 FREEZING AND EVAPORATION MODELING OF PHOENIX WCL SOLUTIONS USING FREZCHEM AND GEOCHEMICAL WORKBENCH. A. Elsenousy1, J. Hanley1, V. Chevrier1. 1. Arkansas Center for Space and Planetary Sciences, 202 Old Museum Building, University of Arkansas, Fayetteville, AR, USA, 72701. amira@uark.edu.  Introduction:  The WCL (Wet Chemistry Laboratory) onboard the NASA's Phoenix lander identified the soluble ionic composition of Mars's Northern region's soil. The ions discovered at the Phoenix landing site included Ca 2+ , Mg 2+ , K +, Na +, Cl , SO4 2- , and the most unexpected ion ClO4 - [1,3]. Although the soluble ionic composition of the soil at phoenix landing site is well known [1-4], this is not the case for the composition of the parent salts in the regolith. Recent studies have been done on modeling the WCL solutions using various thermodynamic codes to understand the salt assemblages at the Phoenix landing site [4, 5] using two different pathways: evaporation at 7º C or freezing of liquid brines [5]. In our study we compare the minerals formed by freezing (T < 0º C using FREZCHEM) with evaporation scenario (T > 0º C using both FREZCHEM and GWB). We also include the presence of chlorate compounds since chlorate is usually associated to perchlorate in natural environments [4, 6].  Experimental Methods:  The WCL solutions evaporation and freezing were modeled with FREZCHEM (modified to include chlorate salts) using initial conditions which reported in Table1 and were taken from [1, 3]. All evaporation runs started with 1000 g of solution and a water decrement of 0.1 g at constant temperature of 283.15 K. Freezing runs started at temperature of 273.15 K down to 173.15 K with temperature decrement of 1 K. Both evaporation and freezing runs were established with an initial pH of 7.7 and initial CO2 of 3 mbar. The Geochemist's Workbench ® (GWB) software package was also updated to include the Pitzer parameters for chlorate salts as reported in [4]. Evaporating 1 kg of water at 7°C took place with initial pH of 7.7 using the composition as defined in [1]. In this case, a fixed calcite concentration of 4.5 wt% [7] was added to the simulation to test for the presence of carbonate.  Table 1: Initial concentrations in mM used in FREZCHEM and GWB CB is for the charge balance  Results and Discussion: When comparing both freezing and evaporation pathways, we note that the highly hydrated salts such as Meridianite (MgSO4. 11H2O) are formed via freezing pathway (Fig. 1 A & B) while the low hydrated phases such as kieserite formed through evaporation (Fig. 2B and 3B). Chlorates are one of the highly soluble minerals that formed via both evaporation and freezing pathways for FREZCHEM and GWB. They are abundant in all runs and dominated by Mg 2+ . Some chlorate salts such as Ca(ClO3)2.2H2O (deposited at 0.78 g water) and NaClO3 (deposited at 0.43 g of water) are observed only with GWB (Fig. 3 A) which is due to some limitation in FREZCHEM where evaporation via FREZCHEM stops running before precipitating Ca(ClO3)2.2H2O or NaClO3.         Figure 1: Results of FREZCHEM model showing minerals precipitation via freezing pathway using model 1 (low sulphate) (A) and model 2 (rich sulphate) (B) initial concentrations.   Species Model 1 Model 2 Model 3 Na +       1.40 1.40 1.40 K +         0.38 0.38 0.40 Ca 2+      0.58 0.58 0.75 Mg 2+     3.30 3.30 6.40 Cl          0.54 0.54 0.75 ClO4     2.40 2.40 2.50 ClO3 -     CB = 6.20 2.40 CB = 2.25 SO4 2-      0.20 CB = 2.10 5.30     Figure 2: Results of FREZCHEM model showing minerals precipitation via evaporation pathway using model 1 (low sulphate) (A) and model 2 (rich sulphate) (B) initial concentrations.   KClO3 is not observed in any of the models neither with FREZCHEM or GWB since all potassium was rather precipitated as potassium perchlorate which is the least soluble perchlorate salt. Therefore KClO4 was among the first precipitated salts through both evaporation and freezing runs (Fig. 1, 2 and 3).  Perchlorates are highly dominated by K +, Mg 2+ and Na + while calcium perchlorate is not observed in any model. The unexpectedly absence of Ca(ClO4)2 is due to presence of the two major sinks KClO4 for ClO4  and gypsum for Ca 2+ . Gypsum is observed as a major sulfate mineral that precipitates in all models via evaporation or freezing. Other sulfates such as epsomite, kieserite, hexahydrite, mirabilite and anhydrite are detected through evaporation pathway mainly with model 2 (Fig. 2B & 3B) due to the high sulfate concentration of 2.10 mmol/kg. Hexahydrite and Mirabilite are not observed with FREZCHEM, because they are not present in the FREZCHEM database. Chlorides are generally observed in minor amounts and in some cases as in evaporation via FREZCHEM they are completely absent. Halite is formed by GWB at 0.12 g of water whereas the FREZCHEM code stops before halite start to precipitate.    Figure 3: Results of GWB showing minerals precipitation via evaporation pathway using model 1 (low sulphate) (A) and model 2 (rich sulphate) (B) initial concentrations.   Conclusion: Modeling the WCL solutions via FREZCHEM and GWB show richness of chlorate salts in all runs which dominated by Mg 2+ . In addition, it illustrates a significant absence of Ca perchlorate in the simulations which indicate either a very arid environment or that Ca-perchlorate remained isolated from other salts, preventing equilibrium to be established. Finally, the simulations show precipitation of the highly hydrated salts via freezing pathway and deposition of the low hydrated slats via evaporation.  References:  [1] Hecht M. H. et al. (2009) Science, 325, 64-67. [2] Kounaves S. P. et al. (2009) J. Geophys. Res., 114, E00A19. [3] Kounaves S. P. et al. (2010) Geophys. Res. Lett., 37, L09201. [4] Hanley J. et al. (2012) Geophys. Res. Lett., 39, L08201.[5] Marion, G. M., et al. (2010) Icarus, 207(2), 675685. [6] Rao, B., et al. (2010) Environ Sci. Technol., 44 (22), pp 8429-8434. [7] Boynton, W. V., et al. (2009) Science, 325(5936), 61-64. 
