Carbon dioxide and water in the crust. Part 1: equation of state for the fluid

H2O–CO2-dominated fluids play a crucial role in most geological phenomena involving fluid–mineral–melt interactions. The equation of state is an essential tool for understanding the phenomena because it predicts the thermodynamic properties of the fluids. The modified Lee–Kesler equation of state for H2O–CO2 mixture fluid developed by Duan and Zhang (2006) is the most accurate at present and is applicable to a wide pressure–temperature range (∼ 2573 K and ∼ 10 GPa). Because of its high accuracy and wide applicable range, the equation has been used for constructing solubility laws in silicate melts. In this paper I review the Duan and Zhang (2006) equation of state and present the calculation procedure. Because the equation for calculating the partial fugacity coefficient is erroneously presented in the original paper, the correct equation is provided here. A C-language code and a Windows executable program for computing thermodynamic properties are provided for the convenience of users. The influence of the nonideal behaviour of the H2O–CO2 mixture fluid on some geological situations is discussed.