HR: 0800h
AN: P21A-0223    [Abstracts]
TI: Br/Cl Partitioning in Halite and Hydrohalite on Mars
AU: * Marion, G M
EM: giles.marion@dri.edu
AF: Desert Research Institute, 2215 Raggio Parkway, Reno, NV 89512, United States
AU: Kargel, J S
EM: jkargel1054@earthlink.net
AF: University of Arizona, 10500 Speedway Blvd., Tucson, AZ 85721, United States
AU: Catling, D C
EM: david.catling@bristol.ac.uk
AF: University of Bristol, 1 Reading Rd., Bristol, BS8 1RJ, United Kingdom
AB: Within Meridiani Planum outcrops, Br may be present in the hundreds of ppm range. Relative to Cl, this suggests that Br is probably 1-10% of Cl; relative to Earth seawater (Br = 0.35% of Cl), these are "high" Br concentrations. Because of low Br concentrations on Earth, Br largely precipitates as a minor constituent in halite(NaCl) crystals rather than as a separate phase. This is also likely to be the case for Mars. However, on Mars much higher ratios of Br/Cl are expected in solid solutions of chloride minerals, and there may exist a greater number of accessory bromide minerals (which are exceedingly rare on Earth) in late-stage crystallization products of brines undergoing extreme drying or freezing. Bromide chemistries were incorporated into the FREZCHEM model that has been frequently used for simulating Martian environments. The Siemann-Schramm model was used to partition Br into halite (or hydrohalite) crystals. We examined evaporation and freezing as the dominant mechanisms for Br incorporation into halite(or hydrohalite). For example, a saturated halite solution with 1% Br(mole basis) begins precipitating a solid solution of 0.046% NaBr in the halite crystal, which increases to 6.1% by the time that the water phase has evaporated to 1% of the initial solution. The evaporation process preferentially increases the solution phase Br concentration relative to Cl, which leads to the increasing Br% incorporation into halite. In addition we also examined the consequences of alternating wetting/drying and thawing/freezing cycles on Br/Cl migration and subsequent reprecipitation and fractionation in Martian environments. Partial dissolution of halite will preferentially remove high Br from upper sediment layers that will lead to reduction in upper sediment halite-Br and a subsequent increase in halite-Br in lower sediments, in agreement with Mars Rover findings. Bromide- rich tracer horizons may be indicative of major transitions in climate (drying or freezing episodes) or late stages of freezing or drying of flood deposits.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
DE: 5470 Surface materials and properties
DE: 6225 Mars
SC: Planetary Sciences [P]
MN: 2007 Fall Meeting