Sulfates

Sulfates form primarily in thick beds by evaporation of sea water. Hydrothermal or diagenetic alteration by acidic groundwater on rocks can also form sulfates. From Earth, we know the maximum of evaporite deposition took place in the Permian and Triassic, when the supercontinent Pangea was assembled and started to break up [Einsele, 2000]. There the rifting and early drifting of proto-oceans with extended arid climate, restricted rapidly subsiding basins caused thick evaporite sequences. Further peaks in salt deposition are Early Cambrian (e.g. Siberian Plateau), Devonian, Permian (e.g. European Zechstein basin) Late Jurassic to Early Cretaceous (e.g. below the Gulf of Mexico), and Miocene (Messinian event; e.g. Red Sea) of the Mediterranean.

On Mars, sulfates were found on Interior Layered Deposits (Fig. 1) in Valles Marineris and chaotic terrains [e.g. Gendrin et al., 2005].

In the saliniferous-formation-cycle carbonates (calcite, dolomite) form first. Then sulphates (anhydrite, gypsum) and at last easy soluble sodium-, potassic- and magnesia salts (halite, sylvite, and carnallite) are formed.

For the formation of evaporites essential conditions are needed [Duff, 1993]:

• a hot and arid climate to ensure the evaporation necessary to maintain a sufficiently high concentration for salts to crystallise out
• an intermittent or continuous supply of salt mainly from seawater flowing into the basin through one-way channels
• subsidence of the basin floor to make room for the growing thickness of salt deposits
• maintenance of the barrier between the subsiding basin and the ocean, possibly by volcanic activity, upward growth of coral reefs, or earth movements
• a final depression, carrying the barrier with it, to make room for the thick and widespread sediments that eventually covered and preserved the salt deposits