Constraints on the fluid alteration of Nakhla meteorite inferred from Fe-rich phyllosilicate characterization

Abstract

The Nakhla Martian meteorite is known to contain secondary minerals, in particular phyllosilicates, that have recorded the conditions of aqueous alteration of the parent rock. A section of this meteorite was analyzed by transmission electron microscopy to characterize the phyllosilicates in veins and mesostasis. High resolution and electron diffraction, combined with chemical data, suggest the presence of veins in olivine filled by carbonates and hisingerite or hisingerite alone. In the mesostasis, phyllosilicates with composition close to that of ferripyrophyllite were observed in rounded pores within feldspars—these phyllosilicates are associated with areas rich in Si likely due to the presence of amorphous silica. Iron oxides/hydroxides were not found in this study. In addition, for the first time, wadsleyite was observed within the vein margins in olivine. Wadsleyite is evidence of shock metamorphism in Nakhla, whereas the veins result from the decompression after the shock wave passed through due to impact(s). The identification of these secondary minerals constrains the temperature, pH, and redox conditions during the aqueous alteration, underlying that these conditions changed over time. For example, hisingerite forms at T = 120–140°C and ferripyrophyllite at 55–65°C, confirming a progressive temperature decrease when the alteration went forward. The occurrence of these Fe-rich phyllosilicates has also implications on possible past life on Mars: H₂-fueled life cannot survive at T > 122°C; thus, it is incompatible with the formation of hisingerite. Life could have been possible only during the last step of aqueous alteration, that is, when the temperature decreased and ferripyrophyllite formed.