Clays and X-ray amorphous material in fine-grained basaltic sediments, Implications for the weathering history of Gale Crater, Mars.

The Mars Science Laboratory (MSL) rover, Curiosity, has been traversing across fluvial, lacustrine, and aeolian sedimentary rocks since it touched down in 2012. The CheMin instrument on board Curiosity has revealed clay minerals in fluvio-lacustrine samples and abundant X-ray amorphous materials in all samples analyzed to date. In terrestrial environments, these phases can provide a detailed history of environmental conditions (e.g., climate, hydrology, and transport distance) as well as record signatures from sedimentary processes (e.g., weathering, sorting, and mixing). However, the formation and behavior of clay and X-ray amorphous phases in sedimentary environments within basaltic terrains on Earth is limited in comparison to its felsic counterpart. Consequently, there remains an incomplete reference frame to interpret the ancient weathering history recorded in the sedimentary rock record of Mars. This work investigates terrestrial analogs in basaltic terrains of Iceland, ultimately focusing on the fine-grained size fraction (<1 μm) of modern fluvio-lacustrine sedimentary deposits, as this fraction host both clay minerals and X-ray amorphous material. With high resolution microscopy techniques, we observe an intimate mixture of these materials and demonstrate that the composition of the fine-grained component is dependent on both sediment sorting by aeolian and fluvial processes as well as the distance from the source. Unraveling the geochemical and structural changes in these weathering products aims to place constraints on the water-rock interactions operating when rivers and lakes were present in Gale Crater, Mars. Furthermore, these results will provide a foundation for interpreting the history of Jezero Crater, the target of the Mars 2020 rover, which from orbit displays a well-defined delta enriched in clay minerals in the bar deposits.