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
International Ocean Discovery Program (IODP) Expedition 357 drilled 17 shallow sites distributed ~10 km in the spreading direction (from west to east) across the Atlantis Massif oceanic core complex ...(Mid-Atlantic Ridge, 30°N). Mantle exposed in the footwall of the Atlantis Massif oceanic core complex is predominantly nearly wholly serpentinized harzburgite with subordinate dunite. Altered peridotites are subdivided into three types: (I) serpentinites, (II) melt-impregnated serpentinites, and (III) metasomatic serpentinites. Type I serpentinites show no evidence of melt-impregnation or metasomatism apart from serpentinization and local oxidation. Type II serpentinites have been intruded by gabbroic melts and are distinguishable in some cases on the basis of macroscopic and microscopic observations, e.g., mm-cm scale mafic-melt veinlets, rare plagioclase (˂0.5 modal % in one sample) or by the local presence of secondary (replacive) olivine after orthopyroxene; in other cases, ‘cryptic’ melt-impregnation is inferred on the basis of incompatible element enrichments. Type III serpentinites are characterized by silica metasomatism manifest by alteration of orthopyroxene to talc and amphibole, and by anomalously high anhydrous SiO2 concentrations (59–61 wt%) and low MgO/SiO2 values (0.48–0.52). Although many chondrite-normalized rare earth element (REE) and primitive mantle-normalized incompatible trace element anomalies, e.g., negative Ce-anomalies, are attributable to serpentinization, other compositional heterogeneities are due to melt-impregnation. On the basis of whole rock incompatible trace elements, a dominant mechanism of melt-impregnation is distinguished in the central and eastern serpentinites from fluid-rock alteration (mostly serpentinization) in the western serpentinites, with increasing melt-impregnation manifest as a west to east increase in enrichment in high-field strength elements and light REE. High degrees of melt extraction are evident in low whole-rock Al2O3/SiO2 values and low concentrations of Al2O3, CaO and incompatible elements. Estimates of the degree of melt extraction based on whole rock REE patterns suggest a maximum of ~20% non-modal fractional melting, with little variation between sites. As some serpentinite samples are ex situ rubble, the magmatic histories observed at each site are consistent with a local source (from the fault zone) rather than rafted rubble that would be expected to show more heterogeneity and no spatial pattern. In this case, the studied sites may provide a record of enhanced melt-rock interactions with time, consistent with proposed geological models. Alternatively, sites may signify heterogeneities in these processes at spatial scales of a few km.
Display omitted
Outcrops of deeply derived ultramafic rocks and gabbros are widespread along slow spreading ridges where they are exposed in the footwall of detachment faults. We report on the microstructural and ...petrological characteristics of a large number of samples from ultramafic exposures in the walls of the Mid‐Atlantic Ridge (MAR) axial valley at three distinct locations at lat. 13°N and 14°45′N. One of these locations corresponds to the footwall beneath a corrugated paleo‐fault surface. Bearing in mind that dredging and ROV sampling may not preserve the most fragile lithologies (fault gouges), this study allows us to document a sequence of deformation, and the magmatic and hydrothermal history recorded in the footwall within a few hundred meters of the axial detachment fault. At the three sampled locations, we find that tremolitic amphiboles have localized deformation in the ultramafic rocks prior to the onset of serpentinization. We interpret these tremolites as hydrothermal alteration products after evolved gabbroic rocks intruded into the peridotites. We also document two types of brittle deformation in the ultramafic rocks, which we infer could produce the sustained low magnitude seismicity recorded at ridge axis detachment faults. The first type of brittle deformation affects fresh peridotite and is associated with the injection of the evolved gabbroic melts, and the second type affects serpentinized peridotites and is associated with the injection of Si‐rich hydrothermal fluids that promote talc crystallization, leading to strain localization in thin talc shear zones. We also observed chlorite + serpentine shear zones but did not identify samples with serpentine‐only shear zones. Although the proportion of magmatic injections in the ultramafic rocks is variable, these characteristics are found at each investigated location and are therefore proposed as fundamental components of the deformation in the footwall of the detachment faults associated with denudation of mantle‐derived rocks at the MAR.
Key Points
Gabbroic protolith for injections in ultramafic rocks
Types of deformation affecting ultramafic rocks
Localization of deformation in hydrous minerals
Abstract
Serpentinized and metasomatized peridotites intruded by gabbros and dolerites have been drilled on the southern wall of the Atlantis Massif (Mid-Atlantic Ridge, 30°N) during International ...Ocean Discovery Program (IODP) Expedition 357. They occur in seven holes from five sites making up an east–west-trending, spreading-parallel profile that crosscuts this exhumed detachment footwall. Here we have taken advantage of this sampling to study heterogeneities of alteration at scales less than a kilometer. We combine textural and mineralogical observations made on 77 samples with in situ major and trace element analyses in primary and serpentine minerals to provide a conceptual model for the development of alteration heterogeneities at the Atlantis Massif. Textural sequences and mineralogical assemblages reveal a transition between an initial pervasive phase of serpentinization and subsequent serpentinization and metasomatism focused along localized pathways preferentially used by hydrothermal fluids. We propose that these localized pathways are interconnected and form 100 m- to 1 km-sized cells in the detachment footwall. This change in fluid pathway distribution is accompanied by variable trace element enrichments in the serpentine textures: deep, syn-serpentinization fluid–peridotite interactions are considered the source of Cu, As, and Sb enrichments, whereas U and Sr enrichments are interpreted as markers of later, shallower fluid–serpentinized peridotite interaction. Alteration of gabbros and dolerites emplaced in the peridotite at different lithospheric levels leads to the development of amphibole-, chlorite- and/or talc-bearing textures as well as enrichments in light rare earth elements, Nb, Y, Th, and Ta in the serpentine textures of the surrounding peridotites. Combining these observations, we propose a model that places the drill holes in a conceptual frame involving mafic intrusions in the peridotites and heterogeneities during progressive alteration and emplacement on the seafloor.
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
We report on the presence of the serpentine-type antigorite in abyssal-serpentinized peridotite. At mid-ocean spreading ridges, antigorite crystallizes under retrograde metamorphic conditions during ...tectonic exhumation of the newly formed oceanic lithosphere. Using optical microscopy and micro-Raman spectroscopy, we identified antigorite in 49 samples drilled at the Hess Deep (East Pacific Rise) and the Atlantis Massif (Mid-Atlantic Ridge, 30°N), and dredged along the Southwest Indian Ridge (62°–65°E). Overall, antigorite is common, but occurs in limited modal amounts. SEM and TEM investigations reveal its frequent crystallization after lizardite and chrysotile via dissolution–recrystallization processes and a local association with olivine or talc. We explain antigorite crystallization by the interaction with seawater-derived hydrothermal fluids moderately enriched in silica (metasomatism). The origin of silica is attributed to alteration of mafic intrusions or pyroxenes. Antigorite can, therefore, be considered a marker of preferential fluid pathways under rock-dominated conditions during exhumation of a portion of the oceanic lithosphere. We also measured the in-situ major and trace-element composition of antigorite and the predating and postdating phases. Most of the elements are immobile during the mineralogical transitions. Other elements (Ni, Ca, Al, and Ti) evolve within the serpentine textures, including antigorite, as a result of chemical exchanges accompanying the development of the sequence of serpentine textures. A further category includes elements that are specifically enriched (Mn, Sn) or depleted (Fluid-Mobile Elements: B, Sr, As, U, Sb, and Cl) in antigorite compared to lizardite and chrysotile. These enrichments and depletions possibly reflect a change of the fluid physicochemical characteristics allowing a change in element mobility during the dissolution–recrystallization accommodating the lizardite/chrysotile-to-antigorite transition. Such depletion in FME is comparable to depletions described in studies of serpentinization and antigorite formation in subduction zone setting, which suggests that the origin of antigorite in some subducted samples could be reevaluated.
Sulfur is one of the main redox sensitive and volatile elements involved in chemical transfers between earth surface and the deep mantle. At mid-oceanic ridges, sulfur cycle is highly influenced by ...serpentinite formation which acts as a sink of sulfur under various oxidation states (S2−, S−, S0 and S6+). Sulfur sequestration in serpentinites is usually attributed to the crystallization of secondary minerals, such as sulfides (e.g. pyrite, pyrrhotite) or sulfates (e.g. anhydrite). However, the role of serpentine minerals as potential sulfur carriers is not constrained. We investigate the distribution and redox state of sulfur at micro-scale combining in situ spectroscopic (X-ray absorption near-edge structure: XANES) and geochemical (SIMS) measurements in abyssal serpentinites from the SWIR (South West Indian Ridge), the Rainbow and the MARK (Mid-Atlantic Ridge, Kane Fracture Zone) areas. These serpentinites are formed in different tectono-metamorphic settings and provide a meaningful database to understand the fate of sulfur during seafloor serpentinization. XANES spectra of serpentinite powders show that the sulfur budget of the studied samples is dominated by oxidized sulfur (S6+/∑S=0.6–1) although sulfate micro-phases, such as barite and anhydrite, are absent. Indeed, μ-XANES analyses of mesh, bastite and antigorite veins in thin sections and of serpentine grains rather suggest the presence of S6+ ions incorporated into serpentine minerals. The structural incorporation of S in serpentine minerals is also supported by X-ray fluorescence mapping revealing large areas (1600 μm2) of serpentinite where S is homogeneously distributed. Our observations show that serpentine minerals can incorporate high S concentrations, from 140 to 1350 ppm, and that this can account for 60 to 100% of the sulfur budget of abyssal serpentinites. Serpentine minerals thus play an important role in S exchanges between the hydrosphere and the mantle at mid-oceanic ridges and may participate to S recycling in subduction zones.
•μ XANES and SIMS measurements of abyssal serpentinites•Structural incorporation of S6+ in serpentine minerals•Serpentine can incorporate high S concentrations (140–1350 ppm)•Serpentine can represent between 60 to 100% of sulfur budget in abyssal serpentinites•Mantle sulfide oxidation can contribute H2 production
PDF
