Geochemical modeling of evaporation processes on Mars: Insight from the sedimentary record at Meridiani Planum
Introduction
The Opportunity rover's analysis of an impure evaporite component present in the martian sedimentary record has revealed an unusual geochemical system. The rich data set returned by the rover during its primary and extended mission phases show that this system is complex, involving periods of evaporation and subsequent diagenesis [1]. The recent characterization of sedimentary rocks several meters thick inside “Endurance” crater, as well as independent spectral and geomorphologic evidence, suggests that evaporites are not limited to the very surface, but occur through a substantial stratigraphic interval over a wide expanse of the martian surface [2], [3]. As a result, fluid evaporation appears to have played a more important role in the martian geologic past than previously thought.
Evaporite mineral assemblages are among the best recorders of paleo-fluid chemistry (see [4]) and an understanding of evaporite geochemistry at Meridiani will place important constraints on the chemistry of ancient aqueous fluids. However, the evaporation of fluids derived exclusively from basaltic weathering is a rare process on Earth and there are few terrestrial environments which can be studied as analogs [5], [6]. Also, because of the complex physical and chemical nature of the sedimentary rocks at Meridiani Planum, the fluid chemistry prior to evaporation is difficult to derive by inverse modeling. An alternative approach is taken here to investigate mineral precipitation during the evaporation of typical basaltic weathering-derived fluids. Data obtained in the laboratory from the weathering of synthetic martian basalt provide a starting point for the investigation of evaporation processes at Meridiani Planum.
Advances in accurately predicting the sequence of mineral precipitation from evaporating solutions allow for an in depth understanding of evaporite geochemistry in many terrestrial environments [7]. The most common approach to modeling evaporative processes is to employ Pitzer's ion interaction equations, which allow for the calculation of mineral solubility in electrolyte solutions of high ionic strength [7], [8], [9]. Based on sound thermodynamic principles and robust datasets, this approach has been successfully applied to a range of geologic problems. The chemical and mineralogical characteristics of the outcrop observed at Meridiani, however, show that adequate modeling of this geochemical system requires that additional components be added to current thermodynamic models, namely Fe2+ and Fe3+. Both Fe2+ and Fe3+ must be added because Fe-sulfates appear to have played an important role in evaporite geochemistry on Mars, and Fe redox disequilibrium is the rule, not the exception in many aqueous environments on Earth [10]. Consequently, some degree of disequilibrium between Fe(II) (a roman numeral will be used to denote all possible aqueous species) and atmospheric oxygen is expected in the Meridiani geochemical system, especially under acidic conditions where Fe oxidation is sluggish. While the available data for adding Fe2+ into such models have improved in recent years [11], [12], the expansion of datasets to include Fe3+ remains problematic, because few experimental data are available to derive various input parameters and test resulting models. The goals of this paper, therefore, are to: (1) develop a thermodynamic dataset suitable for modeling evaporation in the Meridiani system and (2) apply the modeling code to experimental fluid data obtained from weathering synthetic martian basalt. The modeling results provide useful comparisons between what can be expected to form in a closed chemical system upon evaporation and what is observed at the Meridiani Planum landing site. The stability of the resulting evaporite assemblages in contact with later fluids is also modeled, testing hypotheses related to Meridiani diagenesis.
Section snippets
Meridiani Planum: a unique geochemical system
Much of the data returned from the Meridiani landing site relates to the characterization of an impure evaporite unit, with analyses extending from “Eagle” crater (the initial landing site) approximately 750 m to “Endurance” crater and beyond [1]. Available data show that Meridiani outcrop contains four main components: (1) a siliciclastic component, possibly representative of basaltic material and/or other siliceous alteration phases, (2) hematite concentrated in nodules, (3) a phase or phases
Thermodynamic modeling: background and approach
Calculations of mineral solubility based on thermodynamic data can provide important constraints on geochemical systems of interest. The solubility of a specific mineral requires the knowledge of its equilibrium constant (expressed here as log K), which describes the activities of its components at equilibrium with a given fluid. Component activities, effectively thermodynamic concentrations, require accurate calculation of activity coefficients, which correct for non-ideal behavior of
Thermodynamic modeling: assessment of accuracy and limitations
Before applying the model to our input database of solutions, it is necessary to assess the accuracy of the calculated component activities in high ionic strength fluids. To do this, we have calculated the equilibrium state for complete water analyses from the Genna Luas mine site in Sardinia, Italy. Solution analyses were obtained from F. Frau, who collected and analyzed the samples and provided unpublished data accompanying a study done in 2000 [43]. These solutions were chosen because they
Input data
In modeling evaporation processes at Meridiani Planum, we use the solution analysis dataset reported by Tosca et al. [51]. In their study, Tosca et al. [51] synthesized two crystalline basalts of martian composition and two pure basaltic glass compositions which were all reacted with a variety of acid mixtures (sulfuric to hydrochloric in a 4 : 1 molar ratio) for a total of 14 days each at 25 °C. The solution database obtained during that study provides a range of initial solution compositions
Constraints on evaporite geochemistry at Meridiani Planum
Because acidic weathering fluids derived from martian basalt are typically rich in Mg, Fe, Ca, SiO2 and SO4, these components will comprise the majority of the evaporite mineralogy present at the martian surface. The behavior of Fe in evaporitic and diagenetic settings must therefore be well understood. The results discussed above result from the first application of modeling both Fe2+ and Fe3+ as discrete evaporitic components related to Mars. The application of the model to unique fluids
Acknowledgements
We would like to thank the MER science and engineering teams for making this experience a truly unforgettable one. It has been a privilege and a pleasure to work with so many extraordinary individuals. NJT would like to thank Andrew Felmy, Richard Reeder and Martin Schoonen for helpful discussions and to Franco Frau for kindly granting use of his unpublished data. The authors would also like to thank D. Kirk Nordstrom, Giles M. Marion and an anonymous reviewer for thorough and insightful
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