Oceanic detachment faults play a central role in accommodating the plate divergence at slow-ultraslow spreading mid-ocean ridges. Successive flip-flop detachment faults in a nearly-amagmatic region ...of the ultraslow spreading Southwest Indian Ridge (SWIR) at 64°30'E accommodate ~100% of plate divergence, with mostly ultramafic smooth seafloor. Here we present microseismicity data, recorded by ocean bottom seismometers, showing that the axial brittle lithosphere is on the order of 15 km thick under the nearly-amagmatic smooth seafloor, which is no thicker than under nearby volcanic seafloor or at more magmatic SWIR detachment systems. Our data reveal that microearthquakes with normal focal mechanisms are colocated with seismically-imaged damage zones of the active detachment fault and of antithetic hanging-wall faults. The level of the hanging-wall seismicity is significantly higher than that documented at more magmatic detachments of slow-ultraslow ridges, which may be a unique feature of nearly-amagmatic flip-flop detachment systems.
Lost City (mid-Atlantic ridge) is a unique oceanic hydrothermal field where carbonate-brucite chimneys are colonized by a single phylotype of archaeal Methanosarcinales, as well as sulfur- and ...methane-metabolizing bacteria. So far, only one submarine analog of Lost City has been characterized, the Prony Bay hydrothermal field (New Caledonia), which nonetheless shows more microbiological similarities with ecosystems associated with continental ophiolites. This study presents the microbial ecology of the 'Lost City'-type Old City hydrothermal field, recently discovered along the southwest Indian ridge. Five carbonate-brucite chimneys were sampled and subjected to mineralogical and geochemical analyses, microimaging, as well as 16S rRNA-encoding gene and metagenomic sequencing. Dominant taxa and metabolisms vary between chimneys, in conjunction with the predicted redox state, while potential formate- and CO-metabolizing microorganisms as well as sulfur-metabolizing bacteria are always abundant. We hypothesize that the variable environmental conditions resulting from the slow and diffuse hydrothermal fluid discharge that currently characterizes Old City could lead to different microbial populations between chimneys that utilize CO and formate differently as carbon or electron sources. Old City discovery and this first description of its microbial ecology opens up attractive perspectives for understanding environmental factors shaping communities and metabolisms in oceanic serpentinite-hosted ecosystems.
We report on a 3 years monitoring experiment of low to medium temperature diffuse venting at two vent sites (Tour Eiffel and White Castle) of the Lucky Strike, black smoker‐type hydrothermal field, ...Mid‐Atlantic Ridge. Diffuse vents account for a large part of the energy flux of mid‐ocean ridges hydrothermal fields and provide key habitats for the hydrothermal fauna. We document the time and space variability of diffuse venting temperature and chemistry, describe the effect of tidal loading and currents and discuss the extent of mixing, cooling of black smoker fluids, heating of entrained seawater and anhydrite precipitation/dissolution in the substratum. We emphasize the role of a thin (<2 m) volcaniclastic formation capping the brecciated basalt substratum. This formation is porous, but becomes impermeable when indurated by hydrothermal precipitates. It forms an intermediate layer between the vents at the seabed and the fluids as they discharge out of the brecciated basalts. Diffuse fluids inferred to discharge out of meter‐spaced cracks in the brecciated basalts beneath this volcaniclastic layer are hot (>80°C) and contain >10% of the hot endmember fluid component, over distances of up to 25 m from the black smokers. These results provide a geologically integrated framework in which to study site‐scale, near seafloor hydrothermal circulation and associated vent habitats at Lucky Strike and other black smoker‐type hydrothermal fields. They suggest diffuse heat fluxes in the upper range of previously published estimates at the two studied Lucky Strike hydrothermal vent sites.
Plain Language Summary
Mid‐ocean ridges (MOR) are a key feature of plate tectonics, extending some 60,000 km in all the major oceans. MOR hydrothermal circulations transfer heat and chemical compounds from the solid earth to the ocean and provide habitats for the hydrothermal fauna. The vents include black smokers that expel the hottest fluids and diffuse vents that expel lower temperature fluids at lower rates but over larger surfaces. The contribution of diffuse vents to the energy and chemical fluxes of MOR hydrothermal systems is still largely an open question. In this paper, we address it by using data from a 3 years monitoring experiment of diffuse vents at two sites (Tour Eiffel and White Castle) of the Lucky Strike, a black smoker‐type hydrothermal field in the Mid‐Atlantic Ridge. We document the time and space variability of venting temperature and derive chemical constraints on the extent of mixing of black smoker fluids with entrained seawater and of mineral precipitation/dissolution in the substratum of the vents. Our results suggest diffuse heat fluxes in the upper range of previously published Lucky Strike hydrothermal field estimates and provide a geologically integrated framework in which to study diffuse vent habitats at Lucky Strike and other black smoker‐type hydrothermal fields.
Key Points
Time variability of both fluid temperature and fluid chemistry at diffuse vents of the Lucky Strike mid‐ocean ridge hydrothermal field
Hot (>80°C) and hydrothermal endmember‐rich diffuse fluids (>10%) come out of the basalts up to 25 m from the black smokers
Fluids that come out of basalt substratum are modified in volcaniclastic layer before coming out at vents that host the hydrothermal fauna
At slow spreading ridges, axial detachment faults exhume mantle‐derived peridotites and hydrothermal alteration causes serpentinization in a domain extending more than 1 km next to the fault. At the ...microscopic scale, serpentinization progresses from a microfracture network toward the center of olivine relicts and forms a mesh texture. We present a petrographic study (SEM, EBSD, and Raman) of the serpentine mesh texture in a set of 278 abyssal serpentinized peridotites from the Mid‐Atlantic and Southwest Indian Ridges. We show that serpentinization initiated along two intersecting sets of microfractures that have consistent orientations at the sample scale, and in at least one studied location, at the 100 m scale. We propose that these microfractures formed in fresh peridotites due to combined thermal and tectonic stresses and subsequently served as channels for serpentinizing fluids. Additional reaction‐induced cracks developed for serpentinization extents <20%. The resulting microfracture network has a typical spacing of ∼60 µm but most serpentinization occurs next to a subset of these microfractures that define mesh cells 100–400 µm in size. Apparent mesh rim thickness is on average 33 ± 19 µm corresponding to serpentinization extents of 70–80%. Published laboratory experiments suggest that mesh rims formation could be completed in a few years (i.e., quasi instantaneous at the plate tectonic timescale). The depth and extent of the serpentinization domain in the detachment fault's footwall are probably variable in time and space and as a result we expect that the serpentine mesh texture at slow spreading ridges forms at variable rates with a spatially heterogeneous distribution.
Key Points
Serpentinization occurs along thermal, tectonic, and reaction‐induced fractures
Serpentinization may occur quasi instantly at exhumation time scales
The depth and extent of serpentinization are variable in space and time
The kinetics of the reaction (Mg,Fe)‐olivine + H2O → serpentine + magnetite + brucite + H2 were investigated at 500 bars in the 250–350°C range using natural olivine (San Carlos; Fo91) with grain ...sizes between 1 and 150 μm and for run durations up to 514 d. The amount of magnetite produced, which directly relates to reaction progress, was accurately monitored using up to 24 time‐resolved magnetic measurements per experiment. Eighty percent of serpentinization was achieved after 60 d for olivine grain sizes of 5–15μm and after 500 d for grain sizes of 50–63 μm. Serpentinization kinetics were found to be inversely proportional to the geometrical surface area of the starting olivine grains. They were one or two orders of magnitude slower than serpentinization kinetics commonly used for modeling serpentinization‐related processes. The nature of the serpentine mineral product depended on the initial olivine grain size (IGS); for IGS in the 5–150μm range lizardite formed, and olivine dissolution was the rate‐limiting process. At IGS below 5μm, chrysotile crystallized instead of lizardite, and the relationship between olivine surface area and reaction kinetics no longer held. We infer that for such small olivine grain sizes dissolution is no longer the rate‐limiting process. Serpentinization in our experiments was associated with the creation of new reactive surface area according to two cooperative processes: etch pits formation associated with dissolution and grain fracturing for IGS above 20μm. Interestingly, fractures and etch pits with similar geometry and sizes were also observed for residual olivine (with a typical grain size of 50 μm) in serpentinized peridotite samples from the Southwest Indian Ridge. This suggests that the processes governing olivine serpentinization kinetics in our experiments are similar to those prevailing in natural systems. We therefore suggest that the new kinetic data set that we present here, which encompasses a range of olivine grain sizes and reaction temperatures, is relevant to the serpentinization of olivine in the oceanic crust insofar as water is available.
Key Points
Experimental study of the kinetics of olivine serpentinization
Influence of temperature and olivine initial grain size (IGS) on the kinetics
Reactive surface area increases with etch pits and fractures
Extensive outcrops of serpentinized peridotite in melt‐starved spreading corridors of the ultraslow easternmost Southwest Indian Ridge are hypothesized to be due to slip on successive long‐offset ...normal faults that alternate polarity (flip‐flop detachment faults). We investigate the nature of the oceanic crust which forms under these conditions, using seismic reflection data acquired during the SISMOSMOOTH 2014 cruise. Using 3‐D binning, the seismic profiles were binned elastically, while three of the profiles shot closely were merged into one to take advantage of the larger air gun source volume. Using a poststack imaging sequence, we observe several types of reflectors at crustal and infracrustal depths, in the axial valley and off‐axis. Correlating our seismic observations with Residual Mantle Bouguer gravity anomalies and seafloor observations, we find that our results are explicable in the framework of the flip‐flop hypothesis of detachment faulting. Reflectors imaged down to 5 km into the basement and interpreted as due to damaged zones outlining the detachment faults dip 50° at the early stages, while at late stages after developing offsets >10 km, they dip 25°. Other reflectors observed in the crust are interpreted as moderate offset (<200 m) normal faults accommodating deformation and alteration in the hanging wall and channeling the sparse melt to the seafloor. We interpret these and other observed seismic reflectors in the frame of a two‐phase evolutionary sequence over the lifetime of two successive flip‐flop detachment faults: exhumation, footwall flexure, damage, serpentinization, and incipient magmatism in the footwall of one detachment fault; followed by further tectonic damage, alteration, and incipient magmatism in the hanging wall of the next detachment fault.
Key Points
Seismic reflectivity structure of amagmatic spreading corridor reveals signatures of possible flip‐flop detachment faults
Detachment faults are steep‐dipping (>45°) in the upper 5 km of the ultramafic basement at the early stages and more shallow dipping (25°) after developing offsets >10 km
Seismic reflectivity structure consistent with two‐phase accretion history: (1) tectonic exhumation in the footwall of a detachment fault, and (2) further faulting, alteration, and magmatic infiltration in the hanging wall of the next detachment fault
The eastern Southwest Indian Ridge, between 61° and 67°E, has a very low melt supply and comprises several corridors of nearly amagmatic spreading that expose mantle-derived serpentinized peridotite. ...More volcanically active ridge portions separate these corridors. He, Ne, Pb, Nd, Hf and Sr isotopes were analyzed in basalt glasses dredged on two types of seafloor: volcanic and ultramafic. Basalts dredged on on-axis ultramafic seafloor tend to be slightly more depleted for heavy radiogenic isotopes and show slightly higher 3He/4He isotope ratios than basalts dredged on volcanic seafloor, with no systematic difference in neon isotope ratios. We propose that both types of basalts are derived from the same mantle source, but that the basalts dredged on ultramafic seafloor are more affected by melt/mantle reactions, which slightly modify their isotopic signatures. Our dataset also includes a few basalts, dredged on off-axis ultramafic seafloor, that range in age between 2.6 and 8.8 Ma. These few and widely-spaced off-axis samples, erupted at the ridge axis, are a rare opportunity to capture the potential geochemical variability of the mantle source in an ultramafic seafloor corridor over 8.8 Ma. This temporal variability appears to be minor compared to the overall range of isotopic variability of the on-axis lavas from the 61°-67°E region.
The Lucky Strike vent field, located on the Mid‐Atlantic Ridge (MAR), is hosted on enriched mid‐ocean ridge basalt associated with the nearby Azores hotspot. In this study, we present bulk rock ...geochemistry coupled with in situ sulfur isotope analysis of hydrothermal samples from Lucky Strike. We assess the geological controls on the differences in the major and trace element content and sulfur isotopic composition of the hydrothermal deposits within the vent field. The hydrothermal deposits contain elevated concentrations of elements typically enriched in mid‐ocean basalt (E‐MORB), such as Mo, Ba, and Sr, compared to typical values for other hydrothermal deposits hosted on the MAR. The range in sulfur isotope compositions of hydrothermal marcasite and chalcopyrite (−2.5 to 8.7‰) is similar to the range recorded at other sediment‐free basalt‐hosted seafloor hydrothermal sites. However, at Lucky Strike, the Capelinhos vent, situated 1.4 km east of the main field, is enriched in 34S (by ∼3.5‰ for both marcasite and chalcopyrite), relative to the main field. This difference reflects contrasting subseafloor fluid/rock interactions at these two sites, including subseafloor sulfide precipitation at the main field that results in <20% of reduced sulfur within the upwelling hydrothermal fluid reaching the seafloor. We also compare the geochemistry of the hydrothermal deposits at Lucky Strike to other hydrothermal sites along the MAR and show that the average hydrothermal deposit Ba/Co is useful to discriminate between E‐MORB and other mafic/ultramafic hosted deposits.
Plain Language Summary
We investigate the variations in composition of metal‐ and sulfur‐rich hydrothermal deposits that form on the seafloor at a cluster of high‐temperature hot springs called the Lucky Strike hydrothermal vent field, on the Mid‐Atlantic Ridge. We find that the mineralogy and geochemistry of the deposits do not vary spatially within this vent field. However, variations in the relative abundances of different sulfur isotopes within these deposits differ between the central cluster of vents and a newly discovered site called Capelinhos that is located 1.4 km east of the main vent field. Isotopic variations are usually interpreted to indicate differences in sulfur sources, with seawater and sulfur from the mantle as the two primary sources. However, our results instead show that significant mineral precipitation below the seafloor at the main vent cluster is the likely source of these isotopic variations. In addition, we show that the relative abundances of various trace elements within the hydrothermal deposits can be used to fingerprint the composition of the volcanic rocks that host these deposits. In particular, the ratio of Ba to Co can be used to fingerprint specific tectonic settings for different hydrothermal vent sites on mid‐ocean ridges.
Key Points
Spatial variations in in situ sulfur isotope compositions at Lucky Strike indicate differences in fluid/rock interactions in the sub‐surface
In situ sulfur isotope data suggests that >80% of the available H2S in the ascending hydrothermal fluid has precipitated in the subseafloor
The Ba/Co ratio of hydrothermal deposits discriminates those associated with enriched mid‐ocean basalts from other mafic/ultramafic hosted deposits
IODP Expedition 357 used two seabed drills to core 17 shallow holes at 9 sites across Atlantis Massif ocean core complex (Mid-Atlantic Ridge 30°N). The goals of this expedition were to investigate ...serpentinization processes and microbial activity in the shallow subsurface of highly altered ultramafic and mafic sequences that have been uplifted to the seafloor along a major detachment fault zone. More than 57 m of core were recovered, with borehole penetration ranging from 1.3 to 16.4 meters below seafloor, and core recovery as high as 75% of total penetration in one borehole. The cores show highly heterogeneous rock types and alteration associated with changes in bulk rock chemistry that reflect multiple phases of magmatism, fluid-rock interaction and mass transfer within the detachment fault zone. Recovered ultramafic rocks are dominated by pervasively serpentinized harzburgite with intervals of serpentinized dunite and minor pyroxenite veins; gabbroic rocks occur as melt impregnations and veins. Dolerite intrusions and basaltic rocks represent the latest magmatic activity. The proportion of mafic rocks is volumetrically less than the amount of mafic rocks recovered previously by drilling the central dome of Atlantis Massif at IODP Site U1309. This suggests a different mode of melt accumulation in the mantle peridotites at the ridge-transform intersection and/or a tectonic transposition of rock types within a complex detachment fault zone. The cores revealed a high degree of serpentinization and metasomatic alteration dominated by talc-amphibole-chlorite overprinting. Metasomatism is most prevalent at contacts between ultramafic and mafic domains (gabbroic and/or doleritic intrusions) and points to channeled fluid flow and silica mobility during exhumation along the detachment fault. The presence of the mafic lenses within the serpentinites and their alteration to mechanically weak talc, serpentine and chlorite may also be critical in the development of the detachment fault zone and may aid in continued unroofing of the upper mantle peridotite/gabbro sequences.
New technologies were also developed for the seabed drills to enable biogeochemical and microbiological characterization of the environment. An in situ sensor package and water sampling system recorded real-time variations in dissolved methane, oxygen, pH, oxidation reduction potential (Eh), and temperature and during drilling and sampled bottom water after drilling. Systematic excursions in these parameters together with elevated hydrogen and methane concentrations in post-drilling fluids provide evidence for active serpentinization at all sites. In addition, chemical tracers were delivered into the drilling fluids for contamination testing, and a borehole plug system was successfully deployed at some sites for future fluid sampling. A major achievement of IODP Expedition 357 was to obtain microbiological samples along a west–east profile, which will provide a better understanding of how microbial communities evolve as ultramafic and mafic rocks are altered and emplaced on the seafloor. Strict sampling handling protocols allowed for very low limits of microbial cell detection, and our results show that the Atlantis Massif subsurface contains a relatively low density of microbial life.
•Seabed rock drills and real-time fluid monitoring for first time in ocean drilling•First time recovery of continuous sequences along oceanic detachment fault zone•Highly heterogeneous rock type and alteration in shallow detachment fault zone•High methane and hydrogen concentrations in Atlantis Massif shallow basement•Oceanic serpentinites potentially provide important niches for microbial life
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