The interaction between fluid and hydrothermal sulfides, authigenic carbonate rocks and other rocks in the extreme environment of deep sea that represented by hydrothermal and cold seeps, not only records the history of fluid activity in the system, but also provides material and energy sources for the chemical synthetic biota community. Due to the complexity of fluid components and the susceptibility to temperature and pressure in the deep-sea extreme environment, it is difficult to achieve in situ detection of fluid-rock interaction based on the inversion of fluid activity history based on geochemical analysis of rock samples. The essence of fluid-rock interaction is the interaction between fluid and mineral interface, but there are few reports on fluid-rock interaction in situ. Laser Raman spectroscopy has many advantages, such as simple analysis, non-destructive analysis, no sample handling and wide application range, which can be used in studying the in situ detection of fluid-rock interaction. In order to realize in situ detection of fluid-rock interaction by laser Raman spectroscopy, the author has mainly done the following works:
(1) To conduct fluid-rock interaction experiments in the laboratory, corresponding deep-sea environment simulation devices are required. Therefore, the author developed visualized low temperature/high temperature and high pressure reaction chambers, which can be used in conjunction with the confocal microscopic laser Raman spectroscopy detection system. The temperature and pressure cover the deep-sea hydrothermal and cold seep system, which provides equipment support for the in-situ detection of the interaction between fluid and rock in the extreme deep-sea environment.
(2) In order to quantitatively study the fluid-rock interaction process, a series of quantitative analysis models for the key components in the fluid-rock interaction were established based on laser Raman spectroscopy. In this paper, the HSO4- and SO42-Raman quantitative analysis models in hydrothermal HSO4-—SO42- fluid, the quantitative relationship between the Raman shift of main peak of SO42- andtemperature, and the quantitative model of Raman shift and salinity corresponding to the O-H stretching vibration mode of H2O were established respectively by using the simulation system of deep-sea extreme environment.
(3) Laser Raman spectroscopy can effectively reflect the composition and structure of minerals, but laser may also cause thermal oxidation or alteration of minerals. In order to explore the influence of laser thermal effect on mineral composition and structure in the fluid-rock interaction, this study carried out in situ laser Raman spectroscopy observation at the fluid-mineral interface based on the developed micro-visualized reaction chamber, and analyzed the role of fluid in mineral thermal oxidation process. The thermal oxidation process of sulfur minerals and aragonite in extreme deep-sea environments was studied by controlling the laser power to change the heat in the area where the laser spots were located. According to frontier orbit theory, chalcopyrite is more likely to undergo thermal oxidation and eventually oxidize to hematite. Compared with previous studies, pyrite is first converted to marcasite and finally oxidized to hematite during thermal oxidation, which refined the thermal oxidation process of pyrite. Covellite is thermally oxidized to chalcocite. Barite is difficult to be thermally oxidized to form new substances, but the disorder of crystals increases. Aragonite is not easy to absorb the heat generated by laser due to high transparency, and will not occur thermal oxidation. The addition of fluid can inhibit the thermal oxidation alteration of sulfides, which may be related to the high heat dissipation capacity of water or the adsorption reaction at the fluid-mineral interface, which has enlightenment significance for studying the fluid-mineral interface reaction
(4) Laser Raman spectroscopy has established a series of quantitative analysis models for each component of the fluid, and analyzed the mineral composition and structure. In this study, olivine and water were used as the initial reactants, and the Raman analysis system of fluid-rock interaction established in this paper was applied comprehensively in the laboratory. Serpentine reaction, as a typical water-rock reaction, widely exists in deep-sea ultrabasic hydrothermal system and mud volcano region,which provides an important source of material and energy for deep-sea chemical energy synthesis ecosystem. Initial results of in-situ monitoring the serpentinization reaction show that the Raman peak of olivine became wide at the interface between fluid and rock as the reaction progresses, suggesting that olivine structure damage, this may be due to olivine mineral crystal phonon interaction enhanced. The study provides a clue for further research in the serpentinization process at the interface of the olivine and fluid.
(5) On the basis of comprehensive application in the laboratory simulation system, this paper eventually carried out the in situ detection of fluid and rock interaction in the deep sea based on the Raman analysis methods in lab: the author has taken a large scale in-situ detection of the interaction between fluid and authigenic carbonate for the first time in the active seep Site F, Taixinan Basin, South China Sea. In this study, it was found that with the decrease of biomass density, the crystal structure of aragonite was gradually destroyed and the aragonite content decreased. The results of in situ quantitative analysis of fluids at different sites in Site F indicate that biogeochemical reactions such as methane anaerobic oxidation reaction and methane oxidation reaction occur in the lush biological community, which will form fluids with low salinity and low sulfate. Because of the salt effect, these internal flows can effectively inhibit the erosion of authigenic carbonate rocks, whereas the exposed authigenic carbonate rocks are more susceptible to the erosion and weathering of seawater with high salinity and high sulfate concentration, which provides a new understanding for the study of the evolution of authigenic carbonate rocks. Finally, combining with a large number of in-situ Raman spectra and laboratory XRD and SEM analysis results, the author established the distribution pattern diagram of biocenosis, mineral structure and content, and fluid in Site F, which provides a reference for the study of ecosystem and in situ detection of fluid-rock interaction.