PACROFI VI - Electronic Program

Determination of phase equilibria in the system H2O-NaCl-KCl-MgCl2 using synthetic fluid inclusions, with application to fluid inclusion studies

Maxim O. Vityk1, Robert J. Bodnar1, John Hryn2, and John A. Mavrogenes3

1 Fluids Research Laboratory, Department of Geological Sciences, Virginia Polytechnic Institute & State University, Blacksburg, VA, U.S.A. 24061

2 Argonne National Laboratory, 9700S. Cass Avenue, ES/362, Argonne, IL, U.S.A. 60439

3 Research School of Earth Sciences, The Australian National University, Canberra, Australia 0200

The synthetic fluid inclusion technique was used to determine the phase relations in a portion of the system H2O-NaCl-KCl-MgCl2 under vapor saturated conditions. These experiments were designed to evaluate the effect of small amounts (2-10 wt.%) of MgCl2 on the solubilities of halite and sylvite. Compositions studied ranged from 30-50 wt.% NaCl+KCl, with NaCl/KCl ratios of 1:1 and 3:1. Data collected in this study have been regressed to generate a set of equations describing total salinity as a function of salt dissolution temperature and MgCl2 contents.

Addition of small amounts of MgCl2 (2-10 wt.%) to H2O-NaCl-KCl solutions decreases the solubility of halite and sylvite, compared to their solubilities in the H2O-NaCl-KCl system at the same temperature. This in turn leads to overestimation of the relative amounts of NaCl and KCl in solution, if the halite and sylvite dissolution temperatures are interpreted using phase equilibrium data for the H2O-NaCl-KCl system. For example, if an inclusion containing 30 wt.% NaCl+KCl also contains 10 wt.% MgCl2, the NaCl+KCl content in the inclusions may be in error by as much as 7.3 wt.% salt if data for H2O-NaCl-KCl are used to intepret the halite and sylvite dissolution temperatures. For inclusions containing 5 wt.% MgCl2 the error ranges from about 3.8 wt. % for inclusions containing 30 wt.% NaCl + KCl, to about 2 wt.% for the inclusions containing 50 wt.% NaCl + KCl. For a given MgCl2 concentration, the error diminishes as the concentration of NaCl+KCl increases. Our results suggest that for inclusions containing concentrated NaCl-KCl brines (> 50 wt.%), the NaCl+KCl concentrations estimated from halite and sylvite dissolution temperatures may be in error by as much as 4 wt.%. The effect of MgCl2 appears to be stronger in solutions with low NaCl:KCl (1:1) than solutions with higher NaCl:KCl (3:1).