Improved method to determine the molar volume and compositions of the NaCl-H₂O-CO₂ system inclusion

Abstract

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-H₂O-CO₂ (NHC) system inclusion. To use this method, the determination of V _m-X only requires three microthermometric data of a NHC inclusion: partial homogenization 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 CO₂ 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 partial homogenization.