Physics
Scientific paper
Dec 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufm.v34a..07t&link_type=abstract
American Geophysical Union, Fall Meeting 2004, abstract #V34A-07
Physics
5415 Erosion And Weathering, 5470 Surface Materials And Properties, 3672 Planetary Mineralogy And Petrology (5410), 1045 Low-Temperature Geochemistry
Scientific paper
The Opportunity rover's analysis of an impure evaporite component present in the Martian sedimentary record reveals a unique geochemical system. The evaporation of basaltic weathering fluids is a process which is rare on Earth, but is likely to have played a major role in the formation of sedimentary rocks at Meridiani Planum. Adequately modeling the evaporation processes in this system must involve adding additional components to current thermodynamic models, namely Fe(II) and Fe(III). The goals of this study are to: (1) develop a thermodynamic database suitable for modeling evaporation of basaltic weathering fluids in the Meridiani system and (2) to apply the model to experimental fluid data obtained in our laboratory from weathering synthetic Martian basalt, which will allow for the testing of hypotheses related to the geochemical evolution of the Meridiani site. The evaporation of these fluids is simulated using an expanded version of the Harvie-Moller-Weare model which employs Pitzer's ion interaction approach in calculating activity coefficients in high ionic strength solutions. This model has been expanded using recent data to include Fe(II) and Fe(III). Although a full set of experimentally-derived data allowing the inclusion of Fe(III) into such models is not yet available, an adequate set of interaction parameters was built, based on viable assumptions and substitutions using analog data (e.g., Al3+, Ga3+, Cr3+). The accuracy of the thermodynamic model in predicting Fe(II) and Fe(III) activities in a multi-component system can be assessed. This is accomplished by comparing calculated Eh values (proportional to aFe2+/aFe3+) to those measured in the field from high ionic strength acid mine waters containing all of the relevant components of the model. The agreement between calculated and observed values suggests that the model calculations are adequate for reaction path calculations. New thermodynamic data for several Fe(II) and/or Fe(III) containing minerals, including a variety of sulfates have also been incorporated. The resulting model is not only relevant to Mars, but acid mine drainage environments as well. The results of the calculations place constraints on the chemical controls of the evaporation system. For example, using fluids derived from a synthetic olivine-bearing Martian basalt, we predict gypsum (or anhydrite), jarosite, melanterite and hydrated Mg-sulfate as major phases produced upon evaporation. Jarosite has been identified by Moessbauer spectroscopy at Meridiani and Mg-sulfate is a likely outcrop component based on geochemical systematics. The redox conditions are unconstrained in this system and the formation pathways of Fe-containing minerals such as jarosite and hematite remain an open question. However, the inclusion of Fe(II) and Fe(III) in the model allows redox conditions to be systematically varied for any calculation. The stability of evaporite assemblages in contact with later fluids can also be modeled, testing hypotheses related to diagenesis. This may shed light on the origin of possible diagenetic features within the outcrop such as hematitic concretions and vugs that have been interpreted to be crystal moulds. Possible diagenetic reactions may have occurred as a result of groundwater recharge into previously deposited sedimentary layers.
McLennan Scott M.
Tosca Nicholas J.
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