Raman spectroscopic quantitative study of NaCl-CaCl2-H2O system at high temperatures and pressures.

Raman spectra features of the ternary system NaCl-CaCl2-H2O under high temperatures and high pressures were systematically studied in the present work by using hydrothermal diamond anvil cell(HDAC)and Raman shifts of quartz to determine pressures,and it has been obtained for the quantitative relationship between Raman shifts of the O—H stretching band of water,mass fractions of solutes and pressures was obtained.The mass fractions of salts,where salinity of NaCl equal to that of CaCl2,are 4.0 mass%,8.0mass%,and 12.0mass%,respectively.Experimental results indicate that the standardized Raman frequency shift differences of the O—H stretching vibration(Δν(O—H))rise with the increasing temperatures when the mass fractions of salts and pressures of the NaCl-CaCl2-H2O system remain constant.Δν(O—H)increases with the increase in mass fractions of salts in the system when the temperatures and pressures are constant.Linear relationship betweenΔν(O—H)and pressure with similar slopes can be found for the NaCl-CaCl2-H2O system with different salinities.The quantitative relationship betweenΔν(O—H),temperature(T),pressure(P),and mass fraction of solute(M)is P=-31.892Δν(O—H)+10.131T+222.816 M-3 183.567,where the valid PTMrange of the equation is 200MPa≤P≤1 700 MPa,273K≤T≤539Kand M≤12mass%.The equation can be used as a geobarometer in the studies of fluid inclusions of NaCl-CaCl2-H2O system with equal salinities.The method,as a direct geological detecting technique,has a potential application value.