Mid-ocean Ridge System Research Articles

Transfer learning reconstructs submarine topography for global mid-ocean ridges

Mid-ocean ridges are unique, tectonically active geographical units on Earth that profoundly control the ocean environment and dynamics at the global scale. However, high-resolution topographic data from mid-ocean ridges are rarely available due to the difficulty in detecting ocean floors, which further limits ocean research at the global scale. Here, we divide the global mid-ocean ridge system into 2805 tiles and reconstruct their high-resolution topography by using a transfer learning approach with freely available low-resolution digital elevation models (DEMs) and limited high-resolution DEMs. A high-frequency terrain feature-based deep residual network is proposed to generate high-resolution global mid-ocean ridge DEMs. In this network, topographic knowledge related to mid-ocean ridges is integrated and quantified to improve the learning efficiency and reconstruction quality of the network. A series of verifications and evaluations demonstrate the reliability of reconstructed topographies for submarine topography research. We observe that reconstructed topography can achieve good environmental understanding and information acquisition in the global mid-ocean ridge range. We find that the complexity of the previous terrain environment is underestimated by 26.63% in terms of the slope gradient and by 14.95% in terms of terrain relief, while a 101.10% information improvement can be obtained for the reconstructed topography. The reconstructed topography indicates that diverse and intricate topographical environments of mid-ocean ridges exist among different ocean regions. The proposed transfer learning method for reconstructing high-resolution mid-ocean ridge topographies is valuable and can be utilized for reconstructing information in regions that are difficult to observe directly and lack sufficient data.

Prediction of CO2 content in mid-ocean ridge basalts via a machine learning approach

Abstract One of the primary locations of mafic magma production on Earth is the global mid-ocean ridge system. The basalts erupted along ridges probe the upper mantle and can be used to explore the deep carbon cycle. However, mid-ocean ridge basalts (MORBs) degas heavily during magma ascent. Some incompatible-trace-element–depleted and –enriched MORBs avoid heavy degassing, and show a narrow range of CO2/Ba, which have been used to reconstruct the pre-eruptive CO2 content of primitive MORB. With an increasing amount of data, however, it has become apparent that the CO2/Ba ratios of MORBs vary significantly. We compiled a data set of the geochemical compositions of MORB glasses and melt inclusions that are not degassed significantly and used a supervised machine learning model to accurately predict CO2 contents of individual samples from the concentrations of selected elements. This approach reveals that predicted CO2 contents and CO2/Ba ratios of global MORBs are highly variable, highlighting the significance of mantle heterogeneity, which can be attributed to the interactions with deep-sourced plumes or recycled crust (oceanic crust with or without sediments). Our findings underscore the potential of machine learning as a powerful tool for investigating the intricate interplay between carbon, mantle composition, and Earth's long-term geological processes.

Determining mid-ocean ridge geography from upper mantle temperature

In this study, we examine the influence of the mantle and large-scale tectonics on the global mid-ocean ridge (MOR) system. Using solely seismically-inferred upper mantle temperatures below the melting zone (260-600 km) and an interpretable machine learning model (Random Forest and Principal Component Analysis), we can predict a priori the location (ocean basin and ridge system) of global MOR with up to 90% accuracy. Two features provide > 50% of the discriminative power: the temperature difference between the mid-layer (340-500 km) and other depths, and the depth-averaged temperature of the upper mantle. We suggest long-term (100s Myr) tectonic and large-scale convective processes primarily driven by deep subduction left ample, distinct but hidden fingerprints in the mantle that allow us to separate regions at ∼1000 km scale. Our result implies that the large-scale geophysical and geochemical differences observed along the MOR system are reflective, not primarily of shallow processes associated with melting, but of the integrated long-term tectonic, subduction, and convective flow history that determines the present-day upper mantle temperature structure.

Seafloor spreading and the delivery of sulfur and metals to Earth’s oceans

Abstract Circulating fluids within Earth’s mid-ocean ridge system cool and alter the oceanic crust, contribute distinct chemistry to the ocean, and generate economically and geologically important metal-sulfide deposits at the seafloor. Yet, we have few constraints on the characteristics of these fluids at peak subseafloor pressure and temperature conditions or how the primary variable, seafloor spreading, affects these fluids’ delivery of metals and sulfur to seawater. Here, we develop a new, robust technique for estimating the peak endowment of heat and dissolved sulfur, iron, and copper in subseafloor hydrothermal fluids and determining their fate as these superheated fluids rise to the seafloor. Calculations using this technique indicate that >20%–70% of sulfur, iron, and copper dissolved at peak subseafloor conditions are lost during upflow due to cooling and concomitant decreases in sulfide mineral solubility. The interpretation of these estimates within the geologic context of vent fields allows us to demonstrate a strong inverse relationship between seafloor spreading rate and peak pressure-temperature conditions, subseafloor heat loss, and the magnitude of subseafloor sulfide mineralization. Our results demonstrate the extent to which the secular variation of Earth’s mid-ocean ridge system over geologic time has impacted sulfide deposition rates and hydrothermal fluxes of sulfur and metals to the ocean.

South Mid-Atlantic Ridge 25.3–27.7°S segment basalts: Implications for entrainment of the Tristan plume material in the mid-ocean ridge system

We present new major and trace element and Sr-Nd-Pb isotope data for basalts from the 25.3–27.7°S segments of the South Mid-Atlantic Ridge (SMAR). A fractional crystallization model shows that olivine, plagioclase, and clinopyroxene crystals crystallized in the magmatic chamber underlying the SMAR 27.1°S and 27.7°S segments, while the SMAR 25.3°S segment basaltic lavas are dominated by fractional crystallization of olivine and plagioclase. Their Ce/Yb, (Tb/Yb)N, and Fe/Mn ratios suggest a mantle source lithology dominated by spinel-lherzolite. The linear correlations between Mg# and TiO2, Ba, and Zr/Nb indicate that SMAR basalts in this study have primary mantle melts with similar chemical compositions and partial melting extents. The along-ridge variations in the fractionation-corrected incompatible element (Ba, Zr/Nb, (La/Sm)N) and Sr-Nd-Pb isotope compositions of the SMAR 20–40°S basalts show that Tristan mantle plume materials exist in the SMAR asthenosphere and gradually decrease from the southern SMAR 35°S segment in the vicinity of Tristan Island to the northern SMAR 25.3–27.7°S segments. The mixing models of the fractionation-corrected Ba contents, (La/Sm)N ratios and radiogenic Sr-Nd-Pb isotope ratios show that variable degrees of contamination by the Tristan mantle plume contribute to the heterogeneous SMAR 20–40°S mantle source. The proportion of Tristan plume-related materials retained in the SMAR 20–40°S asthenosphere is <10%, which gradually decreases from south to north. By integrating the evolution of the South Atlantic and the historical interaction between the SMAR system and the Tristan mantle plume, we demonstrate that the systematic variations in the geochemical compositions of the SMAR 25.3–27.7°S basalts are induced by the presence of the Tristan mantle plume materials in the southern SMAR asthenosphere.

Mineralogy and trace element geochemistry of hydrothermal sulfides from the Ari vent field, Central Indian Ridge

The Ari vent field (AVF) is an ultramafic-hosted seafloor massive sulfide (SMS) deposit in the middle part of the Central Indian Ridge. In this paper, we describe the detailed mineralogy and geochemistry of hydrothermal sulfide samples from the AVF, which can be classified into Fe–Cu- and Cu-rich types based on the major sulfide minerals. Sulfide mineralisation of the former type comprises: (1) stage I, early deposition of magnetite, pyrrhotite, isocubanite, chalcopyrite, and subhedral–euhedral pyrite under high-temperature fluid conditions (> 335 °C); (2) stage II, deposition of colloform pyrite, sphalerite, galena, and electrum from low-temperature fluids (< 200 °C) during the later mineralisation stage; and (3) stage III, seawater alteration that caused the precipitation of uraninite and chalcocite. This indicates that the fluids in the AVF had decreasing temperature and ƒS2 and increasing ƒO2 as mineralisation proceeded. The Cu-rich sulfide samples have mineral assemblages and a paragenesis similar to those of the Fe–Cu-rich sulfide samples, but the higher proportion of isocubanite is indicative of relatively high-temperatures and reducing conditions during mineralisation. Bulk chemical compositions of the AVF sulfides are characterised by high U contents (up to 51.9 ppm) and a distinct Sn distribution (2.1–86.4 ppm) between the two different types of hydrothermal samples, which differ from those of other ultramafic-hosted sulfide deposits. The U content is controlled mainly by the precipitation of discrete uraninite grains (< 1 μm in size) on altered surfaces of pyrite and hematite. The oxidative alteration of Fe-bearing minerals caused the fixation of seawater-derived U. Laser ablation–inductively coupled plasma–mass spectrometry analysis showed that most trace elements occur in solid solution in the sulfide minerals, mainly controlled by the physicochemical conditions of the hydrothermal fluids (e.g. temperature, ƒS2, and ƒO2). In particular, a comparative analysis of other mid-ocean ridge systems shows that the ultramafic-hosted sphalerite and pyrite are more enriched in Sn as compared with those hosted by basaltic rocks. However, the Fe–Cu-rich sulfide samples of the AVF are Sn-poor (< 10.2 ppm), because pyrite is substantially depleted in Sn (mostly < 1 ppm) as compared with sphalerite, regardless of the effect of the ultramafic-hosted mineralisation. This indicates that in situ trace element analysis of sphalerite and pyrite, especially for Sn, can provide insights into the different hydrothermal mineralisation in basaltic- and ultramafic-hosted systems, which cannot necessarily be inferred from bulk analysis. Our comparison also suggests that the Sn contents of ultramafic-hosted SMS deposits would be a possible source of Sn for the ultramafic-hosted volcanogenic massive sulfide (UM-VMS) deposit. The δ34S values (+ 6.2 to + 8.5‰) of the pyrite record thermochemical sulfate reduction of seawater, which suggests that sulfur and most metals were predominantly leached from the associated host rocks with a contribution (29–40%) from reduced seawater sulfur. In conclusion, the AVF is a rock-dominated system that contains ultramafic-hosted mineralisation in the Central Indian Ridge.

Fluid-assisted grain size reduction leads to strain localization in oceanic transform faults

Oceanic Transform Faults are major plate boundaries representing the most seismogenic part of the mid ocean ridge system. Nonetheless, their structure and deformation mechanisms at depth are largely unknown due to rare exposures of deep sections. Here we study the mineral fabric of deformed mantle peridotites - ultramafic mylonites - collected from the transpressive Atobá ridge, along the northern fault of the St. Paul transform system in the Equatorial Atlantic Ocean. We show that, at pressure and temperature conditions of the lower oceanic lithosphere, the dominant deformation mechanism is fluid-assisted dissolution-precipitation creep. Grain size reduction during deformation is enhanced by dissolution of coarser pyroxene grains in presence of fluid and contextual precipitation of small interstitial ones, leading to strain localization at lower stresses than dislocation creep. This mechanism potentially represents the dominant weakening factor in the oceanic lithosphere and a main driver for the onset and maintenance of oceanic transform faults.

Seamount chains and hotspot tracks: Superficially similar, deeply different

Nearly 80% of the seafloor extension has not been covered by high-resolution bathymetry, impeding direct observation of seamounts. Nevertheless, lists of seamount location and height at a global scale have been produced using different techniques. In this work four of such databases (publicly available) are compared with each other to assess their differences. Results identify large differences among databases that could have exerted strong influences on models of seamount production and associated geodynamic processes. Despite those differences, it is shown that all databases allow the identification of seamount lines both along the present-day Mid Ocean Ridge (MOR) system and on intraplate settings. Notably, those seamount lines do not coincide with the so-called hotspot tracks that commonly were defined by selectively focusing attention on the larger seamounts. Examination of all the databases also shows that distinction based only on seamount size between seamounts produced at Mid-Ocean Ridge (MOR) environments from those associated with mantle-plum fed-hotspot activity has been overestimated. This, combined with the fact that most seamount lines defined by the available databases can be traced back to past locations of MOR indicates that most of the present-day intraplate linear arrays of seamounts, which include some large seamounts, were not produced by the action of underlying mantle anomalies envisaged in the form of mantle plumes. The evidence presented here calls for a reassessment of the form in which volcanic and tectonic activities are conceptually related to each other.

Mantle serpentinization and associated hydrogen flux at North Atlantic magma-poor rifted margins

AbstractMantle serpentinization influences the rheology of altered peridotites and the global fluxes of energy and volatiles, the generation of seafloor and sub-seafloor chemolithotrophic life, and the carbon cycle. As a by-product of serpentinization, molecular hydrogen (H2) is generated, which supports chemosynthetic communities, and this mechanism may have driven the origin of life on early Earth. At continent-ocean transition zones (COTs) of magma-poor rifted margins, the mantle is exposed and hydrated over hundreds of kilometers across the rift, but the H2 fluxes associated with this process are poorly known. Here, we coupled a thermomechanical model with serpentinization reaction equations to estimate associated H2 release during mantle exhumation at COTs. This reproduced a tectonic structure similar to that of the West Iberia margin, one of the best-studied magma-poor margins. We estimated the rate of H2 production from mantle hydration at (7.5 ± 2.5) × 107 mol/(yr × km). By estimating the area of exhumed mantle from wide-angle seismic profiles at North Atlantic magma-poor margins, we calculated that the accumulated H2 production could have been as high as ~4.3 × 1018 mol (~8.6 × 1012 metric tons) prior to opening of the North Atlantic Ocean, at a rate of ~1.4 × 1017 mol/m.y. This is one quarter of the total predicted flux produced by the global system of mid-ocean ridges, thus highlighting the significance of H2 generation at magma-poor margins in global H2 fluxes, to hydrogenothropic microbial life, and, perhaps, as a potential energy source.

The effect of high-temperature alteration of oceanic crust on the potassium isotopic composition of seawater

High-temperature hydrothermal alteration of oceanic crust is one of the two main sources of potassium (K) to the oceans, with modern flux estimates ranging roughly from ∼8 to 30 % of the K flux from rivers. Despite the role of high-temperature hydrothermal fluids in the global seawater K budget, little is known about its effect on the K isotopic composition of the oceans. Here we present stable K isotope measurements (δ41KSRM3141a) of globally distributed high-temperature hydrothermal fluids from three mid-ocean ridge systems: the East Pacific Rise (n = 21), the Juan de Fuca Ridge (n = 34), and the Mid-Atlantic Ridge (n = 18). We find a strong correlation between δ41K and Mg/K ratios, consistent with conservative mixing between a high-temperature hydrothermal fluid (i.e., Mg = 0) and seawater as fluids ascend to the seafloor and/or due to seawater entrainment during sampling. The δ41K of end-member hydrothermal fluids is found to range between −0.80 ‰ and 0.07 ‰, with an average value of −0.36 ± 0.30 ‰ (2σ, n = 38). Most (∼76 %) of the variability in end-member fluid δ41K compositions observed here can be explained by high-temperature fluid-rock K exchange, with little (∼0.2 ‰) to no K isotope fractionation between hydrothermal fluid and altered crust. Larger deviations from the average end-member hydrothermal fluid value are likely to result from processes other than high-temperature fluid-rock exchange, such as (1) low-temperature hydrothermal reactions during fluid recharge, (2) reaction of fluids with local sedimentary sources, and (3) phase separation. The K contents of end-member fluids vary considerably, from ∼1 to 38 mM, thus a K-weighted average of −0.37 ± 0.24 ‰ (2σ, n = 38) is estimated to represent the δ41K composition of the global hydrothermal K flux. Our results suggest that K sourced from axial hydrothermal alteration does not contribute to the elevated 41K/39K of seawater compared to bulk silicate Earth (BSE). In addition, subduction of oceanic crust altered under high-temperature conditions is unlikely to be a significant source of K isotopic heterogeneity to Earth’s mantle.

Oceanic Zircon Records Extreme Fractional Crystallization of MORB to Rhyolite on the Alarcon Rise Mid-Ocean Ridge

Abstract The first known occurrence of rhyolite along the submarine segments of the mid-ocean ridge (MOR) system was discovered on Alarcon Rise, the northernmost segment of the East Pacific Rise (EPR), by the Monterey Bay Aquarium Research Institute in 2012. Zircon trace element and Hf and O isotope patterns indicate that the rhyolite formed by extreme crystal fractionation of primary mid-ocean ridge basalt (MORB) sourced from normal to enriched MOR mantle with little to no addition of continental lithosphere or hydrated oceanic crust. A large range in zircon ɛHf spanning 11 ɛ units is comparable to the range of whole rock ɛHf from the entire EPR. This variability is comparable to continental granitoids that develop over long periods of time from multiple sources. Zircon geochronology from Alarcon Rise suggests that at least 20 kyr was needed for rhyolite petrogenesis. Grain-scale textural discontinuities and trace element trends from zircon cores and rims are consistent with crystal fractionation from a MORB magma with possible perturbations associated with mixing or replenishment events. Comparison of whole rock and zircon oxygen isotopes with modeled fractionation and zircon-melt patterns suggests that, after they formed, rhyolite magmas entrained hydrated mafic crust from conduit walls during ascent and/or were hydrated by seawater in the vent during eruption. These data do not support a model where rhyolites formed directly from partial melts of hydrated oceanic crust or do they require assimilation of such crust during fractional crystallization, both models being commonly invoked for the formation of oceanic plagiogranites and dacites. A spatial association of highly evolved lavas (rhyolites) with an increased number of fault scarps on the northern Alarcon Rise might suggest that low magma flux for ~20 kyr facilitated extended magma residence necessary to generate rhyolite from MORB.

Equilibrium and kinetic controls on molecular hydrogen abundance and hydrogen isotope fractionation in hydrothermal fluids

Molecular hydrogen (H2) is among the main components in hydrothermal fluids and exerts a major role in geochemical and biological processes. Nevertheless, the understanding and quantification of factors controlling H2 abundance and isotope composition remain uncertain. Sub-aerial hydrothermal systems provide a unique opportunity to study sources and reactions of H2. Here, we present hydrogen fugacity (fH2) and δD values of H2 and H2O in hydrothermal fluids from such systems worldwide (Greece, Iceland, Kenya and New Zealand), representative of mid-ocean ridge and arc systems and sourced by seawater and meteoric water. The hydrothermal fluid temperatures are 226-359°C, and the fH2, δD-H2 and δD-H2O values are 0.002-3.3 bar, -646 to -391‰ and -94.1 to +11.3‰, respectively. Comparison of measured data with geochemical modeling reveals that H2 is predominantly sourced from the reduction of H2O, enabled through the oxidation of aqueous Fe+II to Fe+III and precipitation of Fe+III-bearing hydrothermal minerals (e.g., epidote). We argue that fH2 in hydrothermal fluids is controlled by metastable mineral-fluid and fluid-fluid equilibria upon progressive rock alteration and depend on temperature, rock-to-water ratio (r/w), source water composition and volcanic gas input. Consequently, fH2 in these systems is not fixed by a sole redox buffer. Our data demonstrate that δD-H2 values of reservoir fluids at depth are controlled by the δD value of the source water and the equilibrium isotope fractionation characteristic of the hydrothermal reservoir temperatures. Attainment of isotopic equilibrium at reservoir conditions is supported by fast isotopic equilibration times at T > 200°C, on the order of minutes to few hours, relative to hydrothermal fluid residence times within the reservoirs of years to decades or even longer. During its ascent to the surface, the isotopic composition of H2 can be modified due to isotopic re-equilibration with H2O. The extent of re-equilibration depends on the interplay between fluid flow velocity, cooling rate and kinetics of D-H exchange between H2 and H2O. For fast flowing well discharges (∼0.1 to >10 m/s), negligible changes occur, whereas for slowly flowing (<0.06 m/s) fumaroles, complete re-equilibration at surface temperatures (∼100°C) is sometimes observed. Based on the extent of hydrogen isotope disequilibrium relative to reservoir conditions, fluid reservoir-to-surface travel times are estimated to range from few minutes to <3 hrs for well fluids and few hours to days for fumarolic vapor.

Ridge Jumps and Mantle Exhumation in Back-Arc Basins

Back-arc basins in continental settings can develop into oceanic basins, when extension lasts long enough to break up the continental lithosphere and allow mantle melting that generates new oceanic crust. Often, the basement of these basins is not only composed of oceanic crust, but also of exhumed mantle, fragments of continental crust, intrusive magmatic bodies, and a complex mid-ocean ridge system characterised by distinct relocations of the spreading centre. To better understand the dynamics that lead to these characteristic structures in back-arc basins, we performed 2D numerical models of continental extension with asymmetric and time-dependent boundary conditions that simulate episodic trench retreat. We find that, in all models, episodic extension leads to rift and/or ridge jumps. In our parameter space, the length of the jump ranges between 1 and 65 km and the timing necessary to produce a new spreading ridge varies between 0.4 and 7 Myr. With the shortest duration of the first extensional phase, we observe a strong asymmetry in the margins of the basin, with the margin further from trench being characterised by outcropping lithospheric mantle and a long section of thinned continental crust. In other cases, ridge jump creates two consecutive oceanic basins, leaving a continental fragment and exhumed mantle in between the two basins. Finally, when the first extensional phase is long enough to form a well-developed oceanic basin (&gt;35 km long), we observe a very short intra-oceanic ridge jump. Our models are able to reproduce many of the structures observed in back-arc basins today, showing that the transient nature of trench retreat that leads to episodes of fast and slow extension is the cause of ridge jumps, mantle exhumation, and continental fragments formation.

Seawater Chemistry and Hydrothermal Controls on the Cenozoic Osmium Cycle

AbstractOsmium isotope ratios are a key tool to track changes in global weathering and carbon cycle evolution through time. Long‐term changes in seawater Os isotope records over the Cenozoic have been used to argue for changes in weathering from increased uplift, leading to long‐term global cooling. Sulfide mineral precipitation during hydrothermal circulation traps the majority of mantle‐derived Os within mid‐ocean ridge (MOR) hydrothermal systems. Building from evidence that the amount of sulfide mineral precipitation in the subseafloor of MOR systems is related to the amount of sulfate and calcium in seawater, we show that as seawater chemistry changed through time, so too has the extent of hydrothermal sulfide formation and the global Os cycle. With currently estimated changes in seawater Ca/SO4 ratios, the observed progressive increase in seawater Os isotope values through the Cenozoic can be linked to changes in seawater chemistry—instead of a major shift in continental weathering processes.

Beta diversity differs among hydrothermal vent systems: Implications for conservation.

Deep-sea hydrothermal vent habitats are small, rare and support unique species through chemosynthesis. As this vulnerable ecosystem is increasingly threatened by human activities, management approaches should address biodiversity conservation. Diversity distribution data provide a useful basis for management approaches as patterns of β-diversity (the change in diversity from site to site) can guide conservation decisions. Our question is whether such patterns are similar enough across vent systems to support a conservation strategy that can be deployed regardless of location. We compile macrofaunal species occurrence data for vent systems in three geological settings in the North Pacific: volcanic arc, back-arc and mid-ocean ridge. Recent discoveries in the Mariana region provide the opportunity to characterize diversity at many vent sites. We examine the extent to which diversity distribution patterns differ among the systems by comparing pairwise β-diversity, nestedness and their additive components. A null model approach that tests whether species compositions of each site pair are more or less similar than random provides insight into community assembly processes. We resolve several taxonomic uncertainties and find that the Mariana arc and back-arc share only 8% of species despite their proximity. Species overlap, species replacement and richness differences create different diversity distributions within the three vent systems; the arc system exhibits much greater β-diversity than both the back-arc and mid-ocean ridge systems which, instead, show greater nestedness. The influence of nestedness on β-diversity also increased from the arc to back-arc to ridge. Community assembly processes appear more deterministic in the arc and ridge systems while back-arc site pairs deviate little from the null expectation. These analyses reflect the need for a variety of management strategies that consider the character of diversity distribution to protect hydrothermal vents, especially in the context of mining hydrothermal deposits.

Spatiotemporal Relationship between Arctic Mid-Ocean Ridge System and Intraplate Seismicity of the European Arctic

AbstractIn this article, we investigate the influence of the Arctic mid-ocean ridge system (AMORS), including the Gakkel and Mohns ridges, and the Knipovich ridge–Lena trough (KL) segment, on seismicity of the Novaya Zemlya archipelago area (NZ) and the northernmost margin of the East-European Platform (EEP) for 1980–2019. For each individual area, the annual seismic energy was obtained by adding the energies of all earthquakes. To do this, we have converted various types of magnitude by different seismic networks into moment magnitude Mw. We compiled the updated catalog for the NZ, the northern EEP, and the northern part of the Ural fold belt (UFB). As a result, we constructed time distributions of annual seismic energy releases for each composing ridges of AMORS, NZ, and EEP combined with UFB. A model based on the Elsasser’s one describing the transfer of lithospheric stress disturbances in the horizontal direction was built, and quantitative calculations of the disturbance propagations from AMORS were performed. Results are in good agreement with the annual seismic energy time lags between rifts and NZ and EEP together with the UFB. We calculated correlation coefficients between the seismic energy releases over the time for the structures, enabling identification of the characteristic excitation cycles and estimation of the interval of disturbance transfer from AMORS. As a result, disturbances from the Gakkel ridge appear 3 yr later in NZ, from the KL segment in 4 yr, and from the Mona ridge in 8 yr. For the EEP + UBF combined area, we found the following disturbances spreading lags as 7 yr for the Mona ridge, 4 yr for the KL segment, and 5 yr for the Gakkel ridge. The obtained damping amplitudes of the disturbance spreading from the arctic ridges are sufficient to affect the intraplate seismic activity.

Improved Seismic Monitoring with OBS Deployment in the Arctic: A Pilot Study from Offshore Western Svalbard

AbstractThe mid-ocean ridge system is the main source of earthquakes within the Arctic region. The earthquakes are recorded on the permanent land-based stations in the region, although, smaller earthquakes remain undetected. In this study, we make use of three Ocean Bottom Seismographs (OBSs) that were deployed offshore western Svalbard, along the spreading ridges. The OBS arrival times were used to relocate the regional seismicity, using a Bayesian approach, which resulted in a significant improvement with tighter clustering around the spreading ridge. We also extended the regional magnitude scales for the northern Atlantic region for OBSs, by computing site correction terms. Besides location and magnitude improvement, the OBS network was able to detect hundreds of earthquakes, mostly with magnitude below Mw 3, including a swarm activity at the Molloy Deep. Our offshore observations provide further evidence of a low-velocity anomaly offshore Svalbard, at the northern tip of Knipovich ridge that was previously seen in full-waveform inversion. We conclude that even a single permanent OBS near the ridge would make a significant difference to earthquake catalogs and their interpretation.

Cryptic zoning in primitive olivine as an archive of mush fluidization at mid-ocean ridges

Processes operating within magmatic crystal mushes exert a fundamental control on the physical and chemical evolution of mid-ocean ridge systems. It is widely recognized that mush fluidization in response to magma recharge could produce variable compositional zoning in crystals. However, olivines generally show relatively smooth MgFe zoning in comparison with complex and short-length scale zoning in plagioclases. It is still unclear about the proceedings of the olivine before the mush disaggregation immediately prior to eruption and unknown whether plagioclase zoning is genuinely more complex than olivine zoning. Here, using slow-diffusing elements X-ray maps, we identify complex compositional zoning in olivines from MORBs of the Southwest South China Sea (SCS) and thus provide mineralogical evidence for the re-organization within a mush that takes place over an extended period of time. Primitive olivines have homogeneous forsterite concentrations (Fo = 89.5 mol%), but preserve zoning in Al2O3 and Cr2O3. Multiple cores have anomalously low Al2O3 and Cr2O3 contents, with or without high P2O5, Al2O3, and Cr2O3 dendrites, and are surrounded by Al–Cr-rich but P-depleted mantles, in which boundaries between cores and mantles are sharply defined. This compositional and textural zoning indicates dissolution, re-equilibration, and re-growth of evolved olivine within primitive magmas, demonstrating a redistribution of crystals within a mush zone, consistent with mush fluidization. Therefore, primitive olivines, widely-used monitors of mantle processes, may also have undergone mush fluidization, such that apparently primitive olivines may result from the re-equilibration of evolved olivines with primitive melts. PAl zoning patterns in olivine can be used to evaluate the influence of mush processes on olivine mantle probes.

Local rift and intraplate seismicity reveal shallow crustal fluid-related activity and sub-crustal faulting

Seismicity delineating the mid-ocean ridge system in the Panama Basin is mostly concentrated along transform offsets. Most intraplate areas show little-to-no seismic activity. Here we analyze passive recordings from a short-term deployment of 75 ocean-bottom seismographs (OBS) at the Costa Rica Rift (CRR) and across its southern flank along a flowline to Ocean Drilling Program (ODP) Hole 504B. The OBS array recorded 1061 events along the CRR and the Ecuador Fracture Zone, and 127 intraplate events around ODP Hole 504B. The seismic activity along the CRR occurred in clusters, with some events associated with an axial magma lens (AML) located at 3–3.5 km depth below seafloor (bsf), and four events exhibiting normal and reverse faulting focal mechanisms. These deep events are followed by the largest event cluster whose epicenters connect the AML to the seafloor, at a location where a hydrothermal plume was previously reported. During the same period, another event cluster occurs close to the seabed. This spatio-temporal pattern suggests that deeper events close to the AML, which might be related to thermal stresses or stress perturbations due to a volume change in the AML, trigger fluid-related seismicity within the shallower hydrothermal system. The easternmost extent of the seismicity along the CRR corresponds to an overlapping spreading center (OSC). At depth, the seismicity around the OSC is focused beneath one limb, but is spread over a larger area closer to the surface. The focal mechanisms calculated for OSC events show both normal and reverse motions, and might reflect complex stress or fault orientations. Intraplate seismicity around ODP Hole 504B shows both isolated and clustered events from the seafloor to ∼15–25 km bsf. Some of this seismicity is likely associated with the reactivation of east-west trending normal faults under high pore pressure and thermoelastic stresses conditions. Deep seismicity (i.e. between 15 and 20 km bsf) occurs in small, well-defined clusters close to ODP Hole 504B, at the theoretical depth of the brittle-to-plastic transition for a ∼6.9 Ma oceanic lithosphere. Our results show the importance of short-term OBS deployments for improving our understanding of mid-ocean ridge seismicity and hydrothermal processes, as well as intraplate seismicity and deformation mechanisms.

A thin mantle transition zone beneath the equatorial Mid-Atlantic Ridge.

The location and degree of material transfer between the upper and lower mantle are key to the Earth's thermal and chemical evolution. Sinking slabs and rising plumes are generally accepted as locations of transfer1,2, whereas mid-ocean ridges are not typically assumed to have a role3. However, tight constraints from in situ measurements at ridges have proved to be challenging. Here we use receiver functions that reveal the conversion of primary to secondary seismic waves to image the discontinuities that bound the mantle transition zone, using ocean bottom seismic data from the equatorial Mid-Atlantic Ridge. Our images show that the seismic discontinuity at depths ofabout 660kilometres is broadly uplifted by 10±4 kilometres over a swath about 600kilometres wide and that the 410-kilometre discontinuity is depressed by 5±4kilometres. This thinning of the mantle transition zone is coincident with slow shear-wave velocities in the mantle, from global seismic tomography4-7. In addition, seismic velocities in the mantle transition zone beneath the Mid-Atlantic Ridge are on average slower than those beneath older Atlantic Ocean seafloor. The observations imply material transfer from the lower to the upper mantle-either continuous or punctuated-that is linked to the Mid-Atlantic Ridge. Given the length and longevity of the mid-ocean ridge system, this implies that whole-mantle convection may be more prevalent than previously thought, with ridge upwellings having a role in counterbalancing slab downwellings.