Ultraslow-spreading Ridges Research Articles

Temporal and spatial variation of seafloor spreading at ultraslow spreading ridges: Contribution of marine magnetics

The heterogenous magma supply at ultraslow spreading ridges creates diverse seafloor morphologies and lithospheric structures, which in turn generate a large variability in marine magnetic anomalies. The variability brings difficulties to interpret the evolution of oceanic lithosphere. On the other hand, different magnetic signatures of different seafloors provide an opportunity to identify the modes of seafloor spreading on the ultraslow spreading ridges. Here, we modeled several across-axis magnetic profiles selected from the Gakkel Ridge, Southwest Indian Ridge and Mid-Cayman Spreading Center to explore the lithospheric structure and seafloor spreading processes. Considering conjugate flanks, we observed three modes of seafloor spreading, Magmatic vs Magmatic, Magmatic vs Tectonic, and Tectonic vs Tectonic, on the three ultraslow spreading ridges. These three spreading modes reflect a strong, intermediate, and starved magma supply, respectively. Furthermore, four alternances of the different spreading modes were identified including the Magmatic vs Magmatic to Magmatic vs Tectonic, Magmatic vs Tectonic to Tectonic vs Magmatic, Tectonic vs Tectonic to Magmatic vs Magmatic, and Mixed. These alternances of spreading modes in across-axis direction suggest oscillations of the magma supply at different levels. Variation in the modes of seafloor spreading on four nearby profiles over the Mid-Cayman Spreading Center reveals the evolution of magma supply along the axis of this short ridge.

Volcanically hosted venting with indications of ultramafic influence at Aurora hydrothermal field on Gakkel Ridge

The Aurora hydrothermal system, Arctic Ocean, hosts active submarine venting within an extensive field of relict mineral deposits. Here we show the site is associated with a neovolcanic mound located within the Gakkel Ridge rift-valley floor, but deep-tow camera and sidescan surveys reveal the site to be ≥100 m across—unusually large for a volcanically hosted vent on a slow-spreading ridge and more comparable to tectonically hosted systems that require large time-integrated heat-fluxes to form. The hydrothermal plume emanating from Aurora exhibits much higher dissolved CH4/Mn values than typical basalt-hosted hydrothermal systems and, instead, closely resembles those of high-temperature ultramafic-influenced vents at slow-spreading ridges. We hypothesize that deep-penetrating fluid circulation may have sustained the prolonged venting evident at the Aurora hydrothermal field with a hydrothermal convection cell that can access ultramafic lithologies underlying anomalously thin ocean crust at this ultraslow spreading ridge setting. Our findings have implications for ultra-slow ridge cooling, global marine mineral distributions, and the diversity of geologic settings that can host abiotic organic synthesis - pertinent to the search for life beyond Earth.

Metagenomic Features Characterized with Microbial Iron Oxidoreduction and Mineral Interaction in Southwest Indian Ridge.

The Southwest Indian Ridge (SWIR) is one of the typical representatives of deep-sea ultraslow-spreading ridges, and has increasingly become a hot spot of studying subsurface geological activities and deep-sea mining management. However, the understanding of microbial activities is still limited on active hydrothermal vent chimneys in SWIR. In this study, samples from an active black smoker and a diffuse vent located in the Longqi hydrothermal region were collected for deep metagenomic sequencing, which yielded approximately 290 GB clean data and 295 mid-to-high-quality metagenome-assembled genomes (MAGs). Sulfur oxidation conducted by a variety of Gammaproteobacteria, Alphaproteobacteria, and Campylobacterota was presumed to be the major energy source for chemosynthesis in Longqi hydrothermal vents. Diverse iron-related microorganisms were recovered, including iron-oxidizing Zetaproteobacteria, iron-reducing Deferrisoma, and magnetotactic bacterium. Twenty-two bacterial MAGs from 12 uncultured phyla harbored iron oxidase Cyc2 homologs and enzymes for organic carbon degradation, indicated novel chemolithoheterotrophic iron-oxidizing bacteria that affected iron biogeochemistry in hydrothermal vents. Meanwhile, potential interactions between microbial communities and chimney minerals were emphasized as enriched metabolic potential of siderophore transportation, and extracellular electron transfer functioned by multi-heme proteins was discovered. Composition of chimney minerals probably affected microbial iron metabolic potential, as pyrrhotite might provide more available iron for microbial communities. Collectively, this study provides novel insights into microbial activities and potential mineral-microorganism interactions in hydrothermal vents. IMPORTANCE Microbial activities and interactions with minerals and venting fluid in active hydrothermal vents remain unclear in the ultraslow-spreading SWIR (Southwest Indian Ridge). Understanding about how minerals influence microbial metabolism is currently limited given the obstacles in cultivating microorganisms with sulfur or iron oxidoreduction functions. Here, comprehensive descriptions on microbial composition and metabolic profile on 2 hydrothermal vents in SWIR were obtained based on cultivation-free metagenome sequencing. In particular, autotrophic sulfur oxidation supported by minerals was presumed, emphasizing the role of chimney minerals in supporting chemosynthesis. Presence of novel heterotrophic iron-oxidizing bacteria was also indicated, suggesting overlooked biogeochemical pathways directed by microorganisms that connected sulfide mineral dissolution and organic carbon degradation in hydrothermal vents. Our findings offer novel insights into microbial function and biotic interactions on minerals in ultraslow-spreading ridges.

Magmatic Activity and Dynamics of Melt Supply of Volcanic Centers of Ultraslow Spreading Ridges: Hints From Local Earthquake Tomography at the Knipovich Ridge

AbstractAlong ultraslow spreading ridges melt is distributed unequally, but melt focusing guides melt away from amagmatic segments toward volcanic centers. An interplay of tectonism and magmatism is thought to control melt ascent, but the detailed process of melt extraction is not yet understood. We present a detailed image of the seismic velocity structure of the Logachev volcanic center and adjacent region along the Knipovich Ridge. With travel times of P‐ and S‐waves of 3,959 earthquakes we performed a local earthquake tomography. We simultaneously inverted for source locations, velocity structure and the Vp/Vs‐ratio. An extensive low velocity anomaly coincident with high Vp/Vs‐ratios >1.9 lies underneath the volcanic center at depths of 10 km below sea level in an aseismic area. More shallow, tightly clustered earthquake swarms connect the anomaly to a shallow anomaly with high Vp/Vs‐ratio beneath the basaltic seafloor. We consider the deep low‐velocity anomaly to represent an area of partial melt from which melts ascent vertically to the surface and northwards into the adjacent segment. By comparing tomographic studies of the Logachev and Southwest Indian Ridge Segment‐8 volcano we conclude that volcanic centers of ultraslow spreading ridges host spatially confined, circular partial melt areas below 10 km depth, in contrast to the shallow extended melt lenses along fast spreading ridges. Lateral feeding over distances of 35 km is possible at orthogonal spreading segments, but limited at the obliquely spreading Knipovich Ridge.

The heterogeneous mantle massif in south Tibetan ophiolites and its implication for the tectonic evolution of Neo-Tethys

Ophiolitic mantle peridotites show large variations in their geochemical compositions, which provide important information for the histories of mantle melting, melt metasomatism and prior melt depletion in the context of tectonic evolution of ancient oceans. Here we report new petrological and geochemical data for mantle peridotites from the Zhagabu ophiolite in south Tibet, which could represent fossil upper mantle of the Mesozoic Neo-Tethys oceanic lithosphere. The Zhagabu mantle peridotites, including spinel lherzolites and spinel harzburgites, have highly heterogeneous geochemical compositions, with whole-rock Al2O3 of 0.3–2.0 wt% and CaO of 0.3–2.2 wt%, and spinel Cr# [Cr/(Cr + Al)] of 0.21–0.74 and Mg# [Mg/(Mg + Fe2+)] of 0.44–0.74. The clinopyroxenes from the studied samples show extremely depletion in light rare earth element compositions, some of which however are elevated by post-melting melt-rock interactions. Spinel and clinopyroxene compositions collectively suggest that the Zhagabu mantle peridotites have experienced ~9–20% of fractional melting from a depleted mid-ocean ridge basalt mantle source. Trace element modeling of clinopyroxenes further indicates that some of the studied samples follow the refertilization-hydrous melting trends with variable fluid fluxes. Therefore, the heterogeneous compositions of the Zhagabu mantle peridotites are not comparable to those of either abyssal peridotites or forearc peridotites, but most probably record a dynamic transition from the anhydrous condition beneath the Neo-Tethyan mid-ocean ridge to the hydrous condition above the subducted slab. Considering that a forearc spreading is in contradiction with the low magma budget of Yarlung Zangbo ophiolites, the Neo-Tethyan subduction re-initiation was suggested to be achieved along oceanic detachment faults at ultraslow-spreading ridges. The Zhagabu mantle peridotites represent the very heterogeneous mantle massif in south Tibetan ophiolites, and provide an excellent window to investigate the complex mantle dynamics and tectonic evolution of the Neo-Tethys Ocean.

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.

Near-bottom survey of hydrothermal sulfide on an extremely magmatic accretion segment along an ultraslow-spreading ridge

An extremely magmatic accretion could exist along an ultraslow-spreading ridge that has been approved and surveyed, but less attention has been paid to its associated hydrothermal system, as it is supposed to be similar to those of fast/medium-spreading ridges. Here using an autonomous underwater vehicle and a human-occupied vehicle, a systematic investigation was conducted on the hydrothermal system of an extremely magmatic accretion segment along the Southwest Indian Ridge (50°28′ E). A collapsed chimney, microbial mat, and diffused fluid were observed in the old Duanqiao hydrothermal field (DQF), which was earlier a weakly active hydrothermal area. Notably, evident hydrothermal anomalies, including oxidation–reduction potential (ORP), turbidity, and methane, were detected above the new DQF, which lies southwest of the old DQF. Based on this investigation, we concluded that ORP is one of the most effective parameters available to identify vent location by analyzing each type of detected hydrothermal anomaly and the near-bottom magnetic inversion can image the suspected hydrothermal fluid circulation pathways. In general, we believe that the new DQF is controlled by a magmatic chamber different from those found on fast/medium-spreading ridges, and could be a large hydrothermal system.

Geochemistry, sulfur and lead isotopic composition of hydrothermal sulfide from the Duanqiao hydrothermal field on the Southwest Indian Ridge: implications for ore genesis

ABSTRACT Hydrothermal activities on ultraslow-spreading ridges exhibit diverse characteristics, long histories with multiple participants, and might form large-scale, high-grade sulfide deposits. The Duanqiao hydrothermal field (DHF) is located at the segment with the thickest oceanic crust and a large axial magma chamber on the Southwest Indian Ridge, providing unique perspective of sulfide metallogenesis on ultraslow-spreading ridges. Previous studies revealed that DHF sulfide exhibits distinct features of enrichment of ore-forming elements in comparison with those of hydrothermal fields on sediment-starved mid-ocean ridges. However, the genesis and processes responsible for such differences remain poorly constrained. In this study, mineralogical, geochemical and S and Pb isotopic analyses were performed on relict sulfide mound samples to characterize DHF formation. The samples show clear concentric mineral zonation from the interior to the exterior wall. Assemblages of chalcopyrite, sphalerite, and pyrite are distributed mainly in the interior wall, whereas pyrite and marcasite are distributed mainly in the exterior wall. The low Cu content and Pb isotopic composition of the sulfide indicate that the metals are derived mainly from basement basalts. The δ34S values exhibit positive values distributed over a reasonably narrow range (2.42‰–7.97‰), which suggests approximately 62.1%–88.5% of S with basaltic origin. Compared with most hydrothermal fields along the sediment starved mid-ocean ridges, the DHF sulfide shows particularly high contents of Pb (263–2630 ppm), As (234–726 ppm), Sb (7.32–44.3 ppm), and Ag (35.2 to >100 ppm). The δ34S values exhibit an increasing tendency from the sample exterior to the interior. We propose that these features probably reflect the existence of a subsurface zone refining process. Our results provide new insight into the sulfide formation process and contribute to understanding the metallogenic mechanism of hydrothermal sulfides on ultraslow-spreading ridges.

Oxidizing fluids associated with detachment hosted hydrothermal systems: Example from the Suye hydrothermal field on the ultraslow-spreading Southwest Indian Ridge

The redox state of hydrothermal fluids on mid-ocean ridges, which is indirectly affected by the depth of hydrothermal circulation and crustal permeability, plays an important role on the diversity of hydrothermal precipitates and associated ecosystems. Primary hydrothermal fluids that circulate along detachment faults are generally reducing as a result of the serpentinization of ultramafic rocks, while significant seawater infiltration may shift the redox state from reducing to oxidizing. However, the depth of penetration of oxidizing fluids into detachment related systems remain unclear, largely because current observations are based primarily on hydrothermal products that precipitated at the seafloor. Here, we report the first observations of oxidizing mineral assemblages in stockwork samples from the Suye hydrothermal field on the ultraslow spreading Southwest Indian Ridge. This field is hosted by mafic lithologies, while the low As, high Ni and Co contents, and high Au/As, Ag/As, and Ni/As ratios in pyrite from the stockwork samples indicate that the fluids reacted with both mafic and ultramafic rocks in the subseafloor. The high δ34S values (average of 9.8‰) indicate a high proportion (up to 50%) of seawater derived reduced sulfur involved in the stockwork zone formation. The high homogenization temperatures (∼320 °C) and salinity (∼12 wt.% NaCl) of fluid inclusions indicate that the stockwork zone of Suye was formed by a fluid that underwent phase separation deeper in the system that was subsequently diluted by 4–5 times subsurface seawater. The deep penetration of seawater is facilitated by the unique tectonic setting of the Suye hydrothermal field, which occurs between the two stage detachment faults that creates high permeability. Our findings demonstrate that hydrothermal fluid associated with detachment faults could be oxidized below the subsurface stockwork zone, and that deep-rooted detachment faults at ultraslow-spreading ridges can sustain both reducing and oxidizing hydrothermal systems in the same fault system. These results call for a reevaluation of the fate of base metal in ultramafic hosted hydrothermal fields.

Thermal Regime of Slow and Ultraslow Spreading Ridges Controlled by Melt Supply and Modes of Emplacement

AbstractMelt supply at slow‐ultraslow spreading ridges is overall reduced and highly variable. Magma cooling and crystallization substantially shape the axial thermal regime by providing heat that is lost to the ocean through conduction and hydrothermal convection. Geological data suggest that melt is emplaced over a wide depth range, variably accessible to hydrothermal cooling, and that periods of higher and lower melt supply may alternate at a given location. Until now, numerical models focused on steady‐state thermal regimes controlled by either spreading rate or melt supply, falling short at slow‐ultraslow ridges. Here we present results from a 2‐D numerical thermal model that couples repeated melt injections and hydrothermal convection. We first constrain thermal regimes inferred from two ultraslow‐spreading endmembers in melt supply at the Southwest Indian Ridge (SWIR): a magmatically robust endmember at 50°28'E and a nearly amagmatic endmember at 64°30'E. We adjust parameters associated with melt supply (the melt injection frequency and the temperature of the host rocks upon melt emplacement) and hydrothermal circulation (the extent and permeability of the hydrothermal system). Our simulations predict that greater melt injection frequencies unsurprisingly produce hotter thermal regimes. However, at a given frequency, melt emplacement in cooler host rocks (e.g., <800°C) causes cooler thermal regimes with transient black smoker‐type hydrothermal circulation that extracts heat efficiently. Periodic waxing and waning magma supply, as proposed from geological data at the SWIR 50°28'E, induces an oscillatory thermal regime. Transience in the axial thermo‐mechanical state may be an integral part of lower‐crustal construction at magmatically robust sections of slow‐ultraslow ridges.

Genesis of the distal axis East Longjing-2 hydrothermal field on the ultraslow-spreading Southwest Indian Ridge: Evidences from fluid inclusions

Tectonic-related hydrothermal fields constitute about half of the hydrothermal activities on slow-ultraslow spreading ridges, and may form sulfides large in size and rich in precious metals. Ore-forming fluids are critical for element migration progressions, thus are key parameter to reveal their genesis and sulfide mineralization processes. However, previous studies of this kind of hydrothermal activities primarily focused on vent fluids which have transient effects or based on surface samples, while the ore-forming fluid evolution and related sub-seafloor mineralization process are still poorly constrained. In this study, we analyzed the characteristics of fluid inclusions in the stockwork mineralization of the East Longjing-2 hydrothermal field (ELHF-2) on the Southwest Indian Ridge. The results showed that the fluids evolved gradually from medium-high temperatures (260–315 °C, mean of 284 °C) and low salinities (0.70–3.70 wt.% NaCl eq.) in the disseminated pyrite mineralization stage (Stage I) to medium temperatures (239–261 °C, mean of 249 °C) and low salinities (1.05–3.85 wt.% NaCl eq.) in the chalcopyrite–pyrite–quartz veinlet mineralization stage (Stage II). Laser Raman specular analysis indicated the fluid inclusions were mainly composed of H2O. The low salinity (lowest 0.7 wt.% NaCl eq.) and various vapor volume (0.05–0.5) observed in these fluid inclusions suggested the ore-forming fluids were probably mixtures of the low-salinity vapor phase formed by phase separation with large amounts of seawater. The precipitation of the ore-forming elements was likely induced by temperature decreasing. We propose that hydrothermal circulation in ELHF-2 was a result of coupled high-permeability detachment fault and shallow gabbro intrusion. Our results suggest that high temperature hydrothermal activities could be developed on distal axis area on ultraslow spreading ridges.

Variability of Sea‐Surface Magnetic Anomalies at Ultraslow Spreading Centers: Consequence of Detachment Faulting and Contrasted Magmatism?

AbstractThe capacity of oceanic crust to record geomagnetic polarity reversals makes sea‐surface magnetic anomalies an essential tool to study plate tectonics. The anomalies are usually well‐defined at magmatic spreading centers, but are distorted and eventually disappear on magma‐poor mid‐ocean ridges such as the ultraslow Southwest Indian Ridge (SWIR), making their interpretation difficult. We attribute the variability of the SWIR sea‐surface magnetic anomalies to the alternance of magmatic spreading and detachment faulting. A three‐layer magnetic model is used to simulate the influence of such an alternance on the sea‐surface magnetic anomalies. Conversely, observed magnetic profiles at the SWIR are modeled to unravel their off‐axis crustal structure and past mode of spreading. The intruding gabbro bodies on the footwall of detachment faults play a major role in explaining the variability of sea‐surface magnetic anomalies at slow and ultraslow spreading ridges.

Thermochemical anomalies in the upper mantle control Gakkel Ridge accretion

Despite progress in understanding seafloor accretion at ultraslow spreading ridges, the ultimate driving force is still unknown. Here we use 40Ar/39Ar isotopic dating of mid-ocean ridge basalts recovered at variable distances from the axis of the Gakkel Ridge to provide new constraints on the spatial and temporal distribution of volcanic eruptions at various sections of an ultraslow spreading ridge. Our age data show that magmatic-dominated sections of the Gakkel Ridge spread at a steady rate of ~11.1 ± 0.9 mm/yr whereas amagmatic sections have a more widely distributed melt supply yielding ambiguous spreading rate information. These variations in spreading rate and crustal accretion correlate with locations of hotter thermochemical anomalies in the asthenosphere beneath the ridge. We conclude therefore that seafloor generation in ultra-slow spreading centres broadly reflects the distribution of thermochemical anomalies in the upper mantle.

Multi-stage melting of enriched mantle components along the eastern Gakkel Ridge

The global endmember ultra-slow spreading Arctic Gakkel ridge is an ideal place to study mantle melting and the contributions of different mantle components to ridge volcanism. We carried out a high-resolution geochemical study of basalts from a seemingly normal section of the ultraslow-spreading Arctic Gakkel Ridge between 40°E and 60°E, which we refer to as EVZ2. While the majority of volcanics sampled from EVZ2 could be characterized as normal Mid-Ocean Ridge Basalts (NMORB), we identified a group of Isotopically Enriched, Incompatible Element Depleted MORB (IEDMORB) with low MREE/HREE ratios (i.e., “ghost garnet” signature). EVZ2 IEDMORB are mostly found near the rift valley walls away from axial volcanic centers. We propose that IEDMORB are the products of two stages of melting. They are shallow secondary melts of incompatible trace element enriched low-solidus mantle components that had lost some initial melt in the presence of residual garnet at depth. While the deep initial melts are more likely to be focused toward axial volcanic centers and subsequently diluted by normal peridotite melts, some of the shallow secondary melts could erupt as IEDMORB via pre-existing crustal weaknesses, such as deep-rooted high-angle normal faults that are ubiquitous along ultra-slow spreading ridges, thereby preserving their enriched isotopic compositions. While most dredges that sampled IEDMORB also recovered other types of MORB, all the samples from Dredge 55 are IEDMORB with distinctive arc-type trace element signatures, including relative depletion in Nb (and Ta), as well as enrichment in Th and fluid mobile elements (e.g., high Th/Nb, La/Nb, Pb/Ce, and H2O/Ce). These signatures suggest that they sampled recycled metasomatized arc-mantle wedge material. Other EVZ2 IEDMORB also show relative enrichment in fluid mobile elements and depletion in Nb (e.g., high La/Nb), but lack enrichment in Th. As Th has extremely low mobility in aqueous fluids, Th enrichment requires metasomatism involving silicate melts. Thus, we propose that the high Th/Nb, high La/Nb IEDMORB contain silicate-melt metasomatized arc mantle wedge material while the low-Th/Nb, high La/Nb IEDMORB only contain aqueous-fluid metasomatized arc mantle wedge material. Globally, IEDMORB with ghost garnet signatures are mostly found along ridges near mantle plumes where low-solidus, incompatible element and isotopically enriched mantle components that suffered initial melt loss at depth could be entrained in the upwelling subridge mantle and undergo further melting. However, most near-plume IEDMORB do not show arc-type geochemical signatures. Therefore, the discovery of IEDMORB from 20% of dredges along EVZ2, where most ridge volcanics are NMORB with depleted isotopic compositions, reflects the sporadic distribution of recycled arc mantle wedge material in the Arctic mantle and the prevalence of pre-existing crustal conduits, such as high-angle normal faults, along ultra-slow spreading ridges that facilitate melt migration with limited melt pooling and mixing.

Effective global mixing of the highly siderophile elements into Earth’s mantle inferred from oceanic abyssal peridotites

Late accretion occurred through addition of massive impactors to Earth, leading to potential heterogeneity in the distribution of highly siderophile elements (HSE: Os, Ir, Ru, Pt, Pd, Re) within the mantle. Abyssal peridotites sample the present-day convecting mantle, which make them useful for examining the distribution of the HSE within the mantle. Here we report new HSE abundance data and 187Os/188Os ratios, in conjunction with mineral chemistry and bulk rock major- and trace-element compositions for abyssal peridotites from the fast-spreading Pacific Antarctic Ridge (PAR) and East Pacific Rise (Hess Deep), and for slow to intermediate spreading ridges from the Southwest Indian Ridge, Central Indian Ridge and Mid-Atlantic Ridge. These analyses expand the global abyssal peridotite Os isotope and HSE database, enabling evaluation of potential variations with spreading rate, from ultraslow (<20 mm/yr, full spreading rate) to fast (135–150 mm/yr). Accounting for likely effects from seawater modification and serpentinization, the Pacific data reveals heterogeneous and sometimes significant melt depletion for PAR (3–23% melt depletion; 187Os/188Os from 0.1189 to 0.1336, average = 0.1267 ± 0.0065; 2SD) and Hess Deep abyssal peridotites (15–20% melt depletion; 0.1247 ± 0.0027). Abyssal peridotites from fast to intermediate spreading ridges reveal no systematic differences in the distribution and behavior of the HSE or Os isotopes, or in degrees of melt depletion, compared with slow to ultraslow spreading ridges. These observations arise despite significant differences in melt generation processes at mid-ocean ridges, suggesting that the effects of ancient melt depletion are more profound on HSE compositions in abyssal peridotites than modern melting beneath ridges. Using global abyssal peridotites with Al2O3 content > 2 wt.%, the average composition of the primitive mantle is 0.3 ppb Re, 4.9 ppb Pd, 7.1 ppb Pt, 7.2 ppb Ru, 3.8 ppb Ir and Os, showing no Pd/Ir, but a positive Ru/Ir anomaly, relative to chondrites. There is ∼50% variation of the HSE abundances in the oceanic mantle, with much of this variation being observed at small length scales (<1 km) and due entirely to both modern and more ancient partial melting effects. Consequently, any significant HSE heterogeneities formed during late accretion or early Earth differentiation processes are no longer recognizable in the mantle sampled within ocean basins, implying generally efficient mixing of Earth’s mantle for these elements. By contrast, relatively ancient heterogeneity in Os and other radiogenic isotopes has been effectively preserved in the convecting mantle over the last ∼2 Ga, through recycling processes and through preservation and isolation of melt-depleted refractory residues.

Extrusive upper crust formation at slow-spreading ridges: Fault steering of lava flows

The structure of the oceanic upper volcanic crust is less understood at slow-spreading ridges than at faster ones. Its construction is dominated by pillow lavas, reflecting lower effusion rates than those at fast spreading ridges, where sheet and lobate flows are common and flow off-axis while thickening the extrusive volcanic layer. Based on optical and high-resolution bathymetry data from the Lucky Strike segment (Mid-Atlantic Ridge), we document a mode of volcanic emplacement that likely operates at some magmatically robust slow- and ultra-slow spreading ridge segments, in the presence of strong gradients in magma supply. In these settings, sheet flows may efficiently transport melt away from magmatically robust segment centers in the along-axis direction, steered by normal faults, and exploiting along-axis topographic gradients, with limited across-axis flow. Surface lineations of sheet lava flows tend to be subparallel to fault scarps and the overall segment orientation, and show whorls, well-developed lava channels with associated levees, and surface fold structures at flow fronts. This mode of lava emplacement transitions away from the segment center to pillow-dominated seafloor near the segment ends, associated often with hummocks and axial volcanic ridges. This results in local lava emplacement due to a melt supply that is lower than at the segment center. We propose that fault-steering of lava flows along-axis, limiting off-axis transport as observed at fast-spreading systems, may be common at both slow- and ultra-slow spreading ridges with significant along-axis changes in magma supply linked to topographic gradients. As a result, the nature and properties of the extrusive volcanic layer may vary significantly along axis owing to changes in the modes of volcanic emplacement, as transitions from sheet flows to pillow lavas may impact the porosity structure and hence the seismic properties of the extrusive layer in the oceanic upper crust.

An intermittent detachment faulting system with a large sulfide deposit revealed by multi-scale magnetic surveys

Magmatic and tectonic processes can contribute to discontinuous crustal accretion and play an important role in hydrothermal circulation at ultraslow-spreading ridges, however, it is difficult to accurately describe the processes without an age framework to constrain crustal evolution. Here we report on a multi-scale magnetic survey that provides constraints on the fine-scale evolution of a detachment faulting system that hosts hydrothermal activity at 49.7°E on the Southwest Indian Ridge. Reconstruction of the multi-stage detachment faulting history shows a previous episode of detachment faulting took place 0.76~1.48 My BP, while the present fault has been active for the past ~0.33 My and is just in the prime of life. This fault sustains hydrothermal circulation that has the potential for developing a large sulfide deposit. High resolution multiscale magnetics allows us to constrain the relative balance between periods of detachment faulting and magmatism to better describe accretionary processes on an ultraslow spreading ridge.

Behavior of iron isotopes in hydrothermal systems: Beebe and Von Damm vent fields on the Mid-Cayman ultraslow-spreading ridge

It is now clear that, in some parts of the ocean, inputs of hydrothermal iron (Fe) can make a more significant contribution to the Fe inventory than previously thought. While the Fe isotopic signature of seawater has proved useful for distinguishing between inputs of Fe from atmospheric deposition and seafloor sediments, the Fe isotope signature of hydrothermal vent fluids may change during mixing of vent fluids and seawater. To better constrain the processes leading to these changes, the Fe isotopic compositions (δ56Fe) of dissolved and total dissolvable Fe have been determined in high temperature vent fluids and the buoyant hydrothermal plumes at the Beebe and the Von Damm vent fields, which are located along the ultraslow Mid-Cayman spreading center in the Caribbean Sea.Our results show that the δ56Fe value of dissolved Fe in the earliest stages of buoyant plume formation was lower (as low as −4.08‰) than measured in a high temperature, low-Mg, vent fluid sample (−0.28‰). This indicates that the iron isotopic signature of dissolved Fe is principally controlled by oxidation of Fe(II) and precipitation of Fe-(oxyhydr)oxides that preferentially incorporate heavy Fe isotopes. In support of this, the δ56Fe value of labile particulate Fe was higher than the δ56Fe value of dissolved Fe. Nevertheless, at Beebe, the δ56Fe value of total dissolvable Fe increased as the proportion of Fe predicted to have been lost from the plume increased, consistent with preferential fall-out of Fe-sulfides that are enriched in light Fe isotopes. The very low δ56Fe values of dissolved Fe in the Beebe buoyant plume are consistent with (i) the high Fe/H2S ratio of the vent fluids, and (ii) high Fe(II) oxidation rates, relative to other vent sites.

Magma source region size controls along axis variations in crustal thickness, axial depth and relief at plate spreading centers

The amplitude of variations in axial depth and across-axis relief that define spreading center segmentation vary systematically with spreading rate. Seismic and gravity data show that crustal thickness also varies along segments. We derive a highly idealized model to relate axial lithospheric thickness, crustal thickness, and effective magma source region size to segment-scale variation in axial depth and across-axis relief. All magma to build the crust is assumed to pool in a segment-centered magma source region and is distributed along the spreading axis by flow in dikes. The magma source and dike are regarded as a closed system and the magma pressure is taken to decrease linearly with the volume of magma going into dikes. The pressure-volume relation driving lateral magma flow is taken to be proportional to the along-axis gradient of depth. Axial depth variations depend on: (1) local isostatic compensation from crustal thickness variations; and (2) across-axis relief or axial valley depth. The axial valley depth is related to crustal and lithospheric thickness using the recently published model of Liu and Buck (2018). Excellent fits to bathymetric data for 12 segments from intermediate to ultra-slow spreading ridges are found by varying three model parameters (HC0, HL and ΔP/ΔV). The dependence of pressure drop on volume withdrawn is parameterized in terms of an effective radius for a spherical source region in an elastic half-space. A key result is that the effective magma source radius increases with spreading rate.

Seafloor Hydrothermal Activity around a Large Non-Transform Discontinuity along Ultraslow-Spreading Southwest Indian Ridge (48.1–48.7° E)

Non-transform discontinuity (NTD) is one category of tectonic units along slow- and ultraslow-spreading ridges. Some NTD-related hydrothermal fields that may reflect different driving mechanisms have been documented along slow-spreading ridges, but the discrete survey strategy makes it hard to evaluate the incidence of hydrothermal activity. On ultraslow-spreading ridges, fewer NTD-related hydrothermal activities were reported. Factors contributing to the occurrence of hydrothermal activities at NTDs and whether they could be potential targets for hydrothermal exploration are poorly known. Combining turbidity and oxidation reduction potential (ORP) sensors with a near-bottom camera, Chinese Dayang cruises from 2014 to 2018 have conducted systematic towed surveys for hydrothermal activity around a large NTD along the ultraslow-spreading Southwest Indian Ridge (SWIR, 48.1–48.7° E). Five new potential hydrothermal anomaly sites (2 inferred and 3 suspected) of high or low temperature and the previously inferred Sudi hydrothermal field occurred in diverse morphotectonic settings along a 78 km long ridge axis. The calculated vent frequency (Fs, sites/100 km) was ~7.7 over the entire study area, higher than the modified value (Fs ≈ 6.5) between 48 and 52° E of SWIR. Even only for the 54 km long large NTD, three hydrothermal anomaly sites yielded an Fs of ~5.6, which is higher than that of most ridge sections and is comparable to some fast-spreading ridges with high-resolution surveys. This indicates that NTDs along ultraslow-spreading ridges could be promising areas in fertilizing hydrothermal activities. Moreover, the deeply penetrating faults on the rift valley walls and strain-focused areas may contribute to the formation of NTD-related hydrothermal circulations. We suggest that NTDs along ultraslow-spreading ridges may be potential targets for further exploration of hydrothermal activities and seafloor sulfide deposits.