CH 4 -clathrates in Clay Minerals and Sulfate Brines: Application to Gale Crater on Mars
Résumé
The Curiosity rover uncovered new evidences of fluvial-deltaic deposits at the Gale crater, suggesting the existence of a long-lasting lake in the past. Additional observation of methane (CH 4 ) emanations has led to the idea that the past water-rich environment was also conducive to the stabilization of CH 4 -clathrates in the subsurface. If we consider that one of the possible sources of CH 4 can come from subsurface clathrates, it is crucial to determine whether other secondary minerals coexisting in the underground, such as clay minerals and/ or salts, affect their stability. Both clay minerals and salts may affect clathrate thermodynamics and kinetics, as they can decrease gas availability through gas adsorption on clay surfaces or salting-out effects. In this work, we experimentally studied the kinetics of formation and dissociation of CH 4 -clathrates in the presence of MgSO 4 brines and two types of clay minerals, i.e., one montmorillonite (2:1 type), and kaolinite (1:1 type), by differential scanning calorimetry. The results showed that the presence of both sulfates and clay minerals at high concentrations can favor the induction time of clathrate formation, but reduce the final amount of clathrates. Both the salting out effect caused by the salts and the water being strongly bonded to the clay surfaces are the main factors that prevent clathrate growth. These results have important implications for evaluating clathrates as a potential source for the CH 4 detections in Gale, since the experiments demonstrate the inhibition of clathrate crystallization within mineral associations found in the crater.
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