Spectral reflectance characteristics of the Hamar Laghdad hydrothermal sequence, Morocco: Implications for the methane origin on Mars

We analyze and discuss reflectance spectra of carbonate rocks from the Hamar Laghdad area (Morocco), where evidences of interactions with hydrothermal, and, in some cases, methane enriched fluids derived from underlying volcanoclastic rocks, are reported in the literature. Deconvolution of the rock spectra into a sum of Gaussians, using MGM, resolved a number of both vibrational and electronic absorption features, mainly assigned to CO32−, Al–OH, and Fe2+, Fe3+, Cu, CF interaction processes, respectively. The associations of these absorption bands are exhaustive descriptions of the rock spectral properties. A spectral model of the Hamar Laghdad carbonate sequence was therefore delineated, based on the integration of these absorption band associations with XRF and Mössbauer analyses of the rock samples. The model involves: pure limestones and dolostones, limestone with iron carbonates, mixed carbonate and Al-silicate impure limestones, limestones with iron sulfides. The model points out the complexity of the spectral characteristics of rocks that underwent hydrothermal and partly methanogenic processes, but also suggests an alternative geologic scenario plausible for possible methanogenic activity in the Mars geologic past.