In Situ Quantitative Analysis Of Dissolved Gas In Deep-sea Hydrothermal Vent Fluid Based On Laser Raman Spectroscopy

The deep-sea hydrothermal system releases larges number of gas species into the ocean every year,which not only provides an energy source for the hydrothermal ecosystem,but also significantly affects the chemical composition of the surrounding seawater.It is difficult to directly measure the gas content in the high temperature hydrothermal vent fluids because of their harsh temperature and pressure conditions.The detection efficiency of traditional analytical methods involving the collection of hydrothermal fluids using gas-tight samplers and the analysis in laboratory is low.In addition,the loss of gas components,resulting in great measurement errors,in the process of sampling and sample treatment is hard to avoid.Laser Raman spectroscopy is very suitable for the measurement of gas content in deep-sea hydrothermal vent fluids due to its non-invasive,non-destructive and rapid test capabilities as well as the benefit that sample preparation is often not required.In this paper,quantitative analysis methods of Raman spectroscopy for the H₂,CO₂ and CH₄ in hydrothermal fluid were established through high-temperature and high-pressure simulation experiment in laboratory.The gas species of hydrothermal fluids at Okinawa Trough hydrothermal field were analyzed quantitatively based on these methods.Based on the above,the following conclusions can be drawn:(1)The OH stretching band of water can be used to calibrate the temperature of hydrothermal fluid since it is sensitive to the changes of temperature and salinity.The mix proportion of hydrothermal fluids with seawater during Raman detection can be calculated by comparing the difference between the fluids temperatures measured by sensor at hydrothermal vent and derived from OH stretching band of water,which provides reference and guidance for the operation of in situ Raman spectroscopy.(2)The Raman calibration models of H₂,CO₂,CH₄ and SO₄^2-established through high-temperature and high-pressure simulation experiments are linear and have good linear regression,which is suitable for in situ Raman spectroscopy quantitative analysis of hydrothermal vent fluid.The concentrations of SO₄^2-determined by the quantitative calibration model can be used to calculate the endmember concentrations of H₂,CO₂,CH₄ in the hydrothermal vent fluids.(3)The results of in situ Raman detection showed that the hydrothermal fluid at the Okinawa Trough generally suffered from different degrees of phase separation.The content of gas in hydrothermal fluids varies greatly among different vents due to the impacts of subseafloor phase separation.(4)The concentrations of CO₂ and CH₄ measured by Raman spectroscopy are approximately 1.5 to 4.0 times higher than those derived from gas-tight samples collected at the same time and vent,which indicates that the flux of gases released by hydrothermal systems is likely to be significantly underestimated due to the limitations of measurement techniques.In situ Raman measurement has a prominent advantage in the research of the gas flux released by hydrothermal system because of its high efficient detection capabilities.(5)In the Yokosuka site hydrothermal field,in situ Raman measurement was used to determine not only the concentrations of the gas components in the hydrothermal fluid,but also the phase states of the emissions in the inverted lake.Available data suggest that this type of low density emanation hydrothermal system is broadly distributed in diverse geotectonic settings.Given the wide distribution of supercritical and vapor phase hydrothermal systems,the influence of low-density hydrothermal emanations on the recirculation of deep-sea hydrothermal systems and mineralization processes should be given more attention.