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Mantle xenoliths usually represent fragments derived from the depleted and degassed lithospheric mantle with 3He/4He isotope ratios (6 ± 1 RA) lower than those of mid-ocean ridge basalts (8 ± 1 RA). Otherwise, basalts from oceanic islands related to hotspots often have high 3He/4He ratios (>10 RA), suggesting a deep and pristine undegassed mantle source. Here we present a striking high-3He/4He component (up to 27.68 RA) recorded by spinel-facies mantle xenoliths from Patagonia. Remarkably, the highest ratios were found in a long-lived trans-lithospheric suture zone related to the Carboniferous-Permian collision of two continental blocks: the Deseado and the North Patagonian massifs. The mantle xenoliths with notably high-3He/4He ratios are inferred to be fragments of the shallow asthenosphere rising through the eroded and rejuvenated thin lithosphere. The pristine helium component is derived from the western margin of the Karoo mantle plume, related to the initial stages of the Gondwana fragmentation. This study reveals a shallow asthenosphere with high 3He/4He ratios (>20 RA) beneath Patagonia. This primordial composition is linked to the Karoo mantle plume from the early stages of the Gondwana breakup. [ABSTRACT FROM AUTHOR]

The redox state of arc mantle has been considered to be more oxidized and diverse than that of the mid-ocean ridge, but the cause of the variation is debated. We examine the redox state of the Cenozoic global arc mantle by compiling measured/calculated fO2 of olivine-hosted melt inclusions from arc magma and modeled fO2 based on V/Sc and Cu/Zr ratios of arc basaltic rocks. The results indicate that the redox state of Cenozoic arc mantle is latitude dependent, with less oxidized arc mantle in the low latitudes, contrasting with a near constant across-latitude trend in the mid-ocean ridges. We propose that such a latitude-dependent pattern in the arc mantle may be controlled by the variation in the redox state of subducted sediment, possibly related to a latitudinal variation in the primary production of phytoplankton, which results in more organic carbon and sulfide deposited on the low-latitude ocean floor. Our findings provide evidence for the impact of the surface environment on Earth's upper mantle. The fO2 of arc primary magma is latitude dependent with less oxidized magma at low latitudes, which may be related to distribution of ocean primary production resulting in more organic carbon and sulfide in the subducted sediments at low latitudes. [ABSTRACT FROM AUTHOR]

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

The Meseta del Lago Buenos Aires (MLBA) in southern Patagonia, a volcanic plateau formed from ~12 Ma to present, provides an opportunity to investigate the temporal evolution in volcanism as this region transitions from the subduction of the Nazca plate to the formation of the slab window produced by the collision of the Chile Ridge and the Andean subduction zone. Here, we report new major, minor, and trace element contents, as well as Sr, Nd, Pb, and Hf isotopes of the MLBA lavas. Three distinct geochemical endmembers can be distinguished in the MLBA basalts: a subduction-influenced endmember, a transitional component similar to the South Atlantic enriched mid-ocean ridge basalts, and an enriched component akin to the EM1 mantle composition. Lavas older than ~1.5 Ma define a compositional continuum between the subduction-influenced and transitional endmembers; this trend is also present in many other southern Patagonian plateaus regardless of their distance to the trench, eruption age, and the composition of the continental blocks where they are located. In contrast, MLBA basalts younger than ~1.5 Ma uniquely define a transition into the EM1 mantle component at the time when this region was affected by the slab window. The estimated pressures and temperatures of mantle-melt equilibration for the MLBA basalts indicates an increase in both parameters after the formation of the slab window that roughly correlate with the changes in lava composition. The basalts' composition from all southern Patagonia plateaus points to the presence of the South Atlantic mid-ocean ridge basalt mantle influenced by the Discovery, Shona, and Bouvet hotspots rather than the sub-slab mantle, as represented by the Chile Ridge basalts. This observation challenges the hypothesis that the sub-slab mantle within the slab window has had an important role in the composition of the erupted lavas. Instead, it suggests the presence of a South Atlantic mantle beneath southern Patagonia either within the mantle wedge, consistent with a long-lasting South Atlantic convection cell beneath South America, or in the subcontinental lithospheric mantle metasomatized before or just after the opening of the South Atlantic basin, as demonstrated by the composition of southern Patagonia mantle xenoliths. Although it is difficult to precisely distinguish the contributions of the asthenosphere from that of the metasomatized subcontinental lithospheric mantle beneath this region, our work suggests significant contributions from the latter in the composition of the MLBA lavas. [ABSTRACT FROM AUTHOR]

Detachment faults at slow‐ and ultraslow‐spreading mid‐ocean ridges are essential for lithosphere‐hydrosphere interactions. However, their coupling with subseafloor hydrothermal circulation is poorly understood. Here, we investigated shallow hydrothermal fluid circulations in the active Dragon Horn detachment fault system of the Southwest Indian Ridge using combined near‐bottom magnetic anomaly data and rock magnetic analyses. We observed enhanced magnetic anomalies related to the high magnetization of basaltic country rocks and highly serpentinized (>80%) peridotite, whereas the hydrothermally altered basalts and sulfide deposits had weak magnetization and reduced magnetic anomalies. Therefore, the low‐magnetization zone was interpreted as a proxy for the hydrothermal fluid channel. The spatial distribution of the low‐magnetization zone revealed that the inactive Suye field expanded more than 500 m in diameter horizontally and ∼100 m vertically, and the active Longqi hydrothermal field had a >1 km fluid channel, both centered around the detachment fault fracture zone. Our results establish a robust link between detachment faulting and shallow hydrothermal circulation in which the detachment fault system provides critical fluid paths. Therefore, detachment tectonics are important elements for evolving seafloor hydrothermal fields along the Southwest Indian Ridge and possibly other slow‐ and ultraslow‐spreading mid‐ocean ridges globally. Plain Language Summary: The detachment faults of slow‐ and ultraslow‐spreading mid‐ocean ridges are geological structures motivated by seafloor spreading. Intermitted movements of detachment faults exhume deep oceanic crust or mantle rocks and provide pathways for seafloor hydrothermal systems. The magnetic properties of rocks in seafloor detachment fault systems are sensitive to hydrothermal fluids and can be used to study fluid circulation within seafloor detachment fault systems. Using magnetic methods, we found that magnetic minerals in fresh basalt and highly serpentinized peridotite created positive magnetic anomalies, whereas the absence of magnetic minerals in hydrothermally altered basalts and sulfides decreased the magnetic anomalies. As represented by the low‐magnetization zones, the shallow hydrothermal fluid channels sit on the detachment fault, highlighting the close relationship between detachment tectonics and the evolution of seafloor hydrothermal systems. We conclude that the detachment fault system primarily controls the formation of black‐smoker chimneys along the magma‐poor mid‐ocean ridges. Key Points: Altered basalt and sulfide deposits attenuated the magnetic anomaly of the normal‐polarity zone in the Dragon Horn areaSerpentinized peridotite (>80%) produced significant positive magnetic anomalies in the Dragon Horn areaDetachment faulting controls the formation of fluid channels for hydrothermal circulation [ABSTRACT FROM AUTHOR]

The molybdenum (Mo) isotope system is pivotal in reconstructing marine redox changes throughout Earth's history and has emerged as a promising tracer for igneous and metamorphic processes. Understanding its composition and variation across major geochemical reservoirs is essential for its application in investigating high-temperature processes. However, there is debate regarding the δ98/95 Mo value of the Earth's mantle, with estimates ranging from sub-chondritic to super-chondritic values. Recent analyses of global mid-ocean ridge basalt (MORB) glasses revealed significant δ98/95Mo variations attributed to mantle heterogeneity, proposing a two-component mixing model to explain the observed variation. Complementary studies confirmed the sub-chondritic δ98/95Mo of the depleted upper mantle, suggesting remixing of subduction-modified oceanic crust as a plausible mechanism. These findings underscore the role of Mo isotopes as effective tracers for understanding dynamic processes associated with mantle-crustal recycling. [ABSTRACT FROM AUTHOR]

The Dulate arc, located in East Junggar (NW China) in the southern Central Asian orogenic belt, records a Devonian magmatic arc evolution, offering a window to understanding the orogenic processes of the Central Asian orogenic belt. Here we present new geochemical and isotopic data for Late Devonian high-Mg andesite (HMA) and Nb-enriched basalt (NEB) suites from the Qiakuerte area, East Junggar. The HMA samples are typical subduction-related volcanic rocks. They have SiO2 contents ranging from 53.30 to 54.59 wt%, high MgO (5.0-5.26 wt%), and high Mg# values (~55) and show enrichments in large ion lithophile elements (LILEs) and depletions in high field strength elements (HFSEs). The HMA samples have high (La/Yb)N ratios and Sr/Y (~6.5 and 50-59, respectively) with no Eu anomalies. The HMA samples have high Na2O (~3.3 wt%) and low K2O (~2.5 wt%) and Th (~2.4 ppm) contents, combined with positive εNd(t) and low (87Sr/86Sr)i values. These characteristics suggest that the samples were formed mainly through interactions between subducted oceanic melts and mantle peridotites. Compared to normal arc basalts, the NEB samples have higher concentrations of Nb (~20 ppm), higher primitive mantle-normalized Nb/La (0.50-0.58), and higher ratios of Nb/U (9.4-14.6). The NEB samples also have positive εNd(t) and low (87Sr/86Sr)i values, indicating that their source was mantle wedge that had been metasomatized by slab melt. Considering the widespread presence of A-type granites, the abnormally high heat flow, and the tectonic characteristics of East Junggar, we conclude that a slab window created by the subduction of an ocean ridge was responsible for the melting of slab and the formation of the NEB-HMA suites. These processes may have also played a key role in the tectonic evolution processes of East Junggar during the Late Devonian. [ABSTRACT FROM AUTHOR]

The Nagaland–Manipur Hill Ophiolite (NMHO) is an NNE-SSW trending linear ophiolite zone exposed in the northeastern states of Nagaland and Manipur in India. Basaltic rocks of NMHO are geochemically divisible into two broad groups in the Zr/Ti versus Nb/Y classification diagram. Samples with TiO2 < 2 wt% and Zr = 38–118 ppm plot within the basalt field, whereas samples with TiO2 > 2 wt% plot within the alkali basalt field. The latter can be subdivided into the alkali basalt group-1 (AB-1) with Zr = ~ 200 ppm and the alkali basalt group-2 (AB-2) with Zr = ~ 400 ppm. In a normal mid-ocean ridge basalt (N-MORB) normalized trace element pattern, basalt displays a near-horizontal trend from Lu to Pr at rock/N-MORB = ~ 1 and then increases slightly from Pr to Rb, whereas alkali basalt is relatively more enriched than basalt. In the chondrite-normalized rare earth element (REE) pattern, basalt displays near-horizontal trends with (La/Yb)CN ranging between 0.78 and 1.89. On the other hand, alkali basalt displays parallel and steadily increasing enrichment trends from Lu to La [(La/Yb)CN = 11.07–17.61], with a slight drop at Eu and Sm. Mantle melting models suggest: (1) partial melting of N-MORB-like sources at degrees of melting (F) = 4–8% for basalts; and (2) partial melting of ocean island basalt (OIB)-like sources at F = 7.5–17.8% for alkali basalt. Occurrences of basalts with N-MORB-like and and alkali basalt with OIB-like signatures within the NMHO complex suggest their origin from distinct magma batches with direct or indirect involvement of the Kerguelen plume. Chondrite normalised pattern for elements Nb, Zr and REE showing estimated compositions of source S1 and residue R1 for alkali basalt (estimated using DMI modeling) and source S2 for basalt (estimated using NBM modeling) in this study. DMM and OIB (Workman and Hart 2005) values are also shown for comparison. Average abyssal peridotite value is from Niu (2004) database. [ABSTRACT FROM AUTHOR]

14°N on the Mid-Atlantic ridge (MAR) is one of only a few locations worldwide where volatile-saturated, geochemically enriched mid-ocean ridge basalts (E-MORBs) have been recovered. These basaltic glasses are so gas-rich that CO 2 -filled bubbles may "pop" when brought to the surface, due to the pressure and temperature change. Although these "popping rocks" have long been regarded as representative samples of un-degassed magmas sourced from the upper mantle, uncertainties regarding both their generation mechanism(s) and the potential effects of gas loss/accumulation processes have hampered unambiguous quantification of the upper mantle volatile element (water, carbon, nitrogen, noble gas) inventory. Fortunately, the extent and consequences of gas loss/accumulation processes can be tested by studying characteristic changes in volatile elements compositions, including 4He/40Ar* (where 40Ar* is 40Ar corrected for atmospheric contamination). To document the mechanism of popping rock generation and potential effects of degassing and gas accumulation processes on MORB volatile systematics, we present a comprehensive volatile characterization (carbon, nitrogen and noble gas systematics) of popping rocks and associated MORBs (n = 19) recently sampled at 14°N on the MAR, including 2 normal MORBs (N-MORB) from an oceanic core complex (OCC) and 17 E-MORBs. In line with previous studies, we find that PR exhibit the lowest 4He/40Ar* (1.08 ± 0.04) among all MORB samples, lower than the conventional mantle production ratio of 3 ± 1. Such low 4He/40Ar* could either (i) derive from accumulation of first-generated bubbles originating from open-system degassing of underlying magmas, or (ii) represent the actual upper mantle production ratio. We summarize the arguments in favor of each of these two scenarios (including the required accumulation times for radiogenic noble gases accumulation, the K/U and 232Th/238U of the upper mantle, popping rock vesicle size distributions, and physical considerations for vesicle growth and upwelling through a basaltic magma), and discuss their implications for the volatile composition of un-degassed magmas from the upper mantle. We find homogenous N isotope compositions (average δ15N of −4.49 ± 1.40 ‰ at 14°N) but variable δ13C (from −11 ‰ to −3.4 ‰), potentially compatible with the expectations for residual dissolved gas after Rayleigh fractionation. However, explaining the light δ13C signatures of PR via this process appears incompatible with any of the two scenarios proposed for explaining their low 4He/40Ar*, which would predict (i) 13C-enrichements and (ii) no fractionation relative to an initial composition at δ13C ∼ -5 ‰, respectively. After correction for solubility-controlled degassing fractionation and potential gas accumulation processes using 4He/40Ar* systematics, we find relatively homogeneous C/3He ((2.65 ± 0.51) × 109) but variable C/N (from 125 up to 4578), whose potential origins are discussed as part of a companion paper (Part B: Source effects). [ABSTRACT FROM AUTHOR]

Owing to their abundance and relative availability on Earth's seafloor, mid‐ocean ridge basalts (MORBs) have a well‐defined chemical element budget, reflected by the low standard deviation associated with typical normal MORB (N‐MORB) composition. However, the exact mechanisms leading to magma differentiation and MORB generation remain debated, which hinders our ability to evaluate MORB parental magma composition. In this study, we leverage the predictive power of the BDD21 numerical framework to obtain a representative trace element budget of parental MORB magma and assess its ability to fractionate into the N‐MORB composition. Utilizing revised parameterizations for mineralogy, melting, and partitioning, we couple BDD21 with numerical simulations of a MOR system driven by seafloor spreading in which we track the evolution of partial melting, mineral modal abundances, and concentrations of incompatible elements. Parental magma compositions are determined once simulations reach a steady state, and magma chamber replenishment models are employed to predict the trace element budget of the erupted liquid. We explore a range of geophysical and geochemical parameters to evaluate their effect on computed trace element concentrations. Previous magma chamber replenishment models are extended to account for multiple crystallization events and melt‐crystal interaction. Modeling outcomes suggest that petrologically constrained fractionation of parental magma compositions obtained through BDD21 yields glass compositions compatible with the N‐MORB budget. Nevertheless, our results show a systematic underestimation of Sr concentration, indicating the presence of recycled oceanic crust in the MORB source region. Plain Language Summary: Beneath Earth's oceans, tectonic plates spread apart at mid‐ocean ridges, generating volcanic regions whose magmatic output forms Earth's oceanic crust. The rocks composing the oceanic crust represent the solidification of magma produced by partially melting deeper mantle rocks. Accordingly, they carry a message about the nature of Earth's interior. To decipher this message, scientists recover rock samples from the seabed and analyze their composition to infer the nature of that otherwise inaccessible deep layer. Unfortunately, the chemical inventories obtained for these rocks reveal that significant processing of the magma occurs as it ascends toward the surface. Therefore, the relationship between crustal rocks and their deep parental counterparts is indirect. Here, we address this complexity using a two‐step process. First, we simulate the generation and composition of partial melts derived from mantle peridotites (archetypal upper‐mantle rocks) using a mid‐ocean ridge numerical model. Second, we employ models of magma chamber processes to account for the expected magmatic evolution of the generated liquid. By optimizing and extending existing magma chamber models, we obtain an accurate representation of rock compositions sampled from Earth's seabed. Our findings provide a new opportunity to decipher the nature of Earth's interior. Key Points: Refined the BDD21 peridotite melting and melt chemistry framework to reproduce geochemical observations of the mid‐ocean ridge systemImproved calibration of peridotite melting parameterization revises solidus and yields appropriate crustal thickness for mid‐ocean ridgesDepleted mantle compositions underestimate Sr concentration in basalts, requiring a prevalent contribution from recycled oceanic crust [ABSTRACT FROM AUTHOR]

Lamprophyres are mantle-derived rocks characterized by enrichment of incompatible elements and volatiles; however, the origin of their enriched sources remains enigmatic. Here we present zinc isotopic data for contemporary Mesoproterozoic (~ 1.1 Ga) lamprophyres from three localities of the Eastern Dharwar Craton. The Mudigubba and Kadiri lamprophyres with island-arc basalt (IAB)-like trace element features, such as positive Pb and negative Nb–Ta, Zr–Hf and Ti anomalies, have mid-ocean ridge basalt (MORB)-like δ66Zn values ranging from + 0.22‰ to + 0.29‰. In contrast, the Udiripikonda lamprophyre shows higher-than-MORB δ66Zn values of + 0.39‰ to + 0.48‰ and elemental features of intra-plate magmas with a lack of pronounced Nb–Ta-negative anomalies. Based on the covariations between Zn isotopes and trace element ratios, we infer that the Mudigubba and Kadiri lamprophyres with MORB-like Zn isotopes and high Ba/La, K/Nb and low Nb/La, Ce/Pb ratios are inherited from sub-continental lithospheric mantle metasomatized by fluids derived from a subducted slab. On the contrary, the higher-than-MORB Zn isotopic compositions, with low Ba/La, K/Nb and high Nb/La, Ce/Pb, K/U and Ba/Th ratios for the Udiripikonda lamprophyre are inferred to derive from lithospheric mantle enriched by carbonatitic melts or subducted carbonate-bearing sediments within the mantle transition zone. Hence, our study suggests that contemporary lamprophyres can be derived from disparate enriched mantle sources. [ABSTRACT FROM AUTHOR]

Mantle oxidation plays an essential role in volatile cycling between Earth's interior and exterior. However, the conceivable oxidizer remains enigmatic and needs to be explored by geochemical tools. Here, we examine the potential of recycled carbonates in inducing redox changes of the deep mantle through combined isotopic studies of Mg (δ26Mg), Zn (δ66Zn), Fe (δ56Fe), and Cu (δ65Cu) on a suite of intraplate alkaline basalts proposed to have originated from the mantle transition zone (∼440–660 km). Compared with mid‐ocean ridge basalts (MORB), these basalts have variably lower δ26Mg and higher δ66Zn, the typical features of marine carbonates, indicating the presence of various amounts of deeply recycled carbonates in their sources. As the estimated amounts of recycled carbonates in sources increase (i.e., lower δ26Mg and higher δ66Zn), the basalts have higher δ56Fe and lower δ65Cu compared with MORB. These correlations cannot be explained alone by binary mixing between recycled carbonates and the pristine mantle because of the much lower Fe and Cu contents of marine carbonates than those of the mantle and instead require redox‐driven isotope fractionation induced by mantle carbonate metasomatism. During partial melting of an oxidized mantle source, isotopically heavy ferric iron and isotopically light sulfide were preferentially incorporated/dissolved into the melts that are enriched in heavy Fe and light Cu isotopes. These observations highlight that recycled carbonates serve as an important oxidizer for the deep mantle and facilitate the extraction of mantle sulfides, which can be effectively tracked by Fe and Cu isotopic systematics of intraplate basalts. Plain Language Summary: It is widely suggested that the shallow mantle can be oxidized by fluids released from subducted slabs at subarc depths, but the primary oxidizer for the deeper mantle remains enigmatic. We analyze Mg, Zn, Fe, and Cu isotopes for a suite of continental intraplate alkaline basalts proposed to be originated from the deep mantle, which have lower δ26Mg and δ65Cu but higher δ66Zn and δ56Fe values compared to mid‐ocean ridge basalts. These correlations indicate that the mantle oxidation induced by recycled carbonate facilitates the incorporation of ferric iron and dissolution of sulfide into the melts. Thus, recycled carbonates can serve as an important oxidizer for the deep mantle through the reduction of carbonate itself into diamond. The redox state of the mantle affects the efficiency of volatile extraction and provides an important link between oxygen–sulfur cycling and deep carbon–iron redox interactions. Key Points: Large Mg‐Zn‐Fe‐Cu isotopic variations are observed in a suite of Cenozoic intraplate alkaline basaltsBasalts with lower δ26Mg and higher δ66Zn have higher δ56Fe and lower δ65CuRedox‐driven Fe and Cu isotopic variations record oxidation of the deep mantle by recycled carbonate [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]

Stable isotopes of chromium (Cr) and iron (Fe), both multivalent elements in silicate melts, are valuable proxies of redox conditions in magmatic systems. Partial melting and fractional crystallization have been identified as the dominant fractionation processes for Cr and Fe isotopes in high-temperature reservoirs that can lead to small but detectable isotope variations in igneous rocks. More recently, however, the source mineralogy has been suggested to play an important role, which can ultimately affect the magnitude of Cr and Fe isotopic fractionation during partial melting and thus the Cr and Fe isotopic composition of the originating melt. With the aim to investigate the role of magmatic processes and the source Cr and Fe isotope signatures on the isotope composition of these transition metals in normal mid-ocean ridge basalts (N-MORB), we analyzed 21 well-characterized MORB glasses from the southern East-Pacific Rise, Pacific-Antarctic Ridge and Mid-Atlantic Ridge. The Cr isotope compositions of N-MORB show a narrow range from −0.278 to −0.186 ‰ in δ53/52Cr (i.e., difference of a sample's 53Cr/52Cr ratio relative to the international reference material NIST SRM979), with an average value of −0.237 ± 0.050 ‰ (2SD; n = 19). As such, the average N-MORB δ53/52Cr value is significantly lower than that of Bulk Silicate Earth of −0.12 ± 0.06 ‰ and the Cr isotope compositions reported for most ocean island basalts ranging from ∼ −0.23 to 0.00 ‰. Iron isotopic compositions of these MORB range from +0.032 to +0.137 ‰ in δ56/54Fe (i.e., difference of a sample's 56Fe/54Fe ratio relative to the international reference material IRMM014) and are within the range of previously published MORB data. The lack of correlations between N-MORB Cr isotope signatures and indices of magmatic differentiation suggests a negligible control of this process on Cr stable isotopes. Partial melting modeling using pMELTS provides evidence that the significant offset towards lower δ53/52Cr values of N-MORB cannot be produced by decompression partial melting of a lherzolithic source such as the depleted upper mantle alone. Instead, we suggest that the lower δ53/52Cr values of MORB could be linked to intrinsic small-scale 53Cr-depleted pyroxene-rich domains in the upper mantle that influence the melt's Cr isotopic budget during partial melting. [ABSTRACT FROM AUTHOR]

Deyin-1 hydrothermal field is located on the Southern Mid-Atlantic Ridge (SMAR) 15°S. The geochemical characteristics of substrate basalts record the incorporation of Saint Helena plume components in the magma source, which implies that the hydrothermal mineralization characteristics in Deyin-1 might be influenced by the mantle plume indirectly. In this study, the pyrite trace element contents and sulfide in situ Pb and He-Ar isotope compositions of a chimney sample from Deyin-1 field are determined systematically in order to clarify the mantle plume influence on local hydrothermal mineralization process. The higher Ba/Co ratios in Deyin-1 pyrites, compared with those from other mid-ocean ridge hydrothermal fields, may be related to the Saint Helena mantle plume influence. Besides, Deyin-1 pyrites possess lower Pb/As, Ag/As, Ni/As ratios and higher As contents than ultramafic-hosted hydrothermal fields, indicating the Deyin-1 is a mafic-hosted hydrothermal system. The higher Se, Co and lower Zn, As, Pb contents in pyrites from interior layers of the chimney demonstrate that the fluid temperature increases during the chimney growth. The in situ Pb isotope compositions of Deyin-1 sulfides are similar to those of mantle plume-influenced basalts in SMAR 15°S and more radiogenic than that of the plume-free SMAR basalt, which demonstrates that the Saint Helena mantle plume has contributed parts of Pb (and other metal elements) to the Deyin-1 sulfides. The He isotope compositions of Deyin-1 sulfides also prove the mantle plume contribution to the hydrothermal system. End-member mixing calculation of Pb-He isotopes indicates that the Saint Helena plume components account for about 3% of the mineralization material sources in Deyin-1 sulfides. Many hydrothermal fields have been discovered near the hot spots besides Deyin-1. Our study emphasizes that mantle plume influence on hydrothermal mineralization might be ubiquitous and needs to be revaluation. This study might shed some light on modern seafloor hydrothermal mineralization process and lithosphere-seawater mass exchange. [ABSTRACT FROM AUTHOR]

Basalts formed during the early development of an arc are usually buried, but they record critical information relating to the mantle source of the overriding plate prior to the addition of subducting slab components. Basalts recovered at Site U1438 of International Ocean Discovery Program (IODP) Expedition 351, in the Amami Sankaku Basin (ASB), formed during the transition from forearc basalt (FAB) and boninite eruptions to stratovolcano developments in the Izu-Bonin-Mariana (IBM) arc. In this study, we present magnesium (Mg) isotopic data (δ26Mg) for the ASB basalt core samples, formed at arc inception in the absence of down-going slab components, and report the implications of these data for the characteristics of the mantle sources underlying the proto-IBM arc. The δ26Mg values of the ASB basalts range from −0.21‰ to +0.08‰, with an average value of −0.13 ± 0.07‰. These values are systematically higher than those of mid-ocean ridge basalts (MORBs) (−0.25 ± 0.06‰). No obvious effects of post-eruptive alteration, fractional crystallization, partial melting, or subduction component addition can be identified in the Mg isotopic compositions either of the ASB basalts or their mantle source, given the absence of correlations between the δ26Mg values and the proxies of these processes (e.g., K/Nb, Nb/Zr, Ba/La, Nb/Y, etc.). We conclude that the high δ26Mg values of the ASB basalts are inherited from their mantle source that was enriched in heavy Mg isotopes. Melting of an ultra-depleted, refractory spinel peridotite source containing talc-bearing serpentinite components can explain the origin of the heavy Mg isotopic compositions in the ASB basalts. The presence of water-rich serpentinite components in the mantle provides a new perspective on how a refractory mantle source can undergo partial melting at temperatures and pressures similar to the formation of MORBs without the influence of subducted materials, which has important implications for the initiation of plate subduction. Furthermore, the Mg isotopic contributions of subducted additions and mantle source materials should be carefully discussed in future studies due to the heterogeneity of Mg isotopes in the mantle. [ABSTRACT FROM AUTHOR]

As relics of oceanic lithosphere, ophiolites exposed in the East Kunlun region are crucial for deciphering the evolution from the Proto-Tethys to the Paleo-Tethys. Two ophiolite belts have been recognized in the East Kunlun Orogenic Belt: the northern Heishan-Dagele-Qingshuiquan Ophiolite Belt and the southern Muzitag-A'nyemaqen Ophiolite Belt. Most basaltic rocks in the northern ophiolite belt show apparent geochemical features of enriched mid-ocean ridge basalts, with some of suprasubduction zone-type affinity, and probably formed in arc to back-arc settings with ages of 537–406 Ma. Cumulates (wehrlites) and arc-related volcanics in these ophiolite complexes also display a subduction-related signature. However, most basalts in the southern ophiolite belt exhibit characteristics of mid-ocean ridge basalts and plume-related basalts, which are unrelated to subduction zone. The anhydrous troctolite outcrops in the southern belt also support this viewpoint. Statistics of all the available age data for the southern ophiolites indicate two cycles of seafloor spreading or plume activity, which gave rise to two main epochs of magmatic activity at 535–450 Ma and 345–308 Ma, respectively. The northern ophiolite belt recorded the evolution of the Proto-Tethys Ocean from at least the Early Cambrian to the Late Silurian. Based on geochronological and geochemical comparisons, the southern ophiolite belt shows similarities with the ophiolites in the Longmucuo-Shuanghu suture of Qiangtang on the south, which record the closure of Proto-Tethys Ocean and the opening of the Paleo-Tethys Ocean. The spatial and temporal distribution of the two ophiolite belts in the East Kunlun Orogenic Belt reveals the long-lived evolution from the Proto-Tethys to Paleo-Tethys system. [ABSTRACT FROM AUTHOR]

Water is critical in the evolution of the mantle due to its strong influence on the physicochemical properties of mantle rocks. Mid‐ocean ridge basalts (MORBs) are commonly used to study the compositional characteristics of the convecting upper mantle. However, there remains abundant samples in the global MORB data sets without direct measurements of water contents. The commonly observed good correlation between H2O and other incompatible trace components, such as Ce, has been applied to quantify water contents of MORBs. However, this approach assumes constant H2O/Ce in the target samples, which is not always true as the H2O/Ce ratios of MORBs could be rather heterogeneous even in some short ridge segments. Utilizing the present compositional data of global MORB glasses with measured water contents (n = 1,467), we construct a Random Forest Regression model based on machine learning, which can predict water concentrations of samples based on selected major and trace element data, without assuming a ratio between H2O and other trace elements. This model allows us to precisely recover water contents for MORBs with comparable accuracy with traditional analytical methods. The predicted results of MORB glasses from this model (n = 1,931) expand the water content database of global MORBs and indicate a broad existence of high‐H2O MORBs. This new approach allows us to investigate the water content of MORBs from some ridges lacking previous water content measurements (e.g., the Chile Ridge and the Pacific‐Antarctic Ridge) and infer changes in the water content of MORB sources through applying the model to transform fault samples. Plain Language Summary: There are trace amounts of water in the mantle, which can strongly influence the physicochemical properties of the mantle; therefore, exploring the distribution of water is significant for understanding the evolutionary processes of the mantle. Mid‐ocean ridge basalts (MORBs) are derived from the upper mantle, and MORB glasses usually preserve the compositional information (particularly water) of the upper mantle. Based on compiled MORB glass compositional data, we constructed a machine learning‐based regression model, which can precisely predict the water contents of samples by inputting their major and trace element data. Its application largely expands the global MORB geochemical data sets. The new model provides a reliable and low‐cost way to obtain water data from MORB samples and it also provides a new perspective to understand the temporal evolution of transform faults and constrain the water out‐flux through mid‐ocean ridges across the geological time. Key Points: A machine learning‐based model is established to predict water contents of mid‐ocean ridge basalt (MORB) glasses based on their major and trace element dataThe predicted results largely expand the water database of global MORBs with comparable uncertainty of normal instrumental analysis [ABSTRACT FROM AUTHOR]

Oceanic plateaus inevitably interact with trench-arc systems during oceanic subduction, but the subduction mechanisms are poorly understood. The Yap trench-arc system is an important part of the western Pacific subduction zone, and is undergoing ultra-slow subduction of the Caroline Plateau. As such, it is ideal for investigating the magmatic-tectonic processes of oceanic plateau subduction. In this study, we document the mineralogy and geochemistry of serpentinized peridotites from the Yap-Mariana Junction. These peridotites can be subdivided into two groups. Group I samples have intermediate to high whole-rock CaO contents and Al.sub.2O.sub.3/SiO.sub.2 values, low chromite Cr# (as low as 0.16) and high chromite Ga/Fe.sup.3+# values, and low pyroxene Mg# values relative to Mariana Forearc peridotites. These features are consistent with those of abyssal peridotites. Based on La/Sm ratios, two types of clinopyroxene are identified in group 1 samples. Melting models show that the light rare earth element (LREE)-depleted patterns of type 1 clinopyroxene can be produced by ~ 5% fractional melting or ~ 6% open-system melting of depleted upper mantle, while the nearly flat REE patterns of type 2 clinopyroxene are related to LREE-enriched melts. These clinopyroxene compositions record melt impregnation and reactions in the mantle beneath the Yap-Mariana Junction. In contrast, group 2 samples have similar compositions to forearc peridotites, such as low whole-rock Al.sub.2O.sub.3/SiO.sub.2 values, and high chromite Cr# (up to 0.73) and low chromite Ga/Fe.sup.3+# values. The different trends in plots of chromite Cr#-TiO.sub.2 and 100Ti/Fe.sup.3+#-Ga/Fe.sup.3+# indicate that the group 1 samples experienced reactions with mid-ocean ridge basalt-like melts, whereas the group 2 samples experienced reactions with island arc tholeiite-like melts. The Mg#, TiO.sub.2, and Al.sub.2O.sub.3 values of the melts in equilibrium with chromite in the group 1 samples are consistent with those of the Parece Vela Basin basalts, while those from group 2 samples define a compositional trend similar to arc-related volcanic rocks. Therefore, this region experienced a tectonic transition from a back-arc spreading ridge to a subduction zone. Compared with the Mariana Trench that is undergoing normal oceanic crustal subduction, the absence of volcanic rocks at the Yap Trench are the response to ultra-slow subduction of the Caroline Plateau., Author(s): Junhua Yao [sup.1] [sup.2], Guoliang Zhang [sup.1] [sup.2], Ji Zhang [sup.1] [sup.2], De-Feng He [sup.3] Author Affiliations: (1) grid.9227.e, 0000000119573309, Center of Deep Sea Research, Institute of Oceanology, Chinese [...]

The contribution of venting fluids at mid-ocean ridges to global ocean biogeochemical cycles is well recognized. Less is known about the role of magmatically-active intra-plate volcanoes. In this study, new compositional fluid data were acquired from 20 to 50 °C vent fluids at Kamaʻehuakanaloa (previously known as Lōʻihi) seamount (Hawai'ian archipelago) and used to model the wide diversity of reaction conditions capable of producing the Fe-, Si- and CO 2 -rich vent fluids observed. Our conceptual model includes a first step where seawater reacts with increasing proportions of basalt and gas as the temperature increases, and a second step where the resulting hydrothermal fluid mixes with unaltered seawater while continuing to react with basalt until the fluid mixture reaches 20 °C. A series of reaction paths were chosen to vary: the maximum temperature during Step 1 (50 to 400 °C) and the proportions of basalt and gas reacting; the degree, F , of low-temperature basalt alteration during Step 2, which corresponds to the extent to which the hot fluid generated during Step 1 continues to react with more basalt as it ascends to the seafloor. Our model shows that the 20–50 °C vent fluids are greatly dependent on the degree of low-temperature basalt alteration during fluid upwelling. Indeed, the compositions of Kamaʻehuakanaloa vent fluids cannot be reconciled with a general model of subsurface mechanical mixing of high-temperature end-member vent fluid and seawater alone. Instead, they require both subsurface equilibrium mixing between a ≥350 °C hydrothermal fluid end-member and seawater and further basalt alteration that must occur as the fluid mixture rises to the seafloor. Although it involves only ∼4% of the amount of basalt having reacted during Step 1, this low-temperature basalt alteration during Step 2 leads to the characteristic enrichments in Fe observed in the Kamaʻehuakanaloa vent fluids and a concomitant depletion in H 2 S. We hypothesize that low-temperature basalt alteration during an extended path of fluid upwelling through the subseafloor might arise as a direct consequence of the height and steep-sloped topography of Kamaʻehuakanaloa seamount. If correct, this suggests a more general case - that input from magmatically-active intraplate volcanoes, which have been relatively overlooked throughout the history of submarine vent investigations to date, could differ significantly from global mid-ocean ridge fluxes and contribute more substantially than previously recognized to the global ocean Fe cycle. [ABSTRACT FROM AUTHOR]

Zinc (Zn) isotope data for mid-ocean ridge basalts (MORB) are scarce. The possible effects of MORB differentiation and mantle compositional heterogeneity on MORB Zn isotope compositions are poorly understood. In this paper, we report Zn isotope data of MORB lavas from the East Pacific Rise (EPR) at 10°30′N, which are understood to have been derived by a similar extent of melting from a compositionally uniform mantle source but manifest significantly varying extents of magma differentiation (MgO = 7.38–1.76 wt%) to demonstrate unequivocal Zn isotope fractionation during MORB differentiation. These lavas have a small δ66Zn (relative to the JMC-Lyon standard) range of 0.21–0.32‰, with an average of 0.26 ± 0.07‰ (2SD, N = 24), overlapping with literature data. However, δ66Zn values show significant correlations with indices of magma differentiation (e.g., MgO and CaO), with the more differentiated samples having isotopically heavier Zn. We can thus conclude that MORB differentiation can lead to ∼0.1‰ δ66Zn elevation, which is similar to that observed in Hawaiian Kilauea Iki lava lake samples. Fractional crystallization of olivine and clinopyroxene at MgO > ∼3.5 wt% and clinopyroxene and Fe–Ti oxides dominated by ilmenite at MgO < ∼3.5 wt% results in the observed Zn isotope fractionation. The δ66Zn values for primitive N-MORB and MORB source mantle are estimated to be ∼0.22‰ and ∼0.17‰, respectively. Furthermore, we observe a positive correlation between δ66Zn and [La/Sm] N in primitive (MgO = 7.58–9.01 wt%) literature MORB, indicating the existence of possible Zn isotope heterogeneity in the oceanic upper mantle. We thus predict heavy Zn isotope enrichment in mantle sources for E-MORB samples with high [La/Sm] N , which requires future testing. [ABSTRACT FROM AUTHOR]

Accreted basaltic rocks are expected to provide information on intraplate volcanism in the oceans. Basaltic rocks, originating from mid‐ocean ridges, plateaus, and seamounts, have been reported from exhumed accretionary complexes. However, basaltic rocks related to petit‐spot volcanism remain poorly documented. We examined basaltic intrusive rocks in an exhumed accretionary complex on Amami‐Oshima Island, Ryukyu Arc. Basaltic sills intruded mélange composed of seamount‐derived micritic limestone and basaltic rocks, which deformed together with the mudstone‐dominated mélange in the subduction zone. A separate basaltic sill intruded pelagic chert before being incorporated as a mélange block in the mudstone matrix during deformation in the subduction zone. Geochemical discrimination diagrams and trace element patterns indicate that the accreted dolerites and nonintrusive basalts likely originate from mid‐ocean ridges and hotspots, respectively. On the other hand, the geochemical characteristics of basaltic sills are compatible with alkaline basalt with enriched in incompatible trace elements relative to other basaltic rocks of hotspot origin and may represent a low degree of partial melting from a deeper mantle source (>90 km). A higher ratio of gadolinium to ytterbium in the basaltic sills relative to hotspot or mid‐ocean ridge‐related basalt indicates both a deeper melting depth, and a magma source which upwelled through thick oceanic lithosphere far from the mid‐ocean ridge. Based on the timing of basaltic intrusion and geochemical features, we suggest that the basaltic intrusive rocks could record past petit‐spot volcanism in the oceanic plate. Plain Language Summary: Basaltic rocks in exhumed accretionary complexes are crucial for understanding volcanism in oceanic plates. Petit‐spot volcanoes on modern seafloor have been found in the region seaward of the trench, derived from rising basaltic magma from the deep mantle along fractures from the bending of oceanic plates. However, past petit‐spot volcanism on already subducted oceanic plates remains poorly understood. This paper reports on the geological features and geochemical element analyses used to determine that basaltic rocks found in the Chichibu accretionary complex on Amami‐Oshima Island, Ryukyu Arc, originated from the deep mantle and intruded oceanic sedimentary rocks and seamount‐related rocks. We suggest that the basaltic intrusion resulted from petit‐spot volcanism. Our results highlight that basaltic rocks found in outcrops exhumed from subduction zones could contribute to understanding past petit‐spot volcanism in the oceanic plate that has completely subducted to the mantle. Key Points: Alkaline basaltic rocks intruded mélange in an exhumed accretionary complex on Amami‐Oshima IslandGeochemical features suggest that alkaline basaltic intrusive rocks originated from low‐degree partial melts generated in the deep mantleTiming and geochemical signatures of the alkaline basalt intrusions suggest they represent past petit‐spot volcanism [ABSTRACT FROM AUTHOR]

The South Tianshan Orogenic Belt marks the final assembly of the southern Altaids and the Karakum-Tarim Cratons. Integrated petrological, geochemical, and geochronological studies were carried out on the Akeyazi high-alumina basaltic lavas (HABs) and their host sandstones from the southern margin of the Central Tianshan Block. Given their relative high alumina contents (Al2O3 > 16 wt%) and geochemical similarity to basalts from modern arcs (e.g., Aleutian, Kamchatka, Izu, and Honshu arc), the Akeyazi basaltic lavas are classified as HABs. The Akeyazi HABs are distinguished by strong enrichment in large ion lithophile elements (LILE) and negative Nb-Ta anomalies. Furthermore, their mid-oceanic ridge basalt (MORB)-like Sr-Nd-Hf (87Sr/86Sr(i) ~0.705766, eNd(t) ~+2.5, eHf(t) ~+9.5), along with arc-like trace element patterns, indicate a petrogenesis derived from a mantle wedge metasomatized by pelagic, sediment-derived melts (2%-5% melts mixed, as confirmed by melt modeling). Zircon U-Pb ages of Akeyazi HABs and associated arc-related gabbros suggest that they were mainly erupted during ca. 415 Ma. Moreover, host sandstones of Akeyazi HABs exhibit a unimodal distribution with a peak at 480-410 Ma in the detrital zircon spectrum, which is notably distinct from those of the Central Tianshan Block, South Tianshan Accretionary Complex, and North Tarim Craton, each of which is multimodal. The maximum depositional age (MDA) of the meta-sandstones is 351.0 ± 4.3 Ma, with the weighted average of three oldest Paleozoic grains at 502 ± 26 Ma. Collectively, our data, for the first time, discloses an intra-oceanic arc setting from ca. 502-351 Ma during the northward subduction of the South Tianshan Ocean, which further reveals the history of multiple episodes of accretion in the southern Altaids. [ABSTRACT FROM AUTHOR]

The North China Craton records multiple metamorphic events related to supercontinent assembly during the Paleoproterozoic, forming Columbia, and again during the late Paleozoic, forming Pangea. Here we show that the Paleoproterozoic high-pressure granulites (HPGs) formed from enriched mid-ocean ridge basalt protoliths and record a clockwise pressure-temperature-time (P-T-t) path with prograde metamorphism at 7.8-10.0 kbar and 780-820 °C, peak granulite-facies metamorphism at 12-12.3 kbar and ~860-880 °C, and retrograde metamorphism at 8.7-9.1 kbar and 850-855 °C. Subduction initiated prior to 1.90 Ga, with final collision and orogeny at 1.88 Ga, followed by post-collision/exhumation at 1.80 Ga, defining a prolonged exhumation period (~90 m.y.) that occurred at a slow velocity of ~0.16 ± 0.08 mm/y. Late Paleozoic HPGs are normal mid-ocean ridge basalt type and record a near clockwise P-T-t path, with peak/post-peak amphibolitefacies metamorphism at 11.0-12.5 kbar and 860-890 °C, isothermal decompression to 7.2-7.5 kbar and 810-820 °C, and retrogression to 5.5-7.2 kbar and 805-850 °C. Subduction initiated earlier than ca. 340 Ma, exhumation and uplift initiated at 335-309 Ma and continued to 297-287 Ma. The exhumation was short-lived (~50 m.y.) and relatively fast (0.38 ± 0.14 mm/y). When compared to granulite-facies metamorphism documented in many Paleoproterozoic HPGs, late Paleozoic HPGs appear to commonly form with an initial period of steep subduction leading to eclogite-facies metamorphism, with subsequent exhumation to middle/lower levels of the crust. Our results further reveal that the exhumation velocity for supercontinent collision was facilitated and duration shortened through time, and that the exhumation mechanism might have been controlled by subduction angle, compression pressure, and temperature. [ABSTRACT FROM AUTHOR]

The lunar maria are large expanses of basalt that infill antecedent impact basins and show evidence for postemplacement deformation. Landforms within many of these basins suggest a period of compressive tectonics, although their formation mechanism has yet to be established. Previous work for Mare Crisium demonstrated that basin‐circumferential wrinkle ridges, which typically demarcate the inner edge of an annulus of elevated terrain, are the result of deep‐seated thrust faults that preferentially form along the boundary of an elevated superisostatic portion of mantle and a thick, subisostatic collar of crustal material. Here, we show that a similar fault architecture exists for several other mascon‐bearing basins, including Maria Serenitatis, Nectaris, Moscoviense, and, to a lesser degree, Humorum and Imbrium. These deeply penetrating basin‐circumferential thrust faults, as for Mare Crisium, form a (partial) outward‐dipping ring‐fault system that bounds the elevated mantle plug beneath each basin as a geometric consequence of mascon evolution. If this geometric arrangement is unique to the Moon, then some characteristic(s) of lunar mascon evolution enables the formation of such mascon‐bounding faults. Despite the ubiquitous nature of mascon‐bound thrust ring faults at several lunar basins, whether such structures exist at mascon basins on other terrestrial worlds remains an open question. Plain Language Summary: The history of the Moon is violent and characterized by giant impacts, the largest of which are expressed as vast depressions that were later filled with basaltic lavas. The topography within these impact basins suggests that they have been tectonically deformed, with the source of this tectonic deformation long subject to debate. In this study, we examine several large impact basins that host a concentration of mass, or a "mascon," under the ground. This mascon can be observed with the lunar gravity field data and is a result of the impact event itself. Features attributed to compressive stresses called "wrinkle ridges" are typically found in a circular pattern around the boundary of this mascon. We show that these structures sit on top of thrust faults that extend several tens of kilometers into the Moon's interior and outline the mascon in three dimensions. We have discovered these deep faults in several basins on the lunar near‐ and far sides, including Maria Crisium, Serenitatis, Nectaris, Moscoviense, Humorum, and Imbrium. It remains to be seen whether this property of lunar mascon basins is also present on other rocky worlds such as Mercury and Mars. Key Points: Deeply penetrating thrust faults form along the crust–mantle interface beneath numerous lunar mascon basins, comprising (partial) ring fault systemsShortening strain is localized along the crust–mantle boundary by the subsidence of a superisostatic mantle plug and uplift of a thickened crustal collarIn places, the formation of this (partial) ring‐fault system was likely impeded by a combination of basin formation timing and prevailing heat flux [ABSTRACT FROM AUTHOR]

Volcanic clasts from the International Ocean Discovery Program Site U1431 in the South China Sea (SCS) were analyzed for V isotopic compositions in this study. These volcanic clasts represent mantle-derived carbonated melts and record the transition from carbonated melts to alkali basalts. Based on the formation sequence, these clast samples were divided into early-stage (> 8.3 Ma) and late-stage (< 8.3 Ma) clasts. The early-stage volcanic clasts have δ51V values of −0.76‰ to −0.67‰, which provide an estimate for the V isotopic composition of primary carbonated melts. Their V isotopic compositions are slightly heavier than those of mid-ocean ridge basalts (−0.84‰ ± 0.10‰) and bulk silicate Earth (−0.86‰ to −0.91‰), reflecting the control of low-degree partial melting under conditions of high oxygen fugacity. The late-stage volcanic clasts have δ51V values ranging from −0.62‰ to 0.29‰, which are systematically heavier than those of early-stage volcanic clasts. The carbonated melts rose through and reacted with the lithospheric mantle and were transformed into alkali basalts by dissolving orthopyroxene and precipitating olivine and/or clinopyroxene. Because the reactants and reaction products do not lead to significant V isotope fractionation, the V isotopes show limited variations during transformation from carbonated melts to alkali basalts. The alkali basalts were emplaced into shallow crust by multiple pulses, in which the magma experienced variable degrees of magma differentiation, with δ51V values close to those of the initial alkali basalts or increasing due to fractional crystallization of Fe Ti oxides. The change in V isotopic compositions from early-stage volcanic clasts to late-stage volcanic clasts was driven by the varying magnitude of influence exerted by different magmatic processes en route from the deep mantle to the shallow crust. [ABSTRACT FROM AUTHOR]

• Mid-ocean ridge basalt samples exhibit variable and co-related Mg and Fe isotope composition. • The δ'-Δ' diagram proves the Mg isotope variations in MORB samples are dominated by the kinetic rather than equilibrium process. • The Mg and Fe isotope variations in the MORB samples cannot be explained by partial melting or magma differentiation processes. • Mg-Fe inter-diffusion between melt and residual solid is responsible for the Mg and Fe isotope variations in the MORB samples. We analysed forty-nine, hand-picked, mid-ocean ridge basalt (MORB) glasses for their Mg isotopic compositions using a high-precision, critical mixture double-spiking approach. Additionally, the Fe isotopic compositions of a subset (twenty-two) of these samples were measured. Samples from the East Pacific, Mid-Atlantic, South East Indian and East Scotia ridges have average δ26Mg ∼ −0.21 ‰ and δ57Fe ∼0.10 ‰, while those from the ultra-slow spreading Gakkel and South West Indian ridges have lighter Mg isotope compositions, δ26Mg as low as ∼ −0.32 ‰, and heavier Fe isotope compositions, δ57Fe up to 0.25 ‰. Overall, the samples show a striking negative correlation between δ26Mg and δ57Fe. Few MORB have δ26Mg as high as predicted by equilibrium models of melting and differentiation; in complementary fashion, measured δ57Fe are typically higher than modelled, equilibrium values. Furthermore, we show that the slope of 26Mg/24Mg and 25Mg/24Mg covariations in MORB is consistent with kinetic not equilibrium fractionation. These observations identify an important diffusive control on both Mg and Fe isotopic compositions of MORB. Fractional transport of melt beneath ridges juxtaposes low Mg/Fe fractional melts from depth with high Mg/Fe residues at the top of the melting column. We argue that diffusively limited, partial Mg-Fe exchange between melt and peridotite through which it migrates leads to melts becoming isotopically lighter in Mg and heavier in Fe. Low melt rock ratios from sub-adiabatic melting beneath the slowest spreading ridges result in the greatest diffusive exchange between peridotite and melt en route to surface. [ABSTRACT FROM AUTHOR]

Volcanic centers are characteristic features of ultraslow-spreading mid-ocean ridges, the least-explored parts of the global ridge system. Volcanic centers can provide insights into deep magmatic and metamorphic processes at these ridges. Here, we present data from the largest volcanic center on the Gakkel Ridge, the Langseth Ridge, situated at 60–62°E. Langseth is ∼10 km wide, consisting of three peaks that rise to 585 m water depth, some 3–4 km above the surrounding seafloor. It strikes perpendicular to Gakkel's spreading direction and can be traced for ∼40 km, which translates to an age of ∼8 Myr. Seafloor imaging revealed abundant (pillow) basalt but also fissures and geologic faults across the Langseth Ridge. Basaltic rocks were sampled at all summits and diabase at the slope of the northern summit that dips into the rift valley. Our samples are of normal to depleted mid-ocean ridge basalt composition and exhibit a wide range of major and trace element contents, due to magmatic processes, accumulation of macrocrysts, and hydrothermal alteration. Radiogenic isotope ratios, most notably 143Nd/144Nd and 208Pb/206Pb, trend from typical rift valley compositions to isotopically enriched values with increasing distance to the rift valley. This trend may imply melt pooling from different sources, potentially representing a shift from shallow melting beneath the rift valley to deeper melting of enriched sources and higher degrees of melting underneath Langseth. Mineral compositions and plagioclase sieve textures imply prolonged storage of magma at depth prior to eruption. Hydrothermal alteration occurred over a range of conditions. Basalt from the summits is weakly altered at temperatures ≪100 °C, which likely occurred in situ at the summit sites. Diabase samples experienced chloritization and albitization and display epidote and quartz veins, which formed at >300 °C. These assemblages and temperatures are typical for lower crustal levels and imply uplift of the samples of >1 km. Diabase samples from the Afanasenkov Seamount, another volcanic center on the Gakkel Ridge that we investigated for comparison, were altered under comparable conditions. Our findings suggest a combined volcanic–tectonic origin of the studied volcanic centers, potentially implying that such complexes may generally form due to the interplay of magmatism and tectonics. Researching volcanic centers has the potential to further our understanding of both deep and shallow crustal processes at ultraslow-spreading ridges, providing further insights into the role of these centers as linkages between lithosphere and hydrosphere and the (deep) biosphere they sustain. • First geochemical study of a volcanic center on Gakkel Ridge • Mixed volcanic–tectonic processes formed the seamount complex over >8 Myr • Nd–Pb isotope ratios imply different magma sources with distance to the ridge • Epidote–quartz veins in diabase formed at deeper crustal levels at >300 °C • Seamounts link geosphere, hydrosphere, and biosphere in the ice-covered Arctic Ocean [ABSTRACT FROM AUTHOR]

The Mohns Ridge is a very slow‐spreading ridge that, together with the Knipovich Ridge, marks the boundary between the North American and Eurasian plates in the Norwegian‐Greenland Sea. In this study, we report the major and trace element composition of spatially associated basalts and peridotites from a gabbro‐peridotite complex ∼20 km west of the Mohns Ridge rift flank. Formation of the ∼4–5 Myr crustal section involved accretion of normal mid‐ocean ridge basalts with Na‐content suggesting derivation from a depleted mantle source. This is consistent with the degree of partial melting estimated for clinopyroxene poor harzburgites using the Cr‐number of spinel (14%–18%) and rare earth element modeling of orthopyroxene (16%–24%) and reconstructed whole‐rock composition (14%–20%). If all the melting took place beneath the paleo‐Mohns Ridge, a crustal thickness of ∼7–8 km is expected, which is nearly double the observed thickness. Orthopyroxene trace elements are not consistent with typical fractional melting expected for mid‐ocean ridges but rather resemble that seen in supra‐subduction zone peridotites. The geochemistry of both the basalts and the peridotites suggests that a water‐rich slab flux in the past has influenced the mantle source. In turn, this caused hydrous melting which increased the depletion of the pyroxene components, leading to a highly depleted mantle that is now underlying much of the Arctic Mid‐Ocean Ridges and represents the source for the spreading related magmatism. Key Points: We report mantle peridotites and basalts from a 4–5 Myr tectonic window in the Mohns Ridge (Arctic Mid‐Ocean Ridges)The peridotite and basalt compositions reveal a highly depleted mantle with a subduction signatureThe anomalous depleted mantle is likely a consequence of an earlier hydrous melting event [ABSTRACT FROM AUTHOR]

The closure of Tethys Ocean has led to shaping the Central Pontides together with different types of rocks. In the east of Kargı (Gökçedoğan), which is located in the Central Pontides, allochthonous rocks of Late Palaeozoic- Early Cenozoic age and autochthonous units exist together, and allochthonous rocks present various tectono-stratigraphical sequences. Distinctive tectonic slices developed in the area as a consequence of the thrusting of the Permian carbonates from the north towards the south over the Middle Jurassic accretionary Complex (Kunduz Metamorphites) that form the basement of the area, and also because of the thrusting of the units of the Kirazbaşı Complex (ophiolitic melange) and Mesozoic carbonate rocks over this basement from the south towards the north because of the closure of Tethys Ocean. This paper presents whole-rock geochemical data, which suggest that the metabasites and the ophiolitic basalts are tholeiitic. The Jurassic metabasites from the Kunduz Unit and basalts from ophiolitic melange are characterized by the mid-ocean ridge basalt (MORB) and within plate tholeiites. This paper highlights the occurrence of the tectonic slices in the area is closely related to the tectonic evolution of the Tethys Ocean. [ABSTRACT FROM AUTHOR]

We report on the isotopic compositions of the Jurassic supra-subduction zone Evros ophiolite mafic rocks exposed in the eastern Circum-Rhodope Belt in the Thrace region of northeastern Greece. These mafic units consist of low-Ti gabbroic and basaltic rocks, which Nd-Sr-Pb isotopes are compatible with dominant mantle-derived Mid-Ocean Ridge Basalt (MORB) component mixed with a detectable amount of crustal material and/or sediment involved in their melt source in the subduction zone. These isotopic features are consistent with an intra-oceanic arc origin of the mafic ophiolite rocks, and the Evros ophiolite Nd and Pb isotopes are comparable to those of the counterpart mafic rocks from the Mandritsa Unit in the eastern Rhodope Massif of southern Bulgaria. [ABSTRACT FROM AUTHOR]

The basalts of the 2021 Fagradalsfjall eruption were the first erupted on the Reykjanes Peninsula in 781 years and offer a unique opportunity to determine the composition of the mantle underlying Iceland, in particular its oxygen isotope composition (δ18O values). The basalts show compositional variations in Zr/Y, Nb/Zr and Nb/Y values that span roughly half of the previously described range for Icelandic basaltic magmas and signal involvement of Icelandic plume (OIB) and Enriched Mid-Ocean Ridge Basalt (EMORB) in magma genesis. Here we show that Fagradalsfjall δ18O values are invariable (mean δ18O = 5.4 ± 0.3‰ 2 SD, N = 47) and indistinguishable from "normal" upper mantle, in contrast to significantly lower δ18O values reported for erupted materials elsewhere in Iceland (e.g., the 2014–2015 eruption at Holuhraun, Central Iceland). Thus, despite differing trace element characteristics, the melts that supplied the Fagradalsfjall eruption show no evidence for 18O-depleted mantle or interaction with low-δ18O crust and may therefore represent a useful mantle reference value in this part of the Icelandic plume system. The 2021 eruption in the Reykjanes Peninsula of Iceland was the first in 800 years and was supplied by melts from diverse mantle source domains with near-identical oxygen isotope ratios, providing a unique insight into the Icelandic mantle plume. [ABSTRACT FROM AUTHOR]

Chromium (Cr) isotopes represent a powerful tool for tracing the redox conditions during planetary magmatic evolution. However, so far, the systematic investigation of Cr isotope variation has been only performed on ocean island basalts (OIBs) in terrestrial magmatic rocks. Therefore, the Cr stable isotope compositions (expressed as δ53Cr relative to NIST SRM 979) of other magmatic rocks, formed under different oxygen fugacity, have remained unconstrained. In this paper, we present the first Cr stable isotopic data of mid-ocean ridge basalts (MORBs) from the Eastern Pacific Ocean ridge, the Indian ridge, and the Atlantic Ocean ridge. The oxygen fugacity of such basalts is different from that of the above-mentioned OIBs. The Rhyolite-MELTS model shows that the chemical composition variations in the studied basalts are induced by varying extents of fractional crystallization of olivine, clinopyroxene, plagioclase, and spinel. The δ53Cr values of the MORBs range from −0.27 ± 0.03‰ to −0.07 ± 0.02‰ (n = 28), and two distinct groups of basalts are identified based on the correlation between δ53Cr and MgO. On the one hand, the δ53Cr of group I basalts (−0.27 ± 0.03‰ to −0.14 ± 0.03‰; n = 24) are systematically lower than the established average value of the Bulk Silicate Earth (BSE) (δ53Cr = −0.12 ± 0.04‰; 2SD), which are positively correlated with their MgO and Cr concentrations, indicating that Cr isotopes are fractionated during magmatic differentiation. Moreover, Rayleigh fractionation modelling suggests that the crystallization of olivine, clinopyroxene, and spinel gives rise to the Cr depletion and thereby decreases δ53Cr values. On the other hand, group II basalts (−0.10 ± 0.03‰ to −0.07 ± 0.02‰; n = 4) exhibit higher δ53Cr values than group I with identical MgO and Cr concentrations. This is possibly associated with the crystallization of clinopyroxene under low pressure. The average Cr isotope composition (δ53Cr = −0.16 ± 0.02‰, n = 3) of the primitive basalts (MgO > 9%) represents that of the primary MORB melt, which is lighter than the average value of BSE. Using the non-modal melting equations, the δ53Cr of the MORB mantle source is estimated to be −0.12 ± 0.02‰ (2σ), which is consistent with that of BSE. Compared with the Cr isotopic data of OIBs from Fangataufa island, we find that the equilibrium fractionation factors (Δ53Cr crystal-melt = +0.04‰ to +0.13‰) of MORBs during fractional crystallization are larger than that of Fangataufa island lavas (Δ53Cr crystal-melt = 0.010 ± 0.005‰ for low-K suite, and 0.020 ± 0.010‰ for high-K suite; Bonnand et al., 2020a), indicating that the basalts from Fangataufa island have higher oxygen fugacity than those of MORBs analyzed in this study, which is strongly supported by their higher V/Sc ratios. [ABSTRACT FROM AUTHOR]

Ocean island basalts (OIBs) with Zn isotopic ratios higher than the normal mantle (δ66Zn = 0.17 ± 0.08‰) or mid-ocean ridge basalts (MORBs; δ66Zn = 0.27 ± 0.06‰) generally also have an enriched Sr-Nd isotopic signature, suggesting carbonate-bearing eclogites, whose protolith is inferred to be subducting altered oceanic crust, in their mantle source. On the contrary, continental intraplate basalts with high δ66Zn usually show depleted Sr-Nd isotopic signatures (i.e., decoupled Zn-Sr-Nd isotopic composition). To elucidate the origin of the decoupled Zn-Sr-Nd isotopic composition in continental intraplate basalts, we report the discovery of both coupled and decoupled Zn-Sr-Nd isotopic data for a suite of Cenozoic continental intraplate basalts from the Zhejiang province, Southeast China. These basalts display clear spatial and temporal geochemical variations, with early-stage inland low-silica samples presenting moderately enriched Sr-Nd isotopic signatures and high δ66Zn (coupled Zn-Sr-Nd isotopic composition, similar to OIBs), and later-stage coastal high-silica samples that display a pronounced δ66Zn decrease with increasing SiO 2 and 87Sr/86Sr and with decreasing alkali contents and 143Nd/144Nd (decoupled Zn-Sr-Nd isotopic composition). The early-stage basalts with coupled high Zn-Sr-Nd isotopic signatures are also more enriched in incompatible elements than any other basalts from eastern China reported so far. We explain the spatial and temporal geochemical variations of these basalts as the result of two main melting events: 1) the low-silica early-stage magmatism mostly occurs inland and results from high-pressure partial melting of a carbonated eclogite-bearing asthenospheric mantle. Because of the presence of a thick lithosphere limits the melting of the depleted mantle component, the signature of the Zn-Sr-Nd isotopically enriched, and more fusible carbonated eclogite is preserved. 2) At the later stage, magmatism mostly occurs on the coast where the subcontinental lithosphere is thinner. Hence, decompression melting progresses to shallower depth, resulting in an increase of the contribution from the depleted peridotite matrix and a dilution of the signal from the isotopically enriched fusible component. Further upwelling and in-situ melting at the base of the subduction-modified sub-continental lithospheric mantle (SCLM) explains both the decoupled Zn-Sr-Nd isotopic signature of the coastal basalts and their major and trace element variability. We further propose that decompression melting is driven by small-scale convection resulting from variations of lithospheric thickness. Our data highlight the importance of dynamic melting of carbonated eclogite-bearing asthenosphere and subsequent lithospheric melting in preservation and destruction of the coupled enriched Zn-Sr-Nd isotopic signature of carbonated eclogite component and generation of the apparent decoupled Zn-Sr-Nd isotopic signal commonly observed in continental intraplate basalts. [ABSTRACT FROM AUTHOR]

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Subducting oceanic crust is a critical driver of chemical and lithological heterogeneity within the deep mantle. Zinc isotopes (δ66Zn JMC-Lyon) have been widely utilized to trace recycled oceanic crust and surficial materials. However, the lower oceanic crust rocks are not well studied given limited sampling, making it unclear if the δ66Zn estimated from mid-ocean ridge basalts (MORBs) is valid to represent that of the bulk oceanic crust. In this study, we report δ66Zn data for a series of gabbro cumulates (n = 37) from the ultraslow-spreading Southwest Indian Ridge, alongside MORBs (n = 12) from the fast-spreading East Pacific Rise and the slow-spreading South Mid-Atlantic Ridge. MORBs across various spreading rates exhibit homogeneous δ66Zn values (0.27 ± 0.05 ‰, 2sd), in agreement with previous studies. In contrast, the gabbros with several magma episodes exhibit a significant variation of δ66Zn from 0.11 ‰ to 0.34 ‰, with an average of 0.22 ± 0.11 ‰ (2sd). Magma differentiation to form oceanic crust can partly explain such variation (∼ 0.10 ‰), and post-cumulus modification further extends this range. Given the voluminous lower oceanic crust, the weighted δ66Zn estimated for the bulk oceanic crust (0.23 ± 0.03 ‰) is lower than the MORB-based value, which reconciles with the predictions from mantle partial melting. Therefore, recycled oceanic crust with significantly varied Zn isotopic compositions would lead to heterogeneous δ66Zn of the metasomatized mantle. The overall lower δ66Zn value underscores the importance of surficial materials other than oceanic crust, such as carbonates, to explain high δ66Zn of many mantle-derived magmas. • Gabbros show highly variable δ66Zn, contrasting with homogeneous isotopes in MORBs. • Magma differentiation with post-cumulus modification accounts for δ66Zn variation. • The estimated δ66Zn of bulk oceanic crust (0.23 ± 0.03 ‰) is lower than MORBs. • Recycled oceanic crust would lead to heterogeneous δ66Zn of metasomatized mantle. [ABSTRACT FROM AUTHOR]

The timing and origins of the Early Cenozoic magmatic flare-up in southern Tibet remain controversial, mainly because of the lack of available data for the rare, small-volume, ocean island basalt (OIB)-like mafic rocks that can shed insights into this geological event. We have confirmed that Early Cenozoic OIB-like basalts in the Zhongba area, southern Tibet, were closely related to this magmatic flare-up, which occurred in response to deep mantle geodynamics. In this study, we present detailed geochronological and geochemical data for the Zhongba basalts. These Eocene (ca. 52 Ma) OIB-like basalts have negative K, Zr, Hf, and Ti anomalies, low Fe/Mn and high Fe/Zn ratios, and uniform and enriched Sr-Nd-Hf and HIMU-like Pb isotopic compositions (.sup.87Sr/.sup.86Sr.sub.(i) = 0.7052-0.7053; [epsilon]Nd.sub.(t) = - 1.7 to - 1.3; [epsilon]Hf.sub.(t) = - 1.1 to - 0.3; .sup.206Pb/.sup.204Pb = 19.83-19.87), indicative of derivation from carbonated peridotite. Based on the regional tectonic evolution, we propose that the carbonated peridotite source of the Zhongba basalts was generated by the reaction between depleted mid-ocean ridge basalt-like mantle peridotite and melts from subducted Neo-Tethys carbonate-bearing oceanic crust. This carbonated peridotite was possibly preserved at the thermal boundary between the lithosphere and asthenosphere and underwent partial melting to produce the Zhongba basalts in response to slab break-off of the Neo-Tethys oceanic lithosphere at ca. 52 Ma. The slab break-off triggered the magmatic flare-up and other geological processes (e.g., a decrease in the velocity of the Indian Plate) in southern Tibet during the Eocene., Author(s): Ming Lei [sup.1] [sup.2], Jianlin Chen [sup.1] [sup.2], Dan Wang [sup.1] [sup.2], Ying Luo [sup.1] [sup.2], Yixin Liu [sup.1] [sup.2] Author Affiliations: (1) grid.9227.e, 0000000119573309, State Key Laboratory of [...]

Bathymetry and acoustic imagery swath mapping, along with observations and samples from four manned submersible and four ROV dives, confirm that a seafloor slope break on the northern approaches to Kaiwi Channel, between the islands of Oʻahu and Molokaʻi, Hawaiʻi is a former shoreline, now submerged ∼800 m below present sea level. Subaerially emplaced, low-relief basaltic lavas above the slope break transition to submarine morphologies below. The entire region has been tilted about 1° to the SSE (150°), and is cut by an 8–15 m-high, north-facing scarp, 100–400 m south of the slope break. The distribution of platy, table-top, and rarer mounded branching corals indicates the former presence of fringing reefs around low-relief paleo-islands. We infer that the regional tilt resulted from loading by younger Hawaiian volcanoes, compounded by flexural uplift and back tilting away from the unloaded footwall of a flank landslide to the north. Basalt samples collected from both above and below the slope break have petrography, chemical composition, and age (1.64–1.80 Ma) indicating correlation with the (late-shield) Lower Member of the East Molokaʻi Volcanics, rather than with the more proximal volcano of West Molokaʻi. The most likely source of the Kaiwi basalts is a submarine ridge (rift zone) that extends northwest away from ʻĪlio Point on West Molokaʻi. Although the submarine ridge was previously assumed to be an extension of West Molokaʻi's northwest rift, we conclude that regional bathymetry and gravity are consistent with this feature being an extension of the west rift of East Molokaʻi. A corallary of this interpretation is that the shoreline slope break (SSB 7 of Taylor, 2019) in this area is distinct from and younger than the southern SSB 7 formed on West Molokaʻi volcano (∼1.65 Ma vs. ∼1.8 Ma). • A seafloor slope break beneath Kaiwi Channel at ∼800 m below sea level was a shoreline ∼1.65 Ma. • Basalt samples from there are correlative in composition and age with late-shield lavas from East Molokaʻi. • Their most likely source is an extension of the west rift zone of East Molokaʻi along a submarine ridge NW of ʻĪlio Point. [ABSTRACT FROM AUTHOR]

This paper is concerned with new petrological-geochemical, and isotope-geochronological data for the igneous rocks that compose the Khulam volcano-plutonic complex in the central part of the North Caucasus (Kabardino-Balkaria). It is shown that the moderately-alkaline rocks of the Khulam complex are represented by a bimodal association of gabbro + basaltic trachyandesites and trachytes and rhyolites. The fractional crystallization had a leading role in the formation of this bimodal association. Assimilation of the material of host crustal rocks by magmatic melts did not exert any appreciable influence. According to the results of U‒Pb and K‒Ar dating, the Khulam rocks were formed in the Middle Jurassic (Bajocian–Callovian, 167 ± 4 Ma ago). It was found that the Khulam rocks possess geochemical characteristics that are similar to those for intraplate continental rocks, and were erupted in a postcollisional geotectonic setting (when the subduction in the region came to an end) in a local extensional zone that arose in the rear of a former active continental margin. The magma melts that were generated by the mantle source of the Khulam complex had overall geochemical characteristics proper to enriched mid-oceanic ridge basalts (E-MORB). [ABSTRACT FROM AUTHOR]

Recent studies investigating magmatic volatile contents indicate widespread enrichment of carbon, sulfur, and halogens in ocean island basalts (OIBs). At El Hierro in the Western Canary Islands, magmas with exceptionally high CO2 and S contents have been erupting throughout the Holocene. High S content of up to 5200 ppm requires an oxidised mantle source, but estimates of initial magmatic oxygen fugacity (f O2) are sparse. Here, we present estimates of f O2 and magmatic temperature for El Hierro together with a global mantle potential temperature dataset to evaluate redox and temperature conditions in the early stages of melt evolution for volatile-rich OIBs. Oxygen fugacities calculated using vanadium partitioning between melt inclusions (MIs) and their olivine hosts are >FMQ + 2.9 (2.9 log10 units above the fayalite-magnetite-quartz buffer), indicating that El Hierro magmas are highly oxidised. MI and matrix glass sulfur speciation data record f O2 between FMQ-1 to FMQ + 2; these values strongly depend on the position of the S2− to S6+ transition relative to the FMQ buffer. Nonetheless, glass sulfur speciation data record lower oxygen fugacity than V partitioning data, indicating MIs were able to maintain Fe3+/ΣFe and S6+/ΣS equilibrium with the surrounding melt during their evolution. The high f O2 of El Hierro magmas is coupled with an average mantle potential temperature estimate of 1443 ± 66°C (1σ, n  = 17) for the broader Canary Islands, which is slightly higher than the average potential temperature estimated for adjacent mid-ocean ridge segments (1427 ± 33°C, 1σ, n  = 474), albeit the two values are well within error. We find that ~98% of Canary Island rock compositions are not suitable for calculation of mantle potential temperatures using currently available methods. This is caused by the presence of substantial pyroxenite and volatile-enriched peridotite mantle domains under the Canary Islands. A wider compositional calibration of various petrological models is necessary to precisely determine mantle potential temperatures for volatile-rich alkali basalts. Our high oxygen fugacity estimates for El Hierro magmas reflect the fertile, fusible, and volatile-enriched nature of the mantle source beneath the Western Canary Islands. [ABSTRACT FROM AUTHOR]

The mass transfer history of rocks provides direct evidence for fluid–rock interaction within the lithosphere and is recorded by compositional changes, especially in trace elements. The general method adopted for mass transfer analysis is to compare the composition of the protolith/precursor with that of metamorphosed/altered rocks; however, in many cases the protolith cannot be sampled. With the aim of reconstructing the mass transfer history of metabasalt, this study developed protolith reconstruction models (PRMs) for metabasalt using machine-learning algorithms. We designed models to estimate basalt trace-element concentrations from the concentrations of a few (1–9) trace elements, trained with a compositional dataset for fresh basalts, including mid-ocean ridge, ocean-island, and volcanic arc basalts. The developed PRMs were able to estimate basalt trace-element compositions (e.g., Rb, Ba, U, K, Pb, Sr, and rare-earth elements) from only four input elements with a reproducibility of ~ 0.1 log10 units (i.e., ± 25%). As a representative example, we present PRMs where the input elements are Th, Nb, Zr, and Ti, which are typically immobile during metamorphism. Case studies demonstrate the applicability of PRMs to seafloor altered basalt and metabasalt. This method enables us to analyze quantitative mass transfer in regional metamorphic rocks or alteration zones where the protolith is heterogeneous or unknown. [ABSTRACT FROM AUTHOR]

Glasses quenched from relatively undegassed ocean island magmas erupted from volcanoes at Iceland, Hawaii, the Canary Islands, and Erebus have elevated Fe3+/∑Fe ratios compared to glasses quenched from mid-ocean ridge basalts. This has been ascribed to elevated fO2 of their mantle sources, plausibly due to subducted, oxidized near-surface-derived components in their mantle sources. The basaltic magmas from Reunion Island in the Indian ocean have Sr–Nd-Hf-Pb-Os isotopic compositions suggesting that their mantle sources contain little or no subducted near-surface materials and contain the C/FOZO/PREMA mantle component. To constrain the fO2 of the C/FOZO/PREMA mantle component and test the link between oxidized OIB and recycled surface-derived materials in their sources, we measured major and volatile element abundances and Fe3+/∑Fe ratios of naturally glassy, olivine-hosted melt inclusions from Piton de La Fournaise volcano, La Reunion. We conclude that the fO2 of the mantle source of these Reunion lavas is lower than of the mantle sources of primitive, undegassed magmas from Hawaii, Iceland, the Canary Islands, and Mt. Erebus, and indistinguishable from that of the Indian-ocean upper mantle. This finding is consistent with previous suggestions that the source of Reunion lavas (and the C/FOZO/PREMA mantle component) contains little or no recycled materials and with the suggestion that recycled oxidized materials contribute to the high fO2 of some other OIBs, especially those from incompatible-element-enriched mantle sources. Simple mixing models between oxidized melts of EM1 and HIMU components and relatively reduced melts of DMM can explain the isotopic compositions and Fe3+/∑Fe ratios of lavas from Hawaii, Iceland, the Canary Islands, and Mount Erebus; this model can be tested by study of additional OIB magmas, including those rich in the EM2 component. [ABSTRACT FROM AUTHOR]

New geochemical and [.sup.40]Ar/[.sup.39]Ar hornblende and biotite data from the Grenvillian Trenton Prong inlier provide the first constraints for the identification of lithotectonic units, their tectonic setting, and their metamorphic to post-metamorphic history. Gneissic tonalite, diorite, and gabbro compose the Colonial Lake Suite magmatic arc that developed along eastern Laurentia prior to 1.2 Ga. Spatially associated low-and high-Ti[O.sub.2] amphibolites were formed from island-arc basalt proximal to the arc front and mid-ocean ridge basalt-like basalt in a back-arc setting, respectively. Supracrustal paragneisses include meta-arkose derived from a continental sediment source of Laurentian affinity and metagraywacke and metapelite from an arc-like sediment source deposited in a back-arc basin, inboard of the Colonial Lake arc. The Assunpink Creek Granite was emplaced post-tectonically as small bodies of peraluminous syenogranite produced through partial melting of a subduction-modified felsic crustal source. Prograde mineral assemblages reached granulite-to amphibolite-facies metamorphic conditions during the Ottawan phase of the Grenvillian Orogeny. Hornblende [.sup.40]Ar/[.sup.39]Ar ages of 935-923 Ma and a biotite age of 868 Ma record slow cooling in the northern part of the inlier following the metamorphic peak. Elsewhere in the inlier, biotite [.sup.40]Ar/[.sup.39]Ar ages of 440 Ma and 377-341 Ma record partial to complete thermal resetting or new growth during the Taconian and Acadian orogens. The results of this study are consistent with the Trenton Prong being the down-dropped continuation of the Grenvillian New Jersey Highlands on the hanging wall of a major detachment fault. The Trenton Prong therefore correlates to other central and northern Appalachian Grenvillian inliers and to parts of the Grenville Province proper. Key words: Mesoproterozoic, Appalachian Grenvillian inliers, Ottawan Orogeny, Trenton Prong, [.sup.40]Ar/[.sup.39]Ar thermochronology. De nouvelles donnees geochimiques et de [.sup.40]Ar/[.sup.39]Ar dans la hornblende et la biotite de l'enclave de Trenton (<< Trenton Prong>>) d'age grenvillien fournissent les premieres contraintes pour l'identification d'unites lithotectoniques, de leur milieu tectonique et de leur histoire metamorphique et post-metamorphique. Des tonalites, diorites et gabbros gneissiques forment l'arc magmatique de la serie de Colonial Lake qui s'est developpe le long de la Laurentie orientale avant 1,2 Ga. Des amphibolites pauvres et riches en Ti[O.sub.2] qui leur sont associees dans l'espace se sont formees a partir de basalte d'arc magmatique proximal par rapport au front de l'arc et de basalte s'apparentant a du basalte des dorsales medio-oceaniques dans un milieu d'arriere-arc, respectivement. Des paragneiss supracrustaux comprennent des meta-arkoses issues d'une source de sediments continentale d'affinite laurentienne et des metagrauwackes et metapelites issus d'une source de sediments semblable a un arc deposes dans un bassin d'arriere-arc du cote continental de l'arc de Colonial Lake. Le granite d'Assunpink Creek, dont la mise en place est post-tectonique, se presente sous la forme de petits corps de syenogranite hyperalumineuse produite par la fusion partielle d'une source crustale felsique modifiee par la subduction. Les assemblages de mineraux progrades ont atteint des conditions metamorphiques du facies des granulites a celui des amphibolites durant la phase ottavienne de l'orogenese Grenvillienne. Des ages [.sup.40]Ar/[.sup.39]Ar sur hornblende de 935-923 Ma et un age sur biotite de 868 Ma temoignent du refroidissement lent de la partie nord de l'enclave apres le pic du metamorphisme. Ailleurs dans l'enclave, des ages [.sup.40]Ar/[.sup.39]Ar sur biotite de 440 Ma et 377-341 Ma temoignent d'une reinitialisation thermique complete ou d'une nouvelle croissance durant les orogeneses taconique et acadienne. Les resultats de la presente etude concordent avec l'interpretation voulant que l'enclave de Trenton soit le prolongement affaisse des hautes terres du New Jersey, d'age grenvillien, dans le toit d'un important decollement. L'enclave de Trenton est donc correlee a d'autres enclaves du centre et du nord de l'orogenese grenvillienne et a des portions de la province du Grenville proprement dit. [Traduit par la Redaction] Mots-cles: Mesoproterozoique, enclaves grenvilliennes dans les Appalaches, orogenese ottavienne, enclave de Trenton, thermochronologie [.sup.40]Ar/[.sup.39]Ar., Introduction Mesoproterozoic rocks are preserved in a series of inliers that form parallel western and eastern belts in the central Appalachians (Fig. 1). Inliers along the western belt are remnants [...]

Despite progress in understanding seafloor accretion at ultraslow spreading ridges, the ultimate driving force is still unknown. Here we use 40Ar/39Ar isotopic dating of mid-ocean ridge basalts recovered at variable distances from the axis of the Gakkel Ridge to provide new constraints on the spatial and temporal distribution of volcanic eruptions at various sections of an ultraslow spreading ridge. Our age data show that magmatic-dominated sections of the Gakkel Ridge spread at a steady rate of ~11.1 ± 0.9 mm/yr whereas amagmatic sections have a more widely distributed melt supply yielding ambiguous spreading rate information. These variations in spreading rate and crustal accretion correlate with locations of hotter thermochemical anomalies in the asthenosphere beneath the ridge. We conclude therefore that seafloor generation in ultra-slow spreading centres broadly reflects the distribution of thermochemical anomalies in the upper mantle. The ultimate driver of ultraslow spreading ridges is unknown. Here the authors use spreading rates derived directly from isotopic ages of seafloor samples to link magmatic and amagmatic segments with thermochemical variations in the upper mantle. [ABSTRACT FROM AUTHOR]

The compositional difference between subducting slabs and their surrounding lower-mantle can yield the difference in incorporation mechanism of Fe and Al into bridgmanite between both regions, which should cause heterogeneity in physical properties and rheology of the lower mantle. However, the precise cation-distribution has not been examined in bridgmanites with Fe- and Al-contents expected in a mid-ocean ridge basalt component of subducting slabs. Here we report on Mg0.662Fe0.338Si0.662Al0.338O3 bridgmanite single-crystal characterized by a combination of single-crystal X-ray diffraction, synchrotron 57Fe-Mössbauer spectroscopy and electron probe microanalysis. We find that the charge-coupled substitution AMg2+ + BSi4+ ↔ AFe3+(high-spin) + BAl3+ is predominant in the incorporation of Fe and Al into the practically eightfold-coordinated A-site and the sixfold-coordinated B-site in bridgmanite structure. The incorporation of both cations via this substitution enhances the structural distortion due to the tilting of BO6 octahedra, yielding the unusual expansion of mean bond-length due to flexibility of A–O bonds for the structural distortion, in contrast to mean bond-length depending reasonably on the ionic radius effect. Moreover, we imply the phase-transition behavior and the elasticity of bridgmanite in slabs subducting into deeper parts of the lower mantle, in terms of the relative compressibility of AO12 (practically AO8) and BO6 polyhedra. [ABSTRACT FROM AUTHOR]

Carbon contents and isotopic compositions were compared in the basalt groundmass of the oceanic crust of different age in the zone of the East Pacific Rise. In samples the basalt groundmass of the ancient oceanic crust (~270 Ma, ODP Site 801C) in which a carbonate phase was found, the isotopic composition of the oxidized carbon (δ13C = ±1.5‰) indicates that this carbon was formed by the precipitation of seawater dissolved inorganic carbon (DIC). In the samples in which no carbonate phase was identified, the low concentration (<0.1 wt % CO2) of oxidized dispersed carbon and its isotopic composition (δ13C < –7‰) are in the range of values typical of carbon dissolved in basalt glasses without crystallinity. This makes it possible to relate the oxidized dispersed carbon to residual carbon dissolved in the magmatic melt after CO2 degassing. The precipitation of DIC results in a positive correlation between the concentration of total carbon and its δ13C values, along with the formation of a carbonate phase. The application of this criterion to basalt groundmass samples of the young crust (~15 Ma, ODP Site 1256D) showed that oxidized dispersed carbon in the young oceanic crust groundmass was not formed by the precipitation of DIC, contradicting the generally accepted paradigm. Constant concentration and δ13C values of the reduced dispersed carbon in the basalt groundmass of the young and ancient oceanic crusts, including lithological zones where microbial activity has not been recorded, indicate that the most probable model is high-temperature abiogenic generation of reduced dispersed carbon near the ridge axis. The Fischer–Tropsch synthesis and/or Bell–Boudouard reaction provide a possible basis for the abiogenic model. The Bell–Boudouard reaction 2CO = C + CO2 leads to the formation of an adsorbed layer of elemental carbon on the fresh surfaces of minerals during background alteration of the oceanic basalt crust. The CO2–CO gas-phase equilibrium maintains the necessary depletion of the newly formed elemental carbon in the 13C isotope to δ13C < –20‰. Abiogenic models for the origin of the isotopically light reduced dispersed carbon in the basalt groundmass do not assume the presence of carbon depleted in the heavy 13C isotope in the magmatic melt. [ABSTRACT FROM AUTHOR]

Cores recovered by International Ocean Discovery Program Expedition 345 to the Hess Deep Rift (HDR) include lower crustal cumulates from the East Pacific Rise (EPR) and primitive basalts from the Cocos–Nazca Rift (CNR). This study presents major and trace element compositions of channel samples—the continuous strips of rock removed during core splitting—from this expedition. Consistently high Eu/Eu* anomalies (1·37–5·22) and strong correlations among major element oxides in samples of cumulates indicate that rock composition at the meter scale is controlled by the accumulation and segregation of plagioclase and olivine. However, constant Mg# (82·22 ± 0·66) among 13 samples through a ∼50 m interval suggests that this cumulus was host to percolating, replenishing melt(s). Modeling finds this rock composition to be in equilibrium with melts having Mg# = 58–61. This is identical to the mean value of EPR lavas (57·7 ± 6·2) and suggests that melt buffering by permeable crystal mush is a common and important process in controlling mid-ocean ridge basalt compositions at fast-spreading ocean centers. Analyses of the cumulates provide the most comprehensive composition of in situ , fast-spreading lower oceanic crust currently available. These are compiled with analyses of gabbros, dikes, and lavas from across the HDR to calculate the bulk composition of fast-spreading oceanic crust produced at the equatorial EPR. This bulk composition is strikingly similar to the composition of the primitive basalts from the CNR, and these compositions have nearly identical modeled fractional crystallization histories. Lower abundances of incompatible elements in the primitive basalt suggest that CNR magmatism is the result of the resumption of decompression melting in mantle that previously produced EPR crust. However, higher abundances of chalcophile elements in the CNR basalt point to a diversity of mantle melts that is not evident in calculations of the composition of bulk oceanic crust. [ABSTRACT FROM AUTHOR]

Basalts and peridotites from mid-ocean ridges record fO2 near the quartz-fayalite-magnetite buffer (QFM), but peridotite partial melting experiments have mostly been performed in graphite capsules (~ QFM-3), precluding evaluation of ferric iron's behavior during basalt generation. We performed experiments at 1.5 GPa, 1350–1400 °C, and fO2 from about QFM-3 to QFM+3 to investigate the anhydrous partitioning behavior of Fe2O3 between silicate melts and coexisting peridotite mineral phases. We find spinel/melt partitioning of Fe2O3 ( D Fe 2 O 3 spl / melt ) increases as spinel Fe2O3 concentrations increase, independent of increases in fO2, and decreases with temperature, which is consistent with new and previous experiments at 0.1 MPa. We find D Fe 2 O 3 opx / melt = 0.63 ± 0.10 and D Fe 2 O 3 cpx / melt = 0.78 ± 0.30. MORB Fe2O3 and Na2O concentrations are consistent with a modeled MORB source with Fe2O3 = 0.48 ± 0.03 wt% (Fe3+/ΣFe = 0.053 ± 0.003) at potential temperatures (TP) from 1320 to 1440 °C. The temperature-dependence of the D Fe 2 O 3 spl / melt function alone allows ~ 40% of the variation in MORB compositions. If we allow D Fe 2 O 3 opx / melt and D Fe 2 O 3 opx / melt to also vary with temperature by tying them to spinel Fe2O3 through intermineral partitioning, then all the MORB data are within error of the model. Our model Fe2O3 concentration for the MORB source would require that the convecting mantle be more oxidized at a given depth than recorded by continental mantle xenoliths. Our result is supported by thermodynamic models of mantle with Fe3+/ΣFe = 0.03 that predict fO2 of ~ QFM-1 near the garnet-spinel transition, which is inconsistent with fO2 of MORB. Our results support previous suggestions that redox melting may occur between 200 and 250 km depth. [ABSTRACT FROM AUTHOR]

Author(s): Fred A. Davis [sup.1] [sup.2], Elizabeth Cottrell [sup.2] Author Affiliations: (1) grid.266744.5, 0000 0000 9540 9781, Department of Earth and Environmental Sciences, University of Minnesota Duluth, , 229 Heller [...]