FORMATION OF JAROSITE AND MARS-LIKE MINERALS IN A POLAR DESERT: IMPLICATIONS FOR MARS AQUEOUS GEOCHEMISTRY

Alteration crusts formed in intracrater sediments of the Haughton impact structure, Devon Island, share several similarities with rocks studied by the MERs: 1) presence of multiple Ca-Mg-Fe sulfates (gypsum, jarosites, schwertmannite) with Fe-oxides (goethite, hematite); 2) evidence of selective dissolution of soluble phases; 3) presence of nanophase ferric oxides. The crusts form from the oxidation of sedimentary pyrite within carbonate-rich paleolake sediments. In partially oxidized crusts, pyrite grains are typically surrounded by gypsum and fine grained Fe-oxi/hydroxides and (or) schwertmannite. In some cases, selective dissolution of gypsum has produced distinctive honeycombs or rosettes of Fe-oxi/hydroxides. Fine scale heterogeneities suggest that multiple generations of Fe-phases have precipitated. The ferric sulfates jarosite, natrojarosite and hydronium jarosite are only found in the completely oxidized crusts.

The alteration minerals likely formed under water-limited and cold conditions (max. average summer temperatures are 8-10°C). Multiple episodes of liquid water influx and periodic dissolution and precipitation of various phases are likely, possibly as a result of seasonal spring melt. The jarosites likely formed in acidic microenvironments (not buffered by the bulk carbonate sediment), following dissolution of soluble iron minerals, possibly ferrous sulfates (e.g. rozenite), and subsequent release of H⁺ by hydrolysis of ferric iron.

These mineral crusts represent a "process analog" and provide information that can be useful in understanding past Martian aqueous activity and mineralization processes. For example, the formation of jarosite in sedimentary rocks studied by Opportunity has been though to have required widespread acidic solutions. However, the presence of jarosite in a well-buffered carbonate sediment at Haughton suggests that jarosite formation at Meridiani Planum did not require widespread acidic conditions, but merely localized acidic environments. Acidic conditions could have resulted from the oxidation of disseminated sulfides (impact or hydrothermal related) and (or) dissolution of previously formed soluble minerals.