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Seismic anisotropy produced by aligned olivine in oceanic lithosphere offers a window into mid-ocean ridge dynamics and the state of Earth's upper mantle. Yet, interpreting anisotropy in the contex...

In 2018–2020 the research vessel (R/V) Professor Logachev (cruises 39 and 41) carried out geological and geochemical studies in the bottom waters of the Mid-Atlantic Ridge hydrothermal fields at 14°45’ N, 13°07’ N, and 13°09’ N. Two new hydrothermal fields were discovered—the Molodezhnoye and Koralovoye. Standard conductivity, temperature, and depth (CTD) sounding with a methane sensor was accompanied by video surveillance and sampling of rocks and water. The rocks were characterized by a zonal composition with opal and sulfides of copper and zinc. An increase in methane concentration values was accompanied by CTD anomalies in the bottom waters. The methane anomaly was formed within the hydrothermal plume of both high-temperature and low-temperature systems. Methane was almost absent in the plume of neutral buoyancy and was associated in all the studied manifestations with the ascending flow of hot waters over the hydrothermal vents. The hydrothermal plumes were characterized by increased Cu, Zn, and Fe concentrations at background Mn concentrations. Signs of low-temperature hydrothermal activity were also observed. Different sources and mechanisms are required to explain the elevated concentrations of base metals and methane in the hydrothermal plumes.

We present seismic tomographic results from a unique seismic refraction and wide-angle survey along a 600 km long flow-line corridor of oceanic lithosphere ranging in age from 0 to 27 Ma in the equatorial Atlantic Ocean at 2° 43′S. The velocities in the crust near the ridge axis rapidly increase in the first 6 Myr and then change gradually with age. The upper crust (Layer 2) thickness varies between 2 and 2.4 km with an average thickness of 2.2 km and the crustal thickness varies from 5.6 to 6 km along the profile with an average crustal thickness of 5.8 km. At some locations, we observe negative velocity anomalies (∼−0.3 km/s) in the lower crust which could be either due to chemical heterogeneity in gabbroic rocks and/or the effects of fault related deformation zones leading to an increase in porosities up to 1.6% depending on the pore/crack geometry. The existence of a low velocity anomaly beneath the ridge axis suggests the presence of partial melt (∼1.3%) in the lower crust. Upper mantle velocities also remain low (∼7.8 km/s) from ridge axis up to 5 Ma, indicating a high temperature regime associated with mantle melting zone underneath. These results suggest that the evolution of the crust and uppermost mantle at this location occur in the first 10 Ma of its formation and then remains unchanged. Most of the structures in the older crust and upper mantle are fossilized structures and could provide information about past processes at ocean spreading centers. Plain Language Summary Oceanic crust emplaced along mid-ocean ridges shows significant variations in the crustal thickness and velocity structures that might be controlled by spreading rate, aging or changes in ridge crest segmentation. Here, we have analyzed seismic refraction data along a 600 km long profile covering 0–27 Ma to study the evolution of crust and upper mantle in the equatorial Atlantic ocean. The seismic velocities in the crust rapidly increase up to 6 Myr and remain fairly constant up to 27 Myr. Crustal thickness varies between 5.6 and 6 km due to the heterogeneous accretion of crust at slow-spreading ridges. At the ridge-axis, low seismic velocities in the mid-to-lower crust indicate higher temperatures greater than 1,200°C and the presence of partial melt. Our results suggest that most of the evolution in the crust and upper mantle occurs in the first 10 Myr and the structures at older ages beyond 10 Myr could shed light on the history of the past spreading processes and tectonics at mid-ocean ridges.

A statistical analysis of heat flow distribution along nine geotravers crossing the mid-oceanic ridges in the Atlantic, Pacific and Indian oceans is carried out. A significant asymmetry of heat flow distribution is established-its mean values differ on opposite sides of the ridges axis. In geotraverses of the southern Earths hemisphere, their western flank has a higher heat flow mean, and in the geotraverses of the northern hemisphere there is the eastern flank. Various tectonic factors that lead to such a distribution are taken into account, but the universal cause of this regularity is suggested to be the effect of the Coriolis force, which, when the planet rotates, redistributes the magmatic material amount in the asthenospheric reservoir.

Observations of heat flow along nine geotraverses across mid-oceanic ridges in the Atlantic, Pacific, and Indian oceans are analyzed statistically. It is found that there is significant heat flow distribution asymmetry: opposite sides of the axis of the ridges differ in the average heat flow. Geotraverses in the Southern Hemisphere of the Earth exhibit higher average heat flow on the western flanks of the mid-oceanic ridges, while geotraverses in the Northern Hemisphere are characterized by higher average heat flow along the eastern flanks. Various tectonic factors are taken into account when explaining this phenomenon; however, it is suggested that the Coriolis force must be considered the main factor responsible for this asymmetry, causing redistribution of magmatic material in the asthenospheric reservoir.

The Paleo-Tethyan orogenic record is well preserved in the Sanjiang area of SW China. However, ophiolites are commonly dismembered in orogenic belt, and complete ophiolite sequences are rare in the Sanjiang area. In this manuscript, we present a newly discovered ophiolite in the Yakou-Banpo area of the southern Lancangjiang belt in the Sanjiang area, SW China. The ophiolite displays a complete Penrose-type ophiolite sequence, composed of serpentinized peridotite, isotropic and cumulate gabbros, massive and pillow basalts, and plagiogranite. Whole-rock geochemical data indicate that these rocks were formed in an oceanic ridge setting, and they show depletions in Nb, Ta and Ti, and enrichment in Pb, suggesting a supra-subduction zone affinity. Furthermore, positive Ɛ Nd (t) (+4.5 to +6.7) and zircon Ɛ Hf (t) values (+12.4 to +14.3), as well as mantle-like δ 18 O values (~5.5‰), suggest that these rocks were derived from a long-term depleted mantle source. SHRIMP zircon U Pb dating yield weighted mean ages of 305 ± 3 Ma, 310 ± 2 Ma, and 313 ± 6 Ma. Hence, the Yakou-Banpo ophiolite belt formed during the Late Carboniferous (313–305 Ma) and records the existence of a mature ocean basin. We propose that an arc – back-arc system developed in the Sanjiang Paleo-Tethyan orogenic belt of SW China during the Late Carboniferous.

Mid-ocean ridge basalts (MORBs) are characterized by large variations in trace element compositions and isotopic ratios, which are difficult to be interpreted solely by using magmatic process such as partial melting of a peridotitic mantle and subsequently fractional crystallization. Geochemical diversity of MORBs have been attributed to large-scale heterogeneity within the underlying mantle, and the heterogeneity might have been caused by addition of recycled crustal component, subcontinental lithosphere, metasomatized lithosphere and outer core contribution. In this study, we investigated the MORBs along the Mid-Atlantic Ridge (MAR) by estimating the temperature and pressure of partial melting, and comprehensively comparing trace element and isotope ratios. The data for MORBs from areas close to mantle plumes show large variations. Mantle plumes can affect mid-oceanic ridges 1 400 km away, but plume effects did not cover all of the ridge segments, and those segments without plume effects did not have any abnormalities in temperature, trace element or isotope ratios. We ascribed the above phenomena to result from the shapes of the plume flow, which we categorized as “pipelike channels” and “pancake-like channels”. The “pancake-like channels” plumes affected the ambient mantle nondirectionally, but the range of the mantle affected by the “pipe-like channels” plumes were selective. Element ratios of MORBs reveal that the mantle source of the MORBs along the MAR is highly heterogeneous. We suggest that most of source heterogeneities of the MORBs may be due to the presence of subducted slab and delaminated lower crust in the source. In addition, the plume that carried materials from the core-mantle boundary may affect some of the segments.

The Southwest Indian Ridge, which is the slowest-spreading of the main ridges, separates the African and Antarctic plates. The slow expanding rate is associated with less density of hydrothermal vent fields, shorter longevity of hydrothermal activity, cold mantle temperatures and thick lithosphere. However, the microbial communities adapting to such specific characteristics of this area have remained largely unexplored. To study the microbial diversity at the Southwest Indian Ridge, we sampled three sediment cores in a newly found inactive vent field, the Tianzuo field, and used high-throughput sequencing of 16S rRNA genes to reveal the microbial composition. Microbial communities of three sampling sites were very similar at the surface, and underwent a gradient change along depth. Gammaproteobacteria, namely Alteromonadaceae, Nitrosococcus and the JTB255 marine benthic group, were the most dominant bacterial taxa. Marine Group I was the dominant archaeal taxon in our samples. In addition, microbial populations capable of ammonia oxidation, nitrite oxidation, sulfur oxidation and manganese oxidation were detected to be the main chemolithoautotrophs. The enrichment of sulfur-oxidizing and manganese-oxidizing bacteria was observed in deep layers. When compared with other vent fields along different ocean ridges, the Tianzuo field showed distinct composition in both archaeal and bacterial communities. These results provide the first view of microbial communities of the Tianzuo field at the Southwest Indian Ridge, and give a better understanding of metabolic potential possessed by the microbial populations.

We have estimated empirical relationships that associate the age of the Pacific oceanic lithosphere with the seismic energy released on both flanks of the East Pacific Rise. The equations found were tested by comparing other empirical equations based on lithospheric age, heat flow, and bathymetric depth published by other authors. The results of this study are consistent for seafloor ages ranging between 1 and 4 Ma throughout three perpendicular cross-sections. These cross-sections are located between the diverging plate boundary of the northern Pacific Ocean up to the subduction zone of western Mexico. Our results allow us to infer the depth of the 1200 °C isotherm underneath the analyzed plates, from the mid-oceanic ridges up to the subduction zones. The distribution of seismic energy released within the limits of the Rivera and Pacific plates shows a gradual decrease from the ridge axis to the oceanic trench. However, an increase of seismic activity is recorded near the Mesoamerican Trench, where the Rivera Plate subducts under the North American Plate at an angle of ~ 46°. This anomaly is a consequence of the convergence, thrust and deepening of the oceanic plate. Due to the homogeneity of this plate, it is possible to infer the projection of isochrones in the subduction zone. Although the thickening of the thermal boundary layer may be estimated from the square root function of the seismic energy, this brittle–ductile transition in the subduction zone deepens exponentially with relation to the computed energy.

Tectonic melange fragments (metabasalt and metadiabase) in the ophiolitic serpentinites of the Egyptian Eastern Desert show peculiar mineral assemblages and they provide useful insights into metamorphism and metasomatism. New field and mineralogical data indicate the formation of rodingite at the expense of mid-ocean ridge basalt (MORB) at three localities, namely Wadi Sikait, Wadi Abu Rusheid and Hafafit (SRH belt). These rocks occupy a lower structural horizon in the tectonic melange zone(s), whereas ocean-island and arc basalt and diabase (OIB) is relatively younger with no evidence of rodingite formation. Rodingitization starts at slow-ultraslow spreading centers and continued until subduction and exhumation at an accretionary wedge. It is a process that is characterized by Ca-rich fluids and the development of a complicated “blackwall” due to superimposed K+ and Mg2+-metasomatism. Oxygen fugacity is fluctuating where it is high for Hafafit and low for Wadi Abu Rusheid rodingites. The Ta/Yb-Th/Yb ratios suggest that rodingitization is not contemporaneous with serpentinization in an island-arc environment but with serpentinization near seamounts at the spreading centers. Combined field observations, whole-rock geochemistry and mineral chemistry data prove that rodingites are formed at the expense of a MORB (mid-ocean basalt) protolith and occupies a lower structural position than unrodingitized IAT (island-arc tholeiite) metadiabases.

Reaction between mid‐ocean ridge basalt (MORB) and crystal mush in the lower oceanic crust has been invoked to explain chemical variations of both MORB and minerals in the lower oceanic crust. Nonetheless, such reactions have been little studied experimentally. We conducted experiments to investigate the mechanisms and chemical consequences of melt‐mush interaction by reacting molten MORB with troctolite at 0.5 GPa. Isothermal experiments demonstrate that melt infiltrates into troctolite with dissolution of plagioclase and olivine. The reacted melts have higher MgO and Al2O3 and lower TiO2 and Na2O contents and crystallize more primitive olivine and plagioclase compared to those crystallized from the unreacted melts, suggesting melt‐mush reaction could result in the formation of high‐Al basalt. The melt compositional variations induced by reaction also significantly affect the calculated pressures for MORB fractionation, indicating that major element‐based barometers for MORB fractionation can only be used reliably if reaction can be ruled out. After reaction, the troctolite contains olivine with plagioclase inclusions and poikilitic clinopyroxene with partially resorbed olivine and plagioclase chadacrysts, indicating that melt‐mush interaction occurs through dissolution‐reprecipitation mechanisms. Clinopyroxene has high Mg# (>83) and elevated Na2O and TiO2 contents, and olivine has different Fo versus Ni correlations from fractional crystallization models, which provide testable parameters for the effect of melt‐mush reaction in the rock record. By comparison with samples from lower oceanic crust and layered intrusions, we propose that melt‐mush reaction plays an important role during magma transport in the crystal mush in both oceanic and continental magma systems.

Plate tectonics, the principal vehicle for dissipation of planetary energy, is believed to buffer the δ18O of seawater at its near-modern value of 0‰ SMOW (Standard Mean Ocean Water) because the hot and cold cells of hydrothermal circulation at oceanic ridges cancel each other. The persistence of plate tectonics over eons apparently favors attribution of the well-documented oxygen isotope secular trends for carbonates (cherts, phosphates) to progressively warmer oceans, from 40–70 °C in the early Paleozoic to 60–100 °C in the Archean. We argue that these oceanic hydrothermal systems are dominated by low-temperature (

We present here new mineral chemistry and major trace element and Sr-Nd-Pb-O isotope data as well as detailed field and petrographic studies of the Topuk Pluton (TP; ca. 48 Ma) and its hypabyssal counterparts. We aim to constrain the granitic magma genesis and emplacement and magmatic processes within the framework of post-collisional evolution of NW Anatolia. The TP is a shallow level intrusion (1.5–2.1 kbar, 680–760 °C, and 4.5–6.3 km depth) which is characterized by a dominant granodiorite composition and presents holocrystalline porphyritic texture. The pluton includes mafic microgranular enclaves (MMEs) of diorite/quartz diorite composition. During its emplacement, the pluton developed radial and ring fractures around itself, and aplite, porphyry dykes, and stocks were emplaced coeval with the pluton along these ring and radial fractures. The TP shows metaluminous and calc-alkaline I-type characteristics. Normal-mid ocean ridge basalt-normalized trace element patterns show that the samples are enriched in large-ion lithophile elements and light rare earth elements but depleted in Nb, Ti, and Ta elements. Granodiorite, MMEs, and hypabyssal rocks present similar initial 87Sr/86Sr and 143Nd/144Nd isotopic values, which lie in the ranges 0.705860–0.706247 and 0.512455–0.512584, respectively. The 206Pb/204Pb, 207Pb/204Pb, 208Pb/206Pb, and 207Pb/206Pb ratios are 18.831–19.046, 15.673–15.748, 2.0621–2.0725, and 0.82816–0.83321 values, respectively, while the δ18O isotope values range from 9.6 to 10.7. These similar Sr-Nd-Pb-O isotopic features of MMEs and host granodiorites indicate a common igneous precursor as the magma source. We infer that the magma-forming TP originated from enriched mantle type II (EMII) with contribution from the lower crust. The results of numerical modelling indicate that this lower crustal input ranges from 5 to 20%. The Eocene melt generation was most probably caused by the upwelling asthenosphere due to the break-off of Neo-Tethys slab in NW Anatolia.

Diapycnal mixing of nutrients from the thermocline to the surface sunlit ocean is thought to be relatively weak in the world's subtropical gyres as energy inputs from winds are generally low. The interaction of internal tides with rough topography enhances diapycnal mixing, yet the role of tidally induced diapycnal mixing in sustaining nutrient supply to the surface subtropical ocean remains relatively unexplored. During a field campaign in the North Atlantic subtropical gyre, we tested whether tidal interactions with topography enhance diapycnal nitrate fluxes in the upper ocean. We measured an order of magnitude increase in diapycnal nitrate fluxes to the deep chlorophyll maximum (DCM) over the Mid-Atlantic Ridge compared to the adjacent deep ocean. Internal tides drive this enhancement, with diapycnal nitrate supply to the DCM increasing by a factor of 8 between neap and spring tides. Using a global tidal dissipation database, we find that this spring-neap enhancement in diapycnal nitrate fluxes is widespread over ridges and seamounts. Mid-ocean ridges therefore play an important role in sustaining the nutrient supply to the DCM, and these findings may have important implications in a warming global ocean. Plain Language Summary The subtropical gyres cover an extensive area of the global ocean and account for similar to 30% of carbon export to the deep ocean. The pattern of the winds induces downwelling in these gyres and leads to surface waters being relatively nutrient impoverished. Biological production in the subtropical gyres is primarily limited by the availability of nitrate, which can be increased through mixing in the underlying thermocline. Internal tides can enhance mixing in the ocean interior close to steep sloping topography; deep in the ocean interior, this mixing is a key component of ocean physics. In our field study, we reveal the mixing extending up toward the surface and measured a tenfold increase in nitrate fluxes to phytoplankton in the surface ocean over the Mid-Atlantic Ridge compared to in the surface waters in the adjacent deeper ocean. Importantly, nitrate fluxes over the ridge varied fortnightly with an eightfold increase from neap to spring tides. These inferences of enhanced mixing and nutrient supply along ridges and seamounts are relevant for the rest of the global ocean given the ubiquitous nature of the tides.

Fracture-fissure systems found at mid-ocean ridges are dominating conduits for the circulation of metallogenic fluid.Ascertaining the distribution area of active faults on both sides of mid-ocean ridges will provide a useful tool in the search for potential hydrothermal vents,thus guiding the exploration of modem seafloor sulfides.Considering the MidAtlantic Ridge 20°N-24°N (NMAR) and North Chile Rise (NCR) as examples,fault elements such as Fault Spacing (ΔS) and Fault Heave (ΔX) can be identified and quantitatively measured.The methods used include Fourier filtering of the multi -beam bathymetry data,in combination with measurements of the topographic slope,curvature,and slope aspect patterns.According to the Sequential Faulting Model of mid-ocean ridges,the maximal migration distance of an active fault on either side of mid-ocean ridges—that is,the distribution range of active faults—can be measured.Results show that the maximal migration distance of active faults at the NMAR is 0.76-1.01 km (the distance is larger at the center than at the ends of this segment),and at the NCR,the distribution range of active faults is 0.38-1.6 km.The migration distance of active faults on the two study areas is positively related to the axial variation of magma supply.In the NCR study area,where there is an abundant magma input,the number of faults within a certain distance is mainly affected by the variation of lithospheric thickness.Here a large range of faulting clearly corresponds to a high proportion of magmatism to seafloor spreading near mid-ocean ridges (M) value,and in the study area of the NMAR,there is insufficient magmatism,and the number of faults may be controlled by both lithospheric thickness and magma supply,leading to a less obvious positive correlation between the distribution range of active faults and M.

We present the major and trace elements and Sr, Nd, and Pb isotopes in mid-ocean ridge basalts (MORB) from the East Pacific Rise (EPR) at 2.6–3.1°S. These samples are low-K tholeiites and show sign...

International audience; Oceanic detachment faulting is a major mode of seafloor accretion at slow and ultraslow spreading mid-ocean ridges, and is associated with dramatic changes in seafloor morphology. Detachments form expansive dome structures with corrugated surfaces known as oceanic core complexes (OCCs), and often transition to multiple regularly-spaced normal faults that form abyssal hills parallel to the spreading axis. Previous studies have attributed these changes to along-axis gradients in lithospheric strength or magma supply. However, despite the recognition that magma supply can influence fault style and seafloor morphology, the mechanics controlling the transition from oceanic detachment faults to abyssal hill faults and the relationship to along-axis variations in magma supply remain poorly understood. This study investigates this issue using two complementary modeling approaches. The first consists of semi-analytical, two-dimensional (2-D) cross-axis models designed to address the fundamental mechanical controls on the longevity of normal faults. These 2-D model sections are juxtaposed in the along-axis direction to examine the response of the plan-view pattern of faults to along-axis variations in magmatic accretion in the absence of along-axis mechanical coupling. The second approach uses three-dimensional (3-D), time-dependent numerical models that simulate faulting and magma intrusion in a visco-elasto-plastic continuum. The primary variable studied through both approaches is the along-axis gradient in the fraction M of seafloor spreading that is accommodated by magmatism. The 2-D and 3-D results predict different abyssal hill spacing and orientation, however the plan-view geometry of self-emerging detachment faults predicted by the 3-D numerical models are well explained by the juxtaposed 2-D models. This indicates a first-order control by cross-axis effects of changing values of M. These models are also shown to explain the along-axis extent and plan-view curvature of the well-developed 13 • 20 N and Mt. Dent OCCs (Mid-Atlantic Ridge and Cayman Rise) in terms of quantifiable along-axis gradients in magma emplacement rates.

Oceanic plagiogranite was first defined by Coleman in 1975 as an assemblage of felsic rocks in the ocean crust and is the sole rock type in ophiolites for age dating. However, the petrogenesis of these felsic rocks remains controversial. Some consider them as partial melting products of gabbros, while others interpret them as representing highly evolved residual melt of ocean ridge magmatism. To resolve this debate, we focus our study on felsic veins from ODP Hole 735B (Leg 176) in the Southwest Indian Ridge (SWIR). We carried out detailed petrography, mineral compositional analysis, bulk‐rock trace element and Sr‐Nd‐Pb‐Hf isotope analysis, and petrological modeling. These data and observations lead to the conclusion that these felsic vein lithologies (i.e., oceanic plagiogranite) are solidified residual melts after advanced extents of fractional crystallization of ocean ridge basaltic (MORB) magmas, rather than partial melting products of gabbros. At the late stage of MORB magma evolution, Fe‐Ti oxides appear on the liquidus to crystallize, resulting in the residual melts rapidly enriched in SiO2. Such silicic melts are buoyant and can transport through “cracking” within the lithologies of the gabbroic sections of the ocean crust in the form of felsic dykes, veins and veinlets. The volumetrically small (~ 0.5%) but widespread plagiogranite veins and veinlets throughout the gabbroic sections provide evidence that multiple small melt batch intrusion and differentiation are primary mechanism and mode of ocean crust accretion at slow‐spreading ridges.

Observations of the time lag between the last deglaciation and a surge in volcanic activity in Iceland constrain the average melt ascent velocity to be $\geq50$ $\mathrm{m/yr}$. Although existing theoretical work has explained why the surge in eruption rates increased $5$-$30$ fold from the steady-state rates during the last deglaciation, they cannot account for large variations of Rare Earth Element (REE) concentrations in the Icelandic lavas. Lavas erupted during the last deglaciation are depleted in REEs by up to $70\%$; whereas, existing models, which assume instantaneous melt transport, can only produce at most $20\%$ depletion. Here, we develop a numerical model with finite melt ascent velocity and show that the variations of REEs are strongly dependent on the melt ascent velocity. When the average melt ascent velocity is $100$ $\mathrm{m/yr}$, the variation of $\mathrm{La}$ calculated by our model is comparable to that of the observations. In contrast, when the melt ascent velocity is $1,000$ $\mathrm{m/yr}$ or above, the model variation of $\mathrm{La}$ becomes significantly lower than observed, which explains why previous models with instantaneous melt transport did not reproduce the large variations. We provide the first model that takes account of the diachronous response of volcanism to deglaciation. We show by comparing our model calculations of the relative volumes of different eruption types (subglacial, finiglacial and postglacial) and the timing of the bursts in volcanic eruptions with the observations across different volcanic zones that the Icelandic average melt ascent velocity during the last deglaciation is likely to be $\sim100$ $\mathrm{m/yr}$., The Cambridge Trust and the Leverhulme Trust

Highlights • Systematic analysis of ship bathymetry enables segment scale geological mapping. • The Menez Gwen segment experiences magmatic periods every 300 to 500 ka. • Periods of enhanced magmatic activity are a regional phenomenon. • Hydrothermalism at Menez Gwen accompanies a waning, intense magmatic period. • Faulting and along-axis permeability variations focus hydrothermal venting. Abstract 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.

Magmatic processes associated with oceanic crustal accretion at slow-spreading mid-oceanic ridges are less well understood compared with those at fast-spreading ridges. Zoned plagioclase in the basalts might record these magmatic processes as a result of the very slow intra-crystal diffusion of CaAl–NaSi. Plagioclase phenocrysts in plagioclase-phyric basalt from Hole U1433B of International Ocean Discovery Program (IODP) Expedition 349 in the South China Sea show complex zoning patterns (e.g. normal, reverse, oscillatory and patchy). These samples provide a rare opportunity to determine the magma dynamics associated with oceanic crustal accretion at slow-spreading ridges through time. Igneous lithological units in Hole U1433B consist of a series of massive lava flows at the bottom and a thick succession of small pillow lava flows at the top. Most of the plagioclase phenocrysts in the massive lava show core–rim zonation with high-An cores (An ∼85%; in mole fraction; Pl-A) in equilibrium with melts that are more primitive than their host. Some high-An cores of Pl-A phenocrysts contain melt inclusions and are depleted in La, Ce, Y and Ti, but enriched in Sr and Eu; this is interpreted as resulting from dissolution–crystallization processes during reaction of hot melt with pre-existing plagioclase cumulates. In the pillow lavas, most of the plagioclase phenocrysts show normal core–mantle–rim zonation (Pl-B) with An contents decreasing gradually from the core to the mantle to the rim, suggesting extensive magma mixing and differentiation. Reversely zoned plagioclases (Pl-C) are sparsely present throughout the basalts, but mostly occur in the lower part of the drill hole. The cores of euhedral Pl-C phenocrysts are compositionally comparable with the mantles of Pl-B phenocrysts, suggesting that the evolved magma was recharged by a relatively primitive magma. Melt inclusion-bearing Pl-A phenocrysts occur mainly in the massive lava, but rarely in the pillow lava, whereas Pl-B phenocrysts are present dominantly in the pillow lava, which reflects reducing melt–rock interaction and enhanced magma mixing, recharging and differentiation from the bottom to the top of the hole. In addition, the extensive magma mixing and differentiation recorded by Pl-B phenocrysts in the pillow lava require the existence of a melt lens beneath the mid-ocean ridge. Consistently, the plagioclase phenocrysts in the pillow lava mostly lack melt inclusions, corresponding to very weak melt–rock reactions, which indicates that the magma was transported through plagioclase cumulates by channel flow and requires a higher magma supply to the magma conduit. Therefore, the textural and compositional variations of plagioclase phenocrysts in the samples reflect the changes in magma dynamics of the mid-ocean ridge basalt through time with respect to oceanic crustal accretion at slow-spreading ridges. Overall, the oceanic crustal accretion process is sensitive to the magma supply. In the period between two episodes of extension, owing to a low melt supply the primitive melt percolates through and interacts with the mush zone by porous flow, which produces melt inclusion-bearing high-An plagioclase through dissolution–crystallization processes. At the initial stage of a new episode of extension, the melt infiltrates the mush zone and entrains crystal cargoes including melt inclusion-bearing high-An plagioclase. During the major stage of extension, owing to a relatively high melt supply the melt penetrates the mush zone by channel flow and can pool as melt lenses somewhere beneath the dikes; this forms intermediate plagioclases and the reverse zoning of plagioclases by magma mixing, recharging and differentiation in the melt lens. Such magmatic processes might occur repeatedly during the episodic extension that accompanies oceanic crustal accretion at slow-spreading ridges, which enhances the lateral structural heterogeneity of the oceanic crust.

International audience; We present a procedural method for authoring synthetic tectonic planets. Instead of relying on computationally demanding physically-based simulations, we capture the fundamental phenomena into a procedural method that faithfully reproduces large-scale planetary features generated by the movement and collision of the tectonic plates. We approximate complex phenomena such as plate subduction or collisions to deform the lithosphere, including the continental and oceanic crusts. The user can control the movement of the plates, which dynamically evolve and generate a variety of landforms such as continents, oceanic ridges, large scale mountain ranges or island arcs. Finally, we amplify the large-scale planet model with either procedurally-defined or real-world elevation data to synthesize coherent detailed reliefs. Our method allows the user to control the evolution of an entire planet interactively, and to trigger specific events such as catastrophic plate rifting.

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

The tectonic window at Pito Deep, in the southern Pacific Ocean, permits study of the formative processes of uppermost East Pacific Rise (EPR) gabbroic ocean crust. Here we present a detailed microstructural and crystallographic study of 17 gabbroic samples from the uppermost ∼800 m of plutonic crust exposed in the Pito Deep Rift. We integrate two- and three-dimensional measurements of crystal size, shape, spatial distribution and orientation, with petrographic observations and geochemical data to constrain the formation of fast spread gabbroic ocean crust. The shallowest samples, collected < 55 metres below the sheeted dikes (mbsd), have evolved bulk-rock compositions, elongate plagioclase crystals, a clear plagioclase shape- and crystallographic-preferred orientation, and preserve only minor amounts of intracrystalline strain. The characteristics of these rocks and their proximity to the sheeted dike complex, suggests they formed by crystallization at the lateral tip of an axial melt lens that solidified as it moved off axis. Underlying samples from 96–724 mbsd, record more primitive bulk-rock compositions, less elongate plagioclase crystals and exhibit increasing strength of both plagioclase shape- and crystallographic-preferred orientation with depth below the sheeted dikes. These samples host plagioclase crystals that show increasing intracrystalline strain with depth, suggesting magmatic to hypersolidus submagmatic flow within the mush zone beneath the axial melt lens. These observations, together with inclined-to-steeply dipping mineral layering preserved below ∼180 mbsd, are interpreted to record the downward transport of crystal-rich magma originating at the bottom of the melt lens through the uppermost kilometre of the mush zone at the EPR. The location of initial crystallization along the floor of the axial melt lens determines the magmatic processes that affect the crystal-rich magma en route to solidification as lower ocean crust.

There is debate to what degree mid-ocean ridge basalts (MORB) can be used to infer the composition of the sub-ridge mantle and whether the upper mantle contains components that are difficult to recognize by observations on MORB alone. Here we report trace element and isotope data on a fossil mantle section exposed in the ophiolitic Lanzo South massif, Italy. Our findings show the existence of clinopyroxenes in geochemically depleted melt-infiltrated harzburgites with an extreme radiogenic Hf-isotopic composition but MORB-like Nd isotopic compositions. The simplest way of explaining the depleted trace elements in combination with the decoupled isotopic compositions is that the harzburgites were infiltrated by a melt with isotopic compositions inherited from an anciently (>1 Ga) depleted source. The existence of melts from ultra-depleted sectors of the asthenosphere allows constraining of the configuration of the heterogeneities in the sub-ridge mantle. The parallel arrays in Hf–Nd isotope composition space of different Mid-Ocean ridge segments imply the ubiquitous presence of anciently depleted residual mantle as the matrix in which more enriched components occur in pockets.

Ocean ridges are one of the primary connections between Earth's mantle and surface. Therefore, understanding the evolution of ocean ridge processes through Earth history is important for understanding how Earth's surface and mantle have evolved. We combine an analytic model of mantle convection with a petrologic model of mantle melting in the presence of CO2 to compute the mantle temperature, plate speed, melt production, and CO2 outgassing flux at ocean ridges for the past 4 billion years. We explore a large suite of realistic mantle and lithospheric parameters to map out a full range of possible thermal histories. Our results show that in order to satisfy thermal constraints, the Earth must have started in a sluggish-lid mode of plate tectonics and transitioned to an active-lid mode. Furthermore, we show that plate speed and CO2 outgassing flux do not necessarily scale with mantle temperature, and that it is possible to reach present-day mantle temperatures and plate speeds with a simple force balance that does not invoke any feedbacks (e.g. grain size evolution, dehydration stiffening) or a fully stagnant-lid mode of convection in the Precambrian. The solutions show a range of evolutionary behaviors depending on the parameters chosen. The transition from sluggish- to active-lid can be smooth, abrupt, or somewhere in between. A smooth transition is difficult to distinguish from a purely active-lid evolutionary path based on temperature, plate speed, melt production, and CO2 outgassing flux. In contrast, an abrupt transition leads to a large increase in the average plate speed with a corresponding increase in melt production and CO2 outgassing flux. As an abrupt transition would have a much larger effect on CO2 outgassing and melt production than a change in ridge length, we investigate the impact of rapidly changing plate speeds and do not consider changes in ridge length. Finally, we show that carbon recycling is required for a large part of Earth history in order to explain present-day CO2 outgassing. Our model highlights the importance of understanding the style of mantle convection when calculating melt production and volatile fluxes through Earth's history.

The formation of sandstone-style uranium deposits are significantly controlled by the latest tectonic uplift events, especially those generated in an open system by the infiltration of meteoritic water. This study explores the spatiotemporal relationship between global Miocene tectonics and sandstone-style uranium deposits to evaluate the link between tectonic uplift and metallogenic ages. In general, modern landforms were shaped by diverse global tectonic events that began in the Middle and Late Miocene. This includes the formation of continental-scale faults and fold belts, including the uplift of the three broadly contemporaneous ranges, the Alpine–Himalayan Belt, the Cordillera, and the East African Rift. These events were accompanied by intense magmatic activity, as well as an increase in the spreading rates of oceanic ridges and higher sedimentation rates in the Pacific, Indian, and Atlantic oceans. Modern global climate patterns are a direct consequence of these Miocene tectonic events. Studies on sandstone-style uranium deposits report that though their stratigraphic ages extend from the Proterozoic to the Cenozoic, they notably occur from the Late Mesozoic to the Neogene. Uranium mineralization ages are concentrated between 20 and 0.1 Ma, clustering mainly in the Miocene although remobilizations are known to have occurred subsequently, some even in the Holocene. Global tectonics in the Miocene, between 20 and 5 Ma, resulting in the regional uplift of mountain ranges and continental tilts, may have exerted significant control over the migration directions of uranium-bearing fluids that formed these sandstone-style uranium deposits. Climate patterns in the Miocene, specifically subtropical high-pressure belts on both sides of the equator controlled the creation of arid and semi-arid areas, creating characteristic rainfall patterns which would also have influenced the spatial distribution of these uranium deposits. Thus, the spatiotemporal characteristics of most sandstone-style uranium deposits in the world were controlled by global tectonics in the Miocene that uplifted mountain ranges and tilted continents, generating subtropical high-pressure belts and the Asian high pressure region. In addition to new ideas adding greater research value to the study of global sandstone-style uranium mineralization, this paper also provides a new exploration indicator for sandstone-style uranium deposits.

Iceland is a natural laboratory where plate separation and mid-ocean ridge formation can be studied directly in the field. In this setting, it is not fully understood how magma interacts with faulting and fissuring, and how rift zones propagate. In order to improve the understanding of how these processes operate, we conducted a detailed field study about the kinematics and propagation of 33 Holocene faults along the N–S Theistareykir rift (North Volcanic Zone) and mapped all tension fractures. This rift hosts the Holocene Theistareykir central volcano, for which ground deformation data (InSAR and GPS) indicate an inflating shallow magma chamber throughout 2006–2008. Analyses of the cumulative fault slip distribution at 696 sites show two opposite directions of fault/rift propagation: The first, more developed, is directed northwards and affects the area located north of Theistareykir volcano, whereas south of the volcano, the rift propagates southwards. Tension fractures increase in frequency, with respect to faults, outwards from the volcano, along a N–S to NNE–SSW direction. The presence of aligned vents, dykes and topographic bulging along some of the N–S structures, and the active magma chamber below Theistareykir volcano, suggest that faults and tension fractures propagate following repeated dyke intrusions from the magma chamber outward along the plate margin. The fact that the rift is more developed north of the volcano than south of it, is interpreted as the effect of buttressing from another, coterminous magma chamber sitting below a neighbouring central volcano. Other causes for the general observed pattern such as tectonic stresses, mechanical interaction of faults, and changes in the rheological characteristics of rocks, are discussed as well.

Combined Ce and Nd isotope ratios provide a time-integrated record of the light rare earth element (LREE) abundances of their source materials. Here, we present new high precision Ce isotope data for chondrites and basalts from ocean islands (OIB) and mid ocean ridges (MORB). The new Ce isotope ratios in chondritic meteorites define a precise new CHUR reference value. In Ce–Nd isotopic space, the MORB and OIB form a well-defined array that intersects with the Ce–Nd chondritic reference value. The simplest first-order explanation is that the bulk silicate Earth (BSE) has chondritic LREE and Ce–Nd isotope ratios. We show, however, that the intercept and slope of the Ce–Nd isotope mantle array depend on several factors. Perhaps most important are whether the BSE is chondritic and how closely the mantle average reflected in the MORB and OIB data corresponds to the Ce–Nd isotope ratio of the BSE. A significant difference between the accessible mantle's average Ce–Nd isotope ratio and that of BSE could result from the permanent storage of a considerable proportion of Earth's total LREE budget in the continental crust or potential isolated reservoirs. We show that the formation of isolated reservoirs either has a minor effect on the average Ce–Nd composition of the average mantle, or is geochemically and geodynamically implausible. If, due to formation of the continental crust, a significant shift in the average mantle's Ce–Nd isotope composition relative to BSE occurs, this shift is parallel to the Ce–Nd mantle array, and does not affect its chondritic intercept. The chondritic intercept of the Ce–Nd isotope mantle array therefore is strong evidence that BSE's relative LREE and Ce–Nd isotope composition is chondritic. However, an apparent difference between the accessible mantle's average Ce–Nd isotope ratio and that of BSE could also result if MORB and OIB do not sample the accessible mantle in a representative manner. Although we cannot entirely exclude the latter, it would require a fortuitous combination of factors to cause the observed chondritic intercept of the Ce–Nd isotope mantle array. We therefore conclude that the bulk silicate Earth has chondritic LREE and Ce–Nd isotope ratios.

The assumption of isostatic equilibrium is often used in geophysics; e.g., the depth of the lithosphere–asthenosphere boundary (LAB) is determined using data regarding the gravity field and topography, together with the assumption of isostasy. However, isostasy implies a hydrostatic state, which is contrary to the mantle convection hypothesis, which states that most of the mantle matter is in motion. We therefore discuss herein the question of when the assumption of isostasy can be used. It is suggested that isostasy may be used for parts of oceanic plates (except for subduction zones, hotspots, and oceanic ridges) and for many continental regions (except for postglacial regions and regions of intensive volcanic or tectonic activity). Moreover, using the results of a numerical model of convection and calculations of dynamic topography, it is shown that some generalization of isostasy is possible in the form of deep dynamic isostasy (DDI). It is also indicated that, for some regions without isostatic equilibrium (e.g., postglacial regions), it is possible to use the “isostatic” method with some corrections to the results.

An extinct, late Jurassic-to-earliest Cretaceous ridge-and-fracture zone geometry in the western Gulf of Mexico (GOM), and extinct seafloor ridge segments in the eastern Gulf of Mexico (EGOM), were previously identified using the vertical gradient of satellite-derived free-air gravity data. Circular gravity anomaly lows, and magnetic anomaly highs, over the center of spreading ridge segments are interpreted as large volcanic centers that erupted within a late Jurassic-to-earliest Cretaceous, slow-spreading center. Detailed mapping of oceanic basement using oil industry seismic data indicates that the EGOM oceanic ridge system is characterized by 30-60-km-long spreading ridge segments, that include 15-km-wide, 2-km-high axial volcanoes in their centers, and nodal basins at their ends. Stratigraphic evidence from seismic reflection data tied to a deepwater well indicates that volcanism along the spreading ridge ended around the same time (Berriasian), or slightly after (Valanginian), the cessation of seafloor spreading in the EGOM. Flowlines of late Jurassic-to-earliest Cretaceous seafloor, based on a pole of rotation from the geometry of GOM spreading ridges and fracture zones, show a good match with gravity and magnetic anomalies along the Florida and Yucatan conjugate margins of the EGOM. Mapping of age-dated, stratigraphic downlaps onto the oceanic crust is consistent with an interpreted ridge jump at the beginning of seafloor spreading (Kimmeridgian) to the southwest, and in the same southwestward direction of a previously inferred mantle plume in the central GOM. Our 3-D gravity structural inversion of the Moho requires 6.4 km thick oceanic crust in the northwestern EGOM, and 5.5 km thick oceanic crust in southeastern EGOM. We interpret this along-ridge, thickness variation to reflect faster spreading and thicker oceanic crust farther from the opening pole located in the southeastern GOM.

Multistage igneous rocks identified in the newly discovered Fukeshan porphyry Cu Mo deposit, NE China give new insights into the late-stage southwards subduction of the Mongol-Okhotsk oceanic slab (MOOS) and Cu Mo mineralization during the late Mesozoic. The U Pb ages of zircons from medium-grained monzogranite, coarse-grained monzogranite, ore-related quartz diorite porphyry, quartz monzonite, ganodiorite porphyry, diorite porphyry and andesitic porphyry associated with the Fukeshan deposit are 192.7 ± 1.9, 192.2 ± 2.7, 148.7 ± 0.8, 148.8 ± 0.9, 144.1 ± 1.1, 144.9 ± 0.9, and 144.8 ± 1.3 Ma, respectively. The Early Jurassic coarse-grained monzogranite, Late Jurassic quartz monzonite and quartz diorite porphyry, and Early Cretaceous granodiorite porphyry are characterized by low Yb and Y contents, and high Sr/Y ratios, indicating adakite affinities. The whole-rock geochemistry, and Sr Nd and zircon Hf isotopic features of the coarse-grained monzogranite and granodiorite porphyry indicate they were derived from the partial melting of a thickened basaltic lower crust. On the other hand, the quartz monzonite and quartz diorite porphyry probably originated from the melting of oceanic crust together with assimilation of enriched mantle and continental crust. The medium-grained monzogranites were possibly derived from a basaltic lower crust at shallower depths, and the diorite porphyry and andesitic porphyry probably originated from a metasomatized mantle source. Considering the Early Jurassic–Early Cretaceous magmatism and regional tectonic evolution of the Erguna Block, we propose that the MOOS was subducted towards the south during the Early Jurassic (195–170 Ma), slab retreat occurred during the early Late Jurassic (165–155 Ma), oceanic ridge subduction took place during the Late Jurassic (150–147 Ma). The final closure of the Mongol-Okhotsk Ocean occurred during the Early Cretaceous (145–143 Ma). The southwards subduction of a ridge on the MOOS provided a favorable tectonic setting for Cu Mo mineralization, and this is accordance with the discovery of many Late Jurassic–Early Cretaceous porphyry Cu Mo deposits in the eastern part of the Erguna Block.

This study maps hydrothermal alteration of Wadi Tayin Massif of the Sultanate of Oman using the spectral bands of Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) and the image processing methods namely band ratios, false color composite (FCC), spectral angle mapper (SAM), mixture tuned matched filtering (MTMF) and linear spectral unmixing (LSU) to understand alteration of the upper mantle-lower crust transition zone. The study of FCC image of the spectral bands 8, 4, and 1 in Red (R), Green (G), and Blue (B), respectively, showed the presence of the hydrothermal alteration in shades of purple and brown over the mantle peridotites , dunites , and crustal gabbros. The ASTER band ratios R:9/8, G:4/3, B:2/1 used in this study able to show the occurrence and distribution of alteration in orange distinctly. The SAM, LSU and MTMF image processing methods applied over the region confirmed the presence of alteration and altered minerals in the rocks. The assessment of accuracy for the methods provided the high accuracy of 91.86% and Kappa Coefficient = 0.89 in the LSU method. The image interpretations are studied in the field and the occurrence of alterations are validated by presence of strong serpentinization in the peridotites and dunites, and secondary veins in the micro cracks of gabbros. Laboratory studies showed presence of the serpentinization, iddingsitization, chloritization and development of pargasite in the altered rocks. This study shows application of the ASTER sensor to map hydrothermal alteration found near the fast-spreading oceanic ridge.

Late Cretaceous magmatism in the southern Lhasa subterrane, Tibet, provides critical constraints on the subduction processes of the Neo-Tethyan oceanic plate. Here, we provide data on the whole-rock geochemistry, zircon U Pb ages, trace element contents, and Hf isotopes of early Late Cretaceous volcanic rocks from Sangri County to Sangye Temple (SS) along the southern Lhasa subterrane. The SS volcanic rocks include basalts, andesites, and dacites. SIMS and LA–ICP–MS zircon U Pb data indicate that the SS dacites were erupted at ca. 95 Ma, coeval with the early Late Cretaceous “flare-up” event in the southern Lhasa subterrane. The SS volcanic rocks belong to the calc-alkaline to high-K calc-alkaline series and have the geochemical features of subduction-related volcanic rocks with negative Nb, Ta, and Ti anomalies. The basalts have similar rare earth element (REE) patterns to E-MORB and are characterized by high Zr abundances (65–190 ppm) and Zr/Y ratios (3.3–6.5). They were possibly derived by high-degree partial melting of mantle peridotite that had been metasomatized by subduction-related fluids with an input of asthenospheric components. Positive whole-rock eNd(t) (+2.9 to +3.2) values for the andesites, and their variable Mg# values (35–61) and MgO (2.29–7.32 wt%), Cr (12–232 ppm), and Ni (10–127 ppm) contents can be attributed to medium-degree partial melting of mantle peridotite followed by fractional crystallization of clinopyroxene and olivine. The calc-alkaline SS dacites are similar to adakites with high Sr (663–1188 ppm) and low heavy rare earth element (HREE) and Y (10.7–11.5 ppm) contents. These adakitic dacites have positive whole-rock eNd(t) (+2.9 to +3.4) and zircon eHf(t) (+8.4 to +14.9) values with relatively high values of Mg# (36–57) and high contents of compatible elements, indicating that they were derived from the partial melting of subducted oceanic slab, and with the ascending magma interacting with mantle wedge peridotite. Our new data indicate a distinct rock association of coeval asthenosphere-component-involved basalts and slab-derived adakitic dacites. Such an association, together with the coeval high-temperature charnockites and high-temperature granulite-facies metamorphism of the charnockitic country rocks in the southern Lhasa subterrane, indicates an anomalous input of materials and heat during the early Late Cretaceous “flare-up” event. Such an event can probably be attributed to the northwards subduction of a Neo-Tethyan oceanic ridge that allowed for contributions from upwelling asthenosphere that differed from the typical results of normal-angle subduction or low-angle/flat subduction and subsequent slab rollback.

International audience; The current distribution ranges of vent species result from the complex tectonic history of oceanic ridges. A growing number of DNA barcode studies report the presence of cryptic species across geological discontinuities that offset ridge systems and have gradually helped to draw a more precise picture of the historical migration pathways of vent fauna. We reexamined the phylogeny of species within the Lepetodrilus elevatus complex along the East Pacific Rise (EPR) ridge system in the light of new samples from the Galápagos Rift and the Guaymas Basin. Our analyses of mitochondrial cytochrome c oxidase subunit I gene sequences, coupled with morphological data, highlight the occurrence of a distinct lineage along the Galápagos Rift and offer new insight into the current distribution range of this species complex. Due to the absence of clear morphological diagnostic criteria and the potential overlap of these lineages at key locations, we recommend reassigning the taxon L. galriftensis to the subspecies level and maintaining the name L. elevatus for all clades along the EPR/Galápagos Rift system.

The Middle–Late Jurassic formations are regarded to be an important source rock in the Jurassic petroleum system of North Iraq. The major and trace element analysis on the Middle and Late Jurassic shales in the Imbricated Zone, Iraq was analyzed. This study investigated the shales provenance and depositional environment, in addition to evaluate the intensity of weathering in the source area. The chemical index of alteration (CIA), index of compositional variability (ICV) and the Al2O3-CaO + Na2O-K2O diagram of the shales suggest increasing in the intensity of chemical weathering from the Middle to the Late Jurassic due to the climatic change and/or tectonic activity. The elemental ratios: Al2O3/TiO2, Th/Sc, La/Th, Co/Th, (Gd/Yb)n, Eu/Eu*, and REE pattern; and tectonic discrimination diagrams indicated that they were derived from felsic source rocks in the crystalline basement of the northern fringe of the Arabian Shield and Rutba Uplift. The Sargelu, Naokelekan and Barsarin formations are also supplied from the intermediate source rocks from the Mid Oceanic Ridge and deposited in an active setting. The geochemical parameters suggest the lower part of the Middle Jurassic and the shales from upper part of the Late Jurassic were deposited in an oxic shallow marine environment; and other ages were deposited in an anoxic deep marine environment.