•Liquid and vapor phase fluids after phase separation were observed at the same time.•A single source fluid that branches and phase separates is suggested.•It is suggested that the basement rock at the Longqi field is mafic.The Longqi hydrothermal field at 49.6°E on the Southwest Indian Ridge was the first active hydrothermal field found at a bare-rock ultra-slow spreading mid-ocean ridge. Here we report the chemistry of the hydrothermal fluids, for the first time, that were collected from the S zone and the M zone of the Longqi field by gas-tight isobaric samplers by the HOV “Jiaolong” diving cruise in January 2015. According to H2, CH4 and other chemical data of the vent fluid, we suggest that the basement rock at the Longqi field is dominantly mafic. This is consistent with the observation that the host rock of the active Longqi Hydrothermal field is dominated by extensively distributed basaltic rock. It was very interesting to detect simultaneously discharging brine and vapor caused by phase separation at vents DFF6, DFF20, and DFF5 respectively, in a distance of about 400 m. Based on the end-member fluid chemistry and distance between the vents, we propose that there is a single fluid source at the Longqi field. The fluid branches while rising to the seafloor, and two of the branches reach S zone and M zone and phase separate at similar conditions of about 28–30.2 MPa and 400.6–408.3 °C before they discharge from the vents. The end-member fluid compositions of these vents are comparable with or within the range of variation of known global seafloor hydrothermal fluid chemical data from fast, intermediate and slow spreading ridges, which confirms that the spreading rate is not the key factor that directly controls hydrothermal fluid chemistry. The composition of basement rock, water-rock interaction and phase separation are the major factors that control the composition of the vent fluids in the Longqi field.

•Tetraether lipid distributions are different among the three deposit types at SWIR.•GMGTs occurring in metalliferous sediments are absent in background sediments.•GMGTs and high abundance of brGDGTs present in low-temperature hydrothermal deposits.•Different hydrothermal activity has diverse impact on tetraether lipid distributions.The impact of hydrothermal activity on wider ocean geochemistry and microbial ecology remains a topic of much interest. To explore whether hydrothermal microbial signatures are exported to surrounding marine sediments or if such organisms serve as an important source of sedimentary organic matter, we determined the distributions of glycerol dialkyl glycerol tetraether (GDGT) membrane lipids in surficial normal marine sediments, metalliferous sediments and low-temperature hydrothermal deposits at newly discovered hydrothermal fields and adjacent areas at the Southwest Indian Ridge (SWIR). The GDGTs in those samples varied significantly, evidently representing a variable influence of the hydrothermal activity. GDGT compositions of surficial background sediments in SWIR were similar to those commonly observed in marine sediments, dominated by GDGTs associated with marine planktonic archaea and especially GDGT−0 and crenarchaeol. In contrast, the GDGTs of metalliferous sediments strongly impacted by hydrothermal activity and low-temperature hydrothermal deposits were markedly different, characterized by high relative abundances of isoprenoid GDGTs (iGDGTs) bearing multiple rings (yielding a higher ring index), low relative abundances of crenarchaeol, and the presence of glycerol monoalkyl glycerol tetraether lipids (GMGTs; so called ‘H-tetraethers’) that were absent in the normal marine sediments. Sources for these hydrothermal-specific tetraether lipids likely include methanogens and anaerobic methanotrophic archaea (GDGT−0 and GDGT−1–3, respectively), Thermoprotei and Thermoplasmatales (elevated GDGT−3 and 4), and other thermophilic archaea including Methanobacteriales (GMGTs). Deposits influenced by low-temperature hydrothermal activity also contained higher abundances of branched GDGTs (brGDGTs) typically attributed to soil bacteria. The more distal metalliferous sediments influenced by the neutrally buoyant plume did not contain putative hydrothermal GDGTs, having the same GDGT distribution as the background sediments. This suggests that the neutrally buoyant plume has a limited potential to directly influence the organic matter inputs to surrounding sediments, due to a rapidly waning chemosynthetic microbial contribution relative to normal marine contributions as the plume dispersed and was diluted.

The Southwest Indian Ridge (SWIR) is one of the world's slowest spreading ridges with a full spreading rate of ~ 14 mm a− 1. Due to its low thermal budget, high-temperature hydrothermal activity along the SWIR was once considered to be impossible. The Chinese cruise DY115-19 on board R/V Dayang Yihao successfully discovered the first SWIR active hydrothermal field at 37°47′S 49°39′E, located at a magmatically robust spreading segment. Here, the geochemical characteristics of hydrothermal plumes from the hydrothermal field are first reported, and water column anomalies of light transmission, Fe, Mn, Al, both dissolved and particulate, are discussed. The total Fe and dissolved Mn concentrations in the plumes varied from 13.7 to 277.4 nM and 0.47 to 10.41 nM, respectively. The composition of Fe-Mn-Al implied that particles in the plumes were mainly hydrothermal in origin, also included small contributions of resuspended sediments or background particles. Dissolved Fe constituted a considerable fraction of the total Fe, more than 80% in plume samples from station CTD 4. High Fe concentrations might be sustained in the dissolved phase because of the existence of organic complexes and nanoparticles. On board incubation experiments verified the Fe(II) oxidation half-lives for plumes of CTD 4 and CTD 13 were 1.8 and 1.6 h, respectively, which are much longer than the calculated value of ~ 0.5 h based on the deep water pH and oxygen concentration.Highlights► The evidences for the existence of hydrothermal plumes at the SWIR are first reported. ► The Fe, Mn and Al concentrations, both particulate and dissolved, are measured. ► The particle samples in plumes are mainly of hydrothermal origin. ► Dissolved Fe constitute a considerable part of total Fe in samples. ► The Fe(II) half-lives in plumes were measured by shipboard incubation experiments.

Hydrothermal chimney sulfides, vent cap chimney samples, Fe-oxide and basalts from sediment-starving Juan de Fuca Ridge, in the Endeavour segment, exhibit a range of Pb isotope ratios (206Pb/204Pb = 18.658–18.769; 207Pb/204Pb = 15.457–15.566; 208Pb/204Pb = 37.810–38.276). The data array is not parallel to the northern hemisphere mantle reservoirs indicating a possible sediment component within the sulfides. By assuming that the potential end-member sediment component has a 207Pb/204Pb (15.70) similar to Middle Valley sediment, it is suggested the potential end-member sediment component may have 206Pb/204Pb = 18.90; 208Pb/204Pb = 38.82. Basalt-derived Pb for the Endeavour segment hydrothermal system involves about 50/50 leaching of E-MORB and T2-MORB. Detailed observations show the Mothra field derives more Pb from T2-MORB than the Main Endeavour field does. According to the binary mixing model, the results show little Pb (<1.5%) or no Pb derivation from sedimentary sources. However, the high NH4+, CH4 and Br/Cl ratios in hydrothermal fluids are consistent with a sediment component within the segment. Reconciling the Pb isotope data with the chemistry data of hydrothermal fluids, it is suggested that the sediment component may be located in a lower temperature recharge zone where Pb could not be mobilized from the sediment.

A series of samples from the Hine Hina hydrothermal field (HHF) and the Mariner hydrothermal field (MHF) in the Central and Southern Valu Fa Ridge (VFR), Lau Basin were examined to explain the source origin and formation of the hydrothermal Fe–Si–Mn oxide deposits. The mineralogy was studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), Mössbauer spectroscopy, and energy-dispersive spectroscopy (EDS). For the Fe–Mn oxide crusts in the HHF, varying amounts of volcanic fragments and some seawater contributions were recognized, along with higher concentrations of Mn, Al, Co, Ni, Zn, Sr, Mo, elevated ∑REE and negative Ce anomalies. In contrast, the Si-rich oxide samples of the MHF were enriched in Cu, Pb and Ba, indicative of proximity to a hydrothermal jet. Moreover, conductive cooling of hydrothermal fluid evoked the Si-rich deposit formation in the MHF. The Sr, Nd and Pb isotope data provided further constraints regarding the source and formation of the Fe–Si–Mn deposits in the VFR by showing that the samples of the HHF are a mixture of three components, namely, hydrothermal fluid, seawater and volcanic materials, whereas the samples of the MHF were dominated by hydrothermal fluids. The seawater had a minor influence on the Nd isotope data, and the Pb isotope data exhibited a close association with the substrate rock and preformed volcaniclastic layers in this area. The occurrence of relatively high Mn/Fe ratios in the hydrothermal deposits of this area may be a good indicator of the propagating activities of the VFR over geological time.Highlights► The Fe–Mn crust in the HHF has seawater contribution, whereas the Fe–Si oxide in the MHF is dominated by hydrothermal fluid ► The Nd isotope of diffuse flow Fe–Si–Mn deposits indicates the obvious hydrothermal origin. ► The Mn/Fe ratio in hydrothermal deposit may be a good indicator of propagating activities of the Valu Fa Ridge.

Abundance and distribution of total fatty acids (TFAs) were examined along the physicochemical gradient within an active hydrothermal chimney collected from the Main Endeavour segment of Juan de Fuca Ridge. Approximately 27 fatty acids are identified with a chain-length ranging from C12 to C22. From the exterior to the interior of the chimney walls, the total concentrations of TFAs (∑ TFAs) show a trend of evident decrease. The observed compositions of TFAs are rich in bacterial biomarkers especially monounsaturated fatty acids (MUFAs) and minor branched and cyclopropyl FAs. On the basis of the species-specific FAs and bacterial 16SrRNA gene analysis (Li et al., unpublished data), sulfur-based metabolism appears to be the essential metabolic process in the chimney. Furthermore, the sulfur oxidizing bacteria (SOB) are identified as a basic component of microbial communities at the exterior of the hydrothermal chimney, and its proportion shows an inward decrease while the sulfate reducing bacteria (SRB) have an inverse distribution.Research Highlights► Fatty acids were examined in an active vent chimney. ► The value of ∑ TFAs decreases from the exterior to the interior of the chimney wall. ► Microorganism with sulfur-based metabolism were dominant in the microbial communities. ► The proportion of SOB shows an inward decrease. ► The proportion of SRB has an inverse distribution compared to SOB.

Microbial biomineralization in submarine hydrothermal environments provides an insight into the formation of vent microfossils and the interactions between microbes, elements and minerals throughout the geological record. Here, we investigate microbial biomineralization of a deep-sea vent community in the Edmond vent field and provide ultrastructural evidence for the formation of microfossils and biogenic iron-rich minerals related to Archaea and Bacteria. Environmental scanning electron microscopy (ESEM) analysis shows that filamentous and spiral microbes are encrusted by a non-crystalline silica matrix and minor amounts of iron oxides. Examination by transmission electron microscopy (TEM) reveals acicular iron-rich particles and aggregates that occur either intracellularly or extracellularly. A culture-independent molecular phylogenetic analysis demonstrates a diverse range of Bacteria and Archaea, the majority of which are related to sulfur metabolism in the microbial mats. Both Archaea and Bacteria have undergone silicification, in a similar manner to microorganisms in some terrestrial hot springs and indicating that silicification may be driven by silica supersaturation and polymerization. Formation mechanisms of intracellular and extracellular iron oxides associated with microbes are discussed. These results enhance our understanding of microbial mineralization in extreme environments, which may be widespread in the Earth's modern and ancient hydrothermal vent fields.

Hydrothermal Fe–Mn–Si oxides and nontronite are pervasive in the Hine Hina, Vai Lili and Mariner hydrothermal fields along the central Valu Fa Ridge, Lau Basin. Morphometric and mineralogical analyses reveal that the iron-rich filaments are the most important constituents of these Fe–Mn–Si oxide deposits. Both the morphologies and chemical composition of the filaments indicate that neutrophilic Fe-oxidizing bacteria have played a key role in the formation of these deposits. A key process of the formation of these deposits is the creation of a complicated filamentous network in which a series of metabolic activities and passive sorption and nucleation processes occur. The precipitation of dissolved Si in unsaturated and saturated states leads to a “two-generation” growth model in the hydrothermal vents. The precipitation of amorphous opal occurs in a relatively narrow temperature range (41.1–42.9 °C) based on oxygen isotope analyses, indicating a fast precipitation rate of opal-A when conductive cooling of the hydrothermal fluid occurrs. Patchy nontronite in the Mariner fields is a product of the direct precipitation from hydrothermal fluids at a temperature of ∼87.9 °C, whereas the scattered nontronite at the Hine Hina field is the product of the replacement of hydrothermal Fe–Si oxides at a temperature of ∼46.2 °C.Highlights► Neutrophilic Fe-oxidizing bacteria play a key role in the formation of Fe–Si–Mn oxide deposits. ► Precipitation of dissolved Si leads to a “two-generation” model of formation of silica in hydrothermal biogenic mats. ► Nontronite can directly precipitate from hydrothermal fluids and replace hydrothermal Fe–Si oxides.