2010 Annual Science Report
Arizona State University Reporting | SEP 2009 – AUG 2010
Habitability of Water-Rich Environments, Task 1: Improve and Test Codes to Model Water-Rock Interactions
Two new models have been developed in order to calculate 1) phase transitions during concentrating/diluting and cooling/heating in salt-brine-ice systems (from -60°C to 250°C) and 2) the chemical composition of hydrothermal systems. The case of water-granite interaction vs. time has been simulated to test a model of aqueous alteration that combines thermodynamics and kinetics.
Two new codes have been developed in order to calculate chemical equilibria in subfreezing salt-brine-ice, high-temperature (< 250°C) salt-water systems, and hydrothermal systems.
The first code was designed to calculate phase transitions that take place in closed water-salt-gas systems during concentrating/dilution or cooling/heating. It is based on an improved version of the Gibbs free energy minimization routine FREZCHEM2 (Mironenko et al.,1997) combined with a numerical differentiation of Pitzer activity coefficients of solutes and an osmotic coefficient (Mironenko and Polyakov,2009). The thermodynamic database of Marion (2008) is used for the system Na-K-Ca-Mg-FeCl-SO4-CO3-H-H2O over the temperature range –60°C to 25°C, and the database of Greenberg and Møller (1998) is used for the system Na-K-Ca-Cl-SO4-H2O for phase transitions from 25 °C to 250 °C. The model has been verified with experimentally studied systems. For astrobiology applications, the code could be used to calculate speciation and water activity in subfreezing salt-brine-ice-gas aqueous systems in diverse planetary (e.g., Mars, Earth’s polar regions, fluid inclusions in minerals and ices) and satellite (Europa, Enceladus) environments. One possible application of this code is conversion of measured ratios of contents of salts (e.g., in fluid inclusions in minerals of ore deposits) to their absolute concentrations in parent aqueous solutions. In addition to providing the solute concentrations, the model predicts melting transitions not used as input for the calculations (eutectic, peritectic).
The second code is aimed at calculating chemical equilibria in moderately diluted water-rock-gas systems at variable temperature, pressure, and bulk composition without address to thermodynamic databases at point equilibrium computations. This code is designed to be combined with reactive transport hydrological models. The code considers non-ideality of aqueous (Debye-Hückel model), gas (Peng-Robinson model) and solid phases (regular solution model). The code could be used to model fluid chemistry and secondary mineralogy in low-temperature (<120 °C) hydrothermal system (e.g., oceanic hydrothermal systems).
The kinetic-thermodynamic model has been used to perform a case calculation to explore water-rock interaction in time. For this purpose, a water-granite interaction has been developed and tested. A corresponding Mironenko-Zolotov paper has been submitted to Geochemistry International.