SULFATES ON MARS: A PYROCLASTIC AIRFALL MODEL FOR THE ORIGIN, EMPLACEMENT AND INITIAL ALTERATION OF VALLES MARINERIS (VM) INTERIOR LAYERED DEPOSITS (ILD)

VM ILDs show: 1) distinctly layered sulfate deposits, 2) monohydrated atop polyhydrated sulfates, 3) interbedded phyllosilicate/sulfates, and 4) occur in kms-thick eroded floor plateaus. Sulfate formation is generally attributed to Meridiani-like groundwater leaching, upwelling & evaporative deposition. Alternatively: Can volcanic eruptions under martian conditions account for VM ILDs? Observations consistent with this include: 1) Tharsis is the most significant volcanism focus on Mars; 2) Explosive plinian basaltic volcanism is favored on Mars, & with >altitude (Tharsis) and <P_atm (Noachian>Hesperian); 3) Finer ash is favored, enhancing dispersal & creating a profusion of condensation nucleation sites for co-erupted H₂O & S; 4) Airfall products consist of tephra coated with condensed H₂O & S species, producing extensive layered/graded deposits; 5) Magmatic sulfur exsolution is favored by lower P_atm, enhanced by higher altitudes (Tharsis); 6) Petrogenesis favors H₂O & S species co-exsolution; 7) Sulfur speciation & atmospheric chemistry favor H₂SO₄ formation & widespread dispersal; 8) Condensation and ensuing H₂SO₄ precipitation is predicted to melt surface snow and ice, & provide acidic aqueous surface environments favoring sulfate precipitation; 9) Eruption durations readily predict thick accumulations; 10) Eruption condition oscillation & S speciation lead to alternate water-rich/sulfur-rich environments, phyllosilicate/sulfate interbedding.

Tharsis region volcanism appear to meet the necessary requirements for ILD sulfate formation, evolution & preservation, including: 1) S sources, 2) liquid water sources & climates conducive to derivation and/or transport of sulfur, 3) acidic environments resulting from sulfur concentration in aqueous solutions, 4) sufficiently warm conditions to collect S-rich waters, and then to evaporate water and concentrate and deposit sulfates, 5) oscillating climate conditions to permit phyllosilicate/sulfate interbedding, 6) environments to explain their concentration in certain locations & settings (VM), and 7) subsequent dry and cold climatic conditions to preserve ancient sulfate deposits to today. We are developing predictive eruption/tephra/volatile dispersal models to compare to the detailed VM ILD characteristics.