A Combined Synthesis and Modelling Approach to Investigate Formation of Carbonates on Mars

Tomkinson, T.; Wright, I. P.; Hagermann, A.; Needham, A. W. and Grady, M. M. (2009). A Combined Synthesis and Modelling Approach to Investigate Formation of Carbonates on Mars. Meteoritics and Planetary Science Supplement, 44(7) p. 5333.

URL: http://adsabs.harvard.edu/abs/2009M&PSA..72.5333T


Introduction: Prior to material being brought back directly from Mars by a sample return mission, Martian meteorites provide the best source for analysis of the fine-scale mineralogy of the Martian surface. The origins of ALH 84001 carbonates are of great importance for understanding the ancient Martian environment. Thought to have formed ~3.9 Ga [1], they are assumed to have precipitated from fluids with neutral to alkaline pH in contact with CO2 [2]. Approximately 0.6 Ga separates primary crystallization of ALH 84001 from formation of secondary mineral assemblages. The period in which the carbonates formed has been called the Phyllosian era, owing to the outcrops of phyllosilicates discovered by the OMEGA and CRISM NIR spectrometers [3, 4]. Carbonates have also been identified by the orbiters, predominantly in the form of magnesite [2]. Their ancient age, abundance and mineralogical variations make ALH 84001 carbonates ideal candidates to provide insights into early Martian environmental conditions.
Modelling: We are attempting to constrain the carbonate precipitation environment by modelling how changes in fluid and atmospheric composition, oxygen fugacity, temperature, etc. change the final precipitation products. We are using the Geochemist WorkbenchTM program to assist with modelling the Mg-Fe-Ca system. A variety of initial concentrations will be combined with CO2 fugacities and temperatures to assess the effect of each variable on the system. It should be possible to model the evolution of the carbonate assemblage as water evaporates, at set P/T conditions (or with “sliding” variables) with some constraints on either Eh/O2 fugacity and partial pressure/fugacity of CO2.
Synthesis: In order to determine the boundary conditions for
precipitation, without straying into kinetically, or thermodynamically, unviable environments, we are also producing synthetic carbonates from fluids of known composition at known temperatures, following on from the precipitation experiments by Golden et al. [6, 7]. The resulting samples will then be characterized by XRD and SEM for compositional analysis, imaging and mapping. We will compare the synthesized carbonates with carbonate rosettes from ALH 84001 to ensure that our derived environments are realistic for the Martian surface. Solution compositions from our synthetic carbonate production experiments, as well as simulated or approximated compositions taken from literature [2, 8] will define the initial starting conditions for the modelling.
Implications: The results from modelling and carbonate synthesis will help reveal the conditions of the period on Mars that is of greatest interest for future missions when the planet may have had a “warm and wet” environment.

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