Clues to Martian brines based on halogens in salts from nakhlites and MER samples

摘要

Chlorine and Br abundances in fracture-filling secondary salts in Nakhla veins determined in this study by Synchrotron X-ray Microprobe (Br) and Electron Microprobe (Cl) techniques compare well to the halogens determined recently by APXS instruments in soils and rock rinds at the Gusev and Meridiani sites. The salts in these Mars rocks arise from Martian brines that have undergone evaporative or freezing concentration. The halogen abundances in these salts are corrected for “dilution effect” (because of mixing with halogen-poor phases) to obtain their “true” abundances, which seem to be close to the halogen abundances in terrestrial sea brines. Consistent with the petrographic evidence for evaporative salt deposition sequence of carbonate-sulfate-halite in nakhlite meteorites, Nakhla veins yield high Br (～250 ppm) and low Cl/Br ratios (～10–50), suggesting possible salt deposition from concentrated brines, whereas Lafayette iddingsite, with low Br (11 ppm) and high Cl/Br ratios (～250–300), indicates salt deposition from dilute brines on Mars. The low Cl/Br ratios (～20 to 80) in salts from Gusev and Meridiani rock rinds indicate that they presumably originate from concentrated subsurface Martian brines. The high Cl/Br ratio (～270) in salts deposited by dilute solutions in Lafayette and Adirondack is similar to the chondritic Cl/Br ratio, which seems to characterize the dilute brines on Mars (prior to halite precipitation) including “Early Mars Waters” (Noachian or earlier). Our results seem to be consistent with the Burt-Knauth model involving density stratification during evaporation/freezing of brine solutions in Martian regolith.