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The article offers information on the role of fluid overpressures and hydrogeology in the occurrence of slow slip along subduction zones. Through seismic imaging and ocean drilling, the study reveals a previously unknown fluid reservoir within the volcanic upper crust of the subducting Hikurangi Plateau.

The Mascarene Islands in the western Indian Ocean, encompassing La Réunion, Mauritius, and Rodrigues, are the recent (<10 Ma) surface expressions of the Réunion hotspot. Ocean island basalts (OIB) from these islands exhibit a remarkably homogeneous long-lived radiogenic isotopic composition, coinciding with the convergence field of many global OIB trends in the mantle array. Réunion plume-related OIB therefore provide one of Earth's most pristine representations of this "focal zone" component, which may have a primordial heritage. Besides this signature, Mascarene lavas have been suggested to retain contributions from sources with distinct compositions, including: (1) Archaean-aged zircons assimilated from continental crust within the oceanic lithosphere by trachytic magmas from Mauritius; (2) more deeply recycled continental crust components preserved by elevated 87Sr/86Sr and 208Pb/206Pb in lavas from the Piton des Neiges volcano of Réunion; and (3) an isotopically depleted mantle component resulting from interaction with Central Indian Ridge material. In this study we use Sr-Nd-Pb isotope systematics, along with major and trace element compositions of basaltic lavas from all three Mascarene Islands to investigate the relationship of their sources to well-characterized mantle endmembers. Among the Mascarene Islands, Rodrigues lavas are the most enriched in highly incompatible elements, likely reflecting shallower and lower degrees of partial melting than Réunion or Mauritius. Combined Sr-Nd-Pb isotope compositions indicate that lavas from the Older Series of Mauritius resemble those from Réunion, whereas lavas from the Younger and Intermediate Series, together with Rodrigues, are consistent with contributions from an isotopically depleted component. In addition, the Pb isotopic compositions of Rodrigues samples require an additional contribution from a component with a long-term enrichment in its Th/U ratio. Based on isotope mixing models, direct assimilation of continental crust embedded within the oceanic lithosphere is unlikely to account for the Pb isotopic variation of Rodrigues. A metasomatized mantle component, previously envisaged as a "fossil" Réunion plume near the Central Indian Ridge, is partially able to reproduce the trace element signature, but not the observed Sr-Nd-Pb isotopic compositions of Rodrigues. Instead, small proportions (<5%) of an EM1-like component provide a preferred endmember. The composition and origin of this component, which is exclusively reflected within Rodrigues lavas, may constitute a new geochemical feature of the Mascarene Islands and is consistent with geodynamical predictions that small-scale enriched mantle domains may be widespread in the source regions of hotspot volcanoes. [ABSTRACT FROM AUTHOR]

The complex structure of the region has been formed as a result of ​kinematic reorganization of plate boundaries, accompanied by the extinction of old spreading centers, the formation of new ones, and plume magmatic activity in the South Atlantic southeastern part. These processes resulted in the formation of a system of ridges, rises, and plateaus with different morphologies and geochemical characteristics. The results of density modeling of the structure of the crust and tectonosphere along the profiles, which extend from the Falkland Plateau to the Mozambique Ridge and cross a series of rises and ridges divided by deep-water basins, showed that the rises have different crustal structures indicative of their different origins. The formation conditions of various submarine rises have been studied on the basis of physical modeling. The new experimental model of the formation of the lithosphere and submarine plateaus of the region is provided, in which the important role belongs to the breakup of the Agulhas large igneous province on the Agulhas Plateau and Northeast Georgia Rise, the accretion of the oceanic crust on the Agulhas spreading ridge, and a further jump of the Agulhas spreading ridge axis leading to the termination of spreading at this ridge and the formation of a southern segment of the Mid-Atlantic Ridge and conjugated Meteor and Islas Orcadas Rises. Spreading axis jumps, which are accompanied by periodic activity of hotspots, played an important role in the formation of the submarine rises of various genetic types that, in turn, determined the different structures of their crust. [ABSTRACT FROM AUTHOR]

Tertiary alkali basalts in the South-Slovakian Basin contain homogeneous clinopyroxene megacrysts, composite phenocrysts with chemically and optically homogeneous olive-green cores overgrown by oscillatory zoned rims, and fragments of clinopyroxene-rich igneous cumulates. Discrimination based on Ca, Na, Ti and Cr concentrations defined clinopyroxenes with alkalic, tholeiitic and transitional affinities. Single-pyroxene thermobarometry revealed that the incipient clinopyroxene crystallization occurred at 1300-1350 °C and 2.0-2.2 GPa, thus identifying the zone of mantle melting in a depth of >70 km. The highest frequency of pressure data from clinopyroxene-rich mafic cumulates, betwen 0.7 and 1.2 GPa, indicate stagnant basaltic reservoirs located beneath the Moho discontinuity, 30-43 km deep. Late fractionation products of alkali basalt derivatives may have occurred as laccoliths and dykes in the lower crust. Thermodynamic modelling of mineral assemblages of cumulate xenoliths revealed that parental melts for both alkalic and tholeiitic affinities were silica-undersaturated (46-46.5 wt. % SiO2) and moderately alkalic (index of alkalinity 2.0-3.8, Mg# ~50) basalt to basanite, with pre-eruptive water contents between 1 and 2 wt. %. Amphibole-biotite-titanite-ilmenite-plagioclase cumulates recorded the advanced stage of fractionation of more calcic basalt at 900-920 °C and 0.65 GPa. Deep-seated basalt reservoirs have been less alkalic than erupted lavas, the latter showing a Mg depletion and an increasing aluminium saturation index diagnostic of the extensive crystal separation on the way to the surface. Olive-green megacrysts and cores of zoned phenocrysts originated at lower temperatures and higher pressures than their autocrystic rims. The megacrystic assemblage of Fe-diopside-Al-augite, Mg-calcite, apatite, ulvöspinel, and disintegrated amphibole crystallized from evolved, relatively cold (~950-1100 °C), Fe-rich carbonatite-alkalic silicate melt within the depth interval of 26-53 km, corresponding to a lithostatic pressure of 0.7-1.5 GPa. The silicate-carbonate-phosphate melt fraction probably originated in the subducting slab of oceanic crust. [ABSTRACT FROM AUTHOR]

The mid‐ocean ridge system comprises a series of spreading ridges, transform faults, propagating ridges, and other non‐transform offsets. Transform faults remain stable for millions of years leaving long linear scars, or fracture zones, on older seafloor. Propagating ridges migrate in the ridge parallel direction leaving V‐shaped or W‐shaped scars on older seafloor. Vertical gravity gradient maps can now resolve the details of the ridge segmentation. For slow‐ and intermediate‐spreading ridges, there appears to be an offset length threshold above which adjacent ridges do not propagate so remain as stable transform faults. We propose this threshold is due to the yield strength of the lithosphere, and we develop a model framework based on a force balance wherein forces driving propagation must exceed the integrated shear strength of the offset zone. We apply this model framework to 5 major propagating ridges, 55 seesaw propagating ridges, and 69 transform faults. The model correctly predicts the migration of 4 out of 5 major propagating ridges and the stability of transform faults, but the results for seesaw propagators are less accurate. Model predictions for direction of ridge propagation are mixed as well. This model framework simplifies deformation in the shear zone, but can possibly explain why non‐transform deformation is preferred at short offsets. Plain Language Summary: Mid‐ocean ridges are constructive plate boundaries where new crust is created. In map view, the system resembles a stair‐step configuration of alternating spreading ridges and ridge offsets. Some ridges and offsets, typically large ones, remain fixed and maintain their plan‐view shape over many millions of years, while other ridges, usually those bound by shorter offsets, may slowly grow and shrink–such behavior is revealed in maps of the seafloor. The different behavior is possibly due to the material strength of the oceanic crust and upper mantle which, if great, will inhibit ridge growth. To test our hypothesis, we estimate the total material strength at identified ridge offsets and compare this to an estimate of forces contributing to ridge growth. Our estimates can explain why large offsets maintain their shape, and may explain why short offsets do not and allow some segments to grow and shrink. Key Points: Non‐transform offsets at slower‐spreading ridges rarely propagate when offsets exceed 30 km, possibly a result of lithospheric strengthWe develop a model framework that balances material strength at ridge offsets and forces driving ridge propagationGreater strength of the lithosphere as ridge offset increases may limit ridge propagation [ABSTRACT FROM AUTHOR]

Abstract—The vertical movements measured by broadband seismic stations located around the epicenter of the December 26, 2004 Sumatra 2004 mega-earthquake of magnitude 9.1 are analyzed. It is shown that during the five years before the earthquake (1996 to 2000), the COCO station closest to the epicenter, located 1700 km away, recorded quiet daily variations. Step-like distortions in the level of the recorded seismic noise appeared at this station in 2001 and continued up to the time of the earthquake. The station has also recorded pulses lasting a few minutes, with amplitudes gradually increasing to several times the amplitude of diurnal variations, followed by a decrease. The pulses occurred under quiet meteorological and geomagnetic conditions. No such pulses were found in the records of stations more than 2000 km from the epicenter. It is hypothesized that the sharp changes in the low-frequency seismic noise reflect tectonic slips on the faults in the lithosphere of the Indian Ocean. [ABSTRACT FROM AUTHOR]

A 3D model of the Earth's crust for the continental margin of the Laptev Sea and the adjacent part of the Eurasian Basin was developed using the latest seismic and gravity data. The thickness of the consolidated part of the Earth's crust in the study area is estimated at 7–11 km, which corresponds to a highly extended continental or oceanic crust. The formation of the basement and sedimentation in this area most likely began in the Late Jurassic. The southeastern part of the Eurasian Basin is separated from the rest of the basin by a dextral shear zone, the displacement along which during the Paleogene was more than 100 km. [ABSTRACT FROM AUTHOR]

Compositions of arc magmas depend on several factors and are often thought to reflect conditions in the mantle wedge and at the slab-mantle interface. However, in continental arc settings, magmas are also influenced by assimilation of continental crust. Here, we present measurements and modeling of 238U-230Th activity ratios, Sr, Nd, Hf, and Pb isotopic compositions, and major and trace element concentrations in young, historic lavas erupted from Reventador, an active stratovolcano in the Ecuadorian Andes. In arc lavas, 238U-230Th disequilibria are often assumed to reflect processes occurring in the mantle wedge, as U and Th behave differently in this relatively oxidized and fluid-rich environment. Enhanced mobility of hexavalent U in aqueous fluids results in (230Th/238U) < 1 and elevated (238U/232Th), which are common in arc lavas. However, the majority of Reventador lavas have (230Th/238U) = 1.0–1.1 and (238U/232Th) = 0.94–1.12, the latter of which is considerably lower than the depleted mantle ((238U/232Th) ∼ 1.5, Sims and Hart, 2006). While this Th enrichment could be due to melting of subducted oceanic crust, approximately linear trends between (230Th/232Th), wt. % SiO 2 , and radiogenic isotope ratios indicate otherwise. We argue that crustal assimilation lowers long-term, or time-integrated, (238U/232Th) in Reventador magmas. To quantify the effects of assimilation we modeled stepwise assimilation and fractional crystallization. Observed trends between (230Th/232Th), 87Sr/86Sr, ε Nd , ε Hf , and 208Pb/206Pb can be reproduced by up to 10% assimilation, which is consistent with previous regional studies. However, reproducing the full spectrum of isotopic diversity among Reventador lavas requires heterogeneous basalt compositions. In a broader context, this study emphasizes the need to consider crustal processes when examining continental arcs. While 238U-230Th disequilibria develop in the mantle wedge, they can be overwritten by subsequent interaction with continental crust. [ABSTRACT FROM AUTHOR]

The genesis of the subduction mélange in the central Qiangtang terrane has been a long hot debate. However, little research has been conducted on the brittle failure within the accretionary wedge, which is very important to unveil the structural evolution of the mélange. In this study, based on the recognition of multiple deformational phases, we analyse the characteristics and formation history of the vein system in the Gangma Co mélange. Six groups of quartz veins are recognized. Foliation‐parallel extension veins (G1 veins), shear veins (G2 veins) and foliation‐perpendicular extension veins (G4 veins) are supposed to have formed during the subduction of oceanic crust, recording the repeated low‐angle thrust‐sense frictional sliding, tensile fracturing and stress changes generated by subduction‐related earthquakes. Subsequent vertical extension veins (G5 veins) are suggested to be related to the exhumation of the underplated mélange, while the horizontal extension veins (G6 veins) in the last phase represent the final horizontal thrusting. The temperature conditions for shear vein formation were examined by fluid inclusion analysis, ranging from 120 to 200°C, coinciding with the temperature conditions of the slow earthquake region where episodic tremors and slow slip occur. This contribution supports that the Gangma Co mélange represents an in situ subduction zone and that its internal vein system is a response to the tectonic evolution of the Longmu Co‐Shuanghu Tethys Ocean. [ABSTRACT FROM AUTHOR]

This paper reports two types of basalt that have different ages (Carboniferous and Permian) but occur next to each other in the northern part of the Changning–Menglian accretionary wedge, southwestern China. We use the geochronology and geochemical data to evaluate the tectonic evolution of the Palaeo‐Tethys during this period. Zircon grains from the mafic‐ultramafic rocks associated with the Permian basalts in the Xiaomengtai area yield a U–Pb age of 281 Ma and εHf(t) values from +9.2 to +12.8. The Permian mafic‐ultramafic rocks and the associated basalts are all characterized by normal mid‐ocean ridge basalt (N‐MORB)‐like chondrite‐normalized rare earth elements patterns, moderately negative Nb‐Ta anomalies in the mantle‐normalized immobile incompatible trace element patterns, and positive εNd(t) values from +4.2 to +6.5, which are consistent with the geochemical characteristics of mafic‐ultramafic rocks in supra‐subduction zone (SSZ)‐type ophiolite. On the contrary, the Carboniferous basalts, which are associated with marine carbonates, are characterized by light REE enrichments, slightly positive Nb‐Ta anomalies, and εNd(t) values from +2.8 to +4.0. These features are similar to those of typical oceanic island basalts (OIBs) worldwide. Modelling results using REEs show that the parental magmas for the Carboniferous OIBs and the Permian basalts were likely derived from mantle peridotites at the depths of garnet and spinel stability, respectively, consistent with the formation depth of these two different types of basalt globally. The occurrence of these two different types of mafic‐ultramafic rocks with significantly different ages in the same area supports the view that they are the remnants of the accreted Palaeo‐Tethys oceanic crust. The Carboniferous OIBs are considered to be parts of an OIB‐carbonate seamount chain that originally formed in the southern part of the Palaeo‐Tethys. The Permian mafic‐ultramafic rocks are regarded as fragments of SSZ‐type ophiolites that were present in the northern part of the Palaeo‐Tethys. These different pieces of oceanic crust were accreted to the Simao–Indochina continental Block by subduction between the Late Permian and the Triassic. [ABSTRACT FROM AUTHOR]

A tsunami created by the momentary release of a large amount of energy accumulated in the ocean crust destroys coastal structures and generates considerable debris, posing a serious threat to coastal communities. Hence, understanding the movement characteristics of drifting attributed to tsunamis for coastal disaster prevention is necessary. In this study, a color-based Debris mOtion Tracking (DOT) model is developed to understand the behavioral characteristics of drifting caused by solitary waves. The behavioral characteristics of drifting are analyzed quantitatively based on the weight of the debris, scale of solitary waves, and revetment type, which have not been considered previously. The DOT model tracks the drifting behavior more accurately than the existing commercial programs. In a laboratory experiment, the kinetic energy, and maximum debris velocity increase with an increase in the magnitude of solitary waves. An analysis of the drifting characteristics based on revetment type reveals that the initial acceleration of drifting in the wave absorbing revetment (WAR) is higher than that in the vertical revetment (VR). Velocities of vertical and horizontal flow develop in VR and WAR, respectively, and thus the momentum flux acted more strongly. Further, overtopping the wave characteristics based on the revetment type determines the drifting behavior. [ABSTRACT FROM AUTHOR]

[Display omitted] • High precision dating of near-seafloor deep-tow magnetic and IODP constraints on SW Sub-Basin. • Magnetic anomalies of the SW sub-basin are shown to be symmetric, with variable spreading rates. • Spreading rate and roughness of SW Sub-basin influenced by mantle temperature. Marine magnetic lineations play important role in interpreting the age and spreading processes of the oceanic crust. However, it is difficult to identify magnetic lineations from sea surface observations in a deep and narrow low-latitude slow-spreading inactive ocean basin for many reasons, such as the highly filtered low-amplitude magnetic anomalies, highly oblique magnetization, contamination from narrow stripes, etc., which may hamper accurate lineation discrimination and identification. The spreading history of the Southwestern Sub-basin of the South China Sea (SW-SCS), which is located in such a low-latitude tectonic setting, has been debated for a long time. Near-seafloor deep-tow magnetic measurements provide high-resolution reversal records, making accurate lineation discrimination feasible. Corrections, including diurnal variation correction, regional field correction, and de-skewing, are explored. The results, calibrated with drill sites from the International Ocean Discovery Program (IODP) Expedition 349, show that the spreading ages of the SW-SCS range from ∼ 21 to 15 Ma (from C6A to C5B) and the full spreading rate ranges from 26 to 46 mm/yr, with an average rate of 37 mm/yr. Spreading was initially symmetric but later became asymmetric, with a rate differential reaching 10%. Combined with two seismic profiles, it is found that the roughness of the oceanic basement to the south of the extinct ridge is mainly influenced by faulted blocks, whereas to the north it is mainly influenced by magmatism. The RMS seafloor roughness is estimated to be 211 m in the south and 191 m in the north respectively. This indicates that SW-SCS seafloor spreading tends to be a transition type between axial high and axial low at the intermediate-slow spreading rate. There is a large valley in the spreading center, but the roughness far away from the center is low. the RMS basement roughness is obviously lower than that of the typical slow spreading oceanic basin. we conjecture that the larger than normal syn-spreading magma may be the reason for the low roughness. A long history of being subducted might be the deep-seated reason for all these behaviors where, and the mantle is rich in water and carbon. [ABSTRACT FROM AUTHOR]

The Porgera Transfer Zone (PTZ) is a major crustal and probably lithospheric structure across Papua New Guinea recording >50 km offset of ophiolites and very different patterns of geology and topography on either side. In the Late Jurassic, the PTZ probably separated oceanic crust and thick Jurassic Om shales to the west from a continental promontory to the east. During the Late Miocene to Recent orogenesis, the differential compression of these features is interpreted to have created a dextral strike slip fault across the fold belt with pull-apart basins at sites of fault relays. This facilitated the ascent of intrusions and mineralization at Porgera. The acquisition of high-resolution LIDAR data semi-regionally around the Porgera Gold Mine greatly improved interpretation of the regional geology and particularly the recognition of normal faults. By correlating with sparse dip data and paly-dated samples, it was possible to create stratigraphic sections and interpret structural cross-sections using the LIDAR data. As the area involved strike–slip offsets, it was important to construct sections in multiple orientations in order to interpret the 3D geology. Both dips and fault orientation could be directly inferred from the LIDAR data such that sections could be constructed orthogonally to them. A balanced, restored and forward-modelled cross-section illustrates the interaction between thrust faults and normal faults during compression and that it was synchronous with the development of a pull-apart basin. A semi-regional 3D geological model, which was developed mainly from the LIDAR data, supports the hypothesis of inversion of the thick Om beds to the west before or during compression of the continental promontory to the east resulting in dextral strike–slip offsets across the PTZ. A jog or relay in the faults occurred and caused a pull-apart collapse basin to develop in the area of the Porgera mine. Similar pull-apart graben, or negative flower structures, were detected nearby and may be areas for future exploration. [ABSTRACT FROM AUTHOR]

The 110,000 km2 Yucatan Basin in the northern Caribbean Sea is critical for understanding the Late Cretaceous to Recent tectonic evolution of the Caribbean‐North American plate boundary. This study integrates gravity, magnetic, and a 5,500 km grid of 2D seismic data to carry out a tectonostratigraphic analysis of the Yucatan Basin. These data provide the first recognition of 38–102 km‐long spreading ridges that constrain a SW‐NE opening direction in the western Yucatan Basin. The age of this oceanic crust is constrained to be late Paleocene‐middle Eocene (57–42 Ma) based on heat flow measurements, depth‐to‐seafloor, and three sedimentary sequences inferred to be Eocene—Recent in age based on stratigraphic correlations to distant wells. We interpret the Yucatan Basin as a back‐arc basin formed during the northeastward movement of the Caribbean volcanic arc that is now exposed in Cuba, evolved during the early Cretaceous to middle Eocene, and was terminated by collision with the Bahama carbonate platform during the late Paleocene to middle Eocene. We identify regional, left‐lateral strike‐slip faults that extend into the Cuban volcanic arc, as observed in other active back‐arc basins. We propose that the Yucatan back‐arc basin once formed the northwestern extension of age‐equivalent back‐arc basins in Hispaniola, where the basin is inverted, topographically elevated, and strongly shortened, and in the Lesser Antilles where the Paleogene back‐arc basin has remained undeformed and submarine. This once‐continuous back‐arc basin was disrupted and left‐laterally offset by ∼500 km during the Late Eocene‐Recent formation of the Cayman trough strike‐slip system. Plain Language Summary: This study utilizes seismic reflection data to carry out a tectonostratigraphic analysis of the Yucatan Basin. Key Points: We integrate gravimetric, magnetometric, and seismic reflection data to identify oceanic crust in the Yucatan back‐arc basinHeat flow and depth to the seafloor constrain a Late Paleocene‐Middle Eocene age for the oceanic crust adjacent to the Cuban volcanic arcSimilar back‐arc basins nearby were inverted, subaerially exposed, and left‐laterally offset during the late Eocene‐Recent [ABSTRACT FROM AUTHOR]

Construction of oceanic crusts in ultraslow-spreading ridges are controlled by detachment faults, but the specific ways they are determined by detachments remain unclear. Abundant ophiolite massifs discovered along the Yarlung-Tsangpo Suture Zone, southern Tibet, are considered to represent ultraslow-spreading centers of the Cretaceous Neo-Tethys ocean. The presence of various mafic blocks within the ophiolitic mélange beneath the lithospheric mantle provides evidence of the entire processes involved in crustal generation. Therefore, in this study geochemical investigations on these mafic blocks from the southern Luqu (Xigaze) ophiolitic mélanges were conducted to shed new lights on the petrogenesis of mafic crust rocks in ultraslow-spreading settings. Geochronological results via zircon U–Pb dating gave evidence for their coeval formation to other Yarlung-Tsangpo ophiolitic crust rocks in the Early Cretaceous (ca. 120–130 Ma). Geochemistry and Sr–Nd–Hf isotopes document depleted and primitive characteristics, revealing that these mafic rocks were crystallized from residual melts experienced substantial fractional crystallization following along liquid lines of descent of H2O-poor magmas. However, amphiboles are pervasive throughout our samples, which is in contrast with expectations of clinopyroxene and plagioclase as dominant liquidus phases of anhydrous magma. Textures of replacement of clinopyroxene by magnesio-hornblende and their higher contents of trace elements and Eu negative anomalies support that the amphiboles crystallized during transition from dry magma to hydrous one, indicative of water additions to solidifying melts. Based on the tectonic settings of ultraslow-spreading centers, we proposed a model in which seawater-derived fluids induced through detachment faults could be responsible for the generation of ultraslow-spreading crust. [ABSTRACT FROM AUTHOR]

The Azores region straddles the Mid-Atlantic Ridge and comprises volcanic islands and seamounts interspersed with non-magmatic oceanic basins arranged along subparallel slow-spreading systems. The cross-check of CO2-rich fluid inclusions barometry from lavas and tephras erupted during the last 20 ka with the sources of seismicity of the last 22 years traced the deepest magma accumulation zone at a regional scale. These zones are considered to represent the boundary between the dense rocks of magma systems and the lithospheric mantle. This boundary is at a depth range of ~ 17 to 20 km beneath the islands close to the Mid-Atlantic Ridge, which are younger than 500 ka, and up to ~ 29.4 km beneath the eastern island of S. Miguel, younger than 800 ka and the nearby Dom João de Castro Seamount and the Hirondelle Basin. The same method has been applied to the ~ 2 Ma-old volcanic products of Flores, the ~ 1.9 Ma old S. Jorge and the 5 Ma-old lavas of Santa Maria. Results revealed a depth of ~ 30.5 km beneath the Flores and ~ 26 km beneath the ~ 1.9 Ma-old S. Jorge. These two islands are west and east of the Mid-Atlantic Ridge, within 200 km. Finally, the crust-mantle transition has been found below Santa Maria, the easternmost island, ~ 500 km away from the Mid-Atlantic Ridge at a depth of ~ 33 km. Overall, repeated mafic magma injections at the base of each magma system generated vertically stacked layered ultramafic mush bodies, which thickened the crust overtime. [ABSTRACT FROM AUTHOR]

Seafloor hydrothermal vents represent potential sources of Mo and other biologically relevant transition metals to the global ocean, complementing continental runoff. Here, we use a combination of experimental, theoretical, and field-sampling approaches to investigate the behavior of Mo in basalt-hosted seafloor hydrothermal systems to provide insight into the processes controlling Mo concentrations in hydrothermal fluids and to derive estimates of vent fluid Mo concentrations and fluxes. Results of this study demonstrate that reaction fluids generated from 350 °C, 500 bar hydrothermal basalt alteration experiments contain 775–801 nmol/kg Mo and are thus comparable to a recently collected time series of natural seafloor vent fluids that contained 200–220 nmol/kg Mo at 302 °C and 29–30 nmol/kg at 281–282 °C (Evans et al., 2023). Synchrotron-based analyses of experimentally altered basalt produced in this study and additional natural samples of altered oceanic crust originally collected from Pito Deep Rift reveal the presence of Mo-rich particles consistent with trace molybdenite. Comparisons of Mo:Cu ratios in natural vent fluids and near-vent sediment trap samples from Main Endeavour Field indicate that vent fluid Mo is readily incorporated into buoyant plume particles and advected out of the near-vent field, analogous to previous mass balance studies of Cu in this region. Thermodynamic calculations of molybdenite solubility in the context of mineral-buffered hydrothermal fluids and comparisons with natural and experimental hydrothermal fluids suggest that high-temperature vent fluids contain 30–1500 nmol/kg Mo. While a minor component of the modern Mo budget, hydrothermal Mo fluxes are estimated to have constituted 0.3–200× the contemporaneous continental weathering fluxes prior to the ∼2.4 Ga ago "Great Oxidation Event" and widespread oxidative continental weathering. Overall, identification of hydrothermal vents as a source of Mo-rich plume particles with potential for dispersal into the wider marine environment has significant implications for hypotheses regarding the co-evolution of Life and Earth's environments, specifically the form and availability of Mo in anoxic Archean-Eon oceans, where Mo-dependent enzymatic pathways are thought to have emerged and subsequently evolved. [ABSTRACT FROM AUTHOR]

Detachment faulting is one of the main styles of seafloor spreading at slow to intermediate mid-ocean ridges. However, we have limited insight into its role in back-arc basin formation. We surveyed a remnant back-arc spreading center in the Philippine Sea and determined the detailed features and formation processes of the Mado Megamullion (Mado MM) oceanic core complex (OCC). This was undertaken in the context of back-arc evolution, based on the shipborne bathymetry, magnetics, and gravity with radiometric age dating of the rock samples collected. The Mado MM OCC has a typical OCC morphology with prominent corrugations on the domed surface and positive gravity anomalies, suggesting that there has been an exposure of the lower crust and mantle materials by a detachment fault. The downdip side of the detachment continues to the relict axial rift valley, which has indicated that the Mado MM OCC was formed at the end of the back-arc basin opening. The spreading rate of the basin decreased once when the spreading direction changed after six million years of stable trench perpendicular spreading. The rate then further decreased immediately prior to the end of the spreading when the Mado MM OCC was formed. The existence of other OCC-like structures in the neighboring segment and the previously reported OCCs along the Parece Vela Rift have indicated that the melt-poor, tectonic-dominant spreading is a widespread phenomenon at the terminal phase of back-arc spreading. The decrease in spreading rate in the later stage is consistent with the previous numerical modeling because of the decrease in trench retreat. In the Izu–Bonin–Mariana arc trench system, the rotation of the spreading axis and the resultant axis segmentation have enhanced the lithosphere cooling and constrained mantle upwelling, which caused the tectonic-dominant spreading at the final phase of the basin evolution. [ABSTRACT FROM AUTHOR]

Nearly 30% of ocean crust forms at mid-ocean ridges where the spreading rate is less than 20 mm per year. According to the seafloor spreading paradigm, oceanic crust forms along a narrow axial zone and is transported away from the rift valley. However, because quantitative age data of volcanic eruptions are lacking, constructing geological models for the evolution of ultraslow-spreading crust remains a challenge. In this contribution, we use sediment thicknesses acquired from ~4000 km of sub-bottom profiler data combined with 14C ages from sediment cores to determine the age of the ocean floor of the oblique ultraslow-spreading Mohns Ridge to reveal a systematic pattern of young volcanism outside axial volcanic ridges. Here, we present an age map of the upper lava flows within the rift valley of a mid-ocean ridge and find that nearly half of the rift valley floor has been rejuvenated by volcanic activity during the last 25 Kyr. A new age map of the rift valley at an ultraslow-spreading ridge reveals that 50% of the lava flows are <25,000 years old. The study documents a continuous volcanic surface renewal with eruptions occurring throughout the width of the rift valley. [ABSTRACT FROM AUTHOR]

The distribution of water within the Earth's mantle has significant implications for the Earth's dynamics and evolution. Recent mineral physics experiments indicate that dense hydrous magnesium silicates can contain large amounts of water stable up to 60 GPa or even beyond along slab geotherms. Here we perform petrological‐thermomechanical numerical simulations of water transportation by deep slab subduction and related magmatism in the mid‐mantle. Key parameters including those defining the slab thermal parameter and the water storage capacity in the oceanic lithosphere and surrounding mantle are explored. The results show two major dehydration events of ultramafic rocks at around 150 and 750 km by dehydration of serpentine at 600°C and superhydrous phase B in the entrained wet upper mantle, respectively. Large amounts of water, ∼1.5 wt% at least locally, are carried down to the mantle transition zone and lower mantle. We estimate an upper limit of slab water flux into the mid‐mantle of 0.1–0.28 × 1012 kg/yr, which is ∼13%–37% of the input water from the serpentinized mantle. Moreover, a substantial fraction of the water released by the slab is absorbed by the entrained mantle and overlying mid‐mantle portions, such that ∼30%–70% of the water injected at the trench could be delivered to the lower mantle. The deepest magmatism is observed at ∼1,500 km in case of phase H breakdown (MgO‐SiO2‐H2O system), coinciding with the depth of strong seismic attenuation. Overall, these simulations suggest that up to 0.2 ocean mass per billion years could be transported down to the mid‐mantle and beyond. Plain Language Summary: Water plays a crucial role in mantle dynamics as it decreases the effective viscosity and melting temperature. Deep diving slabs can carry water to sub‐arc, mantle transition zone, and lower mantle depths. However, the amount of water that can be entrained by the slab depends on its thermal structure and the water storage capacity of the oceanic crust and surrounding mantle. With the rapid progress of laboratory experiments, many high‐pressure hydrous phases have been reported in recent years. Here we combine different high‐pressure high‐temperature experimental data sets together with geodynamic models to study the water transportation and related magmatism down to the mid‐mantle. With the incorporation of the dense hydrous magnesium silicates, our results show significant slab dehydration due to the breakdown of superhydrous phase B at ∼800 km and of phase H at ∼1,500 km (MgO‐SiO2‐H2O system), which is coincident with the seismic low‐velocity and strong attenuation zones at ∼800–1,500 km globally. Our model further shows that up to 0.2 surface mass ocean can be transported beyond 1,500 km per billion years. Key Points: Water transportation via subduction and related magmatism are numerically simulated down to 2,000 km depthSlab thermal structures and the water storage capacity of the oceanic crust and mid‐mantle control the water transportation processesDehydration melting may explain mid‐mantle seismic scatters [ABSTRACT FROM AUTHOR]

To ascertain the Early-to-Middle Jurassic tectonic setting in the central Great Xing'an Range, this study investigated the Early and Middle Jurassic granitoids exposed in the Chaihe area in the central Great Xing'an Range based on isotopic chronology and petrogeochemistry. The results of this study show that the Early and Middle Jurassic granitoids have emplacement ages of 179–172 Ma. Moreover, the Early and Middle Jurassic granitoids are high-K calc-alkaline unfractionated I-type granitoids and high-K calc-alkaline fractionated I-type granitoids, respectively. The magma sources of the Early and Middle Jurassic granitoids both originated from the partial melting of newly accreted lower crustal basaltic rocks. Meanwhile, the Middle Jurassic magma sources were mixed with mantle-derived materials or ocean-floor sediments formed by the dehydration and metasomatism of subducted slabs. The Early and Middle Jurassic granitoids in the study area were formed in the subduction environment of the oceanic crust, in which the Mongol-Okhotsk oceanic plate was subducted southward beneath the Eerguna and Xing'an blocks. Moreover, the Siberian plate began to collide and converge with northeast China during the Middle Jurassic. [ABSTRACT FROM AUTHOR]

The petrogenesis of late Ordovician–early Silurian adakitic plutons in the North Qilian suture zone (NQSZ) and their copper mineralization potential remain poorly understood. Here we present a detailed study of the Heishishan (HSS) granodiorite–granite pluton, spatially associated with Cu–Au mineralization in the eastern section of the NQSZ. Zircon U–Pb dating confirms that the granodiorite–granite were formed at ca. 438–435 Ma, in association with a continental collision. Geochemically, the granitoids resemble low-Mg adakitic rocks featured by elevated Sr/Y and (La/Yb)N ratios with depleted MgO, Cr, and Ni concentrations, suggesting minimal mantle contribution. They are sodium rich with K2O/Na2O < 1, and have higher and more varied Sr/Y, but lower La/Yb than those from the continental lower crust. The εHf(t) values of zircon grains are positive and vary in a wide range of +2.0–12.7, indicating a heterogeneous source rather than a single arc basaltic source. They show moderately radiogenic Sr and Nd isotope compositions with initial 87Sr/86Sr ratios of 0.705101–0.706312 and εNd(t) values of +0.5–1.0, most likely a mixed source of the oceanic basaltic crust plus ca. 15–20% overlying sediments. The magmatic oxygen fugacity was relatively low as indicated by zircon Ce(IV)/Ce(III) ratios of 32–156, which is unfavorable for a large copper mineralization. [ABSTRACT FROM AUTHOR]

Recycled oceanic crust is believed to have played an important role in the formation of continental flood basalts, whereas the involvement of large amounts of CO2 derived from recycled marine carbonate in their mantle sources is highly debated. Zn isotopes have great potential to trace recycled carbonate due to the distinctly different δ66Zn values between marine carbonates and the mantle. Representative continental flood basalt samples from Keping (Group1) and Yingmai and Shengli (Group 2) in Tarim, NW China, were collected to investigate their mantle sources, and their Zn isotopes were studied systematically for the first time. The Zn isotope values of Keping basalts (between 0.29‰ ± 0.03‰ and 0.32‰ ± 0.05‰) are higher than values in the primitive mantle (δ66Zn = 0.16‰ ± 0.06‰) but similar to those of mid-ocean-ridge basalt (MORB; δ66Zn = 0.24‰-0.31‰). Considering their high fractionation (e.g., Mg# = 0.28-0.37) and potential involvement of crustal contamination (87Sr/86Sri between 0.70720 and 0.70779; εNd between -3.2 and -1.9), their Zn isotopes may not conclusively point to a carbonated mantle source. In contrast, Yingmai and Shengli basalts show heavy Zn isotope values (between 0.32‰ ± 0.03‰ and 0.39‰ ± 0.03‰), nonradiogenic 87Sr/86Sri (0.70459-0.70518) and εNd(t) (between -1.3 and 0.1) signatures, and less fractionation (Mg# = 0.46-0.71), implying the involvement of recycled carbonate components in their mantle source. Nonetheless, the lack of negative Zr, Hf, and Ti anomalies, low CaO/ Al2O3 ratios, and high SiO2 contents preclude direct melting of carbonate-bearing mantle. Alternatively, these features may suggest the melting of decarbonated subducted eclogite and variable interaction with subsolidus peridotite. This assumption is consistent with the positive correlations of δ66Zn with Gd/Yb and Sm/Yb ratios and Zn content as a result of mixing between eclogite-derived high-δ66Zn melt and peridotite-derived mantle-like δ66Zn melt. Our study provides a new model to reconcile the geochemical features of the Tarim continental flood basalts and highlights the potential role of decarbonated eclogite in the formation of continental flood basalts. [ABSTRACT FROM AUTHOR]

U-Pb dating of single detrital zircon grains by laser ablation--inductively coupled plasma--mass spectrometry (LA-ICP-MS) paired with Hf and O isotopic and trace-element analyses provide first-order indicators of the Late Cretaceous--Cenozoic evolution of the southern Ecuadorian magmatic arc. Detrital zircon U-Pb ages define significant clusters that are tentatively interpreted as intense arc magmatism at ca. 72 Ma, ca. 60 Ma, and ca. 43 Ma. A major accretionary event in the Late Cretaceous (75-65 Ma) is marked by a broad range of zircon isotopic values (εHf[t] > 20 and δ18O > 8‰) that suggest melting of both the lower and upper crust (most likely of continental affinity) as well as enriched mantle components. Highly fractionated signatures in trace-element patterns and Eu/Eu* combined with mantle-like δ18O and juvenile εHf values characterize zircons from 60 to 45 Ma, suggesting that the Late Cretaceous--middle Eocene arc originated from an enriched mantle and likely reflects the persistence of overthickened crust previously attributed to the main Late Cretaceous accretionary period. Subsequently, negative shifts in εHf(t) isotopic composition from 45 to 30 Ma are paired with mantle-like δ18O values as well as decreases in U/Yb and Eu/ Eu*. These signatures could be attributed to magma emplacement in a thinner crust and the existence of a broad extensional magmatic arc extending from the current forearc toward areas near the craton; however, other scenarios cannot be excluded. This event was characterized by enriched mantle melt sources with residence times pointing to known crustal events (Sunsás) in the Amazonian craton. From 30 to 10 Ma, the isotopic record slightly evolved toward a depleted mantle signature with a substantial increase in fractionation. Our results combined with previously published isotopic records from detrital zircon grains found in modern rivers suggest that, for at least the last 30 m.y., the southernmost Northern Andes magmatic arc has been segmented, with the emplacement of juvenile magmas to the north and more enriched magmas related to the recycling of ancient continental crust and/or subducted sediments to the south--aspects found in other Northern Andes settings in which the continental arc was constructed in both oceanic and continental crust. [ABSTRACT FROM AUTHOR]

The East Kunlun Orogen (EKO) in the northern Tibetan Plateau records two continental collisional orogenic events and magmatism in early Paleozoic and early Mesozoic. However, possible magmatic additions to the continental crust growth of the EKO in different tectonic stages of early Paleozoic collisional orogeny have been overlooked. Three phases of early Paleozoic plutons from the Xiangride-Kuhai area in the east of the EKO have been chosen for detailed investigation and the results are reported here. The oldest magmatic suite (Stage 1) includes the ca. 471 Ma Qurelong Monzodiorite and ca. 454 Ma granodiorite in the Zhiyu Intrusive Complex. The monzodiorite has a sanukitoid-like composition with high TiO2 and Y contents and is interpreted as being derived from partial melting of metasomatized mantle wedge lherzolite. The granodiorite is typified by its high SiO2 content, high Sr/Y ratio, and depleted Hf isotope, and is interpreted as an adakite-like melt derived from the melting of a subducted Proto-Tethys oceanic crust. The magmatism can be linked to northward subduction of the Proto-Tethys Ocean between 520 and 450 Ma. Stage 2 magmatism is represented by a plutonic suite emplaced during ca. 450-431 Ma with an I-type granitic composition. Of these, the ca. 447 Ma Kengdenongshe Intrusion composed of peraluminous granite with enriched Nd-Hf isotopes is indicative of a Mesoproterozoic igneous source in the orogen. The ca. 450-434 Ma monzogranite and granodiorite in the Walega and Zhiyu intrusive complexes exhibit variable element and isotope compositions. They would have been generated by magma mixing of felsic melts from the old crust and mafic magmas derived from the metasomatized lithospheric mantle, with a mafic melt proportion of >30%. The ca. 431 Ma quartz diorite in the Walega Intrusive Complex is formed through crustal assimilation and fractional crystallization of mafic magmas derived from the metasomatized lithospheric mantle, with a mafic melt proportion >60%. Stage 2 suite was emplaced during the closure of Proto-Tethys oceanic branches and subsequent continental collision during 450-426 Ma. Magmatism diminished between ca. 426 and 410 Ma during exhumation of the continental lithosphere as indicated by the presence of retrograde eclogites in the EKO. Stage 3 magmatic suite includes the ca. 408 Ma Langmuri Intrusion and ca. 403 Ma Niantang Syenogranite. These plutons are adakite-like or have an Atype granitic composition and are enriched in Nd-Hf isotopes. They might have been derived from the remelting of old and juvenile continental crust in a post-collisional extensional setting during 410-390 Ma. Identification of partial melts, derived from the subducted Proto-Tethys oceanic crust and metasomatized lithospheric mantle in stage 1 and 2 plutons, show that the subcrustal materials have been significantly transferred to the overlying continental crust. Hence the magmatism in oceanic subduction (Stage 1) and continental collision (Stage 2) settings contributes to the early Paleozoic juvenile continental crust growth of the EKO. The post-collisional extensional setting (Stage 3) is dominated by the reworking of a pre-existing continental crust. The early Paleozoic continental crust growth processes in the EKO are different from the previous view in which the continental collision orogens have no crust growth, and inconsistent with the proposal that crust growth is significant only in a continental collision setting. [ABSTRACT FROM AUTHOR]

Subduction initiation is a pivotal process in plate tectonics. Models of subduction initiation include the collapse of passive margins, oceanic transform faults, inversion of oceanic core complexes, and ridge failure but have ignored the potential effects of continental crust relicts within the oceanic crust. In this paper, we explore the role of microcontinents on subduction initiation through two-dimensional thermo-mechanical numerical modeling. We consider three scenarios with variable ages of oceanic crust surrounding the microcon-tinent and parametrically examine the microcontinent characteristics (size, crustal thickness, thermal gradient, and rheology), oceanic plate age, and convergence rates. Results suggest that moderate-size (≥300 km) microcontinents can nucleate subduction initiation at the junction between continental and oceanic plates. A large part of the microcontinent would be dragged into the subduction zone, and the subsequent asthenosphere upwellings would incorporate part of the microcontinent. Our numerical models add a new hypothetical scenario for subduction initiation, especially for those places where a young and buoyant plate subducts beneath an older and denser oceanic plate. Moreover, they can explain the origin of exotic crust materials and ultrahigh-pressure minerals in supra-subduction zone ophiolites. [ABSTRACT FROM AUTHOR]

Lawsonite eclogites are major hosts of H2O and trace elements and thus key for long-term deep element cycling in subduction zones. Existing cycling models suggest that the subducting oceanic crust transforms to lawsonite-eclogite assemblages; yet the scarcity of lawsonite eclogites in the rock record questions to what extent the oceanic crust transforms to law-sonite-eclogite assemblages during subduction. Here, we use petrological modeling coupled with geodynamic calculations for a typical subduction zone to show that the occurrence of lawsonite eclogites is controlled by the maturation of the subduction zone. We find that law-sonite eclogite does not form in infant subduction zones; with time, the oceanic crust forms lawsonite assemblages, but prograde heating obliterates lawsonite except in rocks exhumed prior to such heating. Lawsonite-eclogite assemblages in the oceanic crust form and survive prograde metamorphism only in mature and cold subduction zones but still necessitate specific characteristics during exhumation to preserve lawsonite. We show that the stability of lawsonite in mature subduction zones is hindered by hybridization between the mafic crust and the overlaying mantle wedge material; by contrast, lawsonite proportion increases with intense seafloor alteration and Ca-Al metasomatism. These latter processes are thus key for enhancing recycling. We argue that lawsonite-driven mass cycling to the deep mantle is important in mature subduction zones, but the role of lawsonite eclogite in carrying mass deep into Earth in the infant stage of a subduction zone is minor. [ABSTRACT FROM AUTHOR]

[Display omitted] • Eclogitization kinetics of oceanic crust layers controlled Precambrian subduction. • Discrete eclogitization induced flat subduction in Precambrian. • Flat subduction builds short-lived overthickened layered mantle. Flat (shallow) subduction is mainly proposed as an Archean-Paleoproterozoic process, contributing to the growth of continental lithosphere. However, numerical models of Precambrian subduction commonly show steep subduction. Here we investigate the effects of lithology-dependent eclogitization (minor for gabbroic lower crust, strong for basaltic upper crust) of oceanic crust during subduction. We performed 2D petrological-thermomechanical modeling of subduction at potential mantle temperatures up to 250 °C warmer than present, using a numerical approach that accounts for buoyancy effects from both mantle depletion and eclogitization. The modeling reveals that increases in mantle potential temperature and the related thickness of oceanic crust and depleted mantle may induce transition to the flat subduction regime at ΔT ≥ 150 °C but only when a delayed eclogitization of gabbroic compared to basaltic crust is taken into account. Otherwise, subduction operates in the steep slab regime. Flat Precambrian subduction models show episodic bimodal magmatism within arcs located > 400 km from the trench, caused by short-lived (1–4 Myr) subduction transients consisting of three consecutive stages: (1) twisting downward of the eclogitized slab portion, (2) subvertical descent and break-off of this twisted portion, (3) rising of the remnant non-eclogitised slab tip and continuation of flat subduction. Limited formation of TTG-series granitoids is caused by partial melting of the subducting basaltic layer but becomes less pronounced at ΔT = 250 °C. Flat subduction produces an overthickened, layered keel-like continental mantle root, with an intermediate layer of subducted oceanic crust. However, construction of long-lived mantle keels by this mechanism might be limited by peeling off of the gradually eclogitizing oceanic crust and the underlaying mantle when plate convergence ends. At elevated mantle temperatures flat subduction produces voluminous hydration of the mantle wedge, forming a subcrustal serpentinite mélange layer that becomes a potential source of fluids in the subsequent evolution of the overlying continental crust. [ABSTRACT FROM AUTHOR]

NW Canada and Alaska are the continuation of the North American Cordillera through Mexico, western USA and western Canada. I show that they have similar thermal regimes and thermal control of tectonics and seismicity. I first summarize the multiple constraints to crust and upper mantle temperatures and then discuss some consequences. There are bimodal crust and upper mantle temperatures characteristic of most subduction zones: cool forearc, uniformly hot backarc (Yukon Composite Terrane to Southern Brooks Range, and Mackenzie Mountains), and stable cratonic backstops (Arctic Alaska Terrane and Canadian Shield). The main constraints are as follows: (a) Heat flow measurements, (b) Temperature‐dependent upper mantle velocities and seismic attenuation, (c) Temperature‐dependent topographic elevations; thermal isostasy, (d) Depth and temperature of the seismic lithosphere‐asthenosphere boundary (LAB), (e) Origin temperature and depth of craton kimberlite xenoliths, (f) Geochemically inferred source temperature and depth of recent volcanic rocks. (g) Depth to the magnetic Curie temperature, (h) Depth extent of seismicity. The backarc lithosphere is thin, LAB at 50–85 km and ∼1,350°C ± 25°C. Moho temperatures at 35 km are 850°C ± 100°C compared to cool cratonic areas of 400°C–500°C. The consequences include the following: (a) Thin and weak backarc lithosphere that accommodates pervasive tectonic deformation indicated by wide‐spread seismicity and GPS‐defined motions, in contrast to the stable cratonic regions; (b) Weak backarc lower crust that flattens the Moho and allows detachment and thrusting of the upper crust over the cold strong Arctic Alaska Terrane and Canadian Shield. This article provides a model for how to estimate deep temperatures from multiple constraints. Plain Language Summary: Alaska and NW Canada are the continuation of the North American Cordillera mountain belt through Mexico, western USA and western Canada. I show that they have similar high temperatures in the earth's crust and upper mantle, and that these deep temperatures are the principal control of earthquake occurrence and ongoing mountain belt deformation processes. Hot crustal areas are weak and deform easily, whereas cold strong areas like the Canadian Shield deform very little and have fewer earthquakes. I first outline the constraints to deep earth temperatures in Alaska and NW Canada and then the earthquake and mountain building consequences. The key association in this region and globally is that there are uniformly high crustal temperatures in the weak "backarcs," the high mountain belts that extend 100 s of kilometers landward of current and recent subduction zones (where ocean crust is being thrust beneath continents generating volcanoes, and great earthquakes). I summarize eight methods that provide estimates of deep temperatures. They give a consistent picture, with deep temperatures in the backarc mountain belts of central Alaska, the Yukon and western Northwest Territories that are double those in the cold strong cratons (shields) of the northern part of arctic Alaska and the Canadian Shield in NW Territories. Earthquakes and mountain building processes follow this deep temperature difference, with most occurring where crustal temperatures are high and the crust weak. Key Points: The regional distribution of seismicity and tectonic deformation in Alaska/NW Canada is controlled by crust and upper mantle temperaturesEight principal constraints are presented that give consistent deep temperature estimates; bimodal factor of two, backarc versus craton300–800 km wide Cordillera backarc is uniformly hot and tectonically active in contrast to the cold stable Arctic Alaska and Canadian Shield [ABSTRACT FROM AUTHOR]

The incorporation mechanisms of OH groups in garnet were investigated in a suite of high-pressure rocks from the Zermatt–Saas area (Switzerland) using a combination of Fourier transform infrared spectroscopy (FTIR) and electron probe micro-analysis (EPMA). Investigated garnet specimens include grossular–andradite–uvarovite solid solutions in serpentinite and rodingite and almandine–grossular–pyrope–spessartine solid solutions in eclogite, mafic fels and meta-sediment. All rocks experienced the same peak metamorphic conditions corresponding to a burial depth of ∼ 80 km (∼ 540 ∘ C, 2.3 GPa), allowing determination of the OH content in garnet as a function of rock type. The capacity for OH incorporation into garnet strongly depends on its composition. Andradite-rich (400–5000 µ g g -1 H 2 O) and grossular-rich garnet (200–1800 µ g g -1 H 2 O) contain at least 1 order of magnitude more H 2 O than almandine-rich garnet (< 120 µ g g -1 H 2 O). Microscale analyses using FTIR and EPMA profiles and maps reveal the preservation of OH zoning throughout the metamorphic history of the samples. The OH content correlates strongly with Mn, Ca and Ti zoning and produces distinct absorption bands that are characteristic of multiple nano-scale OH environments. The use of 2D diffusion modelling suggests that H diffusion rates in these rocks is as low as log(D [m 2 s -1 ]) = - 24.5 at 540 ∘ C. Data were collected for the main garnet-bearing rock types of the Zermatt–Saas area allowing a mass balance model of H 2 O to be calculated. The result shows that ∼ 3360 kg H 2 O km -1 (section of oceanic crust) yr -1 could be transported by garnet in the subducting slab beyond 80 km depth and contributed to the deep-Earth water cycle during the Eocene subduction of the Piemonte–Liguria Ocean. [ABSTRACT FROM AUTHOR]

The Mohorovičić discontinuity, often known as the Moho, marks the boundary separating the Earth's crust from the mantle. Techniques such as isostatic-gravity and seismic methods can be used to determine this division. The Moho marks the boundary between the continental and oceanic crust and the upper mantle. Simply put, the Moho acts as a physical/chemical boundary between the mantle and the crust and causes significant changes in geophysical properties such as seismic wave velocity, density, pressure, and temperature (Mooney and Masters, 1998; Martinck, 1994; Bagherbandi, 2011 and Dashtbazi et al., 2023). An accurate and high resolution Moho depth model in fields such as geodesy, geology, geophysics, geodynamic modeling, seismic risk assessment, stress field modeling caused by mantle convection (Li et al., 2018; Behr et al., 2022; Singh and Yadav, 2023; Heilman and Becker, 2022; Hashima et al., 2016; Eshagh et al., 2020; Eshagh, 2015 and Gido et al., 2019), and understanding seismic source mechanisms is important, among other applications (Gido et al., 2019 and Dashtbazi et al., 2023). Furthermore, a reliable Moho model can reveal details of crustal structure that provide valuable insights into the complexities of deeper mantle layers; related to the calculations and detailed examination of gravity, geothermal, geomagnetic models (Stalk et al., 2013). Although there are several Moho models, their accuracy and resolution are insufficient in the complex tectonic geometry of the Makran subduction zone (Brizi et al., 2021 and Heilman and Becker, 2022), because these zones show a complex Moho configuration (Shad Manaman et al., 1390; Taghizadeh Farhamand et al., 2015 and Dashtbazi et al., 2023). As a result, the existing models lack the necessary accuracy for the Makran subduction zone, a region approximately 1000 km long located in southeast Iran and southwest Pakistan (Byrne et al., 1992; Shad Manaman et al., 1390, Penney et al., 2017; Dashtbazi et al., 1398; Dashtbazi. et al., 2023). In geophysical and geodetic studies, hybrid methods are mainly used to determine the Moho depth when seismic data with appropriate distribution and abundance are not available. These techniques include the Parker-Oldenberg method and the Wenning-Mines-Moritz method. In an effort to strengthen the existing Moho depth models in the Makran subduction zone, two distinct models named BC and SC through the integration of gravity (VMM) and seismic (CRUST1.0) data, which are processed through the Butterworth filter, spectral combination approaches and the least squares technique, was developed (Bagherbandi, 2011 and Dashtbazi et al., 2023). The resulting models provide a resolution of 5' x 5' degrees of arc, corresponding to a grid size of 9 x 9 km (Dashtbazi et al., 2023). The accuracy of these models was evaluated against four separate regional and local models. The resulting RMS values were 5.28, 1.55, 4.18, and 1.27 km for the BC model and 5.59, 1.17, 3.74, and 3.04 km for the SC model. Also, the Moho depth model obtained for the west Makran region in Iran significantly improved the accuracy and resolution of the Moho depth models in the studied area. The SC Moho model exhibits improved RMS metrics compared to the combined BC model, so we recommend it as the first priority. While the Moho depth models in our research really bring significant improvements to the existing models of the Makran subduction Moho zone, the integration of more detailed seismic data with SC and BC Moho models can improve the developed model for the Makran subduction zone. In the end, we suggest that a similar approach be adopted for the analysis of the Moho model in the eastern Makran region in Pakistan, which allows a comparative evaluation of the Moho depth and structure between the western and eastern parts in order to obtain a better picture of the Moho depth model of the Makran subduction zone. [ABSTRACT FROM AUTHOR]

This study constructs a model to represent the strain–stress field surrounding a splay fault and demonstrates the accumulation processes of both strain and stress around the splay fault. We consider the case of multiple fault dislocations, construct the model as a function of the effective viscosity in the media, and investigate the influence of the effective viscosity on the strain and stress accumulation patterns. The results show that the strain and stress tend to accumulate in the splay fault and the subducting oceanic crust, and the rate of accumulation varies with the effective viscosity. The accumulation and relaxation of strain and stress are simultaneous, and the slower the effective viscosity, the slower the accumulation rate. We discuss the relationship between the splay faults, fluid, and intraslab earthquakes. Finally, the possibility that effective viscosity may contribute to the mode of occurrence of intraslab earthquakes at the Hikurangi subduction margin is discussed. Plain Language Summary: We construct a simple model to explain the evolution of strain and stress over time in a subduction margin. The model constructed in this study is the case where a fault branches from a plate boundary. We investigate the influence of the effective viscosity of its constituents on the temporal evolution of strain and stress. The results show that the rate of strain and stress accumulation varies with the effective viscosity. Our result may suggests the influence of the effective viscosity on the source distribution of the microearthquakes observed at the Hikurangi subduction margin. Key Points: Temporal evolution of strain and stress caused by the dislocation of tri‐materials with different effective viscosities is calculatedStrain and stress accumulation rate depends on the effective viscosityStrain and stress accumulated at the multiple fault edge strongly influence the stress perturbations in the subducting oceanic crust [ABSTRACT FROM AUTHOR]

Uptake of nitrogen (N) during hydrothermal alteration of oceanic crust makes the altered oceanic crust (AOC) a significant N reservoir in parallel to seafloor sediments. While N enrichment in the upper (basaltic) oceanic crust during low-temperature alteration has been widely observed, the N characteristics of moderate- to high-temperature altered gabbroic oceanic crust, which accounts for ∼70 vol% of the crustal material in subducting slabs, have been rarely studied. The lack of these data resulted in large uncertainties in calculating the N input flux of global subducting slabs and modeling the N quantities released through the arc and subducted into the deep mantle. To fill this gap, we examined the N concentrations and isotope compositions of 38 altered gabbroic rocks recovered by ODP/IODP drillings from three oceans, i.e., Hole 735B at the Atlantis Bank in the Southwest Indian Ridge, Hole 1309D at the Atlantis massif in the Mid-Atlantic Ridge, and Hole 1415P at the Hess Deep in the East Pacific Rise. All the gabbroic samples show significant N enrichment with mostly positive δ15N values (in average: 7.4±3.8 ppm and +0.6±2.0‰ for Hole 735B, 5.3±2.1 ppm and +1.4±1.3‰ for Hole 1309D and 7.9±1.9 ppm and +2.0±1.8‰ for Hole 1415P), which are comparable to those of altered basalts from global oceanic crust. The N concentrations and δ15N values of these gabbroic rocks can be readily explained by mixing between minor inherited mantle N and mainly secondary N derived from seawater. The secondary N-hosting minerals in altered gabbroic rocks are likely plagioclase, amphibole and chlorite formed during moderate- to high-temperature alteration stages. Using these new data, we obtained a global nitrogen input flux of 20.6 - 1.3 + 0.9 × 109 mol·yr−1 to 29.9±2.2 × 109 mol·yr−1 for subducting AOC, in which 50% – 64% is contributed by the altered gabbroic oceanic crust. Integrating with previous estimate of N input flux from seafloor sediments, a total N input flux of 74.9 - 1.3 + 0.9 – 84.2±2.2 mol·yr−1 was estimated for global subducting slabs. Mass balance between this input N flux and various estimates of N output flux in global arcs gave a large range (from 47% – 53% to 10% – 20%) for the fraction of slab N to be subducted beyond the sub-arc depth to the deeper mantle. The upper end (47% – 53%) of this range is more consistent with the results from other constraints, such as thermal structural control and comparison between altered gabbros and meta-gabbros. [ABSTRACT FROM AUTHOR]

Troctolites were recovered during Integrated Ocean Drilling Program Expedition 345 at the Hess Deep Rift, next to fast-spreading East Pacific Rise. These troctolites are divided into three groups based on textural differences: coarse-grained (1–10 mm in length) troctolite, fine-grained (~ 2 mm in length) troctolite, and skeletal olivine-bearing troctolite. All troctolites exhibit a magmatic fabric. The major-element compositions of olivine, plagioclase, and clinopyroxene in the troctolites are intermediate between those of Hess Deep gabbros and harzburgites. The trace-element compositions of olivine, plagioclase, and clinopyroxene in the troctolites overlap with those of troctolites from slow-spread crust, but they record no petrographic evidence indicating assimilation of mantle peridotite. Thermodynamic calculation for mineral chemistry showed that fractional crystallization of melt is the dominant process responsible for the formation of the troctolites. The fine-grained troctolite was crystallized with high crystallization rate resulting from hot melt injection into colder wall gabbro. In contrast, interactions between the unsolidified troctolite containing interstitial melt and newly injected melt resulted in the formation of the skeletal olivine-bearing troctolite. While our results demonstrate that the troctolites exhibit multiple melt injections and partial dissolution of a troctolite precursor, fractional crystallization is the dominant process for the creation of the lower crust in the Hess Deep Rift. [ABSTRACT FROM AUTHOR]

The features of the structure formation of the Earth's crust at the early stage of formation of the Southeast Indian Ridge were studied. The process of formation was associated with the separation of Australia from Antarctica and the advance of the rift zone westward towards the ancient oceanic lithosphere and, then, towards the large igneous province of Kerguelen, formed by the activity of the plume of the same name. The separation of Australia and Antarctica spanned a long period of continental rifting (~160‒80 Ma), which was followed by ultraslow spreading (~80‒45 Ma), slow spreading (~45‒40 Ma), and, finally, by recent stationary spreading at average rates (after 40 Ma). The advance of the rift zone towards the ancient oceanic lithosphere gave way to the accretion of young oceanic crust on the emerging Southeast Indian Mid-Oceanic Ridge. The early stages of development of the young spreading ridge are reflected in the modern structural plan of the study region. The advance of the rift zone from the continent into the boundaries of the ancient oceanic lithosphere led to the formation of the Naturalist Plateau and the Bruce Bank near the Antarctic margin. The break-up of the ancient oceanic lithosphere and the formation of the young crust on the Southeast Indian Ridge led to the formation of conjugated Diamantina and Labuan suture zones, fixing the position of the initial rifting. The transition from ultraslow spreading at the initial stage of oceanic crust formation to stationary spreading at average rates is clearly recorded in the change in the ruggedness of the accretionary relief. The Southeast Indian Mid-Oceanic Ridge collided as a result of its propagation westward with a large igneous province during the formation of the Kerguelen Plateau and separated the Broken Range from the plateau. The authors carried out physical modeling of the development conditions of rifting and spreading processes, as well as structure formation within the Southeast Indian Ridge. [ABSTRACT FROM AUTHOR]

Geophysical properties of oceanic crust are strongly influenced by the presence of cracks. We studied the effects of cracks on the physical properties of oceanic crustal rocks collected from the ICDP Oman Drilling Project Hole GT3A. Electrical resistivity and P‐ and S‐wave velocities were measured under dry and brine‐water‐saturated conditions for each sample. The experimental results reveal that electrical resistivity and elastic wave velocities are differently correlated with porosity. We performed joint inversion of the measured electrical and elastic properties combining an effective medium model by Kachanov and a statistical crack fluid flow model by Guéguen and Dienes with percolation theory. As a result, the variations in electrical and elastic properties can be related to the crack microstructural parameters: crack density and aspect ratio, as well as connectivity of cracks evaluated from crack density. To understand the influence of in situ conditions of oceanic crust, the joint inversion using the proposed cross‐property relationship was performed for geophysical properties obtained by logging measurements at IODP Hole 1256D. Results show the depth variations of resistivity and sonic velocities can be successfully interpreted by decreasing crack porosity and connectivity. Our data and analysis can provide new insights into the interpretation of geophysical data from the oceanic crust at which pore‐fluid plays key roles in various geodynamic activities. Plain Language Summary: Cracks in the oceanic crust play key roles in subsurface processes, including fluid transportation, heat and chemical exchange, and microbial activity. Therefore, it is important to understand how cracks are distributed in the oceanic crust. Given that cracks exert an important influence on the physical properties of rocks, electrical and seismological surveys have been conducted at various locations in the oceanic plates. However, to quantitatively interpret the geophysical data obtained from these surveys, laboratory investigations are essential. In this study, we measured the electrical resistivity and elastic wave velocity of oceanic crustal rocks collected from drillcores of the Oman ophiolite, in which tectonic fragments of ancient oceanic plate are preserved on land. Our experimental data show that electrical resistivity and elastic wave velocity are differently correlated with porosity. Analysis of these data indicates that the variations in the electrical and elastic properties can be related to crack density, aspect ratio, and crack connectivity. The cross‐property relationship established is applied successfully to in situ geophysical data obtained by borehole logging in oceanic crust. Our data and proposed cross‐property relationship provide new insights into the interpretation of geophysical data from the oceanic crust. Key Points: We measured the electrical resistivity and elastic wave velocity of the sheeted dike–gabbro transition zone of the Oman Drilling ProjectVariation in the electrical and elastic properties of Hole GT3A was interpreted by combining effective medium model and percolation modelApplying the cross‐property relationship to logging data at IODP Hole 1256D, we estimated the crack parameters of in situ oceanic crust [ABSTRACT FROM AUTHOR]

Subducting oceanic sediments and crusts, originating from the Earth's surface and descending into its deep interior, are important carriers of volatiles. The volatiles have significant effects on materials cycling and the dynamic evolution of the subduction zones. A simplified Al2O3-SiO2-H2O (ASH) ternary system models the relationship of minerals in the hydrated and alumina-silica rich sedimentary layer. Topaz Al2SiO4(F,OH)2 is an important mineral in the ASH system and comprises two volatiles: H2O and fluorine (F). In this study, the thermoelasticity of a natural F-rich topaz was investigated using synchrotron-based single-crystal X-ray diffraction combined with diamond anvil cells up to 29.1 GPa and 750 K. The pressure-volume-temperature data were fitted to a third-order Birch-Murnaghan Equation of state with V0 = 343.15(7) Å3, K0 = 166(1) GPa, K0′= 3.0(1), (∂K0/∂T)P = −0.015(9) GPa/K and α0 = 3.9(5) × 10−5 K−1. The isothermal bulk modulus increases with the F content in topaz, and the various F contents present significant effects on its anisotropic compressibility. Our results further reveal that the isothermal bulk modulus K0 of the minerals in ASH system increases with density. F and H contents in hydrous minerals might greatly affect their properties (e.g., compressibility and stability), providing more comprehensive constraints on the subduction zones. [ABSTRACT FROM AUTHOR]

The kinematic reorganization of plate boundaries in the southeastern Antarctic part of the South Atlantic, expressed in the jump of the spreading axis of the Agulhas Ridge, led to a restructuring of the region's structural plan. This resulted in the formation of the southern segment of the Mid-Atlantic Ridge (MAR), extinction of the previously active Agulhas spreading ridge, and formation of the Meteor and Islas Orcadas rises, marking the location of the MAR and located symmetrically about its axis. Based on the research results, the conditions for the jump in the spreading axis were identified and an experimental model was constructed for the formation of accompanying structures, in which an important role was played by the accretion of oceanic crust on the Agulhas Ridge and westward migration of the Falkland Plateau. This resulted in southward movement of the southern segment of the MAR, formation of the Malvinas microplate, and a jump in the axis of the Agulhas spreading ridge, which led to cessation of spreading on this ridge. An important role in this kinematic restructuring was played by activity of the Shona hotspot. [ABSTRACT FROM AUTHOR]

The structurе of the Olyutorka–Kamchatka accretion region make it possible to reconstruct two paleobasins for the Late Cretaceous–Paleogene, separated and fenced off from the ocean by volcanic arcs. The features of the structure and composition of the complexes characterizing fragments of oceanic crust show that these basins had different natures. The crustal fragments of the Lesnaya–Iruney Basin are Albian–Maastrichtian in age. Volcanic rocks are represented by differentiated N-MORB tholeiites and, rarely, E-MORB intraplate basalts and enriched tholeiites. Crustal fragments of the Vetlovsky Basin are from Campanian–Maastrichtian to Middle Eocene in age. Among the volcanic rocks, mainly N-MORB basalts are widespread; less frequently, E-MORB, and in some structures, oceanic island basalts (OIB). The geodynamics in the Late Cretaceous–Cenozoic for the Kamchatka and Olyutorsky segments differed, starting from the Campanian. [ABSTRACT FROM AUTHOR]

Inversion for seismological structures of the oceanic lithosphere–asthenosphere system is important to understand the mechanisms of plate tectonics. Previous models of the oceanic upper mantle have been primarily obtained via global tomography using surface waveforms. However, besides scarcity of waveform data in the oceanic regions, difficulties in fitting phases for shorter-period components in the previous global tomography have yielded resultant models that possess poor resolutions above |$\sim \, 50$| km depth. Recent developments of broad-band ocean-bottom seismometer (BBOBS) arrays provide larger amount of seismic data with epicentral distances of <20°. In this study, we develop an appropriate method to fully utilize the information contained in the shorter-period components of BBOBS arrays. We first fit the envelopes without phase information to analyse the shorter-period components (8–60 s) which are generally unavailable in the conventional phase fitting. We then use the resultant model as our initial model for waveform inversion of the longer periods (12.5–200 s) to fit the phase, which allows us to infer a continuous structure model from the crust to the asthenosphere. We demonstrate the validity of this combined envelope-fitting and waveform inversion method by analysing the waveform data from a BBOBS array that was deployed in the Northwestern Pacific and has recorded events in the vicinity of the Japan Trench to obtain the average velocity structure between the event and station arrays. We independently resolve the crustal compressional and shear wave velocities, and thickness by analysing the envelopes, which minimizes biases in the subsequent waveform inversion. We also find that the waveform inversion improves the resolution in the asthenosphere. Our results suggest that further extension of this method should improve our knowledge of the oceanic lithosphere–asthenosphere system. [ABSTRACT FROM AUTHOR]

The subduction of the Bangong–Nujiang Ocean is important in the geological evolution of the Tibetan Plateau. In this paper, we report new zircon U-Pb age and Lu-Hf isotopic data and whole-rock elemental and Sr-Nd-Pb isotopic data for Early Cretaceous dacites from the Rena-Co area (RCA) in the southern Qiangtang Terrane (QT), central Tibet and use these data to better understand the tectonic evolution of the Bangong–Nujiang suture. LA–ICP-MS dating of zircons yields ages of 109.5 ± 0.6 Ma to 109.6 ± 0.8 Ma for the dacites from the RCA. Geochemically, these dacites are medium-K calc-alkaline and show high SiO2 contents of 64.79–70.37 wt.%, high Sr contents of 517–598 ppm and low Y contents of 8.45–10.7 ppm, similar to those of typical adakites. Additionally, all the rocks are strongly enriched in light rare earth elements and some large ion lithophile elements (e.g. Rb, U, K and Cs) but significantly depleted in high-field-strength elements (e.g. Nb, Ta and Ti), consistent with the geochemical characteristics of arc-type magmas formed in the subduction zone. Moreover, these adakite-like dacites show whole-rock initial (87Sr/86Sr)i ratios of 0.705119 to 0.705491, (206Pb/204Pb)i ratios of 18.489 to 18.508, (207Pb/204Pb)i ratios of 15.591 to 15.612, (208Pb/204Pb)i ratios of 38.599 to 38.686, ϵ Nd (t) values of −0.28 to +1.25 and single-stage Nd model ages of 642 to 818 Ma, as well as significantly positive zircon ϵ Hf(t) values of 3.9–13.1, with young Hf-depleted mantle ages of 331 to 923 Ma. These geochemical and isotopic data indicate that they are most likely derived from the juvenile thickened mafic lower continental crust, which contains partial melts of metasomatized peridotite and subduction-related fluids in the magma source region. Based on previous studies and our new data, we propose that the RCA adakite-like dacites are most likely a result of the northwards subduction of the Bangong–Nujiang Ocean lithosphere beneath the southern QT during the Early Cretaceous and that a slab rollback model could explain the formation of the RCA adakite-like dacites. [ABSTRACT FROM AUTHOR]

High Ba-Sr granitic rocks are widespread in Phanerozoic orogenic systems, and their petrogenesis is important for revealing the evolutionary process of the Proto-Tethys Ocean in the North Qilian orogenic belt. This paper presents a combination of zircon U-Pb age, whole-rock major and trace element concentrations, and Sr-Nd-Hf isotopic data for Caowa high Ba-Sr dioritic intrusion from the eastern part of the North Qilian orogenic belt, aiming to decipher its petrogenesis and tectonic setting. LA-ICP-MS zircon U-Pb dating yield an emplacement age of 450 ± 2 Ma for the Caowa intrusion, indicating a magmatic activity of the Late Ordovician. The Caowa quartz diorites contain moderate contents of SiO2, MgO, Mg#, and resultant high concentrations of Na2O + K2O, Fe2O3T, and Al2O3, displaying calc-alkaline and metaluminous characteristics. The studied samples have relatively elevated Ba (up to 1165 ppm) and Sr (561 to 646 ppm) contents, with obvious enrichment in LILEs (e.g., Ba, Th, U) and depletions in HFSEs (e.g., Nb, Ta, Ti), resembling those of typical high Ba-Sr granitoids in subduction zones. Together with enriched Sr-Nd isotopic composition [(87Sr/86Sr)i = 0.7082–0.7086, εNd(t) = −5.1 to −4.9], and the wide ranges of zircon εHf(t) values (−13.2 to +8.5), it suggests that these high Ba-Sr quartz diorites were derived from a mixture magma source between the ancient crust materials and the enriched lithospheric mantle metasomatized by fluid released from subducted oceanic crust or sediment. Taking into account the ophiolites, high pressure metamorphic rocks, and arc magmatic rocks in the region, we infer that due to the influence of the northward subduction of the Qilian Proto-Tethys Ocean, the Laohushan oceanic crust of the North Qilian back-arc basin was subducted during the Late Ordovician and resulted in extensive metasomatism of lithospheric mantle by fluids derived from oceanic crust or sediments, and the Caowa high Ba-Sr quartz diorites were generated in the process of crust–mantle interaction during the Late Ordovician. [ABSTRACT FROM AUTHOR]

With the northward subduction and final closure of the Neo-Tethyan oceanic crust, the Indian and Eurasian plates finally collided together and underwent a strong collision orogenic event, resulting in large-scale crust–mantle magmatic interactions. In order to clarify the controversies about tectono-magmatic activities after the Indian–Eurasian continental collision, we report the newly dated Eocene Qiongduojiang gabbro explored in the Tethyan–Himalaya belt, southern Tibet. LA-ICP-MS zircon U-Pb dating shows that the crystallization age of the Qiongduojiang gabbro is 46.1 ± 1.7 Ma. The whole-rock major and trace elements, as well as Rb-Sr, Sm-Nd, and Pb isotopic data results, show that the Qiongduojiang gabbro is apparently depleted in Nd isotopes, is enriched in Pb isotopes, and has maintained a consistent 87Sr/86Sr(t) value. This paper argues that the E-MORB-like Qiongduojiang gabbro originated from asthenosphere upwelling caused by slab breakoff of the Neo-Tethyan oceanic plate. This event caused large-scale magmatic activities, a magmatic mixing process between ancient crust and deep mantle, and wild distribution of Eocene Gangdese plutons along the Yarlung–Tsangpo Suture Zone, and it rendered the subduction-modified Tibetan lithosphere fertile from the Gangdese porphyry Cu deposits. [ABSTRACT FROM AUTHOR]

It is a general tendency that epidote, which is a typical greenschist facies mineral, is scarce in the lower oceanic crust, in spite of the widespread occurrence of the other minerals indicative of similar temperature conditions such as chlorite, actinolite, prehnite and serpentine. To find the cause of this, we carried out petrological analyses of lower crustal rocks of the Oman ophiolite sampled by the Oman Drilling Project of the International Continental Scientific Drilling Program (ICDP). Petrographic observations revealed the tendency, as expected, that the amount of epidote formed by static alteration of plagioclase decreases with depth. Because mineral assemblages indicative of a wide range of temperature conditions from amphibolite to subgreenschist facies occur throughout the cores without systematic variations of abundance, the decrease of epidote amount cannot be explained by the difference of temperature condition of alteration. Petrographic observations also revealed that epidote is absent or rare in rocks containing serpentinized olivine in contrast to prehnite showing a close association with serpentinization of olivine. In an exceptional sample containing both epidote and serpentinized olivine, epidote occurs with chlorite that cuts or replaces plagioclase, mantles adjacent olivine and is connected with chlorite + lizardite veins cutting mesh‐forming serpentine veins. The distribution and mode of occurrence of epidote suggest decoupling of its formation with the main stage of serpentinization. Serpentine veins cutting olivine to form mesh texture are typically lizardite with magnetite ribbons at vein centres and have compositions of lizardite–cronstedtite solid solution at vein margins or in magnetite‐free veins, suggesting a chemical condition with low silica and low oxygen potentials at an early stage of serpentinization. Thermodynamic modelling for olivine and plagioclase alteration at greenschist facies conditions indicates that silica potential for plagioclase alteration to form prehnite + chlorite and epidote + chlorite could be higher than for olivine serpentinization. On the other hand, oxygen potential for the prehnite + chlorite formation is lower than for the epidote + chlorite formation and is comparable with that for olivine serpentinization. From the observations and analyses, it is concluded that epidote formation is inhibited by olivine serpentinization, which maintains a reducing condition for alteration in the lower oceanic crust. [ABSTRACT FROM AUTHOR]

Diamonds can form within eclogite (remnants of ancient subducted oceanic crust) in the subcontinental lithospheric mantle (SCLM), pieces of which may then be transported to the surface as xenoliths in kimberlites. These diamond-bearing xenoliths offer a rare opportunity to study diamonds in their host lithology. The morphology and surface features of diamonds record evidence of attack by diamond-aggressive melts/fluids, which results in resorption, rounding, and destruction of octahedral growth forms. The location and timing of such diamond-destruction processes remain controversial, and the kimberlite magma itself is often considered to be the primary culprit. We used X-ray computed tomography scanning to present a view of diamond morphology and distribution within 24 diamondiferous eclogites in unprecedented detail. These scans clearly capture diamond growth and destruction that occurred within the SCLM, prior to kimberlite entrainment. We show that euhedral diamonds in these eclogites are predominantly step-faced octahedra. This morphology is preserved even when the diamonds are exposed at the surface of the eclogites, indicating that kimberliteinduced resorption was not significant. Six eclogites contain only rounded diamonds with no distinct crystal faces, and their exposed surfaces on the exterior of the xenoliths are highly irregular, indicating diamond-destruction in the SCLM. In three cases, single xenoliths host both resorbed diamonds and step-faced octahedra, indicating multiple metasomatic events, some of which were diamond-aggressive and others diamond-friendly. These diamondiferous xenoliths provide snapshots of diamond growth and destruction in the SCLM, caught in the act. [ABSTRACT FROM AUTHOR]

Highly siderophile elements (HSEs), including Re and Os, are used extensively as geochemical tracers and geochronometers to investigate the formation and evolution of Earth's crust and mantle. Mantle rocks are commonly serpentinized, but the effect of serpentinization on the distribution of HSEs is controversial because HSEs are commonly hosted by rare, micrometerto sub-micrometer-scale grains of platinum group minerals (PGMs) of ambiguous origin that are challenging to identify, characterize, and interpret. In this study, atom probe tomography (APT) is used to characterize two spatially close PGM grains hosted by a partially serpentinized harzburgite from Macquarie Island, Australia. The APT data reveal an extraordinary level of detail that provides insights into the origin of a complex Cu--Pt alloy grain (average composition Cu4Pt). The grain hosts Fe-, Ni-, and Pt-rich sub-grains associated with Rh, variably overlapping networks of Pd- and Cd-enrichment, and OH-rich volumes identified as fluid inclusions. Osmium and Ru are hosted by an idioblastic laurite (RuS2) grain. Compositional, textural, and phase-diagram constraints are consistent with a modified pre-serpentinization origin for the PGMs, and a comparison between observed and calculated grain distributions indicate that while Os isotope ratios were probably unaffected by serpentinization, whole-rock and grain-scale HSE and isotopic ratios may have been decoupled during serpentinization. [ABSTRACT FROM AUTHOR]

To assess the paleo‐permeability structure of oceanic crust, we used 3‐D X‐ray Computed Tomography (XCT) images to quantify the distribution and geometry of mineral veins in core samples from Oman Drilling Project Holes GT1A, GT2A, and GT3A, which correspond to the upper to lower crustal sections of the Samail ophiolite. We developed a new method that automatically detects veins in the XCT core images based on iterative adaption of the two‐step Hough transform combined with multiscale Hessian filtering for identifying an elongate structure. Application of the developed method allowed us to identify the geometry and Computed Tomography number of more than 1500 veins with millimeter‐scale apertures in core sections with a total length of ∼1,200 m. High‐CT (HCT) veins in the drilled cores can be related to relatively high‐temperature fluid circulation near the mid‐ocean ridge, whereas Low‐CT (LCT) veins can be related to subsequent low‐temperature fluid circulation. Applying fracture fluid‐flow models to the geometric information for the detected veins, we found that the HCT and LCT vein systems both yielded bulk permeability of 10−13–10−9 m2 for each hole. This indicates that millimeter‐wide fractures can control crustal‐scale permeability, even in the lower oceanic crust. However, these vein systems show different depth dependencies and anisotropies of permeability, possibly reflecting the different spatial variations of high‐ and low‐temperature fluid circulation in oceanic crust. Plain Language Summary: Water transported through fractures in the oceanic crust plays a key role in various geologic processes, such as global transfers of heat and elements moving with fluid. Such fractures are preserved in crustal rocks as mineral veins; therefore, the geometries of the veins can be used to estimate the paleo‐permeability of the fractured rocks. During this study, we investigated the distribution and geometry of veins in core samples of oceanic crust from the Samail ophiolite in Oman to determine its paleo‐permeability structure. The ophiolite represents fragments of ancient oceanic lithosphere obducted onto the continental margin. We quantified three‐dimensional geometric and material information for the veins using 3‐D X‐ray Computed Tomography images of the core samples. Applying fracture fluid‐flow models to the vein data, we found that the oceanic crust had a markedly high permeability, even at depths of several kilometers immediately after the formation of the fractures. We also found that the hydrothermal systems have different spatial variations and anisotropies depending on the CT numbers of the vein‐filling minerals. Our results indicate that there may be differences in the mechanisms of high‐ and low‐temperature hydrothermal circulation in oceanic crust. Key Points: A new method has been developed to automatically detect vein characteristics from X‐ray Computed Tomography (CT) images of drilled coresThe permeability structure of the crustal section of the Samail ophiolite was estimated from the downhole profiles of detected veinsPermeability structures of High‐CT and Low‐CT vein systems have different spatial variations and anisotropies [ABSTRACT FROM AUTHOR]

Offshore basalts, most commonly found as oceanic crust formed at mid-ocean ridges, are estimated to offer an almost unlimited reservoir for CO2 sequestration and are regarded as one of the most durable locations for carbon sequestration since injected CO2 will mineralize, forming carbonate rock. As part of the Solid Carbon project, the potential of the Cascadia Basin, about 200 km off the west coast of Vancouver Island, Canada, is investigated as a site for geological CO2 sequestration. In anticipation of a demonstration proposed to take place, it is essential to assess the tendency of geologic faults in the area to slip in the presence of CO2 injection, potentially causing seismic events. To understand the viability of the reservoir, a quantitative risk assessment of the proposed site area was conducted. This involved a detailed characterization of the proposed injection site to understand baseline stress and pressure conditions and identify individual faults or fault zones with the potential to slip and thereby generate seismicity. The results indicate that fault slip potential is minimal (less than 1%) for a constant injection of up to ~2.5 MT/yr. This is in part due to the thickness of the basalt aquifer and its permeability. The results provide a reference for assessing the potential earthquake risk from CO2 injection in similar ocean basalt basins. [ABSTRACT FROM AUTHOR]