Aluminum Phyllosilicate and Jarosite Formation Through Alteration of Reworked Al/Si‐Rich Volcaniclastic Sediments in Nili Fossae, Mars

Abstract

Aluminum phyllosilicates occur in 1–10s of km ² exposures of Noachian (∼3.7 Ga) rock across Mars and typically appear stratigraphically above Fe/Mg‐phyllosilicates. These units have been proposed to represent pedogenic basalt weathering sequences. We examine Al‐phyllosilicates in the Nili Fossae region, combining mineral maps generated from imaging spectroscopy with digital elevation models and high‐resolution imagery to determine mineral assemblages, their geologic relationships, and their history of formation. We find that the Al‐phyllosilicate is dominantly kaolinite and lacks associated Fe oxide. The Al‐phyllosilicates typically have a distinct texture relative to Fe/Mg‐phyllosilicate units, indicating an unconformity, and two different protoliths rather than formation as a pedogenic weathering sequence. Layers in some kaolinite deposits and their preferential association with sedimentary basins suggest reworked sedimentary material. The most plausible source is an Al/Si‐enriched (non‐basaltic) airfall volcanic deposit. We detect sparse jarosite closely associated with kaolinite but not Fe/Mg‐phyllosilicate. We interpret that the jarosite, which is metastable under present surface conditions, formed from reaction between Fe‐enriched fluids and S that is found sufficiently only within the kaolinite‐bearing protolith. Collectively, these observations suggest less intense near‐surface oxidative weathering than the basalt pedogenesis hypothesis, namely, an upper bound of several million cumulative years of aqueous activity in Nili Fossae that largely ended after the formation of jarosite. Detection of Al‐phyllosilicate‐bearing float rocks in nearby Jezero Crater by the Perseverance Rover shows that sample return has potential to conclusively determine processes and environmental conditions forming Nili Fossae's distinctive Al‐phyllosilicates.