Southwest Indian Ridge Research Articles

Distribution extension of a vent scale worm Branchinotoglumabipapillata (Polychaeta, Polynoidae) in the Indian Ocean.

Branchinotogluma Pettibone, 1985 is the most species-rich genus within the subfamily Lepidonotopodinae Pettibone, 1983, comprising 18 valid species from chemosynthesis-based ecosystems in the Pacific and Indian Oceans. Here, we report a new distributional record of Branchinotoglumabipapillata Zhou, Wang, Zhang & Wang, 2018, at the hydrothermal vent sites on the northern Central Indian Ridge (nCIR). This record represents the northernmost occurrence of B.bipapillata in the Indian Ocean. We conducted a comparative study of the nCIR population and other documented populations using distributional information, morphological traits, and genetic markers (two mitochondrial [COI, 16S rRNA] and one nuclear [18S rRNA] genes). While most morphological characters of B.bipapillata were consistent with those found in the Southwest Indian Ridge (SWIR), variations were noted in the segment with the last branchiae. Molecular data revealed that all populations of B.bipapillata form a single clade, indicating a wide distribution from the SWIR to nCIR, covering ~4,000 km across various ridges in the Indian Ocean. This study presents extensive distribution of a vent species with well-connected populations throughout the Indian Ocean, distinguishing it from many other vent species affected by the dispersal barrier in the Indian Ocean.

Unraveling abiotic organic synthesis pathways in the mafic crust of mid-ocean ridges

The aqueous alteration of the oceanic lithosphere provides significant energy that impacts the synthesis and diversity of organic compounds, which are crucial for the deep carbon cycle and may have provided the first building blocks for life. Although abiotic organic synthesis has been documented in mantle-derived rocks, the formation mechanisms and complexity of organic compounds in crustal rocks remain largely unknown. Here, we show the specific association of aliphatic carbonaceous matter with Fe oxyhydroxides in mafic crustal rocks of the Southwest Indian Ridge (SWIR). We determine potential Fe-based pathways for abiotic organic synthesis from CO2 and H2 using multimodal and molecular nano-geochemical tools. Quantum mechanical modeling is further employed to constrain the catalytical activity of Fe oxyhydroxides, revealing that the catalytic cycle of hydrogen may play a key role in carbon-carbon bond formation. This approach offers the possibility of interpreting physicochemical organic formation and condensation mechanisms at an atomic scale. The findings expand our knowledge of the existence of abiotic organic carbon in the oceanic crustal rocks and emphasize the mafic oceanic crust of the SWIR as a potential site for low-temperature abiotic organic synthesis.

Zinc isotopic composition of oceanic crust: Insights from oceanic gabbro cumulates and MORBs

Subducting oceanic crust is a critical driver of chemical and lithological heterogeneity within the deep mantle. Zinc isotopes (δ66ZnJMC-Lyon) have been widely utilized to trace recycled oceanic crust and surficial materials. However, the lower oceanic crust rocks are not well studied given limited sampling, making it unclear if the δ66Zn estimated from mid-ocean ridge basalts (MORBs) is valid to represent that of the bulk oceanic crust. In this study, we report δ66Zn data for a series of gabbro cumulates (n = 37) from the ultraslow-spreading Southwest Indian Ridge, alongside MORBs (n = 12) from the fast-spreading East Pacific Rise and the slow-spreading South Mid-Atlantic Ridge. MORBs across various spreading rates exhibit homogeneous δ66Zn values (0.27 ± 0.05 ‰, 2sd), in agreement with previous studies. In contrast, the gabbros with several magma episodes exhibit a significant variation of δ66Zn from 0.11 ‰ to 0.34 ‰, with an average of 0.22 ± 0.11 ‰ (2sd). Magma differentiation to form oceanic crust can partly explain such variation (∼ 0.10 ‰), and post-cumulus modification further extends this range. Given the voluminous lower oceanic crust, the weighted δ66Zn estimated for the bulk oceanic crust (0.23 ± 0.03 ‰) is lower than the MORB-based value, which reconciles with the predictions from mantle partial melting. Therefore, recycled oceanic crust with significantly varied Zn isotopic compositions would lead to heterogeneous δ66Zn of the metasomatized mantle. The overall lower δ66Zn value underscores the importance of surficial materials other than oceanic crust, such as carbonates, to explain high δ66Zn of many mantle-derived magmas.

A 3‐D Seismic Tomographic Study of Spreading Structures and Smooth Seafloor Generated by Detachment Faulting—The Ultra‐Slow Spreading Southwest Indian Ridge at 64°30′E

AbstractAt ultra‐slow spreading ridges, with full spreading rates less than ∼20 mm/yr, spreading is accommodated both by highly spatially and temporally segmented magmatism, and tectonic extension along large‐scale detachment faults that exhume ultramafic material to the seafloor. In the most magma‐poor regions, detachment faulting alternates in polarity over time, producing a “flip‐flopping” effect of subsequent detachment dips. The resulting seafloor in these regions displays a morphology termed “smooth seafloor” comprising elongate, broad ridges with peridotite/serpentinite lithologies. We conducted tomographic travel‐time inversion of a 3‐D wide‐angle seismic data set acquired over a region of smooth seafloor around 64°30′E along the Southwest Indian Ridge (SISMOSMOOTH; Cruise MD199), to produce a seismic velocity volume through the crustal section and into the uppermost mantle. We observe patterns of velocity anomalies that correspond with variations in the bathymetry arising from the mode of spreading and are interpreted as changes in the degree of alteration with depth resulting from spatial and temporal variations in fluid‐rock interaction, controlled by faulting and tectonic damage processes. The detachment faults do not show simple planar structures at depth but instead mirror the shapes of the bathymetric ridges that they exhume. Magmatic input is overall highly limited, but there is one region on the lower part of an exhumed detachment footwall where a thickness of volcanic material is observed that suggests a component of syn‐tectonic volcanism, which could contribute to detachment abandonment.

Spatial variations in the effective elastic thickness of the Indian Ocean lithosphere

The Indian Ocean lithosphere is a complex assemblage of large igneous provinces, seamounts, plateaus and ridges of different loading ages and tectono-thermal evolution. As a proxy for the strength of tectonic plates, effective elastic thickness (Te) illustrates the relationship between surface deformation and lithospheric rheology of the diverse provinces in response to long-term tectonic processes. Mapping the spatial variations in lithospheric rheology can aid in understanding the detailed tectono-thermal history of the Indian Ocean. In this paper, we perform an assessment of the spatial variation of Te for the Indian Ocean from the inversion of the real free-air admittance between free-air gravity anomalies and bathymetry corrected for the effect of density variations within sediments using a continuous wavelet spectral analysis. Incorporating the effect of sediments substantially reduces Te estimates and better corresponds with the tectonic units in the study region. The results show low overall Te over the Indian Ocean attributed to magmatism and temperature during a multistage opening process. We further demonstrate that temperature controls the strength of warm and young oceanic lithosphere, evidenced by the positive correlation between Te and geothermal proxies. Finally, moderately low Te values at the Southwest Indian Ridge suggest a relatively cold ultraslow lithosphere with sparse magmatism compared to typical mid-ocean ridges.

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

Formation of oceanic crust within the Andrew Bain fault zone of the Southwest Indian ridge (Petrological and geochemical data)

A petrogeochemical study of basalts (lithophile elements and Sr-Nd-Pb isotopes, compositions of liquidus olivine and spinel) from the transition zone of the Southwest Indian Ridge in the area of the Du Toit and Andrew Bain faults revealed significant differences in their composition. Within the rift valley adjacent to the faults, tholeiites enriched in Na and depleted in Fe (Na-TOR genetic type) are typical. Deep-type basalts (TOR-1) are present in the western side of the Andrew Bain Fault. The outpouring of these types of magmas reflects a possible change in geodynamic regime during this zone formation: from deeper and higher temperature melting to shallower ones (Sushchevskaya et al., 2022). Differences in the primary melts of tholeiites from the rift valley and the Andrew Bain Transform Fault are also traced in the liquidus olivine compositions. The rift valley olivines are similar to typical Na-TOR olivines with a Mg content of Fo88–87, low Ni and elevated Mn. On the contrary, tholeiite olivines of the Andrew Bain Fault are enriched in Ni and depleted in Mn, which may indicate pyroxenite included in the primary melt formation. This component is either oceanic lithosphere recycled through the deep mantle or fragments of previously formed oceanic crust, which are subsequently involved in melting during the spreading axes jumping. A similar process is typical for the region of the Bouvet Triple Junction, where a significant heterogeneity of the olivine composition in terms of trace-element contents was revealed. The isotope characteristics of the Andrew Bain Fault tholeiites differ in Pb and Sr radiogenic composition and are similar to those of enriched magmas from such Indian Ocean rises as Crozet, Marion and Bouvet, but not from the Konrad and Af. Nikitin Rises. The source of such tholeiite melts is close in composition to the model HIMU type (with high U/Pb), possibly with an admixture of mantle material with EMII characteristics (with elevated Rb/Sr).

Mg and Fe isotope compositions of mid-ocean ridge basalts modified by Mg-Fe inter-diffusion during melt transport

We analysed forty-nine, hand-picked, mid-ocean ridge basalt (MORB) glasses for their Mg isotopic compositions using a high-precision, critical mixture double-spiking approach. Additionally, the Fe isotopic compositions of a subset (twenty-two) of these samples were measured. Samples from the East Pacific, Mid-Atlantic, South East Indian and East Scotia ridges have average δ26Mg ∼ −0.21 ‰ and δ57Fe ∼0.10 ‰, while those from the ultra-slow spreading Gakkel and South West Indian ridges have lighter Mg isotope compositions, δ26Mg as low as ∼ −0.32 ‰, and heavier Fe isotope compositions, δ57Fe up to 0.25 ‰. Overall, the samples show a striking negative correlation between δ26Mg and δ57Fe. Few MORB have δ26Mg as high as predicted by equilibrium models of melting and differentiation; in complementary fashion, measured δ57Fe are typically higher than modelled, equilibrium values. Furthermore, we show that the slope of 26Mg/24Mg and 25Mg/24Mg covariations in MORB is consistent with kinetic not equilibrium fractionation. These observations identify an important diffusive control on both Mg and Fe isotopic compositions of MORB. Fractional transport of melt beneath ridges juxtaposes low Mg/Fe fractional melts from depth with high Mg/Fe residues at the top of the melting column. We argue that diffusively limited, partial Mg-Fe exchange between melt and peridotite through which it migrates leads to melts becoming isotopically lighter in Mg and heavier in Fe. Low melt rock ratios from sub-adiabatic melting beneath the slowest spreading ridges result in the greatest diffusive exchange between peridotite and melt en route to surface.

GOLD ENRICHMENT MECHANISM IN MID-OCEAN RIDGE HYDROTHERMAL SYSTEMS: AN EXAMPLE FROM THE LONGQI HYDROTHERMAL FIELD ON THE ULTRASLOW-SPREADING SOUTHWEST INDIAN RIDGE

Abstract In mid-ocean ridge (MOR) hydrothermal systems, the gold grade of sea-floor massive sulfides (SMSs) is negatively correlated with the spreading rate of the ridge. Previous investigations have addressed the distribution of gold in sulfides from hydrothermal fields hosted by ultramafic rocks. In contrast, the gold enrichment mechanisms in sulfides from hydrothermal fields hosted by mafic rocks in ultraslow-spreading ridge environments are less well constrained. The basalt-hosted Longqi hydrothermal field, located on the classic ultraslow-spreading Southwest Indian Ridge, provides an opportunity to examine gold enrichment mechanisms in such an environment. Two ore-forming stages are identified in chimney fragments: anhydrite + barite + colloidal/porous pyrite (Py1) + marcasite + fine-grained sphalerite (stage 1); euhedral-subhedral pyrite (Py2) + coarse-grained sphalerite + chalcopyrite + isocubanite (stage 2). Py1 is usually overgrown by marcasite, which is in turn enclosed by Py2. Py2 coexists with coarse-grained sphalerite and chalcopyrite. Abundant native gold nanoparticles occur in Py1 or at the transition zone between Py1 and Py2. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis suggests that Py1 contains higher Mo, V, Sn, and Pb and lower As, Co/Ni, and Se/Tl values compared to Py2. In situ LA-multicollector (MC)-ICP-MS analyses show that Py1 has a higher mean δ34S (+7.1‰) value than Py2 (+6.6‰). Sulfur primarily derives from MORB and seawater sulfate, of which the proportion of sulfur from seawater sulfate is between 20.5 and 47.6%. Textures, mineral assemblages, and trace element contents of sulfides indicate that the degree of mixing between hydrothermal fluids and seawater decreases as the chimney grows and is accompanied by a gradual increase in temperature. Based on data compiled from 41 hydrothermal fields hosted by basalt, the large range of sulfide δ34S from slow- and ultraslow-spreading MORs may be attributed to the wide range of sulfur sources (e.g., leaching from MOR basalt, thermochemical reduction of seawater sulfate, magma degassing, and bacterial activity), fluid-basalt interaction, and redox state (CH4/CO2 ratios). Prolonged fluid-basalt interaction and the type of chimneys, such as beehive chimneys, may lead to higher gold grades in hydrothermal fields. Moreover, low H2S content may be an important contributor to gold enrichment in basalt-hosted SMS deposits in ultraslow-spreading MOR environments.

Deep Argo Observations of Antarctic Bottom Water in the Deep Fracture Zones of the Southwest Indian Ridge

AbstractThe Madagascar Basin is the primary pathway for Antarctic Bottom Water to ventilate the entire western Indian Ocean as part of the Global Overturning Circulation. The only way for this water mass to reach this basin is by crossing the Southwest Indian Ridge through its deep fracture zones. However, due to the scarcity of observations, the Antarctic Bottom Water presence has only been well‐established in the Atlantis II fracture zone. In May 2023, the Deep Madagascar Basin Experiment deployed three Deep SOLO Argo floats in the exit of the fracture zones that were more likely to transport Antarctic Bottom Water: Atlantis II, Novara, and Melville. These floats have been collecting temperature and salinity profiles every 3–5 days with high vertical resolution in the deep ocean. In the present paper, we use the first 7 months of float data to characterize the Antarctic Bottom Water in the deep fracture zone area, revisiting a half‐century puzzle about the Melville contribution. We also collected shipboard‐based profiles to calibrate float salinity and show it is within the Deep Argo program target accuracy. We find Antarctic Bottom Water in both Melville and Novara fracture zones, not only in Atlantis II. This is the first time the Novara contribution has been revealed. The floats also uncover their distinct properties, which may result from the different mixing histories.

A hydrothermal plume on the Southwest Indian Ridge revealed by a multi-proxy approach: Impact on iron and manganese distributions (GEOTRACES GS02)

Iron (Fe) and manganese (Mn) are crucial micronutrients that limit oceanic primary productivity in the Southern Ocean. It has been recently suggested that hydrothermal activity may be an important source of oceanic dissolved iron, yet, this contribution is still not fully understood and only one active hydrothermal site has been reported on the Southwest Indian Ridge (SWIR), south of 40°S.Using a multi-proxy approach, this study demonstrates the occurrence of hydrothermal venting on the SWIR in the near vicinity of the location 44°51.690 S, 36°10.460 E, which is likely to be a low or moderately high temperature fluid. Indeed, we report high values of dissolved methane to manganese ratios (up to 11.1 ± 1.2 mol mol−1), low particulate iron (pFe) and manganese (pMn) concentrations (with maximum values of 0.7 nmol L−1 and 0.06 nmol L−1, respectively) associated with the presence of few oxyhydroxides, as well as high 223Radium (Ra) and 224Ra activities near the seafloor. The Fe and Mn data revealed a significant enrichment at depths influenced by hydrothermal circulation on the seafloor, within the Upper Circumpolar Deep Water. Dissolved Fe (dFe) and dissolved Mn (dMn) concentrations were enriched by 3- and 7-fold, respectively, and pFe and pMn by 2- and 1.5-fold, respectively, compared to a reference station located outside the SWIR. They were however lower than concentrations reported so far near high temperature vents, suggesting a weaker influence of this hydrothermal system on deep Fe and Mn reservoirs. We show that a large fraction of the dFe could be stabilized by organic complexation with humic substances (eHS, estimated 27–60% of dFe). High prokaryotic abundance related to the proximity of the hydrothermal vent suggests that other Fe-complexing ligands of biological origin might also stabilize Fe in its dissolved form. Collectively, these measurements integrated within the concept of a “multi-proxy approach”, helped painting a more detailed picture of the complex interactions and processes in this region of the SWIR. Although the system is a source of both dFe and dMn to the deep ocean, the low current velocities and the bathymetry likely limit the fertilization of surface water by dFe and dMn along this section of the SWIR.

Coupled Detachment Faulting and Hydrothermal Circulation at 49.7°E Southwest Indian Ridge Revealed by Seafloor Magnetism

AbstractDetachment faults at slow‐ and ultraslow‐spreading mid‐ocean ridges are essential for lithosphere‐hydrosphere interactions. However, their coupling with subseafloor hydrothermal circulation is poorly understood. Here, we investigated shallow hydrothermal fluid circulations in the active Dragon Horn detachment fault system of the Southwest Indian Ridge using combined near‐bottom magnetic anomaly data and rock magnetic analyses. We observed enhanced magnetic anomalies related to the high magnetization of basaltic country rocks and highly serpentinized (>80%) peridotite, whereas the hydrothermally altered basalts and sulfide deposits had weak magnetization and reduced magnetic anomalies. Therefore, the low‐magnetization zone was interpreted as a proxy for the hydrothermal fluid channel. The spatial distribution of the low‐magnetization zone revealed that the inactive Suye field expanded more than 500 m in diameter horizontally and ∼100 m vertically, and the active Longqi hydrothermal field had a >1 km fluid channel, both centered around the detachment fault fracture zone. Our results establish a robust link between detachment faulting and shallow hydrothermal circulation in which the detachment fault system provides critical fluid paths. Therefore, detachment tectonics are important elements for evolving seafloor hydrothermal fields along the Southwest Indian Ridge and possibly other slow‐ and ultraslow‐spreading mid‐ocean ridges globally.

The Redox State of the Asthenospheric Mantle and the Onset of Melting Beneath Mid‐Ocean Ridges

AbstractThe redox state of the convective asthenospheric mantle governs the speciation of volatile elements such as carbon and, therefore, influences the depth at which (redox) melting can occur, with implications for seismic signals. Geophysical observations suggest the potential presence of carbonatite melts at a depth of 200–250 km. However, thermodynamic models indicate that the onset of (redox) melting would occur at 100–150 km for a mantle with 3%–4% of Fe3+/∑Fe. Here, we present a new oxybarometer that is based on the V/Sc exchange coefficient between olivine and the melt, which is insensitive to surficial alteration, volatile degassing, electron exchange reactions and fractional crystallization. By applying this method to primary mid‐ocean ridge basalts (MORBs) from Southwest Indian Ridge and East Pacific Rise, we demonstrate that the average oxygen fugacity (fo2) of MORBs corrected for the depth of formation is 0.78 ± 0.26 (1σ) log units above the fayalite‐magnetite‐quartz (FMQ) buffer, which is slightly more oxidized than previously estimated (near FMQ buffer). Our findings indicate that the convective asthenospheric mantle exhibits higher oxygen fugacity than the continental lithospheric mantle. Along an adiabat, carbonatitic melts can form from a CO2‐bearing source at a depth of 200–250 km, explaining the asthenospheric mantle's electrical conductivity and seismic velocity anomaly.

Teleseismic Indication of Magmatic and Tectonic Activities at Slow- and Ultraslow-Spreading Ridges

Magmatic and tectonic processes in the formation of oceanic lithosphere at slow–ultraslow-spreading mid-ocean ridges (MORs) are more complicated relative to faster-spreading ridges, as their melt flux is overall low, with highly spatial and temporal variations. Here, we use the teleseismic catalog of magnitudes over 4 between 1995 and 2020 from the International Seismological Center to investigate the characteristics of magmatic and tectonic activities at the ultraslow-spreading Southwest Indian Ridge and Arctic Gakkel Ridge and the slow-spreading North Mid-Atlantic Ridge and Carlsberg Ridge (total length of 14,300 km). Using the single-link cluster analysis technique, we identify 78 seismic swarms (≥8 events), 877 sequences (2–7 events), and 3543 single events. Seismic swarms often occur near the volcanic center of second-order segments, presumably relating to relatively robust magmatism. By comparing the patterns of seismicity between ultraslow- and slow-spreading ridges, and between melt-rich and melt-poor regions of the Southwest Indian Ridge with distinct seafloor morphologies, we demonstrate that a lower spreading rate and a lower melt supply correspond to a higher seismicity rate and a higher potential of large volcano-induced seismic swarms, probably due to a thicker and colder lithosphere with a higher degree of along-axis melt focusing there.

Stable Chromium Isotope Fractionation During the Alteration of Abyssal Peridotite: Implications for Marine Chromium Isotope Mass Balance

AbstractThe stable chromium isotope system has been widely used as a redox proxy to reconstruct the oxygenation history of ocean atmosphere systems. However, the Cr isotope mass balance in modern oceans (i.e., inputs and outputs) remains poorly constrained. To investigate the influence of seawater‐peridotite reaction on the global marine Cr isotope mass balance, we report high‐precision Cr isotope data (δ53Cr) on a series of fresh and altered abyssal peridotites from the Gakkel Ridge and the Southwest Indian Ridge (SWIR). The least altered peridotites give a δ53Cr value of −0.08 ± 0.06‰ (2SD, n = 4) for the oceanic mantle which is consistent with the established δ53Cr of the Bulk Silicate Earth. Compared to fresh peridotites, a subset of altered peridotites exhibit a loss of isotopically light Cr with relatively positive δ53Cr values (up to 0.04‰). These altered peridotites are characterized by significant Cr loss and likely have been subject to serpentinization. By contrast, seafloor weathering has limited influence on the Cr concentrations and isotopic compositions of the altered peridotites. Monte Carlo (MC) simulations of marine alteration suggest a net Cr flux into seawater from altered abyssal peridotites of ∼3.5 × 108 mol/yr, which is on the same order of magnitude as the riverine input flux of 108–109 mol/yr. Furthermore, the MC results suggest that the peridotite‐sourced net Cr flux has a negative δ53Cr signature (−0.33 ± 0.21‰, 2SD). Thus, seawater‐peridotite interactions must be considered when evaluating the modern oceanic Cr isotope mass balance.

Geochemical and Sr‐Nd‐Pb‐Fe Isotopic Constraints on the Formation of Fe‐Si Oxyhydroxide Deposits at the Ultraslow‐Spreading Southwest Indian Ridge

AbstractModern Fe‐Si oxyhydroxide deposits occur in global marine hydrothermal vent sites. Despite their role as biogenic substrates and potential ore resources, much remains unknown about their formation processes. Here, we apply analyses of major and trace elements as well as Sr‐Nd‐Pb‐Fe isotopes combined with 238U‐230Th dating to Fe‐Si oxyhydroxides obtained from several hydrothermal fields along the Southwest Indian Ridge. These mineralized oxyhydroxides primarily consist of poorly crystalline two‐line ferrihydrite and amorphous opal‐A, with lesser amounts of nontronite and birnessite. The ubiquitous and characteristic Fe‐rich ultrastructures in the oxyhydroxides directly indicate microbial activity. The 238U‐230Th dating constrains their crystallization ages from ca. 11,873 to 384 years old. The seawater‐like 87Sr/86Sr and varying 143Nd/144Nd ratios underline a high proportion of seawater mixed with hydrothermal fluids. The radiogenic Pb isotopic patterns suggest a primary derivation of Pb leached from substrate basalts and to a lesser extent Pb from seawater. Stable iron isotopic compositions for different oxyhydroxides display a remarkable range between −1.47 and 0.82‰, which were interpreted as reflecting the fractionation processes during the formation of the deposits under evolving depositional redox conditions. The partial oxidation of Fe(II) and the subsurface removal of isotopically heavy Fe oxyhydroxides are suggested to play a vital role in shifting the Fe isotopic signature toward more negative values. Given that these Fe‐Si oxyhydroxide deposits exhibit features similar to certain ancient iron formations (IFs), Fe isotope systematics of these deposits may hold significant potential for fingerprinting the biological Fe oxidation processes that drove IF deposition on early Earth.

Contrasting Cu isotopes in mid-ocean ridge basalts and lower oceanic crust: Insights into the oceanic crustal magma plumbing systems

Magma plumbing systems exert strong controls on the formation and evolution of the oceanic crust during crust accretion. However, plumbing system dynamics based largely on mid-ocean ridge basalts (MORBs) are extremely difficult to constrain because the MORBs are aggregated melts and original information may have been blurred. Copper isotopic compositions of lower crustal gabbroic cumulates effectively archive early-stage histories because sulfides constitute the dominant Cu budget of gabbroic cumulates and are largely isolated from subsequent magma filtering processes. Here, we present Cu isotopes of a suite of gabbroic cumulates from the ultraslow-spreading Southwest Indian Ridge (Hole U1473A), and MORBs from the South Mid-Atlantic Ridge and East Pacific Rise. The δ65Cu of the MORBs from this study (+0.04 ‰ to +0.19 ‰) match those of previous MORB analyses, indicating uniform δ65Cu at various spreading rates. In contrast, the δ65Cu of gabbroic rocks vary significantly (−1.14 ‰ to 0.87 ‰), exceeding by far the range known for peridotites. The Cu budget of these gabbroic rocks is hosted in sulfides and the mantle-like S isotopic compositions of sulfides indicate their origin of igneous processes. These results thus suggest that magma accumulation and sulfide segregation would lead to notable Cu isotopic fractionation in the lower oceanic crust. We propose that repeated recharging of primitive melts and efficient mixing of melts within the plumbing system can buffer the removal of early 63Cu-rich sulfides from sulfide-saturated MORBs. The discrepancy in δ65Cu between lower oceanic crust and MORBs is a natural consequence of continuous replenishment that occurs concurrently with melt-rock interaction, mixing, crystallization and extraction, irrespective of the oceanic settings. The consistent δ65Cu in MORBs (0.09 ± 0.08 ‰) and komatiites (0.06 ± 0.06 ‰) further indicates that their Cu isotopes reflect the mean mantle source composition, providing a robust δ65Cu for bulk silicate Earth (BSE) of 0.08 ± 0.08 ‰ (2sd). Accordingly, we propose that in open sub-ridge plumbing systems, other incompatible element isotopes and ratios of elements with similar incompatibility in MORBs as Cu isotopes suggest, could also represent mean mantle source compositions.

Tracer Stirring and Variability in the Antarctic Circumpolar Current Near the Southwest Indian Ridge

AbstractOceanic macroturbulence is efficient at stirring and transporting tracers. The dynamical properties of this stirring can be characterized by statistically quantifying tracer structures. Here, we characterize the macroscale (1–100 km) tracer structures observed by two Seagliders downstream of the Southwest Indian Ridge in the Antarctic Circumpolar Current (ACC). These are some of the first glider observations in an energetic standing meander of the ACC, a region associated with enhanced ventilation. The small‐scale density variance in the mixed layer (ML) was relatively enhanced near the surface and base of the ML, while being muted at mid‐depth in the ML, suggesting the formation mechanism to be associated with ML instabilities and eddies. In addition, ML density fronts were formed by comparable contributions from temperature and salinity gradients. In the interior, along‐isopycnal spectra and structure functions of spice indicated that there is relatively lower variance at smaller scales than would be expected based on non‐local stirring, suggesting that flows smaller than the deformation radius play a role in the cascade of tracers to small scales. These interior spice anomalies spanned across isopycnals, and were found to be about 3–5 times flatter than the aspect ratio that would be expected for O(1) Burger number flows like interior QG dynamics, suggesting the ratio of vertical shear to horizontal strain is greater than N/f. This further supports that small‐scale flows, with high‐mode vertical structures, impact tracer distributions.

Illuminating a bacterial adaptation mechanism: Infrared-driven cell division in deep-sea hydrothermal vent environments

<p>Based on Planck's black-body radiation law, deep-sea hydrothermal vent chimneys emit light, predominantly infrared light, which potentially supports bacterial photosynthesis in this ecosystem independently of the solar energy. To investigate the impact of this geothermal light on bacterial growth, we collected samples from the Southwest Indian Ridge and demonstrated that infrared light alone promotes bacterial growth and alters population composition. The mechanism of infrared stimulated growth was analyzed by monitoring cell wall synthesis using the <i>Tepidibacter hydrothermalis</i> strain SWIR-1, which was isolated from cultures enriched through infrared irradiation. The results showed that the elevated hydrostatic pressure inhibited septal peptidoglycan synthesis and cell division, but had less effect on cell elongation, chromosome replication and segregation. The dominant cell shape was filaments with some swelling and inertness in cell wall synthesis depending on the level of pressure applied. Interestingly, irradiation with 880 nm infrared light effectively initiated septal synthesis and alleviated the obstruction. This revelation uncovers a novel adaptation mechanism involving infrared light for bacteria dwelling in deep-sea environments, and sheds light on the potential of infrared-mediated photobiomodulation.</p>

Oceanic Crust Formation within the Andrew Bain Fault Zone, Southwest Indian Ridge: Petrological and Geochemical Evidence

Oceanic Crust Formation within the Andrew Bain Fault Zone, Southwest Indian Ridge: Petrological and Geochemical Evidence