Weathering of iron rich phases in a Martian atmosphere
Résumé
The origin of iron bearing phases in the Martian regolith is usually interpreted as the coexistence of titanomagnetite inherited from the primary magmatic rocks with neoformed phases from Fe2+ solutions. However the use of natural terrestrial analogues does not acknowledge the fundamentally different nature of Earth and Mars atmosphere (replacement of O2 by CO2 and possible presence of peroxides). Moreover a part of the regolith could be of meteoritic origin as a result of accumulation of meteorites and IDPs, which usually contain 10-20 % of iron rich phases (metal, sulfides and magnetite) compared to 1-2% in Martian rocks. Sulfides may also be more common in primary Martian rocks than on Earth. Therefore we undertook an experimental weathering of pure elemental iron and magnetite, as well as natural hexagonal and monoclinic pyrrhotites. Weathering was performed at room temperature and 0.8 atm, in a pure CO2 atmosphere, saturated either with water or hydrogen peroxide vapor. Magnetic properties of neoformed products and primary minerals were investigated and completed using X-ray diffraction, TEM and SEM observations. These one year experiments show that magnetite remains stable in both atmospheres, and thus is likely to be inherited from the primary rocks through weathering processes. Neoformed products are mainly goethite with varying structures according to the atmosphere and substratum. Therefore goethite may be a common precursor to other iron (oxi)hydroxides like hematite or maghemite. But the various textures and structures of the neoformed phases depending on the atmosphere may complicate in situ measurements. Weathering of pyrrhotite induces formation of sulfates and elemental sulfur, which may account for the strong sulfur enrichment of Martian regolith. Metastable siderite is produced from weathering of iron. Lower temperatures occurring on Mars may kinetically stabilise the siderite, which could account for the recent observation of disseminated carbonates in Martian regolith. Thus our experimentally produced phases are well representative for the various in situ measurements, demonstrating that atmospheric weathering may occur on Martian surface, without requiring liquid water and an atmosphere composition different from the present one.