CHARACTERIZING OLIVINE OUTCROPS AND AQUEOUS ALTERATION AT TYRRHENA TERRA, MARS

The Tyrrhena Terra region on Mars, located between the Isidis and Hellas basins features a variety of phyllosilicates, and is mineralogically distinct from the Libya Montes region bordering the Isidis basin. Aqueous alteration minerals are particularly abundant in the eastern central portion of Tyrrhena Terra and include Fe/Mg-smectites, smectite/chlorite mixtures, chlorites, zeolites, hydrated sulfates, and carbonates. The increase in high-temperature clays, including chlorites for this region, follows a change in olivine chemistry as well. Fo₆₅ olivine (where Fo₁₀₀ is the Mg-endmember forsterite and Fo₀ is the Fe-endmember fayalite) and Fe/Mg-smectite are more prevalent in the Libya Montes/South Isidis region, while Fo₅₀ olivine, chlorite, and smectite/chlorite mixtures are more typical of the central Tyrrhena Terra region. Our study coordinates mineralogy from CRISM, TES, and THEMIS spectral data with geomorphology and stratigraphy from HRSC, CTX, CaSSIS, and HiRISE imagery, additional geochemical data from GRS, and magnetic data from MAVEN in order to understand potential formation and alteration processes occurring at Tyrrhena Terra.

Aqueous alteration of the pyroxene-bearing basement rocks likely produced Fe/Mg-smectites across Tyrrhena Terra during the Noachian, similar to the Noachian Fe/Mg-smectite occurrences in the Libya Montes region. Subsequently, olivine-bearing lavas and/or pyroclastics from Syrtis Major volcanoes likely covered the region about 3.1-3.8 Ga. Numerous small craters across the region have ejected olivine, phyllosilicates, and other alteration materials. Geothermal processes appear to have acted on the central portion of Tyrrhena Terra during the late Noachian or early Hesperian to form high-temperature alteration products including chlorite and zeolite. It is surprising that the high-temperature minerals are observed preferentially in this central region rather than closer to the Isidis or Hellas impacts. Increased geothermal flux and diagenesis are favored causes of the high-temperature alteration materials formed in this region.