Ultraslow Ridges Research Articles

EARTHQUAKE SWARMS AS EVIDENCE OF VOLCANO-TECTONIC PROCESSES OF THE SLOWEST SPREADING GAKKEL RIDGE IN THE ARCTIC

Based on the data from the Arctic regional seismic stations, the article presents the results of studying the swarm seismicity of the Gakkel Mid-Ocean Ridge, located in the Arctic Ocean. The active spreading processes of ultraslow ridges with spreading rates of less than 20 mm/yr, which include the Gakkel Ridge, are still poorly understood as compared to the MOR in the Atlantic and Pacific oceans, with spreading rates of more than 25 mm/yr. In 2012–2022, there were identified eight swarms: one within the western volcanic segment of the ridge, others – in the eastern volcanic segment. No earthquake swarms were recorded in the central amagmatic segment of the ridge; the recording therein covers primarily single earthquakes and aftershock sequences. Spatially identified swarms are confied to some volcanic centers revealed earlier from the geological and geophysical data during complex expeditions. The ridge segment at coordinates ∼85…∼93° E is characterized by the most intense manifestations of volcanic processes. The spatial distribution of swarms may also indicate potential presence of volcanic structures that have not yet been identified from the geological, geophysical and geomorphological data. In the temporal domain, there can be preliminarily distinguished a 5-year swarm activation cycle, which, however, requires additional verification over a longer time interval. The swarm seismicity of the slowest spreading Gakkel Ridge cannot be described by a simple model of volcanism and magma intrusion into a symmetrical rift; it is rather a result of a complex interaction between diking and faulting with magma transport along the faults with potentially significant seismic activities. There can probably be suggested a regular combination of the processes of volcanic activation and seismotectonic destruction, which is especially pronounced in the locations of transverse faults. The results described in this article expand our understanding of the manifestation of volcano-tectonic processes occurring within the slowest speading Gakkel Mid-Ocean Ridge.

Современная группирующаяся низкомагнитудная сейсмичность срединно-океанического хребта Гаккеля

Earthquakes at mid-ocean ridges (MOR) reflect the active magmatic and tectonic processes that form new oceanic crust. Active spreading processes of ultraslow ridges, with spreading rates of less than 20 mm/yr, are still poorly understood compared to the MOR in the Atlantic and Pacific oceans, with spreading rates of more than 25 mm/yr. Thanks to the installation of stationary seismic stations in the 21st century on the Arctic archipelagos of Franz Josef Land and Severnaya Zemlya, it became possible to record and study low-magnitude grouping earthquakes within the Gakkel Ridge, including swarm sequences of earthquakes. This article presents the first results of registration, location and study of swarm sequences of low-magnitude earthquakes within the Gakkel Ridge for the period from 2012 to 2022. It is shown that swarm sequences are recorded to a greater extent in the western volcanic and eastern volcanic segments of the ridge, while no such sequences were found within the central amagmatic segment. The structure formation of this part of the ridge is dominated by tectonic rather than magmatic and metamorphic processes. Two large swarm of low-magnitude earthquakes that occurred in the eastern volcanic segment of the ridge are considered in detail.

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.

Self-organization of magma supply controls crustal thickness variation and tectonic pattern along melt-poor mid-ocean ridges

In nature, variations in mantle sources and magmatic processes lead to significant changes of crustal thickness distribution and spreading pattern at mid-ocean ridges. Distributions in crust thicknesses are relatively uniform at fast ridges. On the contrary, great lateral fluctuations with even mantle rocks exhumation are observed at slow and ultraslow ridges. Similarity, the spreading pattern changes from symmetric configuration (at fast ridges) to highly asymmetric plates accretion with detachment faults and oceanic core complexes (at slow ridges). Recent modeling studies suggested that magmatism may play a key role in the variation. Yet, the physical mechanisms controlling spatial-temporal distribution and intensity of the magmatic activity at mid-ocean ridges remain elusive. In this study, using 3D self-consistent magmatic-thermomechanical numerical models, we systematically investigate the effect of mantle potential temperature and spreading rate on the crustal thickness lateral variations and tectonic pattern at spreading ridges of various opening rates. Our numerical results show that along melt-poor slow-ultraslow mid-ocean ridges, there are two fundamentally different types of ridge sections that spontaneously form and alternate. This is due to a tectono-magmatic instability which self-consistently partitions melt supply beneath the ridge. Two geometries form: (1) normal ridge sections (NR) with elevated topography, normal thickness of oceanic crust and a hot thermal structure, and (2) fracture zone sections (FZ) with lowered topography, thin/absent crust, exhumed mantle rocks, and a cold thermal structure. The tectono-magmatic instability along spreading ridges is triggered by the combined effects of a reduced melt supply from the mantle into crustal magma chambers and an increase in brittle layer thickness. Such bimodal structure is then maintained by the combined effects of buoyant melt partitioning along the ridge, localized latent heat release from crustal magma crystallization, and different spreading modes. This results, on one hand, in hotter, thinner and elevated magmatic sections (tectono-magmatic spreading), and, on the other hand, in colder, thicker and subsided amagmatic sections (purely tectonic spreading). This predicted bimodal distribution finds a good agreement with natural observations. This suggests that spontaneous self-organization of magma supply controlled by spreading rate and mantle potential temperature plays a critical role in shaping tectonic and crustal patterns of mid-ocean ridges.

Mantle melting, lithospheric strength and transform fault stability: Insights from the North Atlantic

The offset ridge-transform structure of ocean basins is one of the most prominent expressions of plate tectonics. Yet why this configuration is favored over a continuous divergent boundary has remained unresolved. We examine this issue using mantle Bouguer anomalies (MBAs) from contrasting ridge systems in the North Atlantic. The Reykjanes Ridge north of the Bight transform fault has no transform offsets and is characterized by rapidly propagating melting centers along a linear axis and a continuous MBA low. To the south, the adjoining Mid-Atlantic Ridge has typical ridge segments each underlain by a discrete MBA “bulls-eye” low and offset by transform and non-transform discontinuities. We hypothesize that the pattern of mantle melting, as reflected in the MBAs, creates chemical and rheologic variations in the residual mantle that either favor or hinder transform fault formation. Within ridge segments, mantle melting efficiently extracts water producing dry and strong residual mantle. At segment ends, low extents of melting and inefficient melt extraction preserve damp and weak mantle. On the linear Reykjanes Ridge continuous melting and rapidly propagating melting centers create continuous strong mantle with possibly weak rheologic variations at high angles to the opening direction, not favoring transform faults. In contrast, stable segmented mantle melting on the Mid-Atlantic Ridge forms bands of strong and weak residual mantle aligned in the spreading direction, the latter creating favorable locations for shear deformation and transform faults. Our hypothesis also explains the lack of transform faults at Earth's endmember spreading rates. At ultra-slow ridges, overall melting is limited and irregular and melt extraction is inefficient. At ultra-fast ridges, mantle melting is pervasive and melt extraction is efficient. In both cases, organized spreading-parallel compositional rheological variations do not form and transform faults are not favored. Our model implies that beyond cooling and strengthening with age, the pattern of mantle melting shapes the rheological structure of oceanic lithosphere and the geometry of plate tectonics.

Seismic faults triggered early stage serpentinization of peridotites from the Samail Ophiolite, Oman

Serpentinization of mantle peridotites has first order effects on the rheology and tectonic behavior of the oceanic lithosphere, on the global water cycle, and on the biosphere at mid-oceanic ridges. Investigating serpentinization of abyssal peridotites is limited by the scarce occurrences of peridotites at or close to the ocean floor at slow and ultra-slow ridge environments where peridotite is exposed by long-lived detachments. The processes controlling hydration of the upper mantle below a thick magmatic crust at fast spreading ridges are poorly constrained. Here we present results based on samples from cores drilled in peridotites from the Samail ophiolite obtained during the Oman Drilling Project. We describe an early generation of highly localized brittle faults ubiquitous through all the peridotite cores and investigate their relation to the main serpentinization event represented by mesh-textured serpentinites. We combine microstructural observations with mineral and bulk chemical analyses as well as oxygen isotope microanalyses obtained by secondary ion mass spectrometry (SIMS). Asymmetric wall rock damage, weakening of crystal preferred orientation (CPO) in small fault clasts, and intense fragmentation within the fault zones even in association with very small displacements suggest that the early stage faults represent seismic events and predate mesh formation. Hydration and mesh texture formation follows in the wake of this faulting. Serpentinization is associated with moderate enrichment of fluid mobile elements including B, Li, Rb and U, indicative of fluid rock interaction characterized by relatively low fluid/rock ratios. This is consistent with a scenario where serpentinization took place below a thick magmatic crust following an earthquake-induced permeability increase. The oxygen isotope compositions of mesh serpentine are consistent with off-axis serpentinization at temperatures in the range 200-250°C

Thermo‐Mechanical State of Ultraslow‐Spreading Ridges With a Transient Magma Supply

AbstractThe thermal structure of mid‐ocean ridge axes is a critical control on the mechanical properties of young lithosphere and modulates the faulting styles that shape the Earth's seafloor. Models that balance a steady input of magmatic heat with a vigorous hydrothermal output are generally successful at explaining the thermal state of magmatically robust mid‐ocean ridges. However, the magma supply of ultraslow spreading ridges is often subdued and highly episodic, and the depth‐extent and vigor of hydrothermal circulation in these settings remain poorly known, requiring modifications to the standard magmatic‐hydrothermal paradigm. We develop models that couple repeated magmatic intrusions with hydrothermal convection in a permeable domain whose boundaries are controlled by the extent of grain‐scale thermal cracking. Our simulations reveal decreasing trends of venting temperatures with increasing intrusion periodicity, as well as with increasing permeability. Our model explains the seismically inferred depth (∼10 km) to the brittle‐ductile transition and the depth of MORB crystallization at the Mid‐Cayman Spreading Center by invoking repeated intrusion of sills every 20–50 kyrs, depending on their size. Doubling this periodicity predicts a thicker (∼15 km) seismogenic layer and lower‐temperature venting, as observed at the Southwest Indian Ridge (SWIR) at 64˚E. However, if fluid convection is not possible below ∼6 km, our model requires that high‐temperature venting at ultraslow ridges (e.g., Longqi at SWIR 49.7˚E) be fueled by shallow episodic magma intrusions into the long‐term convective region, which maintain high‐output circulation for at most a few tens of kyrs.

Segment‐Scale Seismicity of the Ultraslow Spreading Knipovich Ridge

AbstractUltraslow spreading ridges form the slowest divergent plate boundaries and exhibit distinct spreading processes in volcanically active magmatic sections and intervening amagmatic sections. Local seismicity studies of ultraslow spreading ridges until now cover only parts of segments and give insight into spreading processes at confined locations. Here, we present a microseismicity data set that allows to study spreading processes on the scale of entire segments. Our network of 26 ocean bottom seismometers covered around 160 km along axis of the ultraslow spreading Knipovich Ridge in the Greenland Sea and recorded earthquakes for a period of about 1 year. We find seismicity varying distinctly along‐axis. The maximum earthquake depths shallow over distances of 70 km toward the Logachev volcanic center. Here, swarm activity occurs in an otherwise aseismic zone. Melts may thus be guided along the subparallel topography of the lithosphere‐asthenosphere boundary toward major volcanic centers explaining the uneven along‐axis melt distribution typical for ultraslow ridges. Absence of shallow seismicity in the upper 8 km of the lithosphere with a band of deep seismicity underneath offsets presumably melt‐poor regions from magma richer sections. Aseismic deformation in these regions may indicate weakening of mantle rocks by alteration. We do not find obvious indications for major detachment faulting that characterizes magma‐poor spreading at some ultraslow spreading segments. The highly oblique spreading of Knipovich Ridge may be the reason for a fine‐scale segmentation of the seismic activity with zones of weak seismicity possibly indicating transform motion on short obliquely oriented faults.

Controls on the magmatic fraction of extension at mid-ocean ridges

The fraction M of plate separation accommodated by magma emplacement at mid-ocean ridges has been recognized as the main control on seafloor spreading modes, yet the factors that control M itself are poorly understood. Here we put forward a simple theoretical framework explaining M in terms of short-term cycles of earthquakes and dike intrusions interacting with one another by modulating the stress state of the ridge axis. Axial lithospheric thickness and the rate of pressure build-up in shallow, replenishing magma sills are the two main parameters controlling M in our simulations. Combined with plausible scenarios for the increase of pressure build-up rate and the decrease of lithospheric thickness with increasing spreading rate, our model appropriately brackets available measurements of M from slow, intermediate, and fast-spreading ridges. Our model further suggests that the transitions between major modes of seafloor spreading (detachment faulting, symmetric faulting, and fully-magmatic) correspond to thresholds in axial lithospheric thickness, and that the great variability in M at slow and ultraslow ridges directly reflects along-axis variability in thermal structure. More generally, this implies that the balance between bottom-up magmatic heating and top-down hydrothermal cooling fully determines the time-averaged rate of magmatic intrusions in the brittle lithosphere, and thus the modes of mid-ocean ridge faulting which shape ∼2/3 of Earth's surface.

On spreading modes and magma supply at slow and ultraslow mid-ocean ridges

The ultraslow eastern Southwest Indian Ridge (SWIR) offers an opportunity to study the effect of magma supply on an ultraslow mid-ocean ridge starting from quasi-melt-free detachment-dominated spreading, and transitioning to volcanic spreading as one nears prominent axial volcanos. Detachments in the quasi-melt-free mode extend along-axis 60 to 95 km and have a lifetime of 0.6 to 1.5 myrs. They cut into their predecessor's footwall with an opposite polarity, causing part of the footwall lithosphere to experience further deformation, hydrothermal alteration, sparse magmatism and possibly thermal rejuvenation, in a hanging wall position. The accretion of the oceanic lithosphere in this context therefore occurs in two distinct stages over the lifetime of two successive detachment faults. We examine the transition from this nearly amagmatic detachment-dominated mode to the more common volcanic mode of spreading, showing that it occurs along-axis over distances ≤30 km. It involves a significant thinning of the axial lithosphere and a gradual decrease of the amount of tectonic displacement on faults, as the magmatic contribution to the divergence of the two plates increases. We develop a conceptual model of this transition, in which magma plays a double role: it fills the space between the diverging plates, thus reducing the need for displacement along faults, and it modifies the thermal state and the rheology of the plate boundary, affecting its thickness and its tectonic response to plate divergence. Based on a comparison of the ultraslow eastern SWIR, with the faster spreading Mid-Atlantic Ridge, we show that the activation of the volcanic, or of the detachment-dominated modes of spreading is connected with the volume of magma supplied per increment of plate separation, over a range of axial lithosphere thickness, and therefore over a range of the M ratio defined by (Buck et al., 2005) as the relative contribution of magma and faults to plate divergence (M is smaller, for a given volume of melt per increment of plate separation, if the plate is thicker). We therefore propose that M does not fully explain the variability in faulting styles observed at slow and ultraslow ridges and propose that rheological changes induced by magma also play a key role (melt itself is weak, hydrothermally altered gabbro-peridotite mixtures are weak, and melt heat sustains more vigorous hydrothermal circulation), resulting in contrasted potentials for strain localization, footwall flexure on faults and the development of detachment faults.

3He along the ultraslow spreading AMOR in the Norwegian-Greenland Seas

The majority of undiscovered hydrothermal vent fields are thought to be located on slow and ultraslow spreading ridges. Locating new vent sites on these ridges is important due to their tendency to host large seafloor massive sulfide deposits. It is also important as it can increase our understanding of the tectonics at ultraslow spreading ridges as well as giving us an understanding of how hydrothermal venting in these regions affect and interact with the surrounding water column. To assess the hydrothermal productivity of the slow to ultraslow spreading Mohns and Knipovich ridge segments of the Arctic Mid-Ocean Ridge we used the primordial isotope 3He. Due to the conservative nature of 3He, this isotope can function as a tracer both for hydrothermal activity and on the ocean current pathways. In addition to assessing the hydrothermal productivity of the ridges we also studied the impact the ridges have on ocean circulation. Between 2006 and 2016, 400 water samples were collected along the Mohns Ridge and the Knipovich Ridge and 117 samples were collected in two transects crossing each ridge. The results show that the surface mixed layer (500–0 m depth) directly above each ridge has higher than equilibrium values of δ3He, which we interpret as an effect of the bottom topography on the vertical mixing. We also observe direct indications of at least two undiscovered vent sites along the Mohns Ridge, making the total count of 7 vent fields along this 550 km ridge segment, which gives a vent field frequency (Fs) of 1.27 sites/100 km. This number is comparable to the vent field frequency on other ultraslow ridges. By comparing the current speed to non-buoyant plume measurements obtained at the previously discovered Loki's Castle and the Jan Mayen vent fields (JMVF) we made an estimate of the 3He flux at these vent fields. This estimate gave a flux of 0.22 mmol/km/mm/yr from Loki's Castle, which is well below the world's average of 0.33 mmol/km/mm/yr. However, the flux rate estimated here is comparable to estimates previously calculated from the ultraslow Gakkel ridge. The northeastern termination of the Mohns Ridge, where Loki's Castle is located, is also the thinnest part of the ridge segment indicating a low magmatic influence. The flux rate of the JMVF was 0.40 mmol/km/mm/yr and thus higher than the global average. This could indicate an influence from the Jan Mayen hot spot as similar flux rates has been found at other hot spot influenced vent sites. This study not only illuminates the hydrothermal productivity of ultraslow spreading ridges, but also adds a considerable dataset of helium isotopes in Nordic Seas.

The influence of spreading rate, basement composition, fluid chemistry and chimney morphology on the formation of gold-rich SMS deposits at slow and ultraslow mid-ocean ridges

Seafloor massive sulphide (SMS) deposits are variably enriched in precious metals including gold. However, the processes invoked to explain the formation of auriferous deposits do not typically apply to mid-ocean ridge settings. Here, we show a statistically significant, negative correlation between the average gold concentration of SMS deposits with spreading rate, at non-sedimented mid-ocean ridges. Deposits located at slow spreading ridges (20–40 mm/a) have average gold concentrations of between 850 and 1600 ppb; however, with increasing spreading rate (up to 140 mm/a), gold concentrations gradually decrease to between ~ 50 and 150 ppb. This correlation of gold content with spreading rate may be controlled by the degree and duration of fluid-rock interaction, which is a function of the heat flux, crustal structure (faulting) and the permeability of the source rocks. Deposits at ultraslow ridges, including ultramafic-hosted deposits, are particularly enriched in gold. This is attributed to the higher permeability of the ultramafic source rocks achieved by serpentinisation and the inherent porosity of serpentine minerals, combined with relatively high gold concentrations in peridotite compared with mid-ocean ridge basalt. Variations in fluid chemistry, such as reducing conditions and the potential for increased sulphur availability at ultramafic-hosted sites, may also contribute to the high concentrations observed. Beehive chimneys, which offer more favourable conditions for gold precipitation, may be more prevalent at ultramafic-hosted sites due to diffuse low-velocity venting compared with more focussed venting at basalt-hosted sites.

Mid-ocean-ridge seismicity reveals extreme types of ocean lithosphere.

Along ultraslow-spreading ridges, where oceanic tectonic plates drift very slowly apart, conductive cooling is thought to limit mantle melting and melt production has been inferred to be highly discontinuous. Along such spreading centres, long ridge sections without any igneous crust alternate with magmatic sections that host massive volcanoes capable of strong earthquakes. Hence melt supply, lithospheric composition and tectonic structure seem to vary considerably along the axis of the slowest-spreading ridges. However, owing to the lack of seismic data, the lithospheric structure of ultraslow ridges is poorly constrained. Here we describe the structure and accretion modes of two end-member types of oceanic lithosphere using a detailed seismicity survey along 390 kilometres of ultraslow-spreading ridge axis. We observe that amagmatic sections lack shallow seismicity in the upper 15 kilometres of the lithosphere, but unusually contain earthquakes down to depths of 35 kilometres. This observation implies a cold, thick lithosphere, with an upper aseismic zone that probably reflects substantial serpentinization. We find that regions of magmatic lithosphere thin dramatically under volcanic centres, and infer that the resulting topography of the lithosphere-asthenosphere boundary could allow along-axis melt flow, explaining the uneven crustal production at ultraslow-spreading ridges. The seismicity data indicate that alteration in ocean lithosphere may reach far deeper than previously thought, with important implications towards seafloor deformation and fluid circulation.

Ultraslow, slow, or fast spreading ridges: Arm wrestling between mantle convection and far-field tectonics

Oceanic spreading rates are highly variable, and these variations are known to correlate to a variety of surface observables, like magmatic production, heat flow or bathymetry. This correlation lead to classify ridges into fast and slow spreading ridges, but also into the more peculiar ultraslow spreading regime. Here we explore the dynamic relationships between spreading ridges, plate tectonics and mantle flow. We first focus on the thermal signature of the mantle, that we infer from the global S-wave seismic tomography model of Debayle and Ricard (2012). We show that the thermal structure of ridges gradually departs from the half-space cooling model for slow, and above all ultraslow spreading ridges. We also infer that the sublithospheric mantle temperature decreases by more than 150 °C from fast to ultraslow spreading regimes. Both observations overall indicate that the mantle convection pattern is increasingly chaotic underneath slow and ultraslow spreading ridges. We suggest that this is due to far-field tectonics at the other ends of lithospheric plates: not only it modulates the spreading rates but it also alters the convection regime by obstructing the circulation of plates, which in turn modifies the surface kinematic conditions for the convecting mantle. We test this hypothesis using a thermo-mechanical model that represents a convection cell carrying a continental lithosphere atop. The continent gradually drifts away from the spreading ridge, from which the oceanic lithosphere grows and cools while the continent eventually collides at the opposite side. In turn, this event drastically modifies the upper kinematic condition for the convecting mantle that evolves from a mobile lid regime to an almost stagnant lid regime. Implications on spreading ridges are prominent: heat advection decreases with respect to thermal conduction, which causes the oceanic lithosphere to thicken faster; the oceanic plates get compressed and destabilized by a growing number of small scale transient plumes, which disrupt the structure of the oceanic lithospheres, lower the heat flow and may even starve ultraslow ridges from partial melting. It follows that the spreading rate of a modern ridge mirrors its status in the global plate tectonics framework within a unique breakup, drift, collision scenario, within the transition from mobile to stagnant lid, and that it is the same mechanism that build mountains at converging boundaries and control spreading rates. Oceanic ridges thus can be regarded as a sensor of the resisting rather than driving forces. Both the model and the seismic structure of the mantle underneath ridges reveal that the temperature variations are largest at shallow depths in the upper mantle, i.e. at the critical depth where the melt supply to the above ridges can be modulated, thereby also explaining why slow and ultraslow ridges are almost exclusively associated to cold mantle. It follows that the chemistry of oceanic ridge basalts may not strictly reveal the mantle potential temperature, but the variations in the sublithospheric temperature field.

Constraints on the accretion of the gabbroic lower oceanic crust from plagioclase lattice preferred orientation in the Samail ophiolite

Oceanic crust represents more than 60% of the earth's surface and despite a large body of knowledge regarding the formation and chemistry of the extrusive upper oceanic crust, there still remains significant debate over how the intrusive gabbroic lower oceanic crust is accreted at the ridge axis. The two proposed end-member models, the Gabbro Glacier and the Sheeted Sills, predict radically different strain accumulation in the lower crust during accretion. In order to determine which of these two hypotheses is most applicable to a well-studied lower crustal section, we present data on plagioclase lattice preferred orientations (LPO) in the Wadi Khafifah section of the Samail ophiolite. We observe no systematic change in the strength of the plagioclase LPO with height above the crust-mantle transition, no dominant orientation of the plagioclase a-axis lineation, and no systematic change in the obliquity of the plagioclase LPO with respect to the modal layering and macroscopic foliation evident in outcrop. These observations are most consistent with the Sheeted Sills hypothesis, in which gabbros are crystallized in situ and fabrics are dominated by compaction and localized extension rather than by systematically increasing shear strain with increasing depth in a Gabbro Glacier. Our data support the hypothesis of MacLeod and Yaouancq (2000) that the rotation of the outcrop-scale layering from sub-horizontal in the layered gabbros to sub-vertical near the sheeted dikes is due to rapid vertical melt migration through upper gabbros close to the axial magma chamber. Additionally, our results support the hypothesis that the majority of extensional strain in fast spreading ridges is accommodated in partially molten regions at the ridge axis, whereas in slow and ultra-slow ridges large shear strains are accommodated by plastic deformation.

Constraints on the mantle mineralogy of an ultra-slow ridge: Hafnium isotopes in abyssal peridotites and basalts from the 9–25°E Southwest Indian Ridge

We report on the Hf isotopic compositions of clinopyroxene mineral separates from eleven abyssal peridotites and Nd and Hf-isotopic compositions of twenty-seven co-located basalts from 9–25°E South West Indian Ridge (SWIR). In Nd–Hf isotope space the SWIR peridotites plot within the global MORB field (εNd=4.5–12.5, εHf=9.6–18.7), with the 15.23°E peridotites being the most radiogenic. The lack of correlation between Hf isotopes and trace or major element systematics including Lu/Hf ratios suggests that the 15.23°E peridotites were recently processed beneath the ridge and therefore participated in the production of the SWIR lavas. The Hf isotopic compositions of 15.23°E peridotites are more radiogenic than all basalts from the 9–25°E ridge, whereas the 9.98°E and 16.64°E peridotites partially overlap with the Hf isotope ratios of the spatially co-located basalts. This indicates the upwelling mantle beneath the SWIR contains material with enriched isotope signatures in addition to an isotopically depleted peridotitic mantle, which is consistent with the SWIR peridotites and basalt Nd isotope systematics from previously published studies. As the enriched isotope signatures are not observed in the peridotites we assume that they are sourced from material with lower solidus temperature than a typical peridotite. This enriched material was consumed during melting, and therefore may be mineralogically distinct (e.g. pyroxenite). Moreover, the variable spatial distribution of the enriched isotope signatures requires preferential sampling of the enriched component at distinct along-axis locations. The Hf–Nd isotope variability of the 9–25°E basalts can be entirely explained by mixing between a depleted peridotitic mantle end-member with the isotope composition of the 15.23°E peridotites and an enriched end-member with the isotope composition of the Narrowgate Segment lavas at 14.6°E. We estimate a maximum of 5% modal abundance of the enriched material in a peridotitic SWIR mantle. Our calculations further suggest that a larger portion (∼40%) of the Hf and Nd budget in the eruptive basalts is dominated by the enriched component. This implies that estimates of the MORB – source composition from the erupted MORB will be biased towards the fertile components and will underestimate the abundance of depleted peridotite in their source. The emerging picture is that of a convective upper mantle that is far more heterogeneous than MORB suggest, and where isotopically depleted peridotitic material may be more abundant than lava compositions suggest. Additional data from abyssal peridotites and other oceanic mantle xenoliths are needed in order to generate more realistic estimates of the upper mantle compositional variability and the processes that produced this picture. In addition, we demonstrate that the contrast in isotopic compositions between the basalts from the magmatic segments and peridotites from the adjacent amagmatic segments is consistent with melt focusing as a response to the variation in lithospheric crustal thickness, where deep low degree melts with larger proportion of enriched component focus in the areas of thin lithosphere.

Sustained volcanically-hosted venting at ultraslow ridges: Piccard Hydrothermal Field, Mid-Cayman Rise

At slow spreading mid-ocean ridges sustained submarine venting and the deposition of large seafloor massive sulfide deposits have previously been ascribed to tectonically-controlled hydrothermal circulation unrelated to young volcanic activity. Here, by contrast, we show that the Piccard Hydrothermal Field (PHF), on the ultraslow spreading Mid-Cayman Rise, represents a site of sustained fluid flow and sulfide formation hosted in a neovolcanic setting. The lateral extent and apparent longevity associated with the PHF are comparable to some of the largest tectonically-hosted vent sites known along the slow-spreading Mid-Atlantic Ridge. If such systems recur along all ultraslow ridges, which comprise ∼20% of the ∼55,000 km global ridge crest, potential implications would include (i) a higher probability of locating large, economically valuable, mineral deposits along ultraslow ridges together with (ii) larger fluxes than previously anticipated of chemicals released from high-temperature venting entering the oceans along the Atlantic–Indian Ocean sectors of the deep-ocean thermohaline conveyor.

Crustal thickness and earthquake distribution south of the Logachev Seamount, Knipovich Ridge

During RV Polarstern cruise ARK‐XXIV/3 (2009), a geophysical study along the ultraslow spreading Knipovich Ridge was conducted. The survey, located in the rift valley south of the Logachev Seamount (∼76°36′N), provides a crustal thickness of ∼4.5 km in the amagmatic parts and 5.7 km underneath the seamount itself. The velocity‐depth function indicates the presence of a thick oceanic layer 2 (4.5 km), but no indication for a thick oceanic layer 3. The only exception is the area underneath the Logachev Seamount, where velocities higher than 6 km/s are detected. This indicates a stronger and focussed melt supply underneath the seamount. Local seismicity was analysed for two days. In total, 191 quakes were identified in the vicinity of the rift valley with magnitudes up to ML = 2.6. At least 48 of them are located in the upper mantle (up to 18 km below sea level), supporting models predicting a cold mantle or conductive cooling underneath ultraslow ridges to explain the reduced melt supply.

First active hydrothermal vents on an ultraslow-spreading center: Southwest Indian Ridge

The ultraslow-spreading Southwest Indian Ridge is a major tectonic province, representing one of the important end-member mid-ocean-ridge types for its very slow and oblique spreading, and providing the only known route for migration of chemosynthetic deep-sea vent fauna between the Atlantic and Indian Oceans. We report the investigation of the first active high-temperature hydrothermal field found on any ultraslow mid-ocean ridge worldwide. Located on Southwest Indian Ridge at 37°47′S, 49°39′E, it consists of three zones extending ∼1000 m laterally, and it is one of four recently discovered active and inactive vent sites within a 250-km-long magmatically robust section. Our results provide the first direct evidence for potentially widespread distribution of hydrothermal activity along ultraslow-spreading ridges—at least along magmatically robust segments. This implies that the segment sections with excess heat from enhanced magmatism and suitable crustal permeability along slow and ultraslow ridges might be the most promising areas for searching for hydrothermal activities. It is surprising that the special vent fauna appear to indicate some complex affinity to those on the Central Indian Ridge, southern Mid-Atlantic Ridge, and the southwest Pacific Ocean.

From slow to ultra-slow: How does spreading rate affect seafloor roughness and crustal thickness?

We examine the relationship of seafloor roughness and gravity-derived crustal thickness to both spreading rate and inferred mantle temperature using statistical analysis of a multibeam bathymetry and gravity data compilation of the axis and flanks between 54°E and 67°E at the Southwest Indian Ridge (southwest Indian Ocean). Our findings indicate that root mean square values of abyssal hill heights increase from 220 ± 20 m to 300 ± 20 m along flow line corridors that transition a well-constrained full spreading rate change from slow (30 mm/yr) to ultra-slow (15 mm/yr). Mantle Bouguer gravity anomalies, however, indicate no significant change in inferred crustal thickness at the spreading rate transition. In the axis-parallel direction, roughness of both slow and ultra-slow seafloor increases from 54°E to 63°E while inferred crustal thickness and/or mantle temperature decrease. These findings have implications for the relationship between spreading rate and melt production: they suggest that mantle temperature at slow and ultra-slow ridges may play a more important role than spreading rate in determining seafloor morphology. The lack of evidence for significant crustal thinning accompanying a change from slow to ultra-slow spreading rate lends support to focused subaxial mantle upwelling models.