Hydrothermal Alteration Constrained by Mineral Assemblages on Mars: Evidence from Craters Near Syrtis Major

Observational evidence for hydrothermal alteration associated with impact craters in the Southern Highlands of Mars is growing [Ehlmann, et al 2008; 2009; Marzo 2008]. Hydrothermal systems are predicted for impacts into volatile-rich targets where fluid flow is typically concentrated along fractures in the walls, central peaks, and beneath the crater floor [e.g. Abramov and Kring, 2005]. The impact process may also excavate pre-existing alteration minerals that were buried at depth, leading to their exposure in rock units comprising the crater central peak, walls, and ejecta [Mustard et al., 2008]. Here we report on the hydrated silicate mineralogy of three impact craters on Syrtis Major that define possible hydrothermal assemblages: Toro Crater (17°N, 71.8°E, 40 km diameter), Schroeter Crater (3°N, 58°E, 64 km diameter), and an unnamed crater (15.5°N, 72.4°E, 50 km diameter). Crater morphology and superposition of ejecta on the Hesperian-aged Syrtis Major lavas shows Toro and Schroeter post-date the emplacement of the lava, while the floor of the unnamed crater is filled by volcanics and thus clearly predates the lavas. Detailed analysis of the overtone and combination tone bands near 1.4 and between 1.9 and 2.5 µm in CRISM near-infrared reflectance spectra leads to identification of a number of hydrated silicate minerals [Ehlmann et al., 2009]. Fe/Mg smectite clay as well as chlorite and prehnite are present in each of the craters, which also exhibit additional mineralogic diversity. Toro crater shows clear evidence for hydrated silica while the unnamed crater and Schroeter show the presence of kaolinite. The central peaks of Toro and Schroeter are exposed and show the most diverse mineralogy, including spectra that are mixtures of numerous alteration minerals. In addition, HiRISE images resolve large breccia blocks and fractures relative to the impact events in these central peaks. Toro shows fractures that are filled with a dark material that also contains small breccia blocks, perhaps emplaced as breccia dikes. The formation of prehnite is constrained to < 3 kbar and 200-350°C. The key question that we are pursuing is does the aqueous alteration pre-date crater formation or is it a consequence of post-impact hydrothermal activity? We will present detailed mapping of lithologic units, their mineral assemblages, and the geologic relationships to assess if the diagnostic mineral assemblages are associated with zones of alteration predicted by hydrothermal models for impact craters. Ehlmann, B. L., et al., 2008. Eos Trans. AGU, 89(53), Fall Meet. Suppl., Abstract P34A-04. Ehlmann, B. L., et al., 2009. (in press) JGR-Planets. Marzo, G. A., et al., 2008. Eos Trans. AGU, 89(53), Fall Meet. Suppl., Abstract P53A-1438. Mustard, J.F. et al., 2008. Nature 454, 305-309.