solid Earth today and over geological time. A diversity of properties and mechanisms contribute to the physics of melt extraction. We review progress of the past ∼25 years of research in this area, with a focus on understanding the speed and style of buoyancy-driven melt extraction. Observations of U-series disequilibria in young lavas and the surge of deglacial volcanism in Iceland suggest this speed is rapid compared to that predicted by the null hypothesis of diffuse porous flow. The discrepancy indicates that the style of extraction is channelized. We discuss how channelization is sensitive to mechanical and thermochemical properties and feedbacks, and to asthenospheric heterogeneity. We review the grain-scale physics that underpins these properties and hence determines the physical behavior at much larger scales. We then discuss how the speed of melt extraction is crucial to predicting the magmatic response to glacial and sea-level variations. Finally, we assess the frontier of current research and identify areas where significant advances are expected over the next 25 years. In particular, we highlight the coupling of melt extraction with more realistic models of mantle thermochemistry and rheological properties. This coupling will be crucial in understanding complex settings such as subduction zones. Mantle melt extraction shapes Earth today and over geological time. Observations, lab experiments, and theory indicate that melt ascends through the mantle at speeds ∼30 m/year by reactively channelized porous flow. Variations in sea level and glacial ice loading can cause significant changes in melt supply to submarine and subaerial volcanoes. Fluid-driven fracture is important in the lithosphere and, perhaps, in the mantle wedge of subduction zones, but remains a challenge to model. [ABSTRACT FROM AUTHOR]

Distinct signatures are present in the circum‐Pacific continental margins (e.g., kinematics, magmatism, and basin evolution), possibly influenced by the input of mid‐ocean ridge. It remains enigmatic why the circum‐Pacific continental margins that have experienced trench‐parallel mid‐ocean ridge subduction show diverse geological evolution. Here we present geodynamic modeling results investigating trench‐parallel mid‐ocean ridge subduction and demonstrate two distinct types of model evolution. Type‐Ⅰ model includes a two‐stage steep subduction and is featured by slab detachment preceding the arrival of ridge at the trench. Type‐Ⅱ model is marked by a continuous flat subduction of mid‐ocean ridge with the opening of a slab window beneath intracontinental lithosphere. These two subduction styles produce diverse tectono‐magmatic responses. Our results could explain the magmatic gap and forearc uplift during the Izanagi‐Pacific ridge subduction and the intraplate magmatic flare‐ups and tectonic uplift during the Nazca‐Antarctic ridge subduction, respectively. Plain Language Summary: Mid‐ocean ridge subduction potentially occurs during the evolution of all subduction zones and has significant influence on subduction dynamics and the related geological evolution. The circum‐Pacific domain that has experienced trench‐parallel mid‐ocean ridge subduction shows contrasting tectono‐magmatic evolution. However, it remains poorly understood how the input of mid‐ocean ridge in subduction zones affects the geological evolution of these continental margins. Here, we conduct a series of numerical experiments and reveal two distinct subduction styles. The modeling results show that an interruption of ridge subduction can induce magmatic quiescence, forearc mid‐ocean ridge basalts, and forearc tectonic uplift, whereas the continuous ridge subduction can induce magmatic flare‐ups and tectonic uplift in intraplate region. These two contrasting models are strongly dependent on subducting slab dips. This study provides a new simple model to explain the complex Cenozoic geological evolution observed in the northwest and southeast Pacific continental margins. Key Points: Geodynamic modeling reveals the dynamics of trench‐parallel mid‐ocean ridge subduction affected by the dip angles of the subducting slabSteep subduction promotes slab detachment along the ridge axis, but flat subduction facilitates continuous subduction of the ridgeModel results could explain the contrasting geological features observed in the northwest and southeast Pacific continental margins [ABSTRACT FROM AUTHOR]

Geodynamic simulations underpin our understanding of upper‐mantle processes, but their predictions require validation against observational data. Widely used geophysical datasets provide limited constraints on dynamic processes into the geological past, whereas under‐exploited geochemical observations from volcanic lavas at Earth's surface constitute a valuable record of mantle processes back in time. Here, we describe a new peridotite‐melting parameterization, BDD21, that can predict the incompatible‐element concentrations of melts within geodynamic simulations, thereby providing a means to validate these simulations against geochemical datasets. Here, BDD21's functionality is illustrated using the Fluidity computational modeling framework, although it is designed so that it can be integrated with other geodynamic software. To validate our melting parameterization and coupled geochemical‐geodynamic approach, we develop 2‐D single‐phase flow simulations of melting associated with passive upwelling beneath mid‐oceanic ridges and edge‐driven convection adjacent to lithospheric steps. We find that melt volumes and compositions calculated for mid‐oceanic ridges at a range of mantle temperatures and plate spreading rates closely match those observed at present‐day ridges with the same conditions. Our lithospheric step simulations predict spatial and temporal melting trends that are consistent with those recorded at intraplate volcanic provinces in similar geologic settings. Taken together, these results suggest that our coupled geochemical‐geodynamic approach can accurately predict a suite of present‐day geochemical observations. Since our results are sensitive to small changes in upper‐mantle thermal and compositional structure, this novel approach provides a means to improve our understanding of the mantle's thermo‐chemical structure and flow regime into the geological past. Plain Language Summary: Earth's mantle is a ∼3,000 km thick layer of hot rock that lies between Earth's crust and core. When anomalously hot mantle arrives at shallow depths it begins to melt. These melts are less dense than the surrounding mantle and ascend to the surface to erupt as volcanic lavas. The mantle's slow, creeping, convective motion over billions of years has been integral to Earth's thermal, chemical, tectonic and geological evolution. However, an inability to reproduce observational constraints derived from the composition of volcanic lavas at Earth's surface limits our capacity to validate models of mantle convection back in time. Here, we present a new framework that can predict the volume and composition of melts generated within the mantle. These predictions compare favorably with those recorded by igneous rocks at Earth's surface in two geologic settings: mid‐oceanic ridges, where plates move apart to drive decompression melting, and lithospheric steps, where instabilities associated with changes in the thickness of Earth's rigid outermost shell generate volcanism far from plate boundaries. The approach and tools presented here will allow scientists to better understand the mantle's past structure, dynamics, evolution and impact on Earth's surface. Key Points: We present the BDD21 peridotite‐melt‐chemistry parameterization that can be coupled with geodynamic models to calculate melt compositionBDD21 is applied to simulations of mid‐oceanic ridges to replicate observed patterns of crustal thickness and melt chemistryGeodynamic simulations that incorporate BDD21 can be used to study melting adjacent to lithospheric steps and in other geologic settings [ABSTRACT FROM AUTHOR]

Plate divergence along mid‐ocean ridges is accommodated through faulting and magmatic accretion, and, at overlapping spreading centers (OSC), is distributed across two curvilinear overlapping ridge axes. One‐meter resolution bathymetry acquired by autonomous underwater vehicles, combined with distribution and ages of lava flows, is used to: (a) analyze the spatial and temporal distribution of flows, faults, and fissures in the OSC between the distal south rift zone of Axial Seamount and the Vance Segment, (b) locate spreading axes, (c) calculate extension, and (d) determine the proportion of extension accommodated at the surface by faults and fissures versus volcanic extrusion over a period of ∼1300–1450 years. Our study reveals that in the recent history of the ridges, extension over a distance of 14 km across the Axial/Vance OSC was asymmetric in proportion and style: faults and fissures across 1–2 km of the Vance axial valley accommodated ∼3/4 of the spreading, whereas dike‐fed eruptions contributed ∼1/4 of the extension and occurred across 4 km of the south rift of Axial Seamount. Plain Language Summary: Along mid‐ocean ridges, oceanic plates separate through the formation and growth of faults and the emplacement of dikes supplying lava flows. Where segments overlap in a zone of separation, these processes are distributed along two spreading axes separated by 2–30 km kilometers. We combine 1‐m resolution bathymetry collected by autonomous underwater vehicles and the age of large lava flows to (a) analyze the distribution of faults and lava flows where Axial Seamount overlaps with the Vance Segment, (b) define the current plate boundary, (c) calculate the speed of plate separation, and (d) determine the proportion and locations of fault extension versus flow emplacement. Our study shows that during the last ∼1300–1450 years, fault formation and growth along the Vance Segment are the main contributor to plate separation. In contrast, the emplacement of dikes and lava flows along Axial Seamount account only for ∼1/4 of the plate separation. Key Points: Autonomous underwater vehicle mapping of an overlapping spreading center reveals the proportion of faulting and eruptions that occurred during the last ∼1300–1450 yearsFaulting at the Vance Segment accommodates ∼3/4 of the spreading and magmatic accretion along Axial Seamount south rift accounts for ∼1/4The spreading axis is <250 m wide along the Vance Segment but ∼4 km wide along the south rift of Axial Seamount [ABSTRACT FROM AUTHOR]

A high‐resolution sampling profile constituted of 52 basalt samples from across the East Pacific Rise (EPR) was investigated. These samples provide a unique opportunity to study the coeval recording of isotopic signals derived from sub‐marine eruptions at a fast spreading ridge over a time interval of ∼320 kyrs. Additionally, the study area is marked by the presence of a neighboring seamount chain that has recently caused the ridge to jump twice toward it. Combining previous geochemical studies and bathymetry, we established a first‐order chronology between analyzed samples, and have reconstructed the evolution of basalt compositions as the ridge and seamounts advance and finally merge. Our data reveal the existence of two distinct types of isotopic variability within the samples. One that has a low amplitude and frequency and is accounted for by the continuous melting of the ambient mantle, indicating a process with a ∼125 kyr periodicity. The other, of higher amplitude, is discontinuous in time, and likely reflects the seamounts source influence on the ridge. Our results support a two‐step hotspot‐ridge interaction including a first stage (≥600 ka) of regional enrichment of the depleted ridge mantle by hotspot material; and a second, more recent (at least 300 ka) even wherein ambient mantle melts mixed with proximal melts from heterogeneous seamounts sourced nearby. We also propose that the ancient gabbroic component previously identified in this region appeared very recently (300 kyrs. We have established a relative chronology between these samples, and propose a paleo‐reconstruction of the area accounting for both the isotopic variations observed and previously reported geophysical results. Our new results also suggest rapid magma transfers within the crust independent of melting of seamounts and/or ridge sources, including periodic injections of enriched mantle material into the ridge‐melting region. Key Points: Ridge‐hotspot interactions were tracked via the isotopic composition of basalts sampled across the East Pacific Rise axisChronological reconstructions of collected mid‐ocean ridge basalts reveal a periodicity of 125 kyrs in the recorded isotopic signalHigh‐resolution sampling allows the integration of geochemical information within the framework of geophysical and geological models [ABSTRACT FROM AUTHOR]

The Shatsky Rise ridge-ridge-ridge triple junction is an ancient triple junction in the Western Pacific Ocean whose initial geodynamic process is poorly understood and can only be inferred based on indirect geological and geophysical constraints. In this paper, we present three-dimensional numerical models that simulate the Shatsky Rise triple junction and calculate its coupled mantle flow and temperature structure. The mantle flow velocity field shows several distinctive features: 1) stronger mantle upwelling closer to the ridge axis and triple junction; 2) greater upwelling velocity at the faster-spreading ridges; and 3) the most significant increase in upwelling velocity for the slowest-spreading ridge toward the triple junction. The calculated mantle temperature field also reveals distinctive characteristics: 1) sharp increases in the mantle temperature with depth and increases toward the spreading ridges and triple junction; 2) the faster-spreading ridges are associated with higher temperatures at depth and identical distances from the triple junction; and 3) the slowest-spreading ridge shows the greatest increase in the along-ridge-axis temperature toward the triple junction. Compared to many present-day triple junctions with slower spreading rates, the along-ridge-axis velocity and thermal fields of the Shatsky Rise are more altered due to the presence of the triple junction. [ABSTRACT FROM AUTHOR]

Recent multi‐channel seismic studies of fast spreading and hot‐spot influenced mid‐ocean ridges reveal magma bodies located beneath the mid‐crustal Axial Magma Lens (AML), embedded within the underlying crustal mush zone. We here present new seismic images from the Juan de Fuca Ridge that show reflections interpreted to be from vertically stacked magma lenses in a number of locations beneath this intermediate‐spreading ridge. The brightest reflections are beneath Northern Symmetric segment, from ∼46°42′‐52′N and Split Seamount, where a small magma body at local Moho depths is also detected, inferred to be a source reservoir for the stacked magma lenses in the crust above. The imaged magma bodies are sub‐horizontal, extend continuously for along‐axis lengths of ∼1–8 km, with the shallowest located at depths of ∼100–1,200 m below the AML, and are similar to sub‐AML bodies found at the East Pacific Rise. At both ridges, stacked sill‐like lenses are detected beneath only a small fraction of the ridge length examined and are inferred to mark local sites of higher melt flux and active replenishment from depth. The imaged magma lenses are focused in the upper part of the lower crust, which coincides with the most melt rich part of the crystal mush zone detected in other geophysical studies and where sub‐vertical fabrics are observed in geologic exposures of oceanic crust. We infer that the multi‐level magma accumulations are ephemeral and may result from porous flow and mush compaction, and that they can be tapped and drained during dike intrusion and eruption events. Key Points: Multi‐channel seismic data reveal stacked magma lenses within the crystal mush zone at the Juan de Fuca RidgeStacked lenses underlie the Axial Magma Lens within the melt‐rich mid crust and span depths of the foliated gabbros in crustal exposuresMid‐crust magma lenses are interpreted to be ephemeral, formed by mush compaction and contributing magmas during eruption events [ABSTRACT FROM AUTHOR]

We quantified the systematic variations in global transform fault morphology, revealing a first-order dependence on the spreading rate. (1) The average age offset of both the full transform and transform sub-segments decrease with increasing spreading rate. (2) The average depth of both the transform valley and adjacent ridges are smaller in the fast compared to the slow systems, reflecting possibly density anomalies associated with warmer mantle at the fast systems and rifting at the slow ridges. However, the average depth difference between the transform valley and adjacent ridges is relatively constant from the fast to slow systems. (3) The nodal basin at a ridge-transform intersection is deeper and dominant at the ultraslow and slow systems, possibly reflecting a lower magma supply and stronger viscous resistance to mantle upwelling near a colder transform wall. In contrast, the nodal high, is most prominent in the fast, intermediate, and hotspot-influenced systems, where robust axial volcanic ridges extend toward the ridge-transform intersection. (4) Statistically, the average transform valley is wider at a transform system of larger age offset, reflecting thicker deforming plates flanking the transform fault. (5) The maximum magnitude of the transform earthquakes increases with age offset owing to an increase in the seismogenic area. Individual transform faults also exhibit significant anomalies owing to the complex local tectonic and magmatic processes. [ABSTRACT FROM AUTHOR]

Ultraslow spreading ridges form the slowest divergent plate boundaries and exhibit distinct spreading processes in volcanically active magmatic sections and intervening amagmatic sections. Local seismicity studies of ultraslow spreading ridges until now cover only parts of segments and give insight into spreading processes at confined locations. Here, we present a microseismicity data set that allows to study spreading processes on the scale of entire segments. Our network of 26 ocean bottom seismometers covered around 160 km along axis of the ultraslow spreading Knipovich Ridge in the Greenland Sea and recorded earthquakes for a period of about 1 year. We find seismicity varying distinctly along‐axis. The maximum earthquake depths shallow over distances of 70 km toward the Logachev volcanic center. Here, swarm activity occurs in an otherwise aseismic zone. Melts may thus be guided along the subparallel topography of the lithosphere‐asthenosphere boundary toward major volcanic centers explaining the uneven along‐axis melt distribution typical for ultraslow ridges. Absence of shallow seismicity in the upper 8 km of the lithosphere with a band of deep seismicity underneath offsets presumably melt‐poor regions from magma richer sections. Aseismic deformation in these regions may indicate weakening of mantle rocks by alteration. We do not find obvious indications for major detachment faulting that characterizes magma‐poor spreading at some ultraslow spreading segments. The highly oblique spreading of Knipovich Ridge may be the reason for a fine‐scale segmentation of the seismic activity with zones of weak seismicity possibly indicating transform motion on short obliquely oriented faults. Plain Language Summary: At mid‐ocean spreading ridges, tectonic plates drift apart and new seafloor is built by upwelling magma. The slowest spreading ridges do not receive enough magma to build new seafloor along the entire ridge. Rather, they show widely spaced volcanic centers with magma‐poor areas in‐between. The study of small earthquakes with seismometers placed on the seafloor has greatly helped to understand how new seafloor forms. Since such studies require substantial logistic effort, only confined ridge sections have been studied and spreading processes operating at segment‐scale remain poorly understood. In this study, we present for the first time observations of earthquakes covering several segments and one major volcanic center along the Knipovich Ridge in the Greenland Sea. Underneath the volcano, earthquake swarms and a gap in seismicity indicate recent magmatic activity. The maximum depth of earthquakes marks the thickness of the mechanically strong lithosphere. It shallows over 70 km toward the volcano such that melts can be channeled over large distances to the prominent volcanoes. Magma‐poor regions have deep earthquakes but do not show earthquake activity in the upper 8 km. We suppose that water reacts with the mantle rocks that become too weak to break in earthquakes. Key Points: Magma‐poor sections are distinguished from magma‐rich sections by deeper hypocenters and an absence of shallow seismicityShallowing maximum earthquake depths over distances of 70 km suggest along‐axis melt focusing toward major volcanic centersMajor detachment faults on the highly oblique spreading Knipovich Ridge were not obvious in the observed seismicity [ABSTRACT FROM AUTHOR]

The existence, or not, of periodicities in abyssal hill morphology has been vigorously debated in recent publications, and some have hypothesized that such periodicities are evidence of the impact of Milankovitch cycle‐caused sea level fluctuations on the volcanic construction process at mid‐ocean ridges. Periodicities are detected by the presence of spectral peaks that rise significantly above the random variations of sample power spectra associated with an aperiodic, continuous spectrum process, typically modeled as a band‐limited fractal (von Kármán model). Here, I formulate and test a new algorithm to "empirically prewhiten" the sample power spectrum which, without needing to model the continuous spectrum, flattens it to a zero‐mean process. This greatly simplifies definition of the null hypothesis, and additional modeling approximates standard deviation levels that provide a conservative basis for detecting peaks that may be indicative of periodicity. The algorithm is applied to extensive bathymetric data flanking the southern East Pacific Rise. Significant periodicities are detected on many profiles analyzed, but the periods vary widely, and do not cluster at Milankovitch periods. The most substantial harmonic signals detected exhibit periods ∼0.082–0.216 my, with root‐mean square (RMS) heights approximately a quarter to a third of the RMS height for the aperiodic signal. It is hypothesized that the dominant aperiodic component of abyssal hills corresponds to morphology constructed by faults that follow a random distribution governed by scaling laws, whereas longer‐scale periodic signals are associated with crustal thickness variations controlled internally by variations in melt supply. Key Points: A new algorithm is formulated to detect periodicities embedded in an aperiodic process using "empirical prewhitening" spectral analysisPeriodicities are detected at the southern East Pacific Rise, but are spatially and temporally inconsistentThe dominant aperiodic signal is likely fault‐constructed, while ephemeral periodic signals may relate to internal melt supply variations [ABSTRACT FROM AUTHOR]

In 2016, temperature recorders were recovered, temperatures were measured, and fluid samples were collected from Vent 1, a high temperature (338°C) hydrothermal discharge site on the southern Cleft Segment of the Juan de Fuca Ridge. Coupled with previous sampling efforts, this collection represents a 32‐year record of discharge from a single chimney structure, the longest record to date. Remarkably, the fluid has remained brine‐dominated for more than three decades. This brine formed during phase separation and segregation prior to initial observations in 1984. Although the chloride concentration of the discharging fluid has decreased with time, the fluid temperature has remained nearly constant for at least 3.3 years and probably for 15 or even 22 years. Compositions of the discharging fluids are consistent with inputs from a deep‐sourced brine, which was last equilibrated at >400°C at a depth consistent with the base of the sheeted dikes and the brittle‐ductile transition. This brine mixed (diffusion or dispersion) with a likely non‐phase‐separated, hydrothermal fluid prior to discharge. A survey of hydrothermal endmember fluids with chlorinities in excess of 700 mmol/kg shows, with the exception of Fe, a single trend between major ion concentrations and chlorinity even though data are from a range of crustal compositions, spreading rates, and water and magma depths. Calculated deep‐sourced brines from hydrothermal fluid data are similar to data based on fluid inclusions and estimates of brine assimilation in magmas. A better understanding of brines is required given their potential duration of discharge and capacity for mobilizing metals. Plain Language Summary: Magma from the mantle ascends forming new oceanic crust that is cooled by hydrothermal processes. Seawater within the permeable crust is heated and reacts with rock before discharging at the seafloor, forming high temperature (>300°C) hydrothermal fluids and metal‐rich deposits. In some cases, circulating seawater is heated to form vapor‐and brine‐dominated phases. The latter, because of its greater density, largely remains within the crust, resulting in a segregation of the two phases. This study focuses on a brine‐dominated hydrothermal system on the Juan de Fuca Ridge (northeastern Pacific Ocean) that has remained relatively uniform for 32 years. During this period, the amount of brine input to a likely non‐phase‐separated hydrothermal fluid has decreased while the temperature (338°C) has remained constant, possibly for the past 15–22 years. We propose that the brine resides at depth and is incorporated into the circulating hydrothermal fluid. We use new data and a compilation of data from many different seafloor hydrothermal systems to constrain the compositions of circulating hydrothermal fluid and brine, which is globally consistent. Brine‐dominated hydrothermal systems have been largely overlooked, but are capable of mobilizing metals from new oceanic crust and can persist for decades or longer. Key Points: Thermal and chemical data provide a 32‐year record of brine‐dominated hydrothermal discharge from the Cleft Segment, Juan de Fuca RidgeThe brine component is a deep‐sourced (>2 km below the seafloor), hot (>400°C) fluid that mixes with hydrothermal fluid before dischargeCompilation of mid‐ocean ridge endmember compositions with >700 mmol Cl/kg shows uniform trends, implying brine composition is globally consistent [ABSTRACT FROM AUTHOR]

Four new hydrothermal vent fields were discovered on the slow spreading Central Indian Ridge (8–12°S; Segments 1–3), all located off‐axis on abyssal hill structures or Ocean Core Complexes (OCCs). Each site was characterized using seafloor observation (towed camera system), plume chemistry (Fe, Mn, and CH4; Conductivity, Temperature, and Depth sensor [CTD]/Miniature Autonomous Plume Recorder [MAPR]), and rock sampling (TVgrab/dredges). Different styles of venting on each segment reflect different geological settings, rock types, likely heat sources, and fluid pathways. The segment 1 field was located on the western flank of the axial valley at the base of OCC‐1‐1. High‐temperature venting was inferred from plume characteristics and extensive seafloor sulfide mineralization, but only diffuse venting was observed. This site appears to be a magmatic‐influenced basaltic‐hosted system despite its off‐axis location. Two low‐temperature diffusely venting sites were located on abyssal hills 6 and 9 km off‐axis on Segment 2. Plume particle, metal, and CH4 concentrations were all very low, suggesting dilution of hydrothermal fluids by intrusion of seawater into the highly permeable flank area fault zone. The "Onnuri Vent Field" (OVF), located at the summit of OCC‐3‐2, vented clear, low‐temperature fluids supporting abundant vent organisms (21 macrofaunal taxa). The plume particle signal was low to absent, but strong ORP anomalies correlated with high CH4 and low metal concentrations. Sulfide mineralization was present, which suggests both serpentinization and magmatic/lithospheric influence on fluid composition. The detachment fault is the likely pathway for hydrothermal fluid circulation at this off‐axis location. These new vent field discoveries, especially the OVF, contribute valuable information toward understanding Indian Ocean hydrothermal systems and their ecology/biogeography. Plain Language Summary: We discovered four new hydrothermal vent fields along the slow spreading Central Indian Ridge. The different styles of hydrothermal venting discovered at each site reflect the different geological settings, types of surrounding rocks, and likely heat sources driving the hydrothermal circulation. Among the new vent fields, the "Onnuri Vent Field" is particularly interesting because the isolated location, unique style of venting, and abundant vent biology provide valuable new information for understanding vent ecology and biogeography of the Indian Ocean. Key Points: Four new hydrothermal vent fields were discovered along the Central Indian Ridge, 8–12°S, all off‐axisA particle‐poor, diffuse vent field with abundant vent fauna was located on an OCC at a nontransform discontinuityChemical compositions of the plumes reveal both basaltic‐ and ultramafic‐hosted hydrothermal systems [ABSTRACT FROM AUTHOR]

Our understanding of oceanic crustal formation is mostly limited to observations of young crust formed in the past several million years, due to the thick sediments on older crust and the remote location of many spreading centers. Here we use 40 m‐resolution bathymetric data collected over hundreds of square kilometers during the search for Malaysia Airlines Flight 370 on the flank of the Southeast Indian Ridge, which provides a record of crustal accretion from 11–23 Ma. Spectra calculated from the data show a characteristic timescale of 300–400 kyr and no evidence for periodicity coinciding with glacial cycles. This characteristic timescale could be explained by fluctuations in melt supply and the amount of faulting, leading to variations in crustal thickness. We show that this timescale of variation is consistent with porosity waves observed in a two‐phase flow model, which persist over millions of years. Plain Language Summary: A 12‐million‐year‐long record of crustal formation is contained within the seafloor mapping data collected during the search for Malaysia Airlines Flight 370, at a resolution that is 15 times higher than previous maps. These data illuminate the structure of a vast area of crust formed on the Southeast Indian Ridge and show that crustal production, rather than being a constant process, has varied in cycles that last hundreds of thousands of years. This pattern can be explained by the varying amount of molten rock that rises from deep in Earth's mantle, arriving in episodic waves. This behavior could be a general feature of mid‐ocean ridges spreading at similar rates, which has not been previously recognized due to a lack of available data. Key Points: Multibeam bathymetric data from MH370 search reveal off‐axis crustal structure of Southeast Indian RidgeCrustal accretion fluctuates at a characteristic timescale of 300–400 kyr, with no evidence for periodicity at glacial cycle intervalsCrustal accretion variability could be explained by the combined effects of melt‐rich porosity waves and mantle heterogeneities [ABSTRACT FROM AUTHOR]

Free hydrogen (H2) is a basal energy source underlying chemosynthetic activity within igneous ocean crust. In an attempt to systematically account for all H2 within young oceanic lithosphere (<10 Ma) near the Mid-Ocean Ridge (MOR), we construct a box model of this environment. Within this control volume, we assess abiotic H2 sources (~6 x 1012 mol H2/y) and sinks (~4 x 1012 mol H2/y) and then attribute the net difference (~2 x 1012 mol H2/y) to microbial consumption in order to balance the H2 budget. Despite poorly constrained details and large uncertainties, our analytical framework allows us to synthesize a vast body of pertinent but currently disparate information in order to propose an initial global estimate for microbial H2 consumption within young ocean crust that is tractable and can be iteratively improved upon as new data and studies become available. Our preliminary investigation suggests that microbes beneath the MOR may be consuming a sizeable portion (at least ~30%) of all produced H2, supporting the widely held notion that subseafloor microbes voraciously consume H2 and play a fundamental role in the geochemistry of Earth's ocean-atmosphere system. [ABSTRACT FROM AUTHOR]

We present results of a study of the morphology, internal structure, and chemical composition of oceanic zircon, which show that zircon is a sensitive indicator of tectonic and physicochemical processes occurring in the lower oceanic crust. Crystallization of magmatic zircon in gabbroids is not an instantaneous process; it proceeds in the course of differentiation of parental melts in the tectonically active mid-ocean ridge (MOR) setting. The main geochemical indicator of crystallization differentiation during magma cooling is an increase in Hf content toward the zircon grain edge. Zoning is also observed in magmatic zircons from oceanic plagiogranites (OPG), but it is weaker, apparently because of the narrower temperature range of zircon crystallization in these rocks. The OPG zircons are depleted in REE as compared with the least altered magmatic zircons of gabbro, which is probably due to the formation of OPG during the partial melting of gabbro with the participation of concentrated water-salt fluid, a derivate of seawater, and due to the co-crystallization of zircon and apatite. High-temperature hydrothermal processes within slow-spreading MORs lead to a partial or complete recrystallization of zircon as a result of dissolution/redeposition. A significantly reduced cerium anomaly and the presence of microinclusions of xenotime, uranium and thorium oxides or silicates, and, sometimes, baddeleyite in zircon alteration zones indicate a reducing type and high alkalinity of the hydrothermal fluid. The fluid, a derivate of seawater, acquires these features during circulation near the axial zone of ridges as a result of phase separation in the system H2O-NaCl and interaction of the fluid with abyssal peridotites of oceanic core complexes. The estimated solubility of zircon in basic melts indicates that even near-solidus crystallization of zircon is highly unlikely in anhydrous basaltic melts but is possible in differentiates of deep-seated hydrous basic magmas. The Ti-in-Zrn geothermometer must be used with caution, because variations in the Ti content in zircon might be controlled not only by temperature but also by other factors, especially when mineral inclusions in zircon testify to a drastic change in its growth (dissolution) conditions. A geothermometer based on the distribution of Zr and Hf between zircon and the host rock has several advantages over indicators of the crystallization temperature of magmatic zircon that are based on the zircon saturation index and the content of Ti in zircon. It does not depend on the composition of melt and on the correct estimates of the SiO2 and TiO2 activity. In addition, reconstruction of the Zr and Hf fractionation trends during crystallization of zircon from granitoid melts makes it possible to evaluate the temperature of separation of more differentiated melt fractions. [ABSTRACT FROM AUTHOR]

Hydrothermal circulation is a process fundamental to all types of mid‐ocean ridges that largely impacts the chemical and physical balance of the World Ocean. However, diversity of geological settings hosting hydrothermal fields complicates the exploration and requires thorough investigation of each individual case study before effective criteria can be established. Analysis of high‐resolution bathymetric and magnetic data, coupled with video and rock samples material, furthers our knowledge about mid‐ocean‐ridge‐hosted venting sites and aid in the interpretation of the interplay between magmatic and tectonic processes along the axial volcanic ridges. The rock‐magnetic data provide constraints on the interpretation of the observed contrasts in crustal magnetization. We map the areal extent of the previously discovered active basalt‐hosted Loki's Castle and inactive sediment‐hosted Mohn's Treasure massive sulfide deposits and infer their subsurface extent. Remarkably, extinct hydrothermal sites have enhanced magnetizations and display clear magnetic signatures allowing their confident identification and delineation. Identified magnetic signatures exert two new fossil hydrothermal deposits, MT‐2 and MT‐3. The Loki's Castle site coincides with negative magnetic anomaly observed in the 2‐D magnetic profile data crossing the deposit. First geophysical investigations in this area reveal the complexity of the geological setting and the variation of the physical properties in the subsurface. Key Points: We present first geophysical observations from the basalt‐hosted active hydrothermal site Loki's Castle, situated at the axial volcanic ridgeSediment‐hosted extinct hydrothermal site Mohn's Treasure, located at the ridge flank, is associated with magnetization highMagnetic data analysis reveals two new fossil hydrothermal sites in the vicinity of the Mohn's Treasure [ABSTRACT FROM AUTHOR]

We investigated the short-period variations in vertical fluxes of hydrothermal plumes at mid-ocean ridges through quantitative analysis of digital video images of plumes using the particle image velocimetry method. The analyzed digital video images of hydrothermal plumes were selected from the V Vent and Biotransect Vent at the fast-spreading East Pacific Rise, the Grotto Mound of the Endeavour Segment at the intermediate fast-spreading Juan de Fuca Ridge, and the TAG vent at the slow-spreading Northern Mid-Atlantic Ridge. The PIV was able to track the motion of turbulent parcels instead of individual particles within a hydrothermal plume. The mean plume vertical flux was shown to increase with increasing height above the vent orifice, revealing significant turbulent disperse of the rising hydrothermal plumes. The dominant periods of oscillation in the mean vertical flux were observed to be within a relatively narrow band width of 0.5–5 s despite diverse geological settings. Such pervasive short-period variations in plume flux are hypothesized to be caused by the pressure-drop oscillation within a network of compressible two-phase fluid within a sub-seafloor hydrothermal source zone. It is further argued that both the period and amplitude of plume flux oscillation might increase with decreasing plume vertical flux. The relatively narrow band width of the observed oscillation periods might reflect common characteristics in the plume dynamics despite diverse geological settings. [ABSTRACT FROM AUTHOR]

Mid‐ocean ridge axes are marked by segmentation of the axes and underlying magmatic systems. Fine‐scale segmentation has mainly been studied along fast‐spreading ridges. Here we offer insight into the third‐ and fourth‐order segmentation of intermediate‐spreading ridges and their temporal evolution. The Alarcón Rise and the Endeavour Segment have similar spreading rates (49 and 52.5 mm/year, respectively) but contrasting morphologies that vary from an axial high with a relatively narrow axial summit trough to an axial valley. One‐meter resolution bathymetry acquired by autonomous underwater vehicles, lava geochemistry, and ages from sediment cores is combined with available seismic reflection profiles to analyze variations in (1) geometry and orientation of the axial summit trough or valley, (2) seafloor depth near the axis, and (3) distribution of hydrothermal vents, (4) lava chemistry, and (5) flow ages between contiguous axes. Along both intermediate‐spreading segments, third‐ and fourth‐order discontinuities and associated segments are similar in dimension to what has been observed along fast‐spreading ridges. The Alarcón Rise and the Endeavour Segment also allow the study of the evolution of fine‐scale segmentation over periods of 300 to 4,000 years. Comparison between old and young axes reveals that the evolution of fine‐scale segmentation depends on the intensity of the magmatic activity. High magmatic periods are associated with rapid evolution of third‐order segments, while low magmatic activity periods, dominated by tectonic deformation and/or hydrothermal activity, are associated with little to no change in segmentation. Plain Language Summary: Structure and morphology of two intermediate‐rate spreading ridge axes were analyzed in 1‐m resolution bathymetry collected using autonomous underwater vehicles. The observations are combined with information on the distribution of hydrothermal vents, lava flow ages, and geochemistry, and, where available, the geometry of the magma lens. The aim is to determine the third‐ and fourth‐order segmentation of the volcanic axis along intermediate‐spreading ridges and their evolution related to cycles in magma supply. The study focuses on two ridge segments, the Alarcón Rise and the Endeavour Segment, that have similar spreading rates but are in magmatic and tectonic phases, respectively, which has resulted in contrasting morphology. Our analysis shows that along both segments the third‐ and fourth‐order segments and their associated discontinuities are similar in geometry to those observed along fast‐spreading ridges, independent of the magmatic activity. However, the intensity of magmatic activity impacts the evolution of segmentation with segments that remain at the same position during tectonic or low magmatic periods and propagate, decrease, or disappear during high magmatic periods. Key Points: Intermediate‐spreading rate Alarcón Rise and Endeavour Segment illustrate past and current fine‐scale segmentationThird‐order segmentation evolves faster throughout the high magmatic phase than through the tectonic phase during which it remains steadyThird‐ and fourth‐order segmentation have similar characteristics along intermediate‐ and fast‐spreading ridges [ABSTRACT FROM AUTHOR]

The flanks of the ultraslow‐spreading Mid‐Cayman Spreading Center (MCSC) are characterized by domal massifs or oceanic core complexes (OCCs). The most prominent of these, Mount Dent, comprises lower‐crustal and upper‐mantle lithologies and hosts the Von Damm vent field ~12 km west of the axial deep. Here, presented autonomous underwater vehicle‐derived swath sonar (multibeam) mapping and deep‐towed side‐scan sonar imagery lead to our interpretation that: (i) slip along the OCC‐bounding detachment fault is ceasing, (ii) the termination zone, where detachment fault meets the hanging wall, is disintegrating, (iii) the domed surface of the OCC is cut by steep north‐south extensional faulting, and (iv) the breakaway zone is cut by outward facing faults. The Von Damm vent field and dispersed pockmarks on the OCC's south flank further suggest that hydrothermal fluid flow is pervasive within the faulted OCC. On the axial floor of the MCSC, bright acoustic backscatter and multibeam bathymetry reveal: (v) a volcanic detachment hanging wall, (vi) a major fault rifting the southern flank of Mount Dent, and (vii) a young axial volcanic ridge intersecting its northern flank. These observations are described by a conceptual model wherein detachment faulting and OCC exhumation are ceasing during an increase in magmatic intrusion, brittle deformation, and hydrothermal circulation within the OCC. Together, this high‐resolution view of the MCSC provides an instructive example of how OCCs, formed within an overall melt‐starved ultraslow spreading center, can undergo magmatism, hydrothermal activity, and faulting in much the same way as expected in magmatically more robust slow‐spreading centers elsewhere. Plain Language Summary: Mid‐ocean ridges create the seafloor of the ocean basins, and the processes involved range from the eruption of volcanoes to the generation of submarine hydrothermal vents. A small spreading center in the Caribbean Sea—the Mid‐Cayman Spreading Center—spreads at such a slow rate that instead of a steady eruption of volcanic material, tectonic faults pull the deeper parts of the Earth to the surface forming Mount Everest‐scale mountains. In 2010 and 2013 we collected data from the most prominent massif in the Cayman Trough using cutting‐edge mapping tools such as an autonomous underwater vehicle, or AUV. Our data show that the massif is complex internally, breaking up by tectonic forces, intruded by magma, and eroded at the surface. All of these processes are related to the flow of fluids through the crust that sustain the Von Damm vent field, a globally distinct vent field in terms of its biology and chemistry. Mount Dent is likely similar to many other such complexes around the world, including other deep, slow‐spreading centers. Key Points: The Mid‐Cayman Spreading Center is an example of how magmatism can drive cycles of ocean core complex development even at ultraslow ratesDecreasing detachment fault slip is marked by crosscutting higher angle faults and patterns of mass wasting and sedimentationThe surface expression of hydrothermal fluid flow in vents and pockmarks is linked with the mechanical evolution of oceanic core complexes [ABSTRACT FROM AUTHOR]

The Early Cretaceous Xigaze ophiolites (XO) exposed along the central segment of the more than 2000 km long Yarlung Zangbo Suture Zone in southern Tibet, preserved the structure of the upper mantle and oceanic crust, is interpreted as a definite record of Neo‐Tethys oceanic lithosphere. Many evolution models of the XO have been proposed since the 1980s (Nicolas et al., 1981; Hébert et al., 2012; Maffione et al., 2015). However, the geodynamic environment of the XO whether at a MOR or SSZ remains controversial. As key evidence for the identification of the SSZ ophiolite, the boninite‐like dolerites from the XO are still controversial and poorly constraint (Chen et al., 2003; Bao et al., 2013; Dai et al., 2013). According to previous reports, dolerites were subdivided to MORB‐like type and boninite‐like type (Chen et al., 2003; Dai et al., 2013). Apart from high‐SiO2 (> 52 wt%), high MgO (>8 wt%) and low‐TiO2 (<0.5 wt%), we found the boninite‐like and MORB‐like dolerites are indistinguishable in outcrop, mineral and chemical. They are sills invaded into mantle sequence of the XO, with 50.35–56.80 wt% SiO2, 0.32–1.19 wt% TiO2, 7.39–8.89 wt%Fe2O3T, 5.50–10.42 wt% MgO and Mg# of 0.49–0.74. MORB‐like dolerites display trace element and REE patterns similar to those of most fore‐arc or/and back‐arc basalts from Izu‐Bonin‐Mariana (IBM), i.e. enrichment in large ion lithophile elements (LILEs, e.g. Cs, Rb, Ba, Pb, U) and depletion in high‐field strength elements (HFSE, Nb, Ta) (Fig. 1). Boninite‐like dolerites resemble the MORB‐like dolerites in trace elements and REE patterns, excepting for lower concentrations in REE, but distinct from the boninites discovered from the IBM or Troodos ophiolite (Fig. 1c, d). Th in both dolerites deviate from the MORB array with negative slopes on the Th/Yb vs. Nb/Yb diagram (Fig. 2a), suggesting constant subduction component added to the mantle source (Pearce et al., 1995). We chose depleted MORB source Mantle (DMM, Workman and Hart, 2005) as the starting composition computing immobile element contents for primary melts using the methods and partition coefficients of Shervais and Jean (2012). The results show that MORB‐like and boninite‐like dolerites can be explained by non‐batch fractional melting (≥15%) of a depleted spinel peridotite with few subduction‐derived components (Fig. 2b). Therefore, the boninite‐like dolerites from the XO cannot be regarded as a product of hydrous melting of hot and highly depleted mantle wedge fluxed by the slab‐derived fluid and the XO cannot be used as analogs for the subduction initiation. [ABSTRACT FROM AUTHOR]

The Early Cretaceous Xigaze ophiolites (XO) exposed along the central segment of the more than 2000 km long Yarlung Zangbo Suture Zone in southern Tibet, preserved the structure of the upper mantle and oceanic crust, is interpreted as a definite record of Neo‐Tethys oceanic lithosphere. Many evolution models of the XO have been proposed since the 1980s (Nicolas et al., 1981; Hébert et al., 2012; Maffione et al., 2015). However, the geodynamic environment of the XO whether at a MOR or SSZ remains controversial. As key evidence for the identification of the SSZ ophiolite, the boninite‐like dolerites from the XO are still controversial and poorly constraint (Chen et al., 2003; Bao et al., 2013; Dai et al., 2013). According to previous reports, dolerites were subdivided to MORB‐like type and boninite‐like type (Chen et al., 2003; Dai et al., 2013). Apart from high‐SiO2 (> 52 wt%), high MgO (>8 wt%) and low‐TiO2 (<0.5 wt%), we found the boninite‐like and MORB‐like dolerites are indistinguishable in outcrop, mineral and chemical. They are sills invaded into mantle sequence of the XO, with 50.35–56.80 wt% SiO2, 0.32–1.19 wt% TiO2, 7.39–8.89 wt%Fe2O3T, 5.50–10.42 wt% MgO and Mg# of 0.49–0.74. MORB‐like dolerites display trace element and REE patterns similar to those of most fore‐arc or/and back‐arc basalts from Izu‐Bonin‐Mariana (IBM), i.e. enrichment in large ion lithophile elements (LILEs, e.g. Cs, Rb, Ba, Pb, U) and depletion in high‐field strength elements (HFSE, Nb, Ta) (Fig. 1). Boninite‐like dolerites resemble the MORB‐like dolerites in trace elements and REE patterns, excepting for lower concentrations in REE, but distinct from the boninites discovered from the IBM or Troodos ophiolite (Fig. 1c, d). Th in both dolerites deviate from the MORB array with negative slopes on the Th/Yb vs. Nb/Yb diagram (Fig. 2a), suggesting constant subduction component added to the mantle source (Pearce et al., 1995). We chose depleted MORB source Mantle (DMM, Workman and Hart, 2005) as the starting composition computing immobile element contents for primary melts using the methods and partition coefficients of Shervais and Jean (2012). The results show that MORB‐like and boninite‐like dolerites can be explained by non‐batch fractional melting (≥15%) of a depleted spinel peridotite with few subduction‐derived components (Fig. 2b). Therefore, the boninite‐like dolerites from the XO cannot be regarded as a product of hydrous melting of hot and highly depleted mantle wedge fluxed by the slab‐derived fluid and the XO cannot be used as analogs for the subduction initiation. [ABSTRACT FROM AUTHOR]

Crystal mush is rapidly emerging as a new paradigm for the evolution of igneous systems. Mid-ocean ridges provide a unique opportunity to study mush processes: geophysical data indicate that, even at the most magmatically robust fast-spreading ridges, the magma plumbing system typically comprises crystal mush. In this paper, we describe some of the consequences of crystal mush for the evolution of the mid-ocean ridge magmatic system. One of these is that melt migration by porous flow plays an important role, in addition to rapid, channelized flow. Facilitated by both buoyancy and (deformation-enhanced) compaction, porous flow leads to reactions between the mush and migrating melts. Reactions between melt and the surrounding crystal framework are also likely to occur upon emplacement of primitive melts into the mush. Furthermore, replenishment facilitates mixing between the replenishing melt and interstitial melts of the mush. Hence, crystal mushes facilitate reaction and mixing, which leads to significant homogenization, and which may account for the geochemical systematics of mid-ocean ridge basalt (MORB). A second consequence is cryptic fractionation. At mido-cean ridges, a plagioclase framework may already have formed when clinopyroxene saturates. As a result, clinopyroxene phenocrysts are rare, despite the fact that the vast majority of MORB records clinopyroxene fractionation. Hence, melts extracted from crystal mush may show a cryptic fractionation signature. Another consequence of a mush-dominated plumbing system is that channelized flow of melts through the crystal mush leads to the occurrence of vertical magmatic fabrics in oceanic gabbros, as well as the entrainment of diverse populations of phenocrysts. Overall, we conclude that the occurrence of crystal mush has a number of fundamental implications for the behaviour and evolution of magmatic systems, and that mid-ocean ridges can serve as a useful template for trans-crustal mush columns elsewhere. [ABSTRACT FROM AUTHOR]

Mid‐ocean ridge eruptions, initiating or revitalizing hydrothermal discharge and disrupting seafloor ecosystems, occur regularly as a consequence of plate spreading. Evaluating their impact on long‐term hydrothermal discharge requires information on the scale and duration of any posteruption enhancement. Here we describe a unique hydrothermal plume time series of annual (or more frequent) observations at Axial Seamount vent fields from 1985 through 2017, missing only 7 years. Axial, a hot spot volcano astride the Juan de Fuca Ridge, experienced eruptions in 1998, 2011, and 2015. In 1998 and 2011 lava flooded the SE caldera and south rift zone, but in 2015 most lava was extruded in a series of flows extending ~20 km down the north rift zone. Response cruises occurred within 18 days (1998) to about 4 months, followed by regular posteruption observations. All 30 cruises measured plume rise height (a proxy for heat flux) and turbidity (indicative of chemical changes in vent discharge) at several vent sites, yielding an integrated view of vent field activity. Venting in the SE caldera area persisted throughout the time series, consistent with the imaged location of the shallowest portion of the melt‐rich magma reservoir. Eruptions produced substantial and diagnostic increases in plume rise and turbidity, and posteruption enhancements lasted 2–5 years, totaling ~10 years over the course of the time series. Estimates of the relative heat flux indicate a sixfold increase during eruption‐enhanced periods, implying that generalizations about mid‐ocean ridge hydrothermal fluxes may be underestimates if based on non–eruption‐enhanced hydrothermal activity alone. Plain Language Summary: We use a three‐decade‐long time series to understand how seafloor volcanic eruptions affect hydrothermal venting. Axial Seamount, 1,400 m deep and about 400 km off the coast of Washington state, has erupted in 1998, 2011, and 2015. Each eruption caused a substantial increase, lasting 2 to 5 years, in the discharge of hot fluids and chemicals from the seafloor. These eruption periods thus contributed more heat and chemicals to the ocean than the 20 years with no eruption activity. Seafloor eruptions may thus play a more important role in hydrothermal venting than previously expected. Key Points: Water column surveys, 1985‐2017, show that hydrothermal plume rise height and turbidity identify the last three Axial Seamount eruptionsPosteruptive enhancement of hydrothermal activity lasted 2‐5 years posteruption, totaling ~10 years over the survey durationPosteruption heat flux increased sixfold, implying that fluxes based on noneruptive activity alone will underestimate the long‐term flux [ABSTRACT FROM AUTHOR]

The Endeavour segment of the Juan de Fuca Ridge is one of the most active and long‐lived hydrothermal areas of the mid‐ocean ridge system. However, the permeability structure that gives rise to long‐term venting at well‐established fields, such as the High Rise, Main Endeavour, and Mothra fields, is not fully understood. Here we jointly invert Pg and Sg traveltimes from a seismic refraction experiment conducted at the Endeavour segment using P‐to‐S coupling constraints. We then calculate porosity and crack density as a function of crack aspect ratio by applying the differential effective medium theory to the seismic velocities. At 1.4‐km depth, average Vp ~5 km off axis increases by ~0.4 km/s compared to the ridge axis. The average Vp/Vs has a minimum of ~1.75 on the ridge axis and increases to a maximum of ~1.84 off axis. The inferred porosity and crack density distributions show that the proportion of thick versus thin cracks decreases from the ridge axis to the flanks, since theoretical models indicate that Vp/Vs increases going from thick to thin cracks (aspect ratio decreasing from 0.1 to 0.01). The dominant presence of thick cracks on the axis may provide long‐term conduits for upflow in high‐temperature hydrothermal circulation potentially forming the vent fields. The increased proportion of thin cracks on the flanks, coupled with the increased seismic velocity, indicates a decrease in permeability caused by progressive clogging of thick cracks due to mineral precipitation likely in the downflow zone of hydrothermal circulation. Key Points: A 3‐D tomographic model of Vp, Vs, and Vp/Vs informs crack aspect ratios in the upper crust at the Endeavour segment, Juan de Fuca RidgeSeafloor spreading produces thick cracks with low Vp/Vs, which fill within a few kilometers of the ridge axis by hydrothermal circulationThick cracks on the axis may provide long‐lived conduits for hydrothermal upflow beneath the high‐temperature vent fields [ABSTRACT FROM AUTHOR]

Models of high‐temperature hydrothermal systems at intermediate‐ to fast‐spreading mid‐ocean ridges have a conductive boundary layer (CBL) separating the magmatic heat source from the convecting hydrothermal fluid. Paleo‐CBLs preserved in the geological record provide a means to test theoretical models of the thermal, mechanical, and petrological evolution of this boundary. CBLs occur as metamorphic contact aureoles at or near the dike‐gabbro boundary where axial magma lenses (now plutonic rocks) intruded into the basal dikes, leading to their transformation into low permeability granulite and hornblende hornfels at >800 °C. Paleo‐CBLs are well documented in the Troodos and Oman ophiolites, and evidence of similar lithologies has been found at every location where the dike‐gabbro transition has been mapped and/or sampled in modern fast‐spreading crust (Hess and Pito Deeps, IODP Hole 1256D). Evaluation of the geothermometers used in studies of the thermal evolution of the CBL shows a lack of consistency that can be understood in terms of compositional controls. Peak temperatures are >900 °C in all areas at the base of the CBL, leading to partial melting, stoping, and disaggregation that facilitates thinning from below as required to maintain the high heat fluxes necessary to drive active black smoker systems. However, such thin CBLs are not the norm; steady state conditions must have thicker CBLs and smaller heat fluxes. In turn, global estimates of properties such as chemical fluxes for normal hydrothermal conditions may lead to substantially different element/heat fluxes than those based on active systems. Key Points: Low permeability paleoconductive boundary layers record peak recrystallization temperatures of >900 °C, ductile behavior, and partial meltingHeat fluxes derived from paleothermal gradients and steady state magmatic construction calculations are much less than those of active vent sitesThin conductive boundary layers form by disaggregation, stoping, and assimilation of altered upper crust during magmatic advance [ABSTRACT FROM AUTHOR]