PHYLLOSILICATE MINERALOGY OF ATACAMA DESERT SOILS: INSIGHT INTO ATACAMA SOIL FORMATION PROCESSES

Mineralogical and geochemical data obtained by Mars Express and the Mars Exploration Rovers suggests that soils in modern, hyper-arid environments on Earth (e.g., Atacama Desert) may serve as analogs to Mars soils and provide insight into potential soil formation processes on Mars. The phyllosilicate mineralogy of Atacama basalt soils was examined to assess the role dust inputs and weathering have in hyper-arid soil formation. Three basalt soils (Catalina Summit, Catalina West, and Buenos Aires) were sampled to depths as deep as 2.6 meters. All soils possessed smectite, kaolinite, mica, chlorite, and sepiolite. In the Catalina soils, smectite, kaolinite, mica, and chlorite levels decreased with depth while sepiolite levels increased. In the deepest Catalina horizons, no chlorite or kaolinite were detected. Similarly, smectite, kaolinte, mica and chlorite levels decreased with depth with chlorite absent in the deepest Buenos Aires horizon. Unlike the Catalina soils, sepiolite decreased with depth and was absent in the deepest Buenos Aires horizon. The greater sepiolite concentrations at depth in the Catalina soils suggest that sepiolite could have formed locally by hydrothermal processes associated with the basalt. The decreasing sepiolite concentrations with depth at Buenos Aires indicate that sepiolite was deposited with dust. Playas in the Chilean Altiplano (90 km to the east) are possible sepiolite sources for the Buenos Aires soil. The decreasing levels of chlorite, mica, kaolinite, smectite with depth at all sites indicate the decreasing influence of dust inputs with depth. If smectite and kaolinite are pedogenic, their decreasing levels with depth may reflect the decreasing influence of aqueous activity with depth. However, the hyper-arid nature of the Atacama suggests that kaolinite was formed elsewhere under more humid conditions and deposited by eolian means. Transient precipitation in the Atacama may provide conditions allowing smectite formation from inherited sepiolite. Results of this work indicate that Atacama phyllosilicates have an eolian origin. However, the possibility exists that some smectite may be derived by weathering.