Slow-spreading Mid-ocean Ridges Research Articles

Widespread diffuse venting and large microbial iron-mounds in the Red Sea

For decades, hydrothermal activity along the Red Sea Rift was only inferred from metalliferous sediments and hot brines. Active hydrothermal fluid discharge was never directly observed from this young ocean basin, but could be key to understanding the evolution of hydrothermal vent fields and associated life. Here we report the discovery of widespread diffuse venting at Hatiba Mons, the largest axial volcano in the Red Sea. The active vent fields are composed of iron-oxyhydroxide mounds, host thriving microbial communities and are larger and more abundant than those known from any other (ultra) slow-spreading mid-ocean ridge. Diffuse venting, controlled by intense faulting, and the lack of vent-specific macrofauna, are likely causes for the abundant microbial mats that dominate and built up the hydrothermal mounds. These microbe-rich hydrothermal vent fields, occurring in a warm ocean, may be analogous to Precambrian environments hosting early life and supporting the formation of large iron deposits.

Metallogenetic process of Xunmei hydrothermal field (26°S), South Mid-Atlantic Ridge: Constraints from in-situ sulfur isotope and trace elements of sulfides

Seafloor hydrothermal sulfides at slow-spreading mid-ocean ridges (MORs) are of economic and scientific significance. The formation of newly discovered submarine hydrothermal sulfides on the South Mid-Atlantic Ridge (SMAR), particularly with respect to the fluid evolution, remains poorly understood. The Xunmei hydrothermal field is a typical volcanic dome-type hydrothermal field at 26°S on the SMAR. Based on the mineralogical zonation and assemblages, six distinct ore-forming stages were identified from low- to high-temperature stages with a seafloor weathering stage. The crystallinity of pyrite/marcasite and grain size of chalcopyrite increase gradually from the outer rim to the inner conduit of a sulfide chimney. The positive δ34SV-CDT values of pyrite/marcasite (2.1‰–8.2‰), chalcopyrite (2.5‰–5.6‰), and sphalerite (2.9‰–7.0‰) suggest 62%–91% of S was derived from the leaching of basement basalts, 9%–38% from reduction of seawater sulfate. In situ trace element data for sulfides show enrichments in Mn, Ag, Tl, and Pb in outer zones A and B, while enrichments in Se, In, and Sn in inner zones C and D, indicating an increase in precipitation temperature during chimney growth. Low pyrite Co contents, low Tl/Pb, low Sb/Pb, low Bi/Pb, high As, Ag, Cu, and Pb concentrations, suggesting that the pyrites precipitated from fluids that may have undergone supercritical phase separation during their ascent. High-temperature, Cl-depleted, vapor-rich hydrothermal fluids discharged into ambient cold seawater over multiple stages and probably evolved from low (<240 °C) to medium (∼ 263 °C) and high (∼ 317 °C) temperatures, reached a highest temperature of ∼335 °C, and then evolved to medium to low temperatures (∼ 270 °C to <240 °C) during the wanning of hydrothermal venting. The sulfur fugacity (fS2) likely evolved from a relatively low to high condition within the scope of intermediate sulfidation, and then decreased to low condition. The redox states (fO2) probably were strongly affected by seawater influx and evolved from relatively oxidized to original reduced conditions. The salinity decreased during chimney growth. Sulfide precipitation in the Xunmei hydrothermal field probably was the result of a combination of fluid-seawater mixing and phase separation. We propose a chimney growth and hydrothermal evolution models for the Xunmei hydrothermal field.

How transform fault shear influences where detachment faults form near mid-ocean ridges

Oceanic detachment faults represent an end-member form of seafloor creation, associated with relatively weak magmatism at slow-spreading mid-ocean ridges. We use 3-D numerical models to investigate the underlying mechanisms for why detachment faults predominantly form on the transform side (inside corner) of a ridge-transform intersection as opposed to the fracture zone side (outside corner). One hypothesis for this behavior is that the slipping, and hence weaker, transform fault allows for the detachment fault to form on the inside corner, and a stronger fracture zone prevents the detachment fault from forming on the outside corner. However, the results of our numerical models, which simulate different frictional strengths in the transform and fracture zone, do not support the first hypothesis. Instead, the model results, combined with evidence from rock physics experiments, suggest that shear-stress on transform fault generates excess lithospheric tension that promotes detachment faulting on the inside corner.

Tracing differences in iron supply to the Mid-Atlantic Ridge valley between hydrothermal vent sites: implications for the addition of iron to the deep ocean

Abstract. Supply of iron (Fe) to the surface ocean supports primary productivity, and while hydrothermal input of Fe to the deep ocean is known to be extensive it remains poorly constrained. Global estimates of hydrothermal Fe supply rely on using dissolved Fe (dFe) to excess He (xs3He) ratios to upscale fluxes, but observational constraints on dFe/xs3He may be sensitive to assumptions linked to sampling and interpolation. We examined the variability in dFe/xs3He using two methods of estimation, for four vent sites with different geochemistry along the Mid-Atlantic Ridge. At both Rainbow and TAG, the plume was sampled repeatedly and the range of dFe/xs3He was 4 to 63 and 4 to 87 nmol:fmol, respectively, primarily due to differences in plume age. To account for background xs3He and shifting plume position, we calibrated He values using contemporaneous dissolved Mn (dMn). Applying this approach more widely, we found dFe/xs3He ratios of 12, 4–8, 4–44, and 4–86 nmol fmol−1 for the Menez Gwen, Lucky Strike, Rainbow, and TAG hydrothermal vent sites, respectively. Differences in plume dFe/xs3He across sites were not simply related to the vent endmember Fe and He fluxes. Within 40 km of the vents, the dFe/xs3He ratios decreased to 3–38 nmol fmol−1, due to the precipitation and subsequent settling of particulates. The ratio of colloidal Fe to dFe was consistently higher (0.67–0.97) than the deep N. Atlantic (0.5) throughout both the TAG and Rainbow plumes, indicative of Fe exchange between dissolved and particulate phases. Our comparison of TAG and Rainbow shows there is a limit to the amount of hydrothermal Fe released from vents that can form colloids in the rising plume. Higher particle loading will enhance the longevity of the Rainbow hydrothermal plume within the deep ocean assuming particles undergo continual dissolution/disaggregation. Future studies examining the length of plume pathways required to escape the ridge valley will be important in determining Fe supply from slow spreading mid-ocean ridges to the deep ocean, along with the frequency of ultramafic sites such as Rainbow. Resolving the ridge valley bathymetry and accounting for variability in vent sources in global biogeochemical models will be key to further constraining the hydrothermal Fe flux.

Weathering-driven porosity generation in altered oceanic peridotites

Ultramafic rocks exposed at slow and ultra-slow spreading mid-ocean ridges represent a significant and extremely reactive portion of the oceanic lithosphere. Thus, mechanistic understanding of the processes by which seawater infiltrates into and reacts with these rocks is essential for constraining their contribution to the chemistry of the oceans and the coupled carbonate-silicate cycle. Recent observations indicate that nanoscale processes contribute to seawater-driven alteration of ultramafic rocks, but conventional petrographic and tomographic observations of the associated physical features are challenging to link to these nanoscale features. Moreover, multiple generations and varying conditions of fluid infiltration often obscure the relative roles of higher-temperature serpentinization, where reactions are mostly isochemical, and lower-temperature weathering reactions, where observations suggest the release of massive amounts of magnesium. Here we bridge these scales and investigate the specific role of weathering processes in dissolution-driven porosity generation by integrating focused ion beam scanning electron microscopy nanotomography and micro-computed X-ray tomography imaging of the pore structures preserved in drill cores of serpentinized oceanic peridotites. Relict olivine crystals in all imaged samples contain abundant etch pits, and those in the higher-resolution FIB-SEM imagery show the presence of channel-like dissolution structures. The pore channels preferentially affect olivine along grain boundaries and show anisotropic distribution likely controlled by crystallographic features. The pores formed via olivine dissolution are interpreted to result from dissolution of serpentinized peridotite at conditions where serpentine and carbonate precipitation are kinetically inhibited, i.e., at weathering conditions. Importantly, the calculated connectivity of the imaged pore structures increases as the scale of investigation increases, suggesting that weathering-driven olivine dissolution facilitates further seawater infiltration and olivine dissolution, a positive feedback that can sustain continued magnesium extraction until the rocks are ultimately cut off from seawater circulation via sedimentation. Thus, while much attention has been directed towards constraining geochemical fluxes from the higher-temperature alteration of ultramafic rocks, our results support literature studies suggesting that mineral dissolution, and hence elemental fluxes, are significant at the lower temperatures of seafloor weathering. Our data thus provide mechanistic evidence of the physical process contributing to the observed elemental loss from weathered oceanic peridotites.

Serpentinization-Driven H2 Production From Continental Break-Up to Mid-Ocean Ridge Spreading: Unexpected High Rates at the West Iberia Margin

Molecular hydrogen (H2) released during serpentinization of mantle rocks is one of the main fuels for chemosynthetic life. Processes of H2 production at slow-spreading mid-ocean ridges (MORs) have received much attention in the past. Less well understood is serpentinization at passive continental margins where different rock types are involved (lherzolite instead of harzburgite/dunite at MORs) and the alteration temperatures tend to be lower (&amp;lt;200°C vs. &amp;gt;200°C). To help closing this knowledge gap we investigated drill core samples from the West Iberia margin. Lherzolitic compositions and spinel geochemistry indicate that the exhumed peridotites resemble sub-continental lithospheric mantle. The rocks are strongly serpentinized, mainly consist of serpentine with little magnetite, and are generally brucite-free. Serpentine can be uncommonly Fe-rich, with XMg = Mg/(Mg + Fe) &amp;lt; 0.8, and shows distinct compositional trends toward a cronstedtite endmember. Bulk rock and silicate fraction Fe(III)/∑Fe ratios are 0.6–0.92 and 0.58–0.8, respectively; our data show that 2/3 of the ferric Fe is accounted for by Fe(III)-serpentine. Mass balance and thermodynamic calculations suggest that the sample’s initial serpentinization produced ∼120 to &amp;gt;300 mmol H2 per kg rock. The cold, late-stage weathering of the serpentinites at the seafloor caused additional H2 formation. These results suggest that the H2 generation potential evolves during the transition from continental break-up to ultraslow and, eventually, slow MOR spreading. Metamorphic phase assemblages systematically vary between these settings, which has consequences for H2 yields during serpentinization. At magma-poor rifted margins and ultraslow-spreading MORs, serpentine hosts most Fe(III). Hydrogen yields of 120 to &amp;gt;300 mmol and 50–150 mmol H2 per kg rock, respectively, may be expected at temperatures of &amp;lt;200°C. At slow-spreading MORs, in contrast, serpentinization may produce 200–350 mmol H2, most of which is related to magnetite formation at &amp;gt;200°C. Since, in comparison to slow-spreading MORs, geothermal gradients at magma-poor margins and ultraslow-spreading MORs are lower, larger volumes of low-temperature serpentinite should form in these settings. Serpentinization of lherzolitic rocks at magma-poor margins should produce particularly high amounts of H2 under conditions within the habitable zone. Magma-poor margins may hence be more relevant environments for hydrogenotrophic microbial life than previously thought.

13 million years of seafloor spreading throughout the Red Sea Basin

The crustal and tectonic structure of the Red Sea and especially the maximum northward extent of the (ultra)slow Red Sea spreading centre has been debated—mainly due to a lack of detailed data. Here, we use a compilation of earthquake and vertical gravity gradient data together with high-resolution bathymetry to show that ocean spreading is occurring throughout the entire basin and is similar in style to that at other (ultra)slow spreading mid-ocean ridges globally, with only one first-order offset along the axis. Off-axis traces of axial volcanic highs, typical features of (ultra)slow-spreading ridges, are clearly visible in gravity data although buried under thick salt and sediments. This allows us to define a minimum off-axis extent of oceanic crust of <55 km off the coast along the complete basin. Hence, the Red Sea is a mature ocean basin in which spreading began along its entire length 13 Ma ago.

Pre-Alpine Tectono-Stratigraphic Reconstruction of the Jurassic Tethys in the High-Pressure Internal Piedmont Zone (Stura di Viù Valley, Western Alps)

We present a detailed description of the tectono-stratigraphic architecture of the eclogite-facies Internal Piedmont Zone (IPZ) metaophiolite, exposed in the Lanzo Valleys (Western Alps), which represents the remnant of the Jurassic Alpine Tethys. Seafloor spreading and mantle exhumation processes related to the Alpine Tethys evolution strongly conditioned the intra-oceanic depositional setting, which resulted in an articulated physiography and a heterogeneous stratigraphic succession above the exhumed serpentinized mantle. “Complete” and “reduced” successions were recognized, reflecting deposition in morphological or structural lows and highs, respectively. The “complete” succession consists of quartzite, followed by marble and calcschist. The “reduced” succession differs for the unconformable contact of the calcschist directly above mantle rocks, lacking quartzite and gray marble. The serpentinite at the base of this succession is intruded by metagabbro and characterized at its top by ophicalcite horizons. Mafic metabreccia grading to metasandstone mark the transition between the “complete” and “reduced” successions. The character of the reconstructed succession and basin floor physiography of the IPZ metaophiolite is well comparable with the Middle Jurassic–Late Cretaceous succession of both the Queyras Complex (External Piedmont Zone) and the Internal Ligurian Units (Northern Apennines) and with modern slow-spreading mid-ocean ridges.

Assessing the impact of sedimentation on fault spacing at the Andaman Sea spreading center

AbstractThe stabilizing effect of surface processes on strain localization, albeit predicted by several decades of geodynamic modeling, remains difficult to document in real tectonic settings. Here we assess whether intense sedimentation can explain the longevity of the normal faults bounding the Andaman Sea spreading center (ASSC). The structure of the ASSC is analogous to a slow-spreading mid-ocean ridge (MOR), with symmetric, evenly spaced axis-facing faults. The average spacing of faults with throws ≥100 m (8.8 km) is however large compared to unsedimented MORs of commensurate spreading rate, suggesting that sedimentation helps focus tectonic strain onto a smaller number of longer-lived faults. We test this idea by simulating a MOR with a specified fraction of magmatic plate separation (M), subjected to a sedimentation rate (s) ranging from 0 to 1 mm/yr. We find that for a given M ≥ 0.7, increasing s increases fault lifespan by ∼50%, and the effect plateaus for s &amp;gt; 0.5 mm/yr. Sedimentation prolongs slip on active faults by leveling seafloor relief and raising the threshold for breaking new faults. The effect is more pronounced for faults with a slower throw rate, which is favored by a greater M. These results suggest that sedimentation-enhanced fault lifespan is a viable explanation for the large spacing of ASSC faults if magmatic input is sufficiently robust. By contrast, longer-lived faults that form under low M are not strongly influenced by sedimentation.

Early-Stage Melt-Rock Reaction in a Cooling Crystal Mush Beneath a Slow-Spreading Mid-Ocean Ridge (IODP Hole U1473A, Atlantis Bank, Southwest Indian Ridge)

Microtextural and chemical evidence from gabbros indicates that melts may react with the crystal framework as they migrate through crystal mushes beneath mid-ocean ridges; however, the importance of this process for the compositional evolution of minerals and melts remains a matter of debate. Here we provide new insights into the extent by which melt-rock reaction process can occur in oceanic gabbros by conducting a detailed study of cryptic reactive melt migration as preserved in an apparently unremarkable, homogeneous olivine gabbro from deep within a section of the plutonic footwall of the Atlantis Bank core complex on the Southwest Indian Ridge (International ocean discovery program Hole U1473A). High-resolution chemical maps reveal that mineral zoning increases toward and becomes extreme within a cm-wide band that is characterized by elevated incompatible trace element concentrations and generates extreme more/less incompatible element ratios. We demonstrate that neither crystallization of trapped melt nor diffusion can account for these observations. Instead, taking the novel approach of correcting mineral-melt partition coefficients for both temperature and composition, we show that these chemical variations can be generated by intergranular reactive porous flow of a melt as it migrated through the mush framework, and whose composition evolved by melt-rock reaction as it progressively localized into a cm-scale reactive channel. We propose that the case reported here may represent, in microcosm, a preserved snapshot of a generic mechanism by which melt can percolate through primitive mafic (olivine gabbro) crystal mushes, and be modified toward more evolved compositions via near-pervasive reactive transport.

Gold and tin mineralisation in the ultramafic-hosted Cheoeum vent field, Central Indian Ridge

The Cheoeum vent field (CVF) is the first example of an inactive ultramafic-hosted seafloor massive sulphide (SMS) deposit identified in the middle part of the Central Indian Ridge. Here, we report on the detailed mineralogy and geochemistry of ultramafic-hosted sulphide sample atop a chimney, together with a few small fragments. Hydrothermal chimneys are characterised by high concentrations of Au (up to 17.8 ppm) and Sn (up to 1720 ppm). The sulphide mineralisation in the CVF shows (1) early precipitation of anhedral sphalerite and pyrite–marcasite aggregates under relatively low-temperature (< 250 °C) fluid conditions; (2) intensive deposition of subhedral pyrrhotite, isocubanite, chalcopyrite, Fe-rich sphalerite (Sp-III), and electrum from high-temperature (250–365 °C) and reduced fluids in the main mineralisation stage; and (3) a seawater alteration stage distinguished by the mineral assemblage of marcasite pseudomorphs, altered isocubanite phase, covellite, amorphous silica, and Fe-oxyhydroxides. Electrum (< 2 μm in size) is the principal form of Au mineralisation and is mainly associated with the main mineralisation stage. The consistently high fineness of electrum (801 to 909‰) is indicative of the selective saturation of Au over Ag in the fluid during high-temperature mineralisation, which differs from the Au mineralisation associated with typical basaltic-hosted hydrothermal systems on mid-ocean ridges. Tin is mainly substituted in structures of sphalerite, isocubanite, and chalcopyrite as a solid solution, and not as mineral inclusions. The continuously ascending hydrothermal fluids enable the early formed Sn-bearing sulphide to be dissolved and reprecipitated, producing significantly Sn-enriched replacement boundaries between isocubanite and Sp-III. This study suggests that Au–Sn mineralisation could be facilitated by the low redox potential of ultramafic-hosted hydrothermal systems such as in the CVF, which may be a common occurrence along slow-spreading mid-ocean ridges.

Transformation and Upwelling of Bottom Water in Fracture Zone Valleys

AbstractClosing the overturning circulation of bottom water requires abyssal transformation to lighter densities and upwelling. Where and how buoyancy is gained and water is transported upward remain topics of debate, not least because the available observations generally show downward-increasing turbulence levels in the abyss, apparently implying mean vertical turbulent buoyancy-flux divergence (densification). Here, we synthesize available observations indicating that bottom water is made less dense and upwelled in fracture zone valleys on the flanks of slow-spreading midocean ridges, which cover more than one-half of the seafloor area in some regions. The fracture zones are filled almost completely with water flowing up-valley and gaining buoyancy. Locally, valley water is transformed to lighter densities both in thin boundary layers that are in contact with the seafloor, where the buoyancy flux must vanish to match the no-flux boundary condition, and in thicker layers associated with downward-decreasing turbulence levels below interior maxima associated with hydraulic overflows and critical-layer interactions. Integrated across the valley, the turbulent buoyancy fluxes show maxima near the sidewall crests, consistent with net convergence below, with little sensitivity of this pattern to the vertical structure of the turbulence profiles, which implies that buoyancy flux convergence in the layers with downward-decreasing turbulence levels dominates over the divergence elsewhere, accounting for the net transformation to lighter densities in fracture zone valleys. We conclude that fracture zone topography likely exerts a controlling influence on the transformation and upwelling of bottom water in many areas of the global ocean.

How do detachment faults form at ultraslow mid-ocean ridges in a thick axial lithosphere?

Slow-spreading mid-ocean ridges develop large offset normal faults, or detachments, that exhume mantle-derived peridotites to the seafloor. Successive detachments that flip polarity (flip-flop detachments) have been documented in corridors of nearly amagmatic spreading at the eastern Southwest Indian Ridge (SWIR). The depth of earthquake hypocenters supports the existence of a very thick axial lithosphere with a brittle lid thicker than 20 km. This is also consistent with our petrological observations on dredged peridotites from the area (plagioclase-free, spinel-bearing dynamically recrystallized assemblages in shear zones) that reveal high stress (80–270 MPa) and high temperature (>800°C) deformation combining brittle and ductile mechanisms. Thermo-mechanical models using strain dependent cohesion weakening to localize deformation do not produce detachments for a brittle lithosphere thicker than 20 km. Here, we explore two other weakening mechanisms observed in situ, and for which we have temperature constraints from rock sampling: serpentinization (at T<350°C) and grain size reduction by dynamic recrystallization (at T∼800–1000°C). Serpentinization and more generally hydrous alteration minerals have been shown to localize strain at detachment-dominated slow-spreading ridge locations. Here we show that grain size reduction due to dynamic recrystallization also occurs in peridotites from the eastern SWIR. Our models explore the effect of implementing these weakening mechanisms separately or together in an amagmatic spreading context, of varying the threshold conditions for their activation, and the thermal conditions on-axis. We show that serpentinization alone does not produce flip-flop detachments, while grain size reduction alone does, albeit with unrealistically high associated topographic relief. Combining the two, we observe the activation of several modes of axial deformation: horst mode, spider mode, and long or short-lived flip-flop detachments. Flip-flop detachment faulting, once initiated, can be activated over long periods of time, with fault topography and offsets that are consistent with geological observations in the eastern SWIR nearly amagmatic study area. The transition between these modes appears to depend on the amount and location of serpentine that has been produced in previous stages of faulting.

Fractional crystallization processes of magma beneath the Carlsberg Ridge (57°–65°E)

Fractional crystallization of basaltic magma at variable depths influences strongly the geochemical compositions of mid-ocean ridge basalts (MORBs), especially at slow-spreading mid-ocean ridges. The Carlsberg Ridge is a typical slow-spreading ridge located in the northwestern Indian Ocean. In this study, we conducted petrological, geochemical and modelling studies of MORBs collected along the Carlsberg Ridge from 57°–65°E to understand the fractional crystallization processes of magma and the controls on variations in MORB geochemistry. Our results show that the mantle sources beneath the Carlsberg Ridge are heterogeneous even on the local scale of a segment; such heterogeneity may be ubiquitous beneath the Carlsberg Ridge. Mantle heterogeneity may be caused by the enriched components resulting in the “DUPAL” anomaly, whereas the effect of pyroxenite on mantle heterogeneity is negligible. The parental melts experienced crystallization of olivine, plagioclase and clinopyroxene prior to eruption, which played a significant role in the major and trace element variations in MORBs from the Carlsberg Ridge. The liquid lines of descent (LLDs), deduced from the forward modelling of three parental magma compositions using the Petrolog3 program at pressures between 1 atm and 10 kbar, demonstrate that clinopyroxene joined the olivine and plagioclase cotectic. The over-enrichment in highly incompatible elements relative to LLDs may be caused by the processes of replenishment-tapping-crystallization in magma chambers. The calculated crystallization pressures suggest that parental magmas beneath the Carlsberg Ridge experienced moderate-to high-pressure crystallization and that crystallization beneath the slow-spreading Carlsberg Ridge may start at upper mantle depths.

Magmatic Processes Associated with Oceanic Crustal Accretion at Slow-spreading Ridges: Evidence from Plagioclase in Mid-ocean Ridge Basalts from the South China Sea

AbstractMagmatic processes associated with oceanic crustal accretion at slow-spreading mid-oceanic ridges are less well understood compared with those at fast-spreading ridges. Zoned plagioclase in the basalts might record these magmatic processes as a result of the very slow intra-crystal diffusion of CaAl–NaSi. Plagioclase phenocrysts in plagioclase-phyric basalt from Hole U1433B of International Ocean Discovery Program (IODP) Expedition 349 in the South China Sea show complex zoning patterns (e.g. normal, reverse, oscillatory and patchy). These samples provide a rare opportunity to determine the magma dynamics associated with oceanic crustal accretion at slow-spreading ridges through time. Igneous lithological units in Hole U1433B consist of a series of massive lava flows at the bottom and a thick succession of small pillow lava flows at the top. Most of the plagioclase phenocrysts in the massive lava show core–rim zonation with high-An cores (An ∼85%; in mole fraction; Pl-A) in equilibrium with melts that are more primitive than their host. Some high-An cores of Pl-A phenocrysts contain melt inclusions and are depleted in La, Ce, Y and Ti, but enriched in Sr and Eu; this is interpreted as resulting from dissolution–crystallization processes during reaction of hot melt with pre-existing plagioclase cumulates. In the pillow lavas, most of the plagioclase phenocrysts show normal core–mantle–rim zonation (Pl-B) with An contents decreasing gradually from the core to the mantle to the rim, suggesting extensive magma mixing and differentiation. Reversely zoned plagioclases (Pl-C) are sparsely present throughout the basalts, but mostly occur in the lower part of the drill hole. The cores of euhedral Pl-C phenocrysts are compositionally comparable with the mantles of Pl-B phenocrysts, suggesting that the evolved magma was recharged by a relatively primitive magma. Melt inclusion-bearing Pl-A phenocrysts occur mainly in the massive lava, but rarely in the pillow lava, whereas Pl-B phenocrysts are present dominantly in the pillow lava, which reflects reducing melt–rock interaction and enhanced magma mixing, recharging and differentiation from the bottom to the top of the hole. In addition, the extensive magma mixing and differentiation recorded by Pl-B phenocrysts in the pillow lava require the existence of a melt lens beneath the mid-ocean ridge. Consistently, the plagioclase phenocrysts in the pillow lava mostly lack melt inclusions, corresponding to very weak melt–rock reactions, which indicates that the magma was transported through plagioclase cumulates by channel flow and requires a higher magma supply to the magma conduit. Therefore, the textural and compositional variations of plagioclase phenocrysts in the samples reflect the changes in magma dynamics of the mid-ocean ridge basalt through time with respect to oceanic crustal accretion at slow-spreading ridges. Overall, the oceanic crustal accretion process is sensitive to the magma supply. In the period between two episodes of extension, owing to a low melt supply the primitive melt percolates through and interacts with the mush zone by porous flow, which produces melt inclusion-bearing high-An plagioclase through dissolution–crystallization processes. At the initial stage of a new episode of extension, the melt infiltrates the mush zone and entrains crystal cargoes including melt inclusion-bearing high-An plagioclase. During the major stage of extension, owing to a relatively high melt supply the melt penetrates the mush zone by channel flow and can pool as melt lenses somewhere beneath the dikes; this forms intermediate plagioclases and the reverse zoning of plagioclases by magma mixing, recharging and differentiation in the melt lens. Such magmatic processes might occur repeatedly during the episodic extension that accompanies oceanic crustal accretion at slow-spreading ridges, which enhances the lateral structural heterogeneity of the oceanic crust.

Seafloor expression of oceanic detachment faulting reflects gradients in mid-ocean ridge magma supply

Oceanic detachment faulting is a major mode of seafloor accretion at slow and ultraslow spreading mid-ocean ridges, and is associated with dramatic changes in seafloor morphology. Detachments form expansive dome structures with corrugated surfaces known as oceanic core complexes (OCCs), and often transition to multiple regularly-spaced normal faults that form abyssal hills parallel to the spreading axis. Previous studies have attributed these changes to along-axis gradients in lithospheric strength or magma supply. However, despite the recognition that magma supply can influence fault style and seafloor morphology, the mechanics controlling the transition from oceanic detachment faults to abyssal hill faults and the relationship to along-axis variations in magma supply remain poorly understood. This study investigates this issue using two complementary modeling approaches. The first consists of semi-analytical, two-dimensional (2-D) cross-axis models designed to address the fundamental mechanical controls on the longevity of normal faults. These 2-D model sections are juxtaposed in the along-axis direction to examine the response of the plan-view pattern of faults to along-axis variations in magmatic accretion in the absence of along-axis mechanical coupling. The second approach uses three-dimensional (3-D), time-dependent numerical models that simulate faulting and magma intrusion in a visco-elasto-plastic continuum. The primary variable studied through both approaches is the along-axis gradient in the fraction M of seafloor spreading that is accommodated by magmatism. The 2-D and 3-D results predict different abyssal hill spacing and orientation, however the plan-view geometry of self-emerging detachment faults predicted by the 3-D numerical models are well explained by the juxtaposed 2-D models. This indicates a first-order control by cross-axis effects of changing values of M. These models are also shown to explain the along-axis extent and plan-view curvature of the well-developed 13°20′N and Mt. Dent OCCs (Mid-Atlantic Ridge and Cayman Rise) in terms of quantifiable along-axis gradients in magma emplacement rates.

Geological mapping of the Menez Gwen segment at 37°50′N on the Mid-Atlantic Ridge: Implications for accretion mechanisms and associated hydrothermal activity at slow-spreading mid-ocean ridges

Slow-spreading mid-ocean ridges have the potential to form large seafloor massive sulphide (SMS) deposits. Current exploration for SMS deposits commonly targets associated active hydrothermal venting on the ridge axis, which makes the discovery of inactive vent sites and SMS deposits in the off-axis regions unlikely. Geological maps of the seafloor, which help understand the timing and location of SMS formation, usually focus on individual hydrothermal vent sites and their immediate surroundings, and are often too small to aid in SMS exploration. This study uses ship-based multibeam echosounder (MBES) data and a systematic classification scheme to produce a segment-scale geological map. When combined with spreading rate, this allows us to not only reconstruct the segment's spreading history, but also reveals important processes that localize hydrothermal venting. Geological mapping around two known hydrothermal vent sites on the Menez Gwen segment at 37°50′N on the slow-spreading Mid-Atlantic Ridge showed that hydrothermal venting accompanies the tectonic break-up of a large, cooling magmatic body. Venting is focussed by faulting and resulting permeability changes. The large magmatic body is associated with an axial volcano that formed as a last stage of a period with intense magmatic accretion. Such magmatic accretion periods occur every 300 to 500 ka at the Menez Gwen segment, with increasing intensity over the past 3.5 Ma years. The most recent, most intense magmatic period appears to be a regional phenomenon, also affecting the neighbouring Lucky Strike and Rifted Hills segments. Understanding the accretional setting and the spatial and temporal constraints of hydrothermal venting enables us to develop criteria in MBES data to aid exploration for inactive SMS deposits.

Hydrothermal Production of H2 and Magnetite From Steel Slags: A Geo-Inspired Approach Based on Olivine Serpentinization

The interaction between ultramafic rocks and hot seawater at slow-spreading mid-oceanic ridges triggers hydrothermal redox reactions which are known to produce magnetite and H2 under appropriate pressure and temperature conditions. Steel slags share some common properties with ultramafic rocks. They are composed of anhydrous and refractory minerals formed at temperatures exceeding 1200°C and they contain ferrous iron in comparable amounts. Consequently, both types of materials, natural and anthropogenic, when submitted to hydrothermal conditions are prone to form magnetite and H2 according the simplified redox reaction: 3[FeO] + H2O => Fe3O4 + H2 (1) where [FeO] is the ferrous iron component of the corresponding material that can be present under different mineralogical forms. Since H2 and magnetite, Fe3O4, especially when occurring as nanomaterial, are two valuable products for new-technology applications, the hydrothermal treatment of steel slags can be seen as a way to valorize a by-product of the steel industry of which a few tenths of billion tons are produced yearly. The hydrothermal behavior of steel slags which arise from basic oxygen furnace (BOF) operations and that of olivine, (Mg,Fe)2SiO4, the main mineral constituent of abyssal peridotites, is described here based on literature data. The thermochemical characteristics of Reaction 1 are reviewed for both types of materials in the perspective of optimizing a process that would valorize BOF steel slags for the production of nanomagnetite (and high-purity H2). In particular the kinetics effect of temperature, pH and solution-to-solid mass ratio on the hydrothermal oxidation of wustite (FeO), considered here as an analogue of ferrous-iron component of steel slags, is modeled. The possible role of additive (impurity) on the hydrothermal oxidation of wustite though the catalysis of the water-splitting reaction is discussed. Finally, the lack of kinetics constraints on nanomagnetite growth under hydrothermal conditions in a wide range of pH is identified has a major gap to understand two important issues, (1) the catalysis of abiotic molecules in the course of serpentinization reactions and (2) tailoring the size of the magnetite produced by hydrothermal treatment of BOF steel slags.

Evidence for archaeal methanogenesis within veins at the onshore serpentinite-hosted Chimaera seeps, Turkey

Serpentinite-hosted ecosystems are potential sites where life may first have evolved on Earth. Serpentinization reactions produce strongly reducing and highly alkaline fluids that are typified by high concentrations of molecular hydrogen (H2) and methane (CH4), which can be used as an energy source by chemosynthetic life. Low-temperature serpentinization at slow-spreading mid-ocean ridges provides an ideal environment for rich microbial communities. Similar environments have also been discovered on land, where present-day low temperature serpentinization occurs during the circulation of groundwater through exposed ophiolites, triggering the production of CH4 and H2, as well as the precipitation of secondary carbonate minerals. The rock samples analyzed here are from the Chimaera seeps in Turkey, representing serpentinized peridotites that are cross-cut by veins composed of brucite and hydromagnesite. Hydromagnesite features a mean δ13C value of −19.8‰ caused by kinetic isotope fractionation during air-groundwater exchange of CO2, followed by CO2 hydroxylation to bicarbonate within the groundwater. Geochemical modeling revealed that mixing of Mg- and Ca-rich groundwaters is required for hydromagnesite formation at the expense of brucite. Within the carbonate-hydroxide veins the lipid biomarkers pentamethylicosane (PMI) and squalane with δ13C values of +10‰ and +14‰, respectively, and unsaturated derivatives thereof were identified. Archaeol, sn2-hydroxyarchaeol, and sn3-hydroxyarchaeol are other prominent archaeal biomarkers in the veins, also revealing high δ13C values from +6 to +13‰. These isotope patterns combined with the absence of crocetane – a biomarker for methanotrophic archaea – reveal that the microbial communities of the Chimaera seeps performed methanogenesis from a CO2-limited pool rather than methanotrophy. Moreover, bacterial dialkyl glycerol diethers (DAGEs) with unusually high δ13C values (−9 to −2‰) and minor monoalkyl glycerol monoethers (MAGEs) were identified, suggesting that bacterial sulfate reduction is also active at the Chimaera site. This study reveals that archaeal methanogenesis and bacterial sulfate reduction may be prominent at onshore peridotite-hosted sites, and that biogenic CH4 may contribute to abiotic CH4 emissions from terrestrial seeps.

Depth-varying seismogenesis on an oceanic detachment fault at 13°20′N on the Mid-Atlantic Ridge

Extension at slow- and intermediate-spreading mid-ocean ridges is commonly accommodated through slip on long-lived faults called oceanic detachments. These curved, convex-upward faults consist of a steeply-dipping section thought to be rooted in the lower crust or upper mantle which rotates to progressively shallower dip-angles at shallower depths. The commonly-observed result is a domed, sub-horizontal oceanic core complex at the seabed. Although it is accepted that detachment faults can accumulate kilometre-scale offsets over millions of years, the mechanism of slip, and their capacity to sustain the shear stresses necessary to produce large earthquakes, remains subject to debate. Here we present a comprehensive seismological study of an active oceanic detachment fault system on the Mid-Atlantic Ridge near 13°20′N, combining the results from a local ocean-bottom seismograph deployment with waveform inversion of a series of larger teleseismically-observed earthquakes. The unique coincidence of these two datasets provides a comprehensive definition of rupture on the fault, from the uppermost mantle to the seabed. Our results demonstrate that although slip on the deep, steeply-dipping portion of detachment faults is accommodated by failure in numerous microearthquakes, the shallow, gently-dipping section of the fault within the upper few kilometres is relatively strong, and is capable of producing large-magnitude earthquakes. This result brings into question the current paradigm that the shallow sections of oceanic detachment faults are dominated by low-friction mineralogies and therefore slip aseismically, but is consistent with observations from continental detachment faults. Slip on the shallow portion of active detachment faults at relatively low angles may therefore account for many more large-magnitude earthquakes at mid-ocean ridges than previously thought, and suggests that the lithospheric strength at slow-spreading mid-ocean ridges may be concentrated at shallow depths.