Improved method to determine the molar volume and compositions of the NaCl-H_2O-CO_2 system inclusion

摘要

On the basis of Parry's method (1986), an improved method was established to determine the molar volume (V_m) and compositions (X) of the NaCl-H_2O-CO_2 (NHC) system inclusion. To use this method, the determination of V_m-X only requires three microthermometric data of a NHC inclusion: partial homog-enization temperature (T_(h,CO_2)), salinity (S) and total homogenization temperature (T_h). Theoretically, four associated equations are needed containing four unknown parameters: X_(CO_2), X_(NaCl), V_m and F (volume fraction of CO_2 phase in total inclusion when occurring partial homogenization). When they are released, the V_m-X are determined. The former three equations, only correlated with T_(h,CO_2), S and F, have simplified expressions:X_(CO_2)= f_1(T_(h,CO_2), S, F), X_(NaCl)=f_2(T_(h,CO_2), S, F), V_m=f_3(T_(h,CO_2), S, F). The last one is the thermodynamic relationship of X_(CO_2), X_(NaCl), V_m and T_h: f_4(X_(CO_2), X_(NaCl), V_m, T_h)=0. Since the above four associated equations are complicated, it is necessary to adopt iterative technique to release them. The technique can be described by: (ⅰ) Freely input a F value (0≤ F ≤ 1), with T_(h, CO_2) and S, into the former three equations. As a result, X_(CO_2), X_(NaCl) and the molar volume value recorded as V_(m1) are derived. (ⅱ) Input the X_(CO_2) and X_(NaCl) gotten in the step above into the last equation, and another molar volume value recorded as V_(m2) is determined. (ⅲ) If V_(m1) is unequal to V_(m2), the calculation will be restarted from "(ⅰ)". The iteration is completed until V_(m1) is equal to V_(m2), which means that the four associated equations are released. Compared to Parry's (1986) solution method, the improved method is more convenient to use, as well as more accurate to determine X_(CO_2). It is available for a NHC inlusion whose partial homogenization temperature is higher than clatherate melting temperature and there are no solid salt crystals in the inclusion at parital homogenization.