Unraveling Geologic Processes and Aqueous Alteration at Tyrrhena Terra, Mars

The central eastern Tyrrhena Terra region of Mars contains abundant aqueously altered minerals including Fe/Mg-smectites, smectite/chlorite mixtures, chlorites, zeolites, and carbonates. This study relates detections of these alteration-generated materials with mafic outcrops, geomorphological units, topography, and broad trends in chemistry and thermal properties to provide a regional view of the geologic history of Tyrrhena Terra. Phyllosilicates forming at elevated temperatures, including chlorites, are significantly more common in central eastern Tyrrhena Terra compared to the Libya Montes region to the north that contains primarily Fe/Mg smectite. Olivine in central Tyrrhena Terra is enriched in Fe compared to olivine outcrops to the north. Olivine in central Tyrrhena Terra is less exposed on the surface compared to nearby regions, likely because the olivine-rich unit is thinner and phyllosilicates were excavated more broadly by impacts. Ejecta containing older, exhumed phyllosilicates largely covers the olivine present at shallower depths. The stratigraphy of mafic and alteration units is evaluated using CRISM mineral detections over 3D HRSC and HiRISE views. Formation of Fe/Mg-rich smectites with some carbonate through alteration of the ancient basaltic basement rock must have occurred prior to the Hellas and Isidis impacts near 4 Ga due to the widespread occurrence of these minerals extending between these large impact basins. According to our model, volcanism from Syrtis Major ~3.6-3.8 Ga or from Hadriaca Patera at ~3.55-3.57 Ga contributed to geothermal processes across the Tyrrhena Terra region. Geothermal flux and diagenesis are favored mechanisms for the formation of chlorite and zeolite under moderately elevated temperatures.