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Examples of plutonic rocks from the Rainbow massif: (a) Coarse-grained gabbronorite. (b) Deformed gabbro. (c) Troctolite. (d) Thin section scan corresponding to the white rectangle in c. (e) Olivine gabbro (yellow-gray) displaying a sharp primary contact with a serpentinized peridotite. (f) Thin section scan corresponding to the white rectangle in e. (g) Undeformed plagiogranite. (h) Plastically deformed plagiogranite. (i) Chromitite. (j) Gabbroic veinlet crosscutting a serpentinized peridotite. Hbl = hornblende; Mgt = magnetite; Ol. = olivine; Plag. = plagioclase; Px = pyroxene; Qz = quartz; Ser. = sericite; Serp. = serpentine.
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Rainbow is a dome-shaped massif at the 36°14’N non-transform offset along the Mid-Atlantic Ridge. It hosts three ultramafic-hosted hydrothermal sites: Rainbow is active and high-temperature; Clamstone and Ghost City are fossil and low-temperature. The MoMARDREAM cruises (2007, 2008) presented here provided extensive rock sampling throughout the mas...
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... plutonic rocks were found in six MoMARDREAM dredges, and along Nautile dive track M7-PL10. They are troctolites (Figures 5c and 5d), olivine gab- bros (Figures 5e and 5f), gab- bros, and gabbronorites (Figures 5a and 5b) and are associated with serpentinized peridotites throughout the core of the massif (Figure 2), except in dredge M8-DR01 where they are exclusively associated with extrusive rocks. Sharp primary contacts between serpentinized peridotites and gabbros ( Figure 5e) or gabbroic veins (Figure 5j) are observed, especially in dredge M8-DR08. ...
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... are troctolites (Figures 5c and 5d), olivine gab- bros (Figures 5e and 5f), gab- bros, and gabbronorites (Figures 5a and 5b) and are associated with serpentinized peridotites throughout the core of the massif (Figure 2), except in dredge M8-DR01 where they are exclusively associated with extrusive rocks. Sharp primary contacts between serpentinized peridotites and gabbros ( Figure 5e) or gabbroic veins (Figure 5j) are observed, especially in dredge M8-DR08. The bulk of these rocks are undeformed, but some gabbros show plastic deformation of pyroxenes, and there are some ductile to brittle deformation bands in the greenschist facies (Figure 5b). ...
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... are troctolites (Figures 5c and 5d), olivine gab- bros (Figures 5e and 5f), gab- bros, and gabbronorites (Figures 5a and 5b) and are associated with serpentinized peridotites throughout the core of the massif (Figure 2), except in dredge M8-DR01 where they are exclusively associated with extrusive rocks. Sharp primary contacts between serpentinized peridotites and gabbros ( Figure 5e) or gabbroic veins (Figure 5j) are observed, especially in dredge M8-DR08. The bulk of these rocks are undeformed, but some gabbros show plastic deformation of pyroxenes, and there are some ductile to brittle deformation bands in the greenschist facies (Figure 5b). ...
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... primary contacts between serpentinized peridotites and gabbros ( Figure 5e) or gabbroic veins (Figure 5j) are observed, especially in dredge M8-DR08. The bulk of these rocks are undeformed, but some gabbros show plastic deformation of pyroxenes, and there are some ductile to brittle deformation bands in the greenschist facies (Figure 5b). Samples with more than 90% chromite ( Figure 5i) were recovered in dredge M8-DR08, some- times in sharp contact with highly altered mantle rocks. ...
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... bulk of these rocks are undeformed, but some gabbros show plastic deformation of pyroxenes, and there are some ductile to brittle deformation bands in the greenschist facies (Figure 5b). Samples with more than 90% chromite ( Figure 5i) were recovered in dredge M8-DR08, some- times in sharp contact with highly altered mantle rocks. Dredge M8-DR01 recovered three granite and pla- giogranite samples (Figures 5g and 5h) displaying variable textures, from macrocrystalline magmatic textures (Figure 5g) to plastically deformed textures exhibiting quartz ribbons that result from high-T solid- ...
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... with more than 90% chromite ( Figure 5i) were recovered in dredge M8-DR08, some- times in sharp contact with highly altered mantle rocks. Dredge M8-DR01 recovered three granite and pla- giogranite samples (Figures 5g and 5h) displaying variable textures, from macrocrystalline magmatic textures (Figure 5g) to plastically deformed textures exhibiting quartz ribbons that result from high-T solid- ...
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... with more than 90% chromite ( Figure 5i) were recovered in dredge M8-DR08, some- times in sharp contact with highly altered mantle rocks. Dredge M8-DR01 recovered three granite and pla- giogranite samples (Figures 5g and 5h) displaying variable textures, from macrocrystalline magmatic textures (Figure 5g) to plastically deformed textures exhibiting quartz ribbons that result from high-T solid- ...
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... ET AL. state deformation (Figure 5h). Plutonic rocks are variably altered under greenschist-facies conditions, with sec- ondary assemblages dominated by actinolite and chlorite. ...
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... lithologies are well preserved from ret- rograde metamorphism, which mainly occurs as a slight albitization of plagioclase. Contact with serpentinized peridotite seems to locally enhance gabbro alteration marked by sericite formation after feldspars (Figure 5f). Gabbroic veins present a partial amphibolite-facies recrystallization of pyroxene to hornblende (e.g., Figure 5j, inset), overprinted by greenschist-facies minerals (mainly chlorite and actinolite). ...
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... with serpentinized peridotite seems to locally enhance gabbro alteration marked by sericite formation after feldspars (Figure 5f). Gabbroic veins present a partial amphibolite-facies recrystallization of pyroxene to hornblende (e.g., Figure 5j, inset), overprinted by greenschist-facies minerals (mainly chlorite and actinolite). Where present, olivine is the most altered mineral. ...
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... present, olivine is the most altered mineral. It is partially replaced by serpentine and magnetite (Figures 5d and 5f), which Figure 6. Examples of mantle rocks from the Rainbow massif: (a) serpentinized harzburgite, (b) serpentinized harzburgite collected at the active Rainbow field (Figure 3e) and displaying an orange-brown alteration crust, (c) serpentinized dunite crosscut by white carbonate veins, (d) highly serpentinized impregnated harzburgite with orange-brown alteration, (e) mylonitic peridotite with relicts of plastically deformed pyroxene (inset), altered to a serpentine and chlorite assemblage, highly enriched in magnetite, (f) foliated serpentinite displaying a first stage of plastic deformation, recorded by elongated pyroxene phenocrysts, and overprinted by a second deformation stage in the semibrittle field of serpentine-chlorite, (g) foliated serpentinite showing slickensided surfaces formed by localized syntectonic crystal growth of chlorite- serpentine, (h) foliated serpentinite deformed in the semibrittle field. ...
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... ET AL. contributes, together with feldspars, to the relatively higher alteration degree of troctolites. In chromitites, chromite is almost fresh (Figure 5i; Cr# (Cr/(Cr 1 Al)) atomic ratio 5 0.51; supporting information Table S3) while plagioclase is fully replaced by chlorite. Granitoids have undergone the least alteration. ...
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... have primarily recov- ered harzburgites (Figures 6a and 6b), and to a lesser extent dunites (Figures 6c and 6d). Some samples are crosscut by gabbroic veins (e.g., Figure 5j) or irregular gabbroic intrusions, which are altered to greenschist- facies assemblages (tremolite-talc-chlorite, e.g., Figure 6d). Mylonitic peridotites displaying evidence of an early phase of plastic deformation, recorded by pyroxene phenocrysts, have been sampled in dredges M8- DR06 and M8-DR15 (Figures 6e and 6f). ...
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... is the most altered lithology, with most of the samples displaying undeformed primary textures overprinted by static serpentinization textures (mesh and bastite textures; Figures 7a and 7b) and late vein- ing (e.g., chrysotile veins; Figure 7a). Serpentinites are commonly green to yellow-green (Figures 5e, 6a-6d, and 7g), with a few relicts of primary minerals (Figures 7a and 7b). Clinopyroxene is uncommon in the stud- ied sample, and is unaffected by hydration. ...
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... is in agreement with the chemical study of Rainbow vent fluids by Seyfried et al. [2011], who propose a concomitant alteration of oli- vine and plagioclase at depth, thought to be more efficient than olivine alteration alone [Andreani et al., 2013a]. This may be related to the observed enhanced alteration of gabbros when in contact with perito- tites (Figures 5e and 5f), in addition to the alteration of olivine-rich mafic units ( Figure 5). ...
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... is in agreement with the chemical study of Rainbow vent fluids by Seyfried et al. [2011], who propose a concomitant alteration of oli- vine and plagioclase at depth, thought to be more efficient than olivine alteration alone [Andreani et al., 2013a]. This may be related to the observed enhanced alteration of gabbros when in contact with perito- tites (Figures 5e and 5f), in addition to the alteration of olivine-rich mafic units ( Figure 5). ...
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We provide a detailed description of a late Carboniferous fold-and-thrust belt in the Lublin Basin based on seismic lines, including new, high-resolution acquisitions, and legacy borehole and gravimetric data. A series of regional cross-sections integrate results of the joint seismic-borehole-gravimetric interpretation. Cross-section restoration pr...
Citations
... The Rainbow massif is an ultramafic OCC located in a non-transform offset of the Mid-Atlantic Ridge 20 (Fig. 1a). Basement outcrops at the centre of the massif expose predominately serpentinites with sporadic gabbros, while basalts have been recovered only around its edges 21 (Fig. 1b). The massif hosts the Rainbow hydrothermal field (HF), a well-studied type II active system venting ~365 °C, H 2 -rich, CH 4 -rich, CO 2 -rich and Fe-rich fluids 22 at high heat and volume fluxes 23 that has been active for ~23 kyr (ref. ...
... Two fossil hydrothermal fields, ~110 kyr-old Ghost City and ~25.5 kyr-old Clamstone, located 1.2 km and 2.5 km from the Rainbow HF, respectively, show characteristics typical of type III systems, such as low-T, metal-poor and alkaline vent fluids 4,26,27 . Additional hydrothermal outflow zones are inferred from mineralized rock samples 21 (Fig. 1b). ...
... It is comparable in size to other seismically imaged and/or drilled OCC gabbroic cores (for example, refs. 14,31,32), which probably formed deeper into the lithosphere and were subsequently uplifted through detachment faulting 21,33 . ...
Deep-sea hydrothermal systems at slow/ultraslow-spreading mid-ocean ridges are often located within ultramafic rocks that are part of oceanic core complexes. These complexes contain lower-crustal and mantle sections exhumed due to detachment faulting. Hydrothermal circulation in these environments leads to massive sulfide deposits, hydration of oceanic lithosphere and conditions resembling early Earth’s life origin. However, the relationship between hydrothermal pathways in these environments and crustal and mantle lithologies, faulting, magmatism, serpentinization and alteration is poorly understood. Here we present seismic models of a Mid-Atlantic Ridge core complex and its ultramafic-hosted hydrothermal system derived from full waveform inversion of controlled-source seismic data and from local earthquake tomography. The models and derived rock properties reveal high-permeability channels within serpentinized peridotite along the flanks of the core complex. These channels converge beneath active and fossil hydrothermal fields and are diverted around mechanically strong, impermeable shallow mafic intrusions (2–3 km wide, ~1 km thick), causing hydrothermal outflow and the formation of massive sulfide deposits around the intrusions’ edges. These mafic intrusions also act as lids that limit fluid downflow—and thus serpentinization—in the centre of the core complex. Our results demonstrate that hydrothermal flow in ultramafic settings is controlled by lithology contacts, with mafic intrusions modulating hydrothermal pathways and extent of mantle serpentinization at depth.
... The influx of vent-derived fluids from high-T hydrothermal fields is further supported by variations in other fluid-mobile and redox-sensitive elements, such as J o u r n a l P r e -p r o o f As, Sb and U(Figs. 3D-F)suggesting a strong influence of redox conditions(Lafay et al., 2016) and high fluid/rock ratios during the hydrothermal activity(Andreani et al., 2014) recorded by sample SG 15-4. The 87 Sr/ 86 Sr ratios of the N. Apennine ophicarbonates characterized by negative δ 11 B range from 0.7065 to 0.7057, values from slightly to significantly lower than the Jurassic-Cretaceous seaweater(ca. ...
In submarine-hydrothermal systems, fluid-rock interactions play a pivotal role in fostering life on rocky planets. Carbonated-ultramafic rocks (i.e., ophicarbonates) represent an important witness of such environments and their study may provide new insights into hydrothermal processes. Here, we report in-situ trace element concentrations and the first in-situ boron isotope analyses (δ11B) of serpentines from the Ligurian N. Apennine (Italy) ophicarbonates, which represent non-subducted remnants of Jurassic ultramafic-hosted hydrothermal system. Based on micro-Raman analyses, lizardite is the dominant stable serpentine polymorph in the studied samples, thus constraining the temperature (T) of serpentinization below ca. 320 °C. By means of U-Pb LA-ICP-MS calcite geochronology, the age of the carbonation event is constrained at 137 ± 3 Ma (2σ, n = 52). The increasing inventory of several fluid-mobile elements (e.g., B, Li, As, Sb, Sr, U) associated with the REE budget variability in serpentines from ophicarbonates compared to those in pure serpentinites suggest that physico-chemical changes during the evolution of the hydrothermal system exert a relevant influence on the geochemistry of serpentines. Pure serpentinites, which are considered the protoliths of the ophicarbonates, show positive δ11B signatures from +15.8 to +35.0‰ (n = 24), thus falling in the range of present-day oceanic and forearc serpentinites and ophiolite-derived serpentinites. The δ11B imprint of serpentines in ophicarbonates clusters at +10.2 ± 2.2‰ (2SD, n = 57) and -5.7± 1.5‰ (2SD, n = 36). Remarkably, this is the first report of oceanic serpentines showing negative B isotope compositions. The positive δ11B data likely reflect inheritance from the precursor serpentinite, while the negative δ11B values are here related to the hydrothermal process(es). We attempt to model how the variation in pH and different δ11B composition of the hydrothermal vent fluids (+25 and +8‰) at different T (100 and 150 °C) can affect the δ11B imprint of serpentines. Our results indicate that either pH condition or δ11B of the hydrothermal vent fluids can shift the δ11B of serpentines from positive to negative values. Our new trace element and δ11B data are integrated and discussed with those from the literature to achieve new constraints in the variations and controls on the physical-chemical conditions of fluid-rock interaction during the progressive evolution of long-lived fossil ultramafic-hosted hydrothermal system. Finally, we discuss how subduction processes can potentially modify the B geochemistry of ophicarbonates and the information that we could achieve by studying B isotopes in such high-P lithologies to unravel the B- and C-cycles into the Earth’s mantle since the onset of modern plate tectonics.
... Only one dredge haul was conducted on the KH-18-2 cruise, and peridotites and serpentinized peridotites were collected ). These observations suggest that this structure is comparable to the NTO massifs including the Rainbow and other massifs along the Mid-Atlantic Ridge (MAR) (Gracia et al. 2000;Andreani et al. 2014;Paulatto et al. 2015;Eason et al. 2016;Dunn et al. 2017), where deep materials are exposed along a detachment. The geometry of the detachment fault is unknown, but the southern edge of the massif is likely the hanging wall cutoff, because the deeper axial basin is located south of the massif and the eastern corrugated seafloor described below appears to be continuous to the massif. ...
Detachment faulting is one of the main styles of seafloor spreading at slow to intermediate mid-ocean ridges. However, we have limited insight into its role in back-arc basin formation. We surveyed a remnant back-arc spreading center in the Philippine Sea and determined the detailed features and formation processes of the Mado Megamullion (Mado MM) oceanic core complex (OCC). This was undertaken in the context of back-arc evolution, based on the shipborne bathymetry, magnetics, and gravity with radiometric age dating of the rock samples collected. The Mado MM OCC has a typical OCC morphology with prominent corrugations on the domed surface and positive gravity anomalies, suggesting that there has been an exposure of the lower crust and mantle materials by a detachment fault. The downdip side of the detachment continues to the relict axial rift valley, which has indicated that the Mado MM OCC was formed at the end of the back-arc basin opening. The spreading rate of the basin decreased once when the spreading direction changed after six million years of stable trench perpendicular spreading. The rate then further decreased immediately prior to the end of the spreading when the Mado MM OCC was formed. The existence of other OCC-like structures in the neighboring segment and the previously reported OCCs along the Parece Vela Rift have indicated that the melt-poor, tectonic-dominant spreading is a widespread phenomenon at the terminal phase of back-arc spreading. The decrease in spreading rate in the later stage is consistent with the previous numerical modeling because of the decrease in trench retreat. In the Izu–Bonin–Mariana arc trench system, the rotation of the spreading axis and the resultant axis segmentation have enhanced the lithosphere cooling and constrained mantle upwelling, which caused the tectonic-dominant spreading at the final phase of the basin evolution.
... This asymmetric accretion mode is now thought to play a key role in the accretion of Atlantic-type slow-spread crust (Bialas et al., 2015;Cann et al., 1997;Escartin et al., 2008). Where tectonic processes dominate and faulting shapes the ridge segment structure, vent sites can be located far off-axis pointing to strong links between tectonic faulting and hydrothermal circulation (Petersen et al., 2009;Andreani et al., 2014;Tao et al., 2020). Fault-controlled hydrothermal systems also tend to have a longer lifetime and host the largest massive sulfide deposits (Jamieson et al., 2014;Hannington et al., 2011;German et al., 2016b), which is typically explained by stable preferential pathways that large offset faults provide for hydrothermal fluid flow. ...
... These vent sites are also located on the hanging wall of presumed detachment faults, and our proposed interplay may also be responsible for the presence of large sulfide accumulations at those sites. However, other detachment-associated vent fields like Rainbow (Andreani et al., 2014) or the von Damm vent field (Haughton et al., 2019) are located on exhumed footwall rocks. While those systems share similarities in terms of driving magmatism, how and if detachment faulting affects the circulation pathways of those vent fields cannot be directly predicted using our proposed flow model for TAG-like systems. ...
Submarine massive sulfide deposits on slow-spreading ridges are larger and longer-lived than deposits at fast-spreading ridges, likely due to more pronounced tectonic faulting creating stable preferential fluid pathways. The TAG hydrothermal mound at 26∘N on the Mid-Atlantic Ridge (MAR) is a typical example located on the hanging wall of a detachment fault. It has formed through distinct phases of high-temperature fluid discharge lasting 10s to 100s of years throughout at least the last 50,000 yrs and is one of the largest sulfide accumulations on the MAR. Yet, the mechanisms that control the episodic behavior, keep the fluid pathways intact, and sustain the observed high heat fluxes of possibly up to 1700 MW remain poorly understood. Previous concepts involved long-distance channelized high-temperature fluid upflow along the detachment but that circulation mode is thermodynamically unfavorable and incompatible with TAG's high discharge fluxes. Here, based on the joint interpretation of hydrothermal flow observations and 3-D flow modeling, we show that the TAG system can be explained by episodic magmatic intrusions into the footwall of a highly permeable detachment surface. These intrusions drive episodes of hydrothermal activity with vertical discharge and recharge along the detachment. The numerical simulations reveal that the high-temperature circulation system at TAG may be confined to a vertical zone of enhanced permeability that channelizes upflow and a recharge system that is hosted by the detachment surface with a high permeability of 2E-13 to 1E-12 m2. This revised flow regime reconciles problematic aspects of previously inferred circulation patterns and allows to identify the prerequisites for generating substantive seafloor mineral systems.
... The rate of accretion of the oceanic crust varies dramatically, especially along ultraslow-slow spreading ridges, both in space and time, due to strongly fluctuating magma supply (Bown & White, 1994;Liu et al., 2022;Morgan & Chen, 1993). Studies from geophysical observations and numerical models indicated that the depth of detachment faults could reach 10-30 km, enhancing the hydrothermal circulation and alteration at spreading centers (e.g., Andreani et al., 2014;Bach et al., 2004;Beard et al., 2009;Bickert et al., 2020;Boschi et al., 2006Boschi et al., , 2013Klein et al., 2009;Maffione et al., 2014;Mével, 2003;Plümper et al., 2012Plümper et al., , 2014Schroeder et al., 2002). At the root of detachment faults, especially at amagmatic sections, strain and stress concentrations lead to dynamic recrystallization, forming grain size reduction zones (Bickert et al., 2021). ...
Although positive buoyancy of young lithosphere near spreading centers does not favor spontaneous subduction, subduction initiation occurs easily near ridges due to their intrinsic rheological weakness when plate motion reverses from extension to compression. It has also been repeatedly proposed that inherited detachment faults may directly control the nucleation of new subduction zones near ridges subjected to forced compression. However, recent 3D numerical experiments suggested that direct inversion of a single detachment fault does not occur. Here we further investigate this controversy numerically by focusing on the influence of brittle‐ductile damage on the dynamics of near‐ridge subduction initiation. We self‐consistently model the inversion of tectonic patterns formed during oceanic spreading using 3D high‐resolution thermomechanical numerical models with strain weakening of faults and grain size evolution. Numerical results show that forced compression predominantly reactivates and rotates inherited extensional faults, shortening and thickening the weakest near‐ridge region of the oceanic lithosphere, thereby producing ridge swellings. As a result, a new megathrust zone is developed, which accommodates further shortening and subduction initiation. Furthermore, brittle/plastic strain weakening has a key impact on the collapse of the thickened ridge and the onset of near‐ridge subduction initiation. In contrast, grain size evolution of the mantle only slightly enhances the localization of shear zones at the brittle‐ductile transition and thus plays a subordinate role. Compared to the geological record, our numerical results provide new helpful insights into possible physical controls and dynamics of natural near‐ridge subduction initiation processes recorded by the Mirdita ophiolite of Albania.
... Further, the high-resolution near seabed data (acoustic, CTD, and photography) provided great insights into the tectonic and magmatic process at the dynamic divergent plate boundaries that are the most potential zones for hosting hydrothermal mineral deposits and home for exotic biological communities (Haase et al. 2007(Haase et al. , 2009). Exploration of OCC over the MAR found that these features host active hydrothermal vents and show extinct hydrothermal deposits or both (e.g., Logatchev, TAG, Semenov, Lost City, Rainbow, Ashadze, and Von Damm) (Andreani et al. 2014;Dyment et al. 2009;Früh-Green et al. 2003;German et al. 2010;McCaig et al. 2007;Ondréas et al. 2012;Pertsev et al. 2012;Petersen et al. 2009;Tivey et al. 2003). The recent microbathymetry data over the OCCs enhanced our knowledge about the processes related to detachment faults and their influence on hydrothermal activity (Escartin et al. 2017). ...
The topographic fabric of the rift valley floor has been analyzed using the multibeam echosounder data obtained by the autonomous underwater vehicle (AUV) Abyss at two locations over a short segment of the slow-spreading Central Indian Ridge between 10°18′ and 10°57′S. The region is influenced by hydrothermal venting in the near vicinity. Two AUV dives D51 and D52 were performed over this segment at two locations that are 30 km apart and covered 5 km2 and 8 km2 seafloor area, respectively. The dive D51 covered the off-axis part of the rift valley floor in the middle part of the segment, and the dive D52 is located near to the non-transform discontinuity that covered the terminal part of an oceanic core complex (OCC). High-resolution seafloor topography as revealed by the AUV-mounted multibeam echosounder system brought out several micro-bathymetric fabric features such as a lava lake, a cratered volcano, an OCC, and the foot wall volcanic complex at the distal part of the OCC. The valley floor imaged in the D51 is marked by a lava flow encompassing an area of 1 km2 and a volcano in the NE corner. The volcano has a diameter of about 800 m with an elevation of about 200 m from the adjacent seafloor, and the partially mapped volcano crater has a relief of about 60 m. A prominent linear fissure running parallel to the ridge axis has been identified; this feature joins with the volcano. Analysis of AUV-mounted CTD data indicated three distinct temperature spikes ranging 0.009 to 0.013 °C in the region of dive D51. The observed temperature spikes appear to be related to the linear fissure on the seafloor and probably represent leaky venting of fluids from the fissure. With respect to the dive D52, the foot wall volcanic features associated with the OCC are prominent. The volcanic seafloor feature covered an area of 3.45 km2 and is conspicuous with rugged topographic fabric at the base of the OCC. These inferences and the morphotectonics of the rift valley floor as revealed by the AUV data suggest moderate hydrothermal venting in this segment of the slow-spreading Central Indian Ridge.
... Low angle detachments are described from oceanic ridges and the inside corners of ridge-transform intersections in slow and intermediate rate spreading regimes (Blackman et al., 2008;Reston and Ranero, 2011;Smith et al., 2014;Sandiford et al., 2021). They exhume a footwall massif of peridotites, gabbros, and serpentinized lower crust rocks (Andreani et al., 2014;Pressling et al., 2012) and detach onto the top of exhumed crust. Their dip typically increases until subvertical at depth and is oriented towards the spreading axis and not the OTF unlike the TNDR-B examples (a 90° strike difference). ...
Oceanic Transform faults are one of manifestations of the three major plate boundaries and a key tectonic feature of oceanic crust. They are broadly considered to accommodate strike-slip displacement along simple vertical faults and to be largely without magmatic addition. We present the first observations from broadband 3D seismic of buried, Cretaceous-aged transform faults in the Gulf of Guinea with complex internal architectures including crustal scale detachments and rotated packages of volcanics within oceanic crust. In the study area, several Oceanic Fracture Zones (OFZ) are described from Top Crust to Moho. OFZ scarps are observed to connect at depth with zones of low angle reflectivity which dip into the OFZ and perpendicular to the spreading orientation. At depth they detach onto the Moho, necking the adjacent crust in the manner of extensional shear zones. Thickly stacked and tilted reflectors, interpreted as extrusive lava flows, are common above the shear zones and infill up to 75% of the crustal thickness. The entire OFZ stratigraphy is overlain and sealed by late-stage lavas that are continuous from the abyssal hills of the trailing spreading ridge. These insights demonstrate complexity previously only predicted in numerical simulations. We propose a model with extension at a high angle to the spreading orientation along a low angle shear zone that acts as a conduit for decompression related melt and volcanism. We conclude that oceanic transforms are non-conservative and not simple strike slip fault zones, contradicting the conventional view.
... The Rainbow field is located at the ultramafic footwall of a detachment fault with masses of seawater penetrating into the host rocks. The hydrothermal circulation at this site was proposed to be driven by an intrusive magmatic unit (Andreani et al., 2014). The Nibelungen field is located on a fault scarp east of the ridge axis. ...
The Daxi Vent Field (DVF) is located on a neovolcanic ridge within a non-transform offset at water depths of ~3500 m, on the Carlsberg Ridge, northwest Indian Ocean. In 2017, we investigated this site using the submersible Jiaolong and collected two fluid samples from orifices of chimneys named “Buddha's Hands” and “A1”, about 37 m apart. Their in-situ measured temperatures are 273 °C and 272 °C, respectively, whereas the Buddha's Hands fluid is highly Cl-enriched (928 mM), while the A1 fluid is Cl-depleted (303 mM). This indicates that they have undergone phase separation. The segregated phases must have remixed during the ascent because the vapor and brine phases sampled cannot be produced by the same phase separation history without other processes. Olivine-rich and/or ultramafic mantle rocks must have been involved during the hydrothermal circulation as evidenced by high dissolved H2 (7.07 mM) and methane (0.884 mM) concentrations, a depletion of B relative to seawater, high Ca and low K, and large positive Eu anomalies. The Fe content in Buddha's Hands fluid is extremely high (11,900 μM) as a result of phase separation, while the Cu concentrations in both fluids are relatively low due to entrainment of seawater which results in precipitation of Cu-rich sulfides in the subseafloor. The concentrations of Zn, Ag, Ga, Sn, Sb, and Cd in A1 vent fluid are significantly elevated due to generation of acidity and remobilization of these elements as Cu-rich sulfides are deposited. The subseafloor processes and associated geochemistry of hydrothermal fluids at Daxi are distinct from other mid-ocean ridge hydrothermal systems due to the specific geologic setting. Hence a hybrid model of hydrothermal circulation is proposed. This study broadens our understanding of the hydrothermal processes occurring in areas of NTO setting and provides more information on mass fluxes discharging from hydrothermal systems and the formation of sulfide deposits.
... A number of hydrothermal systems along the slow-spreading MAR are hosted in ultramafic and gabbroic rocks. These include the Rainbow (36° N), Logatchev (14° 45′ N) and Ashadze vent fields (12° 58′ N) 138,139 ...
Hydrothermal circulation and alteration at mid-ocean ridges and ridge flanks have a key role in regulating seawater chemistry and global chemical fluxes, and support diverse ecosystems in the absence of light. In this Review, we outline tectonic, magmatic and hydrothermal processes that govern crustal architecture, alteration and biogeochemical cycles along mid-ocean ridges with different spreading rates. In general, hydrothermal systems vary from those that are magmatic-dominated with low-pH fluids >300 °C to serpentinizing systems with alkaline fluids <120 °C. Typically, slow-spreading ridges (rates <40 mm yr−1) have greater variability in magmatism, lithology and vent chemistry, which are influenced by detachment faults that expose lower-crustal and serpentinized mantle rocks. Hydrothermal alteration is an important sink for magnesium, sodium, sulfate and bicarbonate, and a net source of volatiles, iron and other nutrients to the deep ocean and vent ecosystems. Magmatic hydrothermal systems sustain a vast, hot and diverse microbial biosphere that represents a deep organic carbon source to ocean carbon budgets. In contrast, high-pH serpentinizing hydrothermal systems harbour a more limited microbial community consisting primarily of methane-metabolizing archaea. Continued advances in monitoring and analytical capabilities coupled with developments in metagenomic technologies will guide future investigations and discoveries in hydrothermal systems. Oceanic spreading centres are sites of extensive tectonic, magmatic and hydrothermal activity that provide nutrients to the ocean and multifaceted habitats for life. This Review explores processes governing variations in hydrothermal vents, microbial ecosystems and global fluxes from ocean ridges. Spreading rates control variations in heat sources, magma input and tectonic processes along mid-ocean ridges and influence crustal architecture and hydrothermal processes, providing multifaceted habitats for life.Approximately one-third (7 × 1012 to 11 × 1012 W) of the global oceanic heat flux (32 × 1012 W) occurs through hydrothermal convection at ridges and ridge flanks. Seawater circulation, hydrothermal alteration and microbial interactions regulate seawater chemistry and change the composition and physical properties of the lithosphere.Roughly 50% of the global mid-ocean ridges are spreading at rates <40 mm yr−1, where major detachment faults expose lower-crustal and upper-mantle rocks, creating asymmetric ridge segments with large variability in structure, hydrothermal processes and vent fluid chemistry.Serpentinization decreases bulk density (<2.9 g cm−3) and seismic velocities (Vp < 6 km s−1) of mantle rocks and weakens the oceanic lithosphere along detachment faults. Serpentinization reactions produce highly reduced fluids with high H2, CH4 and other organic molecules that provide energy for microbial life.Hydrothermal processes govern global chemical fluxes (such as Mg, Fe, Mn and volatiles) and provide nutrients (for example, Fe flux ~4–6 × 109 mol yr−1) to the deep ocean. Approximately 0.05 GtC yr−1 of organic carbon is estimated to be produced through microbial interactions and oxidation of organic compounds within hydrothermal plumes.Basalt-hosted systems support a vast, hot and diverse microbial biosphere, in contrast to serpentinizing systems, which sustain more limited microbial communities primarily dominated by methane-metabolizing archaea. Advanced technologies allow better characterization of the genetic makeup and metabolism of microbes and the role of viruses in shaping diversity. Spreading rates control variations in heat sources, magma input and tectonic processes along mid-ocean ridges and influence crustal architecture and hydrothermal processes, providing multifaceted habitats for life. Approximately one-third (7 × 1012 to 11 × 1012 W) of the global oceanic heat flux (32 × 1012 W) occurs through hydrothermal convection at ridges and ridge flanks. Seawater circulation, hydrothermal alteration and microbial interactions regulate seawater chemistry and change the composition and physical properties of the lithosphere. Roughly 50% of the global mid-ocean ridges are spreading at rates <40 mm yr−1, where major detachment faults expose lower-crustal and upper-mantle rocks, creating asymmetric ridge segments with large variability in structure, hydrothermal processes and vent fluid chemistry. Serpentinization decreases bulk density (<2.9 g cm−3) and seismic velocities (Vp < 6 km s−1) of mantle rocks and weakens the oceanic lithosphere along detachment faults. Serpentinization reactions produce highly reduced fluids with high H2, CH4 and other organic molecules that provide energy for microbial life. Hydrothermal processes govern global chemical fluxes (such as Mg, Fe, Mn and volatiles) and provide nutrients (for example, Fe flux ~4–6 × 109 mol yr−1) to the deep ocean. Approximately 0.05 GtC yr−1 of organic carbon is estimated to be produced through microbial interactions and oxidation of organic compounds within hydrothermal plumes. Basalt-hosted systems support a vast, hot and diverse microbial biosphere, in contrast to serpentinizing systems, which sustain more limited microbial communities primarily dominated by methane-metabolizing archaea. Advanced technologies allow better characterization of the genetic makeup and metabolism of microbes and the role of viruses in shaping diversity.
... Previous work suggested that serpentinization and carbonation of ophiolites, especially peridotites, occurs at the seafloor during and/or after exhumation (Andreani et al., 2014;Noël, 2019;Lafay et al., 2017). Here, we use petrological and isotopic data to constrain the timing of the low-temperature and high-temperature calcites, in order to construct a cooling and/or heating history for the serpentinized peridotites. ...
The ophiolite of Sivas (Turkey) was studied in order to define the chronology of different alteration events related to a series of serpentinization and carbonation episodes. Six samples were investigated, representative of different types of ophicalcite (partially carbonated serpentinite). X-ray diffraction (XRD) and Mössbauer spectroscopy were used to determine the bulk mineralogy and the bulk Fe3+/Fetot ratio, respectively. Electron microprobe and secondary ion mass spectrometer (SIMS) analyses were also conducted to identify the chemical composition of different mineral phases in addition to the carbon and oxygen isotopic compositions of calcite. An initial, i.e. pre-obduction, phase of olivine and pyroxene serpentinization was followed by a brecciation event associated with precipitation of massive serpentine. This first alteration event occurred during exhumation of the peridotites to the ocean seafloor, followed by a carbonation event at temperatures in the range 35‒100°C. A low-temperature (~35°C) carbonation event occurred between 90 and 65 Ma. Finally, a reheating of the system likely occurred after the obduction at 55‒40 Ma, resulting in a carbonation episode followed by late serpentinization. Our study presents the first direct evidence of serpentinization after obduction. In that geological context, the hydrogen produced during the interpreted multiphase serpentinization may have been trapped by the salt deposits overlying the ophiolite but subsurface data will be necessary to define potential traps and reservoirs; further studies are also needed to determine whether the serpentinization process is still ongoing.
