Orals: Mon, 28 Apr | Room D2
At ultraslow-spreading mid-ocean ridges (MORs, spreading rate <20 mm/yr), limited magma supply often results in tectonic extension and the formation of oceanic detachment faults. These faults cut through thick brittle lithosphere (up to 15 km), accommodating tens of kilometers of displacement and exposing heterogeneous rocks altered by seawater-rock interactions. Among these reactions, serpentinization has drawn significant attention for its role in sustaining chemosynthetic microbial life and modulating geological carbon budgets. However, quantitatively determining the extent and distribution of serpentinization within the lithosphere remains challenging, as large-scale estimates rely primarily on seismic observations that struggle to differentiate between serpentinized mantle, gabbro, and fresh mantle at depth. Despite advances in seismic resolution, key uncertainties persist regarding how magmatic, tectonic, and alteration processes shape velocity anomalies in newly formed oceanic lithosphere. Here, we address lithospheric alteration during magma-poor seafloor spreading by coupling a geodynamic model with thermodynamic calculations of alteration reactions and seismic properties as a function of pressure-temperature and mineral assemblages. We focus on the well-documented magma-poor ridge at 64°30′E on the Southwest Indian Ridge, where recent seismic surveys have been conducted. Our model reproduces the “smooth-smooth” seafloor morphology shaped by alternating flip-flop detachments. By coupling water availability and lithosphere alteration with active deformation, we reveal: (i) vertically controlled alteration along detachments, including deep alteration beyond serpentine stability; and (ii) tectonically-induced lateral velocity anomalies caused by variations in alteration mineral assemblages in the detachment footwall. Comparing our thermodynamically-constrained velocity model with seismic observations from 64°30′E SWIR suggests that the imaged alteration boundary along detachment faults likely represents a peak in serpentinization, rather than the traditionally interpreted serpentinization front.
How to cite: Mezri, L., García-Pintado, J., Diehl, A., and Pérez-Gussinyé, M.: Tectonics control seismic velocity anomalies in magma-poor ultraslow-spread oceanic lithospheres, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4386, https://doi.org/10.5194/egusphere-egu25-4386, 2025.
Global ocean crust has an average thickness of 6–7 kilometers, suggesting a globally pervasive, rather uniform mantle composition. At some ultra-slow spreading ridges, crustal thickness is highly variable and mantle peridotite can be found at the surface. The peridotites, however, are mostly recovered in fracture zones that expose the deeper crust, or at the edges of ridge segments where there is a central volcano. The Gakkel Ridge is unusual in this regard because it contains a 400-kilometer-long sparsely magmatic zone (SMZ) with extensive mantle peridotite exposure, negligible crustal thickness and limited basaltic lava outcrops. This segment is also bracketed by two other sections of ridge that have active volcanism, including the adjacent Western Volcanic Zone (WVZ) where no peridotites were recovered. What is the origin of this enigmatic expanse of ridge, and is it simply a curiosity or does it have global implications for ocean ridges and mantle recycling?
We have undertaken systematic geochemical analysis of 267 basaltic glass samples from the WVZ and the few recovered basalts from the SMZ. The WVZ has normal-thickness oceanic crust and predominantly produces depleted normal mid-ocean ridge basalt (N-MORB). Gradients in chemical composition can be accounted for by a combination of more depleted mantle and lower extents of melting as the SMZ is approached. Across an abrupt boundary, the SMZ has negligible crustal thickness and is dominated by exposed mantle peridotite and a few samples of enriched mid-ocean ridge basalt (E-MORB).
Quantitative models suggest the SMZ is the result of cold, ancient ocean mantle lithosphere that has been metasomatized by enriched, low degree melts. While the SMZ is a rare occurrence, simple mass balance considerations suggest such occurrences should instead be very common. While recycled ocean crust is commonly called upon, sometimes as an isolated lower mantle reservoir, the mass of depleted ocean mantle lithosphere would be more than ten times greater. Indeed, using current ridge production rates, over the last 2.5 billion years the total volume of recycled mantle lithosphere would be equivalent to the volume of the entire lower mantle. While vestiges of such lithosphere are frequently invoked from Os isotopes or melt inclusions, almost all of these occurrences are coincident with predominant basalts, and occur in regions with normal crustal thickness. Why are there not vast regions dominated by depleted lithosphere, negligible crust, or common occurrences of basalts that come from highly depleted reservoirs? An obvious solution is that mantle convection is highly efficient at mixing crustal and mantle components on a scale finer than is sampled by melting, permitting relatively uniform crustal thickness and composition on a global basis.
How to cite: Yang, A. Y., Langmuir, C., and Michael, P.: Origin and implications of the amagmatic segment of the Gakkel Ridge, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5117, https://doi.org/10.5194/egusphere-egu25-5117, 2025.
Over one-third of mid-ocean ridges exhibit a spreading rate less than 20 mm per year. The process of crustal accretion, which facilitates the expansion of oceanic plates at mid-ocean ridges, is driven by the combined and interactive efforts of magmatic and tectonic processes. The seafloor morphology along ultraslow mid-ocean ridge flanks serves as a record of the accretion on oceanic crust. However, volcanic eruptions, mass wasting and reverse-faulting earthquakes occurring on mid-ocean ridges, which reshape the seafloor, present a significant obstacle for the precise quantification of oceanic crust accretion and seafloor morphology. Due to the temporal and spatial variability of magma supply, particularly in the Indomed-Gallieni supersegment (46-52°E) of the Southwest Indian Ridge (SWIR), magmatic and tectonic processes exhibit pronounced spatiotemporal variations, along with asymmetric crustal accretion, making it rather difficult to conduct quantitative analysis of the geomorphology of the oceanic crust. By utilizing multibeam bathymetry and gravity data of Indomed-Gallieni supersegment, we calculated several parameters such as the fraction of magmatic accretion (M-value), axial valley depth (D-value), magma supply, melt flux, and strain ratio, as well as fault heave and fault throw, thereby quantifying magmatism and tectonism. The majority of parameters indicative of tectonic accretion exhibit a negative correlation with magmatic parameters. Moreover, we compared the two-dimensional Fourier spectra of seafloor on mid-ocean ridge flank with magma supply. The anisotropy of seafloor is positively correlated with magma supply, with morphology becoming increasingly isotropic as magma supply diminishes. Our research suggests that although tectonic processes account for nearly 50% of oceanic crust accretion at ultraslow spreading mid-ocean ridges, the accretion process and the geomorphic features of the young oceanic crust are predominantly influenced by magma supply.
How to cite: Wang, M., Tao, C., Zhu, Z., and Guo, Z.: Quantifying magmatism and tectonism along the ultraslow-spreading Southwest Indian Ridge (46-52°E) , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9441, https://doi.org/10.5194/egusphere-egu25-9441, 2025.
IODP Expedition 399 drilled a record 1268m hole (U1601C) in the Atlantis Massif oceanic core complex, sampling serpentinised harzburgites and dunites, cut by a gabbro net-vein complex (Lissenberg et al., 2024). The near ridge environment of the Atlantis Massif, and the well constrained exhumation of the section by detachment faulting (Escartin et al., 2022), allows us to constrain the recent (~6 million year) history of this important section of abyssal peridotites exceptionally well. In addition, Site U1601 is located only 800 m from the Lost City hydrothermal field (LCHF), which vents warm (40-115 °C) alkaline fluids rich in H2 and CH4. The section allows direct comparison with the LCHF substrate and reactions occurring deep in the massif, together with extremophile microbiology and abiotic organic synthesis.
Here we focus on the history of the section, beginning with partial melting in the upwelling asthenosphere beneath the mid-Atlantic ridge, inferred to have begun at ~ 60km depth (Olive, 2023) and ~ 6 m.y. ago based on a half-spreading rate of 11.8 mm/yr. It is important to recognise that detachment faulting involves rotation of the fault and footwall. The detachment fault captures part of the mid-ocean ridge corner flow translating vertical upwelling into horizontal plate motion. The current near-vertical section collected by drilling was therefore plunging at a low angle until incorporated into the lithosphere and rotated by faulting. The section contains numerous dunitic veins inferred to be melt pathways forming in the upwelling asthenosphere. Dips of these veins peak at ~45° in the core reference frame, suggesting they were neither vertical nor horizontal in the rotated section. Further upwelling led to incorporation of the section into the lithosphere in the footwall of the nascent detachment fault, at a depth of 7-10 kmbsf. The next event was intrusion of a net vein complex of gabbros, with 265 logged units, mostly < 1 m in thickness. Significant mylonitic deformation is seen along the margins of many of these gabbros. During further uplift towards the seafloor, intense hydrothemal alteration of the gabbros and serpentinisation of the harzburgites and dunites occurred at temperatures < 400 °C, and the section was first exposed on the seafloor at ~ 600 kyr (Escartin et al., 2022), with the detachment fault rotating to a subhorizontal dip. Following this, a local low temperature overprint leading to oxidation of magnetite and locally high uranium contents is observed in the upper 200m of the core.
The history outlined above offers a framework for understanding the full range of magmatic, deformation, alteration and microbiological processes in the upwelling mantle at a slow spreading ridge, including new constraints on processes in the substrate of the LCHF.
Escartin et al., (2022). Tectonic termination of oceanic detachment faults, with constraints on tectonic uplift and mass wasting related erosion rates.Earth and Planetary Science Letters 584, 117449
Lissenberg et al., (2024). A long section of serpentinized depleted mantle peridotite. Science. 623-629 385.6709
Olive (2023) Mid-Ocean Ridges: Geodynamics Written in the Seafloor DOI 10.1016/B978-0-323-85733-8.00018-4
How to cite: McCaig, A., Lissenberg, J., Lang, S., and Peter, B. and the International Ocean Discovery Program Expedition 399 Science Party: IODP Expedition 399: the six million year uplift history of a record-breaking section of depleted mantle, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18332, https://doi.org/10.5194/egusphere-egu25-18332, 2025.
Oceanic core complexes are a common feature along slow-spreading mid-ocean ridges. Serpentinized mantle rocks are exposed at the seafloor in the footwall to large-scale detachment faults. While it is likely that the exposed and rotated footwall has experienced deformation, it is unclear how internal footwall deformation is accommodated by the ultramafic rocks. One example of such an oceanic core complex is the Atlantis Massif at the Mid-Atlantic ridge (30° N) drilled by International Ocean Discovery Program (IODP) Expedition 399. Site U1601 provides the unique opportunity to understand any deformation recorded in serpentinized mantle rocks over >1.2 km depth.
To better understand the depth distribution of deformation and the associated deformation mechanisms, we combine microstructure and crystallographic preferred orientation (CPO) analysis by means of Scanning Electron Microscopy techniques and synchrotron high energy X-ray diffraction. Results show variable microstructures ranging from zoned mesh cells with no CPO, to foliated samples with a strong CPO of both serpentine and magnetite, to serpentinite samples exhibiting deformation microstructures like kinking and dissolution-precipitation features. The origin of characteristic microstructures and CPOs, whether formed due to serpentinization, deformation, or mutual interaction, will be discussed.
How to cite: Kühn, R., Kilian, R., Morales, L., Parsons, A., John, B., and Deans, J. and the IODP Expedition 399 Science Party: Serpentinite microstructure at the Atlantis Massif – serpentinization reaction or deformation?, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16578, https://doi.org/10.5194/egusphere-egu25-16578, 2025.
The use of fluid-mobile elements and their isotopes to track fluid-mediated subduction zone processes requires an accurate estimate of the volatile element systematics of subducting oceanic crust. Near-ridge hydrothermal circulation represents the primary means by which seawater can penetrate the oceanic crust and produce enrichments in fluid-mobile elements (B, Sr, Li, U, Cl etc.), particularly at slow-spreading ridges where hydrated mantle peridotite (ie. serpentinite) is commonly exposed at the seafloor. However no previous drilling has penetrated abyssal serpentinite deeper than 200.8m below seafloor, where late-stage alteration and intense fault-controlled circulation during exhumation might produce anomalous fluid-mobile element signatures. While ophiolites provide a valuable analogue, it is often hard to distinguish geochemical signatures related to interaction with seawater-derived fluid from those acquired during subsequent interaction with subduction-related and/or meteoric fluids.
We present new data from IODP Expedition 399, which recovered 1268m of serpentinized depleted mantle peridotite and variably altered gabbroic rocks (Hole U1601C) from the southern wall of the Atlantis Massif (30°N; Mid-Atlantic Ridge). Peridotites are generally highly serpentinized (80-90%) and display complex pseudomorphic, mesh and vein textures, attesting to a multistage history of alteration. Gabbros range from fresh to completely altered and exhibit a diversity of secondary mineral assemblages (±amphibole ±serpentine ±talc ±chlorite ±sulphides ±prehnite ±secondary plagioclase ±zeolite ±saponite ±carbonate). Our downcore fluid-mobile trace element and B and Sr isotopic profiles provide a comprehensive framework in which to understand physicochemical conditions during serpentinization and metasomatism of the actively metamorphosing basement of the massif, and their relation to current seafloor venting at the Lost City Hydrothermal Field.
B concentrations in serpentinites decrease by an order of magnitude downcore, which we interpret in terms of B depletion of alteration fluid through the serpentinization process. Substantial downcore variation in the B isotopic composition of serpentinite (δ11B of +12‰ to +40‰) reflects local T and pH conditions as well as isotopic evolution of the alteration fluid along the flow path. Serpentinite Sr isotopic compositions vary between seawater and near mantle values (87/86Sr of 0.704 to 0.709); likely reflecting considerable elemental exchange between alteration fluid and gabbroic intrusions. Our results also shed new light on the geochemical influence of late-stage alteration processes (carbonation, oxidation, infilling of reaction porosity etc.) postdating serpentinization.
In addition, we present new B isotope data from (olivine-bearing) gabbroic rocks of the central massif (Hole U1309D) and detachment-proximal serpentinites from the south wall drilled during IODP Expedition 357. Together, these data represent an important step towards quantifying the fluid-mobile element makeup and specifically the B and 11/10 B content of the lower oceanic crust.
How to cite: Osborne, W., Savov, I., McCaig, A., Agostini, S., and Godard, M. and the the International Ocean Discovery Program Expedition 399 Sci Party: The Boron Isotope Record of Fluid-Rock Interaction in Abyssal Serpentinite: Insights from IODP Expedition 399, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1339, https://doi.org/10.5194/egusphere-egu25-1339, 2025.
The specific oceanic crust architecture, magmatism, hydrothermal fluid circulation and lithologies at oceanic core complexes (OCCs) imply different S and metal (e.g. Cu, Zn, Co, Ni) fluxes relative to well-structured oceanic crust at-fast spreading ridges. Extensive hydrothermal fluid circulation within OCCs often leads to seafloor massive sulfide (SMS) deposits formation either hosted in the OCC or in the crustal hanging wall. The S and metal source zones in OCC are nevertheless poorly 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 zones 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 such as at the Atlantis Massif.
How to cite: Patten, C. G. C., Junge, M., Coltat, R., Jesus, A. P., Beranoaguire, A., Tropper, P., and Alt, J.: Sulfur and metal mobilization during the magmatic-hydrothermal evolution of the Atlantis Bank oceanic core complex: implications for seafloor massive sulfide deposits formation at slow and ultra-slow spreading ridges, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1702, https://doi.org/10.5194/egusphere-egu25-1702, 2025.
Hydrothermal vents are oceanic sources of biogeochemical constituents. Some of these constituents, such as iron, significantly contribute to global biogeochemical cycles. Yet, their fate, i.e., transport and mixing through physical processes, and modification of their concentration through bio-geochemical processes, remains poorly quantified. Using state-of-the-art physical-biogeochemical (CROCO-PISCES) model simulations that resolve submesoscale processes, internal gravity waves and parameterized mixing processes, we analyse the physical processes involved in the dispersion of passive tracers (i.e. Helium) released at the Trans-Atlantic Geotraces (TAG) hydrothermal site.
A reference simulation features a horizontal grid spacing of 1 km, 150 terrain-following vertical levels, and includes high-frequency atmospheric and tidal forcing. Helium is initialized and continuously released at TAG, following a distribution that is constrained by observations. We also ran sensitivity experiments, without tides and with a smooth bathymetry designed to investigate the effects of CMIP (Coupled Models Intercomparison Project) model coarse bathymetries on the circulation.
At short spatial and time scales (~20 km, ~10 days), we find that tidal processes are instrumental in the tracer dispersion. Through comparisons between the reference and the no-tides simulations, we show that tidal currents and internal tides drive the dispersion within the TAG surrounding valley, and tidally-induced mixing drives the vertical dispersion of tracers, especially on the flanks of the valley walls and within fracture zones. At longer and larger scales (>20 km, >10 days), submesoscale and mesoscale instabilities catalyzed by the interaction of currents with the ridge topography lead to the formation of eddies that trap tracers and escape from the ridge valley to wander at depth preferentially westward of the ridge. Small-scale topographic structures such as fracture zones and abyssal hills control the dispersion and notably slows down the dispersion of tracers outside of the ridge valley. Simulation with smooth bathymetry hence shows a more isotropic and rapid dispersion. This could lead to biases in the inferred pathways of tracers in global models. Next, we will investigate the fate of active tracers, such as iron, which is impacted by biogeochemical processes, such as scavenging and complexation by ligands.
How to cite: Escobar Franco, M. G., Vic, C., Gorgues, T., and Cathalot, C.: Dispersion of Helium from the TAG hydrothermal vent field: perspectives from coupled physics-geochemistry model experiments, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8921, https://doi.org/10.5194/egusphere-egu25-8921, 2025.
Decompressional melting of the asthenosphere gives rise of mid-ocean ridge basalts (MORB) that are extracted to generate ocean crust, and also leaves mantle residues that are represented by abyssal peridotites. Thus, both MORB and abyssal peridotites can be utilized to constrain the compositional characteristics of the asthenosphere. Numerous studies on MORB have widely demonstrated that they are from a relatively homogenous and geochemically depleted mantle source. The homogeneity of the asthenosphere has been commonly attributed to the efficiency of mantle convection. Nonetheless, geochemical compositions of global abyssal peridotites show highly variable compositions and a wide range of isotopic spectrum, clearly reflecting that the asthenosphere is compositionally heterogeneous. Mantle peridotites memorizing evolutionary histories at different tectonic settings, including sub-continental lithospheric mantle, mantle wedge and oceanic mantle, can be recycled into the asthenosphere, which might be eventually popped up at ocean ridges where they are sampled by abyssal peridotites. Different types of recycled mantle materials can be discriminated using geochemical tools. Our recent studies on abyssal peridotites dredged at different segments along the Southwest Indian Ridge (SWIR) have shown the occurrence of diverse types of recycled mantle, i.e., Archean cratonic mantle in its western segment, mantle wedge in its central segment, and oceanic mantle in this eastern segment. Such a spatial distribution is genetically related to the assembly and breakup of the Gondwana supercontinent since the Cambrian. Therefore, systematic studies on abyssal peridotites outcropped along the ocean ridges can decipher the compositionally characteristics and evolutionary histories of different mantle domains within the asthenosphere.
How to cite: Liu, C.-Z., Zhang, W.-Q., Lin, Y.-Z., Xu, Y., and Zhang, Z.: Abyssal peridotites: Rosetta Stone for recycled mantle materials in the asthenosphere, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1938, https://doi.org/10.5194/egusphere-egu25-1938, 2025.
In the summer of 2024, we conducted the first magnetotelluric (MT) profile survey beneath sea ice at the ultraslow spreading Gakkel Ridge. A total of 17 MT instruments were deployed for approximately 20 days along a 125 km profile across the ridge axis at 100°E. This profile spanned the 0–14 Ma lithosphere north of the Gakkel Ridge. Preliminary analysis reveals a zone of high electrical conductivity at depths of 30–50 km beneath the ridge axis, attributed to a high-degree partial melting zone. A more striking feature is the abrupt deepening of the electrical lithospheric base to ~65 km just north of the ridge axis, beyond which it flattens significantly. The flat lithospheric base likely represents a dehydration boundary, where water content sharply decreases above it due to melting processes. The dehydration could enhance mantle viscosity by 2–3 orders of magnitude, suggesting that the mechanical lithosphere near the ridge axis is governed more by compositions than by thermal structure. The depth of this boundary aligns with the seismic reflection boundary in the Atlantic Ocean, the Gutenberg discontinuity, and the top of the seismic radial anisotropy layer, indicating a possible global significance of this feature.
How to cite: Zhang, T., Li, J., and Team, J.: Magnetotelluric evidence for a compositionally controlled lithosphere at the Gakkel Ridge, Arctic Ocean, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7632, https://doi.org/10.5194/egusphere-egu25-7632, 2025.
Mantle exhumation mechanisms at continent-ocean transitions (COTs) are similar to those at slow and ultraslow spreading ridges, where plate divergence is also accommodated by a combination of magmatic processes and detachment faulting. However, the timescales of exhumation at COTs are poorly constrained because of the thick sediment cover blanketing basement rocks along mature passive margins. IODP Exp. 402 drilled the Tyrrhenian Sea COT and successfully recovered in situ sections of mantle exhumed during Late Cenozoic extension in this back-arc basin. Onedrill site sampled a sequence of variably deformed granitic gneisses intercalated with ~cm-thick slivers of peridotites and basalts, and another drill site sampled a heterogeneous section of heavily serpentinized peridotites with granitoids between the ultramafics. Structural observations and core recovery trends indicate localized deformation along the granitoids, with fabrics varying from undeformed to mylonitic. The presence of both peridotites and felsic granitoids provides a unique opportunity to acquire precise ages for the exhumation and deformation stages that have not yet been resolved in detail.
Zircon and apatite U-Pb geochronology of granitoids yields similar Pliocene ages (<4 Ma), coeval with the biostratigraphic ages of the basal overlying sediments, requiring crystallization at depth followed by rapid exhumation. Thin section microstructures and Electron Backscatter Diffraction data suggest that these granitoids accommodated significant strain during exhumation along a detachment fault. Quartz and feldspar in the mylonites are deformed by dislocation creep, with quartz exhibiting grain boundary rotation and migration, and feldspar displaying bulging, suggesting deformation at temperatures of ~450°C. In contrast, quartz in the protomylonite shows polygonal-shaped grains, indicating static recrystallization at high temperatures with low strain. Ti in quartz analyses yields temperatures of ~400°C for both mylonites and protomylonites, suggesting that the differences in the microstructures are strain dependent and that shear was localized within a ~5-m-thick zone. These chronological and microstructural constraints require >1 cm/year exhumation rates after granitoid emplacement. Lastly, stable isotope constraints from the surrounding peridotites give serpentinization temperatures of ~200°C, with higher temperatures adjacent to granitic intrusions. These results, together with microstructural observations, suggest that serpentinization occurred at shallower depths, after most of the unroofing. Overall, we show that felsic lithologies facilitate most of the exhumation prior to serpentinization and demonstrate that heterogeneous lithologies and pre-existing structures have a major influence on the slip behavior of faults at COTs.
How to cite: Poulaki, E., Bickert, M., Vannucchi, P., Shuck, B., Morishita, T., Sanfilippo, A., Pandey, A., Akizawa, N., Cunningham, E., Tribuzio, R., Barnes, J., Garber, J., Nistor, C., Bernard, R., and Loocke, M. and the IODP Expedition 402 Team: Rapid exhumation of mantle rocks along detachment faults facilitated by felsic granitoid intrusions at a continent-ocean transition drilled in the Tyrrhenian Sea, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5278, https://doi.org/10.5194/egusphere-egu25-5278, 2025.
Hydrothermal activity has been intensively studied at mature mid-ocean ridges and is crucial for the formation of mineral resources, as habitats for chemosynthetic communities, and for the cooling of the newly formed oceanic lithosphere1. However, the role of hydrothermal circulation in the early history of a young emerging ocean basin shortly after continental breakup and the geological expression of its hydrothermal vents, their geochemical characteristics, and their associated ecosystems can only be studied at a few locations. The Red Sea Rift is one of Earth’s youngest ocean basins, but despite ample indications for hydrothermal activity based on rock chemistry, the presence of extinct chimney fields, metalliferous sediments, and high-temperature brine poolssee overview in 2, the first direct observation of active hydrothermal vents was only reported in 20223. These vents at the axial volcano, Hatiba Mons, constitute one of the largest active hydrothermal areas worldwide, hosting 43 individual fields. In contrast to many mature locations, no high-temperature vent nor specialized macro-fauna was observed. Instead, the vents were characterized by low-temperature fluids, numerous Fe-Mn-oxyhydroxide mounds, and thriving microbial mats3. As this was the first active hydrothermal area observed in the Red Sea, the question remains if this is typical for the Red Sea and potentially for young mid-ocean ridges.
Here we present the outcomes of two expeditions in 2023 with the R/V Aegaeo (KRSE5-1) and R/V Meteor (M194)4, which resulted in the discovery of five more hydrothermally active areas distributed along the Red Sea Rift between 17°N and 25°N at water depths between 400-1,800 m. All hydrothermal areas consist of multiple smaller vent fields with similar low-temperature venting as reported from Hatiba Mons. The locations of the vents in their geological context and the expressions of hydrothermal occurrences show some variations ranging from small chimneys along fault lines to larger mounds covering wider areas. We compare the six hydrothermal fields in terms of their geology, geomorphological expression, precipitate chemistry, and fluid characteristics to evaluate their regional differences and similarities to further understand the nature of hydrothermal venting in a young oceanic basin.
1Hannington et al. (2005) In: Economic Geology 100th Anniversary Volume, 111-141
2F. M. van der Zwan et al. (2019) In: Geological Setting, Palaeoenvironment and Archaeology of the Red Sea. Springer, 221-232
3F. M. Van der Zwan et al. (2023) Communications Earth & Environment 4 (1), 496
4N. Augustin (2023) METEOR Short Cruise report, M194. GEOMAR Helmholtz Centre for Ocean Research https://oceanrep.geomar.de/id/eprint/59591
How to cite: van der Zwan, F. M., Augustin, N., Petersen, S., Diercks, I., and Sander, S. G.: Hydrothermal activity along the young, ultra-slow spreading Red Sea Rift – an update from recent discoveries, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8156, https://doi.org/10.5194/egusphere-egu25-8156, 2025.
Since the recognition of seafloor spreading, numerous kinematic and dynamic models for the accretion of oceanic crust and lithosphere have been proposed. Early models were constrained by the interpretation of marine seismic data and the internal structure of ophiolite complexes and predated any direct observations of the oceanic crust. Mapping the extent of axial lava flows and subsurface axial magma chambers established the very limited dimensions of where new oceanic crust is built.
Unlike spreading at slow rates, where faulting and sporadic magmatism result in heterogeneous structures, spreading at intermediate to superfast spreading rates (and higher, more consistent magma budgets) results in a layered upper crustal structure with a complex internal structure. Direct observations from submersibles, ROVs, and deep drill cores provide constraints that allow for the refinement or modification models for oceanic crust accretion at these relatively fast spreading rates.
Key observations reveal structures and processes that are not obvious from surface investigations. These include progressively more steeply inward-dipping (initially horizontal) lava flows, outward-dipping (originally vertical) dikes, downward-increasing brittle deformation and hydrothermal metamorphism of lavas and dikes, and underplating by much-less-faulted and altered gabbroic rocks. The thickness and internal structure of these upper crustal rock units are created by continuous dike intrusion feeding lava flows that cause caldera-like, vertical subsidence of hundreds of meters above an axial magma chamber. Greater subsidence and deformation of upper crustal units occur at intermediate spreading rates (or lower magma budgets) than at the highest rates.
These results have implications for viscous mass redistribution beneath the spreading axis even as additional magma is delivered from the mantle below. Applying observable parameters to dynamic models yields internally consistent results with extremely weak axial lithosphere (effective elastic thickness < 1 km) that strengthens laterally as it ages off axis prior to the formation of abyssal hill faults.
How to cite: Karson, J.: Building the Oceanic Crust at Intermediate to Superfast Mid-Ocean Ridge Spreading Centers: Implications of Complex Internal Structures of the Upper Oceanic Crust, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13464, https://doi.org/10.5194/egusphere-egu25-13464, 2025.
Hydrothermal convection in young oceanic lithosphere accounts for ~25% of the total global heat flow, and thus plays a critical role in Earth's thermal evolution. The permeability structure of the lithosphere is a key factor governing how efficiently heat tapped from magma bodies or hot upwelling mantle can be transferred to the overlying ocean. Drill hole measurements and laboratory experiments unambiguously show that permeability decreases with depth (i.e., pressure), either exponentially or through some power law relations. However, the impact of depth-decreasing permeability on the depth extent and heat output of seafloor hydrothermal systems has not been explored systematically.
Here we present 2-D numerical simulations of hydrothermal convection treated as Darcy porous flow, with fluid properties corresponding to a 3.2 wt% NaCl-H2O mixture, and depth (i.e., pressure)-dependent permeability fields. We consider an empirical exponential dependence as well as a more recently proposed power-law-type dependence rooted in micromechanical modeling of experimental data. In reference simulations with uniform permeability, we find that, for a given basal temperature (TH) imposed at the model bottom, the hydrothermal heat output at the seafloor increases with permeability, but is largely independent of the depth extent of the model domain. On the other hand, in simulations with depth-decreasing permeability, the depth extent of hydrothermal convection (ZH) may be significantly lower than the height of the model domain. In such systems, heat extraction is intuitively more efficient when the heat source lies at a shallower depth. We find that the heat output in these simulations is primarily controlled by the harmonic mean of permeability in the hydrothermal system.
To further quantify this finding, we investigate the relationship between our simulations' Rayleigh number (Ra, estimated from model inputs using the harmonically-averaged permeability) and Nusselt number (Nu, measured from simulation results). We find that the linear relationship Nu=Ra/Rac that is typical of porous convection holds for Ra > 103, with a critical Rayleigh number (Rac) on the order of 102. This relationship allows us to build an analytical model that predicts ZH, given the heat output, basal temperature (TH), and exponentially-decreasing permeability with depth Z: k= k0 e(-cZ). Fitting parameters against observed magma-fueled hydrothermal systems at mid-ocean ridges suggests that permeability at the seafloor (k0) is on the order of 10-12 - 10-11 m2, in agreement with independent estimates based on drill hole measurements and the poro-elastic tidal modulation of venting temperatures, and that the constant c is on the order of 1-4×10-3 m-1. Our findings further suggest that for convection to reach depths > 13 km, as has been proposed near oceanic detachment faults, permeability at the seafloor would need to be extremely large (k0> 10-10 m2). It remains unclear whether such conditions can be attained in the damage zone of a detachment fault.
How to cite: Chen, J., Olive, J.-A., Cannat, M., and Demont, A.: Implications of pressure-dependent permeability for hydrothermal heat transfers, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4307, https://doi.org/10.5194/egusphere-egu25-4307, 2025.
Water-saturated partial melting experiments were carried out using a refractory harzburgite from the Oman Ophiolite as starting material. The experiments were performed at pressures of 100, 200 and 500 MPa using both reducing (corresponding to the FMQ buffer) and oxidizing (FMQ+3) conditions. Specially designed internally heated pressure vessels were used to control oxygen fugacity and allow rapid quenching. Temperatures varied between 980 and 1220°C, and run durations were up to 82 hours. The solidus and clinopyroxene-out curve show significant variation with pressure. As expected, the melts produced were generally SiO2-rich, with SiO2 concentrations ranging between 55 and 65 wt%. These melts exhibit boninitic characteristics. Due to the refractory character of the starting material, the experimental melts are highly depleted in incompatible trace elements, showing chondrite-normalized REE patterns with a characteristic concave-upward shape. Calcium and sodium in the system are mainly derived from the clinopyroxene in the starting harzburgite, resulting in extremely high Ca/Na ratios in the experimental melts. At temperatures above the clinopyroxene breakdown, the residual mineral paragenesis exhibits characteristics similar to extremely refractory harzburgites, with Cr# in Cr-spinel (Cr2O3 /(Al2O3 + Cr2O3), molar) reaching up to 86, reminiscent of ophiolites formed under supra-subduction zone conditions.
The melts produced have compositions of high-Mg andesite and boninite. Our experimental results show that the formation of distinct rock types within the paleocrust of the Oman Ophiolite such as high-Ca boninites, high-Si boninites, high-Mg andesites, depleted gabbronorite cumulate rocks, and extremely refractory harzburgites containing Cr-spinel with Cr# > 80, could, in principle, be attributed to a single process of fluid-induced partial melting of harzburgite below the crust/mantle boundary of the Oman paleocrust. The temperatures for the heating process (> 1040°C) for such a model, could be provided by ascending MORB magmas. The presence of water-rich fluids at the crust/mantle boundary or within the uppermost mantle which are necessary for such a model, could be derived from seawater via deep hydrothermal fault zones. We present amphibole data from deep hydrothermal fault zones in the lowermost gabbros of the Oman Ophiolite, which provide evidence that temperatures of deep hydrothermal fault zones are high enough to trigger the melting of hydrated harzburgites.
How to cite: Koepke, J., Feig, S., and Berndt-Gerdes, J.: Boninites formed in deep hydrothermal fault zones at mid-ocean ridges: experimental evidence , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20531, https://doi.org/10.5194/egusphere-egu25-20531, 2025.
In the current theories of mid-ocean ridges, diking processes have been considered by using simplified models with a single permanently open central dike. Here, I instead consider long-term large-scale rheological effects of multiple dikes emplacement, which lead to rheological weakening of the forming mid-ocean ridge lithosphere. Based on 2D numerical experiments modeling multiple dikes emplacement, I derive rheological expressions representing effective strength of the melt-weakened lithosphere as the function of local melt flux. These expressions are then implemented into 3D visco-elasto-plastic mid-ocean ridge models including mantle decompression melting, crustal growth and melt flux-induced weakening of the spontaneously accreting oceanic lithosphere. Based on 3D numerical experiments, I demonstrate that the newly developed rheological theory explains well the observed mid-ocean ridge topography and faulting pattern variations with spreading rate and oceanic crust thickness. This theory may be further used for other geodynamical situations involving melt transport through oceanic and continental lithosphere such as continental and oceanic rifting, continental breakup and plume-lithosphere interaction processes.
How to cite: Gerya, T.: Melt-induced weakening controls topography and faulting pattern of mid-ocean ridges, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15390, https://doi.org/10.5194/egusphere-egu25-15390, 2025.
Transform faults, key elements of plate tectonics, play a crucial role in shaping transform
margins. Marginal ridges, which are elevated basement highs at margin borders, represent
one of the structural elements occurring at some transform margins.
This study examines marginal ridges and their failed candidates, which occur along the
Zenith-Wallaby-Perth transform fault zone in West Australia, using seismic and gravity data,
and plate reconstructions to investigate their development histories.
Existing models of marginal ridge development often include processes such as thermal
expansion-related uplift, flexural uplift and flower structure development. However, data
from the study area suggest a more complex mechanism, which is related to the formation
of strike-slip faults and pull-apart basins inside the transform fault zone.
This study proposes a model of the marginal ridge formation characterized by the evolving
faulting during continental and continental-oceanic stages of the transform development.
The nucleation and linkage of strike-slip faults along the future transform fault zone lead to
the formation of pull-apart basins, characterized by a complex fault system. In the same
time, (1) initially broad zone of deformation undergoes progressive focusing and (2) fault
activity decays along the transform strike towards the ocean. Depending on the duration of
fault activity, some parts of the initial strike-slip fault zones and pull-aparts develop further,
while others are abandoned. In regions where faults remain active for extended periods
during the continental-oceanic stage of the transform development, marginal ridges may
develop, and even occasionaly evolve into micro-continents separated from the continent.
Further complexity in their development is the effect of the pre-existing anisotropy in
regions of their development.
How to cite: Staniaszek, S. and Nemčok,, M.: Controlling processes of marginal ridge development, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3590, https://doi.org/10.5194/egusphere-egu25-3590, 2025.
The global mid-ocean ridge system produces the oceanic lithosphere accounting for ~70% of the Earth’s surface, while hosting active processes (tectonic, volcanic, hydrothermal circulation). The ridge system is segmented by both transform faults and non-transform offsets, and their geometry can be now re-evaluated with existing multibeam bathymetry (with a resolution of ~100 m or better), both from publicly accessible datasets (e.g., GMRT, NCEI, Pangaea, AWI, among others) and available through published studies. This high-resolution bathymetry is now available for ~25% of the ocean seafloor, but covers a significant proportion of the global mid-ocean ridge system (>70%) and is thus suitable to refine and finely define its geometry.
The MAPRIDGES database (https://doi.org/10.17882/99981) provides a global dataset that includes the newly-defined geometry of individual mid-ocean ridge segments, the most complete catalog to date of transform faults, and identifies non-transform offsets (NTOs). This effort is linked to the World 5M project by CGMW (Commission for the Geological Map of the World). We calculate the lateral offset associated with these NTOs, and determine if they correspond to overlaps of adjacent segments or if they are associated with a gap (underlap). Two different plate models (MORVEL and GSRM) are used to estimate the length of overlaps, underlaps and their links to variations in spreading direction.
Our new database, gives a global, detailed view of the global mid-ocean ridge geometry, and provides the first evaluation of the overall lengths of ridges and associated lateral offsets, both transform and non-transform. Mid ocean ridge segments (1471) show a cumulative length of ~71200 km, with and along-axis distance of ~4800 km of overlapping segments, and ~1700 km of underlap; taking these offsets into account this yields a total length of along-axis segments of ~75300 km. We have also digitized the traces of 262 transform faults to obtain the most complete catalogue to date of these structures. Transform faults account for a cumulative lateral offset of ridges of ~27000 km. We report a first estimate of the lateral offset of 1058 identified NTOs at ~10400 km, accounting for >30% of the cumulative transform fault length. The resulting cumulative lateral offset from both transform and non-tranform segments is thus ~37400 km, and is ~50% of the total ridge length. As in the case of transform faults, these NTOs are associated with deformation of a significant volume of the recently accreted oceanic lithosphere, and thus likely facilitating hydrothermal circulation and alteration of the lithosphere. This study will facilitate the quantification of these processes and provides a basis to better understand their implications on local and global environments (e.g., chemical fluxes associated with alteration at all offsets).
How to cite: Escartín, J., Sautter, B., Gaina, C., Petersen, S., Granot, R., and Pubelier, M.: MAPRIDGES: Geometry of global mid-ocean ridge plate boundaries, and the role of transform faults and non-transform offsets, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18143, https://doi.org/10.5194/egusphere-egu25-18143, 2025.
At ultraslow, magma-poor spreading ridges, plate divergence is controlled by tectonics, leading to the formation of detachment faults. These faults cut through thick, brittle lithosphere (up to 15 km) and accommodate tens of kilometers of displacement, exposing heterogeneous, altered rocks. Among the alteration reactions, serpentinization has garnered significant attention for its role in sustaining chemosynthetic microbial life and influencing the spatial distribution of earthquakes within the lithosphere. Although the influence of serpentinization on seismicity is largely recognized in ultraslow-spread lithospheres, the nature and extent of alteration remain poorly constrained.
To address this, we use a 2D visco-elasto-plastic model with thermodynamic calculations to simulate lithospheric alteration during ultraslow seafloor spreading under a low magma budget. By coupling water availability and lithospheric hydration progress with active deformation, we reveal: (i) a tectonically controlled vertical extent of alteration along detachment faults; (ii) the preservation of amphibole-facies in exhumed serpentinized footwalls, forming kilometer-scale asperity-like features; and (iii) significant lithospheric-scale rheological heterogeneities resulting from tectonically induced spatial variations in alteration mineral assemblage equilibria across the lithosphere. The largest rheological changes occur along the deep hydration front near the brittle-ductile transition zone, where the alteration of exhumed fresh mantle begins to form high-temperature amphibole-bearing assemblages.
By comparing our model results with seismic data from two magma-poor segments—the easternmost Southwest Indian Ridge and the Knipovich Ridge—we observe that sparsely seismically active regions correlate with highly serpentinized domains in the shallow lithosphere, while deeper seismically active zones correspond to areas with low alteration degrees and the presence of amphibole, talc, and chlorite in amphibole-bearing assemblages. These findings support a conceptual model suggesting that tectonics controls the formation of alteration-induced rheological heterogeneities, which play a key role in controlling earthquake depth distribution at mid-ocean ridges and associated transform faults, and also have implications for seismogenesis in subduction zones.
How to cite: Mezri, L., Diehl, A., Ferrand, T. P., Javier García-Pintado, J., Bickert, M., and Pérez-Gussinyé, M.: Tectonics control alteration-induced rheological heterogeneities in magma-poor ultraslow-spread oceanic lithospheres, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4333, https://doi.org/10.5194/egusphere-egu25-4333, 2025.
Oceanic transform faults (OTFs) are one of three classes of plate boundaries representing the most seismogenic part of the global mid-ocean ridge (MOR) system. Their cumulate length represents more than 40% of the global MOR system. In a classical view, OTFs are perpendicular to mid-ocean ridges and considered as pure strike-slip zones where one plate moves past another and no material is added or destroyed. Recent studies show that OTFs are oblique boundaries where extensional tectonics and a two-phase crustal grow, which challenges a major concept of plate tectonics. However, thermal structure and stress pattern that are key to explore geodynamics processes at OTFs remain poorly understood.
We conducted 3D numerical simulations of plate separation and dike injection at a ridge-transform-ridge system by using the geodynamic code LaMEM (Lithosphere and Mantle Evolution Model). Our results reveal three key findings. First, OTFs are always deeper and warmer than fracture zones for all models, which could be well explained by focused brittle deformation that locally reduces viscosity and strength of OTFs, allowing the far-field tectonic stretching to be preferentially partitioned into the transform domain. Mantle upwelling beneath rheologically weaken OTFs is therefore locally enhanced. Second, plate boundaries of ridge-transform intersections (RTIs) at depth are oblique, which is structurally different from its seafloor expressions. Its obliqueness increases with depth and reduced dike injection rate to the inside corner of ridge segments. Third, we found in all models, that strike-slip faulting, which is thought to be a main feature of OTFs only occurs at distances away from the RTIs. Approaching the RTIs, maximal horizontal stress is oblique to OTFs by more than 45, indicating transform-normal extension at the inside corner. These results provide a first-order constraint on thermal and mechanical behaviour of OTFs and are in line with recent bathymetry, gravity and micro-earthquake evidence.
How to cite: Chen, M., Rüpke, L., Grevemeyer, I., Ren, Y., and Liu, S.: Thermal structure and stress pattern of the oceanic transform fault: insights form 3D numerical modelling, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5724, https://doi.org/10.5194/egusphere-egu25-5724, 2025.
The length of oceanic transform faults varies dramatically from near zero-offsets to long-offset mega-transforms that are >1000 km long. However, the formation and development of longer oceanic transform faults (>200 km) remains incomplete and requires further study. We investigate how changes in the plate motion vector impacts plate stress and transform fault development using high resolution 3D geodynamic numerical models in ASPECT (Advanced Solver for Planetary Evolution, Convection, and Tectonics). Specifically, we study how the length of transform faults evolve over time after inducing transpression or transtension across simple and complex stepped rift-transform geometries. We also determine how the angle of oblique extension affects the required tectonic force necessary to develop new tectonic structures, providing insight into real-world plate tectonic processes. Our results show that transpressional deformation along a transform leads to longer, diffuse transforms at higher angles, while transtensional deformation leads to oblique extension across the transform margin. These transpressional model results are also analogous to real world examples such as the Davie (West Somali Basin) and Ungava Fault Zones (Davis Strait), where we also highlight how the contemporaneous alignment of extinct mid-ocean ridges and young oceanic lithosphere can influence where new transform faults develop.
How to cite: Longley, L. and Phethean, J.: Mega-transform fault development: New insights from Geodynamic modelling using ASPECT and real-world examples, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2320, https://doi.org/10.5194/egusphere-egu25-2320, 2025.
Many potential green energy resources are undiscovered on our planet, hidden within crustal structures such as microcontinents, the formation of which is not well-understood. Recent work by Whittaker et al. (2016) suggests formation of microcontinents from plate tectonic reorganisation, where transpression along transform plates causing ridge jumps into rifted continental margins. To test this hypothesis, we aim to globally map transpressive and transtensional oceanic fracture zones. These structures with specific spectral gravity wavelength signatures will be identified using machine learning approaches and the Generic Mapping Tools (GMT). In later work, we will kinematically model the onset and development of these transpressional and transtensional structures to understand their relative timing to kinematic change and decipher the role of lithospheric structures in microcontinent cleaving and the global plate tectonic system.
How to cite: Tranova, T. M. K., Phethean, J., Khan, W. A., and Hussain, M.: Lithospheric controls on plate tectonic motions and microcontinent formation, part 1: Mapping global transpression and transtension using gravity derivatives and machine learning, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13455, https://doi.org/10.5194/egusphere-egu25-13455, 2025.
The refractory mantle contributes little to the genesis of mid-ocean ridge basalts (MORB), thus observations of the component of the asthenospheric mantle based on the MORB alone are incomplete. In this study, we present both major and trace element compositions of ~70 abyssal peridotite samples from the Sparsely Magmatic Zone (SMZ) and Eastern Volcanic Zone (EVZ) of the Gakkel Ridge. Compositional data indicate that they are mantle residues of the asthenosphere after variable degrees of partial melting. Their clinopyroxenes display two different types of REE patterns, i.e., LREE-depleted and LREE-flat. The latter suggests that some Gakkel peridotites have been refertilized by quasi-instantaneous melts that retained in the melting column. The Gakkel peridotites show large geochemical variability along the ridge axis at length-scales which are too short to be thermally driven. Degrees of partial melting modelled by peridotite geochemistry are greater than those inferred seismically by crustal thicknesses in the SMZ and EVZ. This implies that compositional variations in those abyssal peridotites are inherited from prior melting. In addition, the composition of the Gakkel peridotites differs significantly from that of the subduction-related peridotites. Trace element modelling further supports the presence of a geochemically decoupled crust-mantle. We suggest that the strong heterogeneity of theasthenosphere beneath the Gakkel Ridge is the dominant driver of crust-mantle geochemical decoupling. In particular, in the SMZ region, the small amount of enriched mantle domains in the asthenosphere become the source of the enriched MORB, while massive refractory mantle inherited from prior melting hardly contributes to the SMZ basalts. Therefore, compositional signatures of asthenospheric mantle inferred from MORB of amagmatic zones along mid-ocean ridges may considerably overestimate the proportion of enriched mantle.
How to cite: Xu, Y., Liu, C.-Z., and Lin, Y.-Z.: Chemically heterogeneous asthenosphere beneath the Gakkel Ridge constrained by abyssal peridotites, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7696, https://doi.org/10.5194/egusphere-egu25-7696, 2025.
The mid-ocean ridge (MOR) is the longest volcanic chain on the Earth (∼75,000 km), with spreading rates varying from fast (>80 mm/yr) to ultraslow (<20 mm/yr). It is generally believed that mantle beneath MORs upwells passively due to viscous drag from the diverging tectonic plates, leading to pressure-release melting. While passive mantle upwelling models explain the uniform crustal thickness observed at fast-spreading ridges, they fail to account for the complexities at ultraslow-spreading ridges. At these ridges, enhanced conductive cooling and hydrothermal circulation thicken the ocean lithosphere, shrinking the melting zone and inhibiting melt production. The fundamental dynamics governing crustal accretion at ultraslow-spreading ridges remain elusive. In 2021, we conducted a high-resolution active-source ocean-bottom seismometer (OBS) experiment along the eastern ultraslow-spreading Gakkel Ridge between 76° and 100° E using the icebreaker ‘Xuelong 2’, during the Joint Arctic Scientific Mid-ocean ridge Insight Expedition (JASMInE). Our new seismic model reveals highly variable crustal thickness, which ranges from 3.3 km to 8.9 km along the ridge axis. Meanwhile, this thickness increases from ~4.5 km to ~7.5 km over the past 5 Myr across the ridge axis. In addition, the magnetotelluric data reveals prominent low-resistivity zones at depths 20–45 km beneath volcanic centers, but high resistivities (>100 Ω m) down to ~ 50 km at volcanic ends, indicating highly variable electrical lithosphere (eLAB). Microearthquakes recorded by the OBSs occurred at depths of <10 below the seafloor along the ridge axis, suggesting a relatively shallow brittle lithosphere and a high magma supply. These observations contradict the passive upwelling models and are instead consistent with buoyant active mantle flow model that is driven by thermal and compositional density changes due to melt extraction. Active mantle upwelling is predicted to play a more significant role as the spreading rate decreases, which is highly sensitive to the mantle temperature and composition. This implies that the observed variability in crustal and lithospheric thickness is likely an inherent characteristic of ultraslow-spreading ridges.
How to cite: Li, J., Zhang, T., Niu, X., Yu, Z., Wei, X., Zha, C., Jiang, J., Tan, P., Yang, C., Lu, Z., Ding, W., and Fang, Y.: Highly variable lithospheric structure and associated magmatic accretion at the ultraslow-spreading Gakkel Ridge, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4667, https://doi.org/10.5194/egusphere-egu25-4667, 2025.
The mid-ocean ridge forms new oceanic lithosphere, which subsides, thickens, and moves away from the ridge axis. It is generally believed that the lithospheric thickness is dependent on spreading rate. At ultraslow-spreading ridges (<20 mm/yr), the lithosphere is expected to thicken substantially due to strong hydrothermal cooling and limited magma supply. However, this view has been challenged by the observed highly variable crustal thickness at the ultraslow-spreading Southwest Indian Ridge and Gakkel Ridge, where their lithospheric structures are poorly understood due to limited passive seismic observations. In particular, the Gakkel Ridge, located in the Arctic Ocean, is the slowest-spreading mid-ocean ridge in the world, but no onsite microseismicity has been reported due to severe ice conditions. The 2021 JASMInE cruise marked the first deployment of Ocean Bottom Seismometers (OBSs) array in the eastern part of Gakkel Ridge. 43 OBSs with spacings of 5-10 km were set up to record both air-gun source signals and natural seismic signals. These instruments were deployed along and across the ridge axis, with a focus on the volcanic area at 85°E, covering a range from 75°E to 102°E. Analysis of seismic data identified 234 microearthquakes that occurred continuously in August 2021, and ~50% of them have uncertainties of <10 km. Their focal depths are located no deeper than 13 km below the sea floor (bsf), with most events located at 0-10 km bsf. This depth range is much shallower compared to the microseismicity observed by seismic stations installed on the ice floes during the 2007 AGAVE expedition, where most events were found between 7-16 km deep. We reanalyzed the seismic data collected during the 2007 AGAVE expedition, and preliminary results indicate that the seismic phases have a very low signal-to-noise ratio, with poorly picked S-wave phases, which may result in the observed differences. Furthermore, the newly observed deepest depth of these seismic events is consistent with the 600°C isotherm as previously calculated, approximately 12.6 km bsf. It is unexpected that no earthquakes were recorded beneath the volcano center where explosive volcanic eruption was reported in 1999. Seismic source mechanism analysis reveals normal faulting near the volcano center, but no volcanic swarm-like events were observed. Instead, most earthquakes were concentrated near the segment end at around 88°E, likely associated with a normal fault inclined southward within the rift valley. In addition to the JASMInE cruise, a small seismic network consisting of five OBSs was deployed in August 2023 at the 100°E volcanic center. These instruments were operated on the seabed for approximately one week, but no microearthquakes were detected. These observations may suggest that, at ultraslow-spreading ridges, despite robust magma supply in magmatic segments, magmatic activity is not vigorous. Crust accretion and episodic volcanic eruptions appear to be short-lived, and for most of the time, the magmatic system remains in a period of seismic quiet.
How to cite: Li, X., Yu, Z., Li, J., Jia, Y., Liu, Z., Niu, X., Shen, Z., Tong, Z., Tan, P., Zhang, T., Ding, W., and Fang, Y.: Microseismicity of the Eastern Gakkel Ridge, Arctic Ocean , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6973, https://doi.org/10.5194/egusphere-egu25-6973, 2025.
The Mohns Ridge is located in the Norwegian-Greenland Sea, between the Jan Mayen Transform Fault and the Mohns-Knipovich Bend. It is an ultra-slow spreading ridge section with a full spreading rate of 15 to 17 mm/yr. Over its 580 km, the variations in axis depth and crustal thickness attest of the overall decrease of magma supply associated with the distance from the Jan Mayen Hotspot. In parallel seafloor ages based on sediment thickness and sedimentation rate in the axial valley attest of the relatively young volcanic activity (<180 ka) experienced by the entire ridge axis. Utilizing a multi-proxy approach, we aim to provide new insights into the magmato-tectonic interplay along the Mohns Ridge, including its transient nature and controls on hydrothermal circulation. We investigate: (i) the variability in relative tectonic and magmatic extension by deciphering seafloor morphology extracted from the bathymetric data; (ii) the variability in magma supply and volcanic activity by analyzing gravimetry and magnetic anomalies, and (iii) the distribution and intensity of the recent crustal activity affecting the ridge based on the 40 years of seismicity record. The analyses highlight two significant trends. First, a regional trend linked to the Jan Mayen and Iceland plumes controlling the distance between volcanic centers (i.e., axial volcanic ridges – AVRs) and the focus of the volcanic activity. Second, a local trend associated with AVR maturity controlling AVR volume and related faulting patterns. Combining these observations with the location of known hydrothermal vents, we find no evidence of the regional magma budget variability impacting the distribution of hydrothermal vents. Instead, the locations of hydrothermal vents appear to be related to AVRs with recent and voluminous volcanic activity. This suggests that hydrothermal activity is linked to recent phases of the AVR construction over shorter time scales than to overall melt supply and along-axis gradients, over longer timescales. Finaly, although the seismic activity has been stable over the last 40 years, the lack of correlation with the hydrothermal vent distribution or AVR geometry, suggests that it is related to transient processes over shorter time scales than that of the AVR construction and associated hydrothermal activity.
How to cite: Le Saout, M., Barreyre, T., Escartín, J., and Tominaga, M.: Magmato-tectonic variability along the Mohns Ridge: Insights into the controls on hydrothermal circulation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17434, https://doi.org/10.5194/egusphere-egu25-17434, 2025.
Mid-ocean ridges (MORs) are sites of key thermo-chemical transfers between the Earth's interior and the ocean. Heat flow at MORs primarily depends on lithospheric age but is also modulated by various processes including sedimentation, hydrothermal activity, and faulting, which alter the thermal properties of young oceanic lithosphere. Here we quantify this modulation by analyzing heat flow measurements across the ultraslow-spreading Mohn’s Ridge in the Arctic Ocean. The Mohn’s ridge features major asymmetries in tectonic structures, with larger-offset normal faulting occurring on the West side (North American plate), as well as more sedimentation on the East side (Eurasian plate).
Recently acquired measurements of conductive heat flow across Mohn’s Ridge reveal a significant asymmetry. The eastern (sedimented) side shows a typical conductive profile with values exceeding 600 mW/m2 at the axis decreasing off-axis towards an asymptote at ~100 mW/m². By contrast, the western (faulted) side lacks this conductive plateau, with conductive heat flow dropping to near zero off-axis in ~15-Ma seafloor.
We used 2-D numerical models of hydrothermal convection coupled with conductive heat transport to test two hypotheses (1) An asymmetry in the intensity of brittle deformation leads to greater crustal permeability on the faulted west side, enabling cooling by hydrothermal circulation far off-axis. This manifests as very low conductive heat flux in 10+Ma seafloor. (2) Permeability is the same on both sides of the ridge, but a thick, impermeable sediment blanket suppresses off-axis hydrothermal convection in the Eurasian plate to the East. We find that explaining the low Western heat fluxes requires a high off-axis permeability. The Eastern heat fluxes are better explained either by a lower permeability, or the insulating effect of the sediment. Interestingly, the instantaneous addition of a sediment blanket at a prescribed time in our simulations can turn a heat flow profile typical of the West side into a classical conductive profile typical of the East side in a few 100 kyrs. This suggests that a post-glacial input of sediment on the Eurasian plate could have contributed to a rapid onset of the heat flow asymmetry across Mohn’s Ridge.
How to cite: Barreyre, T., Olive, J.-A., Escartin, J., and Jørgensen, S.: Cooling of young Arctic oceanic lithosphere modulated by off-axis fluid circulation and post-glacial sedimentation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17718, https://doi.org/10.5194/egusphere-egu25-17718, 2025.
We report on the composition of gas‑tight hydrothermal vent fluid samples from the Jøtul field at the ultraslow spreading Knipovic ridge, collected during the R/V MARIA S. MERIAN expedition MSM131 in September 2024. The sampled fluids exhibit high pH values and total alkalinities. Elevated methane concentrations–exceeding those at the sediment-hosted Guaymas Basin vent site–suggest fluid-sediment interaction and thermal decomposition of organic matter derived from continental sediments. These fluids also contain high hydrogen concentrations (>14 mM), which surpass typical values for sediment-hosted hydrothermal vent fluids. The elevated hydrogen levels are accompanied by low H2S concentrations (< 2.5 mM), which might point to a heazlewoodite-pentlandite mineral assemblage controlling the concentrations of these compounds. We suggest that the hydrothermal vent fluids at the Jøtul field acquire their distinct chemical signatures through a combination of fluid‑sediment interactions in the recharge and discharge zones, along with fluid rock interactions governed by ultramafic rocks in the high‑temperature reaction zone. This combination of subsurface conditions produces vent fluids that are metal‑poor but enriched in carbon and hydrogen. The high methane concentrations measured in the Jøtul field highlight hydrothermal fluid‑sediment interactions as a yet underestimated source of carbon emissions into the ocean.
How to cite: Diehl, A., Monien, P., Pape, T., Anagnostou, E., Meckel, E.-M., Römer, M., Monien, D., Bach, W., and Bohrmann, G.: The Jøtul field revisited: High carbon and hydrogen fluxes from a sediment‑hosted hydrothermal vent site in the Knipovic Ridge, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8189, https://doi.org/10.5194/egusphere-egu25-8189, 2025.
Motivated by the goal to determine the chemical form, variability, and potential processes that modulate the flux of ecosystem-limiting metals, like hydrothermal iron (Fe) nano-colloids, and to explore their unique catalytic capabilities, we sampled suspended and dissolved matter in the water column above the Rainbow (36°-33°N) hydrothermal vent field at the Mid-Atlantic Ridge. To investigate the (trans)formation of hydrothermal iron-based nanocolloids, we employed a direct sampling approach that bypasses conventional techniques such as filtration and resuspension. Instead, small amounts of plume fluid were immediately drop-cast onto transmission electron microscopy (TEM) grids and plunge-frozen, preserving dissolved compounds and nanocolloids through vitrification. Using an array of microscopic and spectroscopic techniques, combined with machine learning, allowed detailed characterization of the Fe nanocolloids down to the nano-scale and provided insight into their early (trans)formation and bioavailability.
TEM and synchrotron-based spectroscopy show that the Fe colloids suspended in the hydrothermal plume predominantly consist of poorly ordered ferric Fe-oxyhydroxides most similar to 2-line (2L-Fh) and 6-line ferrihydrites (6L-Fh), which contain local enrichments in P, S, and/or Cu phases. Using the machine learning model SIGMA1 allowed us to explore the distribution of distinct Fe phases and revealed local P:Fe ratios of 1:2 for 2L-Fhs and 1:6 for 6L-Fhs. Utilizing nano-scale scanning TEM tomography, we showed that some 2L-Fh aggregates contain ferrous chalcopyrite (CuFeS2) cores. On the outside, the plunge-frozen Fe-nano colloids are covered with the vitrified plume fluid enriched in Mg, Cl, and ± S. Notably, our results do not show associations of Fe with (organic) carbon.
These observations suggest that chalcopyrite forms in the shallow subsurface before venting and acts as a crystallization seed for some fast oxidizing Fe(II) after mixing with seawater. Ferrihydrite (Fh) forms through the formation of Fe13-Keggin clusters2, and we argue that part of the clustering process occurred on the surface of the chalcopyrite, resulting in dendritic textures of some 2L-Fh. In contrast, Fh can also nucleate by clustering of Fe without needing a preexisting template, resulting in a more compact morphology. The larger surface area of the dendritic Fh that utilizes metal sulfides for their nucleation results in higher adsorption of PO4 and, consequently, due to the dehydration of the surface, significantly decreases the dissolution and, therefore, recrystallization, suppressing the transformation into more ordered 6L-Fh. Furthermore, this shows limited interaction between C-rich phases and Fe-bearing precipitates during early (trans)formation in a black smoker system, contrasting previous studies, which suggest that organic compounds play a key role in stabilizing and transporting hydrothermal Fe3.
Our findings shed completely new light on the transport and persistence of vent-derived reduced iron phases, highlighting the role of ferric coatings in protecting nano-scale iron sulfides and challenging the previously proposed importance of complexation with organic matter. Overall, we provide new perspectives on the early (trans)formation processes of vent-derived iron, its interaction with other essential elements, and, eventually, its impact on ocean chemistry.
- Tung, P., et al. Geochem., Geophys., Geosystems 24, (2023).
- Weatherill, J. S., et al. Environ. Sci. Technol. 50, 9333–9342 (2016).
- Toner, B. M. et al. Acc. Chem. Res. 49, 128–137 (2016).
How to cite: Ternieten, L., Preiner, M., Pekker, P., Pósfai, M., Kraal, P., and Plümper, O.: Formation and early transformation of hydrothermal Fe nano-colloids in a black smoker system, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16615, https://doi.org/10.5194/egusphere-egu25-16615, 2025.
Mid-Ocean Ridges (MOR) are accretionary plate boundaries where new seafloor is created by seafloor spreading. In the early 1980s, these features were mapped for the first time in high detail using multi-beam echosounders and researchers found that the ridge crest of this approximately 70.000 km long rift system has many lateral discontinuities that partition its axis into segments. Discontinuities differ in form and behaviour and are often deeper and less active volcanically than the segments they define. As a result, the crest of the MOR undulates up and down by hundreds of meters over distances of several to hundreds of kilometres. The most prominent ridge offsets are the oceanic transform faults which typically offset the ridge axis by over 20 km. Long transform faults generally form deep valleys, while shorter discontinuities (non-transform offsets) displacing the spreading axis by only a few kilometres to tens of kilometres may show more complex tectonic features.
Even 60 years after the plate tectonic revolution and the introduction of seafloor spreading, much of the classification of ridges crest segmentation is still based on the study of fast-spreading ridges dominated by robust magma supply where discontinuities along the spreading axis are readily identified by offsets of the crest-like ridge axis, including overlapping and often migrating Overlapping Spreading Centres (OSC). It is generally believed that slow spreading ridges show analogue features. Yet observations of prominent median valleys at slow spreading ridges show a much more diverse segmentation. Here, we revisit the segmentation of the slow spreading Mid-Atlantic Ridge (MAR) between 29°30’N (south of Atlantis transform) to 35°30’N (north of Oceanographer transform) using data collected in September and October of 2024 aboard the German RV METEOR during the cruise M204 running a swath-mapping survey along the axis of the MAR. In analogy to fast spreading ridges, we find transform faults and overlapping volcanic centres, but we also map large dome-like features, en-échelon spreading segments, and offsets revealing bookshelf faulting. These structures provide insight into both the various styles of non-transform offsets, and the parameters controlling the different shear accommodation styles.
How to cite: Grevemeyer, I., Ruepke, L., Pusok, A., and Escartin, J.: On the segmentation of the slow spreading Mid-Atlantic Ridge between Atlantis and Oceanographer Transform (29.5 N to 35.5 N), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2791, https://doi.org/10.5194/egusphere-egu25-2791, 2025.
Serpentinization is a hydrothermal process that often forms magnetite, significantly altering the magnetic properties of ultramafic rocks at mid-ocean ridges. However, the evolution of these magnetic properties during serpentinization and their stability over geological timescales are not completely understood. The Atlantis Massif, one of the best-studied oceanic core complexes, is an ideal place to study serpentinization's effects on rock magnetism. IODP Expedition 399 drilled a deep (1268m) borehole (Hole U1601C) into uplifted lower crustal and upper mantle rocks on the Mid-Atlantic Ridge, providing an excellent opportunity to study the variation in rock magnetic properties with spatial context at mid-ocean ridges. In-depth magnetic properties were analyzed using facilities at Stanford and the Institute for Rock Magnetism at the University of Minnesota. We measured room temperature hysteresis loops, back field curves, magnetic properties measurement system, first-order reversal curves, low and high-temperature magnetic susceptibility, and anisotropy of magnetic susceptibility. We found that the magnetic carriers for serpentinized peridotites consisted predominantly of stoichiometric magnetite. Magnetic carriers for gabbros were dominated by magnetite and titanomagnetite, with noticeable contributions from monoclinic pyrrhotite in some samples. Most of the serpentinized samples exhibited vortex (pseudo-single domain-like) domain behavior. Ongoing scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy measurements are being used to contextualize the spatial distribution of magnetic minerals in relation to primary phases, secondary minerals (ex. lizardite, brucite), cracks, and void spaces. Tentative results indicate that iron sulfides in gabbros were predominantly located in cracked regions, while SEM-detectable magnetite grains in serpentinized peridotites were typically found along the rims of relict olivine grains.
How to cite: Lopes, E., Ju, O., Tikoo, S., Jung, J., and Burns, D. and the IODP Expedition 399 Science Party: Magnetic Characterization of Borehole Samples from IODP Expedition 399: Atlantis Massif, Mid-Atlantic Ridge, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7472, https://doi.org/10.5194/egusphere-egu25-7472, 2025.
The rock-hosted subseafloor biosphere provides key insights into the limits and origins of life, yet it remains largely unknown due to limited access. Recently, IODP Expedition 399 provided unprecedented access to a 1,268-meter core from the upper mantle of the Atlantis Massif, primarily composed of serpentinized harzburgite. The abundance and composition of indigenous organisms, their metabolic capabilities, physiological activity, and the role of serpentinization in sustaining life are critical, yet unanswered questions. However, the extremely low biomass and high DNA adsorption capacity of these mantle rocks present significant challenges for DNA extraction and contamination control, limiting our exploration of the rock-hosted biosphere. In this study, we made notable progress by distilling and refining DNA extraction protocols. Using 16S rRNA gene amplicon and metagenomic sequencing, we specifically developed the quality control and decontamination workflow tailored to the unique complexities of low-biomass samples. In this context, we characterized candidate microbial residents within the rocks and fluids, including Campylobacteria, Aquificae, Dehalococcoidia, Bathyarchaeia, Hadarchaeia, Methanosarcinia, and Nitrososphaeria, with distinct phylogenies from those typically found in seawater and sediments. These putative microbial residents likely play key roles in mediating the carbon, nitrogen, and sulfur cycles between the mantle rocks and formation fluids. Our findings suggest the presence of a complex metabolic network capable of thriving in the mantle rocks under high-temperature, hydrogen-rich, and alkaline conditions, underscoring the adaptability of microbial life in extreme subsurface environments. These results contribute to a broader understanding of life’s resilience in the deep biosphere and offer new insights into the origins of life and the potential for extraterrestrial life.
How to cite: Wang, Z., Xie, R., Hou, J., Liang, L., Brazelton, W., and Wang, F. and the IODP Expedition 399 Scientists: Microbial Residents in Serpentinized Upper Mantle of the Atlantis Massif, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4754, https://doi.org/10.5194/egusphere-egu25-4754, 2025.
The subseafloor ecosystem includes all life living in marine sediment, crust and the accompany fluids. This ecosystem, also called the deep biosphere, mostly derives its energy source from geological processes, which are cut off from sunlight. Deep-sea hydrothermal vents and cold seeps are regarded as windows of the subsurface life. Still, little is known about the subseafloor life and there is a substantial knowledge gap related to understanding the breadth of their diversity, assemblage, function, and possible ecosystem services to society. These insights are key to understanding the origin of life and evolutionary processes, and also pivotal for evaluating the impact of the proposed ocean-based climate interventions. As part of the efforts to reduce this knowledge deficiency, we initiate a global-scale program “Global Subseafloor Ecosystem and Sustainability” (GSES). This program aims to generate new systematic insights into subseafloor ecosystems with the aim of transforming these datasets for predictive capabilities. As a newly endorsed program of the UN Ocean Decade, the overarching objective of GSES is to significantly advance scientific comprehension, conservation, and sustainable management of Earth's subseafloor ecosystems. Focused on addressing substantial knowledge gaps in microbial life, carbon dynamics, and historical records within this critical, vulnerable and understudied environment, GSES aims to develop internationally standardized protocols, cutting-edge investigation platforms, and ecological indices. A pilot project that targets the microbiome in the oceanic crust, which is the largest by volume but least understood biosystem on Earth, will be showcased and discussed.
How to cite: Wang, F., Hinrichs, K.-U., Takai, K., Makhalanyane, T., Abdulla, M. H., and Jebbar, M.: Global Subseafloor Ecosystem and Sustainability (GSES), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4175, https://doi.org/10.5194/egusphere-egu25-4175, 2025.
The Atlantis Massif is a well-studied oceanic core complex in the Atlantic Ocean that hosts the Lost City Hydrothermal Field (LCHF). The LCHF is a low-moderate temperature, high pH vent system. In gabbroic intrusions within the serpentinite-dominated substrate of the LCHF, a variety of hydrothermal alteration reactions occur, including replacement, dissolution creating macroscopic (mm scale) reaction porosity, and precipitation of secondary minerals including chlorite, amphibole, prehnite and clays.
Many samples recovered from Expedition 399 and earlier expeditions contain zones of reaction porosity. This work presents SEM, EMPA and other analysis of sample: U1601C 18R2 75-78 and U1309D-310R1 92-95 from Exp. 399, as well as several other samples analyzed concurrently, used only for example purposes.
Reaction porosity filled with actinolite is present at several levels in the gabbroic hole U1309D, including in areas that were newly deepened by Expedition 399. We highlight sample U1309D-310R1 92-95, collected at a depth of 1495 meters below seafloor (mbsf), which contains porosity partially filled with amphiboles zoned from edenitic hornblende cores to actinolite rims, suggesting dissolution by relatively higher temperature fluids.
Hole U1601C is dominated by serpentinised peridotite; porosity is widespread in gabbroic intrusions with a wide range of fills including chlorite, tremolite, diopside, serpentine, prehnite and saponite. Sample U1601C 18R2 75-78 consists of a 1 cm wide gabbroic vein (domain 1) within serpentinised peridotite (domain 2). Along the boundary with domain 2, domain 1 contains a ~5 mm zone of porosity partially filled by secondary diopside and serpentine. Relict porosity up to 200 µm in size is common. Domain 2 also contains porosity filled with diopside and serpentine, as well as zoned rosettes, of various stages of hydrogarnet solid solution, moving from pyrope-rich in the inner core, to more definitively hydro-andradite (identified by Raman spectroscopy (Frezzotti et al. 2012) and EPMA) in the rosette rim. The rosettes here may be replacing pyroxene.
We suggest that gabbroic veins acted as conduits for fluid flow during hydrothermal alteration, probably at temperatures of 300-400 °C, and contributed to the intense serpentinisation of the mantle rocks. Magnetite is not observed in this sample, but hydrous andradite rich in Fe3+ offers another potential H2 generating reaction.
Work in progress includes XCT analysis of the porosity. Further work will involve characterising the geological sequence of events, and in some cases their subsequent deformation (through sequence mapping), investigating the arguments for dissolution versus fill reactions (through extensive SEM, EMPA and X-Ray Tomography) and characterising the extent of reaction porosity in the Atlantis Massif.
References
Frezzotti, M.L., Tecce, F. and Casagli, A. (2012) ‘Raman spectroscopy for fluid inclusion analysis’, Journal of Geochemical Exploration, 112, pp. 1–20. Available at: https://doi.org/https://doi.org/10.1016/j.gexplo.2011.09.009.#
How to cite: Matchett, J.: Rotten Rocks at the Heart of the Atlantis Massif – A dive into reaction porosity in the Lost City Hydrothermal Field, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9528, https://doi.org/10.5194/egusphere-egu25-9528, 2025.
Black-smoker-type hydrothermal vent systems are a feature of all mid ocean ridges. They often sit atop developing massive sulfide deposits, such as the TAG mound in the central Atlantic. The measured apparent upper limit for vent fluid temperatures at these sites around 400°C can be explained with the thermodynamic properties of water [1]. However, continuum-scale numerical models of seawater and hydrothermal fluid circulation commonly fail to reproduce these high vent temperatures under realistic assumptions of host rock permeability. While most discharge of circulating seawater does occur diffusively and at low temperatures, an explanation for the extreme focusing of flow at hot vent sites is needed.
One common approach to resolve this is the so-called “clogged shell” model, where the precipitation of mainly anhydrite at the interface of rising hot fluids and entrained seawater locally lowers permeability around the hydrothermal plume, preventing mixing and increasing vent temperatures [2]. This concept has been validated in a number of studies [e.g., 3], but no fully coupled model of hydrothermal fluid flow and fluid-rock interaction in such systems exists.
Using a newly developed coupling of open-source C++ libraries to solve fluid flow in 2D and 3D (OpenFOAM) and local equilibrium thermodynamics (Reaktoro [4]), we investigate feedback between reactive fluid flow, anhydrite precipitation and vent temperatures.
Anhydrite solubility decreases with higher temperatures, leading to precipitation from heated seawater at the interface with rising hot hydrothermal fluids. Solubility also depends on salinity, increasing in saltier fluids [5]. Thus, we vary hydrothermal fluid salinity between 0 and 5 wt%, based on vent fluid measurements.
Our results clearly show that anhydrite precipitation occurs around the plume and inhibits mixing, focusing the hot upflow and increasing vent temperatures over time. These effects are strongly dependent on fluid salinity: Initial vent temperatures are highest with high salinity, linked to thermodynamic properties of water. Over time, lower salinity hydrothermal fluids produce a narrower anhydrite shell, leading to stronger focusing and a steeper vent temperature increase.
Figure 1. Model results: (a) 2D anhydrite shell (b) cut 3D Anhydrite shell (c) vent temperature over time with variable hydrothermal fluid salinity.
References
[1] Jupp, T. and A. Schultz, A thermodynamic explanation for black smoker temperatures. Nature, 2000. 403(6772): p. 880-3.
[2] Cann, J.R. and M.R. Strens, Modeling periodic megaplume emission by black smoker systems. Journal of Geophysical Research: Solid Earth, 1989. 94(B9): p. 12227-12237.
[3] Guo, Z., et al., Anhydrite‐Assisted Hydrothermal Metal Transport to the Ocean Floor—Insights From Thermo‐Hydro‐Chemical Modeling. Journal of Geophysical Research: Solid Earth, 2020. 125(7).
[4] Leal, A.M.M. Reaktoro: An open-source unified framework for modeling chemically reactive systems. 2015; Available from: https://reaktoro.org.
[5] Creaser, E.C., M. Steele-MacInnis, and B.M. Tutolo, A model for the solubility of anhydrite in H2O-NaCl fluids from 25 to 800 °C, 0.1 to 1400 MPa, and 0 to 60 wt% NaCl: Applications to hydrothermal ore-forming systems. Chemical Geology, 2022. 587.
How to cite: Engelmann, J. and Rüpke, L.: The Hydrothermal “Clogged Shell” Model Revisited Using Coupled Reactive Fluid Flow (OpenFOAM + Reaktoro) – Feedback Between Vent Fluid Salinity, Temperature, and Anhydrite Precipitation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16639, https://doi.org/10.5194/egusphere-egu25-16639, 2025.
Felsic plutonic rocks, such as plagiogranites, are commonly found in minor proportions in the lower oceanic crust. The presence of quartz of magmatic origin in the oceanic lithosphere, especially in the mantle, is therefore rarely documented. Here, we present microstructural and petrological observations of a gabbro-peridotite hybrid rock collected in situ by HOV Nautile along the southern wall of the Kane Fracture Zone, at the base of the Kane megamullion, during the KANAUT expedition (Mid-Atlantic ridge, 23°N; Auzende, 1992). This sample, a strongly deformed gabbro containing a peridotite fragment, shows evidence of mantle reacting with hydrous SiO2-rich melt at the contact between both lithologies.
The gabbro is composed of oriented plagioclase-rich layers alternating with polymineralic layers of plagioclase, clinopyroxene, orthopyroxene (Opx) and Fe-Ti oxides, and of mm-thick quartz-rich layers. These gabbroic layers locally enclose an aggregate of weakly deformed olivine grains with few Opx grains (up to 1 mm in size). The high Mg# of both olivine and Opx (up tp 85% for both), and the low TiO2 (< 0.1 wt.%) of Opx and of the rare spinels in the aggregate, support a mantle origin. The contact between the two lithologies is marked by a rim of small, polygonal to interstitial Opx grains, forming bulges into the adjacent olivine grain boundaries. The cusp-shapes of olivine grains at contact with Opx, the bulges of Opx along olivine grain boundaries, and the presence of phlogopite and edenitic amphibole, indicate local dissolution of olivine and precipitation of Opx and phlogopite in presence of a hydrous melt, as documented in peridotite from subcontinental contexts.
Temperatures estimated from geothermometry in Opx, plagioclase-amphibole and quartz all indicate that this melt-rock reaction occurred around 900-1000°C. This is also consistent with the crystallographic preferred orientation (CPO) of plagioclase showing a main direction of [100]. The CPO of all minerals forming the gabbroic layers have a main direction parallel to the foliation, which also follows the contours of the peridotite fragment. By contrast, the olivine CPO in the peridotite fragment, showing a clear [100](010) slip system typical of high temperature, low stress conditions prevailing in the asthenosphere, has a direction orthogonal to the foliation. Taken together, the Mg# of olivine and Opx in the peridotite fragment, and the gabbro foliation orthogonal to the presumed foliation in the peridotite, provide evidence that this peridotite fragment preserved the deep mantle conditions during exhumation, despite its reaction with a hydrous melt. To our knowledge, this is the first time in an abyssal context that the reaction between a mantle component and hydrous Si-rich melt, leading to olivine-quartz association in a same sample, is reported.
AUZENDE Jean-Marie (1992). KANAUT cruise, RV Le Nadir, https://doi.org/10.17600/92003211
How to cite: Bickert, M., Rospabé, M., Kaczmarek, M.-A., and Maia, M.: Olivine-quartz association in a gabbro-peridotite hybrid rock of the Kane Fracture Zone: evidence for hydrous Si-rich melt percolation in abyssal context., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11050, https://doi.org/10.5194/egusphere-egu25-11050, 2025.
Oceanic detachments are deep-rooted, long-lived structures at plate scale, acting as conduits for fluid introduction into the oceanic lithosphere. These processes impact plate rheology and potentially induce oceanic crustal anatexis. However, the mechanisms and extent of fluid ingress and crustal melting during detachment faulting remain poorly constrained. This study investigates felsic veins from the Atlantis Bank oceanic core complex (OCC) on the Southwest Indian Ridge to elucidate controls on crustal anatexis imposed by oceanic detachments.
We report systematic results for mineral chemistry, zircon U-Pb ages and Hf-O-Zr isotopes, and Nd-O isotopes of apatites from 23 felsic rocks retrieved from 50−800 meters below the seafloor in IODP Hole U1473A. Additionally, phase equilibria and zircon trace element modeling for three formation modes of oceanic felsic melts (hydrous partial melting of gabbros, fractional crystallization of MORB, and fractional crystallization of anatectic melts) were performed. These data and models consistently suggest that most U1473A felsic veins were products of advanced mid-ocean ridge basalt (MORB) differentiation.
Further examination of zircon trace element data for the Atlantis Bank OCC indicates that the felsic veins resulted from strong fractionation of either primitive basalts or magmas generated by hydrous melting of gabbros. The presence of anatectic felsic veins near the fault plane suggests that the detachment fault facilitated high-temperature (750–900°C) alteration and hydrous melting of gabbros. Additionally, analyses of felsic rocks from two OCCs on the Mid-Atlantic Ridge, based on published zircon trace element data and models, reveal distinct manifestations of the interplay among faulting, magmatism, and hydrothermal circulation across various OCCs. Our findings underscore the critical role of detachment faulting in fluid ingress and oceanic crust melting, with significant implications for chemical and thermal exchanges between seawater and the oceanic lithosphere.
How to cite: Zhang, W.-Q., Liu, C.-Z., MacLeod, C. J., and Lissenberg, C. J.: Evaluation of the role of detachment faulting in the genesis of felsic melts in the Atlantis Bank oceanic core complex, Southwest Indian Ridge, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1610, https://doi.org/10.5194/egusphere-egu25-1610, 2025.
Oceanic crust is formed by basaltic melt produced through decompression melting of ascending mantle at mid–ocean ridges. This oceanic crust is separated from the residual mantle by the Mohorovičić (Moho) discontinuity. Determining the crustal and mantle velocities and the structure of Moho transition zone is critical for understanding the mantle melting, melt extraction and migration and crustal accretion along mid–ocean ridges.
We used seismic full waveform inversion (FWI) to analyse the ocean bottom seismometer (OBS) data from the 2016 CREST experiment from the South Atlantic Ocean at 31oS that samples the 30.6 Ma crust formed along the Mid–Atlantic Ridge at a slow–spreading rate (half–spreading rate of 24 mm/year). Seven four–component OBSs were deployed at ~10 km interval along the seismic profile, and the airgun array source was shot at 150 m interval. The high–quality OBS data show clear crustal refraction arrivals (Pg) up to ~35 km offsets, strong Moho reflection arrivals (PmP) at ~20–65 km offsets but absence of mantle refraction arrivals (Pn), indicating the presence of a relatively thin Moho transition zone (MTZ) and a negative velocity gradient in the mantle.
We performed two-dimensional elastic FWI of the pressure data recorded by hydrophone to constrain fine–scale crustal and mantle velocity. The starting model for FWI was obtained from a previous study of joint tomography of manually picked travel times of Pg and PmP arrivals. We progressively inverted the OBS seismic data in FWI from 3.0–4.5 Hz data to 3.0–6.5 Hz data to gradually update the subsurface velocity. The preliminary FWI result shows a uniformly thick crust of 7.1 km along the profile, ~1 km thicker than the global mean of oceanic crust. This observation indicates a relatively uniform mantle upwelling along the ridge and ~20oC higher mantle temperature at the time of crustal formation. The lower–upper crustal ratio is ~2.5, suggesting the upper crust was formed by a magma reservoir in the mid–crust. The lower crust is heterogeneous where high and low velocity layering is observed, indicating lower crustal accretion by the in–situ crystallisation of melt sills. Assuming the depths of 7.2 and 8.0 km/s velocity contours as the top and bottom of the MTZ, respectively, the thickness of the MTZ varies between 0.6 and 1.2 km with an average of ~0.9 km. A ~1 km–thick layer with velocity up to 8.2 km/s lies beneath the MTZ, possibly due to the presence of a thin dunite–rich layer. Further below, the upper mantle velocity gradually decreases with depth, which could be due to the mantle anisotropy and/or the presence of frozen gabbroic sills in the mantle.
How to cite: Wang, Z. and Singh, S. C.: Thick crust, thin Moho transition zone and negative velocity gradient in the mantle along a 30.6 Ma segment in the South Atlantic Ocean at 31oS, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9158, https://doi.org/10.5194/egusphere-egu25-9158, 2025.
The Red Sea is one of the youngest ocean basins on Earth and is classified as an ultra-slow spreading ridge, with spreading rates decreasing from 15 mm/year in the Southern Red Sea to 7 mm/year in the Northern Red Sea. The Zabargad Fracture Zone (ZFZ), the largest rift-axis offset (~100 km) in the Red Sea (23.5oN to 26oN), separates the Central and the Northern Red Sea. The proximity of the seismically active ZFZ to coastal cities and infrastructure in the region has implications for the regional seismic hazard. However, thick salt and sedimentary covers in the ZFZ obscure the exact geometry of the oceanic spreading axes, and any potential transform faults or non-transform offsets, resulting in ambiguous interpretations. Seismological studies to date have relied on onshore recordings, yielding limited earthquake location accuracy that has impeded detailed analysis.
We deployed the first-ever broadband ocean-bottom seismometer network in the Red Sea, which was augmented with land-based stations, for a period of 12 months to improve the seismic data coverage in the ZFZ. The deployment resulted in a recovery rate of over 90% for the continuous seismic recordings. Using this new dataset, we applied a deep-learning-based algorithm for automatic earthquake detection and phase picking. The results were manually verified and refined, enabling the development of a high-resolution earthquake catalog. These processing steps yielded over 3,900 local earthquakes, with magnitude ranging from ML -0.4 to ML 2.5. We further optimized a 1-D seismic velocity model for the ZFZ and improved earthquake locations using a double-difference relocation algorithm. Focal mechanisms for selected events were determined using polarity and amplitude ratios.
Our findings reveal two major seismicity clusters in the northern part, near the Mabahiss Deep, a deep with exposed oceanic crust, and in the southern part, around the ZFZ. The hypocenter distribution is consistent with NNW-SSE trending normal faults parallel to the ridge axis, indicating ridge segmentations and at least one ~25 km long NE-SW transform fault with strike-slip mechanisms. Variations in seismicity depth highlight changes in the brittle-ductile transition zone: shallower near Mabahiss Mons, an axial Mid-Oceanic Ridge Basalt volcano, reflecting elevated temperatures, and deeper further south, suggesting lower temperatures due to fluid circulation. These results provide new insights into the ZFZ's tectonic structure and seismic activity, improving our understanding of oceanic spreading dynamics in the northern Red Sea and the associated earthquake hazard.
How to cite: Shiddiqi, H. A., Parisi, L., Cano, E., Fittipaldi, M., Agustin, N., Baby, G., Mai, P. M., and Jónsson, S.: Seismicity in the Zabargad Fracture Zone, Northern Red Sea and its tectonic implications: insights from an Ocean Bottom Seismometers Network, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2367, https://doi.org/10.5194/egusphere-egu25-2367, 2025.
The interaction between transform faults and mid-ocean ridges results in complex magmatic distribution, consequently, intricate crustal accretion processes. In this study, we present magnetic survey data collected over the Argo transform fault in the Central Indian Ocean. Magnetic modeling was conducted along two profiles crossing the adjacent spreading center and one profile over the transform fault. The results reveal the absence of a central magnetic anomaly over the spreading center where it intersects with the transform fault indicating reduced magmatic activity. In this case, plate divergence is alternately driven by magmatic and tectonic processes. Isochron alignment on both sides of the transform fault correlates well, indicating an age offset of 7.5 Myr and a consistent half-spreading rate. The profile over the transform fault and associated fracture zones (FZs) shows strong magmatic signals in the FZ areas near the outside corners, suggesting magma intrusion from the juxtaposed ridge. Conversely, most areas along the transform fault exhibit weak magnetic signals, except for a moderate magnetic anomaly over a transform-parallel serpentinite ridge with gabbro intrusions. These findings further demonstrate that transform faults are not simple conservative plate boundaries and shed light on the dynamics of magmatism and seafloor spreading in ridge-transform systems.
How to cite: Zhou, F., Grevemeyer, I., H. Rüpke, L., and W. Devey, C.: Magmatism distribution and modes of seafloor spreading at a Ridge-transform fault system revealed by marine magnetics, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5906, https://doi.org/10.5194/egusphere-egu25-5906, 2025.
The interaction between the Central Indian Ridge (CIR) and the Réunion hotspot has significantly influenced the formation of notable volcanic features in the Indian Ocean over the past 60 million years, including the Maldive ridge and Chagos bank on the Indian-Australian Plate, as well as the Mascarene Plateau, Mauritius Island, Réunion Island, and Rodrigues Ridge on the African Plate. Plate reconstruction results indicate that the distances between the CIR and Réunion hotspot have varied throughout the geological history, transitioning from off-axis (65-40 Ma) to on-axis (40-20 Ma) and back to off-axis (20-10 Ma) cases, with the current distance exceeding 1,000 km. This makes the CIR-Réunion system an ideal setting for studying both on-axis, off-axis interactions and their transitions. In this study, we utilized the advanced computational geodynamic platform ASPECT to investigate the CIR-Réunion system, focusing on 3-D mantle evolution, deep structures and their connectivity, and the migration pattern of hotspot material towards the ridge and surrounding regions. Our results illustrate the dynamic processes of mantle and crust, the dispersion of temperature anomaly, and the migration of plume material. The model results show that the critical points of the interactions begin and cease are ~50 and ~10 Ma, respectively. There is no direct connection between the ridge and hotspot at present. These indicate that the traces of the ridge-hotspot interaction may show spatial features, but it actually reflects the temporal variations.
How to cite: Luo, Y., Zhang, F., Zhou, Z., and Lin, J.: Historical Interaction of Central Indian Ridge and Réunion hotspot in the Indian Ocean, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14183, https://doi.org/10.5194/egusphere-egu25-14183, 2025.
Extinct spreading ridges are globally widespread and are crucial to understanding the lifespan of oceanic plates. Yet the nature of the LAB beneath extinct ridges remains enigmatic. In this study, we investigate the LAB structure beneath the SCS basin, where a ~700-km-long extinct ridge system stopped spreading at ~15 Ma. A 120 km long marine magnetotelluric (MT) transect perpendicular to the extinct mid-ocean ridge in the southwest sub-basin of SCS was carried out at September 2021. The electrical resistivity model reveals a relatively low-resistivity layer at depths of 50-80 km, potentially corresponding to 0.1%-0.9% partial melts. This low-resistivity layer is heterogeneous and absent directly beneath the extinct ridge axis. This observation supports a model in which melts are efficiently extracted beneath the ridge axis, leaving the central region depleted, while partial melts are retained in the surrounding areas on either flank. Additionally, residual melts at shallower depths have likely solidified due to plate cooling, while deeper melts indicate the depth of the LAB. These findings propose a new mechanism for the emplacement of long-lived partial melts at the LAB and suggest that a discontinuous melt-rich layer may commonly occur near extinct spreading ridges globally.
How to cite: Zhang, F., Yang, B., Lin, J., Zhang, T., Samer, N., Li, J., Uyeshima, M., Liu, C., Ding, W., Zhang, X., Zhang, J., Zha, C., Yang, A. Y., Cheng, Z., Zhou, P., Tian, J., and Lin, W.: Electrical resistivity structure of the lithosphere-asthenosphere boundary beneath the extinct ridge of the South China Sea, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10319, https://doi.org/10.5194/egusphere-egu25-10319, 2025.
The magmatic upper crust is generally divided into Layer 2A and Layer 2B, where Layer 2A is interpreted to consist of lava flows and Layer 2B of dikes, although hydrothermal alteration processes have also been suggested to define the Layer 2A/2B boundary. Using 3D seismic reflection method at the Axial Volcano in the Eastern Pacific, we have recently imaged > 3 km of layered lava flows that dip inwards towards the rift zone and interact with the axial melt lens, hence indicating the absence of a dike sequence. These images also show the injection of melt sills within the lava pile. However, the conventional stacking of wide-angle data (triplication associated with the high velocity gradient zone at the base of Layer 2A) indicates that a classical Layer 2A/2B boundary can be defined in our study area.
Here, we present results of seismic full waveform inversion applied to ultra-long offset (12 km) multi-channel seismic data collected in 2019 during the same survey that yielded the 3D seismic reflection results. In our high-resolution P-wave velocity section and associated velocity gradient section we find layered structures consistent with the 3D seismic image. We also find (1) a low-velocity layer in the upper part, evocative of Layer 2A, (2) a high-velocity gradient zone underlain by (3) a high-velocity but low-gradient zone (similar to Layer 2B) underneath, all within the imaged thick lava pile. We suggest that the uppermost lava flow layer consists of hydrated lava flows whereas the lower layer has undergone dehydration and metamorphism and has been formed by the interaction of lava flows with melt bodies and injected sills. Thus the classical Layer 2A/2B boundary would correspond to the boundary between hydrated and dehydrated lava flows. Our results suggest that the upper oceanic crust is formed by lava flows and their interactions with melt-sills, which resolves the long-standing debate about Layer 2A/2B boundary.
How to cite: Xie, W., Wu, H., Singh, S., Carton, H., Kent, G., and Arnulf, A.: Seismic Evidence of Hydrated/dehydrated Lava Flows at the Layer 2A/2B boundary from Full Waveform Inversion of Ultra-long Offset Multi-channel Seismic Data at the Axial Volcano in the Pacific Ocean, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11076, https://doi.org/10.5194/egusphere-egu25-11076, 2025.
Hydrothermal circulation at the axis of fast-spreading mid-ocean ridges is intrinsically linked to magmatic activity, which typically fluctuates on decadal time scales, i.e., the characteristic recurrence time of eruptions. While hydrothermal vent temperatures are known to fluctuate in response to sudden events such as dike intrusions or seismic swarms, their response to longer-term processes such as the replenishment of an axial melt lens (AML) remain poorly documented. Here we focus on high-temperature vents from the 9°50'N segment of the East Pacific Rise, which experienced eruptions in 1991/1992 and 2005/2006, and has been extensively monitored over the last 3 decades. There, a compilation of legacy data complemented by recently acquired temperature measurements from the Bio9 vent site (cruise AT50-21, February-March 2024) reveal decadal trends where maximum vent temperatures increase by ~30ºC in ~15 yr between eruptions, and drop by a commensurate amount within a few years of each eruption. In this study we use numerical models of hydrothermal convection to test the hypothesis that decadal increases in vent temperatures are caused by AML inflation pressurizing the upper crust and decreasing its permeability.
We simulate 2-D porous convection driven by a constant basal heat flux, where permeability decreases exponentially with pressure, as suggested by rock deformation experiments. We first benchmark the relationship between average maximal vent temperature and mean permeability against the analytical model of Driesner (2010). Then, we perturb the permeability field using a mechanical model of sill inflation that imparts isotropic compression across the upper oceanic crust, resulting in exponentially-decaying permeability above the 1.5 km deep AML. When using a narrow basal heat source, we obtain a single plume of rising hot fluid, whose flow progressively slows down in the basal conductive boundary layer. This creates a positive thermal anomaly which is then advected to the seafloor by the plume. However, when the heat source is broader and the convection geometry more intricate, variations in permeability modify fluid pathways, leading to a more complex response. Lastly, simulating cycles of AML inflation and deflation yields oscillations in vent temperatures with periods representative of the duration of a replenishment cycle, but with a lag strongly modulated by the vigor of the convective system.
How to cite: Moutard, K., Olive, J.-A., Barreyre, T., Fontaine, F. J., Fornari, D. J., McDermott, J., Parnell-Turner, R., Wu, J.-N., and Marjanović, M.: Investigating the response of hydrothermal convection to decadal cycles of magmatic inflation at the East Pacific Rise, 9º50'N, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11184, https://doi.org/10.5194/egusphere-egu25-11184, 2025.
Axial melt lenses (AMLs) are key features of fast and magmatically-robust spreading ridges. These sill-shaped bodies typically sit atop a lower crustal mush zone, and supply magma that gets intruded in the brittle axial lithosphere as dikes, or emplaced at the seafloor as lava flows. The replenishment rate of these shallow reservoirs is thus a critical control on the modes of crustal accretion, the timing of mid-ocean ridge eruptions, and the thermo-chemical output of hydrothermal convection, but remains scarcely documented.
Here we revisit estimates of magmatic inflation rates at the East Pacific Rise, 9º50’N based on measurements of vertical seafloor displacements carried out by Nooner et al. (2014). These measurements revealed seafloor uplift rates as fast as ~7 cm/yr above the AML, decaying over ~10 km in the cross-axis direction, between 2009 and 2011. We model this uplift profile as resulting from the inflation of a 1.5 km-deep, 1-km wide AML in a visco-elastic half-space that includes a viscous mush zone of uniform viscosity.
Our models reveal a tradeoff between the assumed viscosity of the mush zone and the sill inflation rate that is necessary to explain the observed seafloor uplift. Specifically, if we assume a strong mush (viscosity > 1018 Pa.s), the replenishment rate must be ~200 m3/yr per meter along axis. On the other hand, a weaker mush (viscosity < 1016 Pa.s) significantly damps the surface expression of sill inflation, requiring a replenishment rate of ~470 m3/yr/m to match the observations. Further constraints on AML replenishment rates can be obtained by assuming the associated heat flux sustains on-axis hydrothermal venting near 9º50’N (~100 MW). We also find that rapid AML deflation during an eruption can induce a characteristic deformation transient lasting up to a few years, which is akin to the post-seismic phase of the seismic cycle. Depending on the assumed viscosity of the mush zone, this post-eruption signal has the potential to bias estimates of steady AML replenishment rates.
Regardless of the assumed mush viscosity, our modeling yields replenishment rates comparable to the long-term crustal accretion rate (~600 m3/yr/m). This suggests that magmatic inflation is not an unusual event at a fast-spreading ridge like the East-Pacific Rise. By estimating the fraction of the ridge’s magma supply that transits through the AML, our results may also provide new constraints on the modes of accretion of the oceanic lower crust, i.e., help discriminate between the gabbro glacier and multiple-sills endmember models.
How to cite: Olive, J.-A., Boulze, H., and Garaud, J.-D.: Rates of melt lens replenishment at the East Pacific Rise, 9º50’N , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1757, https://doi.org/10.5194/egusphere-egu25-1757, 2025.
Volcanic seamounts found in every ocean are among the most widespread landforms on Earth and their geological evolution provides valuable insights into Earth's interior melting processes. Seamounts form in diverse tectonic settings, including mid-ocean ridges, subduction zones, and intraplate volcanism, with their size and distribution reflecting their tectonic origin. Smaller seamounts typically form on younger seafloor near mid-ocean ridges, while larger seamounts originate from volcanism on older seafloor far from ridge axes. A common height threshold distinguishing small and large seamounts is 1-1.5 km. Using the latest gravity-predictive seamount census, we statistically analyzed 18400 well-surveyed seamounts, integrating geometric data (exposed height above the seafloor, radii, volume, and irregularity) and tectonic features (seafloor age, spreading rate, and hotspot proximity) from GEBCO_2024 and GPlates reconstructions.
Our analyses to date show that 90% of seamounts are under 2 km in height and distribute in all tectonic environments, whereas those above 2 km high are primarily located away from mid-ocean ridges. This height threshold may serve as a new criterion to distinguish small from large seamounts. Additionally, there are no fundamental differences in the distribution and shapes of seamounts across the Atlantic, Indian, and Pacific Oceans. Specifically, seamount height shows no strong correlation with spreading rate but a weak positive trend with seafloor age. Approximately one-third of seamounts in the three major oceans lie within hotspot tracks. Strikingly, nearly all seamounts taller than 4 km are associated with hotspots or large igneous provinces, exemplified by those situated on the "hotspot highway" in the western Pacific.
In a nutshell, seamounts generally grow to heights of up to 2 km regardless of formation setting, but growth to heights exceeding 4 km requires stronger impulse from hotspots or large igneous provinces. This finding suggests that towering seamounts worldwide are likely to be the product of anomalous magmatic activity caused by the upwelling of deep mantle plumes.
How to cite: Liu, S., Rüpke, L., Madrigal, P., and Chen, M.: Global distribution and growth mechanisms of seamounts: Insights from statistical and tectonic analysis, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15659, https://doi.org/10.5194/egusphere-egu25-15659, 2025.
The Troodos ophiolite on Cyprus contains a world-class exposure of a ridge-transform system that developed in a supra-subduction zone setting, making it an ideal location to study the associated tectonic and magmatic processes. On Cyprus, the Arakapas Transform Fault separates the ophiolite into distinct terrains. South of this fault lies the Limassol Forest Complex (LFC), an anomalous domain with stratigraphic and structural contacts that differ markedly from the characteristic Penrose ophiolite stratigraphy.
The LFC was likely formed in an (ultra)slow-spreading environment, dominated by temporally and spatially variable magmatic and amagmatic extension. Evidence of magmatism includes extensive dike intrusions observed throughout the stratigraphy, suggesting a dynamic system with ongoing melt generation and emplacement. The structural contact between the crust and mantle lithologies however indicates episodes of amagmatic tectonic extension, responsible for dismembering the crustal sequence of the LFC, bearing similarities with oceanic core complexes.
To evaluate the resemblance of the LFC to oceanic core complexes, this study focuses on the crust-mantle contact in the northwestern part of the LFC. By integrating high-resolution drone imagery, structural measurements, and detailed geological mapping, we refine our understanding of the stratigraphic contacts, intrusive relationships, and deformation processes. The relative timing of intrusive and tectonic events will help clarify the interactions between magmatic and extensional processes.
The results will be compared to known oceanic core complexes, such as the Monviso ophiolite, active systems along the Mid-Atlantic Ridge, and active supra-subduction zones, such as the Philippine Sea Plate, to identify similarities in mantle exhumation processes, fault dynamics, and magmatic-tectonic interactions. These findings have implications for the evolution of transform margins, the role of magmatism in slow-spreading systems, and the influence of supra-subduction processes on oceanic lithosphere formation. By highlighting the interaction of tectonic, and magmatic processes, this study places the LFC in the larger context of ridge-transform fault systems.
How to cite: van den Ing, S., van den Bosch, M., Beniest, A., and Wessels, R.: An oceanic core complex on Cyprus? Unravelling the Limassol Forest ophiolite., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8689, https://doi.org/10.5194/egusphere-egu25-8689, 2025.
Cyprus exposes a world-class ophiolite containing a fossil ridge-transform system that formed in an (ultra)slow spreading supra-subduction zone setting. The stratigraphic completeness and outcrop quality make it uniquely suited for studying its (de)formation history and associated magmatic and hydrothermal processes. The Arakapas Transform Fault separates two distinct domains of the ophiolite; in the north, the Troodos ophiolite largely conforms to the Penrose stratigraphy, while in the south, the Limassol Forest Complex (LFC) is characterised by anomalous stratigraphic and structural contacts.
In this study, the intrusive history of the sheeted dike complex in the Limassol Forest is unravelled on the basis of field observations, petrology, and geochemistry, and compared with the sheeted dike complex of the Troodos ophiolite. Field descriptions and the relative timing of dike sets in the Limassol Forest and Troodos are expanded with geochemical and petrological characterization of selected samples using optical and scanning electron microscopy combined with whole-rock, trace, and rare-earth elemental analyses.
The geological, geochemical, and petrological data will be used to determine and compare the evolution of the Limassol Forest Complex and the Troodos ophiolite. Their heterogeneous evolution, and the influence of the Arakapas Transform Fault, provide insights into the interplay between tectonic, magmatic, and hydrothermal processes active at slow spreading ridge-transform systems.
How to cite: van den Bosch, M., van den Ing, S., van Grieken, A., Beniest, A., and Wessels, R.: Interplay between tectonics, magmatism, and hydrothermal activity in slow-spreading systems: insights from the sheeted dike complexes of the Limassol Forest and Troodos ophiolites, Cyprus, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8006, https://doi.org/10.5194/egusphere-egu25-8006, 2025.
Posters virtual: Tue, 29 Apr, 14:00–15:45 | vPoster spot 1
EGU25-11736 | Posters virtual | VPS22
Poisson’s ratio structure and three-dimensional P wave velocity structure beneath the profile across the Gakkel ridge 85°E axisTue, 29 Apr, 14:00–15:45 (CEST) | vP1.16
During active-source 2D marine ocean bottom seismic exploration, significant deviations of shot lines from the designed survey lines can introduce errors in 2D structural models, particularly in areas with rough bathymetry, such as mid-ocean ridges. By employing 3D tomography, it is possible to construct a three-dimensional model of the survey area that incorporates the actual shot locations and Ocean Bottom Seismometer (OBS) positions, leading to more accurate velocity structure models.
In 2021, the Joint Arctic Scientific Mid-Ocean Ridge Insight Expedition (JASMInE) acquired high-quality OBS data from the Gakkel Ridge in the Arctic Ocean. However, due to the presence of dense floating ice, significant offsets occurred between the shot lines and the OBS station profiles. Consequently, applying a 3D tomography-based modeling approach is essential for imaging the velocity structure in this region.
This study utilized the JIVE3D software to develop a 3D P-wave velocity model along a profile perpendicular to the 85°E spreading axis of the Gakkel Ridge, based on high-resolution multibeam bathymetry data. Compared to the velocity structure derived from 2D modeling, the P-wave velocities beneath the spreading axis are found to be lower in the 3D model, while lateral velocity variations in the upper oceanic crust are more pronounced away from the spreading axis. Despite these differences, the overall velocity structure and crustal thickness trends are consistent, indirectly validating the reliability of the 2D structural model.
Based on this 2D P-wave model, with data of 1257 S-wave arrival times picked from 9 OBS stations along the profile perpendicular to the mid-ocean ridge, using a forward modeling trial-and-error approach, a preliminary Poisson’s ratio structure beneath the profile was obtained. The Poisson’s ratio in Layer 2 of the oceanic crust ranges from 0.36 to 0.40, with relatively lower values beneath the spreading axis. In Layer 3, the Poisson’s ratio varies from 0.28 to 0.38. The relatively higher Poisson’s ratio values may indicate the presence of abundant fractures or fluids within the oceanic crust in this region.
How to cite: Niu, X., Li, J., Yang, W., Yu, J., Ding, W., and Zhang, T.: Poisson’s ratio structure and three-dimensional P wave velocity structure beneath the profile across the Gakkel ridge 85°E axis, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11736, https://doi.org/10.5194/egusphere-egu25-11736, 2025.
