Sulfates on Mars: A Pyroclastic Airfall Model for the Origin and Emplacement of Valles Marineris Interior Layered Deposits (ILD).

Sulfur is transported to the surface and released in  volcanic effusive and explosive eruptions and is known to be concentrated in both time (acidic aqueous alteration environments in Late Noachian-Early Hesperian) and space (e.g., Valles Marineris-type layered deposits). Requirements necessary for formation, evolution and preservation of sulfates are highly specific due to high sulfate solubility and environmental sensitivity of sulfates to phase transitions (temperature and humidity). Can explosive volcanic eruptions under martian conditions help account for the characteristics of sulfate units in the Valles Marineris Interior Layered Deposits (VM-ILD)?

As a basis for understanding the nature of volcanic eruptions in the martian environment (e.g., low gravity, currently low and historically evolving atmospheric pressure) we developed a theoretical and predictive framework for the generation, ascent and eruption of magma. We have: 1) shown that basaltic plinian eruptions are highly favored (relative to Earth), 2) explored the characteristics/dispersal of tephra/gases in various locations and Patm conditions, and 3) assessed the behavior/fate of S species during eruptions including the role of sulfuric acid precipitates in surface melting and creation of aqueous acidic environments.

Observations consistent with volcanic eruptions under martian conditions accounting for characteristics of units in the VM-ILD include: 1) Volcanism is focused in Tharsis; 2) Explosive plinian basaltic volcanism is favored in general, and with increasing altitude (Tharsis) and decreasing Patm (time); 3) Finer ash is produced relative to Earth, enhancing dispersal; 4) Fine ash creates a profusion of nucleation sites for condensation of co-erupted water and S species; 5) Airfall products are tephra coated with condensed water and S species, producing extensive layered/graded deposits; 6) Tephra distribution is latitudinal (equatorial for Tharsis sources); 7) Temperatures of deposited tephra decrease with distance from vent; 8) Magmatic exsolution of sulfur is favored by lower Patm and enhanced by higher altitude eruption sites (Tharsis); 9) Sulfur speciation and atmospheric chemistry predictions favor sulfuric acid formation and widespread dispersal during and immediately following eruptions; 10) Condensation and ensuing precipitation of sulfuric acid is predicted to melt any existing surface snow and ice, and to provide acidic aqueous surface environments favoring sulfate precipitation; 11) Estimates of eruption duration and continuity readily predict km-thick accumulations; 12) Fluctuating eruption conditions and S speciation can lead to interbedding of phyllosilicates and sulfates. 

Explosive volcanism in the Tharsis region appears to meet the necessary requirements for the formation, evolution and preservation of sulfates in the VM-ILD, including: 1) sources of sulfur; 2) sources of liquid water; 3) cold climates; 4) resulting acidic environments (sulfur concentration in aqueous solutions); 5) mechanism to collect S-rich waters and then to evaporate water and concentrate/deposit sulfates; 6) varying climate conditions to permit observed interbedding of phyllosilicates and sulfates; 7) Tharsis environment accounts for concentration in certain locations; and 8) subsequent dry and cold climatic conditions preserve ancient sulfates to the present.  To test this model we are compiling predictive tephra/volatile dispersal stratigraphies to compare to the detailed characteristics/trends observed in the Valles Marineris ILDs.