Sulfates and Other Hydrated Minerals in Ius Chasma, Valles Marineris and Implications for Water Geochemistry

Ius Chasma is a linear trough in western Valles Marineris containing horst and graben structures, multiple landslides, and light-toned floor deposits. Elsewhere is Valles Marineris, sulfate deposits identified by OMEGA and CRISM are restricted to Interior Layered Deposits or nearby autochthonous loose material (Gendrin et al., 2005; Murchie et al., 2007; Murchie et al., in revision). In Ius Chasma, however, sulfates are found in thin floor units in enclosed depressions at the lowest elevations. Kieserite is at the lowest elevation, with polyhydrated sulfate and an unidentified hydrated phase, at higher elevations. Some poorly defined layering is visible on the HiRISE scale, but is not diagnostic. The total exposed thickness of kieserite is 300 m, with a total range in elevation of -4175 to -4475 m. The kieserite deposit is covered in places by pyroxene-bearing dunes. The polyhydrated sulfate outcrops range from -3515 to -4000 m. The unidentified hydrated phase outcrops between -3770 to -4100 m. The polyhydrated sulfate and unidentified hydrated material occur at the same elevations and do not have a clear stratigraphic relationship. Elsewhere in Ius Chasma, the unidentified hydrated material clearly drapes chasma floor units. The unidentified hydrated material is characterized by absorptions near 1.4 and 1.9 μm, and a narrow doublet at 2.21 and 2.27 μm. The deep ~1.9 μm is due to the combination tone of the OH stretch and H2O bend and the ~1.4 μm absorption is due to the 1st overtone of the OH stretch. Sulfates or other minerals with 2 or more water molecules per unit cell in their structure are necessary to account for deep water and hydroxyl absorptions in many spectra we observe. The doublet doesn't match any known sulfate, phyllosilicate, chloride, hydrated silica library spectra. Possibly a mixture of hydrated phases could explain this phase. The 2.21-2.26 μm region is generally convex in sulfates, but gypsum (CaSO4 2H2O ) and jarosite group members (MFe3(SO4)2(OH)6) have absorptions there. However, neither sulfates' doublet matches the unidentified spectrum's minima. While there is no phyllosilicate that exactly matches the 2.21 and 2.27 μm doublet, a combination of smectite clays, such as nontronite and montmorillonite, produces a similar doublet at 2.21 and 2.29 μm. A third option that is spectrally close but not exact is hydrated silica. Hydrated silica has a wider absorption over 2.21- 2.25 μm and its hydration bands are shifted to ~1.38 and ~1.91 μm (Milliken et al., 2008). The location of kieserite in the lowest points of Ius Chasma, with polyhydrated sulfate exclusively found at higher elevations, suggests they were formed by nearly complete evaporation of a closed basin. Polyhydrated evaporites form first and then kieserite as the basin is drawn down. Originally, Ius Chasma may have been less connected to Melas Chasma than it is now (Peulvast and Masson, 1993). These sulfates may be evidence of an environment that supported isolated, evaporating basins. Better discrimination of the sulfate assemblages present and the stratigraphic relationships within the ILD is critical to understanding the environment during and since their formation.