Slow-spreading Mid-Atlantic Ridge Research Articles

Seismic observation of an active detachment faulting system beneath the Longqi hydrothermal field at the ultraslow spreading southwest Indian ridge

Hydrothermal processes in detachment settings at slow and ultraslow spreading ridges differ greatly from those at melt-rich faster spreading ridges. Active detachment faulting provides the possibility for off-axis high-temperature hydrothermal vents located far away from the heat source beneath the axial volcanic ridge. Seismic data from the slow spreading Mid-Atlantic ridge revealed that hydrothermal fluids may exploit detachment faults to extract heat from a melt zone near the crust-mantle interface. However, knowledge of the subsurface structure and the kinematic processes of detachment faults, and their interaction with hydrothermal fields at the slowest spreading ridges is still insufficient. Here, we report new insights on the seismicity beneath the Longqi hydrothermal field at the ultraslow spreading southwest Indian ridge from three ocean bottom seismometer monitoring experiments. The seismicity outlines the subsurface geometry of a detachment faulting system (DF1 and DF2). The strongly flexed DF1 (45°) is a mature detachment fault with a domed-shaped OCC where ultramafic rocks are exposed on the seafloor. An active young DF2 with intense earthquake activity along the steep subsurface (67°) suggests the initiation phase of rotation. The diversity of hydrothermal activities in the Longqi detachment-hydrothermal systems is closely related to the evolution of detachment faults. Additionally, both the non-transform discontinuity on the western margin of the Longqi-1 field and local faults facilitate hydrothermal circulation. Our study provides baseline observations for a Longqi-type of hydrothermal circulation at the inside corner associated with detachment faulting and non-transform offsets.

Arc signatures in abyssal peridotites and its implications

Investigating mass exchanges between the subduction zones and mid-ocean ridges provides essential clues to understanding recycling process and mantle convection in the Earth's mantle. In this study, we found evidence of recycled subduction signals in the abyssal peridotites from mid-ocean ridges through their extremely low bulk (Dy/Yb)N and bulk Al2O3, which could not be explained by normal decompression melting at mid-ocean ridges, but required a hydrous melting history. We show here that abyssal peridotites from the fast-spreading Pacific Oceanic Ridges (East-Pacific Rise and Pacific-Antarctic Ridge), to the ultraslow-spreading Southwest Indian Ridge and to the slow-spreading Mid-Atlantic Ridge, require increasing amount of slab-derived fluid in their mantle sources. Therefore, the subduction influence on abyssal peridotites is independent of the spreading rates of the overlying ridges. Locations, such as the Fifteen-Twenty Fracture Zone, the Rainbow Massif from the MAR and the Hess Deep from EPR, have abyssal peridotites overlap well with forearc peridotites, suggesting that the prominent characteristics of arc mantle wedge could be preserved in abyssal peridotites when the traveling distance or time is short for the recycled arc mantle migrating in the mantle convection flow. The well-preserved subduction-related signature in ridge areas that are relatively proximal to modern subduction zones imply that mass exchanges between the subduction zones and mid-ocean ridges may be more efficient than previously thought.

ЭКСГУМАЦИЯ ПОРОД ВЕРХНЕЙ МАНТИИ И НИЖНЕЙ КОРЫ В ЦЕНТРАЛЬНОЙ АТЛАНТИКЕ

The article presents examples of exhumation of the upper mantle and lower crust rocks in various areas of the Atlantic Ocean and the mechanisms of translation of these deep formations to the upper crust horizons in the transition zone from the continent to the ocean, in areas of oceanic rises and troughs, in the ridge zone of the slow-spreading Mid-Atlantic ridge. The latter is considered on the basis of original data obtained during many years of geological and geophysical research in the Central Atlantic by the research vessel «Academician Nikolay Strakhov». The main attention is paid to the oceanic core complexes, which are potential sites for the extraction of polymetallic sulfide ores and natural laboratories for studying the processes of ore genesis in the oceanic lithosphere and the formation of abiogenic hydrocarbons. It is shown that the translation of deep rocks to the bottom of the Central Atlantic took place throughout the history of its opening, starting with the rifting of the continental lithosphere in the Early Cretaceous and ending with the formation of oceanic core complexes in the axial part of the Mid-Atlantic Ridge at the present stage. It is concluded that the exhumation of upper mantle and lower crust rocks is a vivid manifestation of the tectonic delamination of the Central Atlantic lithosphere.

Seismic evidence for uniform crustal accretion along slow-spreading ridges in the equatorial Atlantic Ocean

The crustal accretion along mid-ocean ridges is known to be spreading-rate dependent. Along fast-spreading ridges, two-dimensional sheet-like mantle upwelling creates relatively uniform crust. In contrast, the crust formed along slow-spreading ridges shows large along-axis thickness variations with thicker crust at segment centres, which is hypothesised to be due a three-dimensional plume-like mantle upwelling or due to focused melt migration to segment centres. Using wide-angle seismic data acquired from the equatorial Atlantic Ocean, here we show that the crustal thickness is nearly uniform (~5.5 km) across five crustal segments for crust formed at the slow-spreading Mid-Atlantic Ridge with age varying from 8 to 70 Ma. The crustal velocities indicate that this crust is predominantly of magmatic origin. We suggest that this uniform magmatic crustal accretion is due to a two-dimensional sheet-like mantle upwelling facilitated by the long-offset transform faults in the equatorial Atlantic region and the presence of a high concentration of volatiles in the primitive melt in the mantle.

Lower oceanic crust formed by in situ melt crystallization revealed by seismic layering

Oceanic crust forms at mid-ocean spreading centres through a combination of magmatic and tectonic processes, with the magmatic processes creating two distinct layers: the upper and the lower crust. While the upper crust is known to form from lava flows and basaltic dykes based on geophysical and drilling results, the formation of the gabbroic lower crust is still debated. Here we perform a full waveform inversion of wide-angle seismic data from relatively young (7–12-Myr-old) crust formed at the slow-spreading Mid-Atlantic Ridge. The seismic velocity model reveals alternating, 400–500 m thick, high- and low-velocity layers with ±200 m s−1 velocity variations, below ~2 km from the oceanic basement. The uppermost low-velocity layer is consistent with hydrothermal alteration, defining the base of extensive hydrothermal circulation near the ridge axis. The underlying layering supports that the lower crust is formed through the intrusion of melt as sills at different depths, which cool and crystallize in situ. The layering extends up to 5–15 km distance along the seismic profile, covering 300,000–800,000 years, suggesting that this form of lower crustal accretion is a stable process.

Tectonic termination of oceanic detachment faults, with constraints on tectonic uplift and mass wasting related erosion rates

New high-resolution bathymetric data from Atlantis Massif and surrounding seafloor (30°N, Mid-Atlantic Ridge) records avolcanic extension associated with the formation of the axial rift valley floor, following the tectonic truncation of an active corrugated oceanic detachment fault system. The truncated Atlantis detachment is tectonically uplifted by a high-angle valley-bounding normal fault, formed after westward migration of the ridge at ∼0.4 to 0.1 Ma. Detachment fault remnants, with preserved corrugations, lie within the present-day rift valley seafloor, and demonstrate that a ∼20 km ridge section in the immediate vicinity of the Atlantis Fracture zone has not recorded any recent volcanic activity. Avolcanic extension may thus occur locally at the slow-spreading Mid-Atlantic Ridge, albeit for limited periods of time (less than a few hundred thousand years). The new fault dissecting the detachment shows a throw of ∼2800 m, partly due to flexural uplift. Emplacement of the Lost City hydrothermal site occurred at a late stage post-dating the detachment truncation and avolcanic rift valley formation. From the inferred timing of the westward ridge axis shift we calculate uplift rates ≥ 7 mm/yr, possibly as high as 33 mm/yr, which are equivalent to or greater than the fastest vertical uplift rates of active normal faults measured to date on Earth (Gulf of Corinth). Geomorphologic observations also demonstrate that mass wasting efficiently reworks the seafloor topography. We obtain local incision and erosion rates ≥1-2 mm/yr locally, and as high as 4-8 mm/yr, depending on the assumed age for the rift bonding fault (0.4 vs. 0.1 Ma respectively). Our results suggest that (1) avolcanic extension may occur locally at the slow-spreading Mid-Atlantic Ridge, albeit for limited periods of time (less than a few 100s of kyrs), and (2) document that shifts in axial valley location related to the abrupt abandonment of detachment faults is a first-order process in the asymmetric accretion of slow-spread oceanic lithosphere.

Soluble, Colloidal, and Particulate Iron Across the Hydrothermal Vent Mixing Zones in Broken Spur and Rainbow, Mid-Atlantic Ridge.

The slow-spreading Mid-Atlantic Ridge (MAR) forms geological heterogeneity throughout the ridge system by deep crustal faults and their resultant tectonic valleys, which results in the existence of different types of hydrothermal vent fields. Therefore, investigating MAR hydrothermal systems opens a gate to understanding the concentration ranges of ecosystem-limiting metals emanating from compositionally distinct fluids for both near-field chemosynthetic ecosystems and far-field transport into the ocean interiors. Here, we present novel data regarding onboard measured, size-fractionated soluble, colloidal, and particulate iron concentrations from the 2018 R/V L’Atalante – ROV Victor research expedition, during which samples were taken from the mixing zone of black smokers using a ROV-assisted plume sampling. Iron size fractionation (<20, 20–200, and >200nm) data were obtained from onboard sequential filtering, followed by measurement via ferrozine assay and spectrophotometric detection at 562nm. Our results showed the persistent presence of a nanoparticulate/colloidal phase (retained within 20–200nm filtrates) even in high-temperature samples. A significant fraction of this phase was retrievable only under treatment with HNO3 – a strong acid known to attack and dissolve pyrite nanocrystals. Upon mixing with colder bottom waters and removal of iron in the higher parts of the buoyant plume, the larger size fractions became dominant as the total iron levels decreased, but it was still possible to detect significant (micromolar) levels of nanoparticulate Fe even in samples collected 5m above the orifice in the rising plume. The coolest sample (<10°C) still contained more than 1μM of only nitric acid-leachable nanoparticle/colloidal, at least 200 times higher than a typical Fe concentration in the non-buoyant plume. Our results support previous reports of dissolved Fe in MAR vent plumes, and we propose that this recalcitrant Fe pool – surviving immediate precipitation – contributes to maintaining high hydrothermal iron fluxes to the deep ocean.

Geochemical time series of hydrothermal fluids from the slow-spreading Mid-Atlantic Ridge: Implications of medium-term stability

Time series investigations of hydrothermal fluids from different geological settings at the slow-spreading Mid-Atlantic Ridge (MAR) indicate a surprising medium-term stability of their chemical composition over up to 20 years. This does not only apply to tectonically controlled systems with exposure of mantle rocks where magmatic activity is limited, such as the Logatchev-1 hydrothermal field at 14°45′N. We also found the 407 °C hot and extremely metal-rich fluids at the 5°S site Turtle Pits (2986 m water depth) to discharge at constant composition indicating supercritical phase separation for at least 4 years, which contrasts with rather dynamic volcanically driven hydrothermal systems at the fast-spreading East-Pacific Rise (EPR). Possibly, fluid pathways and pressure-temperature conditions are more stable in tectonically controlled settings at MAR than at the EPR where frequent volcanic activity affects permeability within the hydrothermal systems. Despite some spatial variability, hydrothermal fluids discharging at the shallower (ca. 810–870 m) Menez Gwen hydrothermal field at 37°50′N seem to represent a stable discharge of phase-separated vapor-type fluids over 16 years. This demonstrates that medium-term stability of hydrothermal fluid composition seems to be a general feature at slow-spreading ridges. As a consequence, hydrothermal vent organisms at the MAR may live in a more stable chemical environment compared to vent systems at fast-spreading ridges. Considering the fact that slow-spreading ridges account for more than half of the submarine ridge system, such a constant emanation of hydrothermal fluids makes hydrothermal vent systems on slow spreading ridges a medium-term stable source of Fe and other micronutrients into the ocean.

U-Pb dating, oxygen and hafnium isotopic ratios of zircon from rocks of oceanic core complexes at Mid-Atlantic Ridge: evidence for an interaction of young and ancient crusts at spreading of the ocean floor

U-Pb age, oxygen and hafnium isotopic ratios in zircon from rocks of ocean core complexes at Mid-Atlantic Ridge have been studied using SHRIMP and MC-LA-ICP-MS techniques. U-Pb dating revealed four group of zircons: 1) 0,6-1,7 Ma, 2) 6,7-11,2 Ma, 3) 12,9-17,6 Ma, 4) 200 to 2044 Ma. The 18O values range from 4,74 to 7,2 and are distinct for zircon grains of different ages. Hafnium isotopic ratio for zircon aged from 0,6 to 17,6 Ma corresponds or is close to that of MORB from Central Atlantic. The oxygen and hafnium isotopic compositions of zircon elder than 280 Ma correspond to those of the sialic continental crust. A hypothesis of involvement of the ancient pre-Atlantic sialic (280 млн лет) and old Atlantic (7-17 Ma) crusts in a generation of the contemporary (young) oceanic crust during formation of the slow-spreading Mid-Atlantic Ridge has been proposed.

U–Pb Dating, Oxygen and Hafnium Isotope Ratios of Zircon from Rocks of Oceanic Core Complexes at the Mid-Atlantic Ridge: Evidence for the Interaction of Contemporary and Ancient Crusts in the Spreading Center of the Ocean Floor

The U–Pb age and oxygen and hafnium isotope ratios of zircon from rocks of oceanic core complexes along the Mid-Atlantic Ridge have been studied using SHRIMP and MC-LA-ICP-MS techniques. The U–Pb dating revealed four groups of zircon: (1) 0.6–1.7 Ma, (2) 6.7–11.2 Ma, (3) 12.9–17.6 Ma, and (4) ≥ 200 to 2044 Ma. The δ18O values range from 4.74 to 7.2‰ and are distinct for zircon grains of different ages. The hafnium isotope ratio for zircon with an age from 0.6 to 17.6 Ma corresponds or is close to that of the MORB from the Central Atlantic. The oxygen and hafnium isotope compositions of zircon older than 280 Ma correspond to those of the sialic continental crust. The hypothesis of involvement of the ancient “pre-Atlantic” sialic (≥280 Ma) and “old Atlantic” (7–17 Ma) crusts in the generation of the contemporary (“juvenile”) oceanic crust during the formation of the slow-spreading Mid-Atlantic Ridge has been proposed.

Geological mapping of the Menez Gwen segment at 37°50′N on the Mid-Atlantic Ridge: Implications for accretion mechanisms and associated hydrothermal activity at slow-spreading mid-ocean ridges

Slow-spreading mid-ocean ridges have the potential to form large seafloor massive sulphide (SMS) deposits. Current exploration for SMS deposits commonly targets associated active hydrothermal venting on the ridge axis, which makes the discovery of inactive vent sites and SMS deposits in the off-axis regions unlikely. Geological maps of the seafloor, which help understand the timing and location of SMS formation, usually focus on individual hydrothermal vent sites and their immediate surroundings, and are often too small to aid in SMS exploration. This study uses ship-based multibeam echosounder (MBES) data and a systematic classification scheme to produce a segment-scale geological map. When combined with spreading rate, this allows us to not only reconstruct the segment's spreading history, but also reveals important processes that localize hydrothermal venting. Geological mapping around two known hydrothermal vent sites on the Menez Gwen segment at 37°50′N on the slow-spreading Mid-Atlantic Ridge showed that hydrothermal venting accompanies the tectonic break-up of a large, cooling magmatic body. Venting is focussed by faulting and resulting permeability changes. The large magmatic body is associated with an axial volcano that formed as a last stage of a period with intense magmatic accretion. Such magmatic accretion periods occur every 300 to 500 ka at the Menez Gwen segment, with increasing intensity over the past 3.5 Ma years. The most recent, most intense magmatic period appears to be a regional phenomenon, also affecting the neighbouring Lucky Strike and Rifted Hills segments. Understanding the accretional setting and the spatial and temporal constraints of hydrothermal venting enables us to develop criteria in MBES data to aid exploration for inactive SMS deposits.

THE ORIGIN AND STRUCTURE OF THE LOWER CRUST OF OCEANS AND BACK-ARC SEAS: EVIDENCE FROM THE MARKOV DEEP (MID-ATLANTIC RIDGE) AND THE VOIKAR OPHIOLITE ASSOCIATION (POLAR URALS)

The Markov Deep (the axial part of the slow-spreading Mid-Atlantic Ridge, 6°N, Sierra Leone oceanic core complex) and the Paleozoic Voikar ophiolite association (Polar Urals) formed in the back-arc sea conditions. In both cases, the lower crust of a close structure was formed on the basements composed ofdepleted peridotites of the ancient lithospheric mantle. The available data show that the composition of the lower crust of the oceans and back-arc seas is dominated by layeredmafic-ultramafic intrusions originating from the MORB melts, and suggest a similar asthenospheric source of magmas. Sills and dykes formed from other magma sources represent the second structural element of the lower oceanic crust: in case of the ocean, mainly ferrogabbroids originating from specific melts with the OIB involvement, and, in case of the back-sea sea, gabbro-norites of the supra-subduction calc-alkaline series. In both cases, the upper crust originates frombasaltic flows that occurred later and are associated with new episodes in the tectonic development. According to [Sharkov,2012], the development of slow-spreading ridges takes place in discrete impulses and non-simultaneously along their entire length. Furthermore, oceanic core complexes (OCC) in their axial parts are the ridge segments, where spreading is resumed. At the OCC stage, newly formed basalt melts move upwards from the magma generation zone into fractures (dykes) through the lithospheric mantle, and the thickness of the lower crust is built up by sills. As spreading develops in this area, the crust becomes thicker from below due to underplating in form of large layered intrusions. The newly formed restites, in their turn, cause an increase in the lithospheric mantle thickness from below. Apparently, the lower crust formed in the back-arc seas according to a similar scenario, although complicated by the processes taking place in the subduction zone.

Microbial Community Structure of Deep-sea Hydrothermal Vents on the Ultraslow Spreading Southwest Indian Ridge.

Southwest Indian Ridge (SWIR) is a typical oceanic ultraslow spreading ridge with intensive hydrothermal activities. The microbial communities in hydrothermal fields including primary producers to support the entire ecosystem by utilizing geochemical energy generated from rock-seawater interactions. Here we have examined the microbial community structures on four hydrothermal vents from SWIR, representing distinct characteristics in terms of temperature, pH and metal compositions, by using Illumina sequencing of the 16S small subunit ribosomal RNA (rRNA) genes, to correlate bacterial and archaeal populations with the nature of the vents influenced by ultraslow spreading features. Epsilon-, Gamma-, Alpha-, and Deltaproteobacteria and members of the phylum Bacteroidetes and Planctomycetes, as well as Thaumarchaeota, Woesearchaeota, and Euryarchaeota were dominant in all the samples. Both bacterial and archaeal community structures showed distinguished patterns compared to those in the fast-spreading East Pacific Ridge or the slow-spreading Mid-Atlantic Ridge as previously reported. Furthermore, within SWIR, the microbial communities are highly correlated with the local temperatures. For example, the sulfur-oxidizing bacteria were dominant within bacteria from low-temperature vents, but were not represented as the dominating group recovered from high temperature (over 300°C) venting chimneys in SWIR. Meanwhile, Thaumarchaeota, the ammonium oxidizing archaea, only showed high relative abundance of amplicons in the vents with high-temperature in SWIR. These findings provide insights on the microbial community in ultraslow spreading hydrothermal fields, and therefore assist us in the understanding of geochemical cycling therein.

Sulfide geochronology along the Northern Equatorial Mid-Atlantic Ridge

Hydrothermal processes and seafloor massive sulfide (SMS) deposits have different characteristics at fast and slow spreading mid-ocean ridges. One such parameter is the age of a SMS deposit, which differs by 1–2 orders of magnitude between the fast spreading East Pacific Rise (EPR) and the slow spreading Mid-Atlantic Ridge (MAR). The large collection of SMS samples dated from the 18 hydrothermal fields of the northern equatorial part of the Mid-Atlantic Ridge (194 samples) demonstrates a relatively old average age of hydrothermal fields here (~66ka) with the oldest one estimated as ca. 223ka (Peterburgskoye field). Based on geochronological data it was confirmed that hydrothermal discharge has an episodic character: active and inactive periods of the SMS formation alternate. The distribution of events at all hydrothermal fields demonstrates that maximum activity occurred at 38–35, 30–20, and 8–2ka and increased with time. Based on statistical analyses, dating variations can be explained as a superposition of several periods of activity with the duration of ~15, 10 and 5ka.Relationship between the age and distance from the axial rift zone as well as between the age and aerial distribution is different for SMS deposits hosted by basalts and by gabbro-peridotites depending on their geological setting on the particular MAR segment. This difference can be explained by a variety of hydrothermal processes determined by “tectonic” or “magmatic” segment evolution and symmetrical or asymmetrical mode of accretion (Escartin et al., 2008).

Segment‐scale variations in seafloor volcanic and tectonic processes from multibeam sonar imaging, M id‐ A tlantic R idge Rainbow region (35°45′–36°35′N)

Along-axis variations in melt supply and thermal structure can lead to significant variations in the mode of crustal accretion at mid-ocean ridges. We examine variations in seafloor volcanic and tectonic processes on the scale of individual ridge segments in a region of the slow-spreading Mid-Atlantic Ridge (35o45'-36o35'N) centered on the Rainbow non-transform discontinuity (NTD). We use multibeam sonar backscatter amplitude data, taking advantage of multifold and multi-directional coverage from the MARINER geophysical study to create a gridded compilation of seafloor reflectivity, and interpret the sonar image within the context of other data to examine seafloor properties and identify volcanic flow fields and tectonic features. Along the spreading segments, differences in volcanic productivity, faulting, eruption style and frequency correlate with inferred magma supply. Regions of low magma supply are associated with more widely spaced faults, and larger volcanic flow fields that are more easily identified in the backscatter image. Identified flow fields with the highest backscatter occur near the ends of ridge segments. Their relatively smooth topography contrasts with the more hummocky, cone-dominated terrain that dominates most of the neovolcanic zone. Patches of seafloor with high, moderately high, and low backscatter intensity across the Rainbow massif are spatially correlated with observations of basalt, gabbro and serpentinized peridotite, and sediment respectively. Large detachment faults have repeatedly formed along the inside corners of the Rainbow NTD, producing a series of oceanic core complexes along the wake of the NTD. A new detachment fault is currently forming in the ridge segment just north of the now inactive Rainbow massif. This article is protected by copyright. All rights reserved.

Fault geometry and permeability contrast control vent temperatures at the Logatchev 1 hydrothermal field, Mid-Atlantic Ridge

Research Article| January 01, 2015 Fault geometry and permeability contrast control vent temperatures at the Logatchev 1 hydrothermal field, Mid-Atlantic Ridge Christine Andersen; Christine Andersen GEOMAR, Helmholtz Centre for Ocean Research, Wischhofstrasse 1-3, 24148 Kiel, Germany Search for other works by this author on: GSW Google Scholar Lars Rüpke; Lars Rüpke GEOMAR, Helmholtz Centre for Ocean Research, Wischhofstrasse 1-3, 24148 Kiel, Germany Search for other works by this author on: GSW Google Scholar Jörg Hasenclever; Jörg Hasenclever GEOMAR, Helmholtz Centre for Ocean Research, Wischhofstrasse 1-3, 24148 Kiel, Germany Search for other works by this author on: GSW Google Scholar Ingo Grevemeyer; Ingo Grevemeyer GEOMAR, Helmholtz Centre for Ocean Research, Wischhofstrasse 1-3, 24148 Kiel, Germany Search for other works by this author on: GSW Google Scholar Sven Petersen Sven Petersen GEOMAR, Helmholtz Centre for Ocean Research, Wischhofstrasse 1-3, 24148 Kiel, Germany Search for other works by this author on: GSW Google Scholar Geology (2015) 43 (1): 51–54. https://doi.org/10.1130/G36113.1 Article history received: 23 Jul 2014 rev-recd: 22 Oct 2014 accepted: 28 Oct 2014 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Christine Andersen, Lars Rüpke, Jörg Hasenclever, Ingo Grevemeyer, Sven Petersen; Fault geometry and permeability contrast control vent temperatures at the Logatchev 1 hydrothermal field, Mid-Atlantic Ridge. Geology 2015;; 43 (1): 51–54. doi: https://doi.org/10.1130/G36113.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract High-temperature (>300 °C) off-axis hydrothermal systems found along the slow-spreading Mid-Atlantic Ridge are apparently consistently located at outcropping fault zones. While preferential flow of hot fluids along highly permeable, fractured rocks seems intuitive, such efficient flow inevitably leads to the entrainment of cold ambient seawater. The temperature drop this should cause is difficult to reconcile with the observed high-temperature black smoker activity and formation of associated massive sulfide ore deposits. Here we combine newly acquired seismological data from the high-temperature, off-axis Logatchev 1 hydrothermal field (LHF1) with numerical modeling of hydrothermal flow to solve this apparent contradiction. The data show intense off-axis seismicity with focal mechanisms suggesting a fault zone dipping from LHF1 toward the ridge axis. Our simulations predict high-temperature venting at LHF1 only for a limited range of fault widths and permeability contrasts, expressed as the fault’s relative transmissibility (the product of the two parameters). The relative transmissibility must be sufficient to “capture” a rising hydrothermal plume and redirect it toward LHF1 but low enough to prevent extensive mixing with ambient cold fluids. Furthermore, the temperature drop associated with any high permeability zone in heterogeneous crust may explain why a significant part of hydrothermal discharge along slow-spreading ridges occurs at low temperatures. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

High Connectivity of Animal Populations in Deep-Sea Hydrothermal Vent Fields in the Central Indian Ridge Relevant to Its Geological Setting

Dispersal ability plays a key role in the maintenance of species in spatially and temporally discrete niches of deep-sea hydrothermal vent environments. On the basis of population genetic analyses in the eastern Pacific vent fields, dispersal of animals in the mid-oceanic ridge systems generally appears to be constrained by geographical barriers such as trenches, transform faults, and microplates. Four hydrothermal vent fields (the Kairei and Edmond fields near the Rodriguez Triple Junction, and the Dodo and Solitaire fields in the Central Indian Ridge) have been discovered in the mid-oceanic ridge system of the Indian Ocean. In the present study, we monitored the dispersal of four representative animals, Austinograea rodriguezensis, Rimicaris kairei, Alviniconcha and the scaly-foot gastropods, among these vent fields by using indirect methods, i.e., phylogenetic and population genetic analyses. For all four investigated species, we estimated potentially high connectivity, i.e., no genetic difference among the populations present in vent fields located several thousands of kilometers apart; however, the direction of migration appeared to differ among the species, probably because of different dispersal strategies. Comparison of the intermediate-spreading Central Indian Ridge with the fast-spreading East Pacific Rise and slow-spreading Mid-Atlantic Ridge revealed the presence of relatively high connectivity in the intermediate- and slow-spreading ridge systems. We propose that geological background, such as spreading rate which determines distance among vent fields, is related to the larval dispersal and population establishment of vent-endemic animal species, and may play an important role in controlling connectivity among populations within a biogeographical province.

Distribution of large-scale detachment faults on mid-ocean ridges in relation to spreading rates

Large-scale detachment faults on mid-ocean ridges (MORs) provide a window into the deeper earth. They have megamullion on their corrugated surfaces, with exposed lower crustal and upper mantle rocks, relatively high residual Bouguer gravity anomaly and P-wave velocity, and are commonly associated with oceanic core complex. According to 30 detachment faults identified on MORs, we found that their distances to the axis mostly range from 5 to 50 km, half-spreading rates range from 6.8 to 17 mm/a, and activity time ranges from recent to 3 Ma. Most of the detachment faults are developed on the slow spreading Mid-Atlantic Ridge (MAR) and ultra-slow spreading Southwest Indian Ridge (SWIR), with the dominant half-spreading rates of 7-13 mm/a, especially 10-13 mm/a. Furthermore, they mostly occur at the inside corner of one segment end and result in an asymmetric seafloor spreading. The detachment faults on MORs are mainly controlled by the tectonism and influenced by the magmatism. Long-lived detachment faults tend to be formed where the ridge magma supply is at a moderate level, although the tectonism is a first-order controlling factor. At the slow spreading ridges, detachment faults tend to occur where local magma supply is relatively low, whilst at the ultra-slow spreading ridges, they normally occur where local magma supply is relatively high. These faults are accompanied by hydrothermal activities, with their relationships being useful in the study of hydrothermal polymetallic sulfides and their origin.

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.

Os isotopic compositions of MORBs from the ultra-slow spreading Southwest Indian Ridge: Constraints on the assimilation and fractional crystallization (AFC) processes

Normal mid-ocean ridge basalts (MORBs) from the Southwest Indian Ridge (SWIR) (48°E–64°E) have larger ranges of 187Os/188Os ratios from 0.1295 to 0.3023 than previously reported N-MORBs. 187Os/188Os ratios show positive correlations with 1/Os, suggesting possible contamination from a component with higher 187Os/188Os ratios. Mixing modeling between an unradiogenic MORB magma and seawater cannot reproduce the observed Os–Sr isotopic variations, suggesting that seawater alteration is not responsible for such radiogenic Os isotopic compositions. One sample plots between the isochrons of 400ka and 300ka, showing a possible radiogenic origin of 187Os after magma eruption. A group of MORBs have 187Os/188Os ratios negatively correlated with MgO. Such correlations can be modeled with starting 187Os/188Os ratio of 0.1357, bulk DOs of 15.3 and assimilation degree from 3% to 11% with a crystallization to assimilation ratio of 0.8. Os isotopic compositions of some MORBs from the SWIR reflect a significant influence of assimilation and fractional crystallization processes, mostly obscured the Os isotopic characteristics of the mantle sources. N-MORBs from the ultra-slow spreading SWIR and slow-spreading Mid-Atlantic Ridge show larger ranges of 187Os/188Os ratios than those from fast-spreading East Pacific Rise, reflecting different structures and chemical compositions of oceanic crusts due to different spreading rates of the ridges.