Mid-Atlantic Ridge Research Articles

Mercury fluxes from hydrothermal venting at mid-ocean ridges constrained by measurements

Methylmercury is a potent toxin threatening the global population mainly through the consumption of marine fish. Hydrothermal venting directly delivers natural mercury to the ocean, yet its global flux remains poorly constrained. To determine the extent to which anthropogenic inputs have increased oceanic mercury levels, it is crucial to estimate natural mercury levels. Here we combine observations of vent fluids, plume waters, seawater and rock samples to quantify the release of mercury from the Trans-Atlantic Geotraverse hydrothermal vent at the Mid-Atlantic Ridge. The majority (67–95%) of the mercury enriched in the vent fluids (4,966 ± 497 pmol l−1) is rapidly diluted to reach background seawater levels (0.80 pmol l−1). A small Hg fraction (2.6–10%) is scavenged to the Trans-Atlantic Geotraverse mound rocks. Scaling up our findings and previous work, we propose a mercury flux estimate of 1.5–64.7 t per year from mid-ocean ridges. This hydrothermal flux is small in comparison to anthropogenic inputs. This suggests that most of the mercury present in the ocean must be of anthropogenic origin and that the implementation of emissions reduction measures outlined in the Minamata Convention could effectively reduce mercury levels in the global ocean and subsequently in marine fish.

Distinct development trajectories and symbiosis modes in vent shrimps.

Most animal species have a singular developmental pathway and adult ecology, but developmental plasticity is well-known in some such as honeybees where castes display profoundly different morphology and ecology. An intriguing case is the Atlantic deep-sea hydrothermal vent shrimp pair Rimicaris hybisae and R. chacei that share dominant COI haplotypes and could represent very recently diverging lineages or even morphs of the same species. Rimicaris hybisae is symbiont-reliant with a hypertrophied head chamber (in the Mid-Cayman Spreading Centre), while R. chacei is mixotrophic with a narrow head chamber (on the Mid-Atlantic Ridge). Here, we use X-ray micro-computed tomography and fluorescence in situ hybridization to show that key anatomical shifts in both occur during the juvenile-subadult transition, when R. hybisae has fully established symbiosis but not R. chacei. On the Mid-Atlantic Ridge, the diet of R. chacei has been hypothetically linked to competition with the obligatorily symbiotic congener R. exoculata, and we find anatomical evidence that R. exoculata is indeed better adapted for symbiosis. We speculate the possibility that the distinct development trajectories in R. hybisae and R. chacei may be determined by symbiont colonization at a "critical period" before subadulthood, though further genetic studies are warranted to test this hypothesis along with the true relationship between R. hybisae and R. chacei.

Strontium and Neodymium Isotopic Signatures in Basalt Glasses of the Mid-Atlantic Ridge, 12°–31° N

This paper presents the new geochemical isotope (Sr–Nd system, H2O, Cl) data obtained for basalt glasses of the Mid-Atlantic Ridge (MAR) sampled from six areas of the MAR axial zone between 31° and 12° N. The data are consistent with the existing ideas about large-scale geochemical segmentation of the MAR. It is shown that samples from predominantly serpentinite segments have a narrower range of variations of strontium isotopic composition (87Sr/86Sr = 0.7027–0.7032) in comparison with samples collected from the areas where the crustal section is dominated by basalts (87Sr/86Sr = 0.7024–0.7041). The variation ranges of the neodymium isotopic composition in these two groups of samples are almost identical (εNd = +4.9 to +10.9 and +5.9 to +11.6 in serpentinite and basalt segments, respectively), although, in general, serpentinite segments have a slightly more enriched composition. The wide variations of the neodymium isotopic composition and increased contents of Cl, H2O, and U, as well as increased K2O/TiO2 and La/Sm ratios, in samples from serpentinites can most probably be related to the participation of different geochemically heterogeneous sources in the magmatism of the MAR axial zone. The influence of enriched plume-type matter cannot be excluded in some segments. The isotopic composition of noble gases may shed light on the subject.

NEW DATA ON THE COMPOSITION OF BASALTS FROM SEAMOUNT NEAR THE EASTERN FLANK OF THE CHARLIE GIBBS FZ (NORTH ATLANTIC)

Composition of basalts collected from the slopes of a single seamount (guyot) located south of the eastern flank of the Charlie Gibbs FZ in the North Atlantic, was studied. High-titanium and low-titanium groups related respectively to E-MORB and T-MORB oceanic tholeites, were singled out. Basalts of the first group are characterized by higher concentrations of TiO2, Na2O, K2O, P2O5, Fe2O3 and incoherent trace-elements. Basalts of both groups were melted from a mantle substrate of similar composition, which is a mixture of DM and HIMU mantle material and to a lesser extent – EM-2. Their difference in composition is due to the fact that high-titanic melts were formed deeper at a level between the spinel and garnet facies. Basalts similar in petro-geochemical and isotopic parameters were found in the axial zone of the Mid-Atlantic Ridge between the Charlie Gibbs and Maxwell FZ, where they compose large neovolcanic edifices formed with a sharp increase in the intensity of axial magmatism as a result of microplumes rise. Our studies give grounds to believe that the studied seamount was also formed as a large neovolcanic structure 64–67 million years ago under the influence of a microplume, which was branch of the Milne plume of the deep mantle.

Geochronological Study of Hydrothermal Precipitates in the Northern Equatorial Area of the Mid-Atlantic Ridge

Geochronological Study of Hydrothermal Precipitates in the Northern Equatorial Area of the Mid-Atlantic Ridge

Deep segregation and crystallization of ultra-depleted melts in the sub-ridge mantle

Partial melting of mantle peridotite from which considerable amounts of melt have been extracted during prior melting episodes generates melts characterized by low incompatible element contents and very low ratios of highly to moderately incompatible elements, so-called ‘ultra-depleted’ melts. Reaction of peridotite with percolating ultra-depleted melts has been inferred from petrological-geochemical studies of abyssal peridotites and ophiolites. But so far, direct evidence for the existence of ultra-depleted melts, only comes from rare melt inclusions. Here, we show that a pyroxenite layer within abyssal peridotite from the Mid Atlantic Ridge (8°N, Doldrums Fracture Zone) formed by crystallization of a segregated melt that is highly depleted in incompatible elements, at >27 km-depth beneath the ridge axis (at T ∼ 1250 °C), and with little or no modification by interaction with the host harzburgite. During exhumation, the pyroxenite experienced decompression and partial re-equilibration under plagioclase-facies conditions (∼1060 °C and ∼ 15 km depth). The high Hf isotope ratio (εHf = 40.3) of the pyroxenite clinopyroxene is inherited from a melt sourced from an ultra-depleted peridotite that evolved with high Lu/Hf. The associated MORB-like Nd isotope ratios (εNd = 10.6), however, imply a long-term evolution of the source peridotite with composition moderately depleted in incompatible elements (rather low Sm/Nd). These compositions are different from the host harzburgite, but typical for peridotites that have melted and partially reacted with migrating melts in ancient times. Hence, the pyroxenite investigated here is a partial melt from an ultra-depleted peridotite that has become re-enriched in incompatible elements and clinopyroxene. This melt crystallized in the oceanic lithosphere, and partially re-equilibrated at low pressure during exhumation. Overall, our results show that renewed melting of ultra-depleted peridotites with a complex history of prior melting and melt-rock reaction occurs, and that such melts migrate through the sub-ridge mantle, and can thus contribute to mid ocean ridge magmatism, although to a still unknown extent.

The 21 March 2022 Mw 5.1 Tampen Spur Earthquake, North Sea: Location, Moment Tensor, and Context

ABSTRACT The 21 March 2022 Mw 5.1 Tampen Spur earthquake is the largest event in the North Sea over the last 33 yr. The earthquake was recorded by a 10,708 sensors permanent reservoir monitoring system (PRM) deployed at the Snorre field only a few kilometers away from the epicenter. The event was also recorded on regional networks, as well as at teleseismic distances. Here, we take advantage of this large number of observations to relocate this seismic event, estimate a moment tensor, and refine the depth of the earthquake. The different data sets and location methods yield similar locations, suggesting a well-constrained earthquake location with a depth of 22 ± 5 km. Furthermore, a moment tensor is estimated, which indicates a thrust fault mechanism. Deep crustal earthquakes (>20 km) have been documented within several continental rifts, for example, in Basin and Range in the United States and in northern Germany. Studying deep intraplate earthquakes such as the event observed here can offer insights into the rheology and structures in the deeper portions of the crust. This thrust earthquake implies the reactivation of an older fault in the lower crust consistent with the ridge-push forces from the mid-Atlantic ridge. Such brittle failure at depth indicates a lower crust with little to no fluids present at this depth within the Tampen Spur region.

Introduction of isotopically light barium from the Rainbow hydrothermal system into the deep Atlantic Ocean

The marine barium (Ba) cycle is closely connected to the short-timescale carbon cycle, and Ba serves as a valuable paleo proxy for export production, ocean alkalinity, and terrestrial inputs. However, the marine Ba budget is poorly constrained, particularly regarding the fluxes of hydrothermally sourced Ba, which hinders our understanding of the Ba cycle and use of Ba-based proxies. Recent studies have suggested a modern source-sink imbalance of Ba isotopes in the global ocean, with sources being overall isotopically heavier than the sinks, and the hydrothermal Ba inputs were considered isotopically heavy sources.In this study, we present the first investigation of Ba and its isotopes in a non-buoyant hydrothermal plume based on dissolved and particulate samples collected from the Rainbow hydrothermal vent field on the Mid-Atlantic Ridge. Our data reveal strong hydrothermal signals at near-field stations, as evidenced by helium isotopes, accompanied by elevated concentrations of dissolved and particulate Ba. Dissolved Ba isotope compositions (δ138Ba) in hydrothermally influenced deep waters (∼0.3 %0) are lighter than at similar depths of far-field stations (∼0.45 %0) in the Atlantic Ocean. The concentrations and isotopic compositions of dissolved and labile particulate Ba in the non-buoyant hydrothermal plume can be explained by conservative mixing between a Ba-enriched hydrothermal component and North Atlantic Deep Water. By extrapolating the correlations to the vent fluid endmember, our results suggest that there is negligible removal of Ba, and insignificant modification of Ba isotopic signatures, from the vent fluid endmember to the non-buoyant hydrothermal plume. This indicates that the Rainbow hydrothermal system introduces isotopically light Ba (−0.17 ± 0.05 %0) to the deep Atlantic Ocean. We estimate that global hydrothermal inputs of Ba are 4.6 ± 2.2 Gmol/yr. These observations highlight the potential of hydrothermal Ba to be an isotopically light source component of the marine Ba isotope budget.

Fractionation of iron and chromium isotopes in hydrothermal plumes from the northern Mid-Atlantic Ridge

Hydrothermal venting impacts the global-scale biogeochemical cycles of many trace metals and their isotopes. Processes in hydrothermal plumes regulate the dispersal of vent-derived metals and may vary in response to differences in the geologic setting of vent fields and/or the geochemistry of the overlying ocean water. Here we present results of analyses of dissolved Fe and Cr concentrations, and dissolved Fe isotope (δ56Fe) and Cr isotope (δ53Cr) distributions, in seawater samples collected from above TAG and Rainbow vent sites on the Mid-Atlantic Ridge during the GEOTRACES GA13 cruise. We show that profiles of dissolved Fe and Cr isotopes through the near-field hydrothermal plumes are the mirror image of each other. Oxidation of Fe(II) and precipitation of Fe-(oxyhydr)oxides account for the low δ56Fe values of dissolved Fe, as low as −1.83 ‰ at TAG and −6.94 ‰ at Rainbow. Plume samples with low δ56Fe values are associated with elevated δ53Cr values of dissolved Cr compared to background seawater (by up to +0.14 ‰ and +0.69 ‰ at TAG and Rainbow, respectively), while particulate Cr is characterised by relatively low δ53Cr values (−1.02 to −1.22 ‰). This striking result suggests that seawater Cr(VI) is reduced to Cr(III) and precipitates on the surface of Fe(III) particles within the hydrothermal plume. Reduction of Cr(VI) and scavenging of Cr(III) by plume Fe-(oxyhydr)oxide particles mean that high-temperature hydrothermal systems are likely a net sink for seawater Cr at Rainbow (and also at TAG). As the removal flux of Cr is related to the flux of hydrothermal Fe(II) and the rate of Fe(II) oxidation in the hydrothermal plume, it may (i) vary across vent sites at a global scale and (ii) change over glacial-interglacial cycles.

Basalts of a Seamount on the Eastern Flank of the Charlie Gibbs Transform Fracture Zone, North Atlantic: Petrochemical and Isotopic Evidence of a Microplume-Affected Formation in the Axial Zone of the Mid-Atlantic Ridge

The composition of basalts dredged from the slopes of a single seamount (guyot), which is located south of the eastern end of the Charlie Gibbs Fracture Zone in the North Atlantic, is studied. Groups of high- and low-Ti rocks, which belong to oceanic tholeiites of the E-MORB and T-MORB types, respectively, are recognized. The first group of basalts have higher contents of TiO2, Na2O, K2O, P2O5, Fe2O3, and incompatible trace elements. The basalts of both groups are similar in the normalized lithophile element patterns indicating their melting from a similar mantle substrate. The difference in their composition is probably caused by melting of high-Ti volcanic rocks at a deeper intermediate level between spinel and garnet facies. Our studies give grounds to believe that the seamount also formed as a large neovolcanic rise 64–67 Ma ago under the influence of a microplume, which was a branch of the Milne deep mantle plume.

Basalts from MAR at 13°15′–13°40′N: What mixed?

We present new data on chemical composition of basalt glasses collected within the geochemical anomaly of the Mid-Atlantic ridge at 13°25′–13°40′N, where fresh quenched lavas occur dominantly in asymmetric off-axial structures. The collected samples indicate wide compositional ranges from strongly depleted D-MORB with 0.04 wt% K2O, K2O/TiO2 = 0.08 and (La/Sm)N = 0.69 to the most enriched E-MORB with 0.92 wt% K2O, K2O/TiO2 = 0.42 and (La/Sm)N = 2.45. Clear correlations are traced between incompatible element ratios (K2O/TiO2, La/Sm, Nb/Zr), concentrations of MgO, K2O, H2O, and 87Sr/86Sr ratio, suggesting variable contribution to the compositional diversity of the glasses from two distinct source materials, one of which is depleted mantle. Testing the hypothesis of pyroxenite-derived melts as an enriched end-member failed to reproduce the observed REE patterns, (Sr/Zr)N, and (Ba/Nb)N ratios, and the K2O–K2O/TiO2 relations. Two alternative hypotheses – melts derived from melting remnants of continental crust (1) or oceanic core complex (OCC) gabbro (2) – are suggested based on the complementary character of the REE patterns of the basalt glasses and partial melts compositions calculated at 1000 °C/50 MPa from either bulk continental crust as estimated by Taylor and McLennan (1995) (1) or OCC gabbro sampled in the study area (2). Complementarity of the two sources with the D-MORB holds also for the other geochemical parameters: (Sr/Zr)N vs (Ba/Nb)N, (La/Sm)N vs (Rb/Nb)N, and for the K2O–K2O/TiO2 relations. The first hypothesis calls upon partial melting of continental crustal material preserved in the vicinity of ridge-transform fault system, while the second suggests melt-rock interaction within the off-axial structures of the pre-existed gabbro assemblages, with concurrent melting of the wall-rock material and dissolution of the resulting melt in the intruded magma. Both scenarios, operating either separately or jointly, might contribute to the MORB geochemical variability documented in the present study.

Metallogenetic process of Xunmei hydrothermal field (26°S), South Mid-Atlantic Ridge: Constraints from in-situ sulfur isotope and trace elements of sulfides

Seafloor hydrothermal sulfides at slow-spreading mid-ocean ridges (MORs) are of economic and scientific significance. The formation of newly discovered submarine hydrothermal sulfides on the South Mid-Atlantic Ridge (SMAR), particularly with respect to the fluid evolution, remains poorly understood. The Xunmei hydrothermal field is a typical volcanic dome-type hydrothermal field at 26°S on the SMAR. Based on the mineralogical zonation and assemblages, six distinct ore-forming stages were identified from low- to high-temperature stages with a seafloor weathering stage. The crystallinity of pyrite/marcasite and grain size of chalcopyrite increase gradually from the outer rim to the inner conduit of a sulfide chimney. The positive δ34SV-CDT values of pyrite/marcasite (2.1‰–8.2‰), chalcopyrite (2.5‰–5.6‰), and sphalerite (2.9‰–7.0‰) suggest 62%–91% of S was derived from the leaching of basement basalts, 9%–38% from reduction of seawater sulfate. In situ trace element data for sulfides show enrichments in Mn, Ag, Tl, and Pb in outer zones A and B, while enrichments in Se, In, and Sn in inner zones C and D, indicating an increase in precipitation temperature during chimney growth. Low pyrite Co contents, low Tl/Pb, low Sb/Pb, low Bi/Pb, high As, Ag, Cu, and Pb concentrations, suggesting that the pyrites precipitated from fluids that may have undergone supercritical phase separation during their ascent. High-temperature, Cl-depleted, vapor-rich hydrothermal fluids discharged into ambient cold seawater over multiple stages and probably evolved from low (<240 °C) to medium (∼ 263 °C) and high (∼ 317 °C) temperatures, reached a highest temperature of ∼335 °C, and then evolved to medium to low temperatures (∼ 270 °C to <240 °C) during the wanning of hydrothermal venting. The sulfur fugacity (fS2) likely evolved from a relatively low to high condition within the scope of intermediate sulfidation, and then decreased to low condition. The redox states (fO2) probably were strongly affected by seawater influx and evolved from relatively oxidized to original reduced conditions. The salinity decreased during chimney growth. Sulfide precipitation in the Xunmei hydrothermal field probably was the result of a combination of fluid-seawater mixing and phase separation. We propose a chimney growth and hydrothermal evolution models for the Xunmei hydrothermal field.

FIRST DATA ON VERTICAL PARTICLE FLUXES AND ENVIRONMENTAL CONDITIONS IN THE NORTHERN PART OF THE MOHNS RIDGE, NORWEGIAN SEA

For the first time, a continuous two-year study of the particle fluxes and associated environmental parameters in the axial zone of the Arctic segment of the Mid-Atlantic Ridge at the junction of the Mohns and Knipovich Ridges (Norwegian Sea) was carried out. Sediment traps were deployed from the R/V Akademik Mstislav Keldysh in June 2019 in the northeastern part of the Mohns Ridge and realized in July 2020. It was shown that the sedimentation of sinking particles in the study area was influenced by the global transport of water masses in the northeastern direction, with Atlantic waters in the subsurface, and water masses of Arctic origin recirculating deeper in the subsurface. A weak positive temperature trend at a depth of more than 2500 m (0.02°C) is detected. The bimodal vertical distribution of fluxes and changes in the composition of sinking particles corresponded to the pattern of sediment recycling in the ocean. The annual course of fluxes in the subsurface layer was determined by the activity of silicon-concentrating and calcifying plankton, with the maximum bloom period also manifested in the deep layer. At the same time, the main significant moderator of flux deeper than 2000 m is the inflow of lithogenic matter from the benthic nepheloid layer.

Cephalopods sampled near Azores-Biscay rise and along the mid-Atlantic Ridge north of the Azores by the FRV Walther Herwig II in 1982

ABSTRACT In June 1982 F.R.V. Walther Herwig II (WH-II) cruise 52, leg 2, investigated the bathy- and benthopelagic fish fauna along the Azores Biscay Rise and the Mid-Atlantic Ridge (MAR) north of the Azores. In total 678 decapod cephalopods were collected as bycatch and identified belonging to about 33 species in 17 families. The five most abundant families were Histioteuthidae, Cranchiidae, Mastigoteuthidae, Octopoteuthidae, and Chiroteuthidae. A comparison with the cephalopod species captured on a cruise of R.V. G.O. SARS (G.O. SARS) to the North Atlantic between 60°-41°N in June-July 2004 and their distribution patterns and biodiversity have been done. The use of different types of trawls applied on G.O. SARS and WH-II 52(2) regarding selection, species composition and biogeography are discussed. Locations with high species abundance and diversity were observed on WH-II 52(2) along the mid-Atlantic Ridge at about 50°-43°N.

Crustal and uppermost mantle structure near the Gloria Fault, North Atlantic, from ocean bottom seismometer surface wave observations

SUMMARY In this work, we present both 1-D and 3-D shear wave velocity (Vs) models of the oceanic crust and uppermost mantle below the Deep OCean Test ARray area, located ∼ 70 km north of the central section of the Gloria Fault, in the eastern North-Atlantic Ocean. The velocity models are inferred from the dispersion of surface waves recorded on ocean bottom seismometers. Dispersion measurements are obtained from the analysis of ambient seismic noise at short periods (&amp;lt; 14 s) and teleseismic surface waves at long periods (&amp;gt; 14 s) using the two-station method. The 1-D Vs model is inferred from the joint inversion of Rayleigh wave phase velocities and Love wave group and phase velocities. The 3-D tomographic model is obtained by inversion of 2-D Love wave group velocity maps as a function of depth, further constrained by the average of Love wave phase velocities obtained from ambient noise (4–9 s) and the average Rayleigh and Love wave phase velocities calculated from teleseismic data (14–44 s). The 1-D Vs model shows a sediment layer with a low velocity of 1.05 km s−1, similar to previous studies in the region. Below the sediments, we find an oceanic crust with velocities ranging from 3.3 to 4.5 km s−1. The model reaches an unusually high velocity of 4.9 km s−1 in a 20 km thick layer at depths between 16 and 36 km. We interpret this fast velocity layer as indicative of the presence of harzburgite, a residue of enhanced melting that might have been formed by the proximity between the Mid-Atlantic Ridge and the Azores mantle plume. At greater depths the velocity decreases, forming a low-velocity zone that reaches a minimum at ∼ 70 km depth, which we interpret as the maximum depth for the lithosphere–asthenosphere boundary. The 3-D model shows a structure that is mostly horizontally layered, with Vs isocontours at 3.5–4.5 km s−1 highlighting oscillations of the crustal structure with wavelengths of ∼25–30 km. These oscillations may be due to changes in the rate of mantle upwelling and magma supply rate.

The critical role of deformation-assisted melt migration in the formation of oceanic core complexes

Oceanic core complexes provide an accessible window into deep processes occurring at slow and ultra-slow-spreading mid-ocean ridges. We analyse samples from IODP ocean drilling of core complexes at the Atlantis Bank, Atlantis Massif, and near the Kane Transform at the South West Indian and Mid-Atlantic ridges. We correlate secondary minerals, including oxides, with sites of melt migration. We interpret changes to mineral assemblage and microchemistry, reaction textures and melt-pseudomorph microstructures as fingerprints of open-system melt-mediated processes. This micro-scale information is combined with a macro-scale review of legacy mineral chemistry data to show that melt-fluxed rocks share remarkably similar characteristics across the three core complexes investigated. These are rich in oxides and have olivine, orthopyroxene and clinopyroxene grains that are chemically distinct from oxide-poor gabbros. We propose that oceanic crust fluxed with external melt can be recognised by the following key features: (1) high modes of secondary minerals, such as oxides and olivine, (2) microstructural evidence for the former presence of melt, and (3) mineral chemistry differences between primary and secondary olivine, orthopyroxene, clinopyroxene and plagioclase. Importantly, olivine has previously only been reported as primary, that is, having crystallised from magma. However, our results show that gabbros with secondary olivine are reliable indictors of melt–rock interaction during deformation-assisted diffuse melt migration through the gabbroic oceanic crust. Finally, we propose a new model for the formation of oceanic core complexes where deformation-assisted melt migration plays a critical role in strain localisation, exhumation and evolution of the core complex.

Variations in the Composition of MORB Chilled Glasses from the Mid-Atlantic Ridge, 12°–31° N: Reflection of Compositional Evolution of Parental Melts and the Influence of a Hydrothermal Component

Variations in the Composition of MORB Chilled Glasses from the Mid-Atlantic Ridge, 12°–31° N: Reflection of Compositional Evolution of Parental Melts and the Influence of a Hydrothermal Component

Oceanic Crust Formation in the Mid-Atlantic Ridge Segment between Azores and Icelandic Plumes: Results of Geological and Petrogeochemical Studies

Oceanic Crust Formation in the Mid-Atlantic Ridge Segment between Azores and Icelandic Plumes: Results of Geological and Petrogeochemical Studies

Evidence for low Vp/Vs ratios along the eastern Romanche ridge-transform intersection in the equatorial Atlantic Ocean

Very long oceanic transform faults do not just offset mid-oceanic ridges by hundreds of kilometers, but they also induce variations in the lithospheric structure and affect mantle melting, and hence the crustal accretion at the ridge-transform intersection (RTI). However, the deep structure of large offset RTIs remains unknown. Here, we present three-dimensional seismic tomography results (P- and S-wave velocities, Vp, Vs) in the vicinity of the eastern Romanche transform and the Mid-Atlantic Ridge in the equatorial Atlantic Ocean, using arrival times from microearthquakes recorded by a recent temporary array of seafloor seismometers. We find that normal to low Vp/Vs ratios (∼1.7) are observed at the Mid-Atlantic Ridge, representing weak serpentinization and the dominant magmatic processes, whereas high Vp/Vs ratios (∼1.8-1.95) are observed west of the RTI, at the eastern termination of the 2016 Mw 7.1 earthquake. However, unusually low Vp/Vs ratios (∼1.6-1.7) are observed at 0-15 km depth and in the region of the 2016 Mw 7.1 earthquake rupture zone. We suggest that the unusually low Vp/Vs ratios result from the presence of thick cracks at shallow depths (0-8 km) and/or the enrichment of orthopyroxene in the deep mantle (8-15 km).

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.