Fluid-evaporation records preserved in salt assemblages in Meridiani rocks

• Published in: Earth and planetary science letters 2009-09, Vol.286 (3), p.396-403
• Main Authors: Rao, M.N., Nyquist, L.E., Sutton, S.R., Dreibus, G., Garrison, D.H., Herrin, J.
• Format: Article
• Language: English
• Subjects: ANIONS; CATIONS; chemical divide; chlorine and bromine; CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; EVAPORATION; GEOLOGIC HISTORY; GROUNDWATER RECHARGE; Mars; MARS PLANET; Martian fluids; Meridiani Planum; Opportunity APXS data; progressive evaporation; ROCK-FLUID INTERACTIONS; SALT DEPOSITS; sulfate
• ISSN: 0012-821X; 1385-013X
• DOI: 10.1016/j.epsl.2009.07.007

We studied the inter-relationships between the major anions (SO 3, 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 SO 3 versus Cl for the abraded rocks yielded an unusual pattern, whereas the SO 3/Cl ratios versus Cl for the same rocks showed a monotonically decreasing trend represented by a hyperbola. The systematic behavior of the SO 3 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 SO 3 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 SO 3 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 SO 3/Cl diagram for rocks above the Whatanga contact, the cation and anion relationships in this system suggest that the Fe 2+/SO 4 and Ca 2+/SO 4 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 SO 3/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 SO 3 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.