Multiple aqueous processes inferred at Eridania Basin, Mars

Eridania is the name given to a topographically enclosed basin located in the southern highlands of Mars that has been suggested to be the site of an ancient inland sea [1,2]. Stratigraphically, we note three major units composing the basin: 1) a basement unit that forms the original floors of the sub-basins; 2) an overlying escarpment-forming unit, known as the Electris unit [3], which lines the slopes of the basin; and 3) a chaos-forming unit, known as Colles, which sits on the topographic lows of the basin. In our present work, we are conducting a survey of CRISM data of Eridania basin in order to better constrain the aqueous history of the basin. Our analysis has identified three major hydrated mineralogies: Fe/Mg phyllosilicates, kaolin-group minerals, and jarosites. The relative distribution and stratigraphic positions of these minerals suggests that they were emplaced by different processes. The Fe/Mg phyllosilicates dominantly exhibit spectra interpreted to be that of a mixed-layer smectite/chlorite. They are typically exposed at depth in erosional windows of the basement and Electris units, and also in outcrops on the blocks that constitute the Colles units. The kaolin-group minerals overlie the Fe/Mg phyllosilicates, and can extend laterally for many km. HiRISE imagery suggests, on the basis of tonality, that these kaolin group minerals are concentrated in fracture-fill. Finally, the jarosites are found in the Electris and Colles units. Although they do not exhibit a clear association with morphology or tone, they appear to drape topography. Fe/Mg smectites and mixed layer Fe/Mg phyllosilicates can readily form from the alteration of basalt in neutral aqueous conditions either by diagenetic alteration or by direct precipitation from solution. On the other hand, kaolinite-group minerals can form under neutral to acidic conditions. However the critical aspect of kaolinite formation is that it requires extensive leaching, either by large quantities of water, or by acidic waters. The lateral extent, the stratigraphic position of the kaolins over the Fe/Mg phyllosilicates, and their appearance in fracture fill suggest a pedogenic origin. Finally, jarosites form exclusively under acidic conditions. The appearance of the jarosite as a draping unit is interpreted to be associated to the interaction of volcanic SO2 and H2O gases with basaltic rocks. Hence, the three aqueous mineralogies observed point to different aqueous processes in the region. However, the relative timing and duration of these processes is still uncertain. [1] Irwin III, R.P et al. (2002) A large paleolake basin at the head of Ma'adim Vallis, Mars. Science 296, 2209 [2] Irwin III, R.P. et al. (2004) Geomorphology of Ma'adim Vallis, Mars, and associated paleolake basins. J. Geophys. Res. 109 E12009, doi:10.1029/2004JE002287 [3] Grant, J.A., et al. (2009) HiRISE views an enigmatic deposit in the Electris region of Mars: Icarus, 10.1016/j.icarus.2009.04.009.