Mechanisms and kinetics of argon diffusion in hypogene and supergene jarosites: Implications for geochronology and surficial geochemistry on Earth and Mars.

Jarosite [KFe 3 (SO 4) 2 (OH) 6 ] occurs both as a hydrothermal mineral and as the product of weathering and chemical sedimentation. It has been used in 40Ar/39Ar geochronology to date water–rock interaction and weathering processes on the surfaces of Earth and Mars, but the lack of information about Ar diffusivity parameters relevant to specific types of jarosites makes the interpretation of geochronological results tentative. We have filled this gap by investigating Ar diffusion parameters in representative supergene and hypogene jarosites. Detailed diffusion studies were carried out on a hypogene jarosite sample from Gilbert, Nevada, and two supergene jarosite samples from Baiyin, China. The diffusion studies were accompanied by in-situ heating investigations in a transmission electron microscope to directly determine the thermal stability and the phase transformations that jarosite undergoes under progressive heating under ultra-high vacuum. The TEM results suggest that jarosite is stable under vacuum up to ∼400 °C, when it undergoes phase transition to yavapaiite [KFe(SO 4) 2 ] and hematite (Fe 2 O 3). Incremental-heating experiments reveal average diffusion parameters of E a = 138.6 ± 4.2 kJ/mol and ln(D o /a2) = 9.9 ± 0.9 ln(s−1) for hypogene jarosite; E a = 110.3 ± 3.2 kJ/mol and ln(D o /a2) = 5.7 ± 0.7 ln(s−1) for one supergene jarosites; and E a = 141.2 ± 7.9 kJ/mol and ln(D o /a2) = 11.3 ± 1.7 ln(s−1) for the other supergene jarosite sample from the same weathering profile. Jarosite closure temperatures depend strongly on sieve size. For samples between 500–200 µm (grain size usually used for samples in Ar geochronology), at a cooling rate 100 °C·Ma−1, the closure temperatures are 143 ± 18 °C for the hypogene jarosite, and 105 ± 8 and 113 ± 14 °C, respectively, for the two supergene jarosites. Forward modelling of incremental-heating results predicts that coarse-grained hypogene jarosite is retentive of Ar below 50 °C for 100 Ma and below 25 °C for 4 Ga. Densely packed supergene jarosite grains larger than 200 µm are suitable for 40Ar/39Ar geochronology at the timescales suitable for investigating water–rock interaction at the surface of Earth and Mars. Fine-grained, porous jarosites require detailed diffusion analyses prior to geochronology due to possible high Ar losses over Ma timescales. The absence of jarosites older than ∼40 Ma on Earth suggests that jarosite may require continuous exposure to acid oxidizing conditions, and that it does not survive burial and exhumation. Therefore, the occurrence of jarosite on Earth and Mars may identify segments of the planets' surfaces continuously exposed to acid-oxidizing conditions since jarosite precipitation. [ABSTRACT FROM AUTHOR]