| Abstrakt: |
Terra Sirenum, a region of Noachian highlands southwest of the Tharsis volcanic complex, is unique in the number, proximity, and diversity of orbital detections of secondary minerals, as the sole region found to date hosting large‐scale deposits of all of Mars' major salts (chlorides, sulfates, carbonates) as well as diverse hydrated silicates. We combine mineralogical information, high‐resolution imagery, and elevation models to investigate the geologic context of these secondary minerals to understand the sources of water and ions for each type of deposit and their spatial/temporal relationships. Carbonates, where present, are part of Noachian basement rocks exposed through cratering and do not appear associated with evaporative sequences. Numerous small detections of the acid sulfate minerals alunite and jarosite mirror the dominant clay cation in the localities they are found—Al phyllosilicates and Fe phyllosilicates, respectively—suggesting in situ formation. We interpret a previously discovered kaolinite‐rich unit overlying Fe/Mg clays across northeast Terra Sirenum as remnants of a widespread ash unit rather than a pedogenic weathering sequence. Sulfate and chloride detections are decoupled, with sulfates in topographic lows likely precipitated from volcanism‐associated groundwaters, while chloride detections are consistent with surface water runoff, in some instances clearly post‐dating volcanic units capping sulfate detections. Volcanic resurfacing of craters in the region is progressively younger from west to east, and crater statistics‐based ages indicate localized sulfate‐ and chloride‐forming processes continue to occur from ∼3.5 to ∼1.4 Ga. We hypothesize that their decoupling points to disconnected, episodic surface and groundwater reservoirs, perhaps separated by a permafrost layer. Plain Language Summary: A major question is how Mars evolved from a water‐rich planet to a dry planet. We study a region southwest of Mars' largest volcanoes with multiple types of water‐related minerals. We use their detections to infer the properties of the water that deposited them and imagery to understand if/how the minerals are related to each other. Chlorides (e.g., table salt) are found in ponds and channels over a wide elevation range. Sulfates are present only in deep basins with signs of volcanic activity. On Earth, sulfates and chlorides are often together, forming during evaporation of a body of water; however, on Mars, the occurrences seem to be separated, which suggests different water sources and that groundwater with sulfur added from volcanic gases was involved in sulfate formation. The deposits both get progressively younger, as young as ∼2 billion years ago, toward the large volcanic region. Furthermore, aluminum clays on top of iron/magnesium clays in this area have been previously interpreted as weathering sequences, which implies a large volume of water. Instead, we find most aluminum clays are part of a volcanic ash layer deposited on top of older iron/magnesium clays, which needs a smaller amount of water to create the observed layering. Key Points: Thick, layered deposits with Al clays in upper strata thin with distance from Arsia Mons, suggesting volcanic ash parent lithologiesChlorides are decoupled from sulfates in deep basins; sulfates likely precipitated from upwelling groundwater with magmatic volatilesAssociated volcanic units have ages pointing to episodic volcanic springs and surface melt waters into the early Amazonian [ABSTRACT FROM AUTHOR] |
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