Atlantis Bank Research Articles

Sulfur and metal mobilization during the life cycle of an oceanic core complex: Implications for seafloor massive sulfide deposits formation at slow and ultra-slow spreading ridges

Seafloor massive sulfide (SMS) deposits at slow and ultra-slow spreading ridges are often spatially related to, or hosted in oceanic core complexes (OCCs). The specific oceanic crust architecture, magmatism, hydrothermal fluid circulation and lithologies at OCCs, however, imply different S and metal (e.g. Cu, Zn, Co, Ni) fluxes relative to well-structured oceanic crust at-fast spreading ridges and which are not yet fully constrained. The study of S and metal distribution in the ODP Hole 735B deep drill core from the Atlantis bank allows to understand these fluxes along detachment faults and to better constrain the source zone of S and metals for OCC-related SMS deposits. Significant depletion of S, Cu, Zn and Ni are observed within the upper 250 m of the drill core where intense deformation and hydrothermal fluid circulation occurred. During the complex tectono-magmatic-hydrothermal evolution of the Atlantis Bank, four important stages are recognized for S and metal mobilization: 1) magmatic stratification leading to a higher proportion of sulfide-rich and S, Cu, Zn and Co fertile oxide gabbros in the root zone of the Atlantis Bank detachment, 2) high temperature ductile deformation leading to magmatic sulfide reworking and onset of sulfide leaching with limited metal mobilization, 3) extensive sulfide leaching and metal mobilization during amphibolite to greenschist facies metasomatism and, 4) late stage secondary sulfide precipitation and S enrichment during low temperature fluid circulation. Mass balance calculations from the source zones of the Atlantis Bank detachment highlights that metal mobilization during hydrothermal alteration of gabbroic rocks along detachment faults can fully account for the formation of OCC-related SMS deposits at slow and ultraslow spreading ridges. The Atlantis Bank detachment system, however, is gabbroic-dominated and represent the magmatic end-member of OCCs and further work is necessary for understanding metal fluxes in ultramafic-dominated detachment systems.

Development of oxy-symplectites in a slow-spreading lower oceanic crust: Insights from the Atlantis Bank Gabbro Massif, Southwest Indian Ridge

Development of oxy-symplectites in a slow-spreading lower oceanic crust: Insights from the Atlantis Bank Gabbro Massif, Southwest Indian Ridge

The critical role of deformation-assisted melt migration in the formation of oceanic core complexes

Oceanic core complexes provide an accessible window into deep processes occurring at slow and ultra-slow-spreading mid-ocean ridges. We analyse samples from IODP ocean drilling of core complexes at the Atlantis Bank, Atlantis Massif, and near the Kane Transform at the South West Indian and Mid-Atlantic ridges. We correlate secondary minerals, including oxides, with sites of melt migration. We interpret changes to mineral assemblage and microchemistry, reaction textures and melt-pseudomorph microstructures as fingerprints of open-system melt-mediated processes. This micro-scale information is combined with a macro-scale review of legacy mineral chemistry data to show that melt-fluxed rocks share remarkably similar characteristics across the three core complexes investigated. These are rich in oxides and have olivine, orthopyroxene and clinopyroxene grains that are chemically distinct from oxide-poor gabbros. We propose that oceanic crust fluxed with external melt can be recognised by the following key features: (1) high modes of secondary minerals, such as oxides and olivine, (2) microstructural evidence for the former presence of melt, and (3) mineral chemistry differences between primary and secondary olivine, orthopyroxene, clinopyroxene and plagioclase. Importantly, olivine has previously only been reported as primary, that is, having crystallised from magma. However, our results show that gabbros with secondary olivine are reliable indictors of melt–rock interaction during deformation-assisted diffuse melt migration through the gabbroic oceanic crust. Finally, we propose a new model for the formation of oceanic core complexes where deformation-assisted melt migration plays a critical role in strain localisation, exhumation and evolution of the core complex.

Crust-scale reactive porous flow revealed by the brown amphibole in the IODP hole U1473A gabbros, Southwest Indian Ridge

Mid-ocean ridge magma reservoirs are mush-dominated systems, where reactive porous flow is a dominant post-cumulus process. However, the length scale of this process is poorly constrained, especially in ultraslow-spreading ridges. Here, we seek to address this question by presenting a comprehensive brown amphibole dataset of gabbros in IODP Hole U1473A drilled on the Atlantis Bank ocean core complex, Southwest Indian Ridge (SWIR). The results suggest a magmatic origin of the brown amphiboles. Intra-grain chemical variations and compositional zoning of these amphibole grains suggest they were crystallized in a closed system. Nevertheless, geochemical modelings indicate the remarkable Zr/Nd ratio variations in brown amphibole can only be explained by assimilation-fractional crystallization processes triggered by the reactive porous flow. Thus, the brown amphibole in olivine gabbros was crystallized from trapped melts residual after earlier melt-mush interaction. In this light, the reactive porous flow should have ceased prior to the brown amphibole precipitation. This, in turn, suggests the stratigraphic variations in amphibole trace elements reflect the final-stage reactive melt migration. The stratigraphic variations in amphibole trace elements reflect a major episode of reactive melt migration through an ∼800-m section upwardly. This implies that the porosity of the entire gabbroic section remained connected, providing a minimum estimate of the size of the melt-present zones in the lower oceanic crust during the spreading of the 57-60°E magmatic segment of SWIR. Our study reveals that large-scale reactive porous flow can occur in the magmatic segments of ultraslow-spreading ridges, and has important implications for the thermal and rheological evolution of the lower oceanic crust.

Alteration enrichment of nitrogen in the gabbroic oceanic crust: Implications for global subducting nitrogen budget and subduction-zone nitrogen recycling

Uptake of nitrogen (N) during hydrothermal alteration of oceanic crust makes the altered oceanic crust (AOC) a significant N reservoir in parallel to seafloor sediments. While N enrichment in the upper (basaltic) oceanic crust during low-temperature alteration has been widely observed, the N characteristics of moderate- to high-temperature altered gabbroic oceanic crust, which accounts for ∼70 vol% of the crustal material in subducting slabs, have been rarely studied. The lack of these data resulted in large uncertainties in calculating the N input flux of global subducting slabs and modeling the N quantities released through the arc and subducted into the deep mantle. To fill this gap, we examined the N concentrations and isotope compositions of 38 altered gabbroic rocks recovered by ODP/IODP drillings from three oceans, i.e., Hole 735B at the Atlantis Bank in the Southwest Indian Ridge, Hole 1309D at the Atlantis massif in the Mid-Atlantic Ridge, and Hole 1415P at the Hess Deep in the East Pacific Rise. All the gabbroic samples show significant N enrichment with mostly positive δ15N values (in average: 7.4±3.8 ppm and +0.6±2.0‰ for Hole 735B, 5.3±2.1 ppm and +1.4±1.3‰ for Hole 1309D and 7.9±1.9 ppm and +2.0±1.8‰ for Hole 1415P), which are comparable to those of altered basalts from global oceanic crust. The N concentrations and δ15N values of these gabbroic rocks can be readily explained by mixing between minor inherited mantle N and mainly secondary N derived from seawater. The secondary N-hosting minerals in altered gabbroic rocks are likely plagioclase, amphibole and chlorite formed during moderate- to high-temperature alteration stages. Using these new data, we obtained a global nitrogen input flux of 20.6-1.3+0.9 × 109 mol·yr−1 to 29.9±2.2 × 109 mol·yr−1 for subducting AOC, in which 50% – 64% is contributed by the altered gabbroic oceanic crust. Integrating with previous estimate of N input flux from seafloor sediments, a total N input flux of 74.9-1.3+0.9 – 84.2±2.2 mol·yr−1 was estimated for global subducting slabs. Mass balance between this input N flux and various estimates of N output flux in global arcs gave a large range (from 47% – 53% to 10% – 20%) for the fraction of slab N to be subducted beyond the sub-arc depth to the deeper mantle. The upper end (47% – 53%) of this range is more consistent with the results from other constraints, such as thermal structural control and comparison between altered gabbros and meta-gabbros.

Interplay between crystal-plasticity, fracturing and dissolution-precipitation creep in lower-crustal ultramylonite from hole U1473A, Atlantis Bank, Southwest Indian Ridge

Microfabrics of a gabbroic ultramylonitic shear zone from the Atlantis Bank oceanic core complex have been studied to investigate strain localization processes during exhumation of the lower oceanic crust through detachment faults. The ∼0.5 cm wide ultramylonite band is hosted in a coarse-grained deformed gabbro. Microfabric analysis reveal undulatory extinction, kink bands, bent exsolution lamellae or twins and healed microfractures decorated by new grains within porphyroclasts of diopside and plagioclase in the ultramylonite, as well as the host gabbro close to the border of the ultramylonite. These microstructures suggest that strain was initially accommodated by dislocation glide associated with microfracturing, i.e. high-stress crystal plasticity. Heterogenous fluid influx along microfractures is suggested to have led to localized phase transformation, as amphibole is abundant in the fine-grained matrix of the ultramylonite but not in the host gabbro. Fluid-rock interaction resulted in the development of the ultramylonite via grain-size sensitive fluid-assisted granular flow. Strain localization is concomitant to increasing fluid-rock interaction and the replacement of the former deformed anhydrous mineral assemblage into a hydrated ultramylonite composed of a plagioclase-amphibole mixture. Our study highlights the importance of fluid-assisted metamorphic reaction in decreasing the viscosity of the metastable lower oceanic crust and creating zones of extreme rheological weakening that deform by diffusion creep on the long term. Fluid infiltration plays a key role in facilitating ductile deformation and tectonic spreading through large-scale detachment faults in the hot, lower oceanic crust.

Seamount seascape composition and configuration shape Southwest Indian Ridge fish assemblages

Seamounts are commercially important fishing grounds. Yet, little is known about their physical characteristics as fish habitat, important for informing conservation and ecosystem-based management.This study examines how multiscale seabed spatial heterogeneity influences commercially important fish families at three Southwest Indian Ridge seamounts (Coral Seamount, Melville Bank and Atlantis Bank). We quantified seascape heterogeneity from bathymetry and geomorphological habitat maps and identified 15 focal fish families from video data. Fish-habitat associations were examined using spatial pattern metrics that measured terrain morphology, seascape composition (variety and relative abundance of patch types) and seascape configuration (spatial arrangement of patches). Broader seascape context was characterised by geographic location and water depth. Multivariate regression trees and random forests modelled fish-habitat associations and identified the most influential explanatory variables.Assemblage characteristics and individual families were strongly influenced by geographic location and depth, and at finer scales (500 m buffers) seascape composition and configuration helped explain fish-habitat associations. Spatially continuous summit habitat and complex shaped ridge features supported high abundance and diversity of commercial fish families. Metrics of seascape composition and configuration (i.e., habitat size, shape and structural connectivity) had higher predictive power than the terrain metrics commonly used in developing proxies for deep-water fish species and biodiversity.These outcomes indicate that seascape metrics, commonly applied on land and in shallow marine environments, are also relevant environmental predictors of fish distributions in deep-sea environments. We highlight strong context dependency and depth-specific associations that hinder attempts to draw wider generalisations on fish-seascape linkages for seamounts.

Brown Amphibole as Tracer of Tectono-Magmatic Evolution of the Atlantis Bank Oceanic Core Complex (IODP Hole U1473A)

Abstract Brown amphibole is a minor but common mineral component in lower oceanic crust. It is generally interpreted as products of migrating SiO2 and H2O-rich fluids or melts, which can be either residual melts from advanced magmatic differentiation of Mid-Ocean Ridge Basalt (MORB), or hydrothermal fluids including a seawater component. Within the lower oceanic crust exhumed at the Atlantis Bank Oceanic Core Complex, along the ultraslow Southwest Indian Ridge, brown amphibole is ubiquitous in all lithologies from olivine- to oxide-gabbros and diorites, including both undeformed and plastically deformed varieties. We here show the results of a systematic petrological study conceived to unravel the nature of the H2O-rich component recorded in brown amphiboles and document: (i) the evolution of migrating melts during the magmatic stage and (ii) different extents of melt-bearing deformation events recorded throughout the entire crustal transect. The low Cl contents and the light over heavy rare earth elements (LREE/HREE) ratios and high Ti contents in brown amphiboles indicate they crystallized from melts with a magmatic hydrous component. Consistently, their δ18O values are in equilibrium with Mid-Ocean Ridge Basalt (MORB) composition, except for diorite amphiboles that possibly record the local assimilation of altered minerals. In undeformed olivine gabbros, interstitial pargasite crystallized at hypersolidus conditions (~1000°C) from the melt residual after late stages of MORB differentiation. We speculate that before the olivine gabbro crystal mush reached fully solid state, some aliquots of residual melts were extracted and accumulated within discrete intervals. There, ferrobasaltic melts differentiated through the early crystallization of Fe-Ti oxides and clinopyroxene as liquidus phases, ultimately forming the oxide gabbros. This process promoted rapid Si enrichment and depletion in Fe, Ti, V in the residual melt, later extracted to form the crosscutting diorite veins. The mylonitic olivine gabbros record high-temperature plastic deformation (~900°C ± 50°C) under hypersolidus conditions, involving melts residual from previous crystallization of the gabbroic rock. Further solid-state plastic deformation led to substantial grain size reduction and, consequently, to an increase in porosity. This created pathways for subsequent melt focusing, which likely represent late-stage differentiated melts migrating throughout the lower crustal section. This study shows that brown amphibole in the Atlantis Bank lower oceanic crust is the crystallization product of melts residual from advanced magmatic differentiation, which are also locally involved in the plastic deformation events during crustal accretion.

The lower oceanic crust at ultraslow-spreading Southwest Indian Ridge: The inside story

Owing to the lack of persistent magma chamber and the complex interplay between magmatism and tectonism at slow- to ultraslow-spreading ridges, the likeliness of fractional crystallization being the predominant process of evolution of crustal magmas is weak. Here, we report a detailed petro-geochemical investigation from the lower crustal gabbroic rocks recovered from Hole U1473A in Atlantis Bank oceanic core complex, Southwest Indian Ridge that may potentially provide new insights on the dynamic accretion process at ultraslow-spreading ridge. Sampling the entire lithological spectrum encompassing a single geochemical cycle in this study permit us to probe the underlying magmatic processes in ultraslow-spreading lower crust. Grain-size variation is ubiquitously recorded in the gabbroic lithologies of Hole U1473A, where fine-grained intervals represent relatively evolved chemistry than the coarser domains. This suggests that, they were crystallized from a common genetic melt during different degrees of evolution. The forward geochemical modelling approach have not only affirmed the long-established theory of assimilation (of early mushes’ crystals)-fractional crystallization to be the key mechanism in lower crust formation, but also provided explanation for the restricted whole rock geochemical cluster observed throughout all the rock types. We demonstrate for the first time the relationship between the progressive modal mineralogical evolution with respect to the different assimilation/fractionation ratios (r). The observed trend has been successfully replicated for the experiments with high r values = 0.8–0.9, previously predicted from this region. Integrating all results, our study supports the model of lower crustal formation via reactive porous flow of the squeezed-out melt percolating throughout the cumulate pile and segregated into different horizons.

Exhumation and carbonation of the Atlantis Bank core complex constrained by in situ U-Pb dating and Δ47 thermometry of calcite veins, SW Indian Ridge

The Atlantis Bank is a representative example of an oceanic core complex formed on a slow-spreading ridge. It is dominated by olivine gabbros emplaced at ∼12 Ma and exhumed from original depths of more than ∼2500 metres below seafloor (mbsf). In order to investigate the timing of exhumation and low-temperature alteration we investigated twenty-two calcite veins in samples of core recovered from the 809-mbsf Hole U1473A drilled during Expedition 360 of the International Ocean Discovery Program. The calcite veins yielded laser ablation U-Pb and bulk U-Th ages ranging from 10.4±0.3 Ma to 273±5 Ka. They have an average Δ47 formation temperature of 21±4°C and 87Sr/86Sr and rare earth element signatures typical of low-temperature veins dominated by seawater. These data preclude previous cooling models that invoked a 250°C heat pulse related to sill emplacement at ∼9.4 Ma. Instead, the combined Δ47 and U-Pb age data indicate a simple cooling path in which the gabbros were exhumed close to the seafloor and cooled to a mean of 21±4°C by 10.4±0.3 Ma. The timing of the earliest low-temperature veins 1.6±0.7 Myr after accretion is broadly similar to that reported for other oceanic core complexes. However, the 12 Ma age of the Atlantis Bank means it has recorded a longer carbonation history. The vein ages indicate ∼83% of vein growth occurred during a period of 3.1±0.9 Myr between ∼10.4 and ∼7.3 Ma. The Atlantis Bank gabbros are estimated to have a bulk concentration of 0.15±0.04 wt% CO2. Therefore if carbonation and veining occurred proportionally to one another, the initial carbonation rate was ∼400±200 μg/g CO2 per Myr. A single vein with an age of 5.8±0.4 Ma formed between 7.3 and 1 Ma suggesting a virtual hiatus in carbonation. However, resumed vein growth after 0.27-1 Ma is recorded by three samples. The uncertainty in these very young U-Pb ages is substantial, meaning the carbonation rate is only loosely constrained as similar to, or lower than, the initial rate of ∼400 μg/g CO2 per Myr. These data demonstrate that recent carbonation of 12 Ma old ocean crust is significant and probably ongoing. Given that ocean core complexes form on crustal-scale detachments that provide high permeability pathways, and they are bathymetric highs where reactive lithologies can be exposed on the seafloor, oceanic core complexes like the Atlantis Bank could be sites of long-lived alteration on the seafloor.

Oxide enrichment by syntectonic melt-rock interaction

Processes that enrich rocks in oxides, such as ilmenite, are controversial. Current models include magmatic accumulation, crystallisation of veins from immiscible liquids and syntectonic differentiation. In this contribution, we investigate examples of oxide enrichment in both the oceanic and continental crust. The oceanic samples are of oxide gabbros (with up to 45 vol% oxides) from the Atlantis Bank oceanic core complex, Southwest Indian Ridge. The continental sample is from the Cattle Water Pass shear zone (with up to 20 vol% oxides) associated with the intracontinental Alice Springs Orogeny, central Australia. We argue for the occurrence of an open chemical system, with melt rock reactions as a key process involved in oxide enrichment in melt-fluxed shear zones. Our detailed microstructural characterisation reveals that oxides replace silicates and form interstitial grains, grain boundary films and low dihedral angles between silicates often making up an interconnected skeletal texture. Quantitative orientation data reveals that the oxides: 1) have limited internal deformation, 2) form clusters of grains that are connected in 3D, 3) have crystal faces matching the orientation of the grain boundary of nearby newly crystallised diopside (oceanic sample) and 4) form part of the foliation defining assemblage with biotite (continental sample). This evidence suggests the oxides crystallised in the presence of melt and formed during melt-rock interaction. Syntectonic melt migration is known to result in low strain microstructures in shear zones, as the strain is accommodated by the melt that existed in the deforming rock. This produces a high strain rock with silicate and oxide minerals that show limited internal deformation. Microchemical data shows major element variability in silicates and ilmenite at the thin section scale, supporting an open chemical system with local variability in both oceanic and continental settings. It further argues that syntectonic melt migration is important in oxide enrichment. Mineral chemistry data implies that the oceanic tectonic setting involved melt-rock interaction with fractionated gabbroic melt while the continental setting involved peraluminous granite melt driving mineral replacement and enrichment of oxides. We propose that deformation assisted reactive porous flow of near liquidus melt through rocks in any tectonic setting may result in melt-rock interaction induced crystallisation of oxides in preference to silicates and that with high time-integrated melt flux, the accumulation of oxides can be significant.

Sulfide enrichment along igneous layer boundaries in the lower oceanic crust: IODP Hole U1473A, Atlantis Bank, Southwest Indian Ridge

Reactive porous or focused melt flows are common in crystal mushes of mid-ocean ridge magma reservoirs. Although they exert significant control on mid-ocean ridge magmatic differentiation, their role in metal transport between the mantle and the ocean floor remains poorly constrained. Here we aim to improve such knowledge for oceanic crust formed at slow-spreading centers (approximately half of present-day oceanic crust), by focusing on specific igneous features where sulfides are concentrated. International Ocean Discovery Program (IODP) Expedition 360 drilled Hole U1473A 789 m into the lower crust of the Atlantis Bank oceanic core complex, located at the Southwest Indian Ridge. Coarse-grained (5–30 mm) olivine gabbro prevailed throughout the hole, ranging locally from fine- (<1 mm), to very coarse-grained (>30 mm). We studied three distinct intervals of igneous grain size layering at 109.5–110.8, 158.0–158.3, and 593.0–594.4 meters below seafloor to understand the distribution of sulfides. We found that the layer boundaries between the fine- and coarse-grained gabbro were enriched in sulfides and chalcophile elements. On average, sulfide grains throughout the layering were composed of pyrrhotite (81 vol.%; Fe1-xS), chalcopyrite (16 vol.%; CuFeS2), and pentlandite (3 vol.%; [Ni,Fe,Co]9S8), which reflect paragenesis of magmatic origin. The sulfides were most commonly associated with Fe-Ti oxides (titanomagnetites and ilmenites), amphiboles, and apatites located at the interstitial positions between clinopyroxene, plagioclase, and olivine. Pentlandite exsolution textures in pyrrhotite indicate that the sulfides formed from high-temperature sulfide liquid separated from mafic magma that exsolved upon cooling. The relatively homogenous phase proportion within sulfides along with their chemical and isotopic compositions throughout the studied intervals further support the magmatic origin of sulfide enrichment at the layer boundaries. The studied magmatic layers were likely formed as a result of intrusion of more primitive magma (fine-grained gabbro) into the former crystal mush (coarse-grained gabbro). Sulfides from the coarse-grained gabbros are Ir-Platinum Group Element-rich (PGE; i.e., Ir, Os, Ru) but those from the fine-grained gabbros are Pd-PGE-rich (i.e., Pd, Pt, Rh). Notably, the sulfides from the layer boundaries are also enriched in Pd-PGEs, and therefore elevated sulfide contents at the boundaries were likely related to the new intruding melt. Because S concentration at sulfide saturation level is dependent on the Fe content of the melt, sulfide crystallization may have been caused by FeO loss, both via crystallization of late-precipitating oxides at the boundaries, and by exchange of Fe and Mg between melt and Fe-bearing silicates (olivine and clinopyroxene). The increased precipitation of sulfide grains at the layer boundaries might be widespread in the lower oceanic crust, as also observed in the Semail ophiolite and along the Mid-Atlantic Ridge. Therefore, this process might affect the metal budget of the global lower oceanic crust. We estimate that up to ∼20% of the Cu, ∼8% of the S, and ∼84% of the Pb of the oceanic crust inventory is accumulated at the layer boundaries only from the interaction between crystal mush and new magma.

Plastic Deformation of Plagioclase in Oceanic Gabbro Accreted at a Slow‐Spreading Ridge (Hole U1473A, Atlantis Bank, Southwest Indian Ridge)

AbstractCrustal architecture at slow‐spreading oceanic ridges results from complex interactions between magmatism, hydrothermalism, and tectonics. IODP Hole U1473A was drilled during Expeditions 360 and 362T at the summit of the Atlantis Bank, a gabbroic massif exhumed at the Southwest Indian Ridge. In this study, we identify and quantify plastic deformation processes in gabbroic lithologies and active slip systems in plagioclase from 115 microstructural domains throughout Hole U1473A. We describe deformed zones using petrographic observations and electron backscattered diffraction analyses made all along the core. Ductile deformation is widespread, and in places strongly localized in mylonitic and ultramylonitic zones. Plagioclase represents ∼60% of rock's volume and is the dominant phase accommodating deformation in samples. It shows strong dynamic recrystallization accommodated by subgrain rotation in the dislocation creep regime, forming a fine‐grained matrix. Electron backscattered diffraction analyses reveal weak to moderate crystallographic preferred orientations of plagioclase as a result of plastic deformation and strain localization, producing a fabric characterized by (010) parallel to the foliation plane and [100] parallel to the lineation. The fabric strength is first increasing from slightly deformed lithologies to mylonites before decreasing significantly in ultramylonites. This could be explained by orientation scattering after recrystallization, and a change of active slip systems. Subsequent granular flow has likely occurred in some samples. A detailed investigation of intracrystalline misorientations measured at plagioclase subgrain boundaries reveals the activity of four dominant slip systems: [001](010), [100](001), ½[110](001), and ½[](001). These slip systems reflect decreasing temperatures during CPO development and subgrain wall formation.

Magma‐Mush Interactions in the Lower Oceanic Crust: Insights From Atlantis Bank Layered Series (Southwest Indian Ridge)

AbstractMagma migration and differentiation processes are key to understanding the development and evolution of oceanic magma reservoirs. To provide new quantitative geochemical constraints on these processes, we applied a high‐resolution approach to study an interlayered section of the lower oceanic crust sampled at Atlantis Bank, on the (ultra)slow‐spreading Southwest Indian Ridge. The section is characterized by sharp grain‐size layering between fine‐ and coarse‐grained olivine gabbros that is representative of other layered structures described at the Atlantis Bank oceanic core complex. The textures and fabrics of the layers and the nature of their contacts indicate formation by intrusion of a magma (i.e., crystal‐bearing) into an almost solidified coarse‐grained mush. Petrographic observations and in situ incompatible trace element signatures indicate that the fine‐ and coarse‐grained layers record reactive porous migration of melts. Widespread reactive porous flow occurred prior to intrusion within the coarse‐grained gabbro, producing mineral compositions enriched in incompatible elements. The intrusive fine‐grained lithology records a late stage event of localized reactive melt percolation in cm‐scale structures, which lead to strong light rare earth elements depletion relative to heavy rare earth elements. In addition, we highlight the occurrence of interactions at the contacts between layers and partial modification in compositions of the intruded lithology. This layered section likely represents a contact between two larger magma bodies emplaced within the lower crust during accretion, where the type of melt migration (intrusion or porous flow) and the modalities of melt percolation (widespread or localized) strongly govern the composition of the crustal lithologies.

Microbial Abundance and Diversity in Subsurface Lower Oceanic Crust at Atlantis Bank, Southwest Indian Ridge.

International Ocean Discovery Program Expedition 360 drilled Hole U1473A at Atlantis Bank, an oceanic core complex on the Southwest Indian Ridge, with the aim of recovering representative samples of the lower oceanic crust. Recovered cores were primarily gabbro and olivine gabbro. These mineralogies may host serpentinization reactions that have the potential to support microbial life within the recovered rocks or at greater depths beneath Atlantis Bank. We quantified prokaryotic cells and analyzed microbial community composition for rock samples obtained from Hole U1473A and conducted nutrient addition experiments to assess if nutrient supply influences the composition of microbial communities. Microbial abundance was low (≤104 cells cm-3) but positively correlated with the presence of veins in rocks within some depth ranges. Due to the heterogeneous nature of the rocks downhole (alternating stretches of relatively unaltered gabbros and more significantly altered and fractured rocks), the strength of the positive correlations between rock characteristics and microbial abundances was weaker when all depths were considered. Microbial community diversity varied at each depth analyzed. Surprisingly, addition of simple organic acids, ammonium, phosphate, or ammonium plus phosphate in nutrient addition experiments did not affect microbial diversity or methane production in nutrient addition incubation cultures over 60 weeks. The work presented here from Site U1473A, which is representative of basement rock samples at ultraslow spreading ridges and the usually inaccessible lower oceanic crust, increases our understanding of microbial life present in this rarely studied environment and provides an analog for basement below ocean world systems such as Enceladus. IMPORTANCE The lower oceanic crust below the seafloor is one of the most poorly explored habitats on Earth. The rocks from the Southwest Indian Ridge (SWIR) are similar to rock environments on other ocean-bearing planets and moons. Studying this environment helps us increase our understanding of life in other subsurface rocky environments in our solar system that we do not yet have the capability to access. During an expedition to the SWIR, we drilled 780 m into lower oceanic crust and collected over 50 rock samples to count the number of resident microbes and determine who they are. We also selected some of these rocks for an experiment where we provided them with different nutrients to explore energy and carbon sources preferred for growth. We found that the number of resident microbes and community structure varied with depth. Additionally, added nutrients did not shape the microbial diversity in a predictable manner.

Role of compaction in melt extraction and accumulation at a slow spreading center: Microstructures of olivine gabbros from the Atlantis Bank (IODP Hole U1473A, SWIR)

The exposure of gabbroic sequences at Oceanic Core Complexes (OCC) along ultraslow- to slow-spreading ridges permits the study of the processes forming the lower oceanic crust. On top of the Atlantis Bank OCC along the ultraslow-spreading Southwest Indian Ridge, IODP Expedition 360 drilled Hole U1473A, mainly composed of primitive olivine gabbros interspersed with more evolved Ti-Fe oxide-bearing gabbros and minor felsic veins. These rocks record a complex history of protracted magmatism during continuous uplift and deformation of the gabbroic sequence. Extensive crystal-plastic deformation is dominantly recorded in the shallower sections of the drillhole, whereas the deeper sections better preserve primary magmatic features. We focus on microstructures, including intra-crystalline deformation of rock-forming minerals, and plagioclase crystallographic preferred orientations of olivine gabbros lacking evidence for exhumation-related crystal plastic deformation, to gain insights on the relationship between compaction, melt migration and melt accumulation during the early magmatic history of this section of lower oceanic crust. Olivine gabbros are characterized by ubiquitous grain-size variations, from coarse- to fine-grained intervals. Minerals in coarse-grained intervals show intra-crystalline deformation, while fine-grained crystals lack internal strain. Bent coarse-grained plagioclase associated with weak magmatic foliation and lack of lineation suggest that the coarse-grained intervals were deformed under weak compaction. On the other hand, crystallographic preferred orientations of undeformed fine-grained plagioclase show weak lineations, likely indicative of non-coaxial strain. We thereby infer that the coarse-grained intervals underwent ongoing weak compaction from the stage of olivine + plagioclase ± clinopyroxene crystal mush to the melt-poor stage, and that this process likely aided melt extraction and accumulation in discrete melt-rich zones where crystals orientated in the direction of magmatic flow. Crystallization of melts in the melt-rich zones ultimately formed the fine-grained intervals at different depths in Hole U1473A. This indicates that processes of compaction can lead to local chemical and grain-size heterogeneities in a lower crustal section, while had a minor role in the melt movement at larger scales (e.g., the whole crystal mush) within the oceanic crust.

Deformation mechanisms of granulite-facies mafic shear zones from hole U1473A, Atlantis Bank, Southwest Indian Ridge (IODP Expedition 360)

Lower-crustal shear zones from hole U1473A (IODP Expedition 360) were studied via quantitative microstructural analysis and thermodynamic modelling to constrain deformation conditions during detachment faulting. Porphyroclasts of clinopyroxene and orthopyroxene, plagioclase and olivine are included in a fine-grained, polyphase matrix that contains plagioclase-rich layers. Microfractures occur in orthopyroxene, and core-mantle structures are common in all porphyroclasts. Crystallographic fabrics in clinopyroxene clasts indicate activation of (010)[001] slip system, whereas the rimming neoblasts show activity of both (010)[001] and (001)[100]. Fabrics of plagioclase-rich layers suggest the activation of the (010)[100] slip system. Phase mixing and weak crystallographic fabrics in the polyphase matrix point to oriented-growth during diffusion-assisted grain boundary sliding. Thermodynamic modelling indicates that the gabbroic shear zones formed at ~900–920 °C and 2.2–2.7 kbar, under melt-present conditions, and re-equilibrated down to 835 °C during exhumation, as indicated by hornblende–plagioclase thermometry. Our results suggest that deformation in the lower parts of Atlantis Bank was accommodated by a combination of brittle fragmentation and viscous flow during in-situ melt-consumption back-reaction. Such mechanisms effectively resulted in strain localisation in fine-grained, polyphase shear zones that contributed to the weakening of the ocean crust during detachment faulting and subsequent exhumation of the Atlantis Bank core complex.

MORB Melt Transport through Atlantis Bank Oceanic Batholith (SW Indian Ridge)

AbstractThe Atlantis Bank Oceanic Batholith is a 660 km2 gabbro massif representing the plutonic foundation of a major ridge magmatic center. It was continuously accreted, emplaced, and exposed in the rift mountains of the paleo-SW Indian Ridge from 13 to 10·3 Ma. Ocean Drilling Program (ODP) Hole 735B, drilled to 1508 m at Atlantis Bank, recovered evolved intercalated olivine and oxide gabbros representing the upper levels of the lower ocean crust. Within this section, ∼5·6 m of primitive chromian-spinel-bearing troctolites (0·1–2 m thick), with sharp modal contacts with the host gabbros, were cored between 410 and 500 m depth. Here we present new mineral (chromian spinel, clinopyroxene, olivine, and plagioclase) major and trace element and petrographic data for the troctolite suites (i.e. troctolites, clinopyroxene-rich troctolites, troctolitic gabbros) from 410 to 528 mbsf (meters below seafloor) and olivine gabbros in the 0–274 mbsf and 410–500 mbsf intervals, and examine the origin of these troctolite layers. Equilibrium melts for the troctolites are primitive to moderately evolved, with Mg# [= 100Mg/(Mg + Fe), 48–68 mol%] comparable with those of Atlantis Bank basalts. By contrast, equilibrium melts for both the 0–274 mbsf and 410–500 mbsf host olivine gabbros are highly to moderately evolved (Mg# 27–63 mol%). The 410–500 mbsf troctolite suites maintain high clinopyroxene Mg# (81–89 mol%) and mineral concentrations of compatible elements (i.e. clinopyroxene Cr2O3 0·1–1·2 wt%, Ni 151–330 μg g–1 and olivine Ni 1055–1559 μg g–1) with the increase in incompatible trace element abundances (i.e. clinopyroxene TiO2 0·3–2·1 wt%, Zr 5·8–112 μg g–1 and olivine Ti up to 293 μg g–1). Combined with abundant dissolution–reprecipitation textures, our results indicate that the troctolites were formed by the reaction between a spinel-bearing, troctolitic mush, from which they inherited the high Mg#, Ni and Cr, and percolating melts adding incompatible trace elements. Moreover, the spinel (NiO versus TiO2) and olivine (Ti versus Y) trace element compositions indicate that the troctolites were affected by low-degree Fe–Ti-rich melt metasomatism. In contrast, both the 0–274 mbsf and 410–500 mbsf olivine gabbros display a prominent decrease in clinopyroxene Mg# (from 88 to 66) and mineral compatible element concentrations (i.e. clinopyroxene Cr2O3 from 1·1 to ∼0 wt%, Ni from 208 to 34 μg g–1 and olivine Ni from 1200 to 136 μg g–1) with increasing incompatible trace element abundances (e.g. clinopyroxene Zr from 4·8 to 157 μg g–1). These features are compatible with the reactive porous flow of slightly to highly evolved melts through the cooling crystal mush zone. Our results, combined with the literature data, indicate that most olivine gabbros between 410 and 500 mbsf were formed prior to the troctolite layers. We document that the troctolites represent conduits for mid-ocean ridge basalt melt transport through the lower oceanic crust, whereas the olivine gabbros represent crystallization of a large crystal mush, recording initial gabbro emplacement, hyper- and sub-solidus deformation, and melt–rock reaction owing to upward penetrative flow of intercumulus melt.

Grain Size Variations Record Segregation of Residual Melts in Slow‐Spreading Oceanic Crust (Atlantis Bank, 57°E Southwest Indian Ridge)

AbstractBeneath slow‐spreading ridges, melt bodies are generally considered to represent ephemeral magma reservoirs filled with crystal mushes. Formation of the oceanic crust requires at least partial extraction of melts from these crystal mushes. However, melts collection and extraction are processes yet to be fully constrained. We investigate olivine gabbros from the plutonic section recovered at the IODP Hole U1473A, in the Atlantis Bank Oceanic Core Complex (Southwest Indian Ridge), to unravel (i) the process of melt migration through lower crustal crystal mushes, and (ii) the collection and segregation of melts forming discrete microgabbro intervals. Throughout the Hole, fine‐ to coarse‐grained intervals are widespread in olivine gabbros. Along the contacts, coarse‐grained minerals display resorbed grain boundaries against the fine‐grained minerals, suggesting partial dissolution by the melt crystallizing the fine‐grained material. Coarse‐grained plagioclase and clinopyroxene are zoned, showing progressive chemical evolution from more primitive crystal cores to more evolved crystal rims. Fine‐grained minerals are unzoned and chemically similar to rims of coarse‐grained minerals, indicating a genetic relationship. We attribute significant enrichments in the most incompatible elements of plagioclase and clinopyroxene to a magma evolution process associated with reactive melt migration. As temperature decreased, melts residual from the reactive processes were segregated in magma pockets that ultimately crystallize the fine‐grained intervals (microgabbros). We document, for the first time, that these microgabbros are crystallization products of melts modified by reactive melt migration; the melts were extracted from the crystal mush and accumulated into discrete melt‐rich zones. This process could have promoted partial extraction of those melts that in turn potentially contribute to Mid Ocean Ridge Basalts erupted at the seafloor.

Fractional crystallization causes the iron isotope contrast between mid-ocean ridge basalts and abyssal peridotites

The iron isotope contrast between mid-ocean ridge basalts and abyssal peridotites is far greater than can be explained by mantle melting alone. Here we investigate a suite of mid-ocean ridge magma chamber rocks sampled by the Ocean Drilling Project Hole 735B in the Atlantis Bank of the Indian Ocean. We report major and trace element geochemistry from these rocks and measure their iron isotope compositions to investigate the potential role of fractional crystallization during melt evolution. We observe a large range of δ56Fe that defines a significant inverse curvilinear correlation with bulk rock MgO/FeOT. These data confirm that δ56Fe in the melt increases as fractional crystallization proceeds but, contrary to expectation, δ56Fe continues to increase even when oxides begin to crystallize. We conclude that iron isotope fractionation through fractional crystallization during the evolution of mid-ocean ridge basalts from abyssal peridotites reconciles the disparity in isotopic compositions between these two lithologies.