Geochemical and spectral characterization of an altered Antarctic dolerite: Implications for recent weathering on Mars

摘要

We present new mineralogical, chemical and spectral analysis of an alteration profile on the Ferrar dolerite (Dry Valley, Antarctica), complementing a previous study (Chevrier a al., 2006a). The whole profile is about 5 cm long and subdivided into three different layers: a brown surface rind inferior to 1 mm in thickness, followed by a brownish-grey discoloration zone from 1 to 5 mm depth and finally a dark fresh core. Mineralogical (X-Ray Diffraction), chemical (EDAX, LIBS), and spectral (FTIR) measurements indicate the formation of iron (oxy)-hydroxides (maghemite) in the very top millimeter of the alteration profile, resulting from the destabilization of ferromagnesian minerals (pyroxene). This zone also exhibits strong hydration features as evidenced by LIBS hydrogen signal and the 2.80 mu m water band in reflectance spectra. Below this alteration zone (around 5-mm-deep), spectral measurements indicate a discolored zone characterized by an enrichment in pyroxene (1.00 and 2.00 mu m bands), possibly due to the dissolution of the glass component in the matrix. However, despite these spectral changes, the overall chemistry and mineralogy of the sample remains largely unaffected. This suggests that recent cold and dry weathering on the surface of Mars may spectrally modify surfaces of basaltic rocks, by forming iron (and manganese) (oxy)-hydroxides, but essentially in the very uppermost millimeter and that the underlying mineralogy should remain intact.