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Sedimentary manganese (Mn) deposits in the southeastern Yangtze Platform of China have been newly discovered with considerable potential for resource exploration. Such deposits formed due to dynamic changes in the global environment, including the break-up of the supercontinent Rodinia, Snowball Earth glaciation, and the Neoproterozoic Oxygenation Event. The Mn-bearing sequences are hosted in the basal Datangpo Formation and are interbedded with black carbonaceous shale in a series of graben sub-basins in the Nanhua Rift Basin. We investigated the origin of these Mn deposits by analysing the major and trace elements, total organic carbon, and stable carbon and oxygen isotopes. Positive Eu anomalies were observed, indicating that the Mn was derived from a hydrothermal system. Redox-related elements, including Ce, Mo, U, V, S, and P, indicated that the depositional environment of the Mn layers was oxidative and that Mn2+ was initially oxidized to Mn oxides. However, the host rock (i.e. black carbonaceous shale) was deposited under dysoxic – anoxic and even sulfidic water conditions. The alternating distribution of Mn ore and host rock suggested alternating redox conditions in the sub-basins. A negative correlation between the Mn content and δ13Ccarb values (−10‰–−7‰) and a deficiency of sulphide minerals indicate that the diagenetic degradation of organic matter (δ13Corg from − 34‰ to − 31.9‰) took place through Mn oxide reduction rather than microbial sulphate reduction and can contribute to a considerable source of bicarbonates. The approximately homogeneous δ13Ccarb depletion and their apparent discrepancy with δ13Corg indicate that the post-depositional genesis of the Mn carbonate ore occurred in a relatively open early diagenetic pore water system with a considerable contribution from seawater-derived carbon sources. [ABSTRACT FROM AUTHOR]

Compared with the deeply buried marine gas hydrate deposits, gas hydrates in the shallow subsurface, close to and at the seafloor, have attracted more attention owing to their concentrated distribution, high saturation, and easy access. They accumulate at relatively shallow depths <100–120 m and occur as gas hydrate-bearing mounds (also known as hydrate outcrops, pingoes) at the seafloor derived from the growth of hydrates in the shallow subsurface or as pure hydrate chunks formed by gas leakage. This paper reviews and summarizes such gas hydrate systems globally from the perspective of gas sources, migration pathways, and accumulation processes. Here, we divided them into four categories: fault-chimney-controlled, diapir-fault-controlled, fault-controlled, and submarine mud volcano-controlled deposits. Gas chimneys originate immediately above the restricted regions, mostly affected by faults where high gas concentrations trigger elevated pore fluid pressures. Diapirism derives a dendritic network of growth faults facilitating focused gas discharge and hydrate formation near the seafloor. Furthermore, pre-existing faults or fractures created by overpressured gas from greater depths in accretionary tectonics at convergent margins act as preferential pathways channeling free gas upwards to the seafloor. Gas flux rates decrease from the submarine mud volcano center to its margins, creating a concentric pattern of distributing temperature, gas concentrations, and hydrate contents in shallow sediments around the mud volcano. Hydrate-bound hydrocarbons are commonly of thermogenic origin and correspond to high-background geothermal conditions, whereas microbial gas is dominant in a few cases. The presence of heavier hydrocarbons mitigates the inhibition of hydrate formation by salt or heat. Fluid migration and pathways could be compared to the "blood" and "bones" in an organic system, respectively. The root of a pathway serves as the "heart" that gathers and provides considerable free gas concentrations in a restricted area, thereby triggering pore fluid pressures as one important drive force for focused fluid flow in impermeable sediments (the organic system). Besides the suitable temperature and pressure conditions, a prerequisite for the formation and stability of hydrate deposits in the shallow subsurface and at the seafloor is the sufficient supply of gas-rich fluids through the hydrate stability zone. Thus, the proportion of gas migrating from deep sources is significantly larger than that trapped in hydrates. As such, such marine hydrate deposits seem more like temporary carbon storage rather than the main culprit for climate warming at least in a short period. [ABSTRACT FROM AUTHOR]

The Okinawa Trough is characterized by enrichment of Ag in hydrothermal precipitates; however, the distribution of this enrichment remains poorly constrained. This study presents the results of a field‐emission scanning electron microscopy and electron‐microprobe analysis based mineralogical and geochemical investigation of the spatial distribution of Ag within Ag‐rich sulfide samples from the Okinawa Trough. The tetrahedrite, covellite, and galena in these samples contain high concentrations of Ag (average values of 1.60, 0.78, and 0.23 wt%, respectively) and also various Ag sulfosalts. Examination of the Ag budget of these samples indicates that most of the Ag is hosted by tetrahedrite followed by galena. The Ag within tetrahedrite is incorporated by substitution into the Cu site, whereas galena becomes Ag‐enriched by the coupled incorporation of monovalent Ag, Tl, and Cu, and trivalent Sb and Bi into Pb lattice sites. Tetrahedrite and galena containing higher concentrations of Sb favor increased Ag substitution. Four sets of Ag host minerals are identified with distinct ore formation temperatures. Tetrahedrite and galena concentrate the majority of Ag at medium temperatures (150–300°C). Other Ag host minerals concentrate only minor or trace amounts of Ag, including massive sphalerite, chalcopyrite, and pyrite at high temperatures (>300°C), colloform pyrite and sphalerite at low temperatures (<150°C), and Ag‐sulfosalts at even lower temperatures (<100°C). [ABSTRACT FROM AUTHOR]

Magmatic sulfide formation and oxidative dissolution are effective in metal pre-enrichment and remobilization during magmatic processes, and are considered to be a prerequisite for the formation of magmatic–hydrothermal ore deposits in subduction zones. However, this sequential process and sulfide oxidation mechanism are poorly understood. In this study, we report fine-grained sulfides with variable degrees of oxidation distributed within mafic cumulates, brought into felsic magma as mafic magma intrusions in the southwestern Okinawa Trough. Thermodynamic modeling suggests that the oxygen fugacity (f O 2) of primitive mafic magma is ∼FMQ + 2.5, reducing to ∼FMQ + 1 in evolved mafic magma and to ∼FMQ + 0.5 in felsic magma. This variation results from the continuous assimilation of reduced country rocks, as reflected by negative δ34S VCTD values (–0.62‰ ± 0.23‰) of magmatic sulfides and highly radiogenic Sr–Nd isotopic compositions of host dacite. Magmatic reduction and addition of crust-derived sulfur result in early formation of crystalline sulfides, dominantly pyrrhotite, enriched in Cu (0.93 ± 0.38 wt.%). A new injection of oxidized mafic magma triggers partial to complete oxidation of the earlier magmatic sulfides, ultimately transforming them into hematite–magnetite (HM) intergrowths. This means that sulfide oxidation is not directly associated with the mafic magma itself, but more likely with high- f O 2 magmatic fluid exsolved from the primitive mafic magma. The oxidative dissolution process releases >90% of Cu from the primary sulfides, with this Cu preferentially moving into magmatic fluid. This indicates that metal remobilization and transfer is very efficient under HM buffer conditions, in which trisulfur S 3 − ions with high affinities for Au and Cu are probably the dominant sulfur species. We suggest that the periodic injection of oxidized mafic magma into continental crust containing reduced lithologies is essential for the sequential process of early sulfide formation and later oxidative dissolution in the Okinawa Trough and in other subduction zone settings with similar tectono-magmatic conditions. This process could be a precursor to the generation of metal-bearing magmatic fluid, contributing to mineralization in subduction zones. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Carbonates with different morphologies were studied for tracing microenvironments and fluid seepage. • New U-Th dating results reveal the growth order of chimney carbonates. • REE results indicate that the redox environments are various. Abstract Methane-derived carbonates with blocky and chimney morphologies were collected on the northern slope of the South China Sea. The studies of petrography, geochemistry and chronology show that the blocky carbonate has a microcrystalline structure with numerous of biogenic tests, whereas the carbonate chimney has a cryptocrystalline structure with scarce biogenic tests. Aragonite can be detected in the carbonate chimney, which indicates that it formed at the shallow sediments or on the seafloor. The δ13C and δ18O values of the blocky carbonate range from −46.8‰ to −28.8‰ VPDB and 2.2‰ to 3.5‰ VPDB, respectively. The δ13C and δ18O values of the carbonate chimney range from −45.0‰ to −22.9‰ VPDB and −2.8‰ to 3.1‰ VPDB, respectively. The carbon isotopic values indicate that these carbonates are related to the anaerobic oxidation of methane (AOM). The oxygen isotopic values show that the gas hydrate dissociation contributed the 18O-enrich fluid to a certain degree during the authigenic carbonate formation. According to the rare earth element results, the carbonate chimney precipitated in stable anoxic conditions. Because of the La-enriched influence, the formation environment of the blocky carbonate is ambiguous. The U/Th chronology results indicate that these authigenic carbonates precipitated during the last glacial period (primarily Marine Isotopic Stage 3) of low sea level stand. [ABSTRACT FROM AUTHOR]

Lipid biomarkers were investigated to reveal the microbial life preserved in sulfide and Si-rich chimney from the 49.6°E hydrothermal vent field. In sulfide chimney, iso-/anteiso-fatty acids and H-shaped glycerol dialkyl glycerol tetraethers are the main microbial biomarkers. In Si-rich chimney, monounsaturated fatty acids (C16:1n7, C18:1n7) are the main bacterial biomarkers detected, and crenarchaeol and its isomer are relatively abundant (up to 25% of glycerol dialkyl glycerol tetraethers) archaeol biomarkers. Composition of lipid biomarkers reveals the diversity of microbial communities in different types of chimney structures. Sulfate-reducing bacteria and hyperthermophilic archaea were considered to be the majority microbial life in sulfide chimney, and sulfur-oxidizing bacteria were abundant in Si-rich chimney while archaea in Si-rich chimney and mainly attributed to Thaumarchaeota, which were predominately ammonia oxidizers. Our result suggested that fluid temperature and gaseous components could be the main constraints for the diversity of microbial communities in hydrothermal chimney structures in 49.6°E hydrothermal vent field. [ABSTRACT FROM PUBLISHER]

The Sturtian (720–670 Ma) glacial–interglacial transition period was an important interval for sedimentary manganese metallogenesis, including the Mn oxide deposit in the Otjosondu region in Namibia and Mn carbonate deposits in the Datangpo Formation in the south-eastern Yangtze Platform, South China. During this period, Earth experienced the breakup of Rodinia, the Sturtian glaciation, and the Neoproterozoic oxygenation event. In this study, we investigate scenarios that might have provided geologically and geochemically favorable conditions for Mn metallogenesis. In these scenarios, the global recovery of microorganisms enhanced marine primary productivity and O2 levels of the hydrosphere and atmosphere during the Sturtian glacial–interglacial transition. However, the water column was not completely oxidized, maintaining redox stratification. Transgression–regression cycles or O2-rich downwelling drove the exchange of oxygenated topwater and anoxic deep water in rift-related basins that developed due to Rodinia's breakup. The coupling of these processes precipitated existing dissolved Mn(II) at the margins of basins (Otjosondu region) or at their centers (Yangtze Platform). In the latter case, precursor Mn oxides were further converted into Mn carbonates via the reduction of Mn oxides coupled with organic matter oxidation during early diagenesis. A brief review of Mn metallogenesis in the geological record reveals that Mn metallogenic processes typically occur under geo-environmental conditions that, in concert, produce favorable conditions for Mn sourcing, concentration, and sedimentation. [ABSTRACT FROM AUTHOR]

• Fe–Zn-, Zn–Pb-, and Si-rich samples from Tangyin hydrothermal field have low f S 2. • Sulfide minerals have low in situ δ34S values of −1.3‰ to + 5.0‰. • Light S of Tangyin sulfides are derived from biogenic sulfur. The Okinawa Trough (OT), is a back-arc basin where rifting is in its initial stages, and where hydrothermal fields (HTFs) have developed above terrigenous sediments and volcaniclastics layers of variable thickness. The source of ore-forming materials within the hydrothermal systems of the OT is complex, and may include seawater, magmatic rock, magmatic fluid, and sediments. The contribution of magmatic fluids and sediments to the hydrothermal systems of the OT is still uncertain. At least 10 modern seafloor HTFs have been reported from the OT, including the Jade, Hakurei, Iheya North Knoll, CLAM, Tangyin, Yonaguni Knoll IV, Hatoma, and Irabu. Here, we report results of mineralogical, geochemical, and whole-rock Sr–Pb and in situ S isotopic analyses of hydrothermal precipitates from the Tangyin HTF in the southern OT. This field was discovered during the HOBAB3 cruise in 2014 and contains four types of hydrothermal precipitates: Fe–Zn- and Zn–Pb-rich massive sulfides, silica-rich precipitates, and native sulfur-rich samples. Sulfates in Zn–Pb-, Fe–Zn-, and silica-rich samples have 87Sr/86Sr ratios of 0.71068–0.71750, which are higher than those of modern seawater and magmatic rocks, and indicate that the Sr is derived mainly from seawater and sediments. The Zn–Pb-, Fe–Zn-, and silica-rich samples have 206Pb/204Pb (18.5129–18.5307), 207Pb/204Pb (15.6374–15.6489), and 208Pb/204Pb (38.8759–38.9113) ratios that are intermediate between sediments and magmatic rocks, suggesting the Pb is a mixture of sediment- and magmatic-rock-derived Pb. The in situ S isotopic data indicate that the ore-forming fluids associated with all sample types had low δ34S values, with the Fe–Zn- (mean δ34S = 1.27‰ ± 1.38‰; SD, n = 12) and Zn–Pb-rich (mean δ34S = 0.93‰ ± 0.32‰; n = 36) massive sulfides, and silica-rich precipitates (mean δ34S = 2.85‰ ± 1.14‰; n = 58) having lower δ34S values than those of native sulfur-rich samples (mean δ34S = 4.23‰ ± 0.72‰; n = 5). The Zn–Pb- and Fe–Zn-rich massive sulfides, and silica-rich precipitates represent the medium–high temperature (>120 °C) early–middle stages of mineralization, formed under reducing conditions at low sulfur fugacity (f S 2), based on the occurrence of low- f S 2 mineral assemblages (e.g. high-Fe sphalerite ± isocubanite ± pyrrhotite). The low δ34S values of these three types of samples, and of the Hakurei and CLAM HTFs with low- f S 2 mineral assemblages and low concentrations of Cu, Bi, and Au, most likely reflect the incorporation of biological sulfur previously formed by MSR within the sediments. The native sulfur-rich samples of the Tangyin HTF formed at low temperatures (<120 °C), and have slightly higher δ34S values than the Fe–Zn-, Zn–Pb-, and silica-rich samples, may reflect the incorporation of biological sulfur formed mainly by MSR during hydrothermal flow in a seawater recharge zone. Some sulfates in the Jade and Iheya North Knoll HTFs have δ34S values lower than that of modern seawater; this, in combination with high- f S 2 oxidizing hydrothermal fluids, and high concentrations of Cu and Au in sulfide samples, indicates a magmatic fluid contribution to these HTFs. [ABSTRACT FROM AUTHOR]

• Gold enrichment in low-temperature Si-Ba-Fe-rich sulfide, instead of Zn-rich samples. • Low-temperature colloform sulfides and orpiment concentrate majority of gold. • Gold is present as both lattice-bound and submicron inclusions in main carriers. • Dominant Au-HS complexes at near neutral pH probably caused low-T gold enrichment. We assess the distribution of gold and investigate the physicochemical controls on gold enrichment in the Okinawa Trough, an immature back-arc basin. We conducted a systematic study of gold and other trace-element concentrations using in situ laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) for various minerals in hydrothermal sulfide samples from the Iheya North Knoll and the Yonaguni Knoll IV hydrothermal fields. Orpiment, colloform pyrite, and colloform sphalerite are relatively enriched in Au (mean contents of 30.60, 17.48, and 8.68 ppm, respectively) compared with gold-poor massive chalcopyrite, sphalerite, and pyrite. Based on gold budget estimation, colloform pyrite is the most important Au host, followed by colloform sphalerite and orpiment, with these three minerals containing the majority of gold in hydrothermal sulfides. Gold occurs as "invisible" in host minerals: as lattice-bound and submicron inclusions in colloform pyrite, as homogeneously distributed lattice-bound or nano-inclusions in colloform sphalerite, and generally as lattice-bound in orpiment and tetrahedrite. Three sets of gold host minerals are identified with distinct ore-formation temperatures. Colloform sulfides and orpiment incorporate the majority of gold at low temperatures, whereas tetrahedrite and massive sulfides incorporate only minor or trace amounts of gold at medium and high temperatures, respectively. Our results show that gold enrichment in Si–Ba–Fe-rich hydrothermal sulfides at low temperatures is a significant contributor to gold enrichment in hydrothermal sulfide deposits in the Okinawa Trough, along with the more common Zn- and Cu-rich hydrothermal sulfides. Thermodynamic modeling calculations suggest that the near-neutral pH of hydrothermal fluids and the dominant transport species, namely Au–HS complexes rather than Au–Cl complexes, are likely responsible for gold enrichment in low-temperature mineral assemblages in hydrothermal precipitates from the Okinawa Trough. [ABSTRACT FROM AUTHOR]

The nature of upper mantle is important to understand the evolution of the South China Sea (SCS); thus, we need better constrains on its mantle heterogeneity. Magma water concentration is a good indicator, but few data have been reported. However, the rarity of glass and melt inclusions and the special genesis for phenocrysts in SCS basalts present challenges to analyzing magmatic water content. Therefore, it is possible to estimate the water variations through the characteristics of partial melting and magma crystallization. We evaluated variations in Fe depletion, degree of melt fractions, and mantle source composition along the fossil spreading ridge (FSR) using SCS basalt data from published papers. We found that lava from the FSR 116.2° E, FSR 117.7° E, and non-FSR regions can be considered normal lava with normal water content; in contrast, lava from the FSR 117° E-carbonatite and 114.9–115.0° E basalts have higher water content and show evidence of strong Fe depletion during the fractional crystallization after elimination of the effects of plagioclase oversaturation. The enriched water in the 117° E-carbonatite basalts is contained in carbonated silicate melts, and that in the 114.9–115.0° E basalts results from mantle contamination with the lower continental crust. The lava from the 117° E-normal basalt has much lower water content because of the lesser influence of the Hainan plume. Therefore, there must be a mantle source compositional transition area between the southwestern and eastern sub-basins of the SCS, which have different mantle evolution histories. The mantle in the west is more affected by contamination with continental materials, while that in the east is more affected by the Hainan mantle plume. [ABSTRACT FROM AUTHOR]

The Clarion–Clipperton Fracture Zone of the east Pacific contains numerous shallow buried nodules that are in direct contact with pore water in sediment, providing a direct reflection of the interaction between nodules and sediment. However, research on the geochemical behavior of these shallow-buried nodules is limited. This study used laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), high-resolution transmission electron microscopy (HRTEM), and X-ray diffraction (XRD) to compare mineral and element distribution in shallow buried nodules with surface nodules. The shallow buried nodules are products of nodules entering the burial stage. In comparison to surface nodules, shallowly buried nodules develop a fourth oxidized-suboxic diagenetic growth layer after entering the burial stage, in addition to the three main growth inner layers (L1, L2, L3). We suggest that L4 is not influenced by the bottom water source and that the presence of todorokite and the high flux of Mn2+ in the sediment pore water compete with other metal elements to enter the lattice of manganate, resulting in significantly higher Mn, W, and Li contents in L4 compared to L2. However, the content of Ni, Mg, and other hydrogenetic elements is much lower in L4 compared to L2. We suggest that the instantaneous change in surface primary productivity results in a sudden shift in the redox environment of the upper sediment layer. This reaction leads to the reduction of solid-phase Mn, providing growth opportunities for the buried nodules. Simultaneously, this may also be the reason why the growth layer of the nodules is jointly controlled by the sedimentary processes of hydrogenetic, oxic diagenetic, and suboxic diagenetic processes. [ABSTRACT FROM AUTHOR]

The geochronology of deep-sea sediments beneath the carbonate compensation depth (CCD) presents a challenge due to the calcareous microfossils dissolution, low sedimentation rates, sedimentary hiatus, and redeposition. Fish teeth, commonly found in deep-sea sediments, have been utilized for Sr isotope stratigraphic dating. However, continuous Sr isotope exchange with pore water limits the utility of fish teeth in this context. In this study, we employed laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS), scanning electron microscopy (SEM), and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) to obtain accurate stratigraphic chronology results from rare earth element and yttrium (REY)-rich sediments in the Clarion-Clipperton Fracture Zone (CCFZ) of the Pacific Ocean. Our analysis indicated that a fish tooth characterized by a dense structure and low La content exhibited a relatively initial 87Sr/86Sr ratio. Additionally, La contents at the same depth displayed a strong positive correlation with 87Sr/86Sr ratios, suggesting that Sr isotopes exchange with pore water during early diagenesis when La and other elements entered the fish teeth. However, certain fish teeth were able to resist early diagenesis alteration due to their dense structure. Based on in situ Sr-isotope stratigraphy results, we estimated the REY-rich layer age in core GC1901 to be 24–32 Ma, consistent with the REY-rich layers of DSDP/ODP in the CCFZ. Our study thus provides a new method for in situ Sr isotope analysis of fish teeth, which improves the accuracy of Sr isotope stratigraphy of pelagic sediments and could be widely used in deep-sea exploration. • Four types of dentin structure can be distinguished in fish teeth. • 87Sr/86Sr of dense dentin type with low La content tends to be relatively initial. • In-situ Sr isotope analysis of fish teeth can get a feasible isotope stratigraphic age. • The REY-rich sediments in the western CCFZ should deposit in Oligocene. [ABSTRACT FROM AUTHOR]

Ferromanganese nodules are a potential energy resource because of their high contents of economically interesting elements (i.e. Mn, Ni, Cu, and Zn). These are higher in diagenetic layers than in hydrogenetic layers. The study of the causes of elemental accumulation in the diagenetic layer is useful for the exploration metal-rich nodules. A diagenetic-dominant ferromanganese nodule, from the central Clarion–Clipperton Fracture Zone of the eastern Pacific Ocean was studied from core to rim. It was divided into four layers and seven sublayers, of the four typical diagenetic sublayers (L1, L2-1, L2-2, and L3-1). Differences were observed in these diagenetic sublayers. L1 presents the highest Mn/Fe ratio (54), the lowest Co content (2262 ppm), and a positive Ce anomaly. L2-1 exhibits high Co (3122 ppm) and Ba contents (4020 ppm), a positive Ce anomaly, and an obvious peak for 10 Å manganate minerals. L2-2 contains the lowest Ni+Cu contents (3.2 wt%), the highest Ba and Co contents (5110 ppm), and the strongest positive Ce anomaly. In L2-2, the δCe value can be positively correlated to the Mn/Fe ratio and a pronounced peak for 10 Å manganate minerals indicates that this layer has the highest mineral crystallinity. L3-1 shows the highest Ni+Cu contents (5.4 wt%), the lowest Ba (1247ppm), and Co (1725 ppm), a weakly positive Ce anomaly, and the poorest mineral crystallization. Diagenetic- and hydrogenetic-endmember mixing models reveal that hydrogenetic input contributes minimally to these chemical changes, whereas diagenetic input contributes greatly. The changes in diagenetic input may be caused by the changes in primary productivity brought about by movement of tectonic plates and the intense activity of the diagenetic pore fluid. The activity may provide a metal source for the diagenetic sublayer (anomalously high Co and Ce content) via the incorporation of metals released from dissolved buried nodules and micronodules under a suboxic or reducing environments. [ABSTRACT FROM AUTHOR]

Sulphur isotopes can be used as a powerful tool to trace fluid evolution and explore the formation of chimneys. To clarify the in situ S isotopic variations of sulphides at the micro-scale, we analyzed a sulphide chimney collected from the hydrothermal field in the East Pacific Rise 1–2° S using a sensitive high-mass-resolution ion micro-probe for stable isotopes (SHRIMP SI). Three mineral zones can be identified in the chimney: an external outer wall of porous anhydrite and colloform pyrite, an internal middle zone of sub-euhedral pyrite and massive chalcopyrite, and an inner zone of massive pyrite. The δ34SV-CDT values of the sulphides fall within the range 1.83–7.51 ‰ (avg. 4.05 ‰, n = 16), and S isotopic values increase from the core (3.09 ‰, n = 3) to the middle (3.78 ‰, n = 11) to the edge (6.99 ‰, n = 2). These results illustrate mineral crystallization processes and the mixing between seawater-derived S and magmatic–hydrothermal fluids during the growth of the chimney. The zones from the edge to the core are characterized by crystal morphologies of colloform/anhedral pyrite to massive pyrite with decreasing δ34S values, revealing multi-stage mineral deposition and sulphur isotopic fractionation. In contrast to the increase in δ34S values from the core to the edge in one profile (profile A), anomalously low δ34S values in fine-grained pyrite relative to chalcopyrite in another profile (profile B) in the middle zone result from S isotopic exchange between seawater SO42− and fluid H2S due to different fluid–seawater mixing, possibly caused by variations in permeability and porosity across the chimney. [ABSTRACT FROM AUTHOR]

Significant Zn isotope fractionation occurs during seafloor hydrothermal activities. Therefore, exploring variations in Zn isotope composition affected by hydrothermal fluids and oxidative processes would help to better understand hydrothermal fluid cycling and sulfide deposition on mid-ocean ridges. In this paper, the Zn isotope compositions of different types of sulfides and their oxidation products obtained from hydrothermal fields on the South Mid-Atlantic Ridge (13-15°S) were analyzed using a Neptune plus MC-ICP-MS. The δ66Zn ratios range from -0.14‰ to +0.38‰, and the average δ66Zn ratio is +0.12±0.06‰ (n=21, 2 SD) for all the studied sulfides and oxidation products. The Cu-rich sulfides have a slightly heavier Zn isotope composition (average δ66Zn=+0.19±0.07‰, n=6) than the Zn-rich sulfides (average δ66Zn=-0.02±0.06‰, n=5). The Zn isotope compositions of the oxidation products are similar to those of the Cu-rich sulfides, with an average δ66Zn ratio of 0.14±0.06‰ (n=10, 2 SD). The Zn isotope compositions of all the samples are generally within the ranges of sulfides from hydrothermal fields on other mid-ocean ridges, such as the East Pacific Rise (9°N, 21°N) and the Trans-Atlantic Geotraverse. However, the average Zn isotope composition indicates the presence of significantly lighter Zn isotopes relative to those reported in the literature (average δ66Zn=+0.39‰). The significant enrichment of the Zn-rich sulfides with light Zn isotopes reveals that kinetic fractionation likely occurs during mineral deposition. Furthermore, the Zn isotope compositions of the sulfides and their oxidation products (average δ66Zn=+0.12‰) are significantly lighter than the average Zn isotope composition of the ocean (δ66Zn=+0.5‰), which could further constrain the modern Zn isotope cycle in the ocean by serving as a sink for light Zn isotopes. [ABSTRACT FROM AUTHOR]

Carbonate recycling in subduction zones has a strong influence on arc magmatism and carbon reservoirs in the mantle. However, geochemical evidence of carbonate recycling during subduction, especially at sub-arc depths, is rare and the related process is poorly understood. In this study, we document high-Mg olivines (81–91 mol.% forsterite) with extremely low Ni concentrations (mostly <600 ppm), which occur in host dacites recovered from the southwestern Okinawa Trough (SWOT). The temperature–pressure of crystallization had a negligible effect on the Ni contents of the olivines, and reactions between primary basaltic magmas and the lithospheric mantle were also insignificant. Small sulphide droplets are present in the olivines, which suggest that sulphide–silicate melt immiscibility occurred during the early stages of magmatic evolution. Although early segregation of sulphide melts could reduce the Ni contents of the primitive magmas, our forward modelling suggests that sulphide segregation cannot solely explain the low Ni contents. Therefore, the most likely explanation for low Ni concentrations is the interaction between Ca-rich carbonate melts and the mantle wedge, forming an olivine-rich mantle source by consumption of orthopyroxene, as indicated by the extremely low Ni/Mg and high Ca/Fe ratios of magmatic olivines. Given that the SWOT is located just ~100 km above the Wadati–Benioff zone, our results provide rare evidence for carbonate recycling at sub-arc depths in an active subduction zone. [ABSTRACT FROM AUTHOR]

With the rapid development and wide use of fuel cells and scandium (Sc)-aluminum (Al) alloy, the demand for Sc may increase significantly in the future. Due to the lack of independent deposits, Sc is mainly derived from comprehensive utilization at present, which may cause a potential resource risk for the green economy in the future. It has been reported that deep-sea rare earth elements (REE)-rich mud is also enriched in Sc, but the geochemical properties of Sc in pelagic sediments are still poorly understood. In this study, bulk sediments geochemical data of China Ocean Mineral Resources R&D Association cruises at the Western Pacific and the Clarion-Clipperton Fracture Zone combined with in-situ chemical composition of fish teeth (ichthyoliths or bioapatite) and ferromanganese micronodules are used to reveal the geochemical characteristics of Sc in pelagic sediments. The frequency distribution of Sc contents in sediments from the Western Pacific and the CCFZ showed regional variability, with median values of ∼19 ppm and ∼ 24 ppm, respectively. Scandium is commonly enriched in REE-rich sediment layers unassociated with hydrothermal activity, with a median content of ∼38.4 ppm. The Sc contents of the pelagic clay are generally higher than that of calcareous ooze and siliceous ooze. Compared to REE-rich sediments, Sc is concentrated in bioapatites (up to ∼250 ppm) but deficient in micronodules (∼2–20 ppm). Based on the in-situ geochemical results and the correlations between Sc and other major elements, clay minerals and bioapatites are likely to be the main Sc host phases in deep-sea sediments. Considering the high commercial price of Sc and its highest value proportion (>20 %) among REEs, Sc should not be ignored in the resource assessment of REE-rich sediments. • The Sc baseline of deep-sea sediments exhibits remarkable regionality. • Sc content of the REE-rich mud is comparable to other types of Sc deposits. • Bioapatites and clay minerals would be the main host of Sc in pelagic sediment. • Taking Sc into evaluation, the economic value of REE-rich mud increases by >20 %. [ABSTRACT FROM AUTHOR]

Carbonate samples were collected from the northern Okinawa Trough in the East China Sea in 2013. The petrology, mineralogy, carbon and oxygen isotopes, and rare earth elements (REEs) of these samples were analyzed. Aragonite, high-Mg calcite, and dolomite were the main carbonate minerals, the contents of which varied greatly among the carbonate samples. Petrological observations revealed the common occurrence of framboidal pyrites. The δ13C values of carbonates varied from −53.7‰ to −39.39‰ (average of −47.3‰ based on Vienna Pee Dee Belemnite (V-PDB), n=9), and the δ18O values ranged from 0.6‰ to 3.4‰ (average of 1.9‰; V-PDB, n=9). The carbon and oxygen isotope characteristics indicated that the carbonates precipitated during the anaerobic oxidation of methane. The carbon source was a mixture of thermogenic methane and biogenic methane, possibly with a greater contribution from the former. The oxygen isotope data showed that gas hydrate dissociation occurred during carbonate precipitation. The Ce anomalies suggested that the carbonates precipitated in an anoxic environment. A slight enrichment of middle REEs (MREEs) could be attributable to the early diagenesis. The structures, minerals, oxygen isotopes, and MREEs all indicated that the carbonates experienced some degree of early diagenesis. Therefore, the influence of early diagenesis should be considered when using geological and geochemical proxies to reconstruct original methane seepage environments. [ABSTRACT FROM AUTHOR]

The Okinawa Trough is a sediment-covered intra-continental back-arc basin with active hydrothermal activities. The sulfur isotopic systematics of the hydrothermal systems in the Okinawa Trough remain unclear owing to the complex sulfur reservoirs. To address these uncertainties, we conducted in situ S isotopic combined with whole-rock geochemical, Pb isotopic, and barite Sr isotopic analyses of typical hydrothermal precipitates from three hydrothermal fields with varying sediments thickness (Iheya North Knoll (INK) < Yonaguni Knoll IV (YK) < Noho) in the Okinawa Trough. The δ34S values of barite from INK (+16.4‰ to +19.6‰, median + 18.5‰) and YK (+16.2‰ to +22.2‰, median + 19.5‰) mostly indicate lower values than those obtained for seawater (~ +21‰), combined with the observation of high S and O fugacity (fO 2 –fS 2) mineral assemblages (e.g., low-Fe sphalerite ± enargite ± tennantite ± chalcopyrite), and high Cu and Au contents of hydrothermal precipitates, consistent with the addition of magmatic volatile to these two fields. In contrast, the in situ δ34S values of sulfide minerals from INK (+5.6‰ to +10.4‰, median + 9.0‰) and YK (+3.5‰ to +5.5‰, median + 4.7‰) fields were all positive, inconsistent with the S isotopic characteristics of H 2 S formed from SO 2 disproportionation. We conclude that a large proportion of thermochemical SO 4 2− reduction-derived heavy sulfur likely obscure the characteristics of H 2 S sourced from disproportionation. This was likely resulting from (1) sediment controlled long-scale lateral hydrothermal circulation, (2) the existence of early precipitated anhydrite, and (3) high concentrations of H 2 in the high-temperature reaction zone. The Noho field have low δ34S values (−0.2‰ to +5.3‰, median + 0.9‰) of sulfide minerals, combined with high sediment contributions reflected by high Pb isotopic ratios and low-fS 2 –fO 2 mineral assemblages (e.g., high-Fe sphalerite ± pyrrhotite ± isocubanite), which indicates that the low δ34S value sulfur was derived from leaching of biogenic pyrite in thick sedimentary strata by hydrothermal fluid. Near surface, the high-temperature discharging hydrothermal fluids interacted with different thicknesses of sedimentary strata and leached different proportions of biogenetic pyrite, resulting in a negative correlation of sedimentary strata thickness and sulfide δ34S values of the Okinawa Trough hydrothermal fields. • Mineralogy, and S–Pb–Sr isotopes were studied for three hydrothermal fields in OT. • Barite subseawater δ34S and high-fO 2 –fS 2 indicate addition of magmatic volatiles. • Magmatic volatiles-derived light S was obscured by TSR-derived heavy sulfur. • Leached biogenetic S from sedimentary strata is the main source of light S. [ABSTRACT FROM AUTHOR]

Understanding the relationship between sulfide chimney maturity and fluid evolution in submarine hydrothermal systems is essential for deciphering mineralogical changes, sulfur dynamics, and fluid flux over time. We investigate this link through sulfur isotopic analysis of sulfides from three maturity stages of sulfide chimneys at the Niaochao hydrothermal field (East Pacific Rise, 1–2°S), integrated with δ34S values from diverse global hydrothermal fields. Based on mineral associations and sulfur isotopes, the sulfide chimneys are classified into Zn-Fe-rich, Fe-Cu-rich, and Fe-Cu-Zn-rich chimneys, representing the increasing maturity. A systematic increase in δ34S values (from 1.54‰ to 7.22‰) from chimney cores to rims indicates the progressive reduction of seawater sulfate during fluid-seawater mixing in open systems. Fluctuations in δ34S values within chimney zones are attributed to the interaction between hydrothermal fluids and pre-existing minerals. As chimneys grow, rapid sulfur isotopic exchanges occur between seawater sulfate and fluid H 2 S in localized semi-closed systems, leading to a significant drop in δ34S values of pyrites, decreasing by up to 4.82‰. A new chimney evolutionary model has been developed utilizing mineralogical and sulfur isotopic data of sulfide chimneys from global hydrothermal vents. This model encompasses birth, growth, and waning stages, accompanied by an elevation of mean δ34S values from 2.1‰ (n = 66) to 3.2‰ (n = 435) and 3.8‰ (n = 61) due to ongoing seawater sulfate reduction. Conversely, the infusion of magmatic sulfur during chimney growth contributes to a decrease in δ34S values of sulfides. This study underscores the significance of sulfur isotopes as indicators of sulfide chimney maturity, which enhances our understanding of the evolution and formation processes of hydrothermal systems. • Three maturity stages of sulfide chimneys at the Niaochao vent field have been classified. • The evolutionary degree of sulfate reduction in fluids from open to semi-closed systems within the chimney has been deciphered. • A new chimney evolutionary model by variations in δ34S values is established. • The study reveals the link between mineralogy, δ34S values, and maturity of zoned chimneys in submarine hydrothermal systems. [ABSTRACT FROM AUTHOR]

• To characterize the petrographic features of pyrite across the chimney. • To investigate the trace elements in varying types of pyrite and the enrichment factors. • To illustrate the sources of S and evolution of the fluids across the chimney. • To reveal the history of chimney growth. Active black smokers represent important products of seafloor hydrothermal systems and can reveal the fluid evolution and ore genesis in sulfide deposits. The key for understanding the details of fluid circulation and mineralization within sulfide zonation of chimneys lies in the understanding of variation in micro-scale trace elements and sulfur isotopes in sulfide deposits. An active pyrite-dominated chimney collected from the East Pacific Rise 1-2°S hydrothermal field supplies a unique opportunity to exhibit on-going ore-forming processes, especially the detailed fluid processes among the sulfide zonation of chimney, as well as to constrain the mineralization environment that is crucial for metal transport and deposition across the chimney. Pyrites, as the ubiquitous sulfide, present crystal morphologies varying from anhedral though euhedral to massive textures at different mineralization stages. The characterization of trace elements in different types of pyrite, from rim to core, evolves from Zn, Pb, Cd, Tl, Ag, Mn and Ba to Cu, Se, Sn, Bi, Co and Ni in response to increasing mineralization temperature. The dominant control on Co and Ni contents in pyrite from the exterior and interior of the chimney changes from temperature to sulfur fugacity as the hydrothermal circulation becomes active and subsequently wanes. In addition to the physicochemical environment, the chemical variations like As, Sb and Mo in pyrite from different mineral zonation are mostly like related to the temporal evolution of fluid compositions. Seawater-fluid mixing not only influences seawater-derived species of trace elements like V and Mg, and the spatial distributions of temperature-sensitive elements like Co, Cu, Se and Sn, but also increases the S isotope values of pyrite in the mineral sequence of the chimney. However, the incorporation of approximately 8 ∼ 24% bacteria-derived S leads to anhedral pyrite with abnormally light S isotopes in the outer mineral zonation. [ABSTRACT FROM AUTHOR]

The northern Lau back-arc basin (NLB) lavas display a diverse geochemical nature caused by complex geological processes in this region. Independent component (IC) analysis was applied to investigate the nature of mantle sources in the NLB, based on a compiled geochemical data set from the NLB, central-southern Lau Basin (C-SLB), Pacific and Indian Ocean ridges, and Samoan islands. We identified three ICs in the five-dimensional space of Sr-Nd-Pb isotopic ratios, which can account for 96.5% of the isotopic variance. The correlations between the ICs and the incompatible trace elements ratios were further used to examine the origin of these ICs. The first IC (IC1) separates Samoan islands (IC1 < −1) from the other groups, and shows negative correlation with (La/Sm)N ratios. The second IC (IC2) discriminates mid-ocean ridge basalts (MORBs) from Pacific (IC2 > 0) and Indian (IC2 < 0) Ocean ridges as well as the C-SLB (IC2 > 0) and NLB (IC2 < 0) lavas. IC2 correlates positively with Ba/Th ratios. The third IC (IC3) distinguishes MORBs (IC3 > 0) and back-arc basin lavas (IC3 < 0), and displays a negative correlation with Th/Nb values. On a regional scale in the NLB, there is a broad increase in IC1, IC2, and IC3 from the north to the south. Only IC3 presents obvious decrease from the west to the east, i.e. with decreasing distance from the arc. The geochemical nature and the statistical properties of these ICs suggest that IC1 is related to an enriched mantle component most likely from the nearby Samoan plume, and the IC2 corresponds to a fluid-rich component from the subducting Pacific slab. The IC3 may represent partial melt of recycled subducted sediment or recycled continental crustal materials. The geographic distribution of three ICs supports that the relatively recent mixing of Samoan plume materials with the subduction-metasomatized back-arc mantle may be responsible for the observed geochemical diversity in NLB lavas. [ABSTRACT FROM AUTHOR]

The petrographic and geochemical features of the modern hydrothermal sulfides can reveal the growth processes of chimney and the evolution of hydrothermal fluids. To clarify the geochemical variations and ore-forming fluids evolutions during the chimney growth, we performed laser ablation ICP-MS and Nano-SIMS (Nanometer-second ion mass spectrometry) to characterize the variations of trace elements and S isotopes of pyrite across a pyrite-dominated chimney collected from the hydrothermal field on East Pacific Rise. Three types of pyrite were distinguished from exterior to interior of the chimney, which varies from anhedral through euhedral to massive textures. Trace elements of pyrite evolves from high-temperature (Cu, Se, Sn, Bi, Co and Ni) to low-temperature characteristics (Zn, Pb, Cd, Tl, Ag, Mn, Ba, Mg and V) with the decreasing degrees of crystal morphologies from core to rim in response to the changes in temperature and fluid chemistry caused by seawater-fluid mixing. The massive pyrite in core features lower Se/Tl ratios but higher Co/Ni ratios relative to euhedral pyrite, possibly attributed by the decreasing T or ƒS2 of fluids with the waning hydrothermal circulation. The δ34SV-CDT values of pyrite range from -0.51 to 6.43‰ (avg. 3.56‰, n=39). The decreasing S isotopic values in the mineral sequence of chimney result from the decreasing seawater derived-S, whereas the incorporation of approximately 8%–24% of bacteria derived-S lead to partial anhedral pyrite with negative δ34SV-CDT values (down to -0.51‰).The systematic variations in the S isotopic compositions and morphology of the pyrite across the chimney wall reflect multiple stages of mineralization and fluid evolution. Based on in situ sulfur isotope and micro-scale trace elements compositions of pyrite, we propose a chimney growth model that focuses on different degrees of fluid-seawater mixing and variations of the permeability across the chimney. [ABSTRACT FROM AUTHOR]

[Display omitted] • Hydrogenetic Fe-Mn micronodules have statistically high positive Ce anomalies. • REYs (except Ce) contents are lower in hydrogenetic micronodules than in crusts/nodules. • REYs (except Ce) prefer to migrate out from Fe-Mn micronodules to porewater. Ferromanganese micronodule, ubiquitous in aquatic environments, is a key authigenic phase in the rare earth elements and yttrium (REY) enriched deep-sea sediments. This is due to the high porosity and strong scavenging REY capacity of Fe-Mn (oxyhydr)oxides. In this study, micronodules were selected from surface sediments of the NW Pacific inter-seamount basins and the NE Pacific Clarion-Clipperton Zone (CCZ), both distal from oceanic ridges and largely impervious to hydrothermal activities. Hydrogenetic micronodules of this study have commonly consistent and highly positive Ce anomalies. Based on big data statistics, amounts of geochemical data of ferromanganese deposits (n > 1350) were compiled, and those of hydrogenetic ones (including micronodules, (macro)nodules and crusts) were selected. Comparative analyses reveal that both the hydrogenetic crusts (n = 289) and nodules (n = 159) have weaker positive Ce anomalies (avg. 2.23 and 2.97, respectively) than hydrogenetic micronodules (n = 204, avg. 6.27). Combined with the similar Ce content in all three hydrogenetic ferromanganese deposit types, and the lower concentrations of other REYs in hydrogenetic micronodules, the migration of REYs (except for Ce) from micronodules to porewater is indicated. While Ce is likely retained via oxidizing to insoluble Ce(IV), thus forming the highly positive Ce anomalies. Therefore, our study suggests that ferromanganese micronodules could serve as a buffer in the REY enrichment of pelagic sediments. [ABSTRACT FROM AUTHOR]

Analysis of H 2 O concentrations in quenched glass has enabled significant improvements in our understanding of its role in mantle melting, magma differentiation, eruption dynamics, and the origin of mantle heterogeneity. Direct measurements of dissolved H 2 O in glass, however, are not always possible, and we lack robust methods of constraining magmatic H 2 O contents in aphyric, bulk rock samples that lack glass. Here, we present a major element hygrometer for mid-ocean ridge (MOR) and back-arc basin (BAB) basalt magmas based on the sensitivity of phenocryst phase assemblages to magmatic H 2 O contents, which translate into resolvable differences in liquid lines of descent (LLDs) as a function of magmatic H 2 O concentrations. Existing hygrometers lack sufficient resolution to be useful at the low H 2 O concentrations typical of MOR and BAB basalts (<1.0 wt%). We develop the major element proxy, Al 2 O 3 /FeO* (7.0) (fractionation-corrected to 7 wt% MgO), for determining magmatic H 2 O contents using cogenetic suites of oceanic basalts with well-defined LLDs and well-constrained H 2 O contents. H 2 O (7.0) positively correlates with Al 2 O 3 /FeO* (7.0) in the mid-ocean ridge basalt dataset, and this relationship is maintained in back-arc basin basalts with a broader range of water contents (up to 2.0 wt%). The main petrological control over this covariation is the role of H 2 O in suppressing plagioclase crystallization, while crystallization pressure and magmatic oxygen fugacity play lesser roles. Herein, we present an empirical model that uses Al 2 O 3 /FeO* (7.0) to estimate the magmatic water content in plagioclase-saturated oceanic basaltic magmas: H 2 O (7.0) = 1.109 Al 2 O 3 / FeO ∗ (7.0) − 1.111. This model enables the estimation of magmatic H 2 O content using whole-rock major element data, which can be readily determined for aphyric or crystalline lavas that lack quenched glassy rinds, melt inclusions, or appropriate phenocryst assemblages. • An empirical model is developed to estimate magmatic water contents. • Liquid lines of descent demonstrate a covariation of Al 2 O 3 /FeO* (7.0) and H 2 O (7.0). • H 2 O suppressing plagioclase crystallization is the dominant cause of covariation. [ABSTRACT FROM AUTHOR]

Dehydrated fluids expelled from the serpentinised mantle in subducted slabs contribute significantly to arc magmatism and element cycling on Earth. However, the recycling mechanism of serpentinite-derived fluids at various subduction zone depths remains unclear, as does their contribution to the genesis of back-arc basin lavas. In this study, we investigated magnesium (Mg) isotopic compositions of lavas from the Okinawa Trough (OT) and the Lau Basin, because Mg isotopes have shown great potential in tracing the dehydration of slab serpentinites in recent years. After excluding dubious samples affected by alteration, lavas from the OT and the Lau Basin have heavier Mg isotopic compositions than mid-ocean ridge basalt (MORB) mantle, which could be attributed to the involvement of slab serpentinite-derived fluids rather than crustal assimilation or subducted sediment inputs, as indicated by the isotopic modelling results. The δ26Mg values of the southern OT (SOT) and the southern Lau Basin lavas are generally higher than those in the middle OT (MOT) and the northern Lau Basin, respectively, with an average of −0.09 ± 0.02‰ (2SD, n = 6) for the SOT, −0.18 ± 0.10 ‰ (2SD, n = 9) for the MOT, −0.13 ± 0.08 ‰ for the southern Lau Basin (2SD, n = 5), and − 0.19 ± 0.06 ‰ (2SD, n = 10) for the northern Lau Basin. The binary modelling results show that various quantities of serpentinite-derived fluids could explain the variations in Mg isotopic compositions observed in the OT and the Lau Basin. We suggest that the thermal structure of the inter-subduction zone controls the signal of Mg-rich serpentinite-derived fluids, based on both reported δ26Mg values in subduction zones and our findings. In contrast, the contributions of these fluids to different segments in a specific subduction zone may depend on the slab depth beneath sites of magmatic activity. • We report heavier Mg isotopic compositions of the O T and the southern Lau Basin lavas than the mantle for the first time. • The slab serpentinite-derived fluids could explain the higher δ26Mg values. • The slab depth may have exerted the first control on the Mg isotopic compositions in a specific back-arc basin. [ABSTRACT FROM AUTHOR]

[Display omitted] • A model was established to better understand the metal mobilization and sulfur cycles in the shallow seawater-hydrothermal circulation system. • The variations in δ34S values of pyrites are due to the variable sulfur contribution of shallow-origin reduced sulfur from seawater sulfate and reduction of anhydrite. • Pyrite mineralization clarifies the subseafloor mineralization related to shallow seawater-hydrothermal circulation. • PCA results reveal that the control on fluid compositions changes from the early shallow seawater-hydrothermal circulation to later intense hydrothermal activity. The shallow seawater-hydrothermal circulation system plays a crucial role in the subseafloor mineralization of the hydrothermal field. However, its key fluid processes and impacts on the metal mobilization and sulfur cycles in the stockwork mineralization are still poorly understood. We first present the systemic variations in micro-scale trace element and sulfur isotope compositions of pyrite varieties in a stockwork-like sulfide assemblage from the Longqi hydrothermal field to constrain the transport and deposition of metals and the origins and cycles of sulfur in the shallow seawater-hydrothermal circulation system. Pyrites considered as the dominant sulfides can be clarified into disseminated fine-grained (Py-I), euhedral (Py-II), and subhedral-euhedral (Py-III) varieties based on the textures. The wall-rock-derived elements Ti, Cu, and Ni and seawater-derived elements V, U, Mo, Mg, and Mn are concentrated in Py-I in the breccias, which is related to the fluid-rock interaction and fluid-seawater mixing in the shallow seawater-hydrothermal circulation system. Short-lived shallow seawater-hydrothermal circulation results in fluid fluctuation and oscillatory-zoned Py-II with depletion of Co, Ni, Cu, As, and Se in the mantles relative to those in the rims and cores. As the later hydrothermal activity was active, Py-III was overgrown from Py-II rich in hydrothermally inherited metals Se, Te, and Co, possibly implying the hydrothermal field is coming into the main mineralization. The sulfur isotope compositions of pyrites range from 4.30 to 9.98‰ (n = 37), with distinct δ34S variations in the individual Py-I crystals (>1.5 ‰ within a 20 × 20 µm2 region). This variation is attributed to changes in the relative proportion of sulfur sourced from (i) the shallow-origin reduced seawater via reduction by ferrous iron released from basalt, and (ii) the reduction of pre-existing anhydrite by later hydrothermal overprinting in the shallow subseafloor. These findings provide evidence for a model to better understand the effect of the shallow seawater-hydrothermal circulation system on the subseafloor stockwork mineralization of hydrothermal fields. [ABSTRACT FROM AUTHOR]

Luo, X.; Gao, J.; Jin, X.; Li, J., and Chu, F., 2015. A primary research on marine magnetic disturbance correction method using uncombined precise point positioning. Marine magnetic survey is an important mean of geophysical exploration. The magnetic disturbance magnitude, the start and end time, and perfecting magnetic disturbance model are important for magnetic survey. In this paper, on the basis of that dual-frequency GNSS is able to get the ionospheric total electron content, reflect the nature of the magnetic disturbance characteristics, etc. the study of magnetic disturbance correction in marine magnetism measurements using dual-frequency GNSS is carried out. Through quality controlled by the dual-frequency GNSS phase observations, ionospheric total electron content solved by static and dynamic at slant direction. Comparing the solving with geomagnetic observatories and marine magnetic measurements, the solution showed that there is relatively strong correlation between ionospheric total electron content and marine magnetic surveys. This fact helps to perfect magnetic interference correction model and improve the accuracy of magnetic measurement. Also it is a good foundation for the application of GNSS to carry out new areas. [ABSTRACT FROM AUTHOR]

Luo, X.; Gao, J.; Jin, X.; Li, J., and Chu, F., 2015. The latest application to determine tide height in a large-scale sea using shipborne GPS based on precise point positioning and EGM2008. GPS data dedicated to tidal level are usually processed using differential techniques. Unfortunately, the precision of resulting kinematic positions is baseline-length dependent. So, high precision tidal level measurements using differential GPS techniques are limited to coasts, lakes, and rivers. Recent improvements in GPS satellite products (orbits, clocks, and phase biases) make Precise Point Positioning (PPP) technique reach cm-level in three dimensions. At same time, EGM2008 determine the sea level also reached centimeter level. So Precise Point Positioning technique with EGM2008 opens up tidal level measurements without Tidal Gauges. This paper recalls the concept of PPP technique and discusses the precision of GPS shipborne positioning. Tidal data of large-scale sea are calculated from PPP and EGM2008. Some results are gotten. (1) Compliance of tidal height, which obtained from measurements and model, increases in sequence as distance apart from seashore increases. (2) The Cycle, amplitude of the tide calculated by PPP and EGM2008 is more obvious, the performance characteristics more clearly, solving characteristics and theoretical tidal level characteristics match well. (3) Marine gravity measurements result shows that FAA (Free-air anomaly) and BG (Bouguer anomaly) calculated from PPP are similar with those from conventional method, but depth from former is better than that from latter. From above results, PPP technique is a feasible method to determine tidal height. [ABSTRACT FROM AUTHOR]

There are very few studies on the granites formed in Late Permian in Hainan (海南) Island. Petrology, geochronology and geochemistry research on Late Permian granites in Chahe (叉河) area, Hainan Island will be reported in this article. The results show that, Chahe granites formed in 251 Ma, are riched in SiO, alkali, FeO while depleted in MgO and CaO, with low Sr and high Yb, and have all the features consistent with A-type granite. The forming of Chahe granites may suggest that Hainan Island was already in post-orogenic setting in Late Permian. [ABSTRACT FROM AUTHOR]

The weathering processes of seafloor massive sulfide (SMS) deposits would significantly change the mineralogy and geochemistry of sulfides. The key to understanding the oxidation of SMS deposits lies in clarifying the metal mobilization during the oxidation of submarine sulfides by seawater and Fe-Si-rich hydrothermal fluids. We present the morphology, mineralogy, and geochemistry of secondary minerals and oxyhydroxides from the Xunmei hydrothermal field on the Southern Mid-Atlantic Ridge (SMAR) to illuminate their diverse characteristics and origins, further elaborating detailed submarine oxidation processes of sulfide mounds. Under the oxygenated seawater environments, the pyrite/marcasite and chalcopyrite were oxidized to Fe (oxyhydr)oxides and secondary Cu minerals (e.g., covellite, paratacamite/atacamite, and chalcanthite), respectively. The metal speciation in the sulfides (e.g., pyrite, marcasite, and chalcopyrite) controls the metal mobilization during oxidation. Cobalt and As in the crystal lattice of pyrite and marcasite vary over variations in Fe and S, respectively. Zinc, Ag, and Pb that occur as micron-inclusions in the pyrite, marcasite, and chalcopyrite are easily released during the galvanic interaction, of which mechanism is applicable to Cu-inclusions in the pyrite and marcasite. Compared to the enrichment of Zn and Ag in the secondary minerals via adsorption, Pb is depleted because of its high solubility in the seawater. The mineralization of amorphous oxyhydroxides includes low-temperature Si-rich precipitation and later oxidative weathering processes. The Si-rich hydrothermal fluids dissolve the basalt enriched in plagioclase and V-rich clay minerals to precipitate V-rich Si-Al oxyhydroxides. Subsequently, the flowing Si-rich hydrothermal fluids evoke precipitation of amorphous silica due to conductive cooling. As the alteration proceeds, a genetic scenario for the formation of Fe and Si oxyhydroxides is proposed. The Fe-Si-rich hydrothermal fluids alter the sulfides to Fe-Si oxyhydroxides and then Fe oxyhydroxides with an increasing trend of Fe, Cu, Zn, and Co. Our results unravel the metal mobilization during the weathering processes of submarine sulfides, establish a scenario for the formation of oxyhydroxides associated with sulfides, and predict the effect of oxidation on the economic values of sulfides. [Display omitted] • Various secondary minerals were recognized to imply diverse oxidizing environments. • The effect of metal speciation on metal mobilization during oxidation was unraveled. • A scenario for oxyhydroxide formation was proposed in the Xunmei hydrothermal field. • The effect of weathering on the economic values of SMS deposits has been predicted. [ABSTRACT FROM AUTHOR]

[Display omitted] • Sector collapse occurred along the northern flank of Suda Seamount, NW Pacific. • Seamount sector collapse provided sufficient nucleus material for nodule growth. • Nodules are more concentrated along the debris apron than neighboring areas. Manganese nodules are a potential source of critical metals such as Cu, Ni, and Co and are widely distributed on the abyssal plains of the global oceans. A polymetallic nodule metallogenic belt with a heterogeneous and spatially clustered nodule distribution was recently discovered in NW Pacific inter-seamount basin (NPIB) areas. However, the geological processes that regulate the nodule occurrence in that region are unresolved. Here, we report on the characteristics of a high-density field of manganese nodules in the abyssal plain north of Suda Seamount. Ship-borne multibeam bathymetric data reveal a typical seamount sector-collapse topography characterized by radial lineaments of debris channels and ridges formed by rapid debris-avalanche flow. Backscatter data linked with underwater observation indicate that manganese nodules are more concentrated (50%–80% areal coverage) along the main body of the debris apron compared to adjacent neighboring areas (<30%). The extremely high concentrations (~80% areal coverage) characterized by overlapping nodules are apparently associated with downslope movement, possibly triggered by block movement along the fault slip plane or by gravity processes. Our results indicate that seamount sector collapse may have provided sufficient nucleus material for nodule growth and contributed to high nodule concentrations locally. The destruction of submarine volcanic edifices is universal, and the debris aprons and plains around such seamounts are potential prospecting areas for manganese nodule resources throughout the NPIB. [ABSTRACT FROM AUTHOR]

Crustal assimilation and the input of subducted sediment in a subduction zone play vital roles in generating arc and back-arc lavas along a continental margin. However, distinguishing the impacts of these two processes on lava geochemistry is not straightforward or well understood. In this paper, we present new Hf–Nd–Sr–Pb isotopic data for 20 igneous rocks ranging in composition from basalt to rhyolite, which when combined with data from the literature allow us to fingerprint the contributions of crustal and slab-derived materials along the Okinawa Trough (OT). Consistent with previous results, the basaltic lavas were generated by melting of an Indian-type mantle source metasomatised by slab-derived fluids and subducted pelagic/detrital sediments. We confirm that Hf behaves as a non-conservative element during subduction, constrained by Hf-Nd isotopic systematics. Our new data suggest that crustal assimilation processes worked independently of the extent of magma differentiation but were determined by the stage of tectonic development in individual regions. In the southern OT (seafloor spreading stage), crustal assimilation is insignificant, and the addition of subducted sediment is 0.4%–1.5%. In the middle OT (crustal rifting stage), the basaltic and rhyolitic suites do not show distinctive Hf–Nd–Sr–Pb isotopic compositions, and both record moderate crustal assimilation (<10%) and < 0.6% addition of subducted sediment. In contrast, in the southwestern OT (crustal extension stage), the basaltic and dacitic lavas record the highest involvement of 10%–40% crustal material. We conclude that the various stages of back-arc development exerted first-order controls on the extent of crustal assimilation in the OT and had marked effects on the geochemical and isotopic compositions of OT lavas. • A systematic report on Hf isotopes in basaltic to rhyolitic lavas of the Okinawa Trough • Hafnium behaves as a mobile element during input of subducted sediment. • Crustal assimilation process along the Okinawa Trough was evaluated using Hf-Nd isotopes. • Extent of crustal assimilation controlled mainly by the stage of tectonic development. [ABSTRACT FROM AUTHOR]

Sound refraction artifacts are often present in multibeam swath bathymetry data. For a flat array, the artifacts are usually more serious in outer beams than in inner beams. In a 3D topographical mapping they appear as ridges that parallel the tracks of the vessel. To shorten the survey time, the outer beams should be utilized as often as possible. Therefore, the refraction errors should be removed. In this paper, we present a model of reduced sound speed profile that consists of three water layers. The sound speed of the two upper layers has a constant gradient, and the third layer has the same sound speed as the most bottom measured layer. The model parameters can be searched based on the principle of the minimum difference of depth between the overlap of two neighboring swaths. The horizontal position and depth of each beam can be accordingly recalculated using the model parameters. To avoid being trapped in local optimum, the initial search scope is limited according to assumed lunch angle and travel time in each subregion. The method is verified by comparing the simulated and real data. [ABSTRACT FROM AUTHOR]

Highly depleted peridotites from the Yap Trench in the western Pacific Ocean have been studied for Re-Os elements and Re-Os isotopes. These peridotites have a low Re-Os content and variable 187Os/188Os ratios (0.12043–0.14867). The highest 187Os/188Os ratio is far higher than that of the primitive upper mantle and the lowest 187Os/188Os ratio is comparable to the most unradiogenic 187Os/188Os ratio (0.11933) discovered in subduction zone peridotites. The suprachondritic 187Os/188Os ratios of the Yap Trench peridotites results from modification of the mantle wedge by slab-derived fluid and melt. This is consistent with the observation that high 187Os/188Os ratios generally occur in oceanic peridotites with low Os content (<2 ppb) since Os may be reduced during late processes such as fluid alteration and melt refertilization. The sub-chondritic 187Os/188Os ratios of the Yap Trench peridotites correspond to a Re depletion age of 0.24–1.16 billion years, which means that these peridotites represent old mantle residue of ancient melting events. This ancient melting, combined with probable back-arc melting and forearc melting during subduction initiation, indicates that the Yap Trench mantle has a complex evolutionary history. The amount of old mantle residue in the oceanic asthenosphere was underestimated because the 187Os/188Os ratio in mantle peridotites is elevated during late processes. Therefore, old depleted mantle fragments may contribute substantially to the chemical heterogeneity of the oceanic mantle. [ABSTRACT FROM AUTHOR]