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The formation of supergene jarosite at planetary surfaces (i.e. Earth and Mars) requires the presence of water, however preservation of the mineral requires sustained aridity. As a potassium-bearing mineral, jarosite can be dated by the ⁴⁰Ar/³⁹Ar method. Therefore, jarosite ⁴⁰Ar/³⁹Ar ages can be used to date surface processes such as weathering and environmental transitions (i.e. aridification) on Earth and Mars. In preparation for Mars sample return missions and 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/a² of 5.68 ± 0.63 s^-1 corresponding to a closure temperature of 143 ± 28 °C, assuming a cooling rate 100°C/Ma (Figure 1).

Downward extrapolation of these parameters to Martian surface temperatures (≤ 22°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/a² pairs predict that if held at 22°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 (Figure 2).

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.

As a proof of concept for applying jarosite ⁴⁰Ar/³⁹Ar chronometry to constrain the timing and rate of aridification for once-aqueous environments, a vertical profile of jarosite-bearing samples was collected from the top of the Paleocene Fort Union Formation in the Bighorn Basin of Wyoming. The Fort Union Formation exposed at the McDermotts Butte area (Figure 3) consists of interbedded fluvial sheet sandstones and floodplain mudstones (15, 16). Paleosols developed on the fine-grained floodplain deposits contain rhizoliths, relative abundances of goethite and hematite, and preserve jarosite indicating a general history of deposition, water saturation and soil formation, burial with sustained saturation, and finally excavation and drainage. This hydrologic sequence of saturation followed by drainage and sustained aridity (Figure 3) is analogous to that interpreted for the stratigraphy exposed at Burns Cliff at Meridiani Planum (Grotzinger et al., 2005).

Jarosite precipitation during drainage of the Fort Union Formation is a time-transgressive process where the oldest jarosite (first-formed) should be at the top of the section and progressively younger jarosite should be present downsection. The ages measured from 5 jarosite samples collected along a 25 cm vertical profile are in agreement with this prediction. Jarosite ages systematically decrease downsection (Figure 4) indicating a jarosite precipitation front migrated downward through the

section at an average rate of 3.6 cm/Ma. The precipitation front likely reflects slow drainage through this portion of the sedimentary column. This result indicates that jarosite dissolution rates may be slower than those derived from laboratory experiments and that jarosite ages can record geologically slow environmental transitions over very small spatial distributions. Additionally, jarosite-bearing samples retrieved by future Mars sample return missions will afford determining when and how long water existed at the Mars surface.