Formation and destruction of clay minerals in Glen Torridon, Gale crater, Mars

International audienceFrom 2019 to 2021, the Curiosity rover explored the Glen Torridon (GT) region, a topographic troughlocated on the slope of Aeolis Mons, the central mound of Gale crater. GT corresponds to what was previously referred toas the “clay-bearing unit”, due to the detection of smectite signatures by orbital spectro-imagers. One of the objectives ofthis campaign was to better understand the context of formation of the clay minerals, as well as the overall alterationhistory of GT.Here, we used the ChemCam instrument to survey the major-element composition of the bedrock throughout the region.Overall, the GT bedrock is characterized by elevated values of the Chemical Index of Alteration (CIA), which indicatesthat the clay minerals were formed in a context of open-system weathering, i.e., through the interaction of the sediments(or their source rocks) with dilute meteoric fluids at relatively high water-to-rock ratio. We also observed geochemicalvariations correlated with outcrop expression – for example, in the northern part of GT, the spatially dominant type ofbedrock has a distinctive “rubbly” appearance and displays enrichments in both K2O and SiO2, which are best explainedby the presence of illite or a mixed-layer illite/smectite phase.Finally, in the southern part GT, in an area of lighter-toned rocks named the Hutton interval, we found significantly lowerCIA values that are correlated with reduced abundances of clay minerals. We interpret these “anomalous” compositionsas a local diagenetic overprint, and propose that the clay minerals of the Hutton interval have been partially destroyed bythe circulation of concentrated fluids late in the history of Gale