Off-axis inactive hydrothermal fields may harbor significant sulfide deposits. However, their distribution and abundance remain poorly constrained due to the challenges in their detection, as they lack associated hydrothermal plumes. Here, we present the first assessment on the abundance of off-axis inactive fields along four segments of the ultraslow-spreading Southwest Indian Ridge, based on a systematic sediment geochemistry survey. We found distinct dispersion patterns of hydrothermal Cu and Zn between magmatic- and detachment fault- controlled hydrothermal fields. These patterns are likely attributed to multiple factors, including variable physicochemical conditions of the hydrothermal plume,seafloor topography and bottom current. Up to 15 potential fields and their approximate locations were identified by the dispersion distance of hydrothermal Zn, Cu, and Fe. Among these fields, 8-14 are located off-axis, and 3-7 are likely newly identified inactive fields. The latter may have been active within the last 10 ka, with more than 60% spatially associated with detachment faults. The number of inactive fields is at least 2 times greater than previously reported for active fields within 40 km of the axial zone of the studied segments. As the studied sediments represent only the last 10 ka, significantly more buried inactive hydrothermal fields would be expected to be identified over longer time intervals. These findings significantly revise previous estimates of sulfide resources on ultraslow-spreading ridges.

Aerobic methanotrophic bacteria are pivotal in the global carbon cycle by converting methane into biomass and inorganic carbon species. Light rare earth elements (light-REE; La, Ce) are part of the metalloenzymes mediating the biochemical processes in methanotrophs. However, the partitioning of trace metals and REE in chemosymbiotic megafauna with methanotrophic endosymbionts remains largely unknown. Here we determine stable isotope compositions (delta 13C, delta 15N) of soft tissues (gill, mantle, foot) as well as trace metal and REE contents of soft tissues and shells of chemosymbiotic bivalves dwelling at methane seeps (Site F and Haima seeps) of the South China Sea. Isotopic compositions of soft tissues are found to reflect the mode of chemosymbiosis (i.e., methanotrophy vs. thiotrophy). Particularly gill tissues of bivalves with methanotrophic endosymbionts display marked light-REE and trace metal enrichments, which is interpreted to reflect the enzymatic activity of endosymbionts. Additionally, correlations between light-REE and copper (Cu), zinc (Zn), and molybdenum (Mo) contents in soft tissues of chemosymbiotic mussels are ascribed to the uptake of these elements by the methanotrophic symbionts. An observed higher trace metal content in the tissue of the semi-infaunal and infaunal bivalves with thiotrophic endosymbionts is believed to reflect the uptake of metals associated with sulfide particles. This study documents diagnostic enrichment of trace metals and light-REE in the soft tissues of bivalves with chemotrophic endosymbionts and provides new constraints for future identification of chemosymbiosis at ancient seeps. Hydrocarbon seeps, sites on the seafloor where methane-rich fluids leak into seawater, are an important source of methane. Yet, the transfer of this greenhouse gas to the ocean is reduced by microbial activity, particularly aerobic and anaerobic oxidation of methane. Some trace metals and rare earth elements (REE) are key to such methanotrophic activity, representing cofactors of metalloenzymes involved in methane oxidation. Although the actual enzymatic activity is well understood, the modes of biological uptake and the partitioning of these elements in seep ecosystems remain largely unconstrained. Here, chemosymbiotic bivalves, harboring chemotrophic bacteria in their soft tissues, were collected from two active seep provinces in the South China Sea. Comprehensive geochemical analyses have been conducted to characterize nutritional sources and trace element enrichment. The observed element patterns are best explained by simultaneous uptake of the trace elements by methanotrophic endosymbionts. Higher sulfide levels in the environment of bivalves with thiotrophic endosymbionts are held responsible for the observed greater trace metal enrichment in the tissues of bivalves harboring sulfide oxidizers. This study deepens our understanding of the behavior of trace metals and REE in methane-seep ecosystems. It will also help to better understand the adaptation of metazoans to ancient chemosynthesis-based environments. Stable isotopic composition of bivalve tissue reflects nutritional sources Correlations between light-rare earth elements and trace metal enrichment in mussel tissues reflect in vivo uptake by endosymbiotic methanotrophic bacteria Elemental distributions have the potential to identify chemosymbiosis in fossil bivalves

In mid-ocean ridge (MOR) hydrothermal systems, the gold grade of sea-floor massive sulfides (SMSs) is neg-atively correlated with the spreading rate of the ridge. Previous investigations have addressed the distribu-tion of gold in sulfides from hydrothermal fields hosted by ultramafic rocks. In contrast, the gold enrichment mechanisms in sulfides from hydrothermal fields hosted by mafic rocks in ultraslow-spreading ridge environ-ments are less well constrained. The basalt-hosted Longqi hydrothermal field, located on the classic ultra-slow-spreading Southwest Indian Ridge, provides an opportunity to examine gold enrichment mechanisms in such an environment. Two ore-forming stages are identified in chimney fragments: anhydrite + barite + col-loidal/porous pyrite (Py1) + marcasite + fine-grained sphalerite (stage 1); euhedral-subhedral pyrite (Py2) + coarse-grained sphalerite + chalcopyrite + isocubanite (stage 2). Py1 is usually overgrown by marcasite, which is in turn enclosed by Py2. Py2 coexists with coarse-grained sphalerite and chalcopyrite. Abundant native gold nanoparticles occur in Py1 or at the transition zone between Py1 and Py2. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis suggests that Py1 contains higher Mo, V, Sn, and Pb and lower As, Co/Ni, and Se/Tl values compared to Py2. In situ LA-multicollector (MC)-ICP-MS analyses show that Py1 has a higher mean delta 34S (7.1 parts per thousand) value than Py2 (6.6 parts per thousand). Sulfur primarily derives from MOR basalt (MORB) and seawater sulfate, of which the proportion of sulfur from seawater sulfate is between 20.5 and 47.6%. Textures, mineral assemblages, and trace element contents of sulfides indicate that the degree of mixing between hydrothermal fluids and seawater decreases as the chimney grows and is accompanied by a gradual increase in temperature. Based on data compiled from 41 hydrothermal fields hosted by basalt, the large range of sulfide delta 34S from slow- and ultraslow-spreading MORs may be attributed to the wide range of sulfur sources (e.g., leaching from MORB, thermochemical reduction of seawater sulfate, magma degassing, and bacterial activity), fluid-basalt interaction, and redox state (CH4/CO2 ratios). Prolonged fluid-basalt interaction and the type of chimneys, such as beehive chimneys, may lead to higher gold grades in hydrothermal fields. Moreover, low H2S content may be an important contributor to gold enrichment in basalt-hosted SMS deposits in ultraslow-spreading MOR environments.

The increased hydrothermal activity at the inside corners is interpreted to be controlled by the high-permeability detachment faults at mid-ocean ridges. Paradoxically, no hydrothermal activity is confirmed to be located at the detachment terminations, where the permeability is theoretically maximal. Here, we use a numerical model to investigate the stress state of the Longqi hydrothermal field and Trans‐Atlantic Geotraverse (TAG) hydrothermal field, which are both located on the hanging wall of a detachment fault rather than at detachment terminations. Our results show that the maximum horizontal stress SHmax at the inside corner is deviated from the observed spreading direction by more than 45°. Meanwhile, shear strain is much higher at the insider corner and the distributions of maximal shear strain is consistent with locations of hydrothermal field. We speculate that regional stress rotation and shear strain enhance the regional permeability of the shallow crust, facilitate hydrothermal circulation on the hanging wall and therefore control the location of the hydrothermal field. Our model provides a potential mechanism for the location of the hydrothermal activity in similar geological settings and therefore provides an important constrain for the exploration of seafloor massive sulfide deposits in the future.

The Yuhuang hydrothermal field（YHF） is located between the Indomed and Gallieni fracture zones near the top of the off-axis slope on the south rift wall of Segment 29 on the ultraslow Southwest Indian Ridge（SWIR）. Previous studies have shown that sulfides in the YHF formed during different mineralization episodes and the YHF has the greatest potential for the formation of large-scale seafloor massive sulfide deposits. However, the sulfide chronology and hydrothermal activity of the YHF remain poorly constrained. In this study, mineralogical analyses and 230Th/U dating were performed. Hydrothermal activity may start about（35.9 ± 2.3） ka from the southwest part of the YHF and may cease about（708 ± 81） a ago from the northeast part of the YHF. The 74 nonzero chronological data from hydrothermal sulfide samples provide the first quantitative characterization of the spatial and temporal history along the SWIR. Hydrothermal activity in the SWIR has been relatively active over the past20 ka. In contrast, between 40 ka and 100 ka, hydrothermal activity was relatively infrequently and short in duration. The maximum activity occurred at 15–11 ka, 9–7 ka, 6–0.2 ka. There was a slight positive correlation between the maximal age and estimated surface area or estimated tonnage. The minimum mass accumulation rate of YHF is about 278 t/a, which is higher than most HFs related to ultramafic systems. The ultraslow spreading SWIR has the greatest potential to form large-scale seafloor massive sulfides（SMS） deposits. The results of this study provide new insights into the metallogenic mechanism of hydrothermal sulfides along ultraslow-spreading ridges.

The enrichment of precious elements including Au and Ag in submarine hydrothermal sulfide deposits attracts more and more attention. Previous studies indicate that Au and Ag combine distinctly different ligands under the physico-chemical conditions of submarine hydrothermal systems, therefore, the factors controlling their precipitation may be different. In general, silver mineralization has been much less studied than gold mineralization in submarine hydrothermal sulfide deposits. The Ag mineralization process and precipitation mechanism of submarine hydrothermal sulfide deposits formed at Mid-ocean Ridges are still poorly constrained. In this study, we studied the occurrence and precipitation mechanism of Ag in sulfide deposits of Edmond hydrothermal field, located on the intermediate-spreading Central Indian Ridge (CIR). Three ore-forming stages were identified in chimney fragments collected in the Edmond hydrothermal field. The corresponding minerals are (stage I) anhydrite + barite + colloidal/porous pyrite (Py1) + fine-grained sphalerite; (stage II) marcasite; (stage III) euhedral pyrite (Py2) + coarse-grained sphalerite + chalcopyrite + isocubanite. Py1, characterized by colloidal, porous, and micro-sized anhedral morphology, was usually overgrown by marcasite, which is, in turn, surrounded by euhedral-subhedral Py2, usually coexisting with coarsegrained sphalerite, chalcopyrite, and isocubanite. Compared to marcasite and Py2, only Py1 contains abundant native silver nanoparticles. LA-ICP-MS analyses suggest that Py1 contains higher Pb, Cu, Ag, Mn, Tl, Mo, Au, and lower Co as compared to Py2. In-situ LA-ICP-MS and nanoSIMS analyses indicate that Py1 with higher Ag contents (101-586 ppm) has larger variation in sulfur isotopic compositions (0.8 to 6.6%0) than those of Py2 with lower Ag contents (0.22-15.5 ppm; delta 34S values: 2.1 to 4.8%0 for Fe-rich samples and 0 to 2.8%0 for Zn-rich samples). Texture, mineral assemblage, pyrite trace element and sulfur isotopic compositions indicate a progressive decrease on the degree of fluid-seawater mixing as the temperature gradually increases during the growth of the chimneys in Edmond hydrothermal field. Phase diagram analysis indicates that the increase in pH and in particular cooling, due to the mixing of the hot hydrothermal fluid with cold seawater, can significantly decrease the solubility of Ag and be the effective mechanisms of silver precipitation. Our results suggest that mixing of hydrothermal fluid with seawater is the main Ag precipitation mechanism for submarine hydrothermal sulfide deposits formed at Mid-ocean Ridges.

相比于快速和中速扩张洋中脊,慢速和超慢速扩张洋中脊热液区通常含有丰富的金属硫化物资源.近年来的研究表明大洋中脊的扩张速率与矿石中金的品位呈明显的负相关,即超慢速扩张洋中脊热液区矿石中金的含量高.前人对龙旂热液区的构造环境以及硫化物组合进行了详细研究,但是对龙旂热液区硫化物中贵金属金的赋存形式和沉淀机制研究较少.本文对西南印度洋龙旂热液区中的硫化物进行了精细的矿物结构和微量元素分析,并探讨了金的赋存形式和沉淀机制.龙旂热液区的硫化物主要以黄铁矿为主,其次是黄铜矿和闪锌矿,黄铜矿普遍出溶等轴古巴矿,此外还观察到了少量的针钠铁矾和自然金等矿物.根据矿物结构和形态,黄铁矿明显被划分为两期,一期黄铁矿(Py1)自形度低,呈细粒状或胶状,内部多孔洞;二期黄铁矿(Py2)自形度高,呈自形-半自形,且粒径较大.Py1往往存在于Py2内部或以包体的形式被Py2所包裹,Py2则与自形-半自形黄铜矿和闪锌矿等矿物共生.自然金主要存在于Py1的内部孔洞之中,少量存在于Py2以及Py2与其他硫化物之间.相比于Py2,Py1含有更高的Ni、Zn、Pb、Ba、Mn、V、Mg、U、Au、Ag、Cd元素含量,更低的Co、Se、As、Sb元素含量.在龙旂热液区的物理化学条件下,热液流体中金的主要存在形式为Au(HS),HS-浓度的降低和pH值升高均会促进金的沉淀.龙旂热液区早期热液流体与海水的混合造成热液流体pH值升高,而混合作用导致的热液流体温度降低会促使黄铁矿(Py1)的结晶,从而促使热液流体中HS-浓度的降低,热液流体pH值升高和黄铁矿结晶(Py1)引起的硫逸度降低是龙旂热液区自然金沉淀的主要机制.

Hydrothermal activity in the mid-ocean ridge facilitates the chemical exchange of seawater with new oceanic crusts. This activity mostly occurs on the detachment fault of the asymmetric accretion segment in the slow-ultraslow spreading ridge, which is characterised by limited magma supply. Deep faults can readily extract heat from deeper heat sources. Moreover, the repeated movement of faults activates the permeable fluid channels of the overlying oceanic crust, thus driving long-life hydrothermal circulation. Recent studies have found that the response time of the hydrothermal activity of the intermediate-fast spreading ridges differs from that of the slow-spreading ridge to the glacial cycle, and a unified model is expected to explain it. Also, the response of hydrothermal activity to the glacial cycle must consider the differences between oceanic ridges with different spreading rates and types of hydrothermal systems.Here, based on two sediment cores collected near the Yuhuang hydrothermal field (HF)on ultraslow-spreading Southwest Indian ridge, we obtained high-resolution sediment history records spanning three glacial periods, understood the 160 ka history of hydrothermal, volcanic and tectonic activities in the region and attempted to reveal the response mechanism of hydrothermal activities controlled by detachment faults to the glacial cycle. We discovered that in the Yuhuang HF controlled by detachment faults, hydrothermal activity increased significantly during the glacial period, and more active detachment fault activity appeared at the same time. At the end of the glacial period, both activities are reduced at the same time. We believe that in the slow-ultraslow spreading ridge, the magmatism regulated by sea level changes may regulate the evolution of detachment faults and the hydrothermal circulation, which are recorded in the sediments near the hydrothermal field.We established a response model of Sea level change–Magmatism–Detachment fault activity–Hydrothermal activity and concluded that the magmatism of slow-ultraslow spreading ridges is more sensitive to sea level changes; with the synchronous effect of detachment faults, the hydrothermal activity responds faster to the glacial cycle.

The distribution of hydrothermal vents and the biogeography of associated faunal communities in the Indian Ocean are still not well studied, particularly the ultraslow-spreading Southwest Indian Ridge (SWIR). Herein, we present the geological and morphological data for the first reported active hydrothermal field on the ultraslow-spreading center. In this context, we created a detailed seafloor map based on visual data obtained from human-occupied vehicle (HOV) dives. Longqi-1 vent field (LVF) is an off-axis, mafic-hosted but detachment fault-controlled, high-temperature hydrothermal area with at least 28 hydrothermal structures, and their contacting host rocks were identified and mapped. The morphologies of hydrothermal structures in the LVF exhibit unique characteristics, with Zone M characterized by chimneys grown on the relatively flat-lying sulfide mound and Zone S characterized by large sulfide mounds and steep-sided structures hosting large flanges, and isolated beehive structures occurred more often at the boundary between hydrothermal deposits and basalts outcrops in both M and S zone. Microscopic examination showed that anhydrite is the dominant sulfate phase and chalcopyrite, pyrite, pyrrhotite, and sphalerite are the dominant sulfide minerals. Subseafloor hydrothermal circulation controlled by the detachment fault and local fracture zone may have directly determined the morphology of the sulfide structure in the LVF. Our results provide a picture of the LVF as well as complement and expand on previous studies on metal resource evaluation along the SWIR and Indian Ocean ridges, a detailed scenario for studying the surface distribution characteristics of hydrothermal circulation and its relationship with the distribution of hydrothermal fauna, and further enhance our understanding of the formation of modern seafloor hydrothermal systems.

Polymetallic sulfides present in mid-ocean ridges (MORs) have become important strategic resources for humans, and a scientific metallogenic model is necessary for the investigation and exploration of these resources. Compared to fast- and slow-spreading MORs, ultraslow-spreading MORs show substantial differences in magma supply, tectonic activity, and oceanic crust structures. However, information on hydrothermal circulation and a metallogenic model related to sulfides along the ultraslow-spreading ridges is still limited, which hinders further exploration of these resources. In this study, the distribution of hydrothermal activities, as well as the characteristics of the structures, heat sources, fluid pathways, host rock types, fluid properties, and sulfide assemblages in typical hydrothermal fields along the ultraslow-spreading Southwest Indian Ridge (SWIR), have been studied. It is concluded that the hydrothermal systems along the SWIR can be categorized into three types, including local enhanced magma-controlled, one-way detachment/high-angle large-offset fault-controlled, and flip-flop detachment-controlled types, which are further categorized into five subtypes based on their distinct geological backgrounds. Herein, we present a sulfide metallogenic model called Local Enhanced Heat Supply-Deep Faults (eHeat-dFault) for the SWIR. The overall spreading rate remains almost constant (14–18 mm/year), while the magma supply is heterogeneous in the segment scale along the SWIR. Over the past two decades, various hydrothermal systems and sulfide deposits have been identified along the SWIR. A deep magma chamber (4–9 km) is developed in the ridge segment with sufficient magma supply owing to the local enhanced magma supply, while long-lived active deep detachment faults (up to 13 km) with associated metallogenic belts are developed in ridge segments with poor magma supply. Hence, the ultraslow-spreading MORs fulfill the necessary conditions of a sustained heat source and stable hydrothermal pathway for the formation of large-scale polymetallic sulfide deposits. The number of hydrothermal fields detected in the investigation area is 2–3 times that predicted by the traditional Spreading Rate-Magma Flux model, demonstrating its significant endowment for sulfide resources. A balance between magma supply and faulting may influence the type and depth of hydrothermal circulation, the frequency of hydrothermal activity along the axis, and the scale of sulfide deposits. Spreading rate was previously believed to control heat sources, magma supply, and tectonic processes. However, for the SWIR, we suggest that local enhanced heat supply and deep detachment faults have a greater influence than the spreading rate on hydrothermal circulation and sulfide mineralization. The eHeat-dFault sulfide metallogenic model proposed herein could provide guidance for further exploration and research on polymetallic sulfides in ultraslow-spreading SWIR.

The development of hydrothermal activities on mid-ocean ridges is primarily influenced by magmatic and tectonic activities close to the ridge axis. On slow and ultraslow spreading ridges featured by limited magma supply, prolonged hydrothermal activity can also occur in non-transform offsets (NTO), where sustained ultramafic-related hydrothermal circulation happens due to prolonged hydrothermal processes associated with the expose of ultramafic rocks. In contrast, basalt-hosted hydrothermal fields are barely developed in NTOs, especially in off-axis area, due to insufficient magma supply. In this study, we reported the occurrence of a distal axis (similar to 9.5 km) basalt-hosted hydrothermal field (50.63 degrees E) on the NTO of segment 27 of the ultraslow spreading Southwest Indian ridge. The hydrothermal products are characterized by layered crusts mainly composed of oxides, such as limonite and goethite, rich in Fe and depleted in Si. These crusts contain high contents of trace elements derived from seawater, such as P, As, Mo, U, and Sb and show REE pattens with negative Eu and mostly positive Ce anomalies. However, the high Cu content (1.82 +/- 0.89 wt%, N = 27), low absorption ratio of Cu (18.39 +/- 12.71%, N = 9), and residual chalcopyrite altered by goethite suggest prior high-temperature hydrothermal activity. Lead isotopic composition with basaltic rocks signatures that distinct from those of low-temperature crusts of seawater origin also support this conclusion. The Fe-rich crusts appears to be the result of high-temperature sulfides altered by late-stage low-temperature diffuse flow. This field is currently the furthest known distal axis basalt-hosted hydrothermal field located in NTOs of ultraslow spreading ridges. The hydrothermal circulation is likely due to a combination of off-axis normal fault and gabbro diking. Our results emphasize the occurrence of distal axis basalt-hosted high-temperature hydrothermal activities on NTOs of ultraslow spreading ridges, and may lead to an increased estimation of hydrothermal heat and chemical influx to the ocean.

Hydrothermal activities on ultraslow-spreading ridges exhibit diverse characteristics, long histories with multiple participants, and might form large-scale, high-grade sulfide deposits. The Duanqiao hydrothermal field (DHF) is located at the segment with the thickest oceanic crust and a large axial magma chamber on the Southwest Indian Ridge, providing unique perspective of sulfide metallogenesis on ultraslow-spreading ridges. Previous studies revealed that DHF sulfide exhibits distinct features of enrichment of ore-forming elements in comparison with those of hydrothermal fields on sediment-starved mid-ocean ridges. However, the genesis and processes responsible for such differences remain poorly constrained. In this study, mineralogical, geochemical and S and Pb isotopic analyses were performed on relict sulfide mound samples to characterize DHF formation. The samples show clear concentric mineral zonation from the interior to the exterior wall. Assemblages of chalcopyrite, sphalerite, and pyrite are distributed mainly in the interior wall, whereas pyrite and marcasite are distributed mainly in the exterior wall. The low Cu content and Pb isotopic composition of the sulfide indicate that the metals are derived mainly from basement basalts. The delta S-34 values exhibit positive values distributed over a reasonably narrow range (2.42 parts per thousand-7.97 parts per thousand), which suggests approximately 62.1%-88.5% of S with basaltic origin. Compared with most hydrothermal fields along the sediment starved mid-ocean ridges, the DHF sulfide shows particularly high contents of Pb (263-2630 ppm), As (234-726 ppm), Sb (7.32-44.3 ppm), and Ag (35.2 to >100 ppm). The delta S-34 values exhibit an increasing tendency from the sample exterior to the interior. We propose that these features probably reflect the existence of a subsurface zone refining process. Our results provide new insight into the sulfide formation process and contribute to understanding the metallogenic mechanism of hydrothermal sulfides on ultraslow-spreading ridges.

洋中脊多金属硫化物已经成为人类重要的战略资源,科学的成矿模型是对其调查研究和勘探的重要依据.相较快速、慢速扩张洋中脊,超慢速扩张洋中脊在岩浆供给、构造和围岩等特征均存在明显差异,但目前对其热液循环及硫化物成矿模型缺乏系统梳理,制约了其资源勘探评价与研究的有效进程.本文系统总结了超慢速扩张西南印度洋中脊热液活动分布以及典型热液区的构造、热源、热液通道、围岩类型、流体性质和硫化物等特征,根据其成矿地质背景的差异性特点将该洋中脊赋存的热液系统分为局部强岩浆控制型、单向拆离/高角度大偏移距断层控制型以及双向拆离控制型三类,根据岩浆供给率（M值）的大小进一步将其划分为五种类型,从而建立了超慢速扩张西南印度洋中脊的局部强热供给-深大断裂控制硫化物成矿模型.超慢速扩张西南印度洋中脊扩张速率整体变化不大（14～18mm/a）,岩浆供给呈分段不均匀性.通过近20年的调查研究,发现其发育类型多样的热液系统和硫化物.在岩浆供给充足的洋脊段,发育局部强岩浆供给条件下的深部岩浆房（4～9km）.而在岩浆供给贫瘠的洋脊段,发育长期持续活动的深大拆离断层（可达13km）,并沿拆离断层形成成矿带.因而超慢速扩张洋脊具备形成大型多金属硫化物矿床所需的持续热源和稳定热液通道的必要条件.已调查区探测到的热液活动区数量是传统扩张速率-岩浆通量模型预测的2～3倍,具有良好的硫化物资源前景.洋中脊热液循环系统的类型、循环深度、热液活动沿轴发育频率以及硫化物成矿规模可能是岩浆供给和构造活动均衡贡献的结果.通常认为扩张速率控制着热源、岩浆供给和构造过程.对于超慢速扩张西南印度洋中脊,本文认为局部强热供给和深大断裂构造是其热液循环和硫化物成矿更直接的控制因素.局部强热供给-深大断裂控制的硫化物成矿模型有望为超慢速扩张西南印度洋中脊多金属硫化物的勘探与成矿研究提供指示.

Hydrothermal activity on mid-ocean ridges is an important mechanism for the delivery of Zn from the mantle to the surface environment. Zinc isotopic fractionation during hydrothermal activity is mainly controlled by the precipitation of Zn-bearing sulfide minerals, in which isotopically light Zn is preferen-tially retained in solid phases rather than in solution during mineral precipitation. Thus, seafloor hydrothermal activity is expected to supply isotopically heavy Zn to the ocean. Here, we studied sulfide-rich samples from the Duanqiao-1 hydrothermal field, located on the Southwest Indian Ridge. We report that, at the hand-specimen scale, late-stage conduit sulfide material has lower 566Zn values (-0.05 +/- 0.15 %o; n = 19) than early-stage material (+0.13 +/- 0.15 %o; n = 10). These lower values correlate with enrichments in Pb, As, Cd, and Ag, and elevated delta S-34 values. We attribute the low delta Zn-66 values to the remobilization of earlier sub-seafloor Zn-rich mineralization. Based on endmember mass balance calcu-lations, and an assumption of a fractionation factor (alpha(ZnS-Sol.)) of about 0.9997 between sphalerite and its parent solution, the remobilized Zn was found consist of about 1/3 to 2/3 of the total Zn in the fluid that formed the conduit samples. Our study suggests that late-stage subsurface hydrothermal remobilization may release isotopically-light Zn to the ocean, and that this process may be common along mid-ocean ridges, thus increasing the size of the previously identified isotopically light Zn sink in the ocean. (C) 2022 Elsevier Ltd. All rights reserved.

AbstractChanges in sea level caused by glacial cycles may influence the magmatism and hydrothermal activity of oceanic ridges. Recent studies showed that the response time of the hydrothermal activity in the intermediate-fast spreading ridges differs from that in the slow-spreading ridges to the glacial cycles, and a unified model is expected to explain it. Here, we report the 160 ka sediment record adjacent to the Yuhuang hydrothermal field on the Southwest Indian Ridge. Hydrothermal and detachment fault activities were found to enhance or weaken during glacial and interglacial periods, respectively. The magmatism of slow/ultraslow spreading ridges is more sensitive to sea level changes; with the synchronous effect of detachment faults, the hydrothermal activity responds faster to the glacial cycles. We established a model of Sea level change–Magmatism–Detachment fault activity–Hydrothermal activity to explain the different responses of the hydrothermal activity of the mid-ocean ridges to the glacial cycles.

Trace amounts of water in the sub-oceanic mantle play crucial role in the vigor of mantle convection and the production of oceanic crust, and other many geodynamic processes. Consequently, the cycling of H2O between the mantle and the exosphere in the mantle is one of the critical processes governing Earth's geodynamical and geochemical evolution. While the deep cycling of altered oceanic lithosphere was considered as the main way to replenish the water in oceanic upper mantle, the significance of the arc mantle wedge after the genesis of arc magmatism dragged down by the subducting slab concomitantly has been not well constrained. Here, we report that fresh depleted basaltic glasses from the ultraslow-spreading Southwestern Indian Ridge (SWIR), located far from any recent subduction zones, show unusually high H2O/Ce ratios (> 600), water contents and heavy hydrogen isotopic compositions. These results could be best explained by recycling of water through melting of a residual hydrous mantle wedge after early melt extraction. Considering that such mantle wedges residue dragged down to the deeper mantle could occupy a volume one order of magnitude larger than that of the subducted lithosphere in the earth history, we suggest that the potential role of such shallow recycling should be considered in studies of global water recycling and the origin of water in the upper asthenosphere.& nbsp;(c) 2022 Elsevier B.V. All rights reserved.

Tectonic-related hydrothermal fields constitute about half of the hydrothermal activities on slow-ultraslow spreading ridges, and may form sulfides large in size and rich in precious metals. Ore-forming fluids are critical for element migration progressions, thus are key parameter to reveal their genesis and sulfide mineralization processes. However, previous studies of this kind of hydrothermal activities primarily focused on vent fluids which have transient effects or based on surface samples, while the ore-forming fluid evolution and related subseafloor mineralization process are still poorly constrained. In this study, we analyzed the characteristics of fluid inclusions in the stockwork mineralization of the East Longjing-2 hydrothermal field (ELHF-2) on the Southwest Indian Ridge. The results showed that the fluids evolved gradually from medium-high temperatures (260-315 degrees C, mean of 284 degrees C) and low salinities (0.70-3.70 wt.% NaCl eq.) in the disseminated pyrite mineralization stage (Stage I) to medium temperatures (239-261 degrees C, mean of 249 degrees C) and low salinities (1.05-3.85 wt.% NaCl eq.) in the chalcopyrite-pyrite-quartz veinlet mineralization stage (Stage II). Laser Raman specular analysis indicated the fluid inclusions were mainly composed of H2O. The low salinity (lowest 0.7 wt.% NaCl eq.) and various vapor volume (0.05-0.5) observed in these fluid inclusions suggested the ore-forming fluids were probably mixtures of the low-salinity vapor phase formed by phase separation with large amounts of seawater. The precipitation of the ore-forming elements was likely induced by temperature decreasing. We propose that hydrothermal circulation in ELHF-2 was a result of coupled high-permeability detachment fault and shallow gabbro intrusion. Our results suggest that high temperature hydrothermal activities could be developed on distal axis area on ultraslow spreading ridges.

The redox state of hydrothermal fluids on mid-ocean ridges, which is indirectly affected by the depth of hydrothermal cir-culation and crustal permeability, plays an important role on the diversity of hydrothermal precipitates and associated ecosys-tems. Primary hydrothermal fluids that circulate along detachment faults are generally reducing as a result of the serpentinization of ultramafic rocks, while significant seawater infiltration may shift the redox state from reducing to oxidiz-ing. However, the depth of penetration of oxidizing fluids into detachment related systems remain unclear, largely because current observations are based primarily on hydrothermal products that precipitated at the seafloor. Here, we report the first observations of oxidizing mineral assemblages in stockwork samples from the Suye hydrothermal field on the ultraslow spreading Southwest Indian Ridge. This field is hosted by mafic lithologies, while the low As, high Ni and Co contents, and high Au/As, Ag/As, and Ni/As ratios in pyrite from the stockwork samples indicate that the fluids reacted with both mafic and ultramafic rocks in the subseafloor. The high d34S values (average of 9.8 parts per thousand) indicate a high proportion (up to 50%) of seawater derived reduced sulfur involved in the stockwork zone formation. The high homogenization temperatures (similar to 320 degrees C) and salinity (similar to 12 wt.% NaCl) of fluid inclusions indicate that the stockwork zone of Suye was formed by a fluid that underwent phase separation deeper in the system that was subsequently diluted by 4-5 times subsurface seawater. The deep penetration of seawater is facilitated by the unique tectonic setting of the Suye hydrothermal field, which occurs between the two stage detachment faults that creates high permeability. Our findings demonstrate that hydrothermal fluid associated with detachment faults could be oxidized below the subsurface stockwork zone, and that deep-rooted detachment faults at ultraslow-spreading ridges can sustain both reducing and oxidizing hydrothermal systems in the same fault system. These results call for a reevaluation of the fate of base metal in ultramafic hosted hydrothermal fields. (c) 2022 Elsevier Ltd. All rights reserved.

Hydrothermal vent incidence was once thought to be proportional to the spreading rate of the mid-ocean ridges (MORs). However, more and more studies have shown that the ultraslow-spreading ridges (e.g., Southwest Indian Ridge (SWIR)) have a relatively higher incidence of hydrothermal venting fields. The Qiaoyue Seamount (52.1°E) is located at the southern side of segment #25 of the SWIR, to the west of the Gallieni transform fault. The Chinese Dayang cruises conducted eight preliminary deep-towed surveys of hydrothermal activity in the area during 2009 and 2018. Here, through comprehensive analyses of the video and photos obtained by the deep-towed platforms, rock samples, and water column turbidity anomalies, a high-temperature, ultramafic-hosted hydrothermal system is predicted on the northern flank of the Qiaoyue Seamount. We propose that this hydrothermal system is most likely to be driven by gabboric intrusions. Efficient hydrothermal circulation channels appear against a backdrop of high rock permeability related to the detachment fault.

目前有关西南印度洋中脊沉积物的研究多集中在洋脊翼表层沉积物,对洋脊轴部裂谷沉积物的研究较匮乏,制约了对该区沉积物物质来源和沉积环境的深入认识.针对大洋49航次在洋脊轴部裂谷(东经49.58°)获取的沉积物岩心(GC03)进行了全岩样品主量、微量、稀土元素和Y(REY)含量测试分析,探讨研究区沉积物的物质来源和沉积环境.结果表明,GC03岩心样品富CaO、LOI(烧失量)和Sr,指示以钙质生物沉积占主导,并混有玄武岩碎屑.稀土元素总量(ΣREY)低,平均为54.3×10-6,稀土元素球粒陨石标准化配分图显示轻稀土元素富集,Ce和Eu显著负异常特征.其中L2层(83～87 cmbsf)富集Cu、Zn、Fe、Co等金属元素及较低的100?Al/(Al+Fe+Mn)值,揭示存在热液组分输入.Ceanom值与微量元素V/(V+Ni)-U/Th判别指标指示,研究区总体为偏氧化环境,部分层位显示还原特征.