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The plate tectonic cycle produces chemically distinct mid-ocean ridge basalts and arc volcanics, with the latter enriched in elements such as Ba, Rb, Th, Sr and Pb and depleted in Nb owing to the water-rich flux from the subducted slab. Basalts from back-arc basins, with intermediate compositions, show that such a slab flux can be transported behind the volcanic front of the arc and incorporated into mantle flow. Hence it is puzzling why melts of subduction-modified mantle have rarely been recognized in mid-ocean ridge basalts. Here we report the first mid-ocean ridge basalt samples with distinct arc signatures, akin to back-arc basin basalts, from the Arctic Gakkel Ridge. A new high precision dataset for 576 Gakkel samples suggests a pervasive subduction influence in this region. This influence can also be identified in Atlantic and Indian mid-ocean ridge basalts but is nearly absent in Pacific mid-ocean ridge basalts. Such a hemispheric-scale upper mantle heterogeneity reflects subduction modification of the asthenospheric mantle which is incorporated into mantle flow, and whose geographical distribution is controlled dominantly by a “subduction shield” that has surrounded the Pacific Ocean for 180 Myr. Simple modeling suggests that a slab flux equivalent to ~13% of the output at arcs is incorporated into the convecting upper mantle., We discover a pervasive subduction influence in the Arctic, Atlantic and Indian mantle, which is nearly absent in the Pacific mantle. Such a hemispheric-scale upper mantle heterogeneity reflects the control of a “subduction shield” that has surrounded the Pacific Ocean for 180 Myr.

Magma plumbing systems underlying subduction zone volcanoes extend from the mantle through the overlying crust and facilitate protracted fractional crystallisation, assimilation, and mixing, which frequently obscures a clear view of mantle source compositions. In order to see through this crustal noise, we present intracrystal Secondary Ion Mass Spectrometry (SIMS) δ18O values in clinopyroxene from Merapi, Kelut, Batur, and Agung volcanoes in the Sunda arc, Indonesia, under which the thickness of the crust decreases from ca. 30 km at Merapi to ≤20 km at Agung. Here we show that mean clinopyroxene δ18O values decrease concomitantly with crustal thickness and that lavas from Agung possess mantle-like He-Sr-Nd-Pb isotope ratios and clinopyroxene mean equilibrium melt δ18O values of 5.7 ‰ (±0.2 1 SD) indistinguishable from the δ18O range for Mid Ocean Ridge Basalt (MORB). The oxygen isotope composition of the mantle underlying the East Sunda Arc is therefore largely unaffected by subduction-driven metasomatism and may thus represent a sediment-poor arc end-member., Subduction zone volcanoes are underlain by extensive magma plumbing systems, which can obscure original mantle source signals. Here, the authors show that intra-crystal oxygen isotope analysis of clinopyroxenes from the Sunda arc (Indonesia) reveal the δ18 O value of the sub-arc mantle.

The variety of information obtained during scientific research and exploration of seafloor massive sulfide (SMS) deposits include significant data on the morphology of orebodies formed in various geological settings. On the example of hydrothermal vent fields in the Northern Mid-Atlantic Ridge, different hydrothermal ore structures are described, from the most common mound-shaped to rarely encountered “smoking craters.” The processes that govern the morphology of orebodies are analyzed, including alterations during the evolution of hydrothermal ore-forming systems. In contrast to the study of SMS deposits outcropping on the seafloor, the internal structure and formation models are mostly hypothetical. In particular, this concerns assumptions on the presence of massive sulfides and their formation mechanism below the paleo/subseafloor surface. Obviously, the results of drilling planned in the near future will make it possible to confirm or refute existing models and change the resource assessments, which are currently based on surface sampling of SMS deposits.

Accurate knowledge of the composition of Earth’s major chemical reservoirs is fundamental for constraining all modern geochemical cycles. Basaltic rocks provide a direct way of sampling the composition of Earth’s inaccessible interior. Here, we present the first comprehensive neodymium (Nd) stable isotope analyses for a global compilation of mid-ocean ridge, ocean island, continental intraplate and island arc basalts using a double-spike technique. In these primitive magma compositions magmatic differentiation has no resolvable effect on δ146/144Nd. Mid-ocean ridge basalts possess an extremely homogenous δ146/144Nd with an average composition of δ146/144Nd = −0.025 ± 0.013‰ (±2 s.d.; n = 33). Ocean island and continental intraplate magmas possess more variable compositions (δ146/144Nd = 62 ppm) that are related to the variable incorporation of recycled components in their source regions. Island arc basalts from New Britain (δ146/144Nd = 61 ppm) reflect the complex interplay between source composition, degree of melting and slab-fluid inputs. Variations are uncorrelated with indicators of magmatic differentiation or slab-fluid addition, rather increasing δ146/144Nd with slab depth is attributed to a higher proportion of metasomatized sub-arc mantle in the melting region. A partial melting model for Nd stable isotopes has been constructed using Nd–O force constants calculated using the Born-Lande approximation. Melting of typical mantle peridotite will induce no resolvable fractionations of Nd stable isotopes (Δ146/144Ndmelt-mantle

Funding: These field and related experimental studies were supported through US NSF grants: 0549547, 0751771, 0813861, 0961188 and 1736679 (WES). Mid-ocean ridge (MOR) hydrothermal vent fluids are enriched with dissolved barium, but due to barite (BaSO4) precipitation during mixing between Ba-bearing vent fluids and SO4-bearing seawater, the magnitude of hydrothermal Ba input to the ocean remains uncertain. Deep-ocean Ba isotopes show evidence for non-conservative behavior, which might be explained by input of isotopically heavy hydrothermal Ba. In this study we present the first Ba isotope data in mid-ocean ridge hydrothermal vent fluids and particles from systems on the Mid-Atlantic Ridge (Rainbow 36°N and TAG 26°N), the East Pacific Rise (EPR9–10°N and 13°N) and the Juan de Fuca Ridge (MEF and ASHES). The vent fluids display a wide range of dissolved Ba concentrations from 0.43 to 97.9 μmol/kg and δ138/134Ba values from −0.26 to +0.91‰, but are modified relative to initial composition due to precipitation of barite. Calculated endmember vent fluid δ138/134Ba values, prior to barite precipitation, are between −0.17 and +0.09‰, consistent with the values observed in oceanic basalts and pelagic sediments. Water-rock interaction at depth in the oceanic crust appears to occur without Ba isotope fractionation. During subsequent venting and mixing with seawater, barite precipitation preferentially removes isotopically light Ba from vent fluids with a fractionation factor of Δ138/134Bahyd-barite-fluid = −0.35 ± 0.10‰ (2SE, n = 2). Based on knowledge of barite saturation and isotope fractionation during precipitation, the effective hydrothermal Ba component that mixes with seawater after barite precipitation has completed can be calculated: δ138/134Bahyd = +1.7 ± 0.7‰ (2SD). This value is isotopically heavier than deep ocean waters and may explain the observed non-conservative of Ba isotopes in deep waters. These new constraints on hydrothermal Ba compositions enable the hydrothermal input of Ba to Atlantic deep waters to be assessed at ≈3–9% of the observed Ba. Barium isotopes might be used as a tracer to reconstruct the history of hydrothermal Ba inputs and seawater SO4 concentrations in the past. Postprint

The Mako area, located in eastern Senegal, constitutes a segment of Paleoproterozoic (Birimian) formations of the Kedougou-Kenieba Inliers (KKI) in the western part of the West African craton. The basic volcanism of the Birimian formations of the KKI has long been considered to be related to a single magmatic event associated with a Mid Oceanic Ridge Basalts (MORB) setting. The aim of this paper is to demonstrate on the basis of the architectural characteristics of the granitoids dark enclaves, the occurrence of at least two distinct phases of basic Magamtism in the Birimian of the KKI. The methodology consists of a cartography, a petrographic and architectural characterization of the dark enclaves within the Mako granitoids, in order to constrain their spatial and temporal relationships with the granitic magma. The results obtained are compared with those of the bibliography. We have pinpointed two types of enclaves depending on their shapes and the characteristics of their edge with the enclosing rocks: angular enclaves with straight edges (ante-granitoids) and the soft enclaves with uneven edges (syn-granitoids). The enclaves with straight and well-defined rectilinear edges (ante-granitoids) would be linked to a basic magma which is already consolidated before being fragmented, torn and carried away as enclaves in the granitoids. These enclaves would come from enclosing outcrops of metabasalts and metagabbros which are locally cross-cut by the granitoids. The soft enclaves with uneven edges (syn-granitoids) were co-magmatic and not solid during their incorporation into the granitoids. They come from a basic magma which is contemporaneous to the granitic one. Thus, the occurrence of two generations of dark enclaves is related to, at last, two different phases of basic magmatism in the Birimian formations of the KKI.

Modern oceans contain large bathymetric highs (spreading oceanic ridges, aseismic ridges or oceanic plateaus and inactive arc ridges) that, in total, constitute more than 20–30% of the total area of the world’s ocean floor. These bathymetric highs may be subducted, and such processes are commonly referred to as ridge subduction. Such ridge subduction events are not only very common and important geodynamic processes in modern oceanic plate tectonics, they also play an important role in the generation of arc magmatism, material recycling, the growth and evolution of continental crust, the deformation and modification of the overlying plates, and metallogenesis at convergent plate boundaries. Therefore, these events have attracted widespread attention. The perpendicular or high-angle subduction of mid-ocean spreading ridges is commonly characterized by the occurrence of a slab window, and the formation of a distinctive adakite–high-Mg andesite–Nb-enriched basalt-oceanic island basalt (OIB) or a mid-oceanic ridge basalt (MORB)-type rock suite, and is closely associated with Au mineralization. Aseismic ridges or oceanic plateaus are traditionally considered to be difficult to subduct, to typically collide with arcs or continents or to induce flat subduction (low angle of less than 10°) due to the thickness of their underlying normal oceanic crust (> 6–7 km) and high topography. However, the subduction of aseismic ridges and oceanic plateaus occurred on both the western and eastern sides of the Pacific Ocean during the Cenozoic. On the eastern side of the Pacific Ocean, aseismic ridges or oceanic plateaus are being subducted flatly or at low angles beneath South and Central American continents, which may cause a magmatic gap. But slab melting can occur and adakites, or an adakite–high-Mg andesite–adakitic andesite–Nb-enriched basalt suite may be formed during the slab rollback or tearing. Cu-Au mineralization is commonly associated with such flat subduction events. On the western side of the Pacific Ocean, however, aseismic ridges and oceanic plateaus are subducted at relatively high angles (>30°). These subduction processes can generate large scale eruptions of basalts, basaltic andesites and andesites, which may be derived from fractional crystallization of magmas originating from the subduction zone fluid-metasomatized mantle wedge. In addition, some inactive arc ridges are subducted beneath Southwest Japan, and these subduction processes are commonly associated with the production of basalts, high-Mg andesites and adakites and Au mineralization. Besides magmatism and Cu-Au mineralization, ridge subduction may also trigger subduction erosion in subduction zones. Future frontiers of research will include characterizing the spatial and temporal patterns of ridge subduction events, clarifying the associated geodynamic mechanisms, quantifying subduction zone material recycling, establishing the associated deep crustal and mantle events that generate or influence magmatism and Cu-Au mineralization, establishing criteria to recognize pre-Cenozoic ridge subduction, the onset of modern-style plate tectonics and the growth mechanisms for Archean continental crust.

Coexisting alkalic and tholeiitic basalt lavas has been identified in a seamount chain located near the Australian–Antarctic spreading ridge. The KR1 Seamount Trail (KR1 ST) is a series of volcanic...

Mantle evolution is governed by continuous depletion by partial melting and replenishment by recycling oceanic and continental crust. Several important unknowns remain, however, such as the extent of compositional variability of the residual depleted mantle, the timescale, mass flux and composition of recycled oceanic and continental crust. Here, we investigate the Ce-Nd-Hf isotope systematics in a globally representative spectrum of mid ocean ridge and ocean island basalts. Using a Monte Carlo approach for reproducing the observed Ce-Nd-Hf isotope variation shows that the type and age of depleted mantle and recycled crust have the dominant influence on the slope, scatter, and extent of the modeled Ce-Nd-Hf isotope array. The model results suggest a relatively young (

Bangong-Nujiang Suture Zone (BNSZ) in central Tibet plays an important role in evaluating the formation and uplift mechanism of Tibetan Plateau. However, its Mesozoic tectonic evolution is ambiguous and intensely debated. In this study, Early Cretacesous adakites and sodium-rich arc rocks are identified in Western Qiangtang (WQ) and Northern Lhasa (NL) sub-terranes. Forty-four adakite samples from both WQ and NL have akin geochemical features, and are derived from partial melting of subducted oceanic crust with amphibole residual. Nineteen sodium-rich samples originated from a mixed source region between crustal sediment and enriched lithospheric mantle. These two parallel arc belts separated by the Bangong-Nujiang Suture Zone (BNSZ) represent the divergent double subduction of the Bangong-Nujiang Tethyan Ocean (BNTO). Combined with the previous studies, our new data suggest three significant magmatic flare-ups at ∼240–140 Ma, 135–105 Ma and 92–60 Ma in the WQ and BNSZ, and two at 135–105 Ma and 92–60 Ma in the NL. These asymmetrical magmatic activities indicate that the southern subduction may have commenced at about 135 Ma and experienced slab breakoff at the latest Early Cretaceous, and the northern subduction could trace back to L-Triassic (228 Ma) and experienced episodic low-angle subduction, slab rollback (190-140 Ma) and oceanic ridge subduction (135-100 Ma). The 100–92 Ma magmatic gap, 92–60 Ma magmatic flare-up and L-Cretaceous angular unconformities indicate that the double-sided subduction of the BNTO resulted in soft collision with oceanic lithosphere detachment.

Mid-ocean ridge basalts (MORB) reveal large mantle compositional heterogeneity, whose origin remains debated. Here we present a systematic study of molybdenum isotopes on well-characterized MORB glass samples from the East Pacific Rise (EPR) and near-EPR seamounts. Our analyses show significant Mo isotope variations with δ 98 / 95 Mo (relative to NIST SRM3134) ranging from −0.23‰ to −0.06‰. We argue that these Mo isotope variations are not caused by processes of MORB melt generation and evolution but reflect mantle isotopic heterogeneity. Taking together with the literature data, we show that MORB Mo isotope compositions vary systematically with geochemical parameters indicating mantle enrichment. These observations are best explained by two-component mixing between an incompatible element depleted endmember (e.g., low La/Sm, Nb/La, Nb/Zr and Th/Yb, and high Sm/Nd and 143Nd/144Nd) with low δ 98 / 95 Mo (∼−0.21‰) and an incompatible element enriched endmember (e.g., high La/Sm, Nb/La, Nb/Zr and Th/Yb, and low Sm/Nd and 143Nd/144Nd) with high δ 98 / 95 Mo (∼−0.05‰). The association of heavier Mo isotope compositions with the geochemically more enriched MORB is inconsistent with recycled ocean crust with or without sediment being the enriched endmember. Instead, this is consistent with the enriched endmember being of magmatic origin, most likely lithologies of low-degree melt metasomatic origin dispersed in the more depleted peridotite matrix in the MORB mantle. Thus, with MORB Mo isotope systematics, we confirm that recycled oceanic mantle lithosphere metasomatized by low degree melt plays a key role in the formation of E-MORB source lithologies. Our study also highlights Mo isotopes as an effective tool for studying upper mantle processes.

The 4.5km deep IDDP-2 drill hole was drilled at Reykjanes, Iceland, in an active seawater-recharged hydrothermal system on the landward extension the Mid-Atlantic Ridge. Drilling targeted a well-defined hydrothermal up-flow zone feeding the Reykjanes geothermal reservoir, which produces fluids compositionally equivalent to basalt-hosted deep sea hydrothermal vents. Spot cores recovered from depths between 3,648 to 4,659m depths consist of pervasively altered sheeted diabase dikes. The shallowest core has an alteration mineral assemblage similar to the producing geothermal reservoir but is being overprinted by the underlying Hornblende Zone assemblage dominated by labradorite and hornblende. The deepest cored section (4,634–4,659m) retains a diabasic texture, but all primary minerals are replaced or have changed composition. Plagioclase ranges from An30 to An99 and igneous augite is replaced by intergrown hornblende, clinopyroxene, and orthopyroxene. Hydrothermal biotite and potassium feldspar formed locally due to reaction with a phase-separated brine, indicated by co-existing vapor-rich and salt+vapor-rich fluid inclusions. Seawater-derived supercritical hydrothermal fluid entering the bottom of the bore hole actively phase separates due pressure drop controlled by the fluid levels in the drill hole. Felsic veins, present in trace amounts, record a continuous transition from magmatic to hydrothermal conditions, including incipient hydrous melting. The vertical changes observed in mineralogy and mineral chemistry indicate that fluids from the deep high temperature reaction zone can undergo significant cooling and reaction with host rocks along their path to the seafloor.

Volatile elements are concentrated at Earth's surface, forming a rich atmosphere and oceans which enabled the eventual emergence of life. However, volatiles are also abundant in solid Earth reservoirs, such as the crust and mantle, and these reservoirs play a key role in moderating volatile movement throughout the planet. Continental cratons represent a potentially large, yet under-constrained volatile reservoir. When cratonic regions are catastrophically disrupted by large volcanic and/or rifting events, they release massive amounts of volatiles into Earth's atmosphere on geologically-abrupt timescales (e.g., Lowenstern et al., 2014 ; Muirhead et al., 2020 ). Here, we report gas data (He-Ne-N2-Ar-CO2) from seeps along the flanks of the Tanzanian craton, within the western branch of the East African Rift System (EARS) - a region where the stable continental craton is actively being broken apart by rifting and simultaneously heated by plume-induced volcanism. Bulk gas and noble gas isotopic data are reported in seeps from three regions: 1) the Rukwa Rift Basin (RRB), 2) the Lupa Hydrothermal System (LHS) and 3) the Rungwe Volcanic Province (RVP). Seep gases from the RRB are dominantly comprised of N2 and He, with >90% N2 concentrations, high 4He concentrations (2.4–6.9%) and radiogenic He isotopes (0.16–0.20 RA). Seeps in the LHS - located between RRB and RVP - are characterized by little-to-no N2, high CO2 contents (72–84%), relatively low He contents (0.008–0.15%), and higher 3He/4He (0.95–0.99 RA). RVP gases have high CO2 (78%) and low 4He (0.0003%) and more mantle-like He isotopes (3.27–4.00 RA) consistent with previous findings ( Pik et al., 2006 ; Barry et al., 2013 ). All neon isotopes can be explained by mixing between air, high O/F crust and depleted Mid Oceanic Ridge Basalt (MORB) mantle-like signatures. RVP neon isotope seep data potentially suggest a solar-like deep mantle contribution, consistent with findings in rocks from the area ( Halldorsson et al., 2014 ), however we note that this signal is difficult to discern from mass dependent fractionation (MDF). The largest 40Ar/36Ar anomalies occur in RRB, with resolvable excess 40Ar derived from radiogenic production in the crust. Using a noble gas solubility model, we calculate volumetric gas to water ratios (Vg/Vw) and show that Vg/Vw values are low for RRB (0.1), consistent with longer migration distances, whereas Vg/Vw are higher for LHS (Vg/Vw = 0.1–10) and RVP (Vg/Vw = 3–12), suggesting a more direct conduit for volatiles from source to surface. In summary, these data demonstrate interaction between two distinct helium sources, one of which is crustal in origin (most prominent in RRB) and the other being mantle-derived (enriched in RVP). The extent of mixing between the two is shown to be influenced by proximity to rift-related fault structures, groundwater interaction and magmatic heat.

The Oman Ophiolite is regarded as an analogue to modern fast-spreading ocean ridge systems in an environment of subduction zone initiation. In contrast to recent mid-ocean ridge basalts from the East Pacific Rise, parental melts at the Oman paleoridge are assumed to be hydrous in nature. In order to constrain the role of water during magmatic accretion processes in the deep crust at the Oman paleoridge, we evaluated several experimental studies in hydrous tholeiitic systems performed at shallow pressures. We concluded that the wehrlitic phase assemblage (olivine coexisting with clinopyroxene but without plagioclase) is the most significant feature indicative of high prevailing water activities. The stability of the wehrlitic assemblage decreases with decreasing pressure (not stable in the upper plutonic crust) and depends on the chemical system (only stable in primitive MORB systems). We applied these results to plutonic rocks from cores drilled as part of the Oman Drilling Project (OmanDP). A key observation is the presence of coherent wehrlitic layers within the layered gabbro series, which are frequent in the lowermost gabbros (20%), relative sparse in the mid-crust (6%), and absent from the top of the plutonic crust at the dike/gabbro transition. Based on the combined phase relations for hydrous MORB-type systems at shallow pressures, we interpret this as a direct consequence of the presence of a significant water activity during the accretion of the plutonic crust of the Oman paleoridge, and not as a local phenomenon related to variations in temperature or bulk chemistry. These findings have implications for the mechanism of accretion of the lower crust at the Oman paleoridge, supporting a model that significant parts of the plutonic crust were produced by in-situ crystallization of primitive melt sills.

Whether the Earth’s mantle has a chondritic δ56Fe (deviation in 56Fe/54Fe from the IRMM-014 standard in parts per thousand) value or not remains under debate. The current view is that the observed average δ56Fe of mid-ocean ridge basalts (MORB) cannot be explained by partial melting of mantle source with chondritic value alone. Here, we report Fe isotope compositions on 29 MORB glasses sampled along a flowline traverse across the East Pacific Rise (EPR) axis at 10°30′N. These glasses show large MgO variation (1.8–7.4 wt.%) that forms a compositional continuum resulting from varying extent of fractional crystallization, which is accompanied by systematic Fe isotopic variation. Fractional crystallization modeling suggests that early crystallization of olivine, pyroxene and plagioclase gives rise to an iron enrichment trend and an increase in δ56Fe. Once Fe-Ti oxides appear on the liquidus and begin to crystallize, the FeOt and TiO2 contents of the residual melt decrease rapidly, which lead to a slight decrease in δ56Fe. These observations indicate that significant Fe isotope fractionation can indeed take place during MORB melt evolution. Hence, δ56Fe values of variably evolved MORB melts do not represent those of primary MORB melts and thus cannot be used to infer mantle source Fe isotope compositions. Importantly, δ56Fe values of primary MORB melts after correction for the effect of fractional crystallization can be well reproduced by mantle melting. Therefore, our study supports the idea that the Fe isotope composition of the accessible Earth is close to be chondritic. We note that conclusion would assume that the core, which takes up ∼90% of the Earth’s Fe, must have a chondritic Fe isotope composition.

Major processes affecting high field strength (HFSE) and rare earth (REE) element ratios in planetary basalts are degree of melting, separation of metal-sulfide melt fractions, addition and loss of silicate melt, ilmenite fractionation, and subduction. Fractional crystallization of planetary magma oceans has left a surviving imprint on only three bodies for which we have data: the Moon, Vesta, and the angrite parent body. Thorium mobilization in aqueous fluids may account for decoupling of Th and Nb in planetary systems, and this is especially notable on Earth but also possible on Mars, the Moon and some asteroids. On Earth, HFSE and REE ratios in young basalts characterize hydrated (HM), enriched (EM) and depleted (DM) mantle sources, associated with, respectively, subduction, mantle plumes and ocean ridges. Terrestrial hydrated and depleted mantle were in existence by at least 4 Ga and possibly they may have been produced in a stagnant lid tectonic regime before 3 Ga. Also, removal of Nb in metal-sulfide melts can force the composition of silicate restitic material into the hydrated mantle field on HFSE-REE graphs, thus not requiring hydration. Such an origin is probable for “hydrated” mantle in primitive achondrites and plutonic angrites. The record of all three types of mantle in basalts from other bodies in the Solar System indicates the three mantle reservoirs are not diagnostic of plate tectonics, but can be produced in stagnant lid settings. Enriched mantle is thus far recognized only in Earth and possibly Mars. There are at least two enriched mantle reservoirs in Earth: a primordial (>4 Ga) reservoir, perhaps hidden in the D” layer above the core and rarely sampled by basalts, and a recycled plate reservoir ( Although there are several processes by which Nb can be fractionated from Ta in planetary bodies, the low Nb/Ta (

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.

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.

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...

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.

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.

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.

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.

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.

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.

Highlights: • Lake Asal (Afar Rift) is fed by seafloor-type hydrothermal fluids. • An oceanic “embryo” in arid climate is mildly acidic and metal rich. • It has heavy C, O and Ca, and light Zn isotope composition. • Lake chemistry is controlled by hydrothermal discharge and aeolian input. Abstract: At the place where the submarine Aden Ridge encroaches on the African continent and interacts with the East African Rift system, two small basins form: Ghoubbet-al-Kharab and Lake Asal. Whereas Ghoubbet-al-Kharab is connected to the open ocean, Lake Asal is a typical example of oceanic “embryo”, which is defined as a system that is detached from the ocean, but has features of a marine basin with an oceanic type crust and a seawater-based water body. In order to shed light on the source of water, type of hydrothermal activity and hydrothermal deposits, and controls on the water chemistry in an oceanic “embryo”, we undertook a mineralogical-geochemical study of the lake water, hydrothermal fluids and hydrothermal carbonate deposits of Lake Asal. The geochemical analyses of lake water and hydrothermal fluids show that Lake Asal (located in an arid zone with strong evaporation and with no riverine input) is fed by seafloor-type hydrothermal fluids according to the following scenario: percolation of seawater along faults and cracks of extension in the rift, reaction of seawater with the hot basaltic rocks and hydrothermal fluid generation, discharge of the hydrothermal fluid in the Asal depression and accumulation of the Lake Asal water body. The fluid venting at the Lake Asal bottom is a mixture of 97% end-member hydrothermal fluid and 3% lake water. The calculated end-member hydrothermal fluid of this oceanic “embryo” is poorer in metals than the seafloor hydrothermal fluids of an open and evolved ocean. In addition to the seawater/rock interaction, the chemistry of Lake Asal is also controlled by evaporation leading to hyper salinity. In a hyper saline water body a number of hydrothermally supplied metals are stabilized as chloride complexes and accumulate. This results in a metal rich and mildly acidic “embryonic” ocean. Unlike an open and evolved modern ocean, the “embryonic” ocean located in an arid zone has heavy C and O isotope composition and light Zn and Fe isotope composition. Calcium isotope compositions of both types of ocean are similarly heavy. There are two genetically different sources of elements to the Lake Asal that are vertically separated: hydrothermal (lower, or bottom) and aeolian (upper, or surficial). Another important control on the lake water chemistry is the formation of carbonate spires at the lake bottom. Ca‑carbonate precipitation immobilizes substantial amount of hydrothermally supplied Ca and drives up the (Mg/Ca)mol of the lake water. Increasing (Mg/Ca)mol of the evolving lake water leads to changes in the mineralogy of spires: from low-Mg calcite to aragonite. Thus, the spire formation exerts a self-control on its mineralogy. Carbonate spire deposition affects also the Ca, Zn and Fe isotope composition of the lake water through adsorption or/and co-precipitation induced isotope fractionation.

The iron isotope contrast between mid-ocean ridge basalts and abyssal peridotites is far greater than can be explained by mantle melting alone. Here we investigate a suite of mid-ocean ridge magma chamber rocks sampled by the Ocean Drilling Project Hole 735B in the Atlantis Bank of the Indian Ocean. We report major and trace element geochemistry from these rocks and measure their iron isotope compositions to investigate the potential role of fractional crystallization during melt evolution. We observe a large range of δ56Fe that defines a significant inverse curvilinear correlation with bulk rock MgO/FeOT. These data confirm that δ56Fe in the melt increases as fractional crystallization proceeds but, contrary to expectation, δ56Fe continues to increase even when oxides begin to crystallize. We conclude that iron isotope fractionation through fractional crystallization during the evolution of mid-ocean ridge basalts from abyssal peridotites reconciles the disparity in isotopic compositions between these two lithologies. Fractional crystallization during the evolution of mid-ocean ridge basalts can explain why they are so different in iron isotopic composition from abyssal peridotites, according to chemical analyses of drill core samples from the Atlantis Bank.

Two-thirds of the Earth is covered by mid-ocean ridge basalts, which form along a network of divergent plate margins. Basalts along these margins display a chemical diversity, which is consequent to a complex interplay of partial mantle melting in the upper mantle and magmatic differentiation processes in lower crustal levels. Igneous differentiation (crystal fractionation, partial melting) and source heterogeneity, in general, are key drivers creating variable chemistry in mid-ocean ridge basalts. This variability is reflected in iron isotope systematics (expressed as δ57Fe), showing a total range of 0.2 ‰ from δ57Fe = + 0.05 to + 0.25 ‰. Respective contributions of source heterogeneity and magma differentiation leading to this diversity, however, remain elusive. This study investigates the iron isotope systematics in basalts from the ultraslow spreading Gakkel Ridge in the Arctic Ocean and compares them to existing data from the fast spreading East Pacific Rise ridge. Results indicate that Gakkel lavas are driven to heavier iron isotope compositions through partial melting processes, whereas effects of igneous differentiation are minor. This is in stark contrast to fast spreading ridges showing reversed effects of near negligible partial melting effects followed by large isotope fractionation along the liquid line of descent. Gakkel lavas further reveal mantle heterogeneity that is superimposed on the igneous differentiation effects, showing that upper mantle Fe isotope heterogeneity can be transmitted into erupting basalts in the absence of homogenisation processes in sub-oceanic magma chambers.

The crust and mantle both in ophiolites (fossil ocean lithosphere) and in modern oceans are enormously diverse. Along-axis morphology and lower crustal accretion at ultraslow-spreading ocean ridges are fundamentally different from those at faster-spreading ridges, and are critical to understanding how crustal accretion varies with spreading rate and magma supply. Ultraslow-spreading ridges provide analogues for ophiolites, to identify those that may have formed under similar conditions. Parallel studies of modern ocean lithosphere and ophiolites therefore can uniquely inform the origin and genesis of both. Here we report the results of structural and petrological studies on the Xigaze ophiolite in the Tibetan Plateau, and compare it with the morphology and deep drilling results at the ultraslow-spreading Southwest Indian Ridge. The Xigaze ophiolite has a complete but laterally discontinuous crust, with discrete diabase dykes and sills cutting both mantle and lower crust. The gabbro units are thin (c. 350 m) and show upward cyclic chemical variations, supporting for an episodic and intermittent magma supply. These features are comparable with the highly focused magmatism and low magma budget at modern ultraslow-spreading ridges. Thus we suggest that the Xigaze ophiolite represents an on-land analogue of ultraslow-spreading ocean lithosphere. Supplementary material: Single-spot analysis and average compositions for mineral major element compositions (wt.%) of the Jiding gabbro section, Xigaze ophiolite are available at https://doi.org/10.17632/dtzpmwkscp.1

The plate tectonic cycle produces chemically distinct mid-ocean ridge basalts (MORB) and arc volcanics, with the latter enriched in fluid-mobile elements and depleted in Nb owing to fluxes from the subducted slab. Basalts from back-arc basins (BABB), with intermediate compositions, show that the subduction flux can escape the arc. Hence it is puzzling why arc signatures have rarely been recognized in MORB. Here we report the first MORB samples with distinct arc signatures, akin to BABB, from the Arctic Gakkel Ridge. A new high precision dataset for 576 Gakkel samples suggests a pervasive subduction influence. This influence can also be identified in Atlantic and Indian MORB with a “BABB filter”, but is nearly absent in Pacific MORB. This global distribution reflects the control of a “subduction shield” that has surrounded the Pacific Ocean for 180Myr. Statistics suggest that a flux equivalent to ~ 13% of output at arcs is incorporated into the convecting upper mantle.