Raman Spectroscopic Measurements Of The Composition And Pressure Of C-H-O-N-S Volatiles:Influences Of Pressure,Temperature And Fluid Composition

C-H-O-N-S volatiles are important components of geofluids and participate in almost all the geological processes from magmatism to hydrothermal mineralization.Determination of the composition of these volatiles in fluid inclusions is the basis for studying the source and evolution of fluids,as well as the corresponding diagenesis and mineralization.In situ Raman spectroscopy is a powerful method of analyzing the composition and pressure of the volatiles in fluid inclusions,which in turn is a prerequisite for the recovery of the trapping temperature and pressure of fluids.Many experiments were carried out the calibrate the Raman spectroscopic measurements of these volatiles,but the experimental temperature and pressure were limited,and the experimental systems were relatively simple.The influencing factors were not systematically discussed for the Raman quantitative factors of common C-H-O-N-S systems,which severely restricted the application of this in situ and non-destructive analysis method in the field of inclusion research.In this paper,we mainly did the following works:(1)synthesizing reference samples using high-pressure optical cell(HPOC);(2)collecting in situ Raman spectra of the systems including CH4-N2,CH4-N2-H2O,CH4-N2-SO2,CH4-N2-H2,CH4-N2-CO2-CO and CH4-C2H6-C3Hs-H2S-N2 at 21～350℃ and 10～600 bar;(3)investigating the possibility of using in situ Raman spectroscopy to determine the composition and internal pressure of complex C-H-O-N-S volatile-bearing inclusions.Similar to single gas component(such as CH4,CO2),the v1 vibration peak of each component in the mixed gases(v1-2v2 of CO2;v3 of C3H8)shifted to lower wavenumber with increasing pressure at a constant temperature.Therefore,the characteristic Raman peak of each component in complex volatiles is a reliable indicator of fluid pressure.Accordingly,this paper establishes the quantitative relationship between the fluid pressure and peak position of vi(CH4),vi(C2H6),v3+v16(C3H8),v1(SO2),v1(H2S),v1(CH4),v1(N2),v1(CO)and upper v1-2v2(CO2)in some volatiles.The commonly used Raman quantification factors are mainly based on the characteristic peak area or peak height ratio of different components,which are represented by Fi and Gi,respectively.Compared with previous work,this paper measured the Raman quantification factor of common volatiles over a wider T-P range,and evaluated the effects of pressure,temperature and fluid composition.First,the effect of pressure on Fi is most obvious.In the low pressure range,Fi increases with increasing pressure and remains at a relatively stable value after reaching a certain inflection point pressure.For example,the inflection pressures of FCH4,FCO2,FCO,FSO2,FH2S and FC3Hs at room temperature are 100 bar,100 bar,25 bar,75 bar,200 bar and 100 bar,respectively.The FC2H6 gradually decreases with pressure increasing,and becomes stable at a inflection point pressure of 150 bar;FH2 is almostly not affected by the pressure.Secondly,the effect of temperature on Fi is related to the formation and development of hot bands.For CO2,the hot bands is separated from the peak of v1-2v2,so that the spectral intensity of v1-2v2 decreases with increasing temperature,correspondingly leading to the decrease of FCO2.For CH4 and CO,the effect of temperature on Fi is not obvious,especially under high pressure(>100 bar).Finally,the effect of the fluid composition on Fi is relatively weak,and the effect of water on Fi is negligible below 200 ℃.Compared with Fi,Gi is more sensitive to temperature,pressure and composition,and is not suitable for analyzing the composition of volatiles in fluid inclusions.Under certain conditions,it may be more suit to reflect the fluid pressure.The FCH4,FC2H6,FC3H8,FCO2,FCO,FSO2,FH2S and FH2 obtained by different laboratories were compared.The results show that even if the experimental temperature and pressure conditions are similar,the obtained Fi is quite different,indicating that the influence of instrument efficiency factor on Fi can not be ignored.Therefore,it is necessary to calibrate the laboratory’s Fi in advance when applying Raman spectroscopy to quantitatively analyze the composition of complex volatiles;mixed gas standards with known compositions and pressures are required,while HPOC and FSCC are exactly good choice for preparing mixed gas standards.A general method is proposed for quantitative analysis of complex volatile fractions and internal pressures in fluid inclusions by Raman spectroscopy,according to the determination of unknown composition CH4-N2 mixtures in synthetic inclusions(FSCCs).The experimental calibration results were applied to the analysis of fluid inclusions hosted in quartz coexisting with metal sulfides in the Hanzhong Mayuan lead-zinc mine.The molar ratio of volatile in the inclusions such as CH4,H2S and CO2,the solution phase salinity and the inclusion pressure at room temperature were obtained.The petrographic observation shows that the pure gas,gas-rich and liquid-rich inclusions are observed in a same view field.Microthermometry measurements show that the homogenization temperatures of the gas-rich and the liquid-rich inclusions are similar(244.6～289.1℃,indicating a heterogenous trapping of such fluids.Then the homogenization temperature is the trapping temperature,and the homogenization pressure is exactly the trapping pressure.On the basis of accurate calibration of composition and internal pressure of pure gas-phase inclusion,a trapping pressure of 699.6～790.4 bar was obtained.