中国科学院海洋研究所机构知识库

The hydrothermal system in Yokosuka area, southern Okinawa Trough, has a special inverted lake structure. The interior of the lake is dominated by gas phase fluid mixed with the high-temperature steam phase of seawater and CH₄, CO₂ and other gases, topped by a mushroom cap shaped sulfide (MCs) structure. Sulfides formed under different hydrothermal environmental systems vary greatly in mineralization patterns, elemental migration and enrichment. At present, there is no index that can quickly identify the formation environment of MCs when to take the in-situ detection of deep-sea hydrothermal, and the geochemical characteristics of sulfide cannot be preliminarily estimated quickly under the in-situ condition. In this thesis, confocal micro-Raman spectroscopy was used to compare the mushroom-cap sulfide, the non-mushroom-cap sulfide in the southern part and the hydrothermal sulfide in the central part, researching significant changes in the frequency shift and half-peak width of sulfide minerals in different hydrothermal environments. And based on the variation of Raman frequency shift and half peak width, the index of identifying sulfide formed under the influence of this unique gas phase fluid is established. In addition, systematic geochemical (electron microprobe, major-trace element, sulfur-lead isotope, etc.) analyses of sulfides in different environments were also carried out.

The results of Raman spectrum show that the main peaks of chalcopyrite and sphalerite in MCs occurs blue shift and red shift phenomenon respectively, and the chalcopyrite and sphalerite in MCs show a large full width at half maximum (FWHM). The change of Raman shift of minerals in MCs indicates that the elements in chalcopyrite and sphalerite are affected, and the increase of FWHM of Raman spectrum indicates the poor crystallinity of chalcopyrite and sphalerite. In contrast, the Raman spectra of the sulfate mineral barite and elemental sulfur in the MCs do not show much change and are less affected here. The results of electron probe analysis show that there is a difference in the element composition between chalcopyrite and sphalerite, while the element composition of barite does not change, which effectively confirms the conclusion of Raman spectroscopy. According to the variation of chalcopyrite and sphalerite in MCs, it can preliminarily reflect a formation environment of sulfide.

Elemental geochemical analysis can further characterize the particularity of MCs and further explore the practicability of Raman analysis method in sulfide in-situ measurement. The results of the elemental analysis showed that the MCs had a specific elemental composition. MCs 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. In addition, in order to exclude the influence of magma source, overlying rocks, etc., sulfur-lead isotope analysis was performed on the samples. The MCs and non-MCs have the same S-Pb isotopic composition, indicating that hydrothermal source is not the main reason for the MCs difference. Therefore, this thesis suggests that, the main factor affecting the difference of MCs is the unique gas phase fluid in inverted lake.

In this study, the variation of Raman spectra of chalcopyrite and sphalerite is used as an indicator, which can provide preliminary information for in situ detection of hydrothermal sulfides. The geochemical results further confirm the recognition of sulfide by Raman spectroscopy. The Raman spectroscopy method established in this study can provide important guidance for future in situ detection and sampling of hydrothermal sulfides.