Alteration of the Martian Surface Through Time: A Reaction Path Modeling Approach

Details Alteration of the Martian Surface Through Time: A Reaction Path Modeling Approach Alteration of the Martian Surface Through Time: A Reaction Path Modeling Approach

Sep 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30q.565r&link_type=abstract

Meteoritics, vol. 30, no. 5, page 565

Mathematics

Logic

1

Atmospheres, Basalt, Chassignite, Mars, Minerals, Phase Equilibria, Planets

Scientific paper

We have performed preliminary geochemical reaction path calculations [1] that attempt to model reasonable martian geologic processes under various climatic conditions postulated for Mars during its planetary evolution [2]. Weathering and fluid evaporation and freezing are discussed here. Under current Earth conditions, 25 degrees C rainwater in equilibrium with the atmosphere has a calculated pH=5.66, dissolved O2=2.5x10^-4 mole/kg, and dissolved CO2=1.3x10^-5 mole/kg. In contrast, martian rainwater in a postulated early, warm and wet climate would have lower pH,, higher dissolved CO2, and lower dissolved oxygen. For instance, in a martian atmosphere with pCO2=2.2 bar [3] and pO2=10^-5 bar, rainwater is calculated to have pH=3.96, dissolved CO2 of 7.5x10^-2 mole/kg, and dissolved O2=1.3x10^-8 mole/kg. If, under these climatic conditions, there was also extensive basaltic volcanism, then rainwater would be further modified chemically by solution of volcanic gases to form acid rain. For example, we can model this process using a Kilauea gas composition, in mole percent (H2O=57.8; HCl=0.9; SO2=17.4; CO2=21.6; H2S=0.2; H2=1.0), and assume the resultant fluid is buffered by atmospheric CO2. Addition of only 0.1 gram of gas to 1 kgram of ancient rainwater reduces the pH to 3.07 and dramatically increases dissolved SO4- (3.8x10^-4 mole/kg), H2S[aq] (1.3x10^-4 mole/kg) and Cl- (4.4x10^-5 mole/kg). Weathering minerals formed by reaction of the above fluid types with martian basalts can vary significantly depending upon initial fluid acidity and solute content. Acid rain and "normal" rain (buffered under high pCO2 conditions) initially alter martian basalt (modeled as the Chassigny parent composition [4]) to, in order of decreasing abundance: Fe-smectite (+/- ferrihydrite), Mn hydroxide, kaolinite, apatite, chalcedony at pH < 5. As pH increase to > 5 then dolomite, calcite, K-feldspar, and dawsonite also precipitate. An important aspect of this modeling is that acid rain can produce larger volumes of alteration minerals per unit of fluid, and also become greatly enriched in a variety of anion and cation solutes. Reaction of less acidic rainwater (as expected under conditions of low atmospheric pCO2) with basalt is predicted to form mixtures of ferric hydroxides, apatite, carbonates, and smectites. Reduced iron minerals, e.g., chlorite, do not form under any but the most reducing conditions. The paragenesis of minerals precipitated through evaporation of alteration fluids obviously is very sensitive to the evolution of the fluid composition as fluid-rock alteration reactions proceed. For example, water acidified by volcanic SO2 prior to basalt reaction produces large amounts of gypsum as the dominant sulfate mineral with very small degrees of evaporation (< 0.1% H2O removed). Rainwater which obtains sulfate solely through dissolution of basalt precipitates gypsum only after extensive evaporation (> 95% H2O removed). Other minerals predicted to form through evaporative loss of water, include: dolomite, calcite, potassium and sodium feldspars, dawsonite, and apatite. Evaporation of acid rain at sub-zero temperatures (as expected in the present martian climate where atmospheric pressure and composition preclude equilibrium with liquid water) after 95% water loss produces, in decreasing order of abundance: chalcedony, dolomite, Ca-nontronite, dawsonite, kaolinite, K-feldspar, pyrolusite, hydroxyapatite, calcite, gypsum, and chlorapatite. Under conditions of gradually decreasing atmospheric climate, as expected as the early greenhouse effect dissipated, then groundwater would gradually cool to sub-zero temperatures. Cooling of ancient martian rainwater which has reacted with basalt is predicted to form water ice, chalcedony, arcanite, dolomite, and a variety of Mg- silicates. Epsomite might also form near the eutectic. However, at present our models do not account for the possible stability of gas hydrates. References: [1] Spycher N. F. and Reed M. H. (1992) University of Oregon, 64. [2] Clifford S. M. (1993) JGR, 98, 10973-11016. [3] Haberle R. M. et al. (1994) Icarus, 109, 102-120. [4] Longhi J. and Pan V. (1989) Am. Mineral., 76, 785-800.

Affiliated with

Also associated with

No associations

Rating

Alteration of the Martian Surface Through Time: A Reaction Path Modeling Approach does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.

If you have personal experience with Alteration of the Martian Surface Through Time: A Reaction Path Modeling Approach, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Alteration of the Martian Surface Through Time: A Reaction Path Modeling Approach will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFWR-SCP-O-830255