Estimating Duration of Diagenesis at Meridiani Planum from Jarosite Dissolution Rates

Jarosite (K,Na,H)Fe₃(SO₄)₂(OH)₄ is a metastable, ephemeral aqueous alteration mineral observed in acid mine drainage environments as well as fumarole and acid lake environments. Observations of jarosite on Mars at Meridiani Planum provide key constraints on the pH and oxidation state of the fluids involved in jarosite formation. Due to its metastability and therefore propensity to convert to more stable iron oxyhydroxides such as goethite and hematite, as observed in the Meridiani outcrops as well as numerous terrestrial field sites, the presence of jarosite also constrains the temporal extent of liquid water at the site and hence the duration of diagenesis. Laboratory measurements of synthetic endmember K- and Na-jarosites at 293K yield dissolution rates of 8.6 and 7.9 x 10^-9 mol m^-2 sec^-1, respectively. However, temperatures on Mars at the time of diagenesis may have been considerably lower than those easily accessible in the laboratory. Using the Arrhenius equation, we have estimated the effect of lower temperatures on dissolution rates, assuming the activation energy falls within the range of most minerals (20-100 kJ/mol). Applying these corrected rates to a shrinking sphere model results in a maximum duration of liquid water approximately 5 kyr for 1 micron diameter particles at 250K. Less conservative estimates of temperature result in durations from 1 month to 2 years for 1 micron particles at 273K.