The Role of Reducing and Acidic Hydrothermal Fluids in Forming Chloride Deposits in Terra Sirenum, Mars

Orbital remote sensing has shown that some regions of the ancient Martian crust contain hundreds of discrete terrains covered by chloride-rich evaporites. In terrestrial evaporitic systems, evaporite sequences typically begin with the deposition of carbonates, followed by sulfates, and finally chlorides, a depositional sequence that has not yet been found on Mars. Instead, sulfate deposits are always separated spatially and temporally from chlorides, suggesting two different depositional regimes. Here, we present a model driven by the Martian chlorine geochemical cycle that allows the formation of chlorides whilst simultaneously inhibiting sulfate and carbonate precipitation. In this model, the chlorides are produced under reducing and acidic conditions. Chloride deposition was driven by hydrothermal alteration of the Martian crust associated with faults, followed by precipitation from ascending saline solutions along the tectonic conduits. These processes occurred under a relatively thick and reducing atmosphere (1–0.1 bar). The crustal circulation of chloride-precipitating fluids may have been driven by tectonic suction and pumping processes. Parental brines from hydrothermal activity sourcing chloride might also have contributed to the sulfates found in Cross and Columbus craters of Terra Sirenum. Our study integrates orbital imaging, topography, and spectroscopy with geochemical modeling and terrestrial analogs. We propose that the Terra Sirenum chloride deposits derive from subsurface brines, with deposition driven using tectonic and hydrothermal processes. Under inferred reducing and anoxic conditions, chloride formed with minimal co-precipitation of sulfates and carbonates. Unlike isolated chloride deposits confined to topographic lows, the Terra Sirenum chlorides are associated with linear features interpreted as faults.