 THERMODYNAMIC CONSTRAINTS ON LIMITED-WATER INDUCED FRACTIONATION OF MAGNESIUM -PERCHLORATE AND -CHLORIDE: IMPLICATIONS FOR HIGH CLO4-/CL- RATIOS IN MARTIAN POLAR REGIONS. Dongdong Li1, Yu-Yan Sara Zhao2, Zhongchen Wu3, Xiyu Wang4. 1Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining, 810008, China (ddong_li@hotmail.com), 2Center for Lunar and Planetary Sciences, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, 550081, China (zhaoyuyan@mail.gyig.ac.cn), 3Institute of Space Science, Shandong University, Weihai, 264209, China (z.c.wu@sdu.edu.cn), 4University of Chinese Academy of Sciences, Beijing, 100049, China (wangxiyu@mail.gyig.ac.cn). Introduction: In the past decade, perchlorate and/or oxychlorine species have been ubiquitously detected in situ on the Martian surface [1-4] and in Martian meteorites [5]. Based on these observations, Phoenix soils at Mars north polar region are unique, with measured ClO4−/Cl− molar ratio of 4.44 in average [1] and the molar proportion of oxychlorine species to total Cl would be even higher (~0.9) if includes ClO3− which also possibly presents in these samples [6; Table 3]. In contrast, oxychlorine species detected in other Martian samples in-situ [6; Table 3] or in terrestrial analogue, lunar or meteorite samples [5] are usually low, with approximately less than 20% of total Cl in mole. Experimental simulation studies producing oxychlorine species from chloride also show relatively low ClOx/Cl− ratios [6-11], suggesting that such high ClO4−/Cl− ratios (>1) as observed in Phoenix soils are not resulting simply by any known production processes. Currently, it is unknown why such high ClO4-/Cl- ratios are present in Martian polar region. Is it a unique signature solely present in polar environments or it may be present somewhere else on Mars? And yet, any proposed concentration processes of ClO4− through aqueous-related activities such as evaporation, freezethaw cycling, or deliquescence are lack of quantitative constraints. In this study, we examine Mg(ClO4)2 + MgCl2 + H2O system by experimentally measuring solubilities in temperature range 233.15 K to 273.15 K, and conducting comprehensive thermodynamic modeling to construct thermodynamically stable and metastable phase diagrams of the system. By overlapping Phoenix site surface temperature-humidity (T-RH%) data obtained by Thermal and Electrical Conductivity Probe (TECP) [12] on the x(Mg(ClO4)2)-T-RH% phase diagrams, we find that eutectic brines can form by deliquescence of mixtures consisted of magnesium perchlorate (hexahydrate) and -chloride (hexahydrate or octahydrate), or by partially melting of the mixtures if in contact of ice. These eutectic brines contain ClO4−/Cl− ratios varying from 2.5 to 14.4 depending on the hydration states of magnesium chlorides (hexahydrate or octahydrate); but overall, a relative enrichment of perchlorate over chloride signature can be produced. Solubility Measurements: Solubilities were determined isothermally at 273.15 K, 248.15 K and 233.15 K, respectively. MgCl2.6H2O purified by once re-crystallization and Mg(ClO4)2.6H2O without further purification were used. Samples were equilibrated for 5 days at T = 273.15 K, while at least 10 days at T = 248.15 K and 233.15 K. Compositions of the equilibrium liquids were determined by analyzing the content of Mg2+ and Cl− with gravimetric titrations. Then the concentration of ClO4 − was determined by subtraction method according to charge balance. The solid phases were determined by the Schreinemakers' wet residues method [13,14]. Thermodynamic Modeling: Modeling was conducted using the CALPHAD type aqueous-mineral equilibrium code ISLEC developed by Li et al. [15,16]. The latest release is Version 4.2, which enables the calculation of high salinity waters by using a PitzerSimonson-Clegg (PSC) excess Gibbs energy model. Model parameters for the binary system MgCl2 + H2O were taken from [17] and these for Mg(ClO4)2 + H2O were determined in this study. Mixing parameters in the ternary system were regressed from solubility data [18 and this study] and heat capacity data simultaneously as functions of temperature. The model is valid from 200 K to 363 K and its online version is available at http://www.islec.net/islec-web-mg-cl-clo4/. Results and Discussion: Our experiments suggest that the thermodynamically stable magnesium chloride hexahydrate is usually kinetically inhibited and replaced by the metastable octahydrate, which therefore important for understanding chloride salt assemblages and their ability to from cryogenic liquid brine. Our model represents the isothermal solubility curves (stable and metastable) reported in published literature [18] and measured in this study at various temperatures well (Fig. 1). Comparing the model simulated crystallization surfaces of the Mg(ClO4)2 + MgCl2 + H2O system with surface T-RH% conditions at Phoenix site (Fig. 2), we conclude that magnesium perchlorate hexahydrate mixing with either magnesium chloride hexahydrate or octahydrate can form eutectic brines when in contact water-vapor or water-ice under Phoenix site surface T-RH% conditions, and the resulting brines can have elevated ClO4−/Cl− molar     http://www.islec.net/islec-web-mg-cl-clo4/ ratios to 2.5 and 14.4, respectively. Such values are in good agreement with Phoenix soils that ClO4−/Cl− ratios are of 2.7 to 11.3 [1]. Fig. 1. Isothermal solubility curves in the system Mg(ClO4)2 + MgCl2 + H2O at various temperatures. Open symbols are experimental data, solid lines are model generated thermodynamically stable results, and dash lines show model predicted metastable results. Implications for Mars: With cold (T < 233.15 K) and transient wet (RH% > 50) conditions like northern polar region of mars, eutectic brines form by deliquescence of perchlorate and chloride salts or congruent melting with water-ice would preferentially enrich of ClO4− over Cl−, resulting ClO4-/Clfractionation and elevated ClO4− in the brines. Consequent evaporation or freezing of such brines would not fractionate ClO4− and Cl−, so the high ClO4−/Cl− signatures can be preserved in the polar soils. If excess water present, simultaneously leaching of both ClO4− and Cl− are expected, and no ClO4−/Cl− fractionation would occur so the ClO4−/Cl− molar ratio should inherit its production signature. In fact, based on our model, any brine forms with T > 233.15 K and RH% > 50 conditions would simultaneously accumulate both ClO4− and Cl−. With warmer and arid conditions (T > 233.15 K; RH% < 50) the ClO4−/Cl− molar ratio in the brine cannot exceed 0.5 as well. Therefore, the high ClO4−/Cl− molar ratios in Phoenix soils are likely a unique signature resulted by limited-water (e.g., water-film) interaction with salts under a cold environment. Dust/soil particles bear perchlorate and chloride salts formed in-situ or elsewhere, but then preferentially leave perchlorate to the ground when limited eutectic brines forms in the polar region. Elevated ClO4− signature is likely preserved on the top soil layer which continuously being affected by water vapor in the atmosphere. South polar region of Mars may also have elevated ClO4− signature over Cl−, if similar T-RH% conditions are present and eolian processes are dominant and transporting perchlorate and chloride salts in and out of the region. Calcium and Na -perchlorate and -chlorates might also influence the ClOx/Cl patterns in the polar regions. However, currently, phase diagrams of systems consisted of Ca or Na -oxychlorine and -chloride have not been well constructed down to the eutectic points. Further work is needed to determine the low temperature solubility of perchlorate/chloride and chlorate/chloride systems, in order to evaluate their effects on ClOx/Cl ratios on the Martian surface. Fig. 2. Model simulated metastable crystallization surfaces of the Mg(ClO4)2 + MgCl2 + H2O system in the temperature range from 200 K to 300 K with the surface T-RH% conditions at Phoenix site overlapped. The left column (three figures) shows the modeling results with solid phases MgCl2.12H2O and MgCl2.8H2O kinetically inhibited. The right column (three figures) shows the modeling results with solid phase MgCl2.12H2O kinetically inhibited. Acknowledgement: This work was supported by Natural Science Foundation of China (41703064) grants to D. Li and (41573056) grants to Z. Wu. Y.Y.S. 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