Mid-ocean Ridge Research Articles

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.

Upper mantle scale enrichment of Cenozoic intraplate magmatism in Northeast Asia: He-Sr-Nd-Pb-O isotope geochemistry of the basalts around the Korean peninsula

The Earth’s mantle is considered to be geochemically heterogeneous, which is reflected by the diverse composition of oceanic island basalts (OIB). The mantle enrichment resulting in this is attributed primarily to the influx of recycled crustal materials into the mantle through subduction. Additionally, the sub-continental lithospheric mantle (SCLM) complicates the elucidation of mantle heterogeneity. From this perspective, Northeast Asia, where the Pacific stagnant slab in the mantle transition zone and the SCLM distribution are presented, is the suitable site for examining the upper mantle scale enrichment. Here we report He-Sr-Nd-Pb-O isotope values of basalts that erupted around the Korean Peninsula to illustrate the source lithology and components that led to mantle heterogeneity. Our measured helium isotope ratios ranging from 5.7 to 7.3 Ra (3He/4He ratio of air, Ra = 1.39 x 10-6) are mostly within the SCLM range (6.1 ± 0.9 Ra) but lower than the mid-ocean ridge basalt range (MORB; 8 ± 1 Ra). The Sr-Nd-Pb isotope compositions of the basalts generally display a mixture of depleted MORB mantle (DMM), enriched mantle 1 (EM1), and enriched mantle 2 (EM2) components. In addition, the basalts have δ18Oolivine (vs. V-SMOW) values ranging from 4.7 to 5.7 ‰ that deviate from the DMM range (δ18Oolivine = 5.1 ± 0.2 ‰). Our isotopic analysis results highlight the role of the pyroxenite source in the metasomatized SCLM in the genesis of basalts. The low 3He/4He range of the basalts indicates a significant role in the SCLM. Moreover, the delaminated cratonic SCLM and asthenosphere-lithosphere interaction are candidate scenarios for the low 3He/4He ratios. Therefore, we propose that mixing of DMM (high 3He/4He ratio; 7 to 9 Ra) and the metasomatized SCLM (low 3He/4He ratio; 5 to 7 Ra) allowed enrichment within the upper mantle scale for the Cenozoic intraplate magmatism in Northeast Asia.

Do we really need to drill through the intact ocean crust?

We must persevere to drill through the intact ocean crust to fully address fundamental questions towards completion of the plate tectonics theory. The primary questions include: what is the ocean crust made up of, how thick is it and what is the petrological nature of the crust-mantle boundary (i.e., Mohorovičić discontinuity or Moho)? These questions may sound naive because they are widely believed to be well-understood facts, but they are not. Correctly, our current knowledge remains incomplete, and some popular misperceptions come from interpretations based on convenient assumptions. One assumption is that the ocean crust inferred from seismic data is of magmatic origin. Testing this assumption is a principal motivation of Project Mohole (1957–1966), attempting to drill intact ocean crust across the Moho into the mantle. Project Mohole failed because of its high cost, engineering challenges and insufficient tries, but the technologies developed made subsequent ocean drilling successful. However, answers to the original questions remain unsatisfactory. For example, seismic crust interpreted to be of magmatic origin is shown to have globally uniform thickness of 6.0 ± 1.0 km, but crust with such thickness at many slow-spreading ridge segments is dominated by serpentinized mantle peridotites exposed on seafloors. Therefore, the popular view on ocean ridge magmatism must be re-examined, which needs intact ocean crust drilling into the mantle. Drilling at geologically simple sites in the fast-spreading Pacific seafloor is most promising.The US-led D/V JOIDES Resolution that has well served the scientific ocean drilling since 1985 is to retire by the end of 2024, but timely the Chinese geoscience community wishes to continue this international endeavor using the purpose-built D/V Meng Xiang to be in service in 2025. The international community is to gather in November 24–27, 2024, Guangzhou, China, to discuss strategies on where and how to successfully drill intact ocean crust across the Moho in coming years

The geodynamics of plume-influenced mid-ocean ridges: insights from the Foundation Segment of the Pacific-Antarctic Ridge

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 238U/204Pb) OIB (Ocean Island Basalt) component characterized by 206Pb/204Pb of up to 20.5 in its center and a more EM1-like (Enriched Mantle one) OIB component characterized by low U/Pb and 206Pb/204Pb but high Rb/Nb and 87Sr/86Sr 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.

Heavy iron isotopes reveal mantle oxidation in addition to pyroxenite sources for intraplate basalts

The heavier iron isotopic composition of some oceanic island basalts (OIB) relative to primary mid-ocean ridge basalts (MORB) has commonly been interpreted to reflect the heavy bulk isotopic compositions of their sources (e.g., of a pyroxenite-dominated mantle region). Alternatively, comparable Fe isotopic signatures observed in continental intraplate basalts (CIB) may record enhanced isotope fractionation during partial melting of an oxidized source, offering valuable insights into mantle oxidation induced by subducted carbonate. To distinguish between these two independent contributions, here we present high-precision Fe isotopic data (δ57Fe) and Fe3+/∑Fe ratios for a suite of Cenozoic intraplate basalts from Inner Mongolia, East Asia. These basalts are distributed along a northwest-southeast (NW–SE) profile and associated with earlier subduction of the Paleo-Asian oceanic slab. Variations in Fe/Mn ratios and zinc isotopic compositions (δ66Zn) of these basalts reveal that altered oceanic crust has always contributed to their mantle sources but the amounts of recycled carbonate gradually vary. Together with existing data, we demonstrate that all of these basalts generally exhibit heavier δ57Fe (0.07 ‰ to 0.31 ‰), higher Fe3+/∑Femelt (0.08 to 0.39) and Fe/Mn (> 65) relative to primary MORB (δ57Fe ∼0.10 ‰; Fe3+/∑Femelt ∼0.10; Fe/Mn ∼60), pointing to a common origin of heavy Fe isotopes in intraplate basalts that may reflect those of pyroxenite sources (e.g., recycled crust). However, these basalts containing larger amounts of recycled carbonate in their sources (i.e., higher δ66Zn) have heavier δ57Fe at similar Fe/Mn ratios and proportions of pyroxenite melt. Such Fe isotopic signatures exceed the exclusive role of pyroxenite and are best attributed to partial melting at oxidizing conditions (i.e., high Fe3+/∑Fesource) with an inferred enlarged fractionation factor. This oxidized mantle source formed through the reduction of recycled carbonate to graphite/diamond at depths greater than 150 km. A global dataset of Fe isotopes in intraplate basalts is compiled in order to examine this interpretation further. Compared with OIB typically from a pyroxenite/eclogite-dominant source, CIB with similarly heavy δ57Fe and formed by similar melting degrees are commonly more silica-undersaturated, indicating a widespread carbonated mantle source. Thus, these observations highlight that heavy Fe isotopes can record carbonate-induced mantle oxidation in addition to source lithological heterogeneity for intraplate basalts.

Transfer learning reconstructs submarine topography for global mid-ocean ridges

Mid-ocean ridges are unique, tectonically active geographical units on Earth that profoundly control the ocean environment and dynamics at the global scale. However, high-resolution topographic data from mid-ocean ridges are rarely available due to the difficulty in detecting ocean floors, which further limits ocean research at the global scale. Here, we divide the global mid-ocean ridge system into 2805 tiles and reconstruct their high-resolution topography by using a transfer learning approach with freely available low-resolution digital elevation models (DEMs) and limited high-resolution DEMs. A high-frequency terrain feature-based deep residual network is proposed to generate high-resolution global mid-ocean ridge DEMs. In this network, topographic knowledge related to mid-ocean ridges is integrated and quantified to improve the learning efficiency and reconstruction quality of the network. A series of verifications and evaluations demonstrate the reliability of reconstructed topographies for submarine topography research. We observe that reconstructed topography can achieve good environmental understanding and information acquisition in the global mid-ocean ridge range. We find that the complexity of the previous terrain environment is underestimated by 26.63% in terms of the slope gradient and by 14.95% in terms of terrain relief, while a 101.10% information improvement can be obtained for the reconstructed topography. The reconstructed topography indicates that diverse and intricate topographical environments of mid-ocean ridges exist among different ocean regions. The proposed transfer learning method for reconstructing high-resolution mid-ocean ridge topographies is valuable and can be utilized for reconstructing information in regions that are difficult to observe directly and lack sufficient data.

МОРФОЛОГИЯ ОБЛАСТИ СОЧЛЕНЕНИЯ ХРЕБТА РЕЙКЬЯНЕС С РИФТОВЫМИ ЗОНАМИ ИСЛАНДИИ

The joint zone between subaquatic Reykjanes Ridge and Icelandic rift zones known as Reykjanes Rift Zone is a unique transform zone formed in conditions of interaction between the mantle plume and the mid-oceanic ridge. Its morphostructures include a row of en-echelon volcanic systems with oblique extension. The modern tectonic and volcanic relief was formed almost completely during Holocene: the previous landforms were reworked or destroyed by erosion of the Late Pleistocene glaciation. Morphology of the Reykjanes Rift Zone is transitional between the Reykjanes Ridge and the adjacent Western Rift Zone from the northeast. Unlike the Reykjanes Ridge, volcanic activity is not concentrated with in axial volcanic ridges, but is confined by eruptive centers represented by fissure volcanoes. Central volcanoes, in contrast to the Western Rift Zone, are not expressed in topography. Oppositely, tectonic activity increases with distance from the Reykjanes Ridge. Fault scarps reach their largest size within the Western Rift Zone, where Thingvellir graben is located. At the same time, the tectonic and volcanic landforms of the Reykjanes rift zone are in paragenetic association, appearing sequentially during the episodes of riftogenesis.

Mesozoic intraoceanic subduction shaped the lower mantle beneath the East Pacific Rise.

The Pacific large low-shear-velocity province (LLSVP), as revealed by cluster analysis of global tomographic models, hosts multiple internal anomalies, including a notable gap (~20° wide) between the central and eastern Pacific. The cause of the structural gap remains unconstrained. Directly above this structural gap, we identify an anomalously thick mantle transition zone east of the East Pacific Rise, the fastest-spreading ocean ridge in the world, using a dense set of SS precursors. The area of the thickened transition zone exhibits faster-than-average velocities according to recent tomographic images, suggesting perturbed postolivine phase boundaries shifting in response to lowered temperatures. We attribute this observation to episodes of Mesozoic-aged (250 to 120 million years ago) intraoceanic subduction beneath the present-day Nazca Plate. The eastern portion of the Pacific LLSVP was separated by downwelling because of this ancient oceanic slab. Our discovery provides a unique perspective on linking deep Earth structures with surface subduction.