SPECTRAL STRATIGRAPHY CONSTRAINING CARBONATE FORMATION MECHANISMS AT NILI FOSSAE, MARS

The Nili Fossae region of Mars hosts the planet’s largest exposure of olivine-rich rocks that is commonly associated with Mg-rich carbonate. Four hypotheses have been proposed for the formation of these carbonate-bearing rocks: (1) in situ hydrothermal alteration of the ultramafic host rock at slightly elevated temperatures, (2) contact hydrothermal alteration induced by overlying hot lavas, (3) precipitation from transitory shallow lakes, and (4) low-temperature weathering. Building on recent geomorphologic mapping, we focus on the exposure of this olivine-rich unit between the northeastern Syrtis Major volcanic flows and Jezero Crater, at a region informally known as Northeast Syrtis Major. Using co-located CRISM and HiRISE observations, a spectral-stratigraphic analysis was performed collecting spectral signatures in stratigraphic context and in close proximity, on the scale of 10s of meters. The stratigraphy broadly resembles the regional Nili Fossae lithostratigraphic group of phyllosilicate-rich crustal basement, overlain by the olivine-rich unit, and topped by a spectrally bland capping unit. Carbonate spectral signatures are only observed within the olivine-rich unit and not as a separate stratigraphic unit. A feature of variable position at ~2.384 µm is observed in the olivine-rich unit and the down-section crustal basement that could be indicative of Mg-rich smectites, such as saponite, or other phyllosilicates, such as talc, but no sharp 1.39 µm feature is observed in this region that would be indicative of talc. A down-section stratigraphic sequence of olivine, serpentine, Mg-rich carbonate, and talc is not observed that would be indicative of a top-down contact hydrothermal system initiated by the emplacement of hot olivine-rich capping lavas. Therefore, we can conclude that the carbonate observed at Northeast Syrtis Major formed as the result of in situ alteration of the olivine-rich unit, or as a surface-weathering product of this unit.