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• We study the chemistry of the core and ocean of Enceladus based on different core compositions. • We study the hydrothermal products from the ocean-core interaction in the context of the chemistry of the plume of Enceladus. • We evaluate the chemistry of the ocean based on its possible pH values and the saturation indices of mineral products. • We find that the actual composition of the core poses a somewhat wide range of possible compositional scenarios. Measurements made by the Cassini spacecraft instruments were able to reveal the composition of the geysers of the Saturn moon, Enceladus, among which salts (sodium chloride, sodium bicarbonate and/ or sodium carbonate) and traces of silica could be the result of hydrothermal processes from the interaction of an inner liquid ocean with the core of Enceladus. The chemistry of the ocean should be closely connected to the chemistry of the core. Even though. the actual composition of the core is unknown, Enceladus has been characterised as a moon with a silicate core and different authors have estimated the properties of the ocean. A core with a silicate composition alone, does not necessarily justify most of the observed species of the plume. Given the many uncertainties, in the current work, we study the possible chemistry of the core and the ocean in the context of a four-layered Enceladus (dry core, hydrated core, ocean and crust) with three different compositional scenarios for the core: primordial (represented by an ordinary chondrite), composite (represented by a carbonaceous chondrite with an igneous inclusion) and non primordial (epresented by a given peridotite). The scenarios comprise a set of minerals that interact with a primordial ocean (either pure or enriched water) at a given temperature and pressure, producing a series of secondary minerals and compounds, some of them, coincident with the chemical species observed in the geysers. Specifically, we make a chemical speciation for each proposed compositional scenario with the software PhreeqC, however our analysis is limited to the study of interactions that reach a given equilibrium. In particular, output values like the potential of hydrogen of the ocean or the saturation indices of mineral products may give us hints about the chemistry of the ocean and the core. We find that, based on the species observed in the plume, the actual composition of the core (and the ocean as well) poses a somewhat wide range of possible compositional scenarios and each of our proposed scenarios and their products justify, to some extent, the observations of the plume. [ABSTRACT FROM AUTHOR]

La Palma, Canary Islands, had its largest historical eruption in 2021. From January 2022 to May 2023 there were >2,100 seismic events, primarily at depths ≤20 km, prompting us to update the deformation and modeling study, using interferometric synthetic aperture radar observations and a last generation interpretation tool. We detect the evolution of the remaining magmatic body in the SW portion of the island, with arrival of new magma moving into the oceanic crust out to sea, and a pressurized zone in the central‐eastern area, at regions of structural weakness. The current source characteristics have some similarities to the early stage dynamics prior to the 2021 eruption. Operational and multidisciplinary studies must continue to monitor either their stabilization or growth and destabilization. The ability to identify magma ascent using only deformation data over short time periods allows us to characterize unrest patterns and provide new insights into volcanic processes. Plain Language Summary: Following the 2021 eruption at La Palma, Canary Islands, seismic activity has persisted that may be a precursor to future volcanic activity. Here we perform an updated deformation and modeling study that characterizes the evolution of the remaining magma in the SW area of the island. A fresh injection of magma occurs in the oceanic crust off the coast, along with a pressurized region in the central‐eastern area of the island where crustal weakness exists close to recent volcanic eruptions. The 2022–2023.4 process has similar characteristics to the evolution prior to the 2021 eruption. Ongoing studies should focus on monitoring the increase or decrease in this activity. Improved characterization of changing volcanic activity over short time periods can provide new insights into ongoing volcanic processes. Key Points: We determine the current unrest is similar to that prior to the 2021 eruption warranting attention by hazard response decision makersWe detect a shallow magmatic intrusion in the SW of the island migrating W into the submarine crust of the islandWe also detect a shallow pressurized zone located in the central eastern region of the island [ABSTRACT FROM AUTHOR]

The northern Red Sea (NRS) is considered an extended continental region that has resulted in a rift system. Gravity and bathymetry data were used to estimate the Moho depth and the elastic thickness Te of the lithosphere beneath the NRS region to characterize its flexural rigidity and understand its mechanical behavior. Focusing on the Mabahiss Deep in NRS, we analyzed the lithosphere's flexural rigidity. The observed long-wavelength positive Bouguer anomaly is attributed to crustal thinning and lithospheric mantle uplift. The crustal thickness varies from 28 km in coastal areas to 24 km beneath the axial rift, supporting a regional compensation model over the Airy model. Forward modeling suggests that the optimal model explaining the regional Bouguer anomaly is a flexural model with Te equal to 7 km, indicating a weak and irregular continental crust. The primary factor contributing to this weakness is heating activity. Given the weakened state of the crust and the ongoing extension in the region, the NRS rift could evolve into a rupture, potentially leading to the formation of oceanic crust. [ABSTRACT FROM AUTHOR]

We calculated stress drops for 2875 small intraslab earthquakes at intermediate depths beneath Tohoku, Japan. We applied an S-coda-wave spectral ratio method to almost 900,000 event pairs. Detailed velocity values for the oceanic crust (OC) were adopted from previous observational studies. The median stress drops in the OC are about half those in the oceanic mantle (OM). The median stress drop for earthquakes in the OC decreases from depths of 70 to 120 km and increases from 120 to 170 km. Our preferred interpretation is that the rigidity in the OC decreases and then increases with depth due to combined effects of the dehydration associated with the eclogite formation and the increasing temperature with depth. These depth variations are consistent with results of a similar study beneath Hokkaido. The median stress drops in the oceanic plate beneath Tohoku are generally smaller than those beneath Hokkaido. Previous studies imaging the seismic structure at shallow depths and b-value analyses of intraslab earthquakes indicate that the near-trench region of the oceanic plate off Tohoku is more hydrated than that off Hokkaido. Taken together, these results suggest that differences in the degree of hydration of the oceanic plate in the near-trench regions could produce the different behaviors of stress drops of intermediate-depth earthquakes observed in Tohoku and Hokkaido. [ABSTRACT FROM AUTHOR]

Various crustal processes shape both the lower oceanic crust and mid-ocean ridge basalts (MORBs). To better understand how these crustal processes influence MORB compositions, we conducted comprehensive petrographic and geochemical investigations on gabbroic rocks and erupted lavas dredged from a segment of the Central Indian Ridge (CIR) spanning from 7°50′S to 8°30′S. The petrographic and geochemical analyses of the gabbroic rocks revealed evidence of melt-rock reaction through reactive porous flow in olivine gabbro and gabbro. This process resulted in distinctive features in clinopyroxene, including disequilibrium textures with a troctolite/anorthosite matrix, complex variations in Mg#-Cr-Ti [Mg# = molar Mg/(Mg + Fe2+)] relationships, and considerable enrichment and fractionation of incompatible trace elements. A significant finding of our study is the close resemblance of trace element ratios in MORB and olivine-hosted melt inclusions to those of melts in equilibrium with clinopyroxene from olivine gabbro and gabbro with Sr anomaly (Sr/Sr* = SrN/sqrt[PrN*NdN]; N refers to chondrite-normalized values) greater than ~0.7. This observation strongly indicates that the composition of MORB is influenced by the melt-rock reaction taking place in the lower oceanic crust. Furthermore, our findings suggest that evolved melts in equilibrium with clinopyroxene having Sr/Sr* values lower than ~0.7 are less likely to erupt onto the seafloor and are instead trapped within the lower oceanic crust. Oxide gabbronorite is characterized by coarse-granular, pegmatitic textures and exhibits mineralogically and chemically more evolved characteristics compared to olivine gabbro and gabbro. It is inferred that the oxide gabbronorite formed through the in situ freezing of highly evolved melts within a melt-rich layer. Finally, we present a comprehensive model for melt evolution in the lower oceanic crust at the 7°50′S–8°30′S CIR by integrating all petrological and geochemical data obtained from gabbroic rocks, MORB, and olivine-hosted melt inclusions. This holistic model contributes to a better understanding of the intricate processes governing MORB composition in the context of the lower oceanic crust dynamics at slow-spreading ridges. [ABSTRACT FROM AUTHOR]

The Gakkel Ridge in the Eurasian Basin has the slowest seafloor spreading worldwide. The western Gakkel Ridge (3°W–85°E; 14–11 mm/a) alternate between magmatic and sparsely magmatic zones, while the eastern Gakkel Ridge (85–126°E; 11–6 mm/a) appears to be dominated by magmatic zones despite ultraslow spreading. Little is known about the seafloor spreading conditions in the past along the entire ridge. Here, we exploit the residual bathymetry and basement roughness to assess the crustal accretion process of the Gakkel Ridge over time using 23 published regional multichannel seismic reflection profiles. Full seafloor spreading rates were faster (20–24 mm/a) up to ∼45 Ma, and residual bathymetry for the older crust is deeper than the world average in the entire Eurasian Basin. There is a sharp transition to 300–400 m shallower residual bathymetry for seafloor <45 Ma in the eastern Eurasian Basin. The crustal roughness versus spreading rate of the western Eurasian Basin is on the global trend, while that of the eastern is significantly below. Both low roughness and shallow residual bathymetry of the eastern Eurasian Basin is close to that of oceanic crust for spreading rates above 30 mm/a, demonstrating increased magmatic production of the eastern Gakkel Ridge since ∼45 Ma. A recent mantle tomography model predicts partial melting in the upper mantle based on the low Vs anomaly underneath. The sedimentary pattern toward the Lomonosov Ridge indicates that this hot mantle anomaly started to cause dynamic uplift of the area at ∼45 Ma. Plain Language Summary: The Gakkel Ridge has the slowest spreading ridge on earth, though rates were twice the present prior to ∼45 Ma. The sparsity of crustal‐scale seismic data leaves the spreading process uncertain. However, crustal roughness and residual bathymetry offer records of the plate‐spreading history. Lower roughness and shallower residual bathymetry are tied to increased magma production and thicker crust. We analyzed 23 published seismic profiles for these parameters. Seafloor older than ∼45 Ma has a residual bathymetry ∼550 m below the global average in both the eastern and western Eurasian Basin. For seafloor younger than 45 Ma basement depth is 300–400 m shallower in the east, while the west showed minimal change. The western Eurasian Basin's crustal roughness follows the global trend with spreading rate, whereas the eastern basin is much below. This indicates an unexpectedly robust magma production along the eastern Gakkel Ridge since ∼45 Ma. Recent upper mantle tomography models show a low shear‐wave velocity anomaly underneath, which is tied to increased temperature. Systematic shifts in the sedimentary depositional pattern in the east indicate that this anomaly started to cause dynamic uplift at ∼45 Ma, coeval with the increase in melting. Key Points: Residual bathymetry and crustal roughness used as proxy for spreading axis magma productivityThe eastern Eurasian Basin has low crustal roughness and near normal residual bathymetry since ∼45 Ma despite its ultraslow spreading rateThe eastern Gakkel Ridge spreading is magmatically robust since ∼45 Ma, the western variable [ABSTRACT FROM AUTHOR]

Hydrothermal vents serve as a primary interface between the cold deep ocean and the warm oceanic crust. While early research showed that seawater-rock interactions add to or remove elements from seawater during the generation of hydro-thermal fluids, consideration of these fluid fluxes alone does not relay the total impact that hydrothermal systems have on seawater geochemistry. In addition, hydrothermal plumes, areas where hydrothermal fluids mix with ocean waters, are host to a range of particle precipitation and scavenging reactions that further modify gross hydro-thermal fluid fluxes to define the total "net" hydrothermal impact on oceanic inventories. Here, we review the major discoveries made by the international GEOTRACES program regarding the geochemical transformations occurring within hydrothermal plumes. We classify each element into one of five categories based on its behavior in hydrothermal plumes, a spectrum spanning the geochemical mass balance between net hydrothermal source fluxes and net hydrothermal plume scavenging sinks. Overall, we celebrate the role that GEOTRACES has played in defining the extent and dynamics of hydrothermal plume geochemistry, which is a crucial lever for determining global hydrothermal impacts. [ABSTRACT FROM AUTHOR]

The structure of King's Trough and its surroundings (King's mesostructural cluster), located on the eastern flank of the Mid-Atlantic Ridge in the North Atlantic, is described in this paper. This work is based on geological and geophysical data obtained during the 55th expedition of the R/V Akademik Nikolaj Strakhov. Six provinces were identified within the southeastern part of King's Trough, based on the results of bathymetric survey. Each province has its own morphostructural feature resulting from multistage tectonic and volcanic processes, which alternate and conjugate with each other in time. According to seismoacoustic profiling data, three main types of seismic facies have been identified: (a) pelagic complexes; (b) deposits of turbidite flows; and (c) chaotic facies of gravity origin. It is shown that the anomalous magnetic field of the study area is the superposition of linear and isometric anomalies. The first were formed during the formation of the oceanic crust in the axial zone of spreading. The second are associated with volcanic massifs formed under intraplate conditions. The obtained data confirm the assumption that the formation of King's Trough was preceded by the formation of an elongated arched rise, which became a scene of intense intraplate volcanism that increased from southeast to northwest. This stage was followed by subsidence of the axial part of the rise with the formation of King's Trough and the Peake and Freen Troughs. [ABSTRACT FROM AUTHOR]

The Azores archipelago, situated east of the Mid-Atlantic Ridge, comprises volcanic islands arranged along sub-parallel spreading systems and rests on a thick oceanic crust. Magma is supplied directly from the roots of the volcanic systems. Located at or nearby the boundary between the crust and the mantle, they consist of mafic cumulates and mafic mush layers. This work focuses on tephra samples and a submarine lava younger than 40.000 years, collected from both central volcanoes and fissure zones. Our report details a new dataset of major, trace, and volatile elements analysed in glassy melt inclusions trapped in olivine (Fo75.8–85.6) which are extracted from cumulative bodies at the vicinity of the crust-mantle boundary. Their compositions cover a range from subalkaline to mildly alkaline basalt, and trachybasalt, which match those of Azores lavas. They registered a chemical evolution through fractional crystallisation of olivine alone, as well as olivine and clinopyroxene, as both the FeOt/MgO (1.4–3.1) and CaO/Al2O3 (0.4–1.0) ratios of the melt decrease. Incompatible element ratios of Zr (40–352 ppm), Ba (135–612 ppm), and Rb (5–77 ppm), as compared to Nb (5–82 ppm), exhibit variability within a limited but significant range of values. The ranges in the Nb/Zr, Ba/Nb and Rb/Nb ratios recorded by melt inclusions possibly reveal distinct geochemical sources (at least two), and mixing between partial melts as they move upward. The halogen signature is characteristic of the shallow mantle. The majority of melt inclusions show Cl/K ratio (0.06) similar to E-MORB, although some of them are comparable to N-MORB (Cl/K = 0.03). Their F/Nd ratio may achieve a rather high value (27.8). [ABSTRACT FROM AUTHOR]

U-Pb zircon geochronology and isotopic records have played an influential role in our understanding of convergent margin dynamics. Orogenic cyclicity models link tectonic regimes with magmatic isotopic signatures in advancing orogens, relating compressional regimes with evolved signatures and extension with juvenile signatures; however, such frameworks may not apply for retreating orogens, which commonly produce substantial crustal heterogeneities during backarc rifting and ocean spreading. We explore the Mesozoic to Cenozoic Patagonian Andes tectonic evolution, combining U-Pb zircon ages, bulk rock eNd, and new detrital zircon eHf from the retroarc basin to understand the associated magmatic arc evolution during retreat and advance of the margin. Our results reveal a protracted phase of isotopically juvenile magmatism between 150 and 80 Ma, which began during backarc extension and persisted long after the margin switched to a contractional regime. We propose that the prolonged juvenile isotopic trend started mainly due to trenchward migration of the arc during backarc extension (150-120 Ma) and persisted due to partial melting of underthrusted juvenile attenuated and oceanic crust during backarc basin closure (120-80 Ma). This interpretation implies that tectonic stress alone does not predict isotopic trends, and factors like assimilation or the composition of underthrusted crust are important controls on magmatic isotopic composition, especially in retreating and transitional orogens. [ABSTRACT FROM AUTHOR]

Hadean zircons provide a potential record of Earth's earliest subduction 4.3 billion years ago. It remains enigmatic how subduction could be initiated so soon after the presumably Moon‐forming giant impact (MGI). Earlier studies found an increase in Earth's core‐mantle boundary (CMB) temperature due to the accumulation of the impactor's core, and our recent work shows Earth's lower mantle remains largely solid, with some of the impactor's mantle potentially surviving as the large low‐shear velocity provinces (LLSVPs). Here, we show that a hot post‐impact CMB drives the initiation of strong mantle plumes that can induce subduction initiation ∼200 Myr after the MGI. 2D and 3D thermomechanical computations show that a high CMB temperature is the primary factor triggering early subduction, with enrichment of heat‐producing elements in LLSVPs as another potential factor. The models link the earliest subduction to the MGI with implications for understanding the diverse tectonic regimes of rocky planets. Plain Language Summary: Plate tectonics remains unique to Earth, but when and how it started is debated. Earth's oldest minerals indicate a clement surface by 4.3 Ga, resembling the modern Earth with its granitic crust and oceans. Granite is most easily explained as originating from subduction. However, the mechanisms for subduction initiation, especially so soon after the Moon‐forming giant impact, remain elusive. Earlier studies indicate that the core‐mantle boundary (CMB) temperature is increased due to accumulation of the impactor's core during the impact. Our recent work further shows that the lower half of Earth's mantle remains mostly solid after this impact and that parts of the impactor's mantle might have survived as the two seismically‐observed large low‐shear velocity provinces (LLSVPs). In this study, we perform whole‐mantle convection models to illustrate that strong mantle plumes can arise, weaken the lithosphere, and eventually initiate subduction ∼200 Myr after the giant impact. Our systematic computations reveal that the hot CMB temperature after the impact is the primary factor determining whether there is early subduction initiation, with enrichment of heat‐producing elements in LLSVPs as another potential contributor. Our model ties the Moon's formation to incipient subduction, providing insights for understanding the diverse tectonic regimes of rocky planets. Key Points: The mantle thermochemical structure left by the Moon‐forming impact triggers strong plumes that may have initiated the first subductionThe strong mantle plumes may rise from a heated core or from large low‐shear velocity provinces enriched in heat‐producing elementsEarly giant impact events may have a profound influence on the diverse tectonic regimes of rocky planets [ABSTRACT FROM AUTHOR]

Oceanic volcanic islands have a complex internal structure due to their geological evolution. La Palma is the second youngest in the Canary Islands. Historical eruptions occurred in the Cumbre Vieja (CV) volcanic system, including the last 2021 Tajogaite eruption. The seismicity before, during, and after the Tajogaite eruption provides valuable information for a better understanding of deep volcanic processes. We applied the receiver function technique to image the crustal and upper mantle structures beneath the CV volcanic complex. We investigated the first 50 km of the lithosphere, identifying various seismic discontinuities. After a thin layer of recent volcanic rocks, we found a layer corresponding to the old oceanic crust. Beneath the Moho, we found a significant decrease in the seismic velocities until about 37 km of depth. This low‐velocity layer corresponds to a zone of partial melting with interconnected magmatic chambers that feed the volcanic activity in CV. Plain Language Summary: Oceanic volcanic islands have a complex internal structure. Our study is important to understand volcanic activity in its geodynamic context better. The last eruption occurred in the Cumbre Vieja volcanic complex on La Palma Island (2021) and had an unexpected magnitude in terms of lava volume, explosivity, and a significant impact on the economy and society of the island. This study used receiver function analysis to image the lithosphere up to a depth of 50 km. This seismological technique exploits the recordings of teleseismic events (i.e., earthquakes located between 30 and 90° from the stations) for this purpose. We used an advanced inversion technique to study the structure of the crust and the upper mantle beneath the volcano. The most relevant finding is a zone of anomalous low seismic velocities extending from 13 to 37 km depth. This zone is likely to host partial melt and magma chambers and extends over a much wider and thicker volume than expected from previous studies. Previous studies also identified two magma chambers: the upper one, located at about 10–15 km, and the lower one, beneath 25 km depth. These findings provide new insights into the current paradigms about the internal active oceanic volcanic islands. Key Points: Receiver function analysis allowed us to image the crustal and upper mantle seismic structure beneath Cumbre Vieja volcano in La PalmaMain velocity discontinuities in the crust show the contacts between the recent volcanic rocks, the old oceanic crust and the upper mantleA low‐velocity zone extending from the Moho up to about 37 km of depth indicates the presence of a broad zone of partial melting [ABSTRACT FROM AUTHOR]

Potassium (K) isotopic compositions for a suite of crustal rocks (plutonic unit, sheeted dike complex, and volcanic sequence) in the Troodos ophiolite, Cyprus, were determined to investigate fluid-transport processes during subduction inception. The gabbros and plagiogranites in the plutonic unit have restricted K isotopic compositions (0.01–0.20 ‰ and −0.17 to 0.02 ‰, respectively), whereas diabases in the sheeted dike complex have variable δ41K values from −0.24 to 1.13 ‰. The δ41K values of the volcanic sequence range from −0.26 to −0.07 ‰ in the lower pillow lavas, and from −0.71 to 0.34 ‰ in the upper pillow lavas. These δ41K values are clearly distinct from those of subducting sediments and altered oceanic crust, ruling out direct crustal input as the cause of the unusually heavy K isotopic compositions. Rather, the high K/Th, K/U, and Sr/Th ratios of the investigated samples, together with binary mixing modelling, reveal that the anomalously heavy K isotopic compositions in Troodos rocks are attributed to the influx of fluids during subduction inception. Notably, the δ41K values in the crustal rocks of the Troodos ophiolite are more variable and extend to higher values than those of arc-front and rear-arc lavas of the Izu arc and arc lavas of Martinique in the Lesser Antilles, indicating continuous K isotope fractionation during slab dehydration. Thus, the heavy K isotopic compositions of the Troodos ophiolite reflect K isotope fractionation during dehydration of a shallow slab within a newly formed subduction system, where heavy K isotopes preferentially enter into the aqueous fluids. Later, progressive breakdown of K-bearing phases at deeper levels results in isotopically lighter hydrous melts, which can be delivered to the arc. [ABSTRACT FROM AUTHOR]

Tectonic slices of greenschist – epidote blueschist – epidote amphibole schist – garnet epidote amphibole schist are exposed in the northern part of the NOC. Metamorphic P-T conditions and P-T paths of the garnet epidote amphibole schist and epidote blueschist were determined with the help of pseudosection calculations in the MnNCKFMASHTO system using the software Perple_X. The garnet epidote amphibole schist records a pre-peak metamorphic stage at ~12 kbar and ~360°C in the lawsonite blueschist facies field. Lawsonite and glaucophane are not preserved in the rock and are attributed to progressive metamorphic reactions during prograde metamorphism. The peak metamorphic stage is constrained at ~14 kbar and ~570°C in the hornblende eclogite facies field. Post-peak metamorphism shows slight cooling and decompression from ~550°C and ~13 kbar, possibly down to ~350°C and ~7 kbar in the greenschist facies field. These three metamorphic stages reveal a clockwise metamorphic P-T path of evolution for the garnet epidote amphibole schist. The geotherm at metamorphic peak P-T condition is ~12.5°C/km, which is hotter by ~350°C compared to thermal models of mature subduction zones but shows good agreement with models of warm subduction zones. Therefore, the low geothermal gradient of ~8°C/km during the prograde metamorphism possibly reflects the subduction of cold Neo-Tethyan ocean crust when the temperature conditions of the subduction zone were still warm. The epidote blueschist shows peak metamorphism at ~10 kbar and 380°C with a clockwise P-T path. It indicates metamorphism at a more mature stage of subduction. Buoyancy-driven exhumation along the subduction channel due to the serpentinization of the upper mantle wedge played a vital role in the detachment and exhumation of the tectonic slivers of metamorphic rocks from different depth sections in the northern margin of the NOC. [ABSTRACT FROM AUTHOR]

The dynamics and magma transport at the boundary between the upper and lower oceanic crusts (i.e., the dike–gabbro transition) are crucial for understanding the crustal accretion beneath mid-ocean ridges, which however have been studied at quite a few sites such as the East Pacific Rise and ophiolites like Troodos and Oman. Here we present detailed geological, petrological, and geochemical data for the dike–gabbro transition and associated basalts in the Yunzhug ophiolite, central Tibet, to constrain the complex magmatic processes in this specific horizon. The Yunzhug ophiolite contains a large (~ 20 km2) well-exposed sheeted dike complex, which is rooted in a dike–gabbro transition that consists of diverse lithologies, including diabase, gabbro, and minor porphyritic diabase. Petrographically, the Yunzhug gabbros could be grouped into the dominant Plg (plagioclase)-euhedral gabbros (euhedral–subhedral plagioclases enclosed in clinopyroxene oikocrysts) and a small amount of Cpx (clinopyroxene)-euhedral gabbros (with abundant euhedral clinopyroxenes). Plagioclases and their equilibrated melts of the two types of gabbros are similar, whereas clinopyroxenes and their equilibrated melts of the Cpx-euhedral gabbros are more primary and depleted than those of the Plg-euhedral gabbros. These petrographic and geochemical features suggest an earlier crystallization of clinopyroxene for the Cpx-euhedral gabbros, which is best explained by occasional water input in the magmatic system. Nevertheless, the modeled equilibrated melts of the two types of gabbros have compositions indistinguishable from the whole rock compositions of diabases and basalts, indicating a direct genetic linkage between these rocks. The unusual porphyritic diabases, on the other hand, provide evidence supporting for plagioclase accumulation and aggregation during magma upward migration, thus may have served as a unique way for magma to transport from the lower to upper crust. Studies of the Yunzhug ophiolite thus provide some key constraints on the complex magmatic processes in the oceanic dike–gabbro transition, regarding its dynamic accretion and magmatic plumbing mechanisms. [ABSTRACT FROM AUTHOR]

Oceanic cobalt‐rich crusts contain many valuable metals including Co, Ni, Cu, and Mn, of which Co and Ni are scarce metal resources on terrestrial land. Co, Ni, Cu, and Mn in cobalt‐rich crusts can be efficiently extracted by the ammonium sulfate roasting‐water leaching (ARWL) process, but separating and recovering Co and Ni from the leaching solution containing high concentrations of Mn is a challenging problem. In this study, a combination of sulfide precipitation and P507–Cyanex 272 synergistic extraction is proposed for the separation and recovery of Co and Ni from the ARWL solutions of oceanic cobalt‐rich crusts. Under the optimum sulfide precipitation conditions, Co and Ni were precipitated with 99.85% and 99.63% efficiency, respectively, while Mn was coprecipitated with only.79% efficiency. The Co–Ni mixed sulfides were dissolved by dilute sulfuric acid under oxidative conditions to obtain a solution containing 12.584 g/L Co, 11.012 g/L Ni, and a small amount of Mn. Subsequently, Co and Mn were extracted from this solution using P507–Cyanex 272 synergistic extraction, the single‐stage extraction efficiencies of Co and Mn were 95.76% and 99.73%, respectively, and the coextraction efficiency of Ni was 3.02%, under the conditions of a feed solution pH of 3.0, an extractant saponification degree of 70%, a total extractant concentration of 20% (P507 to Cyanex 272 ratio of 3:7), and an O/A ratio of 1.1:1. The results of the thermodynamic study showed that the reaction of Co extraction by this mixed extraction system was exothermic. Ni in the organic phase was washed with an H2SO4 concentration of.12 mol/L, followed by stripping of Co and Mn with an H2SO4 concentration of.6 mol/L. Mn in the stripping solution was removed by oxidative precipitation with (NH4)2S2O8, after which the Co3O4 product was obtained by precipitation with (NH4)2C2O4 and calcination. Similarly, NiC2O4·2H2O can be obtained from the raffinate by (NH4)2C2O4 precipitation. Finally, a flowsheet was developed for the separation and recovery of Co, Ni, Cu, and Mn from the ARWL solutions of oceanic cobalt‐rich crusts. The study provides a promising scheme for the recovery of various valuable metals from deep‐sea cobalt‐rich crusts or manganese nodules. [ABSTRACT FROM AUTHOR]

The increased velocity associated with decaying seismicity in the subducting oceanic crust usually has been attributed to its eclogitization. However, the degree of the eclogitization of subducting oceanic crust at depth remains unclear due to the lack of velocity data for eclogite at pressures of >3 GPa. In this study, the P- and S-wave velocities of eclogite aggregates composed of omphacite and garnet (Omp100, Omp80Grt20, and Omp- 50Grt50) were measured simultaneously at pressures of up to 8 GPa using ultrasonic interferometry. The velocities of the eclogite aggregates increased with rising pressure and garnet content. By determining the velocities at different pressures using the finite strain equation, the elastic moduli of the eclogites and their pressure derivatives were determined to be KS0 = 119-134 GPa, KS0‰ = 5.2-5.3, G0 = 73-80 GPa, and G0‰ = 1.5-1.6. The high-wave velocity and low VP/VS ratio of the eclogites, combined with the seismic tomography and seismicity distribution, jointly constrain the depth and composition of eclogitization in the subducted oceanic crust of Northeast Japan, and a new 1-D velocity structure is proposed. We also compiled and calculated the depth of the eclogitization and the garnet content of the subducting oceanic crusts in 27 typical locations around the Pacific Ocean. The depth of the eclogitization was positively correlated with the age of the subduction zone, and the garnet content was estimated to be 12%-32% in our model. [ABSTRACT FROM AUTHOR]

Transpressional shear zones commonly occur in ancient and modern convergent plate boundaries to accommodate oblique plate convergence. The early Paleozoic Qilian orogen in northeastern Tibet records the subduction of Proto-Tethyan Ocean lithosphere and the accretion-collision of various magmatic arcs and continental terranes. This study focused on the Datong ductile shear zone, which represents the central part of the WNW-ESE-striking ductile shear zone along the northern margin of the Qilian block in the Qilian orogen. This structure bears key information about the evolution of oblique convergence during the early Paleozoic orogeny. The kinematics and timing of the Datong ductile shear zone were investigated via fieldbased, microstructural, and mica 40Ar/39Ar dating analyses. Mesostructural and microstructural data showed predominantly dextral strike-slip shearing within the Datong ductile shear zone. Microstructural features and quartz c-axis crystallographic preferred orientation patterns indicated that dextral ductile shearing occurred under lower-amphibolite-facies conditions (~500-550 °C and ~5.6 kbar) within the shear zone. Microstructures of quartz showed subgrain rotation (SGR) and grain boundary migration (GBM), suggesting dislocation creep-dominated deformation. A strain rate of 10-12 s-1 and a differential stress of 25-39 MPa were estimated by the rheological flow law and quartz paleopiezometry. Finite strain measurements indicated that all deformed rocks of the Datong ductile shear zone exhibit a weakly oblate ellipsoid near the plane strain. Kinematic vorticity (ranging 0.47-0.83) analysis suggested the coexistence of simple shear and pure shear strains within the Datong ductile shear zone, indicating a transpressional setting. Biotite and muscovite 40Ar/39Ar data showed that transpressional shearing deformation started in the Ordovician (before 453 Ma) and lasted to the Silurian (ca. 430 Ma). Our new data combined with regional geological data show that the deformation type, kinematics, and dynamics of the Datong ductile shear zone were controlled by the southward oblique subduction of the Paleo-Qilian Ocean (Proto-Tethyan Ocean) and the following oblique collision between the Qilian block and the Alxa block. The intensive transpressional deformation along the northern Qilian block may reflect strong coupling between the subducting Paleo-Qilian oceanic slab and the overriding Qilian block as well as a high degree of convergence obliquity during the ongoing early Paleozoic convergence. [ABSTRACT FROM AUTHOR]

Despite of the relatively rare occurrence of alkaline igneous rocks on Earth, they are the most diverse group of igneous rocks due to variations in their mineral assemblages and geochemical compositions. Alkaline igneous rocks are generally characterized by Si undersaturation, and K-Na (and in some cases C) enrichment. Although there is no unified alkaline igneous rock classification scheme, diagrams such as total alkalis-SiO2 or K2O-SiO2 can be used to classify alkaline igneous rocks. The origins of silica-saturated alkaline rocks (SiO2>52 wt.%) are relatively complex because multiple episodes of magmatism are often involved, therefore most studies have focused on the origins of silica-undersaturated alkaline magmas. Numerous experimental petrological studies have been conducted in the past few decades. Initially, it was considered that enrichment of trace elements was only achieved by low-degree partial melting of mantle peridotite, but the experimental melts could not reproduce the geochemical composition of natural alkaline igneous rocks. Subsequent studies have focused on carbonate-bearing eclogites that represent the average component of subducted oceanic crust. Although experimental studies indicate that silica-undersaturated and alkali-oversaturated melts can be generated from eclogites, some studies have considered that natural silica-undersaturated alkaline igneous rocks are the result of multi-stage source enrichment by incompatible elements. Low-degree partial melts of the mantle can be emplaced in the lithospheric mantle, forming veins consisting of phlogopite, amphibole, and pyroxene. Melting experiments including these components show that the produced melts better reproduce the naturally occurring silica-undersaturated alkaline magmas. Silica-deficient alkaline igneous rocks occur in various tectonic settings. Those in intraplate and divergent settings (i.e., mainly continental rifts) normally involve contributions from recycled components such as subducted slabs at the mantle transition zone (MTZ) or metasomatized lithospheric mantle. At convergent plate boundaries, silica-undersaturated alkaline magmas can be derived by the melting of mélange or the focused breakdown of phlogopite at back-arc depths. We compiled global data for alkaline igneous rocks and discovered that silica-undersaturated alkaline igneous rocks in continental collisional zones are K-rich and differ from those from other tectonic settings. We suggest that subducted continental sediment is an important K-rich end-member, which contributes a large amount of K to the alkaline magmas in the collision zone. Moreover, the solidus of K-rich minerals in the subducted plate implies that K can only participate in magmatism in the subduction zone. Before the plate reaches a depth of ∼300 km, the mica-group minerals, K-feldspar, evaporitic minerals, and other K-rich minerals are gradually consumed and rarely participate in deeper processes. In contrast, relatively Na-rich minerals such as pyroxene can be transported into the deeper mantle with eclogitic oceanic crust, and the resultant melt can enter the lithosphere owing to convective mantle flow. This forms metasomatic minerals, such as amphibole, in the lithospheric mantle, which melt to generate Na-rich and low-Si alkaline magmas. [ABSTRACT FROM AUTHOR]

Beneath mid‐ocean ridges, mantle upwelling and decompression melting are fundamental processes contributing to the formation of oceanic crust. Previous geodynamic models have suggested that mantle upwelling driven by separating plates can be intensified by thermochemical buoyancy and the thickening of aging lithosphere, resulting in thicker crust. However, the relative contributions of these factors to crustal accretion remain uncertain. We conducted numerical modeling in three scenarios to investigate this: (a) buoyant flow models incorporating age‐dependent lithospheric thickening as a reference; (b) passive flow models incorporating age‐dependent lithospheric thickening to isolate the effect of buoyancy; and (c) passive flow models that neglect age‐dependent lithospheric thickening to isolate its effect. Models are performed under varying potential mantle temperatures (Tp), viscosity structures, and spreading rates. The model‐predicted crustal thickness suggests that both buoyancy and thickening of aging lithosphere increasingly contribute to crustal production as spreading rate decreases. However, given the range of reference mantle viscosities examined here (1019–1020 Pa s), except in ultraslow spreading rates with lower reference viscosity and warmer Tp, the impact of the thickening of aging lithosphere predominates over buoyancy. Furthermore, the relative importance of buoyancy‐induced crustal production increases as spreading rates decrease. Variations in Tp further amplify the variability in crustal thickness compared to that at fast‐spreading ridges where passive upwelling dominates, when combined with 3‐D effects of buoyant flow on altering axial crustal production at ultraslow‐spreading ridge segments. This 4‐D effects of buoyant flow on melt production may explain the observed more variable crustal thickness at ultraslow‐spreading ridges. Plain Language Summary: Mid‐ocean ridges are locations where tectonic plates diverge, leading to the upwelling and melting of mantle material to form oceanic crust. Upwelling raises the isotherm, creating a lithosphere that thickens as it ages. The warmer temperature structure and melting process decrease the mantle density, generating buoyancy. Both buoyancy and thickening of aging lithosphere contribute to increased upwelling rates and melt flux, resulting in a thicker oceanic crust. To understand their relative contributions to crustal production, we conducted numerical experiments in three cases: the first contains both buoyancy and thickening of aging lithosphere, the second contains only thickening of aging lithosphere, and the third ignores both. We distinguished their contributions by comparing the model‐predicted crustal thickness among different configurations. Our results indicate that as spreading rates decrease, both buoyancy and thickening of aging lithosphere contribute to a thicker crust. Moreover, the importance of buoyancy‐induced crustal production increases as spreading rates decrease. However, thickening of aging lithosphere generally has a more dominant effect over buoyancy at almost all spreading rates, except for extremely slow spreading ridges. Key Points: Contributions of buoyancy and thickening of aging lithosphere on oceanic crustal production are distinguished by numerical modelsEffect of thickening of aging lithosphere dominates over buoyancy at nearly all spreading ratesEffect of buoyancy may play a significant role in crustal accretion at ultraslow spreading ridges [ABSTRACT FROM AUTHOR]

The central Inner Mongolia, located at the intersection of the northern margin of the North China Craton (NCC) and the Central Asian Orogenic Belt, is crucial for deciphering the Late Palaeozoic tectonic evolution associated with the subduction and closure of the Palaeo‐Asian Ocean (PAO). Our study focused on petrology, detrital zircon LA–ICP–MS U–Pb geochronology and whole‐rock geochemistry for the Late Carboniferous to Permian sandstones within the Shuanmazhuang, Dahongshan, Naobaogou, and Laowopu formations in Siziwang Banner, central Inner Mongolia. This comprehensive analysis shed light on the dynamic interplay between the NCC and the South Mongolia Block. Detrital zircon U–Pb ages in investigated samples mainly cluster between 250 and 2650 Ma, with significant peaks at 2.4–2.5 Ga, 1.8–2.0 Ga, 400–430 Ma, and 250–320 Ma, respectively. The geochemistry data are characterized by SiO2 contents (56.29–77.95 wt. %), Na2O / K2O ratios (0.45–1.58) and SiO2/Al2O3 ratios between 4.33 and 7.44. Moreover, they exhibit the slight enrichment in large ion lithophile elements (Rb and Ba) and the depletion in high field strength elements (Nb, Ta, Th, and U). These facts indicate that the sedimentary detritus predominantly originates from felsic sources, probably deriving from the Late Carboniferous–Permian continental island arc‐related intermediate‐acid igneous rocks, the Late Ordovician‐Silurian magmatic rocks in the Bainaimiao arc and the basements of the NCC. Furthermore, our present results also suggest that during the Early–Middle Permian, accelerating oceanic crust subduction triggered significant magmatic events in Siziwang Banner, leading to rapid uplift and the erosion of arc magmatic rocks, as well as the abundant corresponding sediments. Subsequently, the gradual convergence and eventual collision between the NCC and the Southern Mongolian Block took place at the end of the Permian, representing final closure of the PAO. [ABSTRACT FROM AUTHOR]

The Ramagiri greenstone terrane (RGT) of Central Dharwar craton, India, is recognized as remnants of Archean oceanic crust squeezed between ancient proto‐continental terranes. The granites and gneisses formed as a result of partial melting, resisted subduction and collided, sinking the intervening ocean basin. The geochemical characteristics of the bimodal metavolcanics of RGT were carefully assessed using suitable proxy. An insight into them suggests that the mafic rocks bear characteristics of island arc tholeiites belonging to suprasubduction zone setting, as corroborated by their low TiO2/Yb (avg 443) and high Th/Nb (avg 0.8). Their high V/Ti (avg 0.3) ratio points towards a mantle source that has been influenced by subduction, whereas low Ti/Yb echoes high degrees of shallow melting with little residual garnet. The felsic rocks on the other hand are metaluminous–peraluminous, mostly calc‐alkaline and geochemically akin to I‐ and S‐type syncollisional and volcanic arc granite emplaced during intercontinent collision. Their immobile element plot with low Nb + Y (avg 25) concentration also points towards a volcanic arc environment. [ABSTRACT FROM AUTHOR]

This paper presents the integration of magnetic susceptibility measurements and whole-rock geochemical compositional and Nd–Sr isotopic ratio analyses for granite samples collected from the Ranong, Lam Pi, Ban Lam Ru, and Phuket granite bodies in the Western Granitoid Belt of Thailand. In addition, U–Pb dating was performed on zircons extracted from the samples. All samples are proper granites based on their mineralogical and geochemical characteristics. Two samples collected from the Lam Pi granite body were classified as magnetite-series and I-type. The remaining granite samples were classified as ilmenite-series and S- or A-type. Furthermore, all granites were classified as syn-collision granites. Excluding the magnetite-series samples from the Lam Pi granite body, the other samples exhibit enrichment in incompatible elements, such as Nb, Sn, Ta, Pb, Bi, Th, U, Ce, Rb, and Cs. Zircon U–Pb dating yielded ages of ca. 60 Ma for the magnetite-series granites from the Lam Pi granite body, whereas ages of 88–84 Ma were obtained for the other granite bodies. Initial Nd–Sr isotopic ratios indicate a higher contribution of mantle material in the Lam Pi magnetite-series granites and a higher contribution of continental crust material in the other granites. Based on these compositional and zircon U–Pb age data, it is inferred that the 88–84 Ma granites formed as a result of the thickening of the continental crust owing to the collision between the Sibumasu and the West Burma blocks. In contrast, the ca. 60 Ma Lam Pi magnetite-series granites are thought to have been generated via partial melting of the mantle wedge associated with the subduction of the Neo-Tethyan oceanic crust beneath the West Burma Block. [ABSTRACT FROM AUTHOR]

• Mapping lithounits of Nidar ophiolites using high-resolution multi-spectral data. • First time characterization of Nidar ophiolites using hyperspectral (PRISMA data) • ML based classification of Nidar Ophiolite & associated rock exposures. • Spectral characterisation of lithounits of Ophiolites and validation from field. Ophiolites represent s a distinctive variety of igneous rock that comprises fragments of oceanic crust with the underlying mantle portion. Their mineral assemblages and distribution relation, both spatial and temporal are in close association with the complex mantle dynamics of the Earth. Nidar ophiolite complex located towards SE of Ladakh is one of such ophiolite assemblages exposed well in the Indus Suture Zone of Himalayas. It represents parts of Neo-Tethys ocean that existed during Mesozoic-Cenozoic period between continental plates of India and Eurasia. The earlier studies for Indian ophiolites are mostly field and analytical based with a limited work carried out using remote sensing techniques and that adds scope to apply this approach for further exploration of ophiolites. Advanced tools and techniques especially, hyperspectral remote sensing and machine learning based approach can be effective operational tools for exploration of ophiolites that provide assessable terrains for characterization of Earth's mantle, serve analogs for planetary crustal rocks and also have economic significance. This study has utilized high-resolution multispectral (Landsat OLI and ASTER) and hyperspectral data from various missions including the recent PRISMA mission which has provided the first hyperspectral datasets for lithological mapping of Nidar ophiolite section. The multiple high-resolution datasets were used for identification and mapping of the various lithounits of the Nidar ophiolites together with the underlying and overlying associated rocks based on band ratioing, derived spectral compositional indices and spectral characterization. The differentiated lithounits of the Nidar ophiolites are interpreted based on spectral response of lithological units in different bands. In addition, classification based on machine learning techniques have been applied to the PRISMA data to generate a detailed classification map of the study area where the various lithological units of the ophiolites and associated rock exposures belonging to various geological formations are clearly discernible. The results have been validated from geological field visit to the study area and spectral analysis of the field samples. [ABSTRACT FROM AUTHOR]

Understanding the subduction-driven recycling of economically important chalcophile metals (e.g., Cu, Au, Ag, and platinum-group elements, PGEs) and volatiles (e.g., sulfur) is critical for assessing models of ore formation. However, whether the sulfide remains stable in recycled altered oceanic crust (AOC) in the mantle is unclear, and the influence of AOC-derived metasomatic agents on the extraction efficiency of chalcophile elements from the deep mantle remains poorly understood. Here we report whole-rock Cu-Ag-Au-Re-PGEs of Cenozoic alkaline basalts at Hannuoba, eastern China, which were derived from the interaction of carbonated eclogite (recycled AOC)-derived melts with lithospheric peridotites. Volatile-rich primitive melts from carbonated eclogite are poor in strongly chalcophile metals (e.g., ∑PGE < 1 ng/g, Au < 1 ng/g, Cu < 30 μg/g). Remarkably, primitive melts show significant fractionation of elements with variable chalcophile affinities, such as lower Cu/Ag ratios than those in primitive mantle and MORBs. These features reflect the sequestration of these metals in residual mono-sulfide solid solution (MSS) during low-degree melting of the AOC in the deep mantle. Given the retention of sulfides in the AOC, partial melts derived from it are rich in sulfur but poor in strongly chalcophile metals. The Hannuoba alkaline melts scavenged some strongly chalcophile metals from peridotites (especially PGEs) during their ascent. However, this did not necessarily lead to the formation of chalcophile metal-enriched melts (e.g., Au < 1 ng/g and Cu < 60 μg/g). These processes appear to be common in eastern China, as evidenced by the low abundances of Cu and Au found in many primitive intraplate alkaline basalts. We propose that strongly chalcophile elements in subducted AOC are efficiently transported into, and stored in, the deep mantle, with a small proportion returning to the exosphere through volcanism. Moreover, the chalcophile metal-poor and volatile-rich melts of subducted AOC could also explain the addition of abundant sulfur (secondary sulfides), but limited chalcophile metals to the metasomatized mantle. [ABSTRACT FROM AUTHOR]

The Qinghai-Tibetan Plateau experienced a unique geological evolution during the Jurassic, driven by the termination of the Palaeotethys and the reduction of the Neotethys. The Indian Plate separated from the northern margin of Gondwana and drifted northward from the Southern Hemisphere. Given that the timing of strata serves as the basis for reconstructing geological history, the present work aimed to develop a new multiple stratigraphic and chronologic framework for the Jurassic strata of the Qinghai-Tibetan Plateau region via a synthesis of the material on lithostratigraphy, palaeontology, iso-radiometric dating, magnetostratigraphy, and other techniques with an emphasis on recent progress and findings. The new framework included the Jurassic System from the four major subdivisions of the plateau: the Baryan Har, Qiangtang, Lhasa-Gandise, and Southern Xizang (Himalaya). Ultimately, a more complete, refined biostratigraphic sequence was proposed, comprising the most common fossils in the plateau and those that are stratigraphically significant for the Jurassic stratigraphy, including ammonites, bivalves, brachiopods, foraminifera, radiolarians, and dinoflagellate cysts for the marine strata, and pollen and spores, and charophytes for the terrestrial sediments. This biostratigraphic framework was correlated with the Jurassic international standard zonation of the Geological Time Scale 2020 via standard or representative species or genera of ammonites. Based on this framework, we constructed a lateral correlation of the Jurassic strata between different basins of the plateau. The palaeontologic correlation in the present work shows that the Lhasa-Gandise Block had a closer relationship with the Qiangtang Block than with the Southern Xizang Himalaya during the Jurassic Period. Meanwhile, the Lhasa-Gandise Block and Qiangtang Block shared similar marine fauna features of the north marginal East Tethys. This contrasts the opinion suggesting that the Yarlung Zangbo Tethys was a small back-arc basin. A combination of stratigraphical, palaeontological, and sedimentological analyses implies that the Bangong Co-Nujiang Tethys may have begun rifting in the Late Triassic, evolving to the birth at the late Early Jurassic with the formation of ocean crust. However, this resulted in failure after it grew into the climax at the end of the Middle Jurassic when the Qiangtang Block began subducting under the Lhasa-Gandise Block. In the Early Cretaceous, the two blocks finally merged. [ABSTRACT FROM AUTHOR]

Ultra‐low velocity zones (ULVZs) are anomalous structures, generally associated with decreased seismic velocity and sometimes an increase in density, that have been detected in some locations atop the Earth's core‐mantle boundary (CMB). A wide range of ULVZ characteristics have been reported by previous studies, leading to many questions regarding their origins. The lowermost mantle beneath Antarctica and surrounding areas is not located near currently active regions of mantle upwelling or downwelling, making it a unique environment in which to study the sources of ULVZs; however, seismic sampling of this portion of the CMB has been sparse. Here, we examine core‐reflected PcP waveforms recorded by seismic stations across Antarctica using a double‐array stacking technique to further elucidate ULVZ structure beneath the southern hemisphere. Our results show widespread, variable ULVZs, some of which can be robustly modeled with 1‐D synthetics; however, others are more complex, which may reflect 2‐D or 3‐D ULVZ structure and/or ULVZs with internal velocity variability. Our findings are consistent with the concept that ULVZs can be largely explained by variable accumulations of subducted oceanic crust along the CMB. Partial melting of subducted crust and other, hydrous subducted materials may also contribute to ULVZ variability. Plain Language Summary: Earth's core‐mantle boundary (CMB), the interface between the solid silicate mantle and the molten iron‐rich outer core, is associated with a range of anomalous structures, including ultra‐low velocity zones (ULVZs). While generally associated with reduced seismic wave velocities and sometimes increased density, prior studies have reported a wide range of ULVZ characteristics, leading to many questions regarding their origins. The lowermost mantle beneath the southern hemisphere provides a unique environment to study ULVZs because it is located away from regions of large‐scale mantle upwelling and downwelling. Our study uses core reflected P‐waves (PcP) recorded by seismic stations in Antarctica to investigate this portion of the CMB for ULVZ presence. We find widespread evidence for variable ULVZ structure. Some of the imaged ULVZs can be modeled with a single layer, but others are more complex. We suggest that the ULVZs beneath the southern hemisphere are predominantly associated with subducted oceanic crust that has variable accumulations along the CMB. In some regions, hydrated subducted materials may also experience partial melting, which may contribute to the complicated ULVZ structures imaged in some locations. Key Points: Core‐reflected P‐waves are used to investigate ultra‐low velocity zones in the lowermost mantle beneath the southern hemisphereResults show widespread evidence for variable ultra‐low velocity zones, some of which may indicate layered or gradational structureVariable accumulations of subducted oceanic crust with localized partial melting can explain these anomalous structures [ABSTRACT FROM AUTHOR]

An eclogite from the Early Palaeozoic Fleur‐de‐Lys Supergroup in Newfoundland was studied because of its biotite porphyroblasts, which very rarely occur in this rock type. Thermodynamic modelling suggests that eclogitic biotite in common metabasite (former basalt–gabbro) is limited to (1) bulk‐rock compositions, which are relatively rich in Fe2+ and K and poor in Fe3+, and (2) the low‐pressure range of the eclogite facies. The latter reason is supported by the determination of the pressure–temperature (P–T) path of the Newfoundland eclogite. Chemical zonation of garnet, presence of phengite with Si contents of ~3.4 per formula unit, Zr contents in rutile and petrographic observations resulted in a P–T trajectory starting at medium‐pressure conditions. Nearly isothermal burial led to a peak pressure of 18–19 kbar at ~575°C, followed by exhumation and slight heating. Deformation occurred at or close to the peak pressure. Subsequent introduction of hydrous fluids caused the formation of porphyroblasts of biotite and Ca–amphibole in the pressure range of 12–17 kbar at peak temperatures of 625–640°C. Retrogression led to very fine‐grained symplectites around omphacite and phengite and marginal replacement of biotite porphyroblasts by plagioclase and titanite. Geodynamic scenarios invoking either a flat subduction of oceanic crust followed by continent–continent collision or intracontinental subduction along a transpressional fault system might best explain the formation of eclogite with biotite porphyroblasts in general. For the Newfoundland eclogite, the latter scenario is preferred. [ABSTRACT FROM AUTHOR]

The Aqishan-Caixiashan polymetallic ore cluster, located in the eastern Tianshan Orogenic Belt, is part of the southern Central Asian Orogenic Belt (CAOB) and experienced extensive tectono-magmatic events with polymetallic mineralization during the Carboniferous. However, the Carboniferous tectonic affinity and magmatic evolution of the ore cluster are still controversial, which limits further understanding of the tectonic evolution and regional metallogeny. With the aim of solving these problems, we have carried out geochronological and geochemical studies on Carboniferous intrusive rocks and volcanic rocks from the ore cluster. The samples of the Carboniferous magmatic rocks are enriched in LREEs and LILEs but depleted in HFSEs, which attest to the I-type affinity and are similar to those formed in a continental arc. The early Carboniferous granodiorites and granites (350–343 Ma) in the Central Tianshan Block (CTB), with high Mg# values of 37–66, low Y values of 2.3–17.2 ppm, and high Sr/Y ratios (34–85), show characteristics of adakites that are formed by partial melting of delaminated oceanic crust and/or lower crust. Additionally, they have a wide range of εHf(t) values (5.4–15.9), high εNd (t) values (1.78–6.31) and low ISr values (0.705–0.706), indicating a mixed source of depleted mantle and lower crust for its origin. The Late Carboniferous volcanic rocks (314–306 Ma) in the Aqishan-Yamansu Tectonic Belt (AYTB) north of the CTB have obviously negative Nb–Ta-Ti anomalies, as well as high Y contents (12–20 ppm) and low Sr/Y ratios (5.6–29.9) compared to primitive mantle, which has a relatively flat chondrite-normalized REE pattern, resembling arc-related magmas. Additionaly, they have positive values of εHf(t) values (8.7–14.7), high εNd(t) values (3.04–4.45) and low ISr ratios (0.704–0.707), showing a depleted mantle source, which is weakly affected by the contamination of ancient crustal material. Combining this study and previous works, we suggest that the continuous southwards subduction of the North Tianshan Ocean during the Carboniferous generated the continental arc and forearc basin in the northern part of the CTB. The expansion of the Aqishan-Yamansu forearc basin formed intrusions and volcanic formations along the arc belt. The early Carboniferous intermediate-felsic mantle-derived adakitic magmas in the CTB are probably related to regional Zn-Pb enrichment and mineralization, and the Late Carboniferous intrusions and volcanic formations in the AYTB are related to large-scale Fe-Cu-Zn-Pb-Ag mineralization. [ABSTRACT FROM AUTHOR]

Crustal structure beneath the major tectonic domains comprising northeast India is investigated using receiver function analyses of teleseismic earthquakes recorded at 19 new broadband seismic stations, combined with Moho depth and Poisson's ratio information from 14 previous studies. Velocity structure inverted from receiver functions are combined with results from H–k stacking and validated using forward modeling of gravity data. The Shillong Plateau in the western Brahmaputra River Valley, later representing the Himalayan foredeep, is inferred to have a thin (33–35 km) felsic crust. A dome-shaped, 5-km-thick high-velocity layer in the upper crust likely accounts for positive Bouguer anomaly over the plateau. The Upper Assam basin in the eastern Brahmaputra River Valley is marked by pronounced gravity low, high Poisson's ratio, and a pair of low- and high-velocity layers in the crust. This narrow segment demarcates the crust of oceanic affinity extending from the Indo-Burmese Wedge. Further to the south of Dauki fault, under the ~ 10-km-thick wedge, a two-layered crust comprising 5–15-km-thick delta sediments overlying a ~ 20-km-thick basement of oceanic affinity extends from Bay of Bengal to the Churachandrapur-Mao Fault, where the deep basement subducts upon collision with the Burmese micro-plate. East of the collision boundary, the overriding Burmese micro-plate having a thin crust of ~ 30 km, including ~ 15-km-thick low-velocity sediments, is imaged beneath Central Myanmar Basin. The findings shed new light on the tectonic reactivation of the different domains of northeast India upon collision with the Himalaya to the north and Burmese micro-plate to the east. [ABSTRACT FROM AUTHOR]

This study focuses on the 3-D velocity structure and thickness of ∼7-Myr-old oceanic crust surrounding borehole 504B, located ∼235 km from the intermediate-spreading Costa Rica Rift (Panama Basin). It investigates how well seismic structure determined by 3-D tomography compares with actual lithology and, consequently, what the origin and cause might be of an amplitude anomaly, the 2A Event, that is observed in multichannel seismic data. Our P -wave model shows an ∼0.3-km-thick sediment layer of velocity between ∼1.6 and 1.9 km s−1 (gradient 1.0 s−1), bound at its base by a velocity step to 4.8 km s−1 at the top of oceanic crustal Layer 2. Layer 2 itself is subdivided into two main units (2A and 2B) by a vertical velocity gradient change at 4.5 km depth, with a gradient of 1.7 s−1 above (4.8–5.8 km s−1) and 0.7 s−1 below (5.8–6.5 km s−1). The base of Layer 2, in turn, is defined by a change in gradient at 5.6 km depth. Below this, Layer 3 has a velocity range of 6.5–7.5 km s−1 and a gradient of ∼0.3 s−1. Corresponding S -wave igneous layer velocities and gradients are: Layer 2A, 2.4–3.1 km s−1 and 1.0 s−1; Layer 2B, 3.1–3.7 km s−1 and 0.5 s−1; Layer 3, 3.7–4.0 km s−1 and 0.1 s−1. The 3-D tomographic models, coupled with gravity modelling, indicate that the crust is ∼6 km thick throughout the region, with a generally flat-lying Moho. Although the P - and S -wave models are smooth, their velocities and gradients are remarkably consistent with the main lithological layering subdivisions logged within 504B. Thus, using the change in velocity gradient as a proxy, Layer 2 is interpreted as ∼1.8 km thick and Layer 3 as ∼3.8 km thick, with little vertical variation throughout the 3-D volume. However, the strike of lateral gradient variation is not Costa Rica Rift-parallel, but instead follows the orientation of the present-day adjacent Ecuador Rift, suggesting a reorientation of the Costa Rica Rift spreading ridge axis. Having determined its consistency with lithological ground-truth, the resulting P -wave model is used as the basis of finite difference calculation of wave propagation to find the origin of the 2A Event. Our modelling shows that no distinct interface, or transition, is required to generate this event. Instead, it is caused by averaging of heterogeneous physical properties by the seismic wave as it propagates through Layer 2 and is scattered. Thus, we conclude that the 2A Event originates and propagates exclusively in the lower part of Layer 2A, above the mean depth to the top of the dykes of Layer 2B. From our synthetic data we conclude that using the 2A Event on seismic reflection profiles as a proxy to determine the Layer 2A/2B boundary's depth will result in an overestimate of up to several hundred metres, the degree of which being dependent on the specific velocity chosen for normal moveout correction prior to stacking. [ABSTRACT FROM AUTHOR]

Seismic surveys along subduction zones have identified anomalously high ratio of P‐ to S‐wave velocity (VP/VS) in the subducting oceanic crust that are possibly due to the presence of pore water. Such interpretations postulate that the pore structure is homogeneous at the scale of the seismic wavelength. Here we present the first statistical evidence of a heterogeneous pore structure in oceanic crust at scales larger than laboratory samples. The spatial correlation of measured bulk density profiles of the crustal section of the Samail ophiolite suggests that the pore structure is heterogeneous at scales smaller than ∼1 m. Wave‐induced fluid flow cannot follow the loading during the seismic wave propagation at this estimated heterogeneity, which implies that fluid‐filled microscopic pores and cracks have a limited impact on the observed high VP/VS anomalies in the subducting oceanic crust. Large‐scale cracks may therefore play an important role in shaping these anomalies. Plain Language Summary: Seismic studies along subduction zones have identified unusually high ratios of P‐ to S‐wave velocity (VP/VS) in the subducting oceanic crust, which indicates the presence of water‐filled cracks and pores. The close link between pore water and local seismic activity highlights the importance of quantitatively interpreting these seismic anomalies in terms of pore characteristics. Previous interpretations have assumed that the microscopic pore structure is quite homogeneous, even at macroscopic scales as large as the seismic wavelength. However, our analysis of a bulk density profile of the crustal section of the Samail ophiolite, Oman, which is a fossilized oceanic plate preserved on land, indicates that the pore structure is more heterogeneous than previously assumed. This means that the fluid flow within the unit volume that represents the macroscopic physical properties of the rock cannot follow the wave‐induced loading during seismic wave propagations. This results in a relatively small impact of water on the seismic velocity, as inferred from theoretical models that predict the effective elastic properties of rock containing fluid‐filled cracks. Therefore, microscopic cracks may not have a large impact on the high VP/VS values of subducting oceanic crust, whereas large‐scale cracks may play a more significant role. Key Points: The bulk density of the crustal section of the Samail ophiolite is more spatially heterogeneous than previously assumedThe effect of fluid‐saturated microcracks on low‐frequency seismic velocities is modeled as an unrelaxed condition for this heterogeneityThe high VP/VS anomaly in the subducting oceanic crust can be explained by both microcracks and large‐scale cracks [ABSTRACT FROM AUTHOR]

The petrogenesis of basaltic arc lavas with heavy Mo isotopic compositions is controversial. Given that oceanic crust preferentially loses isotopically heavy Mo during subduction at forearc depth and the remaining Mo is isotopically light and locked in rutile, fluids derived from such crust at subarc depth cannot account for the heavy Mo isotopic compositions of basaltic arc lavas. Serpentinites can contain substantial Mo and volatiles and are a potential source of isotopically heavy Mo in basaltic arc lavas. However, the Mo isotopic composition of serpentinites and the Mo isotope behavior during serpentinite dehydration are poorly constrained. Here, we present Mo contents and isotope data for serpentinites from the South Sandwich Trench-Fracture Zone intersection, and deserpentinized peridotites and high-pressure meta-serpentinite veins from the western-central Alps and Betic Cordillera, Spain. Most of these samples have heavy Mo isotopic compositions (δ98/95Mo NIST 3134 , ranging from −0.26 ‰ to 1.84 ‰) with low Ce/Mo (0.10–23.98) and Ce/Pb (0.17–3.59) ratios, which are distinct from the compositions of eclogite-facies metabasalts and metapelites. In addition, no systematic variations in Mo contents and Mo isotopic compositions occur for the studied samples, indicating limited Mo isotope fractionation during serpentinite dehydration. The above results suggest that serpentinites and their derived fluids at subarc depths can serve as the main isotopically heavy Mo sources for basaltic arc lavas. The Mo isotope data with Ce/Mo and Ce/Pb systematics suggest that serpentinite-derived fluids or melting of serpentinite diapirs at subarc depths can explain the heavy Mo isotopic compositions of basaltic arc lavas. Furthermore, recycled oceanic slab (oceanic crust + serpentinized mantle) would has limited influence on the Mo systematics of the mantle, which explains why the mantle has maintained a nearly constant Mo isotopic composition since 3.5 Ga. [ABSTRACT FROM AUTHOR]

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 (α sulfide-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. [ABSTRACT FROM AUTHOR]

Orogenic gold systems are arguably the most variable mineral system globally in terms of an extreme range of depositional depths, corresponding P–T conditions and wallrock alteration assemblages, structural controls and styles, and element associations. This diversity has ignited controversy on genetic models for the two decades since orogenic gold became a widely accepted term. From the diverse genetic models proposed, the two groups of fluid-source models that meet most genetic constraints are the following: (1) deposition from crustal fluids via metamorphic devolatilization at the amphibolite-greenschist transition, or potentially even deeper under specific tectonic conditions, and (2) deposition from sub-crustal fluids either by direct devolatilization of subducted oceanic crust and overlying sediment wedge or of previously metasomatized and fertilized mantle lithosphere. Both models normally postulate gold deposition within a geodynamic system that evolves from extension through compression into syn-gold transpression. Crustal metamorphic models normally invoke subduction-driven geodynamic systems that involve advection of crustal metamorphic fluids up crustal-scale faults. In contrast, sub-crustal devolatilization models involve subduction-related processes as both geodynamic drivers and gold sources with fault-controlled fluid conduits extending to below the Moho. The overall lack of orogenic gold and other subduction-related mineral systems during the unique Boring Billion (1.8–0.8 Ga) period provides an important constraint on this genetic debate. Boring Billion orogens had varying geodynamic drivers, asthenosphere upwelling, and low-P metamorphic terranes with crustal-scale faults, all parameters consistent with formation of orogenic gold systems, during subduction-independent accordion-type tectonics. The absence of orogenic gold during the Boring Billion provides critical evidence against the crustal metamorphic model and furthers the sub-crustal model which requires subduction as both the geodynamic driver and auriferous fluid source. [ABSTRACT FROM AUTHOR]

The origin of the carbonatites that appear on Earth is one of the most controversial current topics in the petrogenesis of igneous rocks. Situated in the northern sector of the Basal Complex of Fuerteventura (Canary Islands), the Miocene Esquinzo ultra-alkaline plutonic rock complex is composed of pyroxenites, melteigites-ijolites-urtites, malignites-nepheline syenites, carbonatites, silicocarbonatites, nephelinites and nepheline phonolites. This work tries to establish the genesis of this massif of ultra-alkaline rocks with associated carbonatites from Fuerteventura (which are very rare in the oceans). The geochemical characteristics of these rocks and the minerals that are included in them have allowed us to establish their origin. This complex was generated by three successive magmatic events associated with differentiation of melanephelinite magmas emplaced in the oceanic crust. Silicocarbonatite and calciocarbonatite (sövites) dykes are related to the first magmatic event and were formed by fractional crystallization of H2O- and CO2-rich ijolite magmas. The melanephelinite magmas that formed these plutonic ultra-alkaline rocks were apparently generated as partial melts of asthenospheric mantle, which assimilated enriched lithospheric mantle material as they ascended. The upwelling of this large body of anomalous asthenospheric Miocene material exceeded the deformation associated with plate motions and led to an oceanic rifting event in Fuerteventura. [ABSTRACT FROM AUTHOR]

It is well established that Indian- and Pacific-type mantle domains observed in the present-day upper mantle are both long-lived and can be traced back to at least the Early Paleozoic, with their contrasting isotopic compositions inherited from mantle beneath the Tethys and Panthalassic oceans, respectively. But, the long-term evolution of the Tethyan-Indian oceanic mantle remains loosely constrained due to sparse sampling of the Tethyan oceanic lithosphere, which makes it unclear if there has been temporal variation of its diagnostic DUPAL anomaly throughout the Phanerozoic, and what caused any changes. To address these issues, we report new geochemical data, including bulk-rock major and trace elements and Sr-Nd-Hf-Pb isotopes, for samples from Tethyan ophiolites located in the Tibetan Plateau. These results, combined with existing data for ophiolites across the Tethyan domain and Indian oceanic crust, demonstrate that the DUPAL anomaly in the Tethyan-Indian oceanic mantle is long-lasting, but its strength is weakening through time. The DUPAL anomaly in the Tethyan-Indian upper mantle was most pronounced in the Paleozoic with a homogeneous Indian-type asthenosphere underlying the Proto/Paleo-Tethys oceans, but has become less obvious since the Mesozoic with an isotopically heterogeneous asthenosphere beneath the Neo-Tethys and Indian oceans containing Pacific-type mantle fragments. We propose that the declining DUPAL anomaly in the Tethyan-Indian oceanic mantle since the Mesozoic was most likely associated with inflow of Pacific-type mantle during breakup of Pangea and Gondwana, when interaction between Indian- and Pacific-type mantle was more efficient. On the other hand, Indian-type asthenosphere might have been squeezed out into the Panthalassic-Pacific oceanic mantle during closure of the Tethys Ocean. This transfer may have contributed, at least in part, to the DUPAL isotopic anomaly detected in the Panthalassic-Pacific oceanic mantle. Our findings indicate that tectonically-induced asthenospheric flow permits interactions between Indian- and Pacific-type mantle domains and contributes to evolving mantle compositions. [ABSTRACT FROM AUTHOR]

The western margin of SW Gondwana was a place of active convergence between the Gondwana shield and the oceanic lithosphere of the Panthalassa Ocean during most of the Paleozoic. However, several studies have indicated that especially to the north of latitude 22°S, the Devonian-early Carboniferous was a time of relative quiescence without magmatic activity, metamorphism, or deformation. This interval has been termed the "Devonian Problem." As the Devonian-Carboniferous is extensively well represented by over 4000-m-thick sedimentary sequences in the Tarija Basin in the Southern Bolivian Subandean Zone, provenance analyses--U-Pb isotope analysis on detrital zircon, Sr-Nd whole-rock isotope analysis, and X-ray diffraction (XRD)--were conducted on stratigraphically controlled sedimentary units to attempt to constrain the tectonic setting of the basin during that time. U-Pb on zircon provenance analysis indicates that only the Carboniferous units show input from comparatively young sources (<14%), with ages between 420 Ma and 320 Ma, which can be correlated with a Devonian magmatic arc. The dominant source areas for Devonian-Carboniferous sediment were the Sierras Pampeanas to the southwest and the Arequipa-Antofalla Massif and Famatinian Arc to the west, besides a few zircon grains (<18% of dates) that were possibly derived from pre-Andean inliers with ages typically >1800 Ma. The combined Sr-Nd isotope and XRD results for the pelites and the multidimensional scaling (MDS) analysis indicate that the Carboniferous units most likely represent reworked material from older units, with a minor contribution from the western part of the basin. All currently available data lead us to propose that the Tarija Basin developed during the Devonian-Carboniferous in a foreland basin setting related to the convergent margin, with important glacial sedimentary input from the continent during the Carboniferous. The absence of a contribution from a concomitant magmatic arc for the Devonian units of the Tarija Basin and the scarce magmatic zircon input (<14%) into the Carboniferous units indicate a shift from flat-slab subduction during the Devonian to normal subduction during the Carboniferous following complete delamination of the flat slab. Our comparison of the detrital record for the Tarija Basin with that of the Paraná, Chaco-Paraná, Sauce Grande, Paganzo, Navidad Arizaro, Madre de Dios, and Karoo basins supports an active margin setting for the SW margin of Gondwana during this time interval. [ABSTRACT FROM AUTHOR]

Genesis of the large-volume alkaline crust at active continental margin is still enigmatic for geologists worldwide. The point at issue is whether or not subducted oceanic crusts get involved and how they interact with the mantle source of the alkaline crust. Late Mesozoic juvenile alkaline crusts with high εNd(t)-εHf(t) values are widely distributed in the Great Xing'an Range of NE China, as parts of an arc magmatic belt related to the Mongol-Okhotsk Ocean closure. We carried out multiisotope analyses and 2-D high-resolution numerical modeling to trace the mantle source nature of the alkaline crust. The alkaline rocks show similar trace elements with the I-type enriched mantle and are originated from an upwelling oceanic-island basalt-like mantle. Their high field strength element depleted arc features indicate the crustal material addition in the source region. Low δ18O, mantle-like Sr-Nd-Hf and light Mg isotope compositions, limited δ7Li variations, no Nd-Hf decoupling, and our mixing calculation preclude continental crustal assimilation, marine-sediment melt and/or altered oceanic crust (AOC)-fluid metasomatism, and bulk marine sediment involvement, and provide evidence of the bulk AOC addition in the mantle source. Lower δ18O values than the mantle and relatively low δ7Li values further confirmed the involved AOC to be a high-temperature (high-T) AOC. Our multiisotope tracing successfully fingerprints the recycled high-T AOC into the source region of the alkaline juvenile crust. Then, our 2-D high-resolution numerical modeling reconstructs the high-T AOC recycling processes driven by mélange melting. [ABSTRACT FROM AUTHOR]

The nature and tectonic evolution of the Bangong-Nujiang Tethyan Ocean remain unresolved, which seriously restricts our understanding of the role of this ocean in the Tethys tectonic domain and hinders in-depth study of the geodynamics of the Tethyan tectonic evolution. The study of intraplate oceanic-island fragments in the Bangong-Nujiang Suture Zone of Northern Tibet, which are remnants of the oceanic crust, is important for determining the nature and tectonic evolution of the Bangong-Nujiang Tethyan Ocean. Here, we present a combined analysis of petrological, geochronological, geochemical, and Sr-Nd isotope data for the five intraplate oceanic-island fragments preserved in the Bangong-Nujiang Suture Zone. The oldest intraplate oceanic-island fragment in the Bangong-Nujiang Suture Zone is of Late Permian-Triassic age, and the underlying oceanic lithosphere is Late Permian or older, which indicates the development of the Bangong-Nujiang Tethyan Oceanic basin at this time. The opening time of the Bangong-Nujiang Tethyan Ocean must be even earlier than the Late Permian. At least four Triassic intraplate oceanic-island fragments have been recognized in the Bangong-Nujiang Suture Zone, and their assemblages and geochemistry are mostly similar to present-day oceanicisland basalts that erupted onto the thick lithosphere of mature oceanic basins. All of these observations indicate that the Bangong-Nujiang Tethyan Ocean evolved into a mature oceanic basin during the Triassic. The youngest intraplate oceanic-island fragments are of Late Jurassic-Early Cretaceous ages, which effectively argues for the continued presence of the Bangong-Nujiang Tethyan Ocean at this time. The abundant data of the intraplate oceanic-island fragments provide strong evidence that the Bangong-Nujiang Tethyan Ocean was a broad oceanic basin that separated Cimmeria from Gondwana during the Permian-Early Cretaceous and experienced complex evolution over at least 160 m.y. [ABSTRACT FROM AUTHOR]

Altered oceanic crust (AOC) plays a critical role in geochemical recycling in subduction zones. However, identifying contributions of subducted AOC to arc magmas remains a conundrum due to the lack of effective tracers. Here, we investigate the Ba‐Sr‐Nd isotopic compositions of lavas from the Mariana arc and back‐arc. Based on a statistical analysis of the Sr‐Nd isotopes for global arc volcanoes, we confirm that AOC‐derived fluid (or hydrous melt), rather than sediment‐derived melt or fluid, is responsible for the Sr‐Nd isotope decoupling (i.e., 87Sr/86Sr is "excessively" enriched relative to 143Nd/144Nd when compared to the "normal" mantle derivates) observed in island arc lavas. We show that the arc lavas with increasingly decoupled Sr‐Nd isotopes generally have heavier Ba isotope ratios, which is also a characteristic feature of AOC‐derived fluids. Thus, these results establish an intimate link between subducted AOC, heavy Ba isotope compositions, and Sr‐Nd isotope decoupling signature in island arcs, which provides a powerful tool to trace the AOC recycling in subduction zones. Furthermore, a similar correlation is observed between Sr‐Nd isotope decoupling and heavy B isotope ratios for global arc lavas, implying that the recycling of AOC component is generally linked to serpentinite dehydration in subduction zones. Plain Language Summary: Altered oceanic crust (AOC) is an important subduction component that contributes to the generation of arc magmas. However, identifying this contribution to arc magmas is challenging for the lack of effective tracers. Nonetheless, AOC has two characteristic features: (a) Sr‐Nd isotope decoupling (i.e., enriched in 87Sr/86Sr relative to 143Nd/144Nd in contrast to the "normal" mantle array); and (b) relatively heavy Ba isotope compositions. Here, we analyze the Ba‐Sr‐Nd isotopic compositions of lavas from the Mariana arc and back‐arc, which are combined with a statistical analysis of the Sr‐Nd isotopes for global arc volcanoes. Our results confirm that fluid (or hydrous melt) derived from subducted AOC, rather than sediment, is responsible for the Sr‐Nd isotope decoupling observed in island arc lavas. Strikingly, the magnitude of the Sr‐Nd isotope decoupling correlates well with the Ba isotope compositions of the arc lavas. Thus, our results establish an intimate link between subducted AOC, heavy Ba isotope ratios, and Sr‐Nd isotope decoupling signature in island arcs. This combination of Ba‐Sr‐Nd isotopes can therefore now be used as a powerful tracer to identify the contributions of subducted AOC to arc magmas. Our results also suggest that AOC and serpentinite show synergistic behavior during subduction recycling. Key Points: Global arc data confirm that the Sr‐Nd isotope decoupling of arc lavas is caused by AOC‐derived fluidDecoupled Sr‐Nd isotopes plus heavy Ba isotope is a good tracer for recycled altered oceanic crust (AOC) in arc magmasAOC and serpentinite show synergistic behaviors during subduction recycling [ABSTRACT FROM AUTHOR]

We discuss the tectonic structure, seismic stratigraphy and evolution of the NW Sulu Sea using reprocessed 2D reflection profiles. The NW Sulu Sea is located between the Palawan continental shelf and the Cagayan Ridge and represents the northern part of the Sulu Sea, a marginal sea resulting from Paleogene extension and subsequent Neogene contraction due to convergence between the Palawan and the Philippine blocks. The basin consists of six seismo‐stratigraphic units overlying crystalline basement. Syn‐orogenic depocenters contain calibrated Middle Miocene to, possibly, Lower Miocene units, while rift‐related depocenters consist of uncalibrated but tentatively dated Paleogene to Lower Miocene units. Thickness and depth maps of the main units and bounding horizons differentiate the Piedra‐Blanca and the Rasa domains, separated by the NW‐Sulu‐Break major tectonic structure. Fault‐bounded rift‐related depocenters are strongly segmented. We interpret that NW‐SE and NE‐SW trending zones accommodate shape and trend variations of these depocenters. We suggest that these zones may link rift segments, recording different extensional deformation. Miocene thrusting and folding in the Piedra‐Blanca Domain and mudflow with associated gravitational structures in the Rasa Domain influenced the deposition of syn‐orogenic units. Rift structures inherited from rift segmentation may have conditioned the style and distribution of contractional deformation during the subsequent incipient reactivation during contraction. In the context of SE Asia, our results support that the timing of rifting of the NW Sulu Sea overlaps with the opening of the South China Sea and the North Palawan margin, which may indicate a common geodynamic driving force triggering extension. Plain Language Summary: The SE Asia region includes several basins partially floored by oceanic crust, frequently containing or bounded by volcanic island arcs. These basins are known marginal seas and, although it is critical to decipher the geodynamic evolution of the entire region, the structure and tectonic history of most of these marginal seas are still insufficiently known. The Sulu marginal sea lies between the Sulu Archipelago and the Palawan, Mindanao, Negros and Panay islands. The Cagayan Ridge separates the NW and SE Sulu seas, which show a contrasting seafloor morphology, resulting from Paleogene extension and subsequent Neogene contraction. In this work, we study the structure and tectonic evolution of the NW Sulu Sea based on the re‐processing and interpretation of seismic reflection profiles acquired in 1982 and 1987. We analyzed and calibrated the seismo‐stratigraphy of the sediment infill, determined the main tectonic and gravitational structures and the geometry and spatial distribution of depocenters. Our results show for the first time the basin‐wide overall structure and constrain tectonic models, describing the opening and, particularly, the contractional deformation of the NW Sulu Sea. This provides further information to understand the formation of the Sulu Sea and the evolution of convergent systems in SE Asia. Key Points: A multi‐stage tectonic evolution shaped the Piedra‐Blanca and Rasa structural domains separated by the gentle NW‐Sulu‐Break structureNW‐SE and NE‐SW trending accommodation zones define the geometry and location of the rift‐related oldest basin depocentersThrusts and associated folds deformed the Piedra‐Blanca and mudflows affected the Rasa domain during the contractional basin reactivation [ABSTRACT FROM AUTHOR]

The Comallo volcanic sedimentary Complex in the western North Patagonian Region begins with the development and collapse of Plinian-type eruptive columns of rhyolitic and dacitic composition associated to vent at approximately 192 Ma. Synchronous deposits are originating in alluvial and lagoon environments. The ƐHf(t) values obtained in zircon grains indicate possible derivation by melting Paleoproterozoic and Mesoproterozoic crust. Upwards in the sequence appears basaltic and andesitic lava flows, as well as pyroclastic deposits produced during the eruptions. This volcanism is related to sustained pyroclastic fountains with a U-Pb zircon age of 185 Ma. The volcanic rocks were possibly derived from Paleoproterozoic, Mesoproterozoic, and Neoproterozoic crust affected by juvenile magmas in volcanic-arc environments. Chemical and isotopic variations are mainly due to changes in the local and regional tectonic conditions, the thickness of the continental crust, as well as the dip and age of the subducted oceanic crust. [ABSTRACT FROM AUTHOR]

A small tonalite – dacite body has been discovered in the Nakanogawa Group, Hidaka Belt, northern Japan, which is a Palaeogene subduction complex formed in the palaeo-Japan trench along the northeastern margin of Eurasia. The tonalites are characterized by extremely low K2O (0.2–0.3 wt.%) and relatively flat chondrite-normalized rare earth element patterns (La/Yb[N] ~2.3), similar to plagiogranites in ophiolites. The major-and trace-element characteristics are consistent with low-pressure partial melting of oceanic crust outside the garnet stability field. Zircon U – Pb dating of the tonalite yielded a weighted-mean 206Pb/238U age of 159.1 ± 1.6 Ma. Late Jurassic oceanic plateau-type basaltic rocks with a small amount of plagiogranitic rocks are distributed sporadically in subduction complexes along the palaeo-Japan arc – trench system. A similar zircon U – Pb age (151.6 ± 1.8 Ma) has been reported for greenstone in the Cretaceous subduction complex along the palaeo-Kuril arc – trench system. The tonalite – dacite block was probably derived from the subduction complex at the junction of the palaeo-Japan and -Kuril arc – trench systems. Thus, the greenstones and the tonalite body were likely part of a large oceanic plateau, which formed at the Izanagi – Pacific–Farallon ridge triple junction during the Late Jurassic – Early Cretaceous. [ABSTRACT FROM AUTHOR]

Arc magmas are produced from the mantle wedge, with possible addition of fluids and melts derived from serpentinites and sediments in the subducting slab. Identification of various sources and their relevant contributions to such magmas is challenging; in particular, at continental arcs where crustal assimilation may overprint initial geochemical signatures. This study presents oxygen isotopic compositions of zoned olivine grains from post-caldera basalts and boron contents and isotopes of these basalts and glassy melt inclusions hosted in quartz and clinopyroxene of silicic tuffs in the Toba volcanic system, Indonesia. High-magnesian (≥87 mol% Fo [forsterite]) cores of olivine in the basalts have δ18O values ranging from 5.12‰ to 6.14‰, indicating that the mantle source underneath Toba is variably enriched in 18O. Olivine with <87 mol% Fo has highly variable (4.8-7.2‰), but overall increased, δ18O values, interpreted to reflect assimilation of high δ18O crustal materials during fractional crystallization. Mass balance calculations constrain the overall volume of crustal assimilation for the basalts as ≤13%. The processes responsible for the 18O-enriched basaltic melts are further constrained by boron data that indicate the addition of <0.1 wt% fluids to the mantle, >40% of the fluids being derived from serpentinites and others from altered oceanic crust and sediments. This amount of fluids can increase δ18O of the magma by only ∼0.02‰. Approximately 6-9% sediment-derived melt hybridization in the mantle wedge is further needed to yield basaltic melts with δ18O values in equilibrium with those of the high-Fo olivine cores. The cogenetic silicic tuffs, on the other hand, seem to record a higher proportion of fluid addition dominated by sediment-derived fluids to the mantle source, in addition to crustal assimilation. Our reconnaissance study therefore demonstrates the application of combined B and O isotopes to differentiate between melts and fluids derived from serpentinites and sediments in the subducted slab-an application that can be applied to arc magmas worldwide. [ABSTRACT FROM AUTHOR]

The presence of oceanic crust on the Black Sea bottom indicates that this sea basin was a section - a Pontic Ocean gulf at the northern margin of the Phanerozoic Tethys Ocean. The late Phanerozoic subduction of the Tethys oceanic crust under the European continental massif in this area took place in the subduction zone, whose modern traces are found today south of the Crimean Peninsula. On the surface of the subducted oceanic crust were numerous fragments of the destroyed continental crust on the northern margin of the Gondwana continent. These relatively lighter than oceanic crust crustal fragments (or a significant part of them) were not subducted. They accumulate in the immediate vicinity of the subduction zone on the unsub ducted remnants of oceanic crust. Such is the case with the Black Sea. It is not an imposed subcontinental basin, but a last modern relict of the northern margin of the Late Phanerozoic Tethys Ocean. [ABSTRACT FROM AUTHOR]

Although carbonates are the primary form of carbon subducted into the mantle, their fate during recycling is debated. Here we report the first coupled high‐precision Zn and Mo isotope data for Cenozoic intraplate basalts from western China. The exceptionally high δ66Zn values (+0.39 to +0.50‰) of these lavas require involvement of recycled carbonates in the mantle source. Variable δ98Mo compositions (−0.39 to +0.27‰) are positively correlated with Mo/Ce, best interpreted as mixing between isotopically light Mo from dehydrated oceanic crust and heavy Mo from recycled carbonates, which is also supported by positive coupling between δ66Zn and δ98Mo. Modeling reveals that involvement of ≤5% carbonate‐bearing oceanic crust fully resolves the observed δ66Zn–δ98Mo mantle heterogeneity probed by intracontinental basalts. Our study demonstrates that combined δ66Zn–δ98Mo data sets for mantle‐derived magmas can track recycled surficial carbonates in Earth's interior, providing a powerful geochemical tool for deep carbon science. Plain Language Summary: Carbon is an element of life and studying its geological cycle is crucial for understanding Earth's evolution including formation of a life‐supporting atmosphere. Here we report the first combined high‐precision Zn and Mo isotope data for Cenozoic intraplate lavas from western China, showing that the basalts record ≤5% carbonate‐bearing oceanic crust components in their mantle source. Our results provide new evidence for surficial carbonates being delivered into the deep upper mantle, which adds to the debate about the deepest extent of the terrestrial carbon cycle. Key Points: First combined zinc (Zn) and molybdenum (Mo) isotope data for mantle‐derived magmas to track the fate of subducted carbonatesZn−Mo isotopic compositions of Cenozoic Tarim basalts suggest surficial carbonates being delivered into the deep upper mantleWe highlight the utility of combined Zn−Mo isotope data as a powerful tool in deep carbon science [ABSTRACT FROM AUTHOR]

Petit-spot volcanoes, occurring due to plate flexure, have been reported globally. As the petit-spot melts ascend from the asthenosphere, they provide crucial information of the lithosphere–asthenosphere boundary. Herein, we examined the lava outcrops of six monogenetic volcanoes formed by petit-spot volcanism in the western Pacific. We then analyzed the 40Ar/39Ar ages, major and trace element compositions, and Sr , Nd , and Pb isotopic ratios of the petit-spot basalts. The 40Ar/39Ar ages of two monogenetic volcanoes were ca. 2.6 Ma (million years ago) and ca. 0 Ma. The isotopic compositions of the western Pacific petit-spot basalts suggest geochemically similar melting sources. They were likely derived from a mixture of high- μ (HIMU) mantle-like and enriched mantle (EM)-1-like components related to carbonatitic/carbonated materials and recycled crustal components. The characteristic trace element composition (i.e., Zr , Hf , and Ti depletions) of the western Pacific petit-spot magmas could be explained by the partial melting of ∼ 5 % crust bearing garnet lherzolite, with 10 % carbonatite flux to a given mass of the source, as implied by a mass-balance-based melting model. This result confirms the involvement of carbonatite melt and recycled crust in the source of petit-spot melts. It provides insights into the genesis of tectonic-induced volcanoes, including the Hawaiian North Arch and Samoan petit-spot-like rejuvenated volcanoes that have a similar trace element composition to petit-spot basalts. [ABSTRACT FROM AUTHOR]

Serpentinites, as important reservoirs for volatile and fluid‐mobile elements, have been increasingly recognized as playing a critical role in global geochemical cycles. However, direct evidence for their dominance in the molybdenum (Mo) cycle in subduction zones has been elusive. Here, we address this issue by comprehensively investigating the Mo isotope systematics of serpentinites and accompanying metabasites recovered from the Mariana forearc. Forearc serpentinites under low‐pressure conditions (blue‐ and lizardite‐serpentinites) show substantial enrichments in Mo/Ce ratios (0.42 to 27.34) with high δ98/95Mo values (−0.03‰ to 2.48‰), which arise from not only the incorporation of shallow forearc slab‐derived fluids but also late low‐temperature metasomatism by seawater during clast exhumation. Subducted oceanic crust represented by metabasites has low Mo/Ce ratios (0.004 to 0.049) and δ98/95Mo values (−1.13‰ to −0.61‰), indicating that the loss of Mo during oceanic crust alteration in the early or before subduction stage tends to decrease the Mo isotopic compositions. Combined Mo‐Sr‐Pb isotope systematics indicate that the high δ98/95Mo values in some arc lavas cannot be ascribed simply to the incorporation of slab‐derived fluids at subarc depths; instead, the injection of forearc serpentinite‐derived fluid into their mantle source is noteworthy. We propose that the mobilization of Mo by shallow slab‐derived fluids during the early subduction stage generated forearc lizardite‐bearing serpentinites enriched in heavy Mo isotopes. Subsequently, dehydration and melting of these serpentinites, dragged by the subducting slab down to the subarc region, might have contributed to the Mo systematics in arc lavas. This study demonstrated that forearc serpentinites may be important reservoirs for arc lavas with high Mo isotopic compositions and may contribute to the Mo cycling in subduction zones. Plain Language Summary: Serpentinites, as important reservoirs of volatile elements and mobile elements, play key roles in geochemical cycles. However, direct evidence that these rocks dominate the Mo cycle in subduction zones has been elusive. In this paper, we solve this problem by studying the molybdenum (Mo) isotope system of serpentinites and associated metabasites recovered from the Mariana forearc. The forearc serpentinites have high δ98/95Mo values due to modification by slab‐derived fluids and seawater. The δ98/95Mo values of the metabasites are relatively low, indicating that Mo isotope changes occurred during oceanic crust alteration. The forearc serpentinites with heavy Mo isotopes that experiencing dehydration and melting were dragged below the arc by the subducting slab, which have contributed to the Mo system in arc lava. This study showed that forearc serpentinite may act as an indispensable reservoir in the Mo cycle in subduction zones. Key Points: Forearc serpentinites show high Mo/Ce ratios and δ98/95Mo valuesLizardite‐serpentinites are influenced by shallow slab‐derived fluid, while blue‐serpentinites are influenced by late seawater metasomatismForearc serpentinites may contribute significantly to the Mo systematics in arc lavas [ABSTRACT FROM AUTHOR]

Magnesite (MgCO3) entering the lower mantle together with the subducted oceanic crust is an important carbon carrier. The reaction between magnesite and mantle minerals has been documented, but its influence on the density and velocity profiles of lower mantle remains unexplored. To decipher the deep carbon transportation and its associated effect, here we determined the thermal equations of state of magnesite up to 120 GPa and 2600 K using X‐ray diffraction in laser‐heated diamond anvil cells. The obtained thermal elastic parameters of magnesite facilitated a comprehensive understanding on the influence of magnesite‐SiO2 reaction, variation of carbon and SiO2 content, and temperature on the origin of lower‐mantle scatterers at 1,000–1,800 km depth. Our modeling revealed that the depth of the lower‐mantle VS scatterers is mainly controlled by the Al2O3 content in SiO2, while its magnitude depends on the SiO2 content. Along normal geotherm, the magnesite‐SiO2 reaction would occur before the post‐stishovite transition, consuming substantial SiO2 in the subducted oceanic crust. Depending on the amount of residual SiO2, the post‐stishovite transition can produce a 2.5–5.2 (2)% VS reduction, compatible with the observed seismic scatterers in Izu‐Bonin and Mariana subduction zones. Along slab geotherm, this reaction occurs after the post‐stishovite transition, generating a greater VS reduction of 4.4–6.4 (4)%. We thus propose that the reaction between sinking MgCO3 and SiO2 in the slab is one of the potential factors influencing the magnitude of the lower‐Vs scatterers at 1,000–1,900 km depth. Our results provide new insights into the deep‐mantle carbonate transportation influencing regional geophysics. Plain Language Summary: Carbonates could enter the deep Earth with the subducted oceanic crust. Magnesite is the most likely carbonate to exist stably in the lower mantle. During the subduction process, magnesite would interact with subducted oceanic crust and influence the density and velocity structure of the lower mantle. Here we determined the thermal equations of state (EoS) of magnesite up to 120 GPa and 2600 K. With the obtained EoS parameters, we discussed the influence of magnesite‐SiO2 reaction, variation of carbon, SiO2 content, and temperature on the velocity and density profiles across the post‐stishovite phase transition of SiO2 and explain the origin of lower‐mantle VS scatterers at 1,000–1,900 km depth. Our findings reveal that the depth and magnitude of lower‐mantle VS scatterers depend on Al2O3 content in SiO2 and SiO2 content, respectively. With the variation of temperature and residual SiO2 content of the reaction, the post‐stishovite phase transition of SiO2 can generate a VS reduction in the range of 2.5%–6.4%. Our findings are important in understanding deep‐mantle carbonate transportation and its implications for regional Geophysics. Key Points: The velocity change across the MgCO3‐SiO2 reaction were modeled, with the determined thermal EoS of magnesite up to 120 GPa and 2600 KThe VS reduction during the post‐stv transition could be influence by temperature, CO2 and SiO2 content, and Al content in SiO2The post‐stv transition and the reaction between SiO2 and MgCO3 could explain the lower‐VS scatterers at 1,000–1,900 km depth [ABSTRACT FROM AUTHOR]