Jarosite, argon diffusion, and dating aqueous mineralization on Earth and Mars

Jarosite ⁴⁰Ar/³⁹Ar ages can be used to date surface processes such as weathering and environmental transitions (i.e. aridification) on Earth and Mars. To better interpret jarosite ages from a thermochronological perspective, the diffusion kinetics of argon in jarosite were determined. Incremental fractional loss measurements indicate an activation energy (E) of 37.8±1.5kcal/mol and a log D_o/a² of 5.68±0.63s^-1 corresponding to a closure temperature of 143±28°C, assuming a cooling rate 100°C/Ma. Downward extrapolation of these parameters to Martian surface temperatures (≤22°C) predicts <1% fractional loss of Ar over 4.0Ga. Forward modeling of ⁴⁰Ar/³⁹Ar age spectra using the least retentive E, D_o/a² pairs predict that if held at 22°C or less for 4.0Ga, supergene jarosite would preserve original growth ages manifest as plateau ages consisting of >95% of the gas release. Because of its susceptibility to mineralogical breakdown, ⁴⁰Ar/³⁹Ar ages on preserved Martian jarosite will reflect the time since water was present at a location that has since undergone aridification and remained hydrologically inactive and thermally quiescent.