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Helium isotopes are unrivalled tracers of the origins of melts in the Earth's convecting mantle but their role in determining melt contributions from the shallower and rigid lithospheric mantle is more ambiguous. We have acquired new 3He/4He data for olivine and pyroxene separates from 47 well-characterised mantle xenoliths from global on- and off-craton settings. When combined with existing data they demonstrate a new systematic relationship between fluid-hosted 3He/4He and major and trace element composition of host minerals and whole rock. We show that a significant proportion (>70 %) of mantle peridotites from continental off-craton settings with depleted major element compositions (e.g., olivine Mg# ≥ 89.5) have 3He/4He in the range of modern-day mid-ocean ridge basalt (MORB) source mantle (7–9 R a) and we propose that they represent underplated melt residues, which initially formed in the convecting upper mantle. Furthermore, we observe that off-craton mantle xenoliths with signatures often attributed to enrichment by melts or fluids from 'ancient' subducted oceanic lithosphere have lower 3He/4He (<7 R a). Modest correlations between 3He/4He and whole rock incompatible trace element signatures commonly used as proxies for metasomatism by small-fraction carbonatite and silicate melts or C-O-H fluids characterise lithospheric mantle with 3He/4He ranging from 5 to 8 R a. Using a numerical model that integrates temperature-dependent melt extraction from the upper mantle with in-situ radiogenic ingrowth of 4He in the continental mantle we show that the initial 3He/4He of continental lithosphere mantle has decreased over time. This is consistent with previous observations demonstrating that ancient (2.5–3.5 Ga) cratonic mantle has a depleted mineral chemistry (e.g., olivine Mg# = 91–94) and low 3He/4He (0.5–6.7 R a), while continental off-craton mantle (<2.5 Ga) is more fertile (olivine Mg# = 88–92) and has less radiogenic 3He/4He (4–8.8 R a). This relationship defines a 'global lithospheric mantle array' for intraplate peridotites on plots of 3He/4He vs olivine Mg#. Peridotites influenced by past and present subduction fluids, including those that contain amphibole, plot off this array. Our findings have broad implications for the 3He/4He signatures observed in continental magmas. Many of Earth's deepest melts, i.e. proto-kimberlites, are characterised by relatively low 3He/4He. We attribute this to assimilation and incorporation of low 3He/4He cratonic mantle material during ascent of carbonate-rich melts through thick lithosphere, which overprints the original signatures. Moreover, our findings suggests that the lithospheric mantle acts as a long-term reservoir for other fluid-hosted volatiles (e.g., CO 2 , CH 4 , H 2 O), and in some cases able to sequester these over billion-year timescales until physio-chemical perturbation (e.g., during major rifting or heating events). [ABSTRACT FROM AUTHOR]

Prokaryotes play a crucial role in marine ecosystem health and drive biogeochemical processes. The northern Ninety East Ridge (NER) of the Indian Ocean, a pivotal yet understudied area for these cycles, has been the focus of our study. We employed high-throughput 16S rRNA gene sequencing to analyze 35 water samples from five stations along the ridge, categorized into three depth- and dissolved oxygen-level-based groups. Our approach uncovered a clear stratification of microbial communities, with key bioindicators such as Prochlorococcus MIT9313, Sva0996 marine group, and Candidatus Actinomarina in the upper layer; Ketobacter, Pseudophaeobacter, Nitrospina, and SAR324 clade in the middle layer; and Methylobacterium-Methylorubrum, Sphingomonas, Sphingobium, and Erythrobacter in the deep layer. Methylobacterium-Methylorubrum emerged as the most abundant bacterial genus, while Nitrosopumilaceae predominated among archaeal communities. The spatial and depth-wise distribution patterns revealed that Ketobacter was unique to the northern NER, whereas Methylobacterium-Methylorubrum, UBA10353, SAR324 clade, SAR406, Sva0996_marine_group, Candidatus Actinomarina were ubiquitous across various marine regions, exhibiting niche differentiation at the OTU level. Environmental factors, especially dissolved oxygen (DO), silicate, nitrate, and salinity, significantly influence community structure. These findings not only reveal the novelty and adaptability of the microbial ecosystem in the northern NER but also contribute to the broader understanding of marine microbial diversity and its response to environmental heterogeneity. [ABSTRACT FROM AUTHOR]

Introduction: The Dachaidan ophiolites outcrop within an ultrahigh-pressure metamorphic belt along the northern margin of the Qaidam Basin. However, their age, source, and tectonic setting remain still in debate. Method: In this study, we investigated the geochemistry and geochronology of the Dachaidan ophiolitic gabbros. Results and Discussion: Zircon U-Pb dating yielded a crystallization age of 510.0 ± 2.8 Ma and 510.0 ± 2.9 Ma for the gabbro. The gabbros have low SiO2 contents (47.15-50.10 wt.%) and high MgO contents (6.35-9.04 wt.%) and Mg# values (55-74). The total rare earth element (∑REE) contents are 8.35-28.07 ppm, lower than those of normal-type mid-ocean ridge basalts (MORBs), and the gabbros exhibit light REE depletion or flat REE patterns, with small positive Eu anomalies (Eu/Eu* = 1.06-1.40). Trace element patterns are depleted to enriched in Nb and Ta, similar to island arc rocks and MORB. Clinopyroxene thermobarometry indicates the parental magma of the gabbros formed by high-temperature (1,318°C-1,363°C) and medium-pressure (1.27-1.64 GPa) partial melting in a mantle wedge. The gabbros have depleted Sr-Nd-Pb-Hf isotopic compositions, with (87Sr/86Sr)i = 0.704586-0.707441, εNd(t) = 4.7-6.6, and zircon εHf(t) = 7.6-11.4. The age-corrected Pb isotope ratios of these volcanic rocks are variable, with 206Pb/204Pb(t) = 18.085-18.253, 207Pb/204Pb(t) = 15.595-15.614, and 208Pb/204Pb(t) = 37.880-38.148, which are similar to the isotopic compositions of typical Indian MORBs. The source of the Dachaidan ophiolite is inferred to have been depleted mantle. The Dachaidan ophiolite likely formed in a forearc oceanic setting along the northern margin of the Qaidam Basin, during the initial subduction of an oceanic plate. [ABSTRACT FROM AUTHOR]

The magmatic and metamorphic evolutions of the Muslim Bagh Ophiolite (MBO) can help understand the relationship among supra-subduction ophiolite, subduction initiation, and the spreading centre in the forearc. Here, we investigated the whole-rock chemistry, mineral composition, P–T conditions, and K–Ar ages of the MBO and its associated units. The whole-rock and magmatic clinopyroxene compositions indicate that the MBO amphibolites and tholeiitic basalts of the underlying Bagh Complex originated in a supra-subduction setting and mid-ocean ridges, respectively. Late Cretaceous oceanic island basalts found in the Bagh Complex and the Bibai Formation occurred in oceans and near the passive continental margin of western India, respectively. Metamorphic Mg-hornblendes in the MBO amphibolites record low-P/T type upper amphibolite-facies metamorphism (2.4–2.9 kbar, 612–690 °C), indicating a > 50 ºC/km geothermal gradient. The K–Ar hornblende and plagioclase ages suggest contemporaneous low-P/T type metamorphism at ca. 78–71 Ma in the MBO formed in a forearc and tholeiitic ridge-related magmatism of ca. 78–77 Ma in the Bagh Complex, indicating a spreading ridge was active during the subduction. Our results, combined with plate kinematic reconstructions of the Neotethys Ocean in the previous studies, suggest that the abnormally high geothermal gradient in the MBO could be due to orthogonal ridge subduction near the triple junction after the ca. 80 Ma subduction initiation in the forearc. It is also likely that the geothermal gradient in the MBO dropped from >50 °C/km at 78–71 Ma, through 35–45 °C/km, to <25 °C/km at approximately 65 Ma due to gradual steepening of the subducting slab, followed by the obduction of the MBO during the Paleocene to Eocene period. [ABSTRACT FROM AUTHOR]

The article in the journal "Frontiers in Earth Science" explores contemporary studies on cold seeps, hydrothermal vents, mud volcanoes, and related seafloor features. These features provide insights into deep processes that are typically inaccessible to scientists. The research emphasizes multidisciplinary approaches to studying seafloor fluid emissions, highlighting the importance of geologic, physical, chemical, and biological processes in these environments. The studies cover a wide range of topics, including microbial processes, fluid dynamics, mineralization, and the role of seafloor emissions in global biogeochemical cycles. [Extracted from the article]

We apply the stable potassium isotope system (41K/39K) to well-studied Mariana arc lavas, in which inter-island geochemical variability has been interpreted to reflect near constant addition of an aqueous fluid flux, that dominantly samples the subducting mafic oceanic crust and variable amounts of sediment melt addition to the sub-arc mantle wedge. The nature of the sediment component in the Mariana arc lavas remains enigmatic, with a mixture of melts from all subducted sediment lithologies and the altered-mafic oceanic crust seeming likely, but some interpretations point towards a component from melting of the volcaniclastic sediment alone. We present K isotopic data on a set of well-characterised Mariana arc lavas and sediment samples (from Ocean Drilling Program site 801). Our data show that the majority of Mariana arc lavas are isotopically heavy (ẟ 41K), by up to ∼0.2 ‰, relative to mid-ocean ridge basalt (MORB), but most of the subducting sediment samples are slightly isotopically lighter than, or indistinguishable from MORB. The volumetrically important volcaniclastic sedimentary unit however is significantly isotopically lighter than MORB, by ∼0.8 ‰, which reflects marine diagenetic processes. Thus, volcaniclastic material significantly influences the bulk sediment K isotope composition. We show that ẟ 41K compositions of the Mariana arc lavas can be reproduced by the addition of an aqueous fluid with isotopically heavy K (relative to MORB) and an additional fraction of an incompatible-element-enriched isotopically light melt component. Modelling of K isotopes together with K/La and radiogenic Nd indicate that the melt component is best explained by a mix of melts from bulk sediment and altered-mafic oceanic crust. Our results show that distinctive K isotopic variations in subduction zone inputs and K isotopic fractionation during dehydration reactions makes K a useful tracer of subduction zone processes. [ABSTRACT FROM AUTHOR]

In recent decades, glaciers in the Amundsen Sea Embayment in West Antarctica have made the largest contribution to mass loss from the entire Antarctic Ice Sheet. Glacier retreat and acceleration have led to concerns about the stability of the region and the effects of future climate change. Coastal thinning and near-synchronous increases in ice flux across neighbouring glaciers suggest that ocean-driven melting is one of the main drivers of mass imbalance. However, the response of individual glaciers to changes in ocean conditions varies according to their local geometry. One of the largest and fastest-flowing of these glaciers, Pine Island Glacier (PIG), underwent a retreat from a subglacial ridge in the 1940s following a period of unusually warm conditions. Despite subsequent cooler periods, the glacier failed to recover back to the ridge and continued retreating to its present-day position. Here, we use the ice-flow model Úa to investigate the sensitivity of this retreat to changes in basal melting. We show that a short period of increased basal melt was sufficient to force the glacier from its stable position on the ridge and undergo an irreversible retreat to the next topographic high. Once high melting begins upstream of the ridge, only near-zero melt rates can stop the retreat, indicating a possible hysteresis in the system. Our results suggest that unstable and irreversible responses to warm anomalies are possible and can lead to substantial changes in ice flux over relatively short periods of only a few decades. [ABSTRACT FROM AUTHOR]

In slow spreading environments, oceanic crust is formed by a combination of magmatic and tectonic processes. Using full waveform inversion applied to active‐source ocean bottom seismometer data, we reveal the presence of a strong lateral variability in the 40–48 Ma old oceanic crust formed at the slow‐spreading Mid‐Atlantic Ridge in the equatorial Atlantic Ocean. Over a 120 km‐long section between the St Paul fracture zone (FZ) and the Romanche transform fault (TF), we observe four distinct 20–30 km long crustal segments. The segment affected by the St Paul FZ consists of three layers, an ∼2 km thick layer with a P‐wave velocity <6 km/s, a 1.5 km thick middle crust with a velocity of 6–6.5 km/s, and an underlying layer where velocity is ∼7 km/s, representing the lower crust. The segment associated with an abyssal hill morphology contains a high velocity of ∼7 km/s at 2–2.5 km below the basement, indicating the presence of primitive gabbro or serpenized peridotite. The segment associated with a low basement morphology seems to have 5.5–6.5 km/s velocity starting near the basement extending down to ∼4 km depth, indicating chemically distinct crust. The segment close to the Romanche TF, a velocity 4.5–5 km/s near the seafloor increasing to 7 km/s at 4 km depth indicates a magmatic origin. The four distinct crustal segments have a good correlation with the overlying seafloor morphology. These observed strong crustal heterogeneities could result from alternate tectonic and magmatic processes along the ridge axis, possibly modulated by thermal and/or chemical variations in the mantle during their formation along the ridge segment. Plain Language Summary: Generally magmatically accreted oceanic crust has a nearly uniform crustal structure. The speed of seismic waves, which is widely used as a proxy for the physical properties of the Earth, is relatively low in the upper crust but increases rapidly with depth, while in the lower crust the velocity is high but increases more slowly. Nevertheless, oceanic crust formed at slow‐spreading mid‐ocean ridges can be very heterogeneous. However, it is challenging to quantify the nature of this heterogeneity using conventional travel time based analyses. Here we apply a cutting‐edge full waveform inversion technique to an active‐source ocean bottom seismometer data set from the equatorial Atlantic Ocean. The aim is to create seismic velocity models of oceanic crust with enhanced details. The waveform inversion reveals four distinct zones within a 120 km long crustal section, with boundaries consistent with the seafloor morphology. The section contains two segments of normal oceanic crust, a segment with high velocities at shallow depth, possibly containing primitive gabbro, and another chemically distinct segment with nearly constant velocity in the whole crust. The observed strong crustal heterogeneities seem to result from a combination of magmatic and tectonic accretion processes possibly induced by thermally or chemically heterogeneous mantle. Key Points: We apply full waveform inversion to the crustal turning waves recorded by ocean bottom seismometers in the equatorial Atlantic OceanThe velocity model exhibits strong crustal velocity heterogeneity, containing four distinct segments correlating well with seafloor morphologyThe strong crustal heterogeneity seems to be caused by a combination of magmatic and tectonic processes [ABSTRACT FROM AUTHOR]

Plasmids are one of the key drivers of microbial adaptation and evolution. However, their diversity and role in adaptation, especially in extreme environments, remains largely unexplored. In this study, we aimed to identify, characterize, and compare plasmid sequences originating from samples collected from deep-sea hydrothermal vents located in Arctic Mid-Ocean Ridges. To achieve this, we employed, and benchmarked three recently developed plasmid identification tools—PlasX, GeNomad, and PLASMe—on metagenomic data from this unique ecosystem. To date, this is the first direct comparison of these computational methods in the context of data from extreme environments. Upon recovery of plasmid contigs, we performed a multiapproach analysis, focusing on identifying taxonomic and functional biases within datasets originating from each tool. Next, we implemented a majority voting system to identify high-confidence plasmid contigs, enhancing the reliability of our findings. By analysing the consensus plasmid sequences, we gained insights into their diversity, ecological roles, and adaptive significance. Within the high-confidence sequences, we identified a high abundance of Pseudomonadota and Campylobacterota , as well as multiple toxin–antitoxin systems. Our findings ensure a deeper understanding of how plasmids contribute to shaping microbial communities living under extreme conditions of hydrothermal vents, potentially uncovering novel adaptive mechanisms. [ABSTRACT FROM AUTHOR]

Iceland is a unique example where the rift zone of the Mid-Atlantic Ridge emerges at the surface, and whose morphology and tectonic structure differ considerably from typical rift zones of mid-oceanic ridges. The morphology and geodynamics of the western branch of Icelandic rifts are largely controlled by the thermal influence of the Iceland plume that has created the North Atlantic large igneous province. The western branch of Icelandic rifts is characterized by ceasing tectonic and magmatic activity. Overlapping the Eastern Rift Zone, it forms the rotating Hreppar microplate block, which results in a northward decrease of its tectono-magmatic activity. Based on a morphometric analysis of fault scarps, we identified the degree of present-day activity for individual areas of volcanic systems, and determined its variation during Late Quaternary for some areas. The inferences drawn here demonstrate distinct differences in the present-day tectonic structure and dynamics of rift zones and individual volcanic systems within them. The southernmost, transtensional Reykjanes Rift Zone shows tectono-magmatic activity decreasing to the east, which is due to a lower influence exerted by the Reykjanes Ridge that is adjacent to it to the southwest. We observed its gradual eastward diminution, which is probably due to an analogous southward movement of the most active Eastern Rift Zone and to the formation of a new transtensional zone that combines the present-day Reykjanes Rift Zone and South Iceland Seismic Zone. The Western Rift Zone is developing independently of the Reykjanes Rift Zone, having a major extension center in the area of Lake Thingvallavatn. The Holocene manifestations of tectono-magmatic activity in its northern part, as is the case in the Central Rift Zone, are very weak, being mostly due to glacio-isostatic reactivation of older structures. The identified structural inhomogeneities can also be traced in the morphological aspect of rift zones. As an example, the Western and Central Rift Zones typically contain well-developed shield volcanoes that are largely composed of hyaloclastites, while individual lava shield edifices are observed within fissure swarms. In contrast to this, the Reykjanes Rift Zone is characterized by an absence of central volcanoes expressed in topography, and chains of small volcanic vents are observed within fissure swarms. [ABSTRACT FROM AUTHOR]

In general, mantle-derived volatiles are mainly released into the atmosphere through volcanoes in mid-ocean ridges and subduction zones. However, relatively little attention has been paid to the emission of volatiles from continents on the Earth's surface. It has recently been shown that significant amounts of gases such as carbon dioxide of mantle origin are emitted from continental rifts. Continental degassing has been reported in various ways around the world, however compared to mid-ocean ridges where the depleted upper mantle contributes significantly or arc volcanoes affected by subduction slabs, geochemical generalization is still in progress. In particular, in continental environments, other volatile sources may be added due to the distribution of the subcontinental lithospheric mantle. In the previously reported mantle xenolith samples, components such as noble gases are distinct from the mid-ocean ridge gases, and volcano/fault-related gases on some continents also showed different characteristics. Here, this work proposes representative values of volatiles of the lithosphere by synthesizing the geochemical data of gases emitted from the continent that have been reported until recently. In addition, this study provides a new perspective by considering the recently reported gas results from South Korea. [ABSTRACT FROM AUTHOR]

The alteration of ocean crust through hydrothermal seawater circulation facilitates chemical exchange between Earth's surface and interior. Hydrothermal alteration leads to uranium (U) removal from seawater and net U uptake by the ocean crust, particularly during low temperature alteration that occurs on the vast ocean ridge flanks away from the spreading axes. Determining the timescales of U uptake and its associated 238U/235U signature has important implications for understanding U exchange processes during subduction and recycling into the mantle. Here we study the U systematics of ∼15 million year-old ocean crust drilled at Site 1256 on the eastern flank of the East Pacific Rise. Analysis of cores from the upper ∼1300 m of intact ocean crust at this site, reveal large variability in U concentrations and 238U/235U ratios. Many of the samples from the upper ∼600 m of extrusive lavas have elevated U concentrations and 238U/235U ratios lower than seawater, consistent with mechanisms of U uptake under relatively oxidised conditions. Samples from the underlying sheeted dikes and gabbros show evidence for hydrothermal U mobilisation, but negligible net U uptake. In contrast, in the transition zone between the extrusive lavas and the sheeted dikes, samples revealed large U enrichments and high 238U/235U ratios above seawater. This is consistent with uptake of the reduced U+4 species under relatively reducing conditions from seawater-derived hydrothermal fluids. In addition, large secular disequilibrium in 234U/238U ratios from samples in the lava-dike transition and upper sheeted dikes give evidence for U mobility within the last ∼1.5 million years, likely driven by deep channelled flow of seawater-derived hydrothermal fluids combined with preferential leaching of 234U from the rock matrix. Both the total estimated U uptake and mean 238U/235U at Site 1256 is lower than similar estimates from significantly older (>100 million years) altered ocean crusts at drill Sites 801 and 417/418. This shows the variable total U uptake and 238U/235U ratio in altered ocean crust over time, which needs to be taken into consideration when estimating global U budgets. [ABSTRACT FROM AUTHOR]

Atmospheric circulation has significant impacts on sea ice drifting patterns and mass balance, as wind drag induces pressure ridges and leads on the sea ice surface. In this study, the spatiotemporal distributions of these dynamic sea ice deformation features in the Ross Sea are examined using ICESat‐2 (IS2) ATL10 freeboard data (2019–2022). The temporal variation of the modal sea ice thickness (SIT), caused by thermodynamic ice growth and sea ice advection, varies from 0.7–1.0 m in April to 1.0–1.6 m in July–September and decreases thereafter in the northwest (NW) and northeast (NE) sectors. This temporal variation of modal SIT agrees with the air temperature (correlation coefficients >0.5). The southwest (SW) sector shows a consistently low modal SIT (<1.0 m) because of the production of new ice in polynyas and continuous northward sea ice drift. Meanwhile, the southeast (SE) sector shows the thickest ice in Octobers 2019 and 2020 because of the advection of thick ice from the Amundsen Sea, which was reduced in 2021 and 2022. In terms of dynamic sea ice deformation, the SE sector shows the largest deformation because of the wind‐driven convergence of sea ice movement. However, such intense deformation in the SE sector diminished in 2021 and 2022 due to the dominance of strong southerly wind associated with the Amundsen Sea Low (ASL). This study emphasizes the potential of IS2 sea ice products to assess the role of atmospheric driving forces on thermodynamic and dynamic sea ice changes. Plain Language Summary: The movement of sea ice is primarily driven by wind speed and direction. Furthermore, wind changes sea ice surface topography by forming sea ice deformation features. When sea ice floes collide with each other, the piled‐up sea ice fragments form thicker ice, namely, pressure ridges. Observation of such pressure ridges in the Southern Ocean has been limited to specific seasons or regions because of the lack of high‐resolution remote sensing data. However, high‐resolution observation of pressure ridges is now available regularly through NASA's satellite altimeter ICESat‐2. We investigate the spatiotemporal variations of sea ice topography in the Ross Sea, especially the distribution of pressure ridges, by using ICESat‐2 data. The observation from ICESat‐2 shows that the areal fraction of pressure ridges in the Ross Sea decreased in 2021 and 2022, particularly in the southeast Ross Sea. The comparison with weather data indicates that this decrease might be caused by strong southerly winds associated with the deep Amundsen Sea Low in these 2 years. This study demonstrates that ICESat‐2 can play an important role in understanding the influence of wind on sea ice drift and the formation of pressure ridges. Key Points: Variation of modal ice thickness is characterized by thermodynamic ice growth in the north Ross Sea and ice advection in the south Ross SeaSea ice in the southeast Ross Sea was significantly deformed in 2019–2020 due to the inflow of thick ice and convergent ice driftStrong southerly wind associated with the Amundsen Sea Low decreased sea ice deformation in the southeast Ross Sea in 2021 and 2022 [ABSTRACT FROM AUTHOR]

The Pacific large low-shear-velocity province (LLSVP), as revealed by cluster analysis of global tomographic models, hosts multiple internal anomalies, including a notable gap (~20° wide) between the central and eastern Pacific. The cause of the structural gap remains unconstrained. Directly above this structural gap, we identify an anomalously thick mantle transition zone east of the East Pacific Rise, the fastest-spreading ocean ridge in the world, using a dense set of SS precursors. The area of the thickened transition zone exhibits faster-than-average velocities according to recent tomographic images, suggesting perturbed postolivine phase boundaries shifting in response to lowered temperatures. We attribute this observation to episodes of Mesozoic-aged (250 to 120 million years ago) intraoceanic subduction beneath the present-day Nazca Plate. The eastern portion of the Pacific LLSVP was separated by downwelling because of this ancient oceanic slab. Our discovery provides a unique perspective on linking deep Earth structures with surface subduction. [ABSTRACT FROM AUTHOR]

The eruption of deeply sourced kimberlite magma offers the fastest route to bring deep-seated volatiles back to the Earth's surface. However, the viscosity of kimberlite magma, a factor governing its migration and eruption dynamics within Earth, remains poorly constrained. We conducted synchrotron in situ falling sphere viscometry experiments to examine kimberlite magma with different volatile contents (0 to 5 wt % H2O and 2 to 8 wt % CO2) under high pressure-temperature conditions. The results reveal that the viscosity of volatile-rich kimberlite magma is ~1 to 2 orders lower than that of mid-ocean ridge basalt (MORB) and comparable to the ultramobile pure carbonate melt. Using the measured viscosity values, we simulated the ascent and eruption process of kimberlite magma. We found that a minimum content of ~0.5 wt % water in the primitive magma is necessary to allow the ultrafast eruption process of kimberlite, thereby enabling the preservation of diamonds and high-pressure mineral inclusions transported by the magma. [ABSTRACT FROM AUTHOR]

The ca. 150–135 Ma magmatic belts in the Middle–Lower Yangtze River Valley and its southern adjacent parallel northeastern Jiangnan Orogen in China experienced largescale Cu and W metallogenesis, respectively. The magmatic belts interrupted contemporaneous magmatism along the east China continental margin and are an ideal locality to verify if Cu and W metallogenesis could be linked to a slab window. Oceanic plate subduction, intracontinental extension, lower crustal delamination, or ridge subduction models have been debated for the Cu and W belts. However, these models have weaknesses that cannot explain unique features of the belts such as the belts being coeval and intersecting with the trend of the paleo-trench, or the asymmetric distribution of the magmatic-metallogenic belts along both sides of the Middle–Lower Yangtze River Valley. Based on the configurative, compositional, and isotopic evidences, the magmatic belts record the fingerprints of the slab window generated by disassembly of the paleo-ridge between the Paleo-Pacific and Izanagi plates at ca. 150–135 Ma. During this time, the Paleo-Pacific plate rolled back beneath the eastern margin of the South China block and the Izanagi plate experienced flatslab subduction that reached beneath the intracontinental regions of the North China block. The distinct and contrasting behaviors of the slabs produced a slab window that caused and promoted upwelling mantle to flow farther southward, inducing the asymmetric magmatic-metallogenic belts along the Middle–Lower Yangtze River Valley on both sides. The Middle–Lower Yangtze River Valley Cu-enriched magmatic belt was formed by melting of the lower crust and oceanic slab edge along the ridge by upwelling asthenospheric mantle. However, the Jiangnan Orogen W-enriched magmatic belt was induced by melting of the accretionary belt and mixing the melting of accretionary belt and the same hybrid mantle materials, during which the upwelling asthenospheric mantle tongue infiltrated southward due to the asymmetric structure of the slab window. [ABSTRACT FROM AUTHOR]

The article provides new data on the structure of the Laptev Sea flank of the Gakkel Ridge. The intensive supply of clastic material from the Laptev Sea shelf leads to the development of a thick alluvial fan at the continental rise, which determines the structure of the bottom topography. In the northwestern direction, the influence of the fan decreases and tectonics becomes the main relief-forming factor. The bathymetric survey traced the asymmetrical rift valley of the Gakkel Ridge, the western flank of which is complicated by terraces. The presence of fault structures, bottom subsidence, extensive sediment supply, and the widespread development of subaqueous slump processes indicate the high neotectonic activity of the Laptev Sea flank of the Gakkel Ridge. For the first time in this region, numerous carbonate rocks have been discovered, the authigenic cement of which is represented by magnesian calcite or aragonite with an admixture of terrigenous material. The palynological and micropaleontological analysis of the carbonate rocks indicates the Quaternary formation of authigenic carbonate cement. An important role in the formation of authigenic carbonates was played by diagenetic solutions coming from the sedimentary cover together with methane and oxidation products of gases and organic matter. The authigenic carbonates were precipitated mainly in an isotopic equilibrium with bottom water at a temperature of about 0°C. The negative correlation between 87Sr/86Sr and δ13C indicates the presence of at least two different sources of carbonate-forming solutions. [ABSTRACT FROM AUTHOR]

This study presents a novel approach to map the structural configuration of the northern part of the Central Indian Ridge from satellite-derived gravity data. The proposed approach is based on a combination of derivatives of the horizontal gradient of gravity anomalies. The robustness of the approach is tested on synthetic gravity examples before applying it to satellite gravity data of the northern part of the Central Indian Ridge. The results obtained from the synthetic examples show that the proposed method can map the structural boundaries more precisely and clearly compared to other techniques, and can avoid producing additional structures in output maps. For real application, the structural features determined from the proposed approach are consistent with the fracture zones that have been identified in the area. The obtained result reveals the prominent lineament trend is in the northeast-southwest direction, which is possibly related to plate movement during the Cenozoic. The result demonstrates a high lineament density that is in agreement with the slow spreading rate of the Central Indian Ridge. My findings also demonstrate that the proposed approach can be considered as a valuable tool in mapping structural boundaries. [ABSTRACT FROM AUTHOR]

Areas of the seafloor enriched with seeping fluids host unique chemosynthetic communities, and their interactions not only linked to the presence of oil and gas resources, but directly impact global geochemical cycles. These ecosystems can be found in diverse geological settings, spanning from passive to active continental margins, and encompass environments such as mid-ocean ridges, seamounts, cold brine lakes, mud volcanoes, and carbonate pinnacles. This review aims to examine seep environments in the Western Atlantic, the Caribbean, and Colombia, with a focus on understanding: I) their nature and origin, Itheir associated seabed characteristics, III) the biological communities directly connected to them, and IV) the chemistry and flow of the emitted fluids. The review identifies the close relationship of cold seeps to various geomorphological features, including linear diapir systems, salt diapirs resulting from salt tectonics, regions of mud volcanism, and compact seafloor mounds likely associated with buried mud diapirs. However, existing data on the fauna within these environments predominantly focus on megafauna, such as vestimentiferan tubeworms (Annelida), mussels, and vesicomyid bivalves (Mollusca), overlooking the crucial role of communities of small organisms, including fungi or macrofauna. This review highlights the absence of a consistent consensus among researchers regarding the factors controlling fauna distribution and presence in seep environments. Various authors have put forth divergent factors that influence seep community structures, with some emphasizing water depth, others the geological environment, and some the relationship with geomorphological conditions and fluid emissions. These findings underscore the need for further research into environmental factors and their roles in the observed distribution, presenting a promising avenue for future investigations. [ABSTRACT FROM AUTHOR]

Sulfur forms an immiscible liquid upon saturation in magma, and sulfide droplets are commonly found in fresh mid-ocean ridge basalt (MORB). Scanning electron microscopy (SEM) analysis revealed that the fine-grained and weakly phyric MORB samples exhibited hypocrystalline to vitreous textures. Transmission electron microscopy (TEM) of MORB glasses exhibits nanoscale sulfide droplets (10-15 nm) with rounded shapes and smooth edges, showing crystalline and homogeneous composition. Elemental distribution included S, Fe, Cu, and Ni, while Si, Al, and O were lacking. Prior research clarified the immiscibility between sulfide and silicate melts, impacting the size distribution of sulfide droplets. This is the first report on nanoscale sulfide droplets within MORB glasses, and these results suggest that nanoscale sulfide droplets represent the initial phase of sulfide saturation. Such an insight may prove useful in understanding how siderophile and chalcophile elements behaved during sulfide crystallization. In addition, this study determines the immiscibility of sulfides and silicate melts that occur in the early nanometer stage. Therefore, it is speculated that immiscibility phenomena may occur in the nanometer stage during magma evolution. [ABSTRACT FROM AUTHOR]

Primitive arc magmas are more oxidized and enriched in sulfur-34(34S) compared to mid-ocean ridge basalts. These findings have been linked to the addition of slab-derived volatiles, particularly sulfate, to arc magmas. However, the oxidation state of sulfur in slab fluids and the mechanisms of sulfur transfer in the slab remain inconclusive. Juxtaposed serpentinite and eclogitic metagabbro from the Voltri Massif (Italy) provide evidence for sulfur mobilization and associated redox processes during infiltration of fluids. Using bulk rock and in situ δ34S measurements, combined with thermodynamic calculations, we document the transfer of bisulfide-dominated, 34S-enriched fluids in equilibrium with serpentinite into adjacent metagabbro. We argue that the process documented in this study is pervasive along the subduction interface and infer that subsequent melting of these reacted slab-mantle interface rocks could produce melts that display the characteristic oxygen fugacity and sulfur isotope signatures of arc magmas worldwide. [ABSTRACT FROM AUTHOR]

The Nagaland-Manipur ophiolites (NMO), part of the Phanerozoic (538.8–0 Ma) Tethyan ophiolites, occur in the NNE-SSW trending Indo-Myanmar Orogenic Belt (IMOB), northeast India. The NMO hosts both high-Al (0.46 < Cr# < 0.53) and high-Cr chromitites (0.71 < Cr# < 0.79). These chromitite bodies are hosted in lherzolite, harzburgite, and dunite and show various textures, including massive, disseminated, nodular, and granular. The high-Al chromitite compositions in conjunction with the calculated Al2O3[melt] (15.66–16.39 wt.%), TiO2[melt] (0.65–0.94 wt.%), and FeO/MgO[melt] (0.65–0.83 wt.%) values indicate that they were derived from the tholeiitic melt that formed at the mid-ocean ridge centre through low-degree partial melting. In contrast, the high-Cr chromitites, coupled with the Al2O3[melt] (11.24–12.99 wt.%), TiO2[melt] (0.21–0.33 wt.%), and FeO/MgO[melt] (0.58–1.54) values show similar geochemical affinities to those derived from boninitic melts produced by partial melting of already depleted mantle due to the subduction of oceanic plate in a supra-subduction zone environment. The total platinum group element (PGE) contents (60–190 ppb) of high-Al chromitites are lower than the total PGE contents (118–2341 ppb) in high-Cr chromitites. Chondrite-normalized PGE patterns in high-Al chromitites are flat from Os to Rh and negatively sloping from Rh to Pd, whereas high-Cr chromitites show strongly fractionated chondrite-normalized PGE patterns. Total PGE contents and low Pd/Ir ratios (0.02–0.64) of chromitites are consistent with typical ophiolitic chromitites. Mineral chemistry and PGE systematics suggest that NMO chromitites were generated in two separate tectonic settings. Thus, we argue that the upper mantle of the NMO of the IMOB has been modified by a substantial amount of supra-subduction zone components after initially being formed in a mid-ocean ridge tectonic environment. [ABSTRACT FROM AUTHOR]

The debate regarding whether the Yarlung–Zangbo ophiolite (YZO) on the south of the Qinghai-Tibet Plateau, formed in a mid-ocean ridge (MOR) or a supra-subduction zone (SSZ) setting has remained unresolved. Here we present petrological, mineralogical, and geochemical data associated with modeling melting geodynamics of the mantle peridotites from the Purang ophiolite in the western segment of the Yarlung–Zangbo Suture Zone (YZSZ) to explore its tectonic environment. The Purang lherzolites are characterized by the protogranular texture and have abyssal-peridotite-like mineral compositions, including low Cr# (20–30) and TiO2 contents (<0.1wt%) in spinel, high Al2O3 (2.9wt% – 4.4wt%) and CaO (1.9wt% – 3.7wt%) contents in orthopyroxene and LREE-depletion in clinopyroxene. Compositions of these lherzolites can be modeled by ~11% dynamic melting of the DMM source with a small fraction of melt (~0.5%) entrapped within the source, a similar melting process to typical abyssal peridotites. The Purang harzburgites are characterized by the porphyroclastic texture and exhibit highly refractory mineral compositions such as high spinel Cr# (40–68), low orthopyroxene Al2O3 (<2.2wt%) and CaO (<1.1wt%) contents. Clinopyroxenes in these harzburgites are enriched in Sr (up to 6.0 ppm) and LREE [(Ce)N = 0.02–0.4], but depleted in Ti (200 ppm, on average) and HREE [(Yb)N < 2]. Importantly, the more depleted samples tend to have higher clinopyroxene Sr and LREE contents. These observations indicate an open-system hydrous melting with a continuous influx of slab fluid at a subduction zone. The modeled results show that these harzburgites could be formed by 19% – 23% hydrous melting with the supply rate of slab fluid at 0.1%–1%. The lower clinopyroxene V/Sc ratios in harzburgites than those in lherzolites suggest a high oxidation stage of the melting system of harzburgites, which is consistent with a hydrous melting environment for these harzburgites. It is therefore concluded that the Purang ophiolite has experienced a transformation of tectonic setting from MOR to SSZ. [ABSTRACT FROM AUTHOR]

A nonlocal diffusion single population model with advection and free boundaries is considered. Our aim is to discuss how the advection rate affects dynamic behaviors of species under the case of small advection. Firstly, the well-posed global solution is obtained. Secondly, we apply the eigenvalue problem of integro-differential operator to obtain the dichotomy and sharp criteria for spreading and vanishing, which is determined by initial habitat and initial density. Further, the asymptotic spreading speed of species is estimated when spreading happens. Namely, we get the exact asymptotic spreading speed and find that if kernel function satisfies the certain condition, then the leftward asymptotic spreading speed is less than the rightward one due to the impact of advection rate. Meanwhile, we also observe that accelerated spreading happens if the certain condition does not be satisfied. [ABSTRACT FROM AUTHOR]

The Baoshan deposit is one of the important Cu-Pb-Zn deposits associated with granitic rocks in the Nanling Range, South China. Here, we present He and Ar isotope data for the Baoshan deposit to decipher the contributions of mantle-derived volatiles and heat to its Cu-Pb-Zn mineralization. The ore-forming fluids in sphalerite and pyrite exhibited 3He/4He ratios up to 1.51 Ra. A linear correlation between He and Ar isotopes suggests that the ore-forming fluids were a mixture of a predominantly mantle-derived fluid with a high 3He/4He ratio and a shallow crustal fluid, characterized by a low 3He/4He ratio. The δ34S values of sulfides in the Baoshan deposit ranged from +2.30 to +5.21‰, consistent with the magma-derived sulfur. The calculated 3He/Q ratios for the ore-forming fluid exceeded those of mid-oceanic ridge hydrothermal solutions by 10 to 50 times, indicating that the ore-forming fluids acquired both heat and volatiles in a convective hydrothermal regime rather than a conductive one. Therefore, there is a significant contribution of mantle-derived volatiles, heat, and possibly metals, to the Cu-Pb-Zn mineralization in the Baoshan deposit, and the continuous influx of mantle-derived fluids/melts probably plays a crucial role during the Cu-Pb-Zn mineralization related to granitic rocks. [ABSTRACT FROM AUTHOR]

This study provides a comprehensive literature review of the distribution, the platinum- group elements (PGE) composition, and mineral chemistry of chromitites associated with Mesozoic Tethyan ophiolites in the Mediterranean Basin. These suites outcrop in the northern Italian Apennines, the Balkans, Turkey, and Cyprus. Most chromitites occur in depleted mantle tectonites, with fewer found in the mantle-transition zone (MTZ) and supra-Moho cumulates. Based on their Cr# = (Cr/(Cr + Al)) values, chromitites are primarily classified as high-Cr, with a subordinate presence of high-Al chromitites. Occasionally, high-Al and high-Cr chromitites co-exist within the same ophiolite complex. High-Cr chromitites are formed in supra-subduction zone (SSZ) environments, where depleted mantle interacts with high-Mg boninitic melts. Conversely, high-Al chromitites are typically associated with extensional tectonic regimes and more fertile peridotites. The co-existence of high-Al and high-Cr chromitites within the same ophiolite is attributed to tectonic movements and separate magma intrusions from variably depleted mantle sources, such as mid-ocean ridge basalts (MORB) and back-arc basin basalts. These chromitites formed in different geodynamic settings during the transition of the oceanic lithosphere from a mid-ocean ridge (MOR) to a supra-subduction zone (SSZ) regime or, alternatively, within an SSZ during the differentiation of a single boninitic magma batch. Distinct bimodal distribution and vertical zoning were observed: high-Cr chromitites formed in the deep mantle, while Al-rich counterparts formed at shallower depths near the MTZ. Only a few of the aforementioned chromitites, particularly the high-Cr ones, are enriched in the refractory IPGE (iridium-group PGE: Os, Ir, Ru) relative to PPGE (palladium-group PGE: Rh, Pt, Pd), with an average PPGE/IPGE ratio of 0.66, resulting in well-defined negative slopes in PGE patterns. The IPGE enrichment is attributed to their compatible geochemical behavior during significant degrees of partial melting (up to 30%) of the host mantle. It is suggested that the boninitic melt, which crystallized the high-Cr chromitites, was enriched in IPGE during melt-rock reactions with the mantle source, thus enriching the chromitites in IPGE as well. High-Al chromitites generally exhibit high PPGE/IPGE ratios, up to 3.14, and strongly fractionated chondrite-normalized PGE patterns with positive slopes and significant enrichments in Pt and Pd. The PPGE enrichment in high-Al chromitites is attributed to the lower degree of partial melting of their mantle source and crystallization from a MOR-type melt, which contains fewer IPGE than the boninitic melt above. High-Al chromitites forming at higher stratigraphic levels in the host ophiolite likely derive from progressively evolving parental magma. Thus, the PPGE enrichment in high-Al chromitites is attributed to crystal fractionation processes that consumed part of the IPGE during the early precipitation of co-existing high-Cr chromitites in the deep mantle. Only a few high-Al chromitites show PPGE enrichment due to local sulfur saturation and the potential formation of an immiscible sulfide liquid, which could concentrate the remaining PPGE in the ore-forming system. [ABSTRACT FROM AUTHOR]

Ionospheric effects of powerful seismic events are studied using total electron content (TEC) maps of the ionosphere (http://www.aiub.unibe.ch/download/CODE/) for the northern hemisphere, with the exception of the polar region, in the winter seasons of 2012–2018. It is shown that seismic ionospheric effect is a global effect superimposed by local effects above epicenters of individual earthquakes (EQs). Temporal TEC variations at the time of strong EQs at a large distance from their epicenters (global effect) consist of the two maxima: a precursor maximum and an aftershock maximum. Only a precursor maximum is usually recorded in TEC variations over the EQ epicenter (local effect), the amplitude of which at night (on average 8%) is about twice as high as that observed during day. The reduced amplitude values are observed always (locally and globally) for several days after a positive surge in TEC. The region of the maximum amplitude of the seismic ionospheric effect belongs to the middle latitudes, especially the range of 35° N–40° N latitudes, and, within this range, at longitudes near 30° W (Mid-Atlantic ridge) and 140° E–150° E (Japanese islands and adjacent waters of the Pacific Ocean). Latitudinal amplitude maxima of the seismic ionospheric effect agree well with the latitudinal maxima of the number of EQs in both geographic and geomagnetic coordinate systems. Changes in the number of EQs and, consequently, the ionospheric effect on geomagnetic coordinates are more organized, which is indicative of a substantial impact on seismicity of the same processes at the boundary of the liquid core and lower mantle that form the Earth's magnetic field. In addition to seismic belts and zones of midocean ridges, an increase in TEC has been recorded along the so-called "lineaments" that mark the weakened zones of the Earth's crust with increased flows of deep gases. The correspondence between the spatial features of seismicity and the seismic ionospheric effect gives evidence in favor of the radon mechanism of lithosphere–ionosphere coupling and indirectly confirms the role of deep gases in the formation of planetary features of seismicity. [ABSTRACT FROM AUTHOR]

Chromitites or chromite mineralization of varying degrees has been discovered in the various ophiolites along the east–west trending Yarlung-Zangbo Suture Zone (YZSZ) in Tibet, China. The high-Cr variety dominates the Yarlung-Zangbo chromitites, with rare high-Al chromitites reported in the Zedang, Dongbo, and Purang ophiolites. Using empirical equations, the calculated parental magmas that formed the high-Cr YZSZ chromitites are similar to boninitic melts. 187Os/188Os ratios of chromites from the YZSZ chromitites range from 0.12525 to 0.12933, lower than the proposed present-day 187Os/188Os values for the primitive upper mantle. The TRD age variation of the YZSZ chromitites from late Neo-Proterozoic to early Triassic thus reflects that their parental magmas are derived from depleted mantle sources mixed with diachronous ancient mantle domains. The light Zn isotopic compositions of the YZSZ chromitites indicate that subducted materials (e.g., serpentinites and sediments) have contributed to the parental magma of the YZSZ chromitites. By compiling previously published data on mantle peridotites of the YZSZ ophiolites, we concluded that the YZSZ ophiolites may either have formed initially in an ultraslow-slow mid-ocean ridge environment and were then trapped in a supra-subduction zone environment, or have formed in an ultraslow-slow forearc spreading center in a supra-subduction zone environment. The Luobusa ophiolite hosting the largest chromite deposits is discriminated from the other ophiolites in the YZSZ by a thick dunitic transition zone. Previous theoretical modeling indicates that relative to olivine, only a small amount of cumulus chromites crystallize in cotectic volume ratios of around 100:1 to 100:2 of olivine to chromite, which means that large chromite bodies should always be accompanied by a significantly larger mass of dunites. Therefore, we concluded that a thick dunite transition zone or large masses of dunite of boninitic affinity is an indicator for chromitite prospecting in the future. [ABSTRACT FROM AUTHOR]

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]

Using FY-2G satellite data, Aircraft Meteorological Data Relay (AMDAR) downlink data, and the fifth generation European Centre for Medium-Range Weather Forecasts (ECMWF) atmospheric reanalysis of the global climate (ERA5) dataset, we analyzed the circulation, thermal, and dynamic characteristics of the convective aircraft turbulence over Australia in the Southern Hemisphere. The results show that the near-convective clouds turbulence (NCCT) in the Southern Hemisphere mostly occurred in front of deep warm high-pressure ridges in mid-latitude regions and on the left side of the axis of the subtropical westerly jet stream. The isotherms in this area were relatively dense (i.e., large gradients), and the wind speed was high, with strong horizontal and/or vertical cyclonic wind shear. In addition, the NCCT usually occurred near the zero-divergence or zero-vorticity line and in areas with large vertical wind speed gradients. There were also strong vertical and horizontal wind shears in this area, which could easily trigger severe turbulence. Furthermore, the NCCT in the Southern Hemisphere mostly occurred at the intersection of cold and warm temperature advections (i.e., near the zero-temperature advection line), and the turbulence point was located near the high-altitude frontal zone where there was a strong gradient of cold and warm advections. There was temperature inversion with pseudo-equivalent potential temperatures in the middle and lower troposphere on the warmer side of the turbulence point. The unstable stratification of cold air at the top and warm air at the bottom was conducive to triggering convection from the ground, forming strong convective clouds, and causing severe turbulence. [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]

Serpentinization has been widely documented and investigated at mid-ocean ridges (MOR) and subduction zones. In contrast, at magma-poor rifted margins serpentinization has received much less attention, despite its importance in controlling rheology and mass fluxes during breakup and establishing of a steady-state MOR. In this study, we present new petrological and geochemical data on subcontinental exhumed serpentinized peridotites from the spectacularly exposed Platta, Tasna and Totalp nappes in the Eastern Central Alps in SE Switzerland, belonging to the Alpine Tethys Ocean Continent Transition (OCT). The results testify of a complex history of fluid–rock interactions recorded by several serpentinization events starting with lizardite mesh and bastite textures (S1), subsequently followed by a succession of serpentine-filling veins with distinct textures and serpentine polysomes that include spherical polyhedral serpentine (S2); chrysotile ± polygonal ± lizardite banded veins (S3); lamellar antigorite veins and patches (S4) and chrysotile crack-seal (S5). The serpentinization sequence differs at proximal (i.e. continentwards) and distal (i.e. oceanwards) domains of the OCT. At proximal domains of the OCT (Upper Platta, Tasna) serpentinites record the complete serpentinization sequence (S1 to S5), whereas at distal domains (Lower Platta) serpentinization is restricted to pseudomorphic mesh and bastite (S1) and chrysotile crack-seal (S5). We attribute this discrepancy to contrasted mechanisms of mantle exhumation along the OCT. While at proximal domains mantle is unroofed along continuous and single large offset detachment faults allowing for the formation of all serpentine generations, mantle exhumation at distal domains is a more discontinuous process, controlled by sequential out-of-sequence detachment and flip-flop faults preventing the full development of all serpentine generations. In this frame, the nature and order of formation of the serpentine polysomes are directly controlled by the conditions of serpentinization (i.e. temperature, mantle composition and fluid/rock ratio). We propose that this new conceptual model can be extrapolated to serpentinization at slow to ultra-slow MORs, where close similarities in the serpentinization sequences have been recently reported. [ABSTRACT FROM AUTHOR]

A new hydrothermal field (Tianshi) was discovered on the rift valley wall through plume anomaly surveys and geological work conducted in 2012 and 2018 between 2°35′N and 2°43′N of the slow-spreading Carlsberg Ridge (CR). Here, the results of two expeditions conducted to detect and characterize the new hydrothermal field are reported. Mineralogical and geochemical data, as well as 14C ages of a sediment core collected near the field are presented to reveal the hydrothermal history. Results show that the Tianshi field is a basalt-hosted hydrothermal system. Geochemical data of the sediments collected near the field indicate a strong hydrothermal contribution, and hydrothermal Fe and Cu fluxes range from 30 to 155 mg/(cm2.ka) and 0.59 to 11.49 mg/(cm2.ka), respectively. Temporal variations in the fluxes of hydrothermal Fe indicate that there have been at least three amplified hydrothermal venting events (H1, H2, and H3) in the Tianshi field over the last 35.2 ka, in 28.6–35.2 ka BP, 22.0–27.6 ka BP, and 1.2–11.4 ka BP, respectively. Hydrothermal event H2 was driven by an increased magmatic production associated with sea level fall during the Last Glacial Maximum, while event H3 was promoted by tectonic activity associated with a rapid sea level rise. This study further verified the role of sea level change in modulating hydrothermal activity on mid-ocean ridges. [ABSTRACT FROM AUTHOR]

The Ninety East Ridge in the Indian Ocean has complex and unique characteristics. The concentrations and distribution characteristics of 10 trace metals (V, Cr, Mn, Fe, Co, Ni, Cu, Cd, Pb, and U) in seawater from the Ninety East Ridge in the Indian Ocean were investigated. Results show that the average concentrations of different trace metals in all the collected seawater samples were 1.134 µg/L for V, 0.158 µg/L for Cr, 0.489 µg/L for Mn, 0.427 µg/L for Fe, 0.011 µg/L for Co, 0.395 µg/L for Ni, 0.403 µg/L for Cu, 0.097 µg/L for Cd, 0.139 µg/L for Pb, and 3.470 µg/L for U. Differences in the horizontal and vertical distributions of all measured trace metals were revealed, and the occurrence of high concentrations was nonuniform. In addition, the significant differences in the concentration distribution of different trace metals in seawater on both sides of the Ninety East Ridge present regional segmentation in the area for various trace metals in deep sea water. This study provided basic data for future investigations on the environmental and ecological impact of trace metals in the Indian Ocean and the potential water mass transport mechanism. [ABSTRACT FROM AUTHOR]

Magmatic oxygen fugacity (fO2) is a fundamental property to understanding the long-term evolution of the Earth's atmosphere and the formation of magmatic-hydrothermal mineral deposits. Classically, the magmatic fO2 is estimated using mineral chemistry, such as Fe-Ti oxides, zircon, and hornblende. These methods, however, are only valid within certain limits and/or require a significant amount of a priori knowledge. In this contribution, a new oxybarometer, constructed by data-driven machine learning algorithms using trace elements in zircon and their corresponding independent fO2 constraints, is provided. Seven different algorithms are initially trained and then validated on a data set that was never utilized in the training processes. Results suggest that the oxybarometer constructed by the extremely randomized trees model has the best performance, with the largest R2 value (0.91 ± 0.01), smallest RMSE (0.45 ± 0.03), and low propagated analytical error (~0.10 log units). Feature importance analysis demonstrates that U, Ti, Th, Ce, and Eu in zircon are the key trace elements that preserve fO2 information. This newly developed oxybarometer has been applied in diverse systems, including arc magmas and mid-ocean ridge basalts, fertile and barren porphyry systems, and global S-type detrital zircon, which provide fO2 constraints that are consistent with other independent methods, suggesting that it has wide applicability. To improve accessibility, the oxybarometer was developed into a software application aimed at enabling more consistent and reliable fO2 determinations in magmatic systems, promoting further research. [ABSTRACT FROM AUTHOR]

Cold and diffuse hydrothermal circulation on mid‐ocean ridge flanks impacts heat and fluid fluxes between the seafloor and the ocean. One mode of this circulation is given by outcrop‐to‐outcrop flow, where seawater circulates through a crustal aquifer that connects two or more recharging and discharging seamounts or basement highs that outcrop through the less permeable sediment cover. The physical mechanism driving this flow is a lateral pressure gradient that is sustained by contrasting the hydrological properties of the recharging and discharging outcrops. To investigate the physical controls of this pressure gradient, we performed two‐dimensional numerical simulations of coupled heat transfer and fluid flow. We have modified aquifer permeability, outcrop permeability and width, outcrop distance, and sediment thickness to assess their mutual effects on the lateral pressure differences. We have also investigated how different flow patterns, resulting from changes in these parameters, manifest themselves in seafloor observables such as flow rates, aquifer temperatures, and heat flow. Our models show that outcrop‐to‐outcrop flow generally occurs for aquifer permeabilities ≥10−14 m2, depending on the basal heat input. High aquifer permeabilities correspond to fast flow rates and low fluid temperatures, whereas the maximum lateral pressure differences arise for lower permeabilities. The permeability and the geometric shape of the outcrops determine the flow direction, while the aquifer temperature is also affected by the distance between the outcrops. Thicker sediments increase the lateral pressure difference and the flow rate. Our models thus provide constraints for predicting subseafloor hydrothermal ridge flank flow behavior from regional field data. Key Points: Numerical models identify distinct hydrothermal flow patterns that may arise in outcrop‐to‐outcrop flow on mid‐ocean ridge flanksVigor and direction of the flow are determined for various parameters including crustal permeability, sediment thickness and geometryOutcrop‐to‐outcrop flow manifests itself in a reduced sediment heat flow for high permeabilities, thin sediments and closely spaced outcrops [ABSTRACT FROM AUTHOR]

An animated 100,000‐year‐interval tectonic reconstruction of the Woodlark Basin in the southwest Pacific illustrates how, at intermediate initial spreading rates, orogenic continents break up (dyke model), spreading segments nucleate, transform faults initiate and ocean basins evolve. We refine the location/timing of initial seafloor spreading and Euler poles of rotation back to 6.2 Ma. In the easternmost basin, where spreading younger than 2.6 Ma is not co‐polar with that to the west, we recognize the formation of a Ghizo microplate and Ranongga Transform Fault at ∼2.6 Ma and a 3‐degree rotational opening of the Itina Trough from 2.6 to 1.0 Ma. Allowing for that motion, we show that the 5.2–2.6 Ma seafloor in the easternmost basin formed co‐polar with that to the west. We also identify a ridge jump reorientation at ∼1.0 Ma that formed the NE‐trending Simbo Spreading Segment, whose neovolcanic zone includes Simbo Island and a submarine edifice to its south. Proposed deterministic models of ridge propagation (due to topographic gradients, mantle flow away from hotspots and/or changing plate motion) are not consistent with those observed; mantle chemical heterogeneities and melting anomalies are a potential cause that remains to be tested. We reconstruct the northern conjugate of the oldest extant oceanic crust and estimate the initiation of its subduction at ∼2.6 Ma, concomitant with observed changes in plate motion and segmentation. Where subducted, the young oceanic lithosphere between the conjugate rifted margins appears to be resorbed into the mantle, leaving a slab window where the Pacific subducted slab remains attached. Key Points: An animated tectonic reconstruction of the Woodlark Basin illustrates continental breakup and ocean basin formation and evolutionMost deterministic models of ridge propagation are not consistent with observations; mantle heterogeneities and melting anomalies may beThe Ghizo microplate and Ranongga Transform Fault formed ∼2.6 Ma; the Itina Trough rift graben rotated open by 3‐degrees from 2.6 to 1.0 Ma [ABSTRACT FROM AUTHOR]

At fast-spreading centers, faults develop within the axial summit trough (AST; 0 to 250 m around the axis) primarily by diking-induced deformation originating from the axial magma lens (AML). The formation of the prominent abyssal-hill-bounding faults beyond the axial high (>2,000 m) is typically associated with the unbending of the lithosphere as it cools and spreads away from the AST. The presence of faults is rarely mapped between these two thermally distinct zones, where the lithosphere is still too hot for the faults to be linked with the process of thermal cooling and outside of the AST where the accretional diking process dominates the ridge axis. Here, we reveal a remarkable vertical alignment between the distinct morphological features of the magma body and the orientation of these faults, by comparison of 3-D seismic imagery and bathymetry data collected at the East Pacific Rise (EPR) 9°50'N. The spatial coincidence and asymmetric nucleation mode of the mapped faults represent the most direct evidence for magmatically induced faulting near the ridge axis, providing pathways for hydrothermalism and magma emplacement, helping to build the crust outside of the AST. The high-resolution seafloor and subsurface images also enable revised tectonic strain estimates, which shows that the near-axis tectonic component of seafloor spreading at the EPR is an order of magnitude smaller than previously thought with close to negligible contribution of lava buried faults to spreading. [ABSTRACT FROM AUTHOR]

Earth is the only known habitable planet in the solar system. Understanding how Earth developed its unique habitability has been the frontier of Earth sciences and has become one of the main themes of current deep-space explorations. What are the decisive factors that led to a habitable planet? What is the role of solid Earth processes in the origin of life and in modulating the surface environment? Are Earth's habitability studies relevant to current challenges that human beings face? These questions have attracted the interest of both scientists and the public alike. NSR spoke to Prof. Charles H. Langmuir from Harvard University in the USA, who is a solid Earth geochemist who carries out research on diverse aspects of the plate tectonic geochemical cycle, including ocean ridges, convergent margins and intraplate volcanism. Prof. Langmuir is the author of the book How to Build a Habitable Planet (www.habitableplanet.org), one of the best Earth science books published in 2012. [ABSTRACT FROM AUTHOR]

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

At mid-ocean ridges, melts that formed during adiabatic melting of a heterogeneous mantle migrate upwards and ultimately crystallize the oceanic crust. The lower crustal gabbros represent the first crystallization products of these melts and the processes involved in the accretion of the lowermost crust drive the chemical evolution of the magmas forming two thirds of Earth's surface. At fast-spreading ridges, elevated melt supply leads to the formation of a ⁓6-km-thick layered oceanic crust. Here, we provide a detailed petrochemical characterization of the lower portion of the fast-spread oceanic crust drilled during IODP Expedition 345 at the East Pacific Rise (IODP Holes U1415), together with the processes involved in crustal accretion. The recovered gabbroic rocks are primitive in composition and range from troctolites to olivine gabbros, olivine gabbronorites and gabbros. Although textural evidence of dissolution-precipitation processes is widespread within this gabbroic section, only the most interstitial phases record chemical compositions driven by melt-mush interaction processes during closure of the magmatic system. Comparing mineral compositions from this lower crustal section with its slow-spreading counterparts, we propose that the impact of reactive processes on the chemical evolution of the parental melts is dampened in the lower gabbros from magmatically productive spreading centres. Oceanic accretion thereby seems driven by fractional crystallization in the lower gabbroic layers, followed by upward reactive percolation of melts towards shallower sections. Using the composition of clinopyroxene from these primitive, nearly unmodified gabbros, we estimate the parental melt trace element compositions of Hess Deep, showing that the primary melts of the East Pacific Rise are more depleted in incompatible trace elements compared to those formed at slower spreading rates, as a result of higher melting degrees of the underlying mantle. [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]

Estimates of oxygen fugacity of eclogitic rocks are linked to the redox evolution of the oceanic protolith during subduction and its residence in the lithospheric mantle, and, based on knowledge of pressures and temperatures, allow modelling of the speciation of volatile elements and diamond (or graphite) versus carbonate stability. To date, the oxygen fugacity of mantle eclogites has been shown to vary between −6 (Kasai, Congo and Udachnaya, Siberia) and −0.1 (Udachnaya, Siberia) log units (relative to the fayalite–magnetite–quartz buffer, FMQ), linked to the low Fe3+ contents of garnets. In this study, we investigated the Fe oxidation state of coexisting garnet and clinopyroxene hand-picked out of 17 diamond-free high-MgO and low-MgO mantle eclogites (dated at 2.84 Ga) from the Grib kimberlite pipe (East-European platform). Measured Fe3+/∑Fe values range between 0.03 and 0.19 for garnet and 0.18–0.38 for clinopyroxene, the former being higher than what was measured previously in garnets equilibrated at mantle conditions. The Fe3+/∑Fe of the reconstructed bulk rock ranges between 0.10 and 0.15 for high-MgO eclogites and 0.10 and 0.24 for low-MgO eclogites (with uncertainties of ± 0.02 and ± 0.03 in both cases). Thermobarometric calculations result in equilibration pressures and temperatures of 3.0–5.2 (± 0.4) GPa and 720–1050 (± 60) °C for both high-MgO and low-MgO eclogites, slightly lower than previous P–T estimates of mantle eclogites from the Udachnaya kimberlite pipe (Siberian craton). At these conditions, ∆log f o2 (FMQ) calculated using the available oxythermobarometric model varies from −1.7 to −0.6 log units for high-MgO eclogites and from −2.9 to 0.9 log units for low-MgO eclogites. Samples recording ∆log f o2 (FMQ) ≤ −1 log units overlap with North Slave, West Africa and Udachnaya eclogites, with no difference among eclogite types. The average values of −1.2 (± 0.4) log units for high-MgO and −0.6 (± 1.1) log units for low-MgO eclogites suggest different redox conditions of basaltic protoliths during subduction worldwide. Previous geochemical studies on the same rock samples reported evidence of cryptic metasomatism in both garnet and clinopyroxene that we demonstrate being not recorded by their major elements, while modal metasomatism evidenced by the presence of phlogopite as a product of interaction with a kimberlitic melt only affects the MgO of the bulk rock. Therefore, we suggest that high Fe3+/∑Fe ratios for garnet (> 0.10) and for reconstructed bulk rocks in the case of both low-MgO and high-MgO samples cannot be due to metasomatic interaction with an oxidized fluid, but rather are the consequence of Fe3+ redistribution in an unusually oxidized mafic protolith upon metamorphism. Our results highlight the redox variability of eclogites of Archaean age at conditions more oxidized than present-day mid-ocean ridge basalts (MORBs) and imply an oxidizing nature of the convective mantle source where magma was formed with consequent speciation of C in the form of carbonate fluid explaining, therefore, the lack of eclogitic diamonds in V. Grib kimberlite pipe. [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]

New evidence of the Early Silurian sublithospheric magmatic activity in the eastern part of the Altai–Sayan orogen has been obtained. This activity occurred between large-scale mantle-derived magmatic episodes of the Middle–Late Ordovician and Devonian. It involved high-Mg (15–22 wt % MgO) picritic dolerite dikes of the Naryn complex in the western part of the Tuva–Mongolian Superterrane. The dike complex consists of simple picrodolerite dikes and those combined with a central picrodolerite zone and granitoids at contacts with mingling relationships between contrasting rocks. The picrodolerite geochemical signature is similar to enriched basalts of the mid-ocean ridges or intraplate regions and is indicative of a sublithospheric mantle source. Initial picrodolerite melts were formed at a depth of about 120–140 km, a temperature of 1600–1640°C, and a melting degree of up to 20% at a dry peridotite source. These values are consistent with the conditions of the hot spot magmas origin. Granitoids from the combined dikes had a crustal geochemical signature and were formed due to the anatexis of the Tuva–Mongolian Superterrane host rocks initiated by the picrodolerite intrusion. The U–Pb zircon age (SHRIMP-II) of leucogranite from the combined dike is 439 ± 3 Ma interpreted as the age of the Naryn dike complex. The Early Silurian mantle-derived magmatic activity of the Southeastern Tuva most likely caused the migration of the Siberian paleocontinent over the African mantle hot field. [ABSTRACT FROM AUTHOR]

Calcium (Ca) phosphates are major hosts for phosphorus, halogens and incompatible trace elements in the Earth's crust and mantle and, by supplying almost all phosphorus to the biosphere, are indispensable for sustaining life on Earth. To better understand the role of Ca-phosphates in the deep silicate Earth portion of the global phosphorus cycle at depths of the transition zone, we performed experiments in the P-T range of 15–25 GPa and 1600–2000 °C using a moderately fertile peridotite doped with 3 % synthetic β-Ca 3 (PO 4) 2 and 1 % of a trace element mix containing a range of HFSE, LILE, REEs, Cl and Br. Tuite is stable to at least 25 GPa at which pressure its upper T -stability limit is between 1700 and 1750 °C. At 15 GPa, by comparison, the upper T-stability limit is located between 1750 and 1800 °C, indicating a negative slope of the tuite-out reaction(s). Thus, tuite is stable to at least 700-km depth with a maximum T -stability close to or slightly above the average current mantle adiabat (ACMA) at 20–25 GPa and significantly above the ACMA at 15 GPa. Beyond the P-T stability limit of majoritic garnet, tuite is the principal subsolidus phosphorus carrier in the uppermost lower mantle. Beyond the P-T stability limit of tuite, phosphorus is likely to either (i) enter a Ca 3 (PO 4) 2 polymorph with an even higher density, (ii) form an entirely new phosphate phase or (iii) be accommodated in lower mantle silicates in six-fold coordination. Within the P-T range of our experiments, the phases encountered show the following trend of phosphorus contents:P melt >P majoritic garnet > P olivine ≅ P ringwoodite > P davemaoite ≅ P wadsleyite > P clinoenstatite ≅ P bridgmanite ≅ P ferropericlase. Even when coexisting with tuite, bridgmanite and ferropericlase show phosphorus contents < 100 µg/g, and davemaoite has phosphorus contents that do not exceed 390 µg/g, testifying to the very limited phosphorus storage capacity of the lower mantle. Model calculations based on the phosphorus contents of ocean island basalts (OIBs) compared with those of mid ocean ridge basalts (MORBs) indicate that the OIB source is significantly richer in phosphorus than the MORB source and, for a wide range of OIB phosphorus contents, is likely to contain modal apatite. The relative importance of Ca-phosphates and silicates as phosphorus carriers in the peridotitic upper and uppermost lower mantle is strongly dependent upon pressure and the bulk phosphorus content. With increasing P , phosphorus is progressively transferred from Ca-phosphates to silicates with an ensuing decrease in the modal amount of the Ca-phosphates. Dependent upon the bulk phosphorus content, this may cause the Ca-phosphates to disappear before the upper-to-lower mantle boundary is reached, leaving majoritic garnet as the major phosphorus host. Breakdown of garnet will subsequently cause tuite to re-appear in the uppermost lower mantle and to resume its role of the principal subsolidus phosphorus carrier. For sufficiently high bulk phosphorus contents, tuite will remain stable throughout the transition zone, thereby maintaining phosphorus saturation in the coexisting silicates, and will eventually be joined by the tuite newly generated by garnet breakdown. [ABSTRACT FROM AUTHOR]

The East Kunlun Orogenic Belt (EKOB), northwestern China, recording long-term and multiple accretionary and collisional events of the Tethyan Ocean, belongs to a high-pressure to ultra-high-pressure (HP-UHP) metamorphic belt that underwent complex metamorphic overprinting in the early Paleozoic. In this contribution, we carry out an integrated study, including field investigations, petrographic observations, whole-rock analyses, zircon U-Pb dating, and P-T condition modeling using THERMOCALC in the NCKFMASHTO system for the eclogites, especially for the newly discovered UHP eclogite in the eastern part of EKOB. The eclogites exhibit geochemistry ranging from normal mid-ocean ridge basalt (N-MORB) to enriched mid-ocean ridge basalt (E-MORB). Zircons from the eclogites yield metamorphic ages of 416–413 Ma, indicating the eclogite facies metamorphism. Coesite inclusions in garnet and omphacite and quartz exsolution in omphacite and pseudosection calculation suggest that some eclogites experienced UHP eclogite facies metamorphism. The eclogites from the eastern part of EKOB record peak conditions of 29–33 kbar/705–760 °C, first retrograde conditions of 10 kbar at 9.5–12.5 kbar/610–680 °C, and second retrograde conditions at ~6 kbar/<600 °C. New evidence of the early Paleozoic UHP metamorphism in East Kunlun is identified in our study. Thus, we suggest that these eclogites were produced by the oceanic crust subducting to the depth of 100 km and exhumation. The presence of East Gouli and Gazhima eclogites in this study and other eclogites (430–414 Ma) in East Kunlun record the final closure of the local branch ocean of the Proto-Tethys and the evolution from subduction to collision. [ABSTRACT FROM AUTHOR]

The recycling of subcontinental lithospheric mantle back into the asthenosphere during continental breakup is potentially a major but underappreciated process in plate tectonics. Here, we report major and trace elements for detrital peridotite minerals in carbonate breccias dredged from the Marion Rise, Southwest Indian Ridge. With an emphasis on the trace elements of the relatively abundant orthopyroxene detrital grains, the results show that a portion of pyroxene grains was likely derived from a mantle source that experienced hydrous melting and slab‐derived melt metasomatism above a subduction zone. This finding provides a novel view into the mantle geochemistry at midocean ridges, as well as evidence that Marion Rise is at least partially supported by chemically buoyant recycled refractory mantle wedge material. [ABSTRACT FROM AUTHOR]

Lepidonotopodinae is a subfamily of Polynoidae endemic to deep-sea chemosynthetic ecosystems around the world. Nevertheless, their species composition and phylogeny have only been systematically studied in hydrothermal vents of the Eastern and Western Pacific. Here, we morphologically and genetically examined worms in Lepidonotopodinae from vents across three Indian Ocean ridges, revealing two new Branchinotogluma species (B. jiaolongae sp. nov. and B. kaireiensis sp. nov.) and two new Levensteiniella species (L. pettiboneae sp. nov. and L. longqiensis sp. nov.). Primary morphological characters distinguishing them from other congeners include the number and arrangement of both pharyngeal papillae and ventral papillae. The reconstructed molecular phylogeny of Lepidonotopodinae supports a monophyletic Levensteiniella , with the two new Indian Ocean species recovered as sisters. As revealed in previous studies, a paraphyletic Branchinotogluma was also found, with the three Indian Ocean species separated into distinct clades with sister-relationships to species from the Mid-Atlantic, Alarcon Rise, and Manus Basin, respectively. This indicates three separate historical invasions to Indian Ocean vents. Our findings increase the number of Indian Ocean Lepidonotopodinae worms to seven, now the most diverse annelid group there, and help to elucidate the biodiversity, distribution, and biogeography of this subfamily in the Indian Ocean. [ABSTRACT FROM AUTHOR]

The conditions of pyrite (Py) stability inform the extent of S mobility during prograde metamorphism, the formation of orogenic Au deposits, and the S cycle in subduction zones. The variables that affect Py stability and chalcophile element mobility are investigated in the Pacific Rim Terrane of Vancouver Island, Canada, where sulfide-bearing carbonaceous sediments have been metamorphosed from 230 to 600 °C and 4 kbar by mid-ocean ridge subduction in a hot fore arc setting during the Eocene. The petrographic evidence in the rocks shows Py can coexist with pyrrhotite (Po) over a wide temperature window to >550 °C as preserved in porphyroblasts of andalusite, staurolite, and garnet. Conversely, equilibrium phase diagrams constructed for the rock compositions conflict with observations and suggest the breakdown of primary Py occurs over a narrow temperature range below 400 °C. The phase diagrams are consistent with the coexistence of Py and Po up to lower amphibolite facies only if S locally comprises a much greater proportion involved in a reaction than that of the overall bulk-rock composition used in the calculations. While the chemistry of the bulk rocks and Po included in porphyroblasts show mobilization of H2O and S with increasing metamorphic grade of the forearc, this process appears unrelated to the distribution of chalcophile elements or Au deposits found in the Pacific Rim Terrane. [ABSTRACT FROM AUTHOR]