Mineralogy, geochemistry and occurrences of fougerite in a modern hydrothermal system and its implications for the origin of life
Résumé
Fougerite, the natural green rust, first discovered in soils and universally considered as responsible for the bluegreen colour of gleys and an indicator of reducing conditions, has been recently considered as a key mineral for life's emergence in the alkaline hydrothermal vents theory. It inherits all of the reactive properties of layered double hydroxides in its hydrated interlayer but also the specific reactivity of mixed Fe(II)-Fe(III) compounds, including redox reactivity with metals and metalloids. Along with its structural and compositional analogy with metallo-enzymes, all these properties have stimulated research on the possible role of fougerite as a membrane, and a catalytic engine, especially where gradients of pH, redox potential and temperature favour mixing of chemically contrasted reactants, such as at hydrothermal systems. Although the presence of fougerite, however difficult to detect, has never been reported at alkaline hydrothermal systems, we have thermodynamically evaluated whether the environmental conditions met in such modern oceanic systems are compatible with the formation of fougerite. Data on fluids from the Lost City hydrothermal field (30 • N, Mid Atlantic ridge, Seyfried et al. (2015)) support the reducing nature of this environment, close to the lower limit of stability of water at 90 • C and 80 atm. Calculations show that equilibrium with amakinite, the rare ferrous analogue of brucite, is more likely than equilibrium with brucite. This allows for computing in situ pH values close to 8 and thus mildly alkaline, while pH measured on board on vent fluids at 25 • C is higher than 10. This is in favour of the occurrence of ferrous hydroxide deeper in the root of the hydrothermal system where temperature is higher and pH are lower compared to seafloor vents where the fluids discharge. Secondary oxidation of amakinite, thanks to the recurrent circulation of seawater in the hydrothermal conduits, will necessarily lead to fougerite formation. To deepen this question, several lines of investigation are finally proposed, including e.g., the stability of fougerite at elevated temperatures and pressures, its reactivity with key elements for life such as C, N, P, Mo, Ni, S etc. and its potential role for free energy conversion and basic functions of metabolism.
Origine : Fichiers produits par l'(les) auteur(s)
