DISSOLUTION RATES OF NA-JAROSITE: CONSTRAINING THE DURATION OF AQUEOUS DIAGENESIS ON MARS

Jarosite is a ferric sulfate that forms in acidic, oxidizing, and iron-rich environments on Earth, such as acid mine drainages (AMD). The presence of jarosite in outcrops throughout Meridiani Planum, as observed by the Mars Exploration Rover (MER) Opportunity, and other localities on Mars based on spectral data from orbiting spacecraft indicates that ephemeral, acidic fluids likely existed on Mars. The lifetime of jarosite in aqueous environments, and hence the duration of water in jarosite-bearing sediments on Mars, can be determined by measuring the dissolution rates of jarosite under Mars-relevant conditions. In previous studies, synthetic K- endmember jarosite was used in batch reaction dissolution experiments to constrain the duration of aqueous diagenesis on Mars. K-jarosite dissolution rates ranged from log k= -7.1 at 323 K to -8.6 at 277 K. However, Na-jarosite may be a more common phase in basalt-sourced sediments on Mars. In this current study, the rate of Na-jarosite dissolution was determined and compared to previous K-jarosite results. Na-jarosite dissolution rates ranged from log k = -7.4 at 323 K to log k = -9.5 at 277 K. The dissolution rate of Na-jarosite is slightly slower than the rates obtained from parallel K-jarosite experiments, yielding longer jarosite lifetimes. Experiments examining the effect of pH on the dissolution rates of both K- and Na-jarosite are underway to further constrain the lifetime of jarosite, and hence liquid water in jarosite-bearing sediments on Mars.