Fluid-evaporation Records Preserved in Meridiani Rocks

Introduction: We have shown earlier that the high SO 3 / Cl ratios found in secondary mineral assemblages in shergottite GRIM glasses (Gas-Rich Impact-Melt) likely resulted from interactions of regolith materials with sulfate-rich (and Cl-poor) solutions [1,2]. The low SO 3 /Cl ratios determined in secondary salts in nakhlite fracture-fillings presumably formed by rock interactions with chloride-rich (and SO 4-poor) solutions near Mars surface [1,2]. The SO 3 and Cl abundances determined by APXS in abraded rocks (RAT) from Endurance , Fram and Eagle craters indicate that these salt assemblages likely formed by evaporative concentration of brine fluids at Meridiani [3,4]. The SO 3 /Cl ratios in the abraded rocks are examined here, instead of their absolute abundances, because the abundance ratios might provide better guidelines for tracking the evolution of evaporating fluids at Meridiani. The SO 3 /Cl ratios in these samples, in turn, might provide clues for the mobile element ratios of the altering fluids that infiltrated into the Meridiani rocks. The absolute SO 3 and Cl abundances in these rock samples depend on the modal abundances of the mobile element-bearing phases such as sulfate and chloride and mobile element poor phases such as silica and hematite in a given rock volume analysed by APXS. Differential occurrence/mixing of accessory mineral phases (phase-mixing) in one sample relative to another in the rock could lead sometimes to random variations in elemental concentrations of the salts. Furthermore, unknown geometry effects during sample measurements by APXS could lead to apparent variations in the real salt content of a given sample. Here, we study the interrelationships between the major anions (SO 4 , Cl and Br) and cations (FeO, CaO and MgO), using elemental abundances determined by APXS, in abraded rocks at Endurance, Fram and Eagle craters with a view to characterize the composition and behavior of the fluids that infiltrated into these rocks. Experimental: Mobile element abundances in salt assemblages in abraded Meridiani rocks (RAT) as determined by APXS are taken from [3-6]. Results and Discussion: Sulfate-Chloride system. SO 3 and Cl abundances determined on abraded Meridiani rocks [3-6] are plotted as SO 3 /Cl versus Cl in Fig.1. The concentrations of conservative ions in the evaporating solutions increase in such a way that their ratios remain constant as water is removed from the system. Further, their relative abundances vary along a