Fluid-evaporation records preserved in salt assemblages in Meridiani rocks

We studied the inter-relationships between the major anions (SO3, Cl, and Br) and cations (FeO, CaO and MgO) using elemental abundances determined by APXS in salt assemblages of RATted (abraded) rocks at Meridiani to characterize the behavior of fluids that infiltrated into this region on Mars. A plot of SO3 versus Cl for the abraded rocks yielded an unusual pattern, whereas the SO3/Cl ratios versus Cl for the same rocks showed a monotonically decreasing trend represented by a hyperbola. The systematic behavior of the SO3 and Cl data in the documented rocks at Meridiani suggests that these anions behaved conservatively during fluid–rock interactions. These results further indicate that two kinds of fluids, referred to as SOL-I and SOL-II, infiltrated into Endurance/Eagle/Fram craters, where they underwent progressive evaporative concentration. SOL-I is a low pH fluid consisting of high SO3 and low Cl and high Br, (this fluid infiltrated all the way to the crater-top region), whereas SOL-II fluid of high pH with low SO3 and high Cl and low Br reached only an intermediary level known as the Whatanga contact at Endurance. Based on the FeO/MgO as well as CaO/MgO versus SO3/Cl diagram for rocks above the Whatanga contact, the cation and anion relationships in this system suggest that the Fe2+/SO4 and Ca2+/SO4 ratios in SOL-I fluids at Meridiani were >1 before the onset of evaporation based on the “chemical divide” considerations. Below the Whatanga contact, relatively dilute SOL-II fluids seem to have infiltrated and dissolved/flushed away the easily soluble Mg-sulfate/chloride phases (along with Br) without significantly altering the SO3/Cl ratios in the residual salt assemblages. Further, Cl/Br versus Br in rocks above the Whatanga contact show a hyperbolic trend suggesting that Cl and Br behaved conservatively similar to SO3 and Cl in the SOL-1 fluids at Meridiani. Our results are consistent with a scenario involving two episodes (SOL-I and SOL-II) of groundwater recharge at Meridiani Planum.