Andesite Weathering in a Hyperarid, Periglacial Environment: Implications for Mars

English Abstract:

Widespread phyllosilicates have been identified in Noachian (4.5-3.7 Ga) terrains on Mars inferring the presence of past liquid water on the surface, and thus a habitable environment. To date, investigations of the potential weathering reactions on Mars have focused on basalt given the ubiquity of mafic minerals on the planet. However, there is increasing evidence of more siliceous deposits on Mars, especially in the Noachis Terra dacitic lava in the Nili Patera region and coarse-grained felsic deposits at the Bradbury landing site in Gale Crater. Therefore, there is a need to understand the weathering patterns and reactions of felsic materials on Mars. Full cycle weathering reactions under a hyperarid, periglacial environment were investigated in a dry, isolated depression in the Sur Lipez region of the Bolivian Altiplano (21º25’53.6” S 67º59’08.5” W). Andesitic Quaternary lavas, high sulfur concentrations, high temperature fluctuations, aridity, low ozone concentrations, and high solar radiance dominate the geology and climate of the region making the Altiplano an environmental analogue to early Mars. Preliminary XRD, SEM, and NIR investigations suggest freeze/thaw reactions dominate within the boulders surrounding the basin producing a series of fractures throughout the rocks. Physical degradation of the boulders results in small, highly weathered pebbles around the basin’s edge. Within the pebbles glass is most susceptible to weathering followed by pyroxene > plagioclase laths > plagioclase phenocrysts > Fe-Ti oxides. Evidence of chemical weathering is also present as cavities within the pebbles become lined or filled with alunite from dissolution/ re-precipitation reactions with the aluminosilicates. Visible and near infrared data suggest the < 2 µm fraction collecting in the basin's center is dominated by montmorillonite and hematite. Further results from this study will aid in understanding the weathering processes of siliceous volcanic rocks during early Mars including the chemical trends during alteration and potential clay-forming processes.