The Earth's upper mantle is heterogeneous in lithology and geochemistry, as demonstrated by variations in both abyssal peridotites and fossil oceanic mantle peridotites. The scarcity of spatial relationships between these peridotites, however, hinders further interpretation of the origin of mantle diversity as well as corresponding geodynamic processes. Here, we report the petrographic and chemical data of peridotites from the first fresh drill core (∼1300 m) across a Tibetan ophiolitic mantle sequence. This mantle column shows a primarily heterogeneous lithological structure consisting of repetitive 'layered' lherzolite, harzburgite, and dunite. Lherzolite and harzburgite have experienced 10%-15% and 15%-25% melt depletion, respectively. Such depletion cannot be generated by conventional partial melting models alone, but also requires melt-peridotite interaction in the asthenospheric mantle. Our work provides a high-resolution snapshot of the lithological structure and chemistry of the uppermost oceanic mantle and offers a melt flow model within the asthenosphere to explain the lithological variability of mantle rocks found in both mid-ocean ridges and supra-subduction zones.

Carbon in the asthenosphere is unevenly distributed, reflecting interactions between the depleted mantle and materials added by plumes or subducting slabs. Nevertheless, mid-ocean ridge basalts (MORB) from the Arctic Gakkel Ridge, far from any existent hotspots and subduction zones, are unusually carbon-rich. Here we show that the MORB along the Gakkel Ridge display systematically elevated Zn isotopes (δ66Zn = 0.27‰-0.41‰) relative to typical global MORB (0.26 ± 0.03‰). Their heavy δ66Zn ratios correlate with indicators for mantle source carbon enrichment. We attribute this signature to result from recycled surficial carbonates stored in deep upper mantle under the Arctic Ocean. These carbonate components were likely delivered by ancient oceanic subduction and subsequently entrained into the mantle upwelling feeding the Gakkel Ridge. Our results demonstrate that vestiges of past subduction can dominate carbon budgets beneath plume‑unaffected ridges, and act as a principal driver of large‑scale carbon heterogeneity in the upper mantle.

Mid-ocean ridge basalts (MORBs) have long been used to investigate the composition of the upper mantle. The isotopic heterogeneity of MORB correlates inversely with spreading rate, indicating that enhanced magma mixing at magmatically robust fast-spreading ridges mutes the signature of mantle heterogeneity. It has remained unclear, however, whether this mixing occurs during melt extraction from the mantle or in crustal magma reservoirs. To discriminate between mantle aggregation and crustal magma mixing, we measured the Nd isotopic composition of cumulus plagioclase and clinopyroxene cores within lower crustal gabbros from the fast-spreading crustal section exposed at Hess Deep (equatorial Pacific Ocean). Our data reveal that the mantle is heterogeneous at the scale of melt extraction, and the crystal record from the lower crust shows greater 143Nd/144Nd heterogeneity than the overlying MORB. Hence, Pacific MORBs do not reflect the full heterogeneity of their mantle source, and some aggregation of melts occurs within the crust. However, isotopic heterogeneity in the lower crust at Hess Deep is lower than in slower-spreading settings, suggesting that the extent to which melts aggregate in the mantle versus the crust is controlled by spreading rate.

Oceanic crusts at slow-spreading ridges are created either by symmetric spreading dominated by magmatic accretion or asymmetric spreading controlled by tectonic extension. Consecutive change in the spreading mode at the same ridge has been commonly attributed to variation in magma supply, but the mechanism controlling magma supply remains unclear. Here, we present geochemical analyses of peridotites and basalts from the Mid-Atlantic Ridge at 23°N, a region that has shifted from asymmetric spreading to symmetric spreading over the past 3.3 million years. Our results indicate that the asymmetric phase was characterized by a low magma flux, resulting from inherited ancient melt depletion in the asthenosphere. The subsequent increase in magma supply and shift to symmetric spreading corresponded with the arrival of more fertile mantle material. This work provides evidence of shifts between spreading modes driven by changing mantle compositions, highlighting the crucial role of asthenospheric heterogeneity in controlling the spreading modes at slow-spreading ridges.

Oceanic detachments are deep-rooted, long-lived, plate-scale structures and serve as fluid conduits introducing water into the oceanic lithosphere, impacting plate rheology and potentially inducing oceanic crustal melting. However, the extent and mechanisms of fluid ingress and crustal melting during detachment faulting remains poorly constrained. Here we examine felsic veins from the Atlantis Bank oceanic core complex, Southwest Indian Ridge, to elucidate oceanic detachment controls on crustal melting. We suggest that the felsic veins are products of strong fractionation of either primitive basalts or magmas generated by hydrous melting of gabbros (i.e., anatectic melts). The anatectic felsic veins are proximal to the fault plane, suggesting that detachment fault facilitated high-temperature (750-900 degrees C) seawater infiltration into deep oceanic crusts. Our findings highlight the essential role of detachment faulting played in the fluid ingress and melting of oceanic crust, bearing implications for chemical and heat exchange between seawater and oceanic lithosphere.

Oceanic core complexes (OCCs) are a fundamental component of slow-to-ultraslow spreading mid-ocean ridges, yet the processes that control OCC formation and evolution are poorly understood especially with respect to their high-temperature lithospheric roots. We present detailed analyses of high-temperature ductile deformation preserved in drill-core from IODP Hole U1601C, on the Atlantis Massif OCC (30°N, MAR). We show that gabbroic intrusions within peridotite accommodated significant high-temperature deformation, especially within Fe-Ti oxide-bearing assemblages. This deformation spatially localizes in zones of high lithological heterogeneity created by meter-to-submeter-scale gabbroic intrusions within peridotite. High-temperature ductile deformation often localizes close to, and/or along, intrusive contacts, accompanied by localized, evolved, melt-reactive porous flow (crystallizing Fe-Ti oxides), and followed by fluid-rock reaction that enhanced and sustained further ductile deformation. These spatially controlling relationships between magmatism, deformation, and late melt ± fluid infiltration are a direct consequence of the lithological heterogeneity within moderately-magmatic OCCs, which are the dominant style of OCC along the Mid-Atlantic Ridge and other slow-spreading ridges.

The concentration of dissolved oxygen in the deep oceans has varied over Earth History, but the timing of the transition from anoxic to oxic deep oceans is debated. Under modern-day, oxic, deep ocean conditions, alteration of the upper sections of mafic oceanic crust with U-rich seawater leads to U enrichment, low Th/U ratios, and heterogeneous U-238/U-235 ratios relative to fresh mid-ocean ridge basalt (MORB). Given the solubility behaviour of U, its uptake into altered oceanic crust (AOC) is expected to be smaller and less isotopically fractionated when deep oceans were anoxic and thus U-poor. Determining when, in the geological record, the U elemental and isotopic systematics of ancient oceanic crust first resemble modern day AOC should indicate when deep oceans became oxic. We provide U concentration, Th/U, and U isotopic data on upper-crustal sections of three ophiolites from 750 to 480 Ma, spanning the period inferred for deep ocean oxygenation (similar to 850 to 400 Ma). The ophiolites at 480 and 540 Ma have high U contents, low Th/U ratios, and variability in U-238/U-235 ratios like modern-day AOC, reflecting seawater alteration of oceanic crust under oxygenated seawater conditions. In contrast, the 750 Ma ophiolite does not show the distinctive decreasing Th/U with increasing U concentrations trend of modern AOC and has fewer samples with U-238/U-235 ratios perturbed from mantle values, reflecting alteration under largely anoxic deep ocean conditions. This is also supported by Fe3+/Fe-T ratios in these samples that are like unaltered modern MORB. Thus, our data suggest oxygenated deep oceans at some time between 750 and 540 Ma, either reflecting a full transition or intermittent deep ocean oxygenation events within an otherwise anoxic deep ocean.

Water that flows through permeable ultramafic rocks produces high abundances of molecular hydrogen (H-2), methane (CH4), and other small organic molecules. Such products can fuel life in the rocky subseafloor, be extracted for energy, and may have played a role in pre-biological chemical synthesis on early Earth or other planetary bodies. The International Ocean Discovery Program drilled a new 1268-m-deep borehole (U1601C) into serpentinized mantle with minor gabbroic rocks on the Atlantis Massif, similar to 800-m north of the Lost City hydrothermal field (30 degrees N, Mid-Atlantic Ridge). Measured temperatures of the disturbed borehole reached 91.3 degrees C, and equilibrated temperatures of the deepest section are estimated to be between 110 - 140 degrees C. Water collected every similar to 5-m during drilling operations had H-2 concentrations that were regularly > 200 nM and spiked to > 10 mu M at multiple depths. In these waters, carbon monoxide was only present in deeper, hotter sections, and potentially associated with gabbroic intrusions into the peridotite host. Open borehole fluids were sampled after drilling and samples recovered from the deepest portion contained elevated short-lived 222-Radon and strontium isotope ratios similar to Lost City fluids, pointing to the presence of in situ subseafloor formation waters that have equilibrated with the host rock. The deepest samples were actively degassing upon recovery and contained 740 +/- 360 mu M H-2, 340 +/- 36 mu M CH4, and 25.5 mu M & sum;formate (= formate and formic acid). The shallowest fluids from the open borehole also contain micromolar H-2 and & sum;formate concentrations, the presence of which cannot be attributed to the upward migration of the deeper, higher concentration fluids. We interpret these data as reflecting two distinct and interconnected regimes of fluid flow and composition. Deep waters that are channelized along faults, lithologic contacts, and other high permeability pathways host high H-2 and CH4 concentrations plus micromolar & sum;formate that closely mirror the chemistry and isotopic signatures of LCHF vent fluids. Pervasive fluid flow permeates the mesh texture and microfracture network of the serpentinized peridotite and sustains H-2 and & sum;formate even in the shallowest subseafloor intervals at mild temperatures. These findings demonstrate that both focused and pervasive fluid flow contribute to the transport, and potentially the generation, of reduced volatiles and C1 compounds within the Atlantis Massif.

Much of Earth’s magma is stored as extensive crystal mush systems, yet the prevalence of physical processes operating within mushes and their importance in volcanically active regions remain enigmatic. In this Review, we explore the physical properties and key processes of crystal mush systems. The initiation, evolution and decline of volcanic systems, modulated by heat supply and loss, could generate differences in the prevalence of mush processes through space and time. Additionally, regional tectonics alter mush properties, with mushes in cool wet settings having persistent residual melt, permitting more effective melt segregation than in hot dry settings. Disaggregation of mushes results in crystal mush material being mobilized or entrained into lavas and erupted, presenting opportunities to define the timescales and chemistry of some mush processes in volcanically active regions. Mush systems can be observed on length scales ranging from kilometres (using geological mapping) to micrometres (using crystal textures). Therefore, it is difficult to integrate data and interpretations across different fields. Improved integration of thermodynamics, textural analysis, geochemistry, modelling and experiments, alongside inputs from adjacent fields such as porous media dynamics, engineering and metallurgy will help to advance understanding of mush systems and ultimately improve hazard evaluation at active and dormant volcanic systems. Crystal mush processes are key to magmatic differentiation, volcanic system behaviour and some mineral resources. This Review discusses the processes involved in mush formation, rejuvenation and eruption, and how tectonics and crustal thermal maturity impacts these processes.

Volcanic evolution in ocean island settings is often controlled by variations in the chemistry and volumetric flux of magma from an underlying mantle plume. In locations such as Hawaiʻi or Réunion, this results in predictable variations in magma chemistry, the rate of volcanic activity, and the depth of magma storage with volcanic age and/or distance from the center of plume upwelling. These systems, however, represent outliers in global plume volcanism due to their high buoyancy flux, frequent eruptions, and large distance from any plate boundary. Most mantle plumes display clear interaction with nearby plate boundaries, influencing the dynamics of solid plume material in the upper mantle and the distribution of melt across regions of active volcanism. Yet, the influence of plume-ridge interaction and plume-ridge distance on the structure, characteristics, and evolution of magma storage beneath ocean island volcanoes remains under constrained. In this study, we consider the evolution of magmatic systems in the Galápagos Archipelago, a region of mantle plume volcanism located 150-250 km south of the Galápagos Spreading Centre (GSC), focusing on the depth of magma storage during the eastward transport of volcanic systems away from the centre of plume upwelling. Geochemical analysis of gabbro xenoliths from Isla Floreana in the south-eastern Galápagos suggest that they formed at ~2-2.5 Ma, when the island was located close to the centre of plume upwelling. These nodules, therefore, provide rare insights into the evolution of volcanic systems in the Galápagos Archipelago, tracking variations in the magma system architecture as the Nazca plate carried Isla Floreana eastwards, away from the plume centre. Mineral thermobarometry, thermodynamic modelling, and CO2 fluid inclusion barometry reveal that Isla Floreana’s plume-proximal stage of volcanic activity – recorded in the gabbro xenoliths – was characterized by the presence of high-pressure magma storage (> 25 km), below the base of the crust. In fact, we find no petrological evidence that sustained, crustal-level magma storage ever occurred beneath Isla Floreana. Our results contrast with the characteristics of volcanic systems in the western Galápagos above the current centre of plume upwelling, where mid-crust magma storage has been identified. We propose that this change in magmatic architecture of plume-proximal volcanic centres in the Galápagos – from high-pressure mantle storage at 2.5 Ma to mid-crustal storage at the present day – is controlled by the variations in plume-ridge distance. Owing to the northward migration of the GSC, the distance separating the plume stem and GSC is not constant, and was likely <100 km at 2.5 Ma, significantly less than the current plume-ridge distance of 150 – 250 km. We propose that smaller plume-ridge distances result in greater diversion of plume-material to the GSC, ‘starving’ the eastern Galápagos islands of magma during their initial formation and restricting the ability for these systems to develop long-lived crustal magma reservoirs.

Sanukitoids are unique Archean and early Proterozoic igneous rocks. They contain high amounts of Mg, Ni and Cr, showing they are mantle-derived melts, while they are also enriched in Sr and Ba and have relatively high K contents, requiring the involvement of an incompatible element-enriched component likely derived from recycled crustal material. The appearance of sanukitoids in the geological record coincides with a shift in continental crust composition, and both events have been linked to a change in geodynamic processes on Earth. However, uncertainties remain about sanukitoid petrogenesis, in particular whether their mantle source was metasomatised by a metabasite-derived silicate melt or by an aqueous fluid. Titanium (Ti) stable isotopes can trace magmatic processes where silicate melts are in equilibrium with Fe-Ti oxides and amphibole but are insensitive to fluid-driven processes, making them a suitable tool to investigate not only the formation of sanukitoid magmas but also their subsequent evolution. Here we present Ti isotope data (delta Ti-49) for a series of Neoarchean sanukitoids from the Yilgarn Craton that continuously covers the full compositional range of sanukitoids. These are complemented by Mesoarchean sanukitoids and Paleoarchean "sanukitoid-like" rocks from the Pilbara Craton, and by Paleoproterozoic sanukitoids from the S & atilde;o Francisco Craton/Paleocontinent. In addition, we analysed Paleozoic high Ba-Sr granite suites from Scotland, which are proposed to be Phanerozoic sanukitoid analogues. Evolved sanukitoids, which formed after Fe-Ti oxide saturation, show a more muted delta Ti-49 increase during differentiation compared to currently analysed modern calc-alkaline suites. This difference is best explained by removal of significant proportions of Ti during sanukitoid differentiation by magmatic hornblende, which fractionates Ti isotopes less strongly than Fe-Ti oxides. Combined with early oxide saturation at high Mg#, this suggests that sanukitoid parental magmas had H2O contents and fO(2) at least as high as modern arc magmas. Primitive (pre-oxide saturation) sanukitoids, however, have significantly higher delta Ti-49 (0.11-0.20 parts per thousand) than modern arc basalts, the depleted mantle and the bulk silicate Earth (BSE). Their elevated delta Ti-49 values cannot be explained by aqueous fluids alone in their mantle source, and instead require the involvement of a hydrous eclogite melt component formed in equilibrium with residual rutile. We favour generation of this metasomatic melt by fluid-fluxed eclogite partial melting, demonstrating that both metabasite melts and aqueous fluids are important for sanukitoid formation. The Ti isotope compositions of Archean and Paleoproterozoic sanukitoids therefore favour formation of the sanukitoid mantle source by a subduction-like process at least similar to 2.7 Ga.

The origin of titanium-rich basaltic magmatism on the Moon remains enigmatic. Ilmenite-bearing cumulates in the lunar mantle are often credited as the source, but their partial melts are not a compositional match and are too dense to enable eruption. Here we use petrological reaction experiments to show that partial melts of ilmenite-bearing cumulates react with olivine and orthopyroxene in the lunar mantle, shifting the melt composition to that of the high-Ti suite. New high-precision Mg isotope data confirm that high-Ti basalts have variable and isotopically light Mg isotope compositions that are inconsistent with equilibrium partial melting. We employ a diffusion model to demonstrate that kinetic isotope fractionation during reactive flow of partial melts derived from ilmenite-bearing cumulates can explain these anomalously light Mg isotope compositions, as well as the isotope composition of other elements such as Fe, Ca and Ti. Although this model does not fully replicate lunar melt–solid interaction, we suggest that titanium-rich magmas erupted on the surface of the Moon can be derived through partial melting of ilmenite-bearing cumulates, but melts undergo extensive modification of their elemental and isotopic composition through reactive flow in the lunar mantle. Reactive flow may therefore be the critical process that decreases melt density and allows high-Ti melts to erupt on the lunar surface.

Abstract Enriched mid‐ocean ridge basalts (E‐MORB) commonly erupt at mid‐ocean ridges (MOR) and seamounts, but their relationship to “depleted” MORB (D‐MORB) and the processes controlling their magmatic evolution at MORs are not fully understood, hence raising more general questions about magma generation in the mantle. We here explore this conundrum through an investigation of the Masirah ophiolite (southeast Oman), a near‐unique “true” MOR ophiolite. Unlike most (e.g., Tethyan) ophiolites, it was not affected by subduction and is therefore potentially able to provide valuable geological insights into the magmatic evolution of a full section of oceanic crust. Previous work has shown that the igneous crust at Masirah was thin (1.5–2.0 km) and constructed from both D‐ and E‐MORB magmas, concluding that it formed at a slow‐spreading ridge at ∼150 Ma followed by an episode of “Nb‐enriched” magmatism with trace‐element enrichments exceeding E‐MORB during intraplate rifting ∼20 Ma later. We reinvestigate the geology of Masirah and present new field observations, geochemical data and high‐precision U‐Pb ages to constrain the magmatic history of seafloor spreading and off‐axis magmatism. We found that D‐MORB and E‐MORB magmatism at Masirah was synchronous and overlapped in both composition and time with the Nb‐enriched magmatism (no older than 135 Ma). Both types of magmatism were therefore integral in the formation of the Masirah ocean crust. The relationship between D‐MORB and E‐MORB magmatism described here may be applicable to modern MORs more broadly, but is especially prominent at Masirah due to reduced magmatism and hence a weaker crustal filter.

Unlike their eruptive counterparts, plutonic rocks often experience post-cumulus evolution prior to their final solidification. These processes have the potential to exert significant control on the distributions of elements and their isotopic compositions of plutonic rocks. However, our understanding in these effects remains limited, particularly for metal stable isotopes. Here, we carried out high-precision δ65Cu measurements for a suite of primitive to evolved cumulates from the Kane area, 23oN of Mid-Atlantic Ridge. Most mid-ocean ridge basalts (MORBs) exhibit uniformly bulk silicate Earth (BSE)-like δ65Cu of + 0.09 ± 0.08 ‰, indicating limited Cu isotopic fractionation in crustal magma reservoirs prior to melt extraction. However, our new Cu isotopic data on these cumulates show that large Cu isotopic variations (δ65Cu = -0.6 ‰ to + 0.9 ‰) occur in variably evolved cumulates (MgO/total FeO < 3.0), while the most primitive cumulates (MgO/total FeO > 3.0) exhibit BSE-like δ65Cu (+0.06 ± 0.06 ‰). Petrological and geochemical observations indicate limited hydrothermal effects on Kane cumulate Cu isotopes. Modeling results indicate that the variable δ65Cu in Kane cumulates are unlikely to result from magmatic processes during the cumulus stage. Instead, the large Cu isotopic variation in Kane cumulates requires post-cumulus melt evolution involving fractionation between sulfide and intercumulus melts at relatively low temperatures. Textures and mineral chemistry indicate pervasive post-cumulus modifications of Kane cumulates by porous melt flow. Thermodynamic calculations indicate that intercumulus melts persist at temperatures even to 200 °C lower than the initial liquidus temperatures. Our data indicate that lower temperatures in the post-cumulus melt evolution and lower sulfide Ni contents result in lower sulfide-silicate melt Cu isotope fractionation factor (αsulfide-melt), then causing greater Cu isotopic variations in cumulates. We suggest that post-cumulus melt evolution reconcile both the Cu depletion and large Cu isotope fractionation in oceanic cumulates that are unexpected by a simple process of crystal accumulation from MORBs. This study highlights the essential role of post-cumulus evolution in controlling metal budget and their stable isotope fractionation during intra-crustal differentiation, which might also be applied to island arc and continental settings.

The upper mantle is critical for our understanding of terrestrial magmatism, crust formation, and element cycling between Earth’s solid interior, hydrosphere, atmosphere, and biosphere. Mantle composition and evolution have been primarily inferred by surface sampling and indirect methods. We recovered a long (1268-meter) section of serpentinized abyssal mantle peridotite interleaved with thin gabbroic intrusions. We find depleted compositions with notable variations in mantle mineralogy controlled by melt flow. Dunite zones have predominantly intermediate dips, in contrast to the originally steep mantle fabrics, indicative of oblique melt transport. Extensive hydrothermal fluid-rock interaction is recorded across the full depth of the core and is overprinted by oxidation in the upper 200 meters. Alteration patterns are consistent with vent fluid composition in the nearby Lost City hydrothermal field.

The emergence of the "mush paradigm" has raised several questions for conventional models of magma storage and extraction: how are melts extracted to form eruptible liquid-rich domains? What mechanism controls melt transport in mush-rich systems? Recently, reactive flow has been proposed as a major contributing factor in the formation of high porosity, melt-rich regions. Yet, owing to the absence of accurate geochemical simulations, the influence of reactive flow on the porosity of natural mush systems remains under-constrained. Here, we use a thermodynamically constrained model of melt-mush reaction to simulate the chemical, mineralogical, and physical consequences of reactive flow in a multi-component mush system. Our results demonstrate that reactive flow within troctolitic to gabbroic mushes can drive large changes in mush porosity. For example, primitive magma recharge causes an increase in the system porosity and could trigger melt channelization or mush destabilization, aiding rapid melt transfer through low-porosity mush reservoirs.

Abstract Studies of oceanic crust, which covers a large proportion of the Earth's surface, have provided significant insight into the dynamics of crustal accretion processes at mid‐ocean ridges. It is now recognized that the nature of oceanic crust varies fundamentally as a function of spreading rate. Ocean Drilling Program (ODP) Hole 1256D (eastern Pacific Ocean) was drilled into the crust formed at a superfast spreading rate, and hence represents a crustal end member. Drilling recovered a section through lava and sheeted dykes and into the plutonic sequence, the study of which has yielded abundant insight into magmatic and hydrothermal processes operating at high spreading rates. Here, we present zircon U‐Pb dates for Hole 1256D, which constrain the age of the section, as well as the duration of crustal accretion. We find that the main pulse of zircon crystallization within plutonic rocks occurred at 15.19 Ma, consistent with magnetic anomalies, and lasted tens of thousands of years. During this episode, the main plutonic body intruded, and partial melts of the base of the sheeted dykes crystallized. One sample appears to postdate this episode by up to 0.25 Myr, and may be an off‐axis intrusion. Overall, the duration of crustal accretion was tens to several hundreds of thousands of years, similar to that found at the fast‐spreading East Pacific Rise and the slow‐spreading Mid‐Atlantic Ridge. This indicates that crustal accretion along slow‐ to superfast‐spreading ridges occurs over similar time scales, with substantially longer periods of accretion occurring at ultraslow‐spreading ridges characterized by thick lithosphere.