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Subseafloor phase separation is a common and significant process in hydrothermal systems and may result in a large of fluid composition differences. The temperatures of hydrothermal fluids are generally considered to be below the associated fluid boiling temperature due to mixing with ambient seawater and the phase separation process. However, we report here shimmering water with temperatures up to 383.3 °C observed in a hot overturned lake at the Yokosuka site, Okinawa Trough, East China Sea, where in situ Raman spectra suggest the presence of a superheated vapor phase. Hydrothermal vents similar to the low‐density hydrothermal system found at the Yokosuka site have also been observed in many other regions. Therefore, much more attention should be given to the impacts of low‐density hydrothermal fluid emanations on marine environments and resource distributions. Plain Language Summary: Hydrothermal systems characterized by the emanations of supercritical‐ and vapor‐phase fluids associated with igneous activities have been observed on mid‐ocean ridges. Here, we report a newly found hydrothermal system dominated by low‐density vapor‐phase fluids at the Yokosuka site, a hydrothermal field located in a backarc basin spreading center. Multiple methods, including in situ Raman spectroscopy measurements, thermocouple sensor measurements, and gas‐tight sampling, were used to investigate this vapor‐phase hydrothermal emanation. In situ observations suggest that the highest temperature (383.3 °C) of the hydrothermal fluids at the Yokosuka site is above the two‐phase boundary of seawater at a depth of 2,180 m (378.1 °C) and in situ Raman spectra indicate the fluid of the hot overturned lake features a layered structure. Hydrothermal systems with low‐density emanations are likely to be widespread throughout various tectonic settings since similar phenomena have been observed in other hydrothermal fields with conditions close to the phase separation boundary. Key Points: The low‐density vapor‐phase hydrothermal system is observed in the backarc spreading center for the first timeIn situ Raman spectroscopy measurement was implemented in the vapor‐phase hydrothermal systemImplications of low‐density hydrothermal emanations on the deep ocean and mineralization are underestimated than previously thought [ABSTRACT FROM AUTHOR]

The chlorinity of deep-sea hydrothermal fluids, representing one of the crucial deep-sea hydrothermal indicators, indicates the degree of deep phase separation of hydrothermal fluids and water/rock reactions. However, accurately measuring the chlorinity of high-temperature hydrothermal fluids is still a significant challenge. In this paper, a piecewise chlorinity model to measure the chlorinity of high-temperature hydrothermal fluids was developed based on the OH stretching band of water, exhibiting an accuracy of 96.20%. The peak position, peak area ratio, and F value were selected to establish the chlorinity piecewise calibration model within the temperature ranges of 0–50 ℃, 50–200 ℃, and 200–300 ℃. Compared with that of the chlorinity calibration model built based on a single parameter, the accuracy of this piecewise model increased by approximately 4.83–12.33%. This chlorinity calibration model was applied to determine the concentrations of Cl for high-temperature hydrothermal fluids in the Okinawa Trough hydrothermal field. Graphical Abstract [ABSTRACT FROM AUTHOR]

Supercritical CO 2 appearing as bubbles in hydrothermal vents was identified in the south part of the Okinawa Trough using in situ Raman spectroscopy. Significantly, the N 2 peak in supercritical CO 2 is much larger than those in seawater and vent fluids, indicating that supercritical CO 2 enriches N 2 from the surrounding environment. Considering that the partial pressures of CO 2 and N 2 in the Earth's proto-atmosphere were ~10–20 MPa, supercritical CO 2 with high N 2 was likely the dominant CO 2 phase near the water-air interface in the early history of the Earth, which promoted the synthesis, pre-enrichment and preservation of amino acids and other organic matters that are essential to the origin of life. [ABSTRACT FROM AUTHOR]

[Display omitted] • The special effects of vapor-phase fluids on hydrothermal sulfides are firstly reported. • Raman spectroscopy revealed sulfides composition changed and low degree of crystallinity. • Isotopic combined element analyses confirmed vapor-phases fluids affect sulfide compositions. Inverted lakes filled with shimmering water are unique environments within hydrothermal systems. Recently, vapor-phase fluids have been detected on the top of an inverted lake in a mushroom cap-shaped (MC) sulfide structure based on in situ Raman spectra at the Yokosuka Site in the Okinawa Trough (OT). However, the effects of vapor-phase fluids on the migration and metallogenic characteristics of elements remain unclear. To characterize the influence of such fluids, a comparative study of the micro-Raman spectroscopy, electron microprobe analyses (EMPA), polarizing microscopic, element distribution, and S-Pb isotopic composition among the hydrothermal sulfides collected from MC area, the middle Okinawa Trough (MOT) and southern Okinawa Trough (SOT), was conducted. Raman spectra show that the dominant band of chalcopyrite in MC sulfide generated a blue shift, while sphalerite generated a red shift, indicating that compositional differences exist in MC sulfide samples compared to the other two areas. This was also confirmed by EMPA data for these two minerals. Moreover, the full width(s) at half maximum (FWHM) of the Raman spectra of minerals in the MC sulfide samples showed a lower degree of crystallinity influence by the vapor-phase fluids. Elemental analyses further demonstrated that the MC samples share a particular elemental composition. That is, MC sulfides featured high Mn, Ba, and Sr and low Fe, Cu, Zn, Pb, Bi, Mo, Cd, and Ga. The same enrichment trends were observed among Ba–Sr, Cu–Mo, Cu–Bi, Zn–Pb, Zn–Cd, and Zn–Ga. Moreover, the δ34S of 5.44–5.85‰ in these deposits suggest that S was contributed by the underlying basalt, sediments, and seawater. The Pb isotopic composition of MC sulfides exhibited the same compositional patterns as the other sulfides and the underlying sediments at the SOT site, indicating that the sedimentary contribution of Pb has increased. The S–Pb isotopic composition suggests that a hydrothermal source may not serve as the primary cause of differences in the MC deposits. Instead, the unique vapor-phase fluids and other gaseous components may have predominantly affected the precipitation and migration of elements at this site. Although the shimmering water regions of inverted lakes accompanied by vapor-phase fluids are difficult to observe in deep-sea hydrothermal systems, here, we describe the geochemical characteristics of sulfides affected by this particular vapor-phase fluid, providing a unique perspective on the migration and precipitation occurring during sulfide deposit formation. [ABSTRACT FROM AUTHOR]