Influence of vapor-phase fluids on the geochemical characterization of hydrothermal sulfides in the shimmering waters of the southern Okinawa Trough

Abstract

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.