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 +/- 1.5 kcal/mol and a log D-o/a(2) of 5.68 +/- 0.63 s(-1) corresponding to a closure temperature of 143 +/- 28 degrees C. assuming a cooling rate 100 degrees C/Ma. Downward extrapolation of these parameters to Martian surface temperatures (<= 22 degrees C) predicts <1% fractional loss of Ar over 4.0 Ga. Forward modeling of 40Ar/39Ar age spectra using the least retentive E, D-o/a(2) pairs predict that if held at 22 degrees C or less for 4.0 Ga, 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. (C) 2011 Elsevier B.V. All rights reserved.