Mid-ocean Ridge Research Articles

Deep mantle plumes feeding periodic alignments of asthenospheric fingers beneath the central and southern Atlantic Ocean

High-resolution full waveform seismic tomography of the Earth's mantle beneath the south and central Atlantic Ocean brings into focus a series of asthenospheric low shear velocity channels, or "fingers" on both sides of the southern and central mid-Atlantic ridge (MAR), elongated in the direction of absolute plate motion with a spacing of [Formula: see text]1,800 to 2,000 km, and associated with bands of shallower residual seafloor depth anomalies that suggest channeled flow over thousands of kilometers. Each of the three most clearly resolved fingers on the African side of the MAR corresponds to a separate group of whole mantle plumes rooted in distinct patches at the core-mantle boundary, feeding hotspots, and volcanic lines with distinct isotopic signatures. Plumes of a given group appear to merge at the top of the lower mantle before separating again, suggesting interaction of deep mantle flow with a more vigorous mesoscale circulation in the upper mantle. The corresponding hotspots are generally offset from the location of the deep mantle plume roots. The distinct isotopic signatures of these hotspot groups are also detected in the mid-ocean ridge basalts at the location where the fingers meet the ridge. Meanwhile, at least some of the variability within each plume group could originate in the upper mantle and extended transition zone where plumes in a given group appear to merge and pond. This study also adds to mounting evidence that the African large low shear velocity province is not a uniform, unbroken pile of dense material rising high above the core-mantle boundary, but rather a collection of mantle plumes rooted in patches of distinct composition.

Late Cenozoic intraplate volcanism as a trigger for hydrothermal venting in the Arctic southwestern Eurasia Basin

Intraplate volcanism has occurred for the last 35 million years within Northeast Atlantic and Arctic margins, including the western Barents Sea, Svalbard, and northern Greenland. Earlier studies have suggested that some of this volcanism might be sourced from nearby mid-ocean ridges. However, legacy data does not reveal correlations between the sporadic volcanism, despite comparable setting, ages, and compositions of basalts across the area. Here, we utilize a compilation of geophysical data to document late Cenozoic intraplate volcanism affecting the northeastern Yermak Plateau and southwestern Eurasia Basin located north of Svalbard. The identified seabed and subsurface features include volcanic (Mound-A) and hydrothermal vent systems (Tayrona Vent) formed approximately 10 and 2.6 million years ago, respectively. These intraplate volcanic products are coincident in age and origin with observed hydrothermal systems on Svalbard and Northeast Atlantic. We propose that these magmatic features are the result of intraplate volcanism associated with seismic and thermal anomalies in the mantle beneath northern Svalbard.

Assessing the impact of sea level rise on maritime entitlement and delimitation: an interdisciplinary investigation through legal and technical analysis

This paper delves into the complexities of maritime delimitation in the context of sea level rise (SLR) and ice-covered regions, examining several factors and legal implications. Through academic discussion and technical analysis, the adjustments is required in approaches to the Exclusive Economic Zone and the continental shelf boundaries’ delimitation amidst SLR, while the different models adopted by adjacent and opposite states are presenting. As a result, the paper provides a comprehensive overview of common issues surrounding baseline determination, particularly in relation to SLR and the challenges posed by off-shore features. Legal dynamics concerning ‘submarine ridges’ versus ‘oceanic ridges’ are explored, highlighting the complexities inherent in maritime boundary delineation. Additionally, the dynamics of basepoint selection in ice-covered regions is investigated, emphasizing essential criteria for navigation and offering case studies from the Antarctic and Arctic here. Through this exploration, the paper contributes to a deeper understanding of the challenges and considerations involved in maritime delimitation amidst SLR, offering valuable insights from both technical and legal perspectives.

Geochemical constraints on reaction temperature and pressure and heat- and mass-transfer efficiency at Rainbow Hydrothermal Field

Seafloor vent fluids hosted by oceanic core complexes (OCCs) are thought to represent circulation of seawater-derived hydrothermal fluid along deeply penetrating, low-angle detachment faults. However, estimation of the source temperatures and pressures of such fluids has been limited because geochemical methods typically require vent fluid silica concentrations to be buffered by quartz, a condition not often met at oceanic core complexes. Here, we extend the calculation of the Si-Cl geothermobarometer to enable predictions of fluid Si concentrations in equilibrium with any Si-buffering mineral assemblage, rather than being restricted to quartz. We apply this method to Rainbow Hydrothermal Field vent fluid compositions and find that they are consistent with buffering by a plagioclase+talc+chlorite+tremolite mineral assemblage at conditions ranging from (430 °C, 359 bar) to (470 °C, 468 bar), which correspond to depths of 1.3–2.4 km below the seafloor. These estimates agree well with the locations of seismically imaged magma chambers within the Rainbow Massif. Additionally, calculations of fluxibility and Fe solubility support this relatively shallow origin for Rainbow vent fluids and imply relatively efficient heat and Fe extraction from the seafloor. We estimate that only 24–30 % of the heat content and almost no Fe is lost during upflow of Rainbow vent fluids.Compared to other OCC-hosted seafloor vents, the source region of Rainbow vent fluids is anomalously shallow, an observation consistent with geological interpretations of the Rainbow Massif. Vent fluids at Rainbow Hydrothermal Field have exhibited apparently stable and greater-than-seawater salinity for over two decades. We interpret these vent fluids as vapors derived from a higher-salinity source fluid that developed over multiple cycles of magma injection and phase-separation inherent to the formation of oceanic crust along slow-spreading, non-volcanic segments of oceanic spreading centers. Alternatively, higher-salinity source fluids could be derived from mineral hydration reactions associated with serpentinization of ultramafic rocks. The occurrence of greater-than-seawater salinity vent fluids is thus predicted to be a common feature of OCC-hosted vent fields, as indicated by several known examples, including the TAG, Kairei, and Edmond vent fields.

Magmatic and metamorphic evolution of the Late Cretaceous Muslim Bagh Ophiolite, western Pakistan: implications for ridge subduction after subduction initiation

ABSTRACT The magmatic and metamorphic evolutions of the Muslim Bagh Ophiolite (MBO) can help understand the relationship among supra-subduction ophiolite, subduction initiation, and the spreading centre in the forearc. Here, we investigated the whole-rock chemistry, mineral composition, P–T conditions, and K–Ar ages of the MBO and its associated units. The whole-rock and magmatic clinopyroxene compositions indicate that the MBO amphibolites and tholeiitic basalts of the underlying Bagh Complex originated in a supra-subduction setting and mid-ocean ridges, respectively. Late Cretaceous oceanic island basalts found in the Bagh Complex and the Bibai Formation occurred in oceans and near the passive continental margin of western India, respectively. Metamorphic Mg-hornblendes in the MBO amphibolites record low-P/T type upper amphibolite-facies metamorphism (2.4–2.9 kbar, 612–690 °C), indicating a > 50 ºC/km geothermal gradient. The K–Ar hornblende and plagioclase ages suggest contemporaneous low-P/T type metamorphism at ca. 78–71 Ma in the MBO formed in a forearc and tholeiitic ridge-related magmatism of ca. 78–77 Ma in the Bagh Complex, indicating a spreading ridge was active during the subduction. Our results, combined with plate kinematic reconstructions of the Neotethys Ocean in the previous studies, suggest that the abnormally high geothermal gradient in the MBO could be due to orthogonal ridge subduction near the triple junction after the ca. 80 Ma subduction initiation in the forearc. It is also likely that the geothermal gradient in the MBO dropped from >50 °C/km at 78–71 Ma, through 35–45 °C/km, to <25 °C/km at approximately 65 Ma due to gradual steepening of the subducting slab, followed by the obduction of the MBO during the Paleocene to Eocene period.

Petrogenesis and tectonic setting of mafic magmatism within the Laguna Amarga Metamorphic Complex, Andes of Catamarca, Argentina: Insights into the opening of the Cuyania/Precordillera terrane from the Ouachita rift

Neoproterozoic crystalline basement rocks are exposed as fault-bounded blocks over the high Andes of Catamarca. The crystalline basement is stratigraphically grouped into the unique Laguna Amarga Metamorphic Complex and represents the northern extension of the Cuyania/Precordillera terrane. Tabular bodies of meta-mafic rocks are widespread in the basement interspersed within a thick sequence of meta-sedimentary rocks derived from siliciclastic, calc-silicate, and limestone protoliths. Overall, the geochemical characteristics of meta-mafic rocks are in the compositional range of Normal-Mid Ocean Ridge Basalt (Normal-MORB), reflecting a common depleted-mantle source with varying degrees of partial melting. While preserving the typical bulk chemistry of MORB magmatism, some mafic magma underwent differentiation at emplacement, leading to the development of high-Ti mafic rocks. New U-Pb zircon geochronology reveals three distinct age peaks, with two coinciding with ages identified in the metasedimentary host rocks. The dominant Mesoproterozoic age cluster is linked to inherited zircon crystals assimilated within a single meta-mafic rock. In contrast, zircon ages from the late Ordovician to early Devonian are attributed to metamorphic overgrowths. Notably, the third age cluster, delineates a Late Neoproterozoic magmatic event, indicating the temporal span of mafic magmatism. The finding agrees with the best available age (576 ± 17 Ma) for mafic magmatism on the Precordillera Mafic-Ultramafic Belt. Stratigraphic relationships and geochemical fingerprints enable correlation among the meta-mafic rocks from Laguna Amarga, tracing a belt of mafic magmatism with an oceanic affinity that extends southward. Building upon previous works, this study reaffirms that the rift-drift transition of the Cuyania/Precordillera terrane, linked to the Ouachita rift opening from southeastern Grenville, evolved during the latest Neoproterozoic.

The effective elastic thickness along the Emperor Seamount Chain and its tectonic implications

Summary The Hawaiian-Emperor Seamount Chain, a pre-eminent example of a hotspot-generated intraplate seamount chain, provides key constraints not only on the kinematics of plates but also on their rigidity. Previous studies have shown that the effective elastic thickness, Te, a proxy for the long-term strength of the lithosphere, changes abruptly at the Hawaiian-Emperor ‘bend’ from low values (∼16 km) at the Emperor Seamounts to high values (∼27 km) at the Hawaiian Ridge. To better constrain Te along the poorly explored Emperor Seamounts we have used a free-air gravity anomaly and bathymetry gridded data set, together with fully 3-dimensional elastic plate (flexure) models, to estimate the continuity of Te and volcano load and infill densities along 1000 profiles spaced 2 km apart of the chain. Results show that Te generally decreases northward along the chain. The decrease is most systematic between Ojin and Jimmu guyots where Te depends on the age of the lithosphere at the time of volcano loading and is controlled by the 340°C and 400°C oceanic isotherms. The largest variation from these isotherms occurs at the northern and southern ends of the chain where Te is smaller than expected suggesting the influence of pre-existing, older, loads. We use these results to constrain the subsidence, flexural tilt, rheological properties, and tectonic setting along the seamount chain. We found an excess subsidence in the range 1.2-2.4 km, a tilt as large as 2-3°, oceanic lithosphere that is weaker than it is seaward of the weak zone at subduction zones, and a tectonic setting at Detroit and Koko seamounts that, despite their forming an integral part of the hotspot generated seamount chain, retains a memory of their proximity to earlier loads associated with plume influenced mid-oceanic ridges.

Spreading ridge migration enabled by plume-ridge de-anchoring

It has long been recognised that spreading ridges are kept in place by competing subduction forces that drive plate motions. Asymmetric strain rates pull spreading ridges in the direction of the strongest slab pull force, which partially explains why spreading ridges can migrate vast distances. However, the interaction between mantle plumes and spreading ridges plays a relatively unknown role on the evolution of plate boundaries. Using a numerical model of mantle convection, we show that plumes with high buoyancy flux (>3000 kg/s) can capture spreading ridges within a 1000 km radius and anchor them in place. Exceptionally high buoyancy fluxes may fragment the overriding plate into smaller plates to accommodate more efficient plate motion. If the plume buoyancy flux wanes below 1000 kg/s the ridge may be de-anchored, leading to rapid ridge migration rates when combined with asymmetric plate boundary forces. Our results show that plume-ridge de-anchoring may have contributed to the rapid migration of the SE Indian Ridge from 43 million years ago (Ma) due to waning buoyancy flux from the Kerguelen plume, supported by magma flux estimates and radiogenic isotope geochemistry of eruption products. The plume-ridge de-anchoring mechanism we have identified has global implications for the evolution of plate boundaries near mantle plumes.

Petrogenesis and tectonic setting of the proto-tethyan dachaidan ophiolite, North China: insights from clinopyroxene chemistry and whole-rock Sr–Nd–Pb and zircon Hf isotopes

IntroductionThe Dachaidan ophiolites outcrop within an ultrahigh-pressure metamorphic belt along the northern margin of the Qaidam Basin. However, their age, source, and tectonic setting remain still in debate.MethodIn this study, we investigated the geochemistry and geochronology of the Dachaidan ophiolitic gabbros.Results and DiscussionZircon U–Pb dating yielded a crystallization age of 510.0 ± 2.8 Ma and 510.0 ± 2.9 Ma for the gabbro. The gabbros have low SiO2 contents (47.15–50.10 wt.%) and high MgO contents (6.35–9.04 wt.%) and Mg# values (55–74). The total rare earth element (∑REE) contents are 8.35–28.07 ppm, lower than those of normal-type mid-ocean ridge basalts (MORBs), and the gabbros exhibit light REE depletion or flat REE patterns, with small positive Eu anomalies (Eu/Eu* = 1.06–1.40). Trace element patterns are depleted to enriched in Nb and Ta, similar to island arc rocks and MORB. Clinopyroxene thermobarometry indicates the parental magma of the gabbros formed by high-temperature (1,318°C–1,363°C) and medium-pressure (1.27–1.64 GPa) partial melting in a mantle wedge. The gabbros have depleted Sr–Nd–Pb-Hf isotopic compositions, with (87Sr/86Sr)i = 0.704586–0.707441, εNd(t) = 4.7–6.6, and zircon εHf(t) = 7.6–11.4. The age-corrected Pb isotope ratios of these volcanic rocks are variable, with 206Pb/204Pb(t) = 18.085–18.253, 207Pb/204Pb(t) = 15.595–15.614, and 208Pb/204Pb(t) = 37.880–38.148, which are similar to the isotopic compositions of typical Indian MORBs. The source of the Dachaidan ophiolite is inferred to have been depleted mantle. The Dachaidan ophiolite likely formed in a forearc oceanic setting along the northern margin of the Qaidam Basin, during the initial subduction of an oceanic plate.

A synproportionation-controlled hybrid hydrothermal mineralization system in the Okinawa Trough

Hybrid hydrothermal systems, in which both water-rock reactions and magmatic fluids contribute to sulfide mineralization, have recently been reported in some submarine arcs and mature back-arc basins. Owing to their high mineralization efficiency, hybrid hydrothermal systems have attracted significant attention from geologists. This study provides first clues for the identification of a novel hybrid hydrothermal system (synproportionation-controlled instead of the more common disproportionation-controlled) in the Tangyin field, Southern Okinawa Trough. The studied samples are native sulfur-type precipitates, primarily composed of elemental sulfur with minor sulfide disseminations. The distinct crystallization conditions and S isotope compositions of elemental sulfur and sulfide disseminations indicate their different genesis. The S in sulfide disseminations (δ34S = −2.7 to 1.6 ‰) is derived from water-rock reaction and subsequently influenced by fluid phase separation, which is further demonstrated by the trace element contents of pyrite (low Tl/Pb, Sb/Pb, Te/As, Te/Sb ratios). In contrast, the elemental sulfur (δ34S = 3.9 to 4.3 ‰) is formed by the synproportionation of magmatic SO2 and hydrothermal H2S from the boiled liquid. A simple two-component mixing calculation indicated that the contributions of the water-rock reactions and magmatic fluids to the Tangyin hydrothermal system were approximately equal. The higher temperature (> 350 °C) and oxygen fugacity conditions may be the key factors for the development of a synproportionation-controlled hybrid hydrothermal system in the Tangyin field. Hydrothermal systems in back-arc basins exhibit higher sulfide accumulation rates than those in mid-ocean ridges; this may be attributed to the prevalence of hybrid hydrothermal vents in back-arc settings.

The Mundeck Salt Unit: A Review of Aptian Depositional Context and Hydrocarbon Potential in the Kribi-Campo Sub-Basin (South Cameroon Atlantic Basin)

The Kribi-Campo sub-basin, located in the Gulf of Guinea, constitutes the southeastern segment of the Cameroon Atlantic Margin. Drilling in the Aptian salt unit revealed a sparse hydrocarbon presence, contrasting with modest finds in its counterparts like the Ezanga Salt in Gabon and the Rio Muni Salt in Equatorial Guinea. This discrepancy prompted a reassessment of the depositional context and hydrocarbon potential of the Mundeck salt unit. By integrating 2D seismic reflection and borehole data analysis, this study established the structural and stratigraphic framework of the area, emphasizing the salt unit’s significance. Borehole data indicate a localized salt unit offshore Kribi, with seismic reflection data revealing distinct forms of diapir and pillow. This salt unit displays a substantial lateral extent with thicknesses ranging from 4000 m to 6000 m. The depositional context is linked to the following two major geological events: a significant sea-level drop due to margin uplift during the Aptian and thermodynamic processes driven by transfer faults related to mid-oceanic ridge formation. These events were crucial in forming and evolving the Mundeck Salt. Regarding hydrocarbon prospects, this study identifies the unit as being associated with potential petroleum plays, supported by direct hydrocarbon indicators and fault-related structures. The findings suggest that untapped hydrocarbon resources may still exist, underscoring the need for further exploration and analysis.

Unraveling abiotic organic synthesis pathways in the mafic crust of mid-ocean ridges

The aqueous alteration of the oceanic lithosphere provides significant energy that impacts the synthesis and diversity of organic compounds, which are crucial for the deep carbon cycle and may have provided the first building blocks for life. Although abiotic organic synthesis has been documented in mantle-derived rocks, the formation mechanisms and complexity of organic compounds in crustal rocks remain largely unknown. Here, we show the specific association of aliphatic carbonaceous matter with Fe oxyhydroxides in mafic crustal rocks of the Southwest Indian Ridge (SWIR). We determine potential Fe-based pathways for abiotic organic synthesis from CO2 and H2 using multimodal and molecular nano-geochemical tools. Quantum mechanical modeling is further employed to constrain the catalytical activity of Fe oxyhydroxides, revealing that the catalytic cycle of hydrogen may play a key role in carbon-carbon bond formation. This approach offers the possibility of interpreting physicochemical organic formation and condensation mechanisms at an atomic scale. The findings expand our knowledge of the existence of abiotic organic carbon in the oceanic crustal rocks and emphasize the mafic oceanic crust of the SWIR as a potential site for low-temperature abiotic organic synthesis.

Zinc isotopic composition of oceanic crust: Insights from oceanic gabbro cumulates and MORBs

Subducting oceanic crust is a critical driver of chemical and lithological heterogeneity within the deep mantle. Zinc isotopes (δ66ZnJMC-Lyon) have been widely utilized to trace recycled oceanic crust and surficial materials. However, the lower oceanic crust rocks are not well studied given limited sampling, making it unclear if the δ66Zn estimated from mid-ocean ridge basalts (MORBs) is valid to represent that of the bulk oceanic crust. In this study, we report δ66Zn data for a series of gabbro cumulates (n = 37) from the ultraslow-spreading Southwest Indian Ridge, alongside MORBs (n = 12) from the fast-spreading East Pacific Rise and the slow-spreading South Mid-Atlantic Ridge. MORBs across various spreading rates exhibit homogeneous δ66Zn values (0.27 ± 0.05 ‰, 2sd), in agreement with previous studies. In contrast, the gabbros with several magma episodes exhibit a significant variation of δ66Zn from 0.11 ‰ to 0.34 ‰, with an average of 0.22 ± 0.11 ‰ (2sd). Magma differentiation to form oceanic crust can partly explain such variation (∼ 0.10 ‰), and post-cumulus modification further extends this range. Given the voluminous lower oceanic crust, the weighted δ66Zn estimated for the bulk oceanic crust (0.23 ± 0.03 ‰) is lower than the MORB-based value, which reconciles with the predictions from mantle partial melting. Therefore, recycled oceanic crust with significantly varied Zn isotopic compositions would lead to heterogeneous δ66Zn of the metasomatized mantle. The overall lower δ66Zn value underscores the importance of surficial materials other than oceanic crust, such as carbonates, to explain high δ66Zn of many mantle-derived magmas.

The mid-Variscan suture in the Morais complex of Trás-os-Montes (Portugal): insights from geochemistry and geochronology of oceanic rocks

ABSTRACT The Middle Allochthon of the Galicia-Trás-os-Montes Zone (GTMZ) in NW Iberia is an assemblage of oceanic units representing the Variscan suture of one or more peri-Gondwanan oceans. This contribution brings new geochronological and geochemical data from the Middle Allochthon of the Morais Complex, in northern Portugal. There, the oceanic ensemble consists of a stack of five tectonic units which from bottom to top are the Junqueira, Pombais and Izeda oceanic supracrustals and the Remondes and Morais-Talhinhas ophiolitic units. The Junqueira and Pombais units consist of greenschists and metapelites. No age data are available for these two units, but a correlation is established with a Cambro-Ordovician unit in the Órdenes Complex of Galicia (NW Spain) dated at 500 Ma. The Izeda Unit consists of fine-grained, low-grade amphibolites transitional to epidote-amphibolites and greenschists, for which an age of 484 ± 3 Ma has been obtained for a metabasic tuff. In the ophiolitic ensemble, the Remondes and Morais-Talhinhas units consist of fine-grained amphibolites associated with deformed metagabbros, mafic cumulates and serpentinized ultramafics. In this study, two plagiogranite samples from the Remondes Unit yielded a complex spread of age data with Silurian ages representing the dominant population. This suggests that the newly defined Remondes Unit is younger than the Izeda Unit but older than the previously published age for these rocks, which are considered part of the overlying Morais-Talhinhas Unit, dated as Devonian (406–395 Ma) in a previous study. The geochemistry of the basic rocks in the five units is that of NMORB with a subduction-derived component that varies from weak to null. The ages, together with new and previously published whole rock geochemical data obtained from basic igneous samples, are interpreted to date and reflect the formation of igneous protoliths in oceanic ridge settings associated with the Mid-Variscan Ocean, a part of the Rheic oceanic realm that registered magmatic activity between the late Cambrian and the Lower Devonian.

Dynamic evolution of competing same-dip double subduction: New perspectives of the Neo-Tethyan plate tectonics

Same-dip double-subduction (SDDS) systems are widely reported from present as well as past complex convergent plate tectonic configurations. However, the dynamics of their evolution is poorly understood, which is crucial to conceptualize anomalous subducting slab kinematics and associated observed geological phenomena, such as irregular trench migration rates, high convergence velocities, and slab break-off. To bridge this gap, we develop dynamic thermo-mechanical subduction models and investigate the initiation and evolution of SDDS systems, considering three different initial plate configurations: oceanic, oceanic-continental and multiple continental settings, based on Neo-Tethyan paleo-reconstructions. Each model offers new insights into the complex tectonic history of the major Neo-Tethyan subduction zones, particularly the Indo–Eurasian and Andaman convergent systems. We evaluate the slab-slab interactions, trench and subduction kinematics, inter-plate reorganization, and temporally varying mantle flow patterns involved in the dynamic evolution of these SDDS systems. The oceanic SDDS model simulations reveal that a sizable oceanic plate can initiate two subduction zones synchronously, and they evolve unequally in a competing mode, leading to exceptionally high convergence rates (∼16–17 cm/year) for a prolonged duration (∼7–8 Myr). This finding explains the coeval activity of coupled subduction zones in the Indo-Eurasia convergence during the Cretaceous evolution of the Neo-Tethys. We further implement a corresponding single subduction model to assess the additional effects of competing slab kinematics in an oceanic SDDS setting. The ocean-continent SDDS model, on the other hand, localizes subduction preferentially at passive margins between the oceanic plate and the continental block, forming double subduction zones that grow almost equally to form a spreading centre between the two trenches. These model results allow to reconstruct the Cenozoic evolution of the eastern Neo-Tethyan region, which ultimately led to the development of the Andaman subduction zone. We also show the post-Cretaceous evolution of the Indo–Eurasian collision zone as a consequence of the SDDS dynamics in presence of multiple continental blocks. These dynamics facilitated slab break-off, transforming the SDDS into a single subduction system in a relatively short time frame (∼3 Myr). We finish with a synthesis of the paleo-reconstructions of the Neo-Tethys in the perspective of these SDDS models.

Ridgeward flow of compositionally heterogeneous mantle produces near-ridge seamount chains in the South Pacific

Many near-ridge seamounts and seamount chains in the Pacific Ocean have a nonplume origin. Yet, their origin remains to be fully understood. Our new geochemical study on seamount basalts from the Pukapuka Ridge (PPR) finds a large along-ridge lava compositional variation with a gradual decrease in a geochemically enriched component toward the East Pacific Rise (EPR) axis. This spatial geochemical variation is best understood as resulting from decompression melting of compositionally heterogeneous mantle flowing toward the ridge, where the mantle consists of low-solidus materials of metasomatic origin dispersed within a more refractory peridotite matrix. Far from the ridge axis, preferential melting of enriched lower-solidus materials under thicker lithosphere leaves less enriched residues that undergo further decompression melting as they flow beneath thinner lithosphere toward the ridge axis. This process gives rise to the progressively less enriched lavas along the PPR chain toward the EPR. The residual enriched mantle component became embedded in the mantle beneath the southern EPR (13°S−23°S), forming an along-axis compositional dome at the EPR-PPR intersection (∼17°S−19°S). We predict that nonplume seamounts are best expressed as linear chains near and perpendicular to ocean ridges on fast-spreading plates as long as the flowing mantle is sufficiently heterogeneous. This finding explains widespread seamounts of nonplume origin in the Pacific Ocean, and it also explains the geophysical asymmetries in the mantle electromagnetic tomography (MELT) area.

Explicit role of seamount subduction in upper plate deformation as exemplified in the Ryukyu subduction zone

The Ryukyu subduction zone is typical of the subduction of an oceanic ridge (seamount or plateau) and back-arc rifting. How the upper plate responds to the combined effect of subduction of a bathymetric high and back-arc spreading is not fully understood. We investigate the combined effect by calculating the T-axis from regional focal mechanisms. Three patterns are observed from the outer rise to the back arc, showing a coherent trench-normal T-axis along the strike. They can be readily explained by outer-rise plate bending, forearc upper-plate bending induced by seamount subduction, and back-arc spreading associated with slab rollback. A distinct pattern of trench-parallel T-axes along the strike is also observed in the arc. This can be attributed to the difference in the extension rate between forearc extension and back-arc rifting of the upper plate, where the rate of back-arc extension driven by slab rollback is faster than that of forearc trench-normal extension attributed to seamount subduction. We also examine the T-axis patterns in other systems with seamount subduction, including the southern Manila subduction zone and the southern Central to northern South American subduction zones, displaying similar trench-normal T-axes in the forearc. Therefore, our observations have a broader implication in upper-plate deformation in response to seamount subduction.

A new type of submarine chimneys built of halite

In contrast to the subaquatic sulphide and carbonate chimneys, which are known from Mid Ocean Ridges and abyssal submarine volcanoes, chimneys built of salts have not been described yet. Here we present such halite chimneys as a new form of cold-water smokers in hypersaline environments. The here described structures rise up from the bottom of the Dead Sea and result from the submarine discharge of saturated halite-dissolution brines into the salt lake, which is at halite saturation and holds remarkable chloride excess. At the interface with the lake brine, halite precipitates instantaneously, forming chimneys up to several meters in height. The brines leading to the formation of these chimneys vary in composition, while their generation processes are similar. Fresh groundwater from surrounding aquifers enters the saline lake sediments and considerably leaches halite in the adjacencies of the lake. Simultaneously, it mixes with ancient brines before it emerges from the lake floor. The distinct differences in composition between the Dead Sea and the emerging chimney brines lead to the instantaneous crystallisation of halite and few other mineral phases. The chimney structure result from the buoyancy flow of the chimney brines, which are less dense then the ambient Dead Sea.The chimneys indicate intense cavitation of massive halite bodies in the subsurface of the Dead Sea environment, a process that leads to increasing formation of hazardous sinkholes. Since chimneys are proven in shallow water but may be expected in deeper parts too, they are comfortably mappable by echo-sounding or aerial imaging. They thus provide in the Dead Sea as in any likewise setting a potent predictive tool to locate dangerous subsurface cavitation and hence areas that are at risk of collapse in the near future.

New insight into the velocity and anisotropy structures of the subduction zone in northern Sumatra

In this study, we conducted seismic tomographic inversions to investigate the velocity and anisotropy structures of northern Sumatra, using 9774 P-wave and 8405 S-wave arrivals from regional earthquakes. Isotropic P-wave velocity, isotropic S-wave velocity, P-wave azimuthal anisotropy, and P-wave radial anisotropy models were generated using eikonal equation-based traveltime tomography methods. The study identified low-velocity zones beneath the Toba and Sinabung volcanoes, potentially indicating the presence of magma reservoirs. Furthermore, low-velocity anomalies above the subduction slab were detected, which are likely caused by the dehydration of the slab and interpreted as channels of upwelling flow. The tomographic results revealed a trench-parallel high-velocity belt in the uppermost mantle, representing the subducting slab of the India-Australian plate. The trench-parallel fast velocity directions in the slab suggested that the subducted oceanic slab retains its frozen-in anisotropy formed at the mid-ocean ridge, or that the anisotropy is induced by the lattice-preferred orientation of the B-type olivine. Negative radial anisotropy in the mantle wedge was observed, reflecting hot upwelling flows and transitions of olivine fabrics in the presence of water due to slab dehydration. The results also indicated a multilevel magma plumbing system beneath the Toba Caldera. In summary, the results of this study provide new insights into the structure and dynamic processes of the northern Sumatra subduction zone.

Sulfur and metal mobilization during the life cycle of an oceanic core complex: Implications for seafloor massive sulfide deposits formation at slow and ultra-slow spreading ridges

Seafloor massive sulfide (SMS) deposits at slow and ultra-slow spreading ridges are often spatially related to, or hosted in oceanic core complexes (OCCs). The specific oceanic crust architecture, magmatism, hydrothermal fluid circulation and lithologies at OCCs, however, imply different S and metal (e.g. Cu, Zn, Co, Ni) fluxes relative to well-structured oceanic crust at-fast spreading ridges and which are not yet fully constrained. The study of S and metal distribution in the ODP Hole 735B deep drill core from the Atlantis bank allows to understand these fluxes along detachment faults and to better constrain the source zone of S and metals for OCC-related SMS deposits. Significant depletion of S, Cu, Zn and Ni are observed within the upper 250 m of the drill core where intense deformation and hydrothermal fluid circulation occurred. During the complex tectono-magmatic-hydrothermal evolution of the Atlantis Bank, four important stages are recognized for S and metal mobilization: 1) magmatic stratification leading to a higher proportion of sulfide-rich and S, Cu, Zn and Co fertile oxide gabbros in the root zone of the Atlantis Bank detachment, 2) high temperature ductile deformation leading to magmatic sulfide reworking and onset of sulfide leaching with limited metal mobilization, 3) extensive sulfide leaching and metal mobilization during amphibolite to greenschist facies metasomatism and, 4) late stage secondary sulfide precipitation and S enrichment during low temperature fluid circulation. Mass balance calculations from the source zones of the Atlantis Bank detachment highlights that metal mobilization during hydrothermal alteration of gabbroic rocks along detachment faults can fully account for the formation of OCC-related SMS deposits at slow and ultraslow spreading ridges. The Atlantis Bank detachment system, however, is gabbroic-dominated and represent the magmatic end-member of OCCs and further work is necessary for understanding metal fluxes in ultramafic-dominated detachment systems.