Mineralogy and morphology of geologic units at Libya Montes, Mars: Ancient aqueous outcrops, mafic flows, fluvial features and impacts

Abstract

There is ample evidence of ancient and long-lasting fluvial activity and chemical alteration in the Libya Montes region south of Isidis Basin. The region hosts Noachian to Amazonian aged surface rocks with extensive outcrops of olivine- and pyroxene-bearing material. Libya Montes also features surface outcrops and/or deposits hosting Fe/Mg-smectite, Fe/Mg-smectite mixed with carbonate and/or other Fe/Mg-rich phyllosilicates, and Al-smectite. These aqueous materials likely formed from chemical alteration connected with hydrothermal activity resulting from the formation of the Isidis Basin and/or the pervasive fluvial activity throughout this region. The morphology and stratigraphy of the aqueous and mafic minerals are described using High Resolution Imaging Science Experiment (HiRISE) and High Resolution Stereo Camera (HRSC) derived Digital Terrain Models (DTMs). Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) analyses show variations in the chemistry of the Fe/Mg-smectite from nontronite like exposures with spectral features near 2.29 and 2.4 μm more consistent with Fe3OH groups in the mineral structure, and saponite-like outcrops with spectral features near 2.31 and 2.38 μm characteristic of Mg2OH groups. These Fe/Mg-smectite bearing materials also have bands near 1.9 μm due to H2O and near 2.5 μm that could be due to the smectite, other phyllosilicates, and carbonates. All regions exhibiting carbonate features near 3.4-3.5 μm also have features consistent with the presence of olivine and Fe/Mg-smectite, indicating that the carbonate signatures occur in rocks likely containing a mixture of these minerals. The Al-smectite-bearing rocks have bands near 1.41, 1.91 and 2.19 μm that are more consistent with beidellite than other Al-phyllosilicates, indicating a higher-temperature or diurnally processed origin for this material. Our interpretation of the geologic history of this region is that ancient Noachian basaltic crustal materials experienced extensive aqueous alteration at the time of the Isidis impact, during which the montes were also formed, followed by emplacement of a rough olivine-rich lava or melt and finally the smooth pyroxene-bearing caprock unit.